Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel

Xiaobing Pang; Jiahui Dai; Mingjun Zhang; Yan Zhang

doi:10.3390/photonics8090359

Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel

Pang, Xiaobing;Dai, Jiahui;Zhang, Mingjun;Zhang, Yan 2021-08-28 00:00:00 hv photonics Article Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel 1 , 2 2 3 Xiaobing Pang *, Jiahui Dai , Mingjun Zhang and Yan Zhang College of Mechanical & Electrical Engineering, Changsha University, Changsha 410022, China Key Laboratory of High Performance Intelligent Manufacturing of Mechanical Equipment of Hunan Province, Changsha University of Science & Technology, Changsha 410114, China; daijiahui2018@163.com (J.D.); mj_zhang@csust.edu.cn (M.Z.) Hunan Institute of Science and Technology, College of Mechanical Engineering, Yueyang 414006, China; 12017032@hnist.edu.cn * Correspondence: pangxiaobing55@ccsu.edu.cn; Tel./Fax: +86-731-8426-1492 Abstract: The application bottleneck of laser welding is being gradually highlighted due to a high prevalence of porosity. Although laser welding technology has been well applied in ﬁelds such as vehicle body manufacturing, the suppression of weld porosity in the laser welding of stainless steel containers in the pharmaceutical industry is still challenging. The suppression of bottom porosity was investigated by applying ultrasonic vibration, changing welding positions and optimizing shielding gas in this paper. The results indicate that bottom porosities can be suppressed through application of ultrasonic vibration at an appropriate power. The keyhole in ultrasound-assisted laser welding is easier to penetrate, with better stability. No obvious bulge at the keyhole rear wall is found in vertical down welding, and the keyhole is much more stable than that in ﬂat welding, thus eliminating bottom porosity. The top and bottom shielding gases achieve the minimal total porosities, without bottom porosity. Keywords: ﬁber-laser welding; porosity; suppression; stainless steel Citation: Pang, X.; Dai, J.; Zhang, M.; Zhang, Y. Suppression of Bottom Porosity in Fiber Laser Butt Welding 1. Introduction of Stainless Steel. Photonics 2021, 8, 359. https://doi.org/10.3390/ 304 austenitic stainless steel, due to its good corrosion resistance and mechanical photonics8090359 properties, has been widely used in ﬁelds including pressure vessels and nuclear power equipment [1]. Argon tungsten arc welding (TIG) is currently dominant in manufacturing Received: 8 June 2021 plates of isolators and freeze-dryers in the pharmaceutical ﬁeld. However, this traditional Accepted: 26 August 2021 arc welding method is associated with some disadvantages such as limited penetration Published: 28 August 2021 ability, low welding efﬁciency and poor controllability of heat input [2]. Laser welding is expected to be feasible for welding stainless-steel containers in the pharmaceutical Publisher’s Note: MDPI stays neutral ﬁeld due to its high energy density, high welding speed and small deformation after with regard to jurisdictional claims in welding. However, it is susceptible to porosities, especially the bottom porosities as a published maps and institutional afﬁl- result of the instability of the deep-penetration keyhole, which affects the sealing and anti- iations. pollution ability of the weld seam and restricts its application in welding pharmaceutical containers [3]. Therefore, it is of great signiﬁcance to suppress bottom porosities in laser welding of stainless steel for the application of laser welding technology in pharmaceutical container manufacturing. Copyright: © 2021 by the authors. Considering porosity as a common defect in a laser-welded seam and the lack of Licensee MDPI, Basel, Switzerland. systematic control theoretical basis, laser welding technology is gradually reaching its This article is an open access article application bottleneck [4,5]. Process-induced porosity is the hotspot in research on the distributed under the terms and porosity of laser welding, mainly including keyhole-induced porosity [6,7] and ﬂuid-ﬂow- conditions of the Creative Commons induced porosity [8,9]. Lin et al. [6] found in a numerical simulation that the formation Attribution (CC BY) license (https:// of bubbles in the weld pool of remote laser welding of aluminum alloy depends on the creativecommons.org/licenses/by/ dynamic behaviors of the keyhole and weld pool. Speciﬁcally, the collapse of the keyhole 4.0/). Photonics 2021, 8, 359. https://doi.org/10.3390/photonics8090359 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 359 2 of 11 caused by an intense melt ﬂow and the eddy along the weld pool behind the keyhole are the main contributors to the formation of bubbles. Based on a high-speed photography analysis during the weld test of glass and metal composite specimens, Xu et al. [7] argued that the sharp ﬂuctuation of the keyhole may explain to a great extent the formation and assemblage of bubbles, which resulted in large porosities. Zhang et al. [10] compared the porosity defects of non-penetration and full-penetration laser welding by numerical simulation combined with intuitive observation and revealed that full-penetration laser welding greatly improved the porosity defects and that the formation mechanism of porosity in full-penetration laser welding was consistent with the “hump theory” [11]. Meng et al. [8] studied the formation mechanism of porosity at the T-joint of laser lap welding and proposed that the change of dynamic behaviors of the weld pool is the main reason for the formation of porosities at the lap gap, while the instability of the keyhole is insigniﬁcant in laser lap welding. Panwisawas et al. [9] showed that process-induced porosity is related to plate thickness, laser power and welding speed in laser welding of titanium alloy plates. Especially, the porosity increases with the increase of plate thickness. Meanwhile, it is pointed out that unstable ﬂow and/or porosity escape time relative to local solidiﬁcation time is the key to porosity formation. Numerous research studies have been conducted on the suppression of porosity in laser welding, with many methods proposed, such as pulsed laser welding [12], laser-beam oscillation scanning welding [13,14], ultrasound-assisted laser welding [15,16], changing welding position [17,18] and optimizing shielding gas [19,20]. Shen et al. [12] examined the effects of laser pulse parameters on porosity and ﬂow characteristics of the weld pool and exhibited a negative correlation between pulse frequency and porosity at the joint and the suppression mechanism of porosity in pulsed laser welding. That is, pulsed laser welding results in good stability of the keyhole and limits bubble formation by preventing shielding gas from being involved in the keyhole. Meanwhile, the intermittent impact effect of pulsed laser accelerates the escape of bubbles by stirring the weld pool, thus further reducing porosities. Zhou et al. [13] discovered by utilization of the beam oscillation scanning method in laser welding of aluminum alloy that circular scan is effective in eliminating porosities. Furthermore, improvement in the stability of the keyhole was established as the main reason for the elimination of porosities by recording the ﬂuctuation degree of the entrance of the keyhole during laser beam scanning. Cai et al. [14] explored the swing laser-MAG hybrid welding of carbon steel and pointed out that the swing laser changes the root shape of the hybrid weld seam, facilitating elimination of root porosity. Kim et al. [15] ﬁrst proposed the use of ultrasonic vibration on the bottom of the specimen during laser welding. It was demonstrated that cracks and porosities in the weld seam are greatly suppressed by the ultrasonic vibration due to cavitation effects in the molten pool during ultrasound-assisted laser welding. Lei et al. [16] suggested that the porosity at the weld seam is greatly improved by the cavitation and acoustic streaming of the ultrasonic vibration on the weld pool in ultrasound-assisted laser welding of AZ31B magnesium alloy. Grajczak et al. [21] found that porosity can be eliminated when the weld pool is located in the antinode position during ultrasound-assisted laser welding of nickel-based-alloy round bars. He and Shen [17] analyzed the effects of different welding positions on porosity in laser welding of aluminum alloys and observed the minimal porosities in vertical down welding, with high stability of the keyhole. Miao et al. [18] noted that the number of porosities in vertical welding is smaller than that in ﬂat welding. He and Shen [19] proved that porosity is signiﬁcantly decreased by the addition of side-blowing gas ﬂow in laser welding of aluminum alloy and that the ﬂow rate has a great inﬂuence on porosity. Sun et al. [20] determined that when nitrogen (N2) is used as shielding gas in laser welding of 304 stainless steel, porosity is suppressed and is mainly seen at the bottom of the weld seam. The solubility of N2 in the liquid weld pool contributes to reducing the porosity in laser welding of 304 stainless steel. In conclusion, abundant studies focus on porosity defects in the laser welding process, and the formation mechanism and suppression methods of porosity have been extensively Photonics 2021, 8, 359 3 of 11 Photonics 2021, 8, 359 3 of 11 In conclusion, abundant studies focus on porosity defects in the laser welding pro- cess, and the formation mechanism and suppression methods of porosity have been ex- tensively validated. However, there are few reports on bottom porosity. The suppression validated. However, there are few reports on bottom porosity. The suppression of bottom of bottom porosity was investigated by applying ultrasonic vibration, changing welding porosity was investigated by applying ultrasonic vibration, changing welding positions positions and optimizing shielding gas during laser welding of 304 stainless steel in this and optimizing shielding gas during laser welding of 304 stainless steel in this paper, and paper, and the suppression mechanisms of porosity caused by ultrasonic vibration, weld- the suppression mechanisms of porosity caused by ultrasonic vibration, welding position ing position and shielding gas were revealed by observation of the dynamic laser welding and shielding gas were revealed by observation of the dynamic laser welding process with process with high-speed photography. high-speed photography. 2. Experimental Procedure 2. Experimental Procedure The experimental setup was as illustrated in Figure 1. With a continuous-wave The experimental setup was as illustrated in Figure 1. With a continuous-wave fiber ﬁber laser (YLS-3000-CL) as the laser source, the laser beam emitted from the end of the laser (YLS-3000-CL) as the laser source, the laser beam emitted from the end of the opera- operation ﬁber was collimated and then focused with the aid of a laser welding head tion fiber was collimated and then focused with the aid of a laser welding head (FLW- (FLW-D30), while for the laser beam radiated from the end of the optical ﬁber, collimation D30), while for the laser beam radiated from the end of the optical fiber, collimation was was conducted by a lens of a 100 mm focal length, and focusing on the specimen surface conducted by a lens of a 100 mm focal length, and focusing on the specimen surface was was completed by a focusing unit of a 150 mm focal length. After focusing, the laser beam completed by a focusing unit of a 150 mm focal length. After focusing, the laser beam had had a spot size of about 0.3 mm. The ultrasonic power supply (CSHJ-1000) that has an a spot size of about 0.3 mm. The ultrasonic power supply (CSHJ-1000) that has an ultra- ultrasonic frequency of 20 kHz was applied to provide the power output of 1000 W with a sonic frequency of 20 kHz was applied to provide the power output of 1000 W with a fixed ﬁxed amplitude of 6 m. The ultrasonic amplitude transformer was placed in a sink on amplitude of 6 μm. The ultrasonic amplitude transformer was placed in a sink on the the working platform. The conﬁguration of the bead-on-plate welding with ultrasound is working platform. The configuration of the bead-on-plate welding with ultrasound is il- illustrated in Figure 1c. lustrated in Figure 1c. Figure 1. Experimental setup: (a) on-site layout, (b) schematic diagram of welding with “sandwich” specimen and (c) schematic diagram of bead-on-plate welding with ultrasound. Photonics 2021, 8, 359 4 of 11 The welding materials used were 304 stainless steel plates of 3 mm in thickness. Table 1 overviews the substrate with respect to its chemical composition. The modiﬁed sandwich sample comprises, as displayed in Figure 1b, one sheet of stainless steel and one sheet of GG17 glass, both with a size of 50 mm 3 mm 3 mm [22]. The processing parameters for the laser welding experiments are presented in Table 2. Table 1. Chemical composition of the 304 stainless steel studied. Element C Cr Mn Ni Si P S Fe (Wt.%) 0.039 18.280 1.420 8.150 0.410 0.036 0.015 Bal. Table 2. Parameters used in the welding experiments. Parameters Value Laser power (p ) (W) 2000 laser Welding speed (v) (m/min) 1.2 Defocus (D) (mm) +3 Ultrasonic frequency (kHz) 20 Ultrasonic power (p ) (W) 0, 250, 500, 750, 1000 ultrasonic Ultrasonic amplitude (m) 6 Flat position, Vertical–down position, Welding position Vertical–up position, Horizontal position Shielding gas type N Top shielding gas ﬂow rate (q ) (L/min) 0, 15, 20, 25 top Bottom shielding gas ﬂow rate (q ) (L/min) 0, 15, 20, 25 bottom The welding zone was illuminated by a diode laser (808 nm) with a maximum power of 30 W for general observation of the weld pool, and for selective observation, a ﬁlter was additionally added to the camera lens. To visualize the weld pool and the vapor plume during the experiments, a bandpass ﬁlter with a transmission band of 808 3 nm and a ﬁlter with a transmission band from 350 nm to 650 nm were placed in front of the camera lens, respectively. An X-ray real-time imaging system (XYD-225) was used to detect the pores in the welding test piece upon completion of welding. After being cut by electro-discharge machining (EDM), the longitudinal sections of the welded joints were polished with abrasive paper and polishing cloth. The ﬁnished cross sections of the welded joints were observed under an optical microscope (Leica S9i) upon etching by a solution of aqua regia (HCl:HNO = 3:1) for 15 s. 3. Results and Discussion 3.1. Effect of Ultrasonic Vibration on Bottom Porosity The X-ray nondestructive inspection and longitudinal sectional view of weld seams at different ultrasonic powers are shown in Figure 2. The parameters of laser welding included the laser power of 2000 W, welding speed of 20 mm/s and defocus of +3 mm. Nitrogen was used as the shielding gas, with the ﬂow rate of 20 L/min, the angle of 30 between the shielding gas nozzle and laser beam and the ultrasonic power of 0, 250 W, 500 W, 750 W and 1000 W. Bottom porosities are deﬁned as those located within the depth of 1 mm at the bottom of the weld seam. Figure 2 indicates the maximal total porosities are obtained without ultrasonic vibration, as shown by the blue arrows in Figure 2. The porosities are distributed at the upper, middle and lower parts of the weld seam, with three bottom porosities, as shown by the red arrows in Figure 2. The total and bottom porosities both decrease at the ultrasonic power of 250 W and 500 W. The most signiﬁcant suppression with one pore in total is achieved as the ultrasonic power increases to 750 W. Only a few pores are observed near the upper surface of the weld seam, without bottom porosities. When the ultrasonic power continues to increase to 1000 W, the total number of Photonics 2021, 8, 359 5 of 11 both decrease at the ultrasonic power of 250 W and 500 W. The most significant suppres- Photonics 2021, 8, 359 5 of 11 sion with one pore in total is achieved as the ultrasonic power increases to 750 W. Only a few pores are observed near the upper surface of the weld seam, without bottom porosi- ties. When the ultrasonic power continues to increase to 1000 W, the total number of po- porosities increases to four. The porosities are found at the middle and upper part of the rosities increases to four. The porosities are found at the middle and upper part of the weld seam, without bottom porosities. Therefore, it can be concluded that the potential of weld seam, without bottom porosities. Therefore, it can be concluded that the potential of having fewer porosities is obtained by the application of ultrasonic vibration with a critical having fewer porosities is obtained by the application of ultrasonic vibration with a criti- value of ultrasonic power. cal value of ultrasonic power. Figure 2. Distribution of porosities at different ultrasonic powers: (a) 0, (b)250 W, (c) 500 W, (d) 750 Figure 2. Distribution of porosities at different ultrasonic powers: (a) 0, (b)250 W, (c) 500 W, (d) 750 W, W, (e) 1000 W. (e) 1000 W. The The dynam dynamic ic change proc change process ess of the keyhol of the keyholee and and w weld eld pool pool in in c conventional onventional laser laser welding of “sandwich” specimens is presented in Figure 3. The shape of the keyhole is welding of “sandwich” specimens is presented in Figure 3. The shape of the keyhole is normal, normal, w without ithout obvious fluctuation obvious ﬂuctuation at at tt = = 0.994 0.994 s s (Figur (Figure 3a) e 3a).. Af After ter a a short short while, while, a a local local bulge bulge iis obse s observed rved at the bottom of the at the bottom of the keyhole’s keyhole’s rear wall (Fig rear wall (Figur ure e 3b). 3b). Subsequently Subsequently, the , the brightness inside the keyhole increases sharply, with an obvious local bulge in the middle brightness inside the keyhole increases sharply, with an obvious local bulge in the middle part of the keyhole’s rear wall, and the entrance at the bottom of the keyhole narrows and part of the keyhole’s rear wall, and the entrance at the bottom of the keyhole narrows and necks down (Figure 3c). Soon afterward, a local bulge in the middle part of the keyhole’s necks down (Figure 3c). Soon afterward, a local bulge in the middle part of the keyhole’s rear wall continues to enlarge, with necking down at the bottom of the keyhole (Figure 3d). rear wall continues to enlarge, with necking down at the bottom of the keyhole (Figure Since the local bulge in the middle part of the keyhole’s rear wall enlarges, and more 3d). Since the local bulge in the middle part of the keyhole’s rear wall enlarges, and more importantly, the bottom of the keyhole necks down and gradually folds, an independent importantly, the bottom of the keyhole necks down and gradually folds, an independent bubble is formed, as shown in Figure 3e,f. As the welding process progresses, the keyhole bubble is formed, as shown in Figure 3e,f. As the welding process progresses, the keyhole develops downward and ﬁnally fuses with the bubble (Figure 3g), whereafter the local develops downward and finally fuses with the bubble (Figure 3g), whereafter the local bulge in the middle part of the keyhole’s rear wall becomes smaller (Figure 3h). The bulge in the middle part of the keyhole’s rear wall becomes smaller (Figure 3h). The brightness inside the keyhole increases (Figure 3i), with a local bulge in the middle part brightness inside the keyhole increases (Figure 3i), with a local bulge in the middle part of the keyhole’s front wall and shrinkage of the bottom of the keyhole at t + 2. 4 ms (see of the keyhole’s front wall and shrinkage of the bottom of the keyhole at t + 2. 4 ms (see Figures 3j and 3k, respectively). Figure 3j and Figure 3k, respectively). Photonics 2021, 8, 359 6 of 11 Photonics 2021, 8, 359 6 of 11 Photonics 2021, 8, 359 6 of 11 Figure 3. Dynamic change process of the keyhole and weld pool in conventional laser welding of Figure 3. Dynamic change process of the keyhole and weld pool in conventional laser welding of Figure 3. Dynamic change process of the keyhole and weld pool in conventional laser welding of “sandwich” specimen “sandwich” specimen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). “sandwich” specimen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). (p = 2000 W, v = 1.2 m/min, D = +3 mm, q = 20 L/min (N )). top laser 2 The dynamic change process of the keyhole and weld pool in ultrasound-assisted The dynamic change process of the keyhole and weld pool in ultrasound-assisted The dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” specimens is provided in Figure 4. An obvious bulge is ob- laser welding of “sandwich” specimens is provided in Figure 4. An obvious bulge is ob- laser welding of “sandwich” specimens is provided in Figure 4. An obvious bulge is served at the keyhole’s rear wall, with a narrow outlet of the keyhole at t = 0. 339 s (Figure served at the keyhole’ observed s rear wall, w at the keyhole’s ith a narrow out rear wall, let of the keyhole at with a narrow outlet t = 0. 3 of the 39 s keyhole (Figure at t = 0. 339 s 4a). The brightness inside the keyhole increases sharply, and the keyhole elongates down- 4a). The brightness inside t (Figure 4a). h The e keyhole in brightness creases sh inside the arp keyhole ly, and the ke increases yhole e sharply long , a and tes down- the keyhole elongates ward, with a smaller bulge in the keyhole’s rear wall at t + 0. 3 ms (Figure 4b). Subse- downward, with a smaller bulge in the keyhole’s rear wall at t + 0. 3 ms (Figure 4b). ward, with a smaller bulge in the keyhole’s rear wall at t + 0. 3 ms (Figure 4b). Subse- quently, the outlet of the keyhole opens, with the molten metal spraying downward and quently, the outl Subsequently et of the keyhol , thee opens, outlet ofwi the th the mol keyhole t opens, en metal sprayi with the molten ng downwa metal rd a spraying nd downward the diminution of the keyhole (Figure 3c). At this moment, the keyhole is prone to collapse, and the diminution of the keyhole (Figure 3c). At this moment, the keyhole is prone to the diminution of the keyhole (Figure 3c). At this moment, the keyhole is prone to collapse, since the steam pressure inside it drops significantly (Figure 4d). Hereafter, a new keyhole collapse, since the steam pressure inside it drops signiﬁcantly (Figure 4d). Hereafter, a since the steam pressure inside it drops significantly (Figure 4d). Hereafter, a new keyhole is formed under the action of continuous laser-beam energy, and the newly formed key- new keyhole is formed under the action of continuous laser-beam energy, and the newly is formed under the action of continuous laser-beam energy, and the newly formed key- hole is bright (indicating relatively concentrated energy inside) and gradually moves formed keyhole is bright (indicating relatively concentrated energy inside) and gradually hole is bright (indicating relatively concentrated energy inside) and gradually moves down (Figure 4d and Figure 4e, respectively). As the welding process progresses, the moves down (Figures 4d and 4e, respectively). As the welding process progresses, the down (Figure 4d and Figure 4e, respectively). As the welding process progresses, the newly formed keyhole develops downward and fuses with the collapsed one, with an newly formed keyhole develops downward and fuses with the collapsed one, with an newly formed keyhole develops downward and fuses with the collapsed one, with an obvious local bulge at the rear wall of the fused keyhole (Figure 4f). Afterward, the outlet obvious local bulge at the rear wall of the fused keyhole (Figure 4f). Afterward, the outlet obvious local bulge at the rear wall of the fused keyhole (Figure 4f). Afterward, the outlet of the of the keyho keyhole le opens, w opens, with ith the molten the molten metal metal spraying spraying downwar downwa drd and and t thehdisappearance e disappearance of of the keyhole opens, with the molten metal spraying downward and the disappearance of the the local local bulge bulg at e a the t the keyhol keyhole’se’s rear rear wall wa (Figur ll (Figu e 4 re 4g) g). . of the local bulge at the keyhole’s rear wall (Figure 4g). Figure 4. Dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” spec- Figure 4. Dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” Figure 4. Dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” spec- imen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, p = 750 W, qtop = 20 L/min (N2)). specimen (p = 2000 W, v = 1.2 m/min, D = +3 umm, ltrasonic p = 750 W, q = 20 L/min (N )). imen (plaser = 2000 W, v = 1.2 m/min, laser Δ = +3 mm, p = 750 W, qtop = 20 L/ ultrasonic min (N2)). top 2 ultrasonic Photonics 2021, 8, 359 7 of 11 Photonics 2021, 8, 359 7 of 11 To sum up, the keyhole’s rear wall is always subject to fluctuation in conventional To sum up, the keyhole’s rear wall is always subject to ﬂuctuation in conventional laser laser welding, especially, the bottom of which is prone to local bulges; moreover, poor welding, especially, the bottom of which is prone to local bulges; moreover, poor stability of stability of the bottom of the keyhole can easily lead to necking down of the bottom and the bottom of the keyhole can easily lead to necking down of the bottom and further cause further cause bubbles, thus forming keyhole-induced porosity [6,7]. Under the same weld- bubbles, thus forming keyhole-induced porosity [6,7]. Under the same welding parameters, ing parameters, the keyhole in ultrasound-assisted laser welding is easier to penetrate, the keyhole in ultrasound-assisted laser welding is easier to penetrate, with less local with less local bulging and longer time of stability maintaining at the keyhole’s rear wall, bulging and longer time of stability maintaining at the keyhole’s rear wall, compared with compared with conventional laser welding. Despite the collapse, the formation of the new conventional laser welding. Despite the collapse, the formation of the new keyhole is keyhole is relatively stable. Local bulging at the keyhole’s rear wall is often accompanied relatively stable. Local bulging at the keyhole’s rear wall is often accompanied by the by the penetration of the bottom of the keyhole to ensure its stability. penetration of the bottom of the keyhole to ensure its stability. 3.2. Effect of Welding Position on Bottom Porosity 3.2. Effect of Welding Position on Bottom Porosity The X-ray nondestructive inspection and longitudinal sectional view of weld seams The X-ray nondestructive inspection and longitudinal sectional view of weld seams at different welding positions are displayed in Figure 5. The porosities in the X-ray non- at different welding positions are displayed in Figure 5. The porosities in the X-ray destructive inspection and the bottom porosities in the longitudinal section are shown by nondestructive inspection and the bottom porosities in the longitudinal section are shown the blue and red arrows, respectively. As indicated in Figure 5, the total number of poros- by the blue and red arrows, respectively. As indicated in Figure 5, the total number of ities declines in horizontal position welding, vertical up welding and vertical down weld- porosities declines in horizontal position welding, vertical up welding and vertical down ing, compared with flat welding (Figure 2a). Among them, the largest total number of welding, compared with ﬂat welding (Figure 2a). Among them, the largest total number of porosities is obtained in vertical up welding, with some bottom porosities (Figure 5a). The porosities is obtained in vertical up welding, with some bottom porosities (Figure 5a). The optimum s optimum suppr uppression ession o of f p por orositie osities s is is ac achieved hieved in in vert vertical ical d down own we welding, lding, w without ithout bot bottom tom p por orosit osities ies (Fi (Figur gure e 5b 5b). ). The The p por orosit osities ies in in horizont horizontal al p position osition wel welding ding are are fewe fewer r t than han t those hose in in fl ﬂat at we welding, lding, as as in indicated dicated in in F Figur igure e 5c 5c. . The The res results ults iindicate ndicate tthat hat t the he pot potential ential o of f h having aving fewer poro fewer porosities sities is is ach achieved ieved wit with h t the he we welding lding pos position ition of vert of vertical ical d down own weld welding. ing. Figure 5. Figure 5.Distri Distribution bution of porosities at of porosities at diff different weld erent welding ing posit positions: ions: ((a a) ) vertica vertical l u up p wel welding, ding, ((b b) vert ) vertical ical down welding and (c) horizontal position welding. down welding and (c) horizontal position welding. The dyn The dynamic amic chan change ge proc process ess of the of the ke keyhole yhole an and d w weld eld pool pool in in v vertical ertical down down la laser ser welding of “sandwich” specimens is given in Figure 6. As shown in Figure 6, the ﬂuctuation welding of “sandwich” specimens is given in Figure 6. As shown in Figure 6, the fluctua- of the keyhole in vertical down laser welding is smaller than that in conventional laser tion of the keyhole in vertical down laser welding is smaller than that in conventional welding. The shape of the keyhole is stable with great brightness inside at t = 0. 147 s laser welding. The shape of the keyhole is stable with great brightness inside at t = 0. 147 (Figure 6a). Subsequently, the bottom of the keyhole necks down and collapses successively s (Figure 6a). Subsequently, the bottom of the keyhole necks down and collapses succes- (Figure 6b,c). Soon afterward, the keyhole moves deep to the bottom again, with a complete sively (Figure 6b,c). Soon afterward, the keyhole moves deep to the bottom again, with a new keyhole formed (Figure 6d,e). With the progress of welding, laser energy accumulation complete new keyhole formed (Figure 6d,e). With the progress of welding, laser energy is found successively at the middle and middle-upper parts of the keyhole, accompanied by accumulation is found successively at the middle and middle-upper parts of the keyhole, a local bulge in the keyhole’s rear wall but without collapse, as demonstrated in Figure 6f–j. accompanied by a local bulge in the keyhole’s rear wall but without collapse, as demon- strated in Figure 6f–j. Photonics 2021, 8, 359 8 of 11 Photonics 2021, 8, 359 8 of 11 Photonics 2021, 8, 359 8 of 11 Figure 6. Dynamic change process of the keyhole and weld pool in vertical down laser welding of “sandwich” specimen Figure Figure 6. 6. Dynamic Dynamic c change hange proc process ess of the keyhol of the keyhole e and and weld pool in vert weld pool in vertical ical down laser down laser welding welding of “sa of “sandwich” ndwich” spec specimen imen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). (p = 2000 W, v = 1.2 m/min, D = +3 mm, q = 20 L/min (N )). top laser 2 In summary, no obvious bulge at the keyhole’s rear wall is found in vertical down In summary, no obvious bulge at the keyhole’s rear wall is found in vertical down In summary, no obvious bulge at the keyhole’s rear wall is found in vertical down welding, and its keyhole is more stable than that in flat welding, which thus diminishes welding, and its keyhole is more stable than that in ﬂat welding, which thus diminishes welding, and its keyhole is more stable than that in flat welding, which thus diminishes th the e form formation ation of ke of keyhole-induced yhole-induced ppor oroosity sity at at it its s sou sour rce. Mo ce. Mor reov eover er, com , compar pareed d wit with h fl ﬂat at the formation of keyhole-induced porosity at its source. Moreover, compared with flat welding, the force direction on its weld pool along the welding direction is consistent with welding, the force direction on its weld pool along the welding direction is consistent welding, the force direction on its weld pool along the welding direction is consistent with the gra with the vity, and thus the gravity, and thus wel the d pool weld ipool s length is lengthened, ened, and the escape and the e time of bubb scape time ofles bubbles is prois - the gravity, and thus the weld pool is lengthened, and the escape time of bubbles is pro- longed prolonged [17]. [17 Therefore ]. Therefor , th e, e number the number of porositi of porosities, es, especi especially ally tha that t ofof bottom porosi bottom porosities, ties, is is longed [17]. Therefore, the number of porosities, especially that of bottom porosities, is greatly re greatly reduced. duced. greatly reduced. 3.3. Effect of Shielding Gas on Bottom Porosity 3.3. Effect of Shielding Gas on Bottom Porosity 3.3. Effect of Shielding Gas on Bottom Porosity The effects of ﬂow rate and supply method of shielding gas on the total and bottom The effects of flow rate and supply method of shielding gas on the total and bottom The effects of flow rate and supply method of shielding gas on the total and bottom porosities are illustrated in Figure 7. The highest number of total and bottom porosities is porosities are illustrated in Figure 7. The highest number of total and bottom porosities is porosities are illustrated in Figure 7. The highest number of total and bottom porosities is caused when only top shielding gas is applied, as shown in Figures 7a and 7b, respectively. caused when only top shielding gas is applied, as shown in Figure 7a and Figure 7b, re- caused when only top shielding gas is applied, as shown in Figure 7a and Figure 7b, re- Compared with top shielding gas alone, the total and bottom porosities both decrease spectively. Compared with top shielding gas alone, the total and bottom porosities both spectively. Compared with top shielding gas alone, the total and bottom porosities both when only bottom shielding gas is applied, as shown in Figures 7a and 7b, respectively. The decrease when only bottom shielding gas is applied, as shown in Figure 7a and Figure 7b, decrease when only bottom shielding gas is applied, as shown in Figure 7a and Figure 7b, lowest number of total and bottom porosities is found when the top and bottom shielding respectively. The lowest number of total and bottom porosities is found when the top and respectively. The lowest number of total and bottom porosities is found when the top and gases are both applied, as shown in Figures 7a and 7b, respectively. No bottom porosity bottom shielding gases are both applied, as shown in Figure 7a and Figure 7b, respec- bottom shielding gases are both applied, as shown in Figure 7a and Figure 7b, respec- is formed occasionally at the ﬂow rate of 20 L/min, as shown in Figure 7b. The results tively. No bottom porosity is formed occasionally at the flow rate of 20 L/min, as shown tively. No bottom porosity is formed occasionally at the flow rate of 20 L/min, as shown indicate that the potential of having fewer porosities is obtained when the top and bottom in Figure 7b. The results indicate that the potential of having fewer porosities is obtained in Figure 7b. The results indicate that the potential of having fewer porosities is obtained shielding gases are both applied with a moderate ﬂow rate. when the top and bottom shielding gases are both applied with a moderate flow rate. when the top and bottom shielding gases are both applied with a moderate flow rate. Figure 7. Cont. Photonics 2021, 8, 359 9 of 11 Photonics 2021, 8, 359 9 of 11 Photonics 2021, 8, 359 9 of 11 Figure 7. Effects of flow rate and supply method of shielding gas on the number of porosities. Figure 7. Effects of flow rate and supply method of shielding gas on the number of porosities. Figure 7. Effects of ﬂow rate and supply method of shielding gas on the number of porosities. The high-speed photograph of the upper-surface weld pool and entrance of the key- The high-speed photograph of the upper-surface weld pool and entrance of the The high-speed photograph of the upper-surface weld pool and entrance of the key- hole during laser welding without shielding gas is demonstrated in Figure 8. Figure 9 keyhole during laser welding without shielding gas is demonstrated in Figure 8. Figure 9 hole during laser welding without shielding gas is demonstrated in Figure 8. Figure 9 provides the provides the high-speed high-speed photograph o photograph of f the u the upper pper- -surface surface we weld ld p pool ool and and entrance entrance of of t the he provides the high-speed photograph of the upper-surface weld pool and entrance of the keyhole keyhole whe when n top and bottom shield top and bottom shielding ing gases gases ar ar e e applied applied simultaneously simultaneously. A . As indicated s indicated in keyhole when top and bottom shielding gases are applied simultaneously. As indicated iFigur n Figu e 8 re , 8, without withou shielding t shieldigas, ng ga the s, the lengt leh ng of ththe of t upper he up -surface per-surfweld ace wel pool d p is oo about l is ab 4.0 out mm, 4.0 in Figure 8, without shielding gas, the length of the upper-surface weld pool is about 4.0 mm, wi with a width th a wi of dth of about ab 2.1 out 2.1 mm, a mm, and the nd the op opening enand ing and c closing losing cycle cycle o of thef the entrance entrance of of the mm, with a width of about 2.1 mm, and the opening and closing cycle of the entrance of t keyhole he keyhol is e i about s ab2.4 out 2. ms4 m during s durlaser ing la welding. ser weldThe ing. Th length e length o of the fupper the upper-sur -surface weld face weld pool the keyhole is about 2.4 ms during laser welding. The length of the upper-surface weld is about 7.2 mm, with a width of about 2.6 mm, and the opening and closing cycle of the pool is about 7.2 mm, with a width of about 2.6 mm, and the opening and closing cycle of pool is about 7.2 mm, with a width of about 2.6 mm, and the opening and closing cycle of entrance of the keyhole is about 5.1 ms, when top and bottom shielding gases are applied the entrance of the keyhole is about 5.1 ms, when top and bottom shielding gases are ap- the entrance of the keyhole is about 5.1 ms, when top and bottom shielding gases are ap- simultaneously. Compared to the case without shielding gas, the length and width of the plied simultaneously. Compared to the case without shielding gas, the length and width plied simultaneously. Compared to the case without shielding gas, the length and width upper-surface weld both increase, with the stability-maintaining time of the entrance of of the upper-surface weld both increase, with the stability-maintaining time of the en- of the upper-surface weld both increase, with the stability-maintaining time of the en- the keyhole prolonged, when top and bottom shielding gases are applied simultaneously. trance of the keyhole prolonged, when top and bottom shielding gases are applied simul- trance of the keyhole prolonged, when top and bottom shielding gases are applied simul- Therefore, the increase in the volume of the weld pool and improvement in the stability of taneously. Therefore, the increase in the volume of the weld pool and improvement in the taneously. Therefore, the increase in the volume of the weld pool and improvement in the the keyhole guarantee fewer bubbles caused by the collapse of the keyhole, when top and stability of the keyhole guarantee fewer bubbles caused by the collapse of the keyhole, stability of the keyhole guarantee fewer bubbles caused by the collapse of the keyhole, bottom shielding gases are applied simultaneously. Despite the bubbles formed, the larger when top and bottom shielding gases are applied simultaneously. Despite the bubbles when top and bottom shielding gases are applied simultaneously. Despite the bubbles weld pool provides the bubbles with a longer time to escape upward, and thus the porosity formed, the larger weld pool provides the bubbles with a longer time to escape upward, formed, the larger weld pool provides the bubbles with a longer time to escape upward, defects at the bottom of the weld seam are better improved. and thus the porosity defects at the bottom of the weld seam are better improved. and thus the porosity defects at the bottom of the weld seam are better improved. Figure 8. Dynamic change process of the weld pool and keyhole entrance during laser welding without shielding gas (plaser Figure 8. Dynamic change process of the weld pool and keyhole entrance during laser welding without shielding gas Figure 8. Dynamic change process of the weld pool and keyhole entrance during laser welding without shielding gas (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm). = 2000 W, v = 1.2 m/min, Δ = +3 mm). (p = 2000 W, v = 1.2 m/min, D = +3 mm). laser Photonics 2021, 8, 359 10 of 11 Photonics 2021, 8, 359 10 of 11 Figure Figure 9. 9. Dynamic Dynamic change process of the we change process of the weld ld pool and keyhole en pool and keyhole entrance trance during laser welding with both during laser welding with both top top and botto and bottom m shielding gases (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 L/min (N2), qbottom = 20 L/min (N2)). shielding gases (p = 2000 W, v = 1.2 m/min, D = +3 mm, q = 20 L/min (N ), q = 20 L/min (N )). top laser 2 bottom 2 4. Conclusions 4. Conclusions Given the limited reports on suppression of the porosity defect in the ﬁber-laser Given the limited reports on suppression of the porosity defect in the fiber-laser welding stainless steel, laser welding experiments were conducted on 304 stainless steel welding stainless steel, laser welding experiments were conducted on 304 stainless steel specimens by applying ultrasonic vibration, changing welding position and optimizing specimens by applying ultrasonic vibration, changing welding position and optimizing shielding gas. With a combination of processing experiments and high-speed photography shielding gas. With a combination of processing experiments and high-speed photog- observation of the welding process, the porosity, especially the bottom porosity defects in raphy observation of the welding process, the porosity, especially the bottom porosity the weld, was analyzed. The following conclusions were made based on the experiments. defects in the weld, was analyzed. The following conclusions were made based on the (1) The potential of having fewer porosities occurs when the ultrasonic vibration experiments. is applied with a critical value of ultrasonic power. Compared with conventional laser (1) The potential of having fewer porosities occurs when the ultrasonic vibration is welding, the welding keyhole in ultrasound-assisted laser welding is easier to penetrate, applied with a critical value of ultrasonic power. Compared with conventional laser weld- with less local bulging and longer time of stability maintaining at the keyhole rear wall, ing, the welding keyhole in ultrasound-assisted laser welding is easier to penetrate, with thus leading to reduced porosities. less local bulging and longer time of stability maintaining at the keyhole rear wall, thus (2) Welding position exerts a great inﬂuence on porosity formation in laser welding. leading to reduced porosities. The optimal suppression of porosities is achieved in vertical down welding, followed (2) Welding position exerts a great influence on porosity formation in laser welding. by horizontal position welding. No obvious bulge at the keyhole’s rear wall is observed The optimal suppression of porosities is achieved in vertical down welding, followed by in vertical down welding, and the keyhole is more stable than that in ﬂat welding, thus horizontal position welding. No obvious bulge at the keyhole’s rear wall is observed in suppressing porosities. vertical down welding, and the keyhole is more stable than that in flat welding, thus sup- (3) The potential of having fewer porosities is obtained when the top and bottom pressing porosities. shielding gases are both applied with a moderate ﬂow rate. When top and bottom shielding (3) The potential of having fewer porosities is obtained when the top and bottom gases are applied simultaneously, the length and width of the weld pool increase, with shielding gases are both applied with a moderate flow rate. When top and bottom shield- good stability of the entrance of the keyhole, thus facilitating porosity reduction. ing gases are applied simultaneously, the length and width of the weld pool increase, with good stability of the entrance of the keyhole, thus facilitating porosity reduction. Author Contributions: Conceptualization, X.P.; methodology, X.P.; software, X.P.; validation, M.Z. and Y.Z.; formal analysis, X.P.; investigation, X.P. and J.D.; resources, X.P.; data curation, X.P.; Author Contributions: Conceptualization, X.P.; methodology, X.P.; software, X.P.; validation, M.Z. writing—original draft preparation, X.P. and J.D.; writing—review and editing, Y.Z.; visualization, and Y.Z.; formal analysis, X.P.; investigation, X.P. and J.D.; resources, X.P.; data curation, X.P.; writ- Y.Z.; supervision, Y.Z.; project administration, M.Z. and Y.Z.; funding acquisition, M.Z. and Y.Z. All ing—original draft preparation, X.P. and J.D.; writing—review and editing, Y.Z.; visualization, Y.Z.; authors have read and agreed to the published version of the manuscript. supervision, Y.Z.; project administration, M.Z. and Y.Z.; funding acquisition, M.Z. and Y.Z. All au- Funding: The authors are grateful for the ﬁnancial support from the National Natural Science thors have read and agreed to the published version of the manuscript. Foundation of China (No. 51605045, 51875050), the Natural Science Foundation of Hunan Province Funding: The authors are grateful for the financial support from the National Natural Science Foun- of China (No. 2021JJ30302), and the Science and Technology Plan Project of Changsha City (No. dation of China (No. 51605045, 51875050), the Natural Science Foundation of Hunan Province of kq1907089). China (No. 2021JJ30302), and the Science and Technology Plan Project of Changsha City (No. Institutional Review Board Statement: This study did not involve humans or animals. kq1907089). Informed Institutional Consent Review Board Statement: Statement: Not applicable. This study did not involve humans or animals. Data Informed Consent Availability Statement: StatementThis : Not appl studyic does able.not report any data. 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ISSN: 2304-6732
DOI: 10.3390/photonics8090359
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Abstract

hv photonics Article Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel 1 , 2 2 3 Xiaobing Pang *, Jiahui Dai , Mingjun Zhang and Yan Zhang College of Mechanical & Electrical Engineering, Changsha University, Changsha 410022, China Key Laboratory of High Performance Intelligent Manufacturing of Mechanical Equipment of Hunan Province, Changsha University of Science & Technology, Changsha 410114, China; daijiahui2018@163.com (J.D.); mj_zhang@csust.edu.cn (M.Z.) Hunan Institute of Science and Technology, College of Mechanical Engineering, Yueyang 414006, China; 12017032@hnist.edu.cn * Correspondence: pangxiaobing55@ccsu.edu.cn; Tel./Fax: +86-731-8426-1492 Abstract: The application bottleneck of laser welding is being gradually highlighted due to a high prevalence of porosity. Although laser welding technology has been well applied in ﬁelds such as vehicle body manufacturing, the suppression of weld porosity in the laser welding of stainless steel containers in the pharmaceutical industry is still challenging. The suppression of bottom porosity was investigated by applying ultrasonic vibration, changing welding positions and optimizing shielding gas in this paper. The results indicate that bottom porosities can be suppressed through application of ultrasonic vibration at an appropriate power. The keyhole in ultrasound-assisted laser welding is easier to penetrate, with better stability. No obvious bulge at the keyhole rear wall is found in vertical down welding, and the keyhole is much more stable than that in ﬂat welding, thus eliminating bottom porosity. The top and bottom shielding gases achieve the minimal total porosities, without bottom porosity. Keywords: ﬁber-laser welding; porosity; suppression; stainless steel Citation: Pang, X.; Dai, J.; Zhang, M.; Zhang, Y. Suppression of Bottom Porosity in Fiber Laser Butt Welding 1. Introduction of Stainless Steel. Photonics 2021, 8, 359. https://doi.org/10.3390/ 304 austenitic stainless steel, due to its good corrosion resistance and mechanical photonics8090359 properties, has been widely used in ﬁelds including pressure vessels and nuclear power equipment [1]. Argon tungsten arc welding (TIG) is currently dominant in manufacturing Received: 8 June 2021 plates of isolators and freeze-dryers in the pharmaceutical ﬁeld. However, this traditional Accepted: 26 August 2021 arc welding method is associated with some disadvantages such as limited penetration Published: 28 August 2021 ability, low welding efﬁciency and poor controllability of heat input [2]. Laser welding is expected to be feasible for welding stainless-steel containers in the pharmaceutical Publisher’s Note: MDPI stays neutral ﬁeld due to its high energy density, high welding speed and small deformation after with regard to jurisdictional claims in welding. However, it is susceptible to porosities, especially the bottom porosities as a published maps and institutional afﬁl- result of the instability of the deep-penetration keyhole, which affects the sealing and anti- iations. pollution ability of the weld seam and restricts its application in welding pharmaceutical containers [3]. Therefore, it is of great signiﬁcance to suppress bottom porosities in laser welding of stainless steel for the application of laser welding technology in pharmaceutical container manufacturing. Copyright: © 2021 by the authors. Considering porosity as a common defect in a laser-welded seam and the lack of Licensee MDPI, Basel, Switzerland. systematic control theoretical basis, laser welding technology is gradually reaching its This article is an open access article application bottleneck [4,5]. Process-induced porosity is the hotspot in research on the distributed under the terms and porosity of laser welding, mainly including keyhole-induced porosity [6,7] and ﬂuid-ﬂow- conditions of the Creative Commons induced porosity [8,9]. Lin et al. [6] found in a numerical simulation that the formation Attribution (CC BY) license (https:// of bubbles in the weld pool of remote laser welding of aluminum alloy depends on the creativecommons.org/licenses/by/ dynamic behaviors of the keyhole and weld pool. Speciﬁcally, the collapse of the keyhole 4.0/). Photonics 2021, 8, 359. https://doi.org/10.3390/photonics8090359 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 359 2 of 11 caused by an intense melt ﬂow and the eddy along the weld pool behind the keyhole are the main contributors to the formation of bubbles. Based on a high-speed photography analysis during the weld test of glass and metal composite specimens, Xu et al. [7] argued that the sharp ﬂuctuation of the keyhole may explain to a great extent the formation and assemblage of bubbles, which resulted in large porosities. Zhang et al. [10] compared the porosity defects of non-penetration and full-penetration laser welding by numerical simulation combined with intuitive observation and revealed that full-penetration laser welding greatly improved the porosity defects and that the formation mechanism of porosity in full-penetration laser welding was consistent with the “hump theory” [11]. Meng et al. [8] studied the formation mechanism of porosity at the T-joint of laser lap welding and proposed that the change of dynamic behaviors of the weld pool is the main reason for the formation of porosities at the lap gap, while the instability of the keyhole is insigniﬁcant in laser lap welding. Panwisawas et al. [9] showed that process-induced porosity is related to plate thickness, laser power and welding speed in laser welding of titanium alloy plates. Especially, the porosity increases with the increase of plate thickness. Meanwhile, it is pointed out that unstable ﬂow and/or porosity escape time relative to local solidiﬁcation time is the key to porosity formation. Numerous research studies have been conducted on the suppression of porosity in laser welding, with many methods proposed, such as pulsed laser welding [12], laser-beam oscillation scanning welding [13,14], ultrasound-assisted laser welding [15,16], changing welding position [17,18] and optimizing shielding gas [19,20]. Shen et al. [12] examined the effects of laser pulse parameters on porosity and ﬂow characteristics of the weld pool and exhibited a negative correlation between pulse frequency and porosity at the joint and the suppression mechanism of porosity in pulsed laser welding. That is, pulsed laser welding results in good stability of the keyhole and limits bubble formation by preventing shielding gas from being involved in the keyhole. Meanwhile, the intermittent impact effect of pulsed laser accelerates the escape of bubbles by stirring the weld pool, thus further reducing porosities. Zhou et al. [13] discovered by utilization of the beam oscillation scanning method in laser welding of aluminum alloy that circular scan is effective in eliminating porosities. Furthermore, improvement in the stability of the keyhole was established as the main reason for the elimination of porosities by recording the ﬂuctuation degree of the entrance of the keyhole during laser beam scanning. Cai et al. [14] explored the swing laser-MAG hybrid welding of carbon steel and pointed out that the swing laser changes the root shape of the hybrid weld seam, facilitating elimination of root porosity. Kim et al. [15] ﬁrst proposed the use of ultrasonic vibration on the bottom of the specimen during laser welding. It was demonstrated that cracks and porosities in the weld seam are greatly suppressed by the ultrasonic vibration due to cavitation effects in the molten pool during ultrasound-assisted laser welding. Lei et al. [16] suggested that the porosity at the weld seam is greatly improved by the cavitation and acoustic streaming of the ultrasonic vibration on the weld pool in ultrasound-assisted laser welding of AZ31B magnesium alloy. Grajczak et al. [21] found that porosity can be eliminated when the weld pool is located in the antinode position during ultrasound-assisted laser welding of nickel-based-alloy round bars. He and Shen [17] analyzed the effects of different welding positions on porosity in laser welding of aluminum alloys and observed the minimal porosities in vertical down welding, with high stability of the keyhole. Miao et al. [18] noted that the number of porosities in vertical welding is smaller than that in ﬂat welding. He and Shen [19] proved that porosity is signiﬁcantly decreased by the addition of side-blowing gas ﬂow in laser welding of aluminum alloy and that the ﬂow rate has a great inﬂuence on porosity. Sun et al. [20] determined that when nitrogen (N2) is used as shielding gas in laser welding of 304 stainless steel, porosity is suppressed and is mainly seen at the bottom of the weld seam. The solubility of N2 in the liquid weld pool contributes to reducing the porosity in laser welding of 304 stainless steel. In conclusion, abundant studies focus on porosity defects in the laser welding process, and the formation mechanism and suppression methods of porosity have been extensively Photonics 2021, 8, 359 3 of 11 Photonics 2021, 8, 359 3 of 11 In conclusion, abundant studies focus on porosity defects in the laser welding pro- cess, and the formation mechanism and suppression methods of porosity have been ex- tensively validated. However, there are few reports on bottom porosity. The suppression validated. However, there are few reports on bottom porosity. The suppression of bottom of bottom porosity was investigated by applying ultrasonic vibration, changing welding porosity was investigated by applying ultrasonic vibration, changing welding positions positions and optimizing shielding gas during laser welding of 304 stainless steel in this and optimizing shielding gas during laser welding of 304 stainless steel in this paper, and paper, and the suppression mechanisms of porosity caused by ultrasonic vibration, weld- the suppression mechanisms of porosity caused by ultrasonic vibration, welding position ing position and shielding gas were revealed by observation of the dynamic laser welding and shielding gas were revealed by observation of the dynamic laser welding process with process with high-speed photography. high-speed photography. 2. Experimental Procedure 2. Experimental Procedure The experimental setup was as illustrated in Figure 1. With a continuous-wave The experimental setup was as illustrated in Figure 1. With a continuous-wave fiber ﬁber laser (YLS-3000-CL) as the laser source, the laser beam emitted from the end of the laser (YLS-3000-CL) as the laser source, the laser beam emitted from the end of the opera- operation ﬁber was collimated and then focused with the aid of a laser welding head tion fiber was collimated and then focused with the aid of a laser welding head (FLW- (FLW-D30), while for the laser beam radiated from the end of the optical ﬁber, collimation D30), while for the laser beam radiated from the end of the optical fiber, collimation was was conducted by a lens of a 100 mm focal length, and focusing on the specimen surface conducted by a lens of a 100 mm focal length, and focusing on the specimen surface was was completed by a focusing unit of a 150 mm focal length. After focusing, the laser beam completed by a focusing unit of a 150 mm focal length. After focusing, the laser beam had had a spot size of about 0.3 mm. The ultrasonic power supply (CSHJ-1000) that has an a spot size of about 0.3 mm. The ultrasonic power supply (CSHJ-1000) that has an ultra- ultrasonic frequency of 20 kHz was applied to provide the power output of 1000 W with a sonic frequency of 20 kHz was applied to provide the power output of 1000 W with a fixed ﬁxed amplitude of 6 m. The ultrasonic amplitude transformer was placed in a sink on amplitude of 6 μm. The ultrasonic amplitude transformer was placed in a sink on the the working platform. The conﬁguration of the bead-on-plate welding with ultrasound is working platform. The configuration of the bead-on-plate welding with ultrasound is il- illustrated in Figure 1c. lustrated in Figure 1c. Figure 1. Experimental setup: (a) on-site layout, (b) schematic diagram of welding with “sandwich” specimen and (c) schematic diagram of bead-on-plate welding with ultrasound. Photonics 2021, 8, 359 4 of 11 The welding materials used were 304 stainless steel plates of 3 mm in thickness. Table 1 overviews the substrate with respect to its chemical composition. The modiﬁed sandwich sample comprises, as displayed in Figure 1b, one sheet of stainless steel and one sheet of GG17 glass, both with a size of 50 mm 3 mm 3 mm [22]. The processing parameters for the laser welding experiments are presented in Table 2. Table 1. Chemical composition of the 304 stainless steel studied. Element C Cr Mn Ni Si P S Fe (Wt.%) 0.039 18.280 1.420 8.150 0.410 0.036 0.015 Bal. Table 2. Parameters used in the welding experiments. Parameters Value Laser power (p ) (W) 2000 laser Welding speed (v) (m/min) 1.2 Defocus (D) (mm) +3 Ultrasonic frequency (kHz) 20 Ultrasonic power (p ) (W) 0, 250, 500, 750, 1000 ultrasonic Ultrasonic amplitude (m) 6 Flat position, Vertical–down position, Welding position Vertical–up position, Horizontal position Shielding gas type N Top shielding gas ﬂow rate (q ) (L/min) 0, 15, 20, 25 top Bottom shielding gas ﬂow rate (q ) (L/min) 0, 15, 20, 25 bottom The welding zone was illuminated by a diode laser (808 nm) with a maximum power of 30 W for general observation of the weld pool, and for selective observation, a ﬁlter was additionally added to the camera lens. To visualize the weld pool and the vapor plume during the experiments, a bandpass ﬁlter with a transmission band of 808 3 nm and a ﬁlter with a transmission band from 350 nm to 650 nm were placed in front of the camera lens, respectively. An X-ray real-time imaging system (XYD-225) was used to detect the pores in the welding test piece upon completion of welding. After being cut by electro-discharge machining (EDM), the longitudinal sections of the welded joints were polished with abrasive paper and polishing cloth. The ﬁnished cross sections of the welded joints were observed under an optical microscope (Leica S9i) upon etching by a solution of aqua regia (HCl:HNO = 3:1) for 15 s. 3. Results and Discussion 3.1. Effect of Ultrasonic Vibration on Bottom Porosity The X-ray nondestructive inspection and longitudinal sectional view of weld seams at different ultrasonic powers are shown in Figure 2. The parameters of laser welding included the laser power of 2000 W, welding speed of 20 mm/s and defocus of +3 mm. Nitrogen was used as the shielding gas, with the ﬂow rate of 20 L/min, the angle of 30 between the shielding gas nozzle and laser beam and the ultrasonic power of 0, 250 W, 500 W, 750 W and 1000 W. Bottom porosities are deﬁned as those located within the depth of 1 mm at the bottom of the weld seam. Figure 2 indicates the maximal total porosities are obtained without ultrasonic vibration, as shown by the blue arrows in Figure 2. The porosities are distributed at the upper, middle and lower parts of the weld seam, with three bottom porosities, as shown by the red arrows in Figure 2. The total and bottom porosities both decrease at the ultrasonic power of 250 W and 500 W. The most signiﬁcant suppression with one pore in total is achieved as the ultrasonic power increases to 750 W. Only a few pores are observed near the upper surface of the weld seam, without bottom porosities. When the ultrasonic power continues to increase to 1000 W, the total number of Photonics 2021, 8, 359 5 of 11 both decrease at the ultrasonic power of 250 W and 500 W. The most significant suppres- Photonics 2021, 8, 359 5 of 11 sion with one pore in total is achieved as the ultrasonic power increases to 750 W. Only a few pores are observed near the upper surface of the weld seam, without bottom porosi- ties. When the ultrasonic power continues to increase to 1000 W, the total number of po- porosities increases to four. The porosities are found at the middle and upper part of the rosities increases to four. The porosities are found at the middle and upper part of the weld seam, without bottom porosities. Therefore, it can be concluded that the potential of weld seam, without bottom porosities. Therefore, it can be concluded that the potential of having fewer porosities is obtained by the application of ultrasonic vibration with a critical having fewer porosities is obtained by the application of ultrasonic vibration with a criti- value of ultrasonic power. cal value of ultrasonic power. Figure 2. Distribution of porosities at different ultrasonic powers: (a) 0, (b)250 W, (c) 500 W, (d) 750 Figure 2. Distribution of porosities at different ultrasonic powers: (a) 0, (b)250 W, (c) 500 W, (d) 750 W, W, (e) 1000 W. (e) 1000 W. The The dynam dynamic ic change proc change process ess of the keyhol of the keyholee and and w weld eld pool pool in in c conventional onventional laser laser welding of “sandwich” specimens is presented in Figure 3. The shape of the keyhole is welding of “sandwich” specimens is presented in Figure 3. The shape of the keyhole is normal, normal, w without ithout obvious fluctuation obvious ﬂuctuation at at tt = = 0.994 0.994 s s (Figur (Figure 3a) e 3a).. Af After ter a a short short while, while, a a local local bulge bulge iis obse s observed rved at the bottom of the at the bottom of the keyhole’s keyhole’s rear wall (Fig rear wall (Figur ure e 3b). 3b). Subsequently Subsequently, the , the brightness inside the keyhole increases sharply, with an obvious local bulge in the middle brightness inside the keyhole increases sharply, with an obvious local bulge in the middle part of the keyhole’s rear wall, and the entrance at the bottom of the keyhole narrows and part of the keyhole’s rear wall, and the entrance at the bottom of the keyhole narrows and necks down (Figure 3c). Soon afterward, a local bulge in the middle part of the keyhole’s necks down (Figure 3c). Soon afterward, a local bulge in the middle part of the keyhole’s rear wall continues to enlarge, with necking down at the bottom of the keyhole (Figure 3d). rear wall continues to enlarge, with necking down at the bottom of the keyhole (Figure Since the local bulge in the middle part of the keyhole’s rear wall enlarges, and more 3d). Since the local bulge in the middle part of the keyhole’s rear wall enlarges, and more importantly, the bottom of the keyhole necks down and gradually folds, an independent importantly, the bottom of the keyhole necks down and gradually folds, an independent bubble is formed, as shown in Figure 3e,f. As the welding process progresses, the keyhole bubble is formed, as shown in Figure 3e,f. As the welding process progresses, the keyhole develops downward and ﬁnally fuses with the bubble (Figure 3g), whereafter the local develops downward and finally fuses with the bubble (Figure 3g), whereafter the local bulge in the middle part of the keyhole’s rear wall becomes smaller (Figure 3h). The bulge in the middle part of the keyhole’s rear wall becomes smaller (Figure 3h). The brightness inside the keyhole increases (Figure 3i), with a local bulge in the middle part brightness inside the keyhole increases (Figure 3i), with a local bulge in the middle part of the keyhole’s front wall and shrinkage of the bottom of the keyhole at t + 2. 4 ms (see of the keyhole’s front wall and shrinkage of the bottom of the keyhole at t + 2. 4 ms (see Figures 3j and 3k, respectively). Figure 3j and Figure 3k, respectively). Photonics 2021, 8, 359 6 of 11 Photonics 2021, 8, 359 6 of 11 Photonics 2021, 8, 359 6 of 11 Figure 3. Dynamic change process of the keyhole and weld pool in conventional laser welding of Figure 3. Dynamic change process of the keyhole and weld pool in conventional laser welding of Figure 3. Dynamic change process of the keyhole and weld pool in conventional laser welding of “sandwich” specimen “sandwich” specimen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). “sandwich” specimen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). (p = 2000 W, v = 1.2 m/min, D = +3 mm, q = 20 L/min (N )). top laser 2 The dynamic change process of the keyhole and weld pool in ultrasound-assisted The dynamic change process of the keyhole and weld pool in ultrasound-assisted The dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” specimens is provided in Figure 4. An obvious bulge is ob- laser welding of “sandwich” specimens is provided in Figure 4. An obvious bulge is ob- laser welding of “sandwich” specimens is provided in Figure 4. An obvious bulge is served at the keyhole’s rear wall, with a narrow outlet of the keyhole at t = 0. 339 s (Figure served at the keyhole’ observed s rear wall, w at the keyhole’s ith a narrow out rear wall, let of the keyhole at with a narrow outlet t = 0. 3 of the 39 s keyhole (Figure at t = 0. 339 s 4a). The brightness inside the keyhole increases sharply, and the keyhole elongates down- 4a). The brightness inside t (Figure 4a). h The e keyhole in brightness creases sh inside the arp keyhole ly, and the ke increases yhole e sharply long , a and tes down- the keyhole elongates ward, with a smaller bulge in the keyhole’s rear wall at t + 0. 3 ms (Figure 4b). Subse- downward, with a smaller bulge in the keyhole’s rear wall at t + 0. 3 ms (Figure 4b). ward, with a smaller bulge in the keyhole’s rear wall at t + 0. 3 ms (Figure 4b). Subse- quently, the outlet of the keyhole opens, with the molten metal spraying downward and quently, the outl Subsequently et of the keyhol , thee opens, outlet ofwi the th the mol keyhole t opens, en metal sprayi with the molten ng downwa metal rd a spraying nd downward the diminution of the keyhole (Figure 3c). At this moment, the keyhole is prone to collapse, and the diminution of the keyhole (Figure 3c). At this moment, the keyhole is prone to the diminution of the keyhole (Figure 3c). At this moment, the keyhole is prone to collapse, since the steam pressure inside it drops significantly (Figure 4d). Hereafter, a new keyhole collapse, since the steam pressure inside it drops signiﬁcantly (Figure 4d). Hereafter, a since the steam pressure inside it drops significantly (Figure 4d). Hereafter, a new keyhole is formed under the action of continuous laser-beam energy, and the newly formed key- new keyhole is formed under the action of continuous laser-beam energy, and the newly is formed under the action of continuous laser-beam energy, and the newly formed key- hole is bright (indicating relatively concentrated energy inside) and gradually moves formed keyhole is bright (indicating relatively concentrated energy inside) and gradually hole is bright (indicating relatively concentrated energy inside) and gradually moves down (Figure 4d and Figure 4e, respectively). As the welding process progresses, the moves down (Figures 4d and 4e, respectively). As the welding process progresses, the down (Figure 4d and Figure 4e, respectively). As the welding process progresses, the newly formed keyhole develops downward and fuses with the collapsed one, with an newly formed keyhole develops downward and fuses with the collapsed one, with an newly formed keyhole develops downward and fuses with the collapsed one, with an obvious local bulge at the rear wall of the fused keyhole (Figure 4f). Afterward, the outlet obvious local bulge at the rear wall of the fused keyhole (Figure 4f). Afterward, the outlet obvious local bulge at the rear wall of the fused keyhole (Figure 4f). Afterward, the outlet of the of the keyho keyhole le opens, w opens, with ith the molten the molten metal metal spraying spraying downwar downwa drd and and t thehdisappearance e disappearance of of the keyhole opens, with the molten metal spraying downward and the disappearance of the the local local bulge bulg at e a the t the keyhol keyhole’se’s rear rear wall wa (Figur ll (Figu e 4 re 4g) g). . of the local bulge at the keyhole’s rear wall (Figure 4g). Figure 4. Dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” spec- Figure 4. Dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” Figure 4. Dynamic change process of the keyhole and weld pool in ultrasound-assisted laser welding of “sandwich” spec- imen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, p = 750 W, qtop = 20 L/min (N2)). specimen (p = 2000 W, v = 1.2 m/min, D = +3 umm, ltrasonic p = 750 W, q = 20 L/min (N )). imen (plaser = 2000 W, v = 1.2 m/min, laser Δ = +3 mm, p = 750 W, qtop = 20 L/ ultrasonic min (N2)). top 2 ultrasonic Photonics 2021, 8, 359 7 of 11 Photonics 2021, 8, 359 7 of 11 To sum up, the keyhole’s rear wall is always subject to fluctuation in conventional To sum up, the keyhole’s rear wall is always subject to ﬂuctuation in conventional laser laser welding, especially, the bottom of which is prone to local bulges; moreover, poor welding, especially, the bottom of which is prone to local bulges; moreover, poor stability of stability of the bottom of the keyhole can easily lead to necking down of the bottom and the bottom of the keyhole can easily lead to necking down of the bottom and further cause further cause bubbles, thus forming keyhole-induced porosity [6,7]. Under the same weld- bubbles, thus forming keyhole-induced porosity [6,7]. Under the same welding parameters, ing parameters, the keyhole in ultrasound-assisted laser welding is easier to penetrate, the keyhole in ultrasound-assisted laser welding is easier to penetrate, with less local with less local bulging and longer time of stability maintaining at the keyhole’s rear wall, bulging and longer time of stability maintaining at the keyhole’s rear wall, compared with compared with conventional laser welding. Despite the collapse, the formation of the new conventional laser welding. Despite the collapse, the formation of the new keyhole is keyhole is relatively stable. Local bulging at the keyhole’s rear wall is often accompanied relatively stable. Local bulging at the keyhole’s rear wall is often accompanied by the by the penetration of the bottom of the keyhole to ensure its stability. penetration of the bottom of the keyhole to ensure its stability. 3.2. Effect of Welding Position on Bottom Porosity 3.2. Effect of Welding Position on Bottom Porosity The X-ray nondestructive inspection and longitudinal sectional view of weld seams The X-ray nondestructive inspection and longitudinal sectional view of weld seams at different welding positions are displayed in Figure 5. The porosities in the X-ray non- at different welding positions are displayed in Figure 5. The porosities in the X-ray destructive inspection and the bottom porosities in the longitudinal section are shown by nondestructive inspection and the bottom porosities in the longitudinal section are shown the blue and red arrows, respectively. As indicated in Figure 5, the total number of poros- by the blue and red arrows, respectively. As indicated in Figure 5, the total number of ities declines in horizontal position welding, vertical up welding and vertical down weld- porosities declines in horizontal position welding, vertical up welding and vertical down ing, compared with flat welding (Figure 2a). Among them, the largest total number of welding, compared with ﬂat welding (Figure 2a). Among them, the largest total number of porosities is obtained in vertical up welding, with some bottom porosities (Figure 5a). The porosities is obtained in vertical up welding, with some bottom porosities (Figure 5a). The optimum s optimum suppr uppression ession o of f p por orositie osities s is is ac achieved hieved in in vert vertical ical d down own we welding, lding, w without ithout bot bottom tom p por orosit osities ies (Fi (Figur gure e 5b 5b). ). The The p por orosit osities ies in in horizont horizontal al p position osition wel welding ding are are fewe fewer r t than han t those hose in in fl ﬂat at we welding, lding, as as in indicated dicated in in F Figur igure e 5c 5c. . The The res results ults iindicate ndicate tthat hat t the he pot potential ential o of f h having aving fewer poro fewer porosities sities is is ach achieved ieved wit with h t the he we welding lding pos position ition of vert of vertical ical d down own weld welding. ing. Figure 5. Figure 5.Distri Distribution bution of porosities at of porosities at diff different weld erent welding ing posit positions: ions: ((a a) ) vertica vertical l u up p wel welding, ding, ((b b) vert ) vertical ical down welding and (c) horizontal position welding. down welding and (c) horizontal position welding. The dyn The dynamic amic chan change ge proc process ess of the of the ke keyhole yhole an and d w weld eld pool pool in in v vertical ertical down down la laser ser welding of “sandwich” specimens is given in Figure 6. As shown in Figure 6, the ﬂuctuation welding of “sandwich” specimens is given in Figure 6. As shown in Figure 6, the fluctua- of the keyhole in vertical down laser welding is smaller than that in conventional laser tion of the keyhole in vertical down laser welding is smaller than that in conventional welding. The shape of the keyhole is stable with great brightness inside at t = 0. 147 s laser welding. The shape of the keyhole is stable with great brightness inside at t = 0. 147 (Figure 6a). Subsequently, the bottom of the keyhole necks down and collapses successively s (Figure 6a). Subsequently, the bottom of the keyhole necks down and collapses succes- (Figure 6b,c). Soon afterward, the keyhole moves deep to the bottom again, with a complete sively (Figure 6b,c). Soon afterward, the keyhole moves deep to the bottom again, with a new keyhole formed (Figure 6d,e). With the progress of welding, laser energy accumulation complete new keyhole formed (Figure 6d,e). With the progress of welding, laser energy is found successively at the middle and middle-upper parts of the keyhole, accompanied by accumulation is found successively at the middle and middle-upper parts of the keyhole, a local bulge in the keyhole’s rear wall but without collapse, as demonstrated in Figure 6f–j. accompanied by a local bulge in the keyhole’s rear wall but without collapse, as demon- strated in Figure 6f–j. Photonics 2021, 8, 359 8 of 11 Photonics 2021, 8, 359 8 of 11 Photonics 2021, 8, 359 8 of 11 Figure 6. Dynamic change process of the keyhole and weld pool in vertical down laser welding of “sandwich” specimen Figure Figure 6. 6. Dynamic Dynamic c change hange proc process ess of the keyhol of the keyhole e and and weld pool in vert weld pool in vertical ical down laser down laser welding welding of “sa of “sandwich” ndwich” spec specimen imen (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 l/min (N2)). (p = 2000 W, v = 1.2 m/min, D = +3 mm, q = 20 L/min (N )). top laser 2 In summary, no obvious bulge at the keyhole’s rear wall is found in vertical down In summary, no obvious bulge at the keyhole’s rear wall is found in vertical down In summary, no obvious bulge at the keyhole’s rear wall is found in vertical down welding, and its keyhole is more stable than that in flat welding, which thus diminishes welding, and its keyhole is more stable than that in ﬂat welding, which thus diminishes welding, and its keyhole is more stable than that in flat welding, which thus diminishes th the e form formation ation of ke of keyhole-induced yhole-induced ppor oroosity sity at at it its s sou sour rce. Mo ce. Mor reov eover er, com , compar pareed d wit with h fl ﬂat at the formation of keyhole-induced porosity at its source. Moreover, compared with flat welding, the force direction on its weld pool along the welding direction is consistent with welding, the force direction on its weld pool along the welding direction is consistent welding, the force direction on its weld pool along the welding direction is consistent with the gra with the vity, and thus the gravity, and thus wel the d pool weld ipool s length is lengthened, ened, and the escape and the e time of bubb scape time ofles bubbles is prois - the gravity, and thus the weld pool is lengthened, and the escape time of bubbles is pro- longed prolonged [17]. [17 Therefore ]. Therefor , th e, e number the number of porositi of porosities, es, especi especially ally tha that t ofof bottom porosi bottom porosities, ties, is is longed [17]. Therefore, the number of porosities, especially that of bottom porosities, is greatly re greatly reduced. duced. greatly reduced. 3.3. Effect of Shielding Gas on Bottom Porosity 3.3. Effect of Shielding Gas on Bottom Porosity 3.3. Effect of Shielding Gas on Bottom Porosity The effects of ﬂow rate and supply method of shielding gas on the total and bottom The effects of flow rate and supply method of shielding gas on the total and bottom The effects of flow rate and supply method of shielding gas on the total and bottom porosities are illustrated in Figure 7. The highest number of total and bottom porosities is porosities are illustrated in Figure 7. The highest number of total and bottom porosities is porosities are illustrated in Figure 7. The highest number of total and bottom porosities is caused when only top shielding gas is applied, as shown in Figures 7a and 7b, respectively. caused when only top shielding gas is applied, as shown in Figure 7a and Figure 7b, re- caused when only top shielding gas is applied, as shown in Figure 7a and Figure 7b, re- Compared with top shielding gas alone, the total and bottom porosities both decrease spectively. Compared with top shielding gas alone, the total and bottom porosities both spectively. Compared with top shielding gas alone, the total and bottom porosities both when only bottom shielding gas is applied, as shown in Figures 7a and 7b, respectively. The decrease when only bottom shielding gas is applied, as shown in Figure 7a and Figure 7b, decrease when only bottom shielding gas is applied, as shown in Figure 7a and Figure 7b, lowest number of total and bottom porosities is found when the top and bottom shielding respectively. The lowest number of total and bottom porosities is found when the top and respectively. The lowest number of total and bottom porosities is found when the top and gases are both applied, as shown in Figures 7a and 7b, respectively. No bottom porosity bottom shielding gases are both applied, as shown in Figure 7a and Figure 7b, respec- bottom shielding gases are both applied, as shown in Figure 7a and Figure 7b, respec- is formed occasionally at the ﬂow rate of 20 L/min, as shown in Figure 7b. The results tively. No bottom porosity is formed occasionally at the flow rate of 20 L/min, as shown tively. No bottom porosity is formed occasionally at the flow rate of 20 L/min, as shown indicate that the potential of having fewer porosities is obtained when the top and bottom in Figure 7b. The results indicate that the potential of having fewer porosities is obtained in Figure 7b. The results indicate that the potential of having fewer porosities is obtained shielding gases are both applied with a moderate ﬂow rate. when the top and bottom shielding gases are both applied with a moderate flow rate. when the top and bottom shielding gases are both applied with a moderate flow rate. Figure 7. Cont. Photonics 2021, 8, 359 9 of 11 Photonics 2021, 8, 359 9 of 11 Photonics 2021, 8, 359 9 of 11 Figure 7. Effects of flow rate and supply method of shielding gas on the number of porosities. Figure 7. Effects of flow rate and supply method of shielding gas on the number of porosities. Figure 7. Effects of ﬂow rate and supply method of shielding gas on the number of porosities. The high-speed photograph of the upper-surface weld pool and entrance of the key- The high-speed photograph of the upper-surface weld pool and entrance of the The high-speed photograph of the upper-surface weld pool and entrance of the key- hole during laser welding without shielding gas is demonstrated in Figure 8. Figure 9 keyhole during laser welding without shielding gas is demonstrated in Figure 8. Figure 9 hole during laser welding without shielding gas is demonstrated in Figure 8. Figure 9 provides the provides the high-speed high-speed photograph o photograph of f the u the upper pper- -surface surface we weld ld p pool ool and and entrance entrance of of t the he provides the high-speed photograph of the upper-surface weld pool and entrance of the keyhole keyhole whe when n top and bottom shield top and bottom shielding ing gases gases ar ar e e applied applied simultaneously simultaneously. A . As indicated s indicated in keyhole when top and bottom shielding gases are applied simultaneously. As indicated iFigur n Figu e 8 re , 8, without withou shielding t shieldigas, ng ga the s, the lengt leh ng of ththe of t upper he up -surface per-surfweld ace wel pool d p is oo about l is ab 4.0 out mm, 4.0 in Figure 8, without shielding gas, the length of the upper-surface weld pool is about 4.0 mm, wi with a width th a wi of dth of about ab 2.1 out 2.1 mm, a mm, and the nd the op opening enand ing and c closing losing cycle cycle o of thef the entrance entrance of of the mm, with a width of about 2.1 mm, and the opening and closing cycle of the entrance of t keyhole he keyhol is e i about s ab2.4 out 2. ms4 m during s durlaser ing la welding. ser weldThe ing. Th length e length o of the fupper the upper-sur -surface weld face weld pool the keyhole is about 2.4 ms during laser welding. The length of the upper-surface weld is about 7.2 mm, with a width of about 2.6 mm, and the opening and closing cycle of the pool is about 7.2 mm, with a width of about 2.6 mm, and the opening and closing cycle of pool is about 7.2 mm, with a width of about 2.6 mm, and the opening and closing cycle of entrance of the keyhole is about 5.1 ms, when top and bottom shielding gases are applied the entrance of the keyhole is about 5.1 ms, when top and bottom shielding gases are ap- the entrance of the keyhole is about 5.1 ms, when top and bottom shielding gases are ap- simultaneously. Compared to the case without shielding gas, the length and width of the plied simultaneously. Compared to the case without shielding gas, the length and width plied simultaneously. Compared to the case without shielding gas, the length and width upper-surface weld both increase, with the stability-maintaining time of the entrance of of the upper-surface weld both increase, with the stability-maintaining time of the en- of the upper-surface weld both increase, with the stability-maintaining time of the en- the keyhole prolonged, when top and bottom shielding gases are applied simultaneously. trance of the keyhole prolonged, when top and bottom shielding gases are applied simul- trance of the keyhole prolonged, when top and bottom shielding gases are applied simul- Therefore, the increase in the volume of the weld pool and improvement in the stability of taneously. Therefore, the increase in the volume of the weld pool and improvement in the taneously. Therefore, the increase in the volume of the weld pool and improvement in the the keyhole guarantee fewer bubbles caused by the collapse of the keyhole, when top and stability of the keyhole guarantee fewer bubbles caused by the collapse of the keyhole, stability of the keyhole guarantee fewer bubbles caused by the collapse of the keyhole, bottom shielding gases are applied simultaneously. Despite the bubbles formed, the larger when top and bottom shielding gases are applied simultaneously. Despite the bubbles when top and bottom shielding gases are applied simultaneously. Despite the bubbles weld pool provides the bubbles with a longer time to escape upward, and thus the porosity formed, the larger weld pool provides the bubbles with a longer time to escape upward, formed, the larger weld pool provides the bubbles with a longer time to escape upward, defects at the bottom of the weld seam are better improved. and thus the porosity defects at the bottom of the weld seam are better improved. and thus the porosity defects at the bottom of the weld seam are better improved. Figure 8. Dynamic change process of the weld pool and keyhole entrance during laser welding without shielding gas (plaser Figure 8. Dynamic change process of the weld pool and keyhole entrance during laser welding without shielding gas Figure 8. Dynamic change process of the weld pool and keyhole entrance during laser welding without shielding gas (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm). = 2000 W, v = 1.2 m/min, Δ = +3 mm). (p = 2000 W, v = 1.2 m/min, D = +3 mm). laser Photonics 2021, 8, 359 10 of 11 Photonics 2021, 8, 359 10 of 11 Figure Figure 9. 9. Dynamic Dynamic change process of the we change process of the weld ld pool and keyhole en pool and keyhole entrance trance during laser welding with both during laser welding with both top top and botto and bottom m shielding gases (plaser = 2000 W, v = 1.2 m/min, Δ = +3 mm, qtop = 20 L/min (N2), qbottom = 20 L/min (N2)). shielding gases (p = 2000 W, v = 1.2 m/min, D = +3 mm, q = 20 L/min (N ), q = 20 L/min (N )). top laser 2 bottom 2 4. Conclusions 4. Conclusions Given the limited reports on suppression of the porosity defect in the ﬁber-laser Given the limited reports on suppression of the porosity defect in the fiber-laser welding stainless steel, laser welding experiments were conducted on 304 stainless steel welding stainless steel, laser welding experiments were conducted on 304 stainless steel specimens by applying ultrasonic vibration, changing welding position and optimizing specimens by applying ultrasonic vibration, changing welding position and optimizing shielding gas. With a combination of processing experiments and high-speed photography shielding gas. With a combination of processing experiments and high-speed photog- observation of the welding process, the porosity, especially the bottom porosity defects in raphy observation of the welding process, the porosity, especially the bottom porosity the weld, was analyzed. The following conclusions were made based on the experiments. defects in the weld, was analyzed. The following conclusions were made based on the (1) The potential of having fewer porosities occurs when the ultrasonic vibration experiments. is applied with a critical value of ultrasonic power. Compared with conventional laser (1) The potential of having fewer porosities occurs when the ultrasonic vibration is welding, the welding keyhole in ultrasound-assisted laser welding is easier to penetrate, applied with a critical value of ultrasonic power. Compared with conventional laser weld- with less local bulging and longer time of stability maintaining at the keyhole rear wall, ing, the welding keyhole in ultrasound-assisted laser welding is easier to penetrate, with thus leading to reduced porosities. less local bulging and longer time of stability maintaining at the keyhole rear wall, thus (2) Welding position exerts a great inﬂuence on porosity formation in laser welding. leading to reduced porosities. The optimal suppression of porosities is achieved in vertical down welding, followed (2) Welding position exerts a great influence on porosity formation in laser welding. by horizontal position welding. No obvious bulge at the keyhole’s rear wall is observed The optimal suppression of porosities is achieved in vertical down welding, followed by in vertical down welding, and the keyhole is more stable than that in ﬂat welding, thus horizontal position welding. No obvious bulge at the keyhole’s rear wall is observed in suppressing porosities. vertical down welding, and the keyhole is more stable than that in flat welding, thus sup- (3) The potential of having fewer porosities is obtained when the top and bottom pressing porosities. shielding gases are both applied with a moderate ﬂow rate. When top and bottom shielding (3) The potential of having fewer porosities is obtained when the top and bottom gases are applied simultaneously, the length and width of the weld pool increase, with shielding gases are both applied with a moderate flow rate. When top and bottom shield- good stability of the entrance of the keyhole, thus facilitating porosity reduction. ing gases are applied simultaneously, the length and width of the weld pool increase, with good stability of the entrance of the keyhole, thus facilitating porosity reduction. Author Contributions: Conceptualization, X.P.; methodology, X.P.; software, X.P.; validation, M.Z. and Y.Z.; formal analysis, X.P.; investigation, X.P. and J.D.; resources, X.P.; data curation, X.P.; Author Contributions: Conceptualization, X.P.; methodology, X.P.; software, X.P.; validation, M.Z. writing—original draft preparation, X.P. and J.D.; writing—review and editing, Y.Z.; visualization, and Y.Z.; formal analysis, X.P.; investigation, X.P. and J.D.; resources, X.P.; data curation, X.P.; writ- Y.Z.; supervision, Y.Z.; project administration, M.Z. and Y.Z.; funding acquisition, M.Z. and Y.Z. All ing—original draft preparation, X.P. and J.D.; writing—review and editing, Y.Z.; visualization, Y.Z.; authors have read and agreed to the published version of the manuscript. supervision, Y.Z.; project administration, M.Z. and Y.Z.; funding acquisition, M.Z. and Y.Z. All au- Funding: The authors are grateful for the ﬁnancial support from the National Natural Science thors have read and agreed to the published version of the manuscript. Foundation of China (No. 51605045, 51875050), the Natural Science Foundation of Hunan Province Funding: The authors are grateful for the financial support from the National Natural Science Foun- of China (No. 2021JJ30302), and the Science and Technology Plan Project of Changsha City (No. dation of China (No. 51605045, 51875050), the Natural Science Foundation of Hunan Province of kq1907089). China (No. 2021JJ30302), and the Science and Technology Plan Project of Changsha City (No. Institutional Review Board Statement: This study did not involve humans or animals. kq1907089). Informed Institutional Consent Review Board Statement: Statement: Not applicable. This study did not involve humans or animals. Data Informed Consent Availability Statement: StatementThis : Not appl studyic does able.not report any data. 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Photonics – Multidisciplinary Digital Publishing Institute

Published: Aug 28, 2021

Keywords: fiber-laser welding; porosity; suppression; stainless steel

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Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel

Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel

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Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel

Suppression of Bottom Porosity in Fiber Laser Butt Welding of Stainless Steel

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