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Gas protection is a crucial part of quality control in laser welding, especially for titanium alloy, which oxidizes easily at high temperatures. Substantial experiments concerning shielding gas characteristics in the welding process have been implemented. However, the common analysis conducted is simplistic and lacks a theoretical basis. This paper presented an investigation of the shielding gas behaviors based on numerical simulation and a titanium alloy laser welding experiment. The numerical model was established and validated by experiment. Subsequently, the temperature field and gas flow fields were calculated. By combining the two fields, the threshold temperature of gas protection was determined, and the influence of shielding gas parameters on the protection effect was examined. The results revealed that the protection of the high-temperature zone was primarily influenced by the nozzle height, nozzle inner diameter, and nozzle angle, while the plasma suppression effect was mainly correlated with the nozzle inner diameter and gas flow rate. These initial findings provide scientific guidance for the better quality production of laser beam welded components made of not only titanium alloy but also other metallic materials. [ABSTRACT FROM AUTHOR]

In the narrow-gap multi-layer welding of thick Ti-6Al-4V titanium alloy sheets, reducing the number of filling layers can effectively improve the welding efficiency and reduce the possibility of interlayer defects. In order to explore the changes in the microstructure and properties of the weld after reducing the number of filling layers, Ti-6Al-4V titanium alloy sheets with a thickness of 20 mm were successfully welded using the oscillating laser beam mode by laser narrow-gap multi-pass wire filler welding in eight, six, four, and three layers, and all of the formations were good. To reduce the number of filling layers and increase the welding line energy from 0.4 kJ/mm to 1.2 kJ/mm, the melting depth and width of the single layer were changed from 4.3 mm to 10.6 mm, and 5.7 mm to 10.3 mm. The average grain size of the needle-shaped martensite increased from 1.83 μm to 2.38 μm, while the tensile strength of the filled weld area decreased from 1301.8 MPa to 1169.8 MPa, which was higher than that of the base metal of 902.1 MPa. Since there are more columnar crystals in the center of the weld at low heat input, the impact energy was 20.53 J (60.6% of the base metal) at room temperature and 15.76 J (65.9% of the base metal) at −50 °C. Considering the weld formation, microstructure and mechanical property, welding four layers of fillers obtained with moderate line energy (0.8 kJ/mm) was more suitable. [ABSTRACT FROM AUTHOR]

Highlights • Full penetration laser welding was performed under atmosphere and sub-atmosphere. • There was a significant change of cross section with a decrease in ambient pressure. • The processing window of laser welding under atmosphere was very narrow. • Laser welding under vacuum showed excellent weld quality and wide processing window. Abstract In order to show the advantages of laser welding under vacuum or sub-atmosphere in fabrication of thick section structures, fiber laser welding of thick high-strength steel was performed under atmosphere and sub-atmosphere, respectively. The effect of ambient pressure on penetration depth and cross section was investigated, and the processing window in full penetration welding of 10 mm thick high-strength steel under atmosphere and sub-atmosphere was compared. The results showed that the processing window of full penetration laser welding under atmosphere (101 kPa) was very narrow because of the appearances of various weld defects. Full penetration sound welds could only be achieved in a small processing window near the parameter of 10 kW laser power and 1.5 m/min welding speed. It was found that there was a sharp increase in penetration depth and a significant change of cross section from “nail-head” shape to parallel shape with a decrease in ambient pressure. Full penetration welds joint under sub-atmosphere (0.1 kPa) displayed high weld quality with extreme smooth surface appearances. The processing window under sub-atmosphere with laser power ranging from 4 kW to 6 kW and welding speed ranging from 0.5 m/min to 1.5 m/min was much wider than that under atmosphere. [ABSTRACT FROM AUTHOR]

Double-sided laser beam welding—successive and simultaneous—is a promising method for joining the skin-stringer T-joints in the aircraft industry, because higher production efficiency and lower airframe weight can be achieved. In this work, a comparative study on the microstructure, mechanical properties, and porosity defects of the successive and simultaneous double-sided laser beam-welded AA6056-AA6156 aluminum alloy T-joints was conducted. The results indicate that there are no significant differences in the microstructure near the fusion line in the two cases, but the microstructure characteristics are markedly different at the centre of the fusion zone. The micro-hardness of the simultaneous weld fusion zone is lower than that of the successive weld unaffected fusion zone but higher than that of the successive weld reheat region. Lower head tensile strength and higher hoop tensile strength are obtained for the successively welded specimens. For the two T-joints, the failure propagates along the fusion layer upon tensile loading, but their final fracture positions are distinctly different. The radiographs of the optimized successively and simultaneously welded T-joints show that many porosity defects exist in the successively welded T-joints, whereas there are very few and even no porosity defects exist in the simultaneously welded T-joints. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional model was developed to investigate the temperature fields during a double-sided laser welding process of T-joints, and the correlations between the thermal characteristics and the mechanical properties were researched in details. To verify the modelling results, welding experiments were conducted with two different welding parameters and the geometrical dimensions of the weld pool were measured. It was found that there was a good agreement between the calculated and the measured results. The calculated results showed that the temperature profile was bilateral symmetry along the stringer centre, and the temperature gradient became greater as running far from the stringer centre, especially on the skin side. All of the tensile specimens were fractured along the fusion line on the skin panels for the head and the hoop tensile tests. The loss of the alloying elements near the fusion line on the skin side resulted in the lowest micro-hardness value appeared here, and made it to be the weakest region of the welded T-joints. The calculated thermal cycles indicated that the materials closest to the fusion line on the skin side had been staying at higher temperature for a longer time, and which is the essential reason for the fracture behaviour, micro-hardness distribution and alloying elements distribution of the double-sided laser welded T-joint. [ABSTRACT FROM AUTHOR]

In the current work, T-joints consisting of 2.0 mm thick 2060-T8/2099-T83 aluminum-lithium alloys for aircraft fuselage panels have been fabricated by double-sided fiber laser beam welding with different filler wires. A new type wire CW3 (Al-6.2Cu-5.4Si) was studied and compared with conventional wire AA4047 (Al-12Si) mainly on microstructure and mechanical properties. It was found that the main combined function of Al-6.2%Cu-5.4%Si in CW3 resulted in considerable improvements especially on intergranular strength, hot cracking susceptibility and hoop tensile properties. Typical non-dendritic equiaxed zone (EQZ) was observed along welds’ fusion boundary. Hot cracks and fractures during the load were always located within the EQZ, however, this typical zone could be restrained by CW3, effectively. Furthermore, changing of the main intergranular precipitated phase within the EQZ from T phase by AA4047 to T 2 phase by CW3 also resulted in developments on microscopic intergranular reinforcement and macroscopic hoop tensile properties. In addition, bridging caused by richer substructure dendrites within CW3 weld’s columnar zone resulted in much lower hot cracking susceptibility of the whole weld than AA4047. [ABSTRACT FROM AUTHOR]

In comparison with conventional laser beam welding, double-sided laser beam welding has two laser heat sources simultaneously and symmetrically loaded from both sides makes it to be a more complicated coupled heat transport and fluid flow process. In this work, in order to understand the heat transfer and fluid flow, a three-dimensional model was developed and validated with the experimental results. The temperature field, fluid flow field, and keyhole characteristic were calculated using the developed model by FLUENT software. Calculated results indicated that the temperature and fluid flow fields were bilateral symmetry along the stringer center, and the molten pool maximum length was located near the keyhole intersection position. The skin side had higher temperature and faster cooling speed. Several characteristic flow patterns in the weld pool cross section, including the vortexes flows near the keyhole opening position, the convection flows above the keyhole intersection location, the regularity downward flows at the molten pool bottom. And in the lengthwise section, a distinct vortex flow below the keyhole, and the liquid metal behind the keyhole first flowed to near the molten pool maximum length location and then to the molten pool surface. Perpendicular to and along welding direction the keyhole liquid metal flowed to the weld molten pool surface and around the keyhole, respectively. The special temperature fields and fluid flow patterns were closely related to the effects of the double sides’ laser energy coupling and enhancement. The calculated weld pool geometry basically in good agreement with the experimental results indicated that the developed model was validity and reasonable. [ABSTRACT FROM AUTHOR]

In the aircraft industry, double-sided laser beam welding is an approved method for producing skin-stringer T-joints on aircraft fuselage panels. As for the welding of new generation aluminum-lithium alloys, however, this technique is limited because of high hot cracking susceptibility and strengthening elements’ uneven distributions within weld. In the present study, a new technique of skin embedded wire double-sided laser beam welding (LBW) has been developed to fabricate T-joints consisting of 2.0 mm thick 2060-T8/2099-T83 aluminum-lithium alloys using eutectic alloy AA4047 filler wire. Necessary dimension parameters of the novel groove were reasonably designed for achieving crack-free welds. Comparisons were made between the new technique welded T-joint and conventional T-joint mainly on microstructure, hot crack, elements distribution features and mechanical properties within weld. Excellent crack-free microstructure, uniform distribution of silicon and superior tensile properties within weld were found in the new skin embedded wire double-sided LBW T-joints. [ABSTRACT FROM AUTHOR]

Fiber laser welding of aluminum alloy under vacuum was performed in this study. To understand the increases in weld penetration depth and changes in weld cross section profile in laser welding under vacuum or subatmospheric pressure, the effect of ambient pressure on interaction between laser radiation and plasma plume was studied. The electron temperature, electron density and inverse bremsstrahlung absorption of laser energy were calculated by spectroscopic analyses. The attenuation and refraction of laser energy from plasma plume were studied by probe laser beam measurements. It was found that the inverse bremsstrahlung absorption, attenuation and refraction of laser energy were sensitive to ambient pressure and decreased with the decrease of ambient pressure. Based on the effect of ambient pressure on boiling temperature of materials and interaction between laser radiation and plasma plume, the laser energy absorption mechanisms between atmosphere and vacuum was compared. The finding suggested that the decreasing of plasma plume disturbance, the increasing of the laser energy in keyhole and the decreasing of boiling temperature changed the absorption mechanism of laser energy, resulting in a large aspect ratio with narrow and parallel sides of weld cross section profile under vacuum. [ABSTRACT FROM AUTHOR]

GH3535 superalloy is the candidate structural material for the next-generation nuclear reactors and its related welding problem is inevitable and needs to be solved. In this work, fiber laser welding of 4-mm-thick GH3535 superalloy in continuous wave (CW) and pulsed wave (PW) was comparatively investigated. The weld appearances, dynamic behavior of weld pool, microstructure and mechanical properties in both cases were studied. The results indicated that larger porosities were present in the bottom of the CW joint while few porosities appeared in the PW joint. The laser weld zone of both CW and PW joints was still composed of γ-Ni matrix and M 6 C carbides. Grain size of PW joint was smaller than that of CW joint. More precipitated particles were found in the PW joint. The presence of porosity in the CW joint reduced mechanical properties greatly. The tensile strength decreased to 85% of that of base metal at room temperature, and 82.3% at high temperature while the PW joint strength could reach 97.6% and 95.7% at room and high temperature, respectively. The fracture surface of CW joint had a flat pattern and porosities, while PW joint exhibited dimples on the fracture surface both at room and high temperature. [ABSTRACT FROM AUTHOR]

Pulsed laser spot welding of intersection points of zirconium alloys straps was performed. Weld bead size, microstructure and the corrosion behavior of weld bead were investigated. With the increasing laser peak power or number of shots, the weld width of the beads increased, the protrusion decreased and the dimple increased with further increase in heat input. The fusion zone consisted of a mixture of αZr and residual βZr phases. After annealing treatment, βNb and Zr(Fe, Nb) second phase particles were precipitated inter- and intragranular of αZr grains adequately. The oxide thickness of annealed weld bead was about 3.90 μm, decreased by about 18.1% relative to the 4.76 μm of as-welded specimen corroded at 400 °C and 10.3 MPa for 20 days. The corrosion resistance of annealed specimen was better than that of as-welded specimen, since the second phase particles exerted better corrosion resistance, and the content of Nb in βZr and the fraction of βZr decreased after the annealing treatment. [ABSTRACT FROM AUTHOR]

Microstructure and tensile properties of the laser welded joint of Fe-18.8Mn-0.6C TWIP steel were investigated in this research. The microstructure of fusion zone (FZ) was characterized by means of X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). TEM and in-situ SEM observation were employed to investigate the microstructural evolution and strengthening mechanism of FZ during deformation. The welded joint with a fully austenitic structure was obtained by the laser welding. The granular divorced eutectic phases (Fe, Mn)3C and inclusions formed in the interdendritic regions during the solidification of FZ. The fully austenitic structure and coarse dendrite grains were responsible for the fracture at the weld seam. The FZ exhibited a good combination of strength (e.g. tensile strength up to 1000 MPa) and ductility (e.g. total elongation up to 73%). The microstructural evolution revealed that dislocation slip was the main deformation mechanism at low strains of FZ, while at relatively high strains, mechanical twinning was the domain deformation mechanism and played an important role in improving the strength and ductility as well as the work-hardening effect of FZ. [ABSTRACT FROM AUTHOR]

AA2060/AA2099 Al-Cu-Li alloys were connected via double-sided laser welding using ER4047 Al-Si filler wires, and the joint was subjected to two kinds of post-weld heat treatment: artificial aging (AA, 165 ℃ ×20 h) and solution treatment and AA (STAA, 500 ℃ ×1 h + 165 ℃ ×20 h). The principal strengthening phases of the base metals are the T1 (Al 2 CuLi) and δ' (Al 3 Li) phases. The welding process resulted in the dissolution of T1 and δ' phases, and the formation of T (AlLiSi) and Cu-rich phases in the grain boundary (GB) of the fusion zone (FZ). The δ' phase was reprecipitated in the FZ and the T phase in the GB was characterized by an increase in the number and volume fraction when the as-welded (AW) joint was subjected to AA. In addition, the reduction of the Cu-rich phase in the GB, the reprecipitation of T1 phase in the FZ, and the spheroidization of the T phase with a uniform distribution occurred for the STAA joint. The AW joint had a tensile strength (TS) of 390 MPa and elongation (El) of 0.3%, while the AA joint had a TS of 458 MPa and El of 0.7%. The formation of the δ' phase in the FZ and the increasing in the number and volume fraction of the T phase were responsible for the improvement of the tensile properties. The STAA joint had the highest TS of 490 MPa and El of 2.7%. The reprecipitation of the T1 and δ' phases, and the smallest inter-particle spacing in the FZ led to the best tensile properties when the AW joint was subjected to STAA. [ABSTRACT FROM AUTHOR]

• Dual spot laser beam was more beneficial for the weld formation. • Interface with most homogeneous IMC was obtained under cross dual laser beam. • Joint with tensile strength of 196 MPa was obtained under cross dual laser beam. Joining dissimilar metals Al to steel in butt configuration was successfully realized by laser welding–brazing process with different laser beam models (single laser beam, cross and in–line dual laser beams). Undercut occurred at the joint produced by single laser beam and satisfactory weld formations were obtained in the joints produced by cross and in–line dual laser beams. Dual laser beam was beneficial for the reduction of interfacial intermetallic compound (IMC) thickness and homogenization of interfacial IMC morphology. Numerical models were developed to calculate the thermal cycles along brazing interface. Calculation of interfacial thermal cycles revealed that lowest peak temperatures (1028 °C, 1036 °C and 990 °C respectively for top, middle and bottom regions), shortest time of interaction at high temperature (2.2 S) and smallest temperature gradients (1036–990 °C = 46 °C) were all obtained in the joint produced by cross dual laser beam and these all made contribution to its thinnest and most homogeneous interfacial IMC. Joint with the highest tensile strength was produced when the cross dual laser beam was adopted due to its satisfactory weld formation and most homogeneous of τ 5. [ABSTRACT FROM AUTHOR]

The durable structure and robustness of T-joints in the panel materials for civil aircraft are a crucial matter of importance. In this work, the impact of the post-weld heat treatments (PWHTs) on the microstructure and mechanical properties of laser beam welded T-joints of 2060-T3/2099-T3 Al-Li alloy was analyzed. Heat-treatment of the laser beam welded T-joints was carried out in two different ways, namely, solution treatment and artificial aging (STAA) at varied duration and only artificial aging (AA) at varied duration. The microstructure and mechanical properties of the heat-treated joints were investigated using metallographic and scanning electron microscopic images, micro-hardness test, and tensile test, respectively. The results showed that, in cases of STAA, the eutectic structures on the grain boundary were partially dissolved via solution treatment (ST), and increased dispersed precipitation of the second phase in matrix resulted in significant dispersion strengthening, as well as enhanced strength and plasticity. In contrast, in the AA process, alloy elements in the matrix continued to segregate towards the grain boundary, resulting in significant grain boundary strengthening, enhanced strength, and decreased plasticity. Additionally, joint fractures were micro-porous aggregation transgranular ones in the fusion zone (FZ). The joints treated by STAA exhibited excellent plasticity compared with those treated by AA. Furthermore, the micro-hardness of welded joints treated by AA was higher than that of those treated by STAA. Indeed, the tensile strength of joints treated by STAA and AA ranged from 405 to 475 MPa, which was 81–95% of the base metal 2060-T8, though the elongation of joints treated by STAA was superior to the counterpart AA. [ABSTRACT FROM AUTHOR]

Highlights • Effects of keyhole status on melt flow and porosity formation was studied. • Convection was disappeared and vortex was emerged as keyhole connectivity reduced. • Two vortexes formed between both sides keyhole when keyhole complete asymmetry. • Weakened keyhole connection and symmetry reduced the pores evolution efficiency. • Porosity distributions agreed well with simulated results under test conditions. Abstract Effects of the keyhole status on the melt flow during the double-sided laser welding of aluminium alloy T-joint were numerically investigated. And the influencing mechanisms of the flow-induced porosity formation and evolution were also discussed. When the keyhole status changed from complete connection to complete separation, the convection above the keyhole disappeared and vortex flow was formed. Meanwhile, the vortex flow below the keyhole became more complex. As the beam separation distance increased, the vortex flow near the keyhole opening position emerged and became more and more violent, and two vortexes were formed between both sides of the keyhole. The gradually weakened keyhole connectivity and symmetry reduced the pore evolution efficiency dramatically, and subsequently formed porosity defects in the weld seam. The distribution characteristics of the flow-induced porosity defects were found to be in good agreement with the simulation results under the experimental conditions examined. [ABSTRACT FROM AUTHOR]

To develop a Ti-22Al-25Nb alloy tubular components processing method, U O forming, laser beam welding and hot gas free bulging processes were carried out. Microstructure and texture evolution of β/B2, α 2 and O phases were investigated. As for β/B2 phase, new textures (111) 0 1 - 1 and (111) 1 2 - 1 emerged during the U O forming process at 800 °С. The laser beam welding process was used to build a tube, where the original β/B2-phase rolling texture (001) 1 1 - 0 were dramatically broke up. Additionally, strong {001}〈100〉 textures appeared, which was associated with columnar grains forming during the laser beam welding process. With the bulging temperature rising from 950 °С to 980 °С, dynamic recovery played a predominant role in softening mechanism and a rolling texture (111) 1 1 - 0 emerged. As for α 2 and O phases, lath and lenticular α 2 phase as well as mixture of α 2 and O phases in a lenticular shape were observed in the undeformed joint at 950 °C. Precipitated phases along β/B2-phase grain boundaries decreased, with temperature rising from 950 °C to 980 °C. As the temperature rose, α 2 phase underwent more apparent spheroidization and grain size decreased dramatically, which restricted the deformation of α 2 phase and β/B2-phase matrix. [ABSTRACT FROM AUTHOR]

In this article, the hot deformation mechanism of a laser-welded near α TA15 Ti-alloy joint was studied by hot tensile tests conducted at temperatures ranging from 800 °C to 900 °C with strain rates ranging from 1.0 × 10 − 1 s − 1 to 1.0 × 10 − 3 s − 1 . The results show that the main hot deformation mechanism of the laser-welded TA15 Ti-alloy joint is continuous globularization, but dynamic recrystallization (DRX) plays an important role during the globularization process. The morphology of α phase exerts considerable influence on the DRX mechanism. When the lamella microstructure is dominant in the weld seam, continuous dynamic recrystallization (CDRX) is the active mechanism; whereas with the increasing fraction of equiaxed grains in the weld seam, discontinuous dynamic recrystallization (DDRX) seems to be the dominant mechanism during the tensile deformation. The globularization process can be divided into three stages based on the characteristics of the microstructure. Under such a deformation mechanism, the laser-welded TA15 Ti-alloy joint tends to have a good formability at high temperature and small strain rate. A maximum elongation of 292% was achieved at 900 °C, 0.001 s − 1 , which exhibits superplastic characteristics. [ABSTRACT FROM AUTHOR]

Abstract Bead-on-plate laser welding was performed, and the inner keyhole behavior was observed through a transparent glass using a high-speed camera at various ambient pressures. The keyhole stability was investigated with respect to the keyhole profile area and keyhole depth using image processing. The keyhole states at atmospheric pressure and vacuum were compared according to the pressure balance at the keyhole wall. The weld surface quality and penetration depth improved with decreasing ambient pressure. The keyhole stability was also sensitive to the ambient pressure. The keyhole oscillated and collapsed periodically at atmospheric pressure (101 kPa), whereas it was stable under vacuum. [ABSTRACT FROM AUTHOR]

The local high temperature of laser welding affects the mechanical properties of aluminum alloys. In this paper, a three-dimensional digital image correlation system was used to measure the displacement field of the aluminum alloy-laser welding joint under a uniaxial tensile load. By taking into consideration the metal properties of welded joints, an estimation correction scheme for updating stress was designed for anisotropic materials based on the kinematic hardening process, and the proposed method was then applied to the nonlinear virtual field method, the validity of which was verified by a finite element simulation. The elastic-plastic constitutive parameters of various local regions of the welding joint (fusion zone, heat-affected zone, transition base zone, and base zone) were then retrieved from the measured field data, and the stress–strain relationship of each sub-region was established. The results show that the mechanical properties of the laser-welded joint are weakened while the Poisson's ratio decreases with the increase of the weld distance. Furthermore, the elastic modulus of the heat-affected zone is evidently lower than that of the other sub-regions, while it also has the lowest yield strength and the highest plastic strain rate. All these results are consistent with the trend of the hardness measurements. [ABSTRACT FROM AUTHOR]

The mechanical properties of aluminium alloys can be affected by the local high temperature in laser welding. In this paper, an inversion identification method of local zone-wise elastic-plastic constitutive parameters for laser welding of aluminium alloy 6061 was proposed based on full-field optical measurement data using digital image correlation (DIC). Three regions, i.e., the fusion zone, heat-affected zone, and base zone, of the laser-welded joint were distinguished by means of microstructure optical observation and micrometer hardness measurement. The stress data were obtained using a laser-welded specimen via a uniaxial tensile test. Meanwhile, the local strain data of the laser-welded specimen were obtained by the DIC technique. Thus, the stress–strain relationship for different local regions was established. Finally, the constitutive parameters of the Ramberg–Osgood model were identified by least-square fitting to the experimental stress–strain data. Experimental results revealed that the mechanical properties of the local zones of the welded joints clearly weakened, and these results are consistent with the results of the hardness measurement. [ABSTRACT FROM AUTHOR]

In this study, the microstructure and mechanical properties of laser welds before and after post-weld heat treatment processes were studied. The results show that the tensile strength of the joints can be increased by 90 MPa by a post-weld heat treatment process at 871 °C for 6 h, exceeding the strength of the original state of the base metal. Besides, elongation of the joints are also increased to 43% by the process, whereas the elongation of as-welded joints are only 22%. In addition, the Charpy impact properties of laser welds almost do not change. Second phase precipitates, which were identified as Mo–Si rich M 6 C-type carbides by transmission electron diffraction and scanning electron microscope, were observed at solidification grain boundaries and solidification subgrain boundaries. These carbides can pin dislocations during the following tensile deformation, hence are responsible for the strengthening of tensile properties of the joints. [ABSTRACT FROM AUTHOR]

• The beneficial effect of vacuum on porosity defects was illustrated using 3D micro-CT. • The difference in weld pool behavior for various ambient pressure was visualized. • The flow pattern under low vacuum is favorable to reduce porosity defects. Laser welding trials were conducted under low vacuum to reduce porosity defects. The effect of vacuum on the characteristics of porosity was quantitatively evaluated using 3D micro-CT. To explore the mechanisms of porosity formation at atmospheric pressure and porosity defects mitigation under vacuum, the weld pool and fluid flow behaviors were directly observed by placing a transparent glass plate on one side of the weld pool. The results showed that the vacuum condition was favorable for reducing porosity, especially process porosity. It was also found that the weld pool size and fluid flow were totally different at atmospheric pressure and under vacuum. At atmospheric pressure, bubbles were trapped in the vortex induced by two opposing fluid flows, failed to escape from the weld pool and finally formed pores. Under vacuum, suppression of vapor plume and reduction in the boiling point led to a deeper keyhole and a thinner weld pool, resulting in an upward flow in the rear weld pool. In this case, the bubbles could escape easily from the weld pool with the help of the upward flow. [ABSTRACT FROM AUTHOR]