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The keyhole behaviors were observed directly by laser welding experiment with quartz glass. Based on Fresnel absorption of laser beam and multi-reflection combined with volume of fluid (VOF) method, a three-dimensional mathematical model was established to study the role of welding speed on keyhole behaviors and keyhole-induced porosity formation in laser welding of aluminum alloy. The keyhole behaviors and weld pool fluid flow were discussed, and the result shows that, although the welding speed varies, the mechanism of keyhole collapse was similar. However, the keyhole stability at higher welding speed was improved due to reduced weld depth fluctuations, spatter number, keyhole depth to width ratio, and keyhole collapse frequency. Furthermore, the improved keyhole stability impeded the keyhole collapse, and the lower weld depth and solidification rate facilitated bubble escaping from the weld pool easily, which are the main factors for suppression of porosity at higher welding speed. [ABSTRACT FROM AUTHOR]

Highlights • Two mechanisms are responsible for keyhole-induced bubble formation. • The pressure balance equation is proposed to analyze the breaking of molten bridge by laser beam. • Large bubble and small bubble have high coalescence efficiency. Abstract Laser welding experiment with the aid of glass, and numerical simulation are carried out to study the keyhole behavior and keyhole-induced bubble formation. Two mechanisms are responsible for keyhole-induced bubble formation. The first mechanism is the strong fluid flow inside the weld pool, and the capillary instability of the whole keyhole, causing the collapse between rear keyhole wall and front keyhole wall. This mechanism contributes to most of keyhole-induced bubble formation. The second mechanism is the instability of the rear keyhole wall caused by the increase absorption of laser energy reflected by the bulge at front keyhole wall. The breaking of molten bridge is analyzed based on static pressure balance. The molten bridge with large curvature and low temperature is difficult to be broken. Bubble coalescence can be clearly observed at the bottom of the weld pool. Large bubble and small bubble have high coalescence efficiency. The bubbles at keyhole bottom have more time to escape without broken by laser beam, and the bottom part of rear keyhole wall is more easily depressed, so bubbles are easily formed at the keyhole bottom. [ABSTRACT FROM AUTHOR]

Hybrid laser-MIG (metal inert gas) welding has not been extensively applied in welding of aluminium alloy with thickness more than 10 mm because of the existence of weld defects and low joint strength. In this study, an original hybrid laser-MIG welding plus MIG welding process was developed. Double-sided hybrid laser-MIG welding method was designed and applied to the welding of 30-mm-thick Al 5083 alloy. The process was then optimized by overlaying a MIG welding upon the weld. Analysis on the weld revealed that the overlaid MIG welding played multiple roles, including making up the bead formations, eliminating the keyhole-induced porosities as well as refining grains. Along the longitudinal direction of the weld, the grains became finer from two fringes to the middle, and the mechanical properties improved accordingly. Compared with MIG welding, the hybrid laser-MIG welding plus MIG welding process showed overwhelming superiorities over the MIG welding, which will be promising in industrial applications. [ABSTRACT FROM AUTHOR]

The droplet escape condition in laser welding is established in this paper. A three-dimensional numerical model is developed to study the weld pool convection and spatter formation at full penetration during the fiber laser welding of 5083 aluminum alloy. It is found that when laser power is 9 kW, the bottom of the keyhole is dynamically opened and closed. When the bottom of the keyhole is closed, the molten metal at the bottom of the back keyhole wall flows upwards along the fusion line. When the bottom of the keyhole is opened, few spatters can be seen around the keyhole at the top surface, two flow patterns exists in the rear part of the keyhole: a portion of molten metal flows upwards along the fusion line, other portion of molten metal flows to the bottom of the keyhole, which promote the spatter formation at the bottom of the keyhole rear wall. [ABSTRACT FROM AUTHOR]

The instability of the keyhole and bubble being captured by solidification interface are the main factors that affect the formation of porosity in laser welding of both aluminum alloy and steel. In laser welding of aluminum alloy, the more violent fluid flow behind the keyhole, the larger depth to width ratio can cause the keyhole more unstable, and susceptive to collapse that can be described as three necessary steps. The much larger solidification rate and longer escaping distance make the bubble easily captured by the solidification interface, and easily combined. All of these reasons contribute to the formation of larger number and size of porosity in laser welding of aluminum alloy. The numerical and experimental results are in good agreement. [ABSTRACT FROM AUTHOR]

The droplet escape condition is established, a three-dimensional numerical model is built to investigate the behavior of the weld pool in fiber laser welding of 5083 aluminum alloy, the shear stress caused by the metallic vapor flow that has great influence on spatter formation is considered. A high speed photography system is used to capture the transient images of the weld pool, the spatter formation is discussed. The numerical and experimental results show that the spatters generate mainly around the keyhole. Two factors are responsible for the spatter formation: the surface tension of the molten metal around the keyhole is low. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow accelerate the upward moving melt around the keyhole. Increasing the welding speed and placing the nozzle at the trailing position can decrease the upward momentum of the molten metal around the keyhole, and stabilize the keyhole, which are help to suppress the formation of spatter. [ABSTRACT FROM AUTHOR]

Abstract: The metal transfer in CO2 Laser+GMAW-P hybrid welding by using argon–helium mixtures was investigated and the effect of the laser on the mental transfer is discussed. A 650nm laser, in conjunction with the shadow graph technique, is used to observe the metal transfer process. In order to analyze the heat input to the droplet and the droplet internal current line distribution. An optical emission spectroscopy system was employed to estimate default parameter and optimized plasma temperature, electron number densities distribution. The results indicate that the CO2 plasma plume have a significant impact to the electrode melting, droplet formation, detachment, impingement onto the workpiece and weld morphology. Since the current distribution direction flow changes to the keyhole, to obtain a metal transfer mode of one droplet per pulse, the welding parameters should be adjusted to a higher pulse time (TP) and a lower voltage. [Copyright &y& Elsevier]

• Metal evaporation flux across Knudsen layer is discussed in laser keyhole welding; • A high-gas pressure state is indicated at the keyhole bottom; • Evaporation flux decreases with the keyhole depth due to the high-gas pressure; • The decrease of evaporation flux suppresses the element loss in the fusion zone. This study aims to calculate the metal evaporation flux across the Knudsen layer inside the keyhole in laser welding of Al–Mg alloys. The pressure balance at the keyhole wall was analysed and used as the boundary condition in the calculation. Based on the extracted keyhole shape parameters from high-speed images, the surface temperature and gas pressure inside the keyhole were calculated, and the evaporation fluxes for both Al and Mg were acquired. The results show that the front wall of the keyhole absorbs more laser energy than the rear wall. The gas pressure inside the keyhole increases and the evaporation flux decreases with increasing keyhole depth. In addition, the evaporation flux of Mg is much higher than that of the base element Al, which can result in the loss of Mg inside the melt pool. The distribution of the Mg evaporation flux inside the keyhole agrees with the measured Mg loss in the welded section of the Al–Mg alloy. Thus, the calculation method of the metal evaporation flux inside the keyhole can be applied in future simulations of element loss in laser keyhole welding. [ABSTRACT FROM AUTHOR]

Carbon-steel filler wire was used to solve the coating problem in laser welded blanks of Al-Si-coated 22MnB5 without a decoating procedure. The effect of the filler wire on the fusion-zone microstructure and mechanical properties was investigated. The Al content was diluted by the filler wire. The δ-ferrite phase was also reduced after the reduction in Al and homogenization. The joint strength was ∼1210 MPa, and the elongation was 1.1% without filler wire. When filler wire was used, the joint strength increased to ∼1550 MPa, and the elongation was greater than 3%. The fracture surface of the joint with the filler wire was ductile. The effects of welding speed and feeding speed were also studied. A reduction in the Al content at the higher feeding speed or the use of a slower welding speed was found to be most effective because either approach enabled more filler material to be added to lower the ratio of the Al-Si coating in the fusion zone. In the absence of the filler wire, the δ-ferrite phases were large and long, connecting to a bulk zone. When the filler wire was used, the δ-ferrite phases were broken up and became narrow and small with a uniform distribution in the martensite matrix. [ABSTRACT FROM AUTHOR]

Fe–Cr–Ni alloy is the most potential substitute for Ni-based alloys as consumable in low-temperature nickel steel welding. In this study, six groups of Fe–Cr–Ni weld metals with different chemical composition were fabricated by single-pass laser welding. The volume fraction of retained austenite (RA) in the weld metals in as-welded condition increased from 0 to 30.2% with the increment of amounts of alloy elements (Cr, Ni and Mn). The thermal stability of RA was investigated by deep cryogenic treatment (DCT) to see whether sufficient RA can be maintained at low temperature. The results revealed that if the initial content of RA was < ~ 18%, RA would not transform into martensite after DCT. The surrounding martensite can hinder the transformation of RA, which plays a dominant role in the cryogenic stability of RA. The higher the strength/hardness of the surrounding martensite, the stronger the resistance to the transformation of RA. The carbon content of the surrounding martensite is the crucial factor affecting its strength/hardness. However, if the alloy elements amounts were too large, the thermal stability of RA would decrease and some of RA would transform to martensite after DCT. The newly formed martensite (fresh martensite) increased strain concentration of the weld metal, which enhanced the strain energy of martensite transformation and therefore restrained the further transformation of RA. The content of RA in the Fe-13.497Cr-7.249Ni-0.93Mn and Fe-15.548Cr-7.622Ni-0.961Mn weld metals after DCT dropped to ~ 18%. The optimum of initial content of RA in the weld metals for low-temperature toughness is ~ 18%. [ABSTRACT FROM AUTHOR]

The relationship between the microstructures and cryogenic mechanical properties of the dissimilar laser welded joint between ultra-low carbon 5% Ni steel (SA645) and 304L stainless steel (AISI304L) was investigated to identify the crucial microstructural characteristics determining the low-temperature toughness of the joint. The microstructural constituents of the weld metal were changed by varying the laser beam offset towards AISI304L side. The low-temperature (−120 °C) impact energy of the weld without beam offset which consisted of full lath martensite was only 10.22 J. High-density tangled dislocations in lath martensite resulted in the embrittlement of the weld metal. With increase in the offset of laser beam, the cryogenic toughness of the weld with a dual microstructure of lath martensite and retained austenite (RA) was enhanced more than seven times (72.16 J). Areas with relatively low strain/stress caused by the strain-induced martensite transformation of RA were observed near the crack propagation path, which deflected crack propagation path and increased the resistance of crack propagation. However, the weld metal with a triplex microstructure of martensite, RA and δ-ferrite exhibited a lower cryogenic impact toughness (37.15 J) despite the high volume fraction of RA when the beam offset was high. The thermal stability of RA decreased significantly owing to the weakening of restrain effect of surrounding martensite. The improvement effect of RA on cryogenic toughness was attenuated due to the decreased thermal stability of RA and the occurrence of δ-ferrite. Research results indicate that the volume fraction and stability of RA and the existence of δ-ferrite with increasing the laser beam offset were confirmed as the pivotal microstructural factors determining the cryogenic impact toughness. • The cryogenic impact toughness of the weld was enhanced by the strain-induced martensite transformation of RA. • The restrain effect of surrounding martensite played a dominant role in the thermal stability of RA. • The content and thermal stability of RA were the pivotal microstructural factors determining cryogenic toughness of the weld. • The existence of δ-ferrite was also crucial factor affecting cryogenic toughness of the weld when beam offset was high. [ABSTRACT FROM AUTHOR]

A novel detection method of penetration status was presented for a high-power fiber laser welding. The metallic vapor and molten pool was recorded by a high-speed camera during welding process. The radiation intensity of metallic vapor, as well as the morphology of molten pool end, was calculated by image processing algorithm as image features. Four image features, the radiation intensity of metallic vapor (RIMV), the area of molten pool end (AMPE), the rear angle of molten pool end (RAMPE), and the aspect ratio of molten pool end (ARMPE), were extracted. The mean value, relative range, variation coefficient, and frequency ratio were computed for the four features to obtain the 16 characteristic parameters. Aiming at penetration status, the characteristic parameters were reorganized to form two complex indicators by the principal component analysis. Experimental results showed that the detection method was potential for online detection on the penetration status in a high-power laser welding process. [ABSTRACT FROM AUTHOR]

It is essential to acquire the electron temperature distribution of plasma plume to determine the laser energy loss caused by laser-induced plasma. In this paper, a set of synchronous high-speed cameras were positioned in different angles to obtain plasma images during CO laser welding process shielded by He-Ar mixed gas. The absorption coefficient and refractive index of incident laser was calculated based on the 3D distribution of laser-induced plasma. The results indicated that the temperature field of plasma core region was nonuniform while the temperature distribution was relatively smooth and uniform for plasma periphery portion. It was found that the shielding gas affected the 3D distribution of plasma, as well as the energy interaction between laser and base material. Laser energy loss caused by plasma increased with argon percentage in the shielding gas. Relatively, laser energy loss caused by refraction is slightly higher than that caused by absorption. [ABSTRACT FROM AUTHOR]

After N 2 was added into shielding gas, nitrogen content was increased at upper section and central section of laser weld metal. When shielding gas was pure nitrogen, the microstructure of weld cross-section tended to be homogeneous, its corresponding austenite content was over 40%. The formation of allotriomorphic and penniform austenite was promoted remarkably. The shielding gas of nitrogen was helpful to enhance the corrosion resistance of weldment in 3.5 wt.% sodium chloride (NaCl) solution. [ABSTRACT FROM AUTHOR]

This paper concerns the dynamic variation in plume velocity and keyhole size during the laser welding process of 5083 Al-alloy. The keyhole size and plume velocity were measured from high-speed images through imaging process methods and the particle image velocimetry (PIV) method. The correlation between the oscillation in plume velocity and keyhole size was discussed based on the pressure balance inside the keyhole. Results show that the keyhole keeps opening and collapsing during the welding process. During the time the keyhole is open, the size of the keyhole continuously oscillates until it collapses. Plume velocity increases to a maximum at the initial stage of keyhole's opening, and then varies in synchronisation with the oscillation of the keyhole size. It is considered that local evaporation occurs on the humps of the keyhole front wall, which results in the oscillation in plume velocity. The oscillation in plume velocity then leads to the variation in the pressure balance in the keyhole and drives the keyhole to expand or shrink accordingly. [ABSTRACT FROM AUTHOR]

• Keyhole and plume features were automatically extracted from the image processing method; • Effects of Mg content was helpful to keep the opening of keyhole; • Effects of Mg content leaded to the frequent swelling generation at the rear weldpool. This paper concerns the effects of Mg content on keyhole behaviours during the laser welding process of 5754, 5083 and 5A06 Al-Mg alloys. The features of the vapour plume and keyhole area are extracted from high-speed images with image processing methods. The increase of Mg content leads to the increase of hydrodynamic pressure caused by metal evaporation and decrease of surface tension acting on the keyhole wall. The strong metal evaporation and decrease of surface tension help the formation of keyhole, which both result in a large size of keyhole with deep penetration and low frequency of keyhole collapse. However, the impact of metal vapour could be enhanced by the increase of evaporation and the decrease of both surface tension and viscosity, which leads to the frequent generation of swelling at the rear weldpool and poor formation of the weld bead. [ABSTRACT FROM AUTHOR]