A study on the hot cracking behavior of laser welded aluminum alloy for automotive industry

Abstract

The laser was first introduced into the microelectronics industry in the late 1960s for sealing electronic packages and thin-wire connections. At present, laser welding is a proven joining technique in the automotive, metals (parts supply), shipyard, microelectronics, packaging, and aerospace industries. A laser has a high-power density heat source. Therefore, laser welding is recognized as an advanced process to join materials with a laser beam of high-power, high-energy density. Laser welding promising joining technology with high quality, high precision, high performance, high speed, good flexibility and low distortion. Consequently, application of laser welding is increasing with the development of novel laser apparatuses and joining processes. <br/> On the other hand, the development and commercialization of electric cars and hybrid cars, the application of aluminum to car body is gradually expanding. However, solidification cracking is frequently observed in aluminum welds. The low ductility of a semi-solid in the mushy zone and the high solidification shrinkage of aluminum alloys both increase hot cracking susceptibility. Solidification cracking is initiated by complex interactions between metallurgical and mechanical factors not by one factor. During laser welding, this can be diminished by improving chemical composition, refining solidification structure, optimizing laser pulsing parameters, and/or reducing thermal strains. This is very important that deep understanding for the affecting factors to suppress the hot cracking propagation during laser welding, Since the changing of chemical composition by adding a filler wire is not practical for the laser welding, the effects of thermal cycle and residual stress of the welds have to be considered more seriously. <br/> Various laser welding process was selected to change the thermal cycle. Also, these effects on microstructural and mechanical properties were investigated, too. Therefore, the complex physics during laser welding depending on penetration depth, the finite element method based on the thermal conduction was conducted for the simulation. <br/> Microstructural evolution highly related with thermal cycle resulted from temperature gradient and solidification rate. According to the welding parameters, the width of equiaxed region was varied and the direction of grain growth was fluctuated. Firstly, the microstructural change in the laser welds of Al 6014 alloy is analyzed. High-quality, defect-free welds are successfully produced by ARM laser which can be changed the beam profile freely. An equiaxed structure was formed at the center of the welds, and the equiaxed region compared with fusion zone width was increased as increasing of welding speed. Hot cracking susceptibility was decreased as increasing of equiaxed fraction per unit area. It means that the microstructure formation of the welds affects the hot cracking propagation. <br/> Molten pool size and shape directly influence hot cracking susceptibility. To evaluate the influence of oscillation parameters, laser beam oscillation welding was performed with different beam patterns, widths, and frequencies. The behavior of hot cracking propagation was analyzed by high-speed camera and electron backscatter diffraction. The behavior of crack propagation was observed to be highly correlated with the microstructural evolution of the fusion zone. For an oscillation with an infinite-shaped scanning pattern at 100 Hz and 3.5 m/min welding speed, the bead width, solidification microstructure, and the width of the equiaxed zone at the center of fusion were fluctuating. Furthermore, the equiaxed and columnar regions alternated periodically, which could reduce solidification cracking susceptibility. This originated from the sinuous movement of the laser beam along the longitudinal direction. Consequently, crack propagation was hindered by formation of solidification morphology.<br/> Residual stress at the welds is highly related with the bead appearance. When penetration mode changed from full to partial penetration, length of hot cracking was reduced. To analyze the influence of penetration mode, bead shape factor was analyzed and thermo-mechanical model was developed coupled with the convection heat transfer model. Heat input subjected to the workpiece would affect residual stress behavior during solidification. The strong stress was acted at the bottom place, and it is fairly well agreed with experimental results. From the results, it is confirmed that stress distribution resulted from variation of penetration mode of the welds affects the hot cracking propagation. <br/> From this study, the effects of thermal cycle and residual stress on hot cracking susceptibility, which has not been clear up to now, is described well. Experimental results and the developed model lead to a clearer understanding about hot cracking propagation during laser welding of aluminum alloy sensitive to hot cracking. These results can be used beneficially to arrange an alternative methodology to avoid hot cracking.<br/>