The surface absorption coefficient of S304L stainless steel during Nd:YAG pulse laser welding process was derived by using the inverse engineering technique. The thermal-elastic-plastic finite element method is employed to calculate the temperature distribution, melting pool depth and shape with specific heat source. The derived melting pool depths and diameters with different values of absorption coefficient were compared with experimental results. In this study, the least square error method is used to obtain the equivalent absorption coefficient. The results show that the equivalent absorption coefficient of S304L stainless steel is in the range of 0.1~0.25 for low power intensity Nd:YAG impulse laser welding.

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Both experimental and numerical analyses into the shear toughness parameter for Sn/3.0Ag/0.5Cu and 63Sn/37Pb solder ball joints are performed. The ball shear tests are conducted at the loading speeds of 200μm/s and 300μm/s using ball joint specimens with diameters of 300, 600 and 760 μm. The failure behavior of the solder joints is quantified in terms of their fracture toughness. The results show that the shear toughness increases with an increasing solder ball diameter. Furthermore, it is shown that the Sn/3.0Ag/0.5Cu solder ball has a greater ductility than the eutectic 63Sn/37Pb solder ball. It is found that the shear toughness with great differentiability is relatively insensitive to the loading speed, and is therefore a suitable parameter with which to evaluate the ductility of solder ball joints in ball shear tests. Finally, the finite element analysis is further employed in the simulation with the software of MSC, Marc. Numerical predictions have good agreements compared with experiment ones.

The effects of isothermal aging and the thermal cycling loading on the shear toughness of different solder materials and ball sizes have been explored. The difference between shear toughness values of traditional Sn/37Pb eutectic solder ball joints and the lead free Sn/3.0Ag/0.5Cu solders are chosen for discussion. The experiment measurements under the ball shear test (BST) have been compared and studied for both solder joints. The fracture behaviors of the solder joints under the high temperature aging and thermal cycling testing are examined by scanning electron microscope (SEM). The variation of shear toughness of different ball joints reveals that the high temperature aging and thermal cyclic loading reduce the shear toughness significantly. The measured shear toughness values indicate that the Sn/3.0Ag/0.5Cu solder joints have better ductility for the joints undergoing the high temperature aging and the thermal cycle loadings. Based on the measured results, the better reliability for the Sn/3.0Ag/0.5Cu ball joints is expected, due to the aging and cycling load testing.

The weld bead temperature distribution and shape during pulsed Nd:YAG laser lap welding are studied. A volumetric heat source model is derived to include the surface flux and the keyhole heat transfer effects in the pulsed laser lap welding process. The proposed pulsed laser heat transfer mode is employed in a simulation with the commercial finite element software Marc. The numerically computed results of the weld pool dimensions are compared with the experimental results. The comparison shows a good agreement between the simulated and measurement results, indicating that the proposed model is feasible. The results reveal that the pulse duration and spot pitch have considerable influence on the temperature field distribution and the residual stress distribution.