Investigations on the weld metal composition and associated weld metal cracking in laser beam welded steel copper dissimilar joints

摘要

Cracking as one of the most common weld defects for joining dissimilar metals can lead to challenges as lack of mechanical properties, lack of joint tightness, lowered electrical conductivity or can be a point of attack for corrosion. Joints between stainless steel and copper also suffer from weld metal cracking which is caused by the effect of liquid copper on the microstructure of solidifying steel matrices. The crack formation is dependent on the dilution of copper in the weld metal, the overall metal intermixing and thermal stresses while cooling down. To investigate the crack susceptibility of laser beam welds with that material combination, lap joints with stainless steel on the top side were welded with a wide range of parameter settings to create different weld metal compositions. The joints were evaluated regarding metal intermixing, cracking, micro hardness and shear tensile strength by metallographic cross sections, x-ray spectroscopy line scans, x-ray spectroscopy element mapping and shear tensile tests to create sufficient datasets for analyses. The results show a correlation between the mixing ratio of copper and the formation of hot cracks with a critical amount of approximately 10 wco.% copper in the fusion zone. Based on that, a hypothesis in order to reduce cracks by decreasing the amount of copper in Fe-rich weld metals or to create copper dominated weld metals was put forward. The crack formation has no influence on the tensile shear strength, which is dependent on the connection width reached between the sheets with a maximum shear strength of 9.6 kN at full penetration of both sheets and shows ductile fraction behavior. Consequently, the crack formation in this welding config-uration at copper contents above 10 wco.% has to be weighed against the better mechanical properties at higher connection width and mixing ratios for joining stainless steel and copper. Hardness measurements of the weld metal show results mostly independent of the overall mixing ratio with median values of 170 HV0.1.