Fundamental study of blue wavelength laser for welding low thickness dissimilar Cu and steel materials

Abstract

The present study reports on the application of blue wavelength (450 nm), continuous wave laser welding of Ni-coated copper to mild steel in lap joint configuration. The laser power was varied from 1 kW to 1.5 kW in incremental steps of 0.1 kW whilst the welding speed was kept constant at 6 m/min. Metallographic examination of the weld samples revealed that the weld penetration and width strongly correlated to the laser power, the characteristic of a stable welding process due to the high absorption of the blue wavelength laser. No substantial cracks or a large number of porosities were observed in the welds. The mechanical properties of the weld samples were characterized through tensile testing and microhardness measurements to establish the microstructure property relationship. The maximum tensile strength measured for specified weld geometries was 649 N with corresponding weld efficiency of 91%. The higher-strength welds showed a tensile fracture in the heat-affected zone (HAZ) of Cu, at the periphery of weld nuggets while lower-strength welds showed interfacial fracture due to the lack of fusion. In samples made with sufficient penetration the joint strength was controlled by the HAZ rather than the joint microstructure. A significant increase in the microhardness was measured inside the weld nugget compared to the parent materials, attributed to the formation of Cu-Fe composite microstructure owing to the inter-mixing of Cu and Fe during welding. The highest microhardness was observed in the HAZ of the mild steel due to martensitic microstructure formation in this region.