Microstructural evolution and its effect on joint strength during laser welding of dual phase steel to aluminium alloy

Conduction laser welding of dual phase steel (DP 600) to aluminium alloy (AA 6061) was produced in lap fashion using a high power diode laser. The effect of laser interaction parameters such as power density and interaction time on the quality of the joint was studied. The joint properties such as shear strength and fractural strength of the joint were correlated with the microstructural variation along the weld zone. During welding of steel to aluminium, the formation of Fe-Al intermetallics plays a vital role in determining the shear strength of the joint. Microstructural structural analysis was done along the interface of the weld-aluminum alloy to determine the intermetallic phases (Fe2Al5 and FeAl3). Another significant phenomenon studied was the softening of the heat-affected zone along the weld-steel interface. The softening phenomenon of heat affected zone is important in determining the formability of sheets for post-welding operations such as forming. Mechanical testing such as microhardness and tensile testing was done to analyze the quality of the joint. The microhardness test showed different hardness values along the weld interface zone and the heat-affected zone. The increase in hardness in the interface zone of the weld-aluminum zone was due to the formation of brittle intermetallics. Whereas the reduction in microhardness in the subcritical heat affect zone of the weld-steel side was due to the softening phenomenon, owing to the transition of martensite to tempered martensite during welding. It was observed that a maximum shear strength of 231 N/mm was obtained when the thickness of intermetallics was in the range of 8-11 mu m. The experimental results also showed that the best quality welds were obtained under the conditions of power density and interaction time such as 1.98 kW/mm(2), 0.15 s and 2.26 kW/mm(2), 0.187 s, respectively.