Influence of Post-Weld Processing Techniques on Laser Beam-Welded Al-3Mg-0.25Sc Alloy Sheets

摘要

Fusion welding in aluminium alloys had always remained an issue for aero- and auto industries. An Al-3Mg-0.25Sc alloy is classified as weldable aluminium alloys. In this study, two welding techniques, keyhole and conduction welding, have been used for joining Al-3Mg-0.25Sc sheets of thickness 1.6 mm. Experiments were carried out using different beam diameter (0.16-2 mm dia), beam power (2-4.5 kW) and laser head speed (ranging from 0.5 to 6 m/min). The results for all autogenous keyhole welding clearly demonstrated a drop in ductility (< 0.8%) compared to base metal for all beam power and welding speed combinations although the yield strength was reasonably good (175-180 MPa, i.e. > 50% of base metal). Residual stress-induced distortions affected the final shape of the product, and for that, a suitable stress relief annealing was also required. Hereby, we chose two heat treatment schedules: (1) solution treatment and annealing and (2) retrogression and re-ageing. Retrogression and re-ageing is a precipitation-controlled phenomenon and comprises of an intermediate annealing step between tempering and solutionizing temperature to dissolve the nano-clusters present in the matrix (retrogression) and re-precipitating and growing them by ageing (re-ageing). These two steps were repeated cyclically, and as a result, the strength (190 MPa) and ductility ( 9%) of the material were improved substantially. The improvement of ductility and strength was explained in detail in the light of microstructural studies.