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

Abstract

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.