ORIGINAL ARTICLE Influence of Post-Weld Processing Techniques on Laser Beam- Welded Al–3Mg–0.25Sc Alloy Sheets Kaustav Barat 1 • K. Panbarasu 1 • Ravi Bathe 2 • K. Venkateswarlu 1 Received: 13 December 2019 / Accepted: 29 January 2020 Ó The Indian Institute of Metals - IIM 2020 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 key- hole 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. Retrogres- sion and re-ageing is a precipitation-controlled phe- nomenon and comprises of an intermediate annealing step between tempering and solutionizing temperature to dis- solve the nano-clusters present in the matrix (retrogression) and re-precipitating and growing them by ageing (re-age- ing). 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. Keywords Al-3Mg-0.25Sc alloy Á Post weld treatment Á Strength Á Ductility Á Weldment Á Laser welding 1 Introduction Strength-to-weight ratio, durability and formability desig- nate a material’s candidature for aerospace applications. Various light alloys with different strength and ductility combinations are considered for serving the aerospace sector from the beginning of past century from aluminium (Al), magnesium (Mg), Al–Si alloys, Al–Mg–Si alloys to modern Al–Li alloys [1]. Metal-based airframes (since the inception of German monoplane by Hugo Junkers [2] who retired just 1 year after its fabrication in 1915) are largely made up of AA 2024 alloy. AA 2024 still serves as a workhorse for the aviation industry and probably the best candidate for the airframes comprising of differential structures. Integral structural panels for aerospace have been proposed, and Munroe et al. [3] have done a detailed documentation on this. Integrally stiffened panel [4] con- ceptualizes an idea to eliminate rivets and integrating stringers and longerons by several joining technologies like friction stir welding [5], laser beam welding [6], and fric- tion and laser beam welding [7]. Joinability of aluminium alloys has always remained as an issue, and it is observed that difficulties persist in joining aluminium alloys using fusion process except some aluminium alloys like AA 5024, weldalite Al–Li alloy and other alloys are not weldable by fusion welding. Some specialized solid-state welding techniques like impact welding and friction stir welding are recommended for joining aluminium alloys. & K. Venkateswarlu kvenkat@nal.res.in 1 CSIR-National Aerospace Laboratories, Bangalore 560017, India 2 International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, India 123 Trans Indian Inst Met https://doi.org/10.1007/s12666-020-01891-2