Effect of laser welding parameters on 6061 aluminium alloy

Abstract

Aluminium alloys are widely used in aircraft industry which involves a large number of joint configurations, traditionally fabricated by riveting. Disadvantages of riveting are extra weight to the structure, stress concentration, crevices corrosion and loosening due to vibrations. Moreover, it is a time consuming and manpower intense task. Laser welding can be a potential tool which is superior to other joining techniques, such as, arc welding due its heat input nature and related microstructural effect.The present study is focused on improving metallurgical and mechanical characteristics of aluminium alloy (6061-T6) sheet butt welds obtained by common single pass CO2 laser by refining the microstructure with extra laser pass/remelting. Initially, screening experiments were performed to enumerate the power density requirement for proper coupling of the laser beam in Gaussian as well as Donut beam profile, effect of shielding gas (He, Ar, and without shielding gas) flow rate, and welding speed which can yield stable keyhole, smooth weld surface and minimum weld defects (porosity, under bead, etc.). From the butt welded specimen, weldability of the alloy was studied in terms of joint strength, formability, fatigue and corrosion behaviour. The weld structure was verified with peak temperature and thermal profile analysis. The effect of the additional laser weld pass(bead on bead/remelting) on the existing butt weld was also studied by comparing the results of hardness, tensile tests, formability, fatigue test and corrosion study From the detailed characterization it was observed that with a particular process window only coupling of the laser with the material was possible and Helium showed best result compared to other shielding gas conditions used in the study. The microstructures revealed defect free welds and hardness data confirmed presence of heat affected zone (HAZ) and huge drop in overall hardness after welding. After post weld treatment (aging cycle) the hardness was increased marginally but base hardness could not be achieved and double bead weld showed marginally higher hardness due to finer structure. In both single and double pass welds sharp drop in tensile strength were observed due to cast structure, not availability of age hardening effect and possible micro porosity. Formability value of the welds was low compared to the base material due to the thermal strain in the welds.