Microstructural and mechanical characterization of laser-beam welding of a 8090 Al-Li thin sheet

In extension to a previous study on electron-beam welding (EBW) under vacuum on a 8090 thin sheet, the current paper reports the parallel results of laser-beam welding (LBW) of the same material. Autogenous "bead-on-plate" laser-beam welding was performed by a 3 kW CO₂ LBW machine. The power of the input laser beam, the specimen moving speed, and the focusing condition was varied from 700 to 1300 W, 1500 to 9000 mm min^-1, and 1 to 3 mm below the specimen top surface, respectively. The protection atmosphere and plasma jet were achieved by blowing either Ar or N₂ gas. The effects of using different gases were evaluated in terms of weld-line appearance, fusion-zone dimension, solute evaporation, microhardness, post-weld tensile properties, as well as porosity distribution. In comparing with the EBW results, LBW on the 8090 alloy was characterized with a higher fusion-zone depth/width ratio, cooling rate and porosity amount, and a lower solute loss and post-weld tensile strain. The primary formation mechanism for porosity was thought to be related to the collapsed key-holes during LBW under Ar or N₂ and the hydride-induced gas pores during EBW under vacuum.