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, 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, and postweld tensile properties, as well as porosity distribution. In comparison 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 postweld 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. (Author)