MICROSTRUCTURAL CHARACTERIZATION OF CO2-LASER WELDS IN THE AL-LI BASED ALLOY 8090

The microstructural development of the Al-Li-Cu-Mg-Zr alloy 8090 has b een studied after autogenous CO2 laser welding. Sheets ranging in thic kness from 1-4 mm were welded at speeds of between 20-120 mm s-1 and p owers from 1.5-3.8 kW. Optical microscopy, scanning and transmission e lectron microscopy were used to study the as-received base metal, the heat-affected zone and the solidified fusion zone. The base metal was supplied in a superplastically formable condition and thus had an unre crystallized grain structure containing 1-2 mum sized sub-grains with sub-micrometre delta' and beta' precipitates in the matrix. In the fus ion zone, the as-solidified grain structure was columnar at the interf ace with the base metal but became equiaxed in the central region of t he weld pool. The weld depth and top bead width both increased with de creasing welding speed and increasing beam power within the limits inv estigated. The fusion zone microstructure was cellular-dendritic. Inte rmetallic precipitates, which are rich in copper, magnesium, silicon ( and presumably lithium), formed in the cell/dendrite boundaries. Very fine-scale delta' precipitates were present in the as-solidified alpha -Al matrix but there was no evidence for the beta', S' and T1 phases. The heat-affected zone was only 100 mum wide and was characterized by regions of partial melting. Radiographs of welds reveal that porosity occurred predominantly along the weld centre-line. In partial penetrat ion welds, two types of pores were observed: near spherical and irregu lar. However, in fully penetrating welds, only the spherical type of p orosity was present. Overall volume fractions of porosity were measure d from metallographic sections and were found to vary with welding spe ed and weld type, i.e. partial or full penetration.