The microstructure and impact toughness of vacuum electron beam welded joints of a highly alloyed dual phase titanium alloy

Vacuum electron beam welding of 50 mm thick TC18 titanium alloy plates were carried out at different processing parameters, and full penetration welded joints were obtained. The welded joint presents three typical zones, namely, fusion zone, heat affected zone and base metal zone. It is found that processing parameters have no obvious influence on the width of heat affected zone. By using EBSD technology, the changes of phase composition, grain size and grain boundary density from the fusion zone to heat affected zone and then to base metal zone were clearly observed and quantitatively analyzed. The fusion zone shows coarse equiaxed/columnar beta grains with an average diameter/width of more than 100 mu m. The volume fraction of alpha phase in the heat affected zone decreases with the decrease of the distance to the fusion zone until it is completely dissolved into beta phase. The formation mechanism of microstructure in different zones and its influence on impact toughness were discussed. The beta grain size and undissolved/precipitated fine alpha grain on beta grain boundary are two main factors affecting impact toughness.