Microstructure characterization of laser welded Ti-6Al-4V fusion zones

The as-welded microstructure of laser-welded Ti-6Al-4V is characterized as a function of CO2 key-hole mode laser welding speed. Martensitic alpha' is the predominant phase, with some alpha and retained beta. Phase transformation is affected by the cooling rate through laser welding speed. A higher welding speed of 1.6 to 2.0 m/min produced more martensite alpha' and less retained beta in the welds. 1.4 m/min welding speed produced small amounts of alpha, besides the martensite alpha'. A trace of delta titanium hydride phase seems to have formed in the weld fusion zone. Moire fringes are a common feature in the TEM microstructure, due to abundance of multi-phase interfaces. Tensile twins and clusters of dislocations indicate that plastic deformation has happened in the as-welded microstructure, indicating the local stress levels to be approaching the yield stress on-cooling during laser welding. (C) 2013 Elsevier Inc. All rights reserved.