MICROSTRUCTURAL EVALUATION OF LOW AND HIGH DUTY CYCLE ND-YAG LASER-BEAM WELDS IN 2024-T3 ALUMINUM

The propensity for hot tearing and porosity in pulsed Nd-YAG laser bea m welding of 2024-T3 aluminum was observed to vary with ''beam on'' du ty cycle. High average laser output power of 1 kW and variation in dut y cycle were achieved by multiplexing a system of three 400-W average power Nd:YAG lasers with fiber-optic delivery. Operating this system w ith a greater than 60% duty cycle eliminated the severe hot tearing an d gross porosity that occurred in lower duty cycle welds made under ot herwise identical conditions. Microstructural characterization of the low and high duty cycle welds identified differences in weld morpholog y directly related to hot-crack susceptibility. Solute segregation and the distribution of low melting point phases are dependent upon the s olidification history. In addition, manipulating the input power alter s the solidification-related stress state developed in the weld fusion zone. Appropriate adjustment of the process parameters, e.g., duty cy cle, optimizes the solidification velocity, thermal gradients, and str ess state in the weld fusion zone in order to prevent hot tearing in t his crack-susceptible aluminum alloy.