AN EVALUATION OF CO2-LASER BEAM-WELDING ON A TI3AL-NB ALLOY

The CO2 laser beam weld-ability of Ti-14.3 wt-% Al-21 wt-% Nb, a Ti3Al -Nb (alpha2 aluminide) alloy, was investigated. A sequence of pulsed a nd continuous wave (CW) CO2 laser beam welds was made on 1.7-mm (0.067 -in.) thick coupons of Ti-1421. The weld cross-sections were subjected to micro-hardness evaluation and characterized by optical metallograp hy. The hardness values were plotted with respect to calculated coolin g rates to reveal possible trends in microhardness. Laser welds were p roduced without cracks, porosity or other discontinuities. Laser weldi ng with calculated cooling rates between 195-degrees and 10,400-degree s-C/s produced relatively constant microhardness values. These values ranged between 373 and 432 DPH for the heat-affected zone (HAZ), and b etween 364 and 416 for the fusion zone (FZ). These results are in cont rast to Mascorella's results for gas tungsten arc welding, which showe d microhardness values have an increasing trend with increased cooling rate (from 297 to 488 HAZ and from 292 to 459 FZ for cooling rates fr om 2 to 50-degrees-C/s) Bend ductility values within the unaffected ba se metal (UBM) range were obtained in full penetration longitudinal be nd specimens for laser welds with calculated cooling rates above 3400- degrees-C/s. For cooling rates of 1380-degrees-C/s and lower ductiliti es were well below the UBM range. The slower cooled welds exhibited a large flat cleavage fracture area and lower ductility than the rapidly cooled welds. The results of this investigation show that careful sel ection of CO2 laser welding conditions will result in cooling rates th at will yield more ductile HAZs and FZs when compared to gas tungsten arc welding (GTAW).