Abstract

Due to the low integrated energy input, laser welds are often quenched too rapidly in the fusion region [1]; this results in an untempered martensitic structure in samples with sufficient alloy content to retard formation of ferrite, pearlite and bainite. The high cooling rate of laser welding leads to a variation in the transverse hardness profile; and consequently, a very brittle weld [2,3]. To alleviate these poor mechanical characteristics, this work investigated pre-heating and post-heating of the sample to control the rate of cooling and prevent rapid quenching. Both approaches resulted in considerable improvement in the hardness profiles. Gauge plate (2mm thick) with 0.85% and 0.5% carbon content was welded, sectioned, photographed, and the microstructure was examined. The effect of varying the workpiece translation speed, i.e the degree of post heating and pre-heating, was quantified by measuring the sample’s hardness transverse to the weld direction. The present study used a 1kW CO₂ laser (Ferranti MFKP) and a multi-pass scanning technique to control the sample’s phase transition and to achieve the desired weld characteristics. To control the rate of heating or cooling the workpiece was heated several times with 0.5 kW of unfocussed power. For the pre-heated geometry, 10 passes of the laser were used to heat up the base metal; this was followed by an additional pass to weld the sample with 1 kW laser power. Post-heating was achieved by welding the sample and then making 10 passes of the laser beam over the weld region.

© 1996 IEEE