Structural evolution of pulsed Nd-YAG laser welds of AISI 1006 steel

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Abstract
Although correlations of welding parameters with the metallurgical features of conventional fusion welds in low carbon steels are well established, information on process–structure–property relationships associated with pulsed laser welds is more limited. This paper presents results on the characterisation of weld metal and heat affected zone (HAZ) microstructures observed in laser welded AISI 1006 steel. Pulsed Nd-YAG laser welds in the bead on plate configuration were used for this purpose, both in overlapping and non-overlapping bead configurations. As very rapid heating and cooling cycles occur during laser welding, the microstructures observed in the weld metal are the result of rapid solidification producing thin columnar austenite grains extending from the fusion boundary, which transform to martensite and bainite during fast cooling to ambient temperature. The HAZ structure in the base plate can also be rationalised in terms of the rapid thermal cycling experienced. The HAZ is narrow with the intercritical reheated subzone being dominant. As microstructural development has a critical effect on the mechanical properties of welds, microstructural characterisation plays an integral role not only in the understanding of pulsed laser welding, but also in the selection of optimum welding conditions for the material of interest.