Investigation into properties of laser welded similar and dissimilar steel joints

Abstract

Laser beam welding is currently used in the welding of steels, aluminium alloys, thin sheets, and dissimilar materials. This high power density welding process has unique advantages of cost effectiveness, deep penetration, narrow bead and heat affected zone (HAZ) widths, and low distortion compared to other conventional welding processes. However, the metallurgical and mechanical properties of laser welds and the response of conventional materials to this new process are not yet fully established. The welding process may lead to drastic changes in the microstructure with accompanying effects on the mechanical properties and, hence, on the performance of the joint. The thermal cycles associated with laser beam welding are generally much faster than those involved in the conventional arc welding processes. This leads to the formation of a rather small weld zone that exhibits locally a high hardness in the case of C–Mn structural steels owing to the formation of martensite. It is currently difficult to determine the tensile properties (full stress–strain curves) of the laser welded joint area owing to the small size(∼V 2·3 mm) of the fusion zone. Complete information on the tensile and fracture toughness properties of the fusion zone is essential for prequalification and complete understanding of the joint performance in service, as well as for conducting a defect assessment procedure on such welded joints. Therefore, an experimental investigation into the mechanical properties of laser welded joints was carried out to establish a testing procedure using fiat micro tensile specimens (0·5 mm in thickness, 2 mm in width) for determination of the tensile properties of the weld metal and H AZ of the laser beam welds. Three similar joints, namely St 37–St 37, St 52–St 52, and austenitic–austenitic, and two dissimilar ferritic–austenitic joints were produced by CO2 laser, using 6 mm thickness plates. The mechanical properties have been examined by microhardness survey and testing of conventional transverse tensile, round tensile, and fiat microtensile specimens. The results for the micro tensile specimens were compared with those for standard round tensile specimens and this clearly showed the suitability of the microtensile specimen technique for such joints.