INVESTIGATION INTO PROPERTIES OF LASER WELDED SIMILAR AND DISSIMILAR STEEL JOINTS

Laser beam welding is currently used in the welding of steels, alumini um alloys, thin sheets, and dissimilar materials. This high power dens ity 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 t he metallurgical and mechanical properties of laser welds and the resp onse of conventional materials to this new process are not yet fully e stablished. The welding process may lead to drastic changes in the mic rostructure 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 are welding processes. This leads to the formatio n 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 martensi te. It is currently difficult to determine the tensile properties (ful l stress-strain curves) of the laser welded joint area owing to the sm all size (similar to 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 join t performance in service as well as for conducting a defect assessment procedure on such welded joints. Therefore, an experimental investiga tion into the mechanical properties of laser welded joints was carried out to establish a testing procedure using flat microtensile specimen s (0.5 mm in thickness, 2 mm in width) for determination of the tensil e properties of the weld metal and HAZ of the laser beam welds. Three similar joints, namely St 37-St 37, St 52-St 52, and austenitic-austen itic, and two dissimilar ferritic-austenitic joints were produced by C O2 laser using 6 mm thickness plates. The mechanical properties have b een examined by microhardness survey and testing of conventional trans verse tensile, round tensile, and flat microtensile specimens. The res ults for the microtensile specimens were compared with those for stand ard round tensile specimens and this clearly showed the suitability of the microtensile specimen technique for such joints.