The investigation of joining and mechanic/microstructure properties of the DP600, DP800, MS1200, MS1400 type high strength sheets by CMT method

The recent researches in the automotive industry aim to save fuel and minimize harmful gases emitted to the environment. In the present applications, lighter and higher strength steels have started to be used compared to the past. The materials such as dual phase (DP) and martensitic (MS) steels have been produced in order to minimize the problems that may occur in the welded joining of these special steels and methods such as CMT (Cold Metal Transfer) welding method have been developed to minimize the welding deformation of these steels. In this study, the weldability of 1 mm thick dual phase (DP600 and DP800) and 1.5 mm thick martensitic (MS1200 and MS1400) high strength steel sheets were investigated which are joined by CMT welding method. In the study, welding feed rates of 70, 100, 150 and 200 cm / min for DP600 and DP800 test samples and 70, 130, 160 and 190 cm / min for MS1200 and MS1400 materials were chosen. 0.82 mm diameter SG2 filler wire (for DP600 and DP800 samples) and 0.8 mm diameter ER110 filler wire (for MS1200 and M1400 samples) were used in the welds. In order to determine the mechanical properties of welded joints, tensile tests were applied to the samples and micro hardness values were obtained. Microstructure and SEM images were taken to determine the structural changes occurring in the welding region. In tensile tests, breaks occurred near the weld seam in the heat-affected zone (ITAB). As a result of the hardness measurement, the hardness value of weld seems were compared with the base metal. The hardness results differed according to the properties of the materials used and the filler metal properties. In the microstructure investigations it was observed that heat input generally caused a change in the grain structure of the materials, as the welding speed increased, the heat input decreased and the strength were increased.