Investigation of the microstructural and mechanical properties of DP600 and TRIP800 steels welded by fiber laser welding method

In this study, DP600 and TRIP800 steels were welded as DP600 – DP600 and TRIP800 – TRIP800 by fiber laser welding method using 0 mm and negative focal lenghts and different welding speeds. Beam width, crater width and crater length measurements, RT and PT inspections done for all samples. It was observed that the increase in welding speed and the change in focus length affected the beam width, crater length, crater width and the amount of the shrinkage cavity. In the samples using 0 mm focal length for both material groups, the HAZ width in the both sheets decreased due to the increase in the cooling rate due to the increase in the welding speed; In the samples using negative focal length, there was an increase in the upper sheet HAZ widths due to the change of the cooling profile depending on the focusing geometry of the laser beam. As a result of the tensile shear test for all parameters, the damage areas shifted towards the weld pool area due to the increase in the welding speed, and tensile-shear force values and elongation values of both DP600 and TRIP800 samples decreased due to decreasing penetration depth and brittle fracture surface morphology was determined in the samples using negative focal lenght and the highest welding speed in both material groups. By virtue of the microstructure and microhardness analyzes, the hardness values of the samples obtained by welding DP600 material with a focal lengthof 0 mm increased mainly due to the phase transformations of ferrite, martensite, lath martensite, temper martensite, temper bainite. In the samples welded using - 1.4 mm focal lenght, the hardness values decreased due to the increase in welding speed due to the formation of carbide phases. Similarly, when TRIP800 samples are evaluated, ferrite, residual austenite, bainite, martensite, austenite and lath martensite phases were determined mainly in the samples using 0 mm focal length; Cementite phases precipitated in martensite and bainite phases were detected in samples using - 1.7 mm focal length. According to the microhardness results of the samples made with both focal lengths, it was observed that the microstructures formed with the increase in hardness values due to the increase in welding speed were compatible.