Mechanism of the crack formation and suppression in laser-MAG hybrid welded 30CrMnSiA joints

This paper investigated the crack formation and suppression mechanism in laser-metal active gas (MAG) hybrid welded 30CrMnSiA joints. The cracks were usually found in the weld crater that was located at the end of the weld bead. According to the fracture characteristics, the cracks could be considered as solidification cracks. Shrinkage cavities and intergranular films were found by means of scanning electron microscopy (SEM) observation, confirming the presence of solidification cracks. The energy dispersive spectrometer (EDS) shows severe segregation of impurity elements in the weld metal. The following differential thermal analysis (DTA) shows that there was a sudden change in the differential temperature in the range of 1130 °C-1150 °C, indicating that the low melting point eutectic inducing solidification cracks was most likely to be FeS-MnS eutectic. Finite element models for heat sources were developed. The tensile test results show that the tensile strength of weld metal reached 40 MPa at 1100 °C but dropped to about 24 MPa at 1150 °C, while the calculated highest transverse tensile stress amounted to 430 MPa. Therefore, the tensile stress at the end of the weld seam exceeded the tensile strength by approximately 400 MPa. By adopting double-layer hybrid welding process, crater cracks were successfully suppressed. The electron backscattered diffraction (EBSD) shows that the microstructure of double-layer hybrid welded joints was notably refined. The tensile stress of the weld metal was reduced and intergranular films were absent.