Investigation of the mechanical and metallurgical properties of welded joints of HC420LA steel sheet with laser (RLW) and gas (MAG) techniques

This study aims to minimize the discontinuities and defects detected in laser welding of high strength and low alloy (HSLA) HC420LA steel according to BS EN ISO 13919-1:2019. In order to eliminate the phenomena identified as defects, the applied heat input value of 0.168 kJ/mm for a sheet thickness of 2 mm was changed and HC420LA steel was joined by gas metal arc welding (MAG) and laser welding. Manual laser (ML) and robotic laser (RL) machines were used for laser welding. Three different heat input values were determined for gas metal arc and laser welding and 50x100x2 mm sheet metal parts were joined by butt welding. The microstructures of the welded joints were imaged using optical microscopy and scanning electron microscopy (SEM) and the chemical composition of the weld and heat affected zones were characterized by X-ray diffraction (XRD). The variation of microstructure, hardness and tensile strength were studied as a function of heat input. For all parameters, the weld seams were uniformly formed and no undercutting was observed. In addition, no defects such as lack of fusion, root concavity, excessive penetration or lack of sidewall fusion occurred, except for porosity in some parameters. The tensile strength of the welded specimens decreased with increasing heat input in the gas and robotic laser methods compared to the non-welded condition, while the opposite was observed in the manual laser method. The strength values of the welded specimens decreased by 7% to 33% compared to the non-welded specimen. The highest strength values were 538 MPa at a heat input of 0.096 kJ/mm with gas metal arc welding, 532.5 MPa at a heat input of 0.135 kJ/mm with manual laser welding, and 562.5 MPa at a heat input of 0.108 kJ/mm with robotic laser welding. Microstructure analysis revealed the presence of hard and brittle structures such as Widmanstätten ferrites, which resulted in increased hardness and decreased elongation. XRD analysis of the weld seam of the sample joined by robotic laser at a heat input of 0.108 kJ/mm revealed a unique Mn3O4 phase. The findings show that heat input optimization is very important to improve weld quality and mechanical performance, and robotic welding provides higher strength at lower heat input values compared to gas metal arc welding and manual laser welding.