Effectiveness of Using Low‐Strength‐Grade Filler Wires for 800 MPa Grade Ultra‐High‐Strength Steels Laser Beam Welds

To match the strength of ultra‐high‐strength steel (850 MPa) during laser beam welding (LBW), various filler wires having under‐matching (520–640 MPa) and comparable‐matching (890 MPa) strength grades are used. All‐weld metals with no filler or fillers comprise low‐temperature transformation microstructures such as martensite and bainite, while the base metal (BM) comprises bainite, ferrite, and pearlite. All tensile fractures occur in the BM having the lowest hardness as compared to weld metal and heat‐affected zone. Therefore, the hardness and tensile properties of LBW exhibit insignificant variation regardless of the use of filler and their strength grades. This is because the cooling rate of LBW is fast, where a large dilution of the BM with high hardenability and a small deposition of the under‐matching filler occur at zero gap butt joints. To examine these LBW characteristics, the microstructural and hardness behaviors are successfully discussed through simulation of a continuous‐cooling transformation diagram using JMatPro software and cooling rate T9/3 using Rosenthal's two‐dimensional solution. To obtain the effect of lowering weld metal strength in LBW, it is necessary to increase more deposition of under‐matching filler by creating a gap between BMs, and/or to develop a filler of lower hardenability suitable for LBW. To match the strength of UHSSs during laser welding, filler wires having various strength grades are used. The microstructure and mechanical properties of welds exhibit insignificant variation regardless of various strength grades filler wire. To examine these laser welding characteristics, the microstructural and hardness behaviors are successfully discussed through the continuous‐cooling transformation diagram and the cooling rate using Rosenthal's solution.