D. Lin, M. S. Yang, M. F. Kong, P. D. R. Kovacevic, Southern Methodist University, Dallas, TX; R. Ruokolainen, D. X. (. Gayden, General Motors Corporation, Warren, MI
Dual phase steel DP980, one of the advanced high strength steels, has attracted great attention from automobile industry since it has high strength, high-energy absorption in the crash testing, and good formability. Feasibility study of laser welding of DP980 at butt-joint configuration has demonstrated that the welded joint of this material are characterized by the high strength but with the low toughness and the low formability. Higher welding speed can increase the joint strength since it reduces the size of heat-affected zone, but it cannot effectively improve the toughness of the welded joint. To restore the original material properties at the welded joint, various post-welding heat treatment processes have been tested, including: (1) step quench with heating performed in a furnace followed by quenching in various media, (2) local heat treatment done by direct diode laser beam, and (3) heat treatment using defocused fiber laser beam. The investigations have shown that: (1) step quench can soften the welded joint which results in a decreased joint strength and an extremely high elongation, (2) direct diode laser beam, characterized with a uniform heat input, can soften the weld and the heat affected zone vertically through the thickness of the specimen, so that the strength of the welded joint decreases slightly, but with significantly modified toughness, and (3) a defocused fiber laser beam can re-melt the welded joint such that the joint strength is still high, but with little toughness improvement. Selection of the post-welding heat treatment process strongly depends on the anticipated manufacturing processes following the welding operation. A further investigation has shown that the volume fraction and the distribution of martensite in the weld/heat affected zone play a critical role in the joint strength, and the final tempering of martensite in the weld zone contributes to the improvement of the toughness.
Summary: Advanced high strength steels have attracted the attention from automotive industry in the last decade. However, welding of this type of materials is a challenge since the characteristics of high strength with good formability cannot be sustained under the extensive heating, melting and solidification during welding process. Dual phase steel DP980, for example, can retain high strength after laser welding, but with dramatically lowered toughness. Post-welding heat treatment, such as locally heat-treated technologies by using a direct diode laser and a defocused fiber laser and a step quenching performed in the furnace, can be selected to restore the properties of the welded joints. The selection of heat treatment processes strongly depends on the anticipated manufacturing processes subsequent to the welding operation.
