The role of graphite and carbide nanoparticles on ferrite phase suppre... | Muhammad Shehryar Khan

Abstract: The need for improved vehicle safety is the leading catalyst towards innovation in the automotive industry. The industry has improved vehicle safety by adopting the use of highly optimized structural components made using tailor-welded blanks (TWBs) that use two or more types of ultra-high strength press-hardened steels (PHSs) with different properties that are laser welded together and then hot stamped to form the required part - helping to reduce the weight of vehicles which improves their fuel efficiency while at the same time, ensuring passenger safety [1]. Vehicle parts made using TWBs have improved strength in areas where the preservation of structural integrity under a high dynamic load is required while
also providing high ductility to offer improved energy absorption in the case of a collision. Al-Si coated PHSs are most commonly used to produce TWBs due to their excellent corrosion resistance properties. During laser-welding of Al-Si coated steels, the coating melts and mixes into the fusion zone (FZ) leading to a loss of strength due to the formation of a low strength phase called ferrite that forms due to the ferrite stabilizing nature of aluminum when alloyed in steel [2]. In the absence of the Al-Si coating, the FZ of PHSs would be purely martensitic but removing the coating prior to welding can be problematic, so when welding is done through the coating, the FZ consists of a dual-phase region comprising of a high-strength martensitic matrix interspersed with low-strength ferrite islands which are the primary cause of premature failure in the weld [3]. Laser welding of Al-Si coated steels can also cause concavity defects at the surface of the weld due to the lower melting temperature of the coating, which lowers the strength of the joint. Therefore, the main objective of this research project was to evaluate the effect of introducing austenite stabilizing elementssuch as graphite and carbide nanoparticlesinto the FZ during laser welding to improve the mechanical properties of the weld by suppressing the formation of ferrite by stabilizing the austenite which is the high temperature phase needed to produce a fully martensitic microstructure. The ongoing research has shown that the use of graphite and carbides was effective in helping to reduce the phase volume fraction of ferrite and significantly improved the mechanical performance of the joint [5,6].