Effect of multi-pass friction stir processing on the microstructure and mechanical properties of dual phase steel

摘要

Dual phase (DP) steels have been widely used in the automotive industry due to the excellent engineering properties such as high strength and good formability. However, attempts have recently been ongoing to improve their mechanical and formability properties in order to achieve further weight savings. Mechanical and microstructural properties of DP steel can be improved by severe plastic deformation (SPD) techniques without changing their chemical compositions. Among SPD methods, friction stir processing (FSP) is a new method used to enhance the properties of plate and/or sheet types of metals. Therefore, the effect of multi-pass FSP (M-FSP) on the microstructure and mechanical performance of a DP steel (i.e., DP600) was investigated in the current study. M-FSP was applied to dual phase steel at the 4mm steps. FSP resulted in a refined microstructure which brought about a considerable increase in both hardness and strength values. After FSP, islands of martensite as the secondary phase in the microstructure have been broken and disturbed by the rotational pin. The processed region consists of ferrite, bainite and martensite. The hardness value increased from 210 HV0.2 to about 360 HV0.2 after M-FSP.