Studies On Fatigue Property Of Friction Stir Processed IF Steel

In this paper, cold rolled DC04 IF steel was subjected to friction stir processing(FSP). The microstructure and mechanical property of the FSP materials prepared under different conditions and parameters were analyzed, and then optimized FSP IF steel process. The fatigue behavior of the base metal and FSP material were studied using S-N curves and da/dN-?K curves in air and in 3.5% NaCl solution. The mainly results are as follow:FSP material has good surface in argon gas protection conditions, but size of grains is coarse, about 20μm.The microhardness of surface is 148.6HV;FSP in circulating water could get fine grain size, about 1~5um.The microhardness of surface up to 323.2HV,but surface formability is poor and the tool use wear and incomplete after. After the optimal selection, IF steel was quenched by dry ice and alcohol mixture after processed in argon. The optimal material is obtained with a tool rotation rate of 950 rpm and a welding speed of 60 mm/min. The material can have a good surface formability, and fine grain size of the surface and high microhardness such as processed in circulating water. The size of grain in the processing zone is 5~10μm, at surface could be 1μm; microhardness is 314.8HV.The tensile strength is 474.16 MPa, which is 150.9% of the base metal(BM). Tensile fracture is characterized by dimple fracture.The S-N curve equation obtained in air: BM: lgN=79.332535-31.428751lgσ; FSP: lgN=187.699770-69.444890lgσ. The S-N curve equation obtained in NaCl solution: BM: lgN=25.522309-8.403823lgσ; FSP: lgN=26.892612-8.291334lgσ. The fatigue lives of the FSP specimens exposed in 3.5% NaCl solution are lower than those of the specimens tested in air.The da/dN-?K curve equation obtained in air: BM: da/dN=4.034×10-12(?K)5.688; FSP: da/dN=3.007×10-11(?K)4.880. The da/dN-?K curve equation obtained in NaClsolution: BM: da/dN=6.096×10-18(?K)10.933; FSP: da/dN=2.347×10-14(?K)7.368. Fatigue crack propagation rate test results showed that the da/d N of FSP in 3.5% Na Cl solution is faster than that tested in air.