Effect of Surface Hardened Layer Thickness on Cold Rolling Composite Properties of TA2 Pure Titanium/304 Stainless Steel Dissimilar Sheet

The rolled and annealed （680 ℃×30 min） TA2 pure titanium sheet （thickness 2 mm） and the rolled and annealed （1 050 ℃×3 min） 304 stainless steel sheet （thickness 0.85 mm） were ground to form different surface hardened layer thicknesses （117, 134, 144, 177 μm）. The different total surface hardened layer thicknesses （261, 278, 294, 311 μm） of TA2 pure titanium/304 stainless steel composite sheet were obtained after one pass of cold rolling. The composite plates were then annealed at 500 ℃ for 2 h. The effect of surface hardened layer thickness on the microstructure and tensile properties of the composite plates was studied. The results show that when the surface hardened layer thickness was relatively large, the bonding interface of the composite plate presented a tooth-like structure, with more defects near the interface. When the surface hardened layer thickness was relatively small, the bonding interface of the composite plate was relatively flat, and there were almost no defects near the interface. When the hardened layer thickness was 278 μm, the interface was the flattest, the bonding was the tightest, the yield strength and tensile strength were the largest （844, 905 MPa）, and the percentage elongation after fracture was relatively large （20.4%）. The fracture morphology of the tensile fracture was composed of a few tearing edges and fine craters. When the hardened layer thickness was too small （261 μm）, the strength was relatively small. When the hardened layer thickness was too large （294, 311 μm）, the percentage elongation after fracture was relatively small, and there were more tearing edges and cleavage planes in the fracture, and the fracture mechanism was mainly brittle fracture.