Investigation On Microstructure Regulation And Surface Strengthening Behaviors Of TC4

TC4 titanium alloy is a medium-strength dual-phase titanium alloy with excellent comprehensive properties.It is widely used in aerospace,biomedical,shipbuilding and other fields.Since TC4 titanium alloy is mostly used in harsh environments such as high temperature and high pressure,higher performance and reliability are required on the TC4 titanium alloy.The failure of TC4 titanium alloy often occurs on its surface,and its surface can be strengthened to produce a hardened layer with certain thickness to improve its surface properties and thus improve its overall performance and service life.Based on the above purposes,the surface strengthening treatment of TC4 titanium alloy was carried out by surface spinning strengthening method.The microstructure,properties and formation mechanism of the surface hardened layer were studied and analyzed.In this experiment,TC4 titanium alloys with different microstructures were obtained by different heat treatment systems.Different microstructures were observed by metallographic microscope,scanning electron microscope,and transmission electron microscope.The mechanical properties were compared and analyzed by vickers hardness test and tensile test.The surface strengthening treatment of TC4 titanium alloys with different microstructures was carried out by surface spinning strengthening method.The microstructures and properties of the surface hardened layer were studied and analyzed.The following main conclusions were obtained:(1)The TC4 titanium alloy was annealed below the β phase transition temperature to obtain the equiaxed micro structure.The biomal microstructure was obtained by solution and aging.The widmanstatten microstructure was obtained by annealing above the β phase transition temperature..(2)The yield strength and tensile strength of the equiaxed,bimodal,and widmanstatten microstructures were increased in turn,while the elongation was decreased.Since dislocations were easily entangled at the interface of the sheet,the strength of the microstructure can be improved.The equiaxed α-phase deformed easily.Therefore,the bimodal microstructure had both good strength and plasticity.(3)After the surface strengthening of the different TC4 titanium alloys by surface spinning,the thickness of the hardened layer differs with different alloy.The thickness of the hardened layer was related to the yield strength of the microstructure.The higher the yield strength of the microstructure,the lower the thickness of the hardened layer.The thickness of the hardened layer for the equiaxed microstructure,the biomal microstructure,the original micro structure and the widmanstatten microstructure were 70 μm,56 μm,47 μm and 35 μm,respectively.(4)After surface spinning strengthening,the grains in the surface layer of TC4 titanium alloy were significantly refined.Compared with the matrix,the microhardness of the equiaxed micro structure,the biomal microstructure,the original microstructure and the widmanstatten microstructure was increased by 59%,by 55%,by 39%and by 36%,respectively.The higher the yield strength of the structure,the smaller improvement of the surface microhardness.(5)The penetration depth had a great influence on the surface spinning strengthening effect of the TC4 titanium alloy.With the increase of the penetration depth,grains in the structure were further refined,the thickness of the hardened layer increased and the surface microhardness was significantly improved.(6)After the surface spinning of TC4 titanium alloy,the yield strength and tensile strength of the TC4 titanium alloy were improved,but the elongation was slightly reduced.