Effect of initial residual stress on mechanical properties of ZTC4 titanium alloy

The microstructure， tensile fracture morphology and initial residual stress of ZTC4 titanium alloy repair welding are analyzed by stereomicroscope， optical microscope （OM）， scanning electron microscope （SEM）， X-ray residual stress analyzer and tensile testing machine， and their effects on room temperature/high temperature tensile properties and fatigue properties are studied. The results show that the microstructure of ZTC4 titanium alloy after repair welding is mainly composed of equiaxed grains （with a grain size of about 1.5 mm） and grains with a large aspect ratio （5∶1）. Different repair welding diameters and annealing treatment can affect the properties of residual stress， thus affecting the mechanical properties. The residual stress at the center of the original state of the room temperature/high temperature tensile and fatigue specimens is mainly tensile stress， which is 115.9， -24.6 MPa and 55.6 MPa， respectively. In the high temperature （350 ℃） tensile test， the larger the initial residual tensile stress （173.3 MPa） at the center of the repair welding diameter 4 mm specimens， the lower the tensile strength （710.7 MPa）； the larger the initial residual compressive stress （-159.1 MPa） at the center of the repair welding diameter 4 mm and annealing specimens， the higher the tensile strength （749.3 MPa）.The residual tensile stress significantly shortens the fatigue life of titanium alloy， whereas residual compressive stress increases it. The initial residual stress at the center of the original state， repair welding diameter of 2 mm， repair welding diameter of 2 mm and annealing， repair welding diameter of 4 mm， and repair welding diameter of 4 mm and annealing specimen is 55.6，109.9， -189.1，61.4 MPa， and -64.3 MPa， respectively. The fatigue properties of the specimens with initial residual compressive stress is better.