Study On The Microstructure And Mechanical Properties Of Semi-solid Forging Ti-7Cu Alloy

Titanium alloys has been widely used in various fields because of its high strength, good corrosion resistance and high heat resistance. At present, the titanium alloy is developing in the direction of high performance and low cost. One of the key problems of low cost is the process technology, while Semi-solid processing technology of titanium alloy is one of the key researches. In this paper, the Ti-7Cu alloy, a kind of α-Ti alloy, has been taken as the research object. The relationship among the semi-solid forging temperature, the semi-solid forging deformation, microstructure and properties of the Ti-7Cu alloy is studied. The major research results are as follows:Compared with conventional forging, Semi-solid forging, due to the coordinated deformation mechanism of liquid phase which effectively reduce the deformation resistance of the alloy, shows good formability; during the conditions of forging temperature at 1000℃ or 1050℃, the alloy showed good forming property with the smooth surface and no obvious crack. With the increase of the forging temperature, the grain size shows the trend of growth and the growth is mainly relied on combined growth and Ostwald ripening mechanism; On the other hand, with the increase of the amount of deformation, the dynamic recrystallization occurs, the grain refinement, becomes more round. Meanwhile the results of the microstructure evolution show that: Grain boundary segregation occurred in the situation of liquid phase during semi-solid forging, then eutectoid reaction occurred in the cooling process, which made segregation of Ti2 Cu phase at grain boundaries. The segregation of liquid phase and Ti2 Cu phase has a great relationship with the deformation temperature. That is to say, with the increase of temperature, the amount of precipitated Ti2 Cu phase in grain boundary increased significantly.The results of the mechanical properties show that: The tensile strength of the alloy after semi solid forging is obviously increased, but the ductility decreases. The number, shape and distribution of Ti2 Cu precipitates are changed with the increase of forging temperature. During tensile test, intragranular dislocations of specimen are entanglement or pinning in precipitated phase surface which has a strengthening effect, so that the strength of the alloy increased. In addition, Ti2 Cu is hard phase, the aggregation and pinning of dislocation produces stress concentration, which is easy to crack in tension, leading to the decrease of plasticity. The typical intergranular fracture mode of the conventional forging alloy during the tensile process is showed, and the fracture surface is dominated by small dimples; Hard Ti2 Cu phase was growing along the grain boundary at an angle and the formation of segregation band organization when the alloy is semi-solid forging, then it showed transgranular and along the crystal mixed fracture mode in tensile test and fracture is made up by coarse dimples and cleavage stripes.