The Effect Of Heat Treatment On Microstructure And Mechanical Properties Of TC4 Titanium And The Research On Stress Relief After Heat Treatment

With the development of aviation equipment, the whole complex components with the properties of precision, high performance and lightweight had been used in more and more fields, and has been the key parts of new aviation equipment. Due to low density, high specific strength, heat-resistant, impact-resistant, corrosion-resistant, and well electrochemical compatibility, titanium and titanium alloy have been more and more used in aerospace in Europe and America which have Advanced aerospace industry. To some extent, the usage amount of titanium has been considered as a criterion to evaluate a country's aerospace industry. But because of lack the ability to make high performance and dimensionally stable components, the application in aerospace of titanium alloy has big gap with Europe and America which have Advanced aerospace industry. So it is important to Systematic study titanium alloy.This paper put TC4, the most widely used titanium alloy, as study object. First of all, we studied the effect of heat treatment on mechanical properties and residual stress of TC4 titanium alloy. Then with the assistant of optical microscope, SEM and TEM, studied the transformation of microstructure with the change of heat treatment, invested the root cause of the change of mechanical properties. At last, we studied the effect of cryogenic time, quick heat and quick cold, thermal-cold cycling and prestretching on residual stress of TC4 titanium alloy. At the same time, we also investigated their effect on tensile strength and micro hardness.The research results show that the cooling rate is the critical factor to determine microstructure transformation mechanism. With the cooling rate was increased from low speed to high speed, the dominate transformation mechanism was changed from atomic diffusion(high temperature ? phase transformed to low temperature ? phase) to martensitic transformation(high temperature ? phase transformed to martensite ?' phase). This change had increased tensile strength and reduced ductility and impact toughness. In duplex microstructure,the decrease of equiaxed ? phase volume fraction and the length-width ratio of second ? phase as well as the increase of second ? phase volume fraction had increased the tensile strength. The equiaxed ? phase had positive effect on ductility, but the second ? phase had negative effect. Both the grain size equiaxed ? phase and second ? phase affected the impact toughness and their volume fraction was the critical factor.The difference of microstructure kind had affected the ability of stress relaxation. Even the same kind of microstructure, the difference of microstructure parameters had affected the residual stress significantly. The change of microstructure caused by the diversification of aging temperature and aging time couldn't affect residual considerably.The results of researching on the means to reduce residual stress show that, cryogenic treatment, quick heat and quick cold, thermal-cold cycling and prestretching reduced residual stress by 52.87%, 58.00%, 67.82 and 72.88% respectively. After cycled 3 times of quick heat and quick cold and thermal-cold cycling, the residual stress was reduced to stable level, and it is no need to do more treatment.