| Titanium alloy is commonly used as the manufacturing material of impeller and blade in aviation manufacturing industry,mainly because of its good mechanical properties to meet the high performance requirements of aviation products.However,titanium alloy is a typical difficult-to-process material,so that the manufactured titanium alloy structural parts often have problems such as"poor surface integrity,short life,and low reliability".Therefore,a new rotary ultrasonic machining strategy based on compressive stress manufacturing is proposed to solve the problem of insufficient fatigue resistance of aviation titanium alloy parts.In this paper,TC4titanium alloy is taken as the research object,and the fatigue properties and crack growth after milling are studied in detail through fatigue test.The main work and corresponding research results are as follows:(1)It is clear that the main influencing factor of the residual stress in the rotating ultrasonic machining of titanium alloy is the mechanical effect.Therefore,the focus is on the influence mechanism of ultrasonic machining titanium alloy shear force,plowing force and ultrasonic impact force on the surface residual stress.Subsequently,experiments on common machining of titanium alloy and rotary ultrasonic machining were carried out,and it was found that the surface after milling showed compressive stress.Compared with common machining,rotary ultrasonic machining can significantly increase the residual compressive stress on the surface of titanium alloy by an average of 54.5%.(2)The milling experiments of titanium alloy with and without ultrasonic vibration were carried out,and the effects of ultrasonic vibration on roughness,grain size,micro strain,dislocation density and microhardness were summarized and analyzed.The results showed that:ultrasonic machining caused plastic deformation on the surface,and the plastic deformation continued to accumulate with the increase of energy.The grain size of common machining is12.333nm,and when the ultrasonic machining current is 150m A,the grain size is reduced to6.43nm;the microscopic strain increases from to0.287×10-2 to 0.552×10-2.Similarly,the dislocation density is changed from6.15×101 5/m2 to 22.7×1015/m2.The effect of ultrasonic vibration has significantly improved the hardness of the surface layer,and the microhardness has increased from 350HV to 510HV.In addition,it was found through a laser microscope that fish scale-like surface morphology appeared after rotating ultrasonic machining,while ordinary milling processing showed a furrow shape.By measuring the surface roughness,it was found that the surface roughness Ra and Rz increased significantly after ultrasonic machining.The average surface roughness Ra increased from 0.66?m to 1.76?m,and Rz increased from4.52?m to 9.74?m.(3)Firstly,the tensile fatigue comparative tests of titanium alloy standard parts under high and low loads were carried out.The experimental results show that the fatigue life increases slowly and continuously with the increase of ultrasonic amplitude.The low cycle fatigue life of the fatigue specimen increases by 81%on average,while in the high cycle fatigue test,due to the decrease of stress amplitude and the increase of load frequency,the fatigue life of the rotary ultrasonic machining titanium alloy specimen increases by 24.5%.Secondly,the observation and analysis of fatigue fracture of titanium alloy standard parts were carried out.The fracture measurement results show that the width of fatigue stripe is smaller and the crack propagation rate is slower after ultrasonic machining.The quantitative analysis and calculation of fracture show that the reason for the significant growth of fatigue life is that the fatigue crack initiation has been well restrained,and the fatigue crack initiation life of ultrasonic machining specimens has been increased by 159.7%.Finally,the influence mechanism of high frequency vibration impact effect on the fatigue life of titanium alloy is revealed.It is clear that the improvement of fatigue properties is mainly due to the residual compressive stress and microstructure strengthening on the surface of titanium alloy by ultrasonic machining,which can enhance the mechanical properties of titanium alloy,inhibit the crack initiation and reduce the crack growth rate. |
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