| Due to the high specific strength and good thermal stability,many sorts of titanium alloys are widely applied in aerospace,chemical,petroleum,electric power,medical,etc.However,as a typical high-strength alloy material,titanium alloy is highly sensitive to various surface defects,such as surface micro-cracks,cutting tool marks,etc,which can easily lead to premature fatigue failure of engineering components during service.Therefore,parts of high-end equipment put forward higher requirements on the integrity of the final formed surface.Based on the research of Ultrasonic rolling burnishing(URB)process,Ultrasonic warm rolling burnishing coupled heat treatment(URB/HT)was proposed to realize the surface strengthening of titanium alloy.Comprehensive theoretical analysis,finite element calculation,and experimental research methods,in the ultrasonic warm rolling burnishing coupled heat treatment process and equipment,the effect of ultrasonic warm rolling burnishing coupled heat treatment on the internal stress and strain field of the material,the effect of ultrasonic warm rolling burnishing coupled heat treatment on the surface integrity of the material and the effect of ultrasonic warm rolling burnishing coupled heat treatment on the corrosion resistance and fatigue resistance of materials,four aspects of research work have been carried out,and relevant research conclusions have been drawn.The main research contents and conclusions include:(1)The stress field of ultrasonic rolling burnishing strengthened material is analyzed.Based on the theory of contact mechanics,a theoretical analysis of the stress and strain of the material under a single vibration impact was carried out,and a residual compressive stress calculation model was established when the coverage rate of ultrasonic rolling burnishing was 100%.Based on the finite element analysis method,a microscopic ultrasonic rolling burnishing TC11 finite element model was established,and the correctness of the model was verified by comparison of experimental data.Integrating the stress calculation model and the finite element model,from both theoretical and finite element analysis,the effect of external ultrasonic excitation on the residual stress and strain is analyzed,and then the influence of the medium temperature environment on the instantaneous stress and strain distribution of ultrasonic rolling burnishing was clarified.(2)The influence mechanism of ultrasonic rolling burnishing and ultrasonic warm rolling burnishing coupled heat treatment on the surface integrity of TC11 alloy was studied.Taking the surface micro-morphology,grain size,roughness,microhardness,and residual stress as the characteristic parameters of surface integrity,comparative experiments were carried out on samples under different processing conditions.The results show that both ultrasonic rolling burnishing and ultrasonic warm rolling burnishing coupled heat treatment have different degrees of surface strengthening effects on the material.Compared with ultrasonic rolling burnishing,the surface modification layer produced by ultrasonic warm rolling burnishing coupled heat treatment is thicker,and the surface grain refinement effect is significantly enhanced.The surface roughness of the samples processed by ultrasonic rolling burnishing and ultrasonic warm rolling burnishing coupled heat treatment was significantly reduced.However,compared with ultrasonic rolling burnishing,ultrasonic warm rolling burnishing coupled heat treatment can achieve the same roughness index under a smaller static load.For example,under a static load of 120N,the surface roughness of ultrasonic rolling burnishing is 179nm,and to achieve the same roughness,o and only 40N static load is required for ultrasonic warm rolling burnishing coupled heat treatment to achieve the same roughness,which is of significance for the surface strengthening of thin-walled parts.In addition,the ultrasonic warm rolling burnishing coupled heat treatment has shown better process effects in terms of hardness improvement and uniformity,as well as the introduction of compressive stress.To sum up,due to the synergistic effect of"acoustoplasticity"and"Thermoplasticity",the material is more prone to plastic deformation.ultrasonic warm rolling burnishing coupled heat treatment samples have better surface modification effects and obtain better Surface integrity.(3)The experiment comparatively studied the effect of ultrasonic rolling burnishing and ultrasonic warm rolling burnishing coupled heat treatment on the corrosion resistance and fatigue resistance of titanium alloys.Conducted electrochemical corrosion experiments to compare the corrosion resistance of materials,and found that the corrosion resistance of the ultrasonic rolling burnishing process and the ultrasonic warm rolling burnishing coupled heat treatment process samples have been improved to different degrees,and the ultrasonic warm rolling burnishing coupled heat treatment specimens have improved more significant.The improvement in corrosion resistance is attributed to the refining of the surface grains of the material,the induced residual compressive stress and the reduction of surface roughness by rolling strengthening;in addition to the more significant surface modification of the ultrasonic warm rolling burnishing coupled heat treatment,the medium temperature environment promotes oxidation Layer formation is also the reason why the corrosion resistance of the ultrasonic warm rolling burnishing coupled heat treatment specimens is more significantly improved.The fatigue test results show that the fatigue fracture load cycles of the ultrasonic rolling burnishing and ultrasonic warm rolling burnishing coupled heat treatment specimens are respectively6.3×10~5and 1.2×10~6,which is 16 times and 31 times higher than that of the finely ground sample,respectively.The improvement of the fatigue resistance of the sample is mainly attributed to the combined effect of the formation of a grain refinement layer during the surface strengthening process,which improves the surface hardness and surface finish,and induces the formation of a residual compressive stress layer,that is,a good surface integrity is significantly enhanced.Service performance of components.Compared with ultrasonic rolling,ultrasonic temperature rolling coupled heat treatment processed samples have better surface integrity and show better fatigue resistance.The improvement of the fatigue resistance of the sample is mainly attributed to the combined effect of the surface strengthening process,which formation of a grain refinement layer,improves the surface hardness and surface finish,and induces the formation of a residual compressive stress layer.A good surface integrity is significantly enhanced service performance of components.Compared with ultrasonic rolling burnishing,ultrasonic warm rolling burnishing coupled heat treatment processed samples have better surface integrity and show excellent fatigue resistance. |
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