Investigation On Surface Spinning Strengthening Mechanisms And Fatigue Life Improvement Of Metallic Materials

By improving the microstructure and mechanical properties and introducing compressive residual stress in the surface layer,surface mechanical strengthening becomes an important approach to improve the fatigue properties,and this is because that the fatigue crack initiation is inhibited and the fatigue life of the surface strengthened metallic materials is effectively prolonged.Within them,the microstructure evolution in the surface layer,the mechanical properties in the surface layer,the residual stress in the surface layer,the relationship between the strength and plasticity,and the fatigue properties of the surface strengthened metallic materials have become the core scientific problems.In this thesis,the Q235 carbon steel,316 stainless steel,Cu and Cu-Al alloy,50CrMnMoVNb spring steel and other metallic materials are subjected to the newly developed surface spinning strengthening method.By conducting the microstructure observations,tensile tests and fatigue tests of these surface strengthened metallic materials,the surface strengthening behaviors,the properties of gradient layer,the tensile properties and the mechanisms of prolonging fatigue life were systematically studied to reveal the mechanisms of surface spinning strengthening and fatigue life prolongation of the metallic materials.1.The surface spinning strengthening behaviors of metallic materials with different properties were investigated.In this thesis,a new surface spinning strengthening(3S)method was used,and the shear stress and compressive stress were applied to the surface layer of the metallic materials subjected to the high-speed rotating tool.Firstly,four structural steels with different strength,including the Q235 carbon steel,45 carbon steel,30CrNi2.5MoV steel and 50CrMnMoVNb steel,were carried out by the 3S treatment with the same parameter,and the effect of strength on the surface spinning strengthening behaviors was studied.Secondly,two alloys with different work-hardening capacity,including the 316 stainless steel and TC4 titanium alloy,were carried out by the 3 S treatment with the same parameter,and the effect of work-hardening capacity on surface spinning strengthening behaviors was studied.For the metallic materials with different properties,the gradient microstructures were induced in the surface layers,and the grains in the topmost surface layers were significantly refined or even nanocrystalized.With the change of strength and work-hardening ability capacity of the metallic materials,the gradient microstructure and microhardness distribution in the surface layer showed obvious differences.Based on the characteristics of gradient microstructures and mechanical properties in the gradient layer after the 3S treatment,an exponential model of gradient layer was proposed.In addition,the effects of strength and work-hardening ability on the maximum microhardness,thickness,surface strengthening exponent and surface strengthening energy of gradient layer were studied.2.The properties of gradient layer in the surface strengthened metallic materials were discussed.The mechanical properties of metallic materials are affected by surface mechanical strengthening,and the maximum microhardness and the thickness of gradient layer are two important parameters to describe the properties of gradient layer.Firstly,the pure Cu with different grain sizes was surface strengthened by the 3S treatment with the same parameter,and the gradient microstructure and microhardness distributions in the gradient layers showed some differences.With the increase of grain size of pure Cu,the level of grain refinement and the microhardness at the topmost surface layer were close,but the thickness of the gradient layer of the fine-grain Cu was larger.Secondly,the Cu-Al alloys with different Al contents and grain size were surface strengthened by the 3S treatment with the same parameter.With the change of the composition and microstructure of Cu-Al alloys,the gradient microstructure and microhardness distributions in the surface layer showed obvious differences.The maximum microhardness of the gradient layer of 3S Cu-Al alloy increased with the increase of the Al content,and it was less related to the grain size.The thickness of the gradient layer of 3S Cu-Al alloy decreased with the increase of strength and the decrease of work-hardening ability,respectively,and it was less related to the Al content.Based on the variation rules of the maximum microhardness and thickness of the gradient layer,the relationship between the maximum microhardness and chemical composition,and the relationship between the depth of gradient layer and microstructure were discussed,respectively.3.The effect of gradient structure on the strength and plasticity of the metallic materials was expounded.Tensile properties of metallic materials are affected by microstructure,and the gradient microstructure induced by surface mechanical strengthening can affect the tensile properties of metallic materials.In this thesis,the 304 stainless steel and H62 brass were firstly carried out by the 3S treatment with the same parameter,and the effect of gradient structure on the strength and plasticity of 304 stainless steel and H62 brass was expounded.A method was put forward to calculate the yield strength of the metallic materials after the 3S treatment,and the experimental results agreed well with the calculated results.Secondly,the Cu-Al alloys with different Al contents were carried out by the 3S treatment with the same parameter,the gradient layers with different gradient microstructure and microhardness distributions were constructed.The yield strength of the 3 S Cu-Al alloy with different gradient structure increased,but the uniform elongation decreased slightly.The effect of gradient structure on the strength and ductility of Cu-Al alloy was further clarified according to the variation law of gradient microstructure and microhardness distribution in the gradient layer.Combined with the effects of Al content,grain size,the maximum microhardness of gradient layer,thickness of gradient layer and the gradient layer ratio on the strength and plasticity of Cu-Al alloys,the approaches to improve the strength and plasticity of Cu-Al alloys were further proposed.4.The mechanisms of fatigue life improvement of the 3S metallic materials were revealed.Recent studies on fatigue life improvement of metallic materials show that surface mechanical strengthening may effectively improve the fatigue life.In this thesis,the 50CrMnMoVNb spring steels with different strength were strengthened by shot peening with the same parameter,and the effect of matrix strength on surface integrity was revealed.With the change of the matrix strength,the surface roughness,surface residual stress distribution and surface gradient structure of 50CrMnMoVNb spring steel showed obvious differences.Furthermore,the relationship between the matrix strength and roughness,the maximum residual compressive stress,the depth of residual compressive stress zone,the microhardness in the surface layer and the thickness of gradient layer were established.In addition,the fatigue life of the surface decarburized 50CrMnMoVNb spring steel was significantly improved after the 3S treatment,and the mechanisms of fatigue life improvement were revealed.Combined with the characteristics of gradient microstructure and microhardness distributions of the 50CrMnMoVNb steel before and after the 3S treatment,as well as the fatigue crack sources and fatigue risk factor of the 50CrMnMoVNb spring steel with the different surface states,including the surface decarburization,matrix and 3S,the approaches to improve the fatigue life of 50CrMnMo VNb steel with the surface decarburization layer were proposed.