| Ti-6Al-4V alloy manufactured by HIP had the excellent comprehensive performance and application range.However it had the poor resistance to wear and fatigue properties because of the low plastic shear resistance and work hardening rate and the poor protective effect of surface oxides.Especailly Ti-6Al-4V alloy in the course of using had the large friction coefficient,easy to stick,uneasy to lubricate and prone to wear,which severely limited the use of titanium alloy as the contact pair of the parts.Moreover the fatigue crack source in most cases sprouted on the surface of the material.Therefore surface integrity had an important effect on surface properties of titanium alloys material and parts.Suface strentheing of HIP Ti-6Al-4V alloy before and after heat treatment was carried out by the different surface composite rolling process,and analyzed the strengthening mechanism.The effects of different surface composite rolling parameters on the microstructure of material surface layer,mechanical properties,friction and wear properties and fatigue properties were studied,so as to optimize surface heating assisted ultrasonic composite rolling treatment process.This would guide the actual titanium alloy parts surface strengthening processing,and have the important theoretical significance and strong practical application value.In this paper,a bar with a density of 99.8% was prepared by HIP process using Ti-6Al-4V pre-alloy powder.The mechanical properties and microstructure of no-heat-treated HIP Ti-6Al-4V alloy were analyzed,and the heat treatment scheme of HIP Ti-6Al-4V alloy was determined according to DSC curve.Surface composite rolling strengthening process of HIP Ti-6Al-4V alloy was determined based on the conventional rolling process.The effects of surface ultrasonic rolling process parameters on surface roughness,microhardness and residual stress of the material were analyzed by orthogonal test.The strengthening mechanism of surface heating assisted ultrasonic composite rolling was studied,and discussed the microstructure change of matreial surface layer.On the basis of nonlinear isotropic-kinematic hardening constitutive model,ABAQUS software was used to analyze the change trend of equivalent plastic strain,residual stress and energy in surface layer of the material treated by surface ultrasonic rolling at room temperature.The results indicated that the energy of material surface layer was significantly increased.There was a higher residual compressive stress value,and realized strain hardening.Material microstructure analysis showed that the plastic deformation layer with a certain thickness was produced at the surface of the material treated by surface ultrasonic rolling at room temterature,the thickness of which was gradually increased with the increase of ultrasonic frequency and load.But the change of ultrasonic amplitude had a small effect.The grain of material surface modification layer was obviously refined,and the smallest grain size had reached the submicron level.TEM analysis showed the presence of twin crystals.The plasitc deformation degree of the material treated by surface heating assisted ultrasonic composte rolling was gradually increased with the increase of material heating temperature.The non-heat-treated material had the thicker plastic deformation layer and the better grain refinement effect,and accompanied by high density dislocations,as well as the more dislocations tangle.The smallest grain size had been refined to the nano-level.The effect of surface ultrasonic rolling at different temperatures on material surface roughness,microhardness and residual stress was analyzed.The results indicated that the material treated by surface ultrasonic rolling at room temperature had the better surface morphology and lower surface roughness.In particular,the surface of non-heat-treated material was relatively smooth with no obvious defects,and the minimum surface roughness was 0.13 ?m.Surface heating assisted ultrasonic composte rolling treatment could effectively improve surface microhardness and hardening depth of the material,which was gradually increased with the increse of material heating temperature.The maximum surface microhardness and hardening depth was 511 HV and 250 ?m respectively.Compared to surface heating assisted ultrasonic composite rolling treated material,surface ultrasonic-rolled material at room temperature had the lowest surface compressive redidual stress value(-1173 MPa)because of cold work hardening effect.XRD diffraction analysis indicated that surface heating assisted ultrasonic composite rolled material had the broader diffraction peak,it also meaned that this process had the much better grain refinement and strain hardening.The wear characteristic of surface heating assisted ultrasonic composite rolling material was studied.It was found that surface ultrasonic-rolled material at 25 ? showed the lower CoF,smaller wear scar and less wear volume.The fretting friction and wear properties of heat-treated material treated by surface heating assisted ultrasonic composite rolling were gradually improved with the increase of heating temperature.The best wear resistance was obtained by the heat-treated material treated by 140 ? heating assisted ultrasonic composte roling,and had the slightly better friction and wear properties compared to surface ultrasonic-rolled material at 25 ?.The main performance was the similar fretting wear volume and the better fretting wear scar.When counter material was Gr15,the main wear mode of material was abrasive wear.The debris particles were comparatively small and had a small amount of small peeling blocks.There was some debris bonded to the surface of counter grinding ball.Then the counter material was changed into Si3N4,the main wear form was the massive peeling,and accompanied by abrasive wear.The debris was relatively coarse.The research of tensile and fatigue properties of surface ultrasonic-rolled material were conducted.The yield and tensile strength of heat-treated material after surface ultrasonic rolling at 25 ? were increased to 1005 MPa and 1042 MPa respectively.There were seperately 11.3 % and 8.2 % increase compared to no-heat-treated material.The tensile fracture morphology was characterized by surface modification transition layer with a thickness of about 500 ?m.However the fatigue strength of heat-treated material treated by surface ultrasonic rolling at 25 ? in high cycle fatigue regime was not obviously improved,but the low cycle fatigue life was significantly increased.On the base of 107 cycles,the cycle stress was increased to 570 MPa from the initial 520 MPa,and increased by 24.4% compared to no-heat-treated material.The fatigue life at 700 MPa cycle stress was increased to 97503 times from 13700 times provied by no-heat-treated material,and the fatigue crack initial site was shifted to sub-surface area of the material.The results indicated that surface ultrasonic rolling at 25 ? effectively improved tensile and fatigue properties of HIP Ti-6Al-4V alloy.However suface heating assisted ultrasonic composite rolling process had the better grain refinement effect,thicker modification layer and higher microhardness.So it was reasonable to believe that it could further improve tensile and fatigue properties of the material. |
PDF Full Text Request