Study On The Mechanism Of Electropulsing-assisted Ultrasonic Surface Rolling Processing Of High Strength And High Toughness Titanium Alloy TC27

Titanium alloys are widely used in many areas due to their low density,high specific strength,and high specific stiffness.The titanium alloy TC27(Ti5Al4Mo6V2Nb1Fe)developed in recent years is extensively used as major parts,such as shaft and frame because of its high strength and high toughness.However,TC27 is a difficult-to-machine material and has poor wear resistance limiting its application.In order to promote the wear resistance and fatigue reistance,the direct electric pulse current with high frequency was introduced in surface cold rolling processing(SC RP)and ultrasonic surface rolling processing(USRP)in the present study,respectively.Thus,the composite strengthening processings were developed,namely,electropulsing-assisted surface cold rolling processing(EP-SCRP)and electropulsing-assisted ultrasonic surface rolling processing(EP-USRP).Both were used to treat the surface of TC27 so as to further increase the wear resistance and fatigue resistance.(1)The type of electropulsing power,equipment of EP-SCRP and EP-USRP,the surface rolling equipment,and clamps were chosen according to principle of narrowing the range of closed circuit and decreasing the thermal effect of current on the the machining precision of lathe.The lathe,supporting device,electric pulse power,and ultrasonic surface rolling system were designed and modified.The experiment platforms of EP-SCRP and EP-USRP were set up.The processing parameters were selected,and the experiment scheme was determined for SCRP and USRP.The processing parameters were optimized after the SC RP and USRP.The experiment scheme of EP-SCRP and EP-USRP was determined on the basis of using optimized processing paramters of SCRP and USRP.The EP-SCRP and EP-USRP were carried out for TC27.(2)The study of microstructure evolution and the strengthening mechanism of the plastic deformation surface layer were carried out after SCRP,USRP,EP-SCRP,and EP-USRP for TC27.Every strengthening processing resulted in the serious plastic deformation of the surface layer.The?phase deformed from ball shape to oval shape under the squeezing.The thickness of plastic deformation surface layer caused by EP-SCRP and EP-USRP was relatively greater than that of corresponding SC RP and USRP.The plastic deformation was more severe after the electropulsing was introduced,especially the nanocrystallization of the surface layer occurred after EP-USRP.The nanocrystallized surface layer reached 300 nm and the size of nanocrystallized grains was 30 nm.The mechanism of grain refinement indicated that the twinning segmentation happened in microstructure of TC27 during the SCRP and USRP.It was dislocation multiplication segmentation during EP-SCRP and EP-USRP.The introduction of electropulsing caused the change of grain refinement mechanism.(3)The study of the effect of different processing paramete rs on the surface properties,such as surface roughness,hardness,and compressive residual stress was carried out after SCRP,USRP,EP-SCRP,and EP-USRP for TC27.SCRP promoted the surface properties,and EP-SCRP promoted those of TC27 better than SCRP because of the introduction of the electric pulse current.USRP could further the surface properties compared to the both of the above strengthening processings.EP-USRP that formed after the introduction of electric pulse current in USRP could further enhance the surface properties compared to USRP.The abo ve four processings could cause the mirror effect,the work hardened surface layer,and the compressive residual stress in the surface layer.The introduction of electric pulse current made the plastic deformation of surface layer and work hardening more serious,thus,the surface properties of TC27 material could be further promoted.(4)The study of fretting wear properties and wear mechanism was carried out after SCRP,USRP,EP-SCRP,and EP-USRP for TC27.Under dry condition,the coefficient of friction of TC27 treated by every surface strengthening processing generally decreased.The wear rate of TC27 treated by every strengthening processing also showed the decrease with the order of SCRP,EP-SCRP,USRP,and EP-USRP.Therefore,the wear resistance of treated TC27 increased in the same order.The wear rate under lubricating condition decreased.But the decrease was not distinct.The wear mechanism under dry condition was various including adhesion wear,peeling,gully,and oxidation wear.Sometimes the abrasive wear occurred.The wear mechanism under the lubricating condition was simple and it was abrasive wear.(5)The study of the tensile properties,fatigue resistance,and the fracture mechanism of TC27 was carried out after USRP and EP-USRP.The tensile strength of untreated specimen,USRP specimen,and EP-USRP specimen was 1119 MPa,1135 MPa,and 1167 MPa,respectively.EP-USPR could promote the tensile strength of TC27 more than USRP.EP-USRP promoted the fatigue resistance of high cycles.The fatigue stress treated by EP-USRP at 10~7 cycles was 650 MPa.For USRP,it was 620 MPa.For untreated specimen,it was 600 MPa.The crack source moved to the subsurface layer after treated by USRP and EP-USRP.Fatigue striations occurred at the stable propagation area of crack.Nanocrystallization occurred in the surface layer after EP-USRP,which could hinder the propagation of fatigue striations through transgrainular pattern.So,the fatigue resistance was improved by EP-USRP.