Influence Of Cryogenic Treatment On Mechanical Properties And Friction And Wear Properties Of Ti-6Al-4V Titanium Alloy

Cryogenic treatment is an extension of heat treatment of materials.By improving the microstructure of materials,the comprehensive mechanical properties of materials can be improved.It has the advantages of low cost,environmental protection,safe and simple operation,and has become a hot research direction at home and abroad.The technology of rolling and finishing is a mature means of surface machining for precision parts.It has the advantages of high efficiency,low cost,high dimensional accuracy requirements for precision parts machining,and is widely used in the field of high-end equipment manufacturing,such as military industry,aerospace and so on.In order to combine the cryogenic treatment with the rolling finishing technology,the typical titanium alloy Ti-6Al-4V is selected as the research object in this project,through two aspects of research:(1)the effect of cryogenic treatment on the microstructure and surface mechanical properties of titanium alloy.The influence of cryogenic treatment on the microstructure of titanium alloy was studied.The microstructure of titanium alloy was changed by controlling the holding time,and the change rule of the surface hardness of titanium alloy with the extension of holding time was revealed.The cryogenic treatment time needed for the surface hardness of titanium alloy to be stable was found out.SEM,microhardness tester,universal tensile tester,friction and wear tester were used.The influence mechanism of the change of microstructure on the microhardness and tensile properties of titanium alloy was explored.(2)The friction and wear mechanism between the workpiece after cryogenic treatment and the single media.The complex rolling and polishing process is simplified,and the friction and wear relationship between the workpiece and the media in the rolling and polishing process is established by using the reciprocating friction and wear test mechanism.The dry and wet friction and wear tests of Ti-6Al-4V titanium alloy were carried out at room temperature.The friction coefficient,wear amount,three-dimensional morphology,wear trace surface morphology and surface element change are taken as the main evaluation indexes,so as to reveal the influence mechanism of cryogenic treatment on wear behavior and wear mechanism of titanium alloy under dry and wet wear conditions.Based on the above two aspects,the following five conclusions are obtained:(1)Cryogenic treatment can significantly improve the microhardness of titanium alloy,but it is not the longer the better.The suitable time of cryogenic treatment can effectively improve the microhardness of the material.After cryogenic treatment,the irregular crystal edges are broken,the homogeneity of the structure in the crystal is improved,and the surface mechanical properties are effectively improved.(2)The yield strength and tensile strength of the titanium alloy after the deep cooling treatment in the annealing state are obviously improved.Direct cryogenic treatment is not conducive to improving the comprehensive properties of materials,and the sudden change of elastic modulus makes the internal properties of materials unstable.(3)The change of microstructure after cryogenic treatment makes the surface mechanical properties of titanium alloy improve obviously.Under the action of cold shrinkage stress and internal stress release,the grains rotate in the direction of minimum resistance,and the homogeneity of the structure increases.(4)The wearer under dry conditions is mainly dominated by abrasive wear,oxidative wear and adhesive wear.The wearer under wet conditions is the main reason for abrasive wear,but the effect of oxidative wear and adhesive wear is very small.The main reason is that the grinding fluid has the functions of reducing friction,buffering,lubricating,cleaning,cooling and isolating air.Both the wear rate measured under dry and wet wear conditions can be obtained.(5)Through the sliding friction simulation of a single media,it is found that the change law of equivalent stress and equivalent strain is opposite.In the whole sliding friction process,the change of equivalent stress is from large to small,and the change of equivalent strain is from small to large,this is closely related to the chip formation mechanism of the material.The initial contact stress is large.The wear debris generated in the later period has anti-friction effect.Pressure,a large equivalent strain is produced.