Surface Integrity In Multi-step Machining Of Titanium Alloy Ti6Al4V

Titanium alloy is widely used in aerospace,marine,chemical and other industries,due to its high temperature resistance,corrosion resistance and high strength-to-density ratio.However,due to the inherent characteristics of low thermal conductivity,small elastic modulus,etc.,it is very difficult for actual production and processing.At the same time,because of poor wear resistance,the range of use of titanium alloys is strictly limited.In this thesis,the influence of the change of cutting parameters in the previous step on the surface integrity of the next step was studied in the multi-step machining process,as well as the friction and wear characteristics of the machined surface,and achieving the cutting parameters selection and muti-step regulation of titanium alloy in multi-step machining.In this thesis,the titanium alloy multi-step machining was carried out by designing the single factor experiment.Through the two-step and three-step machining respectively,and the influence of the change of micro-hardness of the surface layer on the machined surface integrity was studied during the multi-step machining.The results showed that the standard deviation of the micro-hardness of the roughing surface layer was greatly affected by the roughing cutting parameters.The larger the standard deviation of micro-hardness of roughing surface layer,the larger the surface roughness after finishing,the worse the surface morphology,the larger the standard deviation of micro-hardness of finishing surface,and the more severe the microstructure of the surface layer that was stretched and twisted.In the three-step machining process,when the roughing depth of cut was large,the semi-finishing would weaken the work hardening of roughing surface layer to a certain extent and weaken the influence on the finishing surface integrity.Friction and wear experiments were performed for multi-step cutting surfaces along the feed direction,and the influence of the machined surface integrity on the friction and wear characteristics was analyzed,and the thesis mainly considered the influence of the standard deviation of the machined surface microhardness on the friction and wear characteristics.The results showed that the time length of the stable friction section Δt and the wear loss were related to the standard deviation of micro-hardness of finishing surface.The standard deviation of the finishing surface micro-hardness was larger,when subjected to the alternating contact stress during the friction process,it was easier to cause fatigue cracks under the cyclic reciprocating condition.The crack expanded continuously and eventually peeled off to form a spalling pit,and even the fatigue wear fracture striations occurred due to fatigue spalling.Finally,according to the friction and wear characteristics of the machined surface and the surface integrity of the titanium alloy multi-step machining,and it was achieved the cutting parameters selection and muti-step regulation of titanium alloy in multi-step machining.In this thesis,muti-step regulation was mainly based on the machining allowance of the sample to be processed.However,the cutting parameters selection depended on the machining allowance of each step,at the same time,referring to the surface friction characteristics and surface integrity.