Study On Element Diffusion And Tool Wear Mechanism Of TiAlN Coated Tool During Milling

Titanium alloy are widely used in aerospace,ship manufacturing,automotive manufacturing due to their low relative density,good heat resistance,and high structural strength.In the manufacturing of engine parts in the aerospace field,titanium alloy materials account for more than 20%of the total and show an increasing trend.However,low thermal conductivity,low modulus of elasticity,and high chemical activity,which makes titanium alloy cause rapid tool wear during machining,resulting in short tool life.TiAlN coated tools are widely used for the cutting of difficult-to-machine materials such as titanium alloys and nickel-based high-temperature alloys by virtue of their good high-temperature resistance and low friction coefficient.During machining,tool wear can cause increased cutting forces,increased cutting temperatures,increased machine vibrations,and deterioration of machined surface quality.This will result in the reduced service life for machined parts.Therefore,the study of tool wear is of great practical importance.It can provide a theoretical basis for cutting parameter optimization and tool selection to improve tool life,part machining accuracy,and surface integrity.In this thesis,the tool wear of TiAlN coated tools during the milling of titanium alloy is investigated by a combination of experiments and simulations.Based on the finite element simulation software,a two-dimensional milling model of titanium alloy was built to study the influence of maximum principal stress,coating-substrate interface temperature and stress on the wear of TiAlN coating tool.It was found that the coating delamination first started in the tool tip area,and then gradually spreads to the rake face and causes a large area of coating delamination.The effects of Al content,tool substrate and coating thickness on tool wear were analyzed.TiAlN coating with low surface roughness,low thermal conductivity,low elastic modulus and 3μm thickness,and the tool substrate with high thermal conductivity and low elastic modulus are beneficial to reduce tool wear.The milling tests are used to research the wear evolution process and wear mechanism of TiAlN coated tools with different Al contents.It is found that the wear form of TiAlN coated tool rake face changes from initial adhesion and microchipping to coating delamination and chipping.The wear form of TiAlN coated tool flank face changes from initial adhesion and grooves to boundary wear,build-up layer,and ablation.Good adhesion（Lc2）,coating hardness,H/E,and H³/E*² result in enhanced wear resistance and reduced delamination of the Ti_0.48Al_0.52N coating.Adhesion wear,diffusion wear,and oxidation wear are the main wear mechanisms during tool milling.The EDS analysis of the tool wear area shows that the rake face has more diffusion wear and oxidation wear in the area near the cutting edge（near the tip）.The flank face is more severely diffusion wear in the area near the cutting edge（near the tip）,and more severely oxidation wear in the area near the cutting edge（near the tip）and at the build-up layer.The effect of tool wear on the machined surface is investigated by milling tests.It is found that the roughness of machined surface gradually increased as the tool wear increased.The machined surface defects are mainly grooves,pits,surface tears and adhesions.Among them,grooves are the main factors affecting the quality of the machined surface.The effect of tool wear on chip morphology is also analyzed.It is found that as the tool wear increased,the chips changed from conical spiral chips to curved chips.The burr of the chips gradually became irregular from serrated.The distance between folds on the free surface of chips increased,and the degree of chip serration increased.By analyzing the cross section of the tool under different wear degrees,it is found that there is a synergistic effect between diffusion wear and adhesion wear.The enhancement of element diffusion increases the bonding strength between the tool and the adhesive material,and the enhancement of adhesion promotes the occurrence of element diffusion.The tool strength is weakened under the combined action of diffusion wear and oxidation wear.Based on molecular dynamics,the mapping relationship between diffusion wear and thermo-mechanical coupling field is established.The diffusion wear increases with the increase of temperature and pressure,and the outward diffusion of tool elements is the main cause of tool diffusion wear.This study better elucidates the wear phenomenon of TiAlN coated tools during titanium alloy cutting.And it provides a reference basis for the design,optimization and selection of TiAlN coated tools,which is conducive to reducing tool wear and extending tool life.