Design And Manufacturing Of Coated Carbide Tools Based On Tool Failure Evolution In High Speed Milling Of Titanium Alloy

Because of the low density, the high strength-to-weight ratio, the favourable stability maintained at elevated temperature, and the exceptional resistance to corrosion, titanium alloys are used extensively in aerospace industry. However, titanium alloys are known as difficult-to-machine materials, due to their several inherent properties, such as low thermal conductivity, high chemical reactivity, and low Young’s modulus. The cutting speed and productivity in machining titanium alloys are adversely restricted because of the low machinability and tool life. With the property requirements of tool failure evolution for coated carbide tools in high speed milling of difficult-to-machine materials, such as titanium alloys, as the guidance and basis, and with the carbide tool material design, tool structure design, and tool coating design as the core, the design theory and methods of coated carbide tools were in-depth investigated and discussed. Finally, the solid carbide coated end mill applied to sidewall surface milling of titanium alloy was successfully developed and its cutting performance was investigated.The multi-view characterization of the chips, which includes free surface, back surface, and cross-sectional surface, was carried out in high-speed milling of Ti-6A1-4V alloy. The variation of characteristics and parameters of chip morphologies were investigated. The results indicated that chip formation takes place by the mechanism of catastrophic thermoplastic shear from the observation of the shear bands using metallurgical analyses of microstructure. There exists the broadening of the diffraction peaks in the back surface of chips. Progressive tool failure processes under different cutting conditions were discussed. The variations of cutting forces components and transient infrared temperature during the machining processes were investigated. The variations of tool failure morphologies and tool failure mechanisms in different positions (rake and flank faces) and different failure stages of the progressive tool failure processes were discussed. The evolution of tool micro-damage to macroscopic failure under the multi-strong thermal-mechanical-chemical fields in high-speed milling was revealed. Tool failure mechanisms were synergistic interaction among coating delamination, abrasive wear, adhesive wear, oxidation wear, diffusion wear and thermal-mechanical fatigue wear. The following requirements, such as good matchability between tool material, coating material and titanium alloy, good wear resistance and fatigue properties for tool material, high interfacial adhesion between coatings and tool substrate, and coating material bearing high temperature to protect tool substrate, are proposed.With regard to the property requirements of progressive tool failure for coated carbide tools in high speed milling, the chemical and tribological matchability of carbide tool material and workpiece material were investigated. The relationships between microstructural parameters (Co content, mean grain size, and WC contiguity) and macroscopic properties were built up. The carbide tool material used in high-speed milling of titanium alloy was selected and designed. The results indicated that the sub-micro fine cemented carbide with10wt.%Co binder (WC grain size in0.6～0.8μm) revealed the smallest friction coefficient and the best wear resistance when sliding against tatinium alloy. The parameterized design software of solid carbide end mill was secondary developed based on UG platform. The software can map or generate two-dimensional engineering drawings, and implement the computer aided rapid design of solid end mills. Three-dimensional finite element simulation of solid carbide end mills in machining titanium alloy was conducted by using Deform-3D. The geometric parameters of the tool structure and cutting edge, such as helix angle, rake angle, and number of teeth, were optimized with low cutting force and cutting temperature as the target. The optimized results are a12mm inner diameter,4teeth, a44°helix angle, a9°side rake angle.The carbide tool materials with various Co content and grain size was sintered, prepared, tested and characterized. The preparation processes, microstructure, mechanical properties, and mechanical fatigue behaviours of carbide tool materials were investigated to verify the reasonableness of material design. The fracture mode can be described as brittle fracture, especially the mixed-mode of intergranular fracture and transgranular fracture of aggregates of WC grains. Grain refinement improves the hardness and transverse rupture strength of carbide tool materials at the expense of reducing fracture toughness. High Co contents of carbide tool materials increase the transverse rupture strength and fracture toughness at the expense of low hardness. The three-point bending fatigue characteristics of WC-Co cemented carbides were investigated using three-point bending specimens. The fatigue fracture typically originates from inhomogeneities or defects such as micropores or aggregates of WC grains near the notch tip. Transverse rupture strength dominated the fatigue behaviour of carbides with low Co content, whilst the fatigue behaviour of carbides with high Co content was determined by fracture toughness.A theoretical method was proposed for the coating design of carbide tools, in order to reduce the possibility of coating delamination in the tool failure processes. The thermal residual stresses of multi-layered coatings were modeled based on equivalent parameters of coating properties, and the calculated results were verified with finite element simulation. This provides theorical basis for the selection and design of coatings of cutting tools in high-speed machining. The resluts show that coatings with a thickness of3μm will produce considerably low thermal residual stresses in the coating deposition process. Dry siding ball-on-disk wear tests of CrN, TiN, TiAIN, AlTiN and AlCrN coatings on flat WC-Co cemented carbide substrate against titanium balls were conducted to evaluate the tribological matchability of tool coatings and workpiece material. TiAIN is strongly recommended as coating of carbide tools when machining titanium alloys due to the lowest friction coefficient and the best wear resistance when sliding against Ti-6A1-4V. Based on the evaluations of matchability of tool substrate and tool coatings, as well as that of tool coatings and workpiece material, the basic principles of tool coating design in high-speed milling were proposed, laying the foundation of the coating design of cutting tools.Cutting performance of the developed solid carbide end mill in high-speed milling of titanium alloy was investigated, comparing with that of a similar end mill. The developed end mill exhibited smaller cutting forces, better chip formation, and longer tool life than the similar tool.