Contact interaction in the cutting of high-temperature alloys

Abstract

1293 It is often assumed that the physicomechanical parameters of the blank’s surface layer and the wear rate of the cutting tool are closely related to the frictional characteristics of the tool‐blank contact [1]. However, no models have been presented for the determination of this relationship. A mathematical model of the formation of the machined blank surface may be derived from research based on the classical theory of elasticity and plasticity, taking account of the plastic deformation of the metal, its hardening and destruction, and current concepts regarding the mechanism by which residual stress is formed [2]. The validity of this model is verified by experiments on the turning of high-temperature nickel alloys by tools made of hard alloys based on tungsten carbide and high-speed steel. A functional relation has been established between the normal p r and tangential τ n stresses at the tool’s working surface and the contacting surface of the blank during mutual slip, on the one hand, and the output parameters of machining (tool wear, surface roughness, depth and degree of surface hardening, residual stress in the surface layer), on the other. It is found that the minimum stress in the tool‐blank contact zone due to the normal and tangential contact stresses, which is characterized by the maximum tangential stress is observed in machining with the optimal temperature in the cutting zone [1]. Consequently, the minimum stress in the tool‐blank contact zone is associated with the formation of a surface layer more suitable for subsequent operation of the part. Calculations by the proposed mathematical model call for numerical values of the normal stress p r and tangential stress τ n . If we adopt the method and equipment developed at Ufa State Aviation-Engineering University for laboratory study of the functional interaction of tool and blank materials—by simulation of the contact between microprojections of the harder material and a soft counterbody (the tool tip and the blank)—we may determine those characteristics over the temperature and stress range observed in machining [3]. This is the best experimental method of determining the contact characteristics of the tool and blank, since it provides reliable and convenient data for calculation of surface quality of the blank on the basis of the proposed mathematical models. τmax In the experiments, we study the turning of KhN60VT, KhN78T, and ZhS6UVI high-temperature alloys, 40 i and 30 iEe A steel, and VT9 titanium alloy by tools made of VK8, VK6M, VK60M, R18, R12F5M, R9K5, R12F2K8M3, and other alloys. The frictional characteristics for the contact of hightemperature KhN60VT nickel alloy and VK6M hard alloy are shown in Fig. 1, where f a is the adhesive component of the frictional coefficient; and τ 0 is the tangential stress in the absence of normal stress p r . As is evident from Fig. 1, the normal stress p r declines with increase in the temperature θ , on account of softening of the blank. The increase in tangential stress τ n with increase in θ to values at which f a is a maximum is associated with the accumulation of excess energy in the course of plastic contact deformation, under the action of normal stress p r .