Force Relationships in Turning Titanium Alloys

M. F. Poletika, A. I. Afonasov

The study of metal-cutting processes is inseparably connected with investigation of the forces that arise during chip formation. The magnitude and direction of the force arising in metal cutting are important characteristics of this process. Knowledge of the magnitudes of the cutting force and its components is necessary for the successful solution of many practical problems. For example, in designing metal-cutting machine tools, tooling, and fixtures; in developing designs of cutting tools; and in developing normative materials for cutting regimes. In addition, accurate determination of the cutting force and knowledge of the nature of its variation as a function of various factors of the cutting process are absolutely essential in many experimental investigations of the metal-cutting process (study of the stressed and deformed state in the cutting zone, study of contact phenomena, investigation of the dynamics of tool wear, etc.). The question of studying force relationships in turning difficult-to-machine steels and alloys therefore becomes all the more urgent.

The high mechanical characteristics of certain titanium alloys, combined with the features characterizing the cutting process of these alloys (short contact length of the chip with the tool, high temperature in the cutting zone, etc.), lead to rapid tool wear. This, in turn, complicates the study of force relationships and, in a number of cases, makes it necessary to use special research methods.

Titanium alloys VT3-1 and VT6 belong to the group of difficult-to-machine alloys. On the basis of data obtained by F. P. Uryvskii and B. S. Korotyn in studying the machinability of titanium alloy VT3-1 [1], and on the basis of our observations, hard alloys of grades VK8 and VK6M were selected as the cutting-tool material for conducting experiments on turning alloys VT3-1 and VT6. The tool geometry was chosen close to optimal and was characterized by the following parameters: rake angle \(+10^\circ\), clearance angle \(+10^\circ\), auxiliary clearance angle \(+10^\circ\), principal cutting-edge angle in plan \(45^\circ\), auxiliary angle in plan \(15^\circ\), inclination angle of the principal cutting edge \(0^\circ \div 5^\circ\). The feed varied from \(0.145\) to \(0.47\) mm/rev. The cutting speed varied from \(4\) m/min up to a speed permitting short-term operation of the cutter,

at the given feed without significant dulling. To measure the components of the cutting force, a three-component elastic-electrical dynamometer with wire strain gauges was used [2]. The dynamometer was used together with a three-channel strain-gauge amplifier IDD-2 and an MPO-2 oscillograph. In addition to the cutting force, the cutting temperature was measured (by the natural thermocouple method), with the thermoelectromotive force recorded on the same MPO-2 oscillograph, as were the length of chip contact with the tool, tool wear, and test duration.

At first the experimental procedure was the usual one. The duration of each test was not controlled. The measurement results showed that, with increasing cutting speed and feed, the changes in the components of the cutting force do not follow a clear pattern, and control tests do not reproduce the preceding ones. To reveal the causes of such an anomaly in the force relationships, tool wear on the flank face was analyzed. It turned out that at different regimes the intensity of tool wear is not the same, and the wear magnitudes are not comparable. In order to obtain stable results when measuring the cutting force, it was necessary to abandon tests with a “sharp” tool and to provide for a certain preliminary natural dulling of the tool at a large feed and cutting speed. This ensured constancy of wear on the specified face at smaller feeds and speeds. A similar method was used by R. F. Makarov [3] and M. F. Poletika [4]. Subsequently, a method of artificial and natural preliminary wear was tested. However, it was not possible to achieve constancy of wear at considerable cutting speeds and large feeds, even during a short-term test.

The results of the preliminary experiments indicate that obtaining clear force relationships under different regimes when turning difficult-to-machine alloys is very difficult because of tool wear. After a series of searches, a new method was developed for investigating force relationships under intensive tool wear. The essence of this method was described earlier [5].

For alloy VT6, the mechanical strength characteristics are somewhat lower than for alloy VT3-1. Alloy VT6 is more ductile than alloy VT3-1. The study of force relationships in turning alloy VT6 was carried out by the same method as for alloy VT3-1.

Since alloy VT6 has better machinability than alloy VT3-1, this made it possible to conduct investigations at higher cutting speeds. To study tool wear and determine its qualitative and quantitative influence on the components of the cutting force, impressions of the cutting edge of the tool were taken at all investigated regimes and degrees of wear. The cutting process for alloy VT6 proceeds analogously to that considered for alloy VT3-1. The predominant tool wear is flank wear. Up to a value of \(h_{\text{fl}} = 0.2\) mm, wear increases smoothly. In full accordance with the amount of wear, the components of the cutting force also change. The horizontal component \(R_{xy}\) is the most sensitive. Experimental data showing the change in the cutting-force components \(P_z\) and \(R_{xy}\) as a function of cutting speed and tool wear are presented in Fig. 1. To save space, data are given only for the feed \(s = 0.26\) mm/rev. From similar relationships for different feeds (from 0.15 to 0.47), an overall picture can be obtained of the change in the cutting-force components under all investigated regimes at some constant flank wear. Figure 2 presents a graph reflecting the change in the components \(P_z\) and \(R_{xy}\) for the case of turning alloy

Fig. 1

Fig. 2

VT6 under the investigated conditions at constant wear on the flank face, $h_{3r}=0.2$ mm. The character of the variation in the cutting-force components remained the same as for alloy VT3-1, but the force level, in comparison with alloy VT3-1 for identical chip cross sections and cutting conditions, decreased considerably. The magnitudes of the forces acting on the flank face of the tool were determined by the method of extrapolating to zero chip thickness the lines of the dependence of force on feed at a constant angle of inclination of the shear plane, $\beta_1$, and at constant wear. In this case the wear should be no more than $0.2$ mm. After separating the forces acting on the rake and flank faces of the tool, it became possible to analyze in greater detail the cutting process in turning titanium alloys VT3-1 and VT6. Curves illustrating the variation of the cutting-force components applied to the rake and flank faces of the tool under different cutting conditions and at constant wear are presented in Fig. 3. As follows from the graphs, the change in the forces on the rake face as a function of speed and feed obeys the regularity for all titanium alloys. The forces on the flank face of the tool have a large specific weight in the overall force balance and determine the character of their variation. This applies especially to the normal compo-

Fig. 3

...which, as the cutting speed increases, increases severalfold. For the case under consideration, we have a threefold increase in the normal force on the flank face. The friction force on the flank face of the tool decreases as the cutting speed increases. This nature of the change in the forces on the flank face agrees well with the data of other authors [3].

The absolute values of the magnitudes of the forces acting on the flank face of the tool are directly dependent on the amount of wear and on the hardness of the material being machined. The decrease in the friction force on the flank face of the tool with increasing cutting speed may be a consequence of the drop in shear resistance in the layer of metal passing along the flank surface (the machined surface). But such an influence is possible at very high temperatures on the flank face. Consequently, the decrease in the friction force is possible not only due to the “softening” of the contact layer of metal. One of the reasons for the decrease in the friction force on the rake face is the activation of titanium with the surrounding...

by the surrounding atmosphere as a result of the rise in temperature with increasing cutting speed [4]. A similar phenomenon should also occur on the flank face. From the positions considered, the increase in the normal component on the flank face of the tool under constant wear is completely inexplicable. All the more so because, when the forces on the flank face of the tool are separated out, the condition of constancy of the geometry of the cutting wedge at certain degrees of dulling is to some extent ensured. Therefore, it appears entirely impossible to explain the significant increase in the normal component on the flank face of the tool by a change in the geometry of the cutting wedge (rounding radius and actual angle on the wear land).

Let us try to explain, from existing positions, the significant increase in the normal force on the flank face with increasing cutting speed in the process of machining low-ductility titanium alloys. An increase in cutting speed, first of all, leads to intensive wear of the tool on the flank face; in this case not only the size of the wear land changes, but also the geometry of the flank surface. This leads to an increase in the normal and tangential forces on the flank face. Such a fact is well known in the theory and practice of metal cutting. At the same time, experiment reveals a significant increase in the normal force on the flank surface of the tool with increasing cutting speed at a constant wear land. Although it is customary to explain the observed phenomenon from the standpoint of a decrease in the coefficient of friction, in our view this question requires a more detailed explanation.

Proceeding from the scheme of interaction of the worn flank surface of the tool with the material being machined and taking into account the physicochemical characteristics of titanium alloys, one should expect that, with an increase in cutting speed, the increase in the normal load on the flank surface will be associated with an increase in the rate of plastic deformation. The question of the influence of the rate of plastic deformation on the mechanical properties of titanium alloys was discussed in the literature [6], where the increased sensitivity of these alloys to the rate of deformation is noted. With an increase in deformation rate, the plastic properties of titanium alloys decrease, and at the same time the resistance to plastic deformation increases. This gives grounds for asserting that the yield strength for titanium alloys should also increase with increasing deformation rate.

The absence of reliable quantitative data on the resistance of titanium alloys to plastic deformation at different deformation rates makes a final solution of this question difficult. However, the reasoning described above appears to us the most probable in explaining the mechanism of growth of the normal force on the flank surface of the tool with increasing cutting speed when machining low-ductility titanium alloys. Analyzing the influence of deformation rate as a whole on the process of cutting titanium alloys, two features may be noted. On the one hand, sensitivity to deformation rate contributes to a decrease in the cutting force (an increase in brittleness in the chip-formation process), and on the other hand, it contributes to an increase in the load on the cutting tool from the side of the flank face. The latter is one of the reasons for intensive wear of the tool on the flank surface and for the growth of the components of the cutting force.

LITERATURE

1. F. P. Uryvsky, B. S. Korotin. Investigation of the machinability of titanium alloy VT3-1 and heat-resistant steel ÉI787. Proceedings of the All-Union Interuniversity Conference, Kuibyshev, 1962.

1. V. I. Vasil’ev. The use of wire sensors for measuring cutting forces in metal machining. Periodic Information. Topic No. 28, No. K-54-83, Moscow, 1963.

2. A. I. Promitov, R. V. Makarov. On the influence of cutting speed and feed on the forces in turning alloy VT3-1. Proceedings of the Irkutsk Polytechnic Institute. Issue 36, Mechanical Engineering Series. Irkutsk, 1967.

3. M. F. Poletika. Investigation of the characteristics of the cutting process of titanium alloy VT-2. News of Higher Educational Institutions of the USSR. “Mechanical Engineering,” No. 11, 1961.

4. G. L. Kufarev, A. I. Afonasov. Investigation of the cutting process of titanium alloys. News of the Tomsk Polytechnic Institute, vol. 147, 1965.

5. L. S. Moroz, B. B. Chechulin et al. Titanium and Its Alloys, vol. 1, Sudpromizdat, Leningrad, 1960.