INVESTIGATION OF THE CUTTING PROCESS OF TITANIUM ALLOYS

G. L. Kufarev, A. I. Afonasov

(Presented by the scientific seminar of the Department of Machine Tools and Metal Cutting)

The cutting process of titanium alloys is characterized by a number of features that distinguish these alloys from steels and many nonferrous metals. The main ones are: high cutting temperature, small contact area of the chip with the cutter, low chip shrinkage (in individual cases less than unity), small coefficients and specific friction forces, weak sensitivity of the cutting process to the action of lubricating-cooling fluids, and active interaction with the surrounding medium. In this connection, the study of the physico-mechanical phenomena arising in the cutting zone, force relationships, tool wear, the influence of the geometric parameters of cutters and their material on cutting forces, the action of the medium (oxidizing, neutral, reducing, lubricating-cooling), and a number of other phenomena arising in the cutting process is a primary task for researchers concerned with questions of the machinability of difficult-to-machine, heat-resistant, and titanium alloys.

The present article gives the results of further investigations on the machining of titanium alloys that are being conducted in the cutting laboratory of TPI.

With respect to machining by cutting, individual titanium alloys may be classified as difficult-to-machine. In particular, the titanium alloy VT3-1, which has a two-phase structure $\alpha + \beta$, is also among the difficult-to-machine alloys. In terms of its mechanical characteristics, it belongs to the low-ductility titanium alloys.

\[ \sigma_{\mathrm{B}} = 105 \ \frac{\mathrm{kg}}{\mathrm{mm}^{2}};\quad \delta = 13\%;\quad HB = 380 \ \frac{\mathrm{kg}}{\mathrm{mm}^{2}}. \]

In experiments on machining the titanium alloy VT3-1, the following questions were investigated:

1. The influence of cutting speed, feed, and depth on cutting forces in relation to cutter wear.
2. The influence of the tool material on cutting forces.
3. The influence of the tool geometry on cutting forces.

Experiments on cutting the titanium alloy VT3-1 were carried out on a model 1D63A lathe. To determine the cutting forces, a three-component elastoelectric dynamometer with wire strain gauges was used. The dynamometer was used together with a three-channel strain-gauge amplifier IDD-2. Recording of forces

cutting was carried out with the aid of an MPO-2 oscillograph. On the basis of the data obtained by F. P. Uryvsky [1] and of our own investigations, the optimal geometry of the cutters for the experiments was adopted as follows: \(\gamma = +10^\circ\); \(\alpha = +10^\circ\); \(\alpha_1 = +10^\circ\); \(\varphi = 45^\circ\); \(\varphi_1 = 15^\circ\); \(\lambda = +5^\circ\). As the cutting material, plates of VK8 hard alloy with mechanical fastening in the head of the dynamometer were used.

The depth of cut was constant and equal to \(3\) mm. The feeds varied from \(0.145 \div 0.47\) mm/rev. The cutting speed varied from the minimum permitted by the machine tool (\(4\) m/min) to the maximum speed that the cutter could withstand without burning out.

The first results of measuring the cutting forces under the above-mentioned conditions showed that, in the course of the experiment, the cutter wears and gives overstated readings for the cutting forces; moreover, at different conditions the intensity of cutter wear, and consequently the magnitude of the cutting forces, are not comparable with the preceding ones. We tried a method of preliminary wear with various modes of wear. However, it was not possible to achieve constancy of wear during even a short-term experiment (\(3 \div 5\) sec). This also explains the intensive increase in forces with increasing cutting speed, which is directly associated with the growth of cutter wear on the flank face and, consequently, with a change in the initial geometry.

After a number of trials, a new procedure was developed for conducting experiments to determine force relationships in cutting hard-to-machine alloys. The essence of this procedure is that, at a constant speed and feed, experiments are carried out with a sharply sharpened and carefully finished cutter of constant geometry over increasing intervals of time, with recording of the cutting forces, temperature, and experimental time on an MPO-2 oscillograph. After each experiment, the contact areas on the rake and flank faces of the tool are measured with an MIR-1 microscope. At low cutting speeds, we were unable to carry the experiment through to complete tool wear because of an insufficient quantity of titanium alloy VT3-1. But even at speeds on the order of \(20 \div 50\) m/min (depending on the feed), the tool lives proved to be insignificant, and the experiments by the above-described procedure were conducted until complete destruction of the cutting edge.

In order to obtain a picture of the force relationships as functions of speed, feed, and tool wear, the experiments were carried out at four feeds (\(0.145\); \(0.265\); \(0.370\); \(0.470\) mm/rev), with at least six speeds taken at each feed. By way of example, Fig. 1 presents experimental data obtained in machining titanium alloy VT3-1 at a feed of \(0.265\) mm/rev. After processing the entire series of experiments, we were able to compare the magnitudes of the cutting forces and the cutting temperature at different stages of tool wear for different cutting speeds and feeds (Fig. 2). The dependences of force on cutting speed were smooth, without the “humps” and “valleys” characteristic of steel, which once again indicates the absence of built-up edge when machining titanium alloys.

The principal component \(P_z\) changes very little with increasing speed; however, with increasing feed it rises considerably (thus, when the feed is increased threefold, the force \(P_z\) increases by a factor of \(2.5\)). Tool wear, which occurs predominantly on the flank face, has a great influence on the magnitude of the cutting force. Thus, with cutter wear on the flank face only up to \(h_{z.?} = 0.15\) mm, the principal component increases by \(15 \div 20\%\), while the resultant \(R_{xy}\) of the horizontal forces \(P_x\) and \(P_y\) increases strongly both with increasing speed and feed and with increasing cutter wear. With cutter wear up to

\(h_{3.2}=0.15\ \mathrm{mm}\), the resultant of the horizontal forces \(R_{xy}\) increases by \(200\text{--}250\%\). In these same figures one can see an intensive increase in the resultant of the horizontal forces \(R_{xy}\) even with constant wear on the flank face. The latter does not agree with the usual picture of the dependence of cutting forces on speed, in which we have a decrease

Fig. 1

in cutting forces with increasing speed. This feature of the increase in the resultant horizontal forces \(R_{xy}\) at constant wear but changing cutting speed is explained by breakdown of the cutting edge of the tool and by a change in the geometry of the rear surface of the tool (this is discussed in detail in the article by V. A. Naumov, “Wear of a Carbide Tool on the Flank Face and Its Influence on Cutting Force,” printed in this same collection).

The method described above made it possible to obtain force dependences at different degrees of tool wear and, for specified regimes, to establish the tool life--speed relationship.

By the same method, the question of the influence of the tool material, as well as of the tool geometry, on the cutting force in machining titanium alloys was investigated. To investigate the influence of the tool material, carbide inserts of grades VK8; VK6M; VK4; T15K6; T5K12V and TT7K12 were selected. Comparative experiments were carried out at one feed \(S=0.265\ \mathrm{mm/rev}\) and speed \(V=11\ \mathrm{m/min}\), with constant tool geometry used in the main series of experiments. From the results of these experiments it may be concluded that

double-carbide and triple-carbide hard alloys are completely unsuitable for machining titanium alloys, which confirms the opinion of a number of researchers [1, 2, 4] concerning the chemical affinity of hard alloys of the titanium–cobalt group with the material being machined—titanium alloys—as a result of which interaction occurs between the contact surfaces of the cutter and the machined metal.

Fig. 2

As tool materials in the machining of titanium alloys, cutters made of high-speed steels are also sometimes used (cutoff, roughing, form tools, etc.). Therefore, analogous experiments were conducted with various grades of high-speed steel: R18, R14F4, R9K10, with mineral ceramics TsM332, and with a cast alloy based on tungsten and cobalt containing tantalum carbides (alloy A). The experiments were carried out at a constant cutting speed \(V = 11\ \text{m/min}\), feed \(S = 0.265\ \text{mm/rev}\), and depth \(t = 3\ \text{mm}\). Of the high-speed steels, cobalt steels and cast alloy A may be considered the most suitable for machining titanium alloys. Mineral-ceramic cutters, in terms of some of their properties (red hardness, wear resistance, absence of affinity in chemical composition), would be a very valuable tool material for machining titanium alloys, but one negative property of mineral ceramics—its increased brittleness—hinders the use of this tool material in the machining of low-plasticity titanium alloys. The poor durability of cutters made of high-speed steel R18 and tungsten–vanadium high-speed steel R14F4 is explained by the fact that, when machining titanium alloys, a large amount of heat is released in the cutting zone and the cutting part of the tool becomes strongly heated—

and under such conditions tungsten–vanadium high-speed steels have no advantage over high-speed steel R18. In these cases cobalt high-speed steels should be used; their hardness at a temperature of \(500 \div 550^\circ\mathrm{C}\) is several Rockwell units higher than that of steel R18, which is also confirmed by our experiments.

In the cutting process, chip removal, friction, and the degree of plastic deformation of the layer being removed—and thus the load on the cutter, the cutting temperature, and its tool life—depend on correctly selected geometric parameters. In the existing literature on the results of machinability of titanium alloys, contradictory recommendations are given for selecting the optimum geometry of carbide cutters.

To clarify the effect of tool geometry on cutter wear and force relationships when machining titanium alloy VT3-1, experiments were carried out according to the above-described procedure. Carbide inserts of grade VK8 from a single batch were used as the tool material. The influence of the rake angle was investigated, since the remaining geometric parameters were not of special interest and were taken as \(\varphi = 45^\circ\), \(\varphi_1 = 15^\circ\), \(\alpha = 10^\circ\), \(\alpha_1 = 10^\circ\), \(\lambda = +5^\circ\). The inserts were sharpened with rake angles \(\gamma = +20^\circ\), \(\gamma = +10^\circ\), \(\gamma = 0^\circ\), \(\gamma = -10^\circ\), \(\gamma = -20^\circ\), and with a chamfer \(\gamma_{\phi} = -10^\circ\), chamfer width \(0.2\ \mathrm{mm}\), the rake angle of the cutter with chamfer being \(\gamma = +10^\circ\).

Comparative tests were carried out at a constant speed \(V = 21.6\ \mathrm{m/min}\) and feed \(S = 0.265\ \mathrm{mm/rev}\). The experiments were conducted only up to the onset of intensive wear of the cutters on the flank face.

It follows from the experiments that wear of cutters with large positive angles proceeds more intensively than with negative ones, but the magnitude of the main component of the cutting force \(P_z\) and the resultant of the horizontal forces \(P_{xy}\) increases substantially in the transition from positive rake angles to negative ones. In this respect, a cutter with a negative rake angle on the chamfer and a positive general rake angle occupies an intermediate position. Hence there follows the reasonable conclusion that it is expedient to use cutters with a negative rake angle on the chamfer and a general positive rake angle when machining difficult-to-cut titanium alloys under roughing conditions. Under finishing conditions, large values of the cutting force at negative rake angles, as well as a substantial increase in forces during intensive blunting at large positive rake angles, can cause impermissible elastic deformations of the workpiece, reducing machining accuracy and increasing residual stresses, which in turn adversely affects the fatigue strength of parts made of titanium alloys. Therefore, under finishing conditions for machining titanium alloys, it is expedient to use cutters with small positive rake angles \(\gamma = 8^\circ \div 10^\circ\).

Conclusions

1. The main component of the cutting force \(P_z\) and the resultant of the horizontal forces \(R_{xy}\) when machining titanium alloy VT3-1 depend noticeably on the feed, but an even more intensive increase in the resultant \(R_{xy}\) is observed with wear of the cutter on the flank face, and the latter increases strongly with increasing speed.

2. For machining titanium alloys as carbide cutters, only inserts of tungsten–co-

the cobalt group (VK). As tool materials, it is possible to use high-speed cobalt steel grades R9K5 and R9K10. However, with the use of high-speed steels, the cutting conditions are considerably reduced.

1. On the basis of an analysis of the influence of the geometry of the cutting tool on the cutting forces, it may be said that for heavy roughing operations it is necessary to use cutters with negative rake angles, or with a negative rake angle on the chamfer in combination with a positive rake angle.

In finishing operations, in order to increase the accuracy of manufacturing parts from titanium alloys and to improve the quality of the machined surface, a cutting tool with positive rake angles \(\gamma = +8^\circ \div +10^\circ\), without a chamfer, should be used.

1. When machining titanium alloys, increased requirements are imposed on the rigidity of the SPID system.

LITERATURE

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

2. M. F. Poletika. Investigation of the features of the cutting process of titanium alloy VT-2. Izv. vuzov SSSR—Mashinostroenie, No. 11, 1961.

3. M. F. Poletika, A. I. Afonasov. Investigation of the cutting process of technical titanium. Izv. TPI, vol. 114, Tomsk, 1964.

4. A. Sh. Shifrin, L. M. Reznitsky. Machining by Cutting of Corrosion-Resistant, Heat-Resistant, and Titanium Steels and Alloys. Mashinostroenie, 1964.