EFFECT OF THE ENVIRONMENT ON THE CUTTING PROCESS OF TITANIUM ALLOYS

A. I. AFONASOV

(Presented by the joint scientific seminar of the departments of machine tools and metal cutting and of mechanical-engineering technology)

In machining titanium and its alloys by cutting, a whole series of interesting features has been observed: extremely small chip shrinkage, a high cutting temperature, a small area of contact between the chip and the tool, low thermal conductivity, and considerable strengthening of the layer being cut; this in turn causes increased specific pressure on the faces of the cutting tool. The heat released during cutting is concentrated over a smaller area, which leads to heating of the tool to a high temperature.

The practice of machining titanium and its alloys shows [1] that the nature of the influence of lubricating-cooling fluids on the cutting process is entirely different from that in cutting carbon and alloy steels.

If, in machining steels, the influence of the medium in which the cutting process takes place is very strong [2], then in machining titanium alloys many investigators have noted a weaker influence of the medium on the cutting process of titanium alloys.

Thus, M. F. Poletika studied the effect of liquid media (water, benzene, and carbon tetrachloride) in free planing of titanium alloy VT-2 at microspeeds. He found an almost complete absence of any effect of lubrication on the cutting process. The features observed in cutting titanium alloys—small chip shrinkage, low coefficients and specific forces of friction, and weak sensitivity of the cutting process to the action of media—M. F. Poletika explains by the high activity of titanium with respect to oxygen and nitrogen of the air. To obtain more convincing evidence for this assumption, he completely isolated the contact zone from the air; for this purpose the cutter and the workpiece plate were immersed in a bath of liquid. But even under these conditions the liquid medium did not affect the friction of the chip on the cutter when cutting the titanium alloy.

V. F. Bobrov carried out experiments to study the influence of lubricating-cooling fluids on the cutting process of technically pure titanium VT-1. Twelve lubricating-cooling fluids were tested (water, oleic acid, spindle oil, activated kerosene, graphitized oil, a 10% emulsion of “Emulsol,” sulfophenol “V,” 15% citric acid, a 5% solution of sodium nitrite in water, a mixture of 60% sulfophenol + 25% kerosene + 15% oleic acid, and a mixture of 80% sulfophenol + 20% carbon tetrachloride—

water). On the basis of the results of the studies, the author drew conclusions about the low effectiveness of the action of surface-active media on the process of cutting titanium alloys, as compared with cutting in air.

Studies of the effect of cutting fluids on friction in machining titanium alloys at high cutting speeds likewise showed a weak lubricating action of liquid media [3, 4, 5].

In view of the foregoing, it was of interest to determine the influence of a lubricating, neutral, oxidizing, and reducing medium on the cutting process of titanium alloys at room temperature (cutting at microspeeds) and at a temperature of about \(800^\circ\mathrm{C}\) (cutting at high speeds).

This choice of media was dictated by the desire to clarify the picture of contact phenomena on the rake face of the tool when machining titanium alloys, in connection with their increased sensitivity to the action of oxygen and nitrogen in air.

Experiments at microspeeds were carried out in free planing of technically pure titanium VT-1 with a broad cutter made of high-speed steel R18 having the following geometry: rake angle \(\gamma = 20^\circ\), clearance angle \(\alpha = 10^\circ\), cutting-edge inclination angle \(\lambda = 0^\circ\).

Specimens of rectangular cross section, of thickness \(b = 3\) mm, were machined at a speed \(V = 0.098\) m/min. The thickness of the layer removed, \(a\), was varied within the limits \(0.05 \div 0.35\) mm. The components of the cutting force \(P_z\) and \(P_y\) were measured with a two-component inductive dynamometer designed by D. V. Kozhevnikov. The forces were recorded by means of an MPO-2 oscillograph. The lubricating fluids used were water, a soap solution, activated kerosene, and carbon tetrachloride; as an oxidizing medium we used 30% hydrogen peroxide, acetone was taken as the neutral medium, and a pyrogallol solution as the reducing medium. All experiments in media, for complete isolation of the tool contact zone, were carried out in a bath, when the cutting zone was completely immersed in the liquid. For comparison, experiments were carried out in air.

The results of the experiments are given in Fig. 1. It is evident from the graphs that the principal component \(P_z\) of the cutting force has insignificant deviations depending on the use of one or another medium, and only carbon tetrachloride is distinguished by the most effective lubricating capacity. A similar picture is also observed for the vertical component of the cutting force \(P_y\). Such behavior of titanium is not anomalous, and other investigators have encountered a similar phenomenon, as was already mentioned earlier [6, 7].

The weak sensitivity of the cutting process to the action of media is explained by the high activity of titanium with respect to oxygen and nitrogen in air. These gases penetrate into the contact zone of the chip with the cutter and react with the juvenile surface of the chip, forming oxide and nitride films that prevent direct contact with the material being machined.

The poor sensitivity of titanium to the action of media under contact isolated from air is apparently explained by the fact that liquids do not completely wet the metallic surface, and near the contact zone there remain separate areas covered with air. It is also possible that the juvenile surface of the chip absorbs oxygen and nitrogen atoms, adsorbed but not chemically bound, from the free surface of the specimen; i.e., when a new surface arises, a certain redistribution of previously attached gas atoms begins.

At high cutting speeds, chip formation and its passage along the rake face of the tool occur in less than

$10^{-4}$ sec. The rapid sliding of the chip along the rake face is accompanied by a high temperature in the contact zone and high specific pressures; moreover, for titanium alloys this is characteristic to a greater degree than for other metals and alloys.

Figure 1.

In-figure labels:

VT-1. Free cutting in a bath
P18. $\gamma = 20^\circ$, $\alpha = 10^\circ$, $v = 0.098\ \mathrm{m/min}$

Axes: $P_z$; $P_y$; $a$, mm

Legend:

• ● — air
• × — water
• ■ — soap solution
• ▲ — kerosene
• ✱ — carbon tetrachloride
• hourglass symbol — 30% hydrogen peroxide
• inverted triangle symbol — acetone
• ○ — pyrogallic acid

For the study of the effect of the environment on the cutting process of titanium alloys at high speeds, carbon tetrachloride was taken as the lubricating medium, since it had proved to be a more effective lubricant in cutting titanium alloys at low speeds. Pure oxygen was chosen as the oxidizing agent. A neutral environment was created with argon and carbon dioxide gas. The experiments were carried out with VT-1 alloy. The workpiece was placed in a special chamber into which carbon tetrachloride or

gas was blown under a pressure of 5 atm through a nozzle directed along the front surface of the tool. The chamber design ensured isolation of the cutting zone from the atmosphere and made it possible, during cutting, to measure the components of the cutting force \(P_z\), \(P_x\), \(P_y\), which were measured by means of an inductive dynamometer designed by V. B. Livshits. The components of the cutting force, temperature, and test time were recorded on an MPO-2 oscillograph. A hard-alloy insert of grade VK6M was used as the cutting tool, with the following geometry: rake angle \(\gamma = +10^\circ\), principal clearance angle \(\alpha = +10^\circ\), principal plan angle \(\varphi_0 = 70^\circ\), cutting-edge inclination angle \(\lambda = 0^\circ\); the depth of cut \(t = 2.5\) mm and feed \(s = 0.26\) mm/rev remained constant in all tests; only the cutting speed \(v\) was varied from 10 to 220 m/min.

Fig. 2

Figure 2 presents the results of the change in the coefficient of friction on the rake face of the tool as a function of cutting speed and medium. The coefficient of friction on the rake face of the tool was calculated without taking into account the forces acting on the flank face of the tool. Separation of the forces on the flank face from the forces measured by the dynamometer was carried out according to the method of Prof. A. M. Rozenberg [8]. The tests were of short duration so that wear would not affect the pattern of forces; on the other hand, sufficient time was allowed for the temperature regime to become established. After each test the cutter was resharpened.

In the same figure, for comparison, the dependence of the coefficient of friction on speed in cutting steel 40 in air is also given, all other conditions being equal.

It is evident from the graphs presented that the coefficient of friction on the rake face of the tool when cutting titanium under ordinary conditions at high speeds is 2.5 times smaller than when cutting steel under the same conditions. The effect of the medium on the cutting process of titanium alloys manifests itself in a distinctive way. As the cutting speed increases, the temperature in the cutting zone rises, and as the temperature rises, the activity of titanium toward the surrounding medium increases.

Thus, tests in oxygen and air showed a decrease in the coefficient of friction with increasing cutting speed; the lubricating medium

(4-chlorinated carbon) did not produce any appreciable reduction in the coefficient of friction compared with an air medium. Cutting in the neutral medium of argon and carbon dioxide, as might have been expected, led to a worsening of the friction conditions on the rake face of the tool; this once again proves that oxide and nitride films serve as the most effective lubricant in the cutting of titanium alloys in an air atmosphere. Similar results were also obtained in classical friction tests on titanium and its alloys. I. S. Kaptyug and V. S. Syshchikov [9] note that the coefficients of friction of titanium on titanium and on other metals are lower than those of other metallic pairs.

Rabinowicz [10] investigated the friction of titanium and its alloys with lubrication and found that liquid lubrication has no effect at all on the coefficient of friction. He sees the reason for this behavior of titanium alloys in the constant presence of a strong oxide film on the surface of titanium.

REFERENCES

1. K. F. Romanov. Theory and practice of the machining of titanium alloys. In: Results of Science, issue 2. Publishing House of the Academy of Sciences of the USSR, 1959.

2. N. N. Zorev. Problems of the mechanics of the metal-cutting process. Mashgiz, 1956.

3. N. I. Reznikov. Productive machining of stainless and heat-resistant materials. Mashgiz, 1960.

4. V. A. Krivoukhov. Machinability by cutting of heat-resistant and titanium alloys. Mashgiz, 1961.

5. Machinability of heat-resistant and titanium alloys. Proceedings of the All-Union Interuniversity Conference. Kuibyshev, 1962.

6. M. F. Poletika. Friction in cutting a titanium alloy at microspeeds. Izvestiya Vuzov SSSR. Mashinostroenie, No. 9, 1960.

7. V. F. Bobrov. On the role of cutting fluids in the cutting of titanium. Vestnik Mashinostroeniya, No. 5, 1961.

8. A. M. Rozenberg, A. N. Eremin. Elements of the theory of the metal-cutting process. Mashgiz, 1956.

9. I. S. Kaptyug, V. I. Syshchikov. Some results of tests of titanium and its alloys for friction and wear. Sudostroenie, No. 8, 1958.

10. Rabinowicz E. Frictional properties of titanium and its alloys. Metal Progress, vol. 65 (1954), No. 2.