SELECTION OF OPTIMAL CUTTING CONDITIONS FOR TURNING TITANIUM ALLOYS

A. I. AFONASOV

(Presented by the scientific seminar of the Departments of Machine Tools and Metal Cutting and of Mechanical-Engineering Technology)

At present, high-strength, heat-resistant, and corrosion-resistant steels and alloys are widely used in many branches of mechanical engineering.

Titanium alloys are among these materials. Along with high service properties, some titanium alloys have poor machinability by cutting. A significant reserve for increasing the productivity of machining titanium alloys lies in the selection of cutting conditions.

The desire to take into account the economic and technological requirements of production in developing rational cutting conditions with minimal expenditure of time and resources has led to the creation of accelerated methods based on the study of the intensity of tool wear.

In recent years A. D. Makarov and his coworkers [1] developed a method for determining the machinability of metals by cutting and a method for determining optimal combinations of feeds and speeds at which maximum tool life and the best quality of the machined surface are ensured.

The present article describes the results of investigations carried out in the metal-cutting laboratory of the Tomsk Polytechnic Institute in cooperation with the Verkhnyaya Salda Metalworking Plant, using A. D. Makarov’s method to determine tool life and optimal cutting conditions in rough and semi-finish turning of difficult-to-machine titanium alloys.

The investigations were carried out on standard titanium alloys of grades VT3-1, VT9, and VT6.

As the tool, cutters with mechanical fastening of VK8 hard-alloy inserts were used. For rough machining the cutters had the geometry: $\gamma = 0^\circ$, $\alpha = 10^\circ$, $\alpha_1 = 10^\circ$, $\varphi = 45^\circ$, $\varphi_1 = 12^\circ$, $\lambda = 0^\circ$, $R = 1$ mm. For semi-finish machining the cutters were sharpened with the following geometry: $\gamma = +7 \div 10^\circ$, $\alpha = 10^\circ$, $\alpha_1 = 10^\circ$, $\varphi = 45^\circ$, $\varphi_1 = 15^\circ$, $\lambda = 0^\circ$, $R = 0.5$ mm. Experiments in roughing bars of titanium alloys with diameters of 120 and 160 mm were carried out under plant conditions on a DIP 500 lathe. The cutting conditions were varied within the following limits: cutting speed—$V =$ from $5 \div$

÷ 40 m/min; feed—\(s =\) from 0.5 ÷ 1.5 mm/rev; cutting depth \(t = 5\) mm.

Semi-finish turning of bars made of titanium alloys with diameters of 120 and 160 mm was carried out on a model 1D63A lathe. The experiments were performed under the following conditions: cutting speed varied—\(V =\) from 4 ÷ 120 m/min, feed—\(s_1 =\) from 0.11 ÷ 0.47 mm/rev, cutting depth—\(t = 2 \div 3\) mm.

The average contact temperature was measured by the natural thermocouple method. Tool wear on the rake and flank faces was measured using an MIR-1M microscope.

The intensity of tool wear was evaluated by the surface relative wear of the cutter on the flank face—\(h_{\text{opz}}\).

\[ h_{\text{opz}} = \frac{h_{\text{zk}} - h_{\text{zn}}}{(l_k - l_n)\cdot s} \cdot 100 \ \text{mm}/10^3\ \text{cm}^2, \]

where

\(h_{\text{zk}}\)—the final or current value of tool wear on the flank face, mm;
\(h_{\text{zn}}\)—the initial wear of the tool on the flank face, mm;
\(l_k\)—the final or current cutting-path length, m;
\(l_n\)—the length of the initial section of the cutting path, m;
\(s\)—the magnitude of the segment, numerically equal to the feed, mm.

Particular empirical relationships showing the influence of cutting speed and feed on tool life were obtained on the basis of the regularities in the change of the surface relative wear of the cutter—\(h_{\text{opz}}\).

The experimental graphs reflecting the relationship speed—tool life in logarithmic coordinates, for cases of machining titanium alloys under the indicated conditions, have two rectilinear sections with a break point. In the sections located to the left of the break points, the influence of cutting speed on tool life is significantly less than it is in the speed range for the right-hand sections. With increasing feed, the bend of the lines \(T = f(V)\) begins at lower speeds. It follows from this that power-law formulas expressing the dependence \(T = f(V \text{ and } s)\) are valid only for definite ranges of cutting speeds and feeds. Calculation of tool life or cutting speed by a single power-law relationship can be permitted only if the left branch of the curve \(T = f(V)\) lies in the zone of very low cutting speeds having no practical significance. The extremal character of the dependence \(T = f(V)\) is caused by a change in the nature and intensity of the prevailing type of wear. As the cutting conditions change, the contact conditions between the work material and the tool material change, and the physico-mechanical characteristics of the contacting surfaces also change.

The use of the relationships \(T = f(V; s)\) for optimizing cutting conditions is associated with a number of shortcomings: conventional tool-life tests are very labor-intensive and require a considerable consumption of the material being machined; the cutting speed \(V_T\) and the tool-life period \(T\) are a function of the adopted blunting criterion; when the function \(T = f(V \cdot s)\) is monotonic, it is impossible to determine the optimal machining conditions that ensure minimum wear intensity.

Thus, the possibility is not excluded of selecting irrational cutting conditions under which, along with low productivity and high machining cost, the tool will have a high wear intensity.

From these standpoints, the most objective characteristic of the machinability of metals by cutting should be considered the intensity of tool wear. The regularity of the change in the intensity of tool wear does not depend on the adopted blunting criterion, which makes it possible to sharply reduce the duration of experiments in studying the machinability of metals and in establishing the most advantageous cutting conditions.

Figure 1 presents the dependence of the wear intensity, expressed through the surface relative wear of the cutter on the flank face, on cutting speed and feed for cases of rough and semi-finish turning of titanium alloys of grades VT3-1 and VT9. The same graph gives experimental values of the average contact temperatures for the cases considered in machining alloys VT3-1 and VT9.

Fig. 1

The dependences \(h_{\mathrm{rel}} = f(V)\) for all investigated feeds have points of minimum, which are observed at different cutting speeds.

The greater the feed and the higher the strength of the alloy being machined, the lower the cutting speed at which the value of the surface relative wear has its minimum value. For each pair—machined material and carbide alloy—at all feeds, the minimum points of the curves \(h_{\mathrm{rel}} = f(V)\) correspond to the same temperature. This temperature will be the optimum contact temperature for the pair “machined alloy—cutter,” at which the wear intensity will be minimal. Thus, each feed corresponds to a definite optimum cutting speed, \(V_0\), during operation at which the minimum intensity of tool wear is attained. Increasing the cutting speed above the optimum value, as well as

its reduction lead to an increase in the intensity of wear. However, the physical causes producing an increase in the intensity of wear at \(V < V_0\) and \(V > V_0\) will be different.

An increase in the intensity of tool wear when the cutting speed and temperature are increased relative to their optimal values occurs as a result of intensification of diffusion processes on the contact surfaces, plastic deformation of the thinnest layers of the tool, and, as a result of these phenomena, changes in the initial geometry of the cutting wedge.

A decrease in cutting speed below the optimal value leads to an increase in the degree of deformation of the layer being cut, and to increased friction on the contact surfaces. A reduction in the rate of deformation and increased friction on the contact surfaces of the tool cause an increase in the volume of plastic deformation of the chip material and of the machined surface. At the same time, the hardness of the machined surface and the depth of the work-hardened layer increase.

These phenomena promote intensive abrasion and microscopic tearing-out of particles of the carbide tool as a result of adhesion phenomena and abrasive wear.

The influence of the thickness of the layer being cut on the intensity of tool wear is a complex dependence, since a change in the chip thickness affects the cutting temperature, as well as the stresses arising on the contact surfaces of the tool.

Under conditions of rough and semi-finishing cutting, the choice of feed is dictated by the desire to obtain the greatest productivity of the cutting process. The productivity of the cutting process may be determined by the volume of metal converted into chips per unit time or by the area of new surface formed per unit time. Rough machining is better characterized by volumetric productivity. Finishing and semi-finishing machining may be characterized by surface productivity.

Fig. 2

As was shown above, the least intensity of tool wear is observed, for each selected feed, at its own optimal cutting speed, which also ensures constancy of the optimal cutting temperature.

If the productivity of the cutting process is calculated for each feed at the optimal cutting speed, then the maximum productivity will be achieved at a certain most advantageous feed.

The graph in Fig. 2 shows the dependence of productivity on feed at the optimal cutting temperature. From the graph it is evident that the maximum area of machined surface, for the case of semi-finishing turning of titanium alloy VT3-1 with a VK8 carbide tool, will be obtained when working at a feed of \(0.3 \div 0.4\) mm/rev.

As a result of the investigations carried out, recommendations can be given for the most advantageous cutting conditions for titanium alloys in roughing and semi-finishing. These recommendations are summarized in Table 1.

Table 1

In this case, the tool-wear criterion should be taken as flank wear under roughing conditions, \(h_{з}=1,2\) mm, and under semi-finishing conditions, \(h_{з}=0,5\) mm.

Conclusions

1. The most objective characteristic of the machinability of metals by cutting should be considered the intensity of tool wear.

2. The study of the regularities governing changes in wear intensity and mean contact temperature in machining metals by cutting under different conditions provides the key to determining optimal conditions.

3. The poorer the machinability of a metal by cutting, the narrower the range of expedient conditions, and the more important it becomes to determine the optimal machining conditions.

4. Using the method of A. D. Makarov, it is possible to determine tool-life relationships and optimal conditions that ensure maximum productivity of the process in roughing and semi-finishing types of metal cutting. In this case, a 15–20-fold saving in time of the workpiece and tool material is achieved in comparison with ordinary tool-life tests.

REFERENCES

1. A. D. Makarov. Wear and Tool Life of Cutting Tools. Moscow, “Mashinostroenie,” 1966.