WEAR PATTERN OF CARBIDE TOOLS IN TURNING DIFFICULT-TO-MACHINE TITANIUM ALLOYS

M. F. POLETIKA, A. I. AFONASOV

Among the most important features of titanium alloys that determine the pattern of tool wear are the following: low ductility combined with high strength, low thermal conductivity, and high chemical activity with respect to metals and gases. These properties of titanium alloys determine the specific nature of the cutting process, which is manifested in the elemental character of the chip, seizure of the removed chip with tool particles, the absence of built-up edge, high contact temperatures and pressures, and small contact areas between the chip and the cutting surface of the tool.

To study the mechanism of cutter wear, a comprehensive investigation was carried out, which can be divided into three stages. The first stage was an analysis of the serviceability of various tool materials and the selection of geometric parameters. At the second stage of the investigations, the pattern of tool wear was studied under different cutting conditions.

The third stage was an investigation of the geometry of worn tool surfaces. The experiments were performed on a rigid lathe, model 1D63A, with stepless spindle-speed control. For a comparative and qualitative assessment of tool serviceability, a three-component rigid piezoelectric dynamometer with wire strain gauges was used [1]. The cutting temperature was measured by the natural thermocouple method. The components of the cutting force and the temperature were recorded on an MPO-2 loop oscillograph.

In addition, in each experiment the following were measured: the length of the contact area of the chip with the tool rake face, and the length of the wear land on the flank face. The measurements were made using MIR-1M portable microscopes with special attachments.

The experiment time was monitored by a stopwatch and by a time marker on the oscillograph tape. The actual tool geometry at different stages of blunting was recorded by the replica method [2]. Since questions concerning the selection of tool material and its geometry in machining titanium alloys by cutting have already been published in the literature, the present article considers the wear pattern of carbide tools of grades VK8 and VK6M in turning titanium alloys VT3-1 and VT6. As for the geometry of the worn tool surfaces, this question has been covered mainly by V. A. Naumov [3].

The wear of the working surfaces of a tool is a complex physicochemical process of destruction of the surface layers, consisting of abrasive, adhesive, diffusion, oxidative, and deformation processes. All these types of wear are very closely interconnected, determining, as a whole, the total wear of the tool. However, the relative share of each of these types in the total wear differs greatly and depends on the properties of the materials of the contacting pairs, as well as on the machining conditions.

The observations and measurements carried out make it possible to present the following picture of the wear of a carbide tool as a function of operating time and cutting conditions for titanium alloys. In the initial stages of operation, the most intensive development of wear occurs on the flank surface, and then the rake surface gradually begins to wear.

Fig. 1

In this case no wear crater is formed; instead, lowering of the cutting edge is observed—a “collapse.” The intensity of wear depends on the magnitude of the cutting speed and feed. Smooth growth of wear in all cases is observed up to a flank-wear land width of the order of 0.2–0.3 mm, after which the wear begins to progress. In the wear process, zonal chipping of the cutting edge is observed. At the places of chipping, centers of more intensive wear are created. The particles of hard alloy carried away by the flowing chip range in size from microscopic to those visible to the unaided eye. Figure 1 shows a photograph of a titanium chip with a piece of hard alloy welded to it. Additional deformation of the chip surface at the place of tearing-out is also clearly visible there.

Abrasion of the flank surface proceeds uniformly, without centers of chipping. After chipping of the cutting edge, wear is transferred also to the flank face of the tool, where deep elongated grooves are formed along a length considerably greater than the main flank-wear land. The moment of this stage of wear is easily detected from scratches on the machined surface and from the appearance of the flowing chip; at the same time, the dynamometer records an increase in the horizontal components of the cutting force.

From this moment the period of intensive wear begins. Zonal chipping of the cutting edge does not always occur. Very often the wear proceeds uniformly, and only when a certain value of the flank-wear land is reached does an instantaneous ...

of the cutting edge along the entire length of the working section. In most cases, chipping of the cutting edge occurs when the wear land on the flank face exceeds 0.4 mm. The rake face then drops sharply, and the stage of catastrophic wear begins. In this connection, for carbide tools of grades VK8 and VK6M when machining difficult-to-machine titanium alloys, the blunting criterion adopted is the width of the wear land along the flank surface, equal to 0.4 mm.

The course of wear over time depends strongly on the cutting speed, feed, and rake angle of the tool, as the principal factors determining the thermal and force conditions under which the tool cutting edges operate. The graph in Fig. 2 shows a significant difference in tool life when the cutting speed and feed are increased.

Figure text: flank wear, \(h_{\text{з.г.}}\); time of operation, \(\tau\), min. Work material: alloy VT3-1. Cutter VK8; \(\gamma = +10^\circ\); \(\alpha = 10^\circ\); \(\varphi = 45^\circ\); \(\varphi_1 = 15^\circ\); \(\lambda = +5^\circ\). Curves marked: \(v = 26\); \(S = 0.37\); \(t = 3\); and \(v = 14\ \text{m/min}\); \(S = 0.265\ \text{mm/rev}\); \(t = 5\ \text{mm}\).

Fig. 2

Developing the concepts described above concerning the nature of tool wear in turning titanium alloys, it is necessary to dwell on a number of causes that determine the specific character of the wear. An important circumstance determining the nature of the wear is the high contact loads on the working surfaces of the tool.

This is associated with the small dimensions of the contact area between the tool and the material being machined. As experimental data show, in cutting titanium alloys the visible contact area is 2–3 times smaller than when cutting steel of the same hardness under analogous conditions. To explain such a phenomenon, it is necessary to take into account one of the features of titanium alloys—their extremely high activity with respect to oxygen and nitrogen in the air. Owing to penetration of air from the side of the less-loaded portion of the contact zone adjacent to the point where the chip separates from the rake face, the area of plastic contact is significantly reduced. As the cutting speed increases, the contact temperature rises, which promotes more intense interaction of the contact side of the chip with the surrounding atmosphere. The area of plastic contact in this case decreases faster than the nominal contact area.

The character of the distribution of temperature and stresses on the contact surfaces of the tool has a substantial influence on wear. According to A. N. Reznikov [4], when turning titanium alloy VT2 with a VK8 carbide tool at a cutting speed of 30 m/min and a feed of 0.2 mm/rev, the temperature maximum \((1000^\circ\text{C})\) is concentrated near the cutting edge. A pronounced nonuniformity is also observed in the distribution of stresses along the cutting edges. Diagrams of the distribution of tangential and normal stresses obtained by V. F. Bobrov [5] for alloy VT-2 show that the intensity of stresses on the cutting edge when turning tita-

of the new alloy is significantly greater than for steel 40, brass L78, and gray cast iron. The specificity of the contact phenomena also determines the geometry of the worn surfaces of the tool. As wear proceeds, a collapse of the cutting edge on the rake-face side is observed, as is the formation of a negative clearance angle on the wear land along the flank face; at the same time, because of self-sharpening, the rounding radius of the cutting blade varies within the range of 15–20 µm.

At the critical stages of wear, plastic deformation of the cutting wedge is observed, manifested in the form of build-ups and bulging of the rake and flank surfaces of the tool beyond the contact areas.

It is evident that, under the action of high contact temperatures, the cobalt binder softens; at the same time, the tangential stresses reach the shear yield point of the contact layers of the tool material. At the stage of intensive wear, the cutting edge becomes blunted and a large rounding radius is formed, which connects the worn rake face of the tool with the negative wear land on the flank face. With further operation of such a tool, the rounding radius of the cutting edge again decreases as a result of self-sharpening.

Thus, the physical picture of tool wear is associated with the specific features of the process of cutting titanium alloys and depends on the machining conditions. At low cutting speeds, abrasive and adhesive wear are observed, which complement one another.

Increasing the cutting speed and feed leads to an increase in the concentration of temperatures and stresses at the cutting edges of the tool.

Under these conditions, the high chemical activity inherent in titanium begins to play the decisive role, promoting intensified adhesion (seizing) and the emergence of diffusion processes, which intensify tool wear. With increasing wear, or with a further increase in machining parameters, the thermal and force load on the contact surfaces of the tool rises, leading to plastic deformation of the cutting edges of the tool or to chipping.

The investigations carried out into the nature of wear of carbide tools in turning difficult-to-machine titanium alloys are of considerable interest for solving the main problem of machinability by cutting of difficult-to-machine steels and alloys—determining the durability and working capacity of the cutting tool.

REFERENCES

1. V. I. Vasil’ev. Application of wire transducers for measuring cutting forces in metal machining. Periodic Information, topic No. 28, No. K-54-83, Moscow, 1953.

2. G. L. Kufarev, V. A. Naumov. The influence of wear on cutting forces in turning. Proceedings of the Tomsk Polytechnic Institute, vol. 147, 165.

3. V. A. Naumov. Specific features of the cutting process with a worn tool. Candidate’s dissertation, Tomsk, 1966.

4. A. N. Reznikov. Heat Transfer in Cutting and Cooling of Tools. Mashgiz, Moscow, 1963.

5. V. F. Bobrov. Determination of stresses in the cutting part of metal-cutting tools. In: Highly Productive Cutting in Mechanical Engineering. Nauka Publishing House, Moscow, 1966.