Chip Formation in Cutting a Low-Ductility Titanium Alloy
Abstract Generated abstract
This paper examines chip formation during cutting of the low-ductility titanium alloy VT3-1, focusing on the transition from continuous to segmented chips and the limitations of conventional chip shrinkage as a process characteristic. Using chip-root microsections obtained by instantaneous stopping, observations of the cutting zone, and comparative measurements, the authors propose estimating the final shear angle of a chip element from the element length and cut thickness. The results indicate that deformation becomes localized in the contact layer at higher cutting speeds, while the main element volume remains comparatively little deformed. The final shear angle increases with decreasing alloy ductility and with increasing feed, and rises initially with cutting speed before becoming relatively insensitive to further speed increases.
Full Text
CHIP FORMATION DURING CUTTING OF A LOW-DUCTILITY TITANIUM ALLOY
M. F. POLETIKA, A. I. AFONASOV
As is known, the process of cutting titanium alloys is characterized by a high temperature in the cutting zone, a short length of contact between the chip and the tool, and a very small chip shrinkage. At high cutting speeds the shrinkage may even become less than unity; moreover, as was shown by one of the authors of the present article [1, 2], as well as by other investigators [3, 4], this is the result of the transition from a continuous chip to a segmented one. This is especially characteristic of the machining of low-ductility alloys, which was previously illustrated by experiments carried out with alloy VT-2 [1]. In the present work we describe certain results of an investigation of chip formation during cutting of alloy VT3-1.
Fig. 1. Microsection of the cutting zone at low speed. \(V=5\) m/min; \(S=0.26\) mm/rev; \(t=3\) mm; \(\gamma=+10^\circ\); \(\alpha=+8^\circ\)
Figures 1 and 2 present microphotographs of chip roots obtained at different speeds by means of a device for instantaneous stopping of the cutting process. In both cases the chip is segmented, and, as is easy to see, with increasing cutting speed the segments become more distinctly separated.
If at low speed the deformation encompasses almost the entire volume of the element being separated, then with increasing speed it becomes localized in the contact layer, which serves as the connecting link between the elements. This is clearly visible in Fig. 2. The contact layer is textured throughout, whereas in the main volume of the element the degree of deformation is small and the initial microstructure is almost not transformed.
In many cases the bond between chip elements proves to be very strong, which, apparently, has misled some investigators [5, 6], who considered that, when cutting titanium alloys, a continuous chip is formed that lengthens while sliding along the rake face (undergoes “negative” shrinkage).
Since the chip soon has an elemental structure, the shrinkage measured by the usual method can no longer be a characteristic of chip formation. As such a characteristic it is more correct to take the limiting angle \(\beta_1\), which determines the position of the plane along which separation of the chip element occurs.
Two methods of measuring the angle \(\beta_1\) are possible: either from microphotographs similar to those shown in Figs. 1, 2; or by continuous observation, during cutting, of the side surface of the chip root. The first method requires a very large number of polished sections of roots, since for each combination of cutting conditions it is necessary to obtain several roots recording different phases of the process of formation of a chip element.
Fig. 2. Microsection of the cutting zone at high cutting speed. \(V = 55.4\ \mathrm{m/min}\), \(S = 0.26\ \mathrm{mm/rev}\); \(t = 3\ \mathrm{mm}\); \(\gamma = +10^\circ\); \(\alpha = +8^\circ\)
The second method is incomparably less laborious, but may lead to errors, since the angle \(\beta_1\), measured at the surface, sometimes differs substantially from the mean value (the average over the chip width).
In search of at least an approximate method for determining the angle \(\beta_1\), we first attempted to calculate it through the angle \(\Psi\), which the long side of the formed element makes with the rake face. In doing so we proceeded from the fact that, when cutting low-ductility titanium alloys, the element, in the course of its displacement along the rake face, is for the most part no longer deformed, and its rotation is also insignificant. In this case the angle \(\beta_1\) is found from the simple formula:
\[ \beta_1 = \frac{\pi}{2} + \gamma^0 - \Psi^0 \ldots \tag{1} \]
A more detailed study showed that rotation of the element during its motion along the rake face nevertheless does occur and is the result of plastic deformation in the contact layer of the chip. The element rotates in the direction of decreasing the angle \(\Psi\), and therefore formula (1) will always give overestimated results.
We shall then replace measurement of the angle \(\Psi\) by measurement of the length of the side of the element. The deformation of this side during displacement of the element is insignificant and is expressed only in some curvature and elongation of its near-contact part. If the length of the element is denoted by \(l\), then the angle \(\beta_1\) is determined by:
\[ \beta_1 = \arcsin \frac{a}{l} \ldots, \tag{2} \]
where
\(a\) is the thickness of the cut.
To verify the proposed procedure, experiments were carried out in which the angle around the element \(\beta_1\) was determined by other known methods. In particular, motion-picture recording was used, as well as visual observation through a microscope with a degree scale and photography with po-
by means of this microscope of the lateral surface of the cutting zone, as well as the determination of the angle \(\beta_1\) at the chip roots. Fig. 3 gives the results of measurements of the angle \(\beta_1\) by the described method for the case of turning titanium alloy VT3-1 under various conditions. For comparison, the same graph also gives data for other titanium alloys with different mechanical properties; among them alloy VT3-1 has low ductility, VT1 belongs to ductile titanium alloys, and alloy VT6 occupies an intermediate position.
Fig. 3. Graph of the dependence of the angle of final shear of the element on cutting speed, feed, and alloy grade
From the graph (Fig. 3) it is seen that, with a decrease in the ductility of the alloy, the magnitude of the angle \(\beta_1\) increases markedly. An increase in feed also leads to an increase in the angle.
An initial increase in cutting speed is accompanied by an increase in the angle of shear of the element, while a further increase in cutting speed has little effect on the change in the angle.
LITERATURE
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