BUILT-UP EDGE FORMATION DURING CUTTING OF ALUMINUM ALLOYS

V. B. Livshits

Presented by Professor, Doctor A. M. Rozenberg

Aluminum alloys are finding ever wider application in industry. A large percentage of parts, in the process of manufacture, are subjected to machining by cutting. It follows from this how important it is to study the distinctive features of the cutting process for aluminum-based alloys.

When cutting aluminum alloys, the formation of a built-up edge is observed to be considerably more active than when cutting steel. A very limited number of experimental works have been devoted to this question, despite the fact that the built-up edge undoubtedly affects the finish of the machined surface, chip formation, and the dynamic characteristics of the process.

The present article sets forth the results of investigations carried out in turning three aluminum alloys, relating directly to the questions of built-up-edge formation:

a) the nature and structure of the built-up edge,

b) the influence of cutting speed, feed, the front angle of the cutter, and the properties of the machined alloy on the geometry of the built-up edge and the activity of its formation.

Research Method

To determine the geometry of the built-up edge, the method developed by A. N. Eremin was selected as the basis; it is founded on measuring the geometry of the built-up edge directly beneath the chip with an optical goniometer-indicator [1].

The structure of the built-up edge was studied from polished sections of chip roots. To obtain the roots, a “falling cutter” device was used [2]. For a more detailed study of the dynamics of built-up-edge formation, in several experiments motion-picture filming was carried out with an SKS-1 motion-picture camera at a speed of 500 frames per second.

As parameters characterizing the geometry of the built-up edge, the height \(H\) and the front angle \(\gamma_n\) were selected, measured in a plane normal to the front face and to the principal cutting edge of the cutter. The value of \(\gamma_n\) was determined along the direction of chip flow within the region of its contact with the built-up edge and the cutter. Observations were conducted

under conditions of non-free cutting with cutters made of VK-8 hard alloy (in the tests for measuring the built-up edge) and of R18 high-speed steel (when obtaining chip roots). The rake angle of the cutters had variable values: 15°, 25°, 35°. The remaining elements of the cutter geometry were kept constant.

The tests were carried out with feeds of 0.07 mm/rev and 0.3 mm/rev, at a constant cutting depth of 3 mm, over a speed range of 1–250 m/min. In addition to the geometry of the built-up edge, the temperature on the rake face of the cutter (by means of a natural thermocouple) and the shrinkage of the chip, which was calculated by the known weight method, were determined in the tests.

The characteristics of the alloys investigated are given in Table 1.

Table 1

All blanks were obtained by sand casting.

Results of the Investigation

Observations under the microscope, as well as motion-picture filming, made it possible to establish the general picture of the process of built-up-edge formation.

The built-up edge forms already at the very lowest cutting speeds, which correspond to a temperature on the rake face of the cutter of about 20°C. In the range of low cutting speeds the built-up edge, gradually increasing, reaches a height that is, for the given regime, somewhat maximal, and then is completely torn off from the rake face. As the cutting speed increases, the separations become increasingly rare. Beginning with speeds corresponding to the most intense formation of the built-up edge, a deposit forms on the rake face (Fig. 1), considerably more stable than the principal mass of the built-up edge, whose separations under these conditions occur along a certain plane $a—а$. Direct measurements during the cutting process indicate that the plane $a—а$ forms a negative rake angle. More

Fig. 1.

¹) Heat treated (quenching from 535°, aging at 180° for 6 hours). In the text it is conventionally designated Al-4T.

distinct built-up edge formation is observed in Al-4, the most ductile and least alloyed of the alloys investigated. In the Al-8 and Al-10 alloys the built-up edge is so insignificant that it became possible to detect it only by means of motion-picture filming.

Within a certain range of cutting speeds the built-up edge disappears. Analysis under a metallographic microscope of polished sections of chip roots obtained under conditions of active built-up-edge formation indicates the following.

The built-up edge has a layered structure (Fig. 2). The direction of the layers in the mass of the built-up edge is variable. At the base the layers are oriented parallel to the rake face of the tool. In the region of contact with the chip there is no clearly

Fig. 2.

defined boundary; the layers of the built-up edge pass into the textured layer of the chip and coincide with it in direction. The layers in the adjacent part of the built-up edge are arranged similarly.

Thus, the built-up edge consists of metal retarded on the rake face of the tool, the grains of which are to a considerable extent deformed and elongated in the direction of the action of frictional forces. The metal forming the built-up edge is more highly deformed than the metal forming the chip, as is indicated by the finer texture of the built-up edge. The hardness of the built-up edge is 1.2–1.3 times greater than the hardness of the chip, which indicates its greater work hardening.

The shape of the built-up edge differs somewhat among the alloys studied. The more highly alloyed the aluminum, the sharper the built-up edge; however, in any case the radius at the tip is comparable with the thickness of the layer being removed.

During cutting, a zone of deformed metal forms ahead of the tool; as the built-up edge develops between detachments, this zone increases sharply, and the action of the built-up edge resembles the penetration of a punch into metal.

Geometry of the Built-Up Edge

The change in the height \(H\) of the built-up edge as a function of cutting speed is described by a curve having a maximum in the region of low speeds.

(Figs. 3, 4). The speeds corresponding to the maximum and minimum of the curves are the lower, the smaller the tool rake angle \(\gamma_p\) and the larger the feed, and they coincide with a quite definite temperature range for each of the alloys (Table 2).

Table 2

The activity of the built-up-edge formation process increases with decreasing rake angle and increasing feed.

The rake angle \(\gamma_n\) characterizes not so much the rake angle of the built-up edge itself as the averaged geometry of the active part of the surface of its contact with the chip. As a rule, contact occurs along the rounded apex and a small portion of the rake face of the built-up edge. The curve expressing the dependence of the angle \(\gamma_n\) on the cutting speed \(V\) is similar to the curve \(H=f(V)\). The speeds, and consequently the cutting temperatures, corresponding to the maxima of these curves are practically identical.

Fig. 3.

Fig. 4.

To illustrate the influence of temperature on built-up-edge formation, the value of the angle \(\gamma_n\) is plotted as a function of the cutting temperature.

(Fig. 5). The region in which the built-up edge disappears is indicated by hatching. The experiments showed that the smaller the front rake angle of the cutter (i.e., the more intensive the formation of the built-up edge), the greater the range of variation of \(\gamma_H\) within the limits of the built-up-edge formation region, and the greater the difference \((\gamma_H-\gamma_p)\) at the maximum built-up edge. It should be noted that the value of \(\gamma_H\), at a given cutting temperature, becomes smaller than the front rake angle of the cutter. In alloy Al-4 (both hardened and not heat-treated), this phenomenon is expressed more sharply than in alloys Al-8 and Al-10, which is apparently explained by the larger rounding radius of the built-up edge in alloy Al-4 and by the formation of adhesion, mentioned above.

Fig. 5.

The work carried out permits the conclusion that the structure of the built-up edge and the process of its formation, both when cutting aluminum alloys and when cutting steel \([1, 2, 3, 4,\ \text{etc.}]\), are fundamentally similar. However, there are also certain differences. Built-up-edge formation in aluminum alloys proceeds more actively; a considerably larger part of the metal deformed during chip formation passes into the built-up edge. Thus, if in the region of the maximum built-up edge (according to the experiments of A. N. Eremin \([1]\)) when cutting steel 1010 the ratio of the maximum height of the built-up edge to the chip thickness is \(0.71\), then when cutting alloys Al-4 and Al-8 this ratio is respectively \(3.6\) and \(5.2\).

As in the cutting of steel, the temperature on the front face is a characteristic of the course of the process, but the maxima of the curves \(H=f(V)\) correspond to considerably lower temperatures. Comparison of the process of built-up-edge formation in the investigated alloys indicates the presence of a number of features.

1. The less alloyed the alloy, the lower the temperature corresponding to \(H_{\max}\), and the higher the cutting temperature at which the built-up edge degenerates (see Table 2).

2. The temperatures of the maximum built-up edge and of its disappearance for alloys Al-4 and Al-4T coincide, despite the fact that the hardness of the alloys is different. On the other hand, in alloy Al-4T, having a hardness \(H_B = 81\ \text{kg/mm}^2\), and in alloy Al-10 \((H_B = 88\ \text{kg/mm}^2)\), built-up-edge formation proceeds differently.

3. At equal cutting temperatures, the maximum height of the built-up edge for each of the alloys is the greater, the higher the specific normal pressure, which, as is known from experiments on steel \([5]\),

increases with increasing chip thickness and decreasing front rake angle of the cutter.

The results obtained make it possible to put forward certain assumptions about the factors that determine the intensity of built-up-edge formation. It is known that aluminum alloys have a high tendency to seizure. The latter, as follows from Semenov’s work [6], depends on the chemical composition of the alloy, heat treatment, and deformation temperature. Seizure proceeds the more actively, the higher the normal pressure, the smaller the amount of impurities present in the solid solution of the alloy, and the more plastic the metal is (with unchanged chemical composition). Raising the deformation temperature up to a certain limit also activates the process.

The seizure of dissimilar metals “is determined by the tendency to seizure of the metal that is more active in this respect” [6, p. 148].

A similar nature of the influence of normal pressure and deformation temperature on seizure (of aluminum and hard alloy, in particular) is confirmed by studies of built-up-edge formation during friction [7].

Built-up-edge formation during cutting takes place under different conditions than the process of built-up-edge formation during friction and seizure under compression. In addition to a different deformation scheme, a major influence is exerted by the friction conditions between the front face of the cutter and the metal forming the chip and the built-up edge, which vary depending on the cutting regime. Nevertheless, it may be assumed that, during cutting, built-up-edge formation depends on the totality of factors determining the conditions of seizure. It is precisely the significantly greater tendency of aluminum alloys to seize that explains the noted differences from built-up-edge formation when cutting steel.

Analysis of the results obtained makes it possible to draw a number of practical conclusions.

1. A decrease in the activity of built-up-edge formation is promoted by reducing the feed and increasing the front rake angle, which, however, is limited by the strength characteristics of the tool.

2. The cutting speed at which the built-up edge disappears decreases with increasing feed.

3. For a given feed and tool geometry, the speed corresponding to cutting without a built-up edge is higher, the less alloyed the alloy is.

4. When previously hardened alloys are machined by cutting, the built-up edge forms less intensively, but disappears completely at the same speeds as in unhardened alloys.

Items 3 and 4 indicate that, when assigning cutting regimes, it is necessary to take into account both the mechanical characteristics and the chemical composition of aluminum alloys.

REFERENCES

1. A. N. Ersmin. The Physical Nature of Phenomena in Steel Cutting. Mashgiz, 1951.

2. T. N. Loladze. Chip Formation in Metal Cutting. Mashgiz, 1952.

3. A. M. Rozenberg, A. N. Eremin. Elements of the Theory of the Metal-Cutting Process. Mashgiz, 1956.

4. A. I. Isaev. The Process of Surface-Layer Formation in the Machining of Metals by Cutting. Mashgiz, 1950.

5. N. N. Zorev. Problems of the Mechanics of the Metal-Cutting Process. Mashgiz, 1956.

6. A. P. Semenov. Seizure of Metals. Mashgiz, 1958.

7. V. D. Kuznetsov. Built-Up Edges in Cutting and Friction. Moscow, 1956.