Hypereutectic aluminum-silicon (Al-Si) alloys are low in specific gravity and coefficient of thermal expansion and have excellent wear resistance; hence they are used extensively in the manufacture of heat-resistant and wear-resistant parts, es- pecially of parts where the alloy is a substitute for cast iron when the engineering advantage of light weight and wear resis- tance in service are considerations.
Machining is generally needed in producing structural parts. Hypereutectic Al-Si alloys are said to be the most diffi- cult to machine among the various aluminum alloys. Tools wear very rapidly (Ref 1). For this reason attempts have been made to optimize selection of cemented carbide tools, cutting conditions (Ref 2, 3), and tool geometry (Ref 3) and to study the effect of the flank buildup (FBU) on the tool wear (Ref 4), the improvement of machinability by adding special elements (Ref 5, 6) and heating workpieces (Ref 7), and the effect of cutting fluids (Ref 8) on the machinability. However, a significant im- provement in machinability has not been achieved. This is be- cause the primary-phase silicon grains are much harder than any other phases in the microstructure and exert an abrasive in- fluence on the tool. Although the refining of primary silicon is effective in improving the mechanical properties and ma- chinability (Ref 9), it is difficult to obtain the optimum micro-structure when the alloy is conventionally processed by ingot metallurgy (I/M) since the growth of proeutectic silicon cannot be prevented.
Conversely, powder metallurgy (P/M) technology using rapid solidification provides aluminum alloys with finely dis- persed silicon grains. In addition, iron (Fe), manganese (Mn), and nickel (Ni) and their compounds can be successfully added in fine dispersive phases to produce novel aluminum alloys with excellent high-temperature properties and wear resistance (Ref 10). Few reports exist on the machinability of P/M proc- essed Al-Si alloys in comparison with I/M processed alloys.
This report concerns the experimental analysis of machinability of P/M and I/M hypereutectic Al-Si alloys in terms of tool wear, surface roughness of machined surface, cutting force, and chip form.
Work materials of P/M and I/M hypereutectic Al-Si alloys, which are nearly equivalent to alloy AC9A (JIS H 5202), were prepared. The chemical composition and the hardness of the work materials are shown in Table 1. The I/M alloys were cast in sand molds without a grain refiner. The P/M alloys (Ref 10) were formed by compressing the atomized and classified hy- pereutectic Al-Si powder by cold isostatic pressing to produce the green compacts. The compacts were placed in 250 mm di- ameter aluminum cans and degassed. The degassed alloys in aluminum cans were hot pressed and hot extruded to a diameter of 90 mm, and the work material was categorized as a wrought alloy. The I/M workpiece had a flange form, which had a diameter of 110 mm and an
effective length of 80 mm.
The tool was a throwaway type (TPGN160308). The turning test on a lathe was done without a cutting fluid .The feed rate ranged from 0.05 to 0.2 mm/rev, and the depth of cut was in a range of 0.5 to 2.5 mm. The maximum width of flank wear land was determined by the use of a measuring microscope as a measure of the machinability.The cutting force and thrust force were measured, and sur- face roughness of Rz (JIS B 0601-1994) was measured by using a stylus instrument. The measuring length was 2.5 mm.
Experimental Results - Tool Wear
Figure 1 shows the effect of depth of cut and feed rate on the maximum flank-wear width of the carbide tool when hypereu- tectic Al-20%Si-2%Cu-1%Mg alloys were turned. When the cast alloy was machined, the flank-wear width increased with increases in the depth of cut and the feed rate. The effect of the depth of cut was greater than that of feed rate. Their effects were much smaller in turning the P/M alloy than the I/M alloy.
Figure 2 illustrates the progress of the flank-wear width. It shows that the wear for the I/M alloys increases gradually until the maximum flank-wear width, VBB max,becomes about 0.2 mm, but with increased cutting times, VBBmax grows rap-idly beyond that value. The wear rate for P/M alloys is much smaller than that for I/M alloys. The tool-life curves for both al- loys were plotted on a log-log graph paper as in Fig. 3. The cut- ting speed versus tool-life diagram produced the following equations. For I/M alloys:
The slopes of the tool-life curves for the alloys are not much different from each other, but the tool life for the P/M alloy is much longer than that for the I/M alloy (e.g. the tool life for the P/M alloy was about 200 times that for the I/M alloy for a cut- ting speed of 600 m/min).
The addition of copper (Cu) and magnesium (Mg) to hypereutectic Al-Si alloys deteriorates the wear resistance of carbide tools in dry cutting, but the tool wear for the P/M alloy.
REFERENCE: JOURNALS OF MATERIALS ENGINEERING AND PERFORMANCE
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