Machinability aspects of heat-treated Al-(6-11)% Si cast alloys: Role of intermetallics and free-cutting elements

This study was conducted with the intention of investigating a new experimental alloy belonging to the Al-Si near-eutectic cast alloy group, containing about 10.8%Si, namely the 396 alloy. In the light of the above, the main purpose of the work is to report on the changes in the machinability criteria resulting from the effects of Fe-intermetallics, namely alpha-Fe, beta-Fe, and sludge; the effects of two levels of Cu, namely 2.25% and 3.5%; and the effects of two levels of Mg, namely 0.3 and 0.6%. In addition to the preceding, the effects of Mg-free alloys and Sr-modification on these same alloys were also investigated together with the effects of free-cutting elements, specifically Sn, Bi, and Pb. Thus, a specific T6 heat treatment was selected to establish the hardness level for the alloys investigated within the range of 110+/-10 BHN, conforming to most of the required hardness levels in the commercial application of aluminum alloys.;The increase in the levels of Cu and/or Mg in the 396-T6 alloy has a detrimental effect on drill life. Such an effect may be attributed to the formation of large amounts of the coarse blocklike Al2Cu phase, together with the formation of thick plates of the Al-Si-Cu-Mg phase. The Mg-free experimental alloy displays the lowest cutting force and moment in addition to producing the highest number of holes in the alloys studied. This observation may be explained by the cooperative precipitation of the Al 2Cu, Mg2Si, Al2CuMg, and Al5Si 6Cu2Mg8 hardening phases in Mg-containing alloys which confer greater strength on the alloy than would be the case with the precipitation of only the Al2Cu phase in the Mg-free alloy. A comparison of the non-modified alloy and the Sr-modified alloy (containing the same level of Mg and Cu additions) in terms of the number of holes drilled, reveals that the morphology of Si particles has a noticeable effect in governing the tool life of near-eutectic Al-Si alloys.;The addition of small but effective amounts of free-machining elements to Al-Si casting alloys significantly improves the machinability of these alloys. The Sn-containing alloy has a beneficial effect on tool life for both carbide drills and HSS taps. On the other hand, the Bi-containing alloys lead to a noticeable coarsening of the eutectic Si particles resulting in a deterioration of tool life. The simultaneous addition of smaller amounts of two or more elements insoluble in aluminum has a greater effect on machinability in terms of reducing the drilling force and moment than individual additions of each element. The addition of Pb, Bi, and Sn appears to have no significant effect on the formation of built-up edge (BUE) or on chip configuration except that the Bi-containing alloy shows a slightly lower tendency to BUE formation, and it also produces finer fan-shaped chips than those observed in the Bi-free alloy.;A visual examination of the chips reveals that the fan shape is by far the predominant form during the drilling of the alloys studied, and also that it is considered to be the ideal chip type for most drilling applications. The chip breakability of alloys containing the Al2Cu phase was superior to that of alloys containing Mg2Si. Thus, combined additions of Cu and Mg are expected to further refine the size of the chips produced. (Abstract shortened by UMI.);The results demonstrate that the presence of sludge in the form of hard spots has a significant effect on cutting forces and tool life, in that it decreases drill life by 50% compared to the base alloy. The formation of the alpha-Fe phase in the M1 base alloy has a beneficial effect on tool life in that this alloy produces the highest number of holes drilled compared to alloys containing sludge or beta-Fe; this result may be explained by the fact that the formation of the alpha-Fe intermetallic with its rounded Chinese script morphology and its presence within alpha-Al dendrites is expected to improve matrix homogeneity via hardening of the soft alpha-Al dendrites. Increasing the Fe-content from 0.5% to 1% in the 396-T6 alloy containing 0.5%Mn produces a distinct improvement in alloy machinability in terms of cutting force and tool life. In tapping tests, it was found that high-speed steel (HSS) tools are considerably more sensitive to the Fe-intermetallic phases than the carbide tools used for drilling. The addition of Fe and/or Mn appears to have no discernible effect on the built-up edge area (BUE) and chip configuration compared to the base alloy.