Machinability of aluminum-(7-11%) silicon casting alloys: Role of free-cutting elements.

摘要

Two of the most widely used aluminum alloys in the aerospace and automotive industries are the 396 and 319 alloys; this may be attributed to ease of casting and their satisfactory ability to meet the requirements imposed by mechanical properties given the fact that the raw material for processing comes mostly from recycled material. These alloys, 396 and 319, belonging to the Al-Si-Cu system are usually heat treated in order to obtain an optimum combination of strength and ductility. The excellent castability and mechanical properties of such Al-Si-Cu-Mg alloys has made them commercially popular for industrial applications. Machining is a common procedure used for the removal of material from a workpiece or casting in the form of chips, and it is also one of the most important of the manufacturing processes. Reducing the machining time and extending cutting tool life both have great economic significance. Actual machining tests are indispensable for determining the machinability characteristics of the workpiece material and, as a result, machinability testing has become an essential activity.;Drilling experiments were performed on a Huron K2X 8 Five vertical machining center at fixed machining conditions which included cutting speed, feed rate, length of cut, tool geometry, tool material, and coolant to investigate the effects of drilling on the machinability of Al-Si casting alloys, namely G2: 396 + 0.15%Sn, G3: 396 + 0.25%Fe + 0.25%Mn, and G12: B319.2 + 0.15%Sn alloys, in the heat-treated condition. The four drills employed were Solid Carbide, Special Solid Carbide, Cobalt Grade and High Precision Solid Carbide drills. It should be mentioned here that the pertinent machinability criteria relate to forces and moments as well as to tool life, chip configuration, and built-up edge (BUE) evolution.;The results obtained from the drilling tests reveal that, for the alloys G2 and G3, the lowest total average drilling force and moment are obtained with the high precision solid carbide drill; whereas for G12 alloy, the cobalt grade drill provides the lowest total average drilling force and moment. The high precision solid carbide drill displays stable behavior when in operation, and is recommended for alloys G2 and G3. Likewise, the cobalt grade drill is recommended for the G12 alloy.;The results also reveal that the G2 and G3 alloys display a rapid increase in the total drilling force and moment with the increase in the number of holes drilled. This can be explained by their higher Si content of 10.8%. The differences in machining behavior of the 396 and B3l9.2 alloys may be attributed mainly to the difference in matrix hardness and alloy chemistry as obtained through additions, and the difference in the silicon contents 10.8%Si in G2 and G3 alloys versus 7.5%Si in G12 alloy.;The present study was undertaken to investigate the effects of drilling tool type and material on the machinability of heat-treated 396 and B319.2 Al-Si casting alloys, containing 10.8%Si and 7.5%Si, respectively, using four different drills. 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. Drilling machining operations were designed to be carried out under fixed conditions in order to examine the following: (i) the effects of Fe-intermetallics namely alpha-Fe, beta-Fe, and sludge; as well as those of free-cutting elements, such as Sn, on the machinability of the selected alloys; (ii) the drilling force, moment, and heat build-up, as well as chip characteristics; (iii) the effects of tool material on tool life and on tool wear behavior; and (iv) an evaluation as to which of the four drills provides a better performance with respect to drilling forces and moments, so as to obtain optimum machining combinations.;The addition of 0.15% Sn to the 396 and B3l9.2 alloys has a beneficial effect on the tool life of the four carbide drills, this may be attributed to the precipitation of fJ-Sn particles which have a low melting point. The presence of sludge in the G3 alloy resulting from the additions of 0.25%Fe and 0.25%Mn to 396 alloy, leads to an extremely rapid increase in the total drilling force and moment, and also has an unfavorable effect on tool life in that the drill life decreases with the progress of the drilled holes of the test and presents more fluctuations in all the results obtained.;An examination of the photographs of the edge build-up (BUE) on the tools indicates that there are minimal changes in the width of the BUE with the number of holes drilled in the course of the drilling process for each alloy and drill. This may be explained by the fact that as the BUE gradually increases in size and exceeds a critical size, it will separate from the cutting face and adhere to the lower surface of the chip, and thus be is removed along with the chip. A visual examination of the chips reveals that the fan shape is by far the predominant form during the drilling of the alloys studied, also, that the fan shape, due to its compact size and shape, is the ideal chip for most drilling applications. The chip breakability of the cobalt grade drill was found to be superior to that of the special solid carbide drill and the high precision solid carbide drill for the alloys G2, G3, and G12.