Machining of aluminium based Metal Matrix Composite (MMC)
The machining of aluminium 2618 particulate reinforced Metal Matrix Composite (MMC) with 18 vol. % silicon carbide (SiC) using cemented carbide cutting tools has been undertaken. Two grades of cemented carbide inserts, uncoated K68 grade and coated KC910 grade (coated with TiC and A1203) having negative and positive rake angles (with and without chip breaker) have been used to machine this material in order to understand the machining process, tool failure modes and wear mechanisms. Turning tests in the speed range 15 - 10 m/min have been carried out at 0.2,0.4 and 0.6 mm/rev feed rates and 2 mm and 4 mm depths of cut. Both cemented carbide tools have been shown to be capable of machining the MMC and give reasonable tool lives. Low speed and high feed rate are found to be a good combination in order to machine this material effectively. Coated KC910 grade inserts with negative rake angle gave the best performance. The use of a chip breaker has no significant effect on the machining process of the NMC because the material is one which inherently short chips due to ductility limitations caused by the particles. Tool failure mode studies showed that the tools failed by flank wear. Tool wear mechanism analysis indicated that abrasion wear was the tool life controlling factor under all cutting conditions. The tool wear is related to the direct contact between the abrasive hard SiC particles and the cutting edge and their relative motion to the rake and clearance face. Hence, the hardness of the SiC particles is a dominant factor for the tool wear. Two separatem odels of abrasio. n haye.b een suggested.B uilt-up edge (BUE) which has a distinct shape was more pro i1ounced at lower cutting speeds, high feed rates and greater depth of cut. The presence of BUE has been found to increase tool life and reduce tool wear but at the expense of surface finish. The increase in tool life or reduction in tool wear is likely due to the protective layer that the BUE formed on the tool surface preventing a direct contact between the tool and chip. Linear regression analysis showed that the value of Taylor exponent n is high (0.8-1.0) compared to the values of n (0.2-0.3) obtained when machining steel. This indicates that the tool life is less sensitive to cutting speed for MMC than it is for steel.