WEDM as a Replacement for Grinding in Machining Ceramic Al2O3-TiC Cutting Inserts

Abstract

Small-size cutting inserts for assembly cutters are widely used to manufacture a variety of parts for the aerospace, automotive and mechanical engineering industries. Due to their high hardness and chemical stability, cutting Al2O3-TiC ceramics significantly outperform hard alloys in machining heat-resistant and difficult-to-machine materials. However, grinding on CNC machines, the most common technology for manufacturing ceramic inserts, is associated with numerous issues when it comes to manufacturing small-size cutting inserts. For example, high cutting forces and high grinding wheel wear rates cause a rapid loss of dimensional accuracy and deterioration of the quality of the surface being machined, while the interference of the grinding wheel with the surface being treated imposes serious limitations on the geometry of the small-size ceramic inserts to be grinded. Here we show that Wire Electrical Discharge Machining (WEDM), which is a contactless and, thus, a more flexible method in terms of the size and geometrical properties of a workpiece to be machined, can be used as a replacement for grinding operations in machining small ceramic inserts. A composite of 70% aluminum oxide and 30% titanium carbide was chosen as a ceramic material because a further increase in the TiC fraction causes a marked decrease in wear resistance, while its decrease results in an undesirable loss of electrical conductivity. While in order to replace grinding with WEDM, WEDM has to be stable in the sense of occurring without frequent wire breakages, achieving WEDM stability is not an easy task due to the low electrical conductivity of Al2O3-TiC ceramics and high operational temperatures, which promote the diffusion of dielectric and electrode products in the surface layer of the cutting inserts being machined. These factors may lower the quality of the final product due to damage to the insert surface, marked increases in the roughness RA and in diffusion in the surface layer, which increases the friction coefficient and, hence, reduces the life of the manufactured cutting inserts. We have increased stability of the WEDM process by identifying and applying rational process conditions that lead to a reduced, by a factor of 2.63, roughness Ra and also a reduced, by a factor of 1.3, depth of craters. Performing a chemical and structural analysis, we found that the application of high energies combined with an increasing interelectrode gap (IG) (technological parameter SSol, a complex indicator that determines the speed of the wire electrode depending on the number of pulses per unit of time and the IG size, is set at 80, EDM3 technology) causes increased surface damage and contamination, while a small IG (SSol = 45, EDM1 technology) reduces the material removal rate due to contamination of the working zone between the surface being machined and the electrodes. After reducing the IG by lowering SSol from 80 to 45, the roughness Ra of 0.344 µm was achieved, which allows for replacing grinding operations with WEDM in machining hardening chamfers, front surfaces and, to a lesser degree, the rear and support surfaces of cutting inserts. In this case, when the IG is reduced to SSol = 45, the electroerosion products in the dielectric promote local breakdowns, which in turn produce a large number of deep craters which adversely affect the performance of cutting inserts. However, we found that a slight increase in SSol from 45 to 55 (EDM3 technology) significantly reduces the number of craters and lowers their depth from 50 μm to 37 μm. Although in this case the roughness grows to 0.534 μm due to increased discharge energy, the improved flushing of the IG and the reduced occurrence of local high-temperature breakdowns—evidenced by a decrease in the depth and number of deep craters formed due to current localization during short circuits—significantly reduced contamination of the surface layer and the crater formation rate. Therefore, WEDM can be recommended for use in machining reinforcing chamfers and, to a lesser degree, front surfaces. These considerations lead us to conclude that WEDM is a viable alternative to grinding in machining Al2O3-TiC ceramic cutting inserts of a small size and a complex shape, and that its application to manufacturing cutting inserts from poorly conductive cutting ceramics should be studied further.