Investigation into the grinding of titanium alloys

Titanium alloys are used extensively in the aerospace industry due to their high specific strength and excellent corrosion resistance. However, their poor thermal conductivity and high chemical reactivity with tool materials make the machining difficult, especially when grinding. During grinding, the excessive heat generated at the wheel-work piece interface may result in poor surface finisil,1_, a transformed surface layer, excessive plastic deformation, thermallyinduced residual tensile stress, burn and micro-cracking on the ground surface. The poor surface integrity and metallurgical changes of the surface and sub-surface may impair the surface sensitive properties such as fatigue life during service. ln order to overcome the thermal problems when grinding titanium alloys, a new cooling strategy, cryogenic grinding, was studied which involves the supply of liquid nitrogen into the grinding zone using a nozzle jet system. lt was found that cryo-cooling with conventional grinding wheel decreased surface roughness values, burn and plastic deformation of Ti-6Al-4V alloys and produced better a cutting mechanism than when using water-based coolant, especially at higher depths of cut. However, the thermal problems still introduced high residual tensile stress which degraded the fatigue life of ground specimens. Another approach to reducing the grinding temperature was to use a superabrasive wheel (diamond), because of its superior thermal conductivity, the integrity of the ground surface was improved and the fatigue life properties of the specimen were maintained at higher values than for conventional grinding wheels. An ultra stiff machine tool, Tetraform C, was also used to grind Ti-6Al-4V alloys and under selected conditions the ground surface reached a good surface finish and the fatigue properties were also retained at lower depth of cut. An ELID system on the Tetraform C was also investigated. It produced a rougher surface finish in this study. However, it is believed that reducing the wheel loading problem when grinding titanium alloys may contribute to maintaining the fatigue properties.