High performance cutting of gamma titanium aluminides: Influence of lubricoolant strategy on tool wear and surface integrity

摘要

Heat resistant gamma titanium aluminides are intermetallic alloys planned to be widely used in high-performance aircraft engines within the next few years. This application field is ascribed to the exceptional material properties, especially the low density and a unique strength-to-weight ratio for titanium-based alloys, good oxidation behaviour and thermal stability, limited ductility and fracture toughness below brittle-to-ductile transition, and good creep resistance. The demanding machinability of gamma titanium aluminides can be traced back to these desirable material properties. Consequently, cutting process adaptation is essential to obtain components suitable to satisfy strong regulations regarding surface integrity, without neglecting an economical production. Previous research activities confirmed that thermal material softening during cutting due to the high speed machining is a key to reach high quality surfaces, but tool wear was identified as the limiting factor. The relatively high cutting speed results in high temperatures in the shear zone and the low thermal conductivity of the γ-TiAl workpiece material leads to an extreme thermal tool load. Furthermore, in combination with the formation of saw-tooth chips and the discontinuous flow of the chip along the rake face, adhesive wear is caused. The influence of conventional flood cooling and high pressure lubricoolant supply (wet conditions), cryogenic cooling with liquid nitrogen, and minimum quantity lubrication (MQL) were investigated in longitudinal external turning operations. Tool wear, cutting forces, chip morphology and surface roughness were evaluated. Surface integrity was analysed in terms of machined surface defects and sub-surface alterations. The investigations indicate that cryogenic cooling is the most promising lubrication strategy, meaning that the thermodynamical impact of the expanding liquid nitrogen applied directly close to the cutting zone successfully counteract the huge thermal load on the tool cutting edges, providing potentially enormous benefits in terms of tool wear reduction and consequent surface quality improvement.