Milling of titanium alloy with cryogenic cooling and minimum quantity lubrication (MQL)

Titanium alloy is being widely used in various applications in aerospace, energy and biomedical industries mainly due to its superior material properties such as high strength even at high temperatures, lightweight and corrosion resistance. However, because of its extremely poor machinability, many enhancement techniques such as minimum quantity lubrication (MQL), cryogenic machining, laser assisted machining (LAM), etc., have been proposed to improve the machinability. This study specifically examined the machinabilities of MQL and cryogenic machining for Ti-6Al-4V and compared to those of dry and wet machining. Liquid nitrogen (LN2) was used for cryogenic machining with the specially designed cryogenic spraying systems. In addition to traditional MQL, a new MQL technique, with the lubricant mixed with a small amount (similar to 0.1%) of exfoliated graphite nano-platelets (xGnPs), was tested to make the comparison against other techniques. The results obtained showed that both cryogenic and MQL machining showed improved performance in comparison to the dry and wet machining. For cryogenic machining, however, the exposure to LN2 causes the thermal gradient on the cutting tools and the hardening of the titanium alloy during the machining, which resulted in excessive tool wear and micro-fracture and increased the cutting forces.