ABSTRACT
Titanium alloys are widely used in a wide number of applications, such as aerospace, medical, structural, and defense, thanks to the outstanding characteristics they offer in terms of high strength-to-weight ratio, mechanical and fatigue resistance, biocompatibility, and corrosion resistance. As a consequence of their elevated strength even at temperatures as high as 600°C, high temperatures are developed during machining operations, which are not dissipated fast enough due to the low thermal conductivity they show. This leads to enhanced wear of the cutting tools, which manifests itself as both diffusion and crater wear, with these wear phenomena thermally activated. At the same time, the high temperatures generated during the cutting process may produce a deterioration of the machined surface integrity, which, if it happens during finishing operations, may be detrimental for the workpiece service life. To this regard, various approaches are used to lower the impact of the generated temperatures on both the tool wear and surface integrity when machining titanium alloys, among which the use of appropriate cutting fluids in form of flood lubrication is the most widespread. However, in recent years, several studies have pointed out the environmental issues of most of the commercially available cutting fluids, making dry machining an attractive technology, even if it is not suitable to be applied to difficult-to-cut alloys, as are the titanium alloys. On the contrary, the application of cryogenic fluids is recognized to offer the advantages of dry machining, together with a rapid cooling effect that is able to reduce the tool wear on one hand and enhance the part surface integrity on the other (Jawahir et al. 2016a).
In this context, this chapter reviews the most significant researches dealing with cryogenic machining of titanium alloys, focusing on the available experimental set-ups applied to the most common machining operations, and the process outcomes in terms of tool wear, machined surface integrity, and functional performances of the machined components. The term “cryogenic” here refers to temperatures below −180°C, as pointed out by the National Institute of Standards and Technology. With liquid nitrogen (LN2) being the most commonly used in cryogenics, this review focuses on machining processes adopting this liquid gas.
