Subsurface deformation micromechanisms induced during machining of titanium alloys at low temperatures, and a novel testing methodology to examine their machining behaviour

The aerospace industry’s drive towards higher productivity has led manufacturers to strive for higher surface speeds in metal cutting. Machining of titanium alloys leads to high temperatures attributed to their low thermal properties, resulting in high tool wear rates. To counter this, large amounts of coolants are used. These contain toxic chemicals, which are harmful to both people and the environment. To reduce these hazards, near-dry strategies such as cryogenic cooling and minimum quantity lubrication (MQL) are investigated in this thesis. A fundamental knowledge gap in the literature was identified, which is the characterisation of the subsurface microstructural evolution during plastic deformation in the machining of titanium alloys. Besides, its impact on surface integrity needs to be investigated in detail. The aims of this PhD research were (1) To determine the “machinability” of titanium alloys by designing a novel and straightforward cutting test. (2) To determine the effect of low temperatures (LTs) on the underlying deformation mechanisms during plastic deformation in the machining of aero-structural Ti-6Al-4V. In particular, during the application of a cryogen media such as LN2 and CO2. (3) To build a constitutive model to predict the experimental flow behaviour. (4) To analyse the imparted subsurface deformation and relate to its subsurface integrity. A material's inherent mechanical, physical and thermal properties strongly influence its machining behaviour. In the uniaxial compression test, it was determined that β annealed Ti-6Al-4V ELI: undergoes shear localisation even at quasi-static strain rates, has a high sensitivity to temperature and a lower sensitivity to strain rate. The higher the temperature, the higher the strain rate sensitivity. Plastic deformation at LTs exhibited higher flow stresses vs ambient temperature. The true strain at the onset of thermal instability (softening) and at fracture was identified, it was c30% smaller at LTs vs room temperature, leading to a reduction in c20% energy for cutting at LTs. The strain-hardening rate during plastic deformation decreases linearly with further imparted strain and decreases faster at LTs. Machining generates a graded subsurface microstructure. Four different regions were identified in this investigation. (1) Severe plastic deformation region (SPD), where a nanocrystalline grain structure was observed through electron microscopy from cryogenic machining under CO2, resulting in a significant increase in strength. (2) Gross plastic deformation region. (3) Twinned region. (4) Undeformed bulk. In conclusion, machining of titanium alloys at cryogenic temperatures is easier as a lower strain is required for shearing, leading to lower energy spent for chip generation. Nevertheless, a larger microstructural damage depth is introduced into the subsurface, leading to more potential sites for crack nucleation. The main challenges that lie ahead are: (1) to determine the extent of the effect the microstructural damage has on the fatigue life during dynamic loading, and (2) to determine whether easy diffusion is allowed to occur under thermal exposure, which would negatively affect their mechanical properties.