Investigation into the grinding performance and surface integrity of advanced aerospace superalloys in creep feed grinding

Abstract

Titanium alloys exhibit exceptional mechanical properties, including high rigidity, favourable strength-to-weight ratio, and excellent resistance to corrosion. Such features have resulted in their widespread application across several industries, more particularly in aerospace and automotive. The increased demand for enhanced burn resistance coupled with low weight has led to the development of more advanced titanium alloys such as gamma titanium aluminide intermetallic (γ-TiAl) and burn-resistant titanium alloy (BuRTi), which are considered potential alternative materials for nickel-based superalloys in aerospace components such as compressors and turbine blades or vanes. However, machining these materials poses significant challenges to the industry in part due to their inherent features, for instance, low thermal conductivity, strong chemical reactivity, and the ability to maintain their tensile strengths at elevated temperatures. The process of grinding plays a crucial role in the manufacturing of mechanical components with high surface quality and dimensional accuracy, and compared to other machining processes, grinding has consistently exhibited improved surface quality when utilised for cutting titanium alloys. In comparison to conventional grinding, creep feed grinding achieves more efficiently in terms of stock removal rate while maintaining good surface finish and tolerances (from ±8µm to ±80µm). The available information in the literature regarding creep feed grinding of the thermo-mechanically produced duplex latest generation gamma titanium aluminide (Ti-45Al-8Nb-0.2C), the patent burn-resistant titanium alloy (Ti-25V-15Cr-2Al-0.2C) and the alpha/beta titanium alloy (Ti-6Al-4V) is scarce and only involving limited investigated machining/grinding conditions. In addition, the existing literature on creep feed grinding of γ-TiAl (Ti–45Al–8Nb–0.2C) lacks information regarding the temperature and residual stresses that are produced during the machining operation, with extremely limited information given for burn-resistant titanium (Ti-25V-15Cr-2Al-0.2C). Moreover, the majority of published work to date has primarily focused on the evaluation of the two-dimensional average surface roughness parameter (Ra), and no details on extreme or three-dimensional surface roughness parameters were reported (Rt, Rz, Sa, St and Sz). Following an extensive literature review on titanium alloy development, grinding fundamentals, surface integrity, experimental design, and grinding of relevant titanium alloys, the thesis details the findings from three main phases of experimental work. The first focused on creep feed grinding of gamma titanium aluminide intermetallic alloy (Ti–45Al–8Nb–0.2C wt%), whilst the second phase provided an assessment of creep feed grinding for burn resistant titanium alloy (Ti-25V-15Cr-2Al-0.2C) and compared its grindability to γ-TiAl material. The last phase is pertinent to creep feed grinding of alpha/beta titanium alloy (Ti-6Al-4V) with benchmarking against the two previously investigated materials. Results provided in each phase/material covered information for both conventional and super-abrasive wheels. Full factorial experimental designs were utilised in each phase to investigate the impact of various operational parameters (wheel speed, depth of cut and feed rate) and their corresponding levels (20 and 30m/s, 0.3 and 1mm, and 150 and 300mm/min) on performance output measures, including the G-ratio, grinding forces, grinding power, specific energy, grinding temperature, and workpiece surface roughness. Furthermore, an assessment of the surface integrity of the machined samples is provided, which encompasses the evaluation of the produced workpiece surface, microstructure, microhardness and residual stresses. The experimental findings demonstrate that γ-TiAl showed more favourable outcomes when compared to BuRTi and Ti-6Al-4V. This was evident from the lower values observed including, 3 times lower wheel wear, ~40% lower grinding forces and power, reduced temperature at the majority of tests and ~50% lower surface roughness. BuRTi and Ti-6Al-4V materials exhibited a moderate degree of similarity in terms of their grinding performance at lower rates of material removal. At increased depths of cuts, the former showed reduced wheel wear (~27µm compared to ~50µm), ~30% lower grinding forces and ~30-40% lower surface roughness. However, these were accompanied by the occurrence of surface burn and crack formation. In terms of grinding temperature, surface integrity, and generated residual stresses, the diamond wheel exhibited superior performance compared to the SiC wheel across all materials that were examined. The results indicate that a significant rise in the G-ratio was only evident in γ-TiAl subjected to diamond wheel machining, while no such improvement was observed in BuRTi and Ti-6Al-4V. Tensile residual stresses were predominantly present on γ-TiAl and BuRTi surfaces ground with a SiC wheel under extreme operating conditions and accompanied with fractures, burn marks, and softening layers on machined surfaces. Compressive residual stresses, on the other hand, were restricted to surfaces free of burn and microstructural deformation, particularly in samples cut with diamond wheels. The current work provides new insights into the generated temperature, residual stresses and 3D surface roughness parameters during creep feed grinding of the thermo-mechanically produced duplex alloy γ-TiAl (Ti–45Al–8Nb–0.2C), the patent burn-resistant titanium alloy (Ti-25V-15Cr-2Al-0.2C) as well as for alpha/beta titanium alloy (Ti-6Al-4V). The findings presented in this thesis demonstrate strong correlations between the output measures pertaining to surface integrity and the generated temperature, 3D surface roughness parameters, and induced residual stress results. These aspects are missing in published literature for investigated materials. The machinability data presented here are extremely useful for future work in understanding the analysis of the performance of grinding processes for titanium alloys including modelling activities.