Grinding of Ti-6Al-4V titanium alloy: an experimental and dimensional analysis evaluation

Ashman, James (2024). Grinding of Ti-6Al-4V titanium alloy: an experimental and dimensional analysis evaluation. University of Birmingham. Ph.D.

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Abstract

The work presented in this thesis deals with the grinding characteristics of aerospace alloy Ti-6Al-4V (Ti64). Firstly, a new and novel approach for evaluating the peak workpiece temperature was conducted and, in addition, the associated cutting forces acting on the grinding wheel were also analysed, alongside the G-ratio and subsequent surface integrity of the ground workpiece, i.e., surface roughness, microstructure analysis, microhardness interrogation and induced residual stresses. Secondly, a further dimensional analysis (DA) study of the test programme was then performed to determine if a theoretical approach could be developed for predicting the experimental test programme findings. In summary, the present research was structured into two phases of work.

Phase 1 involved an experimental test programme to investigate the effect of changes in wheel speed, V\(_s\), feed rate, V\(_w\), and depth of cut, a, values on workpiece temperature, over a range of finishing and larger stock removal conditions when using a SiC grinding wheel. The workpiece temperatures were evaluated through a series of three thermocouples implanted in the workpiece near the entry, middle and exit of the cut, whilst the grinding forces were measured using a piezoelectric dynamometer. A key observation was that significant spikes in workpiece temperature were recorded by the thermocouple positioned near the end of the cut when compared to the first two thermocouples in the series; e.g., a maximum workpiece temperature differential, ΔT, of 72.9%. Moreover, a maximum workpiece temperature of approximately 800°C was measured when using the most aggressive parameters, whilst a minimum workpiece temperature of approximately 75°C was observed when grinding using less aggressive parameters. Film boiling of the coolant was proposed as a possible explanation, in addition to the build-up of latent heat within the system. Similarly, higher cutting forces were observed when using more aggressive grinding parameters, with peak cutting forces reaching 1500N, whilst lower grinding parameters observed force traces as low as 500N, i.e., ΔF of 66.7%. Additional post-grinding surface integrity assessment consisting of surface roughness, optical microscopy of the subsurface, microhardness and residual stresses were evaluated regarding the Ti64 workpieces after the experimental test programme had been conducted. Significantly higher G-ratios were found when using less aggressive grinding parameters, whilst microhardness values were found to be higher further along the grinding pass by 4.1%, indicating that temperatures are higher again due to the lack of coolant fluid within the grinding zone at this point in the pass. Higher wheel speeds were also found to reduce the peak workpiece temperature, and so this should be considered when grinding components that require high precision finishing. The average surface roughness, R\(_a\), observed a maximum 50% reduction when increasing wheel speeds from 20m.s\(^{-1}\) to 30m.s\(^{-1}\) whilst keeping the feed rate and the depth of cut constant.

Phase 2 established DA models using the Buckingham-π method. Six Buckingham-π groups were derived using base unit quantities associated with the workpiece and the tooling material properties, in addition to the machining parameters tested. The Buckingham-π groups were then analysed and compared against the experimental results from Phase 1. Results indicated an excellent correlation for predicting both workpiece temperature and cutting forces when comparing depth of cut, feed rate and wheel speed, with the variance between data sets being less than 5%.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):