Workpiece surface integrity of Ti-6-4 heat-resistant alloy when employing different polishing methods

The use of heat-resistant titanium alloys for the manufacture of gas turbine engines components for aerospace/energy applications has become a routine exercise. However, components with complex designs specifications might pose manufacturing challenges especially when finishing processes are needed to enable their compliance with tight industrial standards for workpiece surface integrity. Information on polishing processes for such sensitive industrial applications is scarce. The paper reports on the influence of polishing methods/strategies on the quality and integrity of workpiece surfaces obtained after different polishing methods on Ti-6-4 heat-resistant alloy. The research focuses on identifying an "optimised" polishing strategy that will enable finishing a family of targeted safety critical aero-engine (TSCA-E) components, on which the simultaneous fulfillment of the following technological/quality criteria is required: (i) tool life to enable polishing of minimum of workpiece surface areas that are related with specific features of TSCA-E components; (ii) removal of pre-machining (i.e. milling) marks while obtaining required surface finish; (iii) generation of damage-free polished surfaces, i.e. high workpiece surface integrity. Two (belt; bob) polishing methods with various media/grades (Al₂O₃, SiC, polycrystalline diamond) of the abrasive materials in conjunction with three cutting media (dry; chilled air; minimum quantity of lubricant) have been tested to address the overall finishing of TSCA-E components. Although significant differences in tool life performance exist between belt and bob polishing methods, both are capable to meet the requirements of minimum workpiece surface coverage if "optimised" operating parameters are employed. When considering surface roughness criteria, Al₂O₃ belts and SiC bob tools were found appropriate. Furthermore, surfaces obtained with these tools when employing cooling media (chilled air for belt polishing and minimum quantity of lubricant (MQL) for bob polishing) showed compliance with the tight requirements of industrial standards for workpiece surface integrity (metallurgical damage and residual stresses). This proved that belt and bob polishing methods can be employed in conjunction as "hybrid" technique to enable automated overall finishing of complex geometrical components.