Ultrasonic analysis: a key driver in manufacturing design cost optimisation for high pressure turbine discs

Abstract

This thesis investigated the best strategy to calculate the unit cost of isothermally forged discs used within large civil gas turbine engines. Research has shown that ultrasonic examination constraints required during the production of forged parts strongly influence the manufacturing design. These constraints have traditionally been ignored during preliminary design analysis due to the unavailability of suitable models. Cost reduction objectives are also commonly simplified using mass as a direct surrogate. These oversights can lead to significantly inaccurate predictions of both cost and material properties.

This thesis assessed the impact of satisfying the ultrasonic constraint and quantified potential cost savings through direct unit cost optimisation. This work created a fast automated method for evaluating ultrasonic examination limitations, and created an optimisation workflow where a detailed analytical cost model can be rapidly evaluated. The inclusion of ultrasonic constraint analysis within an automated design loop increased both the precision of manufacturing designs and unit cost estimates. This improvement has led to detailed manufacturing designs being assessed within the preliminary design environment enabling true concurrent engineering. These new advancements enable a disc designer to obtain manufacturing designs alongside a detailed cost breakdown of the optimised manufacturing route. This approach is now routinely used within Rolls-Royce who have also patented the modelling technique.

Results show that including the ultrasonic constraint increased initial manufacturing geometry mass by at least 3.8% with costs increasing by up to 2.4%. Subsequent unit cost optimisation showed cost increases of between -1.6% and +4.3% with maximum heat treatment depth increasing up to 4.6% compared to the baseline non-ultrasonic constraint scenario. These results prove that ultrasonic constraints are a necessity to predict accurate manufacturing data. Introduction of a detailed analytical cost model also found that direct cost optimisation improved savings by up to 1.9% compared to traditional mass reduction objectives. This confirms the need for cost estimation to achieve effective cost reduction.

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