Control of Residual Stress, Distortion and Mechanical Properties in Wire + Arc Additive Layer Manufactured Ti-6Al-4V Parts

Wire + arc additive manufacturing (WAAM) is a high-deposition-rate ALM process that targets the manufacture of large-scale aerospace parts. Compared to conventional subtractive machining, the main benefits of WAAM are the significant time-, material, and cost savings.

Ti-6Al-4V is the material to benefit most from the advantages of WAAM, due to the high material and process costs, therefore making WAAM especially appealing to the aerospace industry. The subsequent deposition of plasma-welded layers causes large residual stresses in the as-deposited near-net-shape state that result in significant distortion of the final part. Furthermore, the microstructure consists of large, columnar grains, which result in anisotropic and inferior mechanical properties when compared to the parent material.

Cranfield University are researching different cold-working strategies to eliminate these undesired phenomena, namely Cold-Rolling, Global Tensioning and Machine Hammer Peening. The aerospace industry has a high interest in vertical cold-rolling, since the key research findings show a significant reduction of residual stresses and distortion. Moreover, interim cold working automatically causes recrystallization of the microstructure, resulting in isotropic and superior mechanical properties. Current investigations aim to find strategies to further improve the great benefits and the ease of implementing cold-working into the production line. Recently a novel Pinch-Rolling tool was developed that was successfully used to build a zero-distortion part with WAAM for the first time.

This presentation reports the potential of cold-working strategies and the recent achievements at Cranfield University, which have significant, practical benefits and may aid industrialisation of the WAAM process for aerospace parts.