On the Post-printing Heat Treatment of Wire Arc Additively Manufactured Ferrous Alloys

Abstract

Over the past decade, wire arc additive manufacturing has become a promising alternative to conventional manufacturing methods due to its great potential for fabrication of medium to large size components with high deposition rate, low raw material consumption, and flexibility in design. Despite the advantageous features of additive manufacturing technology, its complex thermal cycles result in microstructural heterogeneities and uncertainties in mechanical properties and corrosion performance of the additively manufactured components as compared to conventionally fabricated counterparts. Therefore, with the purpose of microstructural modifications and in-service performance improvements, this thesis aims to investigate the beneficiary effects of post-printing heat treatment on wire arc additively manufactured ferrous alloys, i.e., low-carbon low-alloy steel (ER70S), martensitic stainless steel (ER420) and a precipitation hardening martensitic stainless steel (PH 13-8Mo). The results of microstructural characterizations and mechanical properties evaluations showed that appropriate post-printing austenitizing heat treatments could eliminate the microstructural heterogeneities and minimize the anisotropic mechanical properties of the as-printed thin-wall component of low-carbon low-alloy steel (ER70S), which was characterized by periodic microstructural variations along the building direction with a much lower ductility in vertical direction (~ 12 % el.) as compared to the horizontal direction with around 35 % of elongation. In addition, post-printing austenitizing treatment at 1150 °C on the additively manufactured martensitic stainless steel (ER420) resulted in the removal of undesired δ-ferrite phase, which was formed in the as-printed material due to rapid cooling associated with manufacturing process and high chromium content (13 wt. %) of the feedstock material. Further tempering process led to the formation of chromium carbides with various sizes and distributions over different tempering temperatures, promoting secondary hardening during tempering with the maximum microhardness value of 550 ± 7 H V for the sample tempered at 400 °C. However, the microhardness reduced to the minimum value of 300 ± 1 H V at higher tempering temperatures ascribed to intergranular segregation and coarsening of carbides, leading to excessive softening of the martensitic matrix. Investigations on the effects of post-printing heat treatment on the corrosion properties of additively manufactured PH 13-8Mo stainless steel revealed that solution treatment could significantly improve the corrosion resistance of the material due to the removal of Cr-enriched δ-ferrite phases from the as-printed microstructure, which trigger the sensitization phenomena in Cr-depleted region at the boundaries of δ-ferrite and matrix. However, aging at a high temperature (600 °C) resulted in the formation of Cr-enriched M23C6 carbides that created micro-galvanic coupling sites throughout the microstructure, adversely affecting the corrosion performance of the alloy. Overall, it was concluded that microstructural features, mechanical properties, and corrosion performance of the additively manufactured ferrous alloys can be tailored based on the required in-service condition by implementation of proper post-printing heat treatment cycles.