Microstructural refinement and strengthening of wire arc additively manufactured construction grade low carbon steel through heat treatment

Abstract

The construction industry has always focused on cost-effective manufacturing techniques for fabricating large metal structures. Wire arc additive manufacturing (WAAM) is a promising technology for fabricating large-metal components of moderate complexity at a faster rate than other metal additive manufacturing methods. WAAM-fabricated structures exhibit microstructural heterogeneity and anisotropic mechanical characteristics. Post-deposition heat treatment (PDHT) plays an important role in strengthening fabricated structures by improving microstructural uniformity and reducing inherent anisotropy. This study aims to investigate the effect of PDHT on the mechanical and microstructural characteristics of WAAM-fabricated low carbon steel (LCS) ER70S-6. Optical macrograph results showed that the as-deposited (LCS-AD) and heat-treated (LCS-HT) samples mainly consisted of polygonal ferrite and pearlite phases. However, a microscopic study of both samples reveals the presence of finely polygonal ferrite and laminar pearlite along with traces of martensite and precipitated carbides in LCS-HT. Whereas, LCS-AD exhibits the presence of only polygonal ferrite and laminar pearlite. The average grain size diameter of LCS-HT was found to be less than that of the as-deposited sample. The increment in the distribution of high-angle grain boundaries in LCS-HT confirms grain boundary migration and recrystallization. Mechanical test results show that PDHT improves average microhardness and mechanical strength by reducing the anisotropic nature and interlayer defects of the sample. The average microhardness for LCS-HT was increased by 16.43 HV. Similarly, the yield stress, ultimate tensile stress, and elongation percentage for LCS-HT compared to LCS-AD enhanced on average by 6.39 %, 15.61 %, and 32.72 %, respectively. Fractography analysis shows that PDHT facilitated more uniform tensile fracture for LCS-HT vertical samples. While LCS-AD vertical samples exhibit ductile-brittle fractures due to their interlayer defects.