Microstructural and mechanical analysis of directed energy-deposited mild steel

This study introduces a novel submerged arc additive manufacturing (SAAM) method that uses high-efficiency submerged arc plasma to produce large-scale low-carbon steel components with isotropic microstructures. Using submerged arc welding (SAW) with copper-coated electrode wire and AUTOMELT A55 flux, low-carbon steel parts were produced with high surface quality and dimensional accuracy. The parameters analyzed include the submerged arc welding technique, contact-tip-to-work distance, current and wire feed speed (WFS), welding travel speed, and heat input. The welded part underwent multiple reheating cycles due to the layer-by-layer manufacturing process. As a result, high temperature gradients at the beginning of the weld led to the formation of a fine grain structure. In contrast, as the part's height increased and the temperature gradient decreased, coarser grains were formed. By leveraging an in situ intrinsic heat treatment process (multi-layer-penetration normalizing, full-layer-penetration inter-critical annealing, and tempering), the method achieves a columnar-to-equiaxed grain transition, refining and homogenizing the microstructure layer by layer. Similar trends were observed in the mechanical properties, with an average hardness ranging from 125 to 201 HV1. High yield strength values between 436 and 454 MPa and ultimate tensile strength values between 324 and 340 MPa were achieved. No significant differences were found in the mechanical properties based on specimen orientation, suggesting the isotropy of the material properties. This approach addresses the common issue of mechanical anisotropy in both traditional and additive manufacturing, offering a scalable solution for producing large, high-performance components. The findings of this study provide valuable insights into the effects of thermal cycles on the microstructure and mechanical properties of low-carbon steel produced via wire arc additive manufacturing. This knowledge can guide the optimization of process parameters for specific applications in industries such as aerospace, automotive, and power generation.