Pulsed gas metal arc additive manufacturing of low-carbon steel: Microstructure observations and mechanical properties

Abstract

Gas metal arc additive manufacturing (GMA-AM), also known as wire arc additive manufacturing (WAAM), uses an electric arc to melt a wire electrode and deposit objects layer by layer. This study focuses on creating single-pass wall structures using a low-carbon steel wire (ER70S-6) and examining the relationship between pulse frequency and weld geometry, microstructure, and mechanical properties. Microscopic observations showed a typical columnar microstructure with three distinct regions: acicular ferrite, bainite, and allotriomorphic ferrite in the first and last layers, while the mid-region exhibited homogenous polygonal ferrite grains with some pearlite at the grain boundaries. The tensile test results demonstrated a dependency of strength on the applied pulse frequency, with the highest strength (i.e., the ultimate tensile strength of 522 MPa and yield strength of 375 MPa with ductility of ∼52%) achieved in parts processed at a frequency of 100 Hz. Vickers microhardness values revealed uniform hardness in the middle region, consistent with the microstructure observation. Analyzing thermal cycles, coupled with microstructure analysis and continuous cooling transition diagrams, provided insight into how phase and microstructure evolution occurred in low-carbon low-alloy steels processed through PGMA-AM.