Exploring the Impact of Surface Quality on the Bending Fatigue Strength of Wire Arc Additive Manufactured Carbon Steel

This study delves into the intricate relationship between surface quality, microstructural characteristics, mechanical properties, and fatigue strength of wire arc additive manufactured (WAAM) carbon steel. Firstly, the WAAM-printed components demonstrated remarkable consistency in wall thickness, with a minor variation ranging from 3.6 mm to 4.2 mm and an average thickness of approximately 3.95 mm. Crucially, no discernible pores or significant printing defects were observed, affirming the robustness and effectiveness of the printing process for high-quality outcomes. Microstructure analysis revealed the presence of equiaxed ferrite grains predominantly composed of iron, supplemented with minor carbon and alloying elements. This unique microstructural arrangement endowed the material with excellent ductility and formability, making it suitable for various fabrication processes. Hardness assessments displayed uniformity, with an average hardness of approximately 158 HV observed across the entire WAAM-processed component, emphasizing material property consistency. Surface roughness assessments, encompassing Ra and Rz values, exposed significant variations among different surface treatments. The polished sample boasted superior surface quality with an Ra value of 0.32 um, while the machined sample exhibited slightly higher roughness at 1.71 um, Conversely, the as-built surface displayed pronounced roughness, registering an Ra value of 20.67 um, These findings suggested potential implications for fatigue strength, with the as-built sample anticipated to exhibit compromised fatigue resistance. Tensile tests unveiled impressive mechanical properties, with a maximum tensile strength of 499 MPa, a yield strength of 379 MPa, and a notable elongation of 39%. However, these strengths were relatively lower than those of the welding wire, attributed to variations in heat input between the WAAM process and traditional welding techniques. Bending fatigue tests examined the fatigue limits of the samples, with the machined sample demonstrating a fatigue limit of 250 MPa, and surprisingly, the polished sample exhibiting a slightly lower limit at 200 MPa. Further investigation revealed that the polished sample harbored residual tensile stresses induced by the electropolishing process, negatively affecting its fatigue strength. In contrast, the as-built sample displayed the lowest fatigue strength at approximately 130 MPa, attributed to its inferior surface quality and heightened susceptibility to crack initiation.