Effects of arc waveforms on morphology, microstructure, and properties of unsupported bars in wire and arc additive manufacturing

Large-scale engineering structures are placing greater emphasis on lightweight, high-performance, and multifunctional capabilities. Spatial structures based on bar elements, such as lattice, reinforced concrete, and steel-concrete-steel structures, demonstrate significant application potential. Wire and arc additive manufacturing (WAAM) is suitable for the low-cost and high-efficiency fabrication of such large-scale spatial structures. However, insufficient manufacturing precision and mechanical properties of unsupported bars in WAAM are the primary challenges limiting the application and promotion of this technology. In this study, unsupported bars at 90°, 45°, and 0° were manufactured using various arc waveforms. The effects of arc waveforms on the morphology, microstructure, and mechanical properties of the unsupported bars were systematically analyzed. Additionally, an in-depth investigation into the impact mechanism of arc waveform was conducted through the combination of simulation and theoretical analysis. The results show that the arc waveform regulates the evolution of the molten pool's temperature field, thereby influencing the dynamic flow characteristics of the molten pool and the grain growth behavior. As the fabrication angle decreases, increasing the arc pulse frequency to enhance the molten pool's fluidity becomes crucial for improving the forming quality of unsupported bars. For the 0° unsupported bars, the cross-sectional area fluctuation range and center displacement decreased by 51.0 % and 36.4 %, respectively. In contrast, the constant current arc waveform exhibits insufficient molten pool fluidity, resulting in inadequate manufacturing precision at all angles. Furthermore, as the pulse frequency decreases, the solidification rate and temperature gradient of the molten pool increase significantly, leading to finer grain sizes and enhanced mechanical properties of unsupported bars. Unsupported bars manufactured using the constant current arc waveform exhibit intermediate grain sizes and mechanical properties. These findings establish a systematic framework linking arc waveforms to the evolution of the molten pool's temperature field. Optimizing the arc waveform significantly enhances the manufacturing quality and mechanical properties of unsupported bars at various angle. This provides a theoretical foundation and technical support for the promotion and application of WAAM in the fabrication of large-scale spatial bar structures.