Characterization of arc-based additive manufacturing of low carbon steel deposits

Abstract

Wire arc additive manufacturing (WAAM) is a technique that involves the use of a computer numeric-controlled (CNC) arc welding torch with an integrated wire feed to deposit metal one layer at a time in the fabrication of a near-net-shape part. Advantages of using WAAM over another additive manufacturing process include high deposition rate, large build volume, and reduced need for expensive equipment such as a vacuum chamber that would be used in a laser-based or electron beam-based process. However, little study has been performed to characterize the metallurgical properties of WAAM builds, particularly regarding the evolution of microstructure and mechanical properties along the entire build dimension with respect to length and width. This work characterized the microstructure, composition of alloying elements, and mechanical properties of ER70S6 carbon steel deposits on a layer-to-layer basis to better understand how the WAAM process affects the properties of the part.The microstructure of the build was characterized from layer to layer to determine how microstructural changes take place during the WAAM process. It was found that the as-solidified material formed a variety of ferrite microstructures, mostly acicular ferrite within large columnar prior austenite grains decorated with grain boundary ferrite and Widmanstätten ferrite at the prior austenite grain boundaries. Between subsequent passes, volumes of the prior deposit are heat treated by the process thermal conditions to form a heat affected zone, similar to that found in weldments. Irregular pockets of lath martensite and martensite-austenite-carbide (MAC) microconstituents were found dispersed in the HAZ along the length of the deposit and along the build direction. Also, in the heat affected zones, the as-deposited ferrite would be tempered into polygonal ferrite in the center of the build, due to the heat generated by the arc deposition. The composition of alloying elements was found to increase within each layer, from bottom to top. With remelting of the top regions of each prior layer during the additive manufacturing process, the richer composition near the top of the deposit would be incorporated into the subsequent layer. Arc stirring and pool mixing would uniformize the composition but partition during solidification will again create a composition gradient in this subsequent pass. This process repeats throughout the entire height of the build elevating gradually, but continuously, the concentration of the alloying elements. The mechanical properties were characterized through microhardness measurements to examine fluctuations within the build. Hard regions of martensite were found in the reheated zones of the earlier passes of a build. With subsequent deposits, however, the initially formed hard and soft regions were tempered to result in a more homogenized ferritic microstructure with lower hardness. Except for the initial and end transients, the remainder of a WAAM build would exhibit a relatively uniform microstructure and hardness.