Pass-To-Pass Microstructural Variations in ARC Based AM Deposits in As-Built Conditions

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. However, few studies have been performed to characterize the metallurgical properties of WAAM builds, particularly regarding the evolution of microstructure and mechanical properties along the entire build length. This work characterized the microstructure, composition of alloying elements, and hardness of ER70S-6 carbon steel deposits on a layer-to-layer basis to better understand how the WAAM process affects the properties of the part as a whole. 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 AM 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. 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. For the alloy system investigated, 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. 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.