Abstract
Wire arc additive manufacturing (WAAM) is known to induce a considerable microstructural inhomogeneity and anisotropy in mechanical properties, which can potentially be minimized by adopting appropriate post-printing heat treatment. In this paper, the effects of two heat treatment cycles, including hardening and normalizing on the microstructure and mechanical properties of a WAAM-fabricated low-carbon low-alloy steel (ER70S-6) are studied. The microstructure in the melt pools of the as-printed sample was found to contain a low volume fraction of lamellar pearlite formed along the grain boundaries of polygonal ferrite as the predominant micro-constituents. The grain coarsening in the heat affected zone (HAZ) was also detected at the periphery of each melt pool boundary, leading to a noticeable microstructural inhomogeneity in the as-fabricated sample. In order to modify the nonuniformity of the microstructure, a normalizing treatment was employed to promote a homogenous microstructure with uniform grain size throughout the melt pools and HAZs. Differently, the hardening treatment contributed to the formation of two non-equilibrium micro-constituents, i.e., acicular ferrite and bainite, primarily adjacent to the lamellar pearlite phase. The results of microhardness testing revealed that the normalizing treatment slightly decreases the microhardness of the sample; however, the formation of non-equilibrium phases during hardening process significantly increased the microhardness of the component. Tensile testing of the as-printed part in the building and deposition directions revealed an anisotropic ductility. Although normalizing treatment did not contribute to the tensile strength improvement of the component, it suppressed the observed anisotropy in ductility. On the contrary, the hardening treatment raised the tensile strength, but further intensified the anisotropic behavior of the component.
Highlights
Wire arc additive manufacturing (WAAM) is a novel technology capable of producing metallic components utilizing an arc welding process to additively fabricate engineering parts [1], with various applications such as impeller blades [2], bridge structures [3], shipbuilding plates [4], and wing ribs in the aerospace industry [5]
The dominant microstructure in the center of the melt pool consists of a low volume fraction of lamellar pearlite (P) primarily formed at the grain boundaries of polygonal ferrite (PF) (Figure 2b)
This study aimed to investigate the effect of two post-printing heat treatment cycles, including normalizing and hardening, on the microstructure and mechanical properties of a wire arc additively normalizing and hardening, on the microstructure and mechanical properties of a wire arc additively manufactured low-carbon low-alloy steel (ER70S-6)
Summary
Wire arc additive manufacturing (WAAM) is a novel technology capable of producing metallic components utilizing an arc welding process to additively fabricate engineering parts [1], with various applications such as impeller blades [2], bridge structures [3], shipbuilding plates [4], and wing ribs in the aerospace industry [5]. Wire-based systems are generally suitable for producing large-scale components with less complexity in geometry and design, in contrast to the powder-bed systems, which typically fabricate small and high-definition parts [8]. From another perspective, powder-bed additive manufacturing techniques are limited to a Materials 2020, 13, 2795; doi:10.3390/ma13122795 www.mdpi.com/journal/materials. Since the process is involved with sequential melting and solidification, each region of the component is subjected to periodic fast heating and cooling cycles by the deposition of upper layers Such complex localized thermal cycles lead to heterogeneous microstructure and anisotropic mechanical properties in the AM-fabricated components [1,10,11]. Sridharan et al [12]
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