Additive manufacturing (AM) is changing the way industry produces components beyond simple geometrical models and prototypes [1
]. Each AM technique has rapidly been developed for a variety of industrial applications and to address challenges brought about by subtractive (e.g., machining) processes. For example, metal powder bed fusion has been developed for rapid part manufacturing with specific intricacies, such as internal cooling channels [3
], and specialized alloys, such as high temperature resistant Ni based super alloys [6
]. Directed energy deposition is used for larger parts or repair, due to its flexibility and scalability in parts with large section thickness and moderate geometrical complexity [9
More recently, wire-arc AM has shown promise for large parts, such as tooling dies, and vehicle and aerospace components [2
]. Metals such as Ti-6Al-4V and Aluminum have recently been investigated for aerospace applications, but steels have only recently become of interest [15
]. Unlike other metal AM processes, the wire arc process offers readily available feedstock, high throughput, and uses commercial off the shelf hardware components, resulting in favorable economics [16
A novel wire-arc AM system, Metal Big Area Additive Manufacturing (MBAAM), was developed at the Oak Ridge National Laboratory. This system uses a correction-based approach, where integrated software, in conjunction with a traditional wire feed welding robot, is used to minimize the dynamic nature of welding and compensate for build height. The MBAAM process is comparable with traditional robotic gas tungsten arc welding or gas metal arc welding (GTAW and GMAW, respectively). However, there are some differences between MBAAM and welding. One of the major differences is related to the use of the Oak Ridge National Laboratory (ORNL) developed closed loop control system, to ensure geometrical conformity (e.g., layer height).
In the wire-arc AM process, similar to GMAW, the liquid pool and deposition layer heights depend on many factors like wire composition, wire diameter, wire feed rate, robot motion speed, ambient temperature, shielding gas, shielding nozzle, nozzle condition, gas flow, power, type of deposition/welding mode, part geometry, slicing path, slicing strategy, inter-pass temperature, and many others. For a stable welding process with well controlled joint geometries (e.g., T and butt joints), where the deposition height is constant throughout the build, a smart-closed loop control system may not be necessary. However, due to the dynamic nature of welding with wide ranging geometries, a closed loop system maintains a constant height of the deposit layer and preserves the geometric characteristic of the bead. Frequent starting and stopping, typified in industrial welds, leads to a large scatter in properties [17
]. We also hypothesize that the “print by wire” control in this work substantially reduces the frequency of interruption, resulting in predictable, consistent, thermal conditions experienced by large areas of the build.
In all AM techniques, studies have been conducted on correlating the microstructure to the mechanical properties. Although each AM technique has been rapidly developing over the past decade, material challenges are still present. The overall material challenges in the MBAAM process are expected to be like those reported in welding. These challenges range from local defects, such as gas porosities, lack of fusion, scatter in macro- and microstructure, and consequently, scatter in static and dynamic mechanical properties. The layer-wise deposition leads to continuous and varying numbers of thermal cycles, and, consequently, leads to microstructural heterogeneity and scatter in mechanical properties. These repeated thermal cycles with different heating rates, peak temperatures, and cooling rates causes a composite type microstructure [18
]. The composite microstructure in steels is the result of liquid to solid and solid to liquid transformations, as well as solid-state phase transformations on heating (alpha ferrite to austenite and austenite to delta-ferrite) and cooling (austenite to grain boundary ferrite, Widmanstätten ferrite, acicular ferrite, bainite, and martensite). Solidification involves the epitaxial growth of cellular/columnar delta ferrite grains from the fusion boundary, due to the high thermal gradients involved in welding. The delta ferrite grains have their <001> crystallographic directions oriented to the direction of maximum heat flow. Upon cooling, the delta ferrite (body centered cubic (BCC)) transforms to austenite (face centered cubic (FCC)). Austenite then decomposes to several allotropes of BCC ferrite depending on the local cooling rates. The transformation temperatures for the major constituents during the cooling of C-Mn steels are outlined by Choi and Hill [21
] and discussed in welding textbooks.
Grain boundary nucleated primary ferrite forms between 800 °C and 650 °C at the prior austenite grain boundaries when cooling is moderate.
Ferrite side plates (or often referred as Widmanstätten ferrite) form between 750 °C and 650 °C at the prior austenite grain boundaries when cooling rates increase.
Fine-grained acicular ferrite forms below approximately 650 °C within the prior austenite grains
A lath structure with a significant dislocation substructure forms below 500 °C; this structure is hypothesized to be Bainite when cooling rates are fast (>50 °C/s)
In highly alloyed steels, cooling below 400 °C often leads to a martensitic microstructure.
Upon heating above the Ac1, these transformation products start to re-transform back into FCC austenite. This cyclic transformation may lead to a microstructure with local brittle zones (a mixture of a martensite-austenite-carbide microstructure, also referred as an MA constituent) and may contribute to the scatter in the impact toughness data. Scatter in weld metal toughness of low carbon steel multi-pass welds have been well documented [17
]. In multi-pass welds, constituents, such as upper-bainite and martensite islands, increase the probability of cleavage fracture. On the other hand, microstructural constituents, such as acicular ferrite, lead to superior impact toughness of low carbon welds [25
The focus of this research is to evaluate the mechanical properties of MBAAM manufactured parts and to rationalize the properties with microstructure characterization. The walls built in this study represent the simplest possible geometry that is representative of a larger range of useful geometries. The mechanical properties of different MBAAM geometries will be expectedly different due to cooling rate sensitives and specific geometric aspects. It is important for the future progress of large-scale additive manufacturing to understand how geometrical features affect the properties.