# Correlation of Microstructure and Mechanical Properties of Metal Big Area Additive Manufacturing
## 1 Introduction  
Additive manufacturing (AM) is changing the way industry produces components beyond simple geometrical models and prototypes [1; 2]. 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; 4; 5], and specialized alloys, such as high temperature resistant Ni based super alloys [6; 7; 8]. 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; 10; 11; 12]. More recently, wire-arc AM has shown promise for large parts, such as tooling dies, and vehicle and aerospace components [2; 13; 14]. Metals such as Ti-6 Al-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; 19; 20].