# Correlation of Microstructure and Mechanical Properties of Metal Big Area Additive Manufacturing
## 1 Introduction
The composite microstructure in steels is the result of liquid to solid and solid to liquid transformations, as wel as solid-state phase transformations on heating (alpha ferrite to austenite and austenite to delta-ferrite) and cooling (austenite to grain boundary ferrite, Widmansatten 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 [PERSON] and [PERSON] [21] and discussed in welding textbooks. 1. Grain boundary nucleated primary ferrite forms between 800 \({}^{\circ}\)C and 650 \({}^{\circ}\)C at the prior austenite grain boundaries when cooling is moderate. 2. Ferrite side plates (or often referred as Widmanstatten ferrite) form between 750 \({}^{\circ}\)C and 650 \({}^{\circ}\)C at the prior austenite grain boundaries when cooling rates increase. 3. Fine-grained acicular ferrite forms below approximately 650 \({}^{\circ}\)C within the prior austenite grains
4. A lath structure with a significant dislocation substructure forms below 500 \({}^{\circ}\)C; this structure is hypothesized to be Bainite when cooling rates are fast (\(>\)50 \({}^{\circ}\)C/s)
5. In highly alloyed steels, cooling below 400 \({}^{\circ}\)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; 22; 23; 24]. 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; 26; 27]. 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.