What is the microstructure? And why it is crucial for WAAM?
Simply put, the structure of a material at the micro scale can be considered as “microstructure”. Observing and evaluating the microstructure of a printed material is of crucial importance for its operational functionality. The structures of a material that are seen at the micro level can be considered as “microstructure”.
Wire-arc additive manufacturing (WAAM) is a branch of additive manufacturing technology (AM) allowing the production of large 3D metallic components layer by layer. In fact, the microstructural evaluation allows to relate the processing parameters with the resulting functional properties. Considering the different space-filling features such as grains, subgrains, dispersoids, precipitates and dislocations that form the microstructure of the WAAM part, there is a hierarchy of length scales. The properties of WAAM parts such as strength, fatigue and corrosion resistance are controlled by the interactive response of these microstructural features to external loads in a specific environment.
One of the unique features of WAAM is the opportunity to produce large bimetallic or multimetallic components allowing to optimize the design and enhance the performance. Since different alloys exhibit different chemical compositions, crystal structures and physical properties, a microstructural gradient will be formed across the interface. This microstructural gradient definitely affects the integrity and homogeneity of properties. Therefore, in the case of bimetallic components, it is of crucial importance to characterize in detail the microstructural changes across the interface.
One example of microstructural studies for the Grade 2XL project
Figure 1 below shows the microstructure developed during the WAAM of the Inconel 625/S355 steel bimetal part.
Figure 1 : The image obtained from the EBSD data across the interface of the Inconel 625/S355 steel bimetal part. (BD: building direction, WD: welding direction and TD: transverse direction)
This image has been obtained using electron backscattered diffraction (EBSD) measurements. It can be seen that there is an obvious microstructural change across the interface. The S355 steel side shows a fine (grain size of 11.6 µm) and somehow equiaxed structure, while large and elongated grains along the building direction (BD) can be observed at the Inconel 625 side (major axis ≈ 205 µm and minor axis ≈ 29.4 µm). These different microstructures will result in different properties across the interface. Notably, this is a very simplified interpretation of the EBSD data, and much more detailed qualitative and quantitative analyses can be extracted.
About the authors :
Mahdi Mahmoudiniya (Mahdi.Mahmoudiniya@UGent.be) is a PhD researcher of the group Metal Science and Technology in the Department of Electromechanical, Systems and Metals Engineering at Ghent University, Belgium. The research of this group, under the supervision of Prof.
Leo Kestens (Leo.Kestens@UGent.be), focuses on the study of process-structure-properties relations with a particular interest in microstructural characterization by electron microscopy and orientations contrast imaging.