Additive manufacturing (AM) plays a key role in the field of digital manufacturing (Industry 4.0). Nevertheless, its progress towards industrialization has faced obstacles due to suboptimal mechanical properties stemming from residual stresses resulting from solidification and intrinsic heat treatment. A potential solution considers leveraging volume expansion through a displacive phase transformation, which can offset thermal contraction and, consequently, alleviate the development of residual stress during AM. Here, the challenges are imposed by the complexity of multi-phase transformation phenomena, which cannot be thoroughly assessed using conventional postprocess techniques due to their transient nature. This fact pinpoints the need for in situ characterization methods to comprehensively understand these elementary mechanisms. Moreover, microstructures can be directly tailored by appropriate AM parameters, such as volume energy density, scan pattern, etc. It already has been reported that the specific characteristics of AM microstructures affect well-established parameters, e.g., diffraction elastic constants (DEC) [1]. Here, the microscopic elastic response can significantly deviate from the macroscopic behavior due to the elastic anisotropy. Generally, DEC are determined experimentally through uniaxial tension, compression or bending tests [2,3]. However, the DEC derived from traditionally manufactured components cannot be applied straightforward to material conditions processed by AM. Clearly, DEC are an essential factor in residual stress analysis based on strain measurements by X-Ray diffraction. These considerations point at a critical issue in a case of residual stress analysis and, eventually, the understanding of elastic behavior of AM components.

The main objective of the present study is to provide for an in situ analysis of metastable steels (known as twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) steels) processed by additive manufacturing under uniaxial tension. The focus is on evaluating how the DEC are influenced by the presence of the coarse-grained and textured microstructure. The study specifically concentrates on three different grades of 16Cr-6Mn-XNi steel, each with varying Ni content and, consequently, different phase stabilities. In these metastable and multiphase microstructures, their orientation-dependent behavior has a significant impact on the macroscopic state.

The alloy with the lowest Ni content (3% Ni) exhibits a primarily body-centred cubic (bcc) microstructure upon AM. In contrast, alloys containing 6% and 9% Ni showcase a face-centred cubic (fcc) structure with minor fractions of hexagonal close-packed (hcp) martensite. Due to multiple phase evolutions during AM, variations in residual stress state, grain size, and texture are observed. Leveraging the knowledge of DEC tensors allows the exploitation of general tensor properties to enhance the assessment of such coarse-grained and textured microstructures.

 

[1]    T. Mishurova et al., ASTM International, doi: 10.1520/STP163120190148
[2]    H. Dölle, V. Hauk, Härterei-Tech. Mitt. 31 (1976), 165-168.
[3]    E. Macherauch, P. Müller, Arch. f. d. Eisenhüttenwesen 29 (1958), 257-260.