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Bragg edge tomography characterization of additively manufactured 316L steel

Nikola Kalentics, Christian Leinenbach, Roland Logé, Efthymios Polatidis, Markus Strobl

In this work we perform a neutron Bragg edge tomography of stainless steel 316L additive manufacturing samples, one as built via standard laser powder bed fusion, and one using the novel three-dimensional (3D) laser shock peening technique. First, we consider conventional attenuation tomography of the two samples by integrating the signal for neutron wavelengths beyond the last Bragg edge, to analyze the bulk density properties of the material. This is used to map defects, such as porosities or cracks, which yield a lower density. Second, we obtain strain maps for each of the tomography projections by tracking the wavelength of the strongest Bragg edge corresponding to the {111} lattice plane family. Algebraic reconstruction techniques are used to obtain volumetric 3D maps of the strain in the bulk of the samples. It is found that not only the volume of the sample where the shock peening treatment was carried out yields a higher bulk density, but also a deep and remarkable compressive strain region. Finally, the analysis of the Bragg edge heights as a function of the projection angle is used to describe qualitatively crystallographic texture properties of the samples.

AMER PHYSICAL SOC2022

Effect of post-heat treatment conditions on shape memory property in 4D printed Fe-17Mn-5Si-10Cr-4Ni shape memory alloy

Irene Ferretto, Christian Leinenbach

The influence of post-heat treatment on the microstructure and properties of 4D printed Fe-17Mn-5Si-10Cr-4Ni (wt. %) shape memory alloy (SMA) produced via a laser powder bed fusion process is investigated in this study. It is shown that heat-treatment temperatures lower than 800 ? are not sufficient to completely undergo phase transformation from bcc-delta to fcc-gamma, whereas temperatures higher than 800 ? lead to grain growth and thickening of hcp-epsilon phases. The latter has a severely negative effect on shape memory. Furthermore, heat treatment at 800 C for more than 3 h leads to sigma-phase formation, which also negatively affects the shape memory effect (SME) by changing the chemical composition of the fcc-gamma matrix. The SME along the building direction (BD), is higher than that along the laser scanning direction (SD) and in 45 to the SD (45SD). The higher SME along the BD is attributed to the {110} texture in the plane normal to the BD, whereas almost random but weak {111} and {100} textures are observed in SD and 45SD. Furthermore, transmission electron microscopy revealed that a high stacking fault density and very thin hcp-epsilon phase are formed in the heat-treated sample, which explains the higher SME regardless of direction, compared with conventionally fabricated Fe-based SMAs with a similar composition.

2022

Processability, microstructure and precipitation of a Zr-modified 2618 aluminium alloy fabricated by laser powder bed fusion

Christian Leinenbach, Xavier Maeder

Additive manufacturing offers the opportunity to produce complex geometries from novel alloys with improved properties. Adapting conventional alloys to the process-specific properties can facilitate rapid implementation of these materials in industrial practice. Nevertheless, the processing of conventional alloys by laser powder bed fusion is challenging, particularly in cases of pronounced susceptibility to hot cracking.Previous studies have demonstrated the positive influence of zirconium on the susceptibility to hot cracking of high-strength aluminum alloys, although its influence on precipitation formation, which is immensely important for 2xxx alloys, remains largely unexplored. This work investigates the optimum process parameters and precipitation formation of 2618 modified by zirconium in laser powder bed fusion.The addition of zirconium results in the production of a crack-free, high-density (~99.9%) material. The microstructure is characterized by a trimodal grain size distribution. Very fine (~0.5 mu m) equiaxed grains, nucleated by L12-Al3Zr precipitation at the melt pool boundary, followed by columnar-dendritic grains (5-15 mu m long, 1-3 mu m wide) and coarser equiaxed grains (1-3 mu m) form during solidification. The grain boundaries are populated predominantly by (Al,Cu)9FeNi, but also by Mg2Si, Al2CuMg, and AlCu, which presumably impede grain growth and promote a very fine-grained, low-textured microstructure, even in regions where L12-Al3Zr are absent. The as-built microhardness of 1360 +/- 74 MPa exceeds that of the known high-strength Al alloys tailored to additive manufacturing, AddalloyTM and Scalmalloy (R). The results provide a better understanding of precipitate formation in Zr-modified 2xxx series alloys and pave the way for the commercialization of further 2xxx alloys adapted to additive manufacturing. (c) 2022 The Author(s). Published by Elsevier B.V. CC_BY_NC_ND_4.0

2022