Invar processed by selective laser melting – Tailoring the thermal expansion coefficient by process-induced defects and residual stresses
Final Report Abstract
The main objective of the project reported here was to systematically investigate the influence of process-related defects and residual stress on the thermal expansion behavior of Invar processed by laser-based powder bed fusion, and to specifically analyze the influence of defects and residual stress on the resulting microstructure as well as the mechanical properties under monotonic and cyclic loading. For this purpose, a full factorial experimental design was considered, which comprises 72 parameter combinations and includes specimen with both high and low relative density. The 72 specimens were firstly characterized in terms of their coefficient of thermal expansion (CTE). Subsequently, the microstructure of six selected parameter combinations with low CTE and different relative densities was examined using high-resolution electron microscopy. Furthermore, the influence of process-induced residual stress on the CTE behavior was evaluated for these six conditions on the basis of residual stress depth profiles determined by X-ray diffraction. Finally, the conditions in focus were analyzed with regard to their mechanical properties, in particular their fatigue performance. The results show that a strong correlation between relative density and CTE, which has been reported in various studies in literature, cannot be verified. Depending on the volume energies used, the selected parameter combinations are characterized by different microstructures in terms of grain morphology and orientation as well as prevailing defect types. However, a correlation between defect type and CTE behavior could not be derived. In contrast, a positive influence of process-induced residual stresses on the Invar effect could be demonstrated. Accordingly, a multi-step heat treatment promoting significant reduction of the tensile residual stresses present in all conditions, results in a deterioration of the Invar effect. The fatigue behavior is significantly dominated by the relative density of the respective conditions. Here, specimens with an increased density can already compete with almost completely dense reference specimens from other studies in terms of their number of cycles to failure due to the high damage tolerance of the material. The residual stress states do not appear to have a significant effect on the fatigue behavior in the fatigue regimes investigated. In addition to the results summarized above, further investigations were carried out on Invar specimens produced via wire-based directed energy deposition as part of the project, which were not listed in the work packages of the original application. The investigations focused on the process-microstructure-property relationships and the structural integrity of two different conditions (arc and laser as energy source used).
Publications
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Influence of Defects and Microstructure on the Thermal Expansion Behavior and the Mechanical Properties of Additively Manufactured Fe-36Ni. Materials, 17(17), 4313.
Kahlert, Moritz; Wegener, Thomas; Laabs, Leonard; Vollmer, Malte & Niendorf, Thomas
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On the cyclic deformation behavior of wire-based directed energy deposited Fe-Ni Invar alloy. Materials Science and Engineering: A, 918, 147403.
Wegener, Thomas; Pramanik, Sudipta; Niendorf, Thomas & Günther, Johannes
