A Novel Approach to Robustly Determine Residual Stress in Additively Manufactured Microstructures Using Synchrotron Radiation

Abstract

Introduction 1.1. Microstructure of Additively Manufactured MaterialsAdditive manufacturing (AM) is the subject of many recent publications. [1][2][3] Typical representatives of AM methods for the processing of metallic metals are mainly divided into three categories: wire feeding, powder feeding, and powder bed fusion systems. [3] The latter technique enables the production of metallic structures by layer-wise melting of a thin metal powder bed only using AM equipment and a sliced model generated by a computeraided design (CAD) program. The application of a powder bed in powder-based AM technology facilitates the selective processing of individual layers in complex geometry. The most commonly used powder bed fusion techniques are selective electron beam powder bed fusion (EPBF) and laser powder bed fusion (LPBF). [3,4] EPBF structures are produced in a vacuum atmosphere and at a high temperature, eventually leading to a relatively low residual stress level. In contrast, LPBF processes usually take place at temperatures up to 200 C. The localized energy input and the small melt pool sizes, respectively, in combination with low build chamber temperatures result in high temperature gradients, whereby residual stresses are favored. [5] Generally, the influence of scan vectors and related strategies is a complex topic, which crucially has to be dealt with. Scan vector lengths, orientations, the order of rotation within a specific layer, and its subsequent layers are variables in every building process. Only a very limited number of comprehensive studies have reported on the effect of the scan strategy on the density, microstructure, mechanical properties, and residual stresses of LPBF parts, as clearly stated in previous studies. [5][6][7][8] Already an increased scan path length, e.g., due to a change of cross-section of a part, can result in a severely decreased relative density as well as fundamentally changed microstructure. [9,10] The influence of parameters on the evolution of residual stresses was already numerously studied. [11,12] The authors showed that the residual stresses are mainly influenced by cooling rates, temperature gradients, and melt pool sizes. Also, a minimization of the temperature gradient by powder bed preheating is an option to reduce process-induced stresses. [1,13] The high influence of the laser power and the effect of scan strategies have been characterized.

Understanding the impact of texture on the micromechanical anisotropy of laser powder bed fused Inconel 718

“…In general alignment with this study, their experimental data gave reasonable agreement to the Reuss model when considering the SCEC of the conventional Inconel 625 at room temperature (see [39]). In fact, Aminforoughi et al [72] conducted an in situ tensile experiment on solution heat-treated L-PBF Inconel 718 using synchrotron X-ray diffraction in transmission geometry. In their experiment they showed that stresses calculated on the base of Reuss model elastic constants gave similar values for different lattice planes in the early stages of deformation.…”

Metastable CrMnNi steels processed by laser powder bed fusion: experimental assessment of elementary mechanisms contributing to microstructure, properties and residual stress

“…However, stresses always promote distortion 61 . Numerous studies focus on the reduction of residual stress in PBF-LB/M by alternative scan strategies 4 , 62 , adapted preheating 63 , 64 or specifically designed processing parameters 64 . In the present study, all specimens were manufactured with the same machine and parameter settings, such that similar thermal history and cooling conditions can be expected.…”

“…Process-inherent cooling conditions and melt pool dynamics, respectively, in the PBF-LB/M process often result in unique microstructures; however, advantageous features are usually accompanied by detrimental residual stresses and material defects like porosity 2 – 4 . The high stresses can be attributed to the small melt pool sizes and high cooling rates.…”

Experimental Analysis of the Stability of Retained Austenite in a Low‐Alloy 42CrSi Steel after Different Quenching and Partitioning Heat Treatments

“…For a more detailed explanation of determination of the stress components, the reader is referred to another study. [ 33 ] In both in situ studies, a miniature stress rig (Kammrath & Weiss, see Figure b) with a maximum load capability of 10 kN was used. The specimens were clamped in the stress rig and initially probed in a load‐free state.…”