Alexander Liehr

Poly(lactic acid) (PLA) is one of the most prominent biopolymers and is considered a viable alternative to petroleum‐based polymers. While it exhibits comparable properties to conventional polymers like PET, in certain applications, particularly those involving elevated temperatures, PLA has performance limitations. In addition, the properties of PLA are dependent on the processing parameters in injection molding. Non‐optimal process parameters can lead to defects or undesirable effects that cannot be detected immediately after injection molding. This includes orientation and residual stresses, which significantly influence the material and failure properties. The present study investigates the influence of injection molding machine settings on the residual stress state in PLA components. Test specimens were produced using two different mold tools: an ejector pin and a full‐surface ejector, while varying key machine settings. Residual stress was assessed using a polariscope and the hole drilling method. The polariscope identified distinct isochromatic fringe patterns, particularly near the sprue, indicating regions of elevated residual stress. The hole drilling method confirmed the presence of high residual stress at the specimen edges, extending to a depth of 600 μm, with a peak stress value of 47 MPa. Results revealed that the ejector pin mold induced both tensile and compressive stress states, whereas the full‐surface ejector mold predominantly caused high compressive stresses at the edges. These findings highlight the importance of optimizing injection molding parameters to minimize residual stress and improve the mechanical performance of PLA components.

Every new material needs to be assessed and qualified for an envisaged application. A steadily increasing number of new alloys, designed to address challenges in terms of reliability and sustainability, poses significant demands on well-known analysis methods in terms of their efficiency, e.g., in X-ray diffraction analysis. Particularly in laboratory measurements, where the intensities in diffraction experiments tend to be low, a possibility to adapt the exposure time to the prevailing boundary conditions, i.e., the investigated microstructure, is seen to be a very effective approach. The counting time is decisive for, e.g., complex texture, phase, and residual stress measurements. Traditionally, more measurement points and, thus, longer data collection times lead to more accurate information. Here, too short counting times result in poor signal-to-background ratios and dominant signal noise, respectively, rendering subsequent evaluation more difficult or even impossible. Then, it is necessary to repeat experiments with adjusted, usually significantly longer counting time. To prevent redundant measurements, it is state-of-the-art to always consider the entire measurement range, regardless of whether the investigated points are relevant and contribute to the subsequent materials characterization, respectively. Obviously, this kind of approach is extremely time-consuming and, eventually, not efficient. The present study highlights that specific selection strategies, taking into account the prevailing microstructure of the alloy in focus, can decrease counting times in X-ray energy dispersive diffraction experiments without any detrimental effect on data quality for the subsequent analysis. All relevant data, including the code, are carefully assessed and will be the basis for a widely adapted strategy enabling efficient measurements not only in lab environments but also in large-scale facilities.

Quenching and partitioning (Q&P) steels are characterized by an excellent combination of strength and ductility, opening up great potentials for advanced lightweight components. The Q&P treatment results in microstructures with a martensitic matrix being responsible for increased strength whereas interstitially enriched metastable retained austenite (RA) contributes to excellent ductility. Herein, a comprehensive experimental characterization of microstructure evolution and austenite stability is carried out on a 42CrSi steel being subjected to different Q&P treatments. The microstructure of both conditions is characterized by scanning electron microscopy as well as X‐ray diffraction (XRD) phase analysis. Besides macroscopic standard tensile tests, RA evolution under tensile loading is investigated by in situ XRD using synchrotron and laboratory methods. As a result of different quenching temperatures, the two conditions considered are characterized by different RA contents and morphologies, resulting in different strain hardening behaviors as well as strength and ductility values under tensile loading. In situ synchrotron measurements show differences in the transformation kinetics being rationalized by the different morphologies of the RA. Eventually, the evolution of the phase specific stresses can be explained by the well‐known Masing model.

Corrigendum to: T. Wegener, A. Liehr, A. Bolender, S. Degener, F. Wittich, A. Kroll, T. Niendorf: Calibration and Validation of Micromagnetic Data for Non-Destructive Analysis of Near-Surface Properties after Hard Turning. HTM J. Heat Treatm. Mat. 77 (2022) 2, pp. 156-172, DOI:10.1515/htm-2021-0023.

Micromagnetic non-destructive (NDT) methods offer a great potential for the analysis of near-surface properties after machining due to potential time and cost reduction as well as the option to be implemented into the machining process. As a result, the development of soft sensor concepts including micromagnetic NDT methods is in focus of current investigations in order to eventually improve the surface integrity of machined components and, thus, service life and reliability. However, a preceding calibration based on empirical data as well as a reliable validation is often referred to as one of the main challenges of micromagnetic NDT methods. The present study provides insights into the calibration and validation of a micromagnetic 3MA-II system for NDT analysis of the near-surface properties, with a focus on the residual stress depth profiles after hard turning of 51CrV4 specimens. Different calibration functions as well as a combination of different NDT methods are taken into consideration. The results and the potential of the 3MA system as well as open challenges are critically discussed.

Surface treatments characterized by rapid heating and cooling (e.g. laser hardening) can induce very steep residual stress gradients in the direct vicinity of the area being treated. These gradients cannot be characterized with sufficient accuracy by means of the classical sin²Ψ approach applying angle-dispersive X-ray diffraction. This can be mainly attributed to limitations of the material removal method. In order to resolve residual stress gradients in these regions without affecting the residual stress equilibrium, another angle-dispersive approach, i.e. the universal plot method, can be used. A novel combination of the two approaches (sin²Ψ and universal plot) is introduced in the present work. Prevailing limits with respect to profiles as a function of depth can be overcome and, thus, high-resolution surface layer characterization is enabled. The data obtained are discussed comprehensively in comparison with results elaborated by energy-dispersive X-ray diffraction measurements.

In recent decades additive manufacturing (AM) for years has been in focus of academia and industry as its underlying production principle allows for the realization of designs of unprecedented geometrical complexity. However, often such structures are not realized due to the lack of understanding of structural and mechanical properties, this fact amongst others related to the unique microstructures established by the related processes. In this context, residual stresses, highly affected by the scan strategy and process parameters used, play an essential role. Generally, various methods and approaches can be used to determine residual stress states experimentally. However, especially in case of the unique microstructures formed by AM, most standard procedures cannot be applied reliably. Commonly used methods based on X‐ray diffraction rely on laboratory X‐ray sources and synchrotron radiation. In present work, a novel method is proposed for robustly calculating residual stresses based on the linear regression method (similar to the sin² ψ approach in reflection mode). Data obtained by use of synchrotron radiation in transmission mode are applied. To assess the reliability of the novel procedure, results are validated using simulations and in situ tensile tests. For these tests the well‐known Ni‐base alloy INCONEL 718 processed by laser powder bed fusion (LPBF), being characterized by a complex microstructure, and a conventionally manufactured 100Cr6 steel sample are used.