Open Access

High-strength, fine-grained structural steels are increasingly used in modern steel construction to meet economic and environmental demands through lightweight design and reduced material usage. Additive manufacturing (AM), in particularly direct energy deposition with gas metal arc welding (DED-Arc), offers significant advantages in flexibility, production speed, and cost efficiency. In hybrid AM, conventionally manufactured parts are extended or modified using AM processes. Welding-induced residual stresses, especially in the transition area, pose challenges to structural integrity. This study investigates the influence of substrate design and thermal pretreatment on the residual stress state of hybrid AM components made from base material steel S690QL and a welding filler, a modified solid wire G79. Two substrate geometries (I- and T-substrate) are analysed, which are additively and conventionally manufactured, respectively. Results show that substrate geometry significantly affects local residual stresses, microstructure and hardness, particularly in the transition are. The findings contribute to a better understanding of the evolution of welding stresses in hybrid AM components to achieve reliable, crack-resistant and economic high-strength steel structures.

Ultraschallmessverfahren werden seit Langem erfolgreich für Prüfaufgaben im Bauwesen eingesetzt. Die kontinuierliche Zustandsüberwachung sowie die frühzeitige Erkennung von Schäden an komplexen Neubauten und alternder Infrastruktur können durch Ultraschalltransmissionsmessungen in Kombination mit speziellen Auswertemethoden, wie beispielsweise Korrelationsverfahren oder Codawelleninterferometrie, realisiert werden. Das Ziel des Projekts besteht darin, ein Streckenabdichtungssegment aus Magnesiabaustoff mittels Transmissionsmessungen mit im Baustoff eingebetteten Ultraschallprüfköpfen auf die Ausbildung von Rissen zu überwachen und diese gegebenenfalls zu lokalisieren. Die Eignung der eingesetzten Ultraschallprüfköpfe wurde vorab im Labor geprüft, da während der Aushärtung des Baustoffs hohe Temperaturen von bis zu 110 °C sowie Drücke von bis zu 5 MPa auftreten, die zu signifikanten Belastungen der Sensorik führen können. Vor der Betonage des Bauwerks wurden insgesamt 18 Ultraschallprüfköpfe installiert. Die Instrumentierung erforderte Messkabellängen von bis zu 80 m. Zur Sicherstellung eines möglichst hohen Signal-Rausch-Verhältnisses wurden spezielle Vorverstärker entwickelt, deren Stromversorgung über die Messkabel erfolgt. Dadurch konnten die Verstärker direkt am Prüfkopf positioniert werden. Durch eine spezielle Schaltungsauslegung ist es möglich, jeden Ultraschallprüfkopf bei jeder Messung wahlweise als Sender oder Empfänger zu betreiben. Ein automatisiertes Messsystem führt in definierten Intervallen Messungen durch, um Veränderungen der Ultraschallsignale zu erfassen, die auf Schädigungen des Bauteils, wie beispielsweise Rissbildungen, hinweisen können.

Lightweight construction is a vital approach for reducing CO₂ emissions, as it contributes to the development of more energy-efficient structures and supports the overall goal of achieving carbon neutrality in the transition to sustainable manufacturing. Though, a topology-optimized design often leads to complex geometries. Additive manufacturing (AM) processes such as direct energy deposition with arc (DED-Arc) offer great design freedom due to the buildup of components in layers. Furthermore, DED-Arc enables efficient production due to the high deposition rate, process reliability, and good automation capability. Further efficiency can be achieved through weight optimization, enabled by high-strength steels. However, a major challenge is the process-induced residual stresses (RS) in the component, which arise primarily due to restrained shrinkage. High tensile residual stresses are detrimental as they increase the risk for several types of cracking and failures. Knowledge of residual stress distribution is crucial for predicting the service life of the component and structural integrity assessment, especially for safety-critical applications. Therefore, this study focuses on the use of the contour method (CM) to analyze the full field longitudinal residual stresses in an open hollow cuboid component (dimensions, 120 × 50 × 30 mm 3 ) manufactured by DED-Arc (low-alloyed high-strength steel with yield strength  730 MPa). In CM, the component is cut along a desired plane of interest, and the contour of the deformed cut surface is measured. A finite element model is used to reconstruct the residual stresses field in the 2-dimensional plane of cut. In this paper, a modified cutting strategy was employed. After cutting, the deformed cut surfaces were measured utilizing two surface measurement techniques, i.e., coordinate measuring machine (CMM) and a 3D scanner. The accuracy of the CM was validated against surface stresses measured using X-ray diffraction (XRD). Additionally, a comparison of neutron diffraction experiments was conducted. The residual stresses were further correlated with hardness measurements. The results from surface measurement techniques showed good qualitative and quantitative agreement regarding the measured displacement contour. CM results revealed peak stresses in the DED-Arc walls; bending deformation in the substrate induces tensile stresses at the bottom of the substrate plate and compressive stresses in the middle top region. The residual stresses obtained from diffraction and CM showed a good agreement and correlated qualitatively with the hardness measurements. These results show that practical residual stress mapping and quantification using XRD accompanied by minimal application of CM for values in the bulk are suitable for identifying and minimizing detrimental welding-induced stresses in such high-strength AM lightweight components, even without the use of expensive methods such as neutron diffraction.

Steel structures erected in the late 19th and early 20th centuries still constitute a substantial part of today’s technical infrastructure. A significant proportion of bridge and engineering structures have been in service for more than 80 or even 100 years and are subjected to continuously increasing traffic volumes, axle loads, and dynamic actions. In view of these rising service demands, load-oriented and structurally adequate repair strategies are essential to ensure long-term structural integrity while preserving the existing building stock in a sustainable manner. As historical steel constructions were predominantly designed for riveted or bolted joints, the subsequent implementation of welded repair or strengthening measures represents a particular challenge. In addition to the specific material characteristics of old mild steels, welding-induced residual stresses and microstructural alterations must be carefully considered, as they can significantly affect load-bearing capacity and the overall structure performance. Consequently, repair concepts must not only restore geometry but also be adapted to the actual stress conditions and service requirements of the structure. Against this background, the present study investigates MAG welding trials on mixed joints between old and modern structural steels. The groove opening angle was systematically varied between 50° and 30° in order to analyse the influence of joint geometry on weld metal volume, heat input, and the resulting residual stress state. Furthermore, advanced high-performance arc processes, such as a modified spray arc mode, were applied to enable controlled and reduced heat input. The objective is to minimise welding-induced residual stresses through process and joint design optimisation, thereby providing a technically sound and load-oriented repair strategy for existing old steel structures suitable for long-term service.

The increasing global focus on energy and resource efficiency has stimulated a growing interest in additive manufacturing. AM offers economic advantages and enables an efficient use of materials. However, AM components often require subsequent mechanical post-processing, such as machining (e.g., milling), to achieve the final contours or surfaces. This is a particular challenge due to the heterogeneous and anisotropic nature of AM structures, which affect machining and the resulting component properties. High-performance materials such as iron aluminide represent a promising alternative to conventional high-temperature materials with a significant economic advantage. However, the strength and hardness properties, which are advantageous for applications in highly stressed lightweight components, pose a challenge for economical machining in addition to the AM microstructure properties. The difficult-to-cut material causes accelerated tool wear and insufficient surface quality. This study shows that crack-free additive manufacturing of the three-component system of iron-nickel-aluminum is possible, and advantages in terms of machinability compared to FeAl-AM components are achieved. The more homogeneous microstructure leads to a reduction in cutting forces, with positive effects on the machinability and optimized surface integrity. Ultrasonic assisted milling (USAM) offers great potential to address the major challenges posed by difficult-to-cut materials and additively manufactured weld structures. Therefore, this study focuses on assessing the transferability of previous positive results by USAM to the selected iron aluminide alloys. The machinability of the aluminides is analyzed by varying significant influencing variables in finish milling experiments and evaluated in terms of the loads on the tool and the resulting surface integrity.