13 Zentrale Universitätseinrichtungen
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/14
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Item Open Access Investigation of oxide layer development of X6CrNiNb18-10 stainless steel exposed to high-temperature water(2024) Veile, Georg; Hirpara, Radhika; Lackmann, Simon; Weihe, StefanThe oxide layer development of X6CrNiNb18-10 (AISI 347) during exposure to high-temperature water has been investigated. Stainless steels are known to form a dual oxide layer in corrosive environments. The secondary Fe-rich oxide layer has no significant protective effect. In contrast, the primary Cr-rich oxide layer is known to reach a stabilized state, protecting the base metal from further oxidation. This study’s purpose was to determine the development of oxide layer dimensions over exposure time using SEM, TEM and EDX line scans. While a parabolic development of Cr in the protective primary layer and Fe in the secondary layer was observed, the dimensions of the Ni layer remained constant. Ni required the presence of a pronounced Fe-rich secondary layer before being able to reside on the outer secondary layer. With increasing immersion time, the Ni element fraction surpassed the Cr element fraction in the secondary layer. Oxide growth on the secondary layer could be observed. After 480 h, nearly the entire surface was covered by the outer oxide layer. In the metal matrix, no depletion of Cr or Ni could be observed over time; however, an increased presence of Cr and Ni in the primary layer was found at the expense of Fe content. The Nb-stabilized stainless steel was subject to the formation of Niobium pentoxide (Nb2O5), with the quantity and magnitude of element fraction increasing over exposure time.Item Open Access Effect of weld length on strength, fatigue behaviour and microstructure of intersecting stitch-friction stir welded AA 6016-T4 sheets(2023) Walz, Dominik; Göbel, Robin; Werz, Martin; Weihe, StefanFriction stir welding is a promising joining process for boosting lightweight construction in the industrial and automotive sector by enabling the weldability of high-strength aluminum alloys. However, the high process forces usually result in large and heavy equipment for this joining method, which conflicts with flexible application. In order to circumvent this issue, a friction stir welding gun has been developed which is capable of producing short stitch welds-either stand-alone as an alternative to spot welds or merging into each other appearing like a conventional friction stir weld. In this study, the influence of the stitch seam length on the strength properties of intersecting friction stir welds is investigated, and the weld is characterized. For this purpose, EN AW-6016 T4 sheets were welded in butt joint configuration with varying stitch lengths between 2 and 15 mm. Both the static and dynamic strength properties were investigated, and hardness and temperature measurements were carried out. The results show a scalability of the tensile strength as well as the fatigue strength over the stitch seam length, while the substitute proof strength is not affected. Hereby, the tensile strength reached up 80% of the base materials tensile strength with the chosen parameter setup. Likewise, the stitch weld length influences the hardness characteristics of the welds in the transition area.Item Open Access Development and evaluation of a robot guided friction stir welding gun(2025) Walz, Dominik; Sommer, Korbinian; Werz, Martin; Weihe, StefanFriction stir welding (FSW) generally involves considerable process forces that require the use of heavy and cost-intensive machines like heavy-duty robots with massive clamping and anvil structures, which limits the flexibility of the application. To address this challenge, the Steppwelder FSW gun was developed at the MPA University of Stuttgart. This innovative welding gun enables the production of short stitch welds that can function as stand-alone alternatives to spot welds or merge seamlessly into continuous welds. The C-shaped frame design enables a closed flux of force within the gun, making it suitable for the attachment to industry standard robots used in automotive body construction processes such as clinching, riveting, or resistance spot welding (RSW). This paper presents the overall design of the welding gun, featuring a spindle and a C-frame optimized for FSW, and examines two design iterations with a focus on their stiffness characteristics. The first design, version 1.5, is an early prototype developed to demonstrate the feasibility of the process. The second version 2.0 incorporates enhancements aimed at increasing stiffness and projection while reducing the installation space to prepare it for broader application. Both frame designs are modeled as digital twins (DT) in the ABAQUS simulation software, incorporating a force-time profile of up to 14 kN based on physical models. The elastic deformation behavior and precise deflection values were then qualitatively and quantitatively analyzed at defined measuring points. These DTs were validated and calibrated by using digital image correlation on their physical counterparts under applied force. The optimized design of the welding gun offers a robust system capable of delivering consistent and reliable results for friction stir stitch welding, addressing the growing demand for flexible joining solutions in lightweight materials and mega-casting applications.Item Open Access Enhancement of microstructure, tensile and fatigue performance of EN AW-1050 by wire-based friction stir additive manufacturing(2026) Donaubauer, Stefan; Schmid, Raphael; Weihe, Stefan; Werz, MartinAdditive manufacturing (AM) of aluminium by solid-state routes offers a promising pathway to overcome the limitations of fusion-based processes, such as porosity and hot cracking. This study investigates the potential of wire-based friction stir additive manufacturing (W-FSAM) as an innovative solid-state process. A test specimen made of EN AW-1050 was fabricated and characterised using mechanical testing as well as optical and electron microscopy. Microstructural characterisation revealed a fully consolidated, pore-free build with fine equiaxed grains and partial dynamic recrystallisation (DRX). The average grain size decreased from 13.4 µm near the substrate to 9.7 µm at the top, reflecting the variation in cumulative thermal exposure along the build height. A homogeneous hardness distribution (21.2 HV) and smooth interlayer interfaces were observed. Tensile tests in the travel direction yielded an ultimate tensile strength of approximately 85 MPa and an elongation exceeding 60%, while high-cycle fatigue tests demonstrated a fatigue strength of about 30 MPa at 2×106cycles ( R=0.1) with ductile fracture features. The results confirm that W-FSAM enables the production of fine-grained, defect-free CP-Al structures whose mechanical properties, in terms of strength and ductility, exceed those of the reference material. Thus, W-FSAM represents a promising solid-state additive manufacturing route for the production of high-performance CP-Al components.Item Open Access Oxygen diffusion in a functionally graded material with cracks using a novel probability distribution for varying parameters(2025) Lammert, Robert; Petrova, Vera E.; Weihe, StefanThe elastic modulus profile of a functionally graded material (FGM) is additionally affected by oxygen diffusion. In contrast to earlier work, the reacting material is decreasing in concentration toward the oxygen‐exposed surface, so the rule used to model the elastic modulus is adjusted accordingly. Furthermore, the original distribution function is replaced by a smoother one with similar properties. The resulting oxidation model, including an appropriate fracture toughness rule, is used to calculate critical loads for a set of three parallel edge cracks in the half plane. These loads are then checked against the model without oxygen diffusion.Item Open Access Method for determining the contact and bulk resistance of aluminum alloys in the initial state for resistance spot welding(2025) Fezer, Andreas; Weihe, Stefan; Werz, MartinIn resistance spot welding (RSW), the total electrical resistance (dynamic resistance) as the sum of bulk and contact resistance is a key variable. Both of these respective resistances influence the welding result, but the exact ratio to the total resistance of a real existing sheet is not known. Due to the high scatter in the RSW of aluminum alloys compared to steel, it is of interest to be able to explicitly determine the individual resistance components in order to gain a better understanding of the relationship between the initial state and the lower reproducibility of aluminum welding in the future. So far, only the total resistance and the bulk resistance could be determined experimentally. Due to the different sample shapes, it was not possible to consistently determine the contact resistance from the measurements. In order to realize this, a method was developed that contains the following innovations with the aid of simulation: determination of the absolute bulk resistance at room temperature (RT), determination of the absolute contact resistance at RT and determination of the ratio of bulk and contact resistance. This method makes it possible to compare the resistances of the bulk material and the surface in the initial state quantitatively. This now allows the comparison of batches regarding the surface resistance, especially for welding processes. For the aluminum sheets (EN AW-5182-O, EN AW-6014-T4) investigated, the method showed that the contact resistance dominates and the bulk resistance is less than 20%. These data can also be used to make predictions about the weldability of the alloy using artificial intelligence (AI). If experimental data are available, the method can also be applied to higher temperatures.Item Open Access Influence of stirring pin geometry on weld appearance and microstructure in wire-based friction-stir additive manufacturing of EN AW-6063 aluminium(2025) Donaubauer, Stefan; Weihe, Stefan; Werz, MartinAdditive manufacturing of metal components is predominantly based on fusion-welding processes involving melting and solidification. However, processing high-strength aluminium alloys presents challenges, including reduced mechanical properties and increased susceptibility to hot cracking. To address these issues, alternative solid-state processing methods for aluminium are being explored worldwide. One such method is wire-based friction-stir additive manufacturing, which builds on the principles of friction-stir welding. This study focused on assessing a range of pin tool designs to promote improved mixing between the filler material and substrate. The best results were achieved using a two-stirring-probe configuration, which was then employed to fabricate a multilayer wall made of EN AW-6063 aluminium alloy. The resulting structure showed significant grain refinement, with the deposited layers having an average grain size approximately four times smaller than that of the substrate, indicating dynamic recrystallisation. Tensile testing of the intermediate layer revealed a strength of 147 MPa and 10% elongation, corresponding to 77% of the filler wire strength. These findings highlight the potential of the W-FSAM process for producing near-net-shape, high-quality lightweight metal components with refined microstructures and reliable mechanical performance.Item Open Access Residual stress formation mechanisms in laser powder bed fusion : a numerical evaluation(2023) Kaess, Moritz; Werz, Martin; Weihe, StefanAdditive manufacturing methods, such as the laser powder bed fusion, do not need any special tool or casting mold. This enables the fast realization of complex and individual geometries with integrated functions. However, the local heat input during the manufacturing process often leads to residual stresses and distortion. This in turn causes poor quality, scrap parts or can even terminate a job prematurely if the powder recoating mechanism collides with a distorted part during the process. This study investigates the generation mechanisms of residual stresses and distortion during laser powder bed fusion (LPBF) of stainless steel 316L in order to reduce these effects and thus contribute to improved process safety and efficiency. Therefore, numerical investigations with a finite element model on the scale of a few melt tracks and layers serve to develop a detailed understanding of the mechanisms during production. The work includes an investigation of the build plate temperature, the laser power and speed and the layer thickness. The results show a strong dependency on the build plate preheating and energy per unit length. A higher build plate temperature and a reduction of the energy per unit length both lead to lower residual stresses.Item Open Access A short-time approach for fatigue life evaluation of AISI 347 steel for nuclear power energy applications(2021) Bill, Tobias; Acosta, Ruth; Boller, Christian; Donnerbauer, Kai; Lücker, Lukas; Walther, Frank; Heckmann, Klaus; Sievers, Jürgen; Schopf, Tim; Weihe, Stefan; Starke, PeterAISI 347 austenitic steel is, as an example, used in nuclear energy piping systems. Piping filled with superheated steam or cooled water is particularly exposed to high stresses, whereupon local material properties in the pipes can change significantly, especially in the case of additional corrosive influences, leading to aging of the material. In the absence of appropriate information, such local material property variations are currently covered rather blanketly by safety factors set during the design of those components. An increase in qualified information could improve the assessment of the condition of such aged components. As part of the collaborative project “Microstructure-based assessment of the maximum service life of core materials and components subjected to corrosion and fatigue (MiBaLeB)”, the short-time procedure, StrainLife, was developed and validated by several fatigue tests. With this procedure, a complete S-N curve of a material can be determined on the basis of three fatigue tests only, which reduces the effort compared to a conventional approach significantly and is thus ideal for assessing the condition of aged material, where the material is often rare, and a cost-effective answer is often very needed. The procedure described is not just limited to traditional parameters, such as stress and strain, considered in destructive testing but rather extends into parameters derived from non-destructive testing, which may allow further insight into what may be happening within a material’s microstructure. To evaluate the non-destructive quantities measured within the StrainLife procedure and to correlate them with the aging process in a material, several fatigue tests were performed on unnotched and notched specimens under cyclic loading at room and elevated temperatures, as well as under various media conditions, such as distilled water and reactor pressure vessel boiling water (BWR) conditions.Item Open Access Effects of process parameters, sheet thickness and adhesive on spot diameter during resistance spot welding of aluminum alloys EN AW-5182 and EN AW-6005(2026) Fezer, Andreas; Weihe, Stefan; Werz, MartinResistance spot welding (RSW) is one of the dominant joining processes in body-in-white manufacturing within the automotive industry, while the use of aluminum alloys continues to increase. This study investigates the influence of key process parameters on the spot diameter in RSW of the aluminum alloys EN AW-5182 (AL5-STD) and EN AW-6005 (AL6-HDI). Experiments were performed using industry-standard robotic welding equipment in a partially automated welding cell. Welding current, electrode force, sheet thickness (1-3 mm), and adhesive application were systematically varied. The welded joints were evaluated by destructive testing to determine spot diameter. The results show that higher welding currents increase the spot diameter for both alloys, while higher electrode forces decrease it. EN AW-5182 exhibited a high tendency toward expulsion, whereas no expulsions occurred for EN AW-6005 under identical conditions. The application of the structural adhesive BETAMATE™ 1640 consistently increased the spot diameter.