Papers by Keyword: Microstructure

Abstract: A cold roll-bonding (CRB) process is applied to fabricate a multi-layer Al sheet using AA5052 and AA6061 alloys. The rolling is performed for four-layer sheets in which AA5052 and AA6061 sheets are stacked alternately after surface treatments such as degreasing and wire brushing. The 4-layer sheets with a thickness of 8 mm were roll-bonded to 2 mm by rolling at total reduction of 75%. The as roll-bonded Al sheets are then processed by natural aging (T4) and artificial aging (T6) treatments. T4 and T6 treated specimens showed a typical recrystallization structure over all regions of AA5052 and AA6061. The average grain diameter of T4 and T6 specimens was about 15 μm, which is almost the same. In addition, the Al sheet showed a heterogeneous hardness distribution in thickness direction. After the aging treatments of T4 and T6, the strength rather decreased and the elongation increased. It is found that new multilayer Al sheets made of AA6061 and AA5052 alloys, exhibiting various mechanical properties can be fabricated through the CRB and subsequent aging treatments.

Abstract: This article aims to review the recent advances in the laser powder bed fusion process (L-PBF) of H13 tool steel for the dies used in the high pressure die casting (HPDC) applications. The effect of processing variables is briefly reviewed for the evolution of microstructure (phase transformations, as-built microstructure and carbides precipitation), mechanical properties, and defects. The second part of the review is focused on conformal cooling applications to HPDC dies, which is critical for die life and productivity. Achieving better microstructure without defects, understanding the role of processing variables in L-PBF and their interdependencies remains the key challenge for the as-built part, while the benefits of preheating and post-heat treatments are evident. Significant benefits are realized in the applications of die inserts favoring lower die surface temperature, reduced cycle time and lubrication, and thermo-mechanical stresses. In addition, L-PBF also plays a key role in die remanufacturing where significant benefits are achieved in terms of materials savings and improved performance compared to traditional repair technologies. Overall, L-PBF offers a transformative pathway for high-performance HPDC dies; however, most investigations are trial-based. Long-term studies are needed for performance assessment and establishing failure mechanisms in production environments.

Abstract: High-strength and recycling tolerable aluminum alloys make a significant contribution to weight reduction in modern lightweight construction. The advantages of aluminum alloys in terms of their low density combined with high strength can be significantly improved by the alloy composition. In contrast to the conventionally established process route, high-magnesium alloys can be produced using the twin-roll strip casting process. This allows additional process steps such as hot rolling and annealing to be drastically reduced in the economical production of near-net-shape strips, saving emissions and energy consumption. The strip casting process has already been applied to numerous aluminum alloys and enables their production, although the understanding of advanced alloys in this area is not yet fully understood because of its limited production in industry-related research due to the complexity of the process. However, transferring the high strength generated during rapid solidification into usable sheet performance remains challenging, especially at elevated Mg contents, where segregation, casting-related defects, and solute-affected recrystallization can limit ductility and processability. This study investigates the potential of a high-magnesium aluminum alloy produced by vertical strip casting. The properties of the alloy are correlated with the microstructural and mechanical characteristics and developed on the basis of an industrial reference alloy. For this purpose, an EN AW 5182 and an AlMg10 alloy were processed. The results show that high-magnesium alloys can be produced and processed using strip casting. In terms of the high-magnesium alloy, improved results can be achieved compared to the industrial EN AW 5182 alloy. Key findings: The strength of high-magnesium alloy is significantly above those of the EN-AW 5182 after strip casting enabling nearly 600 N/mm² tensile strength, but the final properties are below this potentially possible characteristic after strip casting, presumably due to non-ideal recrystallization and an insufficiently adapted process route including rolling and annealing parameters.

Abstract: Understanding the relationships between microstructure and (mechanical) properties is inevitable for the design of modern structural metallic materials. A crucial property for most high-strength steels is ductile damage tolerance, since ductile damage can accumulate during cold forming, which either leads to failure in the forming process or subsequently affects the performance. Structure-property relations are often investigated using numerical methods, e.g. crystal plasticity (CP) modeling with representative volume elements (RVE). In a previous study, CP-simulations on 3D-RVE were coupled with surrogate modeling techniques performing a variance-based sensitivity analysis. This analysis enables quantitative descriptions of the relationships between microstructure features with the damage tolerance, quantified by individual indicators for individual damage mechanisms. To investigate the effect of the material model and the corresponding phase properties, 500 sRVE simulations were carried out with different CPparameter sets and the damage tolerance is investigated. All sets stem from the same DP800 but were calibrated with different approaches. Surrogate models were trained on the simulative database to calculate Sobol Indices (SI), which are a measure of how strong damage tolerance is affected by a particular microstructure feature. The SI are compared for the individual material models and damage indicators. The structure-property quantification is heavily influenced by the different material models, resulting in different values for the SI and a different order for the individual microstructure features. The main factor for the pronounced differences is the differently evolving mechanical phase contrast between ferrite and martensite.

Abstract: Predicting the microstructural state during manufacturing is critical, as it directly governs the material's final mechanical properties. Accurate prediction of microstructure evolution in multi-stage industrial hot deformation processes, such as rolling, is limited by the lack of experimental data at intermediate stages, where direct measurement is impractical. To address this, an integrated methodology combining finite element (FE) simulation in QForm UK® software, physical simulation using the Thermo-Mechanical Treatment Simulator (TMTS), and artificial intelligence (AI) is proposed and investigated. The methodology is demonstrated for the 11-pass hot rolling of a 41Cr4 steel bar. Thermomechanical loading histories from an FE model of the industrial process were used to design and simulate a targeted TMTS experiment, generating a synthetic dataset via an analytical JMAK model that combines multiple recrystallisation mechanisms. This data was used to train a recurrent neural network (RNN) with an augmented physics-informed Long Short-Term Memory (LSTM) cell to predict the totally recrystallised fraction (RX) solely from loading history data. The AI model achieved high accuracy when validated within the TMTS simulation domain, successfully capturing different recrystallisation regimes. Implementation within commercial FE software enabled direct prediction in the rolling process simulation, yielding promising predictive capability, particularly in regions with thermal histories similar to the training data, highlighting the critical importance of training data diversity. This work establishes a proof of concept for a novel calibration methodology, where targeted physical simulation bridges the gap between industrial process complexity and data-driven AI model development, offering a practical solution for modelling scenarios where traditional experimental calibration is infeasible.

Abstract: Friction stir welding (FSW) is widely used to bond metals and polymers to create large structures from standardized geometric components. However, FSW has found more limited use in traditional continuous fiber reinforced composites due to the risk of damage to the reinforcing fibers, including fiber misorientation and fragmentation. The process is more amenable to discontinuous fiber composites, which often take the form of injection molding compounds reinforced with short, milled fibers, for which the mechanical performance is significantly degraded. An emerging class of materials, recycled carbon fiber nonwovens, contain long discontinuous fibers that imbue their composites with mechanical performance that bridges the gap between injection molding compounds and continuous fiber laminates, while also decreasing the energy footprint of the materials. This work evaluates this class of materials, alongside injection molding compounds, to develop new insights into the bonding of composite structures using friction stir welding. The bridging effect of discontinuous fibers across the bondline is evaluated using optical microscopy to link processing conditions and fiber length to the resulting performance. Fiber migration was observed in the weld area, though mechanical interlocking was the primary mechanism of bonding in the weld zone. While samples failed at a fraction of the neat materials nominal strength, increased fiber length was found to have a beneficial effect on the apparent tensile and lap shear strength on welds in this study. The new insights gained represent an important step towards the adoption of the FSW process as a means of rapidly manufacturing large composite structures made of carbon fiber/PPS or other fiber-reinforced polymers to enable deployable structures from standardized geometric components.

Abstract: This study provides a comprehensive investigation into the effects of different scanning parameter combinations—specifically scanning speed and hatch distance—on the material properties of IN939 fabricated using the powder bed fusion-laser beam (PBF-LB) process under a constant volumetric energy density (VED). Despite the fixed VED, the fabricated samples experienced different thermal cycles, resulting in distinct microstructural features and corresponding variations in material performance. In-situ infrared monitoring indicated that the sample with the narrowest hatch distance and highest scanning speed (Sample 1) reached the highest normalized temperatures with intense heat accumulation, whereas wider hatch distances (Sample 3) promoted lower and more stable temperature distributions. The results revealed that the intermediate parameter set (Sample 2) achieved the highest relative density (99.29%) and the lowest surface roughness. In contrast, both the narrowest and widest hatch spacing combinations promoted increased porosity, primarily consisting of lack-of-fusion (LoF) and gas pores. Electron backscatter diffraction (EBSD) analysis showed that the area-weighted average grain size increased from 29.5 µm to 36.7 µm as the hatch distance increased. Texture analysis indicated generally weak crystallographic texture development, with only slight intensification of <001>//BD and <111>//BD components, attributed to the 67^o rotation strategy. Furthermore, the microhardness values demonstrated negligible variation across the samples, ranging from 356.7 ± 14.3 HV1 to 360.1 ± 10.5 HV1. This limited variation indicates that the strengthening behavior was predominantly governed by the combined influence of defect density and matrix–defect interactions, rather than being directly correlated with grain size.

Abstract: This paper presents a metallographic and fractographic study of AISI 304 austenitic stainless steel subjected to mechanical loading in the sensitized condition. Static three-point bending tests and impact tests were carried out to evaluate how sensitization affects the mechanical response and fracture behaviour of AISI 304. The study compares the initial state of the material with its condition after sensitization at 700 °C for 10 h, with emphasis on changes in plastic deformation and fracture mechanisms. Microstructural evaluation was performed using light microscopy, while Vickers microhardness measurements provided insight into local mechanical changes. Fractographic analysis using scanning electron microscopy revealed differences in fracture surface morphology. Results demonstrate a decrease in microhardness, reduced impact energy, and noticeable differences in fracture morphology following the sensitization treatment, indicating that the heat treatment influences both the mechanical response and failure behaviour of AISI 304.

Abstract: Face-centered cubic (FCC) medium-entropy alloys (MEAs) are known for their excellent ductility and fracture toughness, but they suffer from relatively low mechanical strength. Alloying elements are added in FCC MEA matrix to promote the formation of hard secondary phase or intermetallic compounds that improve the mechanical performance of the alloys. In this study, the effect of chromium (Cr) and niobium (Nb) additions on the microstructural and corrosion characteristics of the CoNiV MEA matrix was investigated. A scanning electron microscope coupled with energy dispersive spectroscopy was used to analyse the microstructure and composition of the developed alloys. The corrosion properties of the alloys were evaluated using linear polarization. The alloys exhibited a dendritic microstructure with the presence of secondary phases, which is consistent with slow cooling associated with arc melting and the presence of elements with large atomic radii that upset the crystal lattice. Alloy containing Cr possessed better anti-corrosive properties than its Nb counterpart, signalling formation of a more stable Cr₂O₃ passive film. This layer creates a boundary between the corrosive medium and the alloy substrate to prevent further interaction.

Abstract: Austenitic stainless steels are characterised by excellent corrosion resistance and good formability, but their low hardness and fatigue life are limitations in demanding applications. The aim of this study was to analyze the effect of solution annealing and plasma nitriding on the microstructure, hardness and fatigue properties of AISI 304 steel. The experimental material was examined in three states: initial, after solution annealing and after plasma nitriding. Solution annealing resulted in the removal of deformation martensite, giving a homogeneous austenitic structure with a decrease in hardness. On the contrary, plasma nitriding produced a hard nitride layer (1291 HV0.01), while no martensite retransformation took place. The results of the fatigue tests showed that the specimens after plasma nitriding reached the highest fatigue limit (878 MPa), while the specimens in the initial condition had the highest number of cycles to fracture. Fractographic analysis revealed typical fatigue failure characteristics in all conditions. The study highlights the possibility of optimising the fatigue properties of austenitic steels through an appropriate combination of thermal and chemical-heat treatments.