Materials Science Forum Vol. 1174

Abstract: Recently, light-weight and energy-saving requirements for the automobile industry are extremely important in order to protect the environment by a reduction of the emission of CO₂. Hence, high-strength steel (AHSS), even ultrahigh strength steels with tensile strength larger than 1GPa is used. Among AHSS, cold-rolled martensitic steels have attracted much attention due to their superior strength to improve crashworthiness. In this research, the influence of different annealing treatments, especially the auto-tempering, on the phase transformation further affecting the mechanical properties and microstructure was investigated. The result shows that the level of auto-tempering and strength in martensitic steel is dominated by the quenching/auto-tempering temperature. Furthermore, the auto-tempering carbides should be cementite which is fine enough to improve yield strength. The suitable chemical composition combined with auto-tempering method has been implemented to develop cold-rolled martensitic steels with a tensile strength of exceeding 1300MPa. These developed martensitic steels can meet the requirements of bumper reinforcement which has applied in a variety of automobiles.

Abstract: This study investigates how feedstock composition affects the performance of 17-4 PH stainless steel (SS) parts produced by fused deposition modeling (FDM). Feedstocks consisted of 91–93 wt.% SS, with varying amounts of polyethylene glycol (PEG), paraffin wax (PW), and stearic acid (SA). Rheological analysis revealed shear-thinning behavior, with viscosity predominantly affected by PEG content at lower metal concentrations and increasingly governed by metal loading at higher concentrations. Thermal debinding confirmed that feedstocks with at least 91 wt.% metal and 3 wt.% PEG maintained structural integrity. Among tested formulations, 93 wt.% SS provided the best print quality, achieving 4.31 g/cm³ density, 1154 MPa flexural modulus, and 5.8 MPa flexural strength. Overall, the results highlight the importance of balancing metal and binder content for optimal FDM outcomes.

Abstract: Laser surface hardening process is an advanced technology used to enhance metal surface properties through phase transformation. In recent decades, it has gained significant attention for its efficiency and precision. The glassmaking industry has shown interest in this technology as an alternative to laser cladding process, which causes environmental risks and machining challenges. This study aims to enhance AISI 431 stainless steel mold hardness without molten powder deposition. Surface hardening of AISI 431 stainless steel was carried out using continuous wave diode (940-1020 nm) and fiber (1070 nm) 4 kW laser sources. The influence of process parameters such as laser power (500-3040 W), scanning speed (4.5-8.5 mm/s) and number of laser head passes (1-4) were investigated on the hardened zone’s geometry, microhardness, and microstructure. Microstructural analysis was conducted using optical microscopy and scanning electron microscopy (SEM). The current findings revealed a significant increase in microhardness due to martensite formation, which decreased with depth as martensite content reduced. Results showed that beyond a certain limit, the hardness reaches a maximum value that, regardless of the parameters, can no longer increase. This mainly depends on several factors, such as material properties, processing parameters, and cooling conditions. Achieving a target hardness is possible by adjusting the number of passes, while the desired depth will primarily depend on the power and scanning speed.

Abstract: In order to find optimal intercritical annealing treatment (IAT) temperature and alloy composition for simple process route of hot rolling followed by single-step IAT, the effects of IAT on three different medium-manganese steels were investigated. Nominal chemical compositions in wt.% were 1) 6Mn–0.3C, 2) 6Mn–0.4C and 3) 8Mn–0.4C(–2Al–1Si–0.05Nb–Fe). Materials were laboratory hot rolled to a thickness of 6 mm, and IAT was simulated with Gleeble 3800 and Linseis DIL L78 DQT / RITA dilatometer. Different variations of IAT included annealing temperatures of 650 °C, 675 °C, 700 °C and 725 °C, with holding time of 10 minutes, heating rate of 50 °C/s and cooling rate of 10 °C/s. Quasi-static tensile tests were performed parallel to rolling direction. XRD and EBSD phase mappings were performed to assess IAT temperatures effect on volume fraction of retained austenite. Most promising mechanical properties were obtained with material 6Mn–0.4C annealed at 700 °C. Product of strength and elongation well exceeded 40 000 MPa% for above-mentioned IAT-material variation, being distinguishable higher compared to other variations. However, investigated materials, especially 6Mn–0.4C, seems to be very sensitive to IAT temperature, which could inflict some challenges in industrial scale production. Also, all materials experienced some level of serrations during tensile testing, which is frequently encountered phenomena with medium-manganese steels. Further research is required, to evaluate the role of austenite stability on mechanical behavior of these materials and to determine effects of heating and cooling rates.

Abstract: AZ91 magnesium injection molding is suitable for manufacturing complex-shaped electronic product frames or thin plates. However, the strengthening effect of the Mg₁₇Al₁₂ precipitate in AZ91 is limited, and it tends to dissolve during heat treatment, leading to a lack of particles that can pin grain boundaries and prevent grain growth. To address these challenges, the LAZ561Ca alloy has been developed, offering a reduced density (83% of AZ91), AlLi nanoprecipitates with strong strengthening capabilities, and thermally stable Ca-bearing intermetallics that effectively pin grain boundaries, maintaining a fine-grained structure (~8 μm) even after heat treatment. Experimental results demonstrate that AZ91 undergoes abnormal grain growth after solution treatment at 400°C due to a significant reduction in Zener pinning forces. In contrast, the LAZ561Ca alloy, with stable Al₂Ca precipitates, resists such growth during two-stage heat treatment at 370°C – 400°C. Through the coupling between Thermo-Calc and MICRESS software, multiphase field modeling reasonably reproduced the microstructure evolution during injection molding and heat treatment processes, highlighting its value in establishing digital physical metallurgy models. This study reveals the microstructural mechanisms of magnesium alloys, confirming the critical role of Ca-bearing precipitates in grain growth suppression. It provides a foundation for further optimization of alloy compositions and heat treatment conditions, paving the way for advanced magnesium alloys with enhanced performance in injection molding applications.

Abstract: The effect of thermomechanical controlled processing (TMCP) strain and finishing temperature on the microstructure of as-rolled and air-cooled 50mm thick Nb-Ti-V-Ni microalloyed steel plate was investigated through laboratory simulations, incorporating variations in reheating, roughing and finishing practices. Laboratory simulations produced microstructures similar to those observed in industrial rolling owing to comparable total strain in the TMCP region. Larger total strains in the TMCP region promoted sub-grain formation and increased nucleation site density, leading to grain refinement. Recrystallisation was completely suppressed at the commencement of finishing in all TMCP schedules due to sufficiently lower starting temperatures. The extent of recrystallisation during finishing depended on the finishing temperature: i) partial softening after finishing above 910°C and ii) complete recrystallisation below 910°C due to substantial accumulated strain. Finishing below 910°C produced finer polygonal ferrite and pearlite microstructures. Microstructure and mechanical properties were fairly consistent when the finishing temperature was between 925 and 950°C. However, the sub-zero impact toughness can be significantly improved by employing lower finish temperatures or applying larger total TMCP strains. Interrupted accelerated cooling at 5°C/s after finishing significantly refined the microstructure.

Abstract: Laser powder bed fusion (LBPF) is currently the most mature metal additive manufacturing (AM) technology. While it does not have the same flexibility as directed energy deposition techniques to produce compositional gradients, LPBF can still be used to generate bimetallic parts by depositing one metal on a build plate made of another. Here, we print combinations of Ti-6Al-4V with Ta and characterize defects that occur at the interface. We use thermodynamic modeling to explain the formation of keyhole porosity and solidification cracks when Ta is built on a Ti baseplate, and the lack of defects when the materials are reversed. By understanding the mechanisms that lead to defect formation, the methodology demonstrated here can be applied to other material systems to efficiently design bimetallic LPBF processes.

Abstract: This study investigates the bonding properties of transient liquid phase diffusion bonding using Cu/Sn electroplated films. A Cu substrate was electroplated with Cu and Sn films, followed by TLP bonding with a Ni substrate at 280°C under air atmospheric conditions without bonding pressure. Bonding times of 1, 3, and 30 min were employed to evaluate the effect of bonding duration on interfacial microstructure and shear strength. Cross-sectional microstructural analysis using EPMA revealed the formation of a Cu–Ni–Sn reaction layer at the bonded interface, with the thickness of this layer increasing as bonding time increased. Voids were observed at all bonding times, particularly at 30 min, where extensive void formation led to incomplete bonding. Shear test showed that shorter bonding times yielded higher average strengths, while longer bonding times resulted in a reduction due to void-induced degradation. Fracture surface observations confirmed that failure occurred within both the Sn and reaction layer, regardless of bonding time.