Materials Science Forum Vol. 1016

Abstract: Press-hardening is an intensively developing forming technology which is mainly used for the production of car body parts. Because it is a hot forming technology, small forming forces can be utilized and, due to the lower spring-back effect, more accurate products are achieved. In car bodies, materials with high energy absorption and a sufficient hardening coefficient are mainly used in impacted parts. One of these materials is TRIP multiphase steels with different chemical composition. In these steels, it is possible to achieve an ultimate strength up to 1000 MPa with the ductility of 20-30%. In order to achieve the desired properties, it is necessary to select a suitable heat treatment that allows to achieve a multiphase structure. Phase transformations and mechanical properties are influenced by the use of suitable alloying elements. Three low-alloy, multiphase TRIP steels with different chemical compositions with a carbon content of 0.2% were chosen for the experimental program. The first steel was alloyed only with manganese and silicon, in the second niobium was added, and in the third the influence of chromium on increase of hardenability and strength was investigated. Press-hardening was performed in a heated forming tool. To describe the effect of the cooling rate, the forming was carried out in a tool at room temperature and after preheating to 425°C. The influence of holding time in the tool at 425°C to support the formation of bainite and retained austenite stabilization was also investigated. Mixed ferritic-bainitic-martensitic structures with some retained austenite content were obtained.

Abstract: A 2-D finite volume Computational Fluid Dynamic (CFD) model, using Ansys Fluent vR.1 of a vertically oriented upwards continuous casting (VUCC), was investigated for 8 mm, oxygen free copper (OFCu). The simulations enabled the mapping of the cast OFCu solidification front (SF) interface from liquid to solid. Optimisation of the simulation parameters were investigated which included mesh size and the Ansys specific ‘mushy zone’ constant (A_mush), which is used to account for fluid flow dampening at SF within the model. Observations of the SF, the change in fluid volume in the die, the simulation convergence and the total simulation time, revealed that the optimised casting parameters were for mesh size 1×10^-4 m and A_mush 10⁶ kg/m³s. These parameters were compared with the cast rod and highlighted qualitatively the relationship between grain growth direction and SF position during a casting pulse cycle.

Abstract: The aim of this work is to study the effect cold forming rate (CFR) on the mechanical properties and microstructure of a conventional TMCP and a direct-quenched steel in 420 MPa strength level. The microstructure was characterized using FESEM-EBSD. Tensile properties and Charpy-V impact toughness were determined. As the CFR increased, the yield and tensile strength raised quite linearly with both steels. Yield strength values increased from 450 MPa (as-rolled material) to 700 MPa (25 % CFR). However, tensile strength increased less compared to yield strength. Uniform elongation decreased linearly till about 10 % CFR and total elongation till about 15 – 20 % CFR. The impact values decreased quite linearly in -40 °C and -60 °C test temperature when the cold forming rate increased. In longitudinal direction (L-T) the impact values were at high level at -40 °C and -60 °C with both steels with all CFR. In transverse direction (T-L) the impact results were lower. Impact energies were enhanced by direct quenching compared to conventional steel in every CFR stage. EBSD results showed no major difference between steels in the grain sizes in generally. However, cold forming decreased the grain size and increased low-angle grain boundaries in correlation with increasing CFR. Small size of the coarsest grains (d_90%) usually indicate better toughness, however in this case the impact values were decreased even with smaller grain size as cold deformation occurs. On the other hand, the strength level increased with forming rate. Therefore, a brief discussion of the microstructural features controlling the impact toughness is given.

Abstract: Low alloy TRIP steel is expected to be applied to automobile bodies because of its high strength, high ductility, and excellent impact properties and press formability. It has been reported that the low alloy TRIP steel of hydrogen embrittlement resistance is improved by utilizing the hydrogen storage characteristics of highly stable retained austenite. Therefore, for the purpose of increasing the volume fraction of retained austenite, it was produced at various cooling rates below the martensite transformation start temperature. As a result, the volume fraction of retained austenite increased, and then the effect of hydrogen embrittlement decreased. The matrix phase and retained austenite is refined with decrees of the cooling rate. It is considered that the size and surface area of the retained austenite also affected the improvement of hydrogen embrittlement resistance.

Abstract: Friction surfacing (FS), a solid-state joining process, is a coating technology for metallic materials. Frictional and plastic deformation enable the deposition of a consumable material on a substrate. Process temperatures stay below the melting point of the consumable material and are an important factor determining the quality of the resulting deposit. The focus of the current study is the experimental analysis of the flash formation and the temperature evolution in consumable studs during FS deposition of dissimilar aluminum alloys. The main process parameters, axial force, rotational speed and travel speed, were varied while the setting of the process surrounding was kept constant. The temperature evolution for the applied process parameter combinations are investigated for the stud material via infrared camera. The results show that the choice of applied force, rotational speed and travel speed did not lead to significant changes in maximum process temperature values of the consumable stud detectable via infrared camera. However, the flash formation at the tip of the plasticized stud shows significant differences for varied process parameters. Especially reduction of travelspeed or increase in axial force led to formation of larger flashes. Since the material that is pressed out of the process zone into the flash is not deposited on the substrate, the flash formation can be linked to the material efficiency of the FS process.

Abstract: Improvement in efficiency of energy conversion requires the use of high temperature materials in thermal power plants. This has led to the development of new γ' strengthened nickel based superalloy (Haynes 282). This alloy is used for advanced ultra-supercritical (AUSC) plants which are operated under the service conditions of 760 oC temperature and 35 MPa pressure. Bead on plate gas tungsten arc welding experiments were done with optimized process parameters. Thermal cycle in heat affected zone was measured by K-type thermocouple attached to a data acquisition system. Welding simulations were carried out in simufact welding^® by using experimental parameters and thermal field was established. Base metal is characterized with γ solid solution and randomly distributed MC carbides. SEM results showed that the carbides are of MC type. The carbide precipitate distribution correlates with the segregation pattern during solidification of the weld.

Abstract: The study of Hall effect of Kondo semiconductor CeFe₂Al₁₀ is reported as a candidate of thermoelectric material used at low temperatures. Single crystals of CeFe₂Al₁₀ with orthorhombic crystal structure were grown by Al self-flux method. An anisotropy of the Hall effect is clarified by measuring Hall resistance by changing the direction of electrical current, magnetic field, and voltage respect to all the three crystal axes of orthorhombic crystal structure. The Hall effect of CeFe₂Al₁₀ has a strong anisotropy against the direction of magnetic field but weak anisotropy against the directions of current and voltage. The value of carrier concentration indicates that CeFe₂Al₁₀ is matallic, which causes a low performance as a thermoelectric material. In order to improve the value of dimensionless figure of merit, the electrons should be doped to CeFe₂Al₁₀.

Abstract: A high strength austenitic steel is expected as a structural material for cryogenic use because fcc material does not cause a cleavage fracture despite high strength. High manganese steel which is a strong candidate material of the cryogenic high strength austenitic steel was originally famous for the Hadfield steel and widely applicable in actual use. In general, an excellent cryogenic toughness of the high manganese steels is achieved by obtaining stable fcc microstructure with an adequate amount manganese which is a typical austenite former alloy. However, as addition of manganese is not effective for increasing strength, other strengthening alloying elements like carbon and chromium need to be added. In this study, an effect of alloying elements on strength and cryogenic toughness of the high manganese austenitic steel is studied.

Abstract: In recent years, metal injection molding (MIM) has been adopted as a process for manufacturing a solenoid valve which is a component of an electronic fuel injector, and PB permalloy components have been commercialized. However, Ni, an element of PB permalloy, is expensive material, making it difficult to produce the fuel injector component at a low cost. As a solution to this problem, we studied the magnetic and mechanical properties of Fe-Cr-Si alloys by MIM process. These results revealed that the magnetic and mechanical properties were improved by reducing Cr content. However, reducing Cr content is expected to reduce corrosion resistance of the alloys. In this study, Fe-Cr-Si-Mo alloy specimens containing Mo to improve corrosion resistance are manufactured by MIM process, and we investigated the magnetic and mechanical properties. These results revealed that Fe-10Cr-3Si-2Mo alloy is a material with an excellent balance between magnetic and mechanical properties.

Abstract: The development of novel light metal alloys represents an important task in the further optimization of technical materials. Multi-component systems with more than 4 metals are very promising to outperform currently existing alloys, but lack significant research in systems not dominated by transition metals to date. In this work, alloys containing the elements Al, Cu, Mg and Zn were produced using magnetron sputter deposition. A detailed structural investigation using electron microscopy provided valuable insights into the influences of different metals and their relative proportions in the alloy on material properties.