Materials Science Forum Vols. 790-791

Abstract: Shelf-lives of surfactant containing water SiO₂ suspensions were investigated by observing the loss of foamability with time and by making various measurements on the compounds used. Main purposes were to discover the reason of the strong foamability decrease with time and to separate the gravity driven effects. These questions arose in the frame of FOCUS - Foam Casting and Utilisation in Space microgravity experiment preparations (ID: SURE AO-019/PECS 98045, EC contract no: RITA-CT-2006-026069). Foams were prepared using special foaming cartridges, by gas introduction after loading the suspension into the porous structure of the foam generator (FG). Freshly infiltrated FG-s showed superior foamabilities, but this feature was lost after a few days of storage. Several possible chemical or physical reasons were investigated experimentally, including the direction of storage position, composition change, storage temperature, etc. Investigation of gravity-driven sedimentation and creaming tendencies were also examined using dynamic light scattering (DLS) method. These effects found to have significant impact on foamability.

Abstract: In this paper, the thermodynamic basis of ESTPHAD method is briefly shown and the development of the whole ternary calculation can be followed: the first step is the calculation of liquidus and solidus temperatures of the binary phase diagrams (Al-Cu and Al-Fe), the second step is the estimation of the liquidus and solidus surfaces (Al-Cu-Fe system), and the last step is the calculation of liquidus and solidus slopes by using the derivative function of the calculated ESTPHAD equations.

Abstract: This paper includes the binary and ternary liquidus temperature calculations of Sn-Bi-Cd system. The calculation was performed in cases of the surfaces of Sn, Bi and Cd phases too. First of all the liquidus curves were calculated in the binary systems (Bi phase in Bi-Cd and Bi-Sn systems, Sn phase in Sn-Cd and Sn-Bi systems, Cd phase in Cd-Sn and Cd-Bi systems). By using the calculated coefficients of the binary phase diagrams and the data from the digitalized ternary phase diagram, the liquidus temperature of Sn, Bi and the Cd phases were calculated. Finally the eutectic point of the binary liquidus curves and the eutectic valley of the Sn and the Bi surfaces were calculated by means of an iteration method.

Abstract: Whiskers formed on the lead-free tin surfaces pose a serious risk to small electronic devices causing a short circuit and leading to the component/device failure. The present research was focused on the investigation of tin whisker formation on a motor control unit sockets made of tin coated copper, applying to the specimen mechanical load alone or together with heat treatment/electric current. Scanning Electron Microscopic (SEM) imaging was applied in order to study the microstructure of tin whiskers obtained, their length and number at the boundary of each imprint. If the mechanical stress increases from 1000 to 5000 MPa, the average number of whiskers and possible formation spots also increase from 570 to 1300. The length of whiskers varied from 3 μm after 0.5 h to 5.5 μm after 3 hours of exposition. It has been found that heat treatment at 150°C for 1 hour significantly reduces the number of whiskers (on average 6 times as few) formed. Therefore, the threat of failure of the electronic equipment is reduced.

Abstract: There is a continuously developing need for benchmarking of solidification simulations - from the theoretical [1] as well as from the applied [2] points of view. The history of related benchmarking shows differences of the results between different numerical methods, and differences in comparison with the experiments when solving even quite simple solidification situations. The present benchmark test proposes macrosegregation [3] upgrades to the verification benchmark for continuous casting of steel, first presented in [2]. The paper represents guidelines for the presentation of the numerical method, discretisation and results and shows a reasonable comparison between a commercial finite volume based code and our in-house developed meshless method based code.

Abstract: Twin-roll casting (TRC) is an established route to produce sheets of aluminium alloys. Despite enormous potential, severe centreline segregation arising during casting limits the extension of alloy range suitable for commercial applications. At given casting conditions, the centreline segregation in conventional twin-roll-cast strip increases as the solute content in the alloy (and hence freezing range) increases. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed. Enhanced nucleation by melt conditioning favours the advance of an equiaxed solidification front during solidification. It has been demonstrated that the MC-TRC process is capable of producing high quality Al-alloy strips with minimal centreline segregation. In this paper we use binary Al-Mg as a model alloy to investigate the composition limit (the maximum Mg concentration) for the MC-TRC strip without centreline segregation.

Abstract: Twin roll casting (TRC) has been demonstrated to be a process capable of producing Mg sheets at a significantly reduced cost. However, the quality of the Mg sheets produced by the TRC process is limited due to the formation of coarse columnar grains and severe centreline segregation, which reduces both the strength and ductility of Mg-alloys. In this paper, melt conditioned (MC) has been employed prior to twin roll casting (TRC) of AZ31 alloy to understand its effect on the solidification mechanism. For comparison, AZ31 alloy strips were also produced through conventional TRC process. The results showed that the solidification mechanism can be completely altered by introducing MC in TRC process.

Abstract: A complex mathematical model characterizing the centerline segregation level in the mid region of continuously cast slabs was developed. The basic heat transfer and solidification model connected to the semi-empirical liquid feeding model (LMI - Liquid Motion Intensity model) gives the possibility to estimate the centerline segregation parameters of slab cast under industrial circumstances. Solid shell deformation changes the volume of the space available for the liquid inside the slab and hereby also changes the conditions of liquid supply.

Abstract: There is a relationship between pores and inclusions. As hypothesis goes, inclusions have an affinity to gather inside the pores and form clusters. Focus of this paper is how solidified dendritic structure affects the pressure field in the liquid and pore precipitation in austenitic stainless steel. Steel sample is a continuously cast bloom. Temperature profile and width of the mushy zone of the strand is modeled according to a constant temperature at the strands surface. Thermal analysis has been performed with differential thermal analysis (DTA) and differential scanning calorimeter (DSC). Dendrite arm spacing (DAS) is measured with light optical microscopy (LOM) and scanning electron microscopy (SEM). DAS is represented as the weight average of the distance between parallel sets of primary dendrite stems. Pressure field is calculated based on Darcys law. Pore formation is described through segregation of the gas components and pressure field in the liquid.

Abstract: In the frame of ESA-MAP (Microgravity Application Promotion) project entitled XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions), a microgravity (μg) experiment in the XRMON-GF (Gradient Furnace) setup was successfully launched in 2012 on board MASER 12 sounding rocket. During this experiment, in situ and real time observations of the formation of the solidification microstructures in diffusive conditions were carried out for the first time by using X-ray radiography. In addition, two reference experiments with the same control parameters but in ground-based conditions were performed to enable us a direct comparison with the μg experiment and therefore to enlighten the effects of gravity upon microstructure formation. This communication reports on fragmentation phenomenon observed during those experiments. For 1g upward solidification, fragmentations mainly take place in the upper part of the mushy zone. After their detachments, dendrite fragments are carried away by buoyancy force in the bulk liquid where they are gradually remelted. For μg experiment and horizontal solidification, this type of fragmentation is not observed. However, a great number of fragmentations are surprisingly revealed by in situ observation in the deep part of the mushy zone, when the liquid fraction is very small. Moreover, as soon as they are detached, the dendrite fragments move toward the cold part of the mushy zone, even in the case of μg experiment. The observations suggest that sample shrinkage may be at the origin of this fragment motion.