Papers by Keyword: Melting

Abstract: The fraction liquid present during semi-solid processing has a critical effect. Conventionally the process window has been defined by inspecting the liquid fraction versus temperature curve (derived from thermodynamic prediction using a thermodynamic prediction software package for example, or derived from differential scanning calorimetry results). It has been assumed that a freezing range with temperature is required for semi-solid processing to be possible. However, recently a South African group (Curle, Moller and Wilkins) has shown that it is possible to rheo-process both high-purity aluminium and a binary Al-Si eutectic alloy i.e. materials with no freezing range. This behaviour highlights the fact that it takes time for liquid to form i.e. the kinetics of melting are important. Here the liquid fraction vs time for high purity aluminium is derived from experimental results to identify the process window in terms of time rather than temperature. The time sensitivity in thixoforming or rheocasting depends on the sample mass, the heat flux and the phase transformation temperature. It is also important in determining the vulnerability to defects such as hot tears, which tend to occur particularly with the alloys which are conventionally wrought rather than cast such as the 2000 series aluminium alloys.

Abstract: Zirconium alloys have many applications in industry in services too harsh for stainless steels, nickel alloys or where a noteworthy improvement in service life may be achieved, by choosing zirconium alloys instead of other metals, such as high permeability to thermal neutrons and excellent corrosion resistance in nuclear reactor environments. Mixing alloying elements, such as niobium, molybdenum, tin, titanium and tantalum, with zirconium changes its physical and chemical properties, especially its resistance to corrosion. In this study, specimens of Zr-1.0Nb alloy were obtained by melting in a furnace with non-consumable electrodes in argon atmosphere. Different samples were prepared to ensure good homogeneity of the specimens. The melting procedure was tested several times to determine the parameters that ensure proper alloy handling. These parameters include the melting point of the alloys under pressure and the current in the furnace. Using the derived melting parameters and processing parameters, it has been obtained Zr-1.0Nb alloy specimens with appropriate homogeneity, as confirmed by auxiliary characterization techniques, such as optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. For these methods, the requirements for physicochemical properties in the nuclear sector were incorporated into the analyses.

Abstract: We have demonstrated the possibility for epitaxial regrowth of crystalline SiC by laser melt annealing. The quality of the recrystallization is analyzed by XTEM, EELS, electron diffraction and XRD. The annealing guarantees a uniform activation achieved both in melting and solid phase. Carbon graphitization on the top surface and a crystallized silicon layer below is observed as an effect of the high temperature and the melting phase.

Abstract: In this work, we demonstrate the possibility to achieve an ohmic contact using a low thermal budget applicable to backside processing after wafer thinning. The process window for laser annealing as a function of the thinning process is investigated. By laser melt annealing, we demonstrate the possibility for different silicide phases from pure nickel deposition on thinned 4H-SiC, formation of uniform carbon nanoclusters at the metal/SiC interface and recovery of thinning-induced defects. This has been demonstrated as a function of different thinning process and surface conditions.

Abstract: The effects of solution heat treatment of Al6Si2Cu aluminum alloy on incipient melting of θ-Al₂Cu phase have been investigated. Solution heat treatments, in this study, are applied to improve of mechanical properties through a single-step solution heat treatment. The microstructure of as-cast specimen represents a typical dendrite structure having a secondary dendrite arm spacing of 37um. In addition to the Al matrix, a large amount of coarsen eutectic Si, θ-Al₂Cu intermetallic phases and Fe-rich phases are identified. As the solution temperature increases, the Vickers's hardness increases up to 510°C and thereafter hardness decreases at the temperature of 520°C and 530°C. This hardness behavior may closely related with microstructural evolution such as solubility of alloying elements up to 510°C and also melting of θ-Al₂Cu intermetallic phases over 510°C in this study. Consequentially, the optimal single-step solution heat treatment temperature should be 510°C to improve mechanical property.

Abstract: Ion-beam deposition of amorphous Ge layers on different substrates (silicon and quartz) has been performed. Deposited amorphous Ge layers were subjected to pulsed laser annealing (λ = 0.69 μm, τ = 80 ns). Simultaneously the optical probing of the Ge surface was carried out. The computer simulation of heating processes and phase transitions was performed taking into account the temperature dependences of film and substrates’ parameters and phase transition energies. The results of the dynamics of heating, melting, crystallization and plasma formation processes are well described by simulation data. It is shown that the threshold values for radiation power density and phase transition rates are determined mainly by thermophysical parameters of the substrates and thermal contact between Ge melt and substrate.

Abstract: The problem of a planar solidification of a material with an additional nonstationary radiant of heat on a semi-infinite plane has been solved. For a solution the condition of Stefan was used. Results have been compared with an analytical solution in case of the absence of an additional radiant of heat, as well as with a solution obtained by perturbations method. A more complicated two-dimensional nonstationary problem of a solidification of a liquid with interface free-boundary has been also solved. The purpose of this problem solution is to predict position of a material phase boundary, as well as the temperature distribution in a layer of PCM (Phase-Change Material) with boundary conditions of Dirichlet.

Abstract: Phase change problems with natural convection in the liquid phase are of prime importance in accurate technological applications. In this work a melting of pure gallium in a rectangular cavity heated from the left wall is simulated using the finite difference method. Numerical algorithm based on dimensionless variables stream function and vorticity has been checked by experimental data of previously published papers. The effect of the grid size on results of calculations was identified. Motion of phase front and changing parameters of heat and mass-transfer were studied

Abstract: The high pressure casting process is a well-established way for the manufacture of non-ferrous components. Temperature is important technological parameter of production that affects the structure and quality of castings. Using continual exact measurement of alloy temperature is the best approach for predicting structures quality problems.Temperature measurement system also would help to acquire feedback from the manufacturing process so as to prevent possible defects in next phases of manufacturing. For this measurement we used thermocouple type „K“ in protective tube from graphite. It is necessary for correct process to know true values of temperature. For diagnostics we used procedures of SPC (StatisticalProcess Control) and Microsoft Excel 2007 graphic tools.

Abstract: The effect of a moving arc spot on a material of a plasmatron electrode is considered in the present paper. Temperature-dependence of material thermophysical properties is taken into account. Dynamics of temperature and thermal stresses fields are calculated. The characteristic feature of temperatures distribution along radius of an electrode is large gradient of temperatures in a narrow zone near to a surface which can cause large thermostresses and microcracks.