Papers by Keyword: Thermal Cycling

Abstract: In the present study, Cobalt nanoparticles were synthesized and mixed with Sn-3.0Ag-0.5Cu solder (SAC305). The effect of 0.05 wt.%, 0.2 wt.%, 0.5 wt.% and 1.0 wt.% Co nanoparticles on the wettability, the shear strength as well as the growth of intermetallic compounds (IMCs) at composite solder/Cu interface during thermal cycling were investigated. Results show that Co nanoparticle addition can considerably improve wettability, increase the shear strength and suppress the growth of interfacial IMCs. The addition amount of Co nanoparticles have the consistent relation with the improvement extent of the wettability, shear strength and the growth rate of interfacial IMCs. Slight addition (0.05 wt.% and 0.2 wt.%) can significantly improve the properties of nanocomposite solder in all the three aspects. However, with excessive addition amount (0.5 wt.% and 1.0 wt.%) of Co nanoparticles, the improvement extent will decline. The Cross-section images of the interface of nanocomposite solder/Cu joint show that Co can form reinforcement particles in the solder and can act as the sacrificial element to substitute Cu to react with Sn. Excessive Co nanoparticles would form large amount of hard and brittle intermetallics which leads to decreasing shear strength of the composite solder.

Abstract: This study aimed to model numerically the thermal cycling resulting from the steel ASTM A743-CA6NM remelting process. The problem was solved with the support of the commercial software ANSYS / FLUENT ® 14.5 for the three-dimensional case using the finite volume method. The following simplifying assumptions were adopted: heat loss by natural convection, absence of radiation, no phase change, concentrated heat source, and thermophysical properties independent of temperature. The results were analyzed for two different current intensities: 90A and 130A, and compared with experimental measurements. The peak temperatures of the thermocouples near the fusion line for the current of 130A were well represented by the numerical model, with a maximum deviation of 9.62%. In the case of the more remote thermocouples from the fusion line, the best results were obtained for the current of 90A, not exceeding 5% of deviation. In general, it was found that the tested body is heated faster than in simulations. This can be considered as a consequence of the simplification in material properties, which were assumed constants with temperature. The results of this study demonstrate that, given the adopted simplifications, the numerical model was able to satisfactorily reproduce the experimentally measured thermal cycles.

Abstract: Phase change materials (PCMs) as thermal energy storage medium are proven to be effective to enhance the performance of solar thermal system. The degradation of the thermal properties due to thermal cycles is changeable and accordingly the performance of the solar thermal cycle may decline. In this study, the thermal reliability of paraffin wax was investigated to analyse the ability to be used as thermal energy storage (TES) for solar water heating purposes that subjected to many phase change cycles. The mixtures were subjected to 400 phase change cycles and the thermal properties were measured. Two samples were prepared; Sample 1 was paraffin wax without phase change cycles whereas Sample 2 was gone through 400 phase change cycles. Four hundred phase change cycles indicated the phase change cycles for 1 year 35 days as 1 cycle equivalent to 1 day. The comparison of samples with and without 400 phase change cycles showed slight changes in thermal conductivity, specific heat, melting point and solidification point. Fourier Transform Infrared Spectrometer and Thermogravimetric Analysis showed that after 400 phase change cycles there is no weight loss observed. The paraffin wax is hence found reliable to be use without any degradation, without any chemical reaction and slightly improvement of thermophysical properties as TES for solar water heating purposes.

Abstract: Stress, strain and residual energy of the thermal barrier coatings under high temperature shock were simulated by using ABAQUS software. The results show that cooling rate has certain effects on the residual stress. With the increase of cooling rate, the interface stress between the coating and substrate increased. Rapid cooling resulted in the phase transition and larger residual stress in the coating. With increasing of thermal cycles, the residual stress accumulated gradually. And the residual stress of the fourth thermal cycling increased by half compared with the first time. Moreover, the strain presented an increasing tendency with the increase of thermal cycling times and reached to 23.1097×10-3 μm after the fourth thermal cycling. The residual energy increased to 15.5 J after four thermal cycling and increased 1.2 J compared to the first time.

Abstract: Tools for machining are made of hard steels and cemented carbide (WC-Co). For specialized applications, such as aluminium machining, diamond or polycrystalline cubic boron nitride are also used. The main problem with steel, isthat itexhibits a relatively low hardness (below 10 GPa) which strongly decreases upon annealing above about 600 K.Thus, the majority of modern tools are nowadays coated with hard coatings that increase the hardness, decrease the coefficient of friction and protect the tools against oxidation. A similar approach has been recently used to obtain a longer duration of the dies for aluminium die-casting.

Abstract: Corrector lens assembly is used to expand the field of view of a Cassegrain Telescope. There are usually three to five lenses in it. For the assembly of such a kind of high precision optics, decenter and tilt of each optical component and air space between components are key issues to be qualified. In the present study, four lenses were designed. Individual control of centering of the component and air space were achieved by assembling the component into a precision machined subcell. The lens positions were defined by precision spacers according to the designed values. To compensate various thermal expansions of glass and metal, elastomer material was used. Considering the deviation of manufacturing from design data, manufactured data were put into optical software for re-optimization, and air spaces between lenses were obtained. This gives excellent accuracy at low cost. Base on present lens mounting design and lens manufacturing quality, centering error was eliminated by lateral adjustment, which leaded to a good alignment. In this paper, the method for aligning the optics with the barrel assembly is presented and centering error measured by a double autocollimator is listed. In addition, test results of thermal cycling and vibration tests on the corrector lens assembly are discussed.

Abstract: The presence of residual stresses in thermal oxide layers has been recognized for a long time. In the present work, the mechanical fields for chromia oxide are determined either by XRD or Raman spectroscopy. In addition, the microstructure of the chromia films is investigated ant its influence on the evolution of the stress release processes is analyzed.

Abstract: The presence of residual stresses in thermal oxide layers has been recognized for a long time. In the present work, the mechanical fields for chromium oxide are investigated. An extended model is established to take into account the effects of temperature and thermal cycling for the calculation of oxide stress. Numerical results are given in order to predict the influence of different parameters, especially the dependence of some material parameters with temperature. This enables to make comparison with experimental results.

Abstract: In sodium cooled fast breeder reactors, the high operating temperature necessitates hard facing of grid plate to avoid galling and to reduce the wear loss of the base material . Nickel-base cobalt-free alloy called Colmonoy-5 has been chosen as the hard facing material for the Prototype Fast Breeder Reactor (PFBR) grid plate. The grid plate (GP) which is a critical component made of 316 LN SS that holds the core subassemblies are hard faced, to prevent galling and also to minimize wear caused by subassembly insertion/removal and erosion due to high sodium velocity at 670 K. Thermal cycling of hard faced circular grid plate made by Plasma Transferred Arc Welding (PTAW) generates residual stresses due to differential shrinkage of the molten deposit and difference in coefficients of thermal expansion between the deposit and substrate material. In this project the effect of thermal cycling of a nickel-base hardfacing alloy deposited on an austenitic stainless steel grid plate was studied. Finite element analysis of hard faced circular grid plate is performed for obtaining residual stress which includes elasto-plastic analysis. Coupled thermo-mechanical analysis is done for thermal cycling of hardfaced circular gridplate to quantitatively estimate the residual stress.

Abstract: The effects of the thermal cycling on the coefficient of thermal expansion (CTE) and microstructure of the hypereutectic Al-Si alloys were investigated. The hypereutectic Al-Si alloy was produced by spray-forming and extruding process. Experiment results showed that the distribution of Si in Al matrix is uniform for the Al-Si alloy. The primary Si phase grew gradually during the thermal cycle. There is no remarkable change of CTE during thermal cycling for both materials at the same temperature range. The CTE of Al-Si alloy decrease with increasing on thermal cycling temperature up to 300°C due to the compressive thermal stress in the alloys.