The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
S. L. Semiatin - One of the best experts on this subject based on the ideXlab platform.
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the adiabatic correction factor for deformation Heating during the uniaxial compression test
Journal of Materials Engineering and Performance, 2001Co-Authors: R L Goetz, S. L. SemiatinAbstract:The isothermal uniaxial compression test is a common method to determine the flow stress of metals. For accurate flow stress data at strain rates >10−3 s−1, the data must be corrected for flow softening due to deformation Heating. The first step in the correction is to determine the increase in temperature. An adiabatic correction factor, η, is used to determine the temperature between strain rates of 10−3 to 101 s−1. The adiabatic correction factor is the fraction of adiabatic Heat retained in the workpiece after Heat loss to the dies, η=(ΔT ACTUAL)/(ΔT ADIABATIC), where ΔT ADIABATIC=(0.95 f σdɛ)/(ρC p ). The term η is typically taken to be constant with strain and to vary linearly (0 to 1) with log ( $$\dot \varepsilon $$ ) between 10−3) and 101 s−1. However, using the finite element method (FEM) and a one-dimensional, lumped parameter method, η has been found to vary with strain, die and workpiece thermal conductivities, and the Interface Heat-Transfer coefficient (HTC). Using the lumped parameter method, an analytical expression for η was derived. In this expression, η is a function of the die and workpiece thermal conductivities, the Interface Heat-Transfer coefficient, workpiece Heat capacity, strain, and strain rate. The results show that an increase in the HTC or thermal conductivity decreases η.
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Determination of Interface Heat-Transfer coefficients for permanent-mold casting of Ti-6Al-4V
Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science, 2001Co-Authors: Pamela A. Kobryn, S. L. SemiatinAbstract:Interface Heat-Transfer coefficients (h 0) for permanent-mold casting (PMC) of Ti-6Al-4V were established as a function of casting surface temperature using a calibration-curve technique. Because mold geometry has a strong effect on h 0, values were determined for both of the two limiting Interface types, “shrink-off” and “shrink-on.” For this purpose, casting experiments with instrumented molds were performed for cylinder- and pipe-shaped castings. The measured temperature transients were used in conjunction with two-dimensional (2-D) axisymmetric finite-element method (FEM) simulations to determine h 0(T). For the shrink-off Interface type, h 0 was found to decrease linearly from 2000 to 1500 W/m2 K between the liquidus and the solidus, from 1500 to 325 W/m2 K between the solidus and the gap-formation temperature, and at a rate of 0.3 W/m2 K/K thereafter. For the shrink-on Interface type, h 0 was found to increase linearly from 2000 to 2500 W/m2 K between the liquidus and the solidus temperatures, from 2500 to 5000 W/m2 K between the solidus and the gap-formation temperature, and to remain constant thereafter. The shrink-on values were up to 100 times the shrink-off values, indicating the importance of accounting for the Interface geometry in FEM simulations of this process. The FEM-predicted casting and mold temperatures were found to be insensitive to certain changes in the h 0 values and sensitive to others. A comparison to published h 0 values for PMC of aluminum alloys showed some similarities and some differences.
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Thermal Stress Development During Vacuum Arc Remelting and Permanent Mold Casting of Ingots
Journal of Manufacturing Science and Engineering-transactions of The Asme, 1998Co-Authors: M. K. Alam, S. L. Semiatin, Z. AliAbstract:The development of thermal stresses in ingots during the vacuum arc remelting (VAR) as well as specialized permanent mold casting (PMC) process was modeled via numerical solution of the two-dimensional, nonsteady-state Heat conduction and stress equilibrium equations. The numerical analysis was carried out in conjunction with experimental studies of the mechanical properties and microstructure of a cracked VAR titanium aluminide ingot. Numerical solutions were obtained for different values of ingot diameter, crucible-ingot Interface Heat Transfer coefficients, and lengths of the melted-and-resolidified ingot. For both VAR and PMC, model predictions revealed that the maximum tensile thermal stresses are developed at the bottom of the ingot; the magnitude of such stresses increases with ingot diameter and the magnitude of the Interface Heat Transfer coefficients. The microstructural analysis of a cracked ingot indicated that the thermal cracking occurred in the temperature range where the alloy has very little ductility. The predicted development of large tensile stresses correlates well with observations of thermal cracking during VAR of near-gamma titanium aluminide alloy ingots. By contrast, the predicted thermal stresses developed during PMC are lower, thus suggesting an attractive alternative to VAR to obtain sound, crack-free ingots.
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Modeling of thermal stress development during the vacuum arc remelting process
1995Co-Authors: Z. Ali, M. K. Alam, S. L. SemiatinAbstract:The development of thermal stresses during the vacuum arc remelting (VAR) process was investigated through numerical solution of the two-dimensional, non-steady state Heat conduction and stress equilibrium equations. Solutions were obtained for various levels of input power efficiency, values of the crucible-ingot Interface Heat Transfer coefficients, and lengths of the melted and resolidified ingot. Model predictions revealed that the maximum tensile thermal stresses are developed at the bottom of the ingot for cases involving low input power efficiency and high Interface Heat Transfer coefficients. The predicted development of large tensile stresses at the mid-radius position correlates well with observations of thermal cracking during VAR of near-gamma titanium aluminide alloy ingots.
X.j. Zhang - One of the best experts on this subject based on the ideXlab platform.
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Optimization of the Interface Heat Transfer coefficient model based on the dynamic thermo-physical parameters in the pressure-temperature coupled field
International Communications in Heat and Mass Transfer, 2020Co-Authors: Chang Tao, C.m. Zou, H.w. Wang, Zunjie Wei, X.j. ZhangAbstract:Abstract Based on the inverse method, the dynamic thermo-physical parameters of the sand mold in the pressure-temperature coupled field were used for calculation. The optimized inverse method model of Interface Heat Transfer coefficient (IHTC), which combined with the three-layer Dufort-Frankel difference equation, was developed to reveal the effects of pressure on IHTC. The results showed that the IHTC was the maximum at the beginning due to high Heat flux at the Interface under the influence of pressure, then a rapid reduction on account of the forming of air gap and variation of thermo-physical parameters, then slowly stabilize. Compared with the classical inverse model using the static thermo-physical parameters, the accuracy of the calculated temperature field obtained by the optimized inverse model was improved by 18% at most.
David J Browne - One of the best experts on this subject based on the ideXlab platform.
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Interface Heat Transfer in investment casting of aluminum alloys
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2001Co-Authors: David J Browne, Denis OmahoneyAbstract:Via design of experiments and using a newly developed inverse method, the Heat-Transfer boundary conditions in the investment casting process have been studied. It has been shown in the past that these conditions, expressed as Interface Heat Transfer coefficients (HTCs), vary during alloy solidification and cooling. In this work, the authors have studied the additional effects of alloy solidification range, metallostatic head, investment shell thickness, preHeat, and Interface geometry. This provides an improved set of relationships from which to build realistic boundary conditions into computer simulations of shape casting. Using axisymmetric solidification experiments and numerical inverse analysis, it is shown that the effect of metallostatic head is only significant for long freezing-range alloys. Increasing shell mold thickness and preHeat also have effects that are alloy-dependent, and significant differences in thermal behavior are reported between the alloy/mold Interface and the alloy/core Interface. The four alloys used in the experiments are aluminum-based and vary from short freezingrange commercially pure to an alloy with a freezing range of 120 °C.
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use of experiment and an inverse method to study Interface Heat Transfer during solidification in the investment casting process
Experimental Thermal and Fluid Science, 2000Co-Authors: Denis Omahoney, David J BrowneAbstract:Abstract A technique to determine the thermal boundary conditions existing during the solidification of metallic alloys in the investment casting process is presented. Quantitative information about these conditions is needed so that numerical models of Heat Transfer in this process produce accurate results. In particular, the variation of the boundary conditions both spatially and temporally must be known. The method used involves the application of a new inverse Heat conduction method to thermal data recorded during laboratory experiments of aluminium alloy solidification in investment casting shell moulds. The resultant Heat Transfer coefficient for the alloy/mould Interface is calculated. An experimental programme to determine requisite mould thermal properties was also undertaken. It was observed that there is significant variation of the alloy/mould Heat Transfer coefficient during solidification. It is found to be highly dependent on the alloy type and on the vertical position below the initial free surface of the liquid metal. The aluminium casting alloys used in this study were 413, A356, 319 (Aluminum Association designations), and commercially pure aluminium. These alloys have significantly different freezing ranges. In particular, it was found that alloys with a high freezing range solidify with rates of Heat Transfer to the mould which are very sensitive to metallostatic head.
Z. Ali - One of the best experts on this subject based on the ideXlab platform.
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Thermal Stress Development During Vacuum Arc Remelting and Permanent Mold Casting of Ingots
Journal of Manufacturing Science and Engineering-transactions of The Asme, 1998Co-Authors: M. K. Alam, S. L. Semiatin, Z. AliAbstract:The development of thermal stresses in ingots during the vacuum arc remelting (VAR) as well as specialized permanent mold casting (PMC) process was modeled via numerical solution of the two-dimensional, nonsteady-state Heat conduction and stress equilibrium equations. The numerical analysis was carried out in conjunction with experimental studies of the mechanical properties and microstructure of a cracked VAR titanium aluminide ingot. Numerical solutions were obtained for different values of ingot diameter, crucible-ingot Interface Heat Transfer coefficients, and lengths of the melted-and-resolidified ingot. For both VAR and PMC, model predictions revealed that the maximum tensile thermal stresses are developed at the bottom of the ingot; the magnitude of such stresses increases with ingot diameter and the magnitude of the Interface Heat Transfer coefficients. The microstructural analysis of a cracked ingot indicated that the thermal cracking occurred in the temperature range where the alloy has very little ductility. The predicted development of large tensile stresses correlates well with observations of thermal cracking during VAR of near-gamma titanium aluminide alloy ingots. By contrast, the predicted thermal stresses developed during PMC are lower, thus suggesting an attractive alternative to VAR to obtain sound, crack-free ingots.
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Modeling of thermal stress development during the vacuum arc remelting process
1995Co-Authors: Z. Ali, M. K. Alam, S. L. SemiatinAbstract:The development of thermal stresses during the vacuum arc remelting (VAR) process was investigated through numerical solution of the two-dimensional, non-steady state Heat conduction and stress equilibrium equations. Solutions were obtained for various levels of input power efficiency, values of the crucible-ingot Interface Heat Transfer coefficients, and lengths of the melted and resolidified ingot. Model predictions revealed that the maximum tensile thermal stresses are developed at the bottom of the ingot for cases involving low input power efficiency and high Interface Heat Transfer coefficients. The predicted development of large tensile stresses at the mid-radius position correlates well with observations of thermal cracking during VAR of near-gamma titanium aluminide alloy ingots.
J M Urquhart - One of the best experts on this subject based on the ideXlab platform.
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polymer mould Interface Heat Transfer coefficient measurements for polymer processing
Polymer Testing, 2008Co-Authors: A Dawson, Martin Rides, C.r.g. Allen, J M UrquhartAbstract:Abstract Reliable process and product design for plastics through simulation requires reliable Heat Transfer properties data. The thermal contact resistance of Interfaces can have a significant influence in simulation predictions, in particular for micro-moulding, yet the availability of data is limited. Furthermore, the formation of air gaps at the polymer–wall Interface due to shrinkage of the polymer on cooling are not adequately modelled in process simulations. To address these issues, an instrument has been developed for measuring the thermal contact resistance of Interfaces relevant to polymer processing, in particular for injection moulding. It has been used to quantify the thermal resistance of the polymer–mould Interface and also that of air gaps introduced between the mould surface and the polymer, thereby representing the Interfaces occurring in injection moulding. Heat Transfer coefficients (HTC) across a polymer–steel Interface were measured to be of the order of 7000 W/(m 2 K). The thermal contact resistance of the polymer–air–steel Interface were in reasonable agreement with predictions assuming Heat Transfer across an air gap based on thermal conduction through air.
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Polymer–mould Interface Heat Transfer coefficient measurements for polymer processing
Polymer Testing, 2008Co-Authors: A Dawson, Martin Rides, C.r.g. Allen, J M UrquhartAbstract:Abstract Reliable process and product design for plastics through simulation requires reliable Heat Transfer properties data. The thermal contact resistance of Interfaces can have a significant influence in simulation predictions, in particular for micro-moulding, yet the availability of data is limited. Furthermore, the formation of air gaps at the polymer–wall Interface due to shrinkage of the polymer on cooling are not adequately modelled in process simulations. To address these issues, an instrument has been developed for measuring the thermal contact resistance of Interfaces relevant to polymer processing, in particular for injection moulding. It has been used to quantify the thermal resistance of the polymer–mould Interface and also that of air gaps introduced between the mould surface and the polymer, thereby representing the Interfaces occurring in injection moulding. Heat Transfer coefficients (HTC) across a polymer–steel Interface were measured to be of the order of 7000 W/(m 2 K). The thermal contact resistance of the polymer–air–steel Interface were in reasonable agreement with predictions assuming Heat Transfer across an air gap based on thermal conduction through air.