Sensitivity study of IHTC on solidification simulation for automotive casting

Abstract

Interface heat transfer coefficient values between the mould/metal interfaces need to be precisely determined in order to accurately predict the thermal histories at different locations in automotive castings. Thermo-mechanical simulations are carried out for Al–Si alloy casting processes using a commercial code. The simulation results are verified with experimental data from the literature. Sensitivity studies show that the choice of the initial value of the interface heat transfer coefficient (IHTC) between chill/metal as well as the sand mould/metal interfaces has a marked effect on the cooling curves. In addition, having chosen an initial value of the IHTC, the analyses also show differences in the solidification rate of the casting alloy near the sand/metal and chill/metal interfaces, upon further cooling. The gap formation, which results in a change in IHTC from the initial value, does not affect the cooling curves in the vicinity of the sand/metal interface due to lower thermal conductivity of sand. However it is found to have a considerable effect in the chill/metal interfacial regions due to higher thermal conductivity of the chill. Based on these studies we recommend initial IHTC values of 3000 and 7000 W m^–2 K^–1 for sand/metal and chill (steel)/metal interfaces respectively, for application in casting simulations.     

Improved thermal stress analysis for castings

Abstract

Three algorithms are presented to address three problems in stress analysis of castings: difficulties for enmeshment of complicated casting into finite element meshes, coupling of thermal and stress analysis, and determination of inverse deformation, respectively. An algorithm of conversion of finite difference meshes into finite element meshes is achieved by reformatting the data of meshes. An algorithm of the heat transfer coefficient at the casting/mould interface is presented and the coupling of thermal and stress analysis is realised with the feedback of stress and deformation to heat transfer. Machining allowance is applied as a criterion for deformation evaluation of castings. And insufficient machining allowance is transformed to inverse deformation which is fed back to the original casting design for recalculation of stress and deformation. Therefore, the design of casting with appropriate inverse deformation is obtained. Case studies about a cylinder block and a hydro turbine blade casting are illustrated.     

Conference Reports

Abstract

The outcome of a long-term programme on the computer-aided design of castings, carried out at Sharif University of Technology, has been the development of computer simulation software known as SUTCAST. This is currently employed in 16 local foundries. The program is based on a numerical method involving a classical approach to an explicit three-dimensional heat-transfer finite difference method. The software has been designed for the solidification simulation of pure metals, and eutectic and long-freezing-range alloys. It has been written for IBM personal computers and compatibles in the Turbo C version 2.01 programming language.

This paper discusses the computer solidification simulation of an Al—12%Si casting poured in a sand mould and the heat- transfer coefficient at the metal—mould interface. A mathematical model for the estimation of the gap width at the metal—mould interface during solidification based on the plane strain thermoelasticity equations is suggested.

The solidification process for Al—12%Si contained with a sand mould was monitored by measuring temperature at different locations within the casting and the sand mould. An experimental procedure was employed to measure the displacement of the metal and mould walls during solidification. The width of the gap was measured as the difference between the location of the casting and the inner surfaces of the mould, which varies with time.

The computer results are compared with the experimental data and are shown to be in good agreement as regards to cooling curves, solidification time and gap size.     

The Influence of Processing Variables on the Dimensional Integrity of Spheroidal-graphite Iron Castings

Abstract

This investigation was carried out to identify the major factors and their degree of influence on the dimensional accuracy of spheroidal-graphite iron castings produced in chemically- bonded sand moulds. Test castings were poured into furan-resin-bonded zircon sand and silica sand moulds and sodium silicate/ CO₂ bonded silica sand moulds. A comparison of casting sizes with those of the mould cavity into which they were poured showed considerable scatter and overlap. From these data the size that each casting would have had, had it solidified without graphite formation, was calculated and found to depend on mould cavity size for each type of mould. By isolating the differences in casting size due to graphite it was possible to identify the influencing factors. Thus castings poured into furan-resin-bonded zircon sand have the highest contraction and their size depends primarily on the amount of graphite present. The dimensions of castings poured into silica sand moulds show more variation and depend not only on the amount of graphite present and the structure of the metal but also on the thermal expansion of the silica sand moulds.     

Effect of section thickness and modification on thermal analysis parameters of A357 alloy

Abstract

Thermal analysis technique relies on the cooling curve obtained when the sample is cooled in a sampling cup. This may not represent the cooling behaviour of the real casting. The microstructure developed during solidification depends not only on the nucleation and modification potential of the melt but also on the thermal gradient imposed during solidification by the mould. The factors affecting the thermal gradient are the mould material and casting section thickness. In the present investigation the effect of modification melt treatment, cooling rate and casting section thickness on the thermal analysis parameters of A357 alloy was studied. It is found that the dimensionless heat flux parameter is high for small section thickness castings. The metal/mould interfacial heat flux is high in a copper mould. Thermal analysis parameters of A357 alloy are found to be affected significantly by the combined action of modification, chilling and section thickness.     

Enhanced cooling for solidification of aluminium alloys in permanent moulds using advanced heat pipe solutions

Abstract

In order to obtain sound cast components with good properties, a number of measures must be taken to make parts defect free. It is well recognised by the casting industry that it is essential to control cooling rates of permanent mould castings in order to speed up solidification and control the solidification pattern. Each of the traditional controlled cooling techniques (air or water cooling passages and chill inserts) presents certain disadvantages and none offers optimum thermal management. A new cooling method for permanent moulds is proposed. This new technique is based on heat pipe technology that was developed specifically for the cooling of permanent moulds in the casting of light metals where high heat fluxes are normally encountered. The influence of the conductivity of mould coatings on casting solidification and dendrite arm spacing with heat pipe cooling was investigated. Typical experimental results are also presented.     

Prediction of burn-on and mould penetration in steel casting using simulation

Abstract

Burn-on and penetration defects in steel casting are principally caused by localised overheating of the sand mould or cores. Such overheating can cause liquid metal to compromise the mould surface and entrain onto the surface of the mould. A method has been developed to predict likely burn-on and penetration defect locations as part of a standard casting simulation. The method relies on determining, from simulation results, the locations where the mould is above a certain critical temperature. The critical temperature is generally above the temperature at which the steel is fully solidified. By measuring the time periods during which these locations in the mould are above the critical temperature, burn-on and penetration defects can be predicted. The method is validated through comparison with previous experimental data. Several parametric studies are conducted to investigate the sensitivity of the predictions to the choice of the critical temperature, the interfacial heat transfer coefficient between the steel and the mould, the pouring temperature, and the mould material. The results of one case study are presented where burn-on or penetration defects observed on a production steel casting are successfully predicted.     

Effect of cooling rate on air gap formation in aluminium alloy permanent mould casting

Abstract

Displacements of the casting surface and the mould surface at the casting/mould interface were experimentally measured during the solidification of aluminium alloys in a permanent mould. Temperatures of the casting and mould surfaces at this interface were also recorded and correlated with displacement measurements. Four different commercial Al–Si alloys were investigated at varying cooling rates. These results are compared with available data on the effect of cooling rate on solid fraction evolution and consequently strength development during solidification. The temperature of the casting surface at the moment of air gap initiation was found to decrease with increasing cooling rate, although this relationship was confirmed at the 95% confidence level for only one of the alloys, AC601, for which sufficient data points were available. The solid fraction at the casting surface at gap initiation in this alloy is shown not to change with cooling rate. In all hypoeutectic alloys, the gap formed before the solid fraction at the casting surface reached 1·0 at slow cooling rates. For the near eutectic alloy BA401 it occurred at almost 1·0. Casting surface contraction rates following gap formation are also presented both as a function of time and casting surface temperature. It is shown that contractions predicted using the linear thermal expansion coefficient provide a reasonable approximation.     

On the origin of sliver defects in single crystal investment castings

The origin of sliver defects in seeded single crystal castings has been determined to be the deformation of dendrites in the mushy zone through microstructural characterization of a series of castings with a geometry sensitive to these defects. The extent of bending and torsion of the dendrites in the as-cast microstructure and the net misorientation was quantified from electron backscattered diffraction data using a bespoke data analysis method. At the point of initiation of the sliver defects, deformation was localized at the mould wall, indicating that these defects arise as a result of the bending moments generated by differential thermal contraction between the mould and the dendrites. The comparative susceptibilities of dendrites to deformation under converging and diverging growth with respect to the mould wall were distinguished. The experimental observations were supported by continuum finite element simulations using ProCAST?, which confirmed the occurrence of high stresses in the constricted channel where sliver defects form. Contrary to foundry wisdom, ancillary observations demonstrated that the oxide at the seed melt-back interface played no role.     

Towards more reliable investment castings

Abstract

The need for more reliable investment castings to meet the expectations of end users is outlined and the research undertaken during the Fundamentals of Investment Casting (FOCAST) project to meet this requirement is summarised. The traditional gravity poured, top gated mould designs used widely by the investment casting industry are shown to produce the least reliable aluminium alloy and steel castings. Changing to a bottom gated design to minimise surface turbulence during mould filling leads to a significant improvement in reliability, although the mould designs may not be particularly easy to implement in practice. It has been shown that a correctly used tilt pouring technique can also reduce surface turbulence and thereby improve reliability, and it is considered that this process is worthy of further development and evaluation by the investment casting industry. Countergravity mould filling has also been shown to be capable of producing more reliable castings than conventional gravity casting. The three techniques are compared and their industrial implementation discussed.     

Valid mould and process design to cast tensile and fatigue test bars in tilt pour casting process

Abstract

Tilt pour gravity casting technology is increasingly being used for shape casting various components with aluminium alloys. The ASTM B108/B108M-08 standard exists for a metal mould to evaluate the mechanical properties of castings made by gravity permanent mould process, yet there is no standard mould for the tilt pour process. We have designed, developed, tested and validated a standard mould to cast tensile and fatigue test bars in a tilt pour casting process. The new mould has demonstrated abilities to cast sound castings of A356·2 aluminium alloy, and the uniaxial tensile properties were superior to those obtained from conventional direct pour gravity casting process.     

Computer Simulation of Microstructural Evolution in Columnar Castings

Abstract

A computer model has been developed which makes possible a prediction of the as-cast microstructure in unidirectional columnar castings. The primary and secondary dendrite arm spacings and dimensions, concentration profiles across the secondary dendrite arms, and the eutectic fraction can be predicted in every location of the casting, given the pouring temperature and the heat transfer conditions prevailing in the mould. The numerical results are presented for aluminium-4.5 copper alloy. These are shown to agree very well with previous experimental observations. The model is amenable to extensions which allow further predictions demonstrating equiaxed solidification ahead of the columnar front, columnar to equiaxed transition, and fluid flow through the mushy zone.     

The Prediction of Shrinkage Cavities in Cast-Steel Rolls

Abstract

A 2D finite difference program has been written which enables the progress of solidification to be predicted in cylindrical castings with geometries typical of cast-steel rolls. A simple method for predicting the formation of gross shrinkage cavities has been introduced into the program, which assumes that liquid metal flow is instantaneous in regions where the solid fraction is below some critical value and feeding through regions above the critical value is not possible. Using metal/mould heat transfer coefficients determined previously for a variety of mould surface conditions (bare chill, coated chill, sand-lined chill), the computer model has been validated experimentally in terms of solidification times, position and shape of cavities and regions of porosity for a number of geometrical arrangements with different mould surfaces.     

Numerical study of crucial parameters in tilt casting for titanium aluminides

Numerical modeling of the tilt casting process for TiAl alloys was investigated to achieve a tranquil mould filling and TiAl castings free of defects. Titanium alloys are very reactive in molten state, so they are widely melted in cold crucible, e.g. the Induction Skull Melting (ISM) furnace. Then the crucible holding the molten metal together with the mould is rotated to transfer the metal into the mould——ISM+ tilt casting. This paper emphasizes the effect of crucial parameters on mould filling and solidif...     

The tilt casting process

Tilt casting is a process with the unique feature that, in principle, liquid metal can be transferred into a mould by simple mechanical means under the action of gravity, but without surface turbulence. It therefore has the potential to produce very high quality castings. Even so, the process is not often optimised in the industrial environment. This investigation represents an attempt to investigate some fundamental problems associated with the process.

A computer controlled, programmable roll-over casting wheel with a diameter of 1 m was used to produce sand castings in an Al-4.5% Cu alloy. The filling of the mould was studied using realtime X-ray radiography. Real-time X-ray radiography revealed that the molten metal could exhibit tranquil, turbulent or chaotic flow into the mould during tilt casting, depending on (i) the angle of tilt of the mould at the start of casting, and (ii) the tilting speed. Essentially horizontal transfer of the melt could achieve tranquil filling of the mould with minimum surface turbulence by a tilt starting position above the horizontal. The tensile properties of castings made using various starting conditions and rotation rates were measured and the results analysed using Weibull statistics to quantify reliability. Results are summarised on a map of the various operating regimes for tilt casting. An operational window for the production of reliable castings has been defined for the first time.     

Spatial interfacial heat transfer and surface characteristics during gravity casting of A356 alloy

As one of the key boundary conditions during casting solidification process, the interfacial heat transfer coefficient (IHTC) affects the temperature variation and distribution. Based on the improved nonlinear estimation method (NEM), thermal measurements near both bottom and lateral metal-mold interfaces throughout A356 gravity casting process were carried out and applied to solving the inverse heat conduction problem (IHCP). Finite element method (FEM) is employed for modeling transient thermal fields implementing a developed NEM interface program to quantify transient IHTCs. It is found that IHTCs at the lateral interface become stable after the volumetric shrinkage of casting while those of the bottom interface reach the steady period once a surface layer has solidified. The stable value of bottom IHTCs is 750 W/(m²·°C), which is approximately 3 times that at the lateral interface. Further analysis of the interplay between spatial IHTCs and observed surface morphology reveals that spatial heat transfer across casting-mold interfaces is the direct result of different interface evolution during solidification process.     

Metal-Mould Reactions in casting Aluminium-Lithium Alloys in Sodium-silicate-bonded Sand Moulds

Abstract

Metal-mould reactions in casting Al-Li alloys in sodium-silicate-bonded sand moulds have been studied by the modified Gertsman technique. Molten Al-2.7% Li alloy was poured into a bottom-gated vertical cylindrical mould cavity (150 mm x 50 mm dia) made from no-bake organic-binder-based sands. At the bottom of the mould cavity, a standard AFS three-ram sodium-silicate-bonded sand sample (the test sample) was placed vertically to provide the necessary interface for investigation. After cooling, the reaction products formed at the interface and samples from the bottom portion of the castings were collected for investigation. These were analysed to find Li loss from the casting as a result of metal-mould reactions. The casting was vertically sectioned and visually observed for appearance of blow holes, if any, while the sub-surface was studied for microhardness variation. The as-cast surface and the reaction products were also studied by SEM and X-ray diffraction analysis.

The study reveals that the Li in the molten alloy enters into vigorous chemical reaction with the sodium silicate resulting in the release of metallic sodium and formation of reaction products containing αLithium aluminium meta silicate. Li is thus lost from near the surface of the casting. Probably, the sodium released causes the gas blow holes in the sub-surface of castings due to its high vapour pressure at the working temperature.     

Evaluation of the effect of vacuum on mold filling in the magnesium EPC process

The combination of magnesium alloys with the expendable pattern casting (EPC) process will bring a bright future for the application of magnesium alloys. Vacuum is a pre-requisite parameter in the EPC process of magnesium alloys, because without vacuum, the fluidity of the magnesium alloy in the EPC process is too poor to fill the mold completely, especially for the thin-section castings. In this investigation, the effect of vacuum on the fluidity of AZ91 magnesium alloy has been explored. A modified model has been presented to explain the effect of vacuum on mold filling, which was verified by optical microscopy.The results obtained indicate that vacuum is the most effective parameter in improving the fluidity, the effect of vacuum on the fluidity interacting strongly with the pouring temperature and coating. Vacuum greatly changes the mass and heat transfer in the EPC process. Vacuum may not only control the profile of the metal–foam interface, which will influence the mass transfer process, but may also greatly speed up the removal rate of pattern decomposition products at the metal–coating interface. It also changes the primary heat-transfer mode to heat convention, which has a great influence on the distribution of the casting temperature field and solidification process. The microstructures of castings cast with vacuum exhibit a fine grain size and a small amount of precipitated Mg₁₇Al₁₂, but vary insignificantly with the location in the castings.     

Equivalent heat transfer coefficient at casting/Cu mould interface and temperature field simulation

1　INTRODUCTIONThesolidifiedmicrostructuresofalloysdependontheirsolidifyingprocesswhoseprimarycharacteristicsarethetemperaturedropofthesuperheatedmeltandthere leaseofthelatentheat.Sothestudyontheheattransferduringthesolidificationprocessistheessentialprobleminthesolidificationtheorystudy .Theresearchersworkingonthenumericalsimulationofthesolidificationprocessallknowthattheinterfacialheattransfercoefficientatthecasting/mouldisavariablechangingwithtime .Thusthedeterminationoftheinterfacialh…