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.     

Rapid fabricating process of zinc alloy moulds based on vacuum casting process

Abstract

A rapid fabrication process was developed by combining stereolithography prototype, silicone rubber mould and vacuum casting process for zinc alloy moulds. Transfer coating mould was introduced as casting moulds reproduced from silicone rubber moulds. Vacuum casting process was applied to improve filling ability of complicated mould with fine pattern surface. Vacuum reduces the gas counterpressure in the casting mould cavity and then improves the filling ability.     

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.     

The Horizontal Ohno Continuous Casting Process: Process Variables and Their Effects on Casting Stability

Abstract

Temperature profile measurements within a heated mould have been made during continuous casting of pure tin rod of 8.5 mm dia in an attempt to obtain an understanding of the influence of process variables on the position of the solidification front. It has been established that process variables such as casting speed, mould temperature and cooling position have a sensitive effect on the position of the solidification front. It varies linearly with casting speed for a given cooling position and mould temperature. The change in position of the solidification front in turn exerts a significant effect on the surface quality of the cast strand. It has been demonstrated that the solidification front should be brought well within the mould in order to obtain good dimensional and casting stability.     

A Free Thermal Contraction Method for Modelling the Heat-transfer Coefficient at the Casting/Mould Interface

Abstract

This study is intended to explore a simple and inexpensive method that is able to model the variation and distribution of the heat transfer coefficient at the casting/mould interface. It has been assumed that, in a rigid mould, the magnitude of interface gap size primarily depends on the thermal contraction of cast metal as it solidifies. Consequently, a free thermal contraction method has been developed to describe the thermal contact phenomena at the casting/mould interface, based on the above assumptions. This method has been used in the solidification simulations for three sand castings of different shapes. The numerical solutions of the simulations agree closely with the experimental results of the thermocouple measurements in the castings and their moulds. For the casting and mould materials involved, the empirical parameters in the equation of the heat transfer coefficient are the same for each of these cases, i.e. independent of the geometric shape of the castings.     

Refinement of primary carbides in AISI DC 53 cold work tool steel by thin section copper permanent mould casting and water atomization

In this study, as alternative methods to conventional production methods involving thin section copper permanent mould casting (fast solidification) and water atomization (rapid solidification) of AISI DC 53 cold work tool steel have been investigated. Thin slabs obtained by copper mould casting was homogenized at 1150 °C for one hour and then hot rolling was be applied. After hot rolling, conventional heat treatment have been applied to thin slabs. The same heat treatment procedure have been performed for a commercially available AISI DC 53 which is manufactured by thick section ingot casting and rolling. In order to investigate also the effect of rapid solidification on AISI DC 53 tool steel, steel powder was produced by water atomization technique. Experiments have revealed that thin steel slabs in permanent copper mould and rapidly solidified tool steel powder by water atomization have a more refined primary M₇C₃ carbides than commercially available steel.     

Performance of digital patternless freeze-casting sand mould

Digital patternless freeze-casting technology is a new approach for obtaining frozen sand moulds using digital milling technology.The change law of tensile strength and air permeability of frozen sand moulds (100-mesh and 200-mesh silica sand,and zircon sand moulds) under different freezing temperatures and water contents was studied.Results show that with the decrease of freezing temperature and the increase of water contents,the tensile strength and air permeability of the sand moulds are gradually improved.Meanwhile,computed tomography technology was used to characterize the shape and size of the water film between the sand particles mixed with 4wt.% water.The results show that in silica sand moulds,the form of water film is lumpy,and 200-mesh silica sand moulds have more water films and higher proportion of small-sized water films than 100-mesh silica sand moulds,while in zircon sand moulds,the form of water film is membranous.At the same freezing temperature and water content,the tensile strength of zircon sand mould is the highest,and 100-mesh silica sand mould is the lowest.A comparative solidification experiment of A356 aluminum alloy was carried out in frozen sand mould and resin sand mould.The results show that the primary α-Al phase appears in the form of equiaxed and eutectic silicon phase is needle-like in freezing sand mould casting,but the primary α-Al phase grows in the form of dendrites,and the eutectic silicon phase is coarse needle-like in the resin sand mould casting.The difference of microstructure is caused by the different cooling rate.The cooling rate of A356 aluminum alloy in frozen sand mould is higher than that in resin sand mould.     

Casting/mould interfacial heat transfer during solidification in graphite,steel and graphite lined steel moulds

Heat flow between the casting and the mould during solidification of three commercially pure metals, in graphite, steel and graphite lined steel moulds, was assessed using an inverse modelling technique. The analysis yielded the interfacial heat flux (q), heat transfer coefficient (h) and the surface temperatures of the casting and the mould during solidification of the casting. The peak heat flux was incorporated as a dimensionless number and modeled as a function of the thermal diffusivities of the casting and the mould materials. Heat flux transients were normalised with respect to the peak heat flux and modeled as a function of time. The heat flux model proposed was used to estimate the heat flux transients during solidification in graphite lined copper composite moulds.     

Modelling Permanent-mould Casting Processes

Abstract

Numerical and experimental work is carried out to investigate liquid metal flow and heat transfer during the permanent moulding of cast iron. Heat conduction during the pouring and solidification of cast-iron plates, and liquid metal flow and heat transfer during the pouring and solidification of cylindrical cast-iron ingots is numerically and experimentally investigated. The castings are poured in permanent moulds, and temperature distributions in the castings and the moulds are recorded. The mathematical modelling indicates the significance of finite pouring time and liquid metal convection on the temperature distribution. Good agreement is obtained between the experimental data and the predictions of the heat and mass transfer computations. Temperature fields, solidus front shapes and velocity fields during pouring and solidification are simulated for both top and bottom gated cases.     

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.     

热管技术及其在轻合金铸造中的应用

简述了在热管的理论研究比较完善,重点转向广泛开展的应用研究的同时,在材料领域积极、开发和应用热管技术的必要性和重要意义。初步评估了热管在轻合金永久型铸造中的行为、作用、利用潜力和应用的基本形式。特别提出了一种新颖的采用热管技术制备镁合金铸管的方法以及一种轻金属半固态铸锭制备的改进方法,阐明了高温热管技术在永久型铸造冷却控制中的应用价值。     

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.     

Measurements and modelling of air gap formation in Cu-based alloys

Abstract

The formation of an air gap has been experimentally studied during solidification of pure Cu, Cu–Te and Cu–6%Sn in a cylindrical mould. The displacements of the casting and the mould causing an air gap have been measured during solidification and cooling of the casting. The temperature distribution was measured simultaneously. Mathematical modelling has been performed to increase the understanding of the solidification process and the shrinkage of the casting leading to air gap formation. A model, which has been tested in earlier work showing good results for aluminium based alloys, has been applied here to describe air gap formation during solidification of copper based alloys. The model includes the effect of the formation and condensation of vacancies on the solidification process as well as on the material shrinkage resulting in air gap formation. The results from the modelling show a reasonable agreement with the experimental measurements.     

基于Matlab的板坯连铸凝固传热过程及热损失研究

基于Matlab数值计算,对板坯连铸凝固传热问题进行研究,得到随板坯厚度及其与结晶器弯月面距离变化的板坯温度场分布,通过拟合得到板坯凝固点末端位置与二冷总供水流量、过热温度和拉坯速度的关系式,分析二冷区水量分配比对结晶器和二冷区内单位长度板坯热损失率和板坯表面温度梯度的影响。结果表明：板坯温度随冷却阶段的不同其温度变化趋势显著不同;随着过热温度和拉坯速度的增大、二冷总供水流量的减小,板坯凝固点末端位置增大;拉坯速度对板坯凝固点末端位置的影响最为显著,其次是二冷总供水流量,过热温度对其影响较小。通过适当调整二冷区内水量分配比可实现降低板坯表面温度梯度和较少热损失率的折衷,从而在提高板坯质量的同时也提高其蓄能,以实现板坯连铸过程的节能。所得结果能对板坯连铸凝固过程的参数设计和动态运行提供依据和理论指导。     

Effect of particle size variations of gypsum bonded investment powders on metallurgical quality of investment castings

Abstract

In the present work, the effect of particle size distribution parameters of investment powder on the metallurgical characteristics of castings produced by flask mould investment casting process was investigated. The main aim of this study is to establish the relationships between the casting quality and investment powder characteristics. For this purpose, models for tensile, density and surface roughness specimens were prepared in accordance with the standards and these models were moulded by gypsum bonded investment powders. Specimens of sterling silver (92·5Ag–7·5Cu alloy) were subsequently cast into these moulds by centrifugal casting process. After the completion of solidification process, the specimens were tested and results were analysed. The preliminary results illustrate that fineness of investment powders play an important role in determining the metallurgical quality of castings produced by investment casting process.     

Graphite degeneration in surface layer of ductile iron castings

Abstract

In the furan resin moulding technique sulphur in P-toluenesulphonic acid (PTSA), usually used as the hardener, has been identified as an important factor causing graphite degeneration at the metal/mould interface, especially at lower graphite nodularity levels. The greatest surface layer thickness and the lowest graphite nodularity, and shape factors, were obtained with irons solidified in moulds coated with an S bearing material. Uncoated moulds provided better results, but employing a MgO type coating effectively neutralised the sulphur migrating from the mould. In the present solidification conditions, the application of an active mould coating also influenced the graphite phase characteristics in the entire section of the casting, up to its centre. Negative effects were observed using an S bearing coating and positive effects from an MgO based coating.     

The relationship between cooling rate and the structure and properties of hypo-eutectic cast iron

The aim of the present work is to make a correlation between the cooling rate of grey cast iron during solidification and the resulting structure, phases and mechanical properties. Liquid cast iron with a carbon equivalent of 3.92% was cast in sand moulds. Different cooling rates were obtained by changing the casting shape and thickness.

The cooling curves were recorded using a data acquisition system in order to determine the solidification parameters. Some samples were reheated to a temperature above solid state transformation—austenitised (1118 K) and then quenched.

The microstructure was studied in order to determine the phases formed such as graphite, pearlite, ferrite and cementite. Their shape, size and volume fractions were studied. The formation of these phases was correlated with the cooling and solidification parameters. Some mechanical properties, namely hardness and tensile properties, were measured.

Correlation between these properties cooling rate and microstructure parameters are presented.     

Evaluation of melt quality of lead free copper alloy castings using cooling curve

Abstract

The melt quality of bronze castings such as JIS CAC406 has successfully checked by observing a fractured surface of specially designed test sample on the melting spot. Lead free copper alloy castings, however, are not successfully applied by this method because the fractured surface does not show a characteristic feature or colour. In this study, the cooling curve method cast into shell cup mould was applied to evaluate and establish melt quality. Melts, under various kind of atmosphere such as over oxidised, hydrogen absorbed, flux covered, adequately deoxidised and so on, were evaluated to check the cooling curve in a cup. Owing to the melting atmosphere, liquidus temperature and solidification behaviour showed a distinct difference. Another application of cooling curve method was carried out to deoxidising process in industrially clean melt. Phosphorus content necessary for and also during deoxidising process was well determined.