Heat transfer during solidification of chemically modified Al–Si alloys around a copper chill

Abstract

The solidifying metal/chill contour will significantly affect the boundary heat transfer coefficients, and solidification modellers should be aware of the casting conditions for which the heat transfer coefficients are determined. The previous work carried out on solidification of Al–Si alloys in a metallic mould and solidification against bottom/top chills has shown that modification and chilling have synergetic effect resulting in a significant increase in the heat flux transients at the casting/chill interface. In the present work, the heat transfer during solidification of unmodified and chemically modified Al–Si alloys around a cylindrical copper chill was investigated. Heat flux transients were estimated using lumped heat capacitance method. Lower peak heat flux was obtained with chemically modified alloy. This is in contrast to the results reported for alloys solidifying against chills and in metallic moulds. The chill thermal behaviour and heat transfer to the chill material when surrounded by modified and unmodified alloys were explained on the basis of the decrease in the degree of undercooling in the case of modified alloy as compared to unmodified alloy and the change in contact condition and shrinkage characteristics of the alloy due to the addition of chemical modifiers.     

Effect of modification melt treatment on casting/chill interfacial heat transfer and electrical conductivity of Al–13% Si alloy

For successful modelling of the solidification process, a reliable heat transfer boundary condition data is required. These boundary conditions are significantly influenced by the casting and mould parameters. In the present work, the effect of sodium modification melt treatment on casting/chill interfacial heat transfer during upward solidification of an Al–13% Si alloy against metallic chills is investigated using thermal analysis and inverse modelling techniques. In the presence of chills, modification melt treatment resulted in an increase in the cooling rate of the solidifying casting near the casting/chill interfacial region. The corresponding interfacial heat flux transients and electrical conductivities are also found to be higher. This is attributed to (i) improvement in the casting/chill interfacial thermal contact condition brought about by the decrease in the surface tension of the liquid metal on addition of sodium and (ii) increase in the electronic heat conduction in the initial solidified shell due to change in the morphology of silicon from a acicular type to a fine fibrous structure and increase in the ratio of the modification rating to the secondary dendrite arm spacing.     

Some observations on dendritic arm spacing in Al-Si-Mg and Al-Cu alloy chill castings

The heat flux from a cast cylinder to a steel chill was experimentally determined for a commercial Al-Si-Mg alloy (A356) and for Al-Cu alloys having different copper contents. The relationship between variation of the heat flux with time, initial temperature of the chill, and solute concentration was determined. The heat flux from the casting to the chill in the A356 alloy is higher than in an Al-7.5% Si alloy, but the microstructure of the former is coarser. The time dependence of the heat flux in an Al-Cu alloy is similar to that in A356 and in Al-7.5% Si. Calculated values for the temperature of the casting and the local solidification time as functions of the distance from the chill were obtained with the aid of the heat flux data and a program for calculating the temperature field during solidification. A good fit with experimental measurements was achieved. Measurements of the mean secondary dendritic arm spacing at different distances from the chill resulted in the relationship =at _f ^0.43 between the local solidification time (t _f) and the dendtitic arm spacing (), where a is a characteristic of the alloy and of the solute concentration. It is noted from the results that the value depends not only on the solidification time but also on the concentration of solute element. Different aspects of the evolution of structure, and some attention to growth with high temperature gradients in the presence of chill is discussed.     

The effect of varying chill surface roughness on interfacial heat transfer during casting solidification

Experiments to investigate interfacial heat transfer mechanisms during casting solidification were carried out by varying the surface roughness of a Cu chill used to bring about unidirectional solidification of an Al-4.5 wt.% Cu alloy. Little variation in interfacial heat transfer coefficient with varying chill surface roughness was found, confirming previously published results. Examination of the as-cast surface of the casting showed the presence of predendritic contact areas, and also that the casting surface roughness did not form as a replica of the chill surface, as has often been proposed. Rather, the casting surface roughness was consistently greater than that of the chill, but varied little in the experiments. A sum surface roughness parameter was devised to characterise the casting–chill interface that included the roughness of both surfaces. The value of this parameter was strongly influenced by the greater roughness of the casting surface, rather than the chill surface roughness, and therefore also varied little in the experiments. This lack of variation in the casting surface roughness and hence the sum surface roughness parameter showed how interfacial heat transfer should be more strongly influenced by the greater roughness of the casting surface than of the chill surface, and explains why the interfacial heat transfer coefficient was not strongly influenced by the chill surface roughness in these types of experiments. At the time the work was carried out the authors were in the Manchester Materials Science Centre, University of Manchester and UMIST, Manchester M1 7HS, UK.     

Interfacial reactions between AZ91D magnesium alloy and plaster mould material during investment casting

Abstract

Reactions at the mould/metal interface play an important role in determining the quality of investment castings. They are particularly critical in the case of magnesium alloys cast in industrial plaster moulds. In this work, reactions of molten magnesium alloy with plaster mould were studied. First the potential interactions with mould materials (including gases) were examined using thermodynamic considerations. Then thin sheets of AZ91D magnesium alloy were cast in industrial plaster moulds using vacuum assistance: the surface of sheets and plaster mould were characterised. The analysis of reaction products indicates that magnesium vapours diffuse through the plaster and reduce the silica present in the investment material according to the following reaction: 4Mg + SiO₂=2MgO + Mg₂Si. The extent of reactions is controlled by mould temperature and thickness of castings.     

Hydrogen pick-up during mould filling in the lost foam casting of Al alloys

In the Lost Foam casting of Al alloys the foamed polystyrene pattern is broken down by the heat of the advancing liquid metal as it fills the mould. This has led to discussion about the possibility of increased hydrogen pick-up by the liquid metal from the gaseous pattern degradation by-products accumulating at the liquid metal—foam pattern interface, leading to detrimental porosity in the final casting. The results presented here were derived from comparisons of the initial measured hydrogen content of the liquid Al alloy before mould filling, and the hydrogen content of the final castings, coupled with real-time X-ray imaging of the filling of the mould to determine whether entrainment of the foam pattern degradation by-products was occurring. This showed that increased hydrogen content in Al Lost Foam castings was attributable to the entrainment of degrading pattern material, and not due to increased absorption of hydrogen from the interfacial atmosphere.     

Estimation of air gap and heat transfer coefficient at different faces of Al and Al–Si castings solidifying in permanent mould

Abstract

In the casting processes, the heat transfer coefficient at the metal/mould interface is an important controlling factor for the solidification rate and the resulting structure and mechanical properties. Several factors interact to determine its value, among which are the type of metal/alloy, the mould material and surface conditions, the mould and pouring temperatures, casting configuration, and the type of gases at the interfacial air gap formed. It is also time dependent. In this work, the air gap formation was computed using a numerical model of solidification, taking into consideration the shrinkage and expansion of the metal and mould, gas film formation, and the metallostatic pressure. The variation of the air gap formation and heat transfer coefficient at the metal mould interface are studied at the top, bottom, and side surfaces of Al and Al–Si castings in a permanent mould in the form of a simple rectangular parallelepiped. The results show that the air gap formation and the heat transfer coefficient are different for the different casting surfaces. The bottom surface where the metallostatic pressure makes for good contact between the metal and the mould exhibits the highest heat transfer coefficient. For the sidewalls, the air gap was found to depend on the casting thickness as the larger the thickness the larger the air gap. The air gap and heat transfer coefficient also depend on the surface roughness of the mould, the alloy type, and the melt superheat. The air gap is relatively large for low values of melt superheat. The better the surface finish, the higher the heat transfer coefficient in the first few seconds after pouring. For Al–Si alloys, the heat transfer coefficient increases with increasing Si content.     

T4 热处理对石膏型 AZ91 铸造镁合金组织和性能的影响

用石膏型熔模铸造技术,成功制备了AZ91镁合金铸件.用金相显微镜(OM)、扫描电镜(SEM)、能谱(EDS)以及电子万能实验机等,研究了AZ91镁合金铸态及T4热处理态的显微组织演变和力学性能.结果表明,分布在铸态AZ91镁合金晶界的网状β-Mg17Al12相在T4热处理过程中逐渐溶解,铸态和T4热处理态中均存在大量的A18Mn5化合物,T4处理后,其力学性能显著提高.     

Characterization of Al–Mn particles in AZ91D investment castings

Manganese is currently added to Mg–Al alloys in order to improve the corrosion behavior of cast components. A part of this manganese is dissolved in the magnesium matrix and the balance is found as fine Al(Mn,Fe) particles dispersed within castings. For AZ91D specimens prepared using the plaster mould investment casting process, these particles were observed in very large quantity at the surface of castings. These particles were characterized by scanning electron microscopy and electron probe microanalysis. It was found that they consist of Al₈Mn₅ phase and that their morphology and size depend on local solidification conditions. Their presence at the surface of the castings is related to low solidification rates and reduced thermal gradients at the mould/metal interface.     

Modelled heat transfer coefficients for Al–7 wt-%Si alloy castings unidirectionally solidified horizontally and vertically downwards

Abstract

A model is described for the calculation of the interfacial heat transfer coefficient during the unidirectional solidification of Al–7 wt-%Si alloy castings against a water cooled copper chill. The model includes the deformation of the initial solidified skin of the casting into a convex shape within the first seconds of solidification. Thereafter, heat transfer from the casting to the chill takes place through a central contact area and an outer annulus where local separation has occurred. Modelled heat transfer coefficients for solidification horizontally and vertically downwards are compared with experimentally determined values and show broad agreement. Some limitations of the model which prevent better agreement with the experimental values are discussed.     

Effect of melt superheat and chill material on interfacial heat-transfer coefficient in end-chill Al and Al-Cu alloy castings

Solidification of metal castings inside moulds is mainly dependent on the rate of heat removal from the metal to the mould. During casting solidification, an air gap usually develops at the interface between the solidfying metal and the surrounding mould or chill. This condition occurs in most casting geometries, except in some cases such as the cast metal solidifying around a central core. An overall heat-transfer coefficient, which includes all resistances to heat flow from the metal to its surroundings can be determined. The objective of this work was to determine the overall heat-transfer coefficient,h, using experimental and computersimulation results on commercial purity aluminium and Al-4.5 wt% Cu alloy solidifying in a vertical end-chill apparatus. The cast ingots had a cylindrical shape with 12.5 mm diameter and different lengths of 95 and 230 mm. It solidified at different superheats (ranging from 50–110 °C) against two different chill materials: copper, and dry moulding sand. A computer program solving the heat-conduction equation and taking into consideration the convection in the melt, was used to compute the temperature history at numerous points along the ingot length. Differenth values were assumed as a function of time, until agreement between experimental and computed cooling curves was obtained. The variation ofh as a function of time, surface temperature, specimen length for each melt superheat and chill material was found. The thickness of the air gap was also evaluated. The results indicate that the variation of heat-transfer coefficient with time followed a pattern of sudden increase for the first few seconds, followed by a steady state, after whichh decreased and reached another lower constant value. Theh values were also found to decrease rapidly when the liquidus temperature was reached in the melt. For longer specimen and higher melt superheat, the heat-transfer coefficient increased. It was also higher for a copper than for a sand chill.     

差压铸造薄壁铝硅合金铸件的位置效应

采用差压铸造工艺,研究垂直缝隙式浇注系统浇注的铝合金硅铸件不同位置的组织和力学性能变化.采用石英砂型、SiC砂型和冷铁,浇口处铸件的晶粒最细小,致密度高、力学性能最好;铸件冷端的组织和性能次之;位于两者之间的铸件的组织和性能最差.分析表明对于具有垂直缝隙式浇注系统,差压铸造凝固压力对金属的凝固作用具有位置效应,浇口处液态金属温度高,凝固时间长,凝固压力对浇口处金属的凝固作用显著;铸件冷端金属凝固时间短,凝固压力对该处金属的凝固作用不显著,铸型的冷却速度对铸件组织和性能的影响起显著作用.浇口处与冷端之间的金属液体的凝固受压力和冷却速度的影响小,铸件的晶粒尺寸最大、密度最小、性能最低.冷却速度提高,铸件的任意位置的组织和性能都相应得到提高.     

Morphological characteristic of the conventional and melt-spun Al-10Ni-5.6Cu (in wt.%) alloy

The Al-10Ni-5.6Cu alloy was prepared by conventional casting and further processed melt-spinning technique. The resulting conventional cast and melt-spun ribbons were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy together with energy dispersive spectroscopy, differential scanning calorimetry and microhardness techniques. The X-ray diffraction analysis indicated that ingot samples were α-Al, intermetallic Al₃Ni and Al₂Cu phases. The optical microscopy and scanning electron microscopy results show that the microstructures of rapidly solidified ribbons are clearly different from their ingot alloy. Al-10Ni-5.6Cu ribbons reveal a very fine cellular structure with intermetallic Al₃Ni particles. Moreover, at high solidification rates the melt-spun ribbons have a polygonal structure dispersed in a supersaturated aluminum matrix. The differential scanning calorimetry measurements revealed that exothermic reaction was between 290 °C and 440 °C which are more pronounced in the ternary Al-10Ni-5.6Cu alloy.     

Thermal contact at solder/substrate interfaces during solidification

Abstract

Heat flux transients at the solder/substrate interface during the solidification of Sn–37Pb and Sn–3·5Ag solder alloys against metallic substrates were estimated by the lumped heat capacitance model and the contact condition was assessed by scanning electronic microscopy (SEM). Copper substrates yielded maximum contact heat flux followed by brass and aluminium substrates. The SEM study in the solder/substrate interfacial region revealed the existence of a clear gap with the aluminium substrate. A conforming contact was obtained with copper and brass substrates.     

Effect of casting techniques on tensile properties of cast aluminium alloy (Al–Si–Mg) and TiB2 containing metal matrix composite

Abstract

An analysis of the statistical distribution of the tensile strength of a TiB₂ containing aluminium matrix composite and its matrix alloy (Al-7Si-0.35Mg) was carried out using different casting techniques. The scatter of the tensile strength data was assessed by Weibull statistics. Results for the metal matrix composite (MMC) and the matrix alloy in as cast and heat treated conditions were compared. It was found that a low turbulence casting technique resulted in less scatter of tensile values, confirming the greater reliability of the cast material. Fractographic examination of the fractured faces of lower strength specimens showed that entrained oxide films play an important role in failure of the specimens. Heat treatment causes increased scatter in strength values, reflected in the lower Weibull modulus.     

Investigation on Structure and Properties of Brass Casting

In this work, alpha (α) brass was poured in green sand mould and metallic chill mould at about 1050℃. Sand casting method and metallic chill casting method are representing the slow and fast cooling rates of the castings, respectively. The slow cooling rate in the sand mould produces larger grains, while the metallic chill mould produces smaller grains in the castings. As the grain size decreases, the strength of the cast brass increases; micro-porosity in the casting decreases and the tendency for the casting to fracture during solidification decreases. Thus, the faster cooling rate casting offers higher strength, density and hardness compared to the slow cooling rate casting.     

Ti-6Al-4V熔模精密铸造充型及凝固过程计算机模拟

应用自行开发的基于微机上运行的铸件凝固 /充型计算机模拟软件 ,对Ti-6Al-4V钛合金薄壁件精密铸造的充型及凝固传热过程进行了模拟分析 .应用自行安装的多通道钨铼热电耦温度数据计算机采集、分析系统 ,测定了该钛合金起吊接头精密铸件的凝固冷却曲线 ,获取了该合金有关的凝固参数 .对包括上述零件在内的钛合金薄壁件精密铸造的充型过程及凝固传热的温度分布进行了数值模拟 ,模拟计算与实测结果合理吻合 .基于该研究可对其精密铸造工艺进行优化设计 .     

Computer model of unidirectional solidification of single crystals of high temperature alloys

Abstract

I t has been common practice to use mould withdrawal unidirectional solidification to produce single crystal castings. To grow single crystals successfully, it is important to control several solidification parameters, such as the morphology of the solidification front (solid/liquid interface), thermal gradient, and growth rate during solidification. It is the aim of this study to develop a solidification model that can predict such solidification parameters for various design and operating conditions. The solidification phenomena in the process modelled are basically controlled by two heat transfer mechanisms: conduction and radiation. A set of heat transfer equations and boundary conditions were employed to describe mathematically the heat transfer phenomena. Then the finite difference method was used numerically to solve these equations for specified boundary conditions to obtain the temperature distribution and temperature variation in the casting. The solidification parameters can subsequently be deduced from these temperature data. Several thin plate castings were tested using the model developed. The following design and operating conditions were evaluated: susceptor temperature (power input), withdrawal speed, changes of cross-sectional area in the casting, and geometrical arrangement of the casting tree.

MST/1422     

Recent research and development of mould materials for casting TiAl alloys

Intermetallic TiAl alloys are new generation high-temperature material. However, extensive application of TiAl alloys is hindered by some disadvantages, especially the high processing cost. Currently, precision casting is an effective method to manufacture TiAl components with complex shape. However, the interfacial reaction between the TiAl alloy melt and mould affects the quality of the castings and hinders extensive applications of casting TiAl components. In this paper, the research status of mould materials for the casting of TiAl alloys is reviewed. Performances of present used mould materials are compared in details. Reaction mechanisms between mould materials and the melts of TiAl alloys are also summarised. Finally, the future development tendency and prospect are pointed out.     

Influence of mould design on the solidification of heavy forging ingots of low alloy steels by numerical simulation

Ingot casting of a 6-ton, heat-treatable Cr–Mo low alloy steel was simulated using finite element method in three dimensions. Effects of casting parameters including bottom pouring rate, mould slenderness ratio, mould slope, and height and shape of the hot top isolate on solidification behaviour and crack susceptibility during subsequent hot forging of the ingot were investigated. The simulation model was validated against experimental data of two different ingot mould designs. Influences of the casting parameters on the riser efficiency and possible crack formation in the intersection of hot top and ingot body during subsequent open-die forging of the cast steel ingots were discussed. Results showed that pouring the melt under a constant rate, reducing the mould slenderness ratio, and using a proper design for the hot top isolate would all improve the riser efficiency and thereby possibly reduce crack susceptibility during subsequent hot forging.