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.     

Effect of melt superheat on heat transfer coefficient for aluminium solidifying against copper chill

Solidification of metal castings inside moulds is mainly dependent on the heat flow from the metal to the mould which is in turn proportional to an overall heat transfer coefficient h which includes all resistances to heat flow such as the presence of an air gap. In the present work the heat transfer coefficient is determined using a directional solidification set-up with end chill for solidifying commercial-purity aluminium with different superheats (40 K and 115 K) against copper chill. A computer program solving the heat conduction and convection in the solidifying metal is used together with the experimental temperature history in order to determine the heat transfer coefficient at the interface. The variation of h as a function of time, surface temperature and gap temperature for each melt superheat is found. The results indicate that h reaches a maximum value for surface temperature close to the liquidus. The analysis of heat flux from the metal to the mould indicates that it is mainly by conduction. The air gap size is evaluated with time, surface temperature and with melt superheat. It is found that higher h values and smaller gap sizes are obtained with higher superheats.     

Heat Transfer Characteristics of Billet/Die Interface and Measures to Relieve Thermal Stress for Hot Forging Die

The interfacial heat transfer coefficient (IHTC) shows the heat transfer capacity at the billet/die interface during the hot forming process, which affects the temperature gradient in the die that may potentially induce high thermal stress. Consequently, this determines the service life of the die. In this paper, a set of experimental equipments were used to identify the IHTC and the upsetting test of superalloy Inconel718 (GH4169) was carried out on the hot flat die to evaluate the IHTC characteristics after the billet heating and die preheating temperature, holding time, and billet deformation rate. The results indicated that the billet heating temperature has a minimal role in IHTC but the other components have a great impact on IHTC. Among them, the billet deformation rate has influenced the IHTC the most. In the die preheating temperature ranging from \(150\,^{\circ }\hbox {C}~\hbox {to}~400\,^{\circ }\hbox {C}\), it was found that the preheating temperature was proportional to IHTC. A high preheating temperature that leads to a high IHTC was found unfavorable in relieving the die surface thermal stress, and also weakened the die hardness and strength. The IHTC declined with the increase in the holding time as a result of the billet oxidation. Based on these findings, the composite ceramic and polymetallic heat-resistant coatings on the die surface were prepared, respectively, to relieve the thermal stress of die surface by reducing IHTC. It showed that both of the treated dies could effectively reduce the IHTC, blocking the transferred heat from the hot billet and making it applicable to the different hot forging events.     

Determination of Interfacial Heat Transfer Behavior at the Metal/Shot Sleeve of High Pressure Die Casting Process of AZ91D Alloy

The interfacial heat transfer behavior at the metal/shot sleeve interface in the high pressure die casting(HPDC) process of AZ91 D alloy is carefully investigated.Based on the temperature measurements along the shot sleeve,inverse method has been developed to determine the interfacial heat transfer coefficient in the shot sleeve.Under static condition,Interfacial heat transfer coefficient(IHTC) peak values are 11.9,7.3,8.33 k W m~(-2)K~(-1)at pouring zone(S2),middle zone(S5),and end zone(S10),respectively.During the casting process,the IHTC curve displays a second peak of 6.1 k W m~(-2)K~(-1)at middle zone during the casting process at a slow speed of 0.3 m s~(-1).Subsequently,when the high speed started,the IHTC curve reached a second peak of 12.9 k W m~(-2)K~(-1)at end zone.Furthermore,under different slow casting speeds,both the calculated initial temperature(TIDS) and the maximum temperature(Tsimax) of shot sleeve surface first decrease from 0.1 m s~(-1)to 0.3 m s~(-1),but increase again from 0.3 m s~(-1)to 0.6 m s~(-1).This result agrees with the experimental results obtained in a series of "plate-shape" casting experiments under different slow speeds,which reveals that the amount of ESCs decreases to the minimum values at 0.3 m s~(-1)and increase again with the increasing casting slow speed.     

热金属表面多喷嘴风冷过程气-固耦合传热模拟

目的 获得多喷嘴风冷过程的界面换热系数,并研究风冷工艺参数对界面换热的影响规律.方法 基于Fluent软件对三喷嘴强制风冷传热过程进行"气-固"耦合分析,获得高压气流的流速场和钢板表面温度场.基于"气-固"耦合分析得到钢板表面平均温度曲线,利用自开发的反传热软件计算得到"气-固"耦合界面换热系数,并将界面换热系数以第三类边界条件施加在钢板表面进行瞬态传热分析.结果 对于直径为4 mm的喷嘴,当喷嘴间距为10～16 mm时,喷嘴间距对高压气体的流速场影响较大,气流的卷吸效应随着喷嘴距离的增大而增强;喷嘴间距对界面换热系数影响较小,喷嘴至钢板表面的距离对界面换热系数影响较大;随着喷嘴至钢板表面距离的增大,各股气流逐渐汇合为一股,各股气流的滞止区也逐渐汇合,钢板表面温度更加均匀;将界面换热系数以第三类边界条件施加在钢板表面进行瞬态传热分析,得到的钢板表面温度与"气-固"耦合分析得到的钢板表面平均温度曲线吻合得较好.结论 获得的界面换热系数可为多喷嘴风冷过程数值模拟提供可靠的数据,保证温度场的求解精度.     

Effect of casting/mould interfacial heat transfer during solidification of aluminium alloys cast in CO2-sand mould

The ability of heat to flow across the casting and through the interface from the casting to the mold directly affects the evolution of solidification and plays a notable role in determining the freezing conditions within the casting, mainly in foundry systems of high thermal diffusivity such as chill castings. An experimental procedure has been utilized to measure the formation process of an interfacial gap and metal-mould interfacial movement during solidification of hollow cylindrical castings of Al-4.5 % Cu alloy cast in CO₂-sand mould. Heat flow between the casting and the mould during solidification of Al-4.5 % Cu alloy in CO₂-sand mould was assessed using an inverse modeling 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 normalized with respect to the peak heat flux and modeled as a function of time. The heat flux model proposed was to estimate the heat flux transients during solidification of Al-4.5 % Cu alloy cast in CO₂-sand moulds.     

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.     

The effect of heat transfer additive and surface roughness of micro-scale hatched tubes on absorption performance

The objectives of this paper are to investigate the effect of heat transfer additive and surface roughness of micro-scale hatched tubes on the absorption performance and to provide a guideline for the absorber design. Two different micro-scale hatched tubes and a bare tube are tested to quantify the effect of the surface roughness on the absorption performance. The roughness of the micro-scale hatched tubes ranges 0.39–6.97 μm. The working fluid is H₂O/LiBr solution with inlet concentration of 55, 58 and 61 wt.% of LiBr. Normal Octanol is used as the heat transfer additive with the concentration of 400 ppm. The absorber heat exchanger consists of 24 horizontal tubes in a column, liquid distributor at the liquid inlet and the liquid reservoir at the bottom of the absorber. The effect of heat transfer additive on the heat transfer rate is found to be more significant in the bare tube than that in the micro-scale hatched tubes. It is found that the absorption performance for the micro-hatched tube with heat transfer additive becomes up to 4.5 times higher than that for the bare tube without heat transfer additive. It is concluded that the heat transfer enhancement by the heat transfer additive is more significant than that by the micro-scale surface treatment.     

Modeling the impact of a molten metal droplet on a solid surface using variable interfacial thermal contact resistance

An analytical model of the true area of contact between molten metal and a rough, solid surface has been used to calculate thermal contact resistance and to predict how it changes with surface roughness, substrate thermal properties and contact pressure. This analytical model was incorporated into a three-dimensional, time-dependent numerical model of free-surface flows and heat transfer. It was used to simulate impact, spreading and solidification of molten metal droplets on a solid surface while calculating contact resistance distributions at the liquid–solid interface. Simulations were done of the impact of 4 mm diameter molten aluminum alloy droplets and 50 μm diameter plasma sprayed nickel particles on steel plates. Predicted splat shapes were compared with photographs taken in experiments and simulated substrate temperature variation during droplet impact was compared with measurements.     

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.     

Metal foams as compact high performance heat exchangers

Open-cell metal foams with an average cell diameter of 2.3 mm were manufactured from 6101-T6 aluminum alloy and were compressed and fashioned into compact heat exchangers measuring 40.0 mm × 40.0 mm × 2.0 mm high, possessing a surface area to volume ratio on the order of 10,000 m²/m³. They were placed into a forced convection arrangement using water as the coolant. Heat fluxes measured from the heater-foam interface ranged up to 688 kW m⁻², which corresponded to Nusselt numbers up to 134 when calculated based on the heater-foam interface area of 1600 mm² and a Darcian coolant flow velocity of approximately 1.4 m/s. These experiments performed with water were scaled to estimate the heat exchangers’ performance when used with a 50% water–ethylene glycol solution, and were then compared to the performance of commercially available heat exchangers which were designed for the same heat transfer application. The heat exchangers were compared on the basis of required pumping power versus thermal resistance. The compressed open-cell aluminum foam heat exchangers generated thermal resistances that were two to three times lower than the best commercially available heat exchanger tested, while requiring the same pumping power.     

Arc deposited AlSi12 cladding layer on steel plate without zinc coating

ABSTRACT

AlSi12 cladding layer was fabricated on steel plate without zinc coating via cold metal transfer arc deposited technique. The wettability, morphologies, microstructures, and mechanical properties of the cladding layer were investigated. The cladding layer exhibited favourable wettability with steel plate. Large swing amplitude and high swing frequency were the key factors to achieve excellent wettability. The cladding layer was composed of primary Al dendrites and (Al?+?Si) eutectic structure, and the microstructures were refined. Continuous and thin intermetallic compound layer appeared at AlSi12/steel interface, and reliable bonding strength between the cladding layer and steel plate was achieved. The fracture surface exhibited both brittle fracture and ductile fracture.     

Coupled heat and fluid flow model and experimental verification of aluminium plate die casting

Abstract

The present work aims to forecast mould filling, void shape, location and size as well as columnar to equiaxed transition (CET) in commercial pure aluminium casting. A model coupling the momentum equations of the fluid flow and heat transfer equations is presented, in which metallostatic pressure, air gap and oxide layer are considered. Different casting parameters were investigated such as casting configuration by varying the plate thickness from 5 to 20 mm, melt superheat from 40 to 120°C, mould preheat up to 200°C and different pouring heads ranging from 0·3 to 0·6 m. Regarding the microstructure and void formation, the approach based on the Niyama criterion, was considered. The experimental verification of the model was achieved by gravity die casting in the form of a rectangular cavity. Voids inside aluminium plate were investigated by X-ray imaging. Microstructure and CET was investigated microscopically. The supposed model proves its validity for mould filling and in detecting the void features and CET.     

Bone-bonding properties of Ti metal subjected to acid and heat treatments

The effects of surface treatment on the bone-bonding properties of Ti metal were examined by both mechanical detaching test and histological observation after implantation into rabbit tibiae for various periods ranging from?4 to?26?weeks. The bone-bonding ability of Ti metal, which is extremely low as it is abraded, was hardly increased by simple heat treatment at 600?°C or treatment with H₂SO₄/HCl mixed acid alone, but was markedly increased by the heat treatment after the acid treatment. Even Ti metal that had been previously subjected to NaOH treatment showed considerably high bone-bonding ability after acid and heat treatments. Such high bonding abilities were attributed to their high apatite-forming ability in the body environment. Their high apatite-forming abilities were attributed to a high positive surface charge, and not to the type of crystalline phase or specific roughness of their surfaces. The present study has demonstrated that acid and subsequent heat treatments are effective for conferring stable fixation properties on Ti metal implants.     

Ar-Kr低温界面传热过程分子动力学模拟

运用非平衡分子动力学原理和LJ势函数仿真研究氩(Ar)与氪(Kr)之间的界面层传热问题,模拟其界面传热的能量变化过程.仿真结果表明,即使界面粗糙度为0,低温固体界面热阻仍然存在.平均温度为40 K,粗糙度为0时,氩(Ar)与氪(Kr)之间界面热阻为0.15～0.18 Wm2/K,相对误差小于17%.     

Enhancement of thermal performance in a sintered miniature heat pipe

Abstract

The performance of a sintered miniature heat pipe is enhanced. With the capillary limitation, porosity takes priority over the wick structure parameters that would affect the heat transfer capacity. Since sintered dendritic copper powder has higher porosity, it is used to mix with pore former (Na₂CO₃) in experiments for increasing porosity, and hence enhancing the thermal performance. The results show that, for a heat pipe with a 3mm outer diameter and 200 mm effective length, the heat transfer rate is up to 16.5W and the thermal resistance is 0.9°C/W. In comparison with the unmixed case, the performance increases about 40%.     

Near net shape processing of metal and ceramic composite ingot mould

Abstract

A new pressureless infiltration method was developed to place a silicon dioxide particle reinforced layer on the surface of an industrial ingot mould without changing the production process of the ingot mould. The morphology and structure of the composite layer were investigated by optical and scanning electron microscopy. The service life and thermal characteristics of the reinforced mould were tested during their use in practical casting processes. It was found that a 4 mm thickness composite layer can be obtained on the surface of an ingot mould with this method. The reinforced mould can be used directly without any extra machining process. The service life of the new mould is twice that of the original for the same application conditions. The heat resistance and wear resistance of the mould were evidently improved.     

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 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.