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.     

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.     

Mould surface roughness and interfacial heat transfer using heat flow model

Abstract

The heat resistance at the metal/mould interface, represented by the interfacial heat transfer coefficient (IHTC), plays an important role in the rate of heat transfer from the metal to the mould. The objective of the present work was to determine the influence of the mould inner surface roughness on the IHTC using steel moulds of diameter 55 mm and height 56 mm with different surface roughnesses to solidify pure zinc with a superheat of 80 K. A computer program solving the heat conduction equation taking into consideration the convection in the molten zinc was used, together with the experimental temperature history, to determine the IHTC at the metal/mould interface. The results show that IHTC decreases as mould surface roughness increases.     

Aluminium and Al–4·5Cu alloy end chill: structural observation and heat flow analysis

Abstract

End chill experiments were performed on aluminium and Al–4·5Cu (wt-%) in order to study the effect of melt superheat (20–150 K), chill material (copper, iron, or sand), and specimen length (890–230 mm) on the type and size of macrostructure. Increasing melt superheat increases the length of columnar zone, which is shorter for the alloy than for the commercial purity metal. The columnar fraction increases with the thermal conductivity of the chill material and the heat transfer coefficient. The results are correlated with the temperature gradient, solidification rate, and growth rate obtained from a heat flow model. The columnar to equiaxed transition is found to occur at a critical temperature gradient and growth rate. These critical values differ with alloy composition. The grain size of columnar and equiaxed grains is found to follow a power relationship with solidification rate.

MST/1709     

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.     

Influences of solute content, melt superheat and growth direction on the transient metal/mold interfacial heat transfer coefficient during solidification of Sn–Pb alloys

Several factors such as alloy composition, melt superheat, mold material, roughness of inner mold surface, mold coating layer, etc., can affect the transient metal/mold heat transfer coefficient, h_i. An accurate casting solidification model should be able to unequivocally consider these effects on h_i determination. After this previous knowledge on interfacial heat transfer, such models might be used to control the process based on thermal and operational parameters and to predict microstructure which affects casting final properties. In the present work, three different directional solidification systems were designed in such a way that thermal data could be monitored no matter what configuration was tested with respect to the gravity vector: vertical upward and downward or horizontal. Experiments were carried-out with Sn–Pb hypoeutectic alloys (5 wt.% Pb, 10 wt.% Pb, 15 wt.% Pb and 30 wt.% Pb) for investigating the influence of solute content, growth direction and melt superheat on h_i values. The experimentally obtained temperatures were used by a numerical technique in order to determine time-varying h_i values. It was found that h_i rises with decreasing lead content of the alloy, and that h_i profiles can be affected by the initial melt temperature distribution.     

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 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.     

Heat transfer characteristics in centrifuge melt-spinning

Centrifuge melt-spinning (CMS) is a new technique for the production of rapidly solidified metallic ribbons. In CMS, centrifugal forces are used twice: to eject the liquid melt on to the quenching substrate (a copper rim) by rotation of the casting crucible, and to ensure prolonged contact of the solidifying ribbon with the heat extraction sink by making the quenching rim rotate too, in the opposite direction. The heat transport in CMS has a Newtonian nature, as it can be considered as a constant-resistance heat transfer process. Calculated heat transfer coefficients h range between (1.55 to 4.30)×10^–6 W m^–2 sec^–1, a half to one order of magnitude higher than for conventional melt-spinning. Increasing the ejection pressure from 1.8 to 269 kPa causes the apparent heat transfer coefficient to increase by a factor of three. Conversely to conventional melt-spinning, two additional phenomena contribute to the heat transfer characteristics in CMS at high extraction velocities: forced convection and mechanical dragging of the melt. The overall effect is a net improvement of the heat transfer ability in CMS as compared to conventional melt-spinning.     

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.     

An ultrasonic technique for the determination of casting-chill contact during solidification

Heat transfer between a solidifying aluminium alloy casting and a mould is dominated by the thermal resistance created by the interface. Interfacial heat transfer occurs by conduction through the atmosphere between the two surfaces and by conduction through the points of actual contact. (Heat transfer by radiation is probably significant only for ferrous castings.) The extent of real physical contact between two surfaces is difficult to quantify. This paper explains a method, using ultrasonic flaw detection techniques, whereby an estimate of the propagation of an ultrasonic signal through a casting-chill interface is used to infer the degree of actual contact occurring between them.In experiments involving casting and solidification of an aluminium alloy onto a copper chill the technique was found to give information for the first two seconds of the casting process only. In this time a peak in ultrasound transmission was observed, correlating to a maximum in the area of casting-chill contact, followed by a decrease in the ultrasound transmission that corresponded to actual contact areas between the casting and the chill in the region of 5 to 10%.     

An ultrasonic technique for the determination of casting-chill contact during solidification

Heat transfer between a solidifying aluminium alloy casting and a mould is dominated by the thermal resistance created by the interface. Interfacial heat transfer occurs by conduction through the atmosphere between the two surfaces and by conduction through the points of actual contact. (Heat transfer by radiation is probably significant only for ferrous castings.) The extent of real physical contact between two surfaces is difficult to quantify. This paper explains a method, using ultrasonic flaw detection techniques, whereby an estimate of the propagation of an ultrasonic signal through a casting-chill interface is used to infer the degree of actual contact occurring between them.In experiments involving casting and solidification of an aluminium alloy onto a copper chill the technique was found to give information for the first two seconds of the casting process only. In this time a peak in ultrasound transmission was observed, correlating to a maximum in the area of casting-chill contact, followed by a decrease in the ultrasound transmission that corresponded to actual contact areas between the casting and the chill in the region of 5 to 10%.     

直接冷却半连续铸造铝合金圆锭温度场的数值模拟

通过试验方法测定直接冷却半连续铸造铝合金圆锭凝固过程的温度变化曲线,利用逆向法计算出铸锭表面热流和换热系数;然后采用数值方法模拟直接冷却半连续铸造过程温度场,实测结果和模拟数据基本吻合。     

Dendritic solidification microstructure affecting mechanical and corrosion properties of a Zn4Al alloy

In the present article some important trends have been shown regarding the relationship between solidification variables, microstructure, mechanical and corrosion properties of Zn-4 wt%Al alloy castings. The aim of the present work is to investigate the influence of heat transfer solidification variables on the microstructure of Zn-4 wt%Al castings and to develop correlations with mechanical and corrosion properties. Experimental results include transient metal/mould heat transfer coefficient (h_i), secondary dendrite arm spacings (λ₂), corrosion potential (E_Corr), corrosion rate (i_Corr), ultimate tensile strength (σ_u) and yield strength (σ_y) as a function of solidification conditions imposed by the metal/mould system. It was found that a structural dendritic refinement provides both higher corrosion resistance and better mechanical properties for a hypoeutectic Zn4Al alloy.     

The effect of quenching media on the heat transfer coefficient of polycrystalline alumina

Surface heat transfer coefficient values were measured for polycrystalline alumina quenched into water, into oil, and into liquid nitrogen. Since the measurements of the surface heat transfer coefficient h for alumina (and ceramics in general) are very limited, we compare our measurements with calculations of h for the water quench and with measurements of h on non-ceramic materials for the oil and liquid nitrogen quenches.     

Retrieval of multidimensional heat transfer coefficient distributions using an inverse BEM-based regularized algorithm: numerical and experimental results

The surface distribution of heat transfer coefficients (h) is often determined point by point using surface temperature measurements of the tested object, initially at a uniform temperature and impulsively imposed with a convective boundary condition, and the solution to the transient heat conduction equation for a semi-infinite medium. There are many practical cases where this approach fails to adequately model the temperature field and, consequently, leads to erroneous h values. In this paper, we present an inverse BEM-based approach for the retrieval of spatially varying h distributions from surface temperature measurements. In this method, a convolution BEM marching scheme is used to solve the conduction problem. At each time level, a regularized functional is minimized to estimate the current heat flux and simultaneously smooth out uncertainties in calculated h values due to experimental uncertainties in measured temperatures. Newton's cooling law is then invoked to compute h. Results are presented from a numerical simulation and from an experiment. It is also shown that the method can be readily applied to steady-state.     

Digital photocalorimetric measurements of cooling rates in chill block melt spinning of MmNiCoMnAl5 hydride forming alloy

Abstract

A digital photocalorimetric technique has been developed and applied to obtain in situ temperature measurements from chill block melt spun ribbons of a MmNiCoMnAl5 hydride forming alloy. Compared with conventional colour transmission temperature measurements, this technique offers special advantages in terms of high resolutional and positional accuracy, which under the prevailing experimental conditions are found to be +-29 K and +-0.1 mm respectively. Moreover, it is shown that the cooling rate in the solid state is approximately 2.5 times higher than observed during solidification, indicating that the solid ribbon stays in intimate contact with the wheel surface down to very low metal temperatures before the bond is broken. During this contact period, the cooling regime shifts from near ideal in the melt puddle to near Newtonian towards the end, when heat transfer from the solid ribbon to the wheel becomes the rate controlling step.     

Experimental correlation of pool boiling heat transfer for HFC134a on enhanced tubes: Turbo-E

The objectives of this paper are to study the heat transfer characteristics for enhanced surface tubes in the pool boiling and to provide a guideline for the design conditions for the evaporator using HFC134a. The shape of tube surfaces, the wall superheat, and the saturation temperature are considered as the key parameters. Copper tubes (d_o = 19.05 mm) are treated with different helix angles and the saturation temperatures are controlled from 3 to 16 °C. It is found that the pool boiling heat transfer coefficient decreases with increasing the wall superheat. It is also found that boiling heat transfer coefficients for Turbo-II and Turbo-III are 1.5–3.0 times and 1.2–2.0 times higher than that for Turbo-I without the helix angle, respectively. The higher heat transfer performance from Turbo-II and Turbo-III can be explained by the “bubble detention” phenomenon on the surface without the helix angle for the Turbo-I. The experimental correlations for the pool boiling heat transfer on the present enhanced tubes without (Type I) and with the helix angle (Type II and Type III) are developed with the error bands of ±30%, respectively.     

Effect of gravity on pool boiling heat transfer on thermal spray coating

This study deals with heat transfer enhancement surface manufactured by thermal spraying. Two thermal spraying methods using copper as a coating material, wire flame spraying (WFS) and vacuum plasma spraying (VPS), were applied to the outside of copper cylinder with 20 mm OD. The surface structure by WFS was denser than that by VPS. The effect of gravity on boiling heat transfer coeffcient and wall superheat at the onset of boiling were experimentally evaluated under micro- and hyper-gravity condition during a parabolic trajectory flight of an airplane. Pool boiling experiments in saturated liquid of HCFC123 were carried out for heat fluxes between 1.0 and 160 kW/m² and saturated temperature of 30 °C. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at the onset of boiling under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was a little. For the coating, a large difference in heat transfer coefficient to gravity was observed in the moderate heat flux range. The heat transfer coefficinet decreased as gravity changed from the normal to hypergravity, and was improved as gravity changed from the hyperto microgravity. The difference in heat transfer coefficient between the normal and microgravity was a little. Heat transfer enhancement factor was kept over the experimental range of heat flux. It can be said that boiling behavior on thermal spray coating might be influenced by flow convection velocity.     

Abrupt Increase of the Absorption Coefficient of Alumina at Melting by Laser Radiation and Its Decrease at Solidification

Experimental data for the normal-hemispherical reflectivity R of remolded aluminum oxide ceramics for wavelengths of (0.488, 0.6328, 1.15, and 3.39) μm and effective (radiance) temperatures T _eff1 and T _eff2 for wavelengths of 0.55 μm and 0.72 μm were obtained in the process of rapid subsecond heating by CO₂ laser radiation in air and vacuum from room temperature to the formation of thin molten layers of 0.6 mm to 0.7 mm thickness and of subsequent rapid free cooling with solidification of the melt when the laser radiation was blocked. Experimentally and by numerical simulation of combined radiation and conduction heat transfer, the influence of the heating radiation flux on the formation of the thin melt on the surface of ceramics with an abrupt increase of T _eff1 and T _eff2 and on the signal of the spectrometer in the infrared range from 2 μm to 11 μm at melting and on its decrease at solidification were studied. The radiation heat flux varied from 500 W · cm⁻² to 2000 W · cm⁻². It is shown that the determining effect on the temperature field and on the intensity of outgoing radiation is caused by the formation of the isothermal continuous two-phase zone and the abrupt increase (decrease) of the absorption coefficient of the melt. The importance of kinetics in the abrupt change of the absorption coefficient of molten Al₂O₃ is noted.