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%.     

The effect of increased pressure on interfacial heat transfer in the aluminium gravity die casting process

Contraction and distortion of a casting during cooling within a mould can force their respective surfaces together, with the associated increased interfacial pressure resulting in increased interfacial heat transfer. This problem has been examined for the case of gravity and low pressure die casting of an Al alloy, where an insulating coating is applied to the die cavity to assist filling of the mould. The degree of interfacial pressure was estimated to be, for a typical small die casting, at most about 21 MPa. Repeated applications of a compressive load showed that a freshly applied die coating became thinner and smoother, until a stable situation was reached after about ten applications. The interfacial heat transfer coefficient was estimated to be increased by about 20%, with an increase in the applied pressure by a factor of two, from 7 MPa to 14 MPa, and increased by about 40%, with an increase in the applied pressure by a factor of three, from 7 MPa to 21 MPa. The heat transfer mechanisms between the casting and the die surfaces were evaluated to produce a simple model of interfacial heat transfer which included conduction through the points of actual contact, in parallel with conduction through the interfacial gas between the points of actual contact, both mechanisms being in series with the heat transfer by conduction through the die coating. Evaluation of the model produced agreement with experimentally determined values of the interfacial heat transfer coefficient to within about 15%.     

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.     

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.     

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.     

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.     

Metal Pipe Joining with Aluminum Alloy by Ultrasonic Insert Casting

Insert casting of aluminum is widely used in industry. In order to realize better metallurgical bonding between metallic pipe and aluminum alloy castings, a new insert casting method with aided by high-power ultrasound has been developed. The bonding strength between the metal pipe and the cast aluminum alloy was evaluated by shear testing, EPMA analysis at the bonding interface was performed. The experimental results indicate that the application of ultrasonic vibration was effective during insert casting, particularly immediately after the primary a phase began to emerge in the cast aluminum melt. In the case of 6061 Al pipe/AC4C joint, the shear strength was approximately 60 times higher than without ultrasound vibration. This bonding reflects the break up of the oxide film on the aluminum pipe surface by the ultrasound cavitation effect. For the steel pipe/AC4C joint, the bonding shear strength had a several-fold enhancement. Ultrasound may promote a chemical reaction between steel and cast aluminum alloys, leading to excellent bonding.     

Analysis of solidification in Bridgman furnace as simulation of DC casting of aluminium alloy slabs

Abstract

The vertical Bridgman directional solidification equipment has been used in several investigations to simulate direct chill casting of wrought aluminium alloys. As a basis for such investigations and alloy developments, it is important to have an understanding of the performance of the furnace used during simulation of the casting conditions. In this investigation the thermal conditions in the furnace have been analysed in detail, both by measurements and by mathematical modelling. The growth characteristics of the furnace, such as gradient, growth rates and cooling rates have been compared to conditions in large ingots. The direct chill casting conditions, which the simulations have been compared to, are casting of slabs of 330 and 600 mm thickness of an aluminium AA3003 type alloy. The results show that the experiments are able to simulate the cooling conditions in the ingots except from the surface zone. Comparisons of the microstructures have been made and a good agreement has been obtained for structure parameters such as grain size and DAS.     

Effects of ultrasonic treatment on microstructure and mechanical properties of 6016 aluminium alloy

Ultrasonic treatment was applied during solidification forming of 6016 aluminium alloys. The effects of ultrasonic vibration on the microstructure and mechanical properties of the 6016 aluminium alloy ingot were studied. Compared with conventional casting, the ingot prepared with ultrasound applied to the molten metal had a fully refined microstructure and greater material homogeneity. Ultrasonic treatment reduced the amount of porosity developed by embedded gas and also resulted in an obvious decrease in solidification defects, like shrinkage. In addition, component segregation was minimised and the mechanical properties were enhanced.     

Study on casting composite ingot of 4045/3004/4045 aluminum alloys

Two kinds of equipments were designed for casting composite ingots, and three‐layer ingots of 4045/3004/4045 aluminum alloys were prepared by direct‐chill (DC) semi‐continuous casting process, macro‐morphology, microstructure, temperature distribution near the interface, elements distributions and tensile strength of the interface were investigated. The equipment suitable for casting composite ingots was determined, and results show that solid supporting layer with uniform temperature distribution forms near the interface, which ensures the stable casting process. The two different aluminum alloys are well bonded. The criterion for temperature control of solid supporting layer is founded based on the relationship between the interface bonding strength and the temperature of solid supporting layer when contacted by 4045 alloy melt.     

Effect of processing on microstructure and wear characteristics of an Al-4·5Cu-10Pb alloy

An Al-4·5Cu-10Pb alloy was processed by spray forming as well as impeller mixing followed by chill casting methods. The microstructure, mechanical properties and dry sliding wear characteristics of the alloy were evaluated. The spray formed alloy showed an equiaxed grain morphology with a uniform dispersion of lead particles in the matrix phase. In contrast a cellular-dendritic morphology of the primary phase was the characteristic feature of the alloy processed by impeller mixing and chill casting method. The spray formed alloy indicated its superior mechanical properties and low wear rate particularly at higher applied load and sliding velocity. The possible reason for this behaviour is discussed in the light of microstructure of the alloy and the nature of the worn out surfaces of the wear test specimens.     

粗糙表面固态耦合超声检测研究

固态耦合超声检测时两固体粗糙表面接触界面处的超声波不完全耦合,为提升该界面处超声检测的耦合效果,需深入研究其耦合特性。以粗糙表面的弹簧接触模型为基础,结合粗糙表面接触理论推导出固态耦合超声检测的耦合界面理论模型。根据实际情况以及材料参数分析,得到表征界面耦合效果的平均声反射系数与接触载荷以及接触表面当量粗糙度的关系,并与T模型的计算结果进行对比。在不同当量表面粗糙度和不同接触载荷下分别测得接触界面的平均声反射系数,并对比了不同当量表面粗糙度下界面声耦合效果达到最佳时接触载荷的理论值和实验值,计算得最大相对误差为13.04%,表明实验结果与所提出的理论模型结果基本吻合。所建立的固态耦合超声检测界面理论模型形式简洁,实用性强,并可通过针对性地控制相关参数来改善耦合效果。     

Characterisation of Contact Pressure Distribution in Bolted Joints

Abstract: The quantification of contact area and pressure distribution in a bolted joint is essential information, as it determines the integrity of the coupling. Current bolted joint design standards are based on analytical solutions of the pressure distribution, which, because of the inherent assumptions, frequently do not accurately represent the real conditions in a joint. This study uses a non‐intrusive ultrasonic technique to quantify the contact pressure distribution in a bolted connection. The advantage of this experimental technique is that the effect of actual contact conditions can be determined. An ultrasonic wave is focused onto the clamped interface, and the reflected sound signal recorded. In areas where the contact pressure is high, most of the ultrasound is transmitted, and the reflected sound signal is weak. Whereas, when the contact pressure is low, the vast majority of the ultrasound is reflected back. A parallel experimental calibration is then used to find the relationship between the reflected sound signal and contact pressure. In this way, the pressure distribution in a clamped interface is determined for a series of different bolt torques. Two different interfaces were investigated: the first consisted of two ground surfaces clamped together, and the second a turned profile pressed against a ground surface. The effect of a washer underneath the bolt head was also considered. The turned profile was found to cause the contact to spread; there was also a certain degree of fragmentation leading to higher peak pressures than in the ground interface case. With a washer positioned under the bolt head for the turned case, the clamping performance of the bolt was improved. Good agreement was found when comparing the ultrasonic measurements with previous studies, with respect to the spread of the contact pressure distribution. However, in this study, the peak contact pressure was found to occur away from the edge of the bolt hole, and to be influenced by the edge of the bolt head.     

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.     

Controlled effect of ultrasonic cavitation on hydrophobic/hydrophilic surfaces

Controlling cavitation at the solid surface is of increasing interest, as it plays a major role in many physical and chemical processes related to the modification of solid surfaces and formation of multicomponent nanoparticles. Here, we show a selective control of ultrasonic cavitation on metal surfaces with different hydrophobicity. By applying a microcontact printing technique we successfully formed hydrophobic/hydrophilic alternating well-defined microstructures on aluminium surfaces. Fabrication of patterned surfaces provides the unique opportunity to verify a model of heterogeneous nucleation of cavitation bubbles near the solid/water interface by varying the wettability of the surface, temperature and ultrasonic power. At the initial stage of sonication (up to 30 min), microjets and shock waves resulting from the collapsing bubbles preferably impact the hydrophobic surface, whereas the hydrophilic areas of the patterned Al remain unchanged. Longer sonication periods affect both surfaces. These findings confirm the expectation that higher contact angle causes a lower energy barrier, thus cavitation dominates at the hydrophobic surfaces. Experimental results are in good agreement with expectations from nucleation theory. This paper illustrates a new approach to ultrasound induced modification of solid surfaces resulting in the formation of foam-structured metal surfaces.     

Effect of combined application of electromagnetic fields on horizontal direct chill casting of 7050 aluminium alloy

Abstract

Influence of combined electromagnetic field application on horizontal direct chill casting of 7050 aluminium alloy is investigated. Temperature measurement and structure observation are carried out to analyse the casting process under different electromagnetic fields. Combined electromagnetic field can reduce the harmful gravitational thermal effect in the horizontal direct chill casting process and improve the ingot properties. With application of combined electromagnetic field, temperature distribution in the melt is more uniform, sump of the ingot becomes flat and symmetric, surface quality of ingot improves markedly, grain morphology changes from feathery grains to equiaxed grains and grain size decreases. Distribution of copper and zinc in the transverse section of the ingot becomes more uniform.     

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.     

Concept for thermal insulation arrangement within a flexible cryostat for HTS power cables

The successful application of high temperature superconducting power cables requires improved flexible cryostats with respect to their thermal heat load between environment (300 K) and LN₂ (77 K) operating temperature. Heat transfer through the thermal insulation consists of thermal radiation, solid and residual gas conduction. Considering this heat load contributions, a new concept for the thermal insulation arrangement is presented in this paper. The main advantage of this concept consists in a separation of the thermal insulation and the supporting structure. This separation protects the thermal insulation from degradation by mechanical load. The fraction of solid heat conduction through this support structure is minimised by small contact areas and high heat conduction lengths. Heat transfer due to residual gas conduction is reduced by improved evacuation conditions close to the cold wall. Finally the structure allows subdivision of the multilayer insulation ensuring an optimal number of layers with respect to their insulation thickness, i.e. an optimal layer density.     

Fracture analysis of the transition zone between unreinforced alloy and composite

The transition zone between unreinforced alloy and composite is an important problem for selectively reinforced metal-matrix composites manufactured by the squeeze casting technique, mainly because of the weakness of this zone. Developing an understanding of transition zone behaviour is therefore essential. The main purpose of this research was to evaluate the influence of ageing time on the fracture behaviour of such a zone for Saffil (γ-alumina) A356 aluminium alloy composites. Tensile tests were carried out and the fracture surfaces were analysed to evaluate the fracture behaviour. Analysis of microstructure and fracture surfaces was performed using optical and scanning electron microscopies.