Design of mechanical properties of Al-alloys chill castings based on the metal/mold interfacial heat transfer coefficient

https://doi.org/10.1016/j.ijthermalsci.2011.08.014Get rights and content

Abstract

Solidification thermal parameters such as the growth rate and the cooling rate depend on the metal/mold heat transfer efficiency, usually characterized by an interfacial heat transfer coefficient, hi, and determine the arrangement of the solidification microstructure, including its morphology and scale. The mechanical properties can be correlated with the microstructural parameters such as the cellular and dendritic spacings and hence with the instantaneous value of hi. In the present investigation, it is shown that hi varies in time according to an expression of the form hi=atm, where m < 0.5. Directional solidification experiments are carried out, and interrelations of thermal parameters, microstructure and tensile properties are established. Linear relationships between the ultimate tensile strength and hi have been determined for hypoeutectic Al–Fe and Al–Sn alloys.

Highlights

► Solidification thermal parameters depend on the heat transfer coefficient, hi. ► hi determines the solidification microstructure including morphology and scale. ► hi varies in time with an expression hi=atm, m < 0.5 and a depends on alloy composition. ► The mechanical properties can be correlated with the instantaneous value of hi.

Introduction

The boundary conditions imposed at the metal/mold interface of a metallic alloy casting can have a significant role on the solidification kinetics mainly for manufacturing processes such as die casting, permanent mold casting and casting of thin sections by static or continuous processes [1], [2]. Temperature boundary conditions are inherently transient during most commercial casting processes. Several mechanisms of heat transfer occur at the metal/mold interface, which are generally represented by a transient heat transfer coefficient, hi. When metal and mold surfaces are brought into contact an imperfect junction is formed. While uniform temperatures gradients can exist in both metal and mold, the junction between the two surfaces creates a temperature drop, which is dependent upon the thermophysical properties of the contacting materials, the casting and mold geometry, the roughness of mold contacting surface, the presence of gaseous and non-gaseous interstitial media, the melt superheat, contact pressure and initial temperature of the mold. The as –cast microstructures will be affected by certain thermal process parameters, i.e., the growth rate, the thermal gradient and the cooling rate, which are sensitive to hi evolution over time.

The influence of many casting parameters on the interfacial heat transfer coefficients of Al-alloys castings have been investigated and are reported in the literature. Hamasaiid et al. [3] evaluated the influences of coating thickness and coating composition on the interfacial heat transfer coefficient during casting of Al-7Si-0.3Mg and Al-9Si-3Cu alloys in permanent mold castings. The interfacial heat transfer coefficient (hi) was found to decrease as the coating thickness increased, losing about 50 pct of its value when the coating thickness increased from 10 to 100 μm. The peaks of hi are slightly larger during the solidification of the Al-7Si-0.3Mg alloy than the Al-9Si-3Cu alloy, revealing the dependence of hi on the alloy composition. Griffiths proposed a model for the interfacial heat transfer coefficient for an Al casting solidifying against a copper chill, which took into account heat transfer by conduction at the points of interfacial contact and through the interfacial gas [4], [5]. Based on the identification and quantification of the important mechanisms controlling heat transfer between an Al alloy casting and a coated die steel surface, Hallam and Griffiths [6] proposed an accurate model of the interfacial heat transfer coefficient. This model has shown that the principal resistance to heat transfer was due to the presence of a layer of air trapped between the rough casting and coating surfaces. Hamasaiid et al. [7] examined the relationship between in-cavity pressure, heat flux and heat transfer coefficient during high-pressure die casting of an Al-9Si-3Cu alloy. An analytical approach has been recently proposed by Hamasaiid et al. [8] to model hi at the casting–die interface in high-pressure die casting. These authors have shown that that the peak value of hi is primarily determined by both the topography and the mechanisms of contact between the casting and the die, whereas, the transitory phase of hi is further governed by the geometry and composition of casting. Griffiths and Kawai [9] examined the interfacial heat transfer between a solid Al-7Si-0.3Mg casting surface and a coated die surface under conditions of increased interfacial pressure. They reported that hi increased by about 20%, with an increase in the applied pressure by a factor of two, from 7 MPa to14 MPa, and increased by about 40%, with an increase in the applied pressure by a factor of three, from 7 MPa to 21 MPa.

During solidification of alloys, the observed microstructures are diverse but in general can be classified into two basic groups: cells/dendrites and eutectic morphologies. The development of cells during solidification is characterized by a fine corrugate structure, in which the corrugations are roughly parallel to the direction of heat extraction [10], [11]. Dendrite growth is the common mechanism of crystallization from metallic melts, and it is characterized by a morphology which is a result of the growth of long, thin spikes of primary arms in characteristic crystallographic directions with regular branches in other directions. The branching habit can extend to secondary, tertiary and higher orders. The scale of these microstructural patterns, characterized by cellular and dendritic spacings, can significantly affect the properties of as-cast metallic components.

The metal/mold heat transfer coefficient is usually high in the early stages of solidification and tends to drop off with the solidification evolution. Solidification thermal parameters such as the growth rate and the cooling rate depend on hi and determine the arrangement of the solidification microstructure, including its scale and morphology.

The ability to control microstructure it evolves through solidification can enable a casting to be manufactured with appropriate final properties. The effect of microstructure parameters on metallic alloys properties has been highlighted in various studies, particularly considering the influence of the scale of cellular/dendritic arrays upon the mechanical properties and corrosion resistance for a number of alloys [12], [13], [14], [15], [16], [17], [18], [19], [20]. Such influence is particular important for binary alloys which are non heat-treatable. For aluminum-based systems, alloys from two important systems can be classified into this category: hypoeutectic Al–Fe and Al–Sn alloys. Al–Fe alloys are of considerable commercial interest, partially because iron is almost invariably present at significant levels (0.2–1.0 wt%) in foundry aluminum components [21]. Al–Sn alloys are well known for having excellent tribological and mechanical properties making these alloys suitable for engineering applications [22]. In the range of compositions between 15 and 30 wt% Sn these alloys have been widely used as bearing materials [23].

The present study focus on the correlation between the final mechanical properties and the metal/mold heat transfer efficiency during the solidification of chill castings. Al–Fe and Al–Sn alloys, conducive to cellular and dendritic microstructures, respectively, are experimentally examined with a view to determining the transient profile of hi. The effect of heat transfer coefficients on the mechanical properties is established through experimental correlations between the cellular or dendritic scales and the ultimate tensile strength.

Section snippets

The metal/mold heat transfer coefficient (hi)

The way heat flows through the casting and the mold surface affects the evolution of solidification, and is of notable importance in characterizing the casting cooling conditions, mainly for high heat diffusivity metal/mold systems such as chill castings. When the metal comes into contact with the mold, at the metal/mold interface, the solid bodies are only in contact at isolated points and the actual area of contact is only a small fraction of the nominal area, as shown in Fig. 1. Heat flow

Experimental procedure

The casting system used in the unsteady-state solidification experiments consists of a water-cooled mold with heat being extracted only from the bottom, promoting vertical upward directional solidification (Fig. 3). A stainless steel split mold was used having an internal diameter of 60 mm, a height of 150 mm and a wall thickness of 5 mm. The inner vertical surface was covered with a layer of insulating alumina to minimize radial heat losses, and a top cover made of an insulating material was

Results and discussion

Experimental cooling curves were recorded during the directional solidification of all Al-alloys examined. Fig. 5 shows typical examples of experimental cooling curves recorded by thermocouples inserted into the casting and positioned at different vertical positions from the heat-extracting surface at the bottom of the casting, during solidification of Al-1.0wt%Fe and Al-30wt%Sn alloys. Lines appearing in Fig. 5 represent the numerical profiles furnished by a finite difference heat flow model,

Conclusion

Cellular and dendritic growth models and experimental growth laws can be used in order to permit correlations between the scale of cellular and primary dendritic arm spacings and solidification thermal parameters to be established. The mechanical properties can be correlated with these microstructural spacings and hence with hi which was shown to vary in time according to an expression of the form hi=atm, where m < 0.5. In the present study, linear relationships between the ultimate tensile

Acknowledgments

The authors acknowledge the financial support provided by FAPESP (The Scientific Research Foundation of the State of São Paulo, Brazil), CNPq (The Brazilian Research Council) and FAEPEX-UNICAMP.

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