Mechanical properties as a function of microstructure and solidification thermal variables of Al–Si castings

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Abstract

The aim of the present study was to investigate the influence of solidification thermal variables on the as-cast microstructure of hypoeutectic Al–Si alloys and to establish correlations with the casting mechanical properties. Experimental results include transient metal/mold heat transfer coefficients, tip growth rate, local solidification time, secondary dendrite arm spacing, ultimate tensile strength and yield strength as a function of solidification conditions imposed by the metal/mold system. It was found that the ultimate tensile strength increases with increasing alloy solute content and with decreasing secondary dendrite arm spacing. Yield strength seems to be independent of both alloy composition and dendritic arrangement. Such results have permitted general expressions correlating dendrite spacing with transient solidification processing variables to be established. The correlation of such expressions with experimental equations relating the ultimate tensile strength and dendrite spacing provides an insight in the preprogramming of solidification in terms of mechanical strength of Al–Si castings. Predictive theoretical and experimental approaches for dendritic growth have been compared with the present experimental observations.

Introduction

Aluminum alloys with silicon as a major alloying element consist of a class of alloys, which provides the most significant part of all shaped castings manufactured, especially in the aerospace and automotive industries [1]. This is mainly due to the outstanding effect of silicon in the improvement of casting characteristics, combined with other physical properties such as mechanical properties and corrosion resistance. In general, an optimum range of silicon content can be assigned to casting processes. For slow cooling rate processes (sand, plaster, investment) the range is 5–7 wt%, for permanent molds 7–9% and for die-castings 8–12% [2].

Although the metallurgical and micromechanical aspects of the factors controlling microstructure, unsoundness, strength and ductility of as-cast alloys are complex, it is well known that solidification processing variables are of high order of importance. In the as-cast state an alloy may possess within individual grains, a dendritic network of continuously varying solute content, a complex dispersion of second phases, possibly porosity and inclusions. In addition to the above obstacles to slip, there will be the grain boundaries at the perimeters of these grains. It is generally found that as one decreases the grain size, the strength of a metal increases. The well-known Hall–Petch's equation shows that the yield strength is proportional to the reciprocal of the square root of the grain diameter [3]. For cast metals, however, is not always true that the strength improves with decreasing grain size. There appears to be no significant effect of grain size reduction on mechanical properties of as-cast unmodified hypoeutectic Al–Si alloys [2]. Correlations between macrostructural morphology, grain size and the corrosion resistance have also been recently proposed [4].

A number of studies have pointed the effect of microstructure, and particularly of dendrite spacing upon mechanical properties [5], [6], [7], [8], [9], [10]. The dendrite fineness can be of even more importance in the prediction of mechanical properties than grain size. The improved mechanical characteristics of cast structures having smaller dendrite spacings are due largely to the shorter wavelength of the periodicity of the microsegregation. A general model permitting the correlation between ultimate and yield tensile strengths, dendrite arm spacings and solidification processing variables would be useful in the foundry by preprogramming solidification. In recent articles, the influence of solidification conditions on ductility and ultimate tensile strength of Al–Cu and Zn–Al alloys has been examined [9], [10]. The present work is aimed to develop experimental expressions which correlate the ultimate tensile strength with the secondary dendrite arm spacing for Al–Si hypoeutectic alloys. This microstructural parameter has been linked to tip growth rate or local solidification time by dendritic growth models and analytical expressions describing heat flow during transient solidification permitting to connect the final properties with the operational solidification conditions imposed by the metal/mold system.

Section snippets

Dendritic growth models and solidification thermal variables

Numerous solidification studies have been developed with a view to characterize dendrite spacings under experimental circumstances involving solidification in steady-state heat flow and those in the unsteady-state regime [11], [12], [13], [14], [15], [16], [17], [18]. The later case is of prime importance, since this class of heat flow regime encompasses the majority of industrial solidification processes. In this case, which is the focus of this article, secondary dendrite arm spacing (λ2), is

Experimental procedure

The casting assembly used in solidification experiments shown in Fig. 1a has been detailed in previous articles [9], [10]. In order to promote unidirectional heat flow during solidification, a low carbon steel chill was used, with the heat-extracting surface being polished.

Experiments were performed with hypoeutectic Al–5 wt% Si and Al–9 wt% Si alloys. The thermophysical properties of these alloys, pure elements and chill are summarized in Table 1 [24].

Each alloy was melted in an electric

Results and discussion

Temperature was experimentally monitored in the chill at 3 mm from the metal/mold interface and in the casting at six different locations from this interface. In Fig. 2, typical experimental results of such thermal responses are compared to those numerically simulated by using a finite difference heat flow model [26]. The results of experimental thermal analysis in castings were compared with simulations provided by the finite difference heat flow program, and an automatic search has selected

Conclusions

The following conclusions can be drawn from this study:

  • 1.

    The transient metal/mold heat transfer coefficient, hi, can be expressed as a power function of time and rise with decreasing silicon content of the alloy.

  • 2.

    The theoretical approach proposed by Bouchard and Kirkaldy, relating secondary spacings with tip growth rate, appears not to be appropriate for both alloys examined, if the calibration factor a2 = 9 suggested by these authors is adopted.

  • 3.

    The experimental results concerning tensile testing of

Acknowledgements

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

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