Effect of cooling rate on solidified microstructure and mechanical properties of aluminium-A356 alloy

doi:10.1016/j.jmatprotec.2007.12.059

Journal of Materials Processing Technology

Volume 207, Issues 1–3, 16 October 2008, Pages 107-111

https://doi.org/10.1016/j.jmatprotec.2007.12.059 Get rights and content

Abstract

A fast-cooling technology using a copper mould cooled by a phase-transition medium was used to prepare cast aluminium-A356 alloy by solidification of the melt. The cooling rate achieved with this technique is in the order of 10² K/s. As-cast samples with a diameter of 10 mm were produced. The microstructure and mechanical properties of this alloy have been investigated. The results show that both the primary and secondary dendrite arm spacing (DAS) are better refined by using this technology than with a conventional casting method. The cooling rate can be controlled to some extent by changing the amount of cooling medium. The DAS decreases with increasing cooling rate, and the microhardness and strength increase correspondingly.

Introduction

Cast aluminium-A356 alloy is one of the most well-developed aluminium alloys due to its outstanding properties. It is widely employed in numerous automotive and industrial weight sensitive applications, such as aeronautics and space flight, because of its low density and excellent castability.

Actually, in most cases high-level mechanical properties are needed for industrial applications, so the performance of this alloy has been the subject of many micromechanical investigations (López et al., 2003, Gokhale and Patel, 2005, Yu et al., 1999, Yang et al., 2005). Since the strength and hardness of alloys mainly depend on their microstructure, a lot of efforts have been made to refine the microstructure of the castings in order to enhance the mechanical properties of aluminium-A356 alloy. Adding modifier and refiner (Wang et al., 2003, Liao et al., 2002) to the melt is a common way of doing this, and has been adopted by many researchers. Power ultrasound (Jian et al., 2005) and electromagnetic stirring (Jung et al., 2001) have also been used to refine the microstructure of alloys. The methods mentioned above have been used by many researchers in recent years and their effect on refining the microstructure is known to some extent. However, there has been little research on refining the microstructure by improving the bulk melt's cooling rate. To our knowledge, a water-cooled copper mould is an effective fast-cooling method. However, the cooling rate achieved by this method is limited and is difficult to control for use in bulk-casting Al alloy.

Therefore, a novel method, that is a cast copper mould with phase-transition materials as the cooling medium is adopted here to refine the casting of aluminium-A356 alloy. The results show that the dendrites are more refined than in conventional casting methods. The strength and microhardness of A356 alloy solidified in a fine microstructure have been investigated.

Section snippets

Experimental details

Commercial A356 aluminium alloy was used in this experiment, for which the nominal composition (wt.%) was 6.5–7.5Si, 0.25–0.45Mg, 0.08–0.20Ti, Fe < 0.2, with the balance being Al. The alloy ingot was remelted in a vacuum induction furnace with an argon atmosphere and kept for 10 min to make the melt homogeneous. The melt was then poured into copper moulds cooled by a phase-transition medium and water, respectively. Sn was used as the phase-transition cooling medium for its physical properties in

Microstructure observation

Fig. 2 shows optical micrographs of specimens prepared with a copper mould cooled by using a phase-transition medium and water, respectively. The site of the microstructures was shown in Fig. 1 and they were taken in the same place. As can be seen, both them exhibited an ordinary dendrite structure consisting of primary α-Al dendrites and modified eutectic Si particles, but the main difference between them was that both of the primary and secondary dendrite arms of α-Al on the specimen obtained

Conclusions

Copper mould casting with cooling by a phase-transition medium is a potential method for fast solidification. This method can clearly refine the microstructure with a larger cooling rate, and has a superior cooling ability than a water-cooled copper mould, as proved by the microstructure of the as-cast samples. The relationship between the DAS of A356 and the cooling rate agrees with the classical formula. In addition, different cooling rates can be obtained by this technique. The microhardness

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Effect of cooling rate on solidified microstructure and mechanical properties of aluminium-A356 alloy

Abstract

Introduction

Section snippets

Experimental details

Microstructure observation

Conclusions

Mater. Sci. Eng. A

J. Mater. Process. Technol.

Prog. Mater. Sci.

Mater. Lett.

Mater. Lett.

J. Mater. Process. Technol.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Sci. Eng. A