The effect of raw material fabrication process on microstructural characteristics in reheating process for semi-solid forming

doi:10.1016/j.jmatprotec.2005.02.012

Journal of Materials Processing Technology

Volumes 162–163, 15 May 2005, Pages 402-409

https://doi.org/10.1016/j.jmatprotec.2005.02.012 Get rights and content

Abstract

In this paper, the reheating process in the A356 alloy, by electromagnetic stirring (EMS) and extrusion, is discussed. The EMS method is used mainly to manufacture raw materials of a globular microstructure. If relevant reheating is carried out, products with excellent mechanical properties can be manufactured by EMS. However, the cost of raw material for EMS is very high. Contrarily, extruded material has a fine dendrite microstructure and has rarely been assumed to be suitable for the semi-solid forming process. It was found in this study that the microstructure of extruded material could be globularized and applied to the semi-solid forming process only with the relevant reheating parameters. This applicability was estimated by investigating, through image analysis properties such as the solid fraction, the mean equivalent diameter and roundness under various reheating conditions. Under the same reheating conditions, the microstructure of the material by EMS and that of the material by extrusion was compared. Also, the variation of the globular morphology and the coarsening of the primary α-Al phase were investigated according to the final isothermal holding time in the reheating process using a billet diameter 4 in.

Introduction

Recently, in order to obtain lightweight parts for the automobile, aerospace, home appliance industries and others, the use of aluminum and magnesium alloys, has been abruptly increased. To obtain the required mechanical properties of these materials, many foundry processing methods have been applied. Especially, the demand for products by die casting, squeeze casting, and thixocasting has been steadily increased because these casting processes can be near net shape [1]. But, in the die casting process, the mechanical properties are low grade, and heat treatment is impossible. In squeeze casting, the cycle time is longer and the production cost is higher. Thixocasting obtains excellent mechanical properties by the globular microstructure, a longer die life, reduced macrosegregation, reduced porosity and short cycle time [2], [3]. But, raw material cost is high, because the used material in the thixocasting is mainly fabricated by electromagnetic stirring (EMS).

For thixocasting, process of the reheating EMS billets is necessary. The purpose of reheating is not only to obtain the required rheological properties and solid fraction but also to ensure the globular microstructure. In the thixocasting process, a grain controlled microstructure, the globular microstructure, is requested. For the desired microstructure, Loué and Suéry proposed the shape factor that described the influence of initial thermomechanical treatments on microstructural evolution during the partial remelting of AlSi7Mg thixocasting feedstock alloys [4]. It was suggested that various criteria, the solid fraction, the shape factor, the grain size and the contiguity of the solid phase, are necessary for the suitability of the alloys and its microstructure in semi-solid processing [5], [6], [7], [8], [9], [10], [11].

In the reheating method, there are two methods. One is the vertical type, and the other is the horizontal type. In the vertical type, if too much thermal energy and heating time are required to heat the billet to the required temperature, undesirable phenomena such as liquid segregation, “the elephant-foot effect” by the billet's own weight and an “electromagnetic end effect” may occur [12], [13]. On the contrary, in the horizontal type, the “elephant-foot effect” seldom appears and the outflow of the liquid remarkably decreases. Therefore, in this study, the horizontal type is applied to the electromagnetic induction heating system. In electromagnetic induction heating, the temperature of the material abruptly increases for a short time. But, a temperature deviation of the reheated slug may occur according to the position of the deviation within the material. With a small variation of the solid fraction, the viscosity of the slug is abruptly varied. In a high solid fraction, the viscosity is so high that the desired fluidity is unobtainable and incomplete filling inside the cavity may occur. In a low solid fraction, it is difficult to obtain the globular microstructure, and liquid segregation. Therefore, during thixocasting, it is very important to maintain a uniform temperature throughout the slug. Generally, it has been reported that the desired solid fraction ranges from 55 to 65% [14].

Generally, in the thixocasting process with controlled grain, EMS billets have mainly been used. In this study, in order to apply extruded material to the thixocasting process, and due to the higher cost of EMS material, the microstructural characteristics of extruded material were examined during the reheating process. The comparisons of the microstructural characteristics of EMS material and extruded material were carried out in view of the suitability of thixocasting. In particular, the solid fraction, equivalent diameter and roundness were investigated and a statistical analysis was performed.

Section snippets

Materials

The A356 aluminum alloy has been widely used in the foundry processing, especially thixocasting, due to its good fluidity and excellent castablility. The content of Mg2Si in the A356 alloy ranges from 0.5 to 0.6% and by this metastable phase, precipitation occurs. Therefore, this alloy has excellent mechanical properties after T5 or T6 heat treatment. In this study, the A356 EMS alloy, THIXALLOY^® by SAG (Austria), was used and specially adapted to the thixocasting process. The diameter of

Microstructures of feedstock materials

The microstructure of the EMS billet is non-dendritic and fine owing to the high cooling rate during continuous casting as shown in Fig. 2(a). The globular morphology of the primary α-Al phase can only be obtained after reheating. Fig. 2(b) and (c) shows the microstructures of the two extruded materials. The grains are elongated along the extrusion direction. In the case of a higher extrusion ratio, the microstructure is denser because stronger deformation occurred.

Reheating condition of EMS and extruded materials

Fig. 3 shows the temperatures

Conclusions

Reheating experiments were carried out with A356 alloy fabricated by EMS and extrusion. Microstructural characteristics such as the solid fraction, the equivalent diameter and roundness after reheating were analyzed for the desired semi-solid forming process. Also, with the extruded material, microstructural evolution over the isothermal holding time was investigated. The main conclusions can be summarized as follows.

(1)
In the EMS and the HER material after reheating, the morphology of the primary

Acknowledgements

This work has been supported by the National Research Laboratory (NRL) program of Thixo-Rheo Forming. The authors would like to express our deep gratitude to the Ministry of Science and Technology (MOST) for its financial support.

References (15)

N. Saito et al.
JSAE Rev.
(2001)
W.R. Loué et al.
Mater. Sci. Eng. A
(1995)
M. Kiuchi et al.
Ann. CIRP
(2002)
E.J. Zoqui et al.
Mater. Sci. Eng. A
(2002)
E.J. Zoqui
J. Mater. Process. Technol.
(2003)
D. Brabazon et al.
Mater. Sci. Eng. A
(2002)
J.W.K. van Boggelen et al.
Scripta Metall.
(2003)

There are more references available in the full text version of this article.

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The effect of raw material fabrication process on microstructural characteristics in reheating process for semi-solid forming

Abstract

Introduction

Section snippets

Materials

Microstructures of feedstock materials

Reheating condition of EMS and extruded materials

Conclusions

Acknowledgements

JSAE Rev.

Mater. Sci. Eng. A

Ann. CIRP

Mater. Sci. Eng. A

J. Mater. Process. Technol.

Mater. Sci. Eng. A

Scripta Metall.