Alloy development and reheating process exploration of Al-Si casting alloys with globular grains for thixoforming
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Zhong Min Zhang Research and Development Department, Shanghai Junsoft Digital Science & Technology Co., Ltd, Shanghai 200051, China
A.B. Phillion Materials Science and Engineering, McMaster University, Hamilton L8S 4L7, Ontario, Canada
Shu Zhen Shang and Gui Min Lub) The Key Laboratory of Pressure Systems and Safety, Ministry of Education, East China University of Science and Technology, Shanghai 200237, China (Received 1 March 2016; accepted 4 April 2016)
A novel two-stage reheating process with new alloy design has been developed to improve the microstructure morphology of semisolid Al–Si casting aluminum alloy for thixoforming. The process consists of first reheating the material to the liquidus temperature, holding for 5 min, and then lowering to the predetermined two-stage reheating temperature between 843–863 K and holding for 10 min. The experimentally-obtained grain diameter, roundness, and the amount of liquid trapped within the solid phase were characterized, along with the microstructure obtained using the traditional feedstock reheating process. The Wilcox test (with a 5 0.05) was then applied to statistically analyze the measured differences in the microstructures obtained using the two different processing routes. It was found that a refined near-spherical structure with uniform globule size, higher sphericity, lower coarsening rate constant, and less entrapped liquid was obtained via the new two-stage reheating process in comparison with the microstructure obtained using the traditional feedstock reheating process.
I. INTRODUCTION
Thixoforming is a near-net-shape manufacturing process that is performed at temperatures between the solidus and the liquidus,1–3 providing different recrystallization behavior as compared to conventional processes.4–8 The most distinct characteristic of thixoformed microstructure is the co-existence of liquid and solid during processing (since this occurs at semi-solid temperatures), and the formation of a primary phase with near-spherical grain morphology.9–11 This process has received considerable attention recently due to its significant advantages over traditional metal forming process, such as minimization of as-cast porosity and macro-segregation; reduced forming temperatures, forming forces, and machining costs; and high strength and corrosion resistance.12–15 During the thixoforming process, the billet feedstock is reheated into the semisolid state before the forming Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.163
operation. It is well known that semisolid alloys have a thixotropic behavior in which the fluidity is timedependent and strongly shear-thinning, meaning that at rest the material can be handled like a solid, and during shearing it becomes very fluid with liquid-like flow.16,17 This behavior greatly relies on nondendritic and spherical/ globular microstructure being achieved during the feedstock preparation process.1–23 Tzimas and
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