Differential Scanning Calorimetry (DSC) and Thermodynamic Prediction of Liquid Fraction vs Temperature for Two High-Perf
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TRODUCTION
SEMI-SOLID metal (SSM) processing is a powerful technology (with several variants including thixoforming and rheocasting) for forming alloys in the semi-solid state to near net-shaped products (e.g., Reference 1). For thixoforming (which involves reheating billet
DUYAO ZHANG is with the School of Engineering, RMIT University, 115 Queensberry St, Melbourne, VIC, 3053, Australia. HELEN V. ATKINSON is with the Department of Engineering, University of Leicester, University Rd., Leicester, LE1 7RH, UK, and also with the School of Aerospace, Transport and Manufacturing, Cranfield University, College Rd., Bedford, MK43 0AL, UK. Contact e-mails: [email protected], Helen.Atkinson@cranfield.ac.uk HONGBIAO DONG is with the Department of Engineering, University of Leicester. QIANG ZHU is with the Department of Mechanical and Energy Engineering, Southern University of Science and Technology, No 1088, Xueyuan Rd., Shenzhen, Guangdong, 518055, China. Manuscript submitted on March 2, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
material from the solid state into the semisolid state), the liquid fraction for processing is typically between 30 vol pct and 50 vol pct. The process relies on the presence of a microstructure consisting of spheroids (globules) of solid in a liquid matrix when the material is in the semisolid state. With conventionally cast aluminum alloys such as A356 and A357, excellent mechanical properties can be achieved by SSM in comparison with the conventional casting routes. There are more challenges with high strength aluminum alloys (such as the silver-containing 201, or the Al-Si hypoeutectic Cu-containing alloy 319) often due to hot cracking during the final stages of forming. These alloys usually contain copper, a key alloying element which contributes to the strength and age hardening response, but can also increase hot tearing.[2] The excellent castability and good mechanical properties of 319 Al alloy (Al-6Si-3.5Cu-0.1 Mg wt pct) and 201 alloy (Al-5Cu-0.35Mg-0.35Mn-0.7Ag) give them widespread application in the automotive and aerospace industry, respectively.[3,4] Alloy 319s is a primary version of the low-cost foundry alloy 319 to enhance the suitability for
Table I.
Solidification Reactions in 319 Alloy from Ba¨ckerud et al.[3]
Reaction
Suggested Temperature [K (C)]
1. Liq. fi a-Al 2a. Liq. fi Al + Al15(FeMn)3Si2 2b. Liq. fi Al + Si + Al5FeSi 3. Liq. fi Al + Si + Al5FeSi 4. Liq. fi Al + Al2Cu + Si + Al5FeSi 5. Liq. fi Al + Al2Cu + Si + Al5Mg8Cu2Si6
882 863 863 848 798
K K K K K
(609 (590 (590 (575 (525
C) C) C) C) C)
780 K (507 C)
Note that these are for alloy 319 with composition Al-5.7Si3.4Cu-0.62Fe-0.1Mg-0.92Zn-0.14Ti-0.36Mn, wt pct) rather than 319s (see Table III).
Table II. Solidification Reactions in 319 Alloy from Samuel et al.[4]
Reaction 1. 2. 3. 4. 5.
(Al) Dendrite network Precipitation of eutectic Si Precipitation of Al6Mg3FeSi6 + Mg2Si Precipitation of Al2Cu Precipitation of Al5Mg8Cu2Si6
Suggested Temperature [K (C)] 883 835 827 783 763
K K K K K
(610 (562 (554 (
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