Research Papers

Effect of Cooling Rate on Mechanical Properties of 7075 Aluminum Rods Extruded in Semisolid State

[+] Author and Article Information
Mostafa Ketabchi

Department of Mining and Metallurgy,
Amirkabir University of Technology,
424 Hafez Avenue,
Tehran 15875-4413, Iran
e-mail: ketabchi@aut.ac.ir

Mohammad Amin Shafaat

Department of Mining and Metallurgy,
Amirkabir University of Technology,
424 Hafez Avenue,
Tehran 15875-4413, Iran
e-mail: ma_shafaat@aut.ac.ir

Iman Shafaat

P.O. Box 14665-943,
Tehran 1468863331, Iran
e-mail: i.shafaat@binusco.com

Mahmoud Abbasi

Faculty of Engineering,
University of Kashan,
Ravandi Highway,
Kashan 8731751167, Iran
e-mail: m.abbasi@aut.ac.ir

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received November 15, 2012; final manuscript received December 4, 2013; published online January 17, 2014. Assoc. Editor: Ashraf Bastawros.

J. Eng. Mater. Technol 136(2), 021002 (Jan 17, 2014) (8 pages) Paper No: MATS-12-1257; doi: 10.1115/1.4026193 History: Received November 15, 2012; Revised December 04, 2013

Semisolid extrusion of metals involves extrusion of metallic alloys with a microstructure consisting of spherical solids in a liquid matrix. In this research, the effect of cooling rate during forward semisolid extrusion on microstructure and mechanical properties of 7075 aluminum was investigated. Semisolid microstructure was prepared according to the recrystallization and partial melting (RAP) method. Optimum semisolid temperature and holding time which were resulted in a suitable microstructure for specimens were determined at 580 °C for 10 min. Different cooling rates were applied during semisolid extrusion and the resulted mechanical properties were studied. Tensile properties of semisolid extruded rods in T6 condition were also compared with those of conventionally extruded specimen. The results indicate that utilizing optimum values of semisolid extrusion parameters, namely, temperature and time of heating as well as cooling rate severity, brings both the possibility to obtain mechanical properties of conventionally extruded specimens and to get advantages of semisolid forming technique. Experimental results also show that increment of cooling rate and extrusion pressure improves the mechanical properties.

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Fig. 1

Different methods of semisolid microstracture preparation

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Fig. 2

Liquid fraction versus temperature obtained in constant heating rate of 10 °C/min

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Fig. 3

The apparatus used for semisolid extrusion

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Fig. 4

Dimensions of the round, subsized tension test specimens according to the ASTM-E8 standard specifications [10]

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Fig. 5

Optical micrograph of warm extruded specimen along ED before heating to semisolid temperature. Arrows indicate instances of elongated grains and strings of intermetallic particles.

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Fig. 6

Microstructures of the 7075 alloy after heating at different semisolid temperatures for 10 min and 20 min

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Fig. 7

Results of quantitative study of semisolid microstructures holded at different tempratures for 10 min and 20 min: (a) average grain size and (b) shape factor

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Fig. 8

Semisolid extrusion pressure versus the length of the semisolid extruded rod

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Fig. 9

Schematic representation of the load-stroke diagram for conventional forward and backward extrusion [23]

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Fig. 10

Macro-etched cross section of unextruded billets within the die cavity for semisolid extrusion performed under pressure of 23 MPa (a) and 33 MPa (b). In the chilled part, where the material was solidified due to operating the cooling system, dynamically recrystallized (DRX) grains are observable.

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Fig. 11

Comparison of tensile strength (UTS), yield stress (YS), and elongation to fracture (El%) of four groups of specimens listed in Table 3

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Fig. 12

SEM image of fracture surface of the tension test specimen with globular microstructure (Group A)

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Fig. 13

Transverse (a) and longitudinal (b) sections of the fracture surface for the specimen extruded with the extrusion presure of 23 MPa (Group B)

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Fig. 14

Fracture surfaces of extruded specimens: (a) semisolid extruded at 23 MPa (Group B), (b) semisolid extruded at 23 MPa (Group C), and (c) conventionally warm extruded (Group D)




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