Influence of Processing Parameters on Microstructural Evolution and Tensile Properties for 7075 Al Alloy Prepared by an ECAP-Based SIMA Process

doi:10.1007/s40195-017-0672-6

Abstract

Abstract:

A modified strain-induced melt activation (SIMA) process consisting of homogenization, equal-channel angular pressing (ECAP) and subsequent heating to the semisolid temperatures was introduced to prepare the 7075 aluminum alloy with superior thixotropic behaviors. The effects of both the homogenization and the number of ECAP passes, as well as the isothermal temperatures on the microstructural evolution, were investigated. The results indicate that ideal microstructure wherein fine and globular solid grains surrounded by uniform liquid films can be achieved through ECAP deformation-recrystallization mechanism. Increasing the number of ECAP passes accelerates the recrystallization of strained grains, thus reducing the average grain size and improving the grain sphericity. Moreover, higher holding temperatures and prolonged soaking time can improve the growth of the solid grains. Two main coarsening mechanisms, viz. coalescence and Ostwald ripening, contribute to the growth of the solid grains simultaneously and independently. The tensile strength of the 7075 alloys after four-pass ECAP-based SIMA and T6 heat treatment is relatively lower than the as-received billet, while the elongation of SIMA processed samples is much higher than that of as-received ones. Increasing the number of ECAP passes improves the tensile strength for alloys with and without T6 treatment due to the fine grain strengthening mechanism.

Key words: Semisolid, Aluminum, Equal-channel angular pressing, Microstructural evolution, Tensile property

Jin-Long Fu, Hong-Jun Jiang, Kai-Kun Wang. Influence of Processing Parameters on Microstructural Evolution and Tensile Properties for 7075 Al Alloy Prepared by an ECAP-Based SIMA Process[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(4): 337-350.

Figures/Tables 18

Table 1 Chemical composition of as-received 7075 Al alloy (wt%)

Zn	Mg	Cu	Fe	Si	Cr	Mn	Al
6.05	2.48	1.63	0.24	0.15	0.23	0.30	Bal.

Fig. 1 Curve of liquid fraction versus temperature of 7075 alloy

Fig. 2 Schematic illustration of a ECAP die, b pressing route Bc

Fig. 3 OM image of transverse microstructure of as-received 7075 Al alloy

Fig. 4 SEM backscattered electron images of a as-received, b homogenized 7075 Al alloy (L1 and L2 are the lengths of the needle-like MgZn2 precipitates)

Fig. 5 XRD patterns of as-received and homogenized 7075 alloy

Table 2 EDS result of precipitates in homogenized 7075 Al alloy

Element	Region A		Region B
Element	Mass fraction (%)	Atomic fraction (%)	Mass fraction (%)	Atomic fraction (%)
Al	69.38	78.92	74.46	84.44
Zn	18.13	9.53	0.00	0.00
Mg	7.39	9.13	0.00	0.00
Cu	5.10	2.42	3.99	1.92
Fe	0.00	0.00	16.03	8.78
Mn	0.00	0.00	2.18	1.22
Si	0.00	0.00	3.34	3.64
Total	100.00	100.00	100.00	100.00

Fig. 6 a Four-pass ECAP deformed billets, b OM image of the billet

Fig. 7 OM images of four-pass ECAP-processed 7075 Al alloy heating at 600 °C for 5 min (a), 10 min (b), 15 min (c), 20 min (d), 25 min (e), 30 min (f)

Fig. 8 Variations of average grain size and shape factor with increasing holding time of four-pass ECAP-processed samples heated at 600 °C

Fig. 9 a SEM image, b Al, c Zn, d Mg, e Cu, f Fe, g Si, h Cr, i all elements X-ray image analyze of four-pass ECAP-processed 7075 Al alloy reheated at 600 °C for 10 min

Table 3 EDS result for regions A-D shown in Fig. 9a

	Al	Zn	Mg	Cu	Fe	Si
A	94.58	3.40	2.02	-	-	-
B	57.23	12.43	12.61	17.73	-	-
C	82.46	6.64	5.46	5.44	-	-
D	80.87	3.78	3.32	-	4.05	7.99

Fig. 10 OM images of semisolid microstructures for one-pass ECAP-processed 7075 alloys after soaked at 590 °C for 15 min (a), 30 min (b)

Fig. 11 OM images of semisolid microstructures of one-pass ECAP-processed 7075 Al alloy holding for at 570 °C (a), 580 °C (b), 590 °C (c), 600 °C (d) for 15 min

Fig. 12 Effects of extrusion passes on microstructures of ECAP-processed 7075 Al alloy held at 600 °C for 15 min: a zero-pass; b one-pass; c two-pass; d three-pass; e four-pass

Fig. 13 Variations of average grain size and shape factor versus the ECAP passes for samples heated at 600 °C for 15 min

Fig. 14 OM images of semisolid microstructures for ECAP-processed 7075 alloys after isothermal soakage at 570 °C for 5 min: a one-pass ECAP; b three-pass ECAP

Table 4 Tensile properties of 7075 alloy under different processing conditions (dispersion ± 5% for strength and ± 10% for elongation)

Sample processing	Yield strength (MPa)	Ultimate strength (MPa)	Elongation (%)
As-received (As-extruded + T6)	511	546	9.3
One-pass ECAP + SSIT	178	243	-
One-pass ECAP + SSIT + T6	462	499	14.7
Four-pass ECAP + SSIT	201	268	-
Four-pass ECAP + SSIT + T6	493	541	13.3

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