Effects of Solution Treatment on the Microstructure, Tensile Properties, and Impact Toughness of an Al-5.0Mg-3.0Zn-1.0Cu Cast Alloy

doi:10.1007/s40195-020-01077-1

Abstract

Abstract:

This study investigates the effect of solution treatment (at 470 °C for 0-48 h) on the microstructural evolution, tensile properties, and impact properties of an Al-5.0Mg-3.0Zn-1.0Cu (wt%) alloy prepared by permanent gravity casting. The results show that the as-cast microstructure consists of α-Al dendrites and a network-like pattern of T-Mg₃₂(AlZnCu)₄₉ phases. Most of the T-phases were dissolved within 24 h at 470 °C; and a further prolonging of solution time resulted in a rapid growth of α-Al grains. No transformation from the T-phase to the S-Al₂CuMg phase was discovered in this alloy. Both the tensile properties and impact toughness increased quickly, reached a maximum peak value, and decreased gradually as the solution treatment proceeded. The impact toughness is more closely related to the elongation, and the relationship between impact toughness and elongation appears to obey an equation:IT= 8.43EL - 3.46. After optimal solution treatment at 470 °C for 24 h, this alloy exhibits excellent mechanical properties with the ultimate tensile strength, yield strength, elongation and impact toughness being 431.6 MPa, 270.1 MPa, 19.4% and 154.7 kJ/m ², which are comparable to that of a wrought Al-6.0Mg-0.7Mn alloy (5E06, a 5xxx aluminum alloy). Due to its excellent comprehensive combination of mechanical properties, this cast alloy has high potential for use in components which require medium strength, high ductility and high toughness.

Key words: Al-Mg-Zn-Cu cast alloys, T-Mg₃₂(AlZn)₄₉ phase, S-Al₂CuMg phase, Impact toughness, Mechanical properties

Hua-Ping Tang, Qu-Dong Wang, Colin Luo, Chuan Lei, Tian-Wen Liu, Zhong-Yang Li, Kui Wang, Hai-Yan Jiang, Wen-Jiang Ding. Effects of Solution Treatment on the Microstructure, Tensile Properties, and Impact Toughness of an Al-5.0Mg-3.0Zn-1.0Cu Cast Alloy[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(1): 98-110.

Figures/Tables 18

Fig. 1 Schematic diagram of the cast a, dimensions of the tensile specimen b, dimensions of the Charpy V-notch test specimen c (in mm), corresponding tensile and Charpy V-notch impact test sample d

Table 1 Chemical composition of the studied alloy Al-5.0Mg-3.0Zn-1.0Cu (wt%)

Zn	Mg	Cu	Fe	Si	Al
3.10	4.85	0.97	0.16	0.14	Bal.

Fig. 2 DSC heating curve of the as-cast Al-5.0Mg-3.0Zn-1.0Cu alloy

Fig. 3 XRD patterns of the as-cast Al-5.0Mg-3.0Zn-1.0Cu alloys: a under as-cast; b after solution treatment at 470 °C for 24 h

Fig. 4 a SEM microstructure of the as-cast alloy with corresponding elemental maps for Al, Mg, Zn, Cu and Fe; b SEM microstructure after solution treatment at 470 °C for 24 h conditions with corresponding EDS results corresponding to points 4 and 5

Table 2 EDS compositions for the secondary phase points in Fig. 4 (at%)

Point	Al	Mg	Zn	Cu	Fe	Identified phase
Figure 4a, 1	97.5	2.1	0.3	0.1	-	α-Al
Figure 4a, 2	48.5	36.2	10.1	5.2	-	T-Mg₃₂(AlZnCu)₄₉
Figure 4a, 3	71.4	-	0.9	2.5	25.2	Al₃Fe
Figure 4b, 4	92.8	5.6	1.2	0.4	-	α-Al
Figure 4b, 5	79.7	-	1.0	3.7	15.6	Al₃Fe

Fig. 5 SEM images depicting the microstructural evolution of the studied alloy after solution treatment at 470 °C for the following durations: a as-cast conditions, b 0.25 h, c 1 h, d 4 h, e 12 h, f 24 h, g 36 h, h 48 h

Fig. 6 Area fraction of the residual phase vs solution time during solution treatment at 470 °C

Fig. 7 Variation of Al, Mg, Zn and Cu concentrations in the residual phases at different solution time during solution treatment at 470 °C

Fig. 8 DSC curves of the Al-5.0Mg-3.0Zn-1.0Cu alloys after solution treatment at 470 °C for 0 h (as-cast), 1 h, 4 h, 12 h, 24 h, 48 h

Fig. 9 Representative optical micrographs of the studied alloys after solution treatment at 470 °C for different time: a 4 h, b 12 h, c 24 h, d 48 h

Fig. 10 Grain size versus solution treatment time at 470 °C for the studied alloys

Fig. 11 Change in mechanical properties of the Al-5.0Mg-3.0Zn-1.0Cu alloys for increasing solution treatment time at 470 °C, including: a typical tensile curves; b tensile properties; c variations of impact toughness, yield strength and elongation with solution time; d relationship between the impact toughness and the elongation

Table 3 A comparison of mechanical properties of the studied alloy with typical Al-Si casting alloy and Al-Mg wrought alloys [16, 17]

Alloys	State	UTS (MPa)	YS (MPa)	EL (%)	IT (kJ/m²)	Refs.
Al-5.0Mg-3.0Zn-1.0Cu	Gravity casting +T4	431.6	270.1	19.4	154.7	Present study
Al-5.0Mg-3.0Zn-1.0Cu	Gravity casting +T6	490.1	423.3	8.3	104.3	Present study
Al-7.2Si-0.3Mg	Gravity casting +T6	275.0	200.0	6.9	70.0	[16]
Al-6.0Mg-0.7Mn	Cold-rolled	526.2	416.0	10.5	152.0	[17]

Fig. 12 Impact fracture surfaces of the Al-5.0Mg-3.0Zn-1.0Cu alloys under: a-c as-cast; d-f optimal solution treatment conditions

Table 4 EDS compositions for the secondary phase points in Fig. 12 (at%)

Point	Al	Mg	Zn	Cu
Figure 12c, 1	97.5	2.1	0.3	0.1
Figure 12c, 2	48.5	36.2	10.1	5.2
Figure 12c, 3	51.3	31.4	12.6	4.7

Fig. 13 A summary graph indicating the phase transformation from the T-Mg32(AlZnCu)49/η-Mg(ZnCuAl)2 phases to S-Al2CuMg phase of the alloys as functions of Cu content and Zn content [8, 11,12,13,14,15, 18,19,20,21,22,23,24,25,26,27]

Fig. 14 Isothermal cross sections of Al-Zn-Mg-Cu phase diagrams at 470 °C near the Al area with increasing Cu content from a 1.0% to, b 1.5% to, c 2.0%. In the phase diagrams, Fcc, S, C14 and L represent α-Al, S-Al2CuMg, η/T-(MgAlZnCu) and liquid, respectively

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