Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy

doi:10.1016/j.jmst.2018.11.024

Abstract

Abstract:

In this study, Cu was added into the high-pressure die-casting Al-5.5Mg-0.7Mn (wt%) alloy to improve the tensile properties. The effects of Cu addition on the microstructures, mechanical properties of the Al-5.5Mg-0.7Mn alloys under both as-cast and T5 treatment conditions have been investigated. Additions of 0.5 wt%, 0.8 wt% and 1.5 wt% Cu can lead to the formation of irregular-shaped Al₂CuMg particles distributed along the grain boundaries in the as-cast alloys. Furthermore, the rest of Cu can dissolve into the matrixes. The lath-shaped Al₂CuMg precipitates with a size of 15-20 nm × 2-4 nm were generated in the T5-treated Al-5.5Mg-0.7Mn-xCu (x = 0.5, 0.8, 1.5 wt%) alloys. The room temperature tensile and yield strengths of alloys increase with increasing the content of Cu. Increasing Cu content results in more Al₂CuMg phase formation along the grain boundaries, which causes more cracks during tensile deformation and lower ductility. Al-5.5Mg-0.7Mn-0.8Cu alloy exhibits excellent comprehensive tensile properties under both as-cast and T5-treated conditions. The yield strength of 179 MPa, the ultimate tensile strength of 303 MPa and the elongation of 8.7% were achieved in the as-cast Al-5.5Mg-0.7Mn-0.8Cu alloy, while the yield strength significantly was improved to 198 MPa after T5 treatment.

Key words: Die casting, Al-Mg alloys, Cu addition, Al₂CuMg phase, Tensile properties

Lingyang Yuan, Liming Peng, Jingyu Han, Baoliang Liu, Yujuan Wu, Juan Chen. Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy[J]. J. Mater. Sci. Technol., 2019, 35(6): 1017-1026.

Figures/Tables 17

Fig. 1. Schematic of sample castings of Al-5.5Mg-0.7Mn-xCu alloys produced by high-pressure die-casting (unit: mm).

Fig. 2. Casting parameters showing shot filling speed and filling time at different stages.

Table 1 Actual chemical compositions of Al-5.5Mg-0.7 Mn-xCu alloys (wt%).

Alloy	Mg	Mn	Fe	Ti	Cu	Si	Other	Al
Al-5.5Mg-0.7 Mn	5.42	0.68	0.12	0.15	0.03	<0.15	<0.3	Bal.
Al-5.5Mg-0.7 Mn-0.5Cu	5.38	0.67	0.13	0.13	0.46	<0.15	<0.3	Bal.
Al-5.5Mg-0.7 Mn-0.8Cu	5.40	0.65	0.11	0.14	0.78	<0.15	<0.3	Bal.
Al-5.5Mg-0.7 Mn-1.5Cu	5.27	0.71	0.13	0.15	1.43	<0.15	<0.3	Bal.

Fig. 3. Backscattered SEM micrographs of as-cast Al-5.5Mg-0.7Mn-xCu alloys: (a) 0Cu; (b) 0.5Cu; (c) 0.8Cu; (d) 1.5Cu.

Fig. 4. XRD patterns of as-cast Al-5.5Mg-0.7Mn-xCu alloys.

Table 2 SEM-EDX analysis of average chemical compositions of each phase in as-cast Al-5.5Mg-0.7 Mn-xCu alloys (wt%).

Alloy	Identified Phase	Al	Mg	Mn	Fe	Si	Cu
0Cu	Matrix	94.65	4.7	0.65	---	---	0
	(Fe, Mn)_rich	63.5	5.40	9.3	15.3	6.5	---
	(Al, Mg)_rich	84.43	10.2	5.37	---	---	---
0.5Cu	Matrix	94.48	4.64	0.63	---	---	0.25
	(Fe,Mn)_rich	59.38	7.2	11.32	14.3	7.8	---
	(Al,Mg)_rich	80.42	13.38	6.28	---	---	---
	(Al,Cu,Mg)_rich	62.63	20.76	---	---	---	16.61
0.8Cu	Matrix	94.46	4.6	0.61	---	---	0.33
	(Fe,Mn)_rich	57.1	6.9	10.15	18.32	7.62	---
	(Al,Mg)_rich	81.93	13.0	5.07	---	---	---
	(Al,Cu,Mg)_rich	68.23	17.00	---	---	---	14.77
1.5Cu	Matrix	94.53	4.51	0.61	---	---	0.35
	(Fe,Mn)_rich	61.04	6.1	9.2	16.36	7.3	---
	(Al,Mg)_rich	81.64	12.06	6.3	---	---	---
	(Al,Cu,Mg)_rich	63.46	19.98	---	---	---	16.56

Fig. 5. Area fraction (a) and relationship between Al2CuMg size and proportion of as-cast Al-5.5Mg-0.7 Mn-xCu alloys with Cu contents of 0.5 wt% (b), 0.8 wt% (c) and 1.5 wt% (d).

Fig. 6. Aging hardness curves of Al-5.5Mg-0.7 Mn-xCu alloys ageing at 175 °C with different times.

Fig. 7. Backscattered SEM micrographs of Al-5.5Mg-0.7 Mn-0.8Cu alloys aged at 175 °C for 8 h (a), area fraction of Al12Mg17 phase (b), Fe-containing phase (c) and Al2CuMg phase (d).

Fig. 8. TEM analysis of precipitates observed from (a) [001]Al zone, (b) [011]Al zone and (c) [$\bar{1}$ 12]Al zone.

Fig. 9. Yield strength (YS), ultimate tensile strength (UTS) and elongation of as-cast Al-5.5Mg-0.7 Mn-xCu alloys.

Fig. 10. Effect of ageing time on YS, UTS and elongation of Al-5.5Mg-0.7 Mn-xCu alloys at 175 °C: (a) YS; (b) elongation; (c) UTS; (d) variation of tensile properties of Al-5.5Mg-0.7 Mn-xCu alloys at 175 °C for 8 h.

Fig. 11. SEM images of tensile fracture morphologies of as-cast Al-5.5Mg-0.7 Mn-xCu alloys: (a) 0Cu; (b) 0.5Cu; (c) 0.8Cu; (d) 1.5Cu.

Fig. 12. SEM images of longitudinal sections nearby room-temperature tensile fracture of as-cast Al-5.5Mg-0.7 Mn-xCu alloys with Cu contents of (a) 0, (b) 0.5 wt%, (c) 0.8 wt%, (d) 1.5 wt% and high magnification observation of images in (e) Fig. 12(a) and (f) (d).

Fig. 13. Area fraction of phases and cracked phases beneath fracture surface of as-cast Al-5.5Mg-0.7 Mn-xCu alloys.

Fig. 14. Content of Cu within matrix, Al2CuMg phase and increment of YS of as-cast Al-5.5Mg-0.7 Mn-xCu alloys.

Fig. 15. Area number density of Al2CuMg precipitate in Al-5.5Mg-0.7 Mn alloys containing Cu ageing at 175 °C for 8 h.

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