Optimization of Cu content for the development of high-performance T5-treated thixo-cast Al-7Si-0.5Mg-Cu (wt.%) alloy

doi:10.1016/j.jmst.2021.03.036

Abstract

Abstract:

Cu content has been successfully optimized in a thixo-cast and directly aged (T5-treatment) Al-7Si-0.5Mg (wt.%) alloy. Even after the 0.5% Cu addition, plate-like Al₂Cu (θ’) phases, which contribute to high strengths and large plasticity, are precipitated. The formation of a large fraction of Cu-containing brittle phases could be suppressed as well. Then, the Al-7Si-0.5Mg-0.5Cu alloy exhibits high ultimate tensile strength, 0.2% proof stress, and elongation to failure of 296 MPa, 209 MPa, and 8.8%, respectively. This results in a high quality index similar to that of solution-treated and subsequently aged (T6-treated) Al-7Si-Mg alloys. A Cu addition over 1.0% further improves the strengths; however, this leads to poor ductility because of the high fraction of Cu-containing brittle phases.

Key words: Al-Si-Mg-Cu alloy, Thixo-casting, T5-treatment, Mechanical property, Microstructure

K. Yamamoto, M. Takahashi, Y. Kamikubo, Y. Sugiura, S. Iwasawa, T. Nakata, S. Kamado. Optimization of Cu content for the development of high-performance T5-treated thixo-cast Al-7Si-0.5Mg-Cu (wt.%) alloy[J]. J. Mater. Sci. Technol., 2021, 93: 178-190.

Figures/Tables 17

Table 1. Alloy nomenclature and compositions of alloy billets used in this work (wt.%).

Alloy	Si	Mg	Cu	Fe	Ti	Sr	Al
Al-7Si-0.5Mg	7.22	0.52	<0.01	0.11	0.04	0.014	Bal.
Al-7Si-0.5Mg-0.1Cu	7.15	0.50	0.11	0.11	0.05	0.012	Bal.
Al-7Si-0.5Mg-0.2Cu	7.31	0.52	0.26	0.11	0.05	0.010	Bal.
Al-7Si-0.5Mg-0.5Cu	7.01	0.51	0.50	0.12	0.04	0.012	Bal.
Al-7Si-0.5Mg-1.0Cu	7.14	0.53	1.00	0.12	0.04	0.010	Bal.
Al-7Si-0.5Mg-1.5Cu	7.20	0.54	1.52	0.12	0.05	0.009	Bal.

Fig. 1. Change of Vickers hardness in Al-7Si-0.5Mg and Al-7Si-0.5Mg-xCu (x = 0.1-1.5) alloys during the artificial aging at 200 °C. Note that “As-N.A.” was in the naturally aged for 345.6 ks state.

Table 2. Vickers hardness (HV) of the naturally aged (As-N.A.) and peak-aged samples.

Alloy	As-N.A.	Peak-aged
Al-7Si-0.5Mg	67 ± 1	86 ± 1
Al-7Si-0.5Mg-0.1Cu	66 ± 1	88 ± 1
Al-7Si-0.5Mg-0.2Cu	68 ± 1	91 ± 1
Al-7Si-0.5Mg-0.5Cu	70 ± 1	95 ± 1
Al-7Si-0.5Mg-1.0Cu	72 ± 1	98 ± 1
Al-7Si-0.5Mg-1.5Cu	73 ± 1	100 ± 2

Fig. 2. Nominal tensile stress-strain curves obtained from the peak-aged samples.

Table 3. Ultimate tensile strength (U.T.S.), 0.2% proof stress (P.S.), elongation to failure (El.), and work hardening exponent (n-value) of the peak-aged samples.

Alloy	U.T.S. (MPa)	P.S. (MPa)	El. (%)	n-value
Al-7Si-0.5Mg	258 ± 2	184 ± 1	9.8 ± 0.4	0.169 ± 0.002
Al-7Si-0.5Mg-0.1Cu	271 ± 1	193 ± 1	8.6 ± 0.1	0.167 ± 0.001
Al-7Si-0.5Mg-0.2Cu	284 ± 1	204 ± 2	8.6 ± 0.1	0.172 ± 0.002
Al-7Si-0.5Mg-0.5Cu	296 ± 4	209 ± 4	8.8 ± 0.4	0.176 ± 0.001
Al-7Si-0.5Mg-1.0Cu	306 ± 5	216 ± 3	5.8 ± 1.5	0.185 ± 0.003
Al-7Si-0.5Mg-1.5Cu	266 ± 24	214 ± 1	2.1 ± 1.1	0.185 ± 0.006

Fig. 3. (a) El. and P.S. and (b) El. and U.T.S. of various T5-treated Al- (7-10) Si-Mg and Al- (6-10) Si-Mg-Cu alloys produced by gravity-casting [[39], [40], [41], [42], [43], [44], [45], [46]], die-casting [47], [48], [49], [50], [51], [52], [53], [54], and thixo-casting [10,11,55] processing. Note that red symbols represent the data obtained in this work.

Fig. 4. Optical microscope (OM) images of the peak-aged (a) Al-7Si-0.5Mg, (b) Al-7Si-0.5Mg-0.1Cu, (c) Al-7Si-0.5Mg-0.2Cu, (d) Al-7Si-0.5Mg-0.5Cu, (e) Al-7Si-0.5Mg-1.0Cu, and (f) Al-7Si-0.5Mg-1.5Cu alloys.

Fig. 5. Inverse pole figure (IPF) maps of the peak aged (a) Al-7Si-0.5Mg, (b) Al-7Si-0.5Mg-0.1Cu, (c) Al-7Si-0.5Mg-0.2Cu, (d) Al-7Si-0.5Mg-0.5Cu, (e) Al-7Si-0.5Mg-1.0Cu, and (f) Al-7Si-0.5Mg-1.5Cu alloys.

Fig. 6. Backscattered electron (BE) and secondary electron (SE) images of the peak-aged (a) Al-7Si-0.5Mg, (b) Al-7Si-0.5Mg-0.1Cu, (c) Al-7Si-0.5Mg-0.2Cu, (d) Al-7Si-0.5Mg-0.5Cu, (e) Al-7Si-0.5Mg-1.0Cu, and (f) Al-7Si-0.5Mg-1.5Cu alloys.

Fig. 7. BE images and EDX elemental mappings of Al, Si, Mg, Fe, and Cu of the peak-aged (a) Al-7Si-0.5Mg, (b) Al-7Si-0.5Mg-0.1Cu, (c) Al-7Si-0.5Mg-0.2Cu, (d) Al-7Si-0.5Mg-0.5Cu, (e) Al-7Si-0.5Mg-1.0Cu, and (f) Al-7Si-0.5Mg-1.5Cu alloys. Note that marked points in the BE images are locations where EDX point analysis was done.

Table 4. EDX point analysis results of chemical compositions measured from different points indicated in Fig. 7 (at.%).

Alloy	No.	Mg	Al	Si	Fe	Cu
Al-7Si-0.5Mg	1	8.7 ± 0.6	69.2 ± 1.3	19.4 ± 0.3	2.7 ± 0.4	<0.1
Al-7Si-0.5Mg	2	17.9 ± 0.2	69.9 ± 0.2	12.1 ± 0.1	<0.1	<0.1
Al-7Si-0.5Mg-0.1Cu	3	17.1 ± 0.1	72.5 ± 0.2	10.1 ± 0.1	<0.1	0.3 ± 0.1
	4	9.1 ± 0.1	70.0 ± 0.1	17.8 ± 0.1	2.6 ± 0.1	0.5 ± 0.1
	5	13.0 ± 0.1	65.2 ± 0.1	17.9 ± 0.2	<0.1	3.8 ± 0.1
Al-7Si-0.5Mg-0.2Cu	6	18.3 ± 0.4	68.6 ± 0.4	12.6 ± 0.1	<0.1	0.4 ± 0.1
	7	22.9 ± 0.2	48.4 ± 0.4	22.7 ± 0.3	0.2 ± 0.1	5.7 ± 0.3
	8	8.6 ± 0.1	69.7 ± 0.2	18.3 ± 0.2	2.9 ± 0.1	0.4 ± 0.1
Al-7Si-0.5Mg-0.5Cu	9	9.1 ± 0.6	70.7 ± 1.1	17.1 ± 0.6	2.2 ± 0.2	0.9 ± 0.2
	10	1.5 ± 0.0	71.9 ± 0.8	2.9 ± 0.2	0.3 ± 0.1	23.3 ± 0.9
	11	13.3 ± 0.2	69.5 ± 0.4	13.3 ± 0.3	<0.1	3.9 ± 0.2
Al-7Si-0.5Mg-1.0Cu	12	10.9 ± 0.4	66.0 ± 0.6	19.1 ± 0.3	3.3 ± 0.1	0.8 ± 0.1
	13	11.5 ± 0.9	71.7 ± 2.7	13.2 ± 1.4	<0.1	3.6 ± 0.4
	14	0.3 ± 0.1	75.1 ± 1.9	1.4 ± 0.1	0.2 ± 0.0	22.9 ± 1.9
Al-7Si-0.5Mg-1.5Cu	15	20.9 ± 0.2	51.6 ± 0.3	20.7 ± 0.5	<0.1	6.8 ± 0.7
Al-7Si-0.5Mg-1.5Cu	16	0.2 ± 0.1	69.0 ± 1.4	1.3 ± 0.1	0.1 ± 0.1	29.4 ± 1.5

Fig. 8. XRD patterns analyzed from the peak-aged (a) Al-7Si-0.5Mg, (b) Al-7Si-0.5Mg-0.5Cu, and (c) Al-7Si-0.5Mg-1.0Cu alloys.

Fig. 9. (a) A bright-field TEM image and (b) a selected area electron diffraction (SAED) pattern of the peak-aged Al-7Si-0.5Mg alloy. Note that the incident electron beam is parallel to the [001] direction of the α-Al matrix. A high-resolution TEM (HR-TEM) image and the corresponding Fast Fourier Transformation (FFT) image obtained from particle-like precipitates are also given in Fig. 9 (c) and (d).

Fig. 10. (a) A bright-field TEM image and (b) an SAED pattern of the peak-aged Al-7Si-0.5Mg-0.5Cu alloy. Note that the incident electron beam is parallel to the [001] direction of the α-Al matrix. The HR-TEM images and the corresponding FFT images obtained from precipitates indicated by white arrow-heads, red arrow-heads, and yellow arrow-heads in Fig. 10 (a) are also given in Fig. 10 (c-h).

Table 5. Related microstructural factors and resulting σpf, σpo, and σtotal for the Al-7Si-0.5Mg and Al-7Si-0.5Mg-0.5Cu alloy.

Alloy	Precipitates type	V_ff (%)	V_fo (%)	σ_mod (MPa)	σ_coh (MPa)	σ_ord (MPa)	σpf (MPa)	σpo (MPa)	σtotal(MPa)
Al-7Si-0.5Mg	β"	0.14	-	6	89	15	111	-	111
Al-7Si-0.5Mg-0.5Cu	θ'	-	2.00	-	-	-	-	132	156
	β"	0.07	-	5	66	11	82	-
	Q’	-	0.03	-	-	-	-	20

Fig. 11. BE images of the fractured (a, d) Al-7Si-0.5Mg, (b, e) Al-7Si-0.5Mg-0.5Cu, and (c, f) Al-7Si-0.5Mg-1.0Cu alloys. Note that (a-c) are taken at low magnification and (d-f) are taken at high magnification.

Fig. 12. SE images and the corresponding EDX elemental mapping results of the peak-aged (a) Al-7Si-0.5Mg, (b) Al-7Si-0.5Mg-0.5Cu, and (c) Al-7Si-0.5Mg-1.0Cu alloys.

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