Tensile Properties and Deformation Behaviors of a New Aluminum Alloy for High Pressure Die Casting

doi:10.1016/j.jmst.2016.02.013

摘要/Abstract

Abstract:

Effects of natural aging and test temperature on the tensile behaviors have been studied for a high-performance cast aluminum alloy Al-10Si-1.2Cu-0.7Mn. Based on self-strengthening mechanism and spheroidization microstructures, the alloy tested at room temperature (RT) exhibits higher 0.2% proof stress (YS) of 206 MPa, ultimate tensile strength (UTS) of 331 MPa and elongation of 10%. Increasing aging time improves the YS and UTS and reduces the ductility of the alloy. Further increasing aging time beyond 72 h does not significantly increase the tensile strengths. Increasing test temperature significantly decreases the tensile strengths and increases the ductility of the alloy. The UTS of the alloy can be estimated by using the hardness. The Portevin-Le Chatelier effect occurs at RT due to the interactions between solid solution atoms and dislocations. Similar behaviors occurring at 250 °C are attributed to dynamic strain aging mechanism. Increasing aging time leads to decrease in the strain-hardening exponent (n) value and increase in the strain-hardening coefficient (k) value. Increasing test temperature apparently decreases the n andk values. Eutectic phase particles cracking and debonding determine the fracture mechanism of the alloy. Final failure of the alloy mainly depends on the global instability (high temperature, necking) and local instability (RT, shearing). Different tensile behaviors of the alloy are mainly attributed to different matrix strengths, phase particle strengths and damage rate.

Key words: Al-10Si-1.2Cu-0.7Mn aluminum alloy, High pressure die casting, Tensile behavior, Natural aging (self-strengthening), High temperature

. [J]. 材料科学与技术, 2017, 33(4): 367-378.

Zhang Peng, Li Zhenming, Liu Baoliang, Ding Wenjiang. Tensile Properties and Deformation Behaviors of a New Aluminum Alloy for High Pressure Die Casting[J]. J. Mater. Sci. Technol., 2017, 33(4): 367-378.

图/表 20

Fig. 1. Diagram of die casting for the standard tensile testing samples of cast aluminum alloy according to the specification defined in ASTM B557-06. Sample C cut from the Al-10Si-1.2Cu-0.7Mn alloy castings was used for tensile testing.

Fig. 2. Die casting parameters including filling speed (mm/s) and filling time (s) at different stages.

Table 1 Actual chemical composition of the Al-10Si-1.2Cu-0.7Mn aluminum alloy (wt%)

Si	Cu	Mn	Mg	Fe	Ti	Zn	Al
9.96	1.17	0.68	≤0.5	≤0.15	≤0.14	≤0.08	Bal.

Fig. 3. (a) Microstructure of the tensile bar cross-section for the Al-10Si-1.2Cu-0.7Mn alloy sample showing band zone. High magnification images of location A (b), location B (c), and location C (d).

Fig. 4. (a) Detailed observation revealing that the alloy specimens contain a few massive phase particles (marked by the red arrows). EDS analysis of location A (b) and location B (c) in image (a), and XRD curve of the Al-10Si-1.2Cu-0.7Mn alloy (d).

Table 2 Average axis length and area proportion of α-Al in the Al-10Si-1.2Cu-0.7Mn alloy

Locations	Axis length (μm)			Ratio of major axis/minor axis	Fraction of α-Al (%)
Locations	Major (d₁)	Minor (d₂)	Mean	Ratio of major axis/minor axis	Fraction of α-Al (%)
Outside of defect band	7.5	5.6	6.5	1.34	61.5
Defect band	8.9	6.6	7.8	1.35	62.3
Inside of defect band	10.7	7.9	9.3	1.35	68.9

Fig. 5. Average axis length of α-Al distribution fraction of the Al-10Si-1.2Cu-0.7Mn alloy.

Fig. 6. Numerical flow simulation images of thermal field (a) and volume fraction of entrained air (b) based on Flow-3D analysis software showing the band zone formation process.

Fig. 7. (a) Hardness evolution as a function of aging time (h) during nature aging (RT) and (b) tensile properties of the Al-10Si-1.2Cu-0.7Mn alloy, as a function of natural aging time (RT).

Table 3 Tensile properties of the Al-10Si-1.2Cu-0.7Mn alloy with different natural aging time

Time (h)	Symbol	YS (MPa)	UTS (MPa)	A (%)
2	A₁	167	267	11.1
72	A₂	184	312	10.6
144	A₃	189	328	10.1
1200	A₄	206	331	9.8
17,280	A₅	211	342	7.2

Fig. 8. (a) Relationship between the hardness (HV5) and the ultimate tensile strength (σb) of the Al-10Si-1.2Cu-0.7Mn alloy for different natural aging time and (b) comparison between the data based on hardness prediction and the experiment results about the ultimate tensile strength of the alloy.

Fig. 9. Tensile properties of the Al-10Si-1.2Cu-0.7Mn alloy (Group A4 - natural aging for 1200 h), as a function of test temperature.

Table 4 Tensile properties of the Al-10Si-1.2Cu-0.7Mn alloy tested at different temperatures

Test temperature (°C)	Symbol	YS (MPa)	UTS (MPa)	A (%)
-70	B₁	207	302	9.9
20	B₂	206	331	9.8
150	B₃	183	277	11.3
200	B₄	180	241	12.9
250	B₅	148	181	14.7
300	B₆	111	138	18.1
350	B₇	70	78	26.6

Fig. 10. (a) Effect of natural aging time on true stress-logarithmic strain curves of the Al-10Si-1.2Cu-0.7Mn alloy (A1 - 2 h, A2 - 72 h, A3 - 144 h, A4 - 1200 h and A5 - 17,280 h). (b) The magnified curve of location B in (a) showing the Portevin-Le Chatelier effect. σ1 is stress drop amplitude, σ2 is stress increase amplitude; t1is stress decrease time, t2 is reloading time. (c) True stress-logarithmic plastic strain curves showing the effect of natural aging time on flow behavior of the alloy. (d) Strain-hardening rate measured at a plastic strain of 0.0015, as function of natural aging time for the alloy. (e) Determination of n and k values for the alloys tested at different natural aging time by linear fit to the log (true stress)-log (logarithmic plastic strain) curves. The true plastic strain ranges from about 0.01 up to instability. (f) n and (g) k measured at a logarithmic plastic strain range from about 0.01 up to instability, as function of natural aging time for the alloy.

Fig. 11. (a) Effect of test temperature on true stress-logarithmic strain curves of the Al-10Si-1.2Cu-0.7Mn alloy (B1: -70 °C, B2: 20 °C, B3: 150 °C, B4: 200 °C, B5: 250 °C, B6: 300 °C and B7: 350 °C). (b) The magnified curve of location B in (a) showing the Portevin-Le Chatelier effect. (c) True stress-logarithmic plastic strain curves showing the effect of test temperature on flow behavior of the alloy. (d) Strain-hardening rate measured at a plastic strain of 0.0015, as function of test temperatures for the alloy. (e) Determination of nand k values for the alloys tested at different test temperatures by linear fit to the log (true stress)-log (logarithmic plastic strain) curves. The logarithmic plastic strain ranges from about 0.01 up to instability. (f)n and (g) k measured at a logarithmic plastic strain range from about 0.01 up to instability, as function of test temperatures for the alloys.

Fig. 12. SEM fractographs showing the effect of test temperature on the fracture in the Al-10Si-1.2Cu-0.7Mn alloy: (a, b) 20 °C; (c, d) 150 °C; (e, f) 300 °C. The Alx(Fe, Mn)ySiz phase particles and Si eutectic particles are marked by red and blue arrows, respectively.

Fig. 13. Lower magnification images of (a) Fig. 12(a), (b) Fig. 12(b) and (c) Fig. 12(c).

Fig. 14. Tensile instability plots for the Al-10Mg-1.2Cu-0.7Mn alloy for different natural aging time and tested at different test temperatures: (a) A1 - 2 h, A2 - 72 h, A3 - 144 h, A4 - 1200 h and A5 - 17,280 h; (b) B1: -70 °C, B3: 150 °C, B4: 200 °C, B5: 250 °C, B6: 300 °C and B7: 350 °C.

Fig. 15. Ratio of tensile instability strain εi to fracture strain εf of the Al-10Si-1.2Cu-0.7Mn alloy for different natural aging time (a) and tested at different test temperatures (b). εi/εf > 1 indicates that local fracture governs tensile instability, while εi/εf < 1 indicates more uniform damage accumulation where tensile instability occurs at the onset macroscopic necking.

Fig. 16. Macroscpic tensile fracture features of the Al-10Si-1.2Cu-0.7Mn alloy tested at different temperatures (B2: 20 °C, B3: 150 °C, B4: 200 °C, B5: 250 °C, B6: 300 °C and B7: 350 °C).

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