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Acta Metall Sin  2018, Vol. 54 Issue (7): 1059-1067    DOI: 10.11900/0412.1961.2017.00475
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Formation of Second-Phases in a Direct-Chill Casting Al-12Si-0.65Mg-xMn Alloy
Guangdong WANG1, Ni TIAN1,2(), Changshu HE1,2, Gang ZHAO1,2, Liang ZUO2,3
1 School of Materials Science & Engineering, Northeastern University, Shenyang 110819, China;
2 Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China
3 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article: 

Guangdong WANG, Ni TIAN, Changshu HE, Gang ZHAO, Liang ZUO. Formation of Second-Phases in a Direct-Chill Casting Al-12Si-0.65Mg-xMn Alloy. Acta Metall Sin, 2018, 54(7): 1059-1067.

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Abstract  

Mg-containing high Si aluminum alloy that can be heat treatment enhanced is widely used in the fields of engine, vehicle industry and aerospace, because of its high specific strength, high wear resistance, corrosion resistance and low thermal expansion coefficient. At present, the alloying to improve the microstructure of Mg-containing high Si aluminum alloy and improve its mechanical properties is an important research hotspot of this kind of alloy. As an important alloying element in aluminum alloy, Manganese is of great significance to study the type and formation process of Mn-containing second phase in Mg-containing high Si aluminum alloy. The second phases and their formation in a direct-chill casting Al-12Si-0.65Mg-(0~2.27)Mn (mass fraction, %) alloy were investigated by LSCM, XRD, SEM/EDS and TEM/EDS, combined with phase graph analysis. The results show that there are eutectic silicon, Mg2Si and π-(Al8Mg3FeSi6) besides matrix α-Al in the Mn-free Al-12Si-0.65Mg (mass fraction, %) alloy ingot, which are formed by the reactions of L+Al5FeSi→α-Al+Si+Al8Mg3FeSi6, L→α-Al+Si+Mg2Si and L→α-Al+Si+Mg2Si+Al8Mg3FeSi6 at 567, 555 and 550~554 ℃, respectively. The α-Al dendrites are obviously refined, and α-Al(FeMn)Si phase can be observed with the addition of Mn to Al-12Si-0.65Mg-(0.10~2.27)Mn (mass fraction, %) alloy ingot. With the Mn content increasing from 0.10% to 2.27%, the morphology of α-Al dendrites has no obvious change, and the number of α-Al(FeMn)Si increases gradually whereas the size of α-Al(FeMn)Si doesn't change much. There are some Al9(FeMn)4Si3 with the size of about 80 μm in the Al-12Si-0.65Mg-(1.07~2.27)Mn (mass fraction, %) alloy ingot with the Mn content over 1.07%, which are formed by the reaction of L+Al6Mn→α-Al+Al9Mn4Si3 at 647 ℃, and Al9Mn4Si3 turns into Al9(FeMn)4Si3 with Fe dissolved into it. The number of Al9(FeMn)4Si3 increases with the Mn content increasing from 1.07% to 2.27%, whereas the size of Al9(FeMn)4Si3 has no obvious change. Mg2Si entirely dissolves into the matrix. Eutectic silicon, π-(Al8Mg3FeSi6) and α-Al(FeMn)Si spheroidize into granules, whereas the size, the morphology and the number of Al9(FeMn)4Si3 remain unchanged after the Al-12Si-0.65Mg-xMn (mass fraction, %) alloy ingots were homogenized at 550 ℃. Simultaneously, there are many Al9(MnFe)2Si3 at hundreds of nanometer size precipitated out from the Al-12Si-0.65Mg-(0.10~2.27)Mn (mass fraction, %) alloy matrix after homogenization treatment, and the number of them increases with the increasing of Mn content.

Key words:  Mg-containing eutectic Al-Si alloy      Mn      second-phase      direct-chill casting      homogenization     
Received:  13 November 2017     
ZTFLH:  TG146.2  
Fund: Supported by National Natural Science Foundation of China (No.51371045) and National Key Research and Development Program of China (Nos.2016YFB0300801 and 2016YFB1200506-12)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00475     OR     https://www.ams.org.cn/EN/Y2018/V54/I7/1059

No. Mg Si Mn Al
1 0.67 11.7 0.00 Bal.
2 0.66 11.4 0.10 Bal.
3 0.60 11.4 0.46 Bal.
4 0.67 11.7 0.60 Bal.
5 0.65 12.2 1.07 Bal.
6 0.60 11.7 2.27 Bal.
Table 1  Chemical compositions of alloy ingots (mass fraction / %)
Fig.1  Microstructures of Al-12Si-0.65Mg-xMn alloy ingots with x=0 (a), x=0.10 (b), x=0.46 (c), x=0.60 (d), x=1.07 (e) and x=2.27 (f)
Fig.2  SEM-BSE images of Al-12Si-0.65Mg (a) and Al-12Si-0.65Mg-1.07Mn (b, c) alloys
Point Al Si Mn Fe Mg
1 93.88 3.98 0.00 0.09 2.06
2 94.32 3.74 0.05 0.09 1.80
3 69.42 17.90 0.00 4.02 8.65
4 75.04 16.07 5.59 1.23 2.07
5 85.59 11.00 0.00 0.05 3.35
6 63.67 11.74 20.30 2.78 1.50
Table 2  EDS results of the points in Fig.2 (atomic fraction / %)
Fig.3  XRD spectra of Al-12Si-0.65Mg-xMn alloy ingots with x=0, 0.10, 0.46, 0.60, 1.07 and 2.27
Fig.4  Microstructures of Al-12Si-0.65Mg-xMn alloys with x=0 (a), x=0.10 (b), x=0.46 (c), x=0.60 (d), x=1.07 (e) and x=2.27 (f) after 550 ℃ and 24 h homogenization treatment and then water quenched
Fig.5  SEM-BSE images of Al-12Si-0.65Mg (a) and Al-12Si-0.65Mg-1.07Mn (b, c) alloys after 550 ℃ and 24 h homogenization treatment and then water quenched
Point Al Si Mn Fe Mg
1 21.20 78.31 0.00 0.00 0.49
2 58.88 23.20 0.00 6.07 11.84
3 73.87 9.98 12.00 2.21 1.94
4 63.24 11.99 20.46 2.81 1.51
Table 3  EDS results of the points in Fig.5 (atomic fraction / %)
Fig.6  TEM images of Al-12Si-0.65Mg-xMn alloys with x=0 (a), x=0.10 (b), x=0.46 (c), x=0.60 (d), x=1.07 (e), x=2.27 (f), and the SAED patterns of point 1 in Fig.6b (g, h) after 550 ℃ and 24 h homogenization treatment and then water quenched
Point Al Si Mn Fe
1 69.22 14.86 11.90 4.02
2 77.42 11.90 4.58 6.10
Table 4  EDS results of the points in Fig.6 (atomic fraction / %)
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