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Acta Metallurgica Sinica(English letters)  2019, Vol. 32 Issue (12): 1549-1564    DOI: 10.1007/s40195-019-00922-2
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Effect of Cooling Rate on Microstructure and Mechanical Properties of Sand-Casted Al-5.0Mg-0.6Mn-0.25Ce Alloy
Hua-Ping Tang1, Qu-Dong Wang1(), Chuan Lei1, Kui Wang1, Bing Ye1, Hai-Yan Jiang1, Wen-Jiang Ding1,2
1 National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China
2 State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
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Abstract  

This study examines the relationship among cooling rate, microstructure and mechanical properties of a sand-casted Al-5.0Mg-0.6Mn-0.25Ce (wt%) alloy subjected to T4 heat treatment (430 °C × 12 h + natural aging for 5 days), and the tested alloys with wall thickness varying from 5 to 50 mm were prepared. The results show that as the cooling rate increases from 0.22 to 7.65 K/s, the average secondary dendritic arm spacing (SDAS, λ2) decreases from 94.8 to 27.3 μm. The relation between SDAS and cooling rate can be expressed by an equation: $\lambda_{2} = 53.0R_{\text{c}}^{ - 0.345}$. Additionally, an increase in cooling rate was shown not only to reduce the amount of the secondary phases, but also to promote the transition from Al10Mn2Ce to α-Al24(Mn,Fe)6Si2 phase. Tensile tests show that as the cooling rate increases from 0.22 to 7.65 K/s, the ultimate tensile strength (UTS) increases from 146.3 to 241.0 MPa and the elongation (EL) increases sharply from 4.4 to 12.2% for the as-cast alloys. Relations of UTS and EL with SDAS were determined, and both the UTS and EL increase linearly with (1/λ2)0.5 and that these changes can be explained by strengthening mechanisms. Most eutectic Al3Mg2 phases were dissolved during T4 treatment, which in turn further improve the YS, UTS and EL. However, the increment percent of YS, UTS and EL is affected by the cooling rate.

Key words:  Al-Mg-Mn cast alloys      Cooling rate      Microstructure      Al10Mn2Ce      Mechanical properties     
Received:  01 March 2019      Published:  25 November 2019

Cite this article: 

Hua-Ping Tang, Qu-Dong Wang, Chuan Lei, Kui Wang, Bing Ye, Hai-Yan Jiang, Wen-Jiang Ding. Effect of Cooling Rate on Microstructure and Mechanical Properties of Sand-Casted Al-5.0Mg-0.6Mn-0.25Ce Alloy. Acta Metallurgica Sinica(English letters), 2019, 32(12): 1549-1564.

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http://www.amse.org.cn/EN/10.1007/s40195-019-00922-2     OR     http://www.amse.org.cn/EN/Y2019/V32/I12/1549

Fig. 1  Schematic illustration of step shape sand mold a, casting b, the dimension for tensile specimen c (unit: mm)
Mg Mn Ce Si Fe Al
4.91 0.67 0.23 0.29 0.10 Bal.
Table 1  Chemical composition of the as-cast alloy (wt%)
Fig. 2  Cooling curves of the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys with different thicknesses a and the cooling curve (red solid line) and the corresponding first derivative (blue dashed line) of the as-cast alloy with a thickness of 10 mm b
Thickness (mm) Rc (K/s) ?T (°C) ?t (s) Al10Mn2Ce α(Al) Mg2Si
TLiq (°C) tLiq (s) T2 (°C) t2 (s) T3 (°C) t3 (s)
50 0.22 69.7 419.1 654.2 15.2 626.4 31.2 556.7 450.3
25 0.33 69.8 292.3 656.1 11.5 626.5 20.0 556.6 312.3
10 1.39 79.0 71.6 657.1 4.3 629.2 9.6 550.2 81.4
5 7.65 63.9 8.4 - - 614.8 2.5 550.9 10.8
Table 2  Solidification parameters of as-cast alloys with different thicknesses (T2: the formation temperature of α(Al); t2: the formation time of α(Al); T3: the formation temperature of Mg2Si; t3: the formation time of Mg2Si)
Fig. 3  XRD patterns of the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys produced at different cooling rates: a 0.22 K/s; b 0.33 K/s; c 1.39 K/s; d 7.65 K/s
Fig. 4  Optical microstructures of the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys under different cooling rates: a 0.22 K/s; b 0.33 K/s; c 1.39 K/s; d 7.65 K/s
Thickness (mm) Rc (K/s) SDAS (μm) Area fraction (%)
α(Al) Dendrite Al3Mg2 phase Mn-rich phase
50 0.22 94.8?±?13.2 21.5?±?4.6 1.46?±?0.15
25 0.33 75.7?±?12.4 20.8?±?3.8 1.40?±?0.11
10 1.39 44.8?±?9.5 8.7?±?1.3 0.76?±?0.10
5 7.65 27.3?±?5.5 6.6?±?0.9 0.67?±?0.08
Table 3  SDAS of α(Al) dendrites, area fraction (%) of Al3Mg2 and Mn-rich phases in the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys produced at different cooling rates
Fig. 5  Relation of SDAS with the cooling rate for the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys
Fig. 6  SEM micrographs showing effect of the cooling rate on the secondary phases in the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys: a 0.22 K/s; b 0.33 K/s; c 1.39 K/s; d 7.65 K/s; e corresponding EDS results of points 1, 2 and 3
Point Al Mg Mn Ce Fe Si Identified phase
Figure 6a, 1 51.05 32.12 0 0 0.45 16.38 Mg2Si
Figure 6a, 2 77.49 1.39 5.35 0.04 11.41 4.38 α-Al24(Mn,Fe)6Si2
Figure 6a, 3 78.43 7.69 9.83 3.51 0.54 0 Al10Mn2Ce
Figure 6b, 4 77.94 1.74 6.31 0.02 10.35 3.63 α-Al24(Mn,Fe)6Si2
Figure 6b, 5 80.54 7.53 8.00 3.41 0.52 0 Al10Mn2Ce
Figure 6c, 6 78.38 1.65 6.15 0 9.94 3.88 α-Al24(Mn,Fe)6Si2
Figure 6c, 7 80.99 7.53 7.75 3.21 0 0 Al10Mn2Ce
Figure 6d, 8 77.30 3.37 7.38 0.10 8.13 3.72 α-Al24(Mn,Fe)6Si2
Figure 6d, 9 81.88 7.74 7.15 3.23 0 0 Al10Mn2Ce
Table 4  EDS results of the secondary phases shown in Fig. 6 (at%)
Fig. 7  Concentration mapping of Al, Mg, Mn, Ce, Fe and Si in Fig. 6a from the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloy with a cooling rate of 0.22 K/s
Fig. 8  Optical microstructures of the T4-treated Al-5.0Mg-0.6Mn-0.25Ce alloys produced at different cooling rates: a 0.22 K/s; b 0.33 K/s; c 1.39 K/s; d 7.65 K/s
Fig. 9  SEM micrographs and the corresponding EDS mapping of Mg element of Al-5.0Mg-0.6Mn-0.25Ce alloys fabricated at a cooling rate of 0.22 K/s under different states: a, b as-cast; c, d T4-treated
Thickness (mm) Rc (K/s) SDAS (μm) Area fraction (%)
α(Al) dendrites Al3Mg2 phase Mn-rich phase
50 0.22 100.8?±?15.2 ~?0 1.43?±?0.16
25 0.33 79.7?±?12.4 ~?0 1.38?±?0.13
10 1.39 50.8?±?9.4 ~?0 0.72?±?0.10
5 7.65 31.3?±?5.7 ~?0 0.63?±?0.08
Table 5  SDAS of α(Al) dendrites, area fraction (%) of Al3Mg2 and Mn-rich phase in the T4-treated Al-5.0 Mg-0.6Mn-0.25Ce alloys produced at different cooling rates
Fig. 10  TEM images for the T4-treated alloys fabricated at a cooling rate of 7.65 K/s; a, b TEM bright-field images showing dispersoids in matrix and at grain boundary (the insets in a and b show the SAED patterns from the dispersoid and Al matrix, respectively; c, d the EDS results of dispersoids in matrix and at grain boundary)
Fig. 11  Vickers hardness versus the cooling rate for studied alloys under as-cast and T4-treated conditions
Fig. 12  a Typical tensile curves and b tensile properties of the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys produced under different cooling rates
Fig. 13  Relationship between SDAS and a UTS, b EL for as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys
Fig. 14  a Typical tensile curves, b tensile properties of the T4-treated Al-5.0Mg-0.6Mn-0.25Ce alloys under different cooling rates
Fig. 15  Effect of the cooling rate on increment percentage of strength (YS and UTS) and elongation of alloys before and after T4 heat treatment, noting that the ‘increment percentage’ refers to the enhancement of properties compared to those of as-cast materials
Fig. 16  SEM micrographs of fractured tensile specimens from the as-cast Al-5.0Mg-0.6Mn-0.25Ce alloys at different cooling rates: a 0.22 K/s; b 0.33 K/s; c 1.39 K/s; d 7.65 K/s; e EDS results of particles on fracture surfaces
Fig. 17  SEM micrographs of fractured tensile specimens from the T4-treated Al-5.0Mg-0.6Mn-0.25Ce alloys at different cooling rates: a 0.22 K/s; b 0.33 K/s; c 1.39 K/s; d 7.65 K/s; e EDS results of particles on fracture surfaces
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