Effect of Cooling Rate on Microstructure and Mechanical Properties of Sand-Casted Al-5.0Mg-0.6Mn-0.25Ce Alloy

doi:10.1007/s40195-019-00922-2

Abstract

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

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[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(12): 1549-1564.

Figures/Tables 22

Fig. 1 Schematic illustration of step shape sand mold a, casting b, the dimension for tensile specimen c (unit: mm)

Table 1 Chemical composition of the as-cast alloy (wt%)

Mg	Mn	Ce	Si	Fe	Al
4.91	0.67	0.23	0.29	0.10	Bal.

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

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)

Thickness (mm)	R_c (K/s)	?T (°C)	?t (s)	Al₁₀Mn₂Ce		α(Al)		Mg₂Si
Thickness (mm)	R_c (K/s)	?T (°C)	?t (s)	T_Liq (°C)	t_Liq (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

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

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

Thickness (mm)	R_c (K/s)	SDAS (μm)	Area fraction (%)
Thickness (mm)	R_c (K/s)	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

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

Table 4 EDS results of the secondary phases shown in Fig. 6 (at%)

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

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

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

Thickness (mm)	R_c (K/s)	SDAS (μm)	Area fraction (%)
Thickness (mm)	R_c (K/s)	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

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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