Enhancing the bake-hardening responses of a pre-aged Al-Mg-Si alloy by trace Sn additions

doi:10.1016/j.jmst.2019.08.045

Abstract

Abstract:

Effects of additions of trace Sn on the bake-hardening responses of a pre-aged Al-0.85Mg-0.85Si (in wt%) alloy were investigated through mechanical tests, differential scanning calorimetry, electrical resistivity and transmission electron microscopy. Results indicate that trace Sn additions reduced the number density of pre-aging clusters by inhibiting the formation of unstable counterpart during pre-aging treatment, leading to low strength and high supersaturation of solute atoms. In a subsequent paint-bake treatment, the presence of highly supersaturated solute atoms and high concentrated free vacancies moderated the activation energy barrier of βʺ phase and thus kinetically accelerated the formation of βʺ. Consequently, the trace Sn additions enhanced the bake-hardening responses of the pre-aged alloys significantly. The Sn-containing pre-aged Al-Mg-Si alloys with low strength and great bake-hardening responses hold promising potential for automotive body skin applications.

Key words: Al-Mg-Si alloy, Pre-aging, Trace Sn, Bake-hardening responses, Precipitation kinetics

Gang Lu, Shuai Nie, Jianjun Wang, Ying Zhang, Tianhai Wu, Yujie Liu, Chunming Liu. Enhancing the bake-hardening responses of a pre-aged Al-Mg-Si alloy by trace Sn additions[J]. J. Mater. Sci. Technol., 2020, 40: 107-112.

Figures/Tables 10

Table 1 Chemical composition of two alloys investigated (wt%).

Alloys		Mg	Si	Fe	Zn	Cr	Ti	Sn	Al
1#	Sn-free	0.87	0.85	0.09	0.16	0.07	0.06	<0.01	Bal.
2#	Sn-containing	0.86	0.85	0.09	0.16	0.06	0.06	0.037	Bal.

Fig. 1. Mechanical properties of two alloys in different conditions: (a) Vickers hardness; (b) tensile properties.

Table 2 Mechanical properties of two alloys under different conditions.

State	Alloy	Yield strength (MPa)		Ultimate Tensile Strength (MPa)		Elongation (%)		BHR (MPa)
		Before-BH	After-BH	Before-BH	After-BH	Before-BH	After-BH
T4P	1#	147	257	269	334	34.5	24.7	110
T4P	2#	122	257	241	329	34.5	25	135

Fig. 2. DSC traces of two alloys in T4P with heating rate of 10 °C /min.

Table 3 The peak temperatures of βʺ in the DSC curves performed at different heating rates.

Alloy	Heating rate (°C/min)	Peak temperature (°C)
1#	10	270.2
	15	274.3
	20	277.7
2#	10	255.5
	15	260.5
	20	265.3

Fig. 3. lnα/Tp2 versus 1/Tp for βʺ occurred during continuous heating at various heating rates.

Fig. 4. TEM images of two alloys in different states: (a) alloy 1# in T4P; (b) alloy 2# in T4P; (c) alloy 1# in T4P + BH; (d) alloy 2# in T4P + BH.

Fig. 5. HRTEM images and corresponding FFT patterns of precipitates: (a, b) β”; (c, d) GP zone.

Fig. 6. Electrical resistivity of two alloys in different conditions.

Fig. 7. Schematic free energy diagram of precipitation in two alloys.

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