The Effect of Cu Addition on the Precipitation Sequence in the Al-Si-Mg-Cr Alloy

Abstract

In this work, the effect of Cu additives and heat treatment on the precipitation sequence of an Al-Si-Mg-Cr alloy has been systematically studied by means of advanced spherical aberration-corrected electron microscopy. Cu atoms tend to gather at the interface between the precipitates and the matrix at the beginning of the aging process. Then, Cu atoms diffuse into the precipitates. Two types of GP zones are formed in the first stage of precipitation: one is the type I GP zone and the other is the type II GP zone. The type I GP zone βCu″ evolved into the Q′ phase, while the type II GP zone evolved into the θ′ phase during the aging process. The aging sequence of the Al-Si-Mg-Cr alloy can be determined as a supersaturated solid solution (SSSS) → GP zones → β″ → β′/B′(→ β). The aging sequence of the Al-7%Si-0.3%Mg-0.3%Cr-1.5%Cu alloy can be determined as a supersaturated solid solution (SSSS) → GP zone → βCu″ → Q′ + θ′(→ Q + θ).

Microstructural Evolution and Mechanical Properties of Al-Si-Mg-Cu Cast Alloys with Different Cu Contents

“…Al-Si cast aluminum alloys are extensively used in the automobile field due to their superior castability, satisfactory mechanical and physical properties, and low coefficient of thermal expansion [1][2][3][4]. Adding Mg to Al-Si alloys plays a role in solid solution strengthening and precipitation hardening of aging treatment [5][6][7]. Cu can significantly increase the mechanical properties of Al-Si-Mg alloys with the formation of nanoscale θ and Q precipitates during aging [7][8][9][10].…”

“…Adding Mg to Al-Si alloys plays a role in solid solution strengthening and precipitation hardening of aging treatment [5][6][7]. Cu can significantly increase the mechanical properties of Al-Si-Mg alloys with the formation of nanoscale θ and Q precipitates during aging [7][8][9][10]. Unlike that of Al-Si-Mg and Al-Si-Cu cast alloys, the precipitation strengthening of the Al-Si-Mg-Cu alloy is mainly related to the β , θ , and/or Q phases; meanwhile, the type and volume fraction of Mg and/or Cu-rich precipitates are closely related to heat-treatment conditions and Cu level [7,9,10].…”

“…Cu can significantly increase the mechanical properties of Al-Si-Mg alloys with the formation of nanoscale θ and Q precipitates during aging [7][8][9][10]. Unlike that of Al-Si-Mg and Al-Si-Cu cast alloys, the precipitation strengthening of the Al-Si-Mg-Cu alloy is mainly related to the β , θ , and/or Q phases; meanwhile, the type and volume fraction of Mg and/or Cu-rich precipitates are closely related to heat-treatment conditions and Cu level [7,9,10]. However, the enhanced strength of the Al-Si-Mg alloys with Cu addition is usually at the expense of their ductility.…”

“…In addition, the addition of Cu can decrease the melting point and eutectic temperature of Al-Si-Mg alloys, leading to an increase in the solidification range of the alloys and facilitating porosity formation. Simultaneously, with the increase in Cu content, the precipitates in Al-Si-Mg alloys constantly change in type, morphology, quantity, and size [7,[9][10][11][12][13]. Some useful understandings have been reported in the properties and precipitation behavior change with Cu addition in these quaternary alloys [4,7,8].…”

“…Simultaneously, with the increase in Cu content, the precipitates in Al-Si-Mg alloys constantly change in type, morphology, quantity, and size [7,[9][10][11][12][13]. Some useful understandings have been reported in the properties and precipitation behavior change with Cu addition in these quaternary alloys [4,7,8]. Shang [12] systematically analyzed the phase component of these alloys with a wide Cu level (0.01-4.5 wt.%) and discussed the effect of precipitates on mechanical properties.…”