Dissolution of Precipitates During Solution Treatment of Al-Mg-Si-Cu Alloys
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TRODUCTION
AL-MG-SI-CU alloys have experienced a rapid increase of application in automobile industry due to their high strength-to-weight ratio, good weldability and corrosion resistance as a result of the increasing demands for automobile lightening.[1–3] However, their formability and strength are still not as good as those of steels, which limit their applications in large scale, and need to be further enhanced significantly. Generally, the production of outer body sheets by Al-Mg-Si-Cu alloys includes casting, homogenization, hot rolling, intermediate annealing, cold rolling, solution treatment, pre-aging, room-temperature storage, forming, and paint baking.[4] The aim of solution treatment is to re-dissolve precipitates (Mg2Si, AlMgSiCu, and Si particles) formed during the thermomechanical process and to get a supersaturated solid solution by subsequent quenching. Although a long-time solution treatment definitely dissolves the precipitates, yet the recrystallization texture and microstructure may be worse and the mechanical properties cannot be improved further by increasing solution time when exceeding the critical value.[5] Thus, it is very important to control and optimize solution treatment in order to enhance the formability, mechanical properties, and microstructure. The kind and size of precipitates formed during thermomechanical processing change with the change of alloy element kind, concentration, and thermoXUKAI ZHANG, Graduate Student, MINGXING GUO, Associate Professor, and JISHAN ZHANG and LINZHONG ZHUANG, Professors, are with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mails: [email protected]; [email protected] Manuscript submitted May 12, 2015. Article published online November 5, 2015. 608—VOLUME 47B, FEBRUARY 2016
mechanical process. Dissolution time of different precipitates also varies greatly. A dissolution process can be approximately regarded as a reversal of precipitation,[6] but not a complete conjugate process.[7] Therefore, it is possible to predict dissolution time of a precipitate by an appropriate kinetic theory. So far, there have been several diffusion-controlled models related to the dissolution of precipitates. Whelan[8] derived the dissolution equation of a spherical particle in an infinite medium using the Laplace transformation on the assumption that the concentration on the interface remains stable. Nojiri and Enomoto[9] also developed a model for the dissolution of a spherical particle in an infinite medium using Green’s function method. However, the above-mentioned models do not consider the influence of adjacent particles, and therefore the accuracy of these models is questionable. When considering the effect of adjacent particles, Brown[10] proved that the dissolution time should be longer for the same case. Tanzill and Heckel[11] developed a numerical model to solve the dissolution of spherical particles based on a finite cell assumption that both particles an
