Factors Affecting the Nucleation Kinetics of Microporosity Formation in Aluminum Alloy A356

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 43; no. 3; pp. 1004 - 1016
• Main Authors: Yao, Lu, Cockcroft, Steve, Reilly, Carl, Zhu, Jindong
• Format: Journal Article
• Language: English
• Published: Boston Springer US 01.03.2012; Springer; Springer Nature B.V
• Subjects: Aluminum alloys; Aluminum base alloys; Applied sciences; Casting; Castings; Characterization and Evaluation of Materials; Chemistry and Materials Science; Cleanliness; Exact sciences and technology; Kinetics; Materials Science; Mathematical models; Metallic Materials; Metallurgy; Metals. Metallurgy; Microporosity; Nanotechnology; Nucleation; Oxide coatings; Porosity; Porous materials; Structural Materials; Surfaces and Interfaces; Thin Films; Heterogeneous Nucleation Site; Nucleation Site; Oxide Film; Aluminum Alloy Casting; Nucleation Kinetic; Microporosity; Aluminium base alloys; Kinetics; Germination
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-011-1001-8

Metal cleanliness is one of the most critical parameters affecting microporosity formation in aluminum alloy castings. It is generally acknowledged that oxide inclusions in the melt promote microporosity formation by facilitating pore nucleation. In this study, microporosity formation under different casting conditions, which aimed to manipulate the tendency to form and entrain oxide films in small directionally cast A356 samples was investigated. Castings were prepared with and without the aid of argon gas shielding and with a varying pour surface area. Two alloy variants of A356 were tested in which the main difference was Sr content. Porous disc filtration analysis was used to assess the melt cleanliness and identify the inclusions in the castings. The porosity volume fraction and size distribution were measured using X-ray micro-tomography analysis. The measurements show a clear increment in the volume fraction, number density, and pore size in a manner consistent with an increasing tendency to form and entrain oxide films during casting. By fitting the experimental results with a comprehensive pore formation model, an estimate of the pore nucleation population has been made. The model predicts that increasing the tendency to form oxide films increases both the number of nucleation sites and reduces the supersaturation necessary for pore nucleation in A356 castings. Based on the model predictions, Sr modification impacts both the nucleation kinetics and the pore growth kinetics via grain structure.