Fabrication of A1050-A6061 Functionally Graded Aluminum Foam by Friction Stir Processing Route
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Aluminum foam is expected to be used as functional components of automobiles to improve both fuel efficiency and the safety of passengers owing to its light weight and high energy absorption.[1] The functionality of aluminum foam can be increased by varying the properties in the same aluminum foam to realize a functionally graded (FG) foam. Currently, relatively few studies on fabricating FG aluminum foams have been reported.[2–5] In these studies, FG aluminum foams were fabricated by changing the pore size and density distribution in the foam. Closed-cell FG aluminum foam was fabricated from syntactic foam by varying the volume fraction of dispersed low-density hollow particles in an aluminum matrix[2] and by means of a powder metallurgical route involving varying the amount of added blowing agent.[3] Open-cell FG aluminum foam was fabricated by a replication process that tailors the size and packing density of soluble salt space holders[4] and by an investment casting replication method using thermally decomposed polyurethane foam precursors.[5] These FG aluminum foams have varying strength in the same foam with a seamless bonding interface. One advantage of these FG aluminum foams is that the foam can have sufficient strength wherever necessary in a single component. For example, the hood of an YOSHIHIKO HANGAI, Associate Professor, YUTARO OBA, Undergraduate Student, and SHINJI KOYAMA, Assistant Professor, are with the Department of Mechanical System Engineering, Gunma University, Kiryuu 376-8515, Japan. Contact e-mail: [email protected] TAKAO UTSUNOMIYA, Professor, is with the Research Organization for Advanced Engineering, Shibaura Institute of Technology, Saitama 337-8570, Japan. Manuscript submitted April 24, 2011. Article published online October 8, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
automobile with an appropriate strength distribution can improve both the structure of the engine compartment and the safety of pedestrians when they are hit by a car. Another advantage of these FG aluminum foams is that the sequential fracture of structures can be controlled. For example, crush elements on automobiles with sequential fracture properties can provide safer automobiles that reduce the degree of injury to passengers in the event of a collision. Recently, a new processing route for fabricating aluminum foam called the friction stir processing (FSP) route precursor method was developed,[6,7] in which an aluminum alloy and a blowing agent powder are mixed by FSP to fabricate the precursor. The FSP route can be used to fabricate various types of aluminum foam with a range of properties using different types of commercially available aluminum alloy plates as starting materials. FSP was developed from the basic principles of friction stir welding (FSW), which is a solid-state bonding process.[8,9] It is considered that FG aluminum foam can be fabricated by bonding several types of aluminum alloy precursors using FSW.[10] For example, if high- and low-strength aluminum alloys are bonded, aluminum foam with var
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