Direct Bonding Method of Aluminum to Stainless Steel by Pressure Casting of Aluminum
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DIRECT BONDING METHOD OF ALUMINUM TO STAINLESS STEEL BY PRESSURE CASTING OF ALUMINUM
KATSUAKI SUGANUMA National Defense Academy, Department of Materials Science and Engineering, Hashirimizu 1-10-20, Yokosuka 239, Japan. ABSTRACT A new process for joining Al to stainless steel has been established by using pressure casting of Al in air. Blocks of AISI type 304 austenitic stainless steel to be joined were placed in a mold and were preheated at 450 'C - 600 'C in air. Molten Al was poured into the mold and a pressure of 50 MPa was applied immediately. Stainless steel was tightly bonded to Al by the present process. At the optimum preheating condition, the interface strength was higher than that of Al and fracture did not occur at the interface. At the interface, an intermetallic compound layer was formed, and it was analyzed by TEM. Because this process can be carried out in air by casting of Al, the process has a potential for the mass-production of Al/stainless steel joints or clads. INTRODUCTION Al alloys have been expanding their uses in industrial fields primarily because of their light weight character. In many applications, Al alloys are combined with other materials and then joining technology becomes one of the major concerns. Joining Al alloys to dissimilar materials primarily has two difficulties. One is the presence of a stable oxide film on Al alloys. The Al oxide film prevents obtaining good wetting between Al alloys and brazing fillers or materials to be joined. A certain activation treatment of the Al alloy surface is required before joining. The other problem is high reactivity of Al alloys. Once the stable oxide is removed from the interface, Al alloys severely react with most metallic materials at elevated temperature and form brittle intermetallic compounds at interfaces. As the mating metals with Al alloys, low alloy steels and stainless steels are two of the most important materials because they have been used for most structural components in commercial fields. Joining Al alloys to steels/stainless steels is not so easy because of the same reason mentioned above. By brazing or by solid-state bonding, the formation of a thick intermetallic compound layer prevents getting a sound joint. Friction welding and explosive welding are two of the successful methods but they have critical limitations such as limited dimension or as specific joining skill. Then the present work shows the new joining technique to bond Al and stainless steels by a simple method. The method utilizes squeeze casting of Al alloys, a method that is widely used as the mass-production of metallic components in industries. In the present method, liquid Al is cast and joined with mating materials. EXPERIMENTAL PROCEDURE Materials The stainless steel used was a commercial AISI type 304 austenitic stainless steel (Fe-18wt%Cr-8wt%Ni). Hereafter, this alloy is simply called 304 steel. The rod of 20 mm diameter x 15 mm high was prepared and one end became the face to be bonded. This face was finished with 400 mesh abrasive paper. The ro
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