Friction stir welding of recycled A6061 aluminum plates fabricated by hot-extrusion of machined chips

Abstract

An excellent process for saving energy and resources is the ‘solid phase recycling process’ [1–5], since it can process material at lower temperatures and with a higher recycled product yield than the more common remelting processes used currently. This technique consolidates machined chips in the solid phase by a hot-extrusion process. Lightweight metals made by the solid phase recycling process have a fine structure due to severe plastic deformation during the production process. Material shapes, however, are restricted to thin solid cylinders or narrow strips because of processing constraints. Therefore, in order to make large-size components it is necessary to use secondary processing such as joining. Generally, melting-type welding methods are employed for joining lightweight metals. These methods, however, significantly impair the structure because of inevitable grain coarsening and segregation of alloying elements. Friction Stir Welding (FSW) has emerged as a new solid state joining technique [6], especially for aluminum alloys [7–14]. In this process, a rotating tool travels down the length of contacting metal plates, and produces a highly plastic deformed zone through the associated stirring action. Friction between the tool shoulder and the plate top surface produces a localized heating zone, an addition to the plastic deformation of the material brought by contact with the tool. In this research, solid phase recycled materials were friction stir welded in the solid state in order to obtain plate materials with superior mechanical properties. The structure of FSW specimens was analyzed using a scanning ion microscope (SIM, Seiko Instruments Inc. SMI-2000) and a transmission electron microscope (TEM, Hitachi, Ltd. H-800). Fig. 1 shows machined chips of A6061. These were prepared by machining an extruded A6061 rod using a lathe with no lubricant. The machined chips were encapsulated in a pure aluminum can, and extruded with an extrusion ratio 1:4 at 873 K. As a result, cylindrical recycled materials with a diameter of about 20 mm were obtained. The materials were rolled perpendicular to the extrusion direction (in the diametral direction) at 673 K to give plate specimens 100 mm × 60 mm × 4 mm in size. Specimen surfaces that contacted the