Friction stir consolidation for recycling AA6061 chips with its metal flow investigation for billet production

Abstract

This study aims to produce a billet from AA6061 chips recycling by adopting friction stir consolidation (FSC) technique. AA6061 was used to form different shapes of chips with a thickness of 1 mm. The billet was produced by stacking and forming the chips inside the cylindrical die adopting the friction stir consolidation technique with a rotating tool. Three parameters were considered: preheating time, rotating speed of tool, and depth of plunging, with three levels for each parameter. A Taguchi strategy was implemented to design the experiments (DOE) and analyse the billet properties. The billet cross-section was examined by the optical and scanning electron microscope examinations. The billet was successfully produced by recycling the AA6061 chips. The production mechanism occurred by grains elongating, metal flow, and welding the chip’s boundaries with the applied pressure and input heat. Minor defects such as cracks and porosities were found in the entire surfaces of the billets. The presence of the defects relied on the process variables. The billet's surface appearance or quality depended on the tool's depth of plunging. The tool's revolving speed and depth of plunge determined the process's temperature. The depth of plunging was the most influential parameter on the surface quality of the billets. Raising the depth of plunging and rotational speed generated higher heat input, which increased the weldability between the chip’s boundaries and reduced the surface defects. Regions of sufficient input heat during the stir forming process exhibited complete welding between the chip’s boundaries without defects. The best forming condition achieved among all runs was at (1800 rpm rotational speed, 1 min. preheating time, and 35 mm depth of plunging). For the first time, the friction stir consolidation process produced the billets by recycling the AA6061 chips without any material loss.