Mass flash reduction strategies in friction stir processing of aluminum alloys: A review

Friction Stir Processing (FSP) has become a famous solid‐state technology for the fabrication of a wide range of aluminum alloy‐based composites that today find multiple applications across the various metal industries. Generation of revolving, ribbon, bulk, excessive or mass flash as it is generally termed has been a common problem in numerous FSP works. When confronted by this challenge, many researchers apply different experimental and numerical modeling approaches or strategies to reduce the mass flash to practically acceptable limits since it often leads to undesirable loss of material and is also an unwanted defect. This subject is deficiently reviewed, and it therefore becomes the thrust of this paper, to investigate the common trends in mass flash generation during FSP and its commonly employed reduction strategies. Mass flash is caused by high rotational speed at low travel speed and vice versa, flat shoulder, no and low tilt angles, high plunge depth, axial force, and travel force. Mass flash causes material loss, loss of volume fraction control target, material thinning, and leads to poor quality fabrications. Mass flash reduction strategies include the use of high tool tilt angles, concaved tool shoulder, proportional rotational speed and travel speed, and optimal plunge depth, axial force and travel speed as supported by both the experimental and numerical modeling studies. Expulsion of excess flash is a common friction stir processing challenge. Its causes include use of flat‐shouldered tools, no‐to‐low tilt angles, high and low heat input, high plunge depth, and high travel speed and axial forces. Selecting optimal parameters, concave shoulders and lifting high the tool tilt angles when using flat shoulders, reduce the ejected material to acceptable limits.