Describe how Friction Stir Welding successfully joins aluminum alloys without reaching their melting point, detailing the role of the stirring pin's mechanical action.

Friction Stir Welding (FSW) successfully joins aluminum alloys as a solid-state process, meaning the material never reaches its melting point. The process begins with a non-consumable, rotating tool, composed of a shoulder and a pin, plunged into the joint line between the workpieces. Friction between the rotating tool shoulder and the top surface of the aluminum, along with the mechanical agitation from the pin, generates localized heat. This heat, combined with the intense mechanical work from the pin, softens the aluminum alloy to a highly plastic state, below its melting point. In this plasticized state, the aluminum becomes extremely deformable, similar to very stiff dough, rather than a liquid. The rotating pin's mechanical action is crucial. Its specially designed profile, often threaded or fluted, forcibly stirs, deforms, and intermixes the plasticized material. As the tool traverses the joint, the pin mechanically sweeps material from the front and advancing side of the joint, around its rotating body, and then deposits it behind the pin on the retreating side. This continuous shearing and forging action thoroughly mixes the original workpieces' material. Simultaneously, the tool shoulder applies a downward forging force. This force compacts the stirred and mixed plasticized material as it cools in the wake of the pin, forming a strong, metallurgical bond. The complete absence of melting prevents solidification defects common in fusion welding of aluminum, such as porosity or hot cracking, resulting in a fine-grained, consolidated joint.