Friction stir welding (FSW) is a solid state joining technology. It is energy effective, environment friendly and has recently been widely adopted to join various materials. Despite successful applications of FSW in worldwide industries, the fundamental research related to FSW is developing slowly. Broader applications require the science to be explored. This includes material flow, heat generation and process control, which are essential for optimizing welding.

Many researches have been conducted focusing on the weld structure and formation process. Material flow around the tool pin during FSW has been the focus of many investigations. The “stop” action technique by stopping the machine during FSW was the most common technique to freeze the rotating pin into the work piece and used for detecting material flow during the welding process. The results suggested that a rotation zone existed around the pin, and a sticking contact condition existed between the tool and work piece. These observations have been used in modelling systems to estimate material flow velocity and heat generation. However, the delay of the “stop” action technique caused uncertainties with regards to what had been observed and what actually occurred during FSW.

FSW experiments in this research were conducted by using aluminium alloy 5083 and a new instant “stop” action – “pin-breaking” technique to “freeze” a pin into the work piece. This was followed by examining material patterns around the broken pin. Material flows particularly near the pin could then be suggested from the observed patterns. The interfacial contact conditions between tool and work piece were also examined in this research.

The major finding of this study has revealed for the first time the feature of material flow around the pin, mainly along the trailing edge. Shear layers rotated with and then detached from the tool pin along the trailing edge, discontinuously once in each revolution. The deposited layers continued to flow until a complete nugget zone was subsequently formed.

Metallurgical sticking was also identified in this study by identifying the existence of an intermetallic layer on the tool. It was shown that the work piece metallurgically stuck to the tool pin. It was significant that this research started to observe the true nature of the contact condition. Meanwhile this research has produced a more direct estimation of material flow velocity during the FSW process.