when making a butt joint in contrast fssw is generally going to be performed on relatively thinner workpieces thus the pin may generally be shorter than on a tool used for fsw this shorter pin can be used even if the tool is going to penetrate both materials that are being fssw together along with pin length another aspect of the present invention is an understanding that the pin length to shoulder width ratio is important to fssw because of friction stir mixing and welding it is desirable to have a broad area of the workpieces being mixed together fssw of a broader area is more easily accomplished having a shoulder that is relatively broad in the present invention a fssw joint is achieved using a generally solid state process with minimal or no melting of the materials being joined therefore it is important that the tool geometry enables the material of the workpieces to be processed in such a way that the materials mechanically bond for example when a tool having the geometry of the tool shown in fig3 a to 3 d is rotated at 1500 rpm plunges into a lap joint of ahss at a plunge rate of 2 to 8 inches per minute dwells for 1 to 10 seconds and is retracted a fssw joint with a mechanical bond is created it should be understood that these parameters are for illustration purposes only and may be varied to achieve the same or similar results fig3 a is provided as.