This image is just a small edit by me to the image originally uploaded by user Jtmonone~commonswiki as Fsw periaate poikkileikkaus pit paine.png to wikimedia commons. This image shows a schematic of the geometry of a friction stir weld pass. It specifically shows the tool (centered, with threads), along with schematic envelopes for the thermo-mechanically affected zone (TMAZ, in red), and the heat affected zone (HAZ) in pink. Note the small, intentional cant of the tool such that additional pressure is generated on the backside of the weld, improving weld quality. Note that the size and shape of the TMAZ and HAZ will be a function of the welding parameters and material used.