A method of joining at least two parts , the method comprising steps of: angularly orienting the at least two parts relative to each other; forming plasticized material at a joint interface of the at least two parts by rotating a pin tool of a friction stir welding apparatus relative to a spindle housing of the friction stir welding apparatus about an axis , concentric with the pin tool , wherein the pin tool extends through a first shoulder , a second shoulder , and the at least two parts ; and confining the plasticized material between the first shoulder and the second shoulder of the friction stir welding apparatus , wherein the first shoulder and the second shoulder are opposite to each other along the axis and are not substantially rotatable relative to the spindle housing about the axis ; and being characterised in that : the step of angularly orienting the at least two parts relative to each other comprises angularly orienting the at least two parts to have a first joint interface surface and an opposing, second joint interface surface ; the first joint interface surface is non-planar and comprises substantially planar portions, oriented relative to each other at an angle , which is greater than or less than 180 degrees; and the second joint interface surface is non-planar and comprises substantially planar portions, oriented relative to each other at an angle , which is greater than or less than 180 degrees. angularly orienting the at least two parts relative to each other; forming plasticized material at a joint interface of the at least two parts by rotating a pin tool of a friction stir welding apparatus relative to a spindle housing of the friction stir welding apparatus about an axis , concentric with the pin tool , wherein the pin tool extends through a first shoulder , a second shoulder , and the at least two parts ; and confining the plasticized material between the first shoulder and the second shoulder of the friction stir welding apparatus , wherein the first shoulder and the second shoulder are opposite to each other along the axis and are not substantially rotatable relative to the spindle housing about the axis ; and being characterised in that : the step of angularly orienting the at least two parts relative to each other comprises angularly orienting the at least two parts to have a first joint interface surface and an opposing, second joint interface surface ; the first joint interface surface is non-planar and comprises substantially planar portions, oriented relative to each other at an angle , which is greater than or less than 180 degrees; and the second joint interface surface is non-planar and comprises substantially planar portions, oriented relative to each other at an angle , which is greater than or less than 180 degrees. the step of angularly orienting the at least two parts relative to each other comprises angularly orienting the at least two parts to have a first joint interface surface and an opposing, second joint interface surface ; the first joint interface surface is non-planar and comprises substantially planar portions, oriented relative to each other at an angle , which is greater than or less than 180 degrees; and the second joint interface surface is non-planar and comprises substantially planar portions, oriented relative to each other at an angle , which is greater than or less than 180 degrees. <EOS>