A rotary friction welding process including: providing a first axisymmetric workpiece having a first annular weld surface, and a second axisymmetric workpiece having a second annular weld surface, aligning the workpieces on a common axis (A-A) with the weld surfaces facing each other, rotating one workpiece about the axis relative to the other workpiece, and engaging the workpieces at the first and second weld surfaces such that the rotation raises the temperature at the weld surfaces to create a weld interface, and ceasing the rotation and allowing the weld interface to cool to weld the workpieces together at the interface; and being characterised in that: the first annular weld surface is at a radially inward extent of the first workpiece, and the second annular weld surface is at a radially outward extent of the second workpiece, on a longitudinal section through the aligned workpieces, the first and second annular weld surfaces are inclined from the radial direction and are substantially parallel to each other such that the cooled weld interface is correspondingly inclined from the radial direction, one of the first and second annular weld surfaces is an apex region of a convexity which is consumed as the workpieces are engaged, on a longitudinal section through the aligned workpieces the convexity having a profile in which radially inner and outer side surfaces of the convexity taper towards the apex region being formed as an annular weld surface such that, on the longitudinal section, the weld interface increases in length as the convexity is consumed. providing a first axisymmetric workpiece having a first annular weld surface, and a second axisymmetric workpiece having a second annular weld surface, aligning the workpieces on a common axis (A-A) with the weld surfaces facing each other, rotating one workpiece about the axis relative to the other workpiece, and engaging the workpieces at the first and second weld surfaces such that the rotation raises the temperature at the weld surfaces to create a weld interface, and ceasing the rotation and allowing the weld interface to cool to weld the workpieces together at the interface; and being characterised in that: the first annular weld surface is at a radially inward extent of the first workpiece, and the second annular weld surface is at a radially outward extent of the second workpiece, on a longitudinal section through the aligned workpieces, the first and second annular weld surfaces are inclined from the radial direction and are substantially parallel to each other such that the cooled weld interface is correspondingly inclined from the radial direction, one of the first and second annular weld surfaces is an apex region of a convexity which is consumed as the workpieces are engaged, on a longitudinal section through the aligned workpieces the convexity having a profile in which radially inner and outer side surfaces of the convexity taper towards the apex region being formed as an annular weld surface such that, on the longitudinal section, the weld interface increases in length as the convexity is consumed. the first annular weld surface is at a radially inward extent of the first workpiece, and the second annular weld surface is at a radially outward extent of the second workpiece, on a longitudinal section through the aligned workpieces, the first and second annular weld surfaces are inclined from the radial direction and are substantially parallel to each other such that the cooled weld interface is correspondingly inclined from the radial direction, one of the first and second annular weld surfaces is an apex region of a convexity which is consumed as the workpieces are engaged, on a longitudinal section through the aligned workpieces the convexity having a profile in which radially inner and outer side surfaces of the convexity taper towards the apex region being formed as an annular weld surface such that, on the longitudinal section, the weld interface increases in length as the convexity is consumed.