Abstract: A method and apparatus for maintaining quality control in a friction welding operation where two workpieces are relatively rotated while being constantly pressed into engagement at a common interface. One or more operating conditions, including axial upset at the common interface, are monitored and a signal is substantially instantaneously produced to indicate correlation between the monitored operating conditions and respective ranges for those conditions which are predetermined as being representative of an effective bond between the workpieces. In a first embodiment, operating conditions of axial upset, pressure of engagement between the workpieces and relative rotating speed are simultaneously monitored. In a second embodiment, axial upset is monitored in selectively delayed relation to commencement of the weld in order to provide improved quality control over inertia welding.