A method of determining the quality of a friction weld between two components, the method comprising joining two components together using a friction welding process, measuring the loss of length of the components as a function of time during the friction welding process, determining the double differential with respect to time of the loss in length of the components as a function of time, determining the quality of the friction weld by calculating the R2 value of the linear fit to the loss in length of the components as a function of time over a defined time period, determining the end of the defined time period by identifying the start of the final positive peak of the double differential with respect to time of the loss of length of the components as a function of time, calculating R2 from the least square difference of the linear fit over the defined time period of the loss of length of the components as a function of time and comparing the calculated value of R2 with acceptable values of R2 to determine if the quality of the friction weld is acceptable.