Linear friction welding (LFW) has been selected as a screening method to investigate the solid state joining of the Ni-based superalloys: IN718, RR1000, Alloy10, IN100, IN713LC and CMSX4, with the volume fraction (Vf) of γʹ range from 20-70%. Similar welds of each alloy and one dissimilar weld, IN718-IN713, are studied in this project. The extreme thermomechanical history during LFW results in dynamic recrystallisation in the weld zone, dissolution and reprecipitation of the strengthening phase in the HAZ. Sustained load crack growth (SLCG) threshold testing was carried out at 650°C in air for all the similar welds. Weld IN718 which had a low Vf of γʹ+γʹʹ, had the highest threshold (25MPa√m) and highest SLCG resistance. However, there was no such clear effect of γʹ size and volume fraction on threshold value (9-19MPa√m) in welds with a high γʹ content. The crack growth rates of all these welds accelerated sharply from 10-5 to 10-2mm/s over a limited increase in K. A comprehensive mechanical assessment of dissimilar weld IN718-IN713 was carried out, including tensile tests at room temperature and 630ºC, low-cycle and high-cycle fatigue tests at 630ºC on notched specimens, fatigue crack propagation testing at 630ºC, and stress rupture testing at 650ºC.