Causes analysis of crack formation on the friction welding joint for ring 5Ke-leton DT4A and 1Cr18Ni9Ti were analyzed.It was regarded that friction welding parameters were not optimized,so that axial pressure was unloaded ahead and friction welding boundary surface couldn't be welded enough and that coefficients of linear expansion among physicochemical properties between incompatible materials are very different,to result in the highest tensile stresses formation in circumference of the structural stresses...