Dissimilar friction welding of titanium alloys to Alloy 718

A friction welding process was developed for joining titanium alloys (Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V) to nickel-based superalloy Alloy 718. An interlayer system comprised of copper and niobium sheet layers was employed as a diffusion barrier and weld deformation enhancer. A post weld heat treatment (PWHT, 700;The results show that as-welded joint strengths averaged 53.2 Ksi (standard deviation 14.6 Ksi) for welds made with Ti-6Al-2Sn-4Zr-2Mo and 47.3 Ksi (standard deviation 17.2 Ksi) with Ti-6Al-4V. Friction welds with PWHT under pressure had similar joint strength (averaged 51 Ksi, standard deviation 9.7 Ksi for welds made with Ti-6Al-2Sn-4Zr-2Mo, averaged 51 Ksi, standard deviation 12.9 Ksi for welds made with Ti-6Al-4V) but the standard deviation was reduced to 11 Ksi. Joints in welds receiving PWHT had strengths which were consistently near or above the ultimate tensile strength (UTS) of the copper interlayer (32 Ksi). Joint strength increased up to 72 Ksi with increased interlayer deformation due to thin layer constraint effects.;Welds which received a PWHT without pressure developed discontinuities at the copper--Alloy 718 interface and in the copper layer. These welds averaged only 15 Ksi.;Metallographic observations revealed the deformation process of this investigation. The copper layer, having the lowest yield strength, is the first material to undergo plastic deformation in the welds. Niobium has the second lowest yield strength which is the next material to plastically deform. This process of plastic flow and reduction in length continues until the apparent yield strength in the bi-metal interlayer is so high that the titanium alloy begins to plastically deform. This deformation phenomenon is also supported by the results of a computer thermal analysis.;Fractographic examinations indicated that the highest strength joints failed mostly in the copper interlayer by ductile rupture. Lower strength welds exhibited brittle failure at the interface between parent metal and interlayer materials.