Resistance and Solid-State Welding based Repair Technologies

Resistance welding has been demonstrated as a viable technology for repair of aerospace parts and alloys. In particular, a repair technique for bolt or rivet holes in aluminum, titanium, and superalloys has been developed and demonstrated. These developments reflect EWI’s continuous effort to develop processes that help industry solve problems. Current fusion welding techniques are flexible in that they can make welds in almost any location on the part; however, they require a skilled operator and special procedures to achieve acceptable weld quality. Friction welding techniques can eliminate the operator dependence, but require extensive tooling to prevent part movement due to the high torques and forces used in the process. Friction welding is also limited with respect to part edge proximity to the hole.

Several issues are eliminated when a resistance weld repair is used to repair a hole. Operator skill level is dramatically reduced as the machine controls and geometric features of filler materials control the weld quality. The recent improvements in resistance welding controls may also allow for real time quality monitoring. The process is also capable of making the welds with very little supportive tooling as is often required for friction type hole repairs.

Distortion is significantly reduced or virtually eliminated due to balanced heating and melting about both the neutral axis of the hole and the flange or sheet being repaired, as shown in Figure 1. Distortion is typically the result of unbalanced residual stresses in a weld that result in strain developing in either the weld or heat affected zone. These attributes of the process effectively eliminate the uneven heat input related source of distortion.

A second common problem with repair welds is material properties reduction due to either grain growth or base metal alloy dilution as fusion welding often requires the use of dissimilar filler metal additions. In aluminum alloys it is typical to use a near eutectic alloy to prevent hot cracking during fusion weld repairs. This can leave a much weaker material in the repaired area due to lack of proper alloy additions. With conductive heat hole repair, as shown in Figure 2, a matching filler metal slug can be used allowing the repaired area to reach much higher strength levels. The directional solidification developed in conductive heat resistance welding allows the process to produce a defect free microstructure in all tested aluminum alloys including 7075. The matching filler metal may also have positive implications from a corrosion standpoint. In titanium hole repairs, grain growth can reduce fatigue performance significantly. Solid-state resistance weld hole repairs can be made in titanium where low heat input results in base metal static strength and minimal effect on the fatigue performance of the weld repair. (Ref EWI 49916CRP) A cross section of a repair in Ti 6-4 is shown in Figure 3. This technique can also be used to repair superalloys with similar improvements in the repair performance.

Another feature of the processes is that it is possible to forge extra filler material into the hole and actually locally increase the thickness around the repair. This can aid in the post weld machining operations ensuring that the part thickness is maintained after clean up is completed. This reinforcement can be observed in both the aluminum and titanium repairs shown.

In summary, the resistance weld hole repair technique can provide an improved cost performance solution for hole repairs. For aluminum the conductive heat hole repair process has been utilized for most commercial and aerospace grades. The resistance weld repair for non-aluminum materials has been shown to produce solid-state repairs that typically yield better properties than fusion welding techniques. Currently, the process has been demonstrated on metal thicknesses between 0.75 to 10-mm.

These resistance weld process are not limited to hole repair and can be adapted to facilitate the repair of other undesirable geometric characteristics in many aerospace parts.

To learn more about the resistance and solid-state hole repair technologies, please contact David Workman at dworkman@ewi.org or call 614.688.5244.