Friction-based keyhole repair processes

The need for a suitable keyhole repair process for high-strength aluminum alloys is evidenced by the variety of studies published recently in this research field. Several processes were employed, but other than the work published by the author [29-31], no further studies regarding keyhole repair using RFSSW have been conducted by other research groups.

In friction taper plug welding (FTPW, or friction plug welding, FPW), a tapered plug is forced co-axially into a keyhole with a similar taper, Figure 3.3. The conical surface of the plug is friction welded to the surface of the hole. In this method, post-machining is necessary on both sides of the workpiece to remove the unconsumed portions of the plug and the material that is extruded from the plate. Du et al. [32] used FPW to seal the through holes in AA 2219-T87 plates with a 10 mm thickness and found FPW to be feasible only in tapered through holes, not in standard through holes.

The inclinations of the tapered hole and plug must be compatible to ensure that the typical defects in the lower portions of the weld are extruded from the plate. The maximum tensile strength of the friction plug welds was 72.3 % of the base metal (BM) strength with the thermo-mechanically affected zone (TMAZ) located close to the bonding interface, noted as the weakest location of the joints. Metz et al. [33] applied FPW in friction stir-welded AA 2195-T8 aluminum-copper-lithium alloy plates with a 6.36 mm thickness. The major plug diameter was 33 mm, and the minor diameter was 15.9 mm. The weakest area was measured near the plug weld interface, with 65 % of the BM hardness. In a different study, Metz and Barkey [34] found the strength of the same plug welded

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9 samples to be 57 % of the BM strength compared with 68 % of the BM strength in friction stir-welded samples.

Figure 3.3 Schematic illustration of the FPW process. Reprinted from [32], with permission from Elsevier.

Filling friction stir welding (FFSW), as presented by Huang et al. [35], is derived from the plug welding process. A shoulder portion is added to the tapered plug to avoid stress concentration at the interface between the plug and hole. Huang et al. [35] sealed the exit holes with a diameter of 9.8 mm left by FSW using FFSW in Al-Cu-Mg alloy plates with a thickness of 7.8 mm. Additionally, friction stir processing was used to reprocess the sealed keyholes with a rotating non-consumable tool consisting of only a steel shoulder without a probe. The keyhole closure welds reached a tensile strength of 84.3 % of the base welds conducted by FSW. Han et al. [36] used the same approach of filling a keyhole left by FSW with FFSW and subsequently using friction stir processing as a post-weld processing step. Han et al. used a plug with a diameter of 10 mm made of AA 7075-T6 to seal keyholes in an AA 2219-T6 plate with a thickness of 7.8 mm and achieved 96.6 % of the FSW base weld strength. Behmand et al. [13] applied FFSW to remove a 6.5 mm-deep exit hole from friction stir-welded lap joints in AA 5456. The failure load on the coupons with the refilled keyhole reached 91 % of the corresponding defect-free FSW joint.

Zhang et al. [37] modified the FFSW method using a pin-free tool and a T-shaped filler bit to reduce the setup time for replacing the tool between the filling and reprocessing operations. Zhang and colleagues sealed keyholes left by FSW in AA 1060 sheets with a 4.7 mm thickness. To eliminate voids in the lower portion of the weld, Zn braze foil was pre-placed in the keyhole. The ultimate tensile strength of the keyhole closure welds reached 67.3 % of the BM strength. Until now, FFSW has been proven to seal only keyholes left by a conical FSW tool, for which the geometry of the filler bit must be adapted, and has not yet been applied to through holes.

Recently, a method called active-passive filling friction stir repair (A-PFFSR) was introduced by Ji et al. [38] in AZ31B magnesium alloy as schematically shown in Figure 3.4. A-PFFSR is a multistage process that uses different filler bits and non-consumable pinless tools. First, two active filling steps are applied to reshape the keyhole left by FSW. Next, passive filling is performed using a disc-shaped filler material. The filling material is heated by frictional heat generated by the pinless tool.

Additionally, the forging force created by the tool is beneficial to creating a bond between the filling material and the surrounding workpiece. To achieve sound surface formation, the rotating tool must move transversally along the base friction stir weld after the dwelling period. Later, the same authors applied the technique to 7N01-T4 aluminum alloy sheets with a 4 mm thickness [39]. A-PFFSR was

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used to seal keyholes left by the FSW process, reaching 82.1 % of the tensile strength of the FSW base welds and 69.9 % of the BM tensile strength.

Figure 3.4 Schematic illustration of the A-PFFSR process. Reprinted from [39], with permission from Elsevier.

Recently, Chen et al. [40] introduced the method of refilling the exit hole of friction stir spot welds (FSSW) or friction stir welds using the same tool in an additional processing step. For this, the rotating tool penetrates the workpiece in a position close to the exit hole with a smaller penetration depth and travels along a circular path surrounding the keyhole, Figure 3.5. This refills the original keyhole and creates a new but smaller keyhole at a different exit location, Figure 3.6. However, this method accounts for the volume difference between the old and new exit hole by reducing the workpiece thickness in the region of the repair weld because no filler material is used; compare with Figure 3.6 (b). Additionally, this “keyhole refilled friction stir spot welding” process is not suitable for through-hole repair, and as a thickness reduction and a keyhole remain, it is considered unsuitable for high-performance applications.

Figure 3.5 Schematic illustration of the keyhole refilled friction stir spot welding process. Reprinted from [40], with permission from Elsevier.

Figure 3.6 Schematic illustration of the cross-section of the keyhole refilled friction stir spot welding process. Reprinted from [40], with permission from Elsevier.

Self-refilling friction stir welding (SRFSW) was proposed by Zhou et al. [41] to seal the keyhole left by FSW in stainless steel. This multistage process uses a series of non-consumable tools with gradual

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11 changes in pin geometry and size. As the keyhole is merely reshaped, a wide and shallow exit hole remains at the surface because of the lack of filler material. Sajed [42] used the same approach but employed only one refilling step, naming the procedure “two-stage refilled friction stir spot welding”.

The double-acting friction stir spot welding tool and keyhole filling process employed by Uematsu et al. [43] follows the same approach. Similar to other processes that do not use filler material, the disadvantage of the workpiece thickness reduction remains. Additionally, SRFSW is not applicable to through holes.

Additional processes that are not discussed in detail are friction hydro pillar processing (FHPP), which was successfully applied to refill keyholes in steel but is not applicable to aluminum alloys [44], and the modified friction stir spot welding processes employed by Prakash and Muthukumaran [45] and Venukumar et al. [46]. All these processes cannot be applied as a universal keyhole repair process.