Friction plug welding is a solid-state technique used to repair high-performance aluminum alloy structures, particularly thick 2219 aluminum alloy components common in aerospace applications. During the process, a rotating plug is forced into a pre-machined hole in the workpiece. Friction between the plug and the hole’s sidewall generates intense localized heating, softening the material into a thermoplastic state. Once the plug reaches a prescribed depth, rotation stops and a heavy forging load consolidates the bond. Because the material never melts, the classic fusion-welding problems of porosity, segregation, and hot cracking are avoided. The process also produces dynamically recrystallized, fine equiaxed grains near the interface, a narrow heat-affected zone, low residual stress, and requires no filler metal, flux, or radiation shielding.

Yet friction plug welding has an Achilles heel: the quality of the bond is notoriously difficult to verify. The mechanics depend on sidewall friction, so the applied axial force is not transmitted directly perpendicular to the bonding interface, and the softened metal’s plastic flow is hard to control precisely. Local regions can receive too little or too much material flow, producing lack-of-bonding defects that are hidden beneath extruded flash. Worse, different depths within a thick joint experience very different histories of frictional contact and heat accumulation. The lower interface, which engages friction earlier and for longer, is especially prone to solute enrichment and the formation of brittle eutectic phases, while insufficient local deformation can leave microscopic voids known as kissing bonds. These weak spots are nearly invisible to non-destructive testing and can cut tensile strength by 50 to 100 megapascals depending on location through the thickness.

The stakes are illustrated starkly by prior fatigue research. Friction stir welds in 5083 aluminum alloy containing kissing bond defects showed fatigue lives 21 to 43 times shorter than sound welds, with characteristic strength at two million cycles dropping from 100.24 to 65.57 megapascals. In friction stir spot welded aluminum-copper joints, an interfacial Al2Cu eutectic layer provided a preferential crack path and reduced fatigue strength to a mere 2.0 megapascals at ten million cycles. Signal-based monitoring has already proven itself in friction stir welding and additive friction stir deposition, where machine learning models built on force and torque features have achieved defect detection accuracies exceeding 95 percent. The Tianjin-led team set out to bring the same diagnostic power to friction plug welding.

The researchers welded 20-millimeter-thick 2219 aluminum alloy plates using 25 different combinations of plug taper angle, ranging from 12 to 20 degrees, and pre-machined hole taper angle, from 24 to 32 degrees, while recording axial force and torque at 30 hertz. The resulting cross-sections revealed three distinct forming states. Small plug angles paired with large hole angles produced upper lack-of-bonding defects with no visible flash. Large plug angles with small hole angles shifted the defects to the bottom of the joint. Only moderately matched geometries yielded macroscopically defect-free joints with balanced flash extrusion on both surfaces.

The crucial discovery was that each defect type imprints a characteristic signature on the force and torque curves. Joints with upper lack-of-bonding defects showed axial force and torque rising continuously throughout the rotational friction phase, because the frictional contact region never expanded through the full workpiece thickness, so resistance kept climbing as the plug fed deeper. Numerical simulations showed the uncontacted upper region stayed at roughly 333 degrees Celsius while the contact zone heated to nearly 545 degrees, far too cool for adequate plasticization. Lower defects told the opposite story: after force and torque peaked and stabilized, both signals declined. Simulations revealed that the lower plug region, having endured friction longest, reached about 572 degrees Celsius, softened, necked, and locally separated from the workpiece, exactly the contact-separation behavior that produces the rise-stabilization-decrease pattern.

For the ten macroscopically sound joints, the team extracted variables from the steady friction stage and found they correlated tightly with layer-dependent strength. Average axial force during the steady stage, governed mainly by plug taper angle, rose from 34.92 to 50.11 kilonewtons as the angle increased from 12 to 20 degrees, and this force proved decisive for the upper interface. Higher force raised normal stress from roughly 26.7 to 62.7 megapascals and interfacial strain from 0.69 to 1.07 millimeters per millimeter, compressing shut micropores, enabling dynamic recrystallization across the interface, and eliminating kissing bonds entirely once force exceeded about 43.55 kilonewtons. Upper-layer ultimate tensile strength climbed correspondingly from 246.1 to 335.4 megapascals, while middle and lower layers stayed largely indifferent to force.

Heat told the reverse tale at the bottom of the joint. The team defined an effective heat input combining torque and friction duration, which was dominated by the contact angle between plug and hole. Once effective heat input exceeded roughly 0.62 times ten to the fifth joules, Al-Al2Cu eutectic networks began forming at the lower interface and widened up to 14.12 micrometers as heat input rose to its maximum of 1.54 times ten to the fifth joules. Electron backscatter diffraction showed interfacial temperatures climbing from 521 to 562 degrees Celsius, driving fragmentation of Al2Cu particles and copper diffusion toward the interface until the eutectic reaction became thermodynamically favorable. During tensile loading, the hard eutectic phases created stress concentrations and brittle, rock-candy-like fracture surfaces, dragging lower-layer strength down from 323.8 to 238.3 megapascals as heat input increased.

The team then tested whether simply adjusting axial feed could fix defective joints. Increasing feed eliminated upper lack-of-bonding defects and produced a steady friction stage, while decreasing feed suppressed lower defects, confirming the link between plug position and macroscopic quality. But feed adjustment alone proved insufficient: joints freed of upper defects still harbored kissing bonds because steady-stage force remained too low, and joints freed of lower defects accumulated excessive heat and wide eutectic regions. The solution was a synergistic strategy, selecting a joint geometry, a 16-degree plug taper with a 32-degree hole taper and 8-degree contact angle, that promised high steady-stage force with low heat input, then tuning axial feed to 12 millimeters. The optimized joint achieved 43.85 kilonewtons of average steady force with only 0.19 times ten to the fifth joules of heat input, eliminated both kissing bonds and lower-interface eutectic, and delivered tensile strengths above 310 megapascals in every layer.

Beyond its immediate implications for repairing rocket tanks and airframe structures, the framework establishes force and torque signals as quantitative indicators of bonding quality in friction plug welding for the first time, closing a gap that had limited the technique’s adoption. The authors caution that critical thresholds for kissing bonds and eutectic formation will vary with joint thickness, geometry, and alloy system, and that optimal axial feed still requires determination for each configuration. But the trajectory is clear: the same signal-based monitoring that revolutionized friction stir welding could soon enable online quality assessment and dynamic parameter regulation in plug welding, with lessons extending to additive friction stir deposition and other solid-state processes where the decisive action happens far below the surface, hidden from every camera and probe.

Subject of Research: Signal-based monitoring of joint formation and interfacial bonding quality in friction plug welding of thick 2219 aluminum alloy

Article Title: Relationship of axial force–torque responses with joint formation behavior and interfacial bonding quality in friction plug welding of thick aluminum alloy

Article References: Relationship of axial force–torque responses with joint formation behavior and interfacial bonding quality in friction plug welding of thick aluminum alloy. (n.d.). https://doi.org/10.1007/s44500-026-00022-z

Image Credits: AI Generated

DOI: 10.1007/s44500-026-00022-z

Keywords: friction plug welding, 2219 aluminum alloy, axial force, torque, kissing bond, lack-of-bonding defects, Al-Al2Cu eutectic, dynamic recrystallization, solid-state welding, aerospace structures, process monitoring, tensile strength