9.7 High-Frequency, Flash, Stud & Ultrasonic Welding; Surfacing and Thermal Spray

High-Frequency Welding (HFRW and HFIW)

High-frequency welding is the process behind almost all continuously produced welded tube and pipe. A strip is roll-formed into a tube with an open seam, high-frequency current is introduced either by sliding contacts (HFRW) or by an induction coil (HFIW), and pressure rolls forge the heated edges together.

Its efficiency comes from two electromagnetic effects:

• Skin effect. At high frequency, current crowds into a thin surface layer whose depth falls as frequency rises. Operating in the 200 to 500 kHz range confines current to a few tenths of a millimetre.
• Proximity effect. When two conductors carry current in opposite directions, the current concentrates on the facing surfaces. In a tube mill the two strip edges converging on the weld point are exactly such a pair, so the current concentrates precisely where heat is wanted.

The combination heats a very narrow band to forging temperature in milliseconds, and the squeeze rolls expel the softened surface metal with its oxides as flash, which is then trimmed. Line speeds well over 100 metres per minute are routine. The bond is a solid-state forge weld, not a fusion weld, and the classic defect is a cold weld where insufficient heat or upset leaves an unbonded oxide film on the bond line.

Flash Welding (FW) and Upset Welding (UW)

Both are resistance processes that join full cross-sections end to end, and B5.16 lists flash welding among the processes a welding engineer must know.

Flash welding proceeds in three stages:

1. Flashing. The parts are brought into light contact under current. Localised contact points heat, melt and explode, expelling metal as flashing sparks. Repeated advance sustains the flashing and heats the faying surfaces uniformly.
2. Upset. The parts are driven together rapidly under high force. Molten metal, oxides and contaminants are expelled radially into the upset flash.
3. Hold. Force is maintained while the joint cools.

Because the final bond is made between clean, plastically deformed metal after the contaminated layer has been squeezed out, flash welding produces high-integrity joints in rail, chain link, band saw blades, automotive rims and ring forgings. The upset flash is trimmed afterwards.

Upset welding omits the flashing stage: the parts are held in firm contact, resistance heated, and then upset. It suits smaller cross-sections and wire butt joints where surfaces can be prepared clean beforehand.

Stud Welding

Arc stud welding (SW) places a stud in a gun against the work inside a ceramic ferrule, lifts it to draw an arc, melts both stud end and base metal, then plunges the stud into the pool under spring force. The ferrule contains the molten metal, shields the arc and shapes the fillet. Arc time is a fraction of a second. AWS D1.1 governs stud application qualification and production testing, which is done by bend testing or torque testing rather than by radiography.

Capacitor discharge (CD) stud welding stores energy in a capacitor bank and discharges it through a small ignition tip on the stud end in milliseconds. The extremely short cycle means almost no heat reaches the reverse side, which is why CD studs can be attached to sheet as thin as roughly 0.5 mm without marking the show face. The trade-off is a much smaller weld area and lower load capacity.

Ultrasonic Welding (USW)

Ultrasonic welding is a solid-state process. A sonotrode applies shear oscillation at roughly 20 to 40 kHz to the interface while a clamping force presses the parts together. The oscillation breaks up and disperses surface oxides, brings clean metal into intimate contact, and produces a metallurgical bond by interdiffusion and local plastic flow — without melting.

Its natural applications are thin, highly conductive, oxide-forming metals that resist fusion welding: aluminum and copper foils, battery tabs and busbars, wire harness terminations, and dissimilar aluminum-to-copper joints. Because there is no melt, there is no solidification cracking, no porosity and no brittle fusion-zone intermetallic layer of the kind that plagues fusion welding of aluminum to copper — although a thin intermetallic can still form by diffusion at long weld times.

Surfacing (SW) and Thermal Spraying (THSP)

B5.16 groups surfacing and thermal spraying together, and the distinction between them is a favourite exam discrimination.

Weld Surfacing (Overlay, Cladding, Hardfacing)

Weld surfacing deposits filler metal onto a surface by a fusion welding process to restore dimensions or impart different properties:

• Buildup restores worn dimensions with a material similar to the substrate.
• Hardfacing applies a wear-resistant deposit such as a chromium carbide or a cobalt-base alloy.
• Cladding applies a corrosion-resistant layer such as an austenitic stainless or a nickel alloy.
• Buttering deposits a transition layer on one face before making a dissimilar joint.

The defining characteristic is dilution: the deposit melts and mixes with the substrate, so the first layer's chemistry is not the filler's chemistry. Dilution is controlled by process selection, heat input, electrode oscillation and the number of layers, and a corrosion-resistant overlay is normally specified to achieve required chemistry at a stated depth in the second layer.

Thermal Spraying

Thermal spraying melts or softens a consumable and propels the particles onto a prepared substrate, where they flatten and build up a lamellar coating.

The critical difference from weld surfacing is the bond. Thermally sprayed coatings adhere primarily by mechanical interlocking to a grit-blasted surface, with little or no fusion and therefore no dilution. The substrate stays cool, so distortion and metallurgical change are minimal — but bond strength is far lower than a fused overlay and the coating is generally not suitable for high-impact service. The exception is spray-and-fuse, where a self-fluxing nickel-chromium-boron-silicon alloy is sprayed and then fused in a subsequent heating operation to create a metallurgical bond.

Exam Trap 1: Calling thermal spray a welding process with dilution. Thermally sprayed coatings bond mechanically and are not diluted by the substrate. If a question asks how to achieve a specified overlay chemistry at the surface, dilution control points to weld overlay; if it asks how to coat without heating the part, it points to thermal spray.

Exam Trap 2: Confusing flash welding with arc welding. Flash welding is a resistance process. The flashing stage expels metal to clean and heat the faying surfaces, and the actual joint is made by the upset forging stroke after the arc-like flashing has stopped.

Exam Trap 3: Assuming ultrasonic welding melts the interface. Ultrasonic welding is solid state. Shear oscillation disrupts oxides and produces bonding by plastic flow and diffusion below the melting point, which is precisely why it works on aluminum and copper.