---
type: process
name: "Friction Welding"
category: "Joining"
subcategory: "Thermal"
materials: ["Metal", "Plastic"]
tolerances: "Rotary: finished length controlled to about ±0.010 in (±0.25 mm) through upset control; FSW: butt gap under 10% of thickness, plate flatness governed by clamping"
volumes: "500 to 1,000,000+ per year for rotary; FSW is economic from low hundreds of panels upward"
lead_time: "1–30 s cycle for rotary friction welds; 4–12 weeks for machine tooling, clamping fixtures and parameter development"
url: https://manufacturingprocesses.org/processes/joining/friction-welding
---

# Friction Welding

Friction welding rubs two parts together under load until the interface plasticises and forges together, with no melting and no filler metal.

- **Category**: [Joining](https://manufacturingprocesses.org/processes/joining.md)
- **Family**: Thermal
- **Materials**: Metal, Plastic
- **Typical tolerances**: Rotary: finished length controlled to about ±0.010 in (±0.25 mm) through upset control; FSW: butt gap under 10% of thickness, plate flatness governed by clamping
- **Typical volumes**: 500 to 1,000,000+ per year for rotary; FSW is economic from low hundreds of panels upward
- **Lead time**: 1–30 s cycle for rotary friction welds; 4–12 weeks for machine tooling, clamping fixtures and parameter development

## Overview

Friction welding rubs two parts together under load until the interface plasticizes, then forges them together. Nothing melts — peak temperature stays at roughly 0.6–0.9 of the absolute melting point — so the joint is made in the solid state, without filler, flux, shielding gas or an arc.

That has two consequences a design engineer cares about. First, the solidification defects that limit fusion welding (hot cracking, gas porosity, filler dilution) simply do not occur. Second, alloys that are effectively unweldable by arc become joinable: 2024 and 7075 aluminum, aluminum to copper, aluminum to steel.

Four variants cover the field: rotary friction welding (RFW) for round parts, linear (LFW) and orbital (OFW) for non-round sections, and friction stir welding (FSW) for seams in plate and extrusion. FSW joint efficiency in aluminum typically runs 70–90% of parent tensile strength in heat-treatable alloys and approaches 100% in non-heat-treatable 5xxx — against roughly 50–70% for the same alloys arc welded.

## How it works

### Rotary, linear and orbital friction welding

1. **Load and rub.** Rotary drives one part at a surface speed of roughly 3–10 ft/s (1–3 m/s) at the interface; linear reciprocates at 25–125 Hz with 0.04–0.12 in (1–3 mm) amplitude; orbital moves both parts in a small circle so every point on the interface sees the same relative velocity.
2. **Friction phase.** Friction pressure — commonly 4,000–13,000 psi (30–90 MPa) on steel — breaks up oxide and contamination and drives it into the flash while the interface heats and plasticizes. The parts shorten (upset) as material extrudes radially.
3. **Braking or stopping.** Relative motion stops in milliseconds. Direct-drive machines brake the spindle; inertia machines let a flywheel run down, so delivered energy is fixed by flywheel mass and speed.
4. **Forge phase.** Pressure is raised, often to 9,000–29,000 psi (60–200 MPa), and held while the joint consolidates and cools under load. Total cycle is typically 1–30 s.
5. **Flash removal.** Upset flash is machined or trimmed off. Upset length is the process's own quality signal, controlled to about ±0.010 in (±0.25 mm) on production machines.

Rotary welding needs one part round and free to spin, which is why it dominates axles, drive shafts, valves, drill pipe and bimetallic tooling. Linear friction welding removed that constraint and is the production route for integrally bladed rotors (blisks) in titanium and nickel superalloys. Orbital suits non-round sections where neither part can be spun.

### Friction stir welding

1. **Clamp.** FSW generates large lateral and downward forces, so plates are clamped hard against a rigid backing anvil.
2. **Plunge.** A non-consumable tool with a profiled pin and a wider shoulder rotates at 200–2,000 rpm and plunges until the shoulder contacts the surface. Friction under the shoulder supplies most of the heat.
3. **Traverse.** The tool travels along the seam at 2–120 in/min (50–3,000 mm/min), stirring plasticized material from the advancing side around to the retreating side. Peak temperature in aluminum sits around 750–930 °F (400–500 °C) — hot enough to plasticize, well below the melting range.
4. **Retract.** The pin leaves an exit hole at the end of the weld unless a retractable-pin tool or a run-off tab is used. This must be designed for, not discovered.

Production FSW welds aluminum up to roughly 1 in (25 mm) per side, and about 2 in (50 mm) double-sided. The weld nugget is fine-grained and recrystallized; the thermo-mechanically affected zone and HAZ on either side are where heat-treatable tempers soften.

## Design guidelines

### Rotary friction welding: geometry constraints

At least one part must be axisymmetric and free to rotate. Solid bar from about 0.2 to 4 in (5–100 mm) diameter covers most production; tube welds well because interface pressure is uniform. Allow for length loss: upset consumes 0.06–0.4 in (1.5–10 mm) of combined length depending on section, so dimension blanks with an upset allowance and control finished length from the weld. Provide clearance for the flash and a way to machine it off — a relief groove next to the joint is the usual answer.

### Friction stir welding: joint and fixture design

Design for a rigid backing anvil under the entire weld line: FSW pushes down with thousands of pounds of force and the plate cannot deflect. Butt joints need edges machined or sawn square with a gap under about 10% of thickness. Plan the run-on and run-off — either put the exit hole in a sacrificial tab that gets trimmed, or specify a retractable pin tool. Lap joints produce hooking defects at the sheet interface, which must be kept off the tension side.

### Strength and heat effects

FSW does not melt the metal, but it does put the joint through a thermal cycle. In 6061-T6 and 7075-T6 the HAZ over-ages and softens, so joint efficiency lands around 70–90% of parent tensile — still far better than the 50–70% typical of arc welding those alloys, and without hot cracking. In non-heat-treatable 5xxx alloys, where strength comes from work hardening rather than precipitates, joint efficiency is close to 100%.

### Which material pairs work?

| Pair | Process | Notes |
| --- | --- | --- |
| Steel to steel | RFW, LFW | Full base-metal strength; standard for shafts and axles |
| Tool steel to mild steel | RFW | The classic bimetallic economy — expensive alloy only where it cuts |
| Stainless to carbon steel | RFW | No filler dilution and no sensitized weld metal |
| Aluminum to aluminum, same alloy | FSW, RFW | The mainstream FSW case: panels, extrusion seams, battery trays |
| Aluminum 2xxx and 7xxx | FSW | Joinable, where arc welding hot-cracks |
| Aluminum to copper | FSW, RFW | Practical for busbars and battery tabs; intermetallic layer must be kept thin |
| Aluminum to steel | FSW, RFW | Achievable in the solid state; impossible by arc |
| Titanium to titanium | LFW, RFW | Blisk manufacture; no atmospheric contamination since nothing melts |
| Nickel superalloys | LFW | Avoids the strain-age cracking that plagues fusion welding |

Strength and thermal data are on the [material properties chart](/charts/material-properties); melting points that set the 0.6–0.9 Tm working window are on the [metal melting points chart](/charts/metal-melting-points).

### Inspection

Rotary welds are inspected by the flash itself — shape and symmetry are a direct process signature — plus upset length monitoring, ultrasonic testing through the bar, and destructive coupons. FSW is inspected visually for surface flash, galling and root lack-of-penetration, with phased-array ultrasonics for wormhole (tunnel) and root defects. The FSW "kissing bond" — surfaces in contact but not metallurgically joined — is notoriously hard to detect by any NDT method, which makes force and parameter monitoring the primary quality tool.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| RFW: upset allowance on blank length | 0.1–0.4 in (2.5–10 mm) | Section dependent | Material is consumed into flash |
| RFW: flash relief groove at the joint | Provide | — | Flash must be trimmable |
| FSW: joint gap | 0 | 10% of thickness | The tool cannot bridge a gap |
| FSW: backing anvil support | Full length, rigid | — | Downforce deflects unsupported plate and causes root defects |
| FSW: exit hole | Run-off tab or retractable pin | Never in the part | The pin leaves a hole where it retracts |
| FSW: single-pass thickness in aluminum | Up to 1 in (25 mm) per side | ~2 in (50 mm) double-sided | Tool strength and machine force limits |

## Variants

- Rotary Friction Welding (RFW)
- Linear Friction Welding (LFW)
- Orbital Friction Welding (OFW)
- Friction Stir Welding (FSW)

## Cost drivers

Friction welding is machine-dominated. There are no consumables beyond FSW tool wear, and cycle times are short — 1–30 s for rotary — so per-part cost at volume is low. The offsetting costs are a purpose-built machine, dedicated clamping fixtures, and a parameter development program, because each material pair and section needs its own schedule proved on coupons.

Rotary friction welding usually pays for itself through material substitution rather than through the weld: replacing a solid alloy-steel shaft with a mild-steel body friction-welded to a small alloy end removes both expensive stock and the machining that goes with it. FSW pays for itself by replacing riveted panel joints — deleting hundreds of holes, rivets, sealant and inspection steps. FSW machines are large and stiff because the process reacts high loads, which is the main capital barrier and the reason contract capacity is worth checking before designing around it.

1. **Use a bimetallic design.** Put the expensive alloy only where function needs it and friction weld it to a cheap body.
2. **Design flash relief into the part** so trimming is a single facing cut rather than a separate operation.
3. **Detail the FSW exit hole out of the part** with a run-off tab, or budget for a retractable-pin tool.
4. **Consolidate extrusions.** Friction stir welding two profiles is usually cheaper than tooling a single wide extrusion, and can beat casting for panel structures.
5. **Qualify on coupons.** Parameter development destroys parts; do it on representative sections, not finished assemblies.

## FAQ

### How strong is a friction stir weld compared to the base metal?

In heat-treatable aluminum such as 6061-T6 or 7075-T6, joint efficiency is typically 70–90% of parent tensile strength — the loss comes from over-aging in the heat-affected zone, not from the weld nugget. In non-heat-treatable 5xxx alloys the welded joint approaches annealed parent strength, so efficiency is close to 100%. Arc welding the same alloys gives roughly 50–70%.

### Can friction welding join aluminum to steel?

Yes — this is the main reason to use it. Because nothing melts, the thick brittle Fe-Al intermetallic layer that ruins a fusion weld never has time to form. Both rotary friction welding (for round transition pieces) and friction stir welding (for lap and butt seams) are used in production for aluminum-to-steel and aluminum-to-copper joints.

### Does friction stir welding leave a hole at the end of the weld?

Yes. When the tool retracts it leaves an exit hole the diameter of the pin. Production designs either place that hole in a sacrificial run-off tab that gets trimmed away, or use a retractable-pin tool that withdraws the pin gradually while the shoulder closes the surface. This has to be designed in from the start.

### How is friction welding inspected?

Rotary welds are judged first by flash shape and symmetry, which is a direct signature of the parameters, plus upset length monitoring, ultrasonic testing and destructive bend and tensile coupons. FSW uses visual inspection, phased-array ultrasonics for tunnel and root defects, and bend tests. The kissing bond — surfaces touching but not joined — is very hard to detect by NDT, so force and parameter monitoring carry the quality burden.

## Alternative processes

- [Power Beam Welding](https://manufacturingprocesses.org/processes/joining/power-beam-welding.md): Power beam welding fuses metal with a laser or electron beam focused to a very small spot, producing deep, narrow welds with little distortion.
- [Riveting](https://manufacturingprocesses.org/processes/joining/riveting.md): Riveting sets a rivet through aligned holes and upsets its tail so the joint clamps permanently, with no heat and no access to both sides for blind types.
- [Adhesive Bonding](https://manufacturingprocesses.org/processes/joining/adhesive-bonding.md): Adhesive bonding joins parts with a cured polymer layer that spreads load over the whole bond area instead of concentrating it at fastener points.
- [Soldering and Brazing](https://manufacturingprocesses.org/processes/joining/soldering-and-brazing.md): Soldering and brazing join metals with a molten filler that wets the joint by capillary action while the parent metal stays solid.

## Related processes

- [Arc Welding](https://manufacturingprocesses.org/processes/joining/arc-welding.md): Arc welding melts and fuses metal with an electric arc between an electrode and the work, shielded from the atmosphere by gas or flux.
- [Forging](https://manufacturingprocesses.org/processes/forming/forging.md): Forging deforms solid metal under compressive load in a hammer, press or dies, so the grain flow follows the part contour and fatigue strength rises.
- [Metal Extrusion](https://manufacturingprocesses.org/processes/forming/metal-extrusion.md): Metal extrusion pushes heated billet through a shaped die, producing continuous aluminum or copper profile of constant cross-section.
- [Resistance Welding](https://manufacturingprocesses.org/processes/joining/resistance-welding.md): Resistance welding passes a heavy current through the joint so its own electrical resistance melts a nugget between the sheets.

---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/joining/friction-welding)*

*Last updated: August 11, 2026*