Use this guide to evaluate an RFW application
This guide organizes AFW’s practical design guidance for joint types, material combinations, diameter and wall guidelines, flash and upset allowance, tolerances, concentricity, inspection, and quoting requirements.
Start with the Quick Design Reference below, then use the Guide Contents to jump directly to the section that applies to your part.
Quick Design Reference
Use these values as initial design guidelines. Exact requirements depend on material, geometry, tooling, and application.
Process Overview
Rotary Friction Welding (RFW) is a solid-state joining process in which one component rotates relative to another under axial force. Friction generates heat at the interface, plasticizing the material. Rotation stops and forge pressure is applied to create a metallurgical bond.
Advantages
- No filler metal
- No shielding gas
- Minimal heat-affected zone
- Excellent mechanical properties
- Repeatable and highly automated
- Ability to join many dissimilar metals
Best Candidate Parts
RFW is ideal for round, rotationally symmetric components:
Round Components
- Shafts
- Tubes
- Rods
- Spindles
- Pistons
- Axles
- Hydraulic components
Typical Industries
- Aerospace
- Defense
- Oil & Gas
- Heavy Equipment
- Automotive
Basic Joint Types
The most common joint configurations for rotary friction welding are shown below (adapted from ANSI/AWS C6.1):
- Bar-to-Bar (Solid-to-Solid): Two solid cylindrical parts joined end-to-end.
- Tube-to-Tube: Two tubular components joined end-to-end. Wall thicknesses should be reasonably matched.
- Tube-to-Bar: A tube joined to a solid bar. Requires careful evaluation of interface area and flash behavior.
- Tube-to-Disc / Tube-to-Plate / Bar-to-Plate: Common for attaching flanges, ends, or plates; may require specialized fixturing.
Material Considerations
Excellent Combinations (Similar Metals)
| Material | Welded To |
|---|---|
| Carbon Steel | Carbon Steel |
| Stainless Steel | Stainless Steel |
| Titanium | Titanium |
| Aluminum | Aluminum |
| Copper | Copper |
Full Strength Metallurgical Bonds (Excellent Dissimilar)
These combinations produce a full-strength metallurgical bond. In some cases an appropriate post-weld heat treatment may be required to realize the full weld strength of the weaker parent metal.
| Metal A | Metal B |
|---|---|
| Aluminum / Al Alloys | Copper / Cu-Ni |
| Aluminum / Al Alloys | Carbon / Stainless Steel |
| Aluminum / Al Alloys | Magnesium / Mg Alloys |
| Copper | Carbon / Stainless Steel |
| Copper | Nickel / Ni Alloys / Monel |
| Carbon Steel | Stainless Steel |
| Carbon Steel | Nickel Alloys / Monel / Nimonic |
| Carbon Steel | Titanium / Ti Alloys |
| Stainless Steel | Nickel Alloys / Monel / Nimonic |
| Stainless Steel | Copper-Nickel |
| Titanium / Ti Alloys | Copper |
| Steel (various) | Valve Material (Automotive) |
| Steel (various) | Iron / Steel Sintered |
Many combinations within the broader steel family (carbon, alloy, stainless, maraging, tool) and the nickel-alloy family also achieve full-strength bonds and are routinely used in production.
Challenging Dissimilar Metals
These combinations can be friction welded but will not produce a full-strength bond. Strength is limited by brittle intermetallics, free-machining additives, residual porosity, or large differences in hot strength. They are only acceptable for non-critical applications when the reduced strength is accounted for in design.
| Metal A | Metal B | Primary Limitation |
|---|---|---|
| Steel – Free Machining | Most steels, Ni alloys, Monel, Tool Steel | Pb / S / Se / Te additives create weak planes |
| Steel – Free Machining | Copper / Cu-Ni / Aluminum | Additive embrittlement + intermetallics |
| Certain Steels / Ni Alloys | Cemented Carbides / WC Cemented | Hard-particle interface limits ductility |
| Aluminum / Al Alloys | Some Ti / Ti Alloys or refractories | Rapid brittle intermetallic formation |
| Magnesium / Mg Alloys | Many steels or higher-melting alloys | Large difference in hot strength / melting range |
| Lead | Most structural metals | Soft, low-strength interface |
| Sintered Iron / Sintered Steel | Certain wrought steels or alloys | Residual porosity at the interface |
| Ceramic | Most metals | Limited plastic flow on the ceramic side |
| Refractory metals (Mo, W, Ta, Nb) | Al, Cu, or certain steels | Extreme forging-temperature mismatch |
| Silver / Silver Alloys | Certain steels or aluminum | Soft interface or intermetallic layers |
Key Observations
- Similar metals (or closely related alloys) almost always produce full-strength bonds.
- The densest successful (full-strength) regions are among the various steels and between steels and nickel-based alloys.
- Free-machining steels are the most frequent source of reduced-strength joints because of their deliberate free-machining additives (lead, sulfur, selenium, tellurium).
- Very dissimilar pairs involving ceramics, lead, or extreme refractories are generally not recommended or yield only limited-strength bonds.
- Success always depends on optimized RFW parameters (speed, pressure, upset distance, and surface cleanliness). Specific alloy grades and joint designs can shift results.
Unsure whether your material pair is suitable?
Send the alloy designations, heat-treat conditions, drawing, and performance requirements for an application-specific review.
Diameter Guidelines
Solid Parts
Typical diameters:
- 0.250" to 6.000"
- Smaller and larger diameters are possible depending on equipment capacity.
Tubes
Wall thickness should generally be:
- ≥ 0.060"
- Thinner sections may require evaluation and process development.
Length-to-Diameter Ratio
Long, slender parts may require support during welding.
Rule of Thumb
L/D < 20:1 — Preferred when the part is unsupported.
For larger ratios, the following may be required:
- Steady rests
- Special tooling
- Custom fixturing
Flash Formation
Flash is a normal and necessary part of the process. Material displaced during forging creates upset flash at the weld interface.
Typical Flash
Typical flash appearance for a tube-to-tube joint is shown below:
Typical flash appearance for a tube-to-bar joint is shown below:
Designers should determine how flash will be handled:
- Option A — Flash remains on the part (acceptable for many non-critical applications).
- Option B — Flash is machined off in a secondary operation.
Machining Allowance
Material shortening (axial upset) occurs during welding. Design must include adequate upset allowance.
Typical Range (Rule of Thumb)
- Solid bar: Approximately 20% of the interface diameter (axial length reduction).
- Tubes: 70%–100% of the wall cross-section thickness (highly dependent on geometry).
Actual allowance depends on:
- Material
- Diameter / wall thickness
- Weld parameters
Tolerances
Before Welding
Critical surfaces should be machined accurately prior to welding.
Recommended Face Squareness
| Guideline | Value |
|---|---|
| Face squareness (general) | ≤ 0.007" per inch of diameter |
| Example — 1" diameter | 0.007" |
| Example — 2" diameter | 0.014" |
| Example — 3" diameter | 0.021" |
After Welding
Post-weld machining is often recommended for:
- Bearing journals
- Seal diameters
- Threads
- Precision fits
Concentricity Considerations
Welding under axial pressure does not inherently produce tight runout. Expect typical runout between components of:
Better results are possible with optimized tooling, fixturing, and process control. Critical concentricity requirements should be called out on the drawing and discussed during quoting.
Tube Welding Considerations
Recommended
- Wall thicknesses that are reasonably balanced between the two components.
Watch For
- Thin-to-thick transitions
- Collapse risk on thin-wall tubes
- Excessive internal (ID) flash that may require secondary removal
ID flash removal may require secondary operations (machining, boring, or specialized tooling).
Design Features Near the Weld
Avoid placing critical features directly adjacent to the weld interface.
Recommended Clearance
Examples of features to keep clear of the weld zone:
- Threads
- Seal grooves
- O-ring grooves
- Splines
Have a geometry that pushes these guidelines?
AFW can review likely tooling, part support, flash-removal, post-weld machining, and inspection needs before the design is released.
Process Limitations & Special Cases
While RFW is highly versatile, the following limitations and special cases should be considered during design:
- Non-round geometries: Parts that are not rotationally symmetric about the weld axis are generally not suitable.
- Highly asymmetric mass distribution: Can cause imbalance and runout issues; may require special balancing or fixturing.
- Very thin walls or large diameter thin-wall tubes: Risk of collapse or excessive distortion; require process development.
- Parts requiring near-zero runout: Post-weld machining or specialized tooling is typically needed.
- Certain dissimilar combinations: Some material pairs form intermetallic compounds or require intermediate layers; evaluation is essential.
Inspection Methods
Dimensional (Development & Production)
- Length
- Runout
- Concentricity
Mechanical (Development Only)
- Bend testing — Recommended
- Tensile testing — Optional
- Torsion testing — Optional
Metallurgical (Development Only)
- Macroetch — Optional
- Microstructure evaluation — Optional
- Hardness survey — Optional
Non-Destructive Testing (NDT)
- Ultrasonic testing — 10% to 100% during production (as specified)
- Dye penetrant — Application dependent
Information AFW Needs for Quoting
Part Drawings
- PDF drawing
- STEP model
Material Information
- Alloy designation
- Heat-treat condition
- Hardness (if applicable)
Production Information
- Annual volume
- Prototype quantity
- Delivery requirements
Quality Requirements
- PPAP
- FAI
- Material certifications
- Mechanical testing
- NDT requirements
Quick Design Checklist
Use this checklist when designing parts for rotary friction welding:
- Round, rotationally symmetric components
- Flat, square weld faces
- Adequate upset (shortening) allowance included
- Flash accommodation provided (leave or machine)
- Compatible materials selected
- Critical features kept away from weld interface (≥ 0.100")
- Concentricity / runout requirements called out
- Post-weld machining considered for precision features
- Inspection requirements defined
For application-specific guidance or process development support, contact AFW.
Rotary Friction Welding Design Questions
What parts are the best candidates for rotary friction welding?
RFW is ideal for round, rotationally symmetric components, including shafts, tubes, rods, spindles, pistons, axles, and hydraulic components.
Can rotary friction welding join dissimilar metals?
Many dissimilar combinations produce a full-strength metallurgical bond. Suitability depends on exact alloy designation, heat-treat condition, and application requirements.
How much upset allowance should I include?
As a rule of thumb, solid bar requires approximately 20% of the interface diameter in axial length reduction. Tubes require 70%–100% of the wall cross-section thickness, highly dependent on geometry. AFW will help determine the required allowance during process development.
What runout should I expect after welding?
Welding under axial pressure does not inherently produce tight runout. Expect typical runout between components of 0.030" – 0.060" TIR. Critical concentricity requirements should be called out on the drawing and discussed during quoting.
What should I send with an RFQ?
Provide a PDF drawing, STEP model, alloy designation, heat-treat condition, hardness if applicable, annual volume, prototype quantity, delivery requirements, and applicable PPAP, FAI, material certification, mechanical testing, and NDT requirements.
Send the print before the design is locked.
AFW can review material compatibility, interface geometry, upset and flash allowance, expected runout, tooling needs, inspection requirements, prototype quantity, and production volume.
