In short. A forged wheel starts as a solid cylinder of aluminium alloy, most often quoted as 6061, cut to a weight. The billet is heated to between 350 and 550 °C and squeezed in a closed die by a press until it takes the rough shape of a wheel; the barrel is then either formed in the die or spun out on a mandrel. The forging is heat treated to the T6 temper (held at 516 to 579 °C, quenched, then aged for about 8 hours at 177 °C), which is where most of its strength comes from. The design, the offset, the bolt pattern and the centre bore are then cut on CNC machines, which remove a large share of the metal. Last come inspection, finishing and, for a design, the fatigue and impact trials of the Japanese JWL standard, UN Regulation No. 124 or the American SAE J2530.
The main stages at a glance
| Stage | What happens | What it decides | What can go wrong |
|---|---|---|---|
| Billet | A length of 6061 bar is cut to a weight | Alloy, cleanliness, amount of metal | Wrong grade, inclusions, too little metal to fill the die |
| Forging | The billet is heated and pressed in a closed die, often in several steps | Grain flow, rough shape of disc and barrel | Too cold: unfilled die, surface cracks. Too hot: folds, partial melting |
| Barrel forming | The rim is formed in the die or spun out on a mandrel | Barrel thickness, width range | Uneven wall, distortion |
| Heat treatment | Solution, quench, artificial ageing to T6 | Final strength and hardness | Under- or over-ageing, distortion on quenching |
| Machining and finishing | Lathe, milling, drilling; then coating | Design, offset, PCD, centre bore, weight, appearance | Thin sections, runout, a coating cured hot enough to keep hardening the metal |
If you are still deciding between processes, our guide to forged, cast and flow-formed wheels compares them side by side.
Stage 1: the billet
The alloy most often quoted for forged wheels is 6061, an aluminium alloy with magnesium and silicon as its main additions. In the T6 temper its typical tensile strength is 310 MPa and its yield strength 270 MPa (Wikipedia, 6061). The letters after the number describe what was done to the metal: T6 means solution heat-treated and artificially aged, T4 naturally aged, O annealed (Wikipedia, aluminium alloy). So "6061-T6" on a wheel describes the end of the process, not the bar it started from.
ASTM B247, the American specification for aluminium die forgings, hand forgings and rolled ring forgings, refers to a separate specification for "forging stock supplied as rolled, cold-finished bar, extruded bar, or rod" (ASTM B247). In our reading, the billet of a wheel is a slice of such a bar (or of a cast log), cut to a weight.
How heavy is a billet? Our calculation. With a density of 2.7 g/cm³ for 6061 (MakeItFrom), the mass of a slice 100 mm long is the area of the bar times 0.1 m times 2,700 kg/m³. The diameters below are round examples, not those of any particular forge.
| Bar diameter | Area | Mass per 100 mm of length |
|---|---|---|
| 6 in (152.4 mm) | 0.0182 m² | 4.93 kg |
| 8 in (203.2 mm) | 0.0324 m² | 8.76 kg |
| 10 in (254 mm) | 0.0507 m² | 13.68 kg |
Stage 2: heating and the forging press
Wikipedia describes 6061 billet as heated in an induction furnace and forged using a closed-die process (Wikipedia, 6061). In closed-die forging the metal flows to fill the cavities of a die "resembling a mold", usually through a series of cavities from a rough distribution to the finished shape; the excess squeezed out, the flash, is trimmed. Dies for aluminium usually have a draft of at least 3 degrees so that the part can be released (Wikipedia, forging).
The temperature window is narrow. Aluminium is forged between 350 and 550 °C. Above 550 °C the work piece is too close to the solidus, the temperature at which the alloy starts to melt, and the result can be poor surfaces, partial melting and folds; below 350 °C the metal resists more, which can leave the die unfilled, crack the surface and raise the force needed. Because aluminium conducts heat well, the temperature of the dies matters as much as that of the billet (Wikipedia, forging). For 6061 the solidus is 580 °C (MakeItFrom): the top of the window is only 30 °C below it.
What forging changes inside the metal. The same article lists continuous grain flow, a reduced chance of voids, better fatigue resistance and finer grain size among the advantages of forging; it gives that list under open-die forging, and it applies to closed-die work only by our reading (Wikipedia, forging). Those are general properties of the process; a given wheel's strength still depends on its design and is judged by the tests below.
Why the presses are so large: our calculation. The force a press must deliver is roughly the average pressure in the die multiplied by the area of the part seen from above. We take a disc of the wheel's nominal diameter as that area, which is a simplification, and three round pressures that are our assumptions, not measured values. Force in tonnes-force is newtons divided by 9,806.65; one metric tonne-force is about 1.1 US short tons.
| Nominal diameter | Projected area | At 25 MPa | At 50 MPa | At 100 MPa |
|---|---|---|---|---|
| 17 in | 0.146 m² | 373 tf | 747 tf | 1,493 tf |
| 18 in | 0.164 m² | 419 tf | 837 tf | 1,674 tf |
| 19 in | 0.183 m² | 466 tf | 933 tf | 1,865 tf |
| 20 in | 0.203 m² | 517 tf | 1,033 tf | 2,067 tf |
| 21 in | 0.224 m² | 570 tf | 1,139 tf | 2,279 tf |
| 22 in | 0.245 m² | 625 tf | 1,250 tf | 2,501 tf |
Two things follow. The force grows with the square of the diameter, so a 22-inch forging needs two-thirds more than a 17-inch one at the same pressure. And a press's tonnage tells you the size of part it can make, not how good the wheel is. We do not know the real die pressures used by forges.
Stage 3: forming the barrel
The barrel is the part that carries the tyre. On a one-piece forged wheel there are two ways to make it. Either the die forms a deep cup that machining turns into the barrel, or the forging is a thicker, shorter preform whose barrel is then spun out on a mandrel: the part turns, a heater keeps it hot, and rollers press the metal outward and along. Yamaha describes the second method for its motorcycle wheels: a pre-formed aluminium wheel turns on a mandrel, a burner heats it and a roller "begins to shape the rims by pressing against the wheel" (Yamaha). In our reading, the same kind of spinning is also applied to cast wheels, which is why "flow-formed" alone does not tell you whether the centre was cast or forged.
On two- and three-piece wheels the barrel is a separate part joined to the forged centre; Wikipedia's article on alloy wheels says all parts of a modular wheel are held with bolts. Our guides to one-, two- and three-piece wheels and to the parts of a wheel explain how the pieces fit together.
Stage 4: heat treatment, where T6 comes from
6061 is a precipitation-hardened alloy: it gains strength when fine particles of magnesium silicide form inside the metal under controlled heat (Wikipedia, 6061). The T6 temper is reached in three steps. ASTM B918 is the American practice for heat treating wrought aluminium alloys; its scope warns that the times and temperatures in its tables "are typical" and "may not provide the optimum heat treatment for a specific item" (ASTM B918). A diagram on Wikipedia based on that practice gives the values for 6061 (Wikimedia Commons):
- Solution treatment at 516 to 579 °C, long enough for the alloying elements to dissolve evenly.
- Quench to 43 °C or below, quickly, which locks the elements in solution. This is the step in which a part can distort.
- Artificial ageing for 8 hours at 177 ± 6 °C, during which the strengthening particles form. The time depends on the size of the part.
ASTM B918 also notes that some 6xxx alloys can reach the T4 temper by being quenched straight from the hot-working temperature, as when they leave an extrusion die. A separate furnace treatment and a quench straight from the press are different routes, worth asking about.
The temperatures in one ladder, compiled by us from the sources above.
| Temperature | What it is | Source |
|---|---|---|
| 170 °C | Maximum temperature for mechanical use of 6061-T6 | MakeItFrom |
| 171 to 183 °C | Artificial ageing, 8 hours | Commons diagram after ASTM B918 |
| 350 to 550 °C | Forging window for aluminium | Wikipedia, forging |
| 516 to 579 °C | Solution treatment of 6061 | Commons diagram after ASTM B918 |
| 580 °C | Solidus of 6061: melting begins | MakeItFrom |
The forging window and the solution range overlap just under the melting point, so temperature control is the craft of a forge. And ageing happens at the kind of temperature at which coatings are baked. A powder coating maker, IGP, says that precipitation hardening "takes place at temperatures that are in the range of the cross-linking temperatures of powder coatings", and advises against powder coating aluminium wheels again when they are refurbished (IGP, TI 116). Our guide to powder coating and painting wheels gives the curing temperatures.
Stage 5: machining
The forging is a rough shape. The wheel you see is cut from it. A lathe turns the barrel profile, the bead seats and the face; a milling machine cuts the windows between the spokes and the pockets behind them; the bolt holes, the seats for the nuts or bolts, and the centre bore are drilled and bored; the valve hole is drilled last. Much of a forged wheel's design is cut rather than formed, and that is also why its offset, bolt pattern and centre bore can be set for each order: they are dimensions of a cut, not of a die.
How much metal goes? Our illustration, with invented numbers. A finished wheel of 10 kg holds 3.7 litres of aluminium. The share removed depends on the mass of the forging, which no maker we read publishes:
| Forging | Finished wheel | Share removed as chips |
|---|---|---|
| 20 kg | 10 kg | 50 per cent |
| 25 kg | 10 kg | 60 per cent |
| 30 kg | 10 kg | 67 per cent |
Those chips are aluminium of known grade and can be recycled, but the machining time is a large part of the cost of a forged wheel; our guide to what forged wheels cost deals with price.
Stage 6: inspection, finishing and the tests
Inspection. A common check for cracks is dye penetrant inspection: a coloured liquid is drawn into any surface-breaking defect, the surface is cleaned, and a white developer draws the liquid back out so that the defect shows as a "bleed-out" (Wikipedia, dye penetrant inspection). The Japanese JWL trials use the same method to decide whether a wheel has cracked. Dimensions are then checked; what is measured, and on how many wheels, is up to each maker and worth asking.
Finishing. Paint, powder, polishing, brushing or a clear coat. UN Regulation No. 124 asks for a salt-spray test of 384 hours on a coated sample that has been scratched and stone-chipped on purpose: no significant corrosion after 192 hours, and after 384 hours the function of the wheel, its fixings and its bead seat must not be affected (Regulation No. 124, Annex 5).
The tests. The standards judge the finished wheel, not the process. Our reading of the two public texts:
| Trial | JWL (Japan) | UN Regulation No. 124 |
|---|---|---|
| Rotating bending | Moment of 1.5 × F × (0.7 r + d), at least 100,000 revolutions; factor 1.8 or 2.0 with 50,000 revolutions allowed for some aluminium alloys | Two levels: at least 200,000 cycles at 75 per cent and 1,800,000 cycles at 50 per cent of the maximum moment, for aluminium wheels of passenger cars |
| Rolling or radial fatigue | Load 2.25 × F on a drum, at least 500,000 revolutions | For passenger cars, load 2.5 × the wheel's maximum load, 2,000 km on a drum of at least 1.7 m (a footnote allows 2.25 and 1,000 km) |
| Impact | Wheel tilted 13 degrees, mass of 0.6 W + 180 kg dropped from 230 mm | Mass of 0.6 × Fv / g + 180 kg, striker 230 mm above the rim |
| Material | Not part of the three trials | Chemical analysis, and strength and metallurgical defects of test pieces cut from the hub mounting area and the disc-to-rim transition (Annex 4) |
| Pass | No crack shown by dye penetrant, no marked deformation or abnormal loosening; in the impact test, no through-crack, no rim and disc separation, no air leak | No technical crack (more than 1 mm), torque loss limits |
Sources: MLIT, JWL technical standard (in Japanese) and Regulation No. 124. Two details matter for forged wheels. Regulation No. 124 lists the "method of production (welded, forged, cast,…)" among the characteristics that define a wheel type, so a forged wheel and a cast wheel of the same design are different types. And the JWL notice lets the bending test be run at a higher factor, 2.0 with 50,000 revolutions, for aluminium alloys it calls 鋳鍛造材; the term can be read as cast and forged material or as a cast-then-forged hybrid, and we do not settle which.
What the rules say where you live
- United Kingdom. The MOT does not ask how a wheel was made. It lists "any fracture or welding defect on a wheel" as dangerous, a badly distorted wheel as major, and "a wheel and its fixings not compatible" as major (MOT manual, 5.2.2). UN Regulation No. 124 sets the trials for replacement wheels in the countries that apply it.
- United States. SAE J2530 is a Recommended Practice for aftermarket wheels for passenger cars, light trucks and multipurpose vehicles, revised in December 2024; it sets performance requirements, test procedures and marking, and applies only to wheels made of the materials in its tables (SAE J2530). The document is sold, so we do not reproduce its values. The federal standard FMVSS No. 110 deals with rims on new vehicles: each rim must match a rim size the tyre maker lists for the tyre, and must hold a deflated tyre at 97 km/h (60 mph) until a controlled stop (49 CFR 571.110, S4.4).
- Japan, and wheels sold worldwide with its marks. JWL is a technical standard of the Japanese transport ministry. Our guide to reading wheel markings explains both stamps.
- Australia and Canada. We did not read a national text that sets how a wheel must be made, so we give no rule for these two countries. Ask your state or territory registration authority, or your province, before fitting non-original wheels.
What to ask a maker about how a wheel was made
- The alloy and the final temper, written out: "6061-T6", not "aircraft aluminium".
- Whether the centre and the barrel are forged, spun or separate parts.
- Whether the heat treatment is a separate solution and quench after forging.
- Which standard the design was worked to (JWL, Regulation No. 124, SAE J2530), and whether the load rating is stamped on the wheel.
- The weight of one wheel at your diameter and width.
- The finish, and the temperature at which it is cured.
Onyx Forged
Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards. Onyx Forged does not recommend a size or an offset: the dimensions come from the car maker's documents and from the car as you describe it, and the Onyx Forged design office checks the drawing with you before anything is machined.
Frequently asked questions
What are forged wheels?
Wheels shaped from solid aluminium alloy, most often 6061, by pressing a hot billet in a closed die, then heat treated and machined to their final shape. A cast wheel is poured as a liquid into a mould instead.
What does 6061-T6 mean?
6061 is an aluminium alloy with magnesium and silicon. T6 means solution heat-treated and artificially aged; typical values are 310 MPa tensile and 270 MPa yield strength.
Why is the forging temperature so tightly controlled?
Above 550 °C aluminium comes too close to its melting range, with a risk of folds and partial melting; below 350 °C it may not fill the die and can crack. The solidus of 6061 is 580 °C.
Is a forged wheel machined from a solid block?
It is machined from a forging, not from a plain block. The press gives the rough shape and the grain flow; machining cuts the spokes, the barrel profile, the bolt holes and the centre bore.
Is a flow-formed wheel forged?
Only its barrel is worked by spinning. The centre is whatever it started as, cast or forged, so ask about the centre and the barrel separately.
Does the process decide the test results?
No. JWL, Regulation No. 124 and SAE J2530 test the finished wheel. Regulation No. 124 does treat a forged wheel and a cast wheel as different types.
Can a forged wheel be powder coated again?
A powder coating maker, IGP, advises against powder coating aluminium wheels when refurbishing, because curing temperatures overlap those of the alloy's ageing. Ask the wheel maker first.
Where to go next
To see how a set is worked out for your car, read how to customise your wheels.
Sources
- Alloy and process: Wikipedia, 6061 aluminium alloy; Wikipedia, forging; Wikipedia, aluminium alloy, temper designations; Wikipedia, alloy wheel; Wikimedia Commons, 6061 heat treating diagram after ASTM B918M-17a; MakeItFrom, 6061-T6; Yamaha, SpinForged wheels; Wikipedia, dye penetrant inspection.
- Standards: ASTM B247, aluminium die forgings; ASTM B918/B918M, heat treatment of wrought aluminium alloys; MLIT, JWL technical standard for light-alloy disc wheels (in Japanese); UN Regulation No. 124; SAE J2530, December 2024.
- Rules: GOV.UK, MOT inspection manual, section 5.2.2; 49 CFR 571.110, FMVSS No. 110.
- Coating: IGP Powder Coatings, TI 116.
All pages were read on 16 October 2026.
The Onyx Forged design office.