Most humanoid robots are built around an aluminium skeleton: 6061 or 6082 for frames and long links, 7075 for compact, highly loaded joint parts, 5xxx sheet for covers and casting alloys for volume housings. This guide explains which alloy, temper and manufacturing route suits each part of the robot, what tolerances and finishes matter, and how to specify the aluminium stock for prototypes and production.
A humanoid robot is a chain of moving masses. Every gram in a forearm has to be accelerated by the elbow, shoulder, hip and ankle actuators below it, so mass far from the body centre costs motor torque, battery energy and control bandwidth. Aluminium is the default structural metal because it offers about one third the density of steel with strength that, in the 7xxx alloys, approaches that of many structural steels.
The important number is not strength alone but strength and stiffness per unit mass. Aluminium alloys have an elastic modulus of roughly 69–72 GPa, about one third of steel, but because density is also about one third, specific stiffness is almost identical. That means an aluminium link of the same mass as a steel link can be made larger in section and therefore stiffer in bending, which is exactly what a robot arm or leg needs to hold position accuracy under load.
Aluminium also machines several times faster than titanium or steel, conducts heat well enough to act as a heat spreader for motors and drives, takes hard anodic coatings for wear surfaces, and is available in every semi-finished form a robotics team needs: plate, block, bar, flat bar, tube, custom extrusions and forgings. Titanium, magnesium, carbon-fibre composites and engineering plastics all have a place in humanoid design, but they tend to be used selectively where aluminium is not enough, rather than as the main structure.
| Material | Density (g/cm³) | Modulus (GPa) | Typical yield (MPa) | Relative machining speed |
|---|---|---|---|---|
| Aluminium 6061-T6 | 2.70 | 68.9 | 276 | Very fast |
| Aluminium 7075-T6 | 2.81 | 71.7 | 503 | Very fast |
| Titanium Ti-6Al-4V (annealed) | 4.43 | 114 | about 880 | Slow |
| Alloy steel 4140 (Q&T) | 7.85 | 205 | about 655 (varies with temper) | Moderate |
| Magnesium AZ31B | 1.77 | 45 | about 200 | Very fast, fire precautions |
Aluminium values are typical figures from the Aluminum Association and ASM handbooks; titanium, steel and magnesium values are typical published figures for comparison only. Design to the specified minimums of the standard you order to.
The loads, space and production volume differ from one part of the body to the next, so a single robot commonly uses four or five aluminium alloys and three or four manufacturing routes. The table below is a practical starting map; final selection always depends on your load cases and drawings.
| Robot area | Typical parts | Common alloy & temper | Common route |
|---|---|---|---|
| Torso and spine | Main frame, battery bay, spine plates, shoulder and hip mounts | 6061-T6/T651, 6082-T6, 7075-T651 at highly loaded nodes | Plate or extrusion, then CNC machining |
| Pelvis and central nodes | Hip yoke, pelvis block, load-transfer brackets | 7075-T651 or 7050-T7451 plate, 6061-T6 forgings | Machined from plate or block; forging at volume |
| Thigh and shank links | Leg tubes, link beams, ankle struts | 6061-T6, 6082-T6, 7075-T6 | Custom extrusion or tube, machined ends |
| Upper arm and forearm | Arm links, elbow brackets, cable channels | 6061-T6, 6063-T6 for light sections, 7075-T6 | Extrusion + machining, or machined bar |
| Joint and actuator housings | Motor stator housings, bearing carriers, output flanges | 6061-T6, 7075-T6; casting alloys at volume | CNC from bar or forging; die casting + machining |
| Reducer housings | Strain-wave (harmonic) and cycloidal / RV reducer housings | 7075-T6, 6061-T6 | Precision 5-axis CNC, hard anodised |
| Hands and wrists | Finger links, palm plate, wrist yoke | 7075-T6, 6061-T6, 2024-T351 in some designs | Precision machining from small bar and plate |
| Covers and enclosures | Electronics housings, shells, shields, battery covers | 5052-H32, 6061-T6 sheet, 6063 extrusions | Sheet forming, extrusion, or casting |
A useful rule is that anything long with a constant cross-section is a candidate for extrusion, anything compact and highly loaded is machined from 7xxx plate or bar (and later forged), and anything complex, hollow and produced in thousands is a candidate for die casting.
The high-strength choice: typical yield about 503 MPa, nearly twice 6061-T6, with excellent machinability and good fatigue strength. Used for compact joint parts, reducer housings, output flanges, finger links and pelvis nodes where section size is limited. Not weldable by fusion methods and less corrosion resistant, so it is normally anodised.
Similar strength class to 7075 but with much better toughness and resistance to stress-corrosion cracking in thick sections. A good option for thick pelvis blocks and large load-carrying brackets machined from plate over about 50 mm.
The all-rounder: good strength, very good machinability, weldable, anodises well and available everywhere. Often the first choice for frames, brackets, torso plates and actuator housings that are not strength-critical.
The European structural equivalent of 6061 with slightly higher strength in thicker sections. Widely available as bar, tube and custom extrusion, so it suits leg and arm links.
Lower strength but excellent extrudability and surface finish. Used for thin-walled covers, cable trays, heat-sink profiles and lightly loaded internal frames.
Non-heat-treatable sheet alloys that bend and weld well. Used for enclosures, shells, shields and battery covers where formability matters more than strength.
High fatigue strength and toughness. Occasionally used for highly cycled links and finger parts, but it has poor corrosion resistance and needs anodising or coating.
Dedicated foundry alloys for die-cast or sand-cast housings. They are not interchangeable with wrought grades: 7075 and 6061 are not die-casting alloys.
| Alloy & temper | Typical tensile (MPa) | Typical yield (MPa) | Elongation (%) | Fatigue strength* (MPa) | Weldable |
|---|---|---|---|---|---|
| 6061-T6 | 310 | 276 | 12 | 96 | Yes |
| 6082-T6 (bar, EN min.) | 310 | 260 | 8–10 | — | Yes |
| 7075-T6 | 572 | 503 | 11 | 159 | No (fusion) |
| 7050-T7451 | 524 | 469 | 11 | — | No (fusion) |
| 2024-T351 | 469 | 324 | 19 | 138 | No (fusion) |
| 5052-H32 | 228 | 193 | 12 | 117 | Yes |
* Rotating-beam fatigue strength at 5×108 cycles, typical values from the Aluminum Association and ASM. 6082 values are EN 755-2 specified minimums for extruded bar. Where no value is given, consult the mill certificate or published design data for the specific product.
No. 7075 is the obvious answer when a part is strength-limited in a small envelope, but in many robot parts the governing criterion is stiffness or deflection, and the modulus of 7075 is only about 4 % higher than 6061. If a link is deflecting too much, switching from 6061 to 7075 barely helps; increasing the section depth does.
7075 makes most sense where stress is high: thin-walled bearing seats that must not creep under preload, threads in small bosses, clevis lugs, finger links with small cross-sections and output flanges carrying impact loads from falls. It costs more per kilogram, cannot be fusion welded and is more sensitive to corrosion and stress-corrosion cracking, so it is used where it earns its place.
The right route depends on design maturity and volume as much as on the part itself. Humanoid programmes usually move through all four as the design stabilises, and choosing the stock form early can save a redesign later.
| Route | Starting stock | Best for | Strengths | Watch-outs |
|---|---|---|---|---|
| CNC from solid | Plate, block, round bar, flat bar | Prototypes, joint parts, brackets, low volumes | No tooling; any alloy; fast design changes | High material removal; cost per part stays high at volume |
| Extrusion + machining | Custom 6061, 6082, 6063 or 7075 profile | Links, torso rails, beams, heat-sink housings | Near-net section, low cost per metre, integral features | Die cost; tolerances per EN 755-9 or ANSI H35.2 need machining at datums |
| Forging + machining | 6061 or 7075 forging stock, billet | Pelvis nodes, hip and knee yokes, output flanges | Grain flow follows the part, high fatigue and impact strength | Die cost; justified only once the design is frozen |
| Die casting + machining | Casting alloy ingot | Motor, gearbox and joint housings in volume | Complex hollow shapes, ribs and bosses in one part | Porosity, lower ductility; bearing seats must be machined |
Humanoid joints run preloaded bearings, strain-wave gears and encoders, so housing geometry directly affects backlash, noise, efficiency and position accuracy. The figures below are typical design targets engineers set for robot parts; actual requirements always come from your drawing and the bearing or reducer manufacturer's mounting specification.
| Part type | Typical drawing requirements |
|---|---|
| Frame and torso plates | Flatness of a few hundredths of a millimetre over mounting faces; hole position to ±0.01–0.02 mm at datum interfaces |
| Actuator housings | Bearing seats to IT6 or similar fits; coaxiality of bores around 0.01–0.02 mm; bore finish Ra 0.8–1.6 µm |
| Strain-wave and RV reducer housings | Tighter still: coaxiality and roundness in single microns, seat tolerances of a few microns, bore finish around Ra 0.4–0.8 µm |
| Links and arms | Straightness and twist from the extrusion standard, with interfaces machined to datum after cutting |
Achieving these numbers starts with the stock. Stress-relieved tempers, T651 for plate and T6511 for extruded bar, are stretched after quenching so that removing material does not release locked-in stress and distort the part. For thin-walled housings, rough machining, a rest period or low-temperature stabilisation, then finish machining is common practice. Unstretched T6 bar can move noticeably when deep pockets are cut.
Thermal expansion matters too. Aluminium expands about 23 µm/m·K, roughly twice steel, so a steel bearing in an aluminium seat loses interference as the motor warms up. Designers either specify the fit at operating temperature, use steel bearing liners, or choose bearing arrangements that tolerate the change.
The most common mistakes we see in robot material specifications are easy to avoid: specifying 7075 everywhere when stiffness, not strength, is the limit; ordering unstretched T6 bar for thin-walled housings and then fighting distortion; forgetting the growth of hard anodising on bearing seats; and drawing parts around non-standard plate thicknesses that force expensive over-sized stock. Checking each drawing against standard stock sizes and stress-relieved tempers before the first prototype order usually removes these problems, and it makes the later switch to extrusion or forging far smoother because the datums and allowances are already defined.
Finishing protects aluminium against corrosion and wear, provides electrical insulation where needed, and identifies parts. For robot parts it also changes dimensions, which must be designed in.
| Finish | Typical thickness | Purpose | Design notes |
|---|---|---|---|
| Sulphuric anodising (Type II, MIL-A-8625) | 5–25 µm | Corrosion protection, colour, cosmetic black finish | Grows roughly half outward, half inward; mask threads and bearing seats if needed |
| Hard anodising (Type III) | 25–75 µm | Wear resistance on sliding and rolling surfaces, insulation | Adds about half its thickness per surface; finish machining allowance must account for it |
| Chemical conversion (chromate or trivalent) | Sub-micron | Corrosion protection with electrical conductivity for grounding | Used on earthing points and EMI-shielded enclosures |
| Bead blasting + anodising | As above | Uniform matt appearance that hides handling marks | Blasting before anodising; specify media and finish sample |
| Powder coating or paint | 60–120 µm | Outer shells and covers | Not for precision fits |
Alloy matters for the finish. 6061 and 6063 give the most uniform decorative anodising. 7075 anodises well but can look slightly yellow-grey in clear finishes. 2024 and high-silicon casting alloys anodise poorly and unevenly, so casting housings are often painted, powder coated or given a conversion coating instead.
Frameless torque motors and their drives lose heat into the joint housing. Aluminium's thermal conductivity makes the housing itself the main heat path to the outer skin, and that affects alloy choice: 6061-T6 conducts about 167 W/m·K, 6063-T6 about 200 W/m·K, 7075-T6 about 130 W/m·K and typical die-casting alloys about 96–110 W/m·K.
For joints that run hot, a 6xxx housing or a 6063 heat-sink extrusion bonded to the motor can keep winding temperatures lower than a 7075 or cast housing of the same size. Where 7075 is needed for strength, designers often add fins, thermal pads or a 6063 heat spreader. Remember that sustained temperatures above about 100–150 °C gradually over-age 6xxx and 7xxx alloys and reduce their strength.
In battery packs and power electronics bays, the same logic applies: 6063 or 6061 extrusions with integral fins or cooling channels often double as structure and heat sink, which saves both mass and part count compared with a separate frame and cooler.
Robotics has no single aluminium standard, so stock is bought to the general product standards below. Quoting the right one on the drawing fixes chemistry, properties, tolerances and test requirements.
| Product form | ASTM / SAE | EN |
|---|---|---|
| Sheet and plate | ASTM B209; AMS 4027 (6061), AMS 4045 (7075), AMS 4050 (7050) | EN 485-1 to -4 |
| Rolled or drawn bar, rod and wire | ASTM B211; AMS 4117 (6061), AMS 4123 (7075) | EN 754 |
| Extruded bar, profiles and tube | ASTM B221; AMS 4150 (6061), AMS 4154 (7075) | EN 755-1 to -9 |
| Forgings | ASTM B247; AMS 4126 (7075-T6) | EN 586 |
| Die castings | ASTM B85 | EN 1706 |
| Chemical composition | AA Teal Sheets (registration records) | EN 573-3 |
Each delivery from Metline comes with a mill test certificate to EN 10204 3.1 showing the chemistry and mechanical results for the lot, so the material can be traced through your quality system.
Metline supplies the aluminium itself (plate, block, bar, flat bar, tube, custom extrusions and forgings) and can cut or machine it to your drawing. Teams that machine in-house often buy cut blanks; teams that outsource can send part drawings for review.
There is no single best alloy. 7075-T6/T651 suits compact, highly stressed joint parts and reducer housings; 6061 or 6082 suit frames, links and general brackets; 5052 suits formed covers and enclosures; and dedicated casting alloys suit die-cast housings at volume. Most robots use several of these.
Reducer and actuator housings with thin bearing walls, output flanges, finger and wrist links, hip and knee yokes, pelvis nodes and any small part carrying high or impact loads. For thick blocks, 7050-T7451 is often preferred for its toughness.
No. 7075 is a wrought alloy that is rolled, extruded or forged. Die-cast housings use foundry alloys such as A380 or A356-type alloys, which have lower strength and ductility, so bearing seats and highly loaded features are machined and designed accordingly.
Machine from plate or bar while the design is still changing. Once the design is frozen, forge compact high-load nodes for better fatigue and impact properties, and die cast complex housings when volumes justify tooling.
Usually because of residual quench stress in unstretched stock or heavy one-sided material removal. Ordering stress-relieved tempers (T651 plate, T6511 extruded bar), roughing and finishing in separate stages and keeping the part symmetrical all reduce distortion.
Yes. A Type III hard anodic coating grows roughly half outward and half into the metal, so a 50 µm coating adds about 25 µm per surface. Finish-machine bores and fits with that growth in mind, or mask them.
Metline supplies the aluminium stock (plate, block, bar, tube, extrusions and forgings) and can cut or machine it to your drawing. We do not design or assemble robots or actuators.
Send your parts list or drawings with alloy, temper and quantities. We will propose the stock forms and reply with availability, certification and a delivered price.
