Materials for Robotics: The Ultimate Guide

Every robot is a stack of loads looking for a path to ground. Motor torque reacts through a link, the link hands the moment to a joint, the joint dumps it into a frame, the frame carries it to the base or the wheels. The material you choose for each of those members sets three things at once: how much the structure weighs (which the actuators then have to accelerate), how much it deflects under load (which shows up as position error at the tool point), and how long it lasts before a crack finds a stress riser and grows. Get the material wrong and you pay for it in every actuator sizing calculation and every fatigue failure downstream.

The menu is small and the tradeoffs are old. Aluminum, steel, titanium, carbon fiber, and a handful of engineering plastics cover almost everything a robot is built from. What changes between a good design and a bad one is matching the member's actual job (is it stiffness-limited, strength-limited, or fatigue-limited?) to the property that governs that job, then reading the right normalized number instead of the raw one. A drone arm and a gearbox output shaft are both "structure," and they want opposite materials for reasons that fall straight out of the governing equations.

This guide walks the material menu the way a robot mechanical engineer uses it: the physics behind stiffness-to-weight and specific strength, the property table you size against, where each material belongs, how you join and fasten it, what corrosion and fatigue do in service, and where 3D-printed materials fit now that they are load-bearing.

The take: The best robot material is whichever one matches the member's binding constraint. Size stiffness-limited members (arms, frames, anything where deflection is the spec) by specific modulus E/ρ, and here plain 6061 aluminum ties titanium and steel because E/ρ is nearly constant across all three metals, so you drop to carbon fiber or clever geometry to win. Size strength-limited members (fasteners, highly loaded links, impact parts) by specific strength σ/ρ, where 7075 aluminum, titanium, and carbon fiber pull ahead. Then check fatigue (aluminum has no endurance limit, steel does), corrosion, machinability, and cost before you commit. Most robots are correctly built from 6061 and 7075 aluminum with steel where it must be hard and plastic where it must be light, cheap, or electrically quiet.

Companion reading: 3D printing for robotics, humanoid robot hardware, drone/UAV hardware, robot actuators, and linear motion systems.

Table of contents

1. Key takeaways
2. The physics: stiffness, strength, and why you normalize by density
3. The structural material menu
4. The property comparison table
5. Aluminum alloys: 6061 and 7075
6. Steels, titanium, and the specialty metals
7. Carbon fiber and composites
8. Engineering plastics: Delrin, nylon, PC, PEEK
9. Selecting a material: the workflow with numbers
10. Fasteners, joining, and the interfaces that fail
11. Corrosion, fatigue, and environment
12. 3D-printed materials as structure
13. Frequently asked questions

The physics: stiffness, strength, and why you normalize by density

Material selection for a robot structure is a normalization problem. Nature hands you raw properties (modulus, yield strength, density, toughness) and the design problem tells you which combination actually matters. The two properties engineers reach for first are almost never the ones that decide the part.

Stiffness-limited vs strength-limited members

A structural member is limited by one of two things. Either it must not deflect too much (a robot arm whose tip must stay within a positioning tolerance, a frame that must hold two rails parallel, a machine bed) or it must not break/yield (a fastener, a highly loaded link, an impact bracket). These two constraints point to different material properties, and confusing them is the most common material error in robotics.

• A stiffness-limited member is governed by the elastic modulus E (Young's modulus, in GPa). Deflection under load scales as 1/E. You are buying rigidity.
• A strength-limited member is governed by yield or ultimate strength σ (in MPa). Failure happens when stress exceeds the material's limit. You are buying load capacity.

Steel has 3× the modulus of aluminum (200 vs 69 GPa) and roughly 3× the density (7.85 vs ~2.70 g/cm³). That near-perfect proportionality is the whole story of metal selection, and it means the raw numbers deceive.

Why you divide by density

Almost every robot member is weight-constrained, because every kilogram of structure is a kilogram the actuators must accelerate and hold. So the honest metric is performance per unit mass, which means dividing the governing property by density:

Specific modulus (specific stiffness) = E / ρ        [MN·m/kg, or GPa/(g/cm³)]
Specific strength                     = σ / ρ        [kN·m/kg]

Run the numbers for the three structural metals and something surprising falls out:

Steel:     E/ρ = 200 / 7.85 ≈ 25.5 MN·m/kg
Titanium:  E/ρ = 114 / 4.43 ≈ 25.7 MN·m/kg
Aluminum:  E/ρ =  69 / 2.70 ≈ 25.6 MN·m/kg

They are essentially identical. Specific modulus is nearly a universal constant across the common structural metals, because atomic bond stiffness and atomic mass scale together down the periodic table. The practical consequence is blunt: for a stiffness-limited part whose shape is fixed, swapping one metal for another barely changes the weight-for-stiffness result. A steel bracket and an aluminum bracket of identical geometry that carry the same deflection spec weigh nearly the same, because to match the steel's stiffness the aluminum part must be 3× thicker (lower E) but its material is 3× lighter, and the two cancel.

How you actually win: geometry and section

If material choice barely moves specific stiffness, geometry does. Bending stiffness scales with the second moment of area I, and for a member in bending, moving material away from the neutral axis is enormously more effective than changing material:

Bending stiffness ∝ E · I
Solid round:   I = π d⁴ / 64
Hollow tube:   I = π (d_o⁴ − d_i⁴) / 64

The d⁴ dependence is why robot arms are tubes and box sections, not solid bars. A hollow aluminum tube of the same mass as a solid steel rod can be far stiffer in bending, because the aluminum's lower density lets you spend that mass on a larger diameter, and I climbs with the fourth power of diameter. This is the real reason aluminum dominates robot frames: its low density lets you buy section, and section is what buys stiffness. This is the same section-over-material logic that governs linear motion systems, where a screw's critical speed rides on diameter-over-length-squared.

The Ashby view

Michael Ashby's material-selection charts formalize this. Plot modulus against density on log axes and draw lines of constant E/ρ (specific stiffness), E^(1/2)/ρ (light stiff beam in bending), and E^(1/3)/ρ (light stiff panel). The right "material index" depends on the load case and the geometry you are allowed to change. For a beam of fixed shape you want E/ρ; for a beam where you may resize the section you want E^(1/2)/ρ, and on that index carbon fiber beats all metals. Pick the index that matches your load case and design freedom, then read the chart, rather than reaching for the material with the biggest headline number.

Rule of thumb: If the spec is deflection, size by E/ρ (or E^(1/2)/ρ if you can change the section) and reach for geometry first, carbon fiber second. If the spec is breakage, size by σ/ρ and reach for 7075, titanium, or carbon fiber. Never compare raw E or raw σ across materials of different density; you will pick the wrong one.

The structural material menu

The list of materials a robot is actually built from is short. Here is the working menu with the one-line reason each exists.

• 6061 aluminum: the default. Cheap, machinable, weldable, corrosion-resistant, decent strength. Frames, plates, brackets, links, mounts.
• 7075 aluminum: when you need aircraft-grade strength-to-weight and can pay for it and give up weldability and corrosion resistance. Highly loaded links, structural plates, competition parts.
• Steel (mild, alloy, stainless, tool): where you need hardness, wear resistance, fatigue endurance, or maximum stiffness in a small envelope. Shafts, gears, fasteners, bearings, tooling, high-load pins.
• Titanium (Ti-6Al-4V): high specific strength, excellent fatigue and corrosion resistance, biocompatible. Weight-critical high-strength parts, surgical/aerospace robots, springs. Expensive and slow to machine.
• Carbon-fiber composite (CFRP): the specific-stiffness and specific-strength champion. Drone airframes, arm tubes, plates, lightweight links. Anisotropic, brittle, hard to join.
• Delrin / POM (acetal): precise, low-friction, dimensionally stable engineering plastic. Gears, bushings, sliders, small structural parts, cable guides.
• Nylon (PA6, PA66, cast nylon): tough, wear-resistant, impact-absorbing plastic. Impact parts, wear plates, gears, rollers.
• Polycarbonate (PC): transparent, very high impact resistance. Guards, covers, sensor windows, light housings.
• PEEK: high-temperature, chemically inert, strong structural thermoplastic. Parts near actuators/motors, chemical environments, vacuum, medical.
• Fiberglass (GFRP), G10/FR4: cheaper composite for insulating structural plates, jigs, and non-weight-critical panels.

Everything else (magnesium, beryllium, metal-matrix composites, ceramics) shows up in niches: magnesium in weight-obsessed housings, ceramics in bearings and wear surfaces, but the ten above build ninety-plus percent of robots.

The property comparison table

Approximate room-temperature properties for the materials a robot engineer selects from. Treat these as representative middle-of-range values; always check the specific alloy, temper, grade, and layup for a real design.

Material	Density ρ (g/cm³)	Modulus E (GPa)	Yield σ_y (MPa)	Spec. modulus E/ρ	Spec. strength σ_y/ρ	Machinability	Relative cost	Where it belongs
6061-T6 aluminum	2.70	69	276	25.6	102	Excellent	Low	Frames, brackets, links, plates
7075-T6 aluminum	2.81	72	503	25.6	179	Good	Medium	Highly loaded links, aircraft-grade parts
Mild steel (1018)	7.87	205	370	26.1	47	Good	Very low	Shafts, brackets, weldments
4140 alloy steel	7.85	205	655 (Q&T)	26.1	83	Good	Low	Shafts, gears, high-load pins
304/316 stainless	8.00	193	215 to 290	24.1	30 to 36	Fair	Medium	Corrosion/washdown, food, medical
Ti-6Al-4V titanium	4.43	114	880	25.7	199	Poor	Very high	Weight-critical strength, surgical, springs
CFRP (quasi-iso layup)	1.55	50 to 70	500 to 700*	32 to 45	320 to 450*	Poor (abrasive)	High	Drone airframes, arm tubes, plates
CFRP (unidirectional)	1.60	130 to 180	1500+*	80 to 115	900+*	Poor (abrasive)	High	Spars, booms, loaded-along-axis members
Delrin / POM	1.41	3.1	65	2.2	46	Excellent	Low	Gears, bushings, low-friction parts
Nylon (PA66)	1.14	2.5 to 3.5	60 to 85	2.4	60	Good	Low	Impact/wear parts, rollers, gears
Polycarbonate (PC)	1.20	2.3	62	1.9	52	Good	Low	Guards, windows, impact covers
PEEK	1.30	3.6 (neat)	95 to 100	2.8	74	Good	Very high	Hot/chemical structural plastic

*Composite strength is layup- and direction-dependent; the tabulated values are indicative of the fiber-direction or in-plane response and collapse dramatically for off-axis and interlaminar loading.

Two things to read off this table immediately. First, the specific-modulus column confirms the physics: every metal sits near 25 to 26, and only carbon fiber breaks out. Second, the plastics have roughly 1/20th to 1/30th the modulus of metals, so they are almost never chosen for stiffness; they win on the other columns (weight, friction, corrosion, cost, electrical properties) and you design around their compliance.

Aluminum alloys: 6061 and 7075

Aluminum is the backbone of robot structure for one reason above all: its low density lets you buy the section that buys stiffness, at a price and machinability nothing else matches. Two alloys cover almost everything.

6061: the default

6061-T6 is the alloy you reach for unless you have a reason not to. The "T6" temper means solution heat-treated and artificially aged, giving ~276 MPa yield and ~310 MPa ultimate. What makes it the default is the combination:

• Machinable: cuts cleanly, taps well, holds tolerance, forgiving of aggressive feeds.
• Weldable: one of the few high-strength aluminums you can reliably weld (7075 you cannot, practically). Note the penalty below.
• Corrosion-resistant: forms a self-passivating oxide; anodizes beautifully for wear and appearance.
• Available: on every metal supplier's shelf as plate, bar, extrusion, and tube. The 20x20 to 40x40 T-slot extrusion that half the robotics prototypes in the world are bolted from is 6061 or 6063.
• Cheap: among the least expensive engineering metals per part.

The catch with 6061 is the weld heat-affected zone (HAZ). Welding locally re-solutionizes and over-ages the metal, dropping the T6 strength in the weld region to roughly the annealed (T0/W) value, often 40 to 50% of the parent T6 yield, until and unless you re-heat-treat the whole part. A welded 6061 frame is only as strong as its softened weld zones. Design welds away from peak-stress regions, or bolt instead of weld where strength matters.

7075: the aircraft-grade upgrade

7075-T6 is a zinc-alloyed aluminum with roughly 1.8× the yield of 6061 (~503 MPa) at almost the same density and modulus. When a link or plate is strength-limited and weight matters, 7075 is the aluminum answer. It is standard in aircraft and in high-load robot parts where you would otherwise reach for steel and pay the weight.

What you give up:

• Not practically weldable. 7075 is crack-prone in the weld zone; you bolt or bond it.
• Worse corrosion resistance. The zinc content makes it more susceptible, especially to stress-corrosion cracking. T73 and T7351 tempers trade a little strength for much better stress-corrosion resistance and are used where that matters. Anodize or coat 7075 in any humid or salt environment.
• Higher cost, several times 6061 per part.

Rule of thumb: Prototype and general structure in 6061. Move a specific member to 7075 only when a stress or deflection calculation says the 6061 version is too heavy or too big. Do not default the whole robot to 7075: you pay for strength you mostly do not use and inherit corrosion and welding headaches.

Steels, titanium, and the specialty metals

When steel is right

Steel's specific stiffness and specific strength are unremarkable, and its density is the reason robots avoid it for bulk structure. But steel wins decisively in three situations, and you should reach for it without apology when they apply:

• Hardness and wear. Gears, shafts, cams, pins, bearing races, and tool tips need surface hardness aluminum cannot provide. Alloy steels (4140, 4340) through-harden; case-hardening steels (8620) and tool steels (A2, D2, O1) give hard surfaces on tough cores. This is why a robot's gearbox internals and output shafts are steel even when the housing is aluminum.
• Fatigue endurance. Steel has a true endurance limit (see the fatigue section): below ~0.5× its ultimate strength, it survives effectively infinite cycles. Aluminum does not. High-cycle members (a shaft spinning millions of revolutions, a spring, a repeatedly flexed link) often must be steel for this reason alone.
• Stiffness in a tight envelope. When you cannot make the section bigger (a shaft inside a bearing bore, a pin in a clevis), you cannot exploit aluminum's geometry advantage, and steel's 3× modulus wins in the fixed small envelope.

Stainless (304, 316) trades some strength and thermal conductivity for corrosion resistance, and it is the default for washdown, food, marine, and medical robots. 316 (with molybdenum) resists chlorides and is the marine/surgical grade.

Titanium: the premium strength-to-weight metal

Ti-6Al-4V (grade 5) is the workhorse titanium alloy: 880 MPa yield at 4.43 g/cm³, giving a specific strength (199) that rivals or beats 7075 and carbon fiber's off-axis numbers. Titanium also has excellent fatigue resistance, near-total corrosion immunity, biocompatibility (surgical robots, implants), and it keeps strength at temperatures that soften aluminum.

The reasons it is not everywhere: cost (many times aluminum, both material and machining), machinability (work-hardens, holds heat at the cutting edge, wants slow speeds and rigid setups, eats tooling), and the fact that its specific modulus is the same 25 to 26 as every other metal, so it buys you nothing on stiffness. Titanium earns its place where you need high strength and low weight and corrosion/fatigue resistance together and can pay for it: surgical robot arms, aerospace and space mechanisms, high-end drone and humanoid parts, and springs (its low modulus and high strength make good springs).

War story: A team building a lightweight humanoid forearm machined the whole link from Ti-6Al-4V to save weight over 7075. The finished part was barely lighter than the aluminum version would have been, because the link was stiffness-limited, not strength-limited, and titanium's specific modulus is identical to aluminum's. They paid roughly ten times the machining cost and three times the material cost to remove grams that the deflection spec never allowed them to remove. The lesson: check whether the member is stiffness- or strength-limited before you spend money on a strong metal. Titanium only pays when strength (or fatigue, or corrosion, or temperature) is the binding constraint.

Carbon fiber and composites

Carbon-fiber-reinforced polymer (CFRP) is the only common robot material that decisively beats metals on both specific stiffness and specific strength. A unidirectional carbon laminate can hit E/ρ of 80 to 115 against aluminum's 26, and specific strengths several times any metal. That is why serious drone airframes, lightweight arm tubes, and weight-critical links are carbon.

Why it wins, and the catch

The stiffness and strength come from the fibers, aligned filaments of nearly pure carbon a few microns across, carried in an epoxy matrix that transfers load between them. The consequence is anisotropy: the laminate is spectacular along the fibers, weak across them (there you load only the epoxy), and prone to delamination between plies under shear and peel. You engineer around this by choosing the layup: unidirectional for a member loaded along one axis (a spar, a boom), quasi-isotropic (plies at 0/45/90/−45) for parts that see loads from many directions (a plate, a bracket), and balanced layups for torsion. The design freedom is huge, and so is the opportunity to get it wrong.

The real-world liabilities

• Brittle, low damage tolerance. Carbon fiber does not yield; it fails suddenly and can be weakened by impacts that leave no visible mark. Metals dent and warn you; carbon cracks internally and looks fine.
• Terrible at holes, threads, and bearing loads. A bolt hole concentrates stress in a material that cannot yield to redistribute it, and the laminate crushes and delaminates at the bearing surface. You almost never thread carbon fiber; you bond in metal inserts, use through-bolts with metal backing plates, or co-cure fittings.
• Hard to join and repair. Bonding is the primary method, and bonded joints need careful surface prep and are hard to inspect. Field repair is specialist work.
• Machining is nasty. Carbon dust is abrasive (destroys standard tooling, wants diamond or carbide), electrically conductive (fouls electronics), and a respiratory hazard.
• Cost and lead time. Prepreg, tooling, and autoclave or oven cure make one-off carbon parts expensive; the economics favor tubes and plates bought as stock and cut, or volumes that amortize the mold.

Fiberglass (GFRP) and the glass-epoxy laminates G10/FR4 are the cheaper cousins: lower stiffness and strength than carbon, but electrically insulating (unlike conductive carbon), cheaper, and easier to machine. Use them for insulating structural plates, jigs, and panels where carbon's conductivity would be a problem.

Rule of thumb: Use carbon fiber as tubes and flat plates loaded in their strong directions, and hand every concentrated load (bolt, bearing, thread) to a bonded or clamped metal fitting. Design the load to run along the fibers and off the part through metal, never into a hole in the laminate. If a part needs to be threaded, bearing-loaded, or field-repairable, it wants metal.

Engineering plastics: Delrin, nylon, PC, PEEK

Plastics have roughly a twentieth of a metal's modulus, so they are rarely load-bearing structure in the stiffness sense. They earn their place on the other properties: low friction, self-lubrication, corrosion immunity, electrical insulation, low weight, low cost, quiet operation, and ease of machining or molding. In a robot they show up as gears, bushings, sliders, guards, cable management, and light non-structural housings.

Delrin / POM (acetal)

Polyoxymethylene, sold as Delrin (homopolymer) or generic acetal (copolymer), is the machinist's favorite plastic: stiff for a plastic (~3.1 GPa), dimensionally stable, low-friction, wear-resistant, and machines to tight tolerance with a clean finish. It is the default for gears, bushings, cams, sliders, and low-friction wear surfaces, and it holds size because it barely absorbs water (a real advantage over nylon). Weaknesses: limited temperature range, poor adhesive bonding (low surface energy), and flammability.

Nylon (polyamide)

Nylon (PA6, PA66, and cast nylon like MC901/Nylatron) is tougher and more impact-absorbing than acetal, with good wear resistance and self-lubricating filled grades. It is the choice for impact and wear parts, rollers, tough gears, and bushings where toughness beats precision. The major caveat is moisture absorption: nylon takes up water and swells and softens, shifting dimensions by up to a couple of percent, which rules it out for tight-tolerance parts in humid air unless you account for it. Cast nylon is more stable and stronger than extruded.

Polycarbonate (PC)

Polycarbonate's headline property is impact resistance: one of the toughest transparent plastics, far more so than acrylic, which is why it is used for machine guards, safety shields, sensor windows, and light covers. It is transparent, reasonably temperature-tolerant, and easy to machine and thermoform, though it scratches easily and some solvents attack it. Acrylic (PMMA) is the cheaper, clearer, more scratch-resistant but brittle alternative for non-impact windows.

PEEK

Polyether ether ketone is the high-performance structural thermoplastic: strong (~95 MPa), stiff for a plastic, chemically inert, and stable to ~250 C continuous (glass transition ~143 C). Use it near motors where heat kills lesser plastics, in aggressive chemical environments, in vacuum (low outgassing), and in medical and sterilizable parts. Carbon- and glass-filled grades push stiffness higher. The barrier is cost: PEEK stock is very expensive, so it is reserved for parts that genuinely need its temperature or chemical performance. Ultem (PEI) covers some of the same ground cheaper at lower performance.

Rule of thumb: Reach for plastic when the part's job is friction, insulation, corrosion, transparency, weight, or cost, not stiffness. Delrin for precise low-friction parts, nylon for tough impact parts, PC for see-through and impact covers, PEEK when it has to survive heat or chemicals. Design for the plastic's compliance and thermal expansion (many times a metal's); never treat a plastic part as if it were a stiff metal one.

Selecting a material: the workflow with numbers

Here is the actual selection procedure, in order, with a worked example.

1. Classify the member's binding constraint

Ask what fails first. Is the spec a deflection limit (stiffness-limited), a breakage/yield limit (strength-limited), a cycle-life limit (fatigue-limited), or an environmental limit (corrosion, temperature, insulation)? A robot arm tip that must hold position is stiffness-limited. A fastener is strength-limited. A spinning shaft is fatigue-limited. A washdown frame is corrosion-limited. This classification, more than any table, decides the material.

2. Pick the material index and read the ranked list

• Stiffness-limited, fixed shape: maximize E/ρ (metals tie near 26; carbon fiber wins).
• Stiffness-limited, free to resize a beam section: maximize E^(1/2)/ρ (carbon fiber, then aluminum).
• Strength-limited: maximize σ/ρ (carbon fiber, titanium, 7075).
• Fatigue-limited, high cycles: steel (endurance limit) or titanium.
• Environment-limited: stainless, titanium, plastics, coated aluminum.

3. Run the worked example: a robot arm link

Suppose a link between two joints must not deflect past a set amount at the tip, so it is stiffness-limited in bending, and you are free to choose the tube section. The governing quantity is bending stiffness E·I, wanted for minimum mass.

Deflection of a cantilever tip:  δ = F L³ / (3 E I)
Mass of a thin tube:             m = ρ · (π D t) · L
For a thin-wall tube:            I ≈ (π / 8) D³ t

Hold δ, L, and F fixed and you need a target E·I, reachable with a stiff-dense metal in a small tube or a light-compliant material in a bigger one. Minimizing mass shows the light-stiff-beam index E^(1/2)/ρ governs, and on that index:

Aluminum 6061:  E^(1/2)/ρ = 69^0.5 / 2.70  ≈ 3.08
Steel:          E^(1/2)/ρ = 200^0.5 / 7.85 ≈ 1.80
Titanium:       E^(1/2)/ρ = 114^0.5 / 4.43 ≈ 2.41
CFRP (uni):     E^(1/2)/ρ = 150^0.5 / 1.60 ≈ 7.66

Aluminum beats steel by ~1.7× and titanium beats steel too, but carbon fiber beats aluminum by ~2.5× again. So for a stiffness-limited arm link where you can size the tube, the ranking is carbon fiber, then aluminum, then titanium, then steel, and this is exactly the order you see in real robot arms as budget and volume rise. Steel is last for a light stiff link despite its huge raw modulus, because that modulus comes with a density that geometry cannot outrun.

4. Apply the practical filters

The index gives you a shortlist; reality prunes it:

• Machinability / manufacturing: can you make it, at your volume, for your budget? One-off carbon tube: buy stock. One-off aluminum link: machine it. High volume: consider molded plastic or cast/forged metal.
• Joining: how does load get in and out? If the part needs threads, bearings, or welds, that pushes toward metal or metal inserts.
• Environment: humidity, chemicals, temperature, washdown, vacuum, EMI. Pushes toward stainless, titanium, plastics, or coatings.
• Cost and supply: 6061 is on the shelf; aerospace 7075 plate and PEEK stock have lead time and price. The available-and-affordable material usually wins ties.

5. Add the safety factor and check the failure mode

Size against yield (or the fatigue limit for cyclic loads) with a safety factor appropriate to the consequence and the load certainty: roughly 1.5 to 2 for well-characterized static loads on non-critical parts, 3 to 5 or more for impact, uncertainty, or safety-critical members. For brittle materials (carbon fiber, cast metals, ceramics) use larger factors because they give no yielding warning before fracture.

Rule of thumb: Let the index rank the materials and let manufacturability, joining, environment, and cost choose the winner. The most common good answer for a robot structural member is 6061 aluminum, and the second most common is "6061, but this specific part goes to 7075 or carbon fiber because a number said so."

Fasteners, joining, and the interfaces that fail

Structures fail at joints. A member is a continuous piece of well-understood material; a joint is a stress concentration, a mix of materials, and an assembly tolerance stacked together. Design the joint first.

Fasteners

Most robot assembly is bolted, because bolts are serviceable, predictable, and do not soften the parent metal the way welding does. Key points:

• Bolt property class is a strength spec. Steel metric bolts are marked by class (8.8, 10.9, 12.9): the first number is 1/100 of ultimate tensile strength in MPa, the second is the yield-to-ultimate ratio ×10. A 12.9 socket-head cap screw (1200 MPa ultimate) is the robotics default for loaded joints; stainless (A2/A4) is weaker (~500 to 700 MPa) but corrosion-resistant.
• Preload is the point of a bolt. A properly torqued bolt clamps the joint so the parts carry load by friction and the bolt sees little cyclic stress. Under-torqued joints let the bolt take fluctuating load and fail in fatigue, which is why torque specs and thread-locker exist. Torque relates to preload as T ≈ K·F·d (K ≈ 0.2 dry steel), so lubrication changes the achieved preload.
• Threads in soft materials need help. Tapping directly into aluminum, and especially plastic or carbon fiber, gives weak, strippable threads. Use threaded inserts (helical Heli-Coil, or heat-set inserts for plastics and prints) to put steel threads into soft parents, and aim for thread engagement of ~1.5 to 2× bolt diameter in steel, ~2 to 2.5× in aluminum, more in plastic. Never thread carbon fiber directly.

Welding

Welding fuses metal but comes with the HAZ penalty for heat-treatable aluminum (6061 drops to roughly annealed strength in the weld zone). Steel welds well and, with matched filler, can restore near-parent strength. Titanium welds but demands inert-gas shielding of the whole hot zone (it embrittles by absorbing oxygen and nitrogen when hot). You do not weld 7075 or castings reliably, and you cannot weld carbon fiber or plastics (though plastics can be heat- or ultrasonic-welded). Design weld locations away from peak stress, and if you weld heat-treatable aluminum, either accept the softened zone or re-heat-treat.

Bonding and composite joints

Adhesive bonding spreads load over an area instead of concentrating it at a hole, which is why it is the primary way to join carbon fiber. Structural adhesives (epoxy, methacrylate) can exceed the laminate's interlaminar strength if the joint is designed for shear (lap joints) rather than peel. The requirements are ruthless: clean, abraded, correctly prepared surfaces; controlled bond-line thickness; and the understanding that a bad bond is invisible and untestable without destructive or specialized inspection. Metal inserts co-cured or bonded into carbon parts are how you hand bolt and bearing loads to something that can take them.

Rule of thumb: Bolt where you need to service or where welding would soften the metal; weld steel freely and heat-treatable aluminum carefully; bond composites and always back concentrated loads with metal inserts. The interface is where you spend your engineering attention, because that is where the crack starts.

Corrosion, fatigue, and environment

Two failure modes kill robot parts that survived the static load calculation: fatigue (cyclic loading) and corrosion. Both are governed by material choice as much as by stress.

Fatigue: the aluminum endurance-limit problem

Cyclic loading grows cracks at stresses well below the static yield strength. The S-N curve (stress amplitude vs cycles to failure) tells the story, and the two big structural metal families behave fundamentally differently:

• Steel and titanium have a true endurance limit. Below a threshold stress (roughly 0.4 to 0.5× ultimate for steel), the S-N curve goes flat: the part survives effectively infinite cycles. Design below the endurance limit and high-cycle fatigue is not a life-limiting concern.
• Aluminum has no endurance limit. Its S-N curve keeps sloping down forever; there is no stress so low that an aluminum part is safe for infinite cycles. Every aluminum part under cyclic load has a finite fatigue life. You design to a specific cycle count (say 10^7 or 10^8 cycles) and a fatigue strength at that count, then retire or inspect.

This single difference decides material for high-cycle members. A shaft that turns millions of revolutions, a repeatedly flexed link, a spring, a landing-gear leg that cycles every flight: these often must be steel or titanium precisely because aluminum will eventually crack. It is also why aircraft (and drones) have inspection intervals and life limits on aluminum structure. Stress concentrations (sharp internal corners, holes, tool marks, thread roots) are where fatigue cracks start, so generous fillet radii, polished surfaces, and shot-peening (which puts the surface in compression) extend fatigue life more than a material upgrade often does.

War story: A quadcopter arm machined from 6061 flew fine for months, then snapped at the motor mount during an ordinary flight. The break started at a sharp internal corner where the arm stepped down to the motor boss, a textbook fatigue-crack initiation site. Nothing was overloaded; the arm had simply accumulated enough vibration cycles at a stress the material could not survive forever, because aluminum has no endurance limit. The fix was a generous fillet at the corner (to cut the stress concentration) and a periodic inspection. A stronger alloy would not have helped. Fatigue is a geometry and cycle-count problem first, a material problem second.

Corrosion and galvanic pairing

Aluminum and stainless steel passivate (self-protecting oxide) and resist general corrosion well. The failure that catches robot builders is galvanic corrosion: when two dissimilar conductive materials touch in the presence of an electrolyte (humidity, salt, coolant), the more anodic (less noble) one corrodes preferentially. The galvanic series ranks them; the practical robotics traps:

• Aluminum bolted to steel: the aluminum is anodic and sacrifices itself at the interface. Use stainless or coated fasteners, isolate with a coating or washer, or anodize the aluminum.
• Aluminum bolted to carbon fiber: carbon is strongly cathodic (noble), so it drives aggressive corrosion of aluminum fasteners and aluminum inserts in contact with the laminate. This is a well-known aerospace headache. Isolate with a barrier (glass-fiber ply, sealant, coating), use titanium or coated fasteners, or use a compatible insert material.
• Marine, washdown, outdoor: go to stainless (316 for chlorides), anodized or coated aluminum, titanium, or plastics.

Protective measures: anodizing aluminum (hard-anodize for wear), passivating stainless, plating or coating steel (zinc, nickel), and physically isolating dissimilar metals with sealants or non-conductive washers. Anodized aluminum is also electrically insulating on the surface, which matters when the part is a ground path or an EMI concern.

Temperature

Aluminum and plastics lose strength as they warm; near motors, brakes, and power electronics, check the local temperature against the material. This is where PEEK, Ultem, and metals earn their place over commodity plastics, and it connects to the broader problem of getting heat out of a robot, covered in the thermal-management domain. Thermal expansion also matters at interfaces: bolting a plastic part (high expansion) to a metal frame across a temperature swing builds up stress or loosens the joint, so slot the holes or choose matched materials.

3D-printed materials as structure

Additive manufacturing moved from prototypes to load-bearing robot parts over the last decade, and a modern robot often has printed structural components. The material rules change, because a printed part is anisotropic (weaker between layers), and its properties depend on the process as much as the polymer or metal. See the 3D printing for robotics guide for the full process treatment; here is where printed materials sit in the structural menu.

Printed polymers

• PLA: stiff, easy to print, cheap, but brittle and low-temperature (softens near 60 C). Fine for jigs, fixtures, non-structural mounts, and prototypes; wrong for load-bearing or anything near a motor.
• PETG / ABS / ASA: tougher and more temperature-tolerant than PLA; ASA resists UV for outdoor parts. The workhorses for functional printed parts.
• Nylon (PA), often carbon- or glass-filled: the serious FDM structural material. Carbon-filled nylon (Onyx and similar) is stiff, tough, and temperature-tolerant, and is used for real robot brackets, end-effector fingers, and light links.
• Continuous-fiber printing (Markforged and similar) lays continuous carbon, glass, or aramid fiber inside a nylon matrix, reaching a fraction of machined-aluminum strength in the fiber directions, which pushes printed parts into genuinely load-bearing territory.
• PEEK and Ultem (PEI): high-temperature printed thermoplastics for parts near heat or in aggressive environments; they need high-temperature printers and careful process control.

The unavoidable caveat is the layer plane: FDM parts are weakest in the Z direction (between layers), because the inter-layer bond is weaker than the bulk polymer. You orient the print so that the primary load runs within the layer plane, not across layers, and you treat the printed part as anisotropic. Infill percentage, wall count, and orientation matter as much as the material.

Printed metals

Metal additive (laser powder-bed fusion, DMLS) prints aluminum (AlSi10Mg), titanium (Ti-6Al-4V), and stainless (17-4PH, 316L) into near-full-density parts, enabling topology-optimized brackets and complex internal geometry (conformal cooling channels, integrated features) that machining cannot make. Printed metal parts usually need post-processing (heat treatment, HIP to close porosity, machined mating faces) and cost far more than machined stock, so they are reserved for weight-critical, geometrically complex, or low-volume high-value parts (aerospace, medical, motorsport, and increasingly humanoid and drone structure).

Rule of thumb: Use printed plastics for jigs, fixtures, covers, and light functional parts; step to filled or continuous-fiber nylon for load-bearing printed parts; and orient every print so the load runs within the layer plane. Reach for printed metal only when the geometry (topology optimization, internal channels) or the low volume justifies its cost, and post-process it before you trust it structurally.

Frequently asked questions

Why is aluminum used for robot frames if steel is stronger and stiffer? Because the metrics that matter are per unit weight, and there aluminum wins through geometry. Steel has ~3× aluminum's modulus and ~3× its density, so their specific stiffness (E/ρ) is nearly identical. Aluminum's low density lets you spend a given mass on a larger tube or box section, and bending stiffness climbs with the fourth power of section size, so the aluminum part ends up stiffer for the same weight. Steel wins only where you cannot enlarge the section (shafts, pins, gears) or where you need hardness or fatigue endurance.

When should I choose 7075 over 6061 aluminum? When a member is strength-limited and weight matters, and you can accept 7075's downsides. 7075-T6 has roughly 1.8× the yield of 6061 at nearly the same density and modulus, so it makes lighter highly loaded links and plates. The costs are that 7075 is not practically weldable, corrodes more readily (especially stress-corrosion cracking, mitigated by T73 tempers), and costs several times more. For stiffness-limited parts 7075 buys almost nothing over 6061, since their moduli are nearly equal.

Does carbon fiber really beat titanium and aluminum, and why isn't everything made of it? On specific stiffness and specific strength in the fiber direction, yes, decisively. The reasons it is not universal: it is anisotropic (weak across the fibers and at holes), brittle with poor damage tolerance (impacts cause hidden internal damage), very hard to join (you bond it and back every fastener with metal), unfriendly to machine (abrasive, conductive, hazardous dust), and expensive for one-off parts. Carbon fiber wins for tubes, plates, and drone airframes loaded in their strong directions; metal wins wherever you need threads, bearings, welds, toughness, or cheap serviceable parts.

What material should a drone airframe be? Carbon fiber for the load-bearing structure (arms, plates, booms), because a drone is ruthlessly weight- and stiffness-limited and carbon's specific properties are unmatched. Use plates and tubes loaded along the fibers, and hand motor mounts, bearing seats, and bolt loads to metal (aluminum or titanium) inserts and standoffs. Lightly loaded or crash-sacrificial parts can be printed nylon or PETG. Aluminum shows up in fittings and standoffs. See the drone hardware guide.

Why do aluminum parts eventually crack under vibration when steel ones don't? Aluminum has no fatigue endurance limit: its S-N curve keeps sloping down, so there is no stress low enough to guarantee infinite cyclic life. Every aluminum part under cyclic load has a finite life and will eventually crack, usually at a stress concentration (sharp corner, hole, tool mark). Steel and titanium have a true endurance limit below which they survive effectively forever. This is why high-cycle members (shafts, springs, repeatedly flexed links) often must be steel or titanium, and why aluminum aircraft and drone structure carries inspection intervals.

How do I stop galvanic corrosion between my aluminum frame and steel or carbon parts? Break one of the three requirements: dissimilar metals, electrical contact, and an electrolyte. Isolate the metals with a non-conductive coating, washer, or sealant; anodize the aluminum (its oxide is insulating); choose compatible fastener materials (stainless or coated steel into aluminum, titanium or coated fasteners into carbon fiber); and keep water out. Carbon fiber is strongly cathodic and aggressively corrodes aluminum in contact with it, so always isolate aluminum inserts and fasteners from a carbon laminate with a barrier ply or sealant.

Which plastic should I use for a robot gear or bushing? Delrin/POM for precise, low-friction, dimensionally stable parts (it holds tolerance and does not absorb much water); nylon for tougher, higher-impact, higher-wear parts where a little dimensional drift from moisture is acceptable (cast nylon is more stable than extruded). For gears meshing with metal, acetal and nylon both work; add internal lubricant grades for dry running. For high temperature near a motor, step up to PEEK. Design for the plastic's low stiffness and higher thermal expansion, and expect wear rather than a fatigue-limited infinite life.

Can I use 3D-printed parts for load-bearing robot structure? Yes, with the right material and orientation. Filled nylon (carbon- or glass-filled, like Onyx) and continuous-fiber-reinforced prints reach a useful fraction of machined-aluminum strength and are used for real brackets, fingers, and light links. The catch is anisotropy: FDM parts are weakest between layers, so orient the print with the load in the layer plane, and design in generous walls and infill. Printed metal (titanium, aluminum, stainless) handles higher loads and complex topology-optimized geometry but needs post-processing and costs far more than machined stock. See the 3D printing guide.

What's the single most common material mistake in robot design? Comparing raw modulus or raw strength across materials of different density and picking the biggest number, instead of normalizing by density and matching the metric to the member's binding constraint. That mistake makes people over-spec titanium for stiffness-limited parts (where it ties aluminum), reach for steel to make a light stiff arm (where geometry beats material), or thread directly into carbon fiber (which has no business carrying a bolt). Classify the member as stiffness-, strength-, fatigue-, or environment-limited first, then size by the matching normalized property.