30lb Combat: Materials

Metals

Aluminums

2024 Al

2024 Aluminum is mostly the material of choice for many robot frames in iRobotics. It has a good balance of strength and toughness, meaning that it is resistant to fracture and permenant deformation.

Non-weldable

Stats (2024-T351)

• Young's Modulus
• Yield Strength:
• Ultimate Strength:
• Hardness:
• Fracture Toughness:
• Elongation at Break:

7075 Al

High strength and stiffness aluminum alloy. However, it has lower elongation at break comapred to other aluminums, which means it will tend to break and crack more than other aluminums, which will just deform instead. Depending on your application, this might be favorable or unfavorable. For instance, it could be a good choice for a bottom plate that needs to be stiff and should not deform (which could high center the robot). However, 7075 would be less advantageous compared to another alloy like 2024 for exterior aluminum armor panels, as it would tend to break apart easier than other aluminums.

More difficult to machine than 2024 and 6061 but weldable.

Stats (7075-T651)

• Young's Modulus
• Yield Strength:
• Ultimate Strength:
• Hardness:
• Fracture Toughness:
• Elongation at Break:

6061 Al

Cheap, easy to machine, but lower strength than the other options. Can still be a viable material for robots, especially for parts that do not take direct impact. Usually another aluminum is preferred. However, as a counterexample, Roundhouse, A 30lb robot from RoboGym uses a single piece 6061 unibody which has held well over multiple fights

Stats (6061-T6)

• Young's Modulus
• Yield Strength:
• Ultimate Strength:
• Hardness:
• Fracture Toughness:
• Elongation at Break:

Steels

Naming Convention

AISI/SAE Steel Grades

• 4 Digit number
• First two digits represent alloy
  • Ex: 10xx - Pure steel (No alloying)
• Last two digits represent carbon content in units of .01%
  • Ex: xx45 - (0.45% carbon content)

AR Steels (600/500/400)

AR: Abrasion Resistant. The number in the name refers to its hardness. As with most steels, higher hardness leads to higher strength and resistance to damage, but increased brittleness.

Often the material of choice for robot weapons and armor. AR500 is a balance of hardness and toughness, and is easy to source from sources like SendCutSend. AR400 is less commonly used as the extra hardness is often desired. AR600 can be a good option as well if additional hardness is desired and extra strength is wanted to resist bending, but can be somewhat difficult to source.

Can be bent, machined, and welded. Cold forming is preferable as heating ar above 500 can anneal the area, removing its incredible properties. Properly welding AR requires careful setup and planning. It MUST be cleaned with ALL mill scale in the area removed and degreased with acetone. Weld order and heat management is very important or the material will soften and warp. It is recommended to read the following document https://aemach.com/hardox/pdf/011_SSAB_plate_welding_UK.pdf

Titaniums

Ti-6Al-4V “Grade 5”

Twice as strong as mild steel but half the weight. This is the material of choice when aluminum is too soft for a component and steel is too heavy. Commonly used for armor, cleats, and mounting plates. While very strong and tough, it has a hrc rating of 34C which is lower than most weapons. In addition, the material is very expensive and difficult to machine. Machining precautions: Titanium rapidly work hardens when drilled. Use low speeds, moderate pressure, and plenty of coolant. Carbide tooling is preferred

Welding titanium is exceedingly difficult due to its affinity for oxygen. If any portion of the material comes into contact with oxygen while above 400 degrees, it will become brittle and fracture on impact. Cleanliness is a MUST, all parts need to be cleaned with a wire brush then wiped with acetone. Grade 2 filler rods should be used for ductility and wiped with an acetone soaked rag. Weld in half inch sections with a trailing shield, high gas flow rate, and >12 second post flow. Gas coverage on the back of weld surface is absolutely critical. If not welding tube, parts should be welded in an inert chamber. Silver and straw colored welds are acceptable, any other will be brittle.

“Commercially Pure Titaniums”

Encompassing grades 1-4, these materials are easily formable but have less than half the strength of grade 5.

Unsuitable for combat

Composite

Carbon Fiber

Sheets

Formed from layers of carbon fabric in an epoxy matrix. This material boasts the highest stiffness and strength to weight ratio out of all common materials. Robot parts are typically 2d profiles milled out of sheets. Common applications include base and top plates, mounts, and occasionally weapon rails. Carbon fiber does not tolerate impacts and great care must be taken so it is not directly hit.

Notes: Do not waterjet! Try to minimize pocketing as material is highly anisotropic Try not to countersink

Sources: cncmadness.com

Forged

Forged carbon fiber parts are made by packing a mold with chopped tow, pouring in resin, and squeezing excess out. By strategically placing long strands of tow, parts with incredible strength can be made. Hardware can be imbedded in mold to add attachment points. Non impact-resistant and can fail prematurely if improperly made.