Robot Materials

You should be very aware of the weight limit when building your robot. You should try to squeeze in the most power, the thickest armor, and the biggest weapon you can, while staying below the weight limit. One way to do this is by using the highest performance materials available.

At seasons 4 and 5 of BattleBots, BioHazard weighed in at 220 pounds. Following is a list of raw materials, and how much of each I used in the robot:

Item Pounds
Titanium 55
Magnesium 20
Aluminum 25
Steel 13
Total 113

The remaining 107 pounds is made up of the following components:

Item Pounds
Batteries 27
Drive motors 14
Linear actuators 15
Wheels 10
Electronics 5
Other 36
Total 107

The trick is to use the lightest material of sufficient strength for each component. You can use the following table to help you choose your materials. The table lists some moderate and high performance alloys.

"TS" is tensile strength. It is a basic measure of a materials strength. It is listed as thousands of pounds per square inch. For example: a one inch square bar of the highest performance alloy (AerMet) has a breaking strength of 300,000 pounds. It can be used to lift about 80 Cadillac Eldorados! "E" is elongation. It is one measure of a materials ability to absorb an impact and bend rather than break. The higher the number, the less brittle it is. The best materials have high numbers for tensile strength, AND elongation. "D" is density. It is the materials weight in pounds per cubic inch. The price is how much lighter my wallet was for each pound of the material I purchased.

Mateterial Alloy TS E D Price
Magnesium AZ31B-H24 42 15-19 .064 19.00
Magnesium ZK60A-T5 53 11-14 .066 28.00
Aluminum 6061-T6 45 17 .098 6.00
Aluminum 2024-T3 70 18 .100 7.00
Aluminum 7075-T6 83 11 .102 10.00
Titanium TI-6AL-4V 150 12 .160 24.00
Steel 1018 61 20 .283 6.00
Steel 4130 200 20 .283 8.00
Steel AerMet 100 300 12-13 .285 24.00

AZ31B-H24 Magnesium is the strongest mag that is generally available in large plates.

ZK60A-T5 Magnesium is the strongest mag alloy.

6061-T6 Aluminum is a general purpose, strong alloy that is very available.

2024-T3 Aluminum is used in aircraft landing gear and military applications.

7075-T6 Aluminum is harder than 2024-T3, but more brittle.

TI-6AL-4V is the highest strength titanium that is generally available.

1018 steel is cheap, and easy to machine and weld, but it doesn't really fall into "high performance" category.

4130, 4140 steels are the "chromoly" that are used in race car and bicycle frames. Very available.

AerMet 100 is an alloy of Iron, Cobalt, Nickel, Chrome, and Molybdenum. Strongest stuff yet developed. There are steels that have higher tensile strengths, but they are almost as brittle as glass. This alloy needs to be machined in its soft state, then hardened by heat treatment. Use it if you need it, but be prepared to shell out about $450 for a 4" cube!

Materials Facts and FAQs:

What do all those alloy numbers mean?

  • The aluminum alloys that can be heat-treated are the ones that start with 20, 60, or 70. Common examples of these alloys would be 2024-very strong and resistant to cracking, 6061-general purpose, strong alloy, and 7075-very hard. The last two digits in the alloy specification tell you if and how the metal has been heat-treated. Look for 2024-T3 or T4, 6061-T6, or 7075-T6. T6 is heat treatment alone; T3 is heat treatment plus cold working.

What happens if I try to weld heat-treated aluminum or titanium?

  • If you weld T6 material, you will probably lose some strength in the heat-affected zone, but you could probably re-heat treat. If you weld T3 material, you could re-heat treat, but cold working would be impossible, so you will only regain part of the original strength.

OK, so what should I make my robot out of?

  • If you want to use aluminum, then 2024-T3 or T4 is probably your best bet, if you can find it in the shape you need. If you want to use steel for the chassis, you should probably go with 4130, 4140, 4340, or some other "chromoly" alloy. This is available in many different sizes of tubing which have good strength to weight ratio.

  • For high-strength steel, a tool steel known as S-7 is probably your best bet. This steel is also used in chisels, so it is shock resisting, and holds an edge well. You probably wouldn't want to use it as framing material, but for highly stressed parts and weapon blades and spikes, it works very well. You machine it in its soft state, then you have to take it to a heat-treater to harden it to about 54-57 Rockwell. AerMet 100 is the ultimate in high-strength steel, but it is not very available in anything but round bars. It is very difficult to machine, and it also requires heat treatment after machining.

What is the best material for a heat sink?

  • Power electronics and motors can easily be damaged by heat. It is a good idea to mount these items on a material that will conduct that heat away quickly, and keep your expensive components cooler. The best material for your heat sink is pure silver. If you can't afford that, aluminum and magnesium work very well. Copper is about twice as effective as aluminum, but it is also very heavy. Silver is about 50% more effective than copper.

  • In BioHazard, the electronic speed controllers are bolted directly to a magnesium body panel. I also give it a thin coat of heat-sink compound to increase the rate of heat transfer.

BioHazard survives a mighty blow from Nightmare.