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Material Selection
The fact is that there is no
such thing as the "ultimate" material. The properties that make one material
perfect for one type of application may make it completly unsuitable for
another application. Long story short - you need to select your material
based on the planned application.
A few of the criterea you
should ponder when selecting what material to use (in no particular order):
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Application / suitability:
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What am I using this material
for? Does it need to be light weight? Stiff? "Springy"? Tough? Heat
resistant? Water proof?
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Price / availability:
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Can I get hold of this
material? And if so, will it cost me an arm or a leg? Perhaps there is a
suitable alternative to the material, that is easier to find or cheaper to
purchase?
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"Workability":
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Do I have the tools to work
this material? Does it require special drills/bits/files etc? Will it
require special protective gear when I work it, such as breathing gear,
masks, goggles etc?
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> Steel
Steel is a general term for iron containing
small amounts of carbon, manganese and other elements.
Its nice, solid stuff with excellent properties in terms of strength and
fracture toughness. Its fairly easy to work with (depending on
temper), and easy to come by. On the downside its also fairly heavy, and
prone to corrosion and oxydization. Adding other chemicals can change
steel's properties, for instance stainless steel
contains the elements chromium, nickel and molybdenum to make it rust
resistant.
> Typical
characteristics:
Good:
Fairly cheap and easy to
come by. Excellent strength.
Not-so-good:
High weight. Prone to
corrosion. Can be hard to work/machine with "hobby grade" tools.
> Application:
RC vehicle areas where
you might want to consider using steel:
> Nuts, bolts, fasteners, hinge pins:
You want steel fasteners.
'nuff said.
> Drive line / shafts:
Strongest running gear
you can find is typically made from case hardened steel.
> Pinions and gears:
For high torque
applications, steel rules.
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Alloys
> Aluminum
Aluminum is typically the
first choice for many RC vehicle projects. Its lightweight and strong, easy
to come by, it can be cut and bent into a chassis plate or milled into a
suspension arm. In short, its versatile.
Pure aluminium is
very soft, ductile, corrosion resistant and has a
high electrical conductivity. In consequence it is widely used for foil and
conductor cables, but alloying with other elements is necessary to provide
the higher strengths needed for other applications. So
that sheet of "aluminum" your making a chassis from is actually an aluminum
alloy.
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Aluminum alloys
The
details below are most likely a bit "above and way beyond" what you need to
know in order to get started on that chassis. However, it does provide some
insight into all those buzzwords and designations that some companies love
to throw around. 75% of the time you will do just fine with the
no-designation plain-jane aluminum sheet you got from your local source.
However, if you do want to delve deeper into the properties of these alloys
prior to choosing, read on.
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Aluminum alloy designations
The main
alloying elements in aluminum alloys are
copper, zinc, magnesium, silicon, manganese and lithium. There are
over 300 wrought alloys out there, with
about 50 in common use. They are normally
identified by a four digit system,
which you see in all those ads talking about "SPACE AGE MATERIALS" and
such. The figures designate the major elements of the alloy:
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Designation |
Major Alloy Element |
Typical Characteristiscs |
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1000 - series |
None (99%+ Aluminium) |
Very good
atmospheric corrosion resistance. Very good workability.
Low mechanical properties. |
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2000 - series |
Copper |
High strength,
heat treatable alloy. Very good machining characteristics.
High fatigue strength. |
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3000 - series |
Manganese |
Medium
strength alloy.Very good resistance to atmospheric corrosion. Very good
weldability. |
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4000 - series |
Silicon |
- |
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5000 - series |
Magnesium |
Medium
strength alloy. Very good resistance to
atmospheric corrosion. Very good weldability. |
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6000 - series |
Magnesium + Silicon |
Medium high
strength alloy. Very good corrosion
resistance. Very good weldability. Medium
fatigue strength. |
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7000 - series |
Zinc |
Heat treatable
very high strength alloy. Very high fatigue
strength. Corrosion protection is recommended. |
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8000 - series |
Lithium |
- |
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In addition to the 4
digit designation, you will typically see a dash with a "T" something or
another. This designates the type of heat treatment or temper that the alloy
has been subjected to. The temper plays a HUGE role in the overal properties
of the material; in its un-tempered "O"-condition, even 7075 is very soft,
and requires heat treatment to gain its full potential. These are the common
T-designations:
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Designation |
Description |
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| T1 |
Cooled from an elevated temperature
shaping process and naturally aged to a stable condition. |
| T2 |
Cooled from an elevated temperature
shaping process, cold worked and naturally aged to a stable condition. |
| T3 |
Solution heat-treated, cold worked and
naturally aged to a stable condition. |
| T4 |
Solution heat-treated and naturally
aged to a stable condition. |
| T5 |
Cooled from an elevated temperature
shaping process and then artificially aged. |
| T6 |
Solution heat-treated and then
artificially aged. |
| T7 |
Solution heat-treated and
overaged/stabilised |
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> Typical
characteristics for aluminum alloys:
Good:
Fairly cheap and easy to
come by. Easy to work & machine. Good strength-to-weight ratio (depending on
alloy).
Not-so-good:
High strength
applications require better (ie more expensive) alloys such as 7075-T6. High
amount of "material memory", ie the alloy will "remember" all stress cycles
it has been thru and gradually get weaker over time.
> Application:
RC vehicle areas where
you might want to consider using an aluminum alloy:
> Chassis:
Almost regardless of
chassis type, aluminum will fit the bill. Take into account alloy strength
vs material thickness when you design.
> Skid plates:
Not quite as good as TI,
but easier to obtain and machine, aluminum can work great as a skid plate.
Higher temp (T6) will help with scratch resistance as well.
> Shock towers:
Works great as a
shocktower, but again take into account alloy type & strength vs thickness
when you design.
> Engine mounts:
Works great as an engine
mount, and even helps dissapate heat. If you are planning on threading bolts
directly into mount, ensure that the alloy is strong / hard enough for this
purpose.
> Titanium (TI)
Few metals have the shear
sex appeal of titanium and titanium alloys. Lets face it, when the 2 words
"upgrade" and "titanium" are combined; wallets tend to open w-i-i-i-de.
The material does have some truely excellent properties, no doubt about it,
but its not always the "wonder-material" marketing folks would like us to
beleive.
> Typical
characteristics:
Good:
Pure titanium is almost
as strong as steel yet nearly 50% lighter, and very
durable. When added to various alloys, its hardness, toughness and
tensile strength can be increased dramatically.
Titanium is immune to corrosive attacks by saltwater and marine atmosphere.
Not-so-good:
Quite expensive and can
be hard to come by. Very hard to machine and work, it'll dull your drillbits
and files in a jiffy. Likewise hard to weld.
> Application:
Areas where you might
want to consider a titanium alloy:
> Bumpers:
Simple shape means less
machining and small size requires less material. And titanium will do a
great job at absorbing *dings* without deforming/bending.
> Skid plates:
As with bumpers, a simple
shape means easy to machine, TI has great properties when it comes to
scratches and abrasion.
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