- Strong
- Light
- Cheap/easy to source
- Easy to machine
- Hard
- Low thermal conductivity
You may note that several of these are in conflict with each other. For example, "easy to machine" and "hard" are basically exact opposites as are "strong/light" and "cheap".
Wood
I only mention wood as a material since I am an experienced wood worker and love working with it. A great looking USB drive could be made out of wood but for the purpose of a rugged/indestructible drive it is a poor choice. The yield strength of wood is a complex subject but for a rough estimate it is more than ten times weaker than even a low strength Aluminum alloy (yield strength of ~5k psi).Brass
I used Brass for the first prototype and while it is cheap and easy to machine it is also heavy (8 g/cc) and weak (yield strength of ~20 kpsi) and thus not a good candidate for our design.Aluminum
Aluminum seems like a good choice of a material although we'll have to look more closely at which of many alloys in particular. In general, Aluminum is light (2.7 g/cc), easy to machine, and can have yield strengths approaching 100 kpsi in some alloys. We'll look at a few of the most common alloys, particularly those with high strengths.6063
This is a common low cost and readily available alloy but is low strength with a yield strength around 20 kpsi. It is great for quickly testing a prototype design and I've gone through a few feet of 1" diameter 6063 rod over the design process for the VIUD including the second prototype.
Al 7075-T6
7075 is the most common of the so called "aircraft Aluminum" alloys developed for its higher strengths. It is around twice as expensive than 6063 but for our VIUD it still only works out to around $4 of raw material per unit.
The "T6" designation is the "Temper" of the alloy which are standards of how the metal is prepared and is a very important designation of the material. For example a 7075 non-tempered alloy has a yield strength of only 15 kpsi while 7075-T6 is near 80 kpsi. A temper of T6 indicates that the Aluminum is solution heat treated and artificially aged.
Al 7068-T6511
One of the stronger aircraft Aluminums with yield strengths approaching 100 kpsi for the T6511 prepared alloy. Price for 7068 is around twice that of 7075: getting higher but hopefully still acceptable. T6511 is a combination of "T6" solution heat treated and artificially aged) and "T511" (cooled from hot working and artificially aged at elevated temperatures with minor straightening after stretching).Titanium
Titanium is also an obvious choice when creating something that needs to withstand a lot of abuse. It is heavier than Aluminum at 4.5 g/cc making it about half as heavy as Brass and Steel. Where Titanium excels is its strength with certain alloys reaching a yield strength of 140 kpsi. The downside, however, is that due to its hardness, especially with the higher strength alloys, it is difficult to machine which leads to higher production costs.
Another thing to note about Titanium is its lower thermal conductivity of 16.4 W/m-K compared to Aluminum's 173 W/m-K. This means that using Titanium for our VIUD design should render it much more resistant to temperature. Titanium is also much harder than even anodized Aluminum which further increases its usefulness as a"rugged" material.
There are a variety of alloys but we'll only look at the two most common ones, Grade 2 and Grade 5 (6Al-4V).
Ti Grade 2
This is the softer and more common Titanium alloy with a yield strength of the annealed alloy near 50 kpsi. This makes it less than ideal as it is weaker than the 7075 Aluminum despite being more expensive to manufacture, although surprisingly probably not by much. The hardness of Al 7068 is B90 on the Rockwell scale while Grade 2 Titanium is around B98, larger but less than I had originally assumed.
Ti Grade 5 (Ti-6Al-4V)
This is the much stronger alloy with yield strengths nearing 130 kpsi for the annealed version and approaching 170 kpsi for surface treated alloys. The price for this strength is its machining difficulty and high material cost, about four times that of 7068-T6511 making the material cost per unit around $25.
Review
The following table summarizes the important material properties we've been discussing:
| Material | Density g/cc | Yield Strength psi | Heat Conductivity W/m-K | Hardness Rockwell |
|---|---|---|---|---|
| Aluminum 6063 |
2.8
|
20,000
|
200
|
B30
|
| Aluminum 7075-T6 |
2.8
|
65,000
|
200
|
B80
|
| Aluminum 7068-T6511 |
2.8
|
95,000
|
200
|
B90
|
| Brass 360 |
8.5
|
20,000
|
110
|
B25-B80
|
| Titanium Grade 2 Annealed |
4.5
|
40,000
|
16
|
B98
|
| Titanium Grade 5 Annealed |
4.5
|
120,000
|
7
|
C36
|
| Wood, Maple |
0.7
|
6,000
|
0.7
|
N/A
|
The obvious choice of material is the Aluminum 7068-T6511 with 7075-T6 being a close runner up. Grade 5 Titanium is great but will end up being very expensive to use and perhaps is better used for a "special edition" version with a higher price.
Resources
Some of the online resources I found useful for material property references are listed below:
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