Firewall material

Based on the problem statement, I know that the firewall material must be either steel or aluminum. I also know that the manufacturing process for the container will most likely involve bending sheets of the material. It’s also imperative that the firewall material must be resilient to heat, have high tensile strength, and it would be nice for the material to resist corrosion. Using these considerations, I construct a pugh matrix (figure 1) with several aluminum and steel variants offered by McMaster-Carr.

Point Values

McMaster's range of qualitative ratings corresponds to a nice 1, 2, 3 point system, so I used that for all my categories to keep the balancing simple. By using the same 1, 2, 3 range for all categories, I’m able to weigh them in the final tally without first having to balance them with each other. This way, I can assign weight to categories based on how important I deem them to be. For qualitative categories, “Fair” gets a 1 and “Excellent” gets a 3. With the quantitative categories, I plotted the data in bar plots to help divide the data into three ranges and then applied a rating.

Formability

While McMaster does provide their own qualitative measure of formability rated from “Fair” to “Excellent,” I’m not inclined to trust it to its word, and I wanted to dig a little deeper. Thus, I added yield strength, young’s modulus, and elongation at break to get a better sense of the ductility of the materials being considered. Some materials also didn’t have a qualitative measurement of formability, so I was able to use these parameters to fill in my own. I decided to weigh the McMaster rating as 1 while the yield strength, elongation at break, and young’s modulus get a 0.5 weight. I weighted the McMaster rating higher because, despite my reservations, it correlated well with the data I was able to collect on formability, and the lower weight for my data was so that formability isn’t weighted too heavily in the final tally, as I think having a formability qualitative rating and then another three quantitative categories assigns too much point value to formability as a design consideration.

Density

Without knowing exact dimensions of the part, density is the best way to get a sense of which material will be optimal for building an assembly that meets our objective of reducing weight. The two steel alloys perform horribly in this category while all of the aluminum alloys are indistinguishably good.

Strength

Another important consideration for this design is the strength of the material. Metals have only been considered because of their nearly unrivaled toughness, but it’s important to consider how much stress the material can withstand in the ultimate sense; therefore, we consider ultimate tensile strength. Ironically, the two final considerations (AL3003 and AL5083) didn’t do terribly well in this area, but I’m glad nonetheless that it was considered.

Specific Heat Capacity

Since this material is being used to construct a firewall, I decided it may be a good idea to examine the specific heat capacity of all the materials as well. This, next to cost of course, is one of the more heavily weighted categories. Most of the aluminum alloys performed exceptionally compared to steel, and I was glad to see that the two finalists are two of the better performing in this category. A nice indication that I haven’t completely lost my mind—not yet.

Cost

The ugly elephant rears its head as always. As much as I wish the budget was bottomless, cost is a strong consideration for this build as well.