Hey all, I'd like to take you on a deeper dive from a sustainability perspective into the "steel vs. aluminum" design question that's being debated in Episode 4 of Let's Go Design.
Let's start with the punchline from my goofy cameo on LGD (note: you know you're in trouble when the director says to you, "once more… this time, more Shattneresque") – Jeremy and I decided, using SolidWorks Sustainability, that a steel gantry was more environmentally-preferable than an aluminum one.
Pound for Pound
SolidWorks Sustainability will tell you that, pound for pound, steel is better for the environment than aluminum on all four environmental indicators (carbon, energy, air, water). But it's a bit more complex.
First, both steel and aluminum have something going for them that other materials, like plastics, do not: they're essentially infinitely recyclable. Steel that's recycled from your beloved first Cadillac (back when men were men and cars were metal) is pretty much interchangeable with new steel, and aluminum from a soda can could end up in an airplane if the alloys match (determined by the contaminants). This is because both steel and aluminum retain their structural integrity when they go through their respective recycling processes. Not so with plastics; check out how different the "product applications" are from the "products made with recycled content" in this list of plastics sorted by their resin identification codes (you know, those #1-#7 "recycling" stamps).
Behind the data
So steel and aluminum could be used in closed-loop processes — and this is in fact the case for some products, like aluminum cans. The typical aluminum beverage can is made from over 50% recycled content, and completes the cycle from used can tossed in the recycling bin to filled can back on the grocery shelf in as little as 6 weeks. Steel is even more impressive: two out of every three pounds of new steel are made out of old steel!
The truth lies in how these two different sets of data are represented in the LCA model. The data for steel – formally called a life cycle inventory, or LCI, dataset – actually represents a trade mix of primary (virgin) and secondary (recycled) steel. This is because the most common process of manufacturing raw steel from pig iron in a blast furnace incorporates the use of 25-30% steel scrap; that is, the manufacture of steel and the recycling of steel happen at the same time and place, namely in the White Tower of Isengard a blast furnace like the one below.
In contrast, aluminum has separate ecosystems of recyclers — aluminum can recyclers are adept at making new aluminum cans, and auto part recyclers at making new parts. The primary/secondary ratios, and to some extent the processes (e.g. cans have to be "demagged" or have magnesium removed) are different for these different applications, and we can't guess what you'll be using that aluminum for. Therefore, Sustainability uses only a primary aluminum LCI dataset. This (virgin) aluminum is thus several times more impactful than the trade-mix steel, because of all that energy to smelt the bauxite (this rock). As you approach 100% secondary aluminum in the mix, though, the difference in impact between aluminum and steel all but disappears.
Sustainability is about the whole product
So this is why in the LGD video, I told Jeremy that his decision of steel over aluminum as the environmentally-prefered material depended largely on the secondary-aluminum content of his source. That's a discussion you as the designer should definitely have with your raw material suppliers.
Lastly – speaking of design – do remember that Sustainability is indeed a design tool, not just a material tool. You probably wouldn't make the same design — same wall thickness, for example — in steel as you would in aluminum. In the real world you'd want to reengineer the gantry to take best advantage of your material choice. Let's say you wanted to lightweight your new, heavy gantry by thinning the tubes, but only to the point where you were still within your desired factor of safety; this would require iterating between Sustainability and Simulation, and possibly different configurations for your two materials. Environmental impact isn't a decision you can visit just once!
Forty-nine percent of you voted for Aluminum in our last LGD episode, and 51% for Steel. Knowing what you do now about their sustainability stories, would any of you change your votes?