Aluminum is a poor armor. It's a lightweight material, but very heavy for its level of protection. The density of aluminum is 2700-2800 kg/m^3, while the density of steel is 7850 kg/m^3. The densities vary by alloy, of course, but in general steel is about 2.8 times more dense than aluminum.
This is useful because we can do comparisons to existing armor. We can't really compare against modern tanks, because they use highly advanced composite armors which are typically rated in terms of steel equivalency (the M1A1 Abrams is estimated to have an equivalent of over 30 inches of steel worth of stopping power!). However, we can compare against the M4 Sherman from WWII which had 3.7 inches of armor. If we built an aluminum tank with a similar weight, it would have 10.4 inches of aluminum armor. So how well would it fare?
It turns out that the internet has an answer for this question. Demolition ranch took shots at a 8-10inch block of aluminum with varying calibers of guns. It fared well against small arms, but when they moved up into powerful rifles, whole chunks of armor started to fail. When facing basic armor penetrating rounds, they managed to dig about half way through the block. They did not test any of the weapons designed to take on armored naval targets, like 20 mm and up.
The big issue for stopping armor penetrators is that you need a material that is hard and dense to stop them. Aluminum is neither, so it is very weak.
Of course, this is all against modern weapons. If you're talking about stopping ancient weapons it may work better. However, making aluminum is not easy until you could develop the large quantities of electricity used to refine the raw materials. Your nation would have a substantial advantage over other ancient societies simply by having this technology. Copper is easy to work with. Aluminum is not.