I've read a few discussion on the use of aluminum in medieval armor. There are a lot of people saying that aluminum is a bad choice for armor because it's too soft and brittle. There are many different alloys of aluminum available. Each one has properties specific to the task for which it is designed.
The premise of my novel is a 737 is transported into another universe with 12th century technology. The 7075 alloy on the wings and body of an aircraft are strong, nearly the same as hot rolled steel. The material can be annealed at 775 degrees and is then workable with a hammer. The wings are about a quarter inch thick. I'm thinking it can be fairly easily reworked into exceptional armor for the period.