Due to environmental and emissions laws, places in CA, Oregon and WA rely on recycled material to reduce some of the energy overhead of blending their own alloys. A356 is also used in most every automotive application and so getting recycled material domestically isn't tough.
Most raw aluminum in the USA is imported from Canada while bauxite (alumina ore) is imported from many more overseas locations. Bauxite is mined in the USA, but accounts for 1% or less of the market.
