The concern was the depression in prices was cratering local high-quality smelting capacity. However, the DOD issued a memo (penned by Mattis iirc) indicating

The concern was the depression in prices was cratering local high-quality smelting capacity. However, the DOD issued a memo (penned by Mattis iirc) indicating that there was no supply related concerns. The lack of aluminum used to justify the tariffs was in fact just the result of the LME's anti-market-tampering rules creating an incentive for metals traders to stockpile the material in private stores rather than in LME warehouses. There was a nice article about this yesterday, but I don't have the link.