WASHINGTON — When Mandel Metals, an aluminum distributor outside Chicago, found its business threatened by President Trump’s tariffs on foreign metals, it filed hundreds of requests with the administration to exclude its imports from the levies. Again and again, the administration said yes, allowing Mandel to import — if it wanted to — up to 600 million pounds of tariff-free aluminum from Greece, Italy, France, Norway and, perhaps most surprising, China. Trump imposed sweeping tariffs on steel and aluminum to prevent China and other countries from flooding the U.S. market with cheap metals, which he said posed a national security threat by “degrading” the industrial base.