Alcoa has been reluctant to expand aluminum smelting in the U.S., despite recent sky-high prices for the metal. But the Pittsburgh-based company is taking a big swing at making more aluminum overseas by acquiring one of the world's largest smelters in South Africa.

Alcoa plans to buy the Hillside smelter from Australia's South32 as part of a cash and stock deal valued at up to $5.6 billion. The deal also includes South32's stake in an aluminum smelter in Brazil that Alcoa already operates and prized bauxite mining and alumina refining operations in Western Australia near Alcoa's plants.

"These are high quality, well understood assets with strong operating histories," Chief Executive William Oplinger told analysts.

Alcoa's shares finished 8.9% lower Wednesday, with investors likely reacting negatively to the company's plan to use its stock to pay for part of the purchase.