This lengthy article details how the Axis Powers made pure nickel coins as war preparation starting in 1933.
Coins for the conflict: nickel and the Axis, 1933-1945
John Perkins
Sept. 22, 1992
The Historian (Vol. 55, Issue 1.)
Publisher: Phi Alpha Theta, History Honor Society, Inc.
Abstract: The development of applications for nickel to strengthen warship armor, artillery barrels and shell casings caused the stockpiling of nickel by many governments in the years prior to World War II, particularly Japan and Germany. In order to avoid the repercussions of appearing to import nickel for military purposes, both Japan and Germany issued pure nickel coinage in order to assure a ready supply. These coins were circulated until the war began and then recalled in exchange for aluminum or brass coins.
Reference: Perkins, J. (1992). Coins for conflict: Nickel and the axis, 1933–1945. The Historian, 55(1), 85-100. doi:10.1111/j.1540-6563.1992.tb00887.x