PURPOSE:To improve the machinability, formability and surface blackening properties of an aluminum alloy member by mixing an Al alloy with a specified amt. of Mg as well as specified amounts of Fe, Cu, Mn or Cr. CONSTITUTION:The compsn. of an alloy is formed of a one contg., by weight, 1.0 to 3.0% Mg, furthermore contg. at least one kind of element selected from a group constituted of 0.2 to 0.7% Fe, 0.1 to 1.5% Cu, 0.5 to 1.2% Mn and 0.05 to 0.2% Cr and the balance Al with inevitable impurities. Moreover, this surface blackened aluminum alloy member is heated and oxidized to form an oxidized film essentially consisting of Mg on the surface. At the time of heating the aluminum alloy member contg. Mg to a prescribed temp., a blackening phenomenon rapidly progresses on the surface, so that an oxidized film essentially consisting of Mg is formed on the surface. Because this oxidized film is formed by natural oxidation, it is rigid compared to the conventional coated film and anodized film, and moreover, its treating cost is low.
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