Patent ID: 8298357

Claim:
A method of manufacturing an aluminum alloy extruded product, comprising the steps of: extruding a billet of an aluminum alloy having a composition comprising, in mass %, 0.6-1.2% Si, 0.8-1.3% Mg, 1.3-2.1% Cu, 0.04-0.35% Cr and no more than than 0.05% Mn as an impurity, with the balance being aluminum and unavoidable impurities and the following (1)-(4) relationships being satisfied, 3%≦Si %+Mg %+Cu %≦4% (1) Mg %≦1.7×Si % (2) Mg %+Si %≦2.7% (3) Cu %/2≦Mg %≦(Cu %/2)+0.6% (4) into a hollow product by using a porthole die having a ratio, D/W, of chamber depth D to bridge width W of 0.5 or more while setting a ratio of a flow speed of the aluminum alloy in a non-joining section to a flow speed of the aluminum alloy in a joining section in a chamber, where the billet reunites after entering a port section of the die in divided flows and subsequently encircling a mandrel, at 1.5 or less, to obtain a hollow extruded product having a cross-sectional structure which has a recrystallization microstructure with a grain size of no more than 500 μm and, in which, a surface defect is not observed after the hollow extruded product is subjected to a solution heat treatment by heating to 530° C. at a temperature rise rate of 10° C./sec, water quenching within 10 seconds after completion of the solution heat treatment, aged at 180° C. for 10 hours and subjected to 90° bending.