Patent ID: 8152940

Claim:
An aluminum alloy forging material comprising an arm portion comprising, by % by mass, 0.5 to 1.25% of Mg, 0.4 to 1.4% of Si, 0.01 to 0.7% of Cu, 0.05 to 0.4% of Fe, 0.001 to 1.0% of Mn, 0.01 to 0.35% of Cr, 0.005 to 0.1% of Ti, Zr controlled to less than 0.15%, and the balance being Al and inevitable impurities, the material having a substantially H-shaped width-direction sectional form comprising a relatively narrow and thick peripheral rib and a relatively wide central web, wherein in a width-direction sectional structure in a maximum stress producing site of the rib, a density of crystals observed in the sectional structure in the maximum stress producing site is 1.5% or less in terms of an average area ratio, and an average spacing between grain boundary precipitates observed in the sectional structure comprising a parting line which is produced during forging, is 0.7 μm or more, and wherein in the width-direction sectional structure in the maximum stress producing site of the rib, a size of dispersed particles observed in the sectional structure in the maximum stress producing site is 1200 Å or less in terms of an average diameter, a density of the dispersed particles is 4% or more in terms of an average area ratio, an area ratio of recrystallized grains observed in the sectional structure of the rib is 10% or less in terms of an average area ratio, and the area ratio of recrystallized grains observed in a sectional structure of the web adjacent to the sectional structure of the rib in the width direction thereof is 20% or less in terms of an average area ratio.