Aluminum alloy forging and manufacturing method thereof

A method for manufacturing an aluminum alloy forging, comprising:

• a molten metal forming step of preparing a molten alloy;
• a casting step of casting the molten alloy at a cooling speed of 100 to 140° C./sec during casting to obtain a cast aluminum alloy rod;
• and
• a homogenization heat treatment step of subjecting the cast aluminum alloy rod obtained in the casting step to a homogenization heat treatment at a temperature of 370° C. or higher and 470° C. or lower for 4 to 10 hours so that a crystal grain diameter is 110 μm or less in a metallographic structure of an obtained cast rod,
• wherein the molten alloy comprises:
• Cu:
• 0.15% by mass to 1.0% by mass;
• Mg:
• 0.6% by mass to 1.35% by mass;
• Si:
• 0.95% by mass to 1.45% by mass;
• Mn:
• 0.4% by mass to 0.6% by mass;
• Fe:
• 0.2% by mass to 0.7% by mass;
• Cr:
• 0.05% by mass to 0.35% by mass;
• Ti:
• 0.012% by mass to 0.035% by mass;
• B:
• 0.0001% by mass to 0.03% by mass;
• Zn:
• 0.25% by mass or less;
• Zr:
• 0.05% by mass or less;
• and
• a remainder consisting of Al and inevitable impurities,
• a crystal grain diameter in a part of the aluminum alloy forging where a maximum principal stress is applied to the part is 20 to 40 μm,
• the manufactured aluminum alloy forging has a structure in which an average shortest distance from a precipitate having a major axis of 0.1 μm or more to a crystal grain boundary in a cross-sectional structure with a visual field area of 8,000 μm2 is in a range of 0.1 μm or more and 2.0 μm or less, and
• the manufactured aluminum alloy forging has fatigue characteristics in which a fatigue life at a load stress of 150 MPa is 6×106 or more at room temperature.