Ａｌ‐Ｓｉ‐Ｍｇ‐Ｃｕ系鋳造合金の熱疲労寿命に及ぼす析出硬化および共晶Ｓｉ組織の影響

Abstract

The relationship between age-hardening and thermo-mechanical fatigue life was investigated for Al–Si–Mg–Cu cast alloys. The age-hardening behavior of the alloys was almost similar for the Cu-free and Cu containing alloys up to the peak aging, however, the softening behavior by over-aging was different depending on Cu contents. The behavior of decreasing in the loop-edge stress during thermo-mechanical fatigue cycles was also different depending on Cu contents. The Cu-free alloy exhibiting a long fatigue life shows significant decrease in hardness by over-aging and pronounced decrease in the loop-edge stress with increasing fatigue cycles. The Q′ or Q phase is formed in the alloys containing higher Cu contents and retards the hardness decrease during over-aging and the decrease in the loop-edge stress during the initial stage of thermo-mechanical fatigue cycles. These behaviors of the alloys containing higher Cu contents result in the decreased fatigue life. This is explained that the plastic strain localization occurs around the eutectic Si particles when the stress decrease in the initial stage of the fatigue cycles is small and causes micro cracks at around eutectic Si particles. The increase in the calculated area for an age-hardening curve is related to the increase in the fatigue life. Therefore, the thermo-mechanical fatigue life can be predicted based on the calculated area for an age-hardening curve.