Fatigue strength and applications of cast aluminium alloys with different degrees of porosity.

This paper reports on the influence of porosity on constant- and variable-amplitude fatigue of an age-hardened alloy G-Al-7Si-0.6Mg wa and a non-age-hardened alloy G-Al-11Si-Mg-Sr. Both alloys reveal a cycling hardening. The age-hardened alloy is suitable for finite fatigue life and variable-amplitude fatigue applications, but not the non-age-hardened alloy because of the flat course of its S-N curve. Despite the large differences between the static strength properties the endurance limit properties are similar. If the degree of porosity is increased from 0 to 8, fatigue strength is reduced in the unnotched condition by about 17 % for both alloys. However, in the notched condition the age-hardened alloy displays a reduction of 7 % but the non-age-hardened material a drop of 20% due to its lower yield stress. The influence of the thickness of the solidified cross-section on fatigue strength in the porous condition could not be determined. Porosity can be tolerated even in safety parts, but not in highly stressed fatigue-critical areas