Experimental investigation of the size effect in high cycle fatigue: Role of the defect population in cast aluminium alloys

•Effects of porosity on size effect and scatter of fatigue strength are investigated.•Two alloys with different degrees of porosity are used.•Magnitude of size effect and scatter are linked to the population defects.•High density and large size defects lead to minor size effect and less scatter.•Low density and small size defects induce large size effect and high scatter.