During cardiac cycle, aortic semilunar valve closes when 

Aortic semilunar valve guards the opening of the left ventricle into the aorta. Aortic valve opens during ventricular systole allowing ejection of blood into the aorta. During ventricular diastole, as the ventricles relax, pressure within the chambers falls, and blood in the aorta begins to flow backward toward the left ventricle. Backflowing blood catches in the valve cusps and closes the SL valves. The aortic valve closes at a pressure of about 100 mmHg.