The ultrasonic field was introduced into the casting process of the aluminum alloy 7050 flat billet with the size of 500 mm×1 320 mm.The microstructure

distribution of grain and the macrosegregation were investigated comparatively under conventional casting and ultrasonic casting.The results show that

after ultrasonic treatment（UST）

the grain size is refined

and the growth of coarse-dendrites is restrained

meanwhile

the distribution of microstructure of the billet becomes more uniform.The level of macrosegregation along different direction is reduced under ultrasonic field

where the segregation index of Zn

Mg and Cu

along the symmetry axis in the direction of length and width

is decreased from 0.101 3

0.111 7

0.126 0 and 0.111 9

0.111 3

0.128 1 to 0.083 5

0.079 0

0.102 7 and 0.074 3

0.072 5

0.084 1

respectively.With the effects of ultrasonic cavitation and ultrasonic flows

the temperature field and the flow field in the liquid pool are changed

which improves the circumstance of grains formation and promotes the uniform distribution of solute elements

so refining the grain size and weakening macro-segregation behavior.