Effects of different ultrasonic powers on solidification structure of commercial pure magnesium were investigated

and changes of porosity in surface of commercial pure magnesium were observed by OM（optical microscope）.The results show that the coarse columnar dendrites can be observed in the sample without ultrasonic treatment

where the average length and width reach 19.23mm and 5.96mm

respectively.With the increase of ultrasonic power

the coarse columnar crystals in the ingot are refined firstly and then coarsened.The average length of dendritic crystals is changed from 19.23mm to 14.42mm to 12.07mm and to 14.61mm;and the average width is changed from 5.58mm to 3.08mm to 2.21mm and to 4.99mm

corresponding to ultrasonic power of 250

500

750and to 1 000W.In addition

the porosity of the products obtained is changed to 11.2%、3.4%、1.9%、1.48%and to 2.24%

corresponding to ultrasonic power of 0

250

500

750and 1 000 W.The results show that with the increase of ultrasonic power

the porosities in the ingot is decreased rapidly at first and then increased slowly.