The Effect Of Ultrasonic Vibration To Weld Pores In TIG Of Die Casting Magnesium Alloy

It is caused by the intrinsic properties of die-cast magnesium alloy,which is prone to appear porosity defects during welding.This seriously reduces the mechanical properties of welded joints.At present to meet the requirements of the low porosity fusion welding joints can not be obtained,which greatly limits the magnesium alloy engineering applications.Therefore,it is very important to solve the problem of welding pores in die-casting magnesium alloy to expand its engineering application.This paper uses the die-cast magnesium alloy AZ91 D as the object,ultrasonic vibration is applied to the TIG welding process.After welding,the weld porosity and microstructure were investigated to studied the influence of ultrasonic wave on porosity and weld microstructure.The main results are as follows:(1)The porosity of the applied vertical ultrasonic vibration is lower than that of the ordinary without it;and the porosity decreases gradually with the increase of the vertical ultrasonic vibration amplitude.This may be the comprehensive effect of cavitation effect of vertical ultrasonic vibration,acoustic streaming effect,heat effect and mechanical vibration.So that,the bubble rising and escaping in the molten pool are promoted.(2)The grain microstructure of the applied vertical ultrasonic vibration is smaller than that of the ordinary without it,and the grain has a tendency to refine with the increase of the ultrasonic amplitude.The mechanism should be dendrite fragmentation which is caused by the effect of cavitation,effect of vertical ultrasonic vibration,acoustic streaming effect and mechanical vibration during solidification of weld metal.(3)The weld porosity of applying horizontal ultrasonic vibration is lower than the weld without the application of ultrasonic vibrations,but the porosity does not change with the increase of the horizontal ultrasonic vibration amplitude.Its mechanism should be as follows: on the one hand,although the cavitation effect of vertical ultrasonic vibration,acoustic streaming effect,heat effect and mechanical vibration can promote the bubble rising and escaping in the molten pool;However,due to the high damping coefficient and the long propagation distance of magnesium base metal,the effect is weaker than that of the vertical ultrasonic vibration;On the other hand,horizontal shaking pool have reciprocating motion compared with the direction arc so that it can suffer more heat;Therefore,the solidification time of the molten pool is increased,which is favorable for the bubble to escape.However,with the increase of ultrasonic vibration amplitude,the effects of these two aspects are not obvious,so there is no obvious change of porosity.(4)The grain size of the weld microstructure with horizontal ultrasonic vibration is smaller than that without it;Along with the increase of ultrasonic vibration amplitude,there is no obvious change about the grain size.The mechanism may be that horizontal ultrasonic vibration promote grain refinement by breaking dendrites and at the same time it is not conducive to grain refinement by reducing the melting rate of molten pool.But the effect of the former is greater than the latter so that the weld grain has been refined.However,the increase of ultrasonic vibration amplitude will not lead to significant changes in these two aspects,so the grain size does not change much.(5)The weld shape applying horizontal ultrasonic vibration is better than that applying the vertical ultrasonic vibration.This is due to that when applied the vertical ultrasonic vibration,the horn vibrate up and down leading to the distance change between the tungsten and the base metal which affect the shape of the weld by affecting the arc stability.