Abstract: The isolated coupling device designed according to the principles of high-pass filter is used to link the AC arc welding power and high-frequency excitation source in order to excite arc-ultrasonic by modulating the alternating plasma arc.By adding Al-5Ti-5Si as filling material,the effect of alternating current arc ultrasonic frequency on the microstructures of joint of SiC_p/6061Al MMCs in plasma arc in-situ weld-alloying welding is investigated.By comparing the different weld microstructures under different frequencies(0,30,40,50 and 60 kHz),metallographic analyses show that the morphology of Al₃Ti can be improved,from a long strip into intermittent short rod-like;new particle-reinforced phase(AlN,TiN) is increased and more evenly distributed;intergranular segregation of Si-rich phase also is significantly reduced with the frequency increase.