An ultrasonic radiation rod made of Ti-6Al-4V (TC4) titanium alloy can be eroded in an aluminum alloy casting, which can also affect the stable transmission of ultrasonic waves. This erosion is a result of complex, multifactor interaction. Therefore, two groups of experiments were designed: one group did not apply ultrasonic treatment (UST), while the other group did. Five titanium alloy samples removed from the radiation rod were ground on the automatic grinding machine to make their surface roughness similar. The four samples were placed in the molten aluminum for 4.5, 9, 13.5, and 18 h, respectively, without UST. The remaining sample was placed in molten aluminum with UST for 4.5 h. The four samples without UST were cooled and cut through the middle position by a wire-cutting machine. The cross sections of the four samples were ground and observed by a scanning electron microscope (SEM). The generated chemical layer was determined to be titanium trialuminide alloy (TiAl3) by energy dispersion spectrum (EDS) analysis, and its thickness was approximately 20–50, 145, 220, and 345 μm, respectively, corresponding to different processing times. The fifth titanium alloy sample was processed by UST, the residual aluminum on the sample surface was removed by hydrochloric acid, and obvious erosion pits and microcracks were found by SEM. The roughness of the surface was approximately 340 μm in the LY-WN-YH 3-D system. Therefore, the effect of ultrasonic cavitation on TC4 titanium alloy is much larger than that of a chemical reaction.