| Currently the most common welding method of aluminum alloy is alternatingcurrent tungsten inner gas (TIG) welding because that it can provide high quality weldbead with good appearance. But its disadvantage is the shallow welding penetration insingle pass which limits its welding efficiency. Ultrasonic assisted TIG welding (U-TIG)as a new method has so many advantages such as compressing the shape of welding arc,concentrating the energy of welding arc and increasing the welding penetration moreobviously. Many researchers have studied the effects of ultrasonic wave in varied meltmetals. The results of studies show that the frequency of the ultrasonic wave has asignificant impact on the pore, microstructure and mechanical properties of the meltmetals or the welding joints. Since the ultrasonic frequency is so important, therefore itis meaningful to study the effect of ultrasonic frequency in the U-TIG welding process.In order to study the distribution of the ultrasonic field, the ultrasonic field modelwas built using software COMSOL Multiphysics. The curvature radius of the ultrasonichorn whose end surface was concave was21mm according to acoustic pressure in theultrasonic field. Comparing to the ultrasonic horn whose end surface was plane, thesteady and compression level of the welding arc were improved, the width of the fusionzone was decreased, the grain structure in welding joint center was refined, tensilestrength increased9.3MPa when using concave ultrasonic horn.Through the bead on plate experiments the best height of ultrasonic radiation (thedistance between the end of the ultrasonic horn and the work piece) was21mm,28mmin ultrasonic frequency28kHz and40kHz respectively chosen by weld bead appearanceand the ratio of weld depth to width. The results of experiments in different ultrasonicfrequency showed that the soften level of the heat affected zone and the width of thefusion zone were decreased, the weld metal was homogenized and the grain structure inweld center was refined.The size and the distribution of the second phase in weld werechanged when using ultrasonic. In fusion zone and weld center, the size of the secondphase is small and the distribution was uniform. The tensile properties of the jointsusing ultrasonic were improved significantly. The maximum tensile strength is274.9MPa when ultrasonic frequency was28kHz comparing to that of welding jointswithout ultrasonic increased12.85%. So the effect of ultrasonic in welding is mostobvious When the ultrasonic frequency is28kHz.The mechanism of the ultrasonic in TIG welding process was explained simply.According to the ultrasonic field model and the analog result, analyze the law of soundpressure and sound radiation. It showed a periodic distribution in the axial direction. With frequency increasing, the sound pressure and radiation changed periodically andthe ultrasonic attenuation increased. The images of arc under different ultrasonicfrequency were obtained by high-speed camera. The ratio of the arc length to diameterwas measured from the image processed. Comparing to TIG welding without ultrasonic,the area of compression rate was18.36%when the ultrasonic frequency was28kHzwhich was the best. According to the Rayleigh-Plesset equation describing the law ofcavitation bubbles’s motion, the ultrasonic frequency got closer to the resonantfrequency of the cavitation bubble, the energy coupled better and the greater energyreleased by cavitation. When the ultrasonic frequency was higher, cavitation occurredmore difficult because there was no time for cavitation bubble to grow up and debaclewithin the short period of welding process. Acoustic streaming effect is proportional tothe ultrasonic frequency and amplitude. |
PDF Full Text Request