Study On Processing Property And Microstructures In Pulse Ultrasonic TIG Hybrid Welding

Ultrasonic-assisted TIG welding process has avoided the low penetration andinefficiency of conventional TIG process，meanwhile enhancing the stiffness of the arc，increasing depth of penetration and refining weld microstructure by employingapplied acoustic field, but the effects of ultrasound is dependent on the weldingparameter. As vibration source of ultrasonic，radiation shape change of ultrasonic hornhas a lot to do with the transmission of ultrasonic energy. For further study the effect ofthe ultrasound in the hybrid welding，the influence of acoustic emission diameter on theenergy transfer has been studied. The pulse ultrasonic-TIG hybrid welding method isproposed，and the effect of pulse frequency on impact of ultrasonication has beenfurther investigated.The ultrasonic field model was rebuilt by taking arc parameter into considerationon computation model of sound field. Sound pressure amplitude varying with spatialheight under different acoustic emission diameter has been simulated and theexperimental method is used to capture the arc shape and weld macrostructure. Both ofsimulation and experimental results ultimately determine the optimal acoustic emissiondiameter，under which arc is highly compressed and welding section form is centermelting form，achieving the maximum of depth of penetration and efficiency ofultrasonic energy transfer.The influence of pulse frequency on impact of ultrasonication in different pulsedmode has been discussed from the aspects of weld macrostructure and morphology. Theresults shows that with the pulse frequency varied，weld penetration and depth-to-widthratio appear to fluctuations and microstructure evolvement varied. Weld joint obtainedfrom pulse ultrasonic-TIG hybrid welding process is center melting form，and the fusionzone width is narrow. With the pulse frequency increases，weld penetration anddepth-to-width ratio decreases，at the same time equiaxed zone of the upper part ofjoints appears small dendritic crystal and the dendritic crystal number of borders ofwelding seam decreases but the size of cystiform-dendritic grows.Arc force model of ultrasonic-assisted TIG welding is proposed，analysis suggeststhat arc compression and the improvement of axial component of plasma jet force arecomposed of the main factor for molten pool to cause surface depression. Theobservation of morphology of austenite grain boundaries precipitates shows thatprecipitated phases were diffusely distributed within austenite grain inultrasonic-assisted TIG welding process，while they were enriched in austenite grain boundary in conventional TIG process. The introduction of ultrasonic vibration isbeneficial to improve intergranular corrosion resistance of joints.At last，microstructure and mechanical properties of5mm thick butt joint isstudied. the result indicates that arc was steady and shrank dramatically in pulseultrasonic-TIG hybrid welding process，which could realize full penetration in a singlepass. Meanwhile columnar crystal attached the partially molten zone completelyseparate from base metal，the size of that greatly decreases，and there exists regular andfine equiaxed grain in the upper part of the weld seam，hardness value of that haveimproved when compared with conventional pulse TIG welding.