Acoustic Propagation Characteristics And Grain Refinement Mechanism Of Weld During Ultrasonic Assisted TIG Welding Of Aluminum Alloy

The ultrasonic can improve the mechanical properties of welded joint by refine the weld grain and reduce the weld porosity during ultrasonic assisted TIG welding process of aluminum alloy.However,existing researches mainly focus on the effect of ultrasonic on the welding quality of aluminum alloys.The mechanisms of the grain refinement and the porosity defects decrease have not been investigated in depth.The investigation on the influence mechanism of ultrasonic on the weld pool behavior of aluminum alloy is the theoretical basis to the improvement of ultrasonic assisted arc welding technology.Therefore,this dissertation investigates the influence mechanism of ultrasonic on grain refinement in TIG weld of aluminum alloy.At the same time,this dissertation also preliminary explores the influence of ultrasonic on the bubble behavior in the weld pool.The 2195 Al-Li alloy alloy was selected for the experimental material because 2195 Al-Li alloy owns a great application prospect in the aerospace field.Ultrasonics are introduced into the weld pool through the base material.A finite element model was established to investigate the distribution of ultrasonic field in the base material and the weld pool.The attenuation of ultrasonic vibration at the solid-liquid mixing zone was analyzed.The ultrasonic action on the weld pol is divided into three parts: First,the ultrasonic only acts on weld pool before solidification of weld pool;Second,the ultrasonic only acts on weld pool during solidification process of the weld pool;Third,the ultrasonic continuous action on weld pool during the whole process of welding.The grain size changes in TIG weld of Al-Li alloy with different ultrasonic action stages were studied respectively,and the ultrasonic assisted grain refinement mechanism in TIG weld of Al-Li alloy was revealed finally.The expression of heterogeneous nucleation rate was deduced.Finally,the effect of ultrasonic on bubble generation,growth and floating movement in the weld pool were analyzed theoretically,which can lay a foudation for the study of ultrasonic affect on stomatal defect in the weld.The acoustic propagation and acoustic field distribution in base material at room temperature were numerically simulated.The results show that ultrasonic propagates forward in the form of antisymmetrical Lamb waves,and the wavefront is spherical.A stable standing wave field distribution forms after the ultrasonic propagation stabilizes in the base material.The influence of temperature on the ultrasonic propagation in the base material was analyzed,and the acoustic propagation characteristic under the influence of non-uniform temperature field was revealed.The viscoelastic material model at the solid-liquid mixing zone of boundary was established and the attenuation of ultrasonic vibration was analyzed when ultrasound passes through the solid-liquid mixing zone.The acoustic-solid coupling model was established to reveal the acoustic pressure field distribution and influence factors in the weld pool.The results show that the pressure is the maximum at bottom of the weld pool and acoustic pressure gradually decreases from the bottom of the weld pool to the surface of the weld pool.The acoustic pressure in the weld pool is mainly related to the ultrasonic input amplitude and the size of the weld pool.The acoustic cavitation in the weld pool was studied,and the critical ultrasonic input amplitude for acoustic cavitation in the weld pool was obtained.The acoustic cavitation intensity in the weld pool is mainly related to the acoustic pressure amplitude.The ultrasonic is applied before extinguishing the arc under the fixed-point welding condition.The arc and ultrasonic are turned off at the same time.The results show that the grains in weld of Al-Li alloy changes from columnar to equiaxed grain after application of ultrasonic.The comparison experiment of 1A99 pure aluminum shows that the ultrasonic has obvious promoting effect on heterogeneous nucleation in the TIG weld pool of Al-Li alloy.The ultrasonic can no promote the homogeneous nucleation in weld pool.The mechanism of ultrasonic on the nucleation of grain was analyzed.It is shown that ultrasonic could increase the vibration frequency of atoms.The cavitation can increase the solidification temperature and improve the wettability of particle resulting in promoting the heterogeneous nucleation.The acoustic streaming promotes the dispersion of crystal nucleus in weld pool.Finally,the formation mechanism of grain structure in TIG weld of Al-Li alloy with utrasonic before solidification of weld pool was analyzed and experimental verification was carried out.The ultrasonic promotes the crystallization of Al3 Zr in weld pool resulted in an increasing of the heterogeneous nucleation rate.The ultrasonic is applied during solidifacation of weld pool under the fixed-point welding condition.There is a narrow grain breakage zone at the edge of the TIG weld of Al-Li alloy and most grain of the weld is the equiaxed grain after application of ultrasonic.The effect of ultrasonic on grain fracture is investigated by using 1A99 pure aluminum.The grain fracture is mainly caused by ultrasonic cavitation.The solidification rate of the weld pool and the cavitation strength in the weld pool have a great influence on the degree of grain fracture.The internal equiaxed grain zone of the weld is mianly caused by ultrasonic promoting heterogeneous nucleation.The ultrasonic is applied during the whole welding process under the fixed-point welding condition.There is no more grain breakage zone at the edge of the weld,all grains are the equiaxed grain after application of ultrasonic.The grain refinement characteristic is the same as that with the ultrasonic effect before solidification of weld pool,which indicating that the grain refinement is resulted from the ultrasonic promoting heterogeneous nucleatiion instead of ultrasonic breaking grain.The grain refinement mechanism of the Al-Li alloy TIG weld under the ultrasonic arc composite field is proposed.The different of ultrasonic refining grain mechanism between the continuous welding and fixed position welding is analyzed.The formula of heterogeneous nucleation rate in the TIG weld pool of Al-Li alloy with ultrasonic is deduced.Heterogeneous nucleation rate formula is a function of acoustic pressure and welding current in the weld pool.Finally,the effect of ultrasonic on bubble generation,growth and floating movement in the weld pool is analyzed theoretically.The results show that ultrasonic can promote the generation,growth and folating of bubbles.The greater the transient cavitation intenstiy,the more bubbles.