Analysis Of Weld Pool Oscillation In GTAW Based On Laser Vision

Weld pool oscillation is a basic phenomenon of free vibration of the pool surface during weldi ng process;it contains abundant information related to the weld pool.Reliable measurement of the signal of weld pool surface oscillation is of great importance in understanding th e coupling interaction between the arc and weld pool and the penetration st atus and surface tension of molten metal in the weld pool.However,using conventional methods,such as arc light and arc voltage,to measure the oscillation signal in situ during gas tungsten arc(GTA)welding process is challenging due to the small ampli tude of pool surface mo vement.To overcome the disadvantages of poor signal-to-noise ratio(SNR),poor robustness and one-dimension oscillation signal of traditional methods(arc voltage and arc light method),a novel laser vision method was proposed to measure the three-di mensional oscillation of the weld pool surface during GTA welding proces s.The oscillation behavior of the weld pool surface during stationary and traveling GTA welding processes was studied.On the basis of the characteristics of pool su rface oscillation and the principle of laser vision,a sensing s ystem was established to measure the reflected laser line images and electrical signal during welding process,and the corresponding software control program based on Lab VIEW was also developed.The control and coordination of current wav eform were also achieved using this control s ystem.On the basis of the dyn amic variation characteristics of the reflected laser line images,two robust image processing al gorithms,namel y,brightness value an d image centroid algorithms,were proposed to ex tract the oscillation frequency and oscillation amplitude from the reflected laser line images,respectivel y.Th e effectiveness and robustness of the algorithms were stu died.The results show that the oscilla tion signal ex tracted b y the brightness value algori thm has high SNR and strong repeatabilit y,whereas the oscillation amplitude extracted b y the image centroid algorithm can considerabl y amplify the small amplitude oscillation of the weld pool surface,wh ich can b e used to represent the actual oscillation amplitude of the weld pool surface.Furthermore,the oscillation behavior of the weld pool surface in stationary GTA welding process was studied.The results show the three different oscillation behaviors,namel y,partial,critical,and full penetration.A single oscillation frequency occurred in partial and full penetrations.In partial penetration,the oscillation frequency is relativel y high,whereas the oscillation amplitude is relativel y low.The osci llation mode of its weld pool surface can be approximated b y the o scillation of free-surface standing wave.In full penet ration,the oscillation frequenc y is rel ativel y low,whereas the oscillation amplitude is relativel y high.The oscillation mode of its weld pool surface can be approx imated b y the oscillation of two-dimensional thin membrane.An abrupt transition occurred in oscillation frequency and amplitude from partial to full penetration.However,in critical penetration,pool oscillation showed dist inct boundary,with partial and full penetration features.Multiple oscillation frequencies occurred in the frequ ency domain.The free-surface oscillation is basicall y the s ynthetic waves of transverse and longitudinal waves.Using the characteristic of os cillation frequency in critical penetration,a control algorithm and s ystem has been established to sense and control the critical penetration in step GTA welding process i n real time.On the b asis of the free-surface standing and two-dimensional membrane wave theories,the mathematical modes of weld pool oscillation for partial and full penetrations were proposed,respectivel y.A good agreement was proven between the size of the weld pool and oscillation frequency.The oscillation behavior of the weld poo l surface in traveling welding process was also studied.The influence of welding speed on the oscillation behavior in partial penetration was studied b y the several ex periments.The results show that the offset between the el ectrode axis and cent erline of the weld pool graduall y increased with the increase in the welding speed.With the increase in offset,the oscillation mode of pool surface graduall y changes from s ymmetrical mode to as ym metric slosh mode.In as ymmetri c slosh mode,the oscillation frequen cy in partial penetration is considerabl y lower than the s ymm etrical mode and close to the frequenc y range of full penetration.The slosh mode in traveling welding p rocess should be precluded due to the inference on charact eristic frequenc y fro m partial to critical and full penetration.On the basis of the anal ytical mathematical model of three-dimensional transient temperature field of the weld pool,the occurrence of critical welding speed and offset for slosh mode were simulated.Under the range of criti cal welding speed,the oscillation behavior of the weld pool was studied.The results show that the characteristic of oscillation frequency,oscillation amplitude,and oscillation mode in traveling welding process is basicall y in agreement with the station ary welding process.The characteristic of oscillation frequenc y in different penetration levels can be used to sense and control the penetration status in traveling welding process in real time.On the basis of the relationship between the oscillation fr equency and size of the weld pool and the surface tension of the molten metal in the weld pool,a novel surface tension measurem ent method was pro posed.The surface tension of molten metal in the weld pool with and without the activating flux es(i.e.,ox id e and fluoride)was measured,and the mechanisms for A-TIG welding,which increased the penetration depth,were anal yzed.The results show that the oxides can chan ge the temperature coefficient of the surface tension and Marangoni convection in the weld pool.Ox ygen released from the decomposition of ox ides is the dominant factor that contributes to the change of the surface tension.However,ox ygen of small amount or large amount in the weld pool has negligible effect on the sign change of the temperature coefficient.Fluorides can greatl y decrease the absolute value of the s urface tension but have minimal effect on the temperat ure coefficient of surface tension.The surface tensio n and the penetration depth in the welding are affected b y the increase in th e arc temperature induced b y the fluorides.