Spectral Diagnosis On Weld Defects In Laser-Arc Hybrid Welding Of Al Alloys

High power laser-arc hybrid welding is one of the best fusion welding for aluminum(Al)alloy by integrating the advantages of both laser welding and arc welding.However,the weld defects easily occur when the welding parameters are not appropriate.Plasma spectrum diagnosis has the potential to online monitor the weld defects due to the advantages of strong anti-disturbance ability,plentiful information and high sensitivity,but few work have been addressed on this topic.In this paper,the plasma spectrum diagnosis for the weld defects in laser-arc hybrid welding of Al alloys was carried out.The results are listed as follows.The characteristic signals in the spectrum was extracted,and the redundancy were removed by using principal component analysis and threshold segmentation.It was found that the spectrum of laser-arc hybrid welding of Al alloys is mainly composed of two types of principal components,the first principal components,F1 containing 9 metal lines,and the second principal components,F2 containing 10 Ar I lines.Both of the F1 and the F2 contributed 92.6% of the original signal.The parameter of spectral intensity arithmetic mean deviation,Ia was developed to quantify the fluctuation of spectral signal,which was defined as the arithmetic mean of the absolute value of the distance between the point on the contour line along spectral intensity direction and the center line of the contour.The bigger the Ia value,the bigger the spectral fluctuation.The results showed that the spectral characteristic signal was effective to predict the surface defects.The F1 was closely related to the stability of weld pool,while the F2 is closely related to the stability of arc column.The quality of the weld surface met the first level standard when the Ia of F1 and F2 was less than 1×103 and 1×102 counts counts,respectively.Weld porosity was only related to the F1.It was resulted from the fact that the metallic vapor insider the keyhole and that ejecting from the keyhole causes the keyhole instability,and then the porosity within the weld.On the other hand,these behaviors are characterized by the fluctuation of the metallic vapor or the spectrum of metal lines rather than the Ar I lines.This result demonstrated the view that the porosity within laser weld mainly consisted of keyhole-induced process pores again.Moreover,the bigger the Ia of the F1,the bigger the porosity.The porosity met the first level standard when the Ia of the F1 was less than 8×102 counts.The spectral characteristic parameters including spectral intensity,electron temperature and electron density were consistent with the crack susceptibility and the burning rate of Mg element,all of which increased with increasing arc current,and decreased with increasing welding speed.This good consistency suggested that the spectral diagnosis would be effective to monitor the crack susceptibility and Mg burning rate.These phenomena were resulted from the fact that the higher the heat input,the stronger the metallic vapor.On the one hand,it increased the spectral characteristic parameters.On the other hand,it increased the the crack susceptibility and the burning rate of Mg element.