Analysis Of Plasma Electrical Signal In Laser Welding Process Of A304 Stainless Steel

A close attention has been paid to laser welding because of its unique advantages.In the process of laser welding,the real-time detection of various information such as light,sound and electricity,is an effective method to monitor the welding quality.In this paper,a passive electric probe signal acquisition system based on plasma sheath theory is used to detect the plasma electrical signal in the process of laser welding of A304 stainless steel.The electrical signals of different laser power,welding speed,shielding gas flow,gas nozzle location and gas type are detected,and the signal waveform,spectrum and weld formation are compared and analyzed.The signal waveforms under different conditions are analyzed,and the result shows that the mode of thermal conduction welding can be featured by the characteristic of the plasma electrical signal waveform.When a weld is of thermal conduction welding characteristic,the corresponding plasma electrical signal appears at 0V with certain probability.When a weld has the characteristic of deep penetration welding or has a cross section with "nail" shape,the corresponding plasma electrical signal does not appear in the vicinity of 0V,and fluctuates in a certain negative value with a certain magnitude.The plasma electrical signals of the weld with deep penetration and the plasma electrical signals of the weld with "nail" shape can not be distinguished from the electrical signal waveform characteristic.Although there is no characteristic frequency of the electrical signal in different welding mode,the characteristics of the electrical signal spectrum can be used to judge whether the welding process is thermal conduction welding model or not.When a weld has thermal conduction welding characteristic,the corresponding plasma electrical signal spectrum has very high amplitudes in the range of 0~500Hz,and the spectrum mainly distributes on this range.When a weld has the characteristic of deep penetration welding or has a "nail" shape,the corresponding plasma electrical signal spectrum is in a larger frequency range instead of in a narrow distribution.And it is difficult to distinguish deep penetration welding with “nail” shape welding from the electrical signal spectrum because they are similar.The spectrum intensity distribution analysis of electrical signal spectrum in a specific frequency range can be used to determine the welding mode.Under the experimental condition of this paper,the electrical signals' spectrum intensity E in the range of 500~1000Hz is calculated.When E is bigger than 19.52,the corresponding welding mode is deep penetration welding.When E is less than 15.19,the corresponding welding mode is non-deep penetration welding.In the process of non-deep penetration welding,the weld has obvious characteristic of thermal conduction welding or has “nail” shape since the plasma is not effectively suppressed.Thus,it is possible to qualitatively analyze whether the welding process is the thermal conduction welding mode or not by the characteristic of the electrical signal waveform or spectral distribution,and quantitatively determine whether the welding process is the deep penetration welding mode or not by the value of the electrical signal spectral intensity within a specific spectrum.This paper provides a new idea and method for judging laser welding mode.