Measurements Of GTAW Weld Penetration Based On Temperature Field Analytic Model

Weld penetration control is a major challenge in automatic/intelligent welding process.However,the penetration cannot be directly/conventionally measurable during the gas tungsten arc welding(GTAW)process.In this dissertation,we proposed a method to calculate the penetration by using analytic model calibrated by the in-situ measurements from 3D weld pool measure system.The verification experiments verified that this method can compute the accurate penetration.The 3D weld pool surface measure system and welding system is built at first.Because the weld pool has a mirror like surface and can reflect light according to specular reflection.A laser dot matrix pattern is projected into weld pool and intercepted by the imaging plate.The reflected pattern contains the information of the 3D weld pool,extracted it by image processing and reconstruction method.Then the welding system is designed to meet the requirements of experiments.The analytic model based on single-ellipsoid heat source is used to calculate the temperature distribution.A calibration method is proposed to calibrate the temperature distribution by in-stiu measurements.The calibration is conducted in x-y direction first to eliminate the error between weld pool length and width;then use the elevated volume to reduce the error in z direction.After these two calibrations,the penetration is almost same as calibrated and measured from cross-section,the error is only 3.9%.A mathematic model is built to define the accuracy ability of the analytic model with different heat source parameters.With this model,the accuracy ability can be forecasted by prior width error and prior elevated volume error.Since the prior width error and prior elevated volume error both can calculate in real-time,the mathematic model can also calculate the accuracy ability in real-time.As the accuracy ability of analytic model is directly determined by heat source parameters,a model related with calculated width and elevated volume is proposed to compute the optimize heat source parameters.The robustness of this model is verified by using the optimize parameters in 60 A to calculate the case of 65 A and 70 A.Hence,the fixed heat source parameters can be used in the range of 60 A to 70 A.However,the analytic model is constrained by its strict requirements and slow calculation in real control system.The deduction of analytic model is analyzed to discover a more acceptable transformation as changing current during calculation,the temperature distribution is not even and reducing the computing time.Fortunately,the analytic model can separate as an earlier temperature distribution history and the newly applied arc energy.A recursive analytic weld pool model is built in two different time intervals to solve the constrains.Because the earlier temperature distribution used in the recursive analytic weld pool model is composed by discrete points.An analytic description is proposed to fit it.With the analytic description,its convolution with the heat source distribution,given analytically,can be analytically solved to minimize the need computation.Experiments are conducted to verify the accuracy of the recursive analytic weld pool model and the fitting model.The penetration from the cross-section shows an ideal result.At last,the accuracy of the model with an uneven temperature distribution is verified.A step-by-step moving process is proposed to verify this model.The results give a good consistent matching in penetration.The influence of the model is also studied,it is shown that the calibration times and the fitting model will determine the accuracy significantly.