Research On Process Monitoring And Forming Detection Method Of Multi-layer Single-pass GMAW Additive Manufacturing

Arc additive manufacturing is a technology,using welding arc as a heat source,metal welding wire as a deposition material,layered and formed in the form of droplet transfer.In the deposition process,due to the complex process parameters,the difficulty of controlling the transfer mode of the droplets,and the heat accumulation between the layers,it is difficult for the arc additive manufacturing to meet the forming quality requirements.How to realize the process monitoring and forming detection of the arc additive deposition has become a hotspot and difficulty nowadays.Therefore,this paper takes multi-layer single-pass gas metal arc welding(GMAW)additive manufacturing as the subject investigated,and the paper carries out related research work on the process monitoring and forming detection methods of arc additive deposition.First of all,the paper establishes a set of process monitoring and forming detection system for arc additive deposition with the aid of current sensor,temperature sensor and laser vision sensor.According to the principle of triangulation,the laser vision sensing system is calibrated to establish a conversion model between image coordinates and world coordinates.The accuracy of the three-dimensional reconstruction reaches 0.2 mm by linear calibration method of sawtooth target.Based on the analysis and research of the the laser stripe image processing method of the sawtooth target,according to the gray distribution characteristics of the laser stripe,a method of adaptive Region of Interest(ROI)extraction is proposed.In order to preserve the edge details of the image,median filtering is used to reduce the noise of the original image,and on the basis of the method of grayscale center an adaptive threshold grayscale center method based on the maximum value is proposed to achieve accurate extraction of the laser stripe centerline.From the viewpoint of two-dimensional image and three-dimensional point cloud,the three-dimensional measurement method of the multi-layer single-pass arc additive surface is studied.An adaptive ROI extraction method based on boundary constraints is proposed to obtain the ROI area,which only contains the surface characteristic curve of the deposition layer.and then the three-dimensional surface characteristics of the deposition layer are directly obtained through coordinate transformation.The 3D discrete points are fitted by Delaunay triangulation to obtain the three-dimensional solid model of the deposition layer surface.For the three-dimensional point cloud including the surface characteristics of the substrate,the moving platform and the deposition layer,the random sampling consistency(RANSAC)algorithm and the K nearest neighbor(KNN)algorithm are used to eliminate irrelevant interference information,and the surface formation characteristic information of deposition layer is quantified.The distance between the welding wire and the surface of the deposition layer was changed to simulate the change in arc length.During the arc additive deposition process,the arc current stability and the temperature change of the deposition layer were monitored.Then the surface forming characteristics of the multi-layer single-pass deposition layer were analyzed.The K-type thermocouple records the temperature change of the arc additive deposition process,and the current sensor collects the arc current.The empirical mode decomposition is used to decompose the collected current signal into multiple intrinsic mode functions.The empirical mode decomposition energy entropy under different welding conditions is calculated.Through the laser vision sensing system,the distribution of the interlayer on the surface of the accumulation layer,the surface roughness and the change in the deposition height are obtained.The results show that the relatively stable current state can make the interlayer distribution on the surface of the deposition layer uniform and the surface roughness decreased.Unstable current conditions can lead to increased deposition layer height and surface roughness.To further explore the cause of interlayer heat accumulation and the direction of thermal stress,and analyze the effect of heat accumulation on the surface formation of the deposition layer,Simufact welding simulation software is used to establish a multi-layer single-pass arc additive deposition simulation model.The results show that the multi-layer single-pass arc additive deposition process will produce a special heat accumulation effect.The vertical direction shows that the temperature of the middle layer is higher than that of the first and last layers.And the horizontal direction shows that temperature is higher near end of arc.The generation of thermal stress causes the closer the surface of the deposition layer to the end of the arc,the greater the deformation.