Study Of The Effect Of Droplet Oscillation Momentum On The Surface Forming Quality Of Wire And Arc Additive Manufacturing

Wire and Arc Additive Manufacturing(WAAM)can achieve low-cost,efficient and rapid near-net forming of large-size complex metal components.However,the surface quality of WAAM formed parts is difficult to achieve the desired accuracy and application standards,which severely limits its application and development in the manufacturing field.Currently,optimizing surface forming quality by feedback control has become a hot topic of current research.However,considering that feedback control requires too much sensitivity and accuracy of the process monitoring system,it is expected to provide a way to solve the problem if timely adjustments(feed-forward control)are made before deviations occur.Therefore,in order to obtain the characteristic parameters suitable for feed-forward control,this paper takes the droplet oscillation momentum(DOM)as a breakthrough point to carry out research related to WAAM process monitoring and surface forming quality optimization,aiming to analyze the correlation mechanism between welding current,DOM and surface forming quality,and provide a DOM-based"Feed-forward + feedback" control strategy and ideas for WAAM surface forming quality optimization.feedback" control strategy and ideas.Specifically,the following research work was conducted:Firstly,we analyzed the force on the hanging droplet and the effect of droplet oscillation on the geometric deformation of the melt pool,and studied the droplet oscillation momentum.Based on the "mass-spring" model,a theoretical analysis of the droplet oscillation momentum was carried out,and a mathematical model of the droplet oscillation momentum was established.At the same time,the spring and damping coefficients were corrected by combining high-speed camera technology.In addition,the droplet displacement and velocity were corrected by considering the contribution of the wire motion to the droplet displacement,and the calculation procedure of the droplet oscillation momentum model was established.For the problem that there is strong arc light in WAAM deposition process and the droplet cannot be directly observed,a visual sensing system based on high-speed camera technology is designed and built independently.The system fully considers the characteristics of arc light and integrates a COMS high-speed camera and a composite filter system to monitor the droplet transition behavior in real time during the manufacturing process.The experimental results of droplet oscillation momentum model evaluation verify the effectiveness of the visual sensing system designed in this paper.For the problem that the droplet oscillation momentum is difficult to calculate quickly,a droplet feature extraction program is developed.The program integrates image processing techniques such as median filtering,LOG edge detection,and Region-props geometric feature extraction.It can directly output parameters such as axis length,area,and center-of-mass coordinates of droplets,and secondly calculate output volume,displacement,velocity and droplet oscillation momentum.The evaluation results of the droplet feature extraction program verify the effectiveness of the program developed in this paper.For the problem that the surface quality of WAAM formed parts is difficult to be analyzed qualitatively and quantitatively,the surface profile measurement technique of objects based on 3D point cloud technology is studied,and the WAAM surface forming quality evaluation system is established.The principle is to calculate the error distance(C-M distance)between the scanned point cloud of the formed part and the reference surface by the iterative closest point(ICP)algorithm,and characterize the surface forming quality of the thin-walled part by the average error distance.The results of the part surface forming quality analysis verify the effectiveness of the surface forming quality evaluation method in this paper.For the problem of WAAM surface forming quality evolution mechanism,the molten pool fluctuation behavior was studied.Combining theoretical analysis and high-speed visual sensing technology,the intrinsic mechanism of surface forming of WAAM parts is discussed.The experimental results of forming mechanism analysis show that the droplet oscillation momentum has a significant effect on the stability of the molten pool,and a larger droplet oscillation momentum can increase the molten pool fluctuation period and intensify the unevenness of the molten pool fluctuation amplitude.For the accuracy of the DOM model,the accuracy and application value of the DOM model were verified and evaluated by experimentally measuring the droplet oscillation momentum and the surface quality of the WAAM molded parts.the experimental results of the DOM model evaluation showed that the overall trends of the model predictions and experimental values were consistent and could provide guidance for the WAAM process.Finally,the evolution mechanism of WAAM surface forming based on the research line of "current-DOM-molten pool fluctuation distance-surface forming quality" is elucidated.It is shown that in the current range of 120A-180 A,a smaller current corresponds to a smaller DOM,and a smaller DOM can reduce the unevenness of the molten pool fluctuation distance,thus obtaining a better surface forming quality.The above study proves that DOM can be used as a characteristic parameter to characterize the surface forming quality of WAAM,and it can be used as a feed-forward control object to optimize the surface forming quality through the "Feed-fo rward+feedback" method.