Research On Closed-loop Control Of Variable Width Scanning Forming In Inside-laser Powder Feeding

In engineering,the variable wall thickness structures using traditional forming method have multiple procedure and complex process.While laser cladding forming has low forming efficiency and accuracy in thick wall by multi-track overlapping method.The work light spot can be continuously changed through altering defocus by the hollow laser beam internal powder feeding technology and nozzle.The different width layer can be formed by a single scan.The technology was used to form variable wall thickness with higher efficiency and accuracy to avoid overlapping and reduce repeated heating.In this paper,the laser cladding forming system with IPG fiber laser,KUKA robot,GTV powder feeder,hollow laser nozzle and layer height control software are constructed.And the energy distribution and the coupling characteristics of hollow laser spot were studied.Trough single factor wall accumulating experiment,the influence of parameters such as defocus,laser power,scanning speed and feeding rate were studied.The results show that the feeding rate was easy to be interfered,which was not used as a variable.The width of layer was mainly affected by defocus.And scanning speed mainly affects height of layer.Forming the variable wall thickness structures was studied.In this paper,defocus was used to control width of the variable width layer and scanning speed was used to control height of the variable width layer.For the problem of the variable wall thickness structures cannot be directly formed by single factor variable,the piecewise splicing technology was proposed in this paper.The variable wall thickness structures were divided into segments.Assuming equal wall thickness per segment,variable wall thickness structures were gradually approximated to be formed with corresponding parameters.Due to the self-healing effect in laser cladding forming,the defocus of the self-healing interval was selected for forming the structures.The actual height of layer was measured by the layer height control software based on machine vision.Due to variable defocus in forming process,the calculation model of actual height of layer was established.The layer height control software provides height feedback for laser cladding forming.The speed PI controller model based on actual height was established.The proportion coefficient and the integral coefficient were selected rationally.The actual height was compared with expected height to adjust the scanning speed of per segment based on PI controller.The straight wall experiment was designed to test the stability of the speed PI controller.The result shows that the forming system with close-loop control has high stability.The short transition between each segment was set.And the speed control model for transition was designed to solve the transition point bulge effectively.Four structural models of variable wall thickness straight wall,variable thickness eccentric ring,variable wall thickness blade and variable wall thickness twisted blade were designed.Combined with the piecewise splicing technology,the scanning path of variable wall thickness structures was planned.According to actual wall thickness,the four structures were formed successfully using speed controller with suitable initial parameters for each segment.The height curves of variable wall thickness straight wall and variable thickness eccentric ring can fluctuate slightly around the desired height in a steady and convergence condition when the proportion coefficient was-0.003(1/s)and the integral coefficient was-0.00035(1/s).According to the height curve of variable wall thickness blade,the experiment parameters of variable wall thickness twisted blade were adjusted.Finally,the variable wall thickness blade with high accuracy was formed,and the speed controller had good control effect and high accuracy.The microstructure and mechanical properties of the eccentric ring were analyzed.The crystals in different thick wall with similar crystal size were uniform and dense in the fine crystal range.The hardness value fluctuated within the range of 550HV~700HV.And the ring had good mechanical properties.