Forming Characteristics And Dimensional Control Of Multi-layer Multi-bead Deposition Using Gas Metal Arc Additive Manufacturing

Wire and Arc Additive Manufacturing(WAAM)technology is an approach that uses electric arc as the heat source to melt fed wire.The setup for WAAM technology is relatively simple and the equipment can be assembled easily.WAAM is normally used to fabricate metal parts with moderate complexity and large sizes.According to the number of beads in a layer,the basic structure fabricated by WAAM can be divided into two classes,namely multi-layer single-bead(MLSB)structure,and multi-layer multi-bead(MLMB)structure.Recently,scientific research(e.g.path planning,process control)with regard to fabrication of MLSB structure have been significantly reported in the literature.In this paper,MLMB structure was focused,aiming at improving its fabrication accuracy.Therefore,the criteria for process planning,the forming characteristics,and the geometry control strategy for fabrication of various MLMB structures were investigated.Firstly,a robotic WAAM system was implemented,which included various hardware devices and a software system used for planning the manufacturing parameters.Using the implemented system,basic experiments for depositing single weld beads with regard to different manufacturing parameters were carried out.Based on the result of the basic experiments,the relation model between the manufacturing parameters(i.e.voltage,current and deposition speed)and the geometries(i.e.width and height)of bead was established,which forms a basis to predict either the manufacturing parameters of a bead with given geometries or the geometries of a bead deposited under a set of specific manufacturing parameters.Furthermore,in order to eliminate the forming defects of arc striking and extinguishing areas,a hybrid path pattern was applied to form a basic layer,which contains a contour path and several parallelly arranged straight paths.Accordingly,two types of beads-overlapping models were built,including a parallelly overlapping model and a T-shape overlapping model.The conducted validation experiments proved that the proposed overlapping models and deposition strategies enable fabrication of a closed layer with good surface quality.Then,the process models for deposition of various basic MLMB structures were carried out,including fabrication of cubic and inclined structures directly on a flat substrate,and deposition of MLMB structure within a supportive contour.It was found that there are materials shortage areas(MSAs)on the edges of components deposited on a substrate,which have a significant impact on the geometries of the components.Accordingly,solutions were proposed to solve the issues with regard to the MSAs.On the validation side,multiple cubic and inclined components were deposited.It was observed that the deposition order of beads in a layer is also an important factor that influences the formed geometries.In addition,in order to satisfy the requirements for repairing surface defects of metal parts,the relationship between the inclination angle of the supportive contour and the manufacturing parameters used for deposition was investigated.To realize a void-free filling,the inclination angle of the supportive contour should be kept within a given range.Both the upper limit and the lower limit were determined by the geometries of beads used for filling.The proposed process models were validated by various real-life cases.The experimental results showed that the models not only facilitate process automation but also fabrication accuracy.The maximum error of the main part of the repaired region was 0.3mm and the materials utilization rate was 97.3%.An algorithm was developed that enabled automatic measurement of geometries of weld beads from light stripes of a linear structured light sensor.Based on the algorithm,the forming characteristics of MLMB deposition were carried out.The law of geometry changes of beads was investigated with regard to the vertical direction and along the deposition direction.Furthermore,the collapse amount and influencing factors of a negative slope in an inclined structure were also investigated.To validate the forming characteristics,simulation software that represents the shape formed by MLMB deposition was implemented,in which all the built models were included.To prove the validity of the simulation software,also the forming characteristics of MLMB deposition,profiles of various components(e.g.cubic and inclined components)generated by simulation were compared to their actually deposited parts respectively.It was found that the simulations showed good representations of the actually deposited components.Furthermore,in order to ensure the geometries of MLMB deposition are the same as the designed model,an inter-layer controlling strategy was proposed based on fuzzy logic inference.After a layer was deposited,the deviation between the shaped height with regard to the set value was calculated,which formed the basis to infer the change of deposition speed of the bead at the same place but in the layer above.To satisfy the requirements of MLMB deposition,a strategy for processing the control parameters of a process with varying numbers of controlled variables was proposed.Then,the simulation software was applied to optimize the parameters of the fuzzy logic inference machine.To validate the proposed control strategy,various components were deposited.The experimental results indicated that the proposed strategy enabled near-net shaping of MLMB deposition.The formed component achieved high accuracy,good surface flatness.The maximum error of shaped height of a cuboid part was 0.20 mm,while that for an inclined part was 0.26 mm.Lastly,the proposed models and strategies were concerned as the main principles to fabricate a typically complex part that needs MLMB deposition.The target part had many geometric features(e.g.thick-walled,inclined and compositional structure)that this project is focusing.The whole AM process for fabricating the target part was specified,including slicing,path-planning,manufacturing parameters planning,etc.The proposed inter-layer control strategy was applied in the actual deposition process.After deposition,measurements were taken to enable geometrical evaluation.It was observed that the maximum error in height of the part was 0.79 mm,the minimum height error was-0.59 mm,and the mean absolute height error was 0.03 mm.The widths of all layers were bigger than the expected value.Therefore,the fabricated part was with good geometry accuracy and high surface quality,which achieved the design requirement.