Study On In Situ Monitoring And Behavior Of Defects During Direct Laser Deposition Process

Direct laser technology(DLD),one branch of AM,has been widely used in the components production for its advantages in almost net shape and rapid manufacturing.Due to its complex metallurgical reaction and non-equilibrium solidification characteristics during laser direct deposition,defects are generated with improper process parameters,which greatly reducing the performance of the formed parts.Therefore,studying the in situ monitoring system during direct laser deposition process is of high practical significance and value to guarantee the quality of deposition and to increase the production efficiency.The aim of this study is to establish the in-situ monitoring of molten pool and deposition defects during DLD alloy steel process,adopting high accuracy laser displacement sensor(2 μm)and high speed camera dual test system to study the methods and mechanism of in-situ monitoring molten pool and deposition morphology,using macro and micro representations methods to study the relationship between defects such as porosities,cracks,inclusions and the laser depositing parameters.The experiment results show that:Laser displacement sensor has the ability to detect the morphology of deposition with high accuracy,the quadratic function relationship between the scanning speed and cross sectional morphology of deposition can be acquired through data fitting.With the increasing of scanning speed,the width of deposition kept stable while the height of deposition reduced,which lead to lower deposition efficiency,and the surface defects reduced with a relative stable process.The method of identification deposition surface morphology defects was established either by using cloud points reconstruction method,which can clarify the rule of defects and molten pool evolution.The study also clarified the evolution of deposition inner defects and the formation mechanism of defects,the energy area density(EAD)during DLD process was optimized to acquire the deposition with the lowest content of defects,the relative density of samples prepared in this parameters can reach 99.24%;With the increase of EAD,the content of inclusion defects reduced while the crack ratio increased,it was also found that the inclusion defects and thermal stress contributed to the formation of crack defects.Dual test system established in this study and the methods of identifying the defects of surface morphology of deposition and molten pool can established a good foundation for in-situ monitoring and control system during DLD process.