Research On Coupling Phenomenon Between Molten Pool And Powder In Laser Additive Manufacturing Process Of Titanium Alloy

Laser additive manufacturing technology for manufacturing,parts repair,coating and metal rapid prototyping,in this process,involves the interaction of laser and metal powder,laser-substrate interaction,interface evolution,and fusion interaction.It is more important to fully understand the coupling between the laser and the powder.The dynamic characteristics of the titanium alloy powder on the surface of the molten pool and the melting state of the titanium alloy powder in the molten pool directly affect the appearance,defects,and organization of the molded part.Many scholars have conducted numerical simulations of the flow characteristics of molten pools and powder particles added by coaxial feeding powder.However,few scholars currently use experimental methods to precisely describe the flow characteristics of the titanium alloy powder on the surface of the molten pool and to study the melting process of the titanium alloy powder on the surface of the molten pool.The main content of this thesis is the coupling phenomenon of molten pool and titanium alloy powder in the laser additive manufacturing process of titanium alloy.The flow rate of the titanium alloy powder on the surface of the molten pool is analyzed.The melting time of the titanium alloy powder on the surface of the molten pool and the area of the molten pool unit area are analyzed.The phenomenon of coupling between the molten pool and the powder affects the utilization of the powder and the final properties of the molded part.Firstly,this thesis builds a binocular vision system to collect real-time images of the molten pool in the laser additive manufacturing process.The two high-speed camera(MEMRECAM HX-7 and FASTCAM SA4)are installed with the trigger switch,which is used to synchronize the two high-speed cameras.And set the resolution,exposure time,and frame rate of the two high-speed cameras to the same.At the same time,adjust the relative position of the two high-speed cameras to ensure that less disruptive images of the molten pool can be acquired.In addition,dimensional calibration and checkerboard calibration are performed on two high-speed cameras.The purpose of the calibration is to determine the size of the molten pool.The purpose of the checkerboard calibration is to determine the internal and external parameters of two high-speed cameras and the relative positions of two high-speed cameras.Secondly,this thesis deals with the image of the molten pool collected by two high-speed cameras,and extracts the area,length,height and the surface characteristics of the molten pool through a series of mathematical image processing techniques such as image cutting,filtering and threshold segmentation.The average melting time of the powder on the surface of the molten pool and the number of powder particles in the molten pool are counted.At the same time,the powder particles on the surface of the molten pool were extracted from the images collected by two high-speed cameras into the molten pool,and the three-dimensional reconstruction of the powder particles was performed to determine the three-dimensional coordinates of the powder particles.The particle filter tracking algorithm is used to accurately and rapidly track the particles on the surface of the molten pool,and the speed of the particles on the surface of the molten pool is calculated.Finally,in thesis studies the effects of feed rate,laser power and scanning speed on the flow rate of metal powder,the melting time of metal powder and the number of powder particles in the molten pool.The melting process of the metal powders in the molten pool is mainly divided into four types:the powder is directly melted into the molten pool,the powder is melted in a fuzzy area,the powder is melted at the solid-liquid interface,and the powder is not completely integrated with the molten pool.