Study On The Molten Pool Temperature And Forming Defects During Forming Al₂O₃ Ceramics By Laser Direct Material Deposition

Laser direct material deposition(LDMD)utilizes a laser beam to melt Al2O3 ceramics to rapidly fabricate three-dimensional ceramic part without dies.Although the production period is short and the technological process is simple,forming defects of the ceramic deposition seriously restrict the development of this technology.This paper studies the characteristics of molten pool temperature based on process parameters,and then analyzed the correlation between the melt pool temperature and forming defects such as surface size defects,pore defects and crack defects.Finally,it will provide theoretical support for the high quality forming of alumina ceramics by regulating the molten pool temperature.The molten pool temperature under different process parameters was monitored online by colorimetric pyrometer.The results showed that the feeding rate,scanning speed and spot diameter are negatively correlated with the molten pool temperature.And the laser power is directly related to the melt pool temperature,which can be adjusted rapidly and accurately by changing the laser power.The section shape of the single-layer forming parts is rounded from an arc to a circle with the thickness increasing.The heat accumulation in the acceleration stage of the scanning speed increases the width and thickness of the initial part of the single-layer forming parts,resulting in uneven width and thickness of the parts.The thin-walled parts are deposited with single-layer lifting line by line and layer by layer.The surface size defects of single-layer forming parts can lead to deviation of height.When the molten pool temperature is low,the thin-walled parts is shaped like a"saddle"—high in the middle and low in the both ends.On the contrary,when the temperature becomes high,it resembles a "trapezoid" in an opposite way.The molten pool temperature PID closed-loop control system is set up by colorimetric pyrometer,sbRIO-9627 embedded controller and LabVIEW.The optimal PID parameter which is obtained by manual tuning PID parameters is that proportional coefficient,integral time constant and differential time constant are 2.1,1,0 respectively.When the molten pool temperature is set as 2640 ?,a high accuracy has been achieved in shape and size of the thin-walled parts.The inner pore of the thin walled parts is divided into gas pores,interlayer pores and shrinkage cavity by optical microscope.The gas pores are the main pore defect,derived from carrying gas,shielding gas and pores in the powder particles.The porosity decreases by reducing the carrying gas flow and increasing the melt pool temperature properly,and the minimum porosity is about 1.4%.The crack defects of the thin walled parts are divided into macrocrack and microcrack,which are reduced with the melt pool temperature increasing.The correlation between molten pool temperature and forming defects is analyzed.The control of molten pool temperature is able to reduce forming defects.Therefore,the research has major theoretic value and wide application for forming high-quality Al2O3 ceramics by LDMD.