Study On The Behavior Characteristics Of Laser Plasma During Forming Alumina Ceramics By Laser Direct Material Deposition

Alumina ceramics have high melting point,high strength and excellent hardness,abrasion resistance,corrosion resistance and heat resistance and insulation,and chemical properties is stable in complex environment,therefor Alumina ceramics are widely used in aerospace,mechanical industry and other fields.Defects such as porosity,crack and unstable morphology are easy to appear in the parts made by laser direct material(LDMD)desposition of alumina ceramics.There is a close relationship between the plasma in the process and the stability of the desposition process.First,the plume morphology of plasma/ceramic vapor was collected by high-speed photography,and the plume area and height of the center of mass were extracted by image processing.The variation rules of plume area and height of center of mass under the condition of variable power and variable defocusing amount were studied,and the fluctuation of plume area in time domain was analyzed.It was found that the intensity of plume was stronger than metal material,which was related to the oxygen element in alumina.With the increase of power,the intensity of plume gradually increased,and with the increase of absolute value of defocusing amount,the intensity of plume gradually decreased.There is a certain periodicity of plasma plume characteristics in laser direct desposition process.The "powder blocking phenomenon" in the despositiong process is related to vapor eruption.Secondly,the spectral characteristics of plasma are studied.The plasma spectra were obtained by desposition system and spectrum acquisition system.It is found that the plasma phenomenon in the desposition process of alumina ceramics is more violent than that of metal materials.The spectral lines are mainly in the ultraviolet region and visible region.In the power range of 340-900 w,with the increase of power,the intensity of characteristic spectral line keeps increasing,and the electron density keeps increasing.Within the range of the powder feeding rate of 3.5-5.5g/min,the intensity of characteristic spectral line decreases continuously and the electron density increases continuously.Within the range of 280-440mm/min,as the scanning speed increases,the intensity of characteristic spectral line and electron density increase continuously.The spectral intensity of the characteristic spectral line fluctuates periodically in the time domain and is correlated with the fluctuation frequency of the molten pool temperat ure and the correlation coefficient is 0.729,which proves that plasma can influence the molten pool temperature.Finally,the relationship between the morphological features,microdefects with plasma characteristic parameters is analyzed.The morphologica features include cladding width and cladding height;microdefects include pores and cracks.The correlation between defect and plasma was analyzed with process parameters as reference.Plasma is an important factor affecting defects.In the case of variable po wer and variable scanning speed,the cladding width,cladding height,porosity and crack have a certain correlation with plasma electron density and spectral intensity,but there is no obvious correlation with powder feeding rate.Therefore,the desposition defects of thin-wall parts can be monitored by analyzing the characteristics of plasma.By analyzing the plasma system of alumina ceramics,this paper provides a new idea for the quality control of desposition process using plasma in the future and a new method for improving the forming quality.