Forming Mechanism And Experimental Study On Oxide Ceramics Manufactured By Selective Laser Melting

Ceramic materials have a wide range of applications due to their excellent performance.however;it is difficult to rapidly manufacture personalized,lightweight and complex products by traditional ceramic processing methods,thus limiting their applications in various fields such as weapons,aviation and aerospace technology.As a scientifically significant additive manufacturing technology,Selective Laser Melting(SLM)is believed to be an alternate replacement that can overcome many of the deficiencies of traditional ceramic processing methods which can offer great potential for application in aviation,aerospace and weapons.In this dissertation,with the aim of establishing the relationship between the characteristics of the molten pool,and the microstructure,and defect,and obtaining the formation mechanism of defects and solidification structure.From the perspective of the molten pool,a ceramic SLM equipment and a molten pool monitoring system were designed.Furthermore,forming process,and the formation mechanism of processing defects and solidification structures were studied in details.The main highlights of the work are described below:(1)A new manufacturing technique based on slurry by selective laser melting is proposed.The influence of process parameters on the surface morphology was analyzed.The denudation of powders was studied.Based on Bernard-Marangoni convection theory,the formation mechanism of streak and cell solidification on the surface of the Al2O3 parts wasexplained.The correlation diagram between streak convection and laser parameters(i.e.laser power and scanning speed etc.)were established,and the process window was determined.Our analysis have revealed that the pressure difference between high temperature water vapor and the environment will inhibit the powder from splashing.When the scan speed is increased from 60 mm/s to 120 mm/s,and the laser power is increased from 140W to 200W,the strip convection will happen.Al2O3 slurry can be completely melted with laser power of 200W and scanning speed of 90 mm/s,which can obtain a part with no prominent cracks.The Vickers hardness of the part made by SLM is about 14.7 GPa,which is consistent with the traditional sintered parts.(2)A model based on temperature field simulation of the selective laser melting process is successfully constructed.The maximum temperature,the liquid lifetime,the dimensions and the temperature gradient of the molten pool were explored.The simulation data shows that the higher laser power,the higher the maximum temperature of the molten pool and the increase maximum temperature rate.When the laser power is 200W,it can reach 3267.79 K and 4.90×106 K/s.By increasing the scanning speed,the extremism of cooling rate and temperature growing rate increases,while the maximum temperature of the molten pool decreases.As the laser power increases or the scan rate decreases,the size of the molten pool gradually increases.It is shown that the numerical simulation approach can effectively predict the influence of laser parameters on the melting state of alumina powder.It is considered that the temperature gradient is the root cause of a slight different between the stripy solidification structures.Preliminary results show that the growth direction of columnar crystals can be adjusted by changing the scanning speed.(3)A method of photodiode multi-detector divisional detection was proposed.We have developed a software/hardware system to monitor the molten pool light intensity.Defects such as edge effects,cracks,and pores in SLM of ceramic parts were analyzed by building a photodiode signal field.Experimental verification and theoretical calculations show that photodiode multi-detector divisional detection method can effectively improve the detection accuracy.Apply the above laws to find that when the laser power is 260W,the fluctuation range of the photodiode signal value is the largest(about 1.0V-1.4V);when the laser power is 100W,the diode signal is the most stable(about 0.44V-0.57V).(4)The main causes of crack formation and the underlying mechanism of crack propagation are analyzed.The phenomenon of deflection of solid layer was explained.The experimental results show that the samples possess predominantly vertical cracks,parallel cracks and oblique cracks.The analysis affirm that parallel cracks are mainly generated by solidification cracks,and the continuous expansion of tensile stress cracks is the main reason for the formation of vertical cracks and oblique cracks.The crack suppression experiment found that pre-heating,process parameters and defects all have an influence on the crack growth and propagation.