Formation Mechanism And Control Of Porosity During Laser Direct Forming Of Al₂O₃-based Ceramic

Ceramic materials,as the third largest material in the world,are widely used in aerospace,biomedical and basic industries due to their excellent physical and chemical properties.Due to the complicated preparation,mold production,large product shrinkage ratio and the addition of binders,the traditional process of ceramic parts has resulted in low yield and high cost of complex ceramic custom parts.Laser direct forming as a kind of additive manufacturing,which is directly formed by layer-by-layer.Compared with the traditional process,the process of laser direct forming reduces the mold manufacturing and the adding binder,improves the preparation efficiency.This paper studies the alumina-based ceramic single-channel multilayer ceramic structural by using the laser direct forming technology.The formation mechanism of pore defects in the ceramic forming part is analyzed and the pore form model is constructed.The different control method of pore defect are studied.Based on the research foundation of pore defect,the ceramic plunger in blood analyzer is fabricated.The ceramic parts initially meet the appl ication requirement of plunger.The specific research contents and results are as follows:(1)According to the characteristics of laser direct deposition,the interstitial pore,intralayer pore,intercrystalline pore and shrinkage pore of ceramic forming parts were studied.The pore defects of typical ceramic part were classified.Based on the shape characteristic of ceramic part,it was found that the morph ology and distribution characteristics of pore for the three typical ceramic parts were related to the shape characteristics and forming methods of ceramic parts.Based on the property characteristics of ceramic materials and the alumina/zirconia binary phase diagram,pore characteristics and microstructure of ceramic part were analyzed.Pore characteristic in the different regions of ceramic formation part was analyzed.The microstructure evolution and pore distribution characteristics from hypoeutectic,eutectic to hypereutectic ceramic materials were studied.The relationship between the microstructure charqacteristic of the formation parts and the porosity defects was analyzed.It was found that the porosity defects can be reduced by increasing the proportion of zirconia.(2)Accorind to the solidification theory and bubble dynamics,the bubble escape factor was constructed as the basis for the evaluation of pore formation.Based on this,a pore formation model at different regions of different Marangoni convection modes were established.The formation mechanism of the pores was illustrated by the pore formation model.The pore formation model was analyzed by experimental methods.The results show that the upper part of the central region of the molten pool in the outflow mode was easy to form aggregated pore.In the inflow mode,the lower part of the central of the molten pool is easy to form aggregated pores.(3)Based on the pore formation mechanism,the porosity of formation part was analyzed by the process parameter and the process dynamic parameters.The influence of different process parameters on porosity was analyzed by the single factor test.The effect of pore area interval on the porosity was discussed.Lastly,the process parameters were optimized by the Taguchi method.The relationship between molten pool temperature and plasma plume height and porosity was established by analyzing the influence of process parameters such as molten pool temperature and plasma on pore defects.The results show that the number of small pores(0-1000?m~2)in the molded part was the largest.There is a weak negative correlation between molten pool temperature and porosity.The plasma centroid height and porosity are highly positively correlated.(4)The mechanical properties of three structural parts of single-layer,multi-layer and plunger were analyzed.The forming part can be divided into a high porosity area and a low porosity area.In the low porosity region of formation part,the multi-layer structural have the highest hardness of 1500Hv.In the high porosity region of formation part,ceramic plunger pump have the highest hardness.Through the test of the compressive strength of ceramic different structure,the compressive strength of the multi-layer structural member was reach 1373 MPa,and the compressive strength of the ceramic plunger was 474.5 MPa.