3D printing is a manufacturing forming method based on computer three-dimensional model technology. It realizes production by stacking materials layer by layer. It combines advanced technology in many fields such as material science, electromechanical control technology, and computer information technology, changing the traditional manufacturing method and Craftsmanship is a landmark achievement of the "third technological revolution".
Common 3D printing methods include thin material overlay manufacturing (LOM), fused deposition modeling (FDM), light curing (SLA), laser selective melting (SLM), three-dimensional printing (3DP), laser selective sintering (SLS) and other methods. Compared with traditional molding, the above-mentioned 3D printing technologies have shown great advantages in directly molding complex-shaped metal and polymer parts.
However, the aforementioned 3D technology still has many difficulties when forming ceramic parts. Many scholars try to use 3D printing technology to form ceramic parts, combined with high-temperature sintering densification treatment, and some even directly print dense ceramic parts by laser 3D.
3D printing molding technology
1. Laser selective melting (SLM)
The SLM method uses a focused laser beam with a diameter of 30μm~50μm to selectively melt the powder layer by layer, and accumulate it into a metallurgical bonded, densely organized entity, which can be used to directly 3D print ceramic parts without subsequent densification. In the process of SLM forming ceramic parts, the temperature range is large, the speed is fast, and the internal stress generated is large, which makes the ceramic parts extremely easy to produce defects.
2. Thin material overlay manufacturing technology (LOM)
The LOM technology uses laser to cut ceramic sheets, and obtains three-dimensional solid parts through hot pressing or other forms of bonding and superposition. The step effect between the surface layer and the layer of the ceramic body manufactured by the LOM method is obvious, the boundary needs to be polished, and the density difference between the horizontal and height directions is large, which is unfavorable for the subsequent debinding and sintering process, resulting in uneven density of the final ceramic parts.
3. Light curing (SLA)
The SLA process uses ultraviolet light to cure a liquid photosensitive resin material. The ceramic powder is uniformly dispersed in the light curing solution through high-speed stirring to obtain a ceramic slurry with high solid content and low viscosity, and then the slurry is cured layer by layer on the molding machine , It is piled into ceramic parts, and finally the ceramic parts are obtained by densification processes such as drying, degreasing and sintering. However, the photo-curing method requires support for manufacturing ceramic parts, which will also cause harm to the environment, which is not conducive to the application and development of the process.
4. Fused Deposition Modeling (FDM)
FDM technology uses heating and melting nozzles to transform the material into a molten state, squeezing and depositing in a predetermined direction according to the path indicated by the data of each layer, and accumulating the entire part layer by layer. In the method, an organic binder is added to the ceramic powder, processed into a wire by a capillary rheometer or an extruder, and then formed into a ceramic body by an FDM equipment, and finally a higher-density ceramic part is obtained by degreasing and high-temperature sintering.
However, fused deposited ceramics have poor strength and anisotropy in shrinkage. At the same time, due to the large nozzle diameter and low molding accuracy, there are fewer types of plastics suitable for FDM molding, which limits the development of FDM molding of ceramic parts.
5. Three-dimensional printing method (3DP)
3DP technology is a 3D printing method using droplet jet technology. The process is similar to that of a printer. The binder is sprayed on the pre-leveled powder to form a thin layer at the position that needs to be molded, and then stacked layer by layer to form a three-dimensional ceramic. Components. However, when the 3DP method is used to form ceramics, the density of the spray-printed adhesive parts is not high, the print nozzle is easily clogged, and the precision of the parts is also low.
6. Laser selective sintering (SLS)
The working principle of SLS technology is that, first, a thin layer of powder is laid on the worktable, and then the CO2 laser scans the powder to be bonded according to the information of the cross-section of each layer. The powder material in the scanned area is sintered or melted. The powder in the unscanned area is still loose and can be reused. After processing one layer, the workbench is lowered by a layer thickness, and then the next layer is spread and scanned. The layers are sticky. Join together and pile up layer by layer until the entire part is formed, and finally the part is taken out.
SLS/CIP composite molding of ceramics
Ceramic parts manufactured by the above 3D printing technology generally have the shortcomings of low density and poor mechanical properties, and cannot be directly used in actual industrial production. The 3D printed ceramic body needs to be further subjected to subsequent densification post-processing. Liu Kai's team organically combined CIP technology with powder bed-based SLS printing, and then debinding and high-temperature sintering of SLS/CIP blanks, and proposed SLS/CIP composite molding manufacturing technology for ceramic parts. The process flow is as follows:
1. Powder preparation
Use a three-dimensional powder mixer to mix the powder, first coat the ceramic powder with a layer of lubricant PVA polymer by spray granulation method, and then mix the granulated powder and the binder epoxy resin for about 2 hours to obtain a uniform granulation mixture Composite powder, the powder has good SLS bonding performance and CIP pressing performance.
Since the SLS blanks need to be debinding, under the premise of ensuring that the blanks have a certain strength, the less binder content the better. The basic principle of determining the amount of binder is that the smaller the amount of binder, the better without affecting the molding quality of SLS. The composite powder SLS prepared by the granulation mixing method has good moldability, simple preparation method and low cost.
2. Laser selective sintering molding
During the SLS molding process, the laser acts on the powder material, and the powder is heated by the fast-moving point heat source, which is an extremely complex dynamic system. Reasonable SLS process parameters can make the precision, density, and strength of the ceramic achieve a better combination; on the contrary, if the process parameters are unreasonable, the green body will have warping, deformation, poor strength and other problems, which will also affect other links such as subsequent CIP The quality of the treated ceramics.
3. Cold isostatic pressure densification
Under pressure, the density of the ceramic body increases rapidly. Since the SLS sample contains a large number of pores, the process is mainly the recombination of ceramic particles, and the density of the green body increases at the highest rate. As the holding pressure of the CIP process further increases, the bonding neck is broken and the pores are filled, the interaction area between the ceramic powders becomes larger, the organic matter on the ceramic surface also enhances the sliding extrusion between the powders, and the pores of the green body are further reduced.
4. Debinding treatment
Pre-sintering the debinding sample. When the temperature rises to 800℃, the relative density of the sample increases. This is because although the mass of the sample is still reduced, the solid-phase atomic diffusion of alumina particles occurs in the sample during the pre-sintering stage. , The sintering neck is gradually formed, and the specimen shrinks significantly in all directions, so its volume also shrinks and the relative density increases. Increasing the preheating and sintering temperature and extending the holding time are beneficial to the densification of the debinding sample.
5. High temperature sintering
Extending the holding time and reducing the heating rate are beneficial to the densification of alumina samples. When the sintering temperature, heating rate and holding time are 1650℃, 2℃/min and 2h, the relative density of alumina can reach 92% or more.
Liu Kai's team used SLS molding, CIP and high-temperature sintering to densify, and has successfully prepared Al2O3, ZrO2, SiC high-white clay and other high-performance complex structure dense ceramic parts, and the relative density of related parts reached 92%.