OPTIMIZATION OF INERT GAS FLOW INSIDE LASER POWDER BED FUSION CHAMBER WITH COMPUTATIONAL FLUID DYNAMICS

Yu Chen1, a, Guglielmo Vastola1,b and Yong Wei Zhang1,c

1Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis North Tower, Singapore 138632

achenyu@ihpc.a-star.edu.sg, bvastolag@ihpc.a-star.edu.sg, czhangyw@ihpc.a-star.edu.sg

Abstract

It is crucial to maintain a uniform and fast enough inert gas flow inside build chamber to obtain high-quality final products (e.g. low porosity) without oxidation. The current study investigated the behaviors of the inert gas flow inside a chamber with CFD simulations, as well as its evaluation and optimization. The gas flow pattern inside the chamber was evaluated in terms of the uniformity of velocity across the build plate. It was shown that the gas channels and locations of inlet openings significantly affected the flow inside the chamber. So the design of gas channels/inlets and flow rates was carefully adjusted to generate uniform gas flow across the chamber to remove emissions from the melt pool efficiently. Furthermore, the re-circulation of emission inside chamber was significantly reduced to keep the chamber walls clean and minimize the damage to the optical surface. In conclusion, CFD benefits in improving quality of products and reducing life-cycle cost for laser powder-bed fusion process (L-PBF).

Introduction

Selective laser melting process is becoming one important method of powder-bed-fusion based additive manufacturing technologies. However, there are still a lot of unsolved problems within the process, which result in imperfectness in final printed products, such as void, pore and residual stress. So it is essential to predict and avoid these unexpected problems during printing. Inert gas flow over powder bed is one of the techniques ensuring the quality of the final products and efficiency of the printing process. On one hand, the gas protects the metal from oxidation at high temperature during melting under laser. Secondly, the flowing gas carries particles spattering from melt pool away from the chamber, avoiding second deposition on the powder bed and blockage of laser. Thirdly, the emission and metal vapor from melt pools can be carried away by gas flow instead of staying inside chambers then cool down and stick on chamber walls and optical surfaces.

Ferrar et al. (2012) found that poor gas flow induces higher porosity of printed material, resulting in lower compressing strength. Anwar & Pham (2016, 2017) investigated the influence of gas flow velocity and laser scanning direction on tensile strength of part, which is affected by the removal and motions of spattering particles. Ladewig et al. (2016) investigated the mechanism how the by-products of laser melting (spatters and welding plume) induce the lack of fusion and finally the defects of part. As the gas flow is important, Philo et al. (2015, 2017) utilized CFD to numerically simulate the gas flow inside RENISHAW AM250 machine, and compared with experimental results. They showed the capability of CFD in prediction and evaluation of gas flow during design phase. Besides, they also showed the sensitivity of flow uniformity to some design parameters, such gas inlet rail diameter, nozzles’ shape etc. However, there is a lack of explanation of underlying mechanism for these phenomenon and design guidelines to obtain better gas flow. So, it motivated us to study the gas flow inside the RENISHAW AM250 chamber and underlying fluid dynamics, and to summarize design guidelines through step by step optimization of gas channels inside chamber.

Method and Simulation Setup

A virtual RENISHAW AM250 chamber and its gas inlet rail and outlet were built up in ANSYS for simulation and evaluation of the inside gas flow (see Figure 1). The gas firstly enters a 550mm long inlet rail with diameter 40mm, then turns into an array of 13 cylindrical nozzles with 12mm in diameter and 18mm in

Solid Freeform Fabrication 2018: Proceedings of the 29th Annual International

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Solid Freeform Fabrication Symposium – An Additive Manufacturing Conference

length, of which the interval is 20mm. The center of the nozzle array is located 67.5mm above the bottom wall of the chamber, where the 271mm*271mm sized build plate is located.

Figure 1, Sketch of the RENISHAW AM250 chamber

CFD simulation of gas flow inside the chamber was conducted using ANSYS Fluent software with

steady, incompressible, turbulent flow assumption. Standard Navier-Stokes equations and k-e turbulent model were chosen to investigate the gas flow inside print chamber. The inlet was defined as “mass flow rate” of 0.0051kg/s with 2% turbulence intensity was set at the inlet, while “outflow” boundary condition for the outlet. So the velocity of gas coming out from the nozzles is around 5m/s. No slip boundary condition with standard wall function was utilized on other walls of the chamber.

The computational domain was meshed based on Tetrahedron and Hexadedron dominated cells, while

10 inflation layers was added on the surface of bottom wall surface are to capture boundary layer flow precisely. Before examining the gas flow, grid number independence study was conducted to choose the proper mesh resolution for the current study and verify the accuracy of the method. Since the gas flow over the build plate is the main concern, the mean, maximum and minimum velocity and variance of velocity on the plane 3mm above the build plate chosen as criteria for convergence of results. As shown in Figure 2, the value of velocity reaches a good asymptotic status at 3.5 million cells, which was chosen for the rest simulations.

Figure 2, Grid independence study for velocity

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Results and Discussion

Original Design

Before the possible optimizations of gas flow, the gas flow inside the chamber with the original design parameter shall be examined first. Velocity contours on two planes over the bottom wall are presented in Figure 3. It can be seen from the velocity contour on the plane through the inlet nozzles’ center that the jet flow is uniform among the array of nozzles, especially the jet sweep down at the far end of the inlet rail. So the gas flow is strongly non-uniform on the plane 3mm above the chamber’s bottom with a high speed region, as the jet partially sweeps down to the bottom at the far end of inlet rail.

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Figure 3, Velocity contour on the plane through the inlet nozzle’s center a) and the plane 3mm above the bottom of chamber b) for the original design

To check the influence of gas flow on selective laser melting carefully, it is worth to examine the velocity contour over the build plate closely with other parts but off as shown in Figure 4. The high speed spot region can be clearly found located over the left bottom corner of the build plate, leaving other regions without coverage of gas flow. To reveal the underlying mechanism behind the non-uniform velocity of gas flow, a side view of gas flow inside the chamber is demonstrated in Figure 5. It is found that the nozzle is too high compared to the chamber’s bottom, so that jet is too far away from the build plate and becomes unstable and partially sweeps down to form high speed spot over the build plate. So reducing the height of nozzles in the original design is supposed to increase the gas flow quality over the quality, which is investigated in the next part.

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Figure 4, Velocity contour on the plane 3mm above the build plate for the original design

Figure 5, Side view of velocity contour of jets for the original design

Reducing Nozzle height and elongated nozzle length It can be found from Figure 6 that after reducing the height of nozzles, the gas flow velocity over the build plate becomes much more uniform. However, there is still non-uniformity in the region near the nozzles since the nozzle is not long enough for gas to turn direction completely after going through the inlet rail. So, longer nozzles are believed to obtain jet with more unique directions, which is confirmed by the visualization of velocity contour presented in Figure 7.

Figure 6, Velocity contour on the plane 3mm above the build plate for the design with reduced nozzle height

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Figure 6, Velocity contour on the plane 3mm above the build plate for the design with reduced nozzle height

and elongated length However, there is still a high speed spot at the right bottom corner of the build plate. Through the examination of the full picture of the velocity contour on the plane 3mm over the chamber’s wall in Figure 7, it is demonstrated that although the nozzle is close to the chamber bottom the jet still sweeps down to the bottom especially at the two ends of the nozzle array. So expanding the width of nozzle array shall be effective to extend the sweeping down jet outside of the build plate, resulting in more uniform gas flow velocity over the build plate.

Figure 7, Velocity contour on the plane 3mm above the bottom and sweeping down jet at the ends of nozzle

array Extension of nozzle array width After extending the width of nozzle array and increasing the number of nozzles from 13 up to 15, the sweeping down jet on the plane 3mm above the chamber’s bottom close to the end nozzles is shifted outwards. So, the velocity of gas flow over the build plated shown in Figure 9 becomes much more uniform.

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Figure 8, Velocity contour on the plane 3mm above the bottom for the extended nozzle array design

Figure 9, Velocity contour on the plane 3mm above the build plate for the design for the design with extended

nozzle array Secondary flow effect Although the flow over the build plate has been optimized to very uniform pattern, the recirculation also needs to be investigated to avoid emission to stay and stick on the chamber walls and lens surface. As seen from the side view of the velocity in Figure 11, the recirculation is strongly suppressed by the downward secondary gas from the top wall which pumps in as 2/3 of the mas flow rate of the main inlet through the nozzles. Figure 12 demonstrating the path lines of gas flow from main and secondary inlets shows that the secondary flow comes down and join together with the horizontal main jet stream, thus suppressing the recirculation of main jet inside the chamber effectively.

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Figure 10, Side view of velocity contour and recirculation for the optimized design

Figure 11, Side view of velocity contour for the optimized design with downward secondary flow

Figure 12, Path lines of gas flow for the optimized design with downward secondary flow

Summary

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Through the discussion and visualization of gas flow in the above sections, it can be concluded that the flow becomes more uniform with the optimizations step by step. However, it is still worthwhile to evaluate the uniformity quantitatively based on mean, maximum and minimum velocity and variance of velocity. It can be found that through the optimization process (lower the nozzles, elongate nozzles and extend nozzle array/number), the variance of velocity is reduces, and the minimum velocity increases and gets closer to the maximum velocity, resulting in gas flow with higher quality. Furthermore, involvement of secondary downward flow avoids the disturbance of the uniformity while reducing the recirculation.

Figure 12, Evaluation of uniformity of flow velocity for design optimization step by step

Conclusions

In this study, CFD tool was implemented to optimize the gas flow inside RENISHAW AM250 chamber, which is believed to play very important role in the quality of final products. Through the step by step optimization, the uniformity of gas flow velocity was enhanced. It was also shown by the analysis that the instability of jets from nozzles affects the uniformity of gas flow velocity significantly. Several design guidelines were suggested, such as nozzles shall be lower and wider relative to the build plate, and the involvement of downward flow is essential to avoid the recirculation of welding plume inside chamber. As future work, the proposed guidelines shall be verified for other chamber designs besides RENISHAW AM250, and more parameters can be tested the design inlet rail/manifold.

References

1. Ferrar, B., Mullen, L., Jones, E., Stamp, R., & Sutcliffe, C. J. (2012). Gas flow effects on selective laser melting (SLM) manufacturing performance. Journal of Materials Processing Technology, 212(2), 355-364.

2. Anwar, A. B., & Pham, Q. C. (2016). Effect of inert gas flow velocity and unidirectional scanning on the formation and accumulation of spattered powder during selective laser melting. Proceedings of the 2nd International Conference on Progress in Additive Manufacturing (Pro-AM 2016), 531-536.

3. Ladewig, A., Schlick, G., Fisser, M., Schulze, V., & Glatzel, U. (2016). Influence of the shielding gas flow on the removal of process by-products in the selective laser melting process. Additive Manufacturing, 10, 1-9.

4. Anwar, A. B., & Pham, Q. C. (2017). Selective laser melting of AlSi10Mg: Effects of scan direction, part placement and inert gas flow velocity on tensile strength. Journal of Materials Processing Technology, 240, 388-396.

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5. Philo¹, A., Lavery¹, N. P., Brown¹, S. G. R., Cherry¹, J., Sienz¹, J., Joannou, J., & Sutcliffe, C. J. (2015). Comparison and Validation of Gas Flow Models in a Powder Bed Selective Laser Melting Process. Proceedings of the 23rd UK Conference of the Association for Computational Mechanics in Engineering 8 – 10 April 2015, Swansea University, Swansea

6. Philo, A. M., Sutcliffe, C. J., Sillars, S., Sienz, J., Brown, S. G. R., & Lavery, N. P. (2017). A Study into the Effects of Gas Flow Inlet Design of The Renishaw AM250 Laser Powder Bed Fusion Machine Using Computational Modelling. Solid Free. Fabr., Proceedings of the 28th Annual International Solid Freeform Fabrication Symposium, pp. 1203

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WelcomeTitle PagePrefaceOrganizing CommitteePapers to JournalsTable of ContentsBroader ImpactsUsing Additive Manufacturing as a Pathway to Change the Qualification ParadigmTechnology Integration into Existing CompaniesLattice Design Optimization: Crowdsourcing Ideas in the ClassroomEducation of Additive Manufacturing – An Attempt to Inspire ResearchPrinting Orientation and How Implicit It IsMethod for a Software-Based Design Check of Additively Manufactured ComponentsThe Recycling of E-Waste ABS Plastics by Melt Extrusion and 3D Printing Using Solar Powered Devices as a Transformative Tool for Humanitarian Aid

Binder JettingEvaluating the Surface Finish of A356-T6 Cast Parts from Additively Manufactured Sand MoldsEconomies of Complexity of 3D Printed Sand Molds for CastingMitigating Distortion During Sintering of Binder Jet Printed CeramicsBinder Jetting of High Temperature and Thermally Conductive (Aluminum Nitride) CeramicBinder Jetting Additive Manufacturing of Water-Atomized Iron

Data Analytics in AMEffects of Thermal Camera Resolution on Feature Extraction in Selective Laser MeltingArtificial Intelligence-Enhanced Multi-Material Form Measurement for Additive MaterialsMachine Learning for Modeling of Printing Speed in Continuous Projection StereolithographyCurvature-Based Segmentation of Powder Bed Point Clouds for In-Process MonitoringNondestructive Micro-CT Inspection of Additive Parts: How to Beat the BottlenecksNon-Destructive Characterization of Additively Manufactured Components Using X-Ray Micro-Computed TomographyPrecision Enhancement of 3D Printing via In Situ MetrologyLayer-Wise Profile Monitoring of Laser-Based Additive ManufacturingCorrelative Beam Path and Pore Defect Space Analysis for Modulated Powder Bed Laser Fusion Process

Hybrid AMCharacterization of High-Deposition Polymer Extrusion in Hybrid ManufacturingMechanical Properties Evaluation of Ti-6Al-4V Thin-Wall Structure Produced by a Hybrid Manufacturing ProcessDevelopment of Pre-Repair Machining Strategies for Laser-Aided Metallic Component RemanufacturingViscosity Control of Pseudoplastic Polymer Mixtures for Applications in Additive ManufacturingCuring Behavior of Thermosets for the Use in a Combined Selective Laser Sintering Process of PolymersEffect of Porosity on Electrical Insulation and Heat Dissipation of Fused Deposition Modeling Parts Containing Embedded WireA New Digitally Driven Process for the Fabrication of Integrated Flex-Rigid ElectronicsHybrid Manufacturing with FDM Technology for Enabling Power Electronics Component FabricationDigitally-Driven Micro Surface Patterning by Hybrid ManufacturingPotentials and Challenges of Multi-Material Processing by Laser-Based Powder Bed FusionA Digitally Driven Hybrid Manufacturing Process for the Flexible Production of Engineering Ceramic ComponentsStereolithography-Based Manufacturing of Molds for Directionally Solidified CastingsExamination of the Connection between Selective Laser-Melted Components Made of 316L Steel Powder on Conventionally Fabricated Base BodiesCharacterization and Analysis of Geometric Features for the Wire-Arc Additive Process

ApplicationsGeometry/Surface FinishEffect of Inter-Layer Cooling Time on Distortion and Mechanical Properties in Metal Additive ManufacturingEffect of Shield Gas on Surface Finish of Laser Powder Bed Produced PartsMaterial Characterization for Lightweight Thin Wall Structures Using Laser Powder Bed Fusion Additive ManufacturingMetal Additive Manufacturing in the Oil and Gas IndustryDevelopment of a Customized CPAP Mask Using Reverse Engineering and Additive Manufacturing

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Biomedical ApplicationsDesign and Additive Manufacturing of a Patient Specific Polymer Thumb Splint ConceptMandibular Repositioning Appliance following Resection Crossing the Midline- A3D Printed GuideA Sustainable Additive Approach for the Achievement of Tunable PorosityEffects of Electric Field on Selective Laser Sintering of Yttria-Stabilized Zirconia Ceramic PowderFabrication of Ceramic Parts Using a Digital Light Projection System and Tape CastingSlurry-Based Laser Sintering of Alumina CeramicsMaterial Properties of Ceramic Slurries for Applications in Additive Manufacturing Using Stereolithography

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Direct WriteThe Mechanical Behavior of AISI H13 Hot-Work Tool Steel Processed by Selective Laser Melting under Tensile Stress

Broad IssuesAdditive Manufacturing of Metal Functionally Graded Materials: A ReviewUnderstanding Adopting Selective Laser Melting of Metallic MaterialsThe Effect of Processing Parameter on Zirconium Modified Al-Cu-Mg Alloys Fabricated by Selective Laser MeltingSelective Laser Melting of Al6061 Alloy: Processing, Microstructure, and Mechanical PropertiesSmall-Scale Characterization of Additively Manufactured Aluminum Alloys through Depth-Sensing IndentationEffects of Process Parameters and Heat Treatment on the Microstructure and Mechanical Properties of Selective Laser Melted Inconel 718Effects of Design Parameters on Thermal History and Mechanical Behavior of Additively Manufactured 17-4 PH Stainless Steel

AluminumThe Effects of Powder Recycling on the Mechanical Properties of Additively Manufactured 17-4 PH Stainless SteelMechanical Properties of 17-4 Ph Stainless Steel Additively Manufactured Under Ar and N₂ Shielding GasRecyclability of 304L Stainless Steel in the Selective Laser Melting ProcessThe Influence of Build Parameters on the Compressive Properties of Selective Laser Melted 304L Stainless Steel

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17-4PH Stainless SteelIncorporation of Automated Ball Indentation Methodology for Studying Powder Bed Fabricated 304L Stainless SteelEffect of Powder Degradation on the Fatigue Behavior of Additively Manufactured As-Built Ti-6Al-4VVolume Effects on the Fatigue Behavior of Additively Manufactured Ti-6Al-4V Parts

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Polymers for Material ExtrusionReinforcement Learning for Generating Toolpaths in Additive ManufacturingControl System Framework for Using G-Code-Based 3D Printing Paths on a Multi-Degree of Freedom Robotic ArmAnalysis of Build Direction in Deposition-Based Additive Manufacturing of Overhang Structures

Polymers for Powder Bed FusionTopology-Aware Routing of Electric Wires in FDM-Printed ObjectsUsing Autoencoded Voxel Patterns to Predict Part Mass, Required Support Material, and Build TimeContinuous Property Gradation for Multi-Material 3D-Printed ObjectsConvection Heat Transfer Coefficients for Laser Powder Bed FusionLocal Thermal Conductivity Mapping of Selective Laser Melted 316L Stainless Steel

ModelingFinite Element Modeling of the Selective Laser Melting Process for Ti-6Al-4VModelling the Melt Pool of the Laser Sintered Ti6Al4V Layers with Goldak’S Double-Ellipsoidal Heat SourceA Novel Microstructure Simulation Model for Direct Energy Deposition ProcessInfluence of Grain Size and Shape on Mechanical Properties of Metal Am MaterialsExperimental Calibration of Nanoparticle Sintering SimulationSolidification Simulation of Direct Energy Deposition Process by Multi-Phase Field Method Coupled with Thermal Analysis

Physical ModelingThermal Modeling Power BedsEstablishing Property-Performance Relationships through Efficient Thermal Simulation of the Laser-Powder Bed Fusion ProcessAn Investigation into Metallic Powder Thermal Conductivity in Laser Powder-Bed Fusion Additive ManufacturingSimulation of the Thermal Behavior and Analysis of Solidification Process during Selective Laser Melting of AluminaLow Cost Numerical Modeling of Material Jetting-Based Additive ManufacturingScrew Swirling Effects on Fiber Orientation Distribution in Large-Scale Polymer Composite Additive Manufacturing

Multi/Micro-Scale ModelingNumerical Prediction of the Porosity of Parts Fabricated with Fused Deposition ModelingNumerical Modeling of the Material Deposition and Contouring Precision in Fused Deposition ModelingEffect of Environmental Variables on Ti-64 Am Simulation Results

Advanced Thermal ModelingNon-Equilibrium Phase Field Model Using Thermodynamics Data Estimated by Machine Learning for Additive Manufacturing SolidificationMulti-Physics Modeling of Single Track Scanning in Selective Laser Melting: Powder Compaction EffectTowards an Open-Source, Preprocessing Framework for Simulating Material Deposition for a Directed Energy Deposition ProcessQuantifying Uncertainty in Laser Powder Bed Fusion Additive Manufacturing Models and Simulations

Modeling Part PerformanceOptimization of Inert Gas Flow inside Laser Powder-Bed Fusion Chamber with Computational Fluid DynamicsAn Improved Vat Photopolymerization Cure Model Demonstrates Photobleaching EffectsNonlinear and Linearized Gray Box Models of Direct-Write Printing DynamicsInsights into Powder Flow Characterization Methods for Directed Energy Distribution Additive Manufacturing SystemsEffect of Spray-Based Printing Parameters on Cementitious Material Distribution

Modeling InnovationsAligning Material Extrusion Direction with Mechanical Stress via 5-Axis Tool PathsFieldable Platform for Large-Scale Deposition of Concrete StructuresMicroextrusion Based 3D Printing–A Review

ImprovementsTheory and Methodology for High-Performance Material-Extrusion Additive Manufacturing under the Guidance of Force-FlowKnowledge-Based Material Production in the Additive Manufacturing Lifecycle of Fused Deposition ModelingAcoustic Emission Technique for Online Detection of Fusion Defects for Single Tracks during Metal Laser Powder Bed FusionDevelopment of an Acoustic Process Monitoring System for the Selective Laser Melting (SLM)

Process DevelopmentDepositionLow Cost, High Speed Stereovision for Spatter Tracking in Laser Powder Bed FusionMultiple Collaborative Printing Heads in FDM: The Issues in Process PlanningAdditive Manufacturing with Modular Support StructuresPick and Place Robotic Actuator for Big Area Additive Manufacturing

Extrusion3D Printed ElectronicsFiber Traction Printing--A Novel Additive Manufacturing Process of Continuous Fiber Reinforced Metal Matrix CompositeImmiscible-Interface Assisted Direct Metal Drawing

ImagingIn-Situ Optical Emission Spectroscopy during SLM of 304L Stainless SteelTool-Path Generation for Hybrid Additive ManufacturingStructural Health Monitoring of 3D Printed StructuresMechanical Properties of Additively Manufactured Polymer Samples Using a Piezo Controlled Injection Molding Unit and Fused Filament Fabrication Compared with a Conventional Injection Molding ProcessUse of SWIR Imaging to Monitor Layer-To-Layer Part Quality During SLM of 304L Stainless Steel

Novel MethodsDMP Monitoring as a Process Optimization Tool for Direct Metal Printing (DMP) of Ti-6Al-4VEffects of Identical Parts on a Common Build Plate on the Modal Analysis of SLM Created MetalOn the Influence of Thermal Lensing During Selective Laser MeltingDevelopment of Novel High Temperature Laser Powder Bed Fusion System for the Processing of Crack-Susceptible AlloysTowards High Build Rates: Combining Different Layer Thicknesses within One Part in Selective Laser MeltingLaser Heated Electron Beam Gun Optimization to Improve Additive ManufacturingTwo-Dimensional Characterization of Window Contamination in Selective Laser SinteringLaser Metal Additive Manufacturing on Graphite

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Spinning/Pinning/StereolithographyFabrication of Aligned Nanofibers along Z-Axis – A Novel 3D Electrospinning TechniqueZ-Pinning Approach for Reducing Mechanical Anisotropy of 3D Printed PartsStructurally Intelligent 3D Layer Generation for Active-Z PrintingmicroCLIP Ceramic High-Resolution Fabrication and Dimensional Accuracy Requirements

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