Engineering
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Fused Filament Fabrication (FFF) of Metal-Ceramic Components
Summary January 11th, 2019
This study shows multi-material additive manufacturing (AM) using fused filament fabrication (FFF) of stainless steel and zirconia.
Transcript
The aim was to manufacture a long wave infrared heater in an additive manner. This part shows the resulting sintered, two component heating element, made of insulating zirconia, and electrical conductive stainless steel made by FFF. By connecting a power supply, the metal meander is heated up.
This study focuses on IT manufacturing with a mounting material approach to combine a metal with a technical serum. Combining these different materials offers a broad variety of applications due to their different electrical and mechanical properties. This combination can help to answer key questions in medical, automotive, and aerospace fields.
For that purpose, fuse filament fabrication was selected. The main reason was the possibility to process different powders independently of their optical properties. Furthermore, the thermal post processing is similar to well established techniques such as powder injection molding, for which a standard equipment is used.
Fuse filament fabrication becomes economical due to the high material efficiency and the recyclability of the materials. Finally, this technique is easy to upscale for larger parts since the process relies on a moving, printing head on axis. Before beginning the procedure, select a suitable powder couple for the multi-material approach.
For the ceramic grade, select tetragonal yttria stabilized zirconia, due to the coefficient of thermal expansion and the sintering temperature being comparable to special stainless steels as well as the high toughness and flexural strength of this ceramic material. For the specific metal grade, use stainless steel powder as the conductive and ductile metallic material due to its comparable coefficient of thermal expansion, and a similar range of sintering temperatures to those of zirconia under a protective hydrogen atmosphere and a special milling procedure. To achieve a stress free co-sintering, apply attrition milling for 180 minutes to the spherical stainless steel particles to reshape the particles into thin and brittle flakes.
Then, perform planetary ball milling on the brittle flakes for 240 minutes to break the flakes into very fine grained particles with a decreased aspect ratio, but an increased sintering ability. Pre compound the feedstock in a roller rotors mixer. Therefore, the powder has to be mixed with a multi-component binder system obtaining the feedstock with a solid loading of 47 volume percent.
After pre-compounding, the cold, solid material has to be granulated in a cutting mill. Compound the material at high shear rates to improve the dispersion, such as in a co-rotating, twin-screw extruder, like the picture shows. Collect the material with a conveyor belt and cool it down to room temperature.
In the end of the conveyor belt, the two round shaped threads are pelletized. The extrusion line shown in the picture is used to produce the filament. In the single screw extruder, the material is molten and the filament is extruded through a nozzle with a diameter of at least 1.75 millimeters.
Then, a filament is collected with a PTFE conveyor belt. For spooling the material, a unit is placed at the end of the conveyor belt for automatic spooling. Measure and control the dimensions of the filament between pulling and spooling unit.
Filaments with a diameter range of 1.70 to 1.80 millimeters and ovalility smaller than 0.10 millimeters, are required for FFF. For a particular extrusion speed, progressively regulate the conveyor belt and pulling speeds to adjust the dimensions. After creating the CAD file, the G-COD must be generated by using a slicing software.
In the software, the nozzle diameter, the layer heights, printing speed, and printing temperature is defined. In a preview mode, the manufacturing can be demonstrated layer by layer. The ceramic material is blue and the metal is green colored.
For additive manufacturing of the multi material components, first, correct any possible misalignment of the nozzles in the 3D printer software. For the manufacturing of the components, load printhead one with the zirconia filament and printhead two with the stainless steel filament. Using a printhead speed of 10 millimeters per second, and a print bed temperature of 20 degrees Celsius for both filaments.
Then, set the zirconia printhead temperature to 220 degrees Celsius and the stainless steel printhead temperature to 240 degrees Celsius. For multi-material manufacturing, alternate the printhead loading to achieve two or three different layers. For debinding of the components, first submerge the sample in 60 degrees Celsius cyclohexane for eight hours to remove a soluble binder content of an about 7 to 9 weight percentage.
Then, transfer the samples to a high temperature tungsten furnace in a reducing atmoshpere of 80 percent argon and 20 percent hydrogen, for a dwell time of three hours for sintering of the materials, followed by cooling of the kiln to room temperature. While sintering, the parts shrink about 45 percent by volume and, due to the reducing atmosphere, the zirconia turns into black color. The final part properties are achieved after this step by applying an electrical power source.
The metal path acts like a resistance heater while the isolating zirconia covers it. The microstructure was investigated using a scanning electron microscope. The micrograph of the sintered two component part shows the metal microstructure in the upper, and the ceramic one in the lower section.
In between the two materials, mixed phases occur, providing the material bond between metal and ceramic. The best fitting results for stainless steel sintering behavior are obtained with an attrition milling time of 180 minutes, and a planetary ball milling time of 240 minutes. Here, comparison of the sintering behavior of the initial and milled steel powder with the sintering behavior of the zirconia powder, is shown.
Obviously, the milled metal powder shows a good fit in sintering behavior compared to the zirconia one. Compounding of the zirconia feedstock in a twin screw extruder results in a higher ultimate tensile strength and elongation at the ultimate tensile strength of the material. But a lower secant modulus compared to when the material is compounded in a roller rotors mixer.
For zirconia filaments, a good control of the dimensions can be achieved during the extrusion, while for filaments containing the modified stainless steel powder, a higher variability of the average filament diameter is observed. In this figure:a pure zirconia sample, a pure stainless steel sample, and a sintered and well joined steel ceramic composite can be observed. Due to the similar binder system of both materials, it is possible to fuse specific layers to a monolithic composite part.
For example, here a larger, round shaped part with sharp transitions is shown. After its development, this technique paved the way for researches in the manifold field to develop materials to be used to produce in surgical, automotive, or even consumer goods. The results show a permissive approach to manufacture metal semi composites using fuse filament fabrication generating electrically conductive and electrical insulating properties into one component.
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