Proceedings of the 7Th Annual ISC Graduate Research Symposium s2

Proceedings of the 7th Annual ISC Graduate Research Symposium

ISC-GRS 2013

April 24, 2013, Rolla, Missouri

Aaron Thornton

Department of Mechanical and Aerospace Engineering

Missouri University of Science and Technology, Rolla, MO 65409

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Freeze-form Extrusion Fabrication of Functionally Graded Materials

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Abstract

This paper discusses the production of functionally graded material (FGM) parts with a novel additive manufacturing process called freeze-from extrusion fabrication (FEF). The goal of this project is to produce FGM parts with good mechanical properties. After initial trials, poor mechanical strength and poor density resulted in the final fabricated parts achieving only 25 – 30% of the desired values. This paper discusses the study performed to determine why this was occurring and how to fix the problem(s) associated with the poor mechanical properties.

1.  INTRODUCTION

Freeze-form Extrusion Fabrication (FEF) is an environmentally friendly additive manufacturing process which builds 3-D parts layer-by-layer. The build material is in the form of an aqueous-based colloidal paste [1-2]. The build process is computer controlled based off the desired CAD model of the part being fabricated. The organic binder content is only 2-4 vol%, whereas in other similar processes this content is much higher. Paste solids loadings can achieve up to 60 vol%.

While there are many additive manufacturing processes, only a few of them are similar to Freeze-form Extrusion Fabrication. The reason for this is that FEF is capable of fabricating complex ceramic parts. Robocasting [3-4], Extrusion Freeform Fabrication [5], 3-D Printing [6], and Selective Laser Sintering [7-8], are a few additive manufacturing processes which can also produce ceramic parts.

Robocasting is a very similar process which also uses colloidal pastes. The most significant difference between Robocasting and FEF is that Robocasting relies on the extruded paste to dry, whereas FEF relies on the water in the paste to freeze after extrusion. Because the water solidifies after extrusion, much lower amounts of binder content is needed, making FEF much more environmentally friendly.

3-D Printing is another process which can fabricate ceramic parts. 3-D printing works rather differently from FEF. 3-D printing is still a layer-by-layer process but it lays down powder beds and then prints binders to glue different particles together in a desired shape. FEF extrudes paste out of a needle, and can only print one bead track at a time. Unlike 3-D printing, the current FEF machine is capable of printing functionally graded material (FGM) parts. Since 3-D printing uses a large amount of resins to glue particles together to form a green part, it is not as environmentally friendly as FEF.

Typical applications of FGM parts fabricated with FEF are leading edges for hypersonic vehicles, missile nose cones, nozzle throat inserts for propulsion systems. Many of these parts must withstand extreme heat conditions ((> 2000°C), but must also be able to interface and fasten to a structure which is usually made of a metal. One ideal way to make these parts is to grade from a ceramic to a metal. This allows the same part to exhibit both high heat resistances where it is geometrically needed, and to be tightly secured to a structure at the same time. Additive manufacturing is one of the most feasible ways to fabricate FGM parts.

This paper discusses the testing of Zirconium Carbide and Tungsten functionally graded material (FGM) test bars. The resulting mechanical strength of these bars was low. Therefore a study was and is still being performed to obtain a full scientific understanding of why these test bars exhibit such low flexural strength. The study also includes upgrades to machinery to provide better operating conditions for FGM part fabrication.

2.  FEF Equipment setup

The triple-extruder mechanism was designed using three stainless steel cylinders, each containing a paste driven by an individual plunger whose movement is controlled by a DC servo motor (Kollmorgen AKM23D); see Figure 1. The encoder signal from the servo amplifier provided a resolution of 0.62 µm for the plunger’s movement. The paste flow rate in each cylinder was controlled by the plunger’s velocity, and the force exerted on the plunger was measured by a load cell (Omega LC-305). The FEF system used a static mixer to blend the three different pastes and mixed them into a homogeneous stream as they passed a series of mixing blades positioned at alternating angles.

The triple-extruder mechanism was mounted on a gantry system, which consisted of three orthogonal linear drives (Velmex BiSlide), each with a 508 mm travel range. The X-axis consisted of two parallel slides and was used to support the Y-axis. The Z-axis was mounted on the Y-axis, and the extrusion mechanism was mounted on the Z-axis. Four DC servo motors (Pacific Scientific PMA22B), each with a resolver for position feedback at a resolution of 1000 counts per revolution, drove the various axes. Each motion axis had a maximum speed of 127 mm/s and a position sensor resolution of 2.54 μm.

Figure 1. Triple FEF system in a temperature-controlled enclosure. Three servo motors control linear cylinders for extrusion and a three-axis gantry system controls motion.

The part fabrication process was conducted in a freezing environment, which could be controlled to as low as -20°C using a liquid nitrogen injection system. Later in the project, the liquid nitrogen system was replaced with a freezer system. The cold temperatures enabled the aqueous paste to solidify at temperatures below the freezing point of water after it was extruded to solidify the paste, thus avoiding part deformation during the fabrication process and enabling fabrication of larger parts. A heating jacket was used to keep the paste temperature above the freezing point of water until it was deposited. In the present study, the freezer’s temperature was kept at -10°C, while the heating jacket’s temperature was kept at 10°C.

The 3-axis gantry system movement was controlled by a motion control program with the Delta Tau Turbo PMAC PCI board. Paste extrusion was controlled with three servo motors using a National Instruments PXI chassis and LabVIEW Real Time 8.6.

2.1. Zirconium Carbide and Tungsten FGM Test Bars

Zirconium Carbide and tungsten flexure test bars were made via the FEF process. These bars were made from two different sets of paste; 1) 100%vol zirconium carbide and 2) 50%vol zirconium carbide with 50%vol tungsten. Five compositions were made by varying the velocities of the two extrusion rams on the FEF machine. Figure 2 helps explain how this was achieved. For example in order to achieve 87.5%vol ZrC/12.5%vol W, ram 1, containing 100%vol ZrC paste, was set to extrude paste at 75% of the total extrusion velocity. Ram 2, containing 50%volZrC/50%volW paste was set to extrude at 25% total extrusion velocity.

Figure 2. Material Compositions and their correlating Ram Velocities.

In order to achieve good statistical analysis of the mechanical strength, five test specimens were fabricated from each of the compositions totaling 25 test bars. These test bars were cut, ground, polished and broken following ASTM C1161-02b standards.

In order better understand the data from flexural strength of the FEF bars, they were compared to a conventional manufacturing process. ZrC and W powders were mixed into the same ratios as that of the FEF fabricated bars, and were pressed into bars under 30,000 psi in an Isostatic press. These bars were also cut, ground, polished and broken following ASTM C1161-02b standards. The results of these Isostatic pressed bars are compared to the FEF fabricated bars in Fig. 3.

Figure 3. Flexure test results of FEF printed bars compared with Isostatic pressed bars.

As can be seen in Fig 3 the overall flexural strength of FEF fabricated bars is significantly lower than the Isostatic pressed bars. At this point the challenge became to figure out why the FEF fabricated bars resulted with such low density and strength.

Figure 4. Microstructure comparison of a FEF fabricated bar (left) and a iso-pressed bar (right).

Figure 4 is a view of the microsturcure comparing the FEF fabricated bars to the Isostatic pressed bars. The important thing to note in Fig. 4 is the large voids on the FEF bar side. It is hypothesized that these large voids are caused by ice crystals. In order to reduce the size of these large ice crystals, glycerol was introduced into the paste used on the FEF machine. The idea was that the glycerol would increase the number of nucleation sites for the water, therefore creating more ice crystals which would keep the overall size of the ice crystals small.

More test bars were printed from 100%vol ZrC paste containing glycerol on the FEF machine. These bars were once again cut, ground, polished, and broken according to ASTM C1161-02b standards. In order to conserve costly powder, only tests on 100% ZrC were performed.

After these bars were tested, the resulting flexural strength did not improve significantly jumping from 73 mPa to 82 mPa while the density only improved from 62% to 73%. The glycerol helped improve mechanical properties, but not significantly.

2.2. Alumina Test Bars

At this point it was decided to go back to previous materials which had in the past resulted with very good properties such as density and strength. Further testing was performed with alumina.

Single composition alumina bars were fabricated on the FEF machine much in the same way as the ZrC/W bars. These bars were then put into the freeze-dryer to remove all of their water, as per usual FEF post processing. After the freeze-drying process completed, these bars were cut in half to examine their cross-sections. Figure 5 is a picture of one of these cross-sections at 20X magnification.

Figure 5. Top and bottom of the cross-section of an alumina test bar.

Figure 5 shows the top and bottom cross-sections of an alumina bar. The bottom side of the bottom picture is the bottom of the test bar, or in other words it was the first layer that was printed on the FEF machine. Build direction in terms of layer sequence is starting at the bottom and going up. If observed closely, layers and individual bead tracks can be observed in the cross-section of the bar.

After observing this cross-section it is easy to see why this bar, and likely the ZrC bars as well, cannot achieve > 95% density. As can be seen in the bottom picture of Fig. 5 there are large voids which are right in the center of the bar and some cracks throughout.

A density test, Archimedes density, was performed on these bars and despite some of their large pores and cracks they all reached 85% density. If the pores and cracks can be eliminated then it is possible to reach > 95% density, as desired. The next step is to determine why these large pores and cracks occur.

Upon further study of the cross-sections, it can be seen that after about the 7th or 8th layer, no more large pores or cracks appear. In fact if the bar were solid as these upper layers appear to be, then the bar would indeed have greater than 95% density. There are a few reasons this could occur.

The first and most likely cause is that the bars are built upon an aluminum plate which acts as the substrate for part fabrication. Since this plate is sitting in the cryogenic environment, it conducts a lot of heat very rapidly out of the part being built. If this is the case, the aluminum substrate is causing the paste to freeze too fast, not allowing it to fill into the desired shape.

Figure 6 helps to explain what a desired bead shape in this case should be. The desired bead shape for all bead tracks being laid down is a perfect rectangle. This prevents any voids from forming between bead tracks and helps to form a solid green part.

Figure 6. Examples of possible bead cross-sections as printed by the FEF process.

A second possible cause is errors in the gantry system. Since the gantry setup contains two x-axes, a large amount of following error has been observed between the two axes. This along with other observed errors is a likely cause for the gantry to overshoot, or miss the desired absolute position along the toolpath.

A third possible cause is overall poor temperature control within the FEF cabinet. At the time the alumina bars were printed the FEF machine used liquid nitrogen sprayed directly into the box to cool the environment. A commercial PID controller was purchased to control the environment temperature at -100C. After connecting a second thermocouple for comparison, it is easily seen that this is not the case within the FEF box. Figure 7 shows the temperature recording while the environmental chamber is being cooled by liquid nitrogen.