Sintering/Powder Metallurgy: Powder Injection Molding of Titanium Components
Powder injection molding (PIM) is a well-established, cost-effective method to fabricate small-to-moderate size metal components. The process, derived from plastic injection molding, comprises four main steps of mixing of metal powder with a polymer binder, injection molding the mixture, binder removal (via heat treatment or chemical processes) and sintering or densification. Pellets or granules of the metal powder/polymer binder mixture are heated in a cylinder and the resulting melt is forced under pressure into a split-mold cavity where it quickly cools before the mold is opened and the part is ejected onto a conveyor belt or storage bin. In addition to rapid production, by redesigning and replacing the mold, the shape of the component can be readily changed, offering flexibility in part design. Thus, PIM is an economical, net-shape process for manufacturing large volumes of complex-shaped parts.
PIM is used to manufacture a wide variety of products made of stainless steel, nickel-base superalloys and copper alloys. However, titanium and other refractory metal alloys present problems due to alloy impurities directly attributable to the injection molding process. A unique blend of PIM constituents has been developed where only a small volume fraction of binder (~10-15 wt%) is required for injection molding; the remainder of the mixture consists of the metal powder and binder solvent (patent pending). Because of the nature of the decomposition in the binder system and the relatively small amount used, the binder is eliminated almost completely from the presintered component during the initial stage of a two-step heat treatment process. The use of easily removed fugitive phases in the powder mixture and control of the debinding and sintering heat treatments allows optimizing the porosity of the component, which is advantageous in a number of specialized applications including the design of self-lubricating parts and biomedical implants.
Titanium PIMWith titanium, the process is used primarily in the manufacture of jewelry, cases for watches and portable electronics. While these applications take advantage of the high specific strength and excellent corrosion resistance of titanium, penetration of titanium PIM (Ti-PIM) products into automotive, aerospace, chemical production and biomedical applications has been limited because of historical problems with alloy impurities directly attributable to the injection molding process. Specifically, carbon, oxygen and nitrogen are left behind during binder removal and become part of the material's chemistry and microstructure during densification. Even at low concentrations, these impurities can cause severe degradation in titanium and Ti-alloy mechanical properties. Thus, key to the acceptance of Ti-PIM components in advanced engineering applications is the reduction of impurities to an acceptable minimum level in a cost-effective manner. This includes development of high purity titanium powder; design of a binder/solvent system that can be easily removed, does not require burn-out in an oxidizing environment and leaves behind no residual carbon; and development of a controlled method of sintering under a protective environment. U.S. Dept. of Energy's Pacific Northwest National Laboratory (PNNL) is currently investigating low cost methods to manufacture high purity Ti powder. The laboratory and Battelle are investigating new techniques to fabricate high quality components made of these and other powders. Dry blending and extrusions have been performed.
Experimental procedureIn the Ti-PIM material preparation process, a binder material consisting of three different organic substances are combined with different titanium metals and/or alloys. Titanium metal is added to the binder after mixing and heating the binder materials. The binder and titanium metal are mixed for 20 to 30 minutes and allowed to cool, forming a granular powder, or the binder and metal are dry blended and extruded for mixing, and then pelletized for future use in an injection molding operation.
Tensile specimens were fabricated using bench top and 40 ton injection molding machines, and binder removal was done under vacuum and temperature, and the specimens were sintered at different heating and cooling rates, sintering temperatures, and hold times and temperatures. Experimental work consisted of analyzing the rheological behavior under different metal powder loadings in the binder materials, binder removal and sintering, and microscopy of final sintered parts.
Titanium powders were supplied by Zr Energy (Phoenix, Ariz.), including titanium metal, titanium hydride and titanium alloy 6Al-4V. Figure 1 shows the angular geometry of the titanium powder. Angular particles typically influence the rheology compared with spherical particles, but the binder system developed allows for high solid loadings and proper particle size distribution. Larger particle sizes are more detrimental to rheology than smaller sizes. However, smaller particle sizes increase surface area and also can be detrimental to the rheology. The binder system was a proprietary blend of naphthalene, ethylene vinylacetate (EVA) and stearic acid. Naphthalene is a very clean material used for its ability to be easily removed, and it also helps reduce the carbon content of the binder mixture before thermal oxidation.
Naphthalene begins melting at 78°C, sublimes at approximately 80°C and undergoes 83% of its weight loss by 200°C. The EVA component undergoes a melt transition at 140°C and thermal degradation at 390°C and is completely consumed at 450°C and loses 12.5% of its total organic weight.
Different powder loadings ranging from 47-70 vol% were mixed and evaluated for flow and mixing behavior. The following table shows powder and binder weight mixtures for the corresponding ratios.
See "Ti-PIM system powder and binder loadings"
Experimental resultsTorque analysis. The behavior of different feedstock mixtures was determined using torque analysis. Figure 2 shows a typical torque rheometer curve. The curve indicates that torque has come to equilibrium within four minutes of the run, and the material is completely mixed. This particular binder system has the advantage of rapidly wetting the metal materials and quickly dispersing. The torque curve increases in torque once the material crosses the 78°C threshold for solidification of the binder.
Molding trials were performed on each feedstock mix to determine moldability, which was based on mold filling, part flashing, voids in the molded part, release from the mold and the green strength during demolding. The test mold was a 19% scaled up version of the ASTM E8 tensile specimen. The scale up accounted for the part shrinkage that occurred during sintering. As-sintered specimens are shown in Fig. 3. Figure 4 shows other complicated geometries that were molded, illustrating that intricate detail is possible with this feedstock system.
Binder burnout test. Weight loss, dimensional measurements and microscopy were performed on sintered specimens. The ranges of average measurements for different feedstocks are shown in the following table.
See "Burnout and sintering data"
No dimensional changes were observed in the first phase of the binder removal, which indicates that the specimens have much more open porosity for thermal removal and better diffusion rates, as well as a lower carbon content for removal.
Binder removal. The rate of sublimation in pure naphthalene depends on both temperature and pressure; this is the same with the naphthalene impregnated PIM bars. Figure 5 shows a set of weight loss curves for the 67 vol% TiH2-6Al-4V and the 62 vol% Ti-6Al-4V feedstocks as a function of time and temperature. The rate of naphthalene sublimation increases significantly with increasing temperature. Sublimation involves lower surface energies in the drying solvent. High surface energies are typically responsible for deformation and cracking during the process, which means that part distortion can be mitigated during debinding by using a binder that undergoes sublimation. However, it also implies that it is necessary to maximize the initial powder loading in the feedstock to ultimately enhance part sinterability and reduce the chance of void formation. Chemical analysis on the raw powder and sintered parts is shown in the following table.
See "Impurity content before and after PIM"
There is no measurable increase in carbon or oxygen in the sintered Ti-6-4 specimen due to PIM processing.
Microscopy. Figures 6 and 7 show micrographs of a prealloyed feedstock sintered to approximately 92% of theoretical density. Figure 6 shows that there is a higher percentage of porosity towards the surface of the part and is more dense toward the center. Figure 7 shows the microstructure toward the center of the part contains few voids having a much smaller average size of <5 μm. Alloy chemistry is homogeneous across the sample. A small amount of well distributed equiaxed β phase titanium is observed in the β matrix, suggesting that a post-sintering heat treatment is desirable.
Mechanical testing shows that tensile properties of the molded specimens ranged from 445 MPa for the TiH2 powder to 550 MPa for Ti-6Al-4V powder. Tensile specimens were as-sintered without follow on post heat treatments.
Acknowledgements: The authors thank Nat Saenz, Shelly Carlson and Ruby Ermi for assistance on metallography; Jim Coleman and Bruce Arey for helping with scanning electron microscopy; and John Hardy for conducting feedstock debinding studies. Support for this project was provided by Battelle Memorial Institute's Internal R&D program.
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For more information: Eric Lund, IP Development & Licensing; Pacific Northwest National Laboratory, PO Box 999, Battelle Blvd., Richland, WA 99354; tel: 509-375-3764; fax: 509-375-2323; e-mail: firstname.lastname@example.org.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: powder injection molding, PIM, binder, binder burnout, debinding, sintering, rheology.