Binder Optimization for the Production of Tungsten Feedstocks for PIM

Transcription

1 Binder Optimization for the Production of Tungsten Feedstocks for PIM Travis E. Puzz, A. Antonyraj, and Randall M. German Center for Advanced Vehicular Systems Mississippi State University James J. Oakes ATI Alldyne Huntsville, AL ABSTRACT Since the feedstock design is one of the most critical stages in the powder injection molding (PIM) process, a variety of binder formulations have been investigated to produce quality tungsten feedstocks. The present study describes binder optimization for the production of these feedstocks via a multi-faceted design of experiments. The powders used in the experiments were typical, as-received powders with small particle sizes. Experiments were performed to optimize the binder for each feedstock based on various methods including solids loading, mixture homogeneity, handling strength, moldability, and debinding. The experimental results display an interesting system of feedstock opportunities. The results of the experiments also reveal how different binder mixtures perform in the presence of tungsten, tungsten alloys, and tungsten compounds. INTRODUCTION The goal of this paper was to produce a quality, low cost binder system that could be molded or extruded with little or no external heat input from mixing and extrusion. To compound powder injection molding feedstocks, external heat is usually needed to mix the feedstock containing polymers like polypropylene. Usually pure shear mixing will not keep the polymer from hardening. Though external heat is used, mechanical work is usually the main reason for softening the polymers in binder systems. The binder systems described in this paper consist of low melting temperature binders like polyethylene wax, ethylene vinyl acetate, and polyethylene glycol. Plasticizers like polybutene and di-n-butyl phthalate were used to keep the binder from hardening quickly as it cooled. Even though using these low temperature binders could be used in mixers without dependence on external heating, strength of the green compact is subject to be lowered as a result. Since the strength of a binder is detrimental for quality control when the part is subjected to thermal debinding, thermogravimetric analysis results were obtained to account for slower debinding rates to keep thermal stresses to a minimum while the binder is removed

2 []. These results would help in keeping cracks and defects from forming in the debinded part. The further sections will provide mixing, extrusion, and debinding setup along with the different materials used and their resulting properties. MATERIALS The powders were pure tungsten (Grade C20), WC (Grade P22), and WC-0Co (extrusion grade FR0EXT) produced by ATI Alldyne. The powders had different particle sizes, characteristics and shapes which are shown in Table. SEM images were also taken of each powder which is displayed in Figure and 2. Table. Characteristics of W, WC, and WC-0Co powders Powder Supplier Particle Size W Grade C20 ATI Alldyne 6 9 μm WC Grade P22 ATI Alldyne 5 9 μm WC-0Co Grade FR0EXT ATI Alldyne 0. 0 μm Particle Shape Blocky, polygonal Blocky, equiaxed Blocky, agglomerated Pycnometer Density (g/cm 3 ) Figure. Tungsten C20 grade powder (left) and WC P22 grade powder (right) Figure 2. WC-0Co grade FR0 EXT powder

3 The initial idea was to find waxes and thermoplastics with low softening temperature but with enough handling strength after extrusion to hold shape through debinding/pre-sintering. Liquid plasticizers were blended with the waxes and polymers to lower the softening temperature. The difficult task was to use just enough plasticizer to lower the softening temperature but also to retain strength upon cooling to room temperature. The polymer and waxes are located in Table 2. Table 2. Polymer Characteristics Binder Density Softening Binder reference (g/cm 3 ) point ( C) PEW Polyethylene Wax EVA Ethylene Vinyl Acetate PB Polybutene DBT Di-n-butyl phthalate PEG Polyethylene Glycol.2 60 The binder systems as seen in Table 3 show each system and the binder system reference number. Different polymers and waxes were selected on the aspect of thermal properties. The object of selecting these types of binder components was to eliminate the need for external heating while mixing and extrusion. In most thermoplastics and waxes, the lower the melting point means the less strength is has after cooling. Keeping this in mind, the binders needed certain amounts of additives like plasticizers before a suitable handling strength could be compromised. EXPERIMENTAL PROCEDURES Table 3. Binder systems System Binder System reference A DBT-PEW-P EG-SA B PB-PEW-PE G-SA C PB-PEW-EV A-SA Mixing was done purely on the process of heating by shear. Mixing was done in beaker type batches using a 25 ml stainless steel beaker and impellor mixing blade connected to a lab mixer. The powder was mixed with the binder system into a viscous paste. The mixing time for complete addition of powder was recorded at 20 to 30 minutes. The mixing speed was kept constant at an impellor speed of 500 rpm. Sample rods were made by placing the viscous paste into a plunger type extrusion apparatus. Pressure is then applied by a manual press onto the plunger extruding the material from a 2 mm diameter chamber through a 6 mm (0.25") diameter by 9 mm (0.75") in length die orifice. Three to five sample rods were set aside to test green density using Archimedes m ethod. Homogeneity was measured by comparing the experimental green density to theoretical green density [2]. Theoretical green density is given in Equation where ρ m (g/cm 3 3 ) is mixture or green den sity, ρ p (g/cm ) is powder theoretical (pycnometer) density, ρ b (g/cm 3 ) is binder density, and Φ (volume %) is solids loading. m p ( ) b ρ = Φ ρ + Φ ρ () The remaining rods were placed onto a zirconia support tray for debinding and pre-sintering trials. Thermal debinding was done in a retort furnace using a heating rate of 0.5 C/min up to 600 C in a hydrogen atmosphere. Pre-sintering was done using a heating rate of C/min from 600 C to 000 C and cooling of the furnace was done naturally. Thermogravimetric analysis (TGA) and differential scanning

4 calorimeter (DSC) analysis were done to analyze binder burn off for each of the feedstocks. For TGA/DSC analysis, feedstocks consisting of binder system A, B, and C were heated to a temperature of 600 C at a heating rate of 5 C/min in hydrogen and argon atmospheres. This data helped optimize debinding rates and times for each binder system. Data was reported as observations in the pre-sintered compact with regards to cracking, blistering or defects. RESULTS AND DISCUSSION Green density was a helpful way in determining mixture homogeneity of the particular feedstock samples after mixing and extrusion. Three to five extruded rods of each binder system and powder type were tested using Archimedes method. The averaged green density is shown in Table 4. Table 4. Green density of powder-polymer feedstocks Powder Solids loading Binder ρ theoretical ρ green Percent from (vol. %) System (g/cm 3 ) (g/cm 3 ) theoretical WC 55 A % WC-0Co 56.8 A % W 55 B % WC 55 B % WC-0Co 56.8 B % W 55 C % WC 55 C % WC-0Co 56.8 C % The percent in which the actual green density deviates from the theoretical green density gives a good representation on the homogeneity of the powder-polymer mixture []. The mixture deviates less than 4% from theoretical calculations showing the mixture was homogeneous. Thermal debinding was simulated using TGA/DSC f or binder system A, B, and C that are provided in Figures 3, 4, and 5, respectively. From analyzing F igure, the polymer of binder system A shows three distinct areas of decomposition. The steep slope of the first curve from 50 C to 275 C sh ows the fast decomposition of the DBT, oil co ntent in the PEW and ste aric acid. The slope of the second curve from 350 C to 400 C shows the decomposition of the PEG. The final, small slope of the third curve fro m 400 C to 475 C shows the decomposition of the polyethylene content in the PEW Weight Loss (%) Atmosphere: Ar Heating rate: 5 C/min Heat Flow (μw) Temperature ( C) -3.0 Figure 3. TGA (black curve) and DSC (grey curve) results of binder system A

5 Weight Loss(%) Atmosphere: H 2 Heating rate: 5 C/min Temperature ( C) Heat Flow (μw) Figure 4. TGA (black curve) and DSC (grey curve) results of binder system B Weight Loss(%) Atmosphere: Ar Heating rate: 5 C/min Heat Flow (μw) Temperature ( C) Figure 5. TGA (black curve) and DSC (grey curve) results of binder system B From analyzing Figure 4, the polymer of binder system B shows three areas of decomposition. The first curve from 00 C to 350 C shows the decomposition of the PB, oil content in the PEW and stearic acid. The slope of the second curve from 350 C to 400 C shows the decomposition of the PEG. The final slope of the third curve from 400 C to 475 C shows the decomposition of the polyethylene content in the PEW. From analyzing Figure 5, the polymer of binder system C shows three areas of decomposition. The first curve from 00 C to 375 C shows the decomposition of the PB, oil content in the PEW and stearic acid. The slope of the second curve from 375 C to 425 C shows the partial decomposition of the EVA. The final slope of the third curve from 425 C to 475 C shows the decomposition of the EVA and polyethylene content in the PEW. Decomposition of each polymer was analyzed from the standpoint of molecular weight of the polymer, initial weight percent of polymer in the binder system, and heat flow from the DSC curve. After the rods were debound and pre-sintered, they were checked for defects and cracks by observation and the use of a low magnification, light microscope. Table 5 shows the results from debound and presintered rods.

6 Table 5. WC-0Co, WC, and W sample rods debound/presinter data Powder Exp. # Percent from theoretical Results WC 4 % No defects or cracks WC-0Co 4 % No defects or cracks W 49 2% Cracks along length WC 49.6% No defects or cracks WC-0Co % No defects or cracks W % Cracks along length WC % circumference cracking and defects WC-0Co 67.9% No defects or cracks Cracks Figure 6. W rods after debinding from left to right: Binder 67, Binder 49, and enlarged view of crack Defect Figure 7. WC rod after debinding from left to right: Binder 67, Binder 49, and Binder 4 Figure 8. WC-0Co rod after debinding and pre-sintering (on support tray) From left to right: Binder 67, Binder 49, and Binder 4

7 Figure 6 show the W rods after the debinding cycle with excessive cracking down the length of both rods. The cracking was largely due to low solids loading. Typical solids loading for tungsten feedstocks usually range from 55% to 60% by volume. Figure 7 shows WC rods after the debinding cycle with only one binder system encountering with defects. The defects with binder C and the WC powder rods were observed as rough craters on the surface. These defects were caused by non-homogeneity in these sample rods. Figure 8 shows no defects or cracks in the WC-0Co debound and pre-sintered rods. The WC- 0Co powder performed the best in mixing and extrusion with all binder system. C ONCLUSION The experimental results display an interesting system of feedstock opportunities. All binder systems could be recommended for further processing of the WC-0Co rods in the powder-polymer extrusion process. Binder systems 4 and 49 performed well with WC powder and reasonable results were obtained. Due to unsatisfactory results in the W extruded rods and low solids loading, these binder systems are seen as being too weak to support the high density of W during debinding. ACKNOWLEDGEMENTS This research work was supported by ATI Alldyne. The authors would like acknowledge ATI Alldyne for graciously supplying the tungsten, tungsten carbide, and cemented carbide powders. REFERENCES [] Jicheng Zhou, Baiyun Huang, and Enxi Wu, Extrusion moulding of hard-metal powder using a novel binder system, Journal of Materials Processing Technology, 2003, vol. 37, pp [2] John Warren, The Role of Powder Characteristics in Binder Incorporation for Injection Molding Feedstock, 988, M.S. Thesis, Rensselaer Polytechnic Institute, Troy, NY.