Improved Powder Performance Through Binder Treatment of Premixes

Transcription

1 % of Graphite Bonded % of Lubricant Bonded % of Copper Bonded Improved Powder Performance Through Binder Treatment of Premixes C.T. Schade 1 and M. Marucci 1 1 Hoeganaes Corporation, 1001 Taylors Lane, Cinnaminson, NJ 077 USA Abstract Powder premixes used in the PM industry contain ingredients of substantially different particle sizes and specific gravities that have a strong tendency to segregate during handling. Reducing or eliminating this segregation is essential for the part producer to achieve consistent precision and optimum performance. Treating the premixes with various types of binders improves the performance of the premix as a whole. A new binder system will be reviewed and mix properties for this new binder system, such as flow rate, apparent density and lubrication will be related to press performanc e. The bonding capability of the system will be evaluated for various PM additives such as copper, nickel and graphite. Keywords: Alloying; Bonding; Pre-mixing. Introduction Binder treatment or bonding has been well established in the PM industry and is currently used for a variety of applications. Binder treatment has progressed from simply reducing segregation and improving flowability to a process that can be engineered to improve green strength, green density and dimensional change, as well as improving the pressing and ejection of parts. Bonding is used to prevent segregation of powder premixes by attaching smaller additives such as graphite, metallic alloying elements, and lubricant to the base iron [1-3]. While the addition of smaller additives is beneficial due to increased diffusion rates during sintering, it also contributes to segregation, a natural phenomenon that occurs in premixes containing particles of varying size [4]. Bonding helps to maintain a homogeneous premix composition, which prevents variation in structure and mechanical properties from part to part, or within a single pressed part. Figure 1 shows the effect of particle size on the dusting tendency of various additives in a premix. Generally, as the particle size increases, the tendency for the additives to dust or segregate decreases. Due to the electrostatic forces between graphite and iron, the dusting is actually reduced with a finer particle size of graphite. However, coarser graphite particles (typically 8 micron) are typically utilized in PM and the dusting at this particle size tends to be high Particle Size of Graphite (microns) Particle Size of Lubricant (microns) Particle Size of Copper (microns) Figure 1: Effect of particle size of various additives on the dusting resistance. Premixes in PM are generally a mixture of several additives such as lubricant, graphite and metallic additions, typically in the range of 0.50 to 2.0 w/o. In a bonded premix these fine additives all compete for locations on the base iron and the amount and wetting characteristics of the binder play a large role in the overall efficiency in which the additives can be bonded to the base iron. The dust resistance of a mix is usually measured by the retention of fine additives such as graphite, lubricant, copper, and nickel when the mix is subjected to some sort of flow. The dust resistance is measured as the percentage of retained additives in the mix after it is subjected to the flow. Elements such as nickel and copper can be measured separately by chemical analysis, while the dust resistance of the graphite and lubricant are generally combined into a measurement of carbon. Figure 2 shows the bonding efficiency in single and multicomponent systems in which graphite (0. w/o), Nickel (2.0 w/o) and

2 Components in Mix ethylene bis-stearamide-ebs (0.75 w/o) were bonded to Ancorsteel 1000 utilizing a constant amount of binder. In single component systems (i.e. nickel, graphite or EBS only) the bonding efficiency is high, generally ~ %. When both graphite and nickel are added together, the bonding efficiency drops to ~ 70% since both elements are competing for sites on the base powder and a limited amount of binder is available. The amount of binder can be increased, but this is detrimental to the density of the final part. When EBS is added to premixes containing graphite the bonding increases to ~ %. This is most likely due the van der Walls forces between lubricant and the iron powder which is evident when EBS is the only component bonded (bonding efficiency > 90%). Nickel, Graphite and Acrawax Nickel and Acrawax Graphite and Acrawax Graphite and Nickel Acrawax Graphite Nickel Dust Resistance (%) Figure 2: Dust resistance of various additives in bonded premix with constant volume of binder. (Note: Graphite and Lubricant is measured by carbon analysis and is a combined measurement shown in light blue). In this study, a new experimental binder system developed by Hoeganaes Corporation was examined and compared with a regular powder premix. The relative ability of the binder to bond efficiently, as well as improve powder properties, pressing characteristics, and mechanical properties of the finished parts was observed. The increase in powder flowability and apparent density achieved through bonding can result directly in increased productivity and part consistency. Experimental A regular FC-0205 premix and an experimental bonded mix () of similar composition were prepared for this study using commercially available Ancorsteel 1000C iron powder from Hoeganaes Corporation. The two premixes contained 2.1 mass % ACuPowder International 81 Cu, 0.6 mass % Asbury type 3203H graphite, and 0.75 mass % total lubricant content. The nominal compositions and identifications for each premix are shown in Table 1. Table 1: Nominal compositions and identifications of mixes studied (mass %). Iron Balance Balance Copper Graphite Lubricant

3 All laboratory procedures were carried out in accordance with the appropriate MPIF standards [5]. The apparent density and flowability of each powder premix was tested using a Hall Apparatus according to MPIF Standards 3 and 4. The green density and ejection characteristics were observed on rectangular bars with 32 x 12.7 x 12.7 mm dimensions according to MPIF Standard 15. Using a hydraulic compaction press, the initial ejection pressure (strip) and pressure applied as the bar is exiting the die (slide) was measured over time. Figure 3 shows a schematic of a standard pressure vs. time graph, including the points at which strip and slide pressures are recorded. Green density and ejection data were recorded at compaction pressures of 415, 550, 690, and 830 MPa at both room temperature and with a die heated to 75 C. Figure 3: Schematic of ejection measurements [6]. Each powder premix was also tested for dust resistance using an elutriation test. Using the set-up shown in the schematic in Figure 4, nitrogen was forced through a small characteristic sample of each premix. A chemical analysis was performed on powder before and after the elutriation in order to determine any change in the percentage of bonded additions. In this study, the carbon content was measured before and after to determine dust resistance and bonding capacity of the graphite for the mixes tested. The percent bonded graphite for each mix type is a good indication of the dust resistance of other additives in the premix as well. Figure 4: Schematic of the elutriation set-up for testing dust resistance [5]. Mechanical properties were measured using transverse rupture (TR), dogbone tensile, and unnotched Charpy impact bars compacted to green densities of 6. and 7.00 g/cm 3. These samples were sintered at 1120 C for 25 minutes in a mixed atmosphere of 95 v/o nitrogen and 5 v/o hydrogen. Prior to mechanical testing, green and sintered density, apparent hardness, and dimensional change were determined on the TR samples using MPIF Standards 42, 43, and 44, respectively. The mechanical properties were evaluated on sets of five bars for each mix and density combination. TR strength was determined using MPIF Standard 41, tensile properties were found in accordance with MPIF Standard 10, and impact energy was measured using MPIF Standard 40. Sintered carbon values were also measured using a Leco 200 carbon-sulfur combustion gas analyzer with reference standards run before and after test samples. Results Binder Characteristics The stability of powder flow and apparent density is critical to ensure consistency in production of PM parts. Atmospheric conditions such as humidity and temperature as well as storage duration can all influence the performance of a powder mixes. It has also been found that environmental conditions during the compaction process can influence the performance of powder premixes [7]. The binder system in the current study was designed to be hydrophobic and therefore has good stability over time. Figure 5 shows a plot of a bonded premix using the current binder system with Ancorsteel 1000 and 0. w/o graphite. No lubricant was added so that only the binder would influence the mix properties. A 1500 kg bonded premix was made and stored

4 Weight of Lubricant/Binder (%) Temperature/Dewpoint ( o F) in a bulk pack with no desiccant and an open liner in a typical warehouse setting. The apparent density and flow were measured routinely since the bonded premix was made in As can be seen from Figure 5, since the binder is hydrophobic, the apparent density and flow have not significantly degraded over a four year period ( ) AD = 3.62 FLOW = AD = 3.62 FLOW = AD = 3. FLOW = AD = 3.62 FLOW = Week Temperature DewPoint Figure 5: Effect of temperatures and dew point of the apparent density and flow of binder treated mix versus time. Another key aspect of the binder is that it must be removed effectively during the sintering process. Poor de-lubrication leads to defects in the sintered part such as blisters, microporosity, and sooting. To understand performance of the new binder in relation to existing lubricants, a thermogravimetric analysis (TGA) was performed on the new lubricant and compared to EBS and Zinc Stearate (ZnSt). The TGA records the weight change as a function of temperature and lubricants/binders that burn cleanly show a rapid and complete weight loss. Figure 6 shows the new binder system starts to burn off at a lower temperature and more quickly versus EBS and burns more cleanly (no residue) than ZnSt. Considering the lube burn-off zones in most industrial furnaces are operated between o C, the new binder system should burn-off cleanly. 100 Binder EBS ZnSt Temperature ( o C) Figure 6: Weight loss of lubricant versus temperatures (in air) as measured by TGA analysis.

5 Powder Premix Properties Before pressing samples and observing mechanical properties, the properties of the powder premixes themselves were tested. The results, shown in Table 2, highlight the flexibility available through the use of bonding techniques. The regular premix has an apparent density of 3.04 g/cm 3 while the experimental bond () with the new binder resulted in an apparent density of 3.27 g/cm 3. Significant improvements in the flow and dust resistance can be seen as well. While the regularly premixed material lost nearly 50% of the carbon present during the elutriation test, the bonded premix maintained approximately 90% carbon. This bonding effectiveness results in less segregation and better powder homogeneity, lower health risks for press operators, and improved cleanliness of work areas. Powder properties such as apparent density, flow, and dust resistance can be adjusted to meet user specifications through control of bond type and lubricant selection when designing a premix. Table 2: Powder characteristics of the tested premixes. Mix Apparent Density (g/cm 3 ) Flow (s/50g) Dust Resistance (% carbon) Green Properties The green properties for the mixes are shown in Table 3 at room temperature and after pressing with a die heated to 75 C. At room temperature the experimentally bonded mix has higher green density and green strength but slightly higher strip and slide. At 75 o C, the ejection characteristics (strip and slide) of the bonded mix are much better than the premix. Mix Table 3: Green properties of the premixes at room temperature and with a heated die (75 C). Compaction Room Temp. 75 C Pressure GD GS Strip Slide GD GS Strip Slide (MPa) (g/cm 3 ) (MPa) (MPa) (MPa) (g/cm 3 ) (MPa) (MPa) (MPa) Mechanical Properties The mechanical properties found using TR, unnotched Charpy impact and dogbone tensile bars are displayed in Table 4. The bars were pressed for each of the premixes to a green density of 6. and 7.00 g/cm 3 for testing. As shown in the table, there was little difference in properties between the regularly premixed material and the bonded material. Most importantly however, is that the dimensional change of the bonded material matches the premix allowing for direct replacement with the bonded material. Table 4: Mechanical properties of the premixes pressed to green densities of 6. and 7.00 g/cm 3. Green Sintered Dimensional Charpy 0.2% Offset Sintered Mix Density Density Change TRS UTS Elongation HRA Impact YS (MPa) (MPa) (%) Carbon (g/cm 3 ) (g/cm 3 ) (%) (Joules) (MPa) (%)

6 Compaction Study A compaction study was performed at Cloyes Gear & Products, Inc. to determine the performance of each premix in a production environment. Round ABS wheel sensor parts with an approximate height of 2 cm and mass of 130 g were pressed in a Cincinnati Rigid Reflex 200-C-6 mechanical compacting press to a green density of 6. and 7.00 g/cm 3 in runs of 300 parts. The part consistency in terms of density, mass, height, outer diameter, inner diameter, and roundness was measured for 50 consecutive parts in the middle of each run. The consistency of the mass and height for 6. g/cm 3 density green parts is shown in Figures 7 for both mixes. The distributions shown in the graphs were used to determine overall consistency and standard deviation from the mean value. showed excellent consistency in both mass and height due to the bonding methods used. The bonded premix showed an increase in uniformity of the part height and better weight control than the unbonded premix. Similar trends were observed for the parts pressed to 7.00 g/cm 3. Premix Weight Weight Height Height Figure 7: Mass and height variation of green parts pressed from each mix at 6. g/cm 3 density. Following the press study, sintered parts were photographed to determine the presence of any sooting that may have occurred during sintering. Figures 8 and 9 show the cleanliness of the surfaces of a part pressed using each mix at 6. and 7.00 g/cm 3. The figures show that with the current lubricant system employed, sooting was present on parts pressed from the standard premix at both densities. The new bonding technique () reduced the sooting significantly, even at higher density.

7 Sooting (6.8 g/cm 3 ) Figure 8: Sooting observed following sintering for premixes pressed to 6. g/cm 3. Sooting (7.0 g/cm 3 ) Figure 9: Sooting observed following sintering for premixes pressed to 7.00 g/cm 3.

8 Conclusions Bonding can be utilized in most instances to immediately improve powder flow, apparent density, dust resistance, surface finish and ejection characteristics without any degradation of mechanical properties in the sintered product. This study showed the overall flexibility of bonding and how unique bonding techniques can be employed to meet specific goals in terms of powder, mechanical, and pressing properties. In addition, elutriation tests showed a substantial reduction in dust for the bonded mix system studied. A number of binder and lubricant combinations exist that allow parts producers to improve powder properties over a regular premixed material. References 1. G. Poszmik, K. Narasimhan, Segregation-free Premixes for Increased Productivity and Improved Performance, Materials Science Forum, Vols , pg , F. Semel and S. Luk, Continuing Improvements in Binder Treatment Technology, PM2TEC 1996 World Congress, Washington, DC, USA, June 16-21, S. Luk, Advances in Binder-Treatment Technology, Proceedings of 2000 Powder Metallurgy World Congress, Kyoto International Conference Hall, Japan, Nov , S. St-Laurent, C. Gelinas, Y. Thomas, L. Azzi, Using Binder-Treatment Technology for High Performance Steel Powder Mixes, 2005 International Conference on Powder Metallurgy & Particulate Materials, Montréal, Québec, Canada, June 19-23, Standard Test Methods for Metal Powders and Powder Metallurgy Products, published by MPIF, C. Schade, M. Marucci, F. Hanejko, Improved Powder Performance Through Binder Treatment of Premixes, Proceedings of the 2011 International Conference on Powder Metallurgy & Particulate Materials, San Francisco, California, May 18 21, Y. Thomas, S. St-Laurent, S. Pellitier and C. Gelinas Effect of Atmospheric Humidity and Temperature on the Flowability of Lubricated Powder Metallurgy Mixes, 2009 International Conference on Powder Metallurgy & Particulate Materials, Las Vegas, Nevada, USA, June 28- July 1st, 2009.