HIGHER GREEN STRENGTH MATERIALS FOR GREEN HANDLING. Patrick King, George Poszmik, and Robert Causton Hoeganaes Corporation Cinnaminson, NJ 08077

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1 HIGHER GREEN STRENGTH MATERIALS FOR GREEN HANDLING Patrick King, George Poszmik, and Robert Causton Hoeganaes Corporation Cinnaminson, NJ Presented at PM 2 Tec2005 Montréal, Canada June ABSTRACT Proper handling of green components is of great importance to parts manufacturers. Damage to fragile compacts has the potential of being economically catastrophic in a production setting. To counteract this problem, a newly engineered lubricant system was developed that provides enhanced green strengths compared to traditional lubricants. AncorMix HGS, which is applicable to all iron-based compositions, is most effective when tooling temperatures are between 60 and 75 C (140 and 165 F) and requires no powder heating or curing. This manuscript reviews the enhancements in green strengths that are obtained using HGS compared to ethylene bisstearamide. Laboratory as well as production data are presented. INTRODUCTION Green strength plays an important role in powder metallurgy (P/M) processing for several reasons. Vulnerability of fragile green compacts that can be damaged during handling, particularly automated handling, is of great concern to parts manufacturers. Enhanced green strength helps to reduce green scrap and allows more automation in the processing line. High green strength may also permit green machining, that is an attractive alternative to machining sintered parts. The primary source of green strength for powder compacts is particle interlocking [1]. Iron powders that differ in particle shape, size, surface area and the level of alloying elements will develop varying levels of green strength [2]. For example atomized iron, because of its less irregular shape when compared to sponge iron, provides lower green lower green strength. Conventional compaction usually requires a combination of metal powders and non-metallic additives like lubricants and graphite. Admixing these ingredients into the iron powder creates a non-metallic interface between the interlocking metal particles. This interface creates a weak link that will generally lower the green strength of the compact relative to a compact made of pure powder at the same density. Different lubricants yield varying green strengths for parts pressed to a given density [2]. As it was previously reported, some binder treated systems developed by Hoeganaes Corporation, like ANCORBOND can improve on the green strength lowered by the addition of lubricants [3]. A newly ANCORBOND and AncorMix are registered trademarks of Hoeganaes Corporation.

2 developed lubricant system, AncorMix HGS, was specifically designed to provide enhanced green strength. This lubricant system is applicable to all iron-based compositions, is most effective when tooling temperatures are between 60 and 75 C (140 and 165 F) and requires no powder heating or curing. This manuscript reviews the enhancements in green strengths that are obtained using HGS compared to ethylene bisstearamide. Laboratory as well as production data are presented. EXPERIMENTAL PROCEDURE Hybrid alloy FLN was selected as the basis for this study because of its relative familiarity throughout the P/M industry. Laboratory mixes were prepared according to MPIF standards for two compositions: 0.75 wt.% ethylene bisstearamide (EBS) and 0.75 wt.% HGS. All mixes had a nominal 0.60 wt.% graphite level. Test specimens were compacted at pressures ranging from 415 to 690 MPa (30 to 50 tsi). Powders were not heated prior to compaction, though the die temperature was maintained at 63 C (145 F) for the majority of the experiment. Die temperatures ranging from room temperature to 80 C (175 F) were also evaluated using a compaction pressure of 550 MPa (40 tsi). Green strength and ejection pressures were measured on 12.7 mm (0.50 in) tall transverse rupture samples. Transverse rupture and dog-bone tensile samples were sintered in an Abbott belt furnace for 30 minutes at 1120 C (2050 F) in 90N 2-10H 2 (vol.%). The average cooling rates over the range of 650 to 315 C (1200 to 600 F) was 42 C/min (1.3 F/sec). Percent dimensional change, sintered density, and apparent hardness were measured from the 6.4 mm (0.25 in) tall transverse rupture samples using standard MPIF procedures [4]. Tensile testing was performed using a crosshead speed of cm/min (0.025 in/min). The machine is equipped with a 25 mm (1 in) extensometer, which was left on until failure. Production size rings were also compacted using a 220 ton press at Cincinnati, Inc. The rings had nominal dimensions of 38 mm (1.50 in) OD, 25 mm (1.0 in) ID, and 29 mm (1.1 in) height, and were compacted using a die temperature of 63 C (145 F). Radial crushing load measurements were recorded for these components. RESULTS AND DISCUSSION Comparisons of HGS and EBS A plot of green strength as a function of compaction pressure for FLN with 0.75 wt.% EBS or HGS, using die temperatures of 63 C (145 F) or room temperature (RT) is shown in Figure 1 (a). The presence of HGS significantly improves the green strength, with an increase of approximately 60% over the EBS mix. It is critical to note, however, the importance of elevated die temperature when using HGS. Compacting at room temperature provides similar green strengths for HGS and EBS. At 63 C (145 F) the green strength increases only minimally for EBS compared to room temperature, whereas HGS provides a substantial increase. In addition to an increase in green strength, AncorMix HGS also provides the additional benefit of an increase in green density. Figure 1 (b) shows a plot of green density versus compaction pressure for the same samples shown in Figure 1 (a). HGS provides a green density that is about 0.05 g/cm 3 higher than that achieved with EBS for identical compaction pressures in the range of 415 to 690 MPa (30 to 50 tsi).

3 Despite the increase in green density associated with HGS, the enhancement in green strength is primarily due to the presence of the lubricant. This is illustrated in Figure 2, which shows a plot of green strength versus green density using the same data that comprised Figures 1 (a) and (b). At similar green densities, HGS has green strengths that are approximately 40 to 60% higher than those that are achieved with the EBS mix. Figures 3 (a) and (b) show plots of transverse rupture strength and apparent hardness as a function of sintered density for samples of FLN with both 0.75 wt.% HGS and 0.75 wt.% EBS. Both transverse rupture strength and apparent hardness are unaffected by the lubricant type. Figure 1. Plots of green strength and green density as a function of compaction pressure for FLN with 0.75 wt.% EBS or HGS, using die temperatures of 63 C (145 F) or room temperature (RT).

4 Figure 2. Plot of green strength as a function of green density for FLN with 0.75 wt.% EBS or HGS, using die temperatures of 63 C (145 F) or room temperature (RT). Figure 3. Plots of transverse rupture strength and apparent hardness as a function of density for FLN with 0.75 wt.% EBS or HGS, using die temperatures of 63 C (145 F) or room temperature (RT), sintered at 1120 C (2050 F) for 30 minutes.

5 Effect of Die Temperature on HGS As mentioned in the previous section, the use of an elevated die temperature is critical to achieve the enhancements in green strength with HGS. At room temperature compaction, HGS does not have an advantage over EBS with respect to either green density or green strength. Both properties increase with increasing die temperature until 80 C (175 F). At this temperature, green strength appears to decline slightly. The recommended aim temperature for compaction with HGS is 63 C (145 F), with a range of 60 to 75 C (140 to 165 F). It is important to note that although HGS performs best with an elevated die temperature, no powder heating or curing is required for it to be effective. Figure 4. Plots of green density and green strength as a function of die temperature for FLN with 0.75 wt.% HGS using a compaction pressure of 550 MPa (40 tsi). Production Results with HGS In addition to laboratory data shown in the previous two sections, production results were generated using both HGS and EBS versions of FLN The sample geometry used was a ring with a nominal 38 mm (1.50 in) OD, 25 mm (1.0 in) ID, and 29 mm (1.1 in) height, compacted using a die temperature of 63 C (145 F). Figure 5 (a) shows plots of weight variation from average versus part number. Both HGS and EBS showed excellent weight stability. The average weight for HGS rings was ± 0.28 g, while the average weight of EBS rings was ± 0.22 g. Radial compaction load as a function of compaction pressure is shown in Figure 5 (b). These data reinforce the green strength properties shown in Figures 1 and 2. HGS has a radial crush load that is approximately 60-80% higher than that of EBS at identical compaction pressures.

6 Figure 5. Plots of (a) weight variation from average at 550 MPa (40 tsi) and (b) radial crush load versus compaction pressure for a production lot of FLN with either 0.75 wt.% HGS or 0.75 wt.% EBS using a die temperature of 63 C (145 F) on a 220 ton compaction press CONCLUSIONS Data were presented for AncorMix HGS, a newly developed lubricant system that provides enhanced green strengths compared to traditional lubricants. HGS was found to be most effective when tooling temperatures are between 60 and 75 C (140 and 165 F), but requires no powder heating or curing. Comparisons to conventional lubricant ethylene bisstearamide showed that at an elevated die temperature of 63 C (145 F), HGS has approximately 60% more green strength, and provides an increase in green density of 0.05 g/cm 3. Mechanical properties were found to be unaffected by the lubricant. ACKNOWLEDGMENTS The authors wish to thank Ken Cradler of Cincinnati, Inc. for his assistance with data generation. REFERENCES 1. Lund, J.A., Origin of Green Strength in Iron Compacts, Int. J. PowderMetall. Powder Technol., vol. 18 (no. 2), pp , Luk, S., Davala, A., and Kopech, H., Enhanced Green Strength Material System for Ferrous and Stainless P/M Processing, Advances in Powder Metallurgy & Particulate Material, MPIF, Princeton, NJ, Luk, S., Chan, F., and Kuzmicz, V., Enhanced Green Strength Material System for Ferrous and Stainless P/M Processing, Advances in Powder Metallurgy & Particulate Material, MPIF, Princeton, NJ, Standard Test Methods for Metal Powders and Powder Metallurgy Products, MPIF, Princeton, NJ, 2003.