Use of Binder-Treated Ferrous PM Premixes for Improved PM Part Production and Part Density

Transcription

1 Use of Binder-Treated Ferrous PM Premixes for Improved PM Part Production and Part Density Francis Hanejko & William Tambussi Hoeganaes Corporation Cinnaminson, NJ USA Abstract: The benefits of utilizing phosphorous as an alloying element in pure iron are well documented. These include improved DC magnetic performance, higher sintered densities, and good mechanical properties. A major disadvantage of alloying phosphorus in iron is the greater tool wear experienced with using ferro-phosphorous additives. Utilizing an experimental binder treatment process, a reduction in the strip and slide was observed during laboratory testing of both traditional binder treated premixes at sintered densities less than 7.0 g/cm³ and with warm die materials at sintered densities of >7.2 g/cm³. This reduction in strip and slide has the potential to minimize die wear. Both the mechanical and magnetic properties of the new material will be presented. Introduction: Advantages of using phosphorus in PM steels include good magnetic properties, higher magnetic resistivity, and good mechanical properties with excellent elongation and impact toughness. [1,2] Typical applications for phosphorous containing materials (FY-4500) include automotive speed sensors, flux rings for DC electric motors, and electromagnetic actuators. [3] Additionally, phosphorous is added in FC-0205 type steels utilized in automotive main bearing caps because of the unique combination of strength, ductility, and good sintering response. [4] The major disadvantage of phosphorous containing materials is that phosphorous is added as a ferro-phosphorous inter-metallic. Whether Fe 2 P or Fe 3 P, these inter-metallic compounds possess high hardness, which then results in a powder premix consisting of a relatively soft iron matrix incorporating a high hardness second phase particle that can cause significant die wear. The hardness values of Fe 2 P and Fe 3 P are ~1050 HV and 1000 HV, respectively; this is compared to an iron particle hardness of ~100 BHN (no conversion to HV is realistic at this low hardness). [5] These high hardness particles result in abrasive wear of the punches and often the actual tool material. Frayman has reported that the hard particle additions abrade the metallic binder of carbide inserts. [6] The consequence of these hard particles is excessive tool wear with greater frequency of tool repair and replacement. Utilizing advanced die coatings has lessened tool wear but not eliminated it. [7] Thus, users of FY-4500 simply live with the inherent high tool wear and frequent die repair associated with ferro-phosphorus containing materials. Hoeganaes Corporation produces FY-4500 materials as a binder treated premix (ANCORBOND ). Although this binder treatment does not reduce the abrasive nature of ferro-phosphorus, it minimizes segregation thus preventing localized tool wear because of potential segregation. The experimental work described in this report focuses on an experimental enhanced binder treatment system designed to counteract the abrasive characteristic of ferro-phosphorus particles. Although the exact nature of this new binder system is proprietary, it does function with both conventional PM lubricants designed for both conventional and with modifications can be utilized with warm die processes. The work discussed in this report will document the mechanical properties, laboratory measured ejection characteristics of this experimental binder treatment system.

2 Experimental Procedure: In this study, four laboratory premixes were prepared and evaluated; these premixes are listed in Table 1. All premixes were prepared by adding 2.9% Fe 3 P plus the appropriate powder lubricant; thus producing a final sintered phosphorus content of ~0.45%. Table 1 Premixes Evaluated in this Investigation Premix ID Base Iron Type Premix Type 1 High Conventional (ANCORBOND Compressibility Binder treated, FY-4500) Iron Powder cold 2 3 (AncorMax 200 FY-4500) 4 High Compressibility Iron Powder High Compressibility Iron Powder High Compressibility Iron Powder Enhanced binder treated, cold Warm die, conventional Enhanced binder treated, warm die Lubricant added % Lubricant Added Acrawax 0.75% Proprietary 0.75% Proprietary 0.40% Proprietary 0.40% The premixes were evaluated for compressibility, transverse rupture strength, green strength, tensile and impact properties. Compaction conditions for the cold conditions were a die temperature of ~25 C (75 F) and pressures of 415, 550, and 690 MPa. For the warm die premixes, the die temperature was ~93 C (200 F) with pressures of 550, 690, and 830 MPa. In addition to the mechanical property specimens magnetic toroids were pressed from mixes 1 and 2 to a density of 6.9 g/cm³ and from mixes 3 and 4 to a density of 7.25 g/cm³. After, all samples were sintered in a laboratory belt furnace on ceramic trays. Sintering conditions were 1120 C (2050 F) in a 90% nitrogen / 10% hydrogen atmosphere, with a cooling rate after sintering of ~0.6 C per second. The time above 1095 C (2000 F) was approximately 20 minutes. After sintering, the samples were tested via the appropriate MPIF test standard. [8] In addition to the mechanical property test samples, ejection characteristics were quantified. The methodology employed was to compact a green strength bar at 690 MPa, then measure two key characteristics, strip and slide. Strip is defined as the initial break free load required to initiate movement of the test specimen and slide is defined as the load immediately before the specimen exits the die. Strip and slides pressures are then calculated by dividing the measured force by the area of the bar in contact with the die surface. Results: Presented in Table 2 are the powder properties of the four premixes evaluated in this study. The advanced binder treatment utilized in combination with the high compressibility iron does not affect the apparent density (AD) or flow characteristics relative to conventional processing. The data presented in Table 2 was collected on 250 kg laboratory prepared premixes.

3 Table 2 Powder Properties of Materials Evaluated Premix ID Base Iron Type Premix Type AD, g/cm³ Flow, sec / 50 g 1 FY-4500 Conventional, cold FY-4500 Enhanced, cold FY-4500 Warm die, conventional FY-4500 Enhanced, warm die Figure 1: Compressibility data for four materials evaluated, all premixes with 2.9% added ferro phosphorus Compressibility information is presented in Figure 1. As expected, the warm die premixes show ~0.10 g/cm³ to 0.20 g/cm³ improvement in compressibility compared to the conventional premixes. Compressibility of FY-4500 for the standard and experimental enhanced binder systems is identical for both cold and warm die grades. Thus, the experimental binder system had no detrimental effects on the compressibility of the FY alloy system. Figure 2 presents the green strength of the FY-4500 materials compacted at both room temperature and at 93 C (200 F). The experimental binder system has no detrimental effect on the green strength of FY-4500 regardless of conditions. It is noteworthy; warm die gives an approximate 75% increase in green strength over the range of green densities evaluated. This higher green strength can result in a potential reduction in green part damage during part handling prior to sintering.

4 Figure 2: Green Strength of Tested Materials Table 3 Sintered Mechanical Properties of the Six Premixes Evaluated Mix ID Std FY cold Exp FY cold Warm die FY-4500 Exp warm die FY-4500 Sintered Density, g/cm³ Yield Strength, MPa (psi) 218 (31,700) 245 (35,700) 267 (38,800) 216 (31,400) 241 (35,000) 258 (37,500) 232 (33,700) 258 (37,500) 269 (39,100) 229 (33,300) 261 (37,900) 273 (39,700) UTS, MPa (psi) Elongation, % Impact, Joules (ft.lbf) 280 (40.600) (7) (47,100) (17) (51,700) (22) (39,600) (10) (46,800) (14) (51,300) (20) (47,500) (13) (53,400) (25) (55,600) (39) (47,100) (11) (53,900) (22) 38 Hardness, HRA 390 (56,700) (42) 40

5 Table 3 presents the mechanical properties developed for the four alloys investigated. Transverse ruptures test samples were prepared for each of the materials; however, the ductility of the six materials invalidated these data. The TRS samples were utilized to quantify the DC after sintering. Referring to Table 3, premixes 1 and 3 are commercially available materials containing ferro phosphorus. Premixes 2 and 4 represent the experimental binder system materials. Comparing the commercial premix to the experimentally prepared premix, there is no difference in either the measured tensile or impact properties. Figure 3 presents the dimensional change data for the four materials after sintering at 1120 C (2050 F) in a 90% nitrogen / 10% hydrogen atmosphere. The experimental binder system for the cold die showed slightly higher shrinkage by approximately 0.02% at the 6.8 g/cm³ sintered density compared to the commercial material (within experimental measuring error). The sintered dimensional change for the warm die FL-4500 was identical for both the conventional material and the experimental binder system. Figure 3: Dimensional change of the premixes, premixes 3 and 4 are excluded because of carbon pick up during sintering. Table 4 presents the DC magnetic data developed for each premix. From the data two trends are apparent. First, the experimental binder system does not affect the magnetic performance of the either FY-4500 at either 6.9 g/cm³ or 7.25 g/cm³ sintered densities. The data presented in Table 4 appears to suggest that higher density does not improve the magnetic properties of PM materials. This is contradictory to data presented in MPIF standard 35, which indicates that higher part density produces higher magnetic performance. Sintered carbon and sintered oxygen results for 4 materials were identical at <0.01% and ~0.06% respectively. It is expected that the permeability at this ~7.3 g/cm³ sintered density should be ~3500. Ejection characteristics as measured by strip and slide are presented in Table 5. At ambient conditions, the experimental binder system shows a 10% lower stripping pressure and ~5% lower sliding pressure compared to the standard ANCORBOND treated material. At warm die conditions, the standard material has a significantly higher strip and slide compared to the lower density green strength bars. However, the modified experimental binder produces a dramatic decrease in both the strip and slide.

6 Mix ID Sintered Density, g/cm³ Table 4 Sintered DC Magnetic Data Applied Field, Oe Max DC Perm Max Induction, kgauss Hc, Oersteds Br, kgauss Std FY cold Exp FY cold Warm die FY Exp warm die FY Table 5 Ejection Characteristics of FY-4500 Premix ID Std FY-4500 cold Exp FY-4500 cold Warm die FY-4500 Exp warm die FY-4500 Green Density, g/cm³ Stripping Pressure, MPa (psi) Sliding Pressure, MPa (psi) (5630) 19.8 (2870) (5150) 19.1 (2780) (8700) 48.2 (7000) (4800) 29.6 (4300) Discussion: The experimental binder treatment described showed reduced ejection forces for FY-4500 materials as measured by strip and slide stress. In addition to the reduced ejection forces, mechanical property testing demonstrated no deleterious effect on the compressibility or sintered mechanical properties of the FY-4500 material system. To verify the usefulness of the laboratory data, preliminary testing was performed on production PM components. Beta site testing showed reduced ejection pressures, reduced die wear, and an overall improvement in the characteristics of FY-4500 material at green densities ranging from 6.8 to 7.0 g/cm³. A qualitative observation was reduced press adjustments during extended production runs. The fewer number of press adjustments was attributed to reduced entrapment of powder between the punches and die thus enabling the press to return to the fill position with less powder drag between the punches and die assembly. In terms of die wear, measurement of the top punch showed no measurable wear on the top punch after a production run of ~50,000 parts. This represented a considerable improvement from the commercially available material. It was also noted that no differences were observed in either the powder compressibility or sintered dimensional change response.

7 This experimental binder system for the FY-4500 material system has demonstrated that equivalent powder compressibility with identical sintered mechanical properties can be achieved. The implication of this data is reduced die wear and potentially lower die repair and replacement cost. This new development can potentially lead to further utilization of ferro-phosphorus containing premixes in new magnetic applications. Conclusions: From the results presented in this paper, the enhanced binder treatment of ferro-phosphorus containing materials has demonstrated the following: Warm die of FY-4500 type materials gives a 0.1 g/cm³ to 0.2 g/cm³ increase in green density at equivalent pressures. Warm die results in an increase in green strength of ~ 75% at equivalent green densities. The experimental binder system reduces the stripping and sliding stresses of both cold compacted and warm compacted FY This experimental binder system has identical compressibility to the ANCORBOND and AncorMax 200 materials. No difference in DC was observed with the either cold die or warm die compacted materials. Magnetic performance was unaffected; as anticipated, higher sintered densities should produce better magnetic performance. Mechanical properties are identical. The experimental binder system described in this report is still undergoing laboratory development and is not currently commercially available. References: 1. Material Standard for PM Structural Parts (MPIF Standard 35), 2009 Edition, Published by Metal Powders Industries Federation, Princeton, NJ, H. Rutz, F. Hanejko, C. Oliver, Effects of Processing and Materials on Soft Magnetic Performance of Powder Metallurgy Parts, Advances in Powder Metallurgy and Particulate Materials 1992, Vol. 6, pp , Metal Powder Industries Federation, Princeton, NJ, H. Rutz, F. Hanejko, G. Ellis, The Manufacture of Electromagnetic Components by the Powder Metallurgy Process, International Conference on Powder Metallurgy and Particulate Materials, Metal Powders Industry Federation, Princeton NJ, Donald White, Auto Industry Boosts PM Parts, Heat Treat Magazine, January 1993, p J. Nowacki, Phosphorus in iron alloys surface engineering, Journal of Achievements in Materials and Manufacturing Engineering, Vol. 24, Issue 1, September 2007, pp to Leonid Freyman, Advanced Cemented Carbides for PM Tooling Applications, Presented at 2009 PM Parts Compacting/Tooling Seminar, Cleveland OH, Oct 27 28, 2009, Sponsored in Cooperation with the Powder Metallurgy Equipment Association.

8 7. Robert Jacoby, Coating Technology for Metal Forming Operations, Presented at 2009 PM Parts Compacting/Tooling Seminar, Cleveland OH, Oct 27 28, 2009, Sponsored in Cooperation with the Powder Metallurgy Equipment Association. 8. Standard Test Methods for Metal Powders and Powder Metallurgy Products, 2008 Edition, Published by Metal Powders Industries Federation, Princeton, NJ, 2008.