The Effect of Processing and Density on P/M Soft Magnetic Properties. Ian W. Donaldson, GKN Sinter Metals, Worcester, MA, USA

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1 he Effect of Processing and Density on P/M Soft Magnetic Properties Ian W. Donaldson, GKN Sinter Metals, Worcester, MA, USA Fran Hanejko, Hoeganaes Corporation, Cinnaminson, NJ, USA Abstract: With the trend towards more widespread use of automotive electric systems such as electric power steering, new opportunities exist for P/M soft magnetic alloys. hese applications require high density for magnetic properties and precision. o meet density, precision and geometry complexity requirements, secondary operations are usually employed, which degrade magnetic properties. Annealing can be utilized for recovery of the magnetic properties, but with the potential for dimensional changes. hrough the use of an advanced binder system, higher densities with subsequent increases in magnetic properties can be achieved in a single compaction step. he influence of secondary operations, processing methods such as the use of an advanced binder system and annealing are presented for Fe, Fe-P and Fe-Ni materials. Introduction: he powder metallurgy process offers near net shape for magnetic components. his, coupled with the ability to modify and control the chemical composition along with the resultant magnetic properties comparing to wrought materials, have led to growth opportunities. he trend in automotive applications has been with more complex geometries and tighter tolerances, which has allowed for the replacement of low carbon steels. he proper selection of the P/M materials along with the appropriate processing conditions will result in the magnetic properties required for the specific application. Processing and post processing effects on density and microstructure, which in turn affect the soft magnetic response and physical properties, must be understood and controlled. he effects of various processes and microstructures on these properties have been written about earlier [1,2,3,4]. his paper further explores these effects for development of tactics in manufacturing prototypes and production parts. est Methods: est specimens were processed and evaluated as described in each section. For warm compaction, AncorMax D processing was with a heated die at 60 C and with ANCORDENSE processing, the powder and die were heated to 135 C. ensile properties were developed from flat, un-machined dogbone tensile bars according to ASM E8 and MPIF Standard 10 [5]. DC hysteresis loops were generated per ASM A773/A773M-01 on either standard toroid shapes (3.60 cm OD x 2.23 cm ID x 0.62 cm high) or on other samples as described with an OS Walker AMH 20 Hysteresisgraph. After processing, the samples tested for magnetic response were wound with primary and secondary turns of #28 AWG wire. 1

2 RESULS AND DISCUSSION Primary and post processing considerations can be more important for soft magnetic components than structural components due to the significant impact these can have on the soft magnetic response. Figure 1 shows various processing routes that can be utilized in producing magnetic components. Pure Iron Powders Premix Operation Lubricants, Alloys (P, Si) Compaction ANCORDENSE or AncorMax D Process Warm Compaction Pre-Sinter Repress DPDS Process Sinter Secondary Operations Machining, Coining, Sizing, Annealing, etc. Finished Part Figure 1: Processing Routes for P/M Soft Magnetic Components Prototype Processing At a prototype stage, machining of blanks is a common method for testing the feasibility of design. But this can lead to a difference in properties as compared to the production process that may be utilized. A test was performed comparing machined blanks and production parts made with Ancorsteel 45P. he production process was compaction to 7.15 g/cm 3, sintered at 1120 C, then coined to a 7.2 g/cm 3 density as a means to qualify dimensions. he blanks were pressed to a 7.2 g/cm 3 density, sintered at 1120 C, and then machined to the final dimensions. No annealing was performed on either of the sample groups. he samples were wound with 58 primary and secondary #28 AWG wire and tested with a drive field of 1195 A/m. Both samples were measured at <0.01% total carbon. It was found that the machined samples had a 34% lower permeability (1290 versus 1950) and 25% higher coercivity (236.4 A/m versus A/m) than the coined parts. Compaction Magnetic performance improves with increasing density provided the post compaction processing is the same. Figure 2 shows maximum permeability of different materials (Ancorsteel 1000, 1000B, 1000C, 45P and 80P) as a function of density and purity level. Impurities levels are lower for Ancorsteel 1000C as compared to Ancorsteel 1000B, which is lower than Ancorsteel All samples were sintered at 1120 C in v/o N 2 -H 2 atmosphere. As noted, the permeability increases with increasing density level and with increasing purity level. Additions of Fe 3 P improve both the maximum permeability and material resistivity because the phosphorus promotes liquid phase sintering and stabilization of the BCC phase. 2

3 Maximum Permeability Ancorsteel 80P Ancorsteel 45P Ancorsteel 1000C Ancorsteel 1000B Ancorsteel Density (g/cm 3 ) Figure 2: Maximum Permeability as a Function of Density and Purity Level Increasing density at compaction results in an increase in sintered densities with subsequent increase in soft magnetic properties. Figure 3 shows the comparison of Ancorsteel 45P compacted via conventional and warm compaction (AncorMax D and ANCORDENSE) methods. As shown, the soft magnetic properties show an increase with increasing density. Linear regression of the properties for these processing conditions revealed a strong linear relationship with density regardless of compaction method, with R 2 values > AncorMax D ANCORDENSE Maximum Permeability Conventional Conventional AncorMax D Bmax () Density (g/cm 3 ) 0 Max Perm Bmax Figure 3: Permeability and Bmax for Ancorsteel 45P Sintered at 1120 C in 75 v/o Hydrogen and 25 v/o Nitrogen as a Function of Density and Compaction Method 3

4 Sintering he sintering process can have an affect on the magnetic performance. Increasing temperature will provide for an improvement. Interstitials such as carbon, nitrogen and oxygen will also have an influence, such as forming magnetic domain-pinning precipitates or oxides, which degrade the properties. For example, with increasing nitrogen content, a decrease in permeability and increase in coercivity occurs (see Figure 4), with a more pronounced effect occurring at a higher sintering temperature. Control of the sintering process to minimize the effect is of importance. he effect of sintering temperature on 50/50 Fe-Ni with a 0.4% Si content is detailed in able 1. As expected, the soft magnetic properties improve with increasing temperature since grain growth and pore coalescence is enhanced at the higher temperature. With this material, the Si content can affect the sintering kinetics and resultant permeability. A comparison at 1180 C is shown, with a greater difference seen at the higher compaction pressure. Permeability was improved up to 46% at the higher density. Maximum Perm C - Hc 1120 C -µmax 1260 C -µmax 1260 C - Hc Sintered Nitrogen Content (w/o) Coercive (A/m) Figure 4: Effect of Nitrogen and Sintering emperature on 45P Compacted at 7.3 g/cm 3 via ANCORDENSE and Sintered in a N 2 -H 2 Atmosphere Si Content 0.4% 0.3% Compaction Pressure, MPa 415 Sintering emperature C Sintered Density g/cm 3 Permeability Coercive Force A/m Maximum Induction able 1: Effect of Sintering emperature and Si Content for 50/50 Fe-Ni Sintered in a N 2 -H 2 Atmosphere ested at a 1195 A/m Drive Field 4

5 Secondary Operations Various secondary operations can be utilized to achieve the performance requirements of the application. Quite often, sizing or coining is used to qualify the dimensions. he effects of this process will vary based on the amount of cold work that is induced. he plastic deformation associated with sizing strains the iron lattice, which restricts the magnetic domain movement under a magnetic field resulting in a decrease in maximum permeability and coercivity. able 2 shows a comparison of magnetic properties for 45P undergoing full feature sizing at 6.8 g/cm 3. As shown, a significant drop in magnetic properties occurs with sizing. With the addition of annealing, the maximum permeability can be recovered. Condition Max. Permeability HC (A/m) Induction () As-Sintered Sintered and Sized Sintered, Sized and Annealed (815 C for 60 Minutes) able 2: Effect of Sizing on 45P at 6.8 g/cm 3 Measured at 1195 Drive Field With the addition of an annealing operation, the amount of time at temperature can affect the overall cost of the component, so optimization of the process with respect to the performance objectives of the application needs to be realized. Sample parts were compacted at 7.15 g/cm 3 with Ancorsteel 45P, sintered at 1120 C, then sized to 7.20 g/cm 3. hese were then tested as a function of time at temperature. he results are shown in able 3. As can be seen, only a 2% and 9% improvement was seen in permeability at 30 and 60 minutes, respectively, as compared to 15 minutes. Coercivity decreased 2% and 5% at 30 and 60 minutes, respectively, as compared to 15 minutes. Another aspect of the coining process that needs to be considered is the reduction of elongation. For example, this is important when the magnetic component is press fit over a shaft. he effect can be significant. For example, an Fe ring compacted to a 7.0 g/cm 3 density, sintered at 1180 C in an N 2 -H 2 atmosphere had an elongation greater than 6%. Coining the ring to a density of 7.3 g/cm 3 resulted in a reduction of the elongation to 1.6%. Drive A/m Drive A/m Condition Max Perm Hc A/m Bmax Br Max Perm Hc A/m Bmax As Coined Min Min Min able 3: Effect of Annealing ime at 815 C in N 2 -H 2 for 45P at 7.2 g/cm 3 Br 5

6 Another secondary operation that has been utilized on magnetic components as a method to seal porosity prior to a surface coating is steam treatment. he effect of steam treatment on Ancorsteel 45P at 7.2 g/cm 3 density sintered at 1120 C in N 2 -H 2 atmosphere is shown in able 4. An 18% decrease in maximum permeability and an 8% increase in coercivity were measured. Condition Max. Permeability HC (A/m) Induction () As-Sintered Steam reatment able 4: Steam reatment Effect on 45P at 7.2 g/cm 3 Measured at 1195 A/m Drive Field SUMMARY he P/M route provides a flexible and cost effective method for manufacturing parts for soft magnetic applications. Various processing operations that can be utilized for achieving desired part performance objectives, but the proper selection coupled with the appropriate processing method is essential to meet the required performance targets of the specific application. Increasing densities, either by conventional compaction or warm compaction methods, and increasing sintering temperatures provide for improved magnetic properties. he choice of secondary operations needs to be evaluated with respect to the impact it will have on the magnetic properties. he addition of annealing may be required to restore the properties to a level that is necessary for the performance of the component. Each of the process steps employed in the manufacture of the P/M component must be understood and controlled. REFERENCE 1. Narasimhan, K.S., Kasputis, D.J., Fillari, G., Lynn, J.C., Processing of Ferro-Phosphorus- Containing Mixes in Low Hydrogen Atmospheres, Advances in Powder Metallurgy & Particulate Materials-2002, Vol. 14, pp Guo, R., Cheng, C., Lee, J., Developing a Soft Magnetic P/M Component used in Wireless Communication Devices with High Green Strength Lubricants,, Advances in Powder Metallurgy & Particulate Materials-2002, Vol. 14, pp Hanejko, F.G., Rutz, H.G., Oliver, C.G., Effects of Processing and Materials on Soft Magnetic Performance of Powder Metallurgy Parts, Advances in Powder Metallurgy & Particulate Materials- 1992, Vol. 6, pp Lall, C., he Effect of Sintering emperature and Atmosphere on the Soft Magnetic Properties of P/M Materials, Advances in Powder Metallurgy & Particulate Materials-1992, Vol. 3, pp Standard est Methods for Metal Powders and Powder Metallurgy Products, Metal Powder Industries Federation, Princeton, NJ,