Properties of High Density Diffusion Bonded Alloys

Transcription

1 Properties of High Diffusion Bonded Alloys Ian W. Donaldson, (GKN Sinter Metals, Worcester, MA, USA) Michael L. Marucci, (Hoeganaes Corporation, Cinnaminson, NJ, USA) Abstract: For P/M components, overall mechanical properties can be improved by increasing the density coupled with alloy additions. This can be seen by the excellent properties achieved for high performance applications with material compositions based on 1.75% Ni and 4% Ni diffusion bonded steel powders processed to high densities. Through the use of an advanced binder system, higher densities with subsequent increases in mechanical properties can be achieved in a single compaction step. Further densification can be achieved through the use of the double press, double sinter process coupled with the warm compaction process. The static and dynamic mechanical properties of warm compacted and double pressed, double sintered FD-0205 and FD-0405 with densities up to 7.5 are presented. Introduction: Faced with global competitive pressures, a key for continued growth for the P/M industry lies in the ability to improve on the performance characteristics and cost effectiveness of materials and processing. One of the areas that was focused on with these key objectives in mind was to achieve higher densities in a single compaction step through warm compaction via heating the tools and powder which allowed for increased densities of about 0.15 over conventional compaction 1,2,3. This technology, the ANCORDENSE TM lubricant/binder system, required heating of the powder and the tooling to about 150 C. The heating of the powder produced processing complexity, so the objective was to eliminate this aspect in the compaction process. This led to a recent advancement in warm compaction, which was the development of a new binder/lubricant system, AncorMax D TM. This binder/lubricant system does not require powder heating so it eliminates complexity from the compaction process. The effectiveness of this system with processing FLN and a Ni-Mo-Mn-Cu sinterhardening compositions were evaluated and revealed that it provided a means of achieving a density increase between 0.05 at 415 to greater than 0.1 at 825 without the need to heat powder, with no effect on tensile strength as a function of compaction method 4,5. The advantages of processing diffusion bonded materials to high densities have been shown 2,7. This paper provides the results of material compositions based on 1.75% Ni and 4% Ni diffusion bonded steel powders processed to high densities via the new binder/lubricant system. Experimental Procedures: Material, Specimen Preparation and Processing The scope of the testing was carried out on premixes made using Distaloy 4600A and Distaloy 4800A diffusion bonded base materials (1.75% Ni and 4% Ni, respectively). The premixes were made utilizing pilot size equipment to produce 225 kg premixes. The admixed composition was 0.55% graphite and 0.55% AncorMax D lubricant. 1

2 Standard test samples for transverse rupture, Charpy impact, and tensile testing were compacted on tools modified to heat and maintain temperature at +/- 3 C. All samples were compacted with a die temperature between 57 C to 63 C. Compaction of the standard test specimens was performed between 415 and 825. For the specimens compacted for the double press, double sinter (DPDS) processing, compaction pressures of 550, 690 and 825 were used. After compaction, the single press, single sinter (SPSS) test specimens were sintered in a belt furnace at 1120 C or at 1260 C for 20 minutes in a N 2 based atmosphere with 10 v/o H 2. For the DPDS samples, the first sinter was performed at 790 C for 20 minutes in a batch furnace using a 25 v/o N 2-75 v/o H 2 atmosphere. Repressing was performed at pressures of 550, 690 and 825 for each of the first compaction pressures. After repressing, the samples were either sintered in a laboratory belt furnace at 1120 C or 1260 C for 20 minutes in a N 2 based atmosphere with 10 v/o H 2. Testing Test specimens were processed and evaluated according to industry standard test procedures for green density, and sintered TRS and tensile properties 8. Tensile properties were developed from flat, un-machined dogbone tensile bars according to ASTM E8 and MPIF Standard 10. TRS and tensile testing was performed at a crosshead speed of 0.1 in./min. (2.5 mm/min.). Impact testing was performed on un-notched Charpy bars per MPIF Standard 40. A Rockwell hardness tester was used for apparent hardness measurements in the Rockwell A scale. Optical metallurgical analysis was performed on TRS bars processed from each compaction pressure and sintering temperature. RESULTS AND DISCUSSION AncorMax D is an organic binder/lubricant system that does not contain zinc developed to provide increased densification over conventional lubricants when compacted in heated tools (57 C to 63 C). The bonding process is utilized to disperse the lubricant uniformly throughout the powder mix while bonding it to the powder, ensuring that the lubricant stays properly dispersed and improving the flow rate. This combination allowed for a reduction in the amount of lubricant while maintaining good lubrication during compaction. The SPSS and DPDS mechanical properties for both materials developed for the warm compacted specimens at both sintering temperatures along with the densities achieved are shown below in Tables I and II. The green and sintered densities reported were measured on the TRS samples. One aspect of single compaction of P/M powder materials that must be realized is the pore free density. This is the density of the green compact if all the porosity could be removed from the compact, which is dependent on the density and percentage of each of the premix additions. The practical density limit has been determined to be 98% of the pore free density. For the FD-0205 material, the calculated pore free density is 7.478, based on 98.9% base iron at 7.86, 0.55% graphite at 2.3 and 0.55% lubricant/binder at 1.0. For the FD-0405 material, the pore free density is (based on a base iron at 7.9 ). Therefore, the practical density limit of 98% of the pore free density is for FD-0205 and for FD A review of the data reveals that the 98% pore free density was achieved between 690 and 825. The densification of the two materials via both SPSS and DPDS at 1120 C is 2

3 shown in Figure 1. As expected, the higher Ni content material had a slightly higher density. Compaction Pressure Green FD-0205: 1120 C Sinter Sintered TRS Apparent Hardness HRA Tensile 0.2% Yield Elong FD-0205: 1260 C Sinter FD-0405: 1120 C Sinter FD-0405: 1260 C Sinter % Impact Energy J TABLE I: As-Sintered Mechanical Properties for SPSS Warm Compacted FD-0205 and FD-0405 Sintered at 1120 C and 1260 C in a N 2 -H 2 Atmosphere Comp/Repress Pressure Green FD-0205: 1120 C Sinter Sintered TRS Apparent Hardness HRA Tensile 0.2% Yield Elong. 550 / / / FD-0205: 1260 C Sinter 550 / / / FD-0405: 1120 C Sinter 550 / / / FD-0405: 1260 C Sinter 550 / / % Impact Energy J 830 / TABLE III: As-Sintered Mechanical Properties for DPDS Warm Compacted FD-0205 and FD-0405 Sintered at 1120 C and 1260 C in a N 2 -H 2 Atmosphere 3

4 Sintered ( ) Compaction Pressure () FD-0205 SPSS FD-0405 SPSS FD-0205 DPDS FD-0405 DPDS Figure 1: As-Sintered of SPSS Warm Compacted and Sintered FD-0205 and FD at 1120 C in a N 2 -H 2 Atmosphere The results revealed that mechanical properties generally increased with increasing density, regardless of whether it was SPSS or DPDS. Figure 2 shows TRS and UTS as a function of density, with a linear increase with increasing density at 1120 C. Figure 3 shows Charpy Impact energy as a function of density. An exponential relationship with density was shown. There was a decrease in SPSS strength shown at the highest compaction pressures, which is attributed to the higher elastic recovery under removal of the compaction load and ejection from the die, leading to some separation between the particles. It was noted that there was more variation in the high temperature sintering results. Further investigation is required to determine the reason R 2 = R 2 = TRS R 2 = UTS R 2 = Densit y ( ) FD-0205 TRS FD-0405 TRS Figure 2: As-Sintered TRS and UTS as a Function of for Warm Compacted and Sintered FD-0205 and FD-0405 at 1120 C in a N 2 -H 2 Atmosphere 4

5 80 70 Charpy Impact (J) R 2 = R 2 = Sinter ( ) FD-0205 FD-0405 Figure 3: As-Sintered Charpy Impact Energy as a Function of for Warm Compacted and Sintered FD-0205 and FD-0405 at 1120 C in a N 2 -H 2 Atmosphere For the samples sintered at 1120 C, the mechanical properties increase with increasing density. As shown in figures above, a relatively strong linear relationship for TRS, UTS and Charpy Impact as a function of density was determined with R 2 values generally greater than 0.88, regardless of compaction or repress compaction pressure. These results indicate that the densification method did not negatively affect the linearity of density to tensile properties and exponential relationship with impact typically found with PM steels. The microstructural phases present in sintered materials, along with interparticle bonds, porosity morphology and impurity levels affect the mechanical properties. At low density, the main determinant is the interparticle strength. At the high densities that were present in the samples, there is a complex interdependence of the mechanical properties on the pore morphology and interparticle bonds. In the as-polished microstructure, the pore morphology and distribution were examined. More pore rounding was evidenced at the higher sintering temperature. A decrease in pore size with compaction and repress pressures was found as represented in Figure 4. Figure 4: As-Polished FD-0205 Sintered at 1120 C. Left 7.28, right

6 The etched microstructures from Figure 4 at 1120 C for the two densities for FD-0205 are shown in Figure 5. The microstructure consists of divorced pearlite, fine pearlite, nickelrich regions, bainite and martensite around the nickel-rich regions. The presence of martensite around the pores would lead to some loss in toughness. Pore rounding and more pronounced nickel diffusion was evident in the high temperature samples. Figure 5: FD-0205 Sintered at 1120 C from Figure 4. 2% Nital / 4% Picral CONCLUSIONS 1. The new lubricant/binder system is viable for warm compaction of FD-0205 and FD-0405 to achieve high density in a single compaction step. 2. A density greater than 96% of theoretical density can be achieved through the combination of warm compaction and DPDS. 3. Regression analysis revealed that for parts sintered at 1120 C, the mechanical properties increased as a function of density, regardless of densification method. ACKNOWLEDGEMENTS The authors wish to thank Mr. T. Murphy from Hoeganaes Corporation for his support. REFERENCE 1. Rutz, H.G., Hanejko, F.G., High Processing of High Performance Ferrous Materials, Advances in Powder Metallurgy & Particulate Materials, Vol. 5, 1994, Metal Powder Industries Federation, Princeton, NJ, pp Donaldson, I.W., Hanejko, F.G., An Investigation into the Effects of Processing Methods on the Mechanical Characteristics of High Performance Ferrous P/M Materials, Advances in Powder Metallurgy & Particulate Materials, Vol. 2, Part 5, 1995, Metal Powder Industries Federation, Princeton, NJ, pp Rutz, H.G., Rawlings, A.J., Cimino, T.M., Advanced Properties of High Ferrous Powder Metallurgy Materials, Advances in Powder Metallurgy & Particulate Materials, Vol. 3, Part 10, 1995, Metal Powder Industries Federation, Princeton, NJ, pp Donaldson, I.W., Luk, S., Poszmik, G., Narasimhan, K. S., Processing of Hybrid Alloys to High Densities, Advances in Powder Metallurgy & Particulate Materials, 2002, Part 8, pp Marucci, M.L., Baran, M.C., Narasimhan, K. S., Properties of High Sinter-Hardening P/M Steels Processed Using an Advanced Binder System, Advances in Powder Metallurgy & Particulate Materials, 2002, Part 13, pp Rutz, H.G., Hanejko, F.G., Properties of Diffusion Bonded Alloys Processed to High Densities, Advances in Powder Metallurgy & Particulate Materials, Vol. 3, Part 10, 1995, Metal Powder Industries Federation, Princeton, NJ, pp Donaldson, I.W., An Evaluation of High Performance Materials Processed Using Warm Compaction Technology, Advances in Powder Metallurgy & Particulate Materials, Vol. 2, Part 5, 1996, pp Standard Test Methods for Metal Powders and Powder Metallurgy Products, Metal Powder Industries Federation, Princeton, NJ,