Volume 07 - Powder Metal Technologies and Applications Part 15 docx

Note: Heat treatment: 1150 °C (2100 °F) + 760 °C (1400 °F)/8 h Source: Ref Udimet 720 was originally developed as a wrought turbine blade alloy for industrial turbines In the cast and wrought form it is being used as a turbine disk alloy More recently, the alloy has been evaluated as a P/M material (Ref 18, 19) Reportedly, P/M Udimet 720 has excellent fatigue crack growth resistance and is being strongly considered as a disk material in small to medium gas turbine engines as well as aircraft auxiliary power units Udimet 720 can be produced by HIP or by extrusion plus isothermal forging Figure shows a fully machined P/M Udimet 720 turbine disk produced by extrusion plus isothermal forging A comparison of the costs for P/M and cast and wrought Udimet 720 disks showed more than 20% cost reduction with the P/M process (Ref 18) Tensile properties for P/M Udimet 720 are shown in Fig 8, and the results of fatigue crack growth rate tests of P/M and cast and wrought material are shown in Fig Low-cycle fatigue tests (R = 0.0, Kt = 1.0, F = 20 cpm at 425, 540, and 650 °C (800, 1000, and 1200 °F) have shown that the P/M Udimet 720 has mean lives that are higher than cast and wrought Udimet 720 Fig Fully machined P/M Udimet 720 turbine disk produced by extrusion plus isothermal forging Courtesy of Allison Engine Company Fig Tensile properties of P/M Udimet 720 turbine disk produced by extrusion plus isothermal forging (heat treatment: 1090 °C (2000 °F) + two-step age) Source: Ref 18 Fig Fatigue crack growth rates for P/M Udimet 720 turbine disk produced by extrusion plus isothermal forging (heat treatment: 1090 °C (2000 °F) + two-step age) compared to cast and wrought (C/W) Udimet 720 Source: Ref 18 IN-706 is an iron-nickel-base superalloy that is used for very large forged turbine disks in land-based turbine engines for power generation Cast and wrought processing for these applications includes vacuum induction melting plus electroslag remelting plus vacuum arc remelting to minimize melt-related defects and segregation Powder metallurgy is being developed for IN-706 in an effort to further minimize segregation and to lower the cost of these large forgings (Ref 20, 21) In the P/M work, the effects of varying process parameters (argon and nitrogen atomization, powder size, HIP temperature, forging conditions, and heat treatment) have been studied Tables 14 and 15 give the results of room-temperature tensile and Charpy V-notch tests of P/M IN-706 in the HIP plus heat treated and HIP forged plus heat treated conditions, respectively Typical data for cast plus wrought IN-706 are included for comparison Table 16 gives fracture toughness and low-cycle fatigue data for forged P/M IN-706 Compared to cast and wrought material, the P/M material exhibits good fracture toughness and excellent fatigue resistance The work conducted to date indicates that P/M IN-706 ages more rapidly than cast and wrought IN-706 As a result, some heat treatment modifications may be required for the P/M version of IN-706 Table 14 Room-temperature properties of as-HIP P/M 706 Grade(a) Mesh size N706 N706 A706 A706 -60 -60 -140 -140 HIP temperature 0.2% yield strength °C 1130 1065 1130 1065 MPa 1025 1035 1035 1040 °F 2065 1950 2065 1950 ksi 149 150 150 151 Ultimate tensile strength MPa ksi 1325 192 1325 192 1330 193 1345 195 Tensile elongation, % 23 19 20 21 Reduction of area, % 30 25 27 29 Impact energy J 35 27 26 27 ft · lb 26 20 19 20 Note: Heat treatment: 980 °C (1800 °F)/ h + 732 °C (1350 °F)/10 h + 620 °C (1150 °F)/8 h Source: Ref 21 N706, nitrogen atomized; A706, argon atomized (a) Table 15 Room-temperature properties of as-HIP plus forged P/M 706 Grade(a) Mesh size N706 N706 A706 A706 -60 -60 -140 -140 HIP temperature 0.2% yield strength °C 1130 1065 1130 1065 MPa 860 855 855 845 °F 2065 1950 2065 1950 ksi 125 125 124 123 Ultimate tensile strength MPa ksi 1205 177 1206 177 1205 177 1190 175 Tensile elongation, % 23 23 22 23 Reduction of area, % 44 44 40 39 Impact energy J 50 49 41 38 ft · lb 37 36 30 28 Note: Heat treatment: 980 °C (1800 °F)/ h + 732 °C (1350 °F)/10 h + 620 °C (1150 °F)/8 h N706, nitrogen atomized; A706, argon atomized (a) Table 16 Room-temperature fracture toughness and 750 °C (1380 °F) and 900 °C (1650 °F) low-cycle fatigue properties of HIP plus forged P/M 706 Alloy(a) N706 N706 A706 A706 Mesh size HIP temperature °C °F JIc 10-2 · Nm/m2 in · lb/in -60 -60 -140 -140 1130 1065 1130 1065 9.3 7.5 7.4 533 426 420 2065 1950 2065 1950 KIc MPa ksi 145 130 129 133 119 118 Low-cycle fatigue(b) Cycles to failure Cycles to failure at 750 °C at 900 °C (1380 °F) (1650 °F) 34,312 32,820 26,515 52,423 41,846 64,055 27,623 45,916 Note: Heat treatment: 980 °C (1800 °F)/ h + 732 °C (1350 °F)/10 h + 620 °C (1150 °F)/8 h (a) (b) N706, nitrogen atomized; A706, argon atomized Tested at 0.7% strain In addition to excellent mechanical properties, P/M IN-706 is highly resistant to grain growth at elevated temperatures This is shown in Fig 10, which compares the grain growth characteristics of IN-706 with cast and wrought IN-706 The grain growth resistance of P/M IN-706 may permit the use of higher forging temperatures and lower forging forces than that used for cast and wrought material Powder metallurgy would also permit larger finish cross sections and more reliable ultrasonic inspection Fig 10 Grain size of HIP plus forged P/M 706 and cast and wrought (C/W) 706 after heat treating Source: Ref 21 IN-718 is similar to IN-706, but has higher elevated-temperature strength Powder metallurgy IN-718 is being considered as a disk material in the next generation of land-based power generation turbine engines (Ref 22) Research is currently in progress, but no mechanical property data have been reported in the literature The potential advantages of P/M IN-718 are similar to those described above for P/M IN-706 AF115 was developed in the 1970s as a very-high-strength P/M disk alloy for 1400-F service (Ref 23), but the alloy is not currently specified for any engine system However, a lower-carbon version of the alloy (0.05% versus the original 0.15%) continues to be considered for disk application in the as-HIP and HIP plus forged conditions (Ref 24, 25, 26) Typical properties for the alloy in the as-HIP condition are given in Fig 11, 12, 13, and 14 Properties in HIP plus forged condition are discussed below in the section "Dual-Alloy Turbine Disks/Wheels" in this article Fig 11 Tensile properties of as-HIP P/M AF115 HIP: 1180 °C (2155 °F)/3 h/102 MPa (15 ksi) Heat treatment: 1175 °C (2150 °F) + 760 °C (1400 °F) air cool Source: Ref 25 Fig 12 Stress rupture strength of as-HIP P/M AF115 HIP: 1180 °C (2155 °F)/3 h/102 MPa (15 ksi) Heat treatment: 1175 °C (2150 °F) + 760 °C (1400 °F) air cool T, in K; t, in h Source: Ref 25 Fig 13 Low-cycle fatigue life of as-HIP P/M AF115 at 635 °C (1175 °F) HIP: 1180 °C (2155 °F)/3 h/102 MPa (15 ksi) Heat treatment: 1175 °C (2150 °F) + 760 °C (1400 °F) air cool Source: Ref 25 Fig 14 Fatigue crack growth rate of as-HIP P/M AF115 at 635 °C (1175 °F) HIP: 1180 °C (2155 °F)/3 h/102 MPa (15 ksi) Heat treatment: 1175 °C (2150 °F) + 760 °C (1400 °F) air cool Source: Ref 25 AF2-1DA-6 is one of the strongest nickel-base superalloys available for use in the temperature range of 650 to 980 °C (1200 to 1800 °F) Typical tensile rupture properties are given in Tables 17 and 18 Powder metallurgy AF2-1DA-6 has been evaluated as a turbine disk material (Ref 29) However, the alloy is not currently specified for any turbine engine application Table 17 Tensile properties of P/M AF2-1DA-6 produced by extrusion plus isothermal forging Temperature °C °F 70 21 760 1400 870 1600 0.2% yield strength MPa ksi 1138 165 1000 145 793 115 Tensile strength MPa ksi 1586 230 1227 178 1206 175 Elongation, % 12 10 Reduction of area, % 12 10 10 Table 18 Stress rupture properties of P/M AF2-1DA-6 produced by extrusion plus isothermal forging Temperature °C °F 760 1400 816 1500 Stress MPa ksi 662 96 483 70 Rupture life, h 50 55 Elongation, % Reduction area, % 7 PA101 is a hafnium-containing modification of the cast alloy IN-792 (Ref 30) The alloy is not currently specified for any engine system, but it has been successfully evaluated as one component of a dual-alloy turbine wheel for small turbine engines (Ref 31, 32) Tensile and stress rupture properties of as-HIP PA101 in a dual-property wheel are given in Tables 19 and 20 Table 19 Tensile properties of as-HIP P/M PA101 produced as part of a dual-property wheel Temperature °C 20 650 760 °F 70 1200 1400 0.2% yield strength MPa ksi 945 137 896 130 875 127 Tensile strength MPa ksi 1472 213 1315 191 1088 159 Elongation, % Reduction of area, % 15 10 11 14 13 15 Table 20 Stress rupture properties of as-HIP P/M PA101 produced as part of a dual-alloy wheel Temperature °C °F 650 1200 760 1400 Stress MPa ksi 862 125 586 85 Rupture life, h 98 70 Elongation, % 10 MERL 76 is a P/M alloy that was developed for as-HIP fabrication of turbine engine components (Ref 33) It was designed to have properties in the HIP condition similar to those of isothermally forged IN-100 There are no current applications for MERL 76 Figures 15 and 16 give tensile and stress rupture properties, respectively, for the alloy in the as-HIP plus heat treated condition Fig 15 Tensile and yield strength for as-HIP MERL 76 Heat treatment: 1105 °C (2125 °F)/2 h/oil quench + 870 °C (1600 °F)/40 min/air cool + 982 °C (1800 °F)/45 min/air cool + 650 °C (1200 °F)/24 h/air cool + 760 °C (1400 °F)/16 h/air cool Source: Ref 33 Fig 16 Stress rupture properties of as-HIP MERL 76 Heat treatment: 1105 °C (2125 °F)/2 h/oil quench + 870 °C (1600 °F)/40 min/air cool + 982 °C (1800 °F)/45 min/air cool + 650 °C (1200 °F)/24 h/air cool + 760 °C (1400 °F)/16 h/air cool Source: Ref 33 References cited in this section G.S Hoppin, III and W.P Danesi, Manufacturing Processes for Long-Life Gas Turbines, J Metal., July 1986, p 20-23 AlliedSignal Engine Division, private communication, Sept 1997 J.L Bartos and P.S Mathur, Development of Hot Isostatically Pressed (As-HIP) Powder Metallurgy René 95 Turbine Hardware, Superalloys: Metallurgy and Manufacture, Proc of the Third Int Symp., B.H Kear et al., Ed., Claitor's Publishing Division, 1976, p 495-508 General Electric Aircraft Engines Crucible Compaction Metals, Oakdale, PA M.M Allen, RL Athey, and J.B Moore, Nickel-Base Superalloy Powder Metallurgy State of the Art, Progress in Powder Metallurgy, Vol 31, Metal Powder Industries Federation, 1975 10 J.E Coyne, W.H Couts, C.C Chen, and R.P Roehm, Superalloy Turbine Components Which is the Superior Manufacturing Process: As-HIP, HIP + Isoforge or Gatorizing of Extrusion, Powder Metallurgy Superalloys-Aerospace Materials for the 1980's, Vol 1, MPR Publishing, 1980 11 Pratt & Whitney Aircraft, private communication, 1997 12 "Crucible Nickel Base Superalloys; Low Carbon Astroloy," Crucible Compaction Metals, Oakdale, PA 13 C Ducrocq, A Lasalmonie, and Y Honnorat, N 18, A New Damage Tolerant PM Superalloy for High Temperature, Superalloys 1988, The Metallurgical Society, 1988 14 J.H Davidson, G Raisson, and O Faral, The Industrial Development of a New PM Superalloy for Critical Table Mechanical property and fatigue data for iron-copper-carbon alloys with sulfur additions Sintered at 1120 °C (2050 °F) in dissociated ammonia, reheated to 980 °C (1800 °F) in dissociated ammonia, and forged Forging mode Carbon, % Oxygen, ppm Sulfur, % Ultimate tensile strength Manganese sulfide Sulfur 0.59 0.63 270 160 0.13 0.14 MPa 915 840 ksi 133 122 0.2% offset yield strength MPa ksi 620 90 560 81 Elongation in 25 mm (1 in.),% Reduction of area, % 11 12 23.2 21.4 Room-temperature Charpy V-notch impact energy J ft · lbf 6.8 5.0 6.8 5.0 Core hardness, HV30 290 267 Fatigue endurance limit MPa ksi 430 62 415 60 Ratio of fatigue endurance to tensile strength 0.47 0.50 Effects on Mechanical Properties (adapted from Ref 12) In powder forging, porosity occurs near part surfaces where die chilling occurs and also where cracking occurs during the early stages of forging due to the presence of tensile stresses Preform redesign and providing suitable friction conditions aid in eliminating or reducing residual porosity The effects of small amounts of residual porosity on the mechanical properties of P/M forgings were first investigated by Kaufman and Mocarski in the 1970s (Ref 13) Compacted preforms of two different size iron powders and 0.57% graphite were forged at a constant pressure of 440 MPa (32 tsi) Density variations were produced by forging between 700 to 1040 °C (1300 and 1900 °F) Figure 10 shows almost a linear relationship, with density increasing with an increase in forging temperature Figure 11 shows that the yield strength decreases rapidly as porosity concentration increases for normalized steel and for samples held at a temperature below the eutectoid and spheroidized condition Fig 10 Hot compressibility of low-alloy steel powder compacts under a forging pressure of 440 MPa (32 tsi) Source: Ref 13 Fig 11 Effect of porosity on (a) yield strength and (b) ultimate tensile strength Source: Ref 13 Particle size distribution in the initial powder does not have much effect if samples are forged at the same temperature The effects of porosity and second-phase dispersions were additive, but independent of one another Tensile strength was found to monotonically decrease at about the same rate as yield strength with pore concentration (Fig 11) Toughness In steels, the lower initial preform density (75 to 83% of theoretical) results in optimal toughness in fully dense forgings This is attributed to the fact that the lower density preforms have a significantly larger amount of interconnected porosity; during sintering, the oxide reducing gas can penetrate the lower density preforms to a greater extent, thus lowering the final oxide content prior to forging and thereby raising the final toughness after forging Observations of Ferguson et al (Ref 14) have shown that toughness (Charpy energy) is enhanced by lateral flow produced by upsetting in fully dense ferrous powder forgings Fatigue strength and resistance to crack propagation under conditions of cyclic loading also are enhanced by lateral flow at full density An extensive experimental study and analysis were conducted by Ferguson et al (Ref 15, 16) in terms of microstructure and stress state using specimens hot forged in plane-strain conditions Figures 12 and 13 represent the increase in fatigue strength with increased strain (lateral flow) in as-forged and heat treated conditions from axial fatigue tests (Ref 17) Figure 14 represents the increase in endurance ratio in the case of upset versus re-pressed specimens in the forged and heat treated conditions Figure 15 compares the S-N curves for a high-flow P/M prototype part with corresponding parts made by re-pressing and parts machined from a cast and wrought stock Axial and rotating bend fatigue data are discussed in Ref 17 Fig 12 Axial fatigue S-N curves for full-density powder forgings of 4620 as a function of height strain, as-forged condition Source: Ref 17 Fig 13 Axial fatigue S-N curves for full-density powder forgings of 4620 as a function of height strain, heat treated condition Source: Ref 17 Fig 14 Endurance ratio of 4620 powder forgings in axial fatigue as a function of height strain Source: Ref 17 Fig 15 S-N curves for simulated axial fatigue on prototype powder forged parts C and W indicate cast and wrought, respectively Source: Ref 17 Heat treatment of powder forged steels is similar to that of wrought steel For ferrous forgings, conventional quenchand-temper cycles can be used to heat treat parts Figure 16 shows that tensile and yield strengths are comparable for 4640 bar stock and 4640 powder forged material heat treated to the same hardness Hardenability is slightly lower in powder forged steels, owing to the finer grain sizes of powder forged parts as compared to wrought forms Typical hardenability curves for some common powder forged steels are in Fig 17 Fig 16 Tensile properties of P/M forged 4640 and typical properties of wrought 4640 Source: Ref 18 Fig 17 Jominy hardenability curves for (a) P/F-4600, (b) P/F-4200, and (c) Fe-Cu-C materials at various forged carbon levels Vickers hardness was determined at a 30 kgf load Testing was carried out according to ASTM A 255 Specimens were machined from upset forged billets that had been sintered at 1120 °C (2050 °F) in dissociated ammonia Injection Molding Finely divided (1 to 10 m) powders can be mixed with organic binder and injection molded like plastics After binder removal, metal preforms can be sintered to near-full density Mass transport mechanisms during sintering are enhanced by the high surface area of the powder and the elevated temperatures of 1150 to 1315 °C (2100 to 2400 °F) Densification is accompanied by more than 10% linear shrinkage High final densities, often 95 to 99%, result in improved dynamic properties compared to conventionally pressed and sintered materials Current metal injection molded (MIM) ferrous grades include both prealloyed powders and admixtures of iron powder and alloying elements such as nickel, molybdenum, and carbon Chemical compositions of MPIF ferrous grades include: MPIF designation MIM-4600 MIM-4650 Composition, wt % Ni Mo C 1.5-2.5 0.0-0.5 0.0-0.1 1.5-2.5 0.0-0.5 0.4-0.6 Other 2.0 2.0 Fe bal bal Depending on the fineness of the starting powder and the sintering temperature, densities of 93 to 98% of full density are obtained Minimum and typical mechanical property values for 95 and 96% dense MIM low-alloy steels are given in Table 10 As these data indicate, ductility of MIM-processed parts is very high, with elongation values of 30% or higher Table 10 Mechanical properties of MIM low-alloy steels Material designation/condition MIM-4600 as-sintered MIM-4650 as-sintered MIM-4650 quenched and tempered MIM-2700 as-sintered Minimum values Ultimate 0.2% tensile yield strength strength MPa ksi MPa ksi 255 37 110 16 380 55 170 25 1480 215 1310 190 380 55 205 30 0.2% yield strength MPa ksi 125 18 205 30 1480 215 Elongation in 25 mm (1 in.), % Density, g/cm3 Apparent hardness(a) 20.0 11.0 98% of theoretical density) stainless steels Table 11 Typical mechanical properties of nearly dense P/M stainless steel Based on high-temperature sintering Alloy Condition Ultimet 04, 304 Ultimet 16, 316 Sintered Ultimate tensile strength MPa ksi 593 86 MPa 248 Sintered 687 308 99.6 Elongation in 25 mm (1 in.), % Hardness ksi 36 36 80 HRB J 10.8(a) 44.7 26 94 HRB 8.1(a) 0.2% yield strength Density, g/cm3 Theoretical density, g/cm3 ft·lbf 8(a) 7.8 7.9 6(a) 7.7 7.8 Impact strength Ultimet 40C, 440C Hardened and tempered 684 99.3 329 47.7 45 90 HRB 5.4(a) 40(a) 7.7 7.8 2(b) 7.6 7.7 20-30 HRC 50-60 HRC 2.7(b) Solution treated and quenched Sintered 2.7(b) 2(b) 7.6 7.7 Source: Ref 20 (a) (b) Charpy V-notch Unnotched An alternative process for forming stainless steel into mill shapes, such as tubing, is to begin with clean, gas-atomized powder Powder is packed in cans, sealed, cold isostatically pressed to reduce bulk, and extruded into tubing The cold isostatically pressed cans may be hot isostatically pressed to full density, thus eliminating extrusion Table 12 gives mechanical property data for extruded and hot isostatically pressed materials compared to wrought (ingot metallurgy) materials Table 12 Typical mechanical properties of fully dense stainless steel Property P/M Wrought material material Extruded 0.3 by 15.5 mm (0.1 by 0.61 in.) 317LM tube(a) Ultimate tensile strength, MPa (ksi) 693 (100) 693 (100) 324 (47) 353 (51) 0.2% yield strength, MPa (ksi) 71 73 Reduction in area, % 47 50 Elongation in 25 mm (1 in.), % Hot isostatically pressed type 316 Ultimate tensile strength, MPa (ksi) 579 (84) 288 (42) 0.2% yield strength, MPa (ksi) 58 Elongation in 25 mm (1 in.), % Source: Ref 21 (a) Gas-atomized powder, canned, cold isostatically pressed, and extruded References cited in this section W.J Huppmann and L Albano-Muller, Production of Powder Forged Parts of Complex Geometry, Modern Developments in Powder Metallurgy, Vol 12, Metal Powder Industries Federation, 1981, p 631 F Hanejko, Mechanical Properties of Powder Forged 4100 and 1500 Type Alloy Steels, Modern Developments in Powder Metallurgy, Vol 12, Metal Powder Industries Federation, 1981, p 689 R.M Pilliar et al., Fracture Toughness Evaluation of Powder Forged Parts, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 51 P Lindskog, Reduction of Oxide Inclusions in Powder Preforms Prior to Hot Forming, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 285 F Badia, F Heck, and J Tundermann, Effect of Composition and Processing Variations on Properties of Hot Formed Mixed Elemental P/M Nickel Steels, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 255 C Tsumuki et al., Connecting Rods by P/M Hot Forging, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 385 S Mocarski and D.W Hall, Properties of Hot Formed Mo-Ni-Mn P/M Steels with Admixed Copper, Modern Developments in Powder Metallurgy, Vol 9, Metal Powder Industries Federation, 1977, p 467 10 S Saritas, W.B James, and T.J Davies, Influence of Preforging Treatments on the Mechanical Properties of Two Low Alloy Powder Forged Steels, Powder Metall., Vol 3, 1981, p 131 11 T.W Pietrocini and D.A Gustafson, Fatigue and Toughness of Hot Formed Cr-Ni-Mo and Ni-Mo Prealloyed Powders, Modern Developments in Powder Metallurgy, Vol 4, Metal Powder Industries Federation, 1971, p 431 12 R Duggirala and R Shiupuri, Effects of Processing Parameters in P/M Steel Forging on Part Properties: A Review, Part II: Forging of Sintered Compacts, J Mater Eng Perform., Vol (No 4), 1992, p 505-506 13 S.M Kaufman and S Mocarski, Effect of Small Amounts of Residual Porosit y on the Mechanical Properties of P/M Forgings, Int J Powder Metall., Vol 7, 1971, p 19 14 B.L Ferguson, S.K Suh, and A Lawley, Int J Powder Metall Powder Technol., Vol 11, 1978, p 263-275 15 B.L Ferguson, "Toughness and Fatigue of Iron-Base P/M Forgings," Ph.D dissertation, Drexel University, 1976 16 B.L Ferguson, H.A Kuhn, and A Lawley, Modern Developments in Powder Metallurgy, Vol 9, Metal Powder Industries Federation, 1977 17 H.A Kuhn and A Lawley, Powder Metal Processing New Techniques and Analyses, Academic Press, 1978, p 160-165 18 F.T Lally, I.J Toth, and J Dibenedetto, Forged Metal Powder Products, Progress in Powder Metallurgy, Vol 9, Metal Powder Industries Federation, 1972, p 276-302 19 MPIF Standard 35, Materials Standards for Metal Injection Molded Parts, 1993-1994 edition, Metal Powder Industries Federation 20 High Technology Materials, Amstead Research Laboratories, Bensenville, IL, private communication, 1983 21 C Aslund, "Fully Dense Stainless Steel Products Compete Successfully with Forged Products," presented at Metal Powder Industries Federation National Powder Metallurgy Conference (New Orleans), Metal Powder Industries Federation, 1983 Mechanical Properties of High-Performance Powder Metallurgy Parts John C Kosco, Keystone Powdered Metal Company References G.D McAdam, J Iron Steel Inst., Vol 168, 1951, p 346 A Squire, Trans AIME, 1947, p 171, 485 W.J Huppmann and L Albano-Muller, Production of Powder Forged Parts of Complex Geometry, Modern Developments in Powder Metallurgy, Vol 12, Metal Powder Industries Federation, 1981, p 631 F Hanejko, Mechanical Properties of Powder Forged 4100 and 1500 Type Alloy Steels, Modern Developments in Powder Metallurgy, Vol 12, Metal Powder Industries Federation, 1981, p 689 R.M Pilliar et al., Fracture Toughness Evaluation of Powder Forged Parts, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 51 P Lindskog, Reduction of Oxide Inclusions in Powder Preforms Prior to Hot Forming, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 285 F Badia, F Heck, and J Tundermann, Effect of Composition and Processing Variations on Properties of Hot Formed Mixed Elemental P/M Nickel Steels, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 255 C Tsumuki et al., Connecting Rods by P/M Hot Forging, Modern Developments in Powder Metallurgy, Vol 7, Metal Powder Industries Federation, 1974, p 385 S Mocarski and D.W Hall, Properties of Hot Formed Mo-Ni-Mn P/M Steels with Admixed Copper, Modern Developments in Powder Metallurgy, Vol 9, Metal Powder Industries Federation, 1977, p 467 10 S Saritas, W.B James, and T.J Davies, Influence of Preforging Treatments on the Mechanical Properties of Two Low Alloy Powder Forged Steels, Powder Metall., Vol 3, 1981, p 131 11 T.W Pietrocini and D.A Gustafson, Fatigue and Toughness of Hot Formed Cr-Ni-Mo and Ni-Mo Prealloyed Powders, Modern Developments in Powder Metallurgy, Vol 4, Metal Powder Industries Federation, 1971, p 431 12 R Duggirala and R Shiupuri, Effects of Processing Parameters in P/M Steel Forging on Part Properties: A Review, Part II: Forging of Sintered Compacts, J Mater Eng Perform., Vol (No 4), 1992, p 505-506 13 S.M Kaufman and S Mocarski, Effect of Small Amounts of Residual Porosity on the Mechanical Properties of P/M Forgings, Int J Powder Metall., Vol 7, 1971, p 19 14 B.L Ferguson, S.K Suh, and A Lawley, Int J Powder Metall Powder Technol., Vol 11, 1978, p 263-275 15 B.L Ferguson, "Toughness and Fatigue of Iron-Base P/M Forgings," Ph.D dissertation, Drexel University, 1976 16 B.L Ferguson, H.A Kuhn, and A Lawley, Modern Developments in Powder Metallurgy, Vol 9, Metal Powder Industries Federation, 1977 17 H.A Kuhn and A Lawley, Powder Metal Processing New Techniques and Analyses, Academic Press, 1978, p 160-165 18 F.T Lally, I.J Toth, and J Dibenedetto, Forged Metal Powder Products, Progress in Powder Metallurgy, Vol 9, Metal Powder Industries Federation, 1972, p 276-302 19 MPIF Standard 35, Materials Standards for Metal Injection Molded Parts, 1993-1994 edition, Metal Powder Industries Federation 20 High Technology Materials, Amstead Research Laboratories, Bensenville, IL, private communication, 1983 21 C Aslund, "Fully Dense Stainless Steel Products Compete Successfully with Forged Products," presented at Metal Powder Industries Federation National Powder Metallurgy Conference (New Orleans), Metal Powder Industries Federation, 1983 Fatigue and Fracture Control for Powder Metallurgy Components* Randall M German and Richard A Queeney, The Pennsylvania State University Introduction POWDER METALLURGY (P/M) is one of the most diverse approaches to metalworking The main attraction of P/M technology is the ability to fabricate high-quality, complex parts to close tolerances in an economical manner In essence, P/M converts a metal powder from a semifluid state into a strong, precise, high-performance shape Key steps include the shaping or compaction of the powder and the subsequent thermal bonding of the particles by sintering These two steps can be combined into a single operation, for example in hot powder forging or hot isostatic pressing All P/M processes are fairly automated with relatively low energy consumption, high material utilization, and low capital costs These characteristics align P/M with current concerns over manufacturing productivity Consequently, the field is experiencing growth and progressively replacing traditional metalforming operations over a wide range of applications and materials As illustrations, P/M is used in the fabrication of lamp filaments (tungsten), dental restorations (precious metals), self-lubricating bearings (bronze), automotive transmission gears (steels), armor piercing projectiles (tungsten alloys), welding electrodes (copper), nuclear power fuel elements (uranium dioxide), orthopedic implants (cobalt and titanium alloys), high-temperature filters (stainless steels), aircraft brake pads (iron-copper-tin-carbon), rechargeable batteries (nickel), and jet engine components (superalloys) There are three basic approaches to powder metallurgy processing (Ref 1) The most common method, termed pressing and sintering, is to fill a die cavity with loose powder and apply a uniaxial compaction pressure to the powder This pressure deforms and densifies the powder to approximately 85 to 90% of theoretical density Subsequently, the pressed powder is heated to a temperature where atomic diffusion gives rise to interparticle bonding, but with little densification Accordingly, the final product is porous Such a technique is in widespread use for forming moderately complex shapes for mechanical systems using ferrous powders The open continuous pore structures that exist in these P/M products dominate fracture and fatigue behavior Additionally, many filters, electrodes, capacitors, batteries, and other porous structures are formed in a similar manner using low compaction pressures Such high-porosity structures should not be employed in a fatigue-sensitive environment Alternatively, small particles are shaped into useful components at low pressures with the assistance of an organic binder, such as wax, by injection molding, tape casting, or extrusion The particle-packing density is relatively low, typically only 60% of theoretical After shaping, the binder is removed by either heat or solvent extraction, and the powder is densified by sintering at a high temperature These approaches give a final density that is usually between 94 and 100% of theoretical They are slower and more costly than die compaction, but they deliver greater shape complexity and improved mechanical response measures Thus, techniques such as powder injection molding are in widespread use for computer, biomedical, and firearm applications, especially using stainless steels Because the pores are small, closed, and spherical, they have less detrimental effect on fracture and fatigue properties Finally, a powder can be subjected to a combination of heat and stress simultaneously This allows full densification and is widely employed in the fabrication of structural metals, composites, and high-temperature alloys Variations include forging, hot pressing, hot isostatic pressing, extrusion, and roll forming Alloys fabricated this way are usually based on aluminum, titanium, steel, nickel, or refractory metal systems, but include composites and intermetallics Because there is no residual porosity, fracture and fatigue properties are totally dependent on the microstructure, especially any inhomogeneities or contaminants When properly performed, these processes result in full-density P/M products that have mechanical responses superior to those of their wrought equivalents, largely because of the microstructure homogeneity Reference R.M German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994 Note * Adapted from Fatigue and Fracture, Vol 19, ASM Handbook Fatigue and Fracture Control for Powder Metallurgy Components* Randall M German and Richard A Queeney, The Pennsylvania State University P/M Materials Many metals are available via P/M techniques Aluminum and its alloys are highly compressible as powders; green densities of 90% of theoretical are common They can be sintered or hot consolidated using extrusion, forging, hot pressing, and hot isostatic pressing As summarized in Table 1, typical strengths for the press and sinter approach are in the 200 MPa (29 ksi) range with 2% elongation to fracture Higher strengths are available by dispersion strengthening and deformation processing, including hot isostatic pressing and extrusion In the best cases, fatigue endurance limits (or fatigue strength at about 107 cycles) approach about 200 MPa (29 ksi) Some of the high-performance rapidly solidified P/M products provide excellent strength retention to high temperatures Table Properties attainable in aluminum P/M alloys Composition(a), wt% Fabrication(b) Density, g/cm3 4Mg-0.80Si-1.1C 4Cu-1.5Mg-0.80Si-1.1C 0.4Si-0.6Mg 4.4Cu-0.8Si-0.5Mg 0.4Cu-1.0Mg-0.6Si 0.4Cu-1.0Mg-0.6Si 4Ti 8Fe-2Mo MA + forged MA + forged Cold forged P+S P+S P+S MA + HIP HIP 2.66 2.64 2.45 2.58 2.74 2.89 (a) (b) Yield strength, MPa 550 580 90 200 176 230 325 470 Tensile strength, MPa 570 600 180 250 183 238 380 490 Elongation, % 11 11 11 Balance Al MA, mechanically alloyed; HIP, hot isostatically pressed; P+S, pressed and sintered Copper, brass, and bronze are sintered from particles, where the typical applications are not fatigue sensitive Cemented carbides, such as WC-Co and TiC-Ni, are sintered using a liquid phase to deliver a full-density structure, often by the application of high pressure at the end of the sintering cycle The elimination of residual pores has considerable impact on fracture resistance, giving a fracture strength in the 1700 to 3000 MPa (245 to 435 ksi) range Unfortunately, the basic materials are brittle, so fracture toughness is usually in the range of 10 to 20 MPa phase) content and carbide grain size depending on cobalt (or other matrix Stainless steel P/M products are usually selected for their corrosion resistance However, they are capable of highly variable final response measures, depending on the composition, density, and microstructure For precipitation-hardenable alloys such as 17-4 PH, yield strengths of 1100 MPa (160 ksi) with 12% elongation are possible with fatigue endurance limits in the 500 MPa range (72 ksi) Alternatively, for austenitic stainless steels such as 316L, a sintered yield strength of 250 MPa (36 ksi) and considerable ductility (30% or more) are common By far the largest segment of the P/M applications rely on iron-base alloys Generally, a powder is pressed in uniaxial tooling to near-final dimensions, but not to full density Dimensional control during sintering is very important, and usually size can be held to within ±0.025 mm of specification, with concentricity to 0.1 mm, squareness to 0.05 mm, and density to 0.1 g/cm3 Strength typically exhibits a small scatter of ±35 MPa (±5 ksi) and elongation exhibits a scatter of ±2% In most sintered structural steel components, over 90% of the composition is iron Table gives examples of common P/M alloy compositions In all cases, the mechanical properties increase with the final density Table Common ferrous P/M alloy classes Designation Pure iron (steel) Copper steel Iron-nickel Nickel steel Low-alloy steel Infiltrated steel Phosphorus steel Sinter-hardened steel Composition max 1% C 1-22% Cu, max 1% C 1-3% Ni, max 2.5% Cu, max 0.3% C 1-8% Ni, max 2.5% Cu, max 1% C 0.3-2% Ni, 0.5-1% Mo, 0.4-0.8% C 8-25% Cu, max 1% C 0.4-0.8% P, low C 1-3% Cr, 1-2% Mn, 2% Ni, 0.4-0.8% C Iron-copper-carbon compositions are the most common in production, because copper forms a liquid phase during sintering that greatly aids particle bonding This system illustrates the properties possible with P/M Copper and graphite (carbon) are mixed with iron, and during sintering the copper forms a liquid phase Wrought materials of equivalent compositions are not possible due to extensive segregation in the molten state The mechanical properties are degraded by whatever pores remain after sintering Tables and provide examples of the property degradation by listing the hardness, strength, ductility, and impact energy versus density for two Fe-Cu-C alloys Table shows a density effect for an Fe-2Cu-0.8C alloy, while Table includes both density and heat treatment effects for an Fe-10Cu-0.3C alloy In these tables the fatigue life was measured at 107 fully reversed cycles (R = -1) Note that, for example, hardness and strength actually change less with increases in density than does the fatigue endurance strength This reflects the greater sensitivity of the dynamic properties to pore structure as compared with the quasistatic tensile properties Table Sample mechanical properties for Fe-2Cu-0.8C P/M alloys Pressed and sintered, 1120 °C, h, N2-H2 atmosphere Density, g/cm Porosity, % Hardness, HRB Yield strength, MPa Tensile strength, MPa Elongation, % Transverse rupture strength, MPa Fatigue strength, MPa 6.65 14.2 70 365 425 1.3 890 168 6.85 11.8 75 400 495 1.8 1025 198 7.15 7.9 85 415 620 2.5 1325 266 Table Density and heat treatment effects on the properties of an Fe-10Cu-0.3C P/M alloy Density, g/cm3 Thermal condition Hardness (scale) Yield strength, MPa Tensile strength, MPa Elongation, % Fatigue strength, MPa Impact energy, J Elastic modulus, GPa 6.4 As-sintered 50 (HRB) 280 310 0.5 115 90 6.4 Heat treated 25 (HRC) 380 0.5 145 90 7.1 As-sintered 80 (HRB) 395 550 1.5 210 11 130 7.1 Heat treated 40 (HRC) 655 690 0.5 260 130 Nickel is another common addition to ferrous P/M alloys for improved strength In low concentrations, phosphorus is used due to its potent hardening of iron and formation of a liquid phase at temperatures above 1050 °C (1920 °F) The liquid aids sintering, pore spheroidization, and alloy hardening, but usually these additives are selected for magnetic properties, not mechanical properties, with a popular composition containing 0.45% P Most of these alloys are formed by mixing powders that are alloyed as part of the sintering cycle, because of the higher compressibility of the elemental powders as compared with that of prealloyed powders There are several other widely employed P/M materials Tool steels are usually fabricated to full density by liquid-phase sintering or hot isostatic pressing Cobalt-base alloys, titanium alloys, and superalloys are fabricated to full density by hot isostatic pressing Table compares the mechanical properties of Ti-6Al-4V alloys fabricated by three processing routes A very useful group of P/M alloys are the tungsten heavy alloys These are based on W-Ni-Fe mixtures that are densified by liquid-phase sintering Table gives the typical mechanical properties of sintered tungsten heavy alloys These are fulldensity products, but despite the high-sintered density they lack good fatigue properties due to the two-phase microstructure Like tungsten, most of the other refractory metals (molybdenum, tantalum, titanium, chromium, niobium, and rhenium) are fabricated from powders Table Mechanical property comparison for Ti-6Al-4V processed by various P/M Techniques Process Porosity, % Blended elemental P+S Blended elemental HIP Prealloy HIP