Tài liệu ASM Metals HandBook P14 pdf

Fig. 9 Redesign of a MIM part to establish uniform wall thickness. Source: Ref 8 Material Selection. The material systems currently in commercial use include ferrous alloys (low-alloy steel, stainless steels, soft magnetic alloys), nonferrous alloys (brass, bronze), tungsten carbide, pure nickel, electronic alloys (Invar, Kovar), and tungsten-copper composites. The physical and mechanical properties of several MIM engineering alloys (low-alloy steel and stainless steels) have been standardized by MPIF (Ref 10). After sintering, the residual porosity in MIM parts is very low and not interconnected. With densities typically in excess of 96% of theoretical density, the resultant mechanical properties are superior to conventional die-compacted P/M materials and more closely match the properties of investment castings in similar alloys. Therefore, the limitations discussed previously regarding the residual porosity in conventional die-compacted products do not apply to MIM parts. Secondary operations can be performed with no restrictions. References cited in this section 8. Powder Metallurgy Design Manual, 2nd ed., Metal Powder Industries Federation, 1995 10. "Material Standards for Metal Injection Molded Parts," Standard 35 1993-1994 edition, Metal Powder Industries Federation, 1993 Powder Metallurgy Methods and Design * Howard I. Sanderow, Management & Engineering Technologies Powder Forging The design issues in P/F are similar to the requirement of any precision, closed-die forging. The difference is the starting preform; in the case of P/F, the preform is a sintered powder metal part, typically 80 to 85% of theoretical density, with a shape similar to the final part configuration. By contrast, in a precision closed-die forging the preform is a wrought steel blank with very little shape detail. Preform design for P/F fabrication determines the extent of product shape detail required to meet the performance requirements of the finished P/F part. Preform design is a complex, iterative process currently modeled by computer simulation software programs to help reduce design time and development costs. In the forging step, the P/M preform is removed from the reheat (or sintering) furnace, coated with a die lube, and forged in a heated, closed die operation. The forging process reduces the preform height and forces metal into the recesses of the closed die. This step also brings all features to their final tolerances and densities. Configuration guidelines, typical of precision closed-die forged parts, also apply to P/F parts as follows: • Radii on inside corners of the forging as large as possible to promote metal flow around corners in the tool and promote complete fill of all details. • Radii of at least 1 mm (0.040 in.) on all outside corners of the forging to aid in material flow to define features. • Shape of the forging should be such that, when placed in the die, the lateral forces will be balanced. Shapes that are symmetrical along a vertical plane, such as connecting rods and shapes that are axisymmetric (or nearly so), are preferred. • Zero draft is possible on surfaces formed by the die and core rod, but not by the upper punch. • Re-entrant angles (undercuts) cannot be forged. • Axial tolerances--in the direction of forging--are driven by variations in the mass of metal in the preform. Lateral tolerances are driven by metal flow as the cavity fills. Typical axial tolerance of 0.25 to 0.5 mm (0.010 to 0.020 in.) are encountered, with diametric tolerances of 0.003 to 0.005 mm/mm of diameter. • Concentricity of a P/F part is determined by the quality and density distribution in the preform. Concentricity is normally double that of the preform. Powder Metallurgy Methods and Design * Howard I. Sanderow, Management & Engineering Technologies References 1. F.V. Lenel, Powder Metallurgy--Principles and Applications, Metal Powder Industries Federation, 1980, p 426 2. L.F. Pease III and V.C. Potter, Mechanical Properties of P/M Materials, Powder Metallurgy, Vol 7, ASM Handbook (formerly Metals Handbook, 9th ed.), American Society for Metals, 1984, p 467 3. H.I. Sanderow, H. Rodrigues, and J.D. Ruhkamp, New High Strength 4100 Alloy P/M Steels, Prog. Powder Metall., Vol 41, Metal Powder Industries Federation, 1985, p 283 4. D. Gay and H. Sanderow, The Effect of Sintering Conditions on the Magnetic and Mechanical Properties of Warm Compacted Fe-P P/M Steels, Advances in Powder Metallurgy and Particulate Materials--1996, Vol 6, Metal Powder Industries Federation, 1996, p 20-127 5. "Material Standards for P/M Structural Parts," Standard 35 1994 edition, Metal Powder Industries Federation, 1994 6. "Standard Specification for Powder Forged (P/F) Ferrous Structural Parts," B 848-94, Annual Book of ASTM Standards, American Society for Testing and Materials 7. R.M. German, Powder Metallurgy Science, 2nd ed., Metal Powder Industries Federation, 1994, p 193 8. Powder Metallurgy Design Manual, 2nd ed., Metal Powder Industries Federation, 1995 9. "Material Standards for P/M Self-Lubricating Bearings," Standard 35 1991-1992 edition, Metal Powder Industries Federation, 1991 10. "Material Standards for Metal Injection Molded Parts," Standard 35 1993-1994 edition, Metal Powder Industries Federation, 1993 Advances in Powder Metallurgy Applications F.H. "Sam" Froes, Institute for Materials and Advanced Processes (IMAP), University of Idaho, and John Hebeisen, Bodycote IMT, Inc. Introduction POWDER METALLURGY parts can be broadly separated into three categories: those in which the P/M approach allows a lower-cost component to be produced, an intermediate category of cost-effective/high-performance parts, and those in which the P/M approach leads to a part with enhanced mechanical property characteristics (Ref 1, 2, 3). The first approach generally results in a product with lower mechanical properties than wrought product, the third is normally a higher-cost approach. The low-cost P/M product is the more traditional commercial approach, the second has been more recently established commercially, and high-performance parts represent a developing facet of P/M technology. Many P/M parts are now used in a variety of industries, including automobiles, household appliances, yard and garden equipment, computers, fabric industry equipment, and orthodontic devices (Ref 4). The growth in North American metal powder shipments up to 1996 is shown in Table 1 (Ref 5). Table 1 North American metal powder shipments Shipments, tons 1992 1993 1994 1995 1996 Iron and steel 246,300 287,550 337,850 347,172 350,603 Copper and copper base 20,000 22,400 23,100 23,216 22,891 Aluminum 29,700 29,500 43,700 37,044 34,179 Molybdenum 2,500(E) 2,500(E) 2,500(E) 2,500(E) 2,500(E) Tungsten 1,450 1,900 1,450 1,445 1,000(E) Tungsten carbide 4,500 5,200 6,200 10,846 11,200(E) Nickel 9,900 9,600 10,000 10,476 11,600(E) Tin 950 1,100 1,250 1,010 1,010 Total 315,300 359,750 426,050 433,709 434,983 Source: Ref 5 (a) E, estimate. In 1995, North American powder metal shipments totaled 437,774 tons, a 2% increase from the previous year. Iron and steel powder shipments amounted to 347,172 tons, up 2.7% from 1994. Parts applications for iron/steel powders accounted for a record-high 312,974 tons, an increase of 3.1%. However, it was the first time in four years that the parts market improved by a percentage of less than double digits. In the 1992 to 1994 period, the parts market for North American iron and steel powder producers, the biggest market of its kind in the world, grew at an average annual rate of 18.6%. Automotive parts continue to be the leading application of P/M parts. The typical U.S. automobile contains about 14 kg (30 lb) of P/M parts with an increase expected in the next several years (Fig. 1). A slightly lower amount is used in Japanese automobiles (Table 2) (Ref 6), but automotive application of P/M parts is still the dominant use in Japan as well (Fig. 2). The growth of P/M parts in automobiles is due to increased use of P/M components in engines, transmissions, brakes, airbags, and other complex parts. Emerging automotive applications are also described at the end of this article. Table 2 Average weight of P/M components used in each Japanese car Weight Year Cars, ×10 3 P/M part, t kg/car lb/car 1980 11,175 33,923 3.03 6.68 1987 12,350 52,921 4.29 9.46 1988 12,819 60,046 4.68 10.3 1989 12,953 70,138 5.41 11.9 1990 13,592 75,459 5.55 12.2 1991 13,145 75,099 5.71 12.6 Fig. 1 Growth of powder metallurgy in Ford automobiles Fig. 2 Annual production amounts for P/M parts in Japan. Source: Ref 6 References 1. F.H. Froes, C.M. Ward-Close, P.R. Taylor, and W.A. Baeslack, P/M in Aerospace, Defense and Demanding Applications, F.H. Froes, Ed., Metal Powder Industries Federation, 1995, p 3 2. F.H. Froes and P.D. Desai, "Recent Developments in Powder Metallurgy Processing Techniques," Report 5, MIAC, Purdue University, Oct 1994 3. F.H. Froes et al., Light Materials for Transportation Systems, N.J. Kim, Ed., Postech, Pohang, Korea, 1993, p 27 4. P.K. Johnson, Industrial Heating Powder Metallurgy Supplement, June 1996, p 4 5. www.mpif.org/indust.html, 10 Feb 1998, Metal Powder Industries Federation 6. Y. Morioka, Recent Trends in Powder Metallurgy Industry and Technology, J. Jpn. Soc. Powder Powder Metall., Vol 4 (No. 8), Aug 1993, p 755-762 Advances in Powder Metallurgy Applications F.H. "Sam" Froes, Institute for Materials and Advanced Processes (IMAP), University of Idaho, and John Hebeisen, Bodycote IMT, Inc. Net Shape Capability A major advantage of the P/M approach is the net-shape capability, particularly for high-strength materials. This has led to continuing developments in P/M technologies for parts production. Warm Compaction. One recent development in P/M production is warm compaction, which allows the production of higher-density ferrous P/M parts via a single compaction process. The process utilizes heated tooling and powder during the compaction step. The powder and tooling are typically heated between 130 and 150 °C (260 and 300 °F). In order for the powder premix to perform at these temperatures, a proprietary lubricant system has been developed that provides lower die ejection forces than conventional lubricants. This lubricant system also incorporates a polymeric binder system to limit segregation and provide enhanced flow characteristics of the powder premix. By utilizing warm compaction technology, the green density of the consolidated part can be increased from 0.10 to 0.25 g/cm 3 over traditionally processed (single-pressed/single-sintered) materials. The green strength is typically increased between 50 and 100%. Table 3 (Ref 7) compares the green properties of warm-compacted and cold-compacted P/M parts. This increase in green strength provides advantages such as a reduction in green chipping and cracking due to part handling prior to sintering and makes possible the crack-free compaction of complex multilevel parts. Additionally. the higher green strength provides an opportunity to machine the P/M part in the green state. This capability is critical in the use of high- performance alloy systems that achieve high hardness in the as-sintered state. Warm compaction also enables P/M fabricators to single press and single sinter ferrous P/M parts to densities as high as approximately 7.4 g/cm 3 , which is considerably higher than cold-compacted-and-sintered P/M parts. Table 3 Effect of processing on the green properties of ferrous compacts Compaction pressure Green strength Peak die ejection force Base material Processing technique MPa tsi Green density, g/cm 3 MPa psi MPa tsi 415 30 7.14 23.2 3370 29.6 2.15 Ancorsteel 85HP (a) Warm compaction 550 40 7.31 25.4 3685 33.5 2.43 700 50 7.37 24.7 3580 32.0 2.32 415 30 7.00 9.9 1430 37.2 2.70 550 40 7.19 12.2 1770 50.7 3.68 Cold compaction 700 50 7.29 13.4 1950 53.8 3.90 415 30 7.07 28.3 4100 27.4 1.99 550 40 7.29 30.6 4445 31.7 2.30 Warm compaction 700 50 7.36 31.1 4515 32.3 2.37 415 30 6.93 12.2 1770 37.2 2.70 550 40 7.15 15.0 2170 48.5 3.52 Distaloy 4800A (b) Cold compaction 700 50 7.26 16.9 2450 52.0 3.77 Source: Ref 7 (a) Ancorsteel 85HP is a prealloyed steel powder containing 2.0% Ni, 0.85% Mo, 0.4% graphite, and 0.6% lubricant. (b) Distaloy 4800A is a diffusion-alloyed steel powder containing 4% Ni, 1.5% Cu, 0.50% Mo, 0.5% graphite, and 0.6% lubricant. Warm compaction, although applicable to all ferrous material systems, produces the greatest benefits when coupled with high-performance compositions such as diffusion-alloyed steels or molybdenum-prealloyed steels. Achieving densities in excess of 7.25 g/cm 3 using these compositions results in mechanical properties that are comparable to steel forgings and ductile iron castings. Stainless Steels. Other developments in conventional P/M steels include new and improved stainless steels, with the goal of improving compressibility and corrosion resistance. Developing applications include automobile parts such as a solenoid spacer in an electronic fuel injector, sealing washers in the water pump, and brake components. They also include other applications such as bearing holders and pulleys in computers. Metal injection molding (MIM), which is finding increased use in small parts manufacturing, uses fine powders (5- 10 m) that exhibit good sintering densification in combination with binders that hold the particles in place for transportation (Ref 8). The basic five steps involved are feedstock characterization, mixing rheology, injection molding, debinding, and sintering (see Fig. 4 in the article "Powder Metallurgy Methods and Design" in this Volume). With tolerances as low as 0.3%, sizing is generally not required. Densification to a usable article is accomplished in the sintering furnace. However, surrounding these seemingly simple steps there is a great deal of "know-how" that has been developed. Metal injection molding occupies a certain region in the part-size/production-run/part-complexity scenario (Fig. 3). Sizes are generally in the 1 to 200 g range, but parts as large as 1 kg have been made by MIM. The near-net-shape capabilities of the process can reduce machining costs to low levels, especially for long runs ( 10,000 parts). For small complex components, cost savings can be up to 80% compared to conventional approaches. A wide range of geometric options are possible including undercuts, tapered external surfaces, and crossholes. Short production runs can be cost effective, but because of die and powder costs, this only occurs with very complex shapes and/or hard materials. Fig. 3 Section of the part-size/production-run/part-complexity diagram where the MIM process is most effective. Source: Ref 8 Some MIM fabricators produce runs of 2000 to 5000 pieces, particularly on more expensive parts (Ref 9). They can do a short run such as these because there is no danger to the tooling at setup, while in conventional P/M there is more risk. The capital equipment (presses) for injection molding is more economical than that for large-scale P/M presses. Tool life is at least 300,000 pieces. These factors help offset the added short-term material cost. Two of the first MIM production parts in automobiles were parts for an ignition lock (Fig. 4) and a single-part replacement (Fig. 5a) for a two-part turn signal lever assembly (Fig. 5b). Both have been in service since July 1988. Fig. 4 MIM part (upper left) for an automobile ignition lock. The key forces the MIM part into contact with a security switch. Courtesy of SSI Technologies Fig. 5 (a) Single-piece MIM part that replaced (b) a two-piece automobile turn signal lever assembly. The smaller MIM part in (a) was the first version, while the larger MIM part is the finished version that replaced the two-part assembly shown in (b). Courtesy Remington Arms Division of E.I. Du Pont de Nemours & Company, Inc. Figure 4 shows the entire ignition lock and the MIM subcomponent. As the key is inserted in the lock, the cam-shaped MIM part moves away and depresses an electrical switch, which is part of the security system. The initial design of the part was too small and complicated for the model shop to make, and it was prototyped from the MIM tooling. The turn signal indicator lever is an example of the replacement of a two-piece assembly (Fig. 5b) with a single MIM part. The lower portion of Fig. 5(a) shows the first version of the MIM part, and the upper view shows the final 19.0 g (0.670 oz) MIM part that replaced the assembly. The MIM material is iron with 2% Ni, sintered and then case hardened. It replaced AISI 4037 and SAE 1018 case hardened. The MIM part succeeded because of its superior strength compared to the two-piece assembly. The core properties of the materials are 415 MPa (60 ksi) tensile strength and 15% elongation at 60 HRB. Hot powder forging (P/F) continues to be attractive for the production of fully dense P/M parts for demanding applications (Ref 10, 11, 12). In this process a loose powder is blended to the desired composition and pressed to a forging preform having the general shape of the final component. Powder forging has proven to be competitive when the overall economics are improved through some combination of enhanced machining characteristics, mechanical properties, and dimensional or weight tolerances. It was initially believed that P/F products would displace a wide variety of conventionally processed P/M parts, as well as a significant number of conventional forgings. However, the number of high-volume applications has been limited to bearing races, connecting rods, and ring gears. One of the more dramatic applications in the mid-1980s was P/F connecting rods. Compared with conventional forging, P/F technology improved weight and dimensional control (Fig. 6) and reduced machining requirements (Fig. 7). In addition, P/F connecting rods are always forged in one piece, where the rod and cap are separated by "fracture splitting." Use of this technique eliminates several machining operations. In addition, the irregular, mating fracture surfaces (ductile failure mode) provide an intimate interlock between rod and cap. This virtually eliminates both "cap shift"--rotation of the cap relative to the rod--and lateral movement of the cap relative to the rod. Cap shift can lead to accelerated wearing of surfaces and, in extreme cases, bearing seizure. Lateral movement can result in high shear stress on connecting rod bolts at high engine revolutions per minute. . P/M Materials, Powder Metallurgy, Vol 7, ASM Handbook (formerly Metals Handbook, 9th ed.), American Society for Metals, 1984, p 467 3. H.I. Sanderow, H.