Powder Metallurgy Methods and Applications

W. Brian James, Hoeganaes Corporation, retired

Powder metallurgy (PM) is the production ferrous PM structural parts are used). Powder and screw machining. The industry comprises and utilization of metal powders. Powders are metallurgy parts are used in engine, transmis- powder suppliers and parts makers, plus the defined as particles that are usually less than sion, and chassis applications. Sometimes it is a companies that supply the mixing equipment, 1000 nm (1 mm) in size. Most of the metal unique microstructure or property that leads powder handling equipment, compacting particles used in PM are in the range of 5 to to the use of PM processing: for example, porous presses, sintering furnaces, and so forth. 200 mm (0.2 to 7.9 mils). To put this in context, filters, self-lubricating bearings, dispersion- Powder metallurgy processing offers many a human hair is typically in the 100 mm (3.9 mils) strengthened alloys, functionally graded materi- advantages. The PM process is material and range. als (e.g., titanium-hydroxyapatite), and cutting energy efficient compared with other metal The history of PM has already been outlined tools from tungsten carbide or diamond com- forming technologies. Powder metallurgy is in the article “History of Powder Metallurgy” in posites. Captive applications of PM include cost effective for making complex-shaped parts this Volume. This article reviews the various materials that are difficult to process by other and minimizes the need for machining. A wide segments of the PM process from powder pro- techniques, such as refractory metals and reac- range of engineered materials is available, and duction and powder processing through to the tive metals. Other examples in this category are through appropriate material and process selec- characterization of the materials and their prop- special compounds such as molybdenum disili- tion the required microstructure may be devel- erties. It will cover processing methods for con- cide and titanium aluminide, or amorphous oped in the material. Powder metallurgy parts solidating metal powders including options for metals. have good surface finish and they may be heat processing to full density. The metal powder industry is a recognized treated to increase strength or wear resistance. Powders have a high ratio of surface area to metal forming technology that competes dir- The PM process provides part-to-part reproduc- volume and this is taken advantage of in the ectly with other metalworking practices such ibility and is suited to moderate-to-high volume use of metal powders as catalysts or in various as casting, forging, stamping (fine blanking), production. Where necessary, controlled micro- chemical and metallurgical reactions. While porosity can be provided for self-lubrication or this article focuses on the use of powders to filtration. While dimensional precision is good, make functional engineering components, many it typically does not match that of machined metal powders are used in their particulate parts. form. This aspect of PM is covered in the arti- In the case of ferrous PM parts, they have cle “Specialty Applications of Metal Powders” lower ductility and reduced impact resistance in this Volume. compared with wrought steels. Powder technologies are exciting to engi- The majority of PM parts are porous and con- neers because processing options permit the sideration must be given to this when performing selective placement of phases or pores to tailor finishing operations. the component for the application. The capabil- ity of press and sinter processing or metal injection molding (MIM) processing to replicate Metal Powders parts in high volumes is very attractive to design engineers. The ability to fabricate complex Metal powders come in many different shapes shapes to final size and shape or to near-net and sizes (Fig. 2). Their shape, size, and size dis- shape is particularly valuable. Powder metal- tribution depend on the manner in which they lurgy offers the potential to do this in high were produced. Metal powder production is cov- volumes and also for applications where the ered in depth in various articles in the Section, volumes are not so large. “Metal Powder Production” in this Volume. The three main reasons for using PM are eco- There are three main methods of powder nomic, uniqueness, and captive applications, as production: shown in Fig. 1 (Ref 1). For some applications that require high volumes of parts with high Mechanical, including machining, milling, precision, cost is the overarching factor. A good Fig. 1 Three main reasons for choosing powder and mechanical alloying metallurgy shown in the form of a Venn example of this segment is parts for the auto- diagram. The intersection of the three circles represents Chemical, including electrolytic deposition, motive industry (where approximately 70% of an ideal area for applying PM techniques. Source: Ref 1 decomposition of a solid by a gas, thermal 10 / Introduction to Powder Metallurgy

limited plasticity. Rigid die compaction is not suitable for consolidating such powders, and they must be processed by other means such as hot pressing, extrusion, or hot isostatic pressing (HIP), described subsequently in this article. Highly reactive metal powders are also not suitable for rigid die compaction. They generally need to be vacuum hot pressed, or encapsulated and extruded, or HIPed. Rigid die compacted parts and MIM parts are thermally treated to increase their strength in a process known as sintering. The parts are heated, generally in a reducing atmosphere, to a temperature that is below the melting point of the primary constituent of the material, in 5 μm 5 μm order to form metallurgical bonds between the compacted metal powder particles. Sintering Fig. 2 Example of the different particle shapes possible with metal powders is a “shrinkage” process. The system tries to reduce its overall surface area via various diffusion processes. Metallurgical bonds (micro- decomposition, precipitation from a liquid, chemical analysis of the metallic elements in scopic weldments) form between adjacent precipitation from a gas, solid-solid reactive PM materials are provided in MPIF Standard metal particles (after oxides have been reduced synthesis 67 (Ref 2). on the surface of the powder particles), pore Physical, including atomization techniques Complex, multilevel PM parts compacted surfaces become less irregularly shaped, and in rigid dies will not have the same green larger pores grow at the expense of the smaller Most metals are available in powder form. density throughout. While the objective is gen- pores. Sintering is generally carried out using Some may be made by many different methods, erally to achieve a density as uniform as pos- continuous mesh-belt furnaces. For higher while for others only a few options are possible. sible throughout the compacted part, taller temperatures (>1150 C, or 2100 F), pusher, The characteristics of the powder are deter- parts and parts with multiple levels are subject roller hearth, or walking-beam furnaces may mined by the method by which it is produced. to the presence of density differences between be used. Batch furnace processing is used for The shape, size, size distribution, surface area, adjacent regions. This is due to frictional effects special applications (e.g., pressure-assisted apparent density, flow, angle of repose, com- and compacting tool deflections. Taller parts will sintering). More information on sintering may pressibility, and green strength depend on the have a neutral zone or density line—the region befoundintheSection“SinteringBasics”in powder production method. In-depth coverage of the compact that has experienced the least this Volume. of the sampling and testing of metal powders relative movement of powder. The position of is presented in the articles in the Section “Metal the neutral zone may be adjusted by varying Powder Characterization” in this Volume. the pressure exerted by the upper and lower Powder Metallurgy Material punches. Properties Compaction in rigid dies is limited to part Powder Processing shapes that can be ejected from the die cavity. The majority of PM parts contain pores (see Parts with undercuts, reverse tapers, threads, options for processing metal powders to full For the production of PM parts in high and so forth, are not generally practical. Such density later in this article). This is an advan- volumes, compaction is carried out in rigid features are formed by postsintering machining tage when metal powders are used to make dies. In most instances, the metallic powders operations. self-lubricating bearings in which the surface- are mixed with a lubricant (e.g., ethylene bis- There are two main types of compacting connected pores of the parts are impregnated stearamide) to reduce interparticle friction dur- press: mechanical and hydraulic. Some hybrid with oil. When the bearing surface heats up ing compaction and to facilitate ejection of presses offer features of both. A detailed treat- due to frictional heat, oil is released from the the compacted parts by reducing friction at the ment of compaction is provided in the Section pores. When the bearing cools, the oil is sucked die-wall and core-rod interfaces. “Metal Powder Compaction” in this Volume. back into the pore channels by capillary action. The metal powders may be elemental pow- Some PM parts are molded (shaped) rather The porosity in PM parts has an effect on the ders; mixtures of elemental powders; or mix- than compacted. Fine-particle-size metal pow- physical, mechanical, magnetic, thermal, wear, tures of elemental powders with master alloys ders (5 to 20 mm, or 0.2 to 0.8 mils) are mixed and corrosion properties of the parts. or ferroalloys, prealloys, diffusion alloys, or with binders and plasticizers and processed to Thermophysical properties of sintered steels, hybrid alloys. See the article “Ferrous Powder form a feedstock for MIM. Molding is per- in particular their coefficient of thermal expan- Metallurgy Materials” in this Volume for an formed using machines similar to those used sion and their thermal conductivity, are needed in-depth review of the alloying methods used for plastic injection molding. Shrinkage during when designing parts and when modeling heat in ferrous PM. A consequence of the various the subsequent sintering operation is extensive treatment processes. Opinions differ in the PM alloying methods available is that only the PM (15 to 20%) due to the fine-particle-size pow- community as to the effect of density on materials made from prealloyed powders are ders used and the high sintering temperatures. these properties. Danninger has shown, how- chemically homogeneous. The other alloying Because the parts are molded and not com- ever, that the coefficient of thermal expansion methods can result in chemically inhomoge- pacted, they do not contain density gradients up to 1000 C (1832 F), measured through neous materials. The hardenability is determined that lead to distortion or problems with dimen- dilatometry, is virtually independent of porosity by the local chemical composition, and the sional control. The process makes complex- (density) over a density range from 5.97 to resulting microstructures are generally quite shaped, small-to-medium sized PM parts with 7.53 g/cm3 (Ref 3). In addition, thermal con- complex. Chemical analysis can be a challenge high relative densities. ductivity was determined in the same tempera- due to the inhomogeneous nature of the materi- Some metal powders are not very compress- ture range by using laser flash to measure als. Guidelines for sample preparation for the ible. The powder particles are hard and have thermal diffusivity, and specific heat, and then Powder Metallurgy Methods and Applications / 11 the thermal conductivity was calculated from Poisson’s ratio is a weak function of density, to develop strength in the compacts. Rigid die these parameters and the density in accordance and for ferrous PM structural steels it can be compaction falls into this category and is the with: taken as 0.27 ± 0.02. most cost-effective method for the high-volume ÀÁ The mechanical properties of PM materials production of PM structural parts. In order for ¼ l r a Cp (Eq 1) are a function of density: this method to be viable, the metal powders need an irregular shape and good flow character- = ¼ ðÞr=r m where a is thermal diffusivity, l is thermal P P0 0 (Eq 4) istics, they must be compressible, and they must conductivity, r is density, and Cp is specific have good “green” strength. (Green is the term heat at constant pressure. where, P is the property of interest, P0 the value r used to describe an as-pressed compact.) Thermal conductivity was shown to depend for the pore-free material, is the density of r Extremely hard particles with a spherical shape on density. The effect of porosity in the tech- the material, 0 is the density of the pore-free are not appropriate for use in rigid die compac- nically relevant density range was, however, material, and m is an exponent the value of tion. Compaction takes place at high pressure slightly less pronounced than the effect exerted which depends on a given property (Fig. 4) in confined dies (the dies are generally made by the alloying elements; specifically, the vari- (Ref 6–7). While tensile strength increases in from cold work tool steel or cemented carbide). ation observed between different standard PM a linear fashion as density increases, tensile Compacting pressures for ferrous powders are steel grades in the low-to-medium temperature ductility is more dependent on reducing the generally in the range from 400 to 700 MPa range. Both thermophysical properties are, level of porosity. Fatigue performance is even (60 to 100 ksi), from 100 to 400 MPa (14.5 to therefore, significantly less influenced by poros- more influenced by density with an exponent 60 ksi) for aluminum and aluminum alloy pow- ity than by chemical composition. Powder met- m of between 3.5 and 4.5. Impact energy is ders, and approximately 400 MPa (60 ksi) for allurgy steels are more similar to wrought steels the most dependent on density, with an expo- copper and copper-alloy powders. than was generally assumed. nent m of approximately 12. The green density increases as the compact- The elastic constants are also of interest to Magnetic properties of ferrous PM materials ing pressure is increased and levels out at the design engineer. Young’s modulus, Pois- are affected by density. Induction and per- higher compacting pressures. Powder particles son’s ratio, and the shear modulus are related meability increase as the density is increased. work harden as the result of plastic deformation according to: Permeability and coercive field strength are and it requires higher pressures to cause further structure-sensitive properties that are degraded plastic flow. In addition, the lubricant that is by the presence of impurities. The sintering con- E ¼ 2GðÞ1 þ n (Eq 2) typically admixed to aid particle rearrangement ditions are extremely important to keep carbon, and to reduce the fictional forces between the where E is Young’s modulus, G is shear nitrogen, and oxygen contents to low levels powder and the compacting tools eventually modulus, and n is Poisson’s ratio. E and n are (C = 0.03 wt% max; N = 0.01 wt% max; and has no place to go because all the voids determined by resonant frequency and G is O = 0.10 wt% max). Residual stresses from between particles have been closed—either by calculated from Eq 2. operations such as sizing, machining, or shot metal flow or by the presence of lubricant. Beiss (Ref 4) has shown that: peening degrade the magnetic properties. The More lubricant is beneficial at lower compact- properties can be restored through an annealing ing pressures, but there is a transition point at ¼ ðÞr=r m treatment. E E0 0 (Eq 3) which the additional lubricant impedes further densification (Fig. 6) (Ref 9). where E is the Young’s modulus of the pore- 0 Processing Options to Consolidate Warm compaction processing was developed free material, r is the density of the material, to overcome the compressibility constraints of r 0 is the density of the pore-free material, and Metal Powders rigid-die compaction (Ref 10). The powder the exponent m depends on the pore morphology mixture and the compacting tools are heated and varies between 2.5 and 4.5. Nevertheless, There are three basic approaches to the con- over the density range of interest for ferrous solidation of metal powders, as shown in Fig. 5 PM structural materials, 6.4 to 7.4 g/cm3, (Ref 8). Young’s modulus is essentially a linear function Pressure-based compaction establishes of density (Fig. 3) (Ref 5). density via the compaction process then sinters

52 7.5

51 7.4

50 7.3 psi 6 49 7.1

48 7.0

47 6.8 Young’s modulus, GPa Young’s

46 6.7 modulus, 10 Young’s

45 6.5

44 6.4 6.4 6.6 6.8 7.0 7.2 7.4 7.6 Sintered density, g/cm3 Fig. 4 Effect of density on mechanical and physical Fig. 3 Young’s modulus as a function of sintered density. Data from Ref 5 properties of PM materials. Source: Ref 6 12 / Introduction to Powder Metallurgy

Contacting pressure, ksi 58.0 72.5 87.0 101.5 116.0 130.5 7.4

7.3

3 7.2 Transition pressure 7.1

7.0

Green density, g/cm Green density, 6.9 Fig. 5 Three basic approaches to the consolidation of metal powders. Source: Ref 8 Atomized iron 6.8

to approximately 120 C (250 F) and the 6.7 powder is compacted in a single press stroke. 400 500 600 700 800 900 Green densities of up to 7.3 g/cm3 are possible Compacting pressure, MPa with highly compressible ferrous powders. This 0.5% Zn St 0.75% Zn St 1.0% Zn St is approximately 98% of the pore-free density of the powder mixture being compacted (The pore-free density of the powder mixture is the Fig. 6 Effect of lubricant content on the compressibility of metal powders. Source: Ref 9 green density that could be reached if all the porosity was removed from the material. It can be calculated for any mixture based on the density and the amount of each constituent in the mixture and the volume they would occupy in the pore-free condition.) Special lubricants and binder-treated premixes were developed for warm compaction. The efficiency of the special lubricant enabled it to be reduced to 0.6 wt% from the 0.8 wt% more typically used for rigid-die compaction: the pore-free density increases by 0.1 g/cm3 for each 0.2 wt% reduc- tion in the amount of the admixed lubricant. Examples of warm compacted parts are shown in Fig. 7. More recently, warm-die compaction has been introduced. In this instance, only the compacting tooling is heated (the powder is not heated). The optimal die temperature varies according to the specific lubrication system being used. The die temperature is set so that the surface temperature of the green compacts reaches the desired range for the lubricant system in question. Warm-die compaction is ideal for small-to-medium size parts that weigh less than 700 g (1.5 lb), are up to 32 mm (1.3 in.) high, and have wall thicknesses of up to Fig. 7 Examples of warm compacted PM parts. (a) Torque converter hub. Courtesy of Chicago Powder Metal 19 mm (0.75 in.). For larger parts warm com- Products. (b) Transmission output shaft hub. Courtesy of GKN Sinter Metals. (c) Hand tool parts. Courtesy paction processing is required. Green densities of PoriteTaiwan Co. Ltd. of 7.45 g/cm3 have been reached using warm- die compaction with lubricant additions of to final sintering. This “debinding” step is the HIP are used. Hot pressing (often in vacuum) or approximately 0.3 wt% (Ref 11). rate-controlling phase of the MIM process. spark sintering may also be used. In sintering-based densification, the shape Other sintering-based densification processes of the component is formed in a molding oper- that involve the molding or shaping of powders ation (e.g., MIM) and sintering is enhanced by are slip casting and tape casting. Processing to Full Density the use of high temperatures and fine-particle- Hybrid Densification. For some materials, a size powders. While extensive shrinkage occurs hybrid densification process is used in which Options for processing metal powders to full during sintering, it is essentially isotropic in pressure and temperature are applied at the same density are mapped in Fig. 8 (Ref 12). The ver- nature so that good tolerance can still be time. As mentioned previously, some powders tical axis relates to relative stress (the applied achieved. Metal powder loading in the feed- are not suitable for rigid-die compaction; they pressure divided by the in situ yield strength stock used for MIM is approximately 60%. are too hard, are spherical in shape, or are too of the material) and the horizontal axis relative The binders and plasticizers added to make reactive. In this instance, processes such as pow- temperature (based on the melting temperature the mixture moldable must be removed prior der extrusion (typically after encapsulation) or of the material). Powder Metallurgy Methods and Applications / 13

Full-density processing requires the simulta- rely on diffusional creep processes (e.g., hot Hot pressing is performed in a rigid die using neous application of pressure and temperature. pressing or HIP) uniaxially applied pressure: it is a low-strain-rate The approach works because most materials High-stress routes that operate at high strain process. Graphite dies may be used, in which soften as temperature is increased. They also rates and lower temperatures (powder forging case induction heating may be employed. Hot become more ductile and deform with less or extrusion) pressing cycle times are slow compared with work hardening. Routes that achieve high density via the rigid-die compaction. Vacuum is sometimes The processing options fall into the following application of ultrahigh-stress at ambient used to minimize contamination of the compact. categories: temperature (explosive compaction) Diamond-metal-composite cutting tools are often hot pressed. Spark sintering is a process Low-stress processes that operate at high Liquid-phase sintering results in a composite related to hot pressing. In spark sintering, direct temperatures and are dominated by diffusion microstructure that consists of a skeleton of a resistance heating is applied to the punches, processes (e.g., liquid-phase sintering) high-melting-temperature phase in a matrix of a die, and powder mass during consolidation. Processes that apply intermediate stress levels solidified liquid—for example, W-Ni-Fe heavy Hot isostatic pressing applies pressure from and operate at intermediate temperatures and alloys, WC-Co cemented carbides. all directions simultaneously. In order to estab- lish a pressure differential, powders must be processed to the point where they have no surface-connected, interconnected porosity, or they need to be encapsulated prior to the HIP process. Prior to HIP, a container is filled with powder and heated under vacuum to remove volatile contaminants. After evacuation and degassing, the container is sealed. The container may be fabricated from any material that is soft and deformable at the consolidation temperature, for example, glass, steel or stainless steel (the choice depends on compatibility with the powder that is being compacted). A HIP vessel is illustrated in Fig. 9 and the sequence used to make a HIPed part is shown in Fig. 10 (Ref 13). Vacuum sintering then backfilling the sintering furnace with pressurized gas to assist final densification is employed in sinter-HIP processing (a pressure-assisted sintering process). A typical cycle is shown schematically Fig. 8 Options for processing metal powders to full density. Source: Ref 12 in Fig. 11 (Ref 14).

Fig. 9 (a) Typical hot isostatic pressing (HIP) vessel. (b) Schematic of the wire-wound unit. Courtesy of Avure Technologies. Source: Ref 13 14 / Introduction to Powder Metallurgy

Powder forging bridges the gap between con- Freeform Fabrication operations. Sometimes parts need closer ventional pressing and sintering and wrought tolerances than can be held during the pressing steel technology. The process is illustrated Thermal spraying of nickel-base and cobalt- and sintering operation; they can be sized to schematically in Fig. 12 (Ref 15). A PM pre- base alloy powders to form wear-resistant coat- reduce their dimensional variability. form is typically compacted, sintered, and then ings has been practiced for many years. Spray The surface-connected, interconnected poros- reheated before being forged in a single stroke forming is a consolidation process that captures ity in PM parts can be impregnated with oil, in confined dies. A detailed review of the pow- a spray of molten metal or alloy droplets on a and this is the basis for self-lubricating bearings der forging of ferrous materials is given in the moving substrate (Ref 17). Figure 14 illustrates (Ref 20). Conventional bearings can absorb Section “Powder Metallurgy Carbon and Low- billet formation in a vertical mode by spray from 10 to 30% by volume of oil. Alloy Steels” in this Volume. forming. The process can be used to form bil- Pressure tightness can be achieved in PM Extrusion is used to make some PM tool lets, strip, and thick-walled tubing. parts by sealing the surface-connected porosity steels. These materials have better properties The term additive manufacturing of metals is by resin impregnation. Vacuum processing is than similar wrought tool steels because they used to describe freeform processes that offer generally used to impregnate the PM parts. In contain a finer and more uniform dispersion of the possibility to produce complex-shaped PM addition to developing pressure tightness, resin carbides compared with the wrought tool steels. parts without the design constraints of tradi- impregnation of PM parts permits plating (oth- In the latter, the carbides are often banded and tional manufacturing routes (Ref 18). The pro- erwise, plating solutions would be trapped in in the form of stringers due to the rolling process relies on the transfer of a digital file to a the surface-connected pores). Resin impregna- cess used to make them, as shown in Fig. 13 machine that then builds the three-dimensional tion significantly improves the drillability of (Ref 16). component layer by layer from a metal powder PM parts, as shown in Fig. 16 (Ref 21). using a laser or an electron beam to fuse the Machining parameters for PM parts are dif- particles together. Schematic illustrations of ferent from those used for castings or wrought powder-bed and powder-fed systems are shown components. The PM materials contain pores. in Fig. 15 (Ref 19). Depending on the hardness of the material, the material in the vicinity of the cutting tool will densify to a greater or lesser extent. As Finishing Operations the amount of porosity decreases, PM parts machine more like cast or wrought parts with While PM is considered a net or near-net a similar microstructure. Machinability aids shaping process, many PM parts require finishing such as manganese sulfide (MnS) may be added to the PM material prior to compaction to enhance the machinability of the PM parts. Powder metallurgy parts may be turned, milled, drilled, tapped, and ground. Machinabil- ity depends on the density and the microstructure of the material. For a PM material of a given density and microstructure, the machinability will depend on the type of cutting operation being performed, the cutting tool material, and the feeds and speeds being used. Examination of the cutting tool is one of the keys to understanding what is happening during the machining process. Moving to a condition of abrasive wear will lead to greater consis- Fig. 11 Schematic of pressure-assisted sintering process tency and predictability in the machining oper- cycle. Source: Ref 14 ation. A statistical approach to evaluating the

To preheat furnace

Eject from die

Powder fill Press proform

Eject fully dense part

Rapid transfer Fig. 10 (a) Pre-HIP ﬁlled can weighing 2050 kg (4520 lb). Hot forge Heat; controlled (b) Post-HIP. (c) Heat treated and sonic machined atmosphere HIPed part. Courtesy of Carpenter Technologies. Source: Ref 13 Fig. 12 Schematic of the powder forging process. Source: Ref 15 Powder Metallurgy Methods and Applications / 15

Induction heated ladle Particle injector (optional) Atomizer (nitrogen)

Round, spray-deposited billet

Spray chamber

Exhaust

Fig. 13 Extruded T15 tool steel. (a) Wrought. (b) PM. Notice the bands of carbides in the wrought tool steel Fig. 14 Billet formation in a vertical mode by spray compared with the uniform dispersion of ﬁne carbides in the PM tool steel. Source: Ref 16 forming. Source: Ref 17

“Sweet spot” Robust process, able to accept variation “Safe zone” Noncatastrophic, consistent tool Machining process performance

Fig. 17 “Sweet spot” for machinability. Source: Ref 22

Fig. 15 Schematic illustrations of (a) powder-bed and (b) powder-fed additive manufacturing. Source: Ref 18 for neutral hardening. Induction hardening of PM parts is also possible. Gaseous carburizing, nitriding, carbonitriding, and nitrocarburizing processes are applicable. Care is required with ferrous parts at densities below 7.1 g/cm3 (0.26 lb/in.3), because gas penetration to the core of the part can lead to loss of toughness. The use of salt baths is to be avoided because the salt would penetrate the surface-connected pores and lead to subsequent corrosion problems. Microin- dentation hardness testing is used to determine the effective case depth of surface-hardened PM parts (Ref 23). Where there is a clear difference between the hardened layer and the rest of the part, such as with an induction-hardened part, a metallographic estimate may be made of the case depth (Ref 24). Powder metallurgy parts are often tumbled Fig. 16 Relative drillability of various PM materials. Source: Ref 21 in an abrasive medium in rotating barrels or agitated in vibrating tubs to clean them and remove burrs. They are generally resin or oil impregnated before tumbling to minimize water data is extremely beneficial. Abrasive wear is without the tool failing in a catastrophic man- absorption. Rust inhibitors should be added the common and natural mechanism of wear ner. This is the “safe zone” for machining, to the water. Parts may be spun dry or heated during machining—it is the desired mechanism. Fig. 17 (Ref 22). to dry. There are combinations of machining para- Ferrous PM parts may be heat treated to Ferrous PM parts may be furnace blackened meters (cutting tool, feeds, speeds, etc.) that improve their hardness, strength, and wear resis- (steam oxide treated) for indoor corrosion resis- result in consistent machining performance tance. Oil quenching and tempering may be used tance. Afterward, they may be oil dipped for 16 / Introduction to Powder Metallurgy color as well as slightly greater corrosion resis- using PM processes. They are taken from parts been used in an earlier transfer case design and tance (a dry film oil is particularly suitable). that have won awards at the MPIF Design provided 35% cost savings over the forgings. Steam treating forms a coating of magnetite Excellence Competition, which is held annually The variable valve timing (VVT) rotor (Fe3O4) in the surface-connected pores. Parts to highlight the advances made in PM part pro- shown in Fig. 21 consists of an assembly of a are heated to 480 to 570 C (896 to 1060 F) duction (Ref 29). PM steel rotor and an adapter. The parts are and exposed to superheated steam under pres- The carrier and one-way rocker clutch joined by an adhesive, which joins them during sure. This improves the wear resistance of fer- assembly shown in Fig. 19 are used in the Ford the machining of cross-holes and other features rous PM parts and improves their compressive Super Duty TorqShift six-speed automatic on the inside diameter, and seals the joint bet- strength. It does, however, degrade tensile prop- transmission. The hybrid assembly contains ween them. The assembly, used in a Chrysler erties (Ref 25). five PM steel parts weighing a total of 7.7 kg All types of plating processes may be applied (17 lb). The sinter-brazed subassembly consists to PM parts, but the parts should have surface- of four multilevel PM parts, of which three connected porosity sealed by resin impregna- parts (cage, spider, and carrier plate) are made tion prior to plating. Electroless nickel plating to a density of 6.8 g/cm3. In addition, there is applicable to nonimpregnated PM parts. are 17 compacted brazing pellets. The rocker Most conventional welding methods are plate is sinter hardened during the sinter- applicable to PM parts (Ref 26). Care must be brazing phase and has a density of 7.0 g/cm3. taken to avoid residual lubricants, quench oils, The assembly also has a doubled-pressed and machining coolants, plating solutions, impregnat- double-sintered cam plate made to 7.3 g/cm3 ing materials, cleaning or tumbling agents, and density with an ultimate tensile strength of free graphite or residual ash. An example of a 1170 MPa (170 ksi) and a mean tempered hard- PM weldment is shown in Fig. 18 (Ref 27). Care ness exceeding 40 HRC. To form the parts and must be taken with lower-density PM parts, maintain precision tolerances, innovative tooling particularly during fusion welding. Subsequent was developed and used in conjunction with solidification causes high stresses that often result unconventional press motions. Ford subjected in cracks. the assembly to stringent durability testing: ulti- Fig. 19 Carrier and one-way rocker clutch assembly. Furnace brazing can be used to join PM mate torsional torque loading at a minimum of GKN Sinter Metals LLC, courtesy of MPIF parts. When choosing a brazing alloy, the capil- 10.8 kN m (7970 lbf.ft) and fatigue testing at larity of the pores imposes a special condition. a minimum of 299,000 cycles at 2.3 kN m Standard brazing compounds will infiltrate the (1700 lbf.ft). The application provided an esti- adjacent pores, leaving insufficient material to mated 20% cost savings over competing proform a sound brazed joint. A special brazing sys- cesses and represents a new era in the scope tem has been developed for PM materials that and size of PM parts. restricts brazing alloy penetration to the imme- A ball-ramp actuator consisting of a sector gear diately adjacent areas of the part (Ref 28). An and a fixed ring is illustrated in Fig. 20. The actu- example of a brazed carrier and one-way rocker ator applies torque to the front wheels in the clutch assembly is provided in Fig. 19. BMW high-performance X-Drive transfer case that goes into various BMW platforms. Warm compacted from a hybrid low-alloy steel, the parts have a density of 7.2 g/cm3 in the ball ramps and Applications of Powder 7.0 g/cm3 between ramps and on teeth, a typical Metallurgy Parts tensile strength of 1330 MPa (190 ksi), typical yield strength of 1144 MPa (166 ksi), and a sur- The following examples have been selected face hardness of 50 HRC on the ball ramp surface. Fig. 20 Sector gear and fixed ring. Cloyes Gear & to illustrate the wide diversity of the parts made The parts replaced forged components that had Products Inc., courtesy of MPIF

409 Cb wrought tube 409 Cb wrought tube

409 Cb PM flange 409 Cb PM flange TIG weld - no filler Fig. 18 Example of a PM weldment. Source: Ref 27. Reprinted with permission from SAE Technical Paper 930490, Fig. 21 Variable valve timing (VVT) rotor adaptor copyright SAE International assembly. GKN Sinter Metals, courtesy of MPIF Powder Metallurgy Methods and Applications / 17

V-6 engine, is mounted to the engine camshaft. made of 17-4 PH. The extremely complex strength of 565 MPa (82 ksi), yield strength of Formed to a density of 6.8 g/cm3, the rotor has geometry of the blank discharge check, with 450 MPa (65 ksi), and a hardness of 80 HRB an ultimate tensile strength of 415 MPa (60 ksi), the intercrossing of holes, required tooling with before steam oxide treatment. The part is an yield strength of 380 MPa (55 ksi), and a six side cores, three of which move at different original design for PM, because its shape makes 160 MPa (23 ksi) fatigue limit. The adapter is timings. The parts have a minimum density of it impractical for traditional metal cutting meth- formed to a density of 6.9 g/cm3, has a minimum 7.65 g/cm3, an ultimate tensile strength of ods. It is pressed and sintered to net shape, ultimate tensile strength of 400 MPa (58 ksi), 480 MPa (70 ksi), yield strength of 150 MPa requiring no postsintering machining operations. and has a yield strength of 365 MPa (53 ksi). After (22 ksi), an elongation of 45%, and a 100 An example of small, intricate PM parts is sizing and grinding, there is no other machining HRB maximum hardness. This design was provided in Fig. 26. The three parts—bracket, performed on the rotor. The adapter is not judged by the fabricator to be perhaps slide, and removable drop-in hook—used in the machined prior to assembly and is made to net the most complex high-volume part ever made Damon 3MX self-ligation orthodontic tooth- shape with vertical slots for oil feeding. The cus- by MIM. The customer realized cost savings positioning system are made via MIM proces- tomer, however, machines the cross-holes for the of close to 35%, while the pump performance sing. One bracket and one slide go on each oil feed. was improved by modifying the geometry of tooth, with the hook an option for approximately Figure 22 shows a complex PM steel two- the holes to enhance flow dynamics, with the 5% of the teeth. The very tiny, intricate parts are stage helical gear and spur pinion used in a result being a 10 to 20% fuel economy boost. made by MIM from 17-4 PH stainless steel pow- power lift-gate actuator. Made to a nominal Another automotive application is shown in der to a density of 7.5 g/cm3. They have impres- density of 6.85 g/cm3, the combined helical Fig. 24. It is a PM aluminum camshaft-bearing sive physical properties: a tensile strength of gear-and-pinion design features precision jour- cap used in GM’s high-feature V6 engine. 1190 MPa (173 ksi) and yield strength of nals for precise orientation in the actuator Designed originally for PM, the caps—two of 1090 MPa (158 ksi). All of the parts are made assembly. The part has a tensile strength of which go into each engine—operate in engines to a net shape. The customer tumble polishes 450MPa(65ksi)andyieldstrengthof that go into various GM brands, including the them and performs a brazing operation before 380 MPa (55 ksi). The precise elemental gear Cadillac CTS, SRX, and CTX; Buick LaCrosse assembly. data tolerances enable quiet gear performance, and Rendezvous; and Saab 9-3. It is the first decreasing noise, vibration, and harshness. dual overhead cam engine using a single cap Four metal injection molded (MIM) parts across both camshafts. The cap maintains the (a blank discharge check, stop discharge check camshaft position, radially and axially, while valve, valve discharge check, and CRV spring providing integral oil channels for cam lubrica- seat) that go into a device that controls fuel tion and hydraulic control of the variable cam flow in gasoline direct-injection pumps are timing (VCT) system. Made to a net shape, shown in Fig. 23. Three of the parts are made the multiple-level part has a tensile strength of 440C stainless steel, while the fourth is of 117 MPa (17 ksi) and a hardness range of 85 to 90 HRH. Choosing PM over an alternative manufacturing process, such as die casting, provided an estimated 50% cost saving by elim- inating preassembly machining steps. The PM caps require only one line-boring step during installation. In addition to being used in automotive applications, PM parts are also chosen for lawn and garden use. The parking/emergency brake piston shown in Fig. 25 is used in hydraulic transmis- sions in zero-turn-radius lawn maintenance equipment. Made from an FC-0208 iron-copper steel, the piston is compacted with three features Fig. 25 Brake piston for hydraulic transmission used in on top and six on the bottom, using two upper zero-turn-radius lawn maintenance equipment. and three lower punches plus a die shelf. The Lovejoy Powder Metal Group, courtesy of MPIF piston has a density of 6.9 g/cm3, a tensile

Fig. 22 Helical gear and spur pinion. Capstan Atlantic, courtesy of MPIF

Fig. 24 Powder metallurgy aluminum camshaft-bearing Fig. 23 Gasoline direct-injection pump parts. Indo-US cap. Metal Powder Products Co., courtesy of Fig. 26 Orthodontic system bracket, slide, and hook. MIM Tec Pvt. Ltd., courtesy of MPIF MPIF FloMet LLC, courtesy of MPIF 18 / Introduction to Powder Metallurgy

Another example of MIM parts is provided in Fig. 27. It is a high-compression jaw used in laparoscopic vessel fusion. The jaw design has top and bottom jaws, an anchor, and an I-beam. All four components are made from 17-4 PH metal powder and have as-sintered densities greater than 7.6 g/cm3. The parts have very thin walls and highly complex geometries, making them difficult to manufacture economically by any other technology. Top and bottom jaws pivot at the lobes that provide the fulcrum for the assembly. The cutting mechanism on the laparoscopic device is in the shape of an Fig. 29 Nozzle assembly for high-end sound-insulating headphones. Flomet LLC, courtesy of MPIF I-beam. Very high compression is maintained as the blade is advanced from the proximal to the distal end of the jaw. The SurgRx system incorporates smart electrotechnology in a high-compression jaw design to provide rapid vessel fusion without thermal effects. A sound tube used in a hearing aid, the func- Fig. 27 Laparoscopic jaws. Parmatech Corporation, tion of which is to enhance sound frequency courtesy of MPIF and improve hearing, is shown in Fig. 28. Fab- ricated via MIM using 316 stainless steel, the highly complex part achieves all its features in the as-sintered condition, with only glass-bead blasting for a better finish performed as a secondary operation. The tube has a minimum density of 7.65 g/cm3, an ultimate tensile strength of 480 MPa (70 ksi), yield strength of 150 MPa (22 ksi), an elongation of 45%, and a hardness of 100 HRB max. An original design for MIM, it is estimated the part provides 20% Fig. 30 Dipole cryomagnet end cover. Bodycote HIP- Surahammar, courtesy of MPIF cost savings over competing forming processes. A three-piece assembly (nozzle interface, outer nozzle, and metal collar) that goes into high-end sound-isolating earphones that enable user-customizable frequency responses is shown in Fig. 29. Made via MIM from 316L Fig. 28 Hearing aid sound tube. Indo-US MIM Tec stainless steel, the components met the objec- Pvt. Ltd., courtesy of MPIF tive of producing final net-shape parts that not only satisfied the cost demands of the highly competitive professional-audio market but also sheet metal cutting technology, and cutting- maintained a cosmetically perfect surface so edge robotic welding and part manipulation to critical in a consumer product with a clear exte- produce the end covers. This resulted in an rior. The parts have a density >7.6 g/cm3,an increase of more than 50 times over the typical ultimate tensile strength of 520 MPa (75 ksi), production rate of fully dense, HIPed PM near- yield strength of 175 MPa (25 ksi), an elonga- net shapes—an unprecedented breakthrough in tion of 50%, and an apparent hardness of 67 productivity. Approximately 2700 end covers HRB. Metal injection molding was the ideal have been delivered to the European Organiza- choice because alternative fabrication methods, tion for Nuclear Research (CERN). The design such as die casting or machining, could neither of the part features several complex configura- have provided the precision needed at a reason- tions. For example, both the inner and outer able cost nor been able to provide the required surface of the broad face are radiused with the material performance. inner surface approximately parallel to the At the other end of the spectrum are much outer surface. The exterior of the curved surface Fig. 31 Manifold used in offshore oil and gas produc- larger examples. An end cover used in the has either eight or ten projections, depending on tion. Metso Powdermet AB, courtesy of MPIF Large Hadron Collider, the world’s largest and which version of the part is produced. The highest-energy subatomic particle accelerator, design differs slightly depending on which side is shown in Fig. 30. Made from 316LN stain- of the dipole magnet it is located. The PM less steel powder, the part is hot isostatically HIPed part meets the equivalent mechanical forging and conventional machining of these pressed to full density. The superconducting properties of 316LN wrought stainless steel, very large parts, providing an 8% cost savings. dipole-cryomagnets operate in a cryogenic including internal toughness and high ductility. Hot isostatic pressing also reduced the need for environment at –268 C (–450 F). As HIPed A final example (Fig. 31) is a manifold used extensive welding. The manifolds are formed to a near-net shape of 115 kg (253 lb), the fin- in offshore oil and gas production. Formed by close to net shape. The only machining required ished end cover weighs 69.5 kg (153 lb). 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