Powder Metallurgy Methods and Applications

Copyright # 2015 ASM InternationalW All rights reserved asminternational.org ASM Handbook, Volume 7, Powder Metallurgy P. Samal and J. Newkirk, edito...
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Copyright # 2015 ASM InternationalW All rights reserved asminternational.org

ASM Handbook, Volume 7, Powder Metallurgy P. Samal and J. Newkirk, editors

Powder Metallurgy Methods and Applications W. Brian James, Hoeganaes Corporation, retired

Powder metallurgy (PM) is the production and utilization of metal powders. Powders are defined as particles that are usually less than 1000 nm (1 mm) in size. Most of the metal particles used in PM are in the range of 5 to 200 mm (0.2 to 7.9 mils). To put this in context, a human hair is typically in the 100 mm (3.9 mils) range. The history of PM has already been outlined in the article “History of Powder Metallurgy” in this Volume. This article reviews the various segments of the PM process from powder production and powder processing through to the characterization of the materials and their properties. It will cover processing methods for consolidating metal powders including options for processing to full density. Powders have a high ratio of surface area to volume and this is taken advantage of in the use of metal powders as catalysts or in various chemical and metallurgical reactions. While this article focuses on the use of powders to make functional engineering components, many metal powders are used in their particulate form. This aspect of PM is covered in the article “Specialty Applications of Metal Powders” in this Volume. Powder technologies are exciting to engineers because processing options permit the selective placement of phases or pores to tailor the component for the application. The capability of press and sinter processing or metal injection molding (MIM) processing to replicate parts in high volumes is very attractive to design engineers. The ability to fabricate complex shapes to final size and shape or to near-net shape is particularly valuable. Powder metallurgy offers the potential to do this in high volumes and also for applications where the volumes are not so large. The three main reasons for using PM are economic, uniqueness, and captive applications, as shown in Fig. 1 (Ref 1). For some applications that require high volumes of parts with high precision, cost is the overarching factor. A good example of this segment is parts for the automotive industry (where approximately 70% of

ferrous PM structural parts are used). Powder metallurgy parts are used in engine, transmission, and chassis applications. Sometimes it is a unique microstructure or property that leads to the use of PM processing: for example, porous filters, self-lubricating bearings, dispersionstrengthened alloys, functionally graded materials (e.g., titanium-hydroxyapatite), and cutting tools from tungsten carbide or diamond composites. Captive applications of PM include materials that are difficult to process by other techniques, such as refractory metals and reactive metals. Other examples in this category are special compounds such as molybdenum disilicide and titanium aluminide, or amorphous metals. The metal powder industry is a recognized metal forming technology that competes directly with other metalworking practices such as casting, forging, stamping (fine blanking),

and screw machining. The industry comprises powder suppliers and parts makers, plus the companies that supply the mixing equipment, powder handling equipment, compacting presses, sintering furnaces, and so forth. Powder metallurgy processing offers many advantages. The PM process is material and energy efficient compared with other metal forming technologies. Powder metallurgy is cost effective for making complex-shaped parts and minimizes the need for machining. A wide range of engineered materials is available, and through appropriate material and process selection the required microstructure may be developed in the material. Powder metallurgy parts have good surface finish and they may be heat treated to increase strength or wear resistance. The PM process provides part-to-part reproducibility and is suited to moderate-to-high volume production. Where necessary, controlled microporosity can be provided for self-lubrication or filtration. While dimensional precision is good, it typically does not match that of machined parts. In the case of ferrous PM parts, they have lower ductility and reduced impact resistance compared with wrought steels. The majority of PM parts are porous and consideration must be given to this when performing finishing operations.

Metal Powders Metal powders come in many different shapes and sizes (Fig. 2). Their shape, size, and size distribution depend on the manner in which they were produced. Metal powder production is covered in depth in various articles in the Section, “Metal Powder Production” in this Volume. There are three main methods of powder production:

Fig. 1

Three main reasons for choosing powder metallurgy shown in the form of a Venn diagram. The intersection of the three circles represents an ideal area for applying PM techniques. Source: Ref 1

 Mechanical, including machining, milling,

and mechanical alloying

 Chemical, including electrolytic deposition,

decomposition of a solid by a gas, thermal

10 / Introduction to Powder Metallurgy

5 μm

Fig. 2

5 μm

Example of the different particle shapes possible with metal powders

decomposition, precipitation from a liquid, precipitation from a gas, solid-solid reactive synthesis  Physical, including atomization techniques Most metals are available in powder form. Some may be made by many different methods, while for others only a few options are possible. The characteristics of the powder are determined by the method by which it is produced. The shape, size, size distribution, surface area, apparent density, flow, angle of repose, compressibility, and green strength depend on the powder production method. In-depth coverage of the sampling and testing of metal powders is presented in the articles in the Section “Metal Powder Characterization” in this Volume.

Powder Processing For the production of PM parts in high volumes, compaction is carried out in rigid dies. In most instances, the metallic powders are mixed with a lubricant (e.g., ethylene bisstearamide) to reduce interparticle friction during compaction and to facilitate ejection of the compacted parts by reducing friction at the die-wall and core-rod interfaces. The metal powders may be elemental powders; mixtures of elemental powders; or mixtures of elemental powders with master alloys or ferroalloys, prealloys, diffusion alloys, or hybrid alloys. See the article “Ferrous Powder Metallurgy Materials” in this Volume for an in-depth review of the alloying methods used in ferrous PM. A consequence of the various alloying methods available is that only the PM materials made from prealloyed powders are chemically homogeneous. The other alloying methods can result in chemically inhomogeneous materials. The hardenability is determined by the local chemical composition, and the resulting microstructures are generally quite complex. Chemical analysis can be a challenge due to the inhomogeneous nature of the materials. Guidelines for sample preparation for the

chemical analysis of the metallic elements in PM materials are provided in MPIF Standard 67 (Ref 2). Complex, multilevel PM parts compacted in rigid dies will not have the same green density throughout. While the objective is generally to achieve a density as uniform as possible throughout the compacted part, taller parts and parts with multiple levels are subject to the presence of density differences between adjacent regions. This is due to frictional effects and compacting tool deflections. Taller parts will have a neutral zone or density line—the region of the compact that has experienced the least relative movement of powder. The position of the neutral zone may be adjusted by varying the pressure exerted by the upper and lower punches. Compaction in rigid dies is limited to part shapes that can be ejected from the die cavity. Parts with undercuts, reverse tapers, threads, and so forth, are not generally practical. Such features are formed by postsintering machining operations. There are two main types of compacting press: mechanical and hydraulic. Some hybrid presses offer features of both. A detailed treatment of compaction is provided in the Section “Metal Powder Compaction” in this Volume. Some PM parts are molded (shaped) rather than compacted. Fine-particle-size metal powders (5 to 20 mm, or 0.2 to 0.8 mils) are mixed with binders and plasticizers and processed to form a feedstock for MIM. Molding is performed using machines similar to those used for plastic injection molding. Shrinkage during the subsequent sintering operation is extensive (15 to 20%) due to the fine-particle-size powders used and the high sintering temperatures. Because the parts are molded and not compacted, they do not contain density gradients that lead to distortion or problems with dimensional control. The process makes complexshaped, small-to-medium sized PM parts with high relative densities. Some metal powders are not very compressible. The powder particles are hard and have

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 order to form metallurgical bonds between the compacted metal powder particles. Sintering is a “shrinkage” process. The system tries to reduce its overall surface area via various diffusion processes. Metallurgical bonds (microscopic weldments) form between adjacent metal particles (after oxides have been reduced on the surface of the powder particles), pore surfaces become less irregularly shaped, and larger pores grow at the expense of the smaller pores. Sintering is generally carried out using continuous mesh-belt furnaces. For higher temperatures (>1150  C, or 2100  F), pusher, roller hearth, or walking-beam furnaces may be used. Batch furnace processing is used for special applications (e.g., pressure-assisted sintering). More information on sintering may be found in the Section “Sintering Basics” in this Volume.

Powder Metallurgy Material Properties The majority of PM parts contain pores (see options for processing metal powders to full density later in this article). This is an advantage when metal powders are used to make self-lubricating bearings in which the surfaceconnected pores of the parts are impregnated with oil. When the bearing surface heats up due to frictional heat, oil is released from the pores. When the bearing cools, the oil is sucked back into the pore channels by capillary action. The porosity in PM parts has an effect on the physical, mechanical, magnetic, thermal, wear, and corrosion properties of the parts. Thermophysical properties of sintered steels, in particular their coefficient of thermal expansion and their thermal conductivity, are needed when designing parts and when modeling heat treatment processes. Opinions differ in the PM community as to the effect of density on these properties. Danninger has shown, however, that the coefficient of thermal expansion up to 1000  C (1832  F), measured through dilatometry, is virtually independent of porosity (density) over a density range from 5.97 to 7.53 g/cm3 (Ref 3). In addition, thermal conductivity was determined in the same temperature range by using laser flash to measure thermal diffusivity, and specific heat, and then

Powder Metallurgy Methods and Applications / 11

  a ¼ l rCp

(Eq 1)

where a is thermal diffusivity, l is thermal conductivity, r is density, and Cp is specific heat at constant pressure. Thermal conductivity was shown to depend on density. The effect of porosity in the technically relevant density range was, however, slightly less pronounced than the effect exerted by the alloying elements; specifically, the variation observed between different standard PM steel grades in the low-to-medium temperature range. Both thermophysical properties are, therefore, significantly less influenced by porosity than by chemical composition. Powder metallurgy steels are more similar to wrought steels than was generally assumed. The elastic constants are also of interest to the design engineer. Young’s modulus, Poisson’s ratio, and the shear modulus are related according to: E ¼ 2Gð1 þ nÞ

(Eq 2)

where E is Young’s modulus, G is shear modulus, and n is Poisson’s ratio. E and n are determined by resonant frequency and G is calculated from Eq 2. Beiss (Ref 4) has shown that: E ¼ E0 ðr=r0 Þm

(Eq 3)

Young’s modulus, GPa

where E0 is the Young’s modulus of the porefree material, r is the density of the material, r0 is the density of the pore-free material, and the exponent m depends on the pore morphology and varies between 2.5 and 4.5. Nevertheless, over the density range of interest for ferrous PM structural materials, 6.4 to 7.4 g/cm3, Young’s modulus is essentially a linear function of density (Fig. 3) (Ref 5).

P=P0 ¼ ðr=r0 Þm

(Eq 4)

where, P is the property of interest, P0 the value for the pore-free material, r is the density of the material, r0 is the density of the pore-free material, and m is an exponent the value of which depends on a given property (Fig. 4) (Ref 6–7). While tensile strength increases in a linear fashion as density increases, tensile ductility is more dependent on reducing the level of porosity. Fatigue performance is even more influenced by density with an exponent m of between 3.5 and 4.5. Impact energy is the most dependent on density, with an exponent m of approximately 12. Magnetic properties of ferrous PM materials are affected by density. Induction and permeability increase as the density is increased. Permeability and coercive field strength are structure-sensitive properties that are degraded by the presence of impurities. The sintering conditions are extremely important to keep carbon, nitrogen, and oxygen contents to low levels (C = 0.03 wt% max; N = 0.01 wt% max; and O = 0.10 wt% max). Residual stresses from operations such as sizing, machining, or shot peening degrade the magnetic properties. The properties can be restored through an annealing treatment.

Processing Options to Consolidate Metal Powders There are three basic approaches to the consolidation of metal powders, as shown in Fig. 5 (Ref 8). Pressure-based compaction establishes density via the compaction process then sinters

52

7.5

51

7.4

50

7.3

49

7.1

48

7.0

47

6.8

46

6.7

45

6.5

44 6.4

Fig. 3

Poisson’s ratio is a weak function of density, and for ferrous PM structural steels it can be taken as 0.27 ± 0.02. The mechanical properties of PM materials are a function of density:

6.6

6.8 7.0 7.2 Sintered density, g/cm3

Young’s modulus as a function of sintered density. Data from Ref 5

7.4

to develop strength in the compacts. Rigid die compaction falls into this category and is the most cost-effective method for the high-volume production of PM structural parts. In order for this method to be viable, the metal powders need an irregular shape and good flow characteristics, they must be compressible, and they must have good “green” strength. (Green is the term used to describe an as-pressed compact.) Extremely hard particles with a spherical shape are not appropriate for use in rigid die compaction. Compaction takes place at high pressure in confined dies (the dies are generally made from cold work tool steel or cemented carbide). Compacting pressures for ferrous powders are generally in the range from 400 to 700 MPa (60 to 100 ksi), from 100 to 400 MPa (14.5 to 60 ksi) for aluminum and aluminum alloy powders, and approximately 400 MPa (60 ksi) for copper and copper-alloy powders. The green density increases as the compacting pressure is increased and levels out at higher compacting pressures. Powder particles work harden as the result of plastic deformation and it requires higher pressures to cause further plastic flow. In addition, the lubricant that is typically admixed to aid particle rearrangement and to reduce the fictional forces between the powder and the compacting tools eventually has no place to go because all the voids between particles have been closed—either by metal flow or by the presence of lubricant. More lubricant is beneficial at lower compacting pressures, but there is a transition point at which the additional lubricant impedes further densification (Fig. 6) (Ref 9). Warm compaction processing was developed to overcome the compressibility constraints of rigid-die compaction (Ref 10). The powder mixture and the compacting tools are heated

Young’s modulus, 106psi

the thermal conductivity was calculated from these parameters and the density in accordance with:

6.4 7.6

Fig. 4

Effect of density on mechanical and physical properties of PM materials. Source: Ref 6

12 / Introduction to Powder Metallurgy Contacting pressure, ksi 58.0 7.4

72.5

87.0

101.5

116.0

130.5

Green density, g/cm3

7.3

Fig. 5

Three basic approaches to the consolidation of metal powders. Source: Ref 8

to approximately 120  C (250  F) and the powder is compacted in a single press stroke. Green densities of up to 7.3 g/cm3 are possible with highly compressible ferrous powders. This is approximately 98% of the pore-free density of the powder mixture being compacted (The pore-free density of the powder mixture is the 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% reduction 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 19 mm (0.75 in.). For larger parts warm compaction processing is required. Green densities of 7.45 g/cm3 have been reached using warmdie compaction with lubricant additions of approximately 0.3 wt% (Ref 11). In sintering-based densification, the shape of the component is formed in a molding operation (e.g., MIM) and sintering is enhanced by the use of high temperatures and fine-particlesize powders. While extensive shrinkage occurs during sintering, it is essentially isotropic in nature so that good tolerance can still be achieved. Metal powder loading in the feedstock used for MIM is approximately 60%. The binders and plasticizers added to make the mixture moldable must be removed prior

7.2 Transition pressure 7.1 7.0 6.9 Atomized iron 6.8 6.7 400

500

600

700

800

900

Compacting pressure, MPa 0.5% Zn St

0.75% Zn St

1.0% Zn St

Fig. 6

Effect of lubricant content on the compressibility of metal powders. Source: Ref 9

Fig. 7

Examples of warm compacted PM parts. (a) Torque converter hub. Courtesy of Chicago Powder Metal Products. (b) Transmission output shaft hub. Courtesy of GKN Sinter Metals. (c) Hand tool parts. Courtesy of PoriteTaiwan Co. Ltd.

to final sintering. This “debinding” step is the rate-controlling phase of the MIM process. Other sintering-based densification processes that involve the molding or shaping of powders are slip casting and tape casting. Hybrid Densification. For some materials, a hybrid densification process is used in which pressure and temperature are applied at the same time. As mentioned previously, some powders are not suitable for rigid-die compaction; they are too hard, are spherical in shape, or are too reactive. In this instance, processes such as powder extrusion (typically after encapsulation) or

HIP are used. Hot pressing (often in vacuum) or spark sintering may also be used.

Processing to Full Density Options for processing metal powders to full density are mapped in Fig. 8 (Ref 12). The vertical axis relates to relative stress (the applied pressure divided by the in situ yield strength of the material) and the horizontal axis relative temperature (based on the melting temperature of the material).

Powder Metallurgy Methods and Applications / 13 Full-density processing requires the simultaneous application of pressure and temperature. The approach works because most materials soften as temperature is increased. They also become more ductile and deform with less work hardening. The processing options fall into the following categories:  Low-stress processes that operate at high

temperatures and are dominated by diffusion processes (e.g., liquid-phase sintering)  Processes that apply intermediate stress levels and operate at intermediate temperatures and

rely on diffusional creep processes (e.g., hot pressing or HIP)  High-stress routes that operate at high strain rates and lower temperatures (powder forging or extrusion)  Routes that achieve high density via the application of ultrahigh-stress at ambient temperature (explosive compaction) Liquid-phase sintering results in a composite microstructure that consists of a skeleton of a high-melting-temperature phase in a matrix of a solidified liquid—for example, W-Ni-Fe heavy alloys, WC-Co cemented carbides.

Hot pressing is performed in a rigid die using uniaxially applied pressure: it is a low-strain-rate process. Graphite dies may be used, in which case induction heating may be employed. Hot pressing cycle times are slow compared with rigid-die compaction. Vacuum is sometimes used to minimize contamination of the compact. Diamond-metal-composite cutting tools are often hot pressed. Spark sintering is a process related to hot pressing. In spark sintering, direct resistance heating is applied to the punches, die, and powder mass during consolidation. Hot isostatic pressing applies pressure from all directions simultaneously. In order to establish 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 in Fig. 11 (Ref 14).

Fig. 8

Options for processing metal powders to full density. Source: Ref 12

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 conventional pressing and sintering and wrought steel technology. The process is illustrated schematically in Fig. 12 (Ref 15). A PM preform is typically compacted, sintered, and then reheated before being forged in a single stroke in confined dies. A detailed review of the powder forging of ferrous materials is given in the Section “Powder Metallurgy Carbon and LowAlloy Steels” in this Volume. Extrusion is used to make some PM tool steels. These materials have better properties than similar wrought tool steels because they contain a finer and more uniform dispersion of carbides compared with the wrought tool steels. In the latter, the carbides are often banded and in the form of stringers due to the rolling process used to make them, as shown in Fig. 13 (Ref 16).

Freeform Fabrication Thermal spraying of nickel-base and cobaltbase alloy powders to form wear-resistant coatings has been practiced for many years. Spray forming is a consolidation process that captures a spray of molten metal or alloy droplets on a moving substrate (Ref 17). Figure 14 illustrates billet formation in a vertical mode by spray forming. The process can be used to form billets, strip, and thick-walled tubing. The term additive manufacturing of metals is used to describe freeform processes that offer the possibility to produce complex-shaped PM parts without the design constraints of traditional manufacturing routes (Ref 18). The process relies on the transfer of a digital file to a machine that then builds the three-dimensional component layer by layer from a metal powder using a laser or an electron beam to fuse the particles together. Schematic illustrations of powder-bed and powder-fed systems are shown in Fig. 15 (Ref 19).

Finishing Operations While PM is considered a net or near-net shaping process, many PM parts require finishing

Fig. 11

Schematic of pressure-assisted sintering process cycle. Source: Ref 14

operations. Sometimes parts need closer tolerances than can be held during the pressing and sintering operation; they can be sized to reduce their dimensional variability. The surface-connected, interconnected porosity in PM parts can be impregnated with oil, and this is the basis for self-lubricating bearings (Ref 20). Conventional bearings can absorb from 10 to 30% by volume of oil. Pressure tightness can be achieved in PM parts by sealing the surface-connected porosity by resin impregnation. Vacuum processing is generally used to impregnate the PM parts. In addition to developing pressure tightness, resin impregnation of PM parts permits plating (otherwise, plating solutions would be trapped in the surface-connected pores). Resin impregnation significantly improves the drillability of PM parts, as shown in Fig. 16 (Ref 21). Machining parameters for PM parts are different from those used for castings or wrought components. The PM materials contain pores. 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 the amount of porosity decreases, PM parts machine more like cast or wrought parts with a similar microstructure. Machinability aids 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. Machinability 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 consistency and predictability in the machining operation. A statistical approach to evaluating the

To preheat furnace

Eject from die Powder fill

Press proform

Eject fully dense part Hot forge

Fig. 10

(a) Pre-HIP filled can weighing 2050 kg (4520 lb). (b) Post-HIP. (c) Heat treated and sonic machined HIPed part. Courtesy of Carpenter Technologies. Source: Ref 13

Fig. 12

Rapid transfer

Schematic of the powder forging process. Source: Ref 15

Heat; controlled atmosphere

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 compared with the uniform dispersion of fine carbides in the PM tool steel. Source: Ref 16

Fig. 14

Billet formation in a vertical mode by spray forming. Source: Ref 17

“Sweet spot” Robust process, able to accept variation

Ma

Fig. 17 Fig. 15

Schematic illustrations of (a) powder-bed and (b) powder-fed additive manufacturing. Source: Ref 18

Fig. 16

Relative drillability of various PM materials. Source: Ref 21

data is extremely beneficial. Abrasive wear is the common and natural mechanism of wear during machining—it is the desired mechanism. There are combinations of machining parameters (cutting tool, feeds, speeds, etc.) that result in consistent machining performance

without the tool failing in a catastrophic manner. This is the “safe zone” for machining, Fig. 17 (Ref 22). Ferrous PM parts may be heat treated to improve their hardness, strength, and wear resistance. Oil quenching and tempering may be used

ch

in

p ing

roc

es

s

“Safe zone” Noncatastrophic, consistent tool performance

“Sweet spot” for machinability. Source: Ref 22

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. Microindentation 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 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 absorption. Rust inhibitors should be added to the water. Parts may be spun dry or heated to dry. Ferrous PM parts may be furnace blackened (steam oxide treated) for indoor corrosion resistance. Afterward, they may be oil dipped for

16 / Introduction to Powder Metallurgy color as well as slightly greater corrosion resistance (a dry film oil is particularly suitable). Steam treating forms a coating of magnetite (Fe3O4) in the surface-connected pores. Parts are heated to 480 to 570  C (896 to 1060  F) and exposed to superheated steam under pressure. This improves the wear resistance of ferrous PM parts and improves their compressive strength. It does, however, degrade tensile properties (Ref 25). All types of plating processes may be applied to PM parts, but the parts should have surfaceconnected porosity sealed by resin impregnation prior to plating. Electroless nickel plating is applicable to nonimpregnated PM parts. Most conventional welding methods are applicable to PM parts (Ref 26). Care must be taken to avoid residual lubricants, quench oils, machining coolants, plating solutions, impregnating materials, cleaning or tumbling agents, and free graphite or residual ash. An example of a PM weldment is shown in Fig. 18 (Ref 27). Care must be taken with lower-density PM parts, particularly during fusion welding. Subsequent solidification causes high stresses that often result in cracks. Furnace brazing can be used to join PM parts. When choosing a brazing alloy, the capillarity of the pores imposes a special condition. Standard brazing compounds will infiltrate the adjacent pores, leaving insufficient material to form a sound brazed joint. A special brazing system has been developed for PM materials that restricts brazing alloy penetration to the immediately adjacent areas of the part (Ref 28). An example of a brazed carrier and one-way rocker clutch assembly is provided in Fig. 19.

Applications of Powder Metallurgy Parts The following examples have been selected to illustrate the wide diversity of the parts made

using PM processes. They are taken from parts that have won awards at the MPIF Design Excellence Competition, which is held annually to highlight the advances made in PM part production (Ref 29). The carrier and one-way rocker clutch assembly shown in Fig. 19 are used in the Ford Super Duty TorqShift six-speed automatic transmission. The hybrid assembly contains five PM steel parts weighing a total of 7.7 kg (17 lb). The sinter-brazed subassembly consists of four multilevel PM parts, of which three parts (cage, spider, and carrier plate) are made to a density of 6.8 g/cm3. In addition, there are 17 compacted brazing pellets. The rocker plate is sinter hardened during the sinterbrazing phase and has a density of 7.0 g/cm3. The assembly also has a doubled-pressed and double-sintered cam plate made to 7.3 g/cm3 density with an ultimate tensile strength of 1170 MPa (170 ksi) and a mean tempered hardness exceeding 40 HRC. To form the parts and maintain precision tolerances, innovative tooling was developed and used in conjunction with unconventional press motions. Ford subjected the assembly to stringent durability testing: ultimate torsional torque loading at a minimum of 10.8 kN  m (7970 lbf.ft) and fatigue testing at a minimum of 299,000 cycles at 2.3 kN  m (1700 lbf.ft). The application provided an estimated 20% cost savings over competing processes and represents a new era in the scope and size of PM parts. A ball-ramp actuator consisting of a sector gear and a fixed ring is illustrated in Fig. 20. The actuator applies torque to the front wheels in the 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 7.0 g/cm3 between ramps and on teeth, a typical tensile strength of 1330 MPa (190 ksi), typical yield strength of 1144 MPa (166 ksi), and a surface hardness of 50 HRC on the ball ramp surface. The parts replaced forged components that had

been used in an earlier transfer case design and provided 35% cost savings over the forgings. The variable valve timing (VVT) rotor shown in Fig. 21 consists of an assembly of a PM steel rotor and an adapter. The parts are joined by an adhesive, which joins them during the machining of cross-holes and other features on the inside diameter, and seals the joint between them. The assembly, used in a Chrysler

Fig. 19

Carrier and one-way rocker clutch assembly. GKN Sinter Metals LLC, courtesy of MPIF

Fig. 20

Sector gear and fixed ring. Cloyes Gear & Products Inc., courtesy of MPIF

Fig. 21

Variable valve timing (VVT) rotor adaptor assembly. GKN Sinter Metals, 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, copyright SAE International

Powder Metallurgy Methods and Applications / 17 V-6 engine, is mounted to the engine camshaft. Formed to a density of 6.8 g/cm3, the rotor has an ultimate tensile strength of 415 MPa (60 ksi), yield strength of 380 MPa (55 ksi), and a 160 MPa (23 ksi) fatigue limit. The adapter is formed to a density of 6.9 g/cm3, has a minimum ultimate tensile strength of 400 MPa (58 ksi), and has a yield strength of 365 MPa (53 ksi). After sizing and grinding, there is no other machining performed on the rotor. The adapter is not machined prior to assembly and is made to net shape with vertical slots for oil feeding. The customer, however, machines the cross-holes for the oil feed. Figure 22 shows a complex PM steel twostage helical gear and spur pinion used in a power lift-gate actuator. Made to a nominal density of 6.85 g/cm3, the combined helical gear-and-pinion design features precision journals for precise orientation in the actuator assembly. The part has a tensile strength of 450 MPa (65 ksi) and yield strength of 380 MPa (55 ksi). The precise elemental gear data tolerances enable quiet gear performance, decreasing noise, vibration, and harshness. Four metal injection molded (MIM) parts (a blank discharge check, stop discharge check valve, valve discharge check, and CRV spring seat) that go into a device that controls fuel flow in gasoline direct-injection pumps are shown in Fig. 23. Three of the parts are made of 440C stainless steel, while the fourth is

Fig. 22

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

Fig. 23

Gasoline direct-injection pump parts. Indo-US MIM Tec Pvt. Ltd., courtesy of MPIF

made of 17-4 PH. The extremely complex geometry of the blank discharge check, with the intercrossing of holes, required tooling with six side cores, three of which move at different timings. The parts have 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 100 HRB maximum hardness. This design was judged by the fabricator to be perhaps the most complex high-volume part ever made by MIM. The customer realized cost savings of close to 35%, while the pump performance was improved by modifying the geometry of the holes to enhance flow dynamics, with the result being a 10 to 20% fuel economy boost. Another automotive application is shown in Fig. 24. It is a PM aluminum camshaft-bearing cap used in GM’s high-feature V6 engine. Designed originally for PM, the caps—two of which go into each engine—operate in engines that go into various GM brands, including the Cadillac CTS, SRX, and CTX; Buick LaCrosse and Rendezvous; and Saab 9-3. It is the first dual overhead cam engine using a single cap across both camshafts. The cap maintains the camshaft position, radially and axially, while providing integral oil channels for cam lubrication and hydraulic control of the variable cam timing (VCT) system. Made to a net shape, the multiple-level part has a tensile strength 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 eliminating 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 transmissions in zero-turn-radius lawn maintenance equipment. Made from an FC-0208 iron-copper steel, the piston is compacted with three features on top and six on the bottom, using two upper and three lower punches plus a die shelf. The piston has a density of 6.9 g/cm3, a tensile

Fig. 24

Powder metallurgy aluminum camshaft-bearing cap. Metal Powder Products Co., courtesy of MPIF

strength of 565 MPa (82 ksi), yield strength of 450 MPa (65 ksi), and a hardness of 80 HRB before steam oxide treatment. The part is an original design for PM, because its shape makes it impractical for traditional metal cutting methods. It is pressed and sintered to net shape, requiring no postsintering machining operations. An example of small, intricate PM parts is provided in Fig. 26. The three parts—bracket, slide, and removable drop-in hook—used in the Damon 3MX self-ligation orthodontic toothpositioning system are made via MIM processing. One bracket and one slide go on each tooth, with the hook an option for approximately 5% of the teeth. The very tiny, intricate parts are made by MIM from 17-4 PH stainless steel powder to a density of 7.5 g/cm3. They have impressive physical properties: a tensile strength of 1190 MPa (173 ksi) and yield strength of 1090 MPa (158 ksi). All of the parts are made to a net shape. The customer tumble polishes them and performs a brazing operation before assembly.

Fig. 25

Brake piston for hydraulic transmission used in zero-turn-radius lawn maintenance equipment. Lovejoy Powder Metal Group, courtesy of MPIF

Fig. 26

Orthodontic system bracket, slide, and hook. 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 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 function of which is to enhance sound frequency and improve hearing, is shown in Fig. 28. Fabricated 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% 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 stainless steel, the components met the objective of producing final net-shape parts that not only satisfied the cost demands of the highly competitive professional-audio market but also maintained a cosmetically perfect surface so critical in a consumer product with a clear exterior. The parts have a density >7.6 g/cm3, an ultimate tensile strength of 520 MPa (75 ksi), yield strength of 175 MPa (25 ksi), an elongation of 50%, and an apparent hardness of 67 HRB. Metal injection molding was the ideal choice because alternative fabrication methods, such as die casting or machining, could neither have provided the precision needed at a reasonable cost nor been able to provide the required material performance. At the other end of the spectrum are much larger examples. An end cover used in the Large Hadron Collider, the world’s largest and highest-energy subatomic particle accelerator, is shown in Fig. 30. Made from 316LN stainless steel powder, the part is hot isostatically pressed to full density. The superconducting dipole-cryomagnets operate in a cryogenic environment at –268  C (–450  F). As HIPed to a near-net shape of 115 kg (253 lb), the finished end cover weighs 69.5 kg (153 lb). The fabricator incorporated finite-element analysis, computer-aided design, numerically controlled

Fig. 27

Fig. 28

Fig. 29

Nozzle assembly for high-end sound-insulating headphones. Flomet LLC, courtesy of MPIF

Fig. 30

Dipole cryomagnet end cover. Bodycote HIPSurahammar, courtesy of MPIF

Fig. 31

Manifold used in offshore oil and gas production. Metso Powdermet AB, courtesy of MPIF

Laparoscopic jaws. Parmatech Corporation, courtesy of MPIF

Hearing aid sound tube. Indo-US MIM Tec Pvt. Ltd., courtesy of MPIF

sheet metal cutting technology, and cuttingedge robotic welding and part manipulation to produce the end covers. This resulted in an increase of more than 50 times over the typical production rate of fully dense, HIPed PM nearnet shapes—an unprecedented breakthrough in productivity. Approximately 2700 end covers have been delivered to the European Organization for Nuclear Research (CERN). The design of the part features several complex configurations. For example, both the inner and outer surface of the broad face are radiused with the inner surface approximately parallel to the outer surface. The exterior of the curved surface has either eight or ten projections, depending on which version of the part is produced. The design differs slightly depending on which side of the dipole magnet it is located. The PM HIPed part meets the equivalent mechanical properties of 316LN wrought stainless steel, including internal toughness and high ductility. A final example (Fig. 31) is a manifold used in offshore oil and gas production. Formed by HIPing from a duplex stainless steel material, the manifold weighs approximately 10,000 kg (22,050 lb). Hot isostatic pressing replaced

forging and conventional machining of these very large parts, providing an 8% cost savings. Hot isostatic pressing also reduced the need for extensive welding. The manifolds are formed close to net shape. The only machining required is preparing weld bevels for circumferential welds of the header outside diameter and sealing areas of the connecting flanges. The manifold

Powder Metallurgy Methods and Applications / 19 sections are produced to an average length of 2.5 m (98 in.). The manifold collects oil or gas from wellheads and is also used for water injection.

9.

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