1. TOPIC 7
POWDER METALLURGY
Prepared by: Dr SAO

2. Chapter Outline Introduction Production of Metal Powders
Compaction of Metal Powders
Sintering
Secondary and Finishing Operations

3. 1.0 Introduction
The powder metallurgy (P/M) process, in which metal powders are compacted into desired and often complex shapes and sintered (heated without melting) to form a solid piece.
Powder metallurgy has become competitive with processes (such as casting, forging, and machining), particularly for relatively complex parts made of high strength and hard alloys.
The most commonly used metals in P/M are iron, copper, aluminum, tin, nickel, titanium, and the refractory metals.
The powder-metallurgy process consists of the following operations, in sequence:
Powder production
Blending
Compaction
Sintering
Finishing operations

4. Fig 1 (a) shows the examples of typical parts made by powder-metallurgy processes. (b) Upper trip lever for a commercial irrigation sprinkler made by P/M. This part is made of an unleaded brass alloy; it replaces a die-cast part with a 60% cost savings. (c) Main-bearing metal-powder caps for 3.8 and 3.1 liter General Motors automotive engines.

5. Why Powder Metallurgy is Important ?
PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining
PM process wastes very little material - ~ 97% of starting powders are converted to product
PM parts can be made with a specified level of porosity, to produce porous metal parts
Examples: filters, oil‑impregnated bearings and gears

6. Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy
Tungsten filaments for incandescent lamp bulbs are made by PM
Certain alloy combinations and cermets made by PM cannot be produced in other ways
PM compares favorably to most casting processes in dimensional control
PM production methods can be automated for economical production

7. Advantages of PM
Although the cost of metal powder is high, there is no loss of material. The parts can be produced clean & bright, ready for use.- Net shape and near shape products.
Composition of product can be controlled. No risk of contamination.
Close dimensional tolerances can be maintained.
Non-metallic substances can be produced and in any proportion to get the final product.
A wide range of properties such as density, porosity and particle size can be obtained for particular applications.
It is possible to unite materials that cannot be alloyed in the normal sense or would not yield the desired characteristics.
Useful for magnetic core having special desirable properties.
Reduction in the production time.
No skill labor is required.
Saving material and 97 % is possible.
Composition, structure and properties can be controlled more easily and closely than any other fabricating process.

8. Limitations and disadvantages
Pure metal powders are very expensive to produce.
Size of the products to be produced is limited because of the large presses are required.
Lack of metals powder like steels, bronzes, brasses etc.
Strength properties are lower than those of similar article produced by conventional methods.
Poor plastic properties – impact strength and elongation.
Die design limit the size of products.
Dies required are very expensive and needed large quantities of products. Volume must be justified.

9. Fig 2 Outline of processes and operations involved in producing powder metallurgy parts.

10. 2.0 Powder Production
The choice depends on the requirements of the end product.
The microstructure, bulk and surface properties, chemical purity, porosity, shape, and size distribution of the particles depend on the particular process used.
Fig 3 shows the particle shapes in metal powders, and the processes by which they are produced. iron powders are produced by many of these processes.

11. FIGURE 3 Particle shapes in metal powders, and the processes by which they are produced; iron powders are produced by many of these processes (see also Fig. 17.4).

12. FIGURE 4 (a) Scanning-electron microscope image of iron-powder particles made by atomization. (b) Nickel-based superalloy (Udimet 700) powder particles made by the rotating electrode process;. Source: Courtesy of P.G. Nash, Illinois Institute of Technology, Chicago.

13. 2.1 Method of Metal Powder 2.1.1 Atomization
Atomization produces a liquid-metal stream by injecting molten metal through a small orifice.
Fig 5 show the methods of metal-powder production by atomization: (a) gas atomization; (b) water atomization; (c) atomization with a rotating consumable electrode; and (d) centrifugal atomization with a spinning disk or cup.
In centrifugal atomization, the molten-metal stream drops onto a rapidly rotating disk or cup, so that centrifugal forces break up the molten-metal stream and generate particles.

14. FIGURE 5 Methods of metal powder production by atomization: (a) gas atomization; (b) water atomization; (c) centrifugal atomization with a spinning disk or cup; and (d) atomization with a rotating consumable electrode.

15. 2.1.3 Electrolytic deposition
Reduction
The reduction of metal oxides (i.e., removal of oxygen) uses gases, such as hydrogen and carbon monoxide, as reducing agents.
By this means, very fine metallic oxides are reduced to the metallic state.
The powders produced are spongy and porous and have uniformly sized spherical or angular shapes.
Electrolytic deposition
Electrolytic deposition utilizes either aqueous solutions or fused salts. The powders produced are among the purest available.

16. Carbonyls
Metal carbonyls, such as iron carbonyl and nickel carbonyl are formed by letting iron or nickel react with carbon monoxide.
The reaction products are then decomposed to iron and nickel, and they turn into small, dense, uniformly spherical particles of high purity.

17. Comminution
Mechanical comminution (pulverization) involves crushing (Fig. 6), milling in a ball mill, or grinding of brittle or less ductile metals into small particles.
A ball mill (Fig. 6b) is a machine with a rotating hollow cylinder partly filled with steel or white cast-iron balls.
With brittle materials, the powder particles produced have angular shapes; with ductile metals, they are flaky and are not particularly suitable for powder-metallurgy applications.

18. FIGURE Methods of mechanical comminution to obtain fine particles: (a) roll crushing; (b) ball mill; and (c) hammer milling.

19. 2.1.6 Mechanical alloying
In mechanical alloying, powders of two or more pure metals are mixed in a ball mill, as illustrated in Fig. 7.
Under the impact of the hard balls, the powders fracture and bond together by diffusion, forming alloy powders.
The dispersed phase can result in strengthening of the particles or can impart special electrical or magnetic properties of the powder.
FIGURE 7 Sequence of mechanical alloying of nickel particles with dispersed smaller particles. As nickel particles are flattened between two balls, the second, smaller phase is impressed into the nickel surface and eventually is dispersed throughout the particle due to successive flattening, fracture, and welding.

20. 2.1.7 Miscellaneous methods
Other less commonly used methods for making powders are:
Precipitation from a chemical solution
Production of fine metal chips by machining
Vapor condensation
More recent developments include techniques based on high- temperature extractive metallurgical processess—based on the reaction of volatile halides (a compound of halogen and an electropositive element) with liquid metals and the controlled reduction and reduction/carburization of solid oxides.

21. 2.1.8 Nanopowders
Most recent developments include the production of nanopowders of copper, aluminum, iron, titanium, and various other metals.
Because these powders are pyrophoric (ignite spontaneously) or are contaminated readily when exposed to air, they are shipped as thick slurries under hexane gas (which itself is highly volatile and combustible).

22. 2.1.9 Microencapsulated powders
These metal powders are coated completely with a binder. For electrical applications (such as magnetic components of ignition coils and other pulsed AC and DC applications), the binder acts like an insulator, preventing electricity from flowing between particles and thus reducing eddy-current losses.
The powders are compacted by warm pressing, and they are used with the binder still in place

23. 2.2 Particle size, shape and distribution
Particle size usually is measured by screening—that is, by passing the metal powder through screens (sieves) of various mesh sizes.
In addition to screen analysis, several other methods are available for particle size analysis:
Sedimentation, which involves measuring the rate at which particles settle in a fluid.
Microscopic analysis, which may include the use of transmission and scanning electron microscopy.

24. Light scattering from a laser that illuminates a sample consisting of particles suspended in a liquid medium. The particles cause the light to be scattered, and a detector then digitizes the signals and computes the particle-size distribution.
Optical (such as particles blocking a beam of light), which is then sensed by a photocell.
Suspending particles in a liquid and then detecting particle size and distribution by electrical sensors.

25. Particle shape
A major influence on processing characteristics, particle shape usually is described in terms of aspect ratio or shape factor.
Aspect ratio is the ratio of the largest dimension to the smallest dimension of the particle.
This ratio ranges from unity (for a spherical particle) to about 10 for flake- like or needle-like particles.

26. Shape factor (SF)
Also called the shape index, this is a measure of the ratio of the surface area of the particle to its volume—normalized by reference to a spherical particle of equivalent volume.
Thus, the shape factor for a flake is higher than that for a sphere.

27. 2.2.3 Size distribution
The size distribution of particles is an important consideration, because it affects the processing characteristics of the powder.
The distribution of particle size is given in terms of a frequency-distribution plot.
The maximum is called the mode size.
Other properties of metal powders that have an effect on their behavior in processing them are:
(a) flow properties when filled into dies
(b) compressibility when being compacted,
(c) density, as defined in various terms such as theoretical density, apparent density, and the density when the powder is shaken or tapped in the die cavity.

28. 2.3 Blending Metal Powders
Blending (mixing) powders is the next step in powder-metallurgy processing.
It is carried out for the following purposes:
Powders of different metals and other materials can be mixed in order to impart special physical and mechanical properties and characteristics to the P/M product.
Even when a single metal is used, the powders may vary significantly in size and shape, hence they must be blended to obtain uniformity from part to part.
Lubricants can be mixed with the powders to improve their flow characteristics. They reduce friction between the metal particles, improve flow of the powder metals into the dies, and improve die life.

29. Other additives—binders (as in sand molds) are used to develop sufficient green strength and additives also can be used to facilitate sintering.
Powder mixing must be carried out under controlled conditions in order to avoid contamination or deterioration.
HAZZARD
Because of their high surface area-to-volume ratio, metal powders can be explosive, particularly aluminum, magnesium, titanium, zirconium, and thorium.
Great care must be exercised both during blending and in storage and handling.
Precautions include (a) grounding equipment, (b) preventing sparks (by using non-sparking tools) and avoiding friction as a source of heat, and (c) avoiding dust clouds, open flames, and chemical reactions.

30. FIGURE 8 (a)–(d) Some common bowl geometries for mixing or blending powders. (e) A mixer suitable for blending metal powders. Since metal powders are abrasive, mixers rely on the rotation or tumbling of enclosed geometries, as opposed to using aggressive agitators. Source: Courtesy of Kemutec Group, Inc.

31. 3.0 Compaction of Metal Powder
Compaction is the step in which the blended powders are pressed into various shapes in dies.
The purposes of compaction are to obtain the required shape, density, and particle-to-particle contact and to make the part sufficiently strong for further processing.
Fig 9(a) shows the compaction of metal powder to form a bushing. The pressed powder part is called green compact. (b) Typical tool and die set for compacting a spur gear.

32. FIGURE 9 (a) Compaction of metal powder to form a bushing; the pressed powder part is called green compact. (b) A typical tool and die set for compacting a spur gear. Source: Reprinted with permission from Metal Powder Industries Federation, Princeton, NJ, USA.

33. The pressed powder is known as green compact, since it has a low strength just as is seen in green parts in slip casting.
The density of the green compact depends on the pressure applied.
Fig 10(a) shows the a) Density of copper- and iron-powder compacts as a function of compacting pressure. Density greatly influences the mechanical and physical properties of P/M parts. (b) Effect of density on tensile strength, elongation, and electrical conductivity of copper powder.

34. FIGURE 10 (a) Density of copper- and iron-powder compacts as a function of compacting pressure; density greatly influences the mechanical and physical properties of PM parts. (b) Effect of density on tensile strength, elongation, and electrical conductivity of copper powder. Source: (a) After F.V. Lenel. (b) After the International Annealed Copper Standard (IACS) for electrical conductivity.

35. FIGURE 11 Compaction of metal powders; at low compaction pressures, the powder rearranges without deforming, leading to a high rate of density increase. Once the powders are more closely packed, plastic deformation occurs at their interfaces, leading to further density increases but at lower rates. At very high densities, the powder behaves like a bulk solid.

36. The higher the density of the compacted part, the higher is its strength and elastic modulus.
The reason is that the higher the density, the higher the amount of solid metal in the same volume, and hence the greater its strength (resistance to external forces).
Fig 12 shows the density variation in compacting metal powders in various dies: (a) and (c) single-action press; (b) and (d) double-action press. Note in (d) the greater uniformity of density from pressing with two punches with separate movements when compared with (c). (e) Pressure contours in compacted copper powder in a single-action press.

37. FIGURE 12 Density variation in compacting metal powders in various dies.

38. 3.1.1 Equipment
TABLE 17.1 & Fig 13 (a) Compacting Pressures for Various Powders (b) 7.3-MN mechanical press for compacting metal powder.

39. 3.1.2 Isostatic pressing (a) CIP
Green compacts may be subjected to hydrostatic pressure in order to achieve more uniform compaction and, hence, density.
In cold isostatic pressing (CIP), the metal powder is placed in a flexible rubber mold typically made of neoprene rubber, urethane, polyvinyl chloride, or another elastomer.
Fig 14 shows the Schematic diagram of cold isostatic pressing, as applied to forming a tube. The powder is enclosed in a flexible container around a solid-core rod. Pressure is applied isostatically to the assembly inside a high-pressure chamber.

40. FIGURE 14 Schematic diagrams of cold isostatic pressing; pressure is applied isostatically inside a high-pressure chamber. (a) The wet bag process to form a cup-shaped part; the powder is enclosed in a flexible container around a solid-core rod. (b) The dry bag process used to form a PM cylinder.

41. P/F means powder forging.
FIGURE 15 Capabilities, with respect to part size and shape complexity, available from various PM operations. PF = powder forging. Source: Reprinted with permission from Metal Powder Industries Federation, Princeton, NJ, USA.

42. (b) HIP
In hot isostatic pressing (HIP), the container generally is made of a high- melting- point sheet metal, and the pressurizing medium is high- temperature inert gas or a vitreous (glasslike) fluid.
Fig 16 shows the schematic illustration of hot isostatic pressing. The pressure and temperature variation versus time are shown in the diagram

43. The main advantages of isostatic pressing are the following:
Because of the uniformity of pressure from all directions and the absence of die-wall friction, it produces fully dense compacts of practically uniform grain structure and density (hence, isotropic properties), irrespective of part shape.
HIP is capable of handling much larger parts than those in other compacting processes.
The limitations of HIP are as follows:
Wider dimensional tolerances than those obtained in other compacting processes.
Higher equipment cost and production time than are required by other processes.
Applicability only to relatively small production quantities, typically less than 10,000 parts per year.

44. 3.1.3 Miscellaneous compacting and shaping processes
Powder-injection molding (PIM). In this process (also called metal-injection molding (MIM)).
Generally, metals that are suitable for powder-injection molding are those that melt at temperatures above 1000°C.
The major advantages of powder-injection molding over conventional compaction are:
Complex shapes having wall thicknesses as small as 5 mm can be molded and then removed easily from the dies.
Mechanical properties are nearly equal to those of wrought products.
Dimensional tolerances are good.
High production rates can be achieved by using multicavity dies.
Parts produced by the PIM process compete well against small investment-cast parts, small forgings, and complex machined parts. However, it does not compete well with zinc and aluminum die casting or with screw machining.

45. Rolling
In powder rolling (also called roll compaction), the metal powder is fed into the roll gap in a two-high rolling mill and is compacted into a continuous strip at speeds of up to 0.5 m/s.
The rolling process can be carried out at room or at elevated temperature.
Sheet metal for electrical and electronic components and for coins can be made by this process.
FIGURE 17 An illustration of metal powder rolling.

46. 3.1.3.3 Pressureless compaction
Extrusion
Powders can be compacted by extrusion, whereby the powder is encased in a metal container and hot extruded.
After sintering, preformed P/M parts may be reheated and forged in a closed die to their final shape.
Superalloy powders, for example, are hot extruded for enhanced properties.
Pressureless compaction
In pressureless compaction, the die is filled with metal powder by gravity, and the powder is sintered directly in the die.
Because of the resulting low density, pressureless compaction is used principally for porous metal parts, such as filters.

47. Spray Deposition
Spray deposition is a shape-generation process.
The basic components of the spray-deposition process for metal powders are
(a) An atomizer,
(b) a spray chamber with inert atmosphere, and (c) a mold for producing preforms.
The mold may be made in various shapes, such as billets, tubes, disks, and cylinders.

48. FIGURE 18 Spray deposition (Osprey process) in which molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe.

49. Ceramic molds
Ceramic molds for shaping metal powders are made by the technique used in investment casting.
After the mold is made, it is filled with metal powder and placed in a steel container.
The space between the mold and the container is filled with particulate material. The container then is evacuated, sealed, and subjected to hot isostatic pressing.
Titanium-alloy compressor rotors for missile engines have been made by this process.

50. 3.1.3.6 Punch and die materials
The selection of punch and die materials for powder metallurgy depends on the abrasiveness of the powder metal and the number of parts to be produced.
Because of their higher hardness and wear resistance, tungsten-carbide dies are used for more severe applications.
Punches generally are made of similar materials.
Close control of die and punch dimensions is essential for proper compaction and die life.

51. 3.1.3.7 Dynamic and Explosive Compaction
Some metal powders that are difficult to compact with sufficient green strength can be compacted rapidly to near full-density .
The explosive drives a mass into green powder at high velocities, generating a shock wave that develops pressures up to 30GPa
Pre-heating of the powder is often practiced to prevent fracture
FIGURE 19 Schematic illustration of explosive compaction. (a) A tube filled with powder is surrounded by explosive media inside a container, typically cardboard or wood. (b) After detonation, a compression wave follows the detonation wave, resulting in a compacted metal powder part.

52. 4.0 Sintering
Sintering is the process whereby green compacts are heated in a controlled-atmosphere furnace to a temperature below the melting point but sufficiently high to allow bonding (fusion) of the individual particles.
Table 17.2 shows the sintering temperature and time for various metals.

53. Continuous-sintering furnaces, which are used for most production, have three chambers:
Burn-off chamber for volatilizing the lubricants in the green compact in order to improve bond strength and prevent cracking.
High-temperature chamber for sintering.
Cooling chamber.
Sintering mechanisms are complex and depend on the composition of the metal particles as well as on the processing parameters.
The sintering mechanisms are diffusion, vapor-phase transport, and liquid-phase sintering. As temperature increases, two adjacent powder particles begin to form a bond by a diffusion mechanism

54. A second sintering mechanism is vapor-phase transport.
Fig20 shows the schematic illustration of two mechanisms for sintering metal powders: (a) solid-state material transport; and (b) vapor-phase material transport.
In spark sintering (an experimental process), loose metal powders are placed in a graphite mold, heated by electric current, subjected to a high- energy discharge, and compacted—all in one step.

55. FIGURE 21 Schematic illustration of liquid phase sintering using a mixture of two powders. (a) Green compact of a higher melting point base metal and lower temperature additive; (b) liquid melting, wetting and re-precipitation on surfaces; and (c) fully sintered solid material.

56. Mechanical Properties
Depending on temperature, time, and the processing history, different structures and porosities can be obtained in a sintered compact and, thus, affect its properties.
Typical mechanical properties for several sintered P/M alloys are given in Table 17.3.
The differences in mechanical properties of wrought versus P/M metals are given in Table 17.4.
Table 17.5 shows the mechanical property comparisons

57. TABLE 17.3 Mechanical Properties of Selected PM Materials

58. TABLE 17.3 (continued) Mechanical Properties of Selected PM Materials

59. TABLE Comparison of Mechanical Properties of Selected Wrought and Equivalent PM Metals (as Sintered)

60. TABLE 17.5 Mechanical Property Comparisons for Ti-6AL-4V Titanium Alloy

61. 5.0 Secondary and finishing operations
In order to further improve the properties of sintered P/M products, or to impart special characteristics, several additional operations may be carried out after sintering.
Coining and sizing are compacting operations, performed under high pressure in presses. The purposes of these operations are to impart dimensional accuracy to the sintered part and to improve its strength and surface finish by further densification.
Preformed and sintered alloy-powder compacts subsequently may be cold or hot forged to the desired final shapes and sometimes by impact forging.
Powder-metal parts may be subjected to other finishing operations such as:
• Machining: for producing various geometric features by milling, drilling, and tapping (to produce threaded holes).
• Grinding: for improved dimensional accuracy and surface finish.
• Plating: for improved appearance and resistance to wear and corrosion.
• Heat treating: for improved hardness and strength.

62. 4. The inherent porosity of P/M components can be utilized by impregnating them with a fluid. Bearings and bushings that are lubricated internally with up to 30% oil by volume are made by immersing the sintered bearing in heated oil. These bearings have a continuous supply of lubricant (due to capillary action) during their service lives.
Infiltration is a process whereby a slug of a lower-melting-point metal is placed in contact the sintered part. The assembly then is heated to a temperature sufficiently high to melt the slug. The molten metal infiltrates the pores by capillary action and produces a relatively pore-free part having good density and strength.