MEM560MANUFACTURING PROCESSES

9.1 IntroductionWhat is Powder Metallurgy(PM)? Process for forming metal parts by heating compacted metal powders to just

below their melting points Can be mass produced to net shape or near net shape, eliminating or

reducing the need for subsequent machining. Common PM powders: iron,copper,aluminium,tin,nickel,titanium and

refractory metals Brass,bronze,steel,stainless steel-prealloyed powders (powder particle itself is

alloy) Commonly for parts weighs less than 2kg can go up to 50 kg parts.

Applications Net-shape or near-net shape parts made of expensive materials. PM process

is capable of less than 3% scrap losses. Parts with porosity such as filters can be made. Bearings especially so-called permanently lubricated bearings, in which air

pores in the PM parts are filled with oil (process of impregnation) parts of certain metals and metal alloys that are difficult to fabricate by other

methods (carbide tool inserts, tungsten, ceramics, etc.) parts of materials with special and unique properties (alloys that cannot be

produced by other processes)

APPLICATIONS

Advantages: Less waste (97% starting powders are used) Can control degree of porosity Reduce requirement for further machining (because near net shape) Can produce parts for difficult to fabricate material e.g tungsten filaments of

incandescent lamp Good dimensional control compared to most casting +0.13mm Can be automated - economical production Electrical, thermal & magnetic properties vary with density Porosity promotes good sound & vibration damping Combine with certain metal alloy and cermets

Disadvantages/limitations: High tooling and equipment costs Metallic powders are expensive Problems in storing and handling metal powders

- Degradation over time, fire hazards with certain metals Limitations on part geometry because metal powders do not readily flow

laterally in the die during pressing Variations in density throughout part may be a problem, especially for

complex geometries

Engineering powders:

Classification of powders:

• The starting material in PM consist of fine particles of uniform sizes so-called the engineering powders.

• Produced from raw metallic or nonmetallic powders, which contains particles of different sizes by separation of particles according to their size.

• The procedure of separating the powders by size is called classification of powders.

Particle size:

• Powders are classified by passing them through a series of screens of progressively smaller mesh size.

• The particle size is defined by the so-called mesh count, term that refers to the number of openings per linear inch of mesh.

• Particle shape -describe in terms of aspect ratio or shape factor

Aspect ratio: largest dimensions/smallest dimension of particle

9.2 Production of metal powderMethod of production Depends on requirement of products(microstructure,bulk and surface

properties,chemical purity,porosity,shape,particle size distributions) Method:

Atomization Reduction Electrolytic deposition Carbonyls Comminution Mechanical alloying

• Atomization• Atomization produces a

liquid-metal stream by injecting molten metal through a small orifice.

• Figure 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.

• Reductionreduction of metal oxides (i.e., removal of oxygen) uses gases, such as hydrogen and carbon monoxide, as reducing agents.

- very fine metallic oxides metallic powderPowders –spongy, porous and uniformly sized spherical or angular shapes.

• Electrolytic deposition (Electrolysis)• In this method, an electrolytic cell is set up in which the source of desired

metal is the anode. • It is slowly dissolved and deposited on the cathode from where the deposit

is removed, washed and dried.-Purest powder produced

• CarbonylMetal carbonyls, such as iron carbonyl and nickel carbonyl are formed by

letting iron or nickel react with carbon monoxide-reaction products are decomposed to iron and nickel - small, dense, uniformly spherical particles of high purity.

• Comminutionmechanically crushing, milling in mill ball or grinding brittle material into small particles.

• Mechanical alloying- Powders of two or more pure metals are mixed in a ball mill.- Under the impact of the hard balls, the powders fracture and bond

together by diffusion, forming alloy powders.

9.3 Blending of metal powderBlending and mixing Blending : mixing powder of the same chemical composition but different

sizes Mixing : combining powders of different chemistries

Purpose for blending and mixing To impart special physical and mechanical properties and characteristics

to the P/M product. To obtain uniformity in size and shape of powders. To improve powder flow characteristics by mixing with lubricant. They

reduce friction between the metal particles, improve flow of the powder metals into the dies, and improve die life.

To develop sufficient green strength and facilitate sintering by adding additives—binders (as in sand molds)

Powder mixing must be carried out under controlled conditions in order to avoid contamination or deterioration.

9.4 Compaction of metal powderWhat is compaction? Step which blended powders are

pressed in die Purpose:

- To obtain required shape,density and particle-to-particle contact- To make part sufficiently strong for next process

Density of green compact depends on pressure applied-higher density,higher strength,highers modulus of elasticity

Figure (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.

Since density can vary (due to friction between 1)metal particle & powder particle 2)punch surface & die wall),proper punch & die design and friction control are needed to control density variation

Density variation in compacting metal powders in different dies: (a) and (c) singleactionpress; (b) and (d) double-action press,where the punches have separate movements.Note the greater uniformity of density in (d) as compared with (c). Generally, uniformity ofdensity is preferred, although there are situations in which density variation, and hence variation of properties, within a part may be desirable. Source: After P. Duwezand L. Zwell.

• Miscellaneous compacting and shaping processes Powder injection molding

- also known as metal injection molding- suitable for metal melts above 100°C- fine metal powder + polymer/wax based binder inject in mold

(similar to die casting)- products:watches components,surgical knives,door hinges- advantages:1) Complex shapes having wall thicknesses as small as 5 mm can be

molded and then removed easily from the dies. 2) Mechanical properties are nearly equal to those of wrought

products.3) Dimensional tolerances are good.4) High production rates can be achieved by using multicavity dies.5) 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.

Rolling- 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. - Sheet metal for electrical and electronic components and for coins can be made by this process.

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- the die is filled with metal powder by gravity, and the powder is sintered directly in the die.-low density- is used principally for porous metal parts, such as filters.

Spray decomposition

9.5 Sintering of metal powderWhat is sintering? Step whereby green compacts are heated in controllable atmosphere

furnace to temperature below melting point but sufficient enough to allow bonding (fusion) of individual particle

Part shrinkage occurs during sintering due to pore size reduction .

Sintering process sequence

Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions.

9.6 Secondary and finishing operation Additional operations:1. Coining and sizing- to impart dimensional accuracy,improve strength & surface finish2. Forging (hot or cold) on preformed and sintered alloy powder3. Machining-milling,drilling,tapping4. Grinding5. Plating-improve appearance,resistance to wear and corrosion6. Heat treating-improve hardness and strength7. Impregnation-immerse sintered bearing in heated oil-no need traditional

grease fittings8. Infiltration- pores of the PM part are filled with a molten metal -heating the

filler metal in contact with the sintered component so capillary action draws the filler into the pores -Resulting structure is relatively nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strength

POROSITY• Porosity is a characteristic trait of powder processed materials. • In some cases the goal is to mitigate or eliminate porosity. In

other cases a certain level of porosity is desired. As discussed, porosity exists within the green compact.

• Amount of porosity in the green compact can be controlled to some extent by the level of pressure used to press the compact. If the compact is not fully pressed, more porosity will occur than with complete compaction.

• In fact, in loose sintering the powder is not pressed at all, achieving very high porosity for special components such as metal filters.

INFILTRATION• Infiltration is the filling of a metal's pores with another metal

of lower melting point than the base material. • The infiltration metal is heated to a temperature above its

melting point but below that of the porous metal part. Liquid metal is allowed to enter into the porous network and solidifies, filling the pores with solid metal.

• Infiltration can produce parts with special mechanical properties. Iron infiltrated with copper is a common example of this process in manufacturing industry.

IMPREGNATION• Impregnation is the filling of the pores in a metal with a fluid. • A common application of this is in the production of self

lubricating components such as bearings and gears. In these cases, the powder processed part is usually soaked in hot oil.

• Parts are typically 10%-30% oil impregnated by volume. • Sometimes a part will be impregnated with polymer resin to

prevent other substances from entering the pores or to assist with further processing.

9.7 Process design and capabilities• Design considerations1. The shape of the parts must be as simple as possible.2. Parts should be made with the widest tolerances. The PM process is

capable of achieving tolerances of bigger than 0.1 mm.3. Hole and grooves must be parallel to the direction of ejection

4. Sharp corners, radii, thin section must be avoided. Minimum wall thickness is 1.5 mm. Corners radii and chamfers are still possible, but certain rules should be observed:

• Process capabilitiesThe process capabilities of powder metallurgy may be summarized as

follows:1. It is a technique for making parts from high-melting-point refractory

metals, and parts which may be difficult or uneconomical to produce by other methods.

2. High production rates are possible on relatively complex parts using automated equipment and requiring little labor.

3. Powder-metal processing offers good dimensional control and (in many instances) the elimination of machining and finishing operations; in this way, it reduces scrap and waste and saves energy.

4. The availability of a wide range of compositions makes it possible to obtain special mechanical and physical properties, such as stiffness, vibration damping, hardness, density, toughness, and specific electrical and magnetic properties. Some of the newer highly alloyed superalloys can be manufactured into parts only by P/M processing.

5. It offers the capability of impregnation and infiltration for specific applications.

Case study-Polarizing keysSize: 2.05 mm (0.080 in.) Weight: 0.5 g (0.001 lb.) Alloy: Nickel Silver (Copper + Nickel + Zinc) Tensile Strength: 230 MPa (34,000 psi) Yield Strength: 140 MPa (20,000 psi) Elongation: 14% Apparent Hardness: 85 HRH Density: 7.9 g/cm³ (0.285 lb./in.³) Secondary Operations: None Alternative Process: None Annual Production: 200,000Description: Used in electrical rack and panel connectors on aircraft flight data recorders, these thin-walled parts have an ultimate tensile strength of 230 MPa (34,000 psi), a yieldstrength of 140 MPa (20,000 psi), 85 HRH hardness and a 14% elongation. Designed to a net shape, the parts require no secondary operations. The tooling system for the three-level part includes an upper punch, auxiliary die, lower punch, die and core to form the through hole and a 30° angle. The parts withstand a 500 connect/disconnect cycle test without measurable wear and are also tested for electrical conductivity. Polarizing keys are used on multiple types of connectorson aircraft aviation systems. They were designed specifically for powder metallurgy, as other manufacturing techniques could not provide the shape and properties required at a reasonable cost.

SUMMARY• PM is net-shape forming process-compaction can be cold or hot isostatic

pressing for improved properties-can produce complex parts economically,close dimensional tolerance,wide variety of metal and alloy powders

• Secondary process may be done to improve dimensional accuracy,surface finish ,mechanical & physical properties and appearance

• Control of powder shape and quality, process variables,and sintering atmosphere are important for product quality-density and mechanical properties can be controlled by tooling design and compacting pressure

• Design consideration for PM :sharp of part,ability to eject green compact from die, dimensional tolerance

• PM is suitable for medium-high volume production, small parts