Processing of Powder Metals, Ceramics, Glass & Superconductors

Powder Metals

• Commonly used metals in P/M– Iron,Tin, Copper, Aluminum, and Nickel

• It is a completive process with forging and machining

• Parts can weigh as much as little as 2.5Kg or up to 50Kg

Powder Metallurgy

a

b

c

Fig: (a)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 unleaded brass alloy; it replaces a die-cast part, with a 60% savings. (c) Main-bearing powder metal caps for 3.8 and 3.1 liter General Motors automotive engines.

Production of Metal Powders

• Powder production• Blending• Compaction• Sintering• Finishing operations

Particle Size, Distribution, and shape• Particle size is measured by

screening

• In addition to screen analysis one can use:– Sedimentation – measuring the

rate that particles settle in a fluid

– Microscopic analysis – using a scanning electron microscope– Light scattering– Optical – particles blocking a

beam of light that is sensed by a photocell

– Suspending particles in a liquid & detecting particle size and

distribution Fig: Particle shapes in metal powders,and the processes by which they are produced.Iron powders are produced by many of these processes

Powder Particles

Fig : (a) Scanning electron-microscopy photograph of iron-powder particles made by atomization. (b) Nickel-based superalloy powder particles made by the rotating electrode process.

Methods of Powder Production

Fig : Methods of metal-powder production by atomization;(a) melt atomization; (b) atomization with a rotating consumable electrode

Fig: Methods of mechanical communication, to obtain fine particles: (a) roll crushing, (b) ball mill, & (c) hammer milling

Blending Powders

• Blending powders is the second step in the P/M process

• Powders made by different processes have different sizes and shapes and must be well mixed

• Powders of different metals can be mixed together

• Lubricants can be mixed with the

powders to improve their flow characteristics

Fig: Some common equipment geometries for mixing or blending powders. (a) cylindrical, (b) rotating cube, (c) double cone, and (d) twin shell.

Compaction of Metal Powders

• Blended powders are pressed together

• The powder must flow easily into the die

• Size distribution is an important fact– They should not be all the same

size– Should be a mixture of large and

small particles

• The higher the density the higher the strength Fig: 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

Equipment

• Uses 100-300 ton press

• Selection of the press depends on the part and the configuration of the part

Fig: A7.3 MN (825 ton) mechanical press for compacting metal powder.

Isostatic Pressing

• Cold isostatic Pressing (CIP)– Metal powder is placed in a

flexible rubber mold– Pressurized hydrostatically– Uses pressures up to 150

KSI– Typical application is

automotive cylinder liners

Fig: Schematic diagram, of cold isostatic, as applied to forming a tube.The powder is enclosed in a flexible container around a solid core rod.Pressure is applied iso-statically to the assembly inside a high-pressure chamber.

Isostatic Pressing

• Hot Isostatic pressing– Container is made of high-

melting-point sheet metal– Uses a inert gas as the

pressurizing medium– Common conditions for

HIP are 15KSI at 2000F– Mainly used for super

alloy casting

Fig: Schematic illustration of hot isostatic pressing.The pressure and temperature variation vs.time are shown in the diagram

Punch and Die Materials

• Depends on the abrasiveness of the powder metal

• Tungsten-carbide dies are used

• Punches are generally made of the similar materials

• Dimensions are watched very close

Metal Injection Molding

• MIM uses very fine metal powders blended with a polymer

• The molded greens are then placed in a furnace to burn off the plastics

• Advantages of injection molding– Produces complex shapes– Mechanical properties are nearly equal to

those of wrought products

Other Shaping Processes• Rolling – powder is fed though the roll gap and is used to make coins

and sheet metal

• Extrusion – has improved properties and parts my be forged in a closed die to get final shape

• Pressureless compaction – gravity filled die and used to make porous parts

• Ceramic molds – molds are made by made by investment casting and the powder is compressed by hot isostatic pressing

• Spray deposition – shape-generation process

An example of powder rolling

Sintering• Sintering - Green compacts are heated in a furnace to a temperature

below melting point• Improves the strength of the material• Proper furnace control is important for optimum properties

Fig: Schematic illustration of two mechanism for sintering metal powders: (a) solid-state material transport; (b) liquid-phase material transport.R= particle radius, r=neck radius, and p=neck profile radius

Sintering

• Particles start forming a bond by diffusion• Vapor-phase transport – heated very close to melting

temperature allows metal atoms to release to the vapor phase

Mechanical Properties

Secondary & Finishing Operations

• To improve the properties of sintered P/M products several additional operations may be used:

– Coining and sizing – compaction operations

– Impact forging – cold or hot forging may be used

• Parts may be impregnated with a fluid to reduce the porosity

Fig: Examples of P/M parts,showing poor designs and good ones.Notes that sharp radii and re entry corners should be avoided and that threads and transverse holes have to be produced separately by additional machining operations.

Secondary & Finishing Operations

• Infiltration – metal infiltrates the pores of a sintered part to produce a stronger part and produces a pore free part

• Other finishing operations– Heat treating– Machining– Grinding– Plating

Design Considerations for P/M

• Design principles to consider– Shape of the compact must be simple and

uniform– Provision must be made for the ejection of the

part– Wide tolerances should be used when ever

possible

Process Capabilities

• It is a technique for making parts from high melting point refractory metals

• High production rates

• Good dimensional control

• Wide range of compositions for obtaining special mechanical and physical properties

Process Capabilities

• Limitations

– High cost– Tooling cost for short production runs– Limitations on part size and shape– Mechanical properties of the part• Strength • Ductility

Economics of Powder Metallurgy

• Competitive with casting and forging

• High initial cost

• Economical for quantities over 10,000 pieces

• Reduces or eliminates scraps

Shaping Ceramics

Processing ceramics– Crushing or grinding the raw materials in to very fine particles– Mixing with additives– Shaping, drying , and firing the material

SLIP CASTING

Processing steps involved in making ceramic parts