powder-metal processing and equipment

D1

University of Rome Tor Vergata

Manufacturing Technologies Faculty of Engineering

Bachelor's Degree in Engineering Sciences

Powder-Metal Processing and Equipment

• Availability of a wide range of metal-powder compositions

• Net-shape forming and overall economics

• Automotive 70% of PM market

• The most commonly used metals in PM are iron, copper, aluminum, tin, nickel, titanium,

and the refractory metals. For parts made of brass, bronze, steels, and stainless steels,

prealloyed powders are used

• Competitive with processes such as casting, forging, and machining, particularly for relatively

complex parts made of high strength and hard alloys

• Although most parts weigh less than 2.5 kg, they can weigh as much as 50 kg

http://web.uniroma2.it/

D2




Production of metal powders

1. Powder production

2. Blending

3. Compaction

4. Sintering

5. Finishing operations

• Many processes depending on

requirements (more processes for same

powders)

• Microstructure, bulk and surface

properties, chemical purity, porosity, shape,

and size distribution

• Particle sizes produced range from 0.1 to

1000 mm


D3




• Atomization

• Liquid-metal stream

• The stream is broken up by jets of

inert gas or air

• Parameters: temperature of the molten

metal, rate of flow, nozzle size, and jet

characteristics

• More spherical particles

• Rotating consumable electrode in He


D4




• Reduction

• Metal oxide in gas (H2, CO), particelle fine ridotte

• Spongy and porous, sized spherical or angular

shapes

• Electrolytic Deposition

• Aqueous solutions or fused salts.

• Among the purest available powders

• Carbonyls

• E.g. Fe(CO)5 or Ni(CO)4 , reaction with CO

• Small, dense, uniformly spherical particles of high

purity

• Comminution

• Crushing, milling in a ball mill, or grinding of brittle

or less ductile metals into small particles

• With brittle materials, particles have angular shapes;

with ductile metals, they are flaky not particularly

suitable for powder-metallurgy applications

• Mechanical Alloying

• powders of two or more pure metals are mixed in a

ball mill

• Miscellaneous Methods

• Precipitation from a chemical solution, Production of

fine metal chips by machining, Vapor condensation.

• Nanopowders, Microencapsulated Powders


D5




Blending and mixing

• Screen analysis

• Mesh size of 30 has an opening of 600 µm, size 100 has 150 µm

• Sedimentation, Microscopic analysis, Light scattering, Optical

methods, Suspending particles

• Particle Shape

• Aspect ratio (10 for flake-like or needle-like)

• Shape Factor

• Shape index, shape factor (SF) S/V

• Size Distribution, flow properties,

compressibility, density

• Different powders, homogeneity,

lubricants (0.25-5%), binders and additives

• Risks and practices

• Contamination or deterioration

• Work-hardening

• Mixing in air, in inert atmospheres (to avoid oxidation), or in

liquids (lubricants)

• Hazards

• Aluminum, magnesium, titanium, zirconium, and thorium


D6




Compaction

• The presses used are actuated either hydraulically or mechanically

• Generally room temperature

• Green compact (very fragile)

• Correct size distribution

(if all particles are of the same size,

theoretical porosity at least 24 vol%)

• Effect of pressure and friction

(a) Compaction of metal powder to form a

bushing. The pressed-owder part is called

green compact. (b) Typical tool and die set

for compacting a spur gear.


D7





D8




A 7.3-MN mechanical press for

compacting metal powder


D9




• Pressure ranges from 70 MPa for aluminumto 800 MPa for high-density iron parts

• Effect of the characteristics and shape of the particles, the method of blending, and the lubricant

• Press capacities are usually around 1.8 to 2.7 MN

• For small tonnage, crank- or eccentric-type mechanical presses are used

• For higher capacities, toggle or knuckle-joint presses are used

• Hydraulic presses with capacities as high as 45 MN can be used for large parts

• The higher the pressing speed, the greater is the tendency to trap air in the die cavity


D10




Isostatic Pressing

• Cold isostatic pressing (CIP): flexible rubber

mold typically made of neoprene rubber,

urethane, polyvinyl chloride, or another

elastomer – Generally use of water – Most

common pressure is 400 MPa (up to 1000 MPa)

• Hot isostatic pressing (HIP): container

generally in high-melting-point sheet metal –

Pressurizing medium is high-temperature inert

gas or a vitreous (glass-like) fluid – Common

conditions 100 MPa and 1200°C – Almost 100%

density – Superalloys and Aerospace – High

costs and low productivity (less than 10.000

parts/year)


D11




Miscellaneous Compacting and Shaping Processes

• Powder-injection Molding: The molded green parts are placed in

a low-temperature oven to burn off the plastic (debinding), or the

binder is removed by solvent extraction – Complex shapes, small

thickness (down to 5 mm)

• Powder forging: powder is fed into the roll gap in a two-high

rolling mill

• Extrusion: Hot process

• Pressureless Compaction: Porous metal parts

• Spray Deposition: Shape-generation process

Capabilities, with respect to part size and shape

complexity, available from various PM operations.

PF = powder forging.


http://www.ameteknickelstrip.com/images/Roll.jpg

D12




Punch and Die Materials

• Most common die materials are air- or oil-hardening tool steels with a hardness range from 60 to 64 HRC

• Tungsten-carbide dies are used for more severe applications

• Too large clearance between punch and die allows the metal powder to enter the gap (wear, < 25 µm )

• Die and punch surfaces must be lapped or polished for increasing life

Sintering

• Green compacts are heated in a controlled atmosphere furnace to a temperature below the melting point

• Most common gases: H2, N2, hydrocarbons, NH3

• An oxygen-free atmosphere is essential for iron and iron-based compacts

• A vacuum generally is used for sintering refractory-metal alloys and stainless steels


D13




• The sintering mechanisms are diffusion, vapor-phase transport, and liquid-phase sintering

• In spark sintering loose metal powders are placed in a graphite mold, heated by electric current

•

• The sintering mechanisms are diffusion, vapor-phase transport, and liquid-phase sintering

• In spark sintering loose metal powders are placed in a graphite mold, heated by electric current

• Depending on temperature, time, and processing history different structures and porosities can be obtained

• Temperature is 70-90% of Tm, time from 10 min (Cu, Fe) up to 8 h (Ti)

• Generally, with density less than 80% of its theoretical density, the pores are interconnected.

• Continuous-sintering furnaces: Burn-off chamber, High-temperature chamber and Cooling chamber


D14





D15





D16




Secondary and Finishing Operations

• Coining and sizing: dimensional accuracy and improvement of strength and surface finish

• Impact forging: good surface finish, good dimensional tolerances, and a uniform and fine grain size

• Machining, Grinding, Plating, Heat treating

• Fluid impregnation: 30 vol% of oil in bearings

• Infiltration (slug of a lower-melting-point metal is placed in contact with the sintered part)

• Elctroplating

Design Considerations

• The shape of the compact must be

kept as simple and uniform as

possible

• Provision must be made for ejection

• PM parts should be made with the

widest acceptable dimensional

tolerances for tool life

• Part walls generally should not be

less than 1.5 mm thick

• Simple steps if the size doesn’t

exceed 15% of the overall part length


D17




• Letters can be pressed if they are

oriented perpendicular to the direction of

pressing

• Flanges or overhangs can be produced

but long flanges can be broken upon

ejection

• A true radius cannot be pressed into the:

chamfers or flats are preferred

• Keys, keyways, and holes can be formed

during powder compaction

• Notches and grooves can be made if they

are oriented perpendicular to the pressing

direction.

• Dimensional tolerances of sintered PM

parts are usually on the order of ±0.05 to

0.1 mm


D18




• Near-net shape

• High initial cost of punches, dies, and equipment

• Generally economical over 10,000 pieces

• Cost increases significantly with HIP and PIM

• Labor costs are not as high in other processes

• Skills required are not as high

Economics

Yearly global flow of ferrous powder for structural applications. Sankey diagram, the material flows from left to right and the

width of the lines are proportional to the mass. Most data sources used to make the diagram are from 2010 to 2015


powder-metal processing and equipment

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