powder metallurgy part2

Powder Metallurgy (P/M)

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• Competitive with processes such as casting, forging, and machining.

• Used when•melting point is too high (W,

Mo).•reaction occurs at melting (Zr).•too hard to machine.•very large quantity.

• Near 70% of the P/M part production is for automotive applications.

• Good dimensional accuracy.• Controllable porosity.• Size range from tiny balls for ball-

point pens to parts weighing 100 lb. Most are around 5 lb.

Basic Steps In Powder Metallurgy

• Powder Production

• Blending or Mixing

• Powder Consolidation

• Sintering

• Finishing

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1. Powder Production

• Many methods: extraction from compounds, deposition, atomization, fiber production, mechanical powder production, etc.

• Atomization is the dominant process

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(a) (b) (c)

(a) Water or gas atomization; (b) Centrifugal atomization; (c) Rotating electrode

Characterization of Powders

Size of powders 0.1 um – 1 mmSieve size quoted as mesh numberParticle D = 15/mesh number (mm)325 mesh 45 um

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2. Blending or Mixing• Blending a coarser fraction with a finer fraction ensures that the

interstices between large particles will be filled out.

• Powders of different metals and other materials may be mixed in order to impart special physical and mechanical properties through metallic alloying.

• Lubricants may be mixed to improve the powders’ flow characteristics.

• Binders such as wax or thermoplastic polymers are added to improve green strength.

• Sintering aids are added to accelerate densification on heating.

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3. Powder Consolidation• Cold compaction with 100 – 900

MPa to produce a “Green body”.– Die pressing

– Cold isostatic pressing

– Rolling

– Gravity

• Injection Molding small, complex parts.

6Die pressing

Friction problem in cold compaction

W. Li 7

• The effectiveness of pressing with a single-acting punch is limited. Wall friction opposes compaction.

• The pressure tapers off rapidly and density diminishes away from the punch.

• Floating container and two counteracting punches help alleviate the problem.

4. Sintering

• Parts are heated to 0.7~0.9 Tm.

• Transforms compacted mechanical bonds to much stronger metallic bonds.

• Shrinkage always occurs:

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5. Finishing

• The porosity of a fully sintered part is still significant (4-15%).

• Density is often kept intentionally low to preserve interconnected porosity for bearings, filters, acoustic barriers, and battery electrodes.

• However, to improve properties, finishing processes are needed:

– Cold restriking, resintering, and heat treatment.

– Impregnation of heated oil.

– Infiltration with metal (e.g., Cu for ferrous parts).

– Machining to tighter tolerance.

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Special Process: Hot compaction• Advantages can be gained by combining consolidation and sintering,

• High pressure is applied at the sintering temperature to bring the particles together and thus accelerate sintering.

• Methods include

– Hot pressing

– Spark sintering

– Hot isostatic pressing (HIP)

– Hot rolling and extrusion

– Hot forging of powder preform

– Spray deposition

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Process Capabilities

Con’tional HIP Injection Molding (IM)

Precision IM PreformForging

Metal All All (SA, SS)

All (Steel, SS) All Steel, SA

Surface detail B B-C B A A

Mass, kg 0.01-5(30) 0.1-10

10-7000 (e)

0.01-0.2 0.005-0.2 0.1-3

Min. section, mm 1.5 1 0.1 3

Min. core diam. mm 4-6 1 0.2 5

Tolerance +/-% 0.1 2 0.3 0.1 0.25

Throughput (pc/h) 100-1000 5-20 100-2000 100-2000 200-2000

Min. quantity 1000-50,000 1-100 10,000 10,000 100,000

Eq. Cost B-C A A-B A-B A-B

11A: highest, B: median, C: lowest

Design Aspects

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(a) Length to thickness ratio limited to 2-4; (b) Steps limited to avoid density variation; (c) Radii provided to extend die life, sleeves greater than 1 mm, through hole greater than 5 mm; (d) Feather-edged punches with flat face; (e) Internal cavity requires a draft; (f) Sharp corner should be avoided; (g) Large wall thickness difference should be avoided; (h) Wall thickness should be larger than 1 mm.

Advantages and Disadvantages of P/M

• Virtually unlimited choice of alloys, composites, and associated properties. – Refractory materials are popular by this process.

• Controlled porosity for self lubrication or filtration uses.

• Can be very economical at large run sizes (100,000 parts).

• Long term reliability through close control of dimensions and physical properties.

• Very good material utilization.

• Limited part size and complexity

• High cost of powder material.

• High cost of tooling.

• Less strong parts than wrought ones.

• Less well known process.13