powder metallurgy
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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.
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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
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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
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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.
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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
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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.
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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