Powder Metallurgy


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Powder Metallurgy

  1. 1. Powder Metallurgy (P/M) ME 355 Sp’06 W. Li <ul><li>Competitive with processes such as casting, forging, and machining. </li></ul><ul><li>Used when </li></ul><ul><ul><li>melting point is too high (W, Mo). </li></ul></ul><ul><ul><li>reaction occurs at melting (Zr). </li></ul></ul><ul><ul><li>too hard to machine. </li></ul></ul><ul><ul><li>very large quantity. </li></ul></ul><ul><li>Near 70% of the P/M part production is for automotive applications. </li></ul><ul><li>Good dimensional accuracy. </li></ul><ul><li>Controllable porosity. </li></ul><ul><li>Size range from tiny balls for ball-point pens to parts weighing 100 lb. Most are around 5 lb. </li></ul>
  2. 2. Basic Steps In Powder Metallurgy <ul><li>Powder Production </li></ul><ul><li>Blending or Mixing </li></ul><ul><li>Powder Consolidation </li></ul><ul><li>Sintering </li></ul><ul><li>Finishing </li></ul>ME 355 Sp’06 W. Li
  3. 3. 1. Powder Production <ul><li>Many methods: extraction from compounds, deposition, atomization, fiber production, mechanical powder production, etc. </li></ul><ul><li>Atomization is the dominant process </li></ul>ME 355 Sp’06 W. Li (a) (b) (c) (a) Water or gas atomization; (b) Centrifugal atomization; (c) Rotating electrode
  4. 4. Characterization of Powders <ul><li>Size of powders 0.1 um – 1 mm </li></ul><ul><li>Sieve size quoted as mesh number </li></ul><ul><li>Particle D = 15/mesh number (mm) </li></ul><ul><li>325 mesh  45 um </li></ul>ME 355 Sp’06 W. Li
  5. 5. 2. Blending or Mixing <ul><li>Blending a coarser fraction with a finer fraction ensures that the interstices between large particles will be filled out. </li></ul><ul><li>Powders of different metals and other materials may be mixed in order to impart special physical and mechanical properties through metallic alloying. </li></ul><ul><li>Lubricants may be mixed to improve the powders’ flow characteristics. </li></ul><ul><li>Binders such as wax or thermoplastic polymers are added to improve green strength. </li></ul><ul><li>Sintering aids are added to accelerate densification on heating. </li></ul>ME 355 Sp’06 W. Li
  6. 6. 3. Powder Consolidation <ul><li>Cold compaction with 100 – 900 MPa to produce a “Green body”. </li></ul><ul><ul><li>Die pressing </li></ul></ul><ul><ul><li>Cold isostatic pressing </li></ul></ul><ul><ul><li>Rolling </li></ul></ul><ul><ul><li>Gravity </li></ul></ul><ul><li>Injection Molding small, complex parts. </li></ul>ME 355 Sp’06 W. Li Die pressing
  7. 7. Friction problem in cold compaction ME 355 Sp’06 W. Li <ul><li>The effectiveness of pressing with a single-acting punch is limited. Wall friction opposes compaction. </li></ul><ul><li>The pressure tapers off rapidly and density diminishes away from the punch. </li></ul><ul><li>Floating container and two counteracting punches help alleviate the problem. </li></ul>
  8. 8. 4. Sintering <ul><li>Parts are heated to 0.7~0.9 T m . </li></ul><ul><li>Transforms compacted mechanical bonds to much stronger metallic bonds. </li></ul><ul><li>Shrinkage always occurs: </li></ul>ME 355 Sp’06 W. Li
  9. 9. 5. Finishing <ul><li>The porosity of a fully sintered part is still significant (4-15%). </li></ul><ul><li>Density is often kept intentionally low to preserve interconnected porosity for bearings, filters, acoustic barriers, and battery electrodes. </li></ul><ul><li>However, to improve properties, finishing processes are needed: </li></ul><ul><ul><li>Cold restriking, resintering, and heat treatment. </li></ul></ul><ul><ul><li>Impregnation of heated oil. </li></ul></ul><ul><ul><li>Infiltration with metal (e.g., Cu for ferrous parts). </li></ul></ul><ul><ul><li>Machining to tighter tolerance. </li></ul></ul>ME 355 Sp’06 W. Li
  10. 10. Special Process: Hot compaction <ul><li>Advantages can be gained by combining consolidation and sintering, </li></ul><ul><li>High pressure is applied at the sintering temperature to bring the particles together and thus accelerate sintering. </li></ul><ul><li>Methods include </li></ul><ul><ul><li>Hot pressing </li></ul></ul><ul><ul><li>Spark sintering </li></ul></ul><ul><ul><li>Hot isostatic pressing (HIP) </li></ul></ul><ul><ul><li>Hot rolling and extrusion </li></ul></ul><ul><ul><li>Hot forging of powder preform </li></ul></ul><ul><ul><li>Spray deposition </li></ul></ul>ME 355 Sp’06 W. Li
  11. 11. Process Capabilities ME 355 Sp’06 W. Li A: highest, B: median, C: lowest Con’tional HIP Injection Molding (IM) Precision IM Preform Forging 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
  12. 12. Design Aspects ME 355 Sp’06 W. Li (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.
  13. 13. Advantages and Disadvantages of P/M <ul><li>Virtually unlimited choice of alloys, composites, and associated properties. </li></ul><ul><ul><li>Refractory materials are popular by this process. </li></ul></ul><ul><li>Controlled porosity for self lubrication or filtration uses. </li></ul><ul><li>Can be very economical at large run sizes (100,000 parts). </li></ul><ul><li>Long term reliability through close control of dimensions and physical properties. </li></ul><ul><li>Very good material utilization. </li></ul><ul><li>Limited part size and complexity </li></ul><ul><li>High cost of powder material. </li></ul><ul><li>High cost of tooling. </li></ul><ul><li>Less strong parts than wrought ones. </li></ul><ul><li>Less well known process. </li></ul>ME 355 Sp’06 W. Li