Fall 2005

IMSE 302 Powder Processing

A. Basics

Powder Processing and in particular, powder metallurgy is a near net shape forming process

Powder metallurgy was used by the Inca Indians for making jewelry as they used platinum and there were no containers to hold it in the liquid state until the 1800’s as Platinum melts at 1770 C, or 3218 F. Modern powder metallurgy started about 1910 with the production of ductile tungsten for light bulb filaments.

B. Unusual Properties

  1. Density can be varied by controlling porosity.
  2. Strength and toughness will both increase with density.
  3. Can form alloy compositions that may not form under normal liquid solidification
  4. Can form materials which cannot be formed by casting due to high melting temperatures(Tpowder processing is about ½ Tmelt(absolute))
  5. Impregnation–can impregnate voids with oil or low melting alloys

C. General Properties

  1. Produce Powders

a. Grinding(mechanical)d. Electrolytic Deposition

b. Liquid Spray/Atomizatione. Precipitation

c. Reduction(of oxides)

Critical Factors – powder size & distribution

  1. Mix Powders and any Lubricants(wax, graphite)
  2. Form Shape
  3. Cold ( 5-50 tons/in2)
  4. Hot(HIP Hot Isostatic Pressing - thousands of atmospheres)

(do not sinter if hot isostatic pressing)

  1. Sintering(solid state fusion)

Primary control factors are temperature, atmosphere(prevent oxide formation), and time

  1. Secondary Finishing & Manufacturing Operations

(Impregnation is a operation unique to Power Processing)

D. Part Size & Shape Factors

  1. Generally intricate shaped parts, less than 50 pounds and usually less than 1 pound.
  2. Long thin shapes, 2 dimensional are best(gears, knives, etc). It is difficult to compact thick sections to a uniform density.
  3. Avoid parts with undercuts, re-entrant tapers, and threads as powder cannot be compacted easily in more than one direction.

E. Design Considerations

  1. Linear Shrinkage
  2. Materials Useage
  1. Linear Shrinkage – due to density changes(increases) during sintering.

i = initial density before sintering and f =final density after sintering

Vf = Vi x [i / f ]or Lf3 = Li3 x [i / f ]

or Lf = Li [i / f ]1/3

  1. Materials Useage

VT = VTotal from Design = Total Volume of Part

VT = VA + VB + VC + … + Vvoid

VT - Vvoid = VA + VB + VC

= VT x (1 – Void Fraction)

= VT x (fraction dense)

but MT = MA + MB + MC

MA = MT x WFA

Where MA = Mass Fraction A and WFA = weight fraction A in mix

VT x (fraction dense) = VA + VB + VC

= MA / A + MB / B + MC/C

= MT xWFA/A + MT xWFB/B +MT xWFC/C

= MT x [WFA/A + WFB/B + WFC/C]

Thus

MT = VT x (fraction dense) / [WFA/A + WFB/B + WFC/C]

MA = MT x WFA and similar for B and C

= MT / VT

The results can also be adjusted for Yield and Scrap, but the Yield is often close to 100%, typically 95-98%.

Mc = ( MA + MB + ) x (1/Y) x (1/(1-SR)) = MT x (1/Y) x (1/(1-SR))

(Scrap cannot be reused, but the powder from the yield can be reused)

Return Powder =RP =MT x ((1-Y)/Y) x (1/(1-SR)

Scrap = S= MT x (SR/(1-SR))

  1. Material Properties

For composites and mixtures, the mechanical and material properties vary in proportion to the volume fraction and not the mass fraction.

composite = A x VFA + B x VFB + +

where VFA = (MA/A ) / (MA/A + MB/B + … + void space)

= (MA/A)/ (Total Part Volume which includes voids)

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