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Introduction - Michigan State University Flipbook PDF
6 ME477 Kwon 31 Semi-dry Pressing Uses high pressure to overcome the clay’s low plasticity and force it into a die cavit
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Introduction •
POWDER METALLURGY (P/M)
Feasible when 1. the melting point of a metal is too high such as W, Ta, Mo 2. the reaction occurs when melting such as Zr and for superhard tool materials
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Powder Metallurgy (PM) (around 1800s) –
1. Characterization of Powder 2. Production of Metallic Powder 3. Conventional Processing and Sintering 4. Alternative Processing 5. Materials and Products for PM 6. Design Consideration ME477
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Pressing – Powders are compressed into the desired shape in a press-type machine using punch-and-die tooling designed specifically for the part. Sintering - Heating at a temperature well below melting.
Advantage – Near-net shape, No waste, controlled porosity, Dimension control better than casting
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Disadvantage – High cost of tooling and powder, Powder Harder to handle Geometric & Size limitation, Density variation
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1. Characterization of Powders
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Particle Shapes and sizes • Particle Shape
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P/M materials – Alloys of iron, steel, and aluminum, copper, nickel, and refractory metals such as molybdenum and tungsten and metallic carbides
• Geometric feature – – – –
Particle shape Particle size Particle distribution They affects surface area, packing density, porosity, interparticle friction (Flow Characteristics), Green Strength
Mesh count refers to the number of openings per linear inch of screen. Thus, higher mesh count 1 means smaller particle size
• Particle Size (PS)
PS = − tw MC where PS = particle size
MC = mesh count t w = wire thickness
• Other Factors – (Chemistry and Surface Film) • Surface Area (SA)
For spherical powder, A = πD 2 ;
V=
πD 3 6
A 6 = V D
;
How do SA and PS affect the final product? ME477
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2. Production of Metallic Powders
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Gas and Water Atomization Collection Chamber
• Atomization
Nozzle Gas
– The molten metal is injected in a stream of Gas, Water or Centrifugal forces (e.g. rotating disk)
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Metal Powder
Siphon
• Chemical Reduction – Liberation of metals from oxides. (Iron, Tungsten and Copper) • Precipitation of metallic elements from the salts dissolved in water • Electrolytic: Anode made of desired metal is dissolved into the solution. Cathode collects the deposit. (Beryllium, Copper, Iron, Silver, Tantalum and Titanium ME477
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Molten Metal
Molten Metal Water
Metal powder 5
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Water
Collection Chamber Kwon
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Friction
Packing Factor (PF) • True density
• Interparticle Friction & Flow Characteristics
– density of the true volume of the sintered material
– Friction between particles affects ability of a powder to flow readily and pack tightly – A common test of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel – Smaller particle sizes generally • greater friction and steeper angles
• Bulk density – density of the powders in the loose state after pouring – bulk density ? true density
Funnel
• Typical PF for loose powders: 0.5 and 0.7
– Spherical shapes
– powders of various sizes vs. uniform size – Compaction Pressure increases packing
• the lowest interpartical friction
– As shape deviates from spherical, friction between particles tends to increase
• Porosity (P) - Ratio of the volume of the empty spaces in the powder to the bulk volume. (P+PF=1)
Pile of powders
Angle of Repose ME477
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3. Conventional Pressing & Sintering
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Blending, compacting & sintering (Pressing)
• Blending and Mixing
– Blending – intermingling of powders – Mixing – combining powders of different chemistries – Additives – Lubricants, binders and deflocculants
• Pressing (Compaction) – green compact, low density and strength • Sintering – increases strength and density – Reduction of surface energy – Necking, reduction of pore and grain growth
• Secondary Operation – Densification & sizing, Impregnation & Infiltration, Heat Treating & Finishing ME477 Kwon
Die
Loose powder 9
a green compact
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Sintering on a microscopic scale
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po
⎛ πD 2 ⎞ ⎛ πD 2 ⎞ ⎟⎟( p x + dp x ) − (πD )(µσ r )dx = 0 ⎟⎟ p x − ⎜⎜ ⎜⎜ 4 ⎝ 4 ⎠ ⎠ ⎝ Ddp x + 4 µσ r dx = 0 p Assuming dx
L
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σ r = kp x (interparticle friction ) dp x 4 µkdx px
px
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Pressure Distribution in Compaction
Sintering at between 70% and 90% of the metal's melting point (absolute scale)
(1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions
A Fully sintered part
D
For
µσr
σr
p x = po
=−
D
at x = 0 p x = p0 e − 4 µkx / D
px+dpx
x 11
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Densification and Sizing
Impregnation & Infiltration • Impregnation - when oil or other fluid is permeated into the pores of a sintered part.
• Secondary operations to increase density, improve accuracy, or accomplish additional shaping – Repressing - pressing the sintered part in a closed die to increase density and improve properties – Sizing - pressing a sintered part to improve dimensional accuracy – Coining - pressworking operation on a sintered part to press details into its surface – Machining - creates geometric features that cannot be achieved by pressing, such as threads, side holes, and other details ME477
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– oil-impregnated bearings, gears, and similar components – An alternative application is when parts are impregnated with polymer resins to create a pressure tight part
• Infiltration - An operation in which the pores of the PM part are filled with a molten metal with a lower melting point – capillary action draws the filler into the pores – Relatively nonporous, and more uniform density, as well as improved toughness and strength ME477
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PM Parts Classification System The Metal Powder Industries Federation (MPIF) defines four classes of powder metallurgy part by level of difficulty
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4. Alternative Pressing • Isostatic Pressing – Cold Isostatic Pressing (CIP) – Hot Isostatic Pressing (HIP)
• Powder Injection Molding (Metal & Ceramics) – Powder mixed with binder to form granular pellets – Pellets are heated to remove binder – Sintering and Secondary operation (a) (b) (c) (d)
Class I - simple thin shapes, pressed from one direction; Class II - simple but thicker shapes require pressing from two directions; Class III - two levels of thickness, pressed from two directions; and Class IV - multiple levels of thickness, pressed from two directions, with separate controls for each level
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• Powder Rolling, Powder Extrusion (powder in a container) and Powder Forging • Hot pressing and Spark pressing (high electric current) • Liquid-phase sintering ME477
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Design Guidelines
Processing Capabilities
• Economics usually require large quantities to justify cost of equipment and special tooling • PM is unique in its capability to fabricate parts with a controlled level of porosity (up to 50%) • PM can make parts out of materials that would be difficult if not impossible to produce by other means • The part geometry must permit ejection from die after pressing. • Wall thickness should be a minimum of 1.5 mm (0.060 in) between holes or a hole and outside wall • Minimum recommended hole diameter is 1.5 mm (0.060 in) Kwon
L/D= 2 (single action press) 4 (double action press)
L
– Minimum quantities of 10,000 units are suggested
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D x L
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If x>L/4, high density gradient breaks the compact If x25%) – Plastic forming (15 to 25%) – Semidry pressing (10 to 15%) – Dry pressing (