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Level 26

16 A - POWDER METALLURGY


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Powder metallurgy
METAL POWDER PROCESSING TECHNOLOGY - metal powder is compressed ($pressed$) into a certain form, usually with some additives and heated up until the it fuses into a solid block ($sintering$). $$--Advantages--$$ [1] Net shape or near net shape products, [2] Great dimensional control (0.13mm), [3] Can process unique materials like tungstens that other processes have a problem processing, [3] Very little waste material, [5] Can make materials no other process can - cermets, [6] production can be automated. $$--Disadvantages--$$ [1] Expensive tools and equipment, [2] Metal powders are expensive and hard to store - some oxidize and some burn, [4] Limitations on part geometry, [5] Variation of powder density in some parts. $$---$$ ACCURATELY UNIQUE POWDERS BURN EXPENSIVE GEOMETRIES.
CHARACTERIZATION OF ENGINEERING POWDERS
CHARACTERIZATION OF ENGINEERING POWDERS is done by: [1] Geometric features of the particle such as shape and porosity and, [2] Other features, such as chemistry, flow characteristics, and porosity.
Powder geometric features
Geometry of individual powders is defined by: [1] Particle size, [2] Particle shape and internal structure, [3] Surface area. >>> ASS: AREA, SIZE, SHAPE
Particle size
MESHES CLASSIFY PARTICLES BY SIZE, the meshes classified by mesh count - the number of opening for a linear inch
Particle shape and structure
PARTICLE SHAPE AND STRUCTURE DIFFERS BETWEEN MATERIALS, since particles come in many shapes, thus a simple ratio between their maximum and minimum size used to measure shape, for example, a sphere has a ratio of 1, furthermore the particle can have internal voids and form external voids - open pores for metal, oil, water to penetrate, when packed with other particles. PARTICLE ASPECT RATIO SAYS NOTHING ABOUT OPEN AND CLOSED PORES.
Particle aspect ratio
The aspect ratio of a particle is the ratio of the maximum dimension to the minimum dimension of the given particle.
Open pores
Air spaces between particles
Closed pores
Pores enclosed inside of the particle
Surface area
The more surface area to volume ratio a particle has, the more area it has to adhere to other particles, producing better the shrinkage and the mechanical properties will be during production. THE SMALLER THE PARTICLE IS, THE HIGHER IT'S SURFACE ARE TO VOLUME IS.
Other particle features
Other particle features include: [1] Interparticle features and flow characteristics, [2] packing, density, and porosity, [3] Chemistry, and surface films >>> FeeD Poo: FRICTION, DENSITY, POROSITY
Interparticle friction and flow characteristics
PARTICLES NEED TO FLOW IN DIE FILLING AND PRESSING, thus the lower the friction between particles, the easier it is to for them to flow. Interparticle friction measured by angle of repose - making a pile of particles and measuring it's angle, their ability to flow are measured by mass flow rate through standard sized funnel. ROUGH PARTICLES FLOW INTO REPOSING PILE
Packing and density and porosity
Powder's density relates directly to it's packing and porosity - if the powder is in a loose state, then it has more pores between the particle, meaning that the density decreases. INCREASE DENSITY BY PACKING POROUS POWDER
True density
True density of the true volume of the material, the density of the melted and solidified part, THE DENSITY OF THE ORIGINAL MATERIAL FOR THE POWDER.
Bulk density
Density of powder in loose state after pouring, in a state when all the voids between particles form. DENSITY OF PARTICLES WITH INNER AND OUTER VOIDS.
Packing factor
Packing factor is the ratio between bulk density divided by true density, usually around 0.5 to 0.7 $$PF = \frac{Bulk.Density}{True.Density} \approx 0.5 \div 0.7 $$
Porosity
Ratio of pore volume to the bulk volume, an alternative way to consider packing characteristics. In principle: $Porosity + Packing \, Factor = 1.0$
Chemistry of metal powders
EITHER PURE METAL OR ALLOY. Charaterization of metal powders by their chemistry: either elemental, consisiting of pure metal or pre-alloyed, each particle being an alloy in itself. CHEMICAL POWDERS CUM ON PURE ELEMENTS OR ALLOYS.
Surface films
OXIDE FILMS FORM ON PARTICLE SURFACES, oxide, silica, adsorbed organic materials films removed before processing.
PRODUCTION OF METALLIC POWDERS
Production of metallic powders is achieved by following means: [1] Atomization, [2] Chemical, [3] Electrolytic. >>> ACE: ATOMIZATION, CHEMICAL, ELECTROLYTIC
Atomization
REDUCTION OF MOLTEN METAL INTO SMALL PARTICLES by following means: [1] GAS ATOMIZATION - high velocity gas breaks up molten metal into small round particles. [2] WATER ATOMIZATION - the most common method of atomization, sprays molten metal by water jets, producing irregular , non spherical, strange and grotesque in their ugly shape, particles with their size being inversly related to velocity, and their surface a bit oxidated. Synthetic oils sometimes used instead water. [3] CENTRIFUGAL ATOMIZATION - metal drops drop on rotating disk, disk sprays drops all over in form of small particles. >>> ROUND WIND ROTATES MOIST and UGLY AND GROTESQUE RUSTY PARTICLES
Gas atomization
Gas atomizes by blowing on liquid metal and produces round particles
Water atomization
Water atomizes by spraying on liquid metal, producing irregularly shaped and somewhat rusty particles.
Oil atomization
Oil atomizes by spraying on liquid metal.
Centrifugal atomization
Rotating disk atomizes by throwing around liquid metal.
Other powder production methods
Other powder production methods include: [1] Chemical reduction, [2] Precipitation methods, [3] electrolysis >>> Cee Poo Eject: CHEMICAL, PRECIPITATION, ELECTROLYSIS.
Chemical reduction
REDUCING METALLIC OXIDES BY USE OF REDUCING AGENTS which combine with the oxygen to free the metals in the form of powders, a process used to produce powders of iron, tungsten, and copper.
Precipitation
Precipitation of metallic elements from salts dissolved in water.
Electrolysis
CATHODE TAKES METAL POWDER FROM ANODE, the anode slowly disolving and giving up it's berrilioum, copper, iron, silver, tantalum, titanium, ions, forming a very high purity substance on the cathode.