Powder metallurgy History 1829 Woolaston- paper published Edison-electric light-filament 1909 Coolidge tungsten worked at elevated temperature New method of fabrication-refractory metals 1
Advantages of P/M The processes of powder metallurgy are quite and clean. Hard to process materials such as diamond can be converted into usable components and tools through this process. High production rates can be easily achieved. The phase diagram constraints, which do not allow an alloy formation between mutually insoluble constituents in liquid state, such as in case of copper and lead are removed in this process and mixtures of such metal powders can be easily processed and shaped through this process. This process facilitates production of many such parts, which cannot be produced through other methods, such as sintered carbides and self-lubricating bearings. The process enables an effective control over several properties such as purity, density, porosity, particle size, etc., in the parts produced through this process. It enables production of parts from such alloys, which possess poor cast ability. It is possible to ensure uniformity of composition, since exact proportions of constituent metal powders can be used. Porous parts can be produced that could not be made in any other way. Parts with wide variations in compositions and materials can be produced. Super-hard cutting tool bits, which are impossible to produce by other manufacturing processes, can be easily manufactured using this process. Components shapes obtained possess excellent reproducibility. defect such voids and blowholes in structure can be eliminated. 2
Limitations of P/M Powder metallurgy process is not economical for small-scale production. The size of products as compared to casting is limited because of the requirement of large presses and expensive tools which would be required for compacting. Metal powders are expensive and in some cases difficult to store without some deterioration. Intricate or complex shapes produced by casting cannot be made by powder metallurgy because metallic powders lack the ability to flow to the extent of molten metals. It may be difficult sometimes to obtain particular alloy powders Parts pressed from the top tend to be less dense at the bottom. Applications of P/M Porous products such as bearings and filters. Various machine parts are produced from tungsten powder. Highly heat and wear resistant cutting tools from tungsten carbide powder Refractory parts such as components made out of tungsten, tantalum and molybdenum are used in electric bulbs, radio valves, oscillator valves, X-ray tubes in the form of filament, cathode, anode, control grids, electric contact points etc. Products of complex shapes that require considerable machining when made by other processes namely toothed components such as gears. Products where the combined properties of two metals or metals and non-metals are desired such as non-porous bearings, electric motor brushes, etc. Porous metal bearings made which are later impregnated with lubricants. The combinations of metals and ceramics, which are bonded by similar process as metal powders, are called cermets. They combine in them useful properties of high refractoriness of ceramics and toughness of metals. 3
Processes Powder preparation Compacting Sintering Presentering Sizing Machining Preparation of metal powders oatomizing Process Particle size Uniformity of particle size 4
oreduction of metal oxides Chemically produced oxides reduction with carbon monoxide/hydrogen Raw material particle size Irregular shape Apparent density Weight of loosely heaped quantity of powder necessary to fill a given die cavity completely Packing is influenced by shape 5
Size of particle and distribution Method Passing through mesh Mesh count -200 means there are 200 openings per linear inch of square screen. High mesh count smaller particle size Interparticle friction Influences flow of powder Angle of repose Large angle- high friction Smaller angle- low friction 6
Flow of particles Imp in die filling and pressing Resistance to flow increases density variations Measurement-time required to flow certain amount of powder through standard sized funnel Lubricants eases flow Blending/mixing Homogenization is required Blending-same chemical composition but different particle size are intermingled Mixing- powders of different chemistries being combined Advantage of powder metallurgy alloying which is difficult by other means 7
Blending/Mixing Lubricants - friction between particles and at the die walls Binders adequate strength in unsintered part Diflocculants agglomerations of powders for better flow Compacting Desired shape Final dimensions Taking account dimensional changes because of sintering Type of porosity Strength adequate for handling but far less than final part strength Pressure techniques Pressure less techniques 8
Required pressure depends on projected area of PM part Compaction pressure -70 Mpafor aluminium to 700 Mpafor iron 9
Die compaction Mechanical presses Hydraulic presses Die having two part Isostatic compaction Simultaneous, equal pressure Uniform green density Gravity compaction Sintering in die Porous filters 10
Sintering Bonding of metallic particles Temperature should be 0.7 to 0.9 of melting temperature Shrinkage 11
Secondary operations Impregnation fluid is permeated to pores of P/M parts Oil impregnated bearings, gears Self lubricated bearings Infilteration Porosity filled with molten metal Melting point should be lower than the base metal Uniform density Improved toughness, strength 12
Secondary operations Densification and sizing Repressing after sintering Sizing to improve dimensional accuracy Coining Machining for creating features like threads, side holes Alternative methods Powder injection molding Powder rolling Powder extrusion Powder forging Liquid phase sintering Mixture of two metal Powder heated to temp Which is melting point Of low melting metal powder Examples: Fe-Cu, W-Cu, Cu-Co 13
Design guidelines for P/M 14