Powder Metallurgy - √دنیـــــــای متـــــــــــالورژی فقـــــــط از ایـــــــنجا
به "با سابقه ترين و محبوب ترين وبلاگ متالورژي در ايران" خوش آمديد

تاريخ : سه‌شنبه ۱۳ دی ۱۳۸٤

Powder Metallurgy

Introduction .powder metallurgy is the process of molding metal parts from metal powders to precise shapes through the application of high pressures .Compaction of the metal powders is followed by a high temperature sintering operation in an atmosphere –controlled furance in which the cold-welded-metal compact is bonded into a strong homogeneous structure .Physical properties of the sintered compact are similar to those of the parent metal. sintering is usually accomplished at about 80 percent of the melting point of the principal constituent to allow particle bonding and and recrystallization across the powders particle interface . Compacting of the powdered metals in the die is accomplished by simultaneously pressing on it with the upper and lower punches at pressures of about 30tons per square inch. Typical parts produced by P/M , the accepted acronym for powder metallurgy , include cutting tools, machine components, automotive parts, self-lubricating bearings , porous filters , magnetic materials , and composite metal-nonmetals .The P/M technique is also useful in forming of metals with low ductility and extremely high melting points such as tungsten , and some refractory metals.

The major applications of powder metallurgy today may be grouped into five areas:

Alloying of Essentially Non-alloyable metals.

Combining metals and Non-metallic materials .

Unitizing Metals with Melting Points too High for Casting .

Fabrication of Metals for Unique Structural Properties.

Vibration damping is another unusual and often desirable property peculiar to the P/M part.

Economical Production of Precision Parts.

Even with all of his important discoveries,the modern metallurgist cannot claim credit for the discovery of the powder metallurgy process metallic powders and solid from metallic powders have been made for many years . powder metallurgy was used in Europe at the end of the 18th century for working platinum into useful forms . platinum was infusible at that time .however it is known that the early incas in Ecuador manufactured shapes from platinum powders long before columbus made his famous voyage . At the present time , the powder metallurgy process is used for the manufacture of shapes of up to about 350 pounds in weight ,but some 1600 years ago , the famous iron pillar in Delhi,India weighting 6½ tons,was made from iron particles or sponge iron similar to an iron used in the modern process.

Wollaston , by 1829 ,had developed a technique that proved very successful in the manufacture of a malleable platinum from platinum powder that permitted forging of the resultant solid like any other metal.his published paer on the process is considered the first scientific work in powder metallurgy and laid the foundation for modern techniques .

The first modern application of the powder metallurgy process was the making of filaments for incandescent lamps.The first successful metal filament was made made from tantalum ,but with the discovery in 1909 by Coolidge that tungsten sintered from tungsten powder could be worked within a certain temperature range and then retain its ductility at room temperature ,tungsten became the most important filament material. Finely divided tungsten powder was compressed intosmall briquettes which were sintered below the melting point of tungsten . Those sintered briquettes were brittle at room temperature but could be worked at 4530-4890°F .subsequent hot working improved ductility until a stage was reached where the metal was ductile at room temperature and could be drawn into fine wire with tensile strengths approaching 600,000 psi.

                   

powder metallurgy is a competitive process to die casting and injection molding of precision parts.when large quantities of high strength parts of iron ,steel ,or nickel are wanted , P/M offers an excellent solution for economical production of bushings , cams, gears, and other parts.
A feature unique to P/M is the ability to produce self-lubricating bearings containing a network of small interconnected pores filled with lubricant .porous filters for diffusion , separation and regulation of flow may also be made by P/M.
The melting points of the refractory metals such as tungsten , tantalum, and molybdenum are so high that it is difficult to melt and cast them , whereas with powder metal techniques, they are sintered below their melting points. powder metal compacting and sintered is the only feasible method of forming super hard tool materials such as cemented carbides and sintered oxide .
Examples of metal-nonmetal combinations bearings made from iron and Teflon , and heat resistant metal-ceramics combinations such as Al2o3 and nickel . The metal-ceramic combination is frequently referred to as a cermet and finds applications in corrosion resistant chemical apparatus , pumps for severe service , and nuclear energy equipment.
A number of metals which are not mutually soluble in either the liquid or solid states can be combined by powder metallurgy . This is particularly true for the electrical industry where motor brushes and contact points are made from powders of copper and graphite , and where contact parts are made from silver and nickel , or tungsten and copper .


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