5. powder metallurgy

Powder Metallurgy

ME 312 Manufacturing TechnologyVikrant Sharma, Mechanical Engineering Department. FET. MITS

Introduction:Powder metallurgy can be described as an art of

manufacturing commercial articles from powdered metals by placing these powders in moulds and applying pressure. These compressed parts are then heated to bind the particles together and improve their strength and other properties.

Vikrant Sharma , FET. MITS ME 312 Manufacturing Technology

The products made through this process are very costly on account of he high cost of metal powders as well as the die used.

The application of the process is, therefore economically feasible only when the number of required products is very high.

A few typical examples of such products are tungsten carbide cutting tools, self lubricating bearings, turbine blades having high temperature strength.

Small, intricate parts with high precision are required.


Historical Note Powders of metals such as gold and copper, as well as some

of the metallic oxides, have been used for decorative purposes since ancient times.

It is believed that the Egyptians used PM to make tools as far back as 3000 BC.

Around 1815, Englishman William Wollaston developed a technique for preparing platinum powders, compacting them under high pressure, and baking (sintering) them at red heat. The Wollaston process marks the beginning of powder metallurgy as it is practiced today.

U.S. patents were issued in 1870 to S. Gwynn that relate to PM self- lubricating bearings. He used a mixture of 99% powdered tin and 1% petroleum, mixing, heating, and finally subjecting the mixture to extreme pressures to form it into the desired shape inside a mold cavity.


In the 1920s, cemented carbide tools were being fabricated by PM techniques.

Powder metal gears and other components were mass produced in the 1960s and 1970s, especially in the automotive industry.

in the 1980s, PM parts for aircraft turbine engines were developed.


The powder metallurgy process:The entire powder metallurgy process mainly consists of the

following basic stages,

1. Production of metal powders.2. Mixing and blending of metal powders in required

proportions. 3. Pressing and compacting the powders into desired shapes

and sizes.4. Sintering the compacted parts in a controlled furnace

atmosphere.5. Subjecting the sintered parts to secondary processing, if

required.


Main characteristics of metal powders:1. Particle shape: There will be a variation in the particle

shapes in a collection of powders. It effects compactness, porosity and strength of the part.


2. Particle Size and Distribution: Particle size refers to the dimensions of the individual powders. There are various methods available to obtain particle size data. The most common method uses screens of different mesh sizes. The term mesh count is used to refer to the number of openings per linear inch of screen. Higher mesh count indicates smaller particle size. A mesh count of 200 means there are 200 openings per linear inch.

It has an appreciable influence over compressibility, density, porosity and also shrinkage during sintering. Use of particle of similar size increases porosity and of dissimilar size decreases it.


3. Interparticle Friction and Flow Characteristics: Friction between particles affects the ability of a powder to flow readily and pack tightly. A common measure 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.

Flow characteristics are important in die filling and pressing. Automatic die filling depends on easy and consistent flow of the powders. In pressing, resistance to flow increases density variations in the compacted part; these density gradients are generally undesirable. A common measure of flow is the time required for a certain amount of powder (by weight) to flow through a standard-sized funnel.


Production of metallic powder:1. Atomization: This method involves the conversion of molten metal into a spray of

droplets that solidify into powders. It is the most versatile and popular method for producing metal powders today, applicable to almost all metals, alloys as well as pure metals.


2. Reduction process: It includes a variety of chemical reactions by which metallic

compounds are reduced to elemental metal powders. A common process involves liberation of metals from their oxides by use of reducing agents such as hydrogen or carbon monoxide. The reducing agent is made to combine with the oxygen in the compound to free the metallic element. This approach is used to produce powders of iron, tungsten, and copper.

3. Mechanical methods:


Blending and mixing of powders:To achieve successful results in compaction and sintering, the

metallic powders must be thoroughly homogenized. Blending refers to when powders of the same chemical

composition but possibly different particle sizes are intermingled. Different particle sizes are often blended to reduce porosity.

Mixing refers to powders of different chemistries being combined. An advantage of PM technology is the opportunity to mix various metals into alloys that would be difficult or impossible to produce by other means.


Compacting:It is the process of converting loose powder into a green

compact of accurate shape and size. It is done in steel dies and punches.


The dies and punches used should be highly polished and the clearance between them should be kept at the minimum in order to maintain proper alignment.

The clearance should be sufficient to allow a free movement.

High carbon steel and tungsten carbide are the principal die material.


As a result of compaction, the density of the part, called the green density is much greater than the starting material density, but is not uniform in the green. The density and therefore mechanical properties vary across the part volume and depend on pressure in compaction.


Sintering:Sintering is a heat treatment operation performed on the

compact to bond its metallic particles, thereby increasing strength and hardness. The treatment is usually carried out at temperatures between 0.7 and 0.9 of the metal’s melting point. Compressed metal powder is heated in a controlled-atmosphere furnace.

Because PM applications usually involve medium-to-high production, most sintering furnaces are designed with mechanized flow-through capability for the work parts. The heat treatment consists of three steps, accomplished in three chambers in these continuous furnaces: (1) preheat, in which lubricants and binders are burned off; (2) sinter; and (3) cool down.


Main objectives of sintering1. Achieving high strength.2. Achieving good bonding of powder particles.3. Producing a dense and compact structure.4. Producing parts free of oxides.5. Obtaining desired structure and improved mechanical

properties.


Design Considerations The shape of the parts must be as simple as possible. Hole and grooves must be parallel to the direction of

ejection.


undercuts, threads, knurling and other similar impression should not be included in die, but produced later through machining.

The designed dimensions of parts should carry adequate allowances to compensate for the likely changes in dimensions due to shrinkage during sintering.


5. powder metallurgy

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