30-05-2013, 02:46 PM
POWDER METAL POWDER
POWDER METAL.docx (Size: 929.32 KB / Downloads: 24)
INTRODUCTION
Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape or form (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering). It uses sintering process for making various parts out of metal powder. The metal powder is compacted by placing in a closed metal cavity (the die) under pressure. This compacted material is placed in an oven and sintered in a controlled atmosphere at high temperatures and the metal powders coalesce and form a solid. A second pressing operation, repressing, can be done prior to sintering to improve the compaction and the material properties.
CHARACTERSTICS OF POWDER METAL POWDER
The performance of metal powders during processing and the properties of powder metallurgy are
dependent upon the characteristics of the metal powders that are used. Following are the important
characteristics of metal powders.
(a) Particle shape
(b) Particle size
© Particle size distribution
(d) Flow rate
(e) Compressibility
(f) Apparent density
(g) Purity
(a) Particle Shape: The particle shape depends largely on the method of powder manufacture.The shape may be special nodular, irregular, angular, and dendritic. The particle shape influences the flow characteristics of powders. Special particles have excellent sintering properties. However, irregular shaped particles are good at green strength because they will interlook on computing.
(b) Particle Size: The particle size influences the control of porosity, compressibility and amount of shrinkage. It is determined by passing the powder through standard sieves or by microscopic measurement.
© Particle Size Distribution: It is specified in term of a sieve analysis, the amount of powder passing through 100, 200 etc., mess sieves. Particle size distribution influences the packing of powder and its behaviour during moulding and sintering.
(d) Flow Rate: It is the ability of powder to flow readily and confirm to the mould cavity. It determines the rate of production and economy.
(e) Compressibility: It is defined as volume of initial powder (powder loosely filled in cavity) to the volume of compact part. It depends on particle size, distribution and shape.
(f) Apparent Density: It depends on particle size and is defined as the ratio of volume to weight of loosely filled mixture.
(g) Purity: Metal powders should be free from impurities as theimpurities reduces the life of dies and effect sintering process. The oxides and the gaseous impurities can be removed from the part during sintering by use of reducing atmosphere.
Atomization:
In this method a molten metal is forced through a small orifice and is disintegrated by a powerful jet of compressed air, inert gas or water jet. These small particles are then allowed to solidify. These are generally spherical in shape. Automation is used mostly for low melting point metals/alloy such as brass, bronze, zinc, tin, lead and aluminium powders.
Electrolytic Deposition:
This method is commonly used for producing iron and copper
powders. This process is similar to electroplating. For making copper powder, copper plates are placed as anodes in the tank of electrolyte, whereas the aluminium plates are placed into electrolyte to act as anode. When D. C. current is passed through the electrolyte, the copper gets deposited on cathode. The cathode plates are taken out from electrolyte tank and the deposited powder is scrapped off. The powder is washed, dried and pulverised to produce powder of the desired grain size. The powder is further subjected to heat treatment to remove work hardness effect. The cost of manufacturing is high.
Shotting:
In this method, the molten metal is poured through aorifice and is cooled by dropping into water. This produces spherical particles of large size. This method is commonly used for metals of law melting points.
Condensation:
In this method, metals are boiled to produce metal vapours and then condensed
to obtain metal powders, This process is applied to volatile metals such as zinc, magnesiumand cadmium.
(b) Blending and Mixing of Powders
Powder blending and mixing of the powders are essential for uniformity of the product. Lubricants are added to the blending of powders before mixing. The function of lubricant is to minimise the wear, to reduce friction. Different powder in correct proportions are thoroughly mixed either wet or in a ball mill.
© Compacting
The main purpose of compacting is converting loose powder into a green compact of accurate shape and size. The following methods are adopted for compacting:
1. Pressing 2. Centrifugal compacting
3. Slip casting4. Extrusion
5. Gravity sintering 6. Rolling
7. Isostatic moulding 8. Explosive moulding
Gravity Sintering:
This process is used for making sheets for controlled porosity. In this process the powder is poured on ceramic tray to form a uniform layer and is then sintered up to
48 hours in ammonia gas at high temperature. The sheets are then rolled to desired thickness. Porous sheet of stainless steel are made by this process and popularly used for fitters.
Rolling:
This method is used for making continuous strips and rods having controlled porosity
with uniform mechanical properties. In this method, the metal powder is fed between two rolls which compress and interlock the powder particles to form a sheet of sufficient strength. It then situated, rerolled and heat treated if necessary. The metals that can be rolled are Cu, Brass, Bronze, Ni, Stainless steel.
ADVANTAGES OF POWDER METALLURGY
1. Although the cost of making powder is high there is no loss of material. The components produced are clean, bright and ready for use.
2. The greatest advantage of this process is the control of the composition of the product.
3. Components can be produced with good surface finish and close tolerance.
4. High production rates.
5. Complex shapes can be produced.
6. Wide range of properties such as density, porosity and particle size can be obtained for particular applications.
7. There is usually no need for subsequent machining or finishing operations.
8. This process facilitates mixing of both metallic and non-metallic powders to give products of special characteristics.
9. Porous parts can be produced that could not be made any other way.
10. Impossible parts (cutting tool bits) can be produced.
11. Highly qualified or skilled labour is not required.