Seminar on Pm

POWDER METALLURGY

POWDER METALLURGY

CHAPTER-1INTRODUCTION:Powder Metallurgy is the process of blending fine powdered materials, pressing them into a desired shape or form, and then heating the compressed material in a controlled atmosphere to bond the material. It may also be referred to as powder processing considering that non-metal powders can be involved. Powders are compacted into a certain geometry then heated, (sintered), to solidify the part.The first consideration in powder metallurgy is the powders used for the manufacturing process. Several different measures are used to quantify the properties of a certain powder. Powders can be pure elements or alloys. A powder might be a mixture of different kinds of powders. It could be a combination of elemental powders, alloy powders, or both elemental and alloy powders together. Material and the method of powder production are critical factors in determining the properties of a powder.The powder metallurgy process generally consists of four basic steps: Powder manufacture, Powder blending, Compacting Sintering.

CHAPTER-2POWDER MANUFACTURE:Several techniques have been developed which permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. Powders may be prepared byfollowing methods. Atomization Centrifugal disintegration Sponge Iron process

Atomization: Atomization is accomplished by forcing a molten metal stream through an orifice at moderate pressures. A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. Air and powder streams are segregated using gravity orcyclonic separation.There are three types of atomization:1. Liquid atomization2. Gas atomization3. Centrifugal atomization

Liquid Atomization: (Fig a) It is the process in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow, but at higher velocities the stream becomes turbulent and breaks into droplets. Pumping energy is applied to droplet formation with very low efficiency (on the order of 1%) and control over the size distribution of the metal particles produced is rather poor. Other techniques such as nozzle vibration, nozzle asymmetry, multiple impinging streams, or molten-metal injection into ambient gas are all available to increase atomization efficiency, produce finer grains, and to narrow the particle size distribution. Unfortunately, it is difficult to eject metals through orifices smaller than a few millimeters in diameter, which in practice limits the minimum size of powder grains to approximately 10 m. Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and remelting a significant fraction of the grain boundary.

Gas Atomization: (Fig b) A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. Air and powder streams are segregated using gravity orcyclonic separation. Most atomized powders are annealed, which helps reduce the oxide and carbon content. The water atomized particles are smaller, cleaner, and nonporous and have a greater breadth of size, which allows better compacting. The particles produced through this method are normally of spherical or pear shape. Usually, they also carry a layer of oxide over them.

Centrifugal Atomization: Metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle. Opposite the spindle tip is an electrode from which an arc is established which heats the metal rod. As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls. A circulating gas sweeps particles from the chamber. Similar techniques could be employed in space or on the Moon. The chamber wall could be rotated to force new powders into remote collection vesselsand the electrode could be replaced by a solar mirror focused at the end of the rod.

CHAPTER-3Particle Structure: The structure, or shape, of particles is a major factor in a powder processing operation. Material and method of powder production are the main variables determining powder shape. Particles of a certain powder may have similar shapes but no particle shapes are exactly the same. Hence, there will exist a shape distribution within a powder. Different types of powders combined together may also have significant differences in particle shape, which will show in the shape distribution.Particle shape plays a large role in powder density and flow characteristics; it is also a major factor in pressing and sintering. There are several types of basic powder particle shapes. These are ideal shapes; particles in reality are imperfect and may exhibit characteristics of more than one shape type.

CHAPTER-4

POWDER BLENDING:POWDER BLENDING:Blending: mixing powder of the same chemical composition but different sizes.Mixing: combining powders of different chemistries

Blending and mixing are accomplished by mechanical means:

The above figures shows several blending and mixing devices:

(a) rotating drum, (b) rotating double cone, (c) screw mixer, (d)blade mixer

Except for powders, some other ingredients are usually added:

Lubricants: To reduce the particles-die frictionBinders: To achieve enough strength before sinteringDeflocculants: To improve the flow characteristics during feeding

CHAPTER-5

Compaction:

Compaction of the powder within the die with punches to form the compact. Generally, compaction pressure is applied through punches from both ends of the toolset in order to reduce the level of density gradient within the compact.

Blended powers are pressed in dies under high pressure to form them into the required shape. The work part after compaction is called a green compact or simply a green, the word green meaning not yet fully processed.

Typical steps in compaction and a typical press are as shown in figure

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Pressure and density distributions after compaction:

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:

Effect of applied pressure during compaction: (1) initial loose powders after filling, (2) repacking, and (3) deformation of particles.

There are different ways to improve the density distribution:Application of double acting press and two moving punches in conventional compaction

Fig: Compaction with a Single Punch Fig: Compaction obtained through double acting punch

Isostatic pressing:Pressure is applied from all directions against the powder, which is placed in a flexible mold:

Cold isostatic pressing: (1) powders are placed in the flexible mold; (2) hydrostatic pressure is applied against the mold to compact the powders; and (3) pressure is reduced and the part is removed

CHAPTER-7Sintering:Compressed metal powder is heated in a controlled-atmosphere furnace to a temperature below its melting point, but high enough to allow bounding of the particles.

Solid state sintering is the process of taking metal in the form of a powder and placing it into a mold or die. Once compacted into the mold the material is placed under a high heat for a long period of time. Under heat, bonding takes place between the porous aggregate particles and once cooled the powder has bonded to form a solid piece.

Sintering can be considered to proceed in three stages. During the first, neck growth proceeds rapidly but powder particles remain discrete. During the second, most densification occurs, the structure recrystallizes and particles diffuse into each other. During the third, isolated pores tend to become spheroidal and densification continues at a much lower rate. The words solid state in solid state sintering simply refer to the state the material is in when it bonds, solid meaning the material was not turned molten to bond together as alloys are formed.[10]

One recently developed technique for high-speed sintering involves passing high electrical current through a powder to preferentially heat the asperities. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders.

To allow efficient stacking of product in the furnace during sintering and prevent parts sticking together, many manufacturers separate ware using Ceramic Powder Separator Sheets. These sheets are available in various materials such as alumina, zirconia and magnesia. They are also available in fine medium and coarse particle sizes. By matching the material and particle size to the ware being sintered, surface damage and contamination can be reduced while maximizing furnace loading.

(a) Typical heat treatment cycle in sintering; and (b) schematic cross-section of a continuous sintering furnace.

The primary driving force for sintering is not the fusion of material, but formation and growth of bonds between the particles, as illustrated in a series of sketches showing on a microscopic scale the changes that occur during sintering of metallic powders.

Sintering on a microscopic scale. The illustration shows different stages in development of grain boundaries between particles.

CHAPTER-8Continuous powder processing:The phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however, the term refers to processes whose products are much longer in one physical dimension than in the other two. Compression, rolling, and extrusion are the most common examples.

In a simple compression process, powder flows from a bin onto a two-walled channel and is repeatedly compressed vertically by a horizontally stationary punch. After stripping the compress from the conveyor the compact is introduced into a sintering furnace. An even easier approach is to spray powder onto a moving belt and sinter it without compression. Good methods for stripping cold-pressed materials from moving belts are hard to find. One alternative that avoids the belt-stripping difficulty altogether is the manufacture of metal sheets using opposed hydraulic rams, although weakness lines across the sheet may arise during successive press operations.

Powders can also be rolled to produce sheets. The powdered metal is fed into a two-high rolling mill and is compacted into strip at up to 100 feet per minute (0.5 m/s).[11] The strip is then sintered and subjected to another rolling and sintering. Rolling is commonly used to produce sheet metal for electrical and electronic components as well as coins.[11] Considerable work also has been done on rolling multiple layers of different materials simultaneously into sheets.

Extrusion processes are of two general types. In one type, the powder is mixed with a binder or plasticizer at room temperature; in the other, the powder is extruded at elevated temperatures without fortification. Extrusions with binders are used extensively in the preparation of tungsten-carbide composites. Tubes, complex sections, and spiral drill shapes are manufactured in extended lengths and diameters varying from 0.5300 mm. Hard metal wires of 0.1 mm diameter have been drawn from powder stock. At the opposite extreme, large extrusions on a tonnage basis may be feasible.

There appears to be no limitation to the variety of metals and alloys that can be extruded, provided the temperatures and pressures involved are within the capabilities of die materials. Extrusion lengths may range from 330 m and diameters from 0.21 m.

CHAPTER-9Finishing operations:A number of secondary and finishing operations can be applied after sintering, some of them are: Sizing: cold pressing to improve dimensional accuracy Coining: cold pressing to press details into surface Impregnation: oil fills the pores of the part Infiltration: pores are filled with a molten metal Heat treating, plating, painting

CHAPTER-10Powder Metallurgy Products:The high precision forming capability of Powder Metallurgy generates components with near net shape, intricate features and close dimensional precision, finished without the need of machining.Powder Metallurgy is especially suited to the production of large series of pieces with narrow tolerances. By producing parts with a homogeneous structure Powder Metallurgy enables manufacturers to make products that are more consistent and predictable in their behavior across a wide range of applications.Thanks to its process flexibility Powder Metallurgy allows the tailoring of the physical characteristics of a product to suit your specific property and performance requirements. Good performance in stress and absorbing of vibrations as well as special properties such as hardness and wear resistance are a feature of Powder Metallurgy components.In the overall material process technology industry, there are a variety of products utilizing Powder Metallurgy. Currently this process is extensively used in production of Structural Parts, Tribiological part and Magnetic Parts. This process is also used in the development of High performance next generation parts.

The above figure shows structural parts manufactured from Powder Metallurgy Technique.

The above figure shows Tribological part manufactured from Powder Metallurgy.

The above figure shows Soft Magnets Manufactured from Powder Metallurgy.Powder Metallurgy allows the processing, in an intimate mixed form, of combinations of materials that would be conventionally regarded as immiscible. Well-established examples of this type of Powder Metallurgy application are:

Friction materials for brake linings and clutch facings in which a range of non-metallic materials, to impart wear resistance or to control friction levels, are embedded in a copper-based or iron-based matrix.

Cutting tools, Indexable Inserts, Hard metals or cemented carbides used for cutting tools, forming tools or wear parts. These comprise a hard phase bonded with a metallic phase, a microstructure that can only be generated through liquid phase sintering at a temperature above the melting point of the binder. Tungsten carbide bonded with cobalt is the predominant example of such a material, but other hard metals are available that include a range of other carbides, nitrides, carbonitrides or oxides and metals other than cobalt can be used as the binder (Ni, Ni-Cr, Ni-Co etc.)

Diamond cutting tool materials, in which fine diamond grit is uniformly dispersed in a metallic matrix. Again, liquid phase sintering is employed in the processing of these materials. Electrical contact materials e.g. copper/tungsten, silver/cadmium oxide.

Processing of materials with very high melting points:Powder Metallurgy enables the processing of materials with very high melting points, including refractory metals such as tungsten, molybdenum and tantalum. Such metals are very difficult to produce by melting and casting and are often very brittle in the cast state. The production of tungsten billet, for subsequent drawing to wire for incandescent lamps, was one of Powder Metallurgys very early application areas.

Products with controlled levels of porosity: Powder Metallurgy enables the manufacture of products with controlled levels of porosity in their structure. Sintered filter elements are examples of such an application. The other prime example is the oil-retaining or self-lubricating bearing, one of Powder Metallurgys longest established applications, in which the interconnected porosity in the sintered structure is used to hold a reservoir of oil.

Products with superior properties:In some specific applications, the generation of superior properties, often through superior control over microstructure, is possible by Powder Metallurgy processing as opposed to conventional casting or wrought routes. Good examples in this category of application are:

Magnetic materials:Virtually all hard (permanent) magnets and around 30% of soft magnets are processed from powder feed stocks.

High speed steels: The finer and more controlled microstructure from a Powder Metallurgy processed material provides superior toughness and cutting performance than wrought products.

Nickel- or cobalt-based superalloys:Nickel- or cobalt-based superalloys are used for aero-engine applications, in which Powder Metallurgy processing can deliver compositional ranges and microstructural control not achievable conventionally and therefore an enhancement in operating temperature and performance.

Defense Applications:Metal powders play an important role in military and national defense systems.They find use in missiles, rockets, cartridge cases, bullets, etc.Also used in military pyrotechnics like tracers, incendiaries, etc.

CHAPTER-11Advantages and Disadvantages of Powder Metallurgy: Advantages:1. The vast majority of refractory metals and their compounds, false alloy, porous materials can only be made from powder metallurgy method.2. Due to the powder metallurgical method can be compressed into the final size of the compact, and don't need or rarely need subsequent mechanical processing, so can greatly save metal, reduce product cost. With products manufactured by the powder metallurgy, metal loss is only 1-5%, and with the general casting method production, metal loss could reach 80%.3. Due to powder metallurgy technology in material does not melt during production, are not afraid with impurities from crucible and deoxidizer, and sintering in a vacuum and restore general atmosphere, is not afraid of oxidation, also did not give any material pollution, therefore, likely to preparing high purity materials.4. Powder metallurgy method can guarantee the correctness of the material composition ratio and uniformity.5. Powder metallurgy is suitable for the production of the same shape and quantity of the products, especially the gear etc. The high cost of processing products, manufactured by powder metallurgy method can greatly reduce the production cost.6. Sintering is a crucial process in the powder metallurgy process. After forming compact required is obtained by sintering the final physical and mechanical properties.7. After sintering processing, can be different according to product requirements, adopt a variety of ways. Such as finishing, oiled, machining, heat treatment and plating. In addition, in recent years, some new technology such as rolling, forging is used in sintering of powder metallurgy materials after processing to obtain ideal effect.Disadvantages:1. The mold cost is relatively higher.2. The tooling and equipments are very expensive, therefore becomes main issue with low production volume. - Limited shapes and features.3. Powders cannot flow round corners.4. Complex shapes requires several punches, otherwise density will not be uniform.5. Difficult to produce large and complex shaped parts with powder metallurgy.6. Difficult to handle low melting point metals as they tend to melt when sintered - Slight shrinkage on sintering and cooling to room temperature.7. Cannot be bent or cold worked due to brittleness - Threaded feature can only be produced during secondary operations - 1mm