GTU EXAMINATION QUESTIONS 1. What is powder metallurgy? Describe various steps involved in powder metallurgy with each step controlling properties of final sintered component.(7) 2. Give advantages, limitation & applications of powder metallurgy.(7) 3. Which are merit and demerit and application of powder metallurgy? (7) 4. Define Powder Metallurgy. State advantages, limitations and applications of Powder Metallurgy.(7) 5. Explain in brief: Sintering Process.(5) 6. Discuss the advantages, disadvantages and limitations of powder metallurgy.(7) 7. Enlist methods of manufacturing metal powder. Discuss any one in detail.(7) 8. Enlist the products made from powder metallurgy. Explain all four steps of power metallurgy.(7) 9. Explain any two methods for production of metal powders.(7)
INTRODUCTION • Manufacturing: is the process to convert the raw material to finish products. • It encompasses – The design of the product – The selection of the raw materials – The sequence of the process through which the raw material is converted into finish product There are numbers of manufacturing process which are used to manufacture product. There are some examples for manufacturing. Powder metallurgy comes into the categories of forming and shaping process. Casting Forming and shaping ( rolling, forging, powder
HISTORY OF P/M Powder metallurgy recognized today as one of the most important manufacturing process for making several industrial component. Dates back to a long time in history one of the earliest examples of P/M techniques practiced in the ancient days were the reduction of the iron ore with charcoal to produce sponge iron powders. These powders were subsequently formed into useful shapes by forging. Typical example of ancient powder metallurgy artifacts includes the ASHOKA PILLAR of Delhi. The important of p/m as an important manufacturing process became evident as early as 19th century. When tungsten powders were produced and strengthened with THORIA for use in electric lamp filaments. The development of tungsten carbides tool was another mildstone. Production of porous bearing and bushes by p/m further spurred its growth. Important examples in this category include use of metal powders in printing, catalysis in chemicals industries and in food industry. Powder metallurgy, one of the most important techniques not only for many commercials but also for several advanced applications in aerospace, nuclear and defense components. Importance of P/M:
The methods of powder metallurgy have permitted the attainment of compositions and properties not possible by the conventional methods of melting and casting. Powder metallurgy is an alternative, economically viable mass production method for structural components to very close tolerance. Powder metallurgy techniques produce some parts which can’t be made by any other method. The process of P/M is the process of producing metallic parts from metallic powders of a single metal, of several metals or of a combination of metals and non-metals by applying pressure. The powders are mixed mechanically, compacted into a particular shape and then heated at elevated temperature below the melting point of the main constituent. PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining PM process wastes very little material - about 97% of the starting powders are converted to product PM parts can be made with a specified level of porosity, to produce porous metal parts impregnated bearings and Gears Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy Examples: filters, oil-
− Example: Tungsten filaments for incandescent lamp bulbs are made by PM Certain alloy combinations and cermets made by PM cannot be produced in other ways PM compares favorably to most casting processes in dimensional control PM production methods can be automated for economical production.
Why powder metallurgy? • Used for metals that are too hard to machine • Used when very large quantity is expected • Reaction occurs at melting point. • Melting point of the metal to be used for making a product is too high (W , Mo)
POWDER METALLURGY • Powder metallurgy is defined as the art and science of producing fine metal powders and using them to make serviceable products. • It may also be defined as “material processing technique used to consolidate particulate matter i.e. powders both metal and/or non-metals • “Chip-less” process.
Components are produced in their final form by pressing metal powders into the desired shape, usually in a metal mold, and then heating the compacted powders, either concurrently or subsequently, for a period of the time at a temperature below the melting point of the major consequent of time at a temperature below the melting point of the major constituent.
APPLICATION OF POWDER METALLURGY Cemented carbides tool = the resistance offered by
a materials to a cutting tool during machining is many times non-uniform. Also, a variation in this resistance exerts non-uniform forces on cutting tools and becomes responsible for shock loads and impacts. This is the case especially when rough cuts are taken at high speeds during machining. At high cutting speeds, the heat is generated due to friction between work piece and cutting tools, this softens the cutting edge of tool and therefore the cutting tool loses its ability to cut properly. Thus, tool becomes blunt which starts chattering or rubbing on the work piece instead of cutting.
The cemented carbide tools consist of extremely hard phase well distributed within a tough matrix. The hard particle generally includes tungsten, titanium or tantalum. They retain their hardness and hence cutting edge at high speed and even at high temperatures. At the same time they are very brittle. Thus a single piece cutting tool made of this hard materials is usually not capable to withstand shocks and so breaks easily. Due to above limitations a continuous matrix phase of relatively soft and ductile materials holds and binds the particles together to form a tool. The cutting edge are fastened mechanically or brazed to the shank of the tool in the form of a tip made of hard carbide particles, which cuts the materials without failure fro longer period of time and at a faster rate. Self lubricating bearing: All bearing required lubrication (oil, wax) in such case external bearing require frequently lubrication and maintenance. Besides such a bearing used in Food, chemical drug and cement industries, they get contaminated by undesirable
particles or chemicals. This reduces the life of the bearing. In such case bearing needs to be sealed or protected.
This disadvantage can be eliminated by using self lubricating bearing. They are manufactured by powder metallurgy by Impregnating lubricating oil under the temperature of 77 0C to 82 0C by immersing them for some time in oil. The oils enter the pores which are interconnected. Bearing becomes like a sponge soaked with oil. Example of this type of bearing is porous bronze bearings. When the shaft is stationary the bearing load of shaft forms a thin film of oil at the shaft bearing interface and this provides initial lubricating effect at the beginning of rotation.But during the subsequent rotations, heat is generated due to friction as the initial film cannot overcome this friction. The generated heat raises the temperature which causes volumetric expansion of oil and reduction in its viscosity. Due to this oil comes out of the pores and provides required lubrications at the interface. When the rotating shaft stops and subsequently the bearing cool down, oil again occupies the pores by capillary actions. Some of the following example for powder metallurgy as: Porous products Tungsten wires
Diamond tools Magnetics materials Oil pump gears Electrical contracts materials Babbitt bearings for automobiles ADVANTAGE OF POWDER METALLURGY Clean and smooth operations Close dimensional tolerances and smooth surfaces can be achieved. High purity raw material can be used and this purity can be maintained till the end of the process by controlling the fabrication steps. No defects such as gas pockets, blow holes etc., are produced whereas these are common in metal casting. New combination can be achieved. For example METAL and NON-METAL can be mixed which quite impossible by other methods. Highly qualified or skilled person are not required. Machining operation is almost eliminated. The parts have an excellent finish and high dimensional accuracy.
There is overall economy as material wastage is negligible. Diamond tools and tungsten carbides are made possible only by powder metallurgy . Super –hard cutting tools bits and refractory materials which can never be manufactured by any other method, are made by powder metallurgy. Tungsten(3422 °C), tantalum or molybdenum(electric bulbs, radio valves, X-ray tube, oscillator valves) Metal+ Non-metal (clutch facing , brake lining) Copper +graphite (motor brushes) Ni + Cd (batteries) Titanium + zirconium (gauge pins) LIMITATIONS Size and complexity limitations High cost of powder metals compared to other raw materials High cost of tooling and equipment for small production runs There are numbers of limitations of Powder Metallurgy process as given below:
(i)In general, the principal limitations of the process are those imposed by the size and shape of the part, the compacting pressure required and the material used. (ii)The process is capital intensive and initial high costs mean that the production ranges in excess of 10,000 are necessary for economic viability (cost of dies is very high). (iii)The configuration of the component should be such that it can be easily formed and ejected from a die, undercuts and re-entrant angles cannot be molded and have to be machined subsequently The capacity and stroke of the compacting press and the compacting pressure required limit the cross-sectional area and length of the component. (v) Spheres cannot be molded and hence a central cylindrical portion is required. (vi) All materials which can be satisfactorily cold-worked by conventional methods have been produced (e.g. brass up-to 30 % Zn and bronzes up-to 10 % tin). Copper-based materials which are hot-worked have not so far been made by P/M successfully.
PRODUCTION OF METAL POWDERS 1) Mechanical methods of powder production:
i) Chopping or Cutting ii) Abrasion methods iii) Machining methods iv) Milling v) Cold-stream Process. 2) Chemical methods of powder production: i) Reduction of oxides ii) Precipitation from solutions iii) Thermal decomposition of compounds iv) Hydride decomposition v) Thermite reaction vi) Electro- chemical methods 3) Physical methods of powder production i) Water atomization ii) Gas atomization iii) Special atomization methods • The choice of a specific technique for powder production depends on particle size, shape, microstructure and chemistry of powder and also on the cost of the process.
1. Chopping or Cutting: this process, strands of hard steel wire, in diameter as small as 0.0313 inches are cut up into small pieces by means of a milling cutter.
manufacturing of cut wire shots which are used for peening or shot cleaning.
Limitations: It would, however, be difficult and costly to make powders by this method and for this reason it is not profitable to discuss the technique in detail. 2. Rubbing or Abrasion Methods: These are all sorts of ways in which a mass of metal might be attacked by some form of abrasion. a) Rubbing of Two Surfaces:
When we rub two surfaces against each other, hard surface removes the material from the surface of soft material. b)Filling:
Filing as a production method has been frequently employed, especially to alloy powders, when supplies from conventional sources have been unobtainable. Such methods are also used for manufacture of coarse powders of dental alloys. Filing can also be used to produce finer powder if its teeth are smaller. * commercially not feasible. c) Scratching: If a hard pin is rubbed on some soft metal the powder flakes are produced. Scratching is a technique actually used on a large scale for the preparation of coarse magnesium powders. * scratching a slab of magnesium with hardened steel pins. * a revolving metal drum to the surface of which is fixed a scratching belt. The drum, which is about 8 inches in diameter, rotates at a peripheral speed of approximately 2500 ft./min. The slab of magnesium metal, 14 in. wide by 1.75 in. thick enters through a gland in the drum casing and presses against the steel pins. d) Machining:
A machining process, using for example a lathe or a milling cutter in which something more than just scratching is involved, since the attacking tool actually digs under the surface of the metal and tears it off. On lathe machine by applying small force we get fine chips. A large amount of machining scrap is produced in machining operations. This scrap in the form of chips and turnings can be further reduced in size by grinding. * Small scale production.
COMMERCIAL METHODS These are the methods used for high production rate. Best examples of mechanical production methods are the Milling Process and Cold Stream Process. Milling: The basic principal of milling process is the application of impact and shear forces between two materials, a hard and a soft, causing soft material to be ground into fine particles. Milling techniques are suitable for brittle materials. Two types of milling are; i) Ball Milling
ii) Attrition Milling Ball Milling: Ball milling is an old and relatively simple method for grinding large lumps of materials into smaller pieces and powder form. Principle of the process: The principle is simple and is based on the impact and shear forces. Hard balls are used for mechanical combination of brittle materials and producing powders. Milling Unit: The basic apparatus consists of the following; • A ball mill or jar mill which mainly consists of a rotating drum lined from inside with a hard material. • Hard balls, as a grinding medium, which continue to impact the material inside the drum as it rotates/rolls.
Attrition mill: is the term which means to wear or rub away. It is a process of grinding down by friction.
Milling Unit: In attrition milling a very high efficiency ball mill is agitated (sudden force) by a vertical rotating shaft with horizontal arms. In these mills the rotational speeds are nearly 6 – 80 rpm while the size of medium (balls) used is 3 – 6 mm. Power is used to rotate the agitator and not the vessel as in case of ball mills. The central rotating shaft of attrition mill is equipped with several horizontal arms. When rotated, it exerts the stirring action to tumble the grinding medium randomly throughout the entire chamber. COLD STREAM PROCESS This process is based on impact phenomenon caused by impingement of high velocity particles against a cemented carbide plate. The unit consists of:
velocity stream of air (56 m3/min.) operating at 7 MPa (1000 psi);
nozzle and target generally are made of cemented tungsten carbide. Mechanism of the Process: The material to be powdered is fed in the chamber and from there falls in front of high velocity stream of air. This air causes the impingement of material against target plate, where material due to impaction is shattered into powder form. This powder is sucked and is classified in the classifying chamber. Oversize is recycled and fine powder is removed from discharge area. * Rapidly expanding gases leaving the nozzle create a strong cooling effect through adiabatic expansion. This effect is greater than the heat produced by pulverization. CHEMICAL METHODS REDUCTION OF METAL OXIDES Manufacturing of metal powder by reduction of oxides is extensively employed, particularly for Fe, Cu, W and Mo. As a manufacturing technique, oxide reduction may exhibit certain advantages and disadvantages. These are listed below; Advantages:
economical when carbon is used. --- because oxides are generally friable, easily pulverized and easily graded by sieving.
and either batch or continuous processes Limitations:
circumstances where this is economically available may be limited; in some cases, however, costs may be reduced by recirculation of the gas.
upon the purity of the raw material, and economic or technical considerations may set a limitation to that which can be attained.
• The compound of metals (oxides, iron oxide) is reduced with CO, H2O at temperatures below the melting point of the metal in the controlled furnace.
• Iron powder is produce Fe3O4 + 4C =3Fe + 4CO Fe3O4 + 4CO =3Fe + 4CO2 Copper powder is produce Cu2O + H2 =2Cu + H2O • Largest volume of metal powder is made by the process of oxide reduction. Tungsten (W) and molybdenum (MO), iron, Nickel (Ni), cobalt (CO), are the example of this process Electro chemical methods These methods are based on the electrolysis of molten solutions of metals. The metals are electrically deposited on the cathode of an electrolytic cell as a sponge or powder or at least in a physical form in which it can be easily disintegrated into a powder. Advantages of the process: The technique has a number of advantages, e.g. The product is usually of a high commercial purity. A considerable range of powder qualities can be obtained by varying bath compositions.
Frequently the product has excellent pressing and sintering properties. The cost of the operation may in some cases be low. STEPS FOR POWDER METALLURGY Production of Metal Powder Mixing – Blending Compacting Sintering Operations Finishing Operation 2-Mixing or Blending Blending : blending is an operation of through intermingling of different powders of the same composition or various grades of the same powders. Mixing : refers to through intermingling of powders of more than one materials. Various types of blender and mills are used for mixing. Ball Mill and Rod Mill 3 COMPACTING Pressure Less Shaping Technique o Without external pressure.
1-loose shaping 2-slip casting 3-slurry casting Cold Pressure Shaping o Cold die compaction o Isostatic pressing o Powder rolling o Vibratory compacting o Forging o Extrusion Pressure shaping with heat o Hot pressing o Powder of sinter forging o Hot rolling 4-Sintering Operations • Sintering process is carried out at a temperature below the highest melting point. • Sintering is essentially a process of bonding solid bodies by atomic force.(intermolecular force) • Sintering imparts the required properties such as more strength, densification, and dimensional control to the green compacts due to the formation of strong bond between the particles
• The process whereby compressed metal powder is heated in a controlled atmosphere furnace to a temperature below its melting point, but high enough to allow bonding of the particles. • Sintering temperatures are generally within 70 to 90% of the melting point of the metal or alloy. • Times range from 10 minutes for iron and copper to 8 hours for tungsten and tantalum
• Chemical change • Electrical properties change • Phase change • Relief of internal stress • Alloying
electrical and thermal conductivity increase with increase amount of sintering.