powder metallurgy.pptx

321

1. POWDER ME T A L LU R GY

2. Powder Metallurgy Essentially, Powder Metallurgy (PM) is an art & science of producing metal or metallic powders, and using them to make finished or semi-finished products. Particulate technology is probably the oldest forming technique known to man. There are archeological evidences to prove that the ancient man knew something about it. Powder Metallurgy

3. Powder Metallurgy An important point that comes out : The entire material need not be melted to fuse it. The working temperature is well below the melting point of the major constituent, making it a very suitable method to work with refractory materials, such as: W, Mo, Ta, Nb, oxides, carbides, etc. It began with Platinum technology about 4 centuries ago… in those days, Platinum, [MP = 1774°C], was "refractory", and could not be melted. Powder Metallurgy

4. Powder Metallurgy Process Powder production Blending or mixing Powder compaction Sintering Finishing Operations Powder Metallurgy

5. POWDER PREPARATION: This is a first and basic step for producing an object by powder metallurgy process. Any material can convert into powder. There are various processes of producing powder such as  atomization, grinding, chemical reaction,  electrolysis process , etc. MIXING AND BLENDING: As the name implies, this step involves the mixing of two or more material powder to produce a high strength alloy material according to the product requirement. This process ensures even distribution of powder with additives, binders, etc. Sometimes lubricants also added in the blending process to improve flow characteristic of powder.

6. COMPACTING Compacting means compressed the prepared powder mixture into pre-defined dies. This step ensures to reduce voids and increase the density of the product. The powder is compacted into the mold by the application of pressure to form a product which is called green compact (the product gets by compacting). It involves pressure range from 80 to 1600 MPa. This pressure depends on the properties of metal powder and binders. For soft powder compacting pressure is about 100 – 350 MPa. For steel, iron, etc. the pressure is between 400 – 700 MPa.

7. SINTERING The green compact, produced by compressing, is not very strong and can’t be used as a final product. This step involves heating of green compact at an elevated temperature which ensures a permanent strong bond between adjacent particles. This process provides strength to green compact and converts it into a final product. The sintering temperature is generally about 70 to 90 percent of the melting temperature of metal powder.

8. SECONDARY OPERATION The sintered object is more porous compared to fully dense material. The density of the product depends upon press capacity, sintering temperature, compressing pressure, etc. Sometimes, the product does not require high density and the sintered product is directly used as a final product. But sometimes, a highly dense product is required (for example manufacturing bearing, etc.) Where a sintered product cannot be used as a finished product. That’s why a secondary operation required to obtain high density and high dimensional accuracy. The most common secondary operation used is  sizing ,  hot forging , coining, infiltration, impregnation,  etc.

9. Powder Metallurgy Process Powder Metallurgy

10. PM Advantages & Disadvantges The parts can be produced clean, bright and ready for use. The composition of the product can be controlled effectively. Articles of any intricate shape can be manufactured. Close dimensional tolerance can be achieved. The machining operation is almost eliminated. Parts have excellent finish and high dimensional accuracy. There is the overall economy as material wastage is negligible. Metals and non-metals can be mixed in any proportion. A wide range of properties such as porosity, density, etc. The high initial cost of metal powder. The equipment used for the operation is costly. Pressure up to 100 tonnes capacity is used even for a small product. Some power may present explosion hazards. Dies used must be of high accuracy and capable of withstanding high pressure and temperature . The metal powder is expensive and in some case difficult to store

11. POWDER PRODUCTION Mechanical Processes Physical Processes Chemical Processes Electrochemical processes i )Machining ii)Crushing iii)Milling iv)Shooting v)Graining vi) Atomization Condensation Thermal Decomposition (Gaseous pyrolysis) i ) Reduction ii) Intergranular corrosion iii) Precipitation

12. MACHINING As the name suggests this method employs use of machine tools to produce powder in the form of chips, filings etc. The chip so obtained are hammered and made in the form of powder.

13. CRUSHING In this method the small pieces of material are collected and crushed together The method employs use of stamps, hammer, jaw crusher and gyratory crusher etc.

14. MILLING It employs use of rotating ball mill machine The raw material are feed with balls or rod and then the mill is allowed to rotate so that the ball or rod hits the raw material and crushing it and slowly time comes when the raw material is converted to powder

15. SHOTTING In this method molten metal is poured on vibrating screen and the liquid droplets are solidifies either in air or neutral gas. The size and character of the powder depends on the temperature of molten metal, size of openings in the screen and frequency of vibration of the screen shape of particles is nearly spherical. GRAINING Graining involves the same procedure as the shotting, the only difference being the solidification of molten metal droplets is done in water

16. 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.

17. PHYSICAL PROCESSES CONDENSATION In this method metal vapours are condensed to obtain metal powders. This method is highly suitable for volatile metals because they get easily transformed to their vapours. Large quantities of Zn, Mg and Cd powders are manufactured by this method. Due care must be taken to avoid the formation of metal oxides. The powder shape is nearly spherical.

18. Thermal decomposition Fine metal powders of some metal like Fe,Ni,W,Mo,Co , Mg etc are manufactured by thermal decomposition of their respective carbonyl vapours. This method is highly suitable for the manufacture of Fe and Ni powder. Fe & Ni carbonyls are produced by passing CO over a spongy or powdered metal at some suitable temperature (200 to 270 C)& pressure (70 to 200 atmosphere) These carbonyl are volatile liquids and their vapours decompose at one atmosphere pressure and temp of 150 to 400 degree centigrade as shown below Fe(CO)5 Fe+5 CO Ni(CO)4 Ni+ 4 CO This decomposition gives metal powder of very high purity and the carbon monoxide can be recycled. Fe powder is perfectly spherical whereas Ni powder is irregular in shape. This powder is used for the manufacture of high permeable cores which are used in long distance communication systems.

19. CHEMICAL PROCESSES REDUCTION In this method, a suitable compound of metal such as oxide, oxalate, formate or halide is reduced by suitable reducing agent to obtain the metal in the powder form. The reducing agent may be a solid or a gas. It is extensively used for the manufacture of Fe,Cu , Ni,W,Mo and Co powder; and to a lesser extent for Zr,Ti , Ta,Th , Al and Cr powders. The usual reducing agents employed are carbon, hydrogen, ammonia & carbon monoxide. W and Mo powders are produced from WO3 and MoO2 respectively by reduction with carbon for their use in manufacture of sintered carbide cutting tools. However they are reduced by H2 for their use in manufacture of incandescent lamp filaments, X-ray targets and electrical contact materials. The powders obtained by this method are fine & the shape of particles is irregular.

20. Intergranular corrosion It is fact that the grain boundaries of any metal corrode faster than the grains. In this method, grain boundary area of the metal under interest is corroded by a suitable electrolyte so as to separate out the grains from the polycrystalline metal. The powder of stainless steel is made by this process as below Stainless steel is an alloy of Fe and Cr with some amount of carbon. When the steel is heated in the temperature range of 500 to 800 degree centigrade for long time, the Cr combines with cardon and forms complex chromium carbides. These carbides separate out along the grain boundaries and due to this, the steel becomes sensitive to corrosion This method was extensively used in the old days for preparing the stainless steel powder. However, today it is obsolete and atomization process is used for the same.

21. Particle Shapes in Metal Powders Powder Metallurgy

22. Blending or Mixing 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 methods Powders of different metals and other materials may be mixed in order to impart special physical and mechanical properties through metallic alloying. Lubricants may be mixed to improve the powder’s flow characteristics. Bin d ers su c h a s w a x o r th e r moplastic poly m e r s ar e add e d to improve green strength. Powder Metallurgy

23. Blending Combining is generally carried out in Air or inert gases to avoid oxidation Liqu i ds for be t t e r mixi ng , elimina t i o n of dusts an d re d uced explosion hazards MIXING MACHINE MANUFACTURES – YouTube mezclador en v lleal – YouTube Twin Shell Blender – YouTube Powder Metallurgy

24. Bowl Geometries for Blending Powders A m i x er s uit a bl e f or blendin g m etal powders. Some common equipment geometries used for blending powders (a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell Powder Metallurgy

25. Compaction Applic ati o n of hi g h pres s ure to the p o w de r s to fo r m them into the required shape. Co n v ent i o na l co m p a ction method i s pr e ssin g , i n whi c h opposing punches squeeze the powders contained in a die. The work part after pressing is called a green compact , the word green meaning not yet fully processed. The green strength of the part when pressed is adequate for handling but far less than after sintering. Powder Metallurgy

26. Pr ess p o w der into the desired shape an d size in dies using a hydraulic or mechanical press Pressed powder is known as “green compact” Stages of metal powder compaction: Compacting Powder Metallurgy

27. P o w ders do n o t f l o w li k e liqui d, th e y simply c omp r ess u n til an equal and opposing force is created. – This opposing force is created from a combination of resistance by the bottom punch and friction between the particles and die surface Compa c ting consolidates an d d andi f ies the co m pone n t for transportation to the sintering furnace. Compacting consists of automatically feeding a controlled amount of mixed powder into a precision die, after which it is compacted . Compacting Powder Metallurgy

28. Compacting Compacting is usually performed at room temperature. Pressures range from 10 tons per square inch (tons/in 2 ) (138 MPa) to 60 tons/in 2 (827 MPa), or more. Powder Metallurgy

29. Figure: (Left) Typical press for the compacting of metal powders. A removable die set (right) allows the machine to be producing parts with one die set while another is being fitted to produce a second product. Powder Metallurgy

30. Compaction Sequence Figure: Typical compaction sequence for a single-level part, showing the functions of the feed shoe, die core rod, and upper and lower punches. Loose powder is shaded; compacted powder is solid black. Powder Metallurgy

31. Sintering Hea t treat m ent t o bon d the metall i c pa r ti cle s , th e re b y increa sing strength and hardness. Usually carried out at between 70% and 90% of the metal's melting point (absolute scale) Generally agreed among researchers that the primary driving force for sintering is reduction of surface energy Pa r t shrink a g e oc c urs du r ing sint e rin g du e t o por e size reduction Powder Metallurgy

32. Parts are heated to ~80% of melting temperature. Transforms compacted mechanical bonds to much stronger metal bonds. Many parts are done at this stage. Some will require additional processing. Sintering Powder Metallurgy

33. Figure: Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions. Sintering Sequence stronger Parts are heated to 0.7~0.9 T m . T r a ns f o r ms compact e d me c hanic a l bond s t o m u c h metallic bonds. Powder Metallurgy

34. Third stage: Sintered product is cooled in a controlled atmosphere. – Prevents oxidation and thermal shock Gases commonly used for sintering: H 2 , N 2 , inert gases or vacuum Sintering Powder Metallurgy

35. Sintering Cycle and Furnace Powder Metallurgy

36. Powder Rolling Powder Metallurgy

37. Powder Metallurgy Merits Near Nett Shape is possible, thereby reducing the post-production costs, therefore: 🠞 Precision parts can be produced 🠞 The production can be fully automated, therefore, 🠞 Mass production is possible 🠞 Production rate is high 🠞 Over-head costs are low 🠞 Break even point is not too large 🠞 Material loss is small 12/1/2014 Powder Metallurgy

38. Limitations and Disadvantages High tooling and equipment costs. Metallic powders are expensive. Problems in storing and handling metal powders. Degradation over time, fire hazards with certain metals Lim i ta t ions o n pa r t g eomet r y beca u se metal p o w ders do n ot readily flow laterally in the die during pressing. Variations in density throughout part may be a problem, especially for complex geometries. Powder Metallurgy

39. Powder Metallurgy Disadvantages Porous !! Not always desired. Large components cannot be produced on a large scale. Some shapes are difficult to be produced by the conventional p/m route. Whatever, the merits are so many that P/M, as a forming technique, is gaining popularity Powder Metallurgy

40. PM Parts Powder Metallurgy

41. Connecting Rods: Forged on left; P/M on right Powdered Metal Transmission Gear Warm compaction method with 1650-ton press Teeth are molded net shape: No machining UTS = 155,000 psi 30% cost savings over the original forged part Powder Metallurgy

42. Powder Metallurgy