Processing of Powder Metals and Ceramics

Transcription

1 Processing of Powder Metals and Ceramics Session delivered by: Dr. Srikari S. 1

2 Session Objectives At the end of this session the delegate should be able to analyze Fabrication of ceramic parts Slip casting Powder metallurgy Powder processing for different materials Application of powder processing techniques 2

3 Fabrication Techniques of Ceramics Particulate forming processes: Powder Pressing Hot, uniaxial, Isostatic Hydroplastic Forming Slip casting Tape casting Cementation Drying - Firing 3

4 Hydroplastic Forming Adopted for clay-based compositions Ceramic is mixed with water to form a stiff plastic ceramic mass The mass is forced through a die orifice having the required cross-sectional geometry (extruded) Air is removed in a vacuum chamber to enhance the density Bricks, ceramic blocks and tiles are all commonly fabricated by this technique. 4

5 Slip Casting A slip is a suspension of clay or other non-plastic materials in water. The slip must have proper solid to water ratio. The slip is formed and poured into a porous mold commonly made of Plaster of Paris. Water from the slip will be absorbed into the mold, leaving behind a solid layer on the mold wall, the thickness of which depends on time. The process is continued until the entire mold cavity becomes solid or stopped at the required thickness. Eg.: Clay products, Sanitary wares, art objects, radomes 5

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7 Drying and Firing Ceramic piece which is processed by Hydroplastic or Slip casting techniques is dried and fired. Before firing the ceramic piece is said to be green (Machining is done preferably at this stage) Drying is a process in which the water content in the ceramic is removed. As the ceramic dries it also experiences shrinkage. The drying rate is controlled by temperature and humidity. After drying the ceramic piece is fired at high temperatures between 900 and 1400 o C. The firing temperature depends on the composition and the desired property of the finished piece. During firing the density of the ceramic piece increases. 7

8 Tape Casting Forming technique for producing thin, flat ceramics. The method was originally developed for producing electronic ceramics (insulating substrates and packages and multilayer capacitors) Structural laminates, knives, membranes and solid oxide fuel cells are examples of other applications for thin ceramics formed by tape casting. The tape thickness that can be achieved is generally in the range of 25 µm up to 1 mm but it is possible to produce tapes down to 5 µm. 8

9 Tape Casting The slip is made by a suspension of ceramic particles in an organic liquid that also contains binders and plasticizers. The slip is poured onto a flat surface (of stainless steel, glass or a polymeric film) A Doctor Blade spreads the slip into a thin tape of uniform thickness. The tape is dried cut into required sizes and then fired. To avoid air bubbles it is de-aired in vacuum before drying. 9

10 Tape Casting PEMP Top Schematic : Illustration of tape casting; Bottom Schematic: Different steps during the processing: The slip consisting of water, ceramic particles and binder; the cast, dried green sheet, microstructure of the sintered material 10 [12]

11 Gel Casting Gelcasting, an advanced forming method for ceramics, was invented in 1987 by Mark A. Janney and Ogbemi O. Omatete It allows a ceramic part to be machined before, not after, the part is heated to make it hard AlliedSignal Ceramic Components and other companies are using this technology for manufacturing airplane and automobile engine parts that are light and resistant to corrosion and high temperatures 11

12 Powder Metallurgy: Introduction Powder Metallurgy is the art as well as science of producing metal, ceramic powders and using them to make useful objects. Products in powder metallurgy are often made with a mix of different metal powders or may sometimes even contain non-metallic constituents in order to improve the bonding qualities of the particles and properties of the final product. 12

13 Process in Powder Metallurgy Production of metallic powders Conditioning of powders Compaction Pre-sintering Sintering Secondary operations 13

14 Basic Processing Steps PEMP [9] 14

15 Production of Metal Powders METAL POWDERS ATOMIZATION REDUCTION ELECTROLYSIS MILLING/GRINDING Atomization Molten metal is forced through a small orifice/nozzle into stream of water at high pressure The liquid metal from the orifice gets atomized and solidifies into finely divided particles [3] 15

16 Similar to electroplating. Copper plates act as anode and aluminum plates as cathode. On applying high amperage electric current to the electrolyte, a powdery deposit of the anode metal results on the cathode. Electrolysis 16

17 Particularly suitable for brittle materials. This process utilizes various types of crushers, rotary mills, stamping mills and grinders in order to break down the metals by crushing and impact. Main advantage is the extent of fineness obtained. Milling / Grinding 17

18 Reduction Production of powders from metals from their respective oxide form. The oxides are reduced with carbon monoxide or hydrogen and the reduced powder is subsequently ground. High degree of uniformity in size could be obtained in the reduced powder if the oxide powder is graded before reduction - Particles produced are sponge like in structure Powders of refractory metals such as tungsten and molybdenum can be obtained through this process. Fe 3 O 4 + 4CO Fe +4CO 2 Cu 2 O + H 2 2Cu + H 2 O 18

19 Characteristics of Metallic Powder Properties of metallurgical components are greatly affected by a number of characteristics such as particle size, shape, fineness, particle size distribution, flowability, compressibility, apparent density, sintering capacity etc. Shape: Depends upon how it is produced. Could be spherical, rugged, dendritic, flat/angular. Fineness: Particle size and is determined through standard sieve analysis (micro to nano meters) 19

20 Characteristic of Metallic Powder Particle size distribution refers to the amount of each standard particle size in the powder. Flowability is the characteristic of a powder which permits it to flow readily and occupy the nooks and corners of the mould cavity. Compressibility is the ratio of the volume of the initial powder to the volume of the compressed piece. Sintering capacity is the ability of the particles to bond upon application of heat. 20

21 Various types of metallic powders produced by atomization, electrolysis, reduction etc. Metal Powders 21

22 Conditioning of Metal Powders Conditioning of metallic powders involves blending and mixing of metal powders. Blend of the powders determines the composition and also the different properties that can be obtained later in the components. Blending is done to obtain desired properties such as heat resistance, wear resistance, toughness etc. Mixing of metal powders can be done either in wet state or in dry state. Mixers are used to obtain homogenous mixture. The type mixer used depends upon amount of powder to be handled and type of powder. For efficient mixing lubricants, binders and volatizing agents are used. 22

23 Compaction Metal powders are pressed into objects of desired shape and approximately to the final dimensions. Dimensions are taken into account to limit any changes after sintering. This process also imparts certain amount of porosity and adequate strength. Powders are pressed using high pressures, the pressure required depends upon the desired density of final product and ease of adhering of the powders. Different techniques employed are: Die pressing, Roll pressing, Extrusion 23

24 Powder Pressing Procedures PEMP The ceramic or metal in the form of fine powder is mixed with small amount of water or other binder and is compacted into the desired shape by applying high pressure. Structural, electronic and magnetic ceramics are processed by this technique. Three basic powder pressing procedures are: Uniaxial pressing (pressure is applied in one direction) Isostatic pressing (pressure is applied uniformly in all directions through a fluid, by placing the powder mixture in a rubber envelope) Cold (CIP) or hot isostatic pressing (HIP) depending on the application of temperature Hot pressing (pressing and heating is performed simultaneously) 24

25 Powder Compaction Metal powders are pressed to required shape [9] 25

26 Presintering and Sintering Presintering It is a process of heating the compacted object to temperature below sintering temperature Lubricants, binders etc added during blending is removed It is carried out on such components that require some sort of machining after compaction Sintering The green compact obtained after compaction is heated to elevated temperature, usually below the melting point of the base metal. With sintering the particles adhere/fuse together thus increasing the strength of the sintered component 26

27 Sintering Liquid phase sintering Sintering carried out with one of the constituent of the component in its liquid state. 27

28 Sintering PEMP pore size 28

29 Sizing Infiltration Impregnation Heat treatment Plating Secondary Operations 29

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31 Powder Metallurgy Advantages PEMP High strength parts with low ductility metals and metals with very high melting temperatures. High tolerance parts possible with minimum processing. High alloy contents possible; often alloy content exceeds solubility limits of conventional wrought metallurgical processing. Relatively low processing temperatures. Sintering is generally a diffusion driven process rather than a melting process, although some alloy metals may become molten at sintering temperatures. 31

32 Limitations Complicated shapes, such as those produced by casting cannot be easily produced (due to die design) Size of components is limited because of the large presses, dies, expensive tooling Not economical for small scale production. Metal powders are expensive to produce. Susceptible to oxidation due to larger surface areas. Strength and stiffness may be inferior to wrought alloys of similar composition Porosity and low ductility may impair durability Fracture Toughness may be low. 32

33 Examples Typical metals used to take advantage of P/M technology include: Fe-based alloys (plain-carbon, low alloy, high alloy and stainless steels) Al-based alloys Cu-based alloys Co-based alloys Ni-based alloys Ti-based alloys W-based alloys Refractory metal alloys (Rhenium, tantalum). 33

34 Typical Stress-Strain Behaviour PEMP 1000 x FN-0205-T Stress (MPa) x x FC-0200-P F S SAE1030 Steel (Ref) x Strain 34

35 Effect of Density on Strength PEMP 35

36 Refinement Treatments Fe- based P/M parts can be subsequently treated to improve properties Steam Treatment is one method Improves Density, and Corrosion Resistance Mechanical properties also improved 36

37 Metal Injection Moulding Marriage between powder metallurgy and plastic injection moulding Metal injection moulding The need for complex shaped parts in large volumes with short lead times triggered the MIM technology Defense industry was the mother of MIM. Other industries are electronic, consumer, automotive also use MIM extensively 37

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39 MIM vs. P/M 39

40 Fine metal powder is mixed with a polymer binder, which is granulated to form Feedstock Feedstock is heated to form viscous slurry and injection molded to form green part. Part of the binders are removed through solvent debinding to get brown part. The brown part is densified through sintering to get finished part. MIM Process Flow 40

41 Aluminum Based Powder Metallurgy Alloys P/M used with aluminum alloys to give improved corrosion resistance and mechanical properties in comparison with wrought metal alloys. T m = 660 C, Al-alloys not normally considered for use in elevated temperature applications. However, P/M technology can push the limits of application of Al-alloys to temperatures of 300 C or better. E.g. Al-Fe-Ce alloys (7-9% Fe, 4-6% Ce) that withstand 1000 hours of continuous exposure at 225 C with no reduction of ambient-temperature strength. Similar results occur after 100 hrs at 310 C. Alloys 7090 and 7091 are two commercially available P/M alloys with good high temperature properties. 41

42 Applications Porous and self lubricating bearings Refractory parts such as filaments, cathodes, anodes used in electric bulbs, x-ray tubes, radio valves Toothed components like gears, carbide tip tools used in machining and cutting Fe alloys Welding rods and electrodes which comprise of a mixture of powdered metal and flux The HIP process was first used for space and aeronautic components Now also used for automotive parts, such as pistons, rods, crankshafts, and engine blocks and also in medical implants, turbine blades, pump impeller, and boat propellers A cast titanium blade for a helicopter rotor for helicopter was HIPed. It was processed at 1680 F and 15,000psi for two hours. The casing s properties were similar to the forging previously used, but the cost was much less. 42

43 HIP Applications PEMP Pratt & Whitney reported that HIPing increased the tensile strength of Inconel 718 to 80% that of the wrought material, compared to the 60 70% for as-cast. At the same time, fatigue strength increased from 55-85% of the wrought metal. HIP reduced the reject rate of Rene 120 high-pressure turbine blades from 28% to only 4% at the dovetail grind. The cost of manufacturing an aircraft fitting was compared using three different methods. These methods were machining from a bar stock, a titanium forging, and using titaniumpowder metal technology. The fitting was finally made from P/M by cold isostatic pressing, the vacuum sintering to a density exceeding 95%. Material utilization was 90% by P/M, 30% as forging, and 5% machined from bar stock 43

44 Hot Isostatic Pressing [9] 44

45 Hipped Components 45

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47 High end automotive component Complex component with Accurate tolerance. Component for automobile and motorcycles 47

48 Bearing for steering pinion,column & master jacket Connection bearings and thrust washers 48

49 Summary Different processing techniques like Hydroplastic forming, slip casting, tape casting can be used to produce ceramics. Powder metallurgy is a powder processing procedure where the powders are consolidated by the application of pressure and temperature. 49