Powder Metallurgy Science of producing metal powders and making finished/semifinished objects from mixed or alloyed powders with or without the addition of nonmetallic constituents The powder metallurgy technique finds use in various industries and manufacturing processes. It has become very popular in a very short short span of time because of its efficiency, durability and reliable output. Some of them are mentioned here. Applications of Powder Metallurgy in Industries
Manufacturing metal bonded diamond tools and materials. Manufacturing power tools and modern home appliances. Aerospace and Automobile Industry have a huge scope for power metallurgy for making large equipment and machine parts. Manufacturing of friction materials, refractory metals, switch materials and electric contact materials. Production and processing of metals with high melting points like Tungsten and Molybdenum that are used in electronics industry.
Parts with irregular curves or recesses that are hard to machine can be manufactured using the powder metallurgy techniques. It is suitable for high-volume and mass production with practically negligible wastage of the manufacturing material. The process of secondary machining is virtually eliminated or reduced to negligible extent by the technique of powder metallurgy and it helps in improving efficiency by a huge margin. Cams, sprockets, pawls, iron bearings, sintered bronze, ratchets and carbide tool tips are the most commonly manufactured items with the help of powder technology. The advantages are: • Efficient material utilization • Enables close dimensional tolerances – near near net shape possible • Good surface finish • Manufacture of complex shapes possible • Hard materials used to make components that a re difficult to machine can be readily made – made – tungsten tungsten wires for incandescent lamps • Environment friendly, energy efficient • Suited for moderate tto o high volume component production • Powders of uniform chemical composition => reflected in the finished part • wide variety of materials => misci ble, ble, immiscible systems; refractory metals • Parts with controlled porosity can be made • High cost of powder material & tooling • Less strong parts than wrought ones • Less well known process
Processes of Powder Technology The basic steps involved in powder metallurgy are the following: Blending and Mixing: This is carried out to achieve uniformity of the product manufactured. Distribution of properly sized particles is attained by mixing elementary powder with alloy powders to obtain a homogeneous homogeneous mixture. Lubricants are also mixed with powders to minimize the wear of dies and reduce friction between the surfaces of dies and particles of powder during compaction. Mixing time will depend upon the results desired, and over-
mixing should be prevented, or otherwise the size of particles will be decreased and they will be hardened. Pressing: The cavity of the die is filled with a specified quantity of blended powder, necessary pressure is applied, and then the compacted part is ejected. Pressing is performed at room temperature, while the pressure is dependent upon the material, properties of the powder used, and the density required of the compaction. Friction between the powder and the wall of the die opposes application of a proper pressure that decreases with depth and thus causes uneven density in the compact. Thus the ratio of length and diameter is kept low to prevent substantial variations in density. Sintering: Changes occur during sintering, including changes in size, configuration, and the nature of pores. Commonly used atmospheres for sintering are hydrogen, carbon monoxide, and ammonia. Sintering operation ensures that powder particles are bonded strongly and that better alloying is achieved. Flow chart of PM activities
Properties of Metal Powders Properties of metal powder depend upon the process employed for its production. Therefore, it is essential to determine the physical and chemical properties of powders to prevent
variations in the desired characteristics of the compactions. Significant properties of metal powders are:
Chemical composition that is determined by chemical analysis. Shape of particles that is affected by methods employed for production of powder. Particle size influences the properties of flow and density of powder metal. It can be measured by a microscope, sieve, or by sedimentation. Distribution of particle size has a significant effect on physical properties of powder, and can be determined by sieving test. Flowability is the relative ease of the flow of powder through an orifice. Bulk density can be measured by filling a pot whose volume is known with powder, and then obtaining the weight of the powder. Other properties include compressibility, compatibility, sintering ability, and specific surface.
Powder synthesis techniques Atomization • • •
Produce a liquid-metal stream by injecting molten metal through a small orifice Stream is broken by jets of inert gas, air, or water The size of the particle formed depends on the temperature of the metal, metal flowrate through the orifice, nozzle size and jet characteristics
Reduction
• •
Reduce metal oxides with H 2/CO Powders are spongy and porous and they have uniformly sized spherical or angular shapes
Electrolytic deposition
• •
Metal powder deposits at the cathode from aqueous solution Powders are among the purest available
Carbonyls
• • •
React high purity Fe or Ni with CO to form gaseous carbonyls Carbonyl decomposes to Fe and Ni Small, dense, uniformly spherical powders of high purity
Comminution
• • • • •
Crushing Milling in a ball mill Powder produced Brittle: Angular Ductile: flaky and not particularly suitable for P/M operations
Mechanical Alloying
• •
Powders of two or more metals are mixed in a ball mill Under the impact of hard balls, powders fracture and join together by diffusion Products manufactured by powder technique
Blending
• • • • • •
To make a homogeneous mass with uniform distribution of particle size and composition Powders made by different processes have different sizes and shapes Mixing powders of different metals/materials Add lubricants (<5%), such as graphite and stearic acid, to improve the flow characteristics and compressibility of mixtures Combining is generally carried out in air or inert gases to avoid oxidation Liquids for better mixing, elimination of dusts and reduced explosion hazards Hazards
•
Metal powders, because of high surface area to volume ratio are explosive, particularly Al, Mg, Ti, Zr, Th
Compaction •
Press powder into the desired shape and size in dies using a hydraulic or mechanical press • Pressed powder is known as “green compact” • Stages of metal powder compaction:
Cold Uniaxial Pressing
Components or articles are produced by forming a mass of powder into a shape, then consolidating to form inter-particle metallurgical bonds. An elevated temperature diffusion process referred to as sintering, sometimes assisted by external pressure, accomplishes this. The material is never fully molten, although there mi ght be a small volume fraction of liquid present during the sintering process. Sintering can be regarded as welding the particles present in the initial useful shape. As a general rule both mechanical and physical properties improve with increasing density. Therefore the method selected for the fabrication of a component by powder metallurgy will depend on the level of performance required from the part. Many components are adequate when produced at 85-90% of theoretical full density (T.D.) whilst others require full density for satisfactory performance. Some components, in particular bush type bearings often made from copper and its alloys, are produced with significant and controlled levels of porosity, the porosity being subsequently filled with a lubricant.
Cold Isostatic Pressing
• • •
Metal powder placed in a flexible rubber mold Assembly pressurized hydrostatically by water (400 – 1000 MPa) Typical: Automotive cylinder liners
Elemental metal, or an atomised prealloyed, powder is mixed with a lubricant, typically lithium stearate (0.75 wt.%), and pressed at pressures of say, 600 MPa (87,000 lb/in 2) in metal dies. Cold compaction ensures that the as-compacted, or „green‟, component is dimensionally very accurate, as it is moulded precisely to the size and shape of the die. Irregularly shaped particles are required to ensure that the as-pressed component has a high green strength from the interlocking and plastic deformation of individual particles with their neighbours. One disadvantage of this technique is the differences in pressed density that can occur in different parts of the component due to particle/particle and die wall/particle frictional effects. Typical as-pressed densities for soft iron components would be 7.0 g/cc, i.e. about 90% of theoretical density. Compaction pressure rises significantly if higher as-pressed densities are required, and this practice becomes uneconomic due to higher costs for the larger presses and stronger tools to withstand the higher pressures.
Sintering Sintering is the process whereby powder compacts are heated so that adjacent particles fuse together, thus resulting in a solid article with improved mechanical strength compared to the powder compact. This “fusing” of particles results in an increase in the density of the part and hence the process is sometimes called densification. There are some processes such as hot
isostatic pressing which combine the compaction and sintering processes i nto a single step. After compaction the components pass through a sintering furnace. This typically has two heating zones, the first removes the lubricant, and the second higher temperature zone allows diffusion and bonding between powder particles. A range of atmospheres, including vacuum, are used to sinter different materials depending on their chemical compositions. As an example, precise atmosphere control allows iron/carbon materials to be produced with specific carbon compositions and mechanical properties. The density of the component can also change during sintering, depending on the materials and the sintering temperature. These dimensional changes can be controlle d by an understanding and control of the pressing and sintering parameters, and components can be produced with dimensions that need little or no rectification to meet the dimensional tolerances. Note that in many cases all of the powder used is present in the finished product, scrap losses will only occur when secondary machining operations are necessary First stage: Temperature is slowly increased so that all volatile materials in the green compact that would interfere with good bonding is removed Rapid heating in this stage may entrap gases and produce high internal pressure which may fracture the compact Second stage: High temperature stage Promotes solid-state bonding by diffusion. Diffusion is timetemperature sensitive. Needs sufficient time Promotes vapour-phase transport. Because material heated very close to MP, metal atoms will be released in the vapour phase from the particles. Vapour phase resolidifies at the interface
HOT ISOSTATIC PRESSING (HIP)
Powders are usually encapsulated in a metallic container but sometimes in glass. The container is evacuated, the powder out-gassed to avoid contamination of the materials by any residual gas during the consolidation stage and sealed-off. It is then heated and subjected to isostatic pressure sufficient to plastically deform both the container and the powder. The rate of densification of the powder depends upon the yield strength of the powder at the temperatures and pressures chosen. At moderate temperature the yield strength of the powder can still be high and require high pressure to produce densification in an economic time. Typical values might be 1120°C and 100 MPa for ferrous alloys. By pressing at very much higher temperatures lower pressures are required as the yield strength of the material is lower. Using a glass enclosure atmospheric pressure (15 psi) is used to consolidate bars and larger billets. The technique requires considerable financial investment as the pressure vessel has to withstand the internal gas pressure and allow the powder to be heated to high temperatures. As with cold isostatic pressing only semifinished products are produced, either for subsequent working to smaller sizes, or for machining to finished dimensions.
•
Simultaneous compaction + sintering
•
Container: High MP sheet metal
•
Container subjected to elevated temperature and a very high vacuum to remove air and moisture from the powder
•
Pressurizing medium: Inert gas
•
Operating conditions
– 100 MPa at 1100 C – Produces compacts with almost 100% density – Good metallurgical bonding between particles and good mechanical strength – Uses – Superalloy components for aerospace industries – Final densification step for WC cutting tools and P/M tool steels
