Chapter 7 Powder Processing PDF

Chapter 7 Powder Processing  Introduction  Production of metal powders  Compaction of metal powders  Sintering  Secondary and finishing operations  Powder metallurgy (PM) process involves metal powders compacted into desired shapes and sintered (heated without melting) to form a so...

Chapter 7 Powder Processing  Introduction  Production of metal powders  Compaction of metal powders  Sintering  Secondary and finishing operations  Powder metallurgy (PM) process involves metal powders compacted into desired shapes and sintered (heated without melting) to form a solid piece.  The most commonly used metals in P/M are iron, copper, aluminum, tin, nickel, titanium, and the refractory metals. For parts made of brass, bronze, steels, prealloyed powders are used.  Powder-metallurgy process consists of: 1. Powder production 2. Blending 3. Compaction 4. Sintering 5. Finishing operations  The choice of producing metal powders depends on the requirements of the end product.  The microstructure, bulk and surface properties, chemical purity, porosity, shape, and size distribution of the particles depend on the particular process used.  Particle sizes produced range from 0.1 to 1000 μm. Atomization  Involves a liquid-metal stream produced by injecting molten metal through a small orifice  Stream is broken up by jets of inert gas or air or water known as gas or water atomization  Size and shape of the particles formed depend on the temperature of the molten metal, rate of flow, nozzle size, and jet characteristics.  In centrifugal atomization, the molten metal stream drops onto a rapidly rotating disk or cup, the centrifugal forces break up the stream and generate particles. Gas atomization Water atomization  In atomization with rotation consumable electrode, the electrode is rotated rapidly ( about 15000 rev/min) in a helium filled chamber.  The centrifugal force breaks up the molten tip of the electrode into metal particles. Iron powder particles Nickel based superalloy powder particles made by rotating electrode Reduction  The reduction of metal oxides uses gases, such as hydrogen and carbon monoxide, as reducing agents.  By this means, very fine metallic oxides are reduced to the metallic state.  The powders produced are spongy and porous and have uniformly sized spherical or angular shapes. Electrolytic Deposition  Used either aqueous solutions or fused salts. The powders produced are among the purest available. Carbonyls  Metal carbonyls are formed by letting iron or nickel react with carbon monoxide  Reaction products are decomposed to iron and nickel, and they turn into small, dense, uniformly spherical particles of high purity. Comminution  Involves crushing milling in a ball mill, or grinding of brittle or less ductile metals into small particles  A ball mill is a machine with a rotating hollow cylinder partly filled with steel or white cast-iron balls Mechanical Alloying  Powders of two or more pure metals are mixed in a ball mill  Under the impact of the hard balls, the powders fracture and bond together by diffusion ( movement and exchange of atom across the interface), entrapping the second phase and forming alloy powders.  Blending (mixing) powders is the next step in powder-metallurgy processing.  It is carried out for the following purposes: 1. Powders of different metals and other materials can be mixed in order to impart special physical and mechanical properties and characteristics to the P/M product. 2. Even when a single metal is used, the powders may vary significantly in size and shape, hence they must be blended to obtain uniformity from part to part. 3. Lubricants can be mixed with the powders to improve their flow characteristics. They reduce friction between the metal particles, improve flow of the powder metals into the dies, and improve die life. 4. Other additives—binders (as in sand molds) are used to develop sufficient green strength and additives also can be used to facilitate sintering.  Powder mixing must be carried out under controlled conditions in order to avoid contamination or deterioration. Some common bowl geometries for mixing and blending powder Mixer for blending metal powders  Compaction is the step in which the blended powders are pressed into various shapes in dies.  The purposes of compaction are: (a) to obtain the required shape, density, and particle-to-particle contact (b) to make the part sufficiently strong for further processing. The compaction of metal powder to form a bushing. The pressed powder part is called green compact. (b) Typical tool and die set for compacting a spur gear.  The pressed powder is known as green compact, since it has a low strength just as is seen in green parts in slip casting.  The density of the green compact depends on the pressure applied.  Important factor in density is the size distribution of particles. ( same size and different size powder, which one has higher density?) a) Density of copper- and iron-powder compacts as a function of compacting pressure. Density greatly influences the mechanical and physical properties of P/M parts. (b) Effect of density on tensile strength, elongation, and electrical conductivity of copper powder  Green compacts is subjected to hydrostatic pressure to achieve more uniform compaction and density  In cold isostatic pressing (CIP), the metal powder is placed in a flexible rubber mold  The assembly then is pressurized hydrostatically in a chamber. The most common pressure is 400 MPa. Schematic diagrams of cold isostatic pressing; pressure is applied isostatically inside a high-pressure chamber. (a) The wet bag process to form a cup-shaped part; the powder is enclosed in a flexible container around a solid-core rod. (b) The dry bag process used to form a PM cylinder. In hot isostatic pressing (HIP), the container generally is made of a high-melting- point sheet metal, and the pressurizing medium is high-temperature inert gas or a vitreous (glasslike) fluid. Powder-injection Molding  Also called metal-injection molding  Very fine metal powders are blended with a 25 to 45% polymer or a wax-based binder ( for holding the powder together)  Mixture undergoes a process similar to die casting, where it is injected into the mold, at the temperature of 135 degree to 200 degree Celsius.  Parts generally have sprues and runners.  The molded green parts are placed in low temperature oven, to burn off the plastic ( debinding)  Advantages of powder-injection molding are: 1. Complex shapes 2. Good dimensional tolerances 3. High production rates A single shot of four metal injection molded components, with sprue, runners, and gates  Also called roll compaction Metal powder is fed into the roll gap in a two-high rolling mill and compacted into a continuous strip , at a speed of up to 0.5 m/s.  It is a shape-generation process die  Basic components are 1. An atomizer 2. A spray chamber with an inert atmosphere 3. A mold for producing preforms Molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe.  Sintering is the process whereby green compacts are heated in a controlled-atmosphere furnace to a temperature below the melting point but sufficiently high to allow bonding (fusion) of the individual particles.  Continuous-sintering furnaces have three chambers: 1. Burn-off chamber - for volatilizing the lubricants in the green compact in order to improve bond strength and prevent cracking. 2. High-temperature chamber -for sintering. 3. Cooling chamber.  Sintering mechanisms are complex and depend on the composition of the metal particles as well as on the processing parameters. 32  The sintering mechanisms are diffusion, vapor-phase transport, and liquid-phase sintering. As temperature increases, two adjacent powder particles begin to form a bond by a diffusion mechanism  A second sintering mechanism is vapor-phase transport. Two mechanisms for sintering metal powders: (a) solid-state material transport; and (b) vapor-phase material transport. 33  Liquid phase sintering – if two particles are of different metals, alloying can take place. Schematic illustration of liquid phase sintering using a mixture of two powders. (a) Green compact of a higher melting point base metal and lower temperature additive; (b) liquid melting, wetting and reprecipitation on surfaces; and (c) fully sintered solid material. Mechanical Properties  Depending on temperature, time, and the processing history, different structures and porosities can be obtained in a sintered compact and, thus, affect its properties.  Porosity cannot be eliminated completely because: (a) Voids remain after compaction (b) Gases evolve during sintering Porosity 35  In order to further improve the properties of sintered P/M products, or to impart special characteristics, several additional operations may be carried out after sintering. 1. Coining and sizing - compacting operations to impart dimensional accuracy, and to improve its strength and surface finish by further densification. 2. Preformed and sintered alloy-powder compacts subsequently may be cold or hot forged to the desired final shapes and sometimes by impact forging. 36 3. Powder-metal parts may be subjected to other finishing operations such as: Machining: for producing various geometric features by milling, drilling, and tapping (to produce threaded holes). Grinding: for improving dimensional accuracy and surface finish. Plating: for improving appearance and resistance to wear and corrosion. Heat treating: for improving hardness and strength. 37 4. Impregnating – reduce porosity with a fluid 5. Infiltration - a slug of a lower-melting-point metal is placed in contact the sintered part. The assembly then is heated to a temperature sufficiently high to melt the slug. The molten metal infiltrates the pores by capillary action and produces a relatively pore-free part having good density and strength. 38