Chapter 10IT 2081 Powder Methods of Change of Form Chapter 10

IT Powder Methods of Change of Form Competencies List the advantages of, and the products made from Powder Metal List and describe the order of operations in Powder Metallurgy

Chapter 10 IT Powder Methods of Change of Form Power metallurgy - the process of compacting metal powders in suitable dies and sintering them. Net shape parts of fairly complex shape can be produced economically Competitive with casting, forging and machining Good dimensional accuracy and size; from balls for ball point pens to parts weighing 200 lbs.

Chapter 10 IT Powder Methods of Change of Form Parts such as Self lubricating bearings impregnated with oil Break pads with embedded ceramic fibers Machine tool cutting instruments; cermets (ceramic-metals) higher heat absorption Commonly made out of iron, copper, aluminum, tin, and nickel Order of operation: Powder production, blending, compacting, sintering, finishing

Chapter 10 IT Powder Methods of Change of Form Powder Preparation Virtually any metal can be made into powder form. There are three principal methods by which metallic powders are commercially produced, each of which involves energy input to increase the surface area of the metal.

Chapter 10 IT Powder Methods of Change of Form Atomization – involves the conversion of molten metal into a spray of droplets that solidify into powder. It is the most versatile and popular methods for producing metal powders today Gas atomization – in which a high velocity gas stream is utilized to atomize the liquid metal. Water atomization – a high-velocity water stream is used instead of air. Chemical

Chapter 10 IT Powder Methods of Change of Form Electrolytic – an electrolytic cell is set up in which the source of the desired metal is the anode. The anode is slowly dissolved under an applied voltage, transported through the electrolyte, and deposited on the cathode. The deposit is removed, washed, and dried to yield a metallic powder of very high purity. In addition, mechanical methods are occasionally used to reduce powder sizes; however, these methods are much more commonly associated with ceramic powder production.

Chapter 10 IT Powder Methods of Change of Form Comminution, a term used for the techniques for reducing particles size in ceramics processing, deliver mechanical energy in various forms. Two general types of communition operations are distinguished: crushing and grinding

Chapter 10 IT Powder Methods of Change of Form Crushing – the reduction of large lumps from the mine to smaller sizes for subsequent further reduction. Several stages may be required (e.g. primary crushing, secondary crushing)

Chapter 10 IT Powder Methods of Change of Form Grinding – refers to the operation of reducing the small pieces after crushing to a fine powder. Grinding is accomplished by abrasion and impact of the crushed mineral by the free motion of unconnected hard media such as balls, pebbles, or rods. Ball Mill – hard spheres mixed with the stock to be comminuted are rotated inside a large cylindrical container. Roller mill – stock is compressed against a flat horizontal grinding table by rollers riding over the table surface Impact grinding – particles of stock are thrown against a hard flat surface, either in high velocity air stream or in a high-speed slurry. The impact fractures the pieces into smaller particles.

Chapter 10 IT Powder Methods of Change of Form 2.Blending Mixing several powders of different sizes, a dry lubricant or an antioxidant for uniform compaction. (done carefully to avoid explosions) Can add lubricants- do not stick to mold walls Can add binders- so green strength is adequate Can add sintering aids- acceleration of densification upon heating

Chapter 10 IT Powder Methods of Change of Form 3.Compaction Bringing the materials into required shape. The workpart after pressing is called a green compact, the word green meaning not yet fully processed. Briquetting - Compression of powder in the die cavity from both the top and the bottom. Roller Compaction - compacted between two rollers to produce sheet or plate stock. Extrusion Compacting - powder packed into a mild steel tube, then forced through a die.

Chapter 10 IT Powder Methods of Change of Form 4.Sintering Process of heating compressed powdered metals to within 70 – 90 % of its melting point. Often called (solid-state sintering, or solid-phase sintering) because the metal remains unmelted Used for materials such as ceramics and cermets that cannot be melted and cast by other methods.

Chapter 10 IT Powder Methods of Change of Form Sintering involves mass transport to create the necks and transform them into grain boundaries. The principal mechanism by which this occurs is diffusion; other possible mechanisms include plastic flow. Sintering – the heat treatment consists of three steps Preheat, in which lubricants and binders are burned off Sinter Cool down

Chapter 10 IT Powder Methods of Change of Form 5. Finishing of Sintered Parts (Secondary Operations) Densification and Sizing Repressing – the part is squeezed in a closed die to increase density and improve physical properties. Sizing - forcing the part through a finish die to provide dimensional accuracy Coining – pressing details into its surface Machining

Chapter 10 IT Powder Methods of Change of Form Infiltration and Impregnation - Because powder-formed parts can be very porous, other materials can be placed in the voids to enhance the properties of the product. Prevents moisture penetration Lowers frictional properties Infiltration – pores are filled with a molten metal Impregnation - Impregnating the sintered part with oil to create a “self lubricating” bearing.

Chapter 10 IT Powder Methods of Change of Form Advantages Wide range of mech. & phys. properties Parts made from high melting point metals High production rates on relatively complex parts Good dimensional control Impregnating and infiltration

Chapter 10 IT Powder Methods of Change of Form Limitations Size of parts, complexity of shapes of parts High cost of powdered metal compared to other materials High tooling cost for small production runs Lower strength and ductility than forging