Powder Methods of Change of Form

1. Powder Methods of Change of Form Chapter 10 IT 208

2. 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 IT 208

3. 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. IT 208

4. 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 IT 208

5. 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. IT 208

6. 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 IT 208

7. 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. IT 208

8. 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 IT 208

9. 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) IT 208

10. 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. IT 208

11. Powder Methods of Change of Form • 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 IT 208

12. Powder Methods of Change of Form • 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. IT 208

13. Powder Methods of Change of Form • 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. IT 208

14. 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 IT 208

15. Powder Methods of Change of Form • 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 IT 208

16. 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. IT 208

17. 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 IT 208

18. 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 IT 208