Production of Metallic Powder Machinery's Handbook, 31st Edition
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In principle:
(10) The issue is complicated by the possible existence of closed pores in some of the parti cles. If internal pore volumes are included in the above porosity value, then the equation is exact. Production of Metallic Powder A powder can be defined as a finely divided particulate solid. It can be produced in the form of sponges or atomized powders. Generally, any metal can be made into a powder. There are three basic methods for producing metallic powder: atomization, chemical, and electrolytic. Each method can be further subdivided, and powders can also be made by various combinations from each group. As a result, there is a wide range of production routes available, each producing powders with their own particular characteristics. Which powder production process is used depends on the required production rate, the desired powder properties, and the properties desired in the final part. Chemical and elec trolytic methods are used to produce high-purity powders. Mechanical milling is widely used for the production of hard metals and oxides. Atomization.— Atomization involves either spraying or smashing molten metal into smaller particles. It is the most versatile method for metal powder production. The main operational pa- rameters and physical conditions for a suitable atomization process for different appli cations in powder production are the following: • melting (surface tension, viscosity, and temperature range–solidification temperature) • process type (aimed throughput, energy efficiency) • product type (particle size distribution). Atomization is the dominant method for producing metal and pre-alloying powders from aluminum, brass, iron, low-alloy steel, stainless steel, superalloy, titanium alloy, and other alloys. Atomization is the best method because it yields high production rates and favors economies of scale, and because pre-alloying powders can only be produced by atomization. There are many different processes employed industrially for atomization, several of which are: Water Atomization: In water atomization (Fig. 5) the raw material is melted, and then the liquid metal is broken into individual particles. To accomplish this, the melt stock, in the form of elemental, multi-element metallic alloys, is melted in an induction, arc, or other type of furnace. After the metal is molten and homogenous, it is transferred to a tundish, which is a reservoir used to supply a constant, controlled flow of metal into the atomizing chamber. As the metal stream exits the tundish, it is struck by a high-velocity stream of the water. The water is sprayed at pressures that range from 145 to 1450 psi (1 MPa to 10 MPa). The molten metal stream is disintegrated into fine droplets, which solidify and cool dur - ing their fall through the atomizing tank. Particles are collected at the bottom of the tank. Water atomization cools the metal very rapidly, and the process can cause the formation of irregular particles. It can also corrode some metals. • water atomization • gas/air atomization • centrifugal atomization. Porosity Packing factor + = 1
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