Machinery's Handbook, 31st Edition
1534 Powder Manufacturing Processes Hybrid atomization efficiently combines gas atomization with centrifugal atomiza - tion. The gas atomization breaks molten metal into pieces of several tens to several hun- dred micrometers in size using a gas jet. A rotating disk located beneath the spray is driven at high speed up to 66,000 rpm, and the molten metal is spread uniformly over the rotating disk using a gas spray flow, thereby forming a thin liquid film of 0.0004 in. (0.01 mm) or less in thickness. Then, the fine droplets are scattered from the edge of the rotating disk to result in a fine spherical powder. Powder Manufacturing Processes for Metal Parts The first modern PM product, the tungsten filament used in electric light bulbs, was developed in the early 1900s. It was followed by tungsten carbide cutting tool materials in the 1930s, automobile parts in the 1960s and 1970s, aircraft turbine engine parts in the 1980s, and, finally, parts made by powder forging, metal injection molding, and warm compacting in the 1990s. An outline of the manufacturing processes and operations involved in making metal powder parts is shown in Fig. 11.
Cold Compaction
Sintering
Metal Powders
Secondary Operations
Blending
Hot Compaction
Finished PM Product
Additives Lubricants
Fig. 11. Outline of Manufacturing Processes of Metal Parts. After metallic powder is produced, it can be converted to a solid in different ways. But there are three basic steps in producing PM components that are common to most tech niques. Whether the final result is sintered brass, sintered stainless steel, sintered bronze, or any other powder-based element, the three production steps are the same: 1) blending and mixing of the powders 2) compaction 3) sintering. Blending and Mixing of the Powders.— Blending and mixing is carried out for the fol lowing purposes: 1) powders made by various processes have different sizes and shapes, so they must be mixed to obtain uniformity; 2) powder of different metals and other mate rials can be mixed in order to impart special physical and mechanical properties and char acteristics to the PM parts; 3) lubricants can be mixed within powders to improve their flow characteristics (reducing friction between metal particles, improving flow of powder materials into the dies, and lengthening die life). Many of the alloy powders are available premixed—the powder already has the correct material characteristics. Additives, such as admixed lubricants, which coat each powder particle and reduce interparticle friction, usually are added to the mix. Mixing time depends upon the results desired, and overmixing should be prevented; otherwise the size of particles will be decreased, and they will be hardened. This makes the subsequent compacting operation more difficult. The mixing operation can be carried out in air-inert atmospheres or in liquids, which act as lubricants and make the mixture more uniform. Blending and mixing is done in a mixer device; some of these are schematically illus trated in Fig. 12; they include the horizontal rotating drum, the rotating double cone, the screw mixer, and the blade mixer.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online