Machinery's Handbook, 31st Edition
Powder Manufacturing Processes 1537 Pressing Force: The required force for pressing depends on the cross-sectional area of the PM part (area in the horizontal plane for a vertical press) and the pressure needed to compact given metal powders. This force may be calculated by the following formula: (11) where F = required force, lb (N) A p = cross-sectional area, in 2 (m 2 ) p c = compaction pressure, psi (MPa) Compaction pressures typically range from 1007 to 116030 psi (69 MPa to 800 MPa); however, 20305 to 60191 psi (140 to 415 MPa) is the most common range. Table 1 shows compaction pressures for various metal powders. Table 1. Compaction Pressure for Various Metal Powders F A p p c =
Compaction Pressures psi
Material
MPa 40–70 70–200 145–350 275–690 250–275
Porous metals and filters Refractory metals and carbides
5802–10153 10298–29008 21030–50763 39885–100076 36259–39885 100076–239312
Porous bearings
Machine parts (medium-density iron and steel) High-density copper and aluminum parts High-density iron and steel parts
690–1650 Most conventional PM compacting is done with mechanical presses, although hydraulic and hybrid (combinations of mechanical, hydraulic, or pneumatic) presses are often used. The mechanical presses may be of the eccentric or crank type. Most presses have capacities of less than 100 tons. Mechanical presses with capacities of the magnitude of a few hun dred tons are usually adequate for most PM processing operations. Hydraulic presses with a capacity of several thousand tons are sometimes used for compacting parts that require more force. Double action presses are commonly used; for more complex parts, multiple action presses may be employed. Punch speed must be regulated. Faster compaction of the workpiece can result in higher productivity; however, if the punch speed is too high, air may become trapped in the parts and prevent the parts from compacting correctly. Die: Punch and die surfaces are very important in the powder compacting operation. Some clearance between the punch and die must exist in order for the punch to move within the die. However, in a die with a large clearance, powder particles can become stuck in this clearance, causing problems with the proper movement of the punch. In order to prevent this problem, clearance is designed to be extremely low, typically less than the size of the powder particle. Most punches and dies are made from hardened tool steels, and the surfaces of the die and punch in the direction of the punch movement are grounded and polished or lapped. Sintering.— Sintering is the final step in the PM process. Green powder compacts are heated in a controlled-atmosphere furnace so that adjacent particles fuse together, thus resulting in a solid part with improved mechanical strength compared to the powder com pact. The nature and strength of the bond between the particles and the sintered compact depend on the mechanisms of diffusion, the plastic flow, the evaporation of volatile mate rials in the compact, the recrystallization, the grain growth, and the pore shrinkage. The main variables in sintering are: • temperature • time • furnace atmosphere.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online