Machinery's Handbook, 31st Edition
Powder Manufacturing Processes 1539 Joining: Larger parts and very complex shapes can be obtained by joining. Several tech niques exist for joining, such as diffusion bonding, sinter brazing, and laser welding. Infiltration: The interconnected porosity is filled with an alloy having a melting point lower than the sintering temperature of the metal of which the component is made, e.g., copper-based alloys infiltrate ferrous parts, usually during the sintering phase. Infil - tration makes the components impermeable, and there is some increase in mechanical properties, but at the expense of dimensional accuracy. Infiltration simplifies some heat treatments. For instance, it is easier to obtain a defined case depth without intercon - nected porosity. Impregnation : Oil or other fluid is permeated into the parts of a sintered PM part. Sintered parts achieve greater protection against corrosion by being impregnated by oil or another nonmetallic material such as polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure-tight part. Common products are gears and bearings. Self-lubricating bearings are manufactured by impregnating po- rous sintered bearings with lubricants; these bearings can only be produced by powder metallurgy. Liquid Phase Sintering.— Liquid phase sintering (LPS) is a subclass of the sintering process and can be defined as sintering involving a coexisting liquid and particulate solid during some part of the thermal cycle. The most common way to obtain the liquid phase is to use a system involving a mixture of two powder metals in which there is a difference in the melting temperatures between the metals. The interaction of the two powders leads to formation of a liquid during sintering. The melted metal thoroughly wets the solid particles, leading to rapid consolidation and giving rapid compact den- sification without the need for an external force. It is, of course, essential to restrict the amount of liquid phase in order to avoid impairing the shape of the part. Depending on the metals involved, prolonged heating may lead to diffusion of the liquid metal into the solid or the dissolution of solid particles into the liquid melt. In either case, the resulting part is fully dense (having no pores) and strong. Isostatic Pressing.— Isostatic pressing is generally used to produce large PM parts to near-net shapes of varied complexity. Unlike conventional PM, in which the powder is compacted uniaxially through direct contact with tooling, isostatic pressing confines the metal powder within a flexible membrane or hermetic metal or glass container, which acts as a pressure barrier between the powder and the pressurizing medium, whether liquid or gas, that surrounds it. The use of this pressurizing system ensures a uniform compaction pressure from all directions and the absence of die wall friction; it produces a fully homogeneous and uniform grain structure and density irrespective of the shape of the final parts. Other advantages of isostatic pressing are that parts with high length-to-diameter ratios have been produced with very uniform density, strength, toughness, and good surface detail; and that it is possible to compact much larger parts than are possible with other compacting processes. Limitations of isostatic pressing include wider dimensional tolerances than are caused by other compacting processes; greater cost and time than are required by other processes; applicability only to rela- tively small production quantities (fewer than 10,000). Two of the most commonly used methods are cold isostatic pressing and hot isostatic pressing. Cold Isostatic Pressing: In cold isostatic pressing (CIP), metal powders are contained in a flexible mold that is typically made of rubber or another elastomer material and pressed to the green compact by a fluid. Water or oil is usually used to provide hy - drostatic pressure against the mold inside the chamber. The most common pressure is 58,000 psi (400 MPa). As the pressure is isostatic, the resulting pressed component is of uniform density. In CIP processing, the part must be sintered after removal from the mold. Fig. 16 illustrates the processing sequence in cold isostatic pressing. The advantages of CIP include more uniform density, less expensive tooling, and greater applicability to shorter production runs, typically fewer than 10,000 parts per year.
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