Machinery's Handbook, 31st Edition
1548 Powder Metallurgy Materials and blended elementals in die pressing or isostatic pressing); spraying (high-speed project- ing of powder onto a substrate); and laser melting (powder feed or powder layer melted with a laser. Titanium PM alloys have a combination of low density 2.6 oz/in 3 (4.5 g/cm 3 ) and high strength. The strengths vary from 70,000 psi (480 MPa) for some grades of commercial titanium to about 160,000 psi (1100 MPa) for structural titanium alloy products and over 250,000 psi (1725 MPa) for special forms such as wires and springs. Another important characteristic of titanium-based materials is the reversible transformation of the crystal structure from alpha (hexagonal close-packed) structure to beta (body-centered cubic) structure when the temperature exceeds 1634°F (890°C). The use of PM titanium metal and its alloys continues to increase since PM provides a near-net shape product as compared to more expensive cast, forged, and machined pro cessing. Depending on the application and production quantities, titanium PM near-net products are processed either by conventional press-and-sinter techniques or by hot isos tatic pressing. The latter exceeds the minimum wrought alloy specifications. If the final product is made net-shape by application of hot isostatic pressing (HIP), a lack of texture can result, thus giving equal properties in all directions. Product applica- tions include aerospace/aircraft components, sporting goods, chemical processing equip- ment, and biomedical systems. Design Considerations in Powder Metallurgy The powder metallurgy manufacturer is often confronted by a drawing for a component designed with a conventional manufacturing process in mind. It is not desirable for the powder metallurgy component manufacturer or designer to attempt to follow these draw ings; it is far better to redesign the component so that it can fulfill its design function, while taking full advantage of the powder metallurgy process, in particular the cost-effective manufacture to near-net shape with close dimensional tolerances. The PM part and the complexity of tooling used to produce it are closely related. Two major factors in the compacting operation influence or control part design: the flow char acteristic in the die and the degree of brittleness of the green parts. Although powder metallurgy industry standards (MPIF Standard 35) provide useful physical and mechanical property design data for engineers familiar with the PM process, those less experienced with this manufacturing process may benefit from additional guid ance. In this section, design considerations are divided into three broad areas: size and shapes, design details, and design of typical parts. Sizes and Shapes.— Part Sizes: The size limitation of PM parts is based on powder com pressibility and press tonnage. Since compaction occurs in the vertical direction, using only top and bottom motions, part lengths in the pressing direction are limited. The com pression ratio—the ratio of the height of the loose powder filling the die to the height of the compacted part—also tends to limit vertical part lengths. A length-to-wall thickness ratio greater than 8:1 is difficult to press, and density variations are virtually unavoidable. Shapes: The shape of the part must permit ejection from the die. Part geometry must be compatible with a uniaxial compaction motion in the vertical direction. Significant varia tions in part length in the cross section require different tooling motions in the compaction press. Cam, gears, and sprockets are readily made. The shape of the part should permit the construction of strong tooling. The part should be designed with as few changes in section thickness as possible: the number of levels in the part should not exceed the number of pressing actions available in the compacting press. With too many levels, density varies considerably and part quality becomes a problem. Face forms on upper or lower punches can provide bosses, pads, lettering, countersinks, and other features. Parts weighing a few grams to about 26 lb (around 12 kg) or more are possible.
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