Machinery's Handbook, 31st Edition
Powder Metallurgy Materials 1547 The hot isostatic process has also been used in the production of PM tool steels to near- net tool shapes, such as hobs and shaper cutters. Tool steel parts can also be made by con ventional cold compacting combined with high-temperature sintering. Refractory Metals: Refractory metals are a class of metals that are extraordinarily resis tant to heat and wear. The expression is mostly used in the contexts of materials science, metallurgy, and engineering. The definition of the elements that belong to this group dif fers. By one definition, a melting point above 4000°F (2200°C) is necessary to qualify an element as refractory. Five elements, niobium, molybdenum, tantalum, tungsten and rhenium, are included in all definitions. The refractory metals, alloys, and composites are produced using powder metallurgy techniques. Powders of the pure metal are compacted into rectangular bars, sintered by heating using electric current, and further fabricated by cold-working with annealing steps. Refractory metals can be worked into wire, ingots, sheets, or foil. Some of their applications include tools for working metals at high temperatures, casting molds, chemical-reaction vessels for corrosive environments, nuclear reaction control rods, tantalum capacitors, and tungsten-silver composite circuit breaker contacts. Refractory met- als are stable against creep deformation to very high temperatures. Beryllium: As a free element, beryllium is a steel-gray, strong, lightweight and brit- tle metal. Beryllium increases hardness and resistance to corrosion when alloyed with aluminum, cobalt, copper, iron, and nickel. Powder metallurgy is the main processing route to produce beryllium components. The vacuum hot processing method results in a fine-grained machinable form of the material. The random orientation and fine grain size of powder metallurgy ingots enable beryllium to be used in structural applications with relatively uniform mechanical properties in all directions. Vacuum hot pressing has been a standard consolidation technique throughout the devel- opment of beryllium technology, and hot isostatic pressing powder is the major form of beryllium in use today. Sometimes, hot isostatic pressing is substituted for the hot pressing step. Due to its light weight, beryllium is used in structural applications; its high flexural rigidity, thermal stability, thermal conductivity, and low density make beryllium a qual- ity aerospace material for high-speed aircraft, missiles, spacecraft, and communication satellites. Because of its low density and atomic mass, beryllium is relatively transparent to X-rays and other forms of ionizing radiation; therefore, it is the most common window material for X-ray equipment and in particle physics experiments. The high thermal con- ductivities of beryllium and beryllium oxide have led to their use in heat transport and heat sinking applications. Beryllium is used commercially in the manufacture of telecommunications infrastruc ture equipment, computers, and cellular phones, thereby helping people around the world to keep in touch. Also, medical lasers made with beryllia ceramics help ophthalmologists to restore or improve eyesight for millions. Beryllium ceramic is the only material that offers the thermal conductivity, strength, and dielectric properties required to contain and control these tiny, high-powered gas laser bores. Copper beryllium connectors transmit precise electrical signals to the delicate surgical instruments and monitoring devices used in the newest, non-invasive surgical techniques. Such techniques reduce patient trauma and infection risk, while speeding the process of healing and recovery. Titanium: Titanium alloys are among the most important of the advanced materials that are key to improved performance in aerospace and conventional systems. This is because of the excellent combination of specific mechanical properties and outstanding corrosion behavior exhibited by titanium alloys. However, discouraging widespread use is the high cost of titanium alloys compared to competing materials. This has led to numerous inves tigations of various potentially lower cost processes, including powder metallurgy (PM). Titanium powder metallurgy can be divided into categories such as powder injection mold- ing (use of a binder to produce complex small parts); near-net shaping (use of pre-alloyed
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online