(1-5 ^m), the large size difference creates agglomeration of the particulates in the blend. Ultrasonic agitation can be used to break up the agglomerations. Dry blending is normally conducted in an inert environment. The blending stage is followed by cold pressing to produce what is called a green body, which is about 75-80% dense and can be easily handled.
The next step is hot pressing, uniaxial or isostatic, to produce a fully dense billet. The hot pressing temperature can be either below or above that of the matrix alloy solidus. The powder blend is loaded into a metal can and evacuated at 750-930° F to remove all air and volatiles. Evacuation times are long; times as long as 10-30 h are not unusual. Out-gassing removes adsorbed water from the reinforcement and matrix powder, chemically combined water, and other volatile species.10 After out-gassing, the can is sealed. If a vacuum hot press is used, the canning and sealing processes are not necessary and the powder can be loaded directly into the press. Hot pressing or hot isostatic pressing is normally conducted at as high a temperature as possible so that the matrix will be in its softest condition. Although liquid-phase sintering is normally not used due to grain boundary segregation and reinforcement degradation, some small amount of liquid phase allows the use of lower pressures. Another method is to consolidate the blend in a can to about 95% density by hot pressing, and then remove the can, and HIP to produce full density. Consolidation occurs by creep of the matrix material into the interstices between the reinforcement particles. When all the interconnected porosity is closed at about 90% density, diffusion closes the remaining porosity at triple points and grain boundaries.
PM processed billets are then normally extruded, forged, and/or hot rolled to produce useful shapes.
The PM technique generally produces properties superior to those obtained by casting and liquid metal infiltration (squeeze casting) techniques. Because no melting and casting are involved, the powder process offers several advantages. Conventional wrought alloys such as 2XXX, 6XXX, and 7XXX can be processed. A lower temperature can be used compared to casting. This results in less interaction between the matrix and the reinforcement, minimizing undesirable interfacial reactions, which leads to improved mechanical properties. The distribution of particulate or whiskers is usually better when the PM method is used than in casting processes. It is popular because it is reliable compared with alternative methods, but it also has some disadvantages. The blending step is a time-consuming, expensive, and a potentially dangerous operation. In addition, it is difficult to achieve an even distribution of particulate throughout the product, and the use of powders requires a high level of cleanliness; otherwise, inclusions will be incorporated into the product, with a deleterious effect on fracture toughness and fatigue life.
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