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Mechanical alloying is a powder metalworking technique that crushes base materials into a fine powder form in a ball mill, followed by cold welding. This process creates designer superalloyed metals with desirable physical properties used in spacecraft and satellite equipment. Cold welding involves placing pre-alloy component powders together under extreme pressure of a vacuum and heating them to high temperatures to create molecular bonds without melting the metal. The use of hot isostatic pressure (HIP) ensures a uniform grain size throughout the alloy and produces heat-resistant metals and super alloys.
Mechanical alloying is a powder metalworking technique used to create alloy metals from elemental powders and pre-alloys. This two-part process is accomplished by crushing the base materials in a ball mill, followed by cold welding. The mechanical alloying process can produce designer superalloyed metals that inherit desirable physical properties of the various elemental powders used in their creation. Thanks to this ability, the process has been used to create superalloy metals used in the construction of spacecraft and satellite equipment.
The first stage of the process is called fracturing. This essentially involves crushing the metals or pre-alloyed elements into a fine powder form. By crushing the base materials into such a powder, better molecular bonds are formed in the subsequent cold welding process. To carry out the fracturing task, a special type of industrial grinder is used.
The ball mill is a barrel-shaped rotating grinder that works in the same way as a rock tumbler. In mechanical alloying, cold welded metals are inserted into the barrel along with the grinding media. While grinding media can vary, common options include flint pebbles and ceramic or stainless steel balls. When the ball mill is activated, the barrel begins to rotate and rolls the combination of metal and grinding material inside. As the medium cascades, it reduces the metal to a powder to prepare it for the next stage of the process.
The second part of the mechanical alloying process is cold welding. This involves placing the pre-alloy component powders together under the extreme pressure of a vacuum. The metal powders are then heated to high temperatures at which a process called sintering occurs and molecular bonds can form. These high temperatures combined with the vacuum pressure essentially melt, or weld, the metal powder without the need to melt it.
Cold welding is so called because, unlike traditional welding, metals never reach a molten stage. Instead of melting metals to achieve fusion, cold weld metal is heated to less than a degree away from its melting temperature using hot isostatic pressure (HIP). This temperature is called the recrystallization point. The use of HIP in the mechanical alloying process aids in the creation of a uniform grain size throughout the alloy. It also produces heat resistant metals and super alloys. The use of HIP ensures that any molecular stress from the cold welding process is negated.
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