Mechanical alloying is a metallurgical powder processing technique used to create alloyed metals from elemental and pre-alloy powders. 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 super alloy metals that inherit desirable physical properties of the various elemental powders used in their creation. Due to this ability, the process has been used to create super alloy metals used in the construction of spacecraft and satellite equipment.
The first stage of the process is called fracturing. This essentially involves crushing the pre-alloy metals or elements into a fine powdered form. By crushing the base materials into such a powder, better molecular bonds are formed in the cold welding process that follows. To accomplish the task of fracturing, a special type of industrial grinder is used.
The ball mill is a rotating barrel-shaped grinder that functions in the same way as a rock tumbler. In mechanical alloying, the cold-welded metals are placed in the barrel along with grinding media. While the grinding media may vary, common options include flint pebbles and ceramic or stainless steel balls. When the ball mill is activated, the barrel begins to rotate and tumbles the metal and grinding media combination inside. As the media cascades and falls, it crushes the metal into powder to prepare it for the next step 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 pressure of the vacuum essentially fuse, or weld, the metal powder together without the need to melt it.
Cold welding is so named because, unlike traditional welding, the metals never reach a molten stage. Instead of melting the metals to achieve fusion, the cold welded metal is heated to less than one degree away from its melting temperature using hot isostatic pressure (HIP). This temperature is called the point of re-crystallization. The use of HIP in the mechanical alloying process helps 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.
