Advanced Technology--Rapid Solidification

Rapid solidification is a metal forming process that spews molten, atomized metal at a rotating substrate to form a metal ingot or billet. The high solidification rate of the process results in a well-defined microstructure. The process produces material similar to that achieved by powder metallurgy at a fraction of the cost. Billets or bars of different shapes are possible, depending on the substrate shape. The process is applicable to a wide variety of alloys.
Due to their unique properties, metals have been manipulated by mankind for thousands of years in the making of implements, tools, vessels, and various pieces of equipment or weapons. Modern tools and instruments require metal parts that fit together tightly and move smoothly against each other. Additionally, these precision parts must exhibit high strength and hardness, functions of the microstructure of the solid metal.
By careful control of the rapid solidification process, the droplet size, shape, and composition can be manipulated to create desired finish properties. For example, carbides, carbon-metal compounds, are a significant factor in the final hardness of steel used in high-stress environments, such as in space or underwater. In microscopic examination of cast steel sheets, carbide crystals are seen as long chains, increasing the brittleness of the final product. Most of these carbides are chromium compounds. In similar spray-formed steel sheets, the carbides are smaller, evenly distributed, and are compounds of vanadium, a stronger metal.
Rapid solidification is less expensive than powder metallurgy. In this alternate technology, the powder is first prepared from one of several techniques. Then the powder is blended, compacted in a die, and sintered in a furnace. The sintering allows a slow coalescence of the particles as they melt and run together. The grain growth is microscopic. Powder metallurgy produces very dense, strong materials.
Rapid solidification products are also very dense, but require two processing steps. The alloy is melted, atomized through a nozzle, and impacts the substrate. By cooling the nitrogen gas that is blown counter-current to the droplets, the droplets are kept separate and distinct, but cool faster. Segregation of the alloy’s metals is delayed by not allowing the droplets to mix. The droplets are partially molten when they hit the substrate; in powder metallurgy, the metal is in solid particulate form.