Solidification, rapid melt spinning

The alloys are first produced by rapid solidification and are amorphous in nature. They are either directly fabricated as powders, by a process such as high-pressure gas atomisation (HPGA), or by melt-spinning of ribbons, which are subsequently pulverised to form a powder (<150 /im). The powders are then consolidated by hot extrusion between 950-1050°C where the initial amorphous structure breaks down and forms a fine dispersion of stable borides in a ductile Fe-based matrix. 

The polymerized product is an extremely insoluble material and must be melt-spun, as discussed later. Therefore, should a delustered or precolored fiber be desired, it is necessary to add the titanium dioxide or colored pigment to the polymerization batch prior to solidification. For ease of handling, the batch of nylon polymer may be extruded from the autoclave to form a thin ribbon, which is easily broken down into chips after rapid cooling. But, whenever possible, the liquid polymer is pumped directly to the fiber melt spinning operation (see Fig. 12.14). 

Rapid solidification processing (RSP), for example, by melt spinning onto a cooled copper drum, which is capable of retaining many alloys in a glassy (amorphous) state or in nanocrystalline form ... 

Transmet Corporation manufactures flakes by a Rapid Solidification Technology. There are two variations of this method Melt spin and Spinning cup methods. In the Melt spin method, molten metal of any composition (pure metal or alloy) is driven through an orifice and the shape formed in the orifice (continuous sheet) is rapidly cooled on a chilling block. This metal sheet is cut into segments in the form of flakes (square and rectangular), flat fibers, and ribbons of desired... 

Melts of metals as well as crystalline ceramic oxides have low viscosities. All solidify at a sharp melting point and their viscosity above the melting point increases rapidly and then reaches a viscosity comparable to that of motor oil at room temperature, i.e., log <0.2 poise. Yet, fiber formation from the liquid phase requires a viscosity of log 2.5 to log 3.0 poise. Only liquid droplets are formed if a low viscosity liquid is extruded at a normal quench rate of -10" K/s through an orifice, spinneret hole or bushing tip. To facilitate fiber formation, the viscosity must be raised from log <0.2 poise to log 2.5 to log 3.0 poise. This can be achieved by one of two generic routes, i.e., by a rapid solidification (RS) or an inviscid melt spinning (IMS) process. 

Rapid solidification and inviscid melt spinning suppress crystallization, which would otherwise occur with aluminate and YAG melts which are derived from highly crystalline materials. Fibers from liquefied gases, fibers from inviscid melt spun oxides and ribbons from rapidly solidified metal alloy melts are amorphous. Fibers spun from inviscid metal melts are predominantly amorphous but contain a minor crystalline phase. 

Commercial wire drawing processes produce metal wires with round cross sections but they are highly energy and labor intensive. Wire drawing falls outside the scope of this book. Commercial rapid solidification processes yield amorphous metallic ribbons. Inviscid melt spinning yields metal fibers by a chemically assisted jet stabilization process. 

A viable process for the formation of continuous, self-supporting fibers such as hydrogen from liquefied gases has emerged over the past two decades [74]. Like all prior process iterations [74], it appears to be an inviscid melt spinning process (IMS) and not a rapid solidification (RS) process. 

Commercial melt spinning processes involve substantial filament cooling, and control of the quench profile is one of the practical considerations for design and operation. Laboratory experiments are often designed to operate isothermally, however, typically by spinning in a temperature-controlled chamber with rapid solidification effected at a fixed position (by spinning into a water bath, for example) or by taking... 

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