Production, forming and joining of metals

Increasing use is now being made of alternative processing routes. In powder metallurgy the liquid metal is atomised into small droplets which solidify to a fine powder. The powder is then hot pressed to shape (as we shall see in Chapter 19, hot-pressing is 

It is not our intention here to give a comprehensive survey of the forming processes listed in Fig. 14.1. This would itself take up a whole book, and details can be found in the many books on production technology. Instead, we look at the underlying principles, and relate them to the characteristics of the materials that we are dealing with. 

Many of these problems can be solved by using continuous casting (Fig. 14.3). Contraction cavities do not form because the mould is continuously topped up with liquid metal. Segregation is reduced because the columnar grains grow over smaller distances. And, because the product has a small cross-section, little work is needed to roll it to a finished section. 

During firing the wax burns out of the ceramic mould to leave a perfectly shaped mould cavity. 

We get a quite different answer if we include the friction between the die and the forging. The extreme case is one of sticking friction the coefficient of friction is so high that a shear stress k is needed to cause sliding between die and forging. The total area between the dies and piece c is given by 

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Eabrication techniques must take into account the metallurgical properties of the metals to be joined and the possibiUty of undesirable diffusion at the interface during hot forming, heat treating, and welding. Compatible alloys, ie, those that do not form intermetaUic compounds upon alloying, eg, nickel and nickel alloys (qv), copper and copper alloys (qv), and stainless steel alloys clad to steel, may be treated by the traditional techniques developed for clads produced by other processes. On the other hand, incompatible combinations, eg, titanium, zirconium, or aluminum to steel, require special techniques designed to limit the production at the interface of undesirable intermetaUics which would jeopardize bond ductihty. 

Aluminum alloys are commercially available in a wide variety of cast forms and in wrought mill products produced by rolling, extmsion, drawing, or forging. The mill products may be further shaped by a variety of metal working and forming processes and assembled by conventional joining procedures into more complex components and stmctures. 

Ultrasonic head forming and welding is a fast assembly technique. It is a very rapid operation of about 2 seconds or less and lends itself to full automation. In this process high-frequency vibrations and pressure are applied to the products to be joined, heat is generated at the plastic causing it to flow, and, when the vibrations cease, the melt solidifies. The heart of the ultrasonic system is the horn, which is made of a metal that can be carefully tuned to the frequency of the system. The manufacture of the horn and its shape is normally developed by the manufacturer of the equipment. The results from this operation are not only economical, but also most satisfactory from a quality control standpoint. 

A different type of bridging occurs in hydrolysis complexes of tho-rium(IV) (219) and uranium(IV) (130). Here a distinct peak at 3.94(2) A in the hydrolyzed solutions can be ascribed to the metal-metal distances in the hydrolysis complexes. Discrete dinuclear complexes with a very similar metal-metal distance, 3.988(2) A, in which the metal atoms are joined by double hydroxo bridges have been found in crystals ofTh2(OH2)(N03)6(H20)8 (229). The same type of bridging, therefore, must occur in solution. When hydrolysis is increased, however, the number of metal-metal distances per metal atom increases beyond a value of 0.5, valid for a dinuclear complex, and larger hydrolysis complexes are obviously formed. These structures are unknown but an extensive X-ray investigation of highly hydrolyzed thorium(IV) solutions has shown that there is probably no close relation between the structures of the hydrolysis complexes in solution and the structure of thorium dioxide, which is the ultimate product of the hydrolysis process (230). 

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