Joining metal components

Braze fluidity can be assured only if the joining temperature is higher than the braze liquidus. The viscosities of liquid metals are relatively low, only a few mPa.s, Table 10.1, and decrease slowly as the temperature is raised. A dramatic increase in viscosity can occur, however, if the temperature drops below the 

While the use of pure metals and eutectic alloys as brazes is generally good practice, exceptions do occur. Thus it may be necessary to use brazes that deviate from eutectic compositions to minimise the concentration of an embrittling temperature depressant, or to complicate the melting characteristics and accept degraded fluidity by introducing elements that enhance the corrosion resistance of the solidified braze. Thus recommended Al-Si brazes have hypo-eutectic compositions and Cr is introduced into Ni-P and Ni-Si-B braze alloys. 

The capillary attraction exerted by clean metal surfaces on metallic brazes is high. Indeed the contact angles of liquid metals on clean metal substrates is invariably less than, and usually much less than, 90° even for chemically inert and mutually insoluble systems, as discussed in Chapter 5. 

Of even more practical importance in determining the behaviour of systems is the tendency of many component materials, and some brazes, to form chemically stable and physically tenacious oxide films when their surfaces are exposed to even mildly oxidising environments. (Note that these oxides are in turn covered by absorbed layers of organic contaminants, but these can be removed relatively easily by washing in solvents, vapour degreasing or baking in a vacuum). Oxide films can 

The fluxes used when brazing are alkali halide and borate mixtures and compounds, and they have two main functions first, to dissolve the oxide film on the component surface or at least to degrade its adhesion by penetration of naturally occurring flaws and electrolytic action at the oxide-substrate interface, and secondly to prevent formation on the liquid surface of oxide skins which would restrict braze flow. Fluxes can be contained in a bath held at the brazing temperature in which the, usually aluminium, component is placed or else applied as a paste to surfaces of the component or braze. 

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This special class of brazes reacts chemically with the surfaces of ceramic components to produce wettable products with metallic characteristics, such as TiO, TiC x or TiN x as described in Sections 6.3 and 7.2. Thus the wetting is due to an in situ metallization . By definition, the brazes must contain chemically reactive elements such as Ti that are often added to eutectic brazes similar to those developed for joining metal components. Many sessile drop experiments have shown that active metal brazes can wet a wide range of ceramics when a suitable inert environment is used. Particularly high standards of environmental inertness... 

Why are special precautions necessary when joining ceramic components to metal components What methods are available for the satisfactory joining of ceramics to metals ... 

Mass transfer This phenomenon manifests itself as the physical transport of a metal from one portion of the system to another, and may occur when there is an alloy compositional difference or a temperature gradient between parts of the unit joined by the flowing liquid phase. An exceedingly small solubility of the metal component or corrosion product in the molten metal or salt appears sufficient to permit mass transfer to proceed at a fairly rapid pace. 

Cadmium alloyed with silver forms a type of solder with a low melting point. It is used to join electrical junctions and other specialized metallic components. Precautions are required... 

Most complete insulators comprise metal components cemented to the porcelain insulator to join it to the line structure, or to build up a string of insulators. Various cements are used depending on application, although conventional Portland cement is most common. Galvanized malleable cast iron is preferred for the metal since its thermal expansion is closer to that of porcelain than most metals and it is comparatively inexpensive. None the less, the cost of the metal components when used is often 40-80% of the cost of the insulator. The form of the widely used rod-insulator is shown in Fig. 5.17. 

In high-frequency induction sealing a metallic component is located at the point of joining in such a manner as not to inhibit the flow of molten material the inset is designed to ensure a flow of material sufficient to give a seal of the strength required. The method is used mainly for the assembly of moulded components, or for the sealing of plastic bottles in instances where a tamper-evident hermetic seal is needed. 

It is a technique suitable for automatic equipment since there is no contact between the heating device and the materials to be sealed, there are none of the problems of sticking that can arise with sealing dies or electrodes. On modern machines, even with relatively thick-walled sections, sealing times can be as little as a fraction of a second. The method is ideal for work in which thermoplastics and metal components have to be joined—as in the assembly of medical syringes and the sealing of tinplate cans. 

JOINING CERAMIC COMPONENTS CERAMIC-CERAMIC AND CERAMIC-METAL JOINTS... 

The same capillary phenomena affect brazing practice for joining both ceramic and metal components, but the relative importance of the phenomena differs, and this makes it convenient to discuss their effects in a different sequence. Further, most joining of ceramics is to metals and the different thermal expansion and mechanical characteristics of these two families of materials, as exemplified in Table 10.4, have a profound effect on joint design that is not related to capillarity. 

By definition, brazes have a different composition from the components they are used to join and hence interdiffusion will occur during and after interface creation. Reference has been made already to the detrimental effects of the growth of thick reaction products at metal-ceramic interfaces and similar effects can occur with metal-metal systems. Thus it is not good practice to use A1 brazes for the joining of steel or Cu components or to use Ni brazes containing Si for the joining of refractory metal components because of the rapid formation of fragile layers of intermetallic compounds. 

At the present time, the most likely concept of the mechanism of a heterogeneous polymerization catalyzed by a Ziegler-Natta catalyst involves a complex in which the organometallic component and the transition metal component—i.e., the A1 and Ti atoms—are joined by electron-deficient bonds. Natta, Corradini, and Bassi (13) have reported such a structure for the active catalyst prepared from bis (cyclopentadienyl) titanium dichloride and aluminum triethyl. Natta and Pasquon (14), Patat and Sinn (18), and Furukawa and Tsuruta (2) have proposed mechanisms for the stereospecific polymerization of a-olefins in terms of such electron-deficient complexes. 

NijAl and NiAl can be produced through a termite reaction between nickel and aluminium at a relatively low temperature. Nickel aluminides were produced between the zirconia and the metallic component to achieve the vacuum-tight joining. The morphology, composition, and density of the metal-zirconia joint have been optimized to minimize the GTE mismatch at the metal-ceranuc interface, and they are worth considering in detail as they could be used for sensor applications at temperatures up to 1000°C. 

Two types of pipe joint are required for cable installations. One type is used to join connecting lengths of GRE pipe the second type joins GRE piping to such metal components as expansion bellows, cable splice boxes, and pothead terminations. The development of each type of joint was undertaken in this program, and results are described here. 

The current steel wheel is composed of two components, the disc and the rim, joined together by force fitting one into the other and then welded, as schematized in figure 12. Welding is a classic method to join steel components together, but often it is not possible to weld two different metals or joining metals to ceramic, composite or polymeric materials. The exploitation of new... 

The various options for sealing and joining the ceramic and metal components in pSOFCs can be broadly classified into one of three categories rigid bonded seals, compressive seals, and bonded compliant seals. As outiined in Table 11.2, rigid... 

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