Metals, bonded joints

Note In the case of metal bonded joints, the resistance to moisture and climate can decisively be influenced by the kind of surface pretreatment apart from the adhesive selection, especially in the areas adjacent to the adhesive surface (e.g., primer, sealing of gluelines). 

H. D. Steffens and W. Brockmann, Aging resistance of light metal bonded joints using new surface treatment methods. Royal Aircraft Estab. Library Trans. No. 1662 (1972). 

Conventional X-ray techniques are of little use on metal-to-metal bonded joints since the polymeric adhesive is much less dense than the adherends. Metallic fillers can be used to enhance the contrast and show tapering or voids. However, the density of fibre reinforced plastics adherends is of a similar order to the adhesive and so X-rays can be used, by choosing a suitable energy and flux. For honeycomb-cored panels. X-rays are used for checking the position of the core and whether it has been locally crushed or otherwise damaged. 

Evaluation of Bonding Process in Diffusion Bonding Joints of Dissimilar Metals using Ultrasonic Testing Method. 

The practical result of epitaxy is a very high degree of adhesion between coating and substrate. The force needed to separate the interface is similar to that needed to break the metals on either side. Where a true metallic bond forms at an epitaxial interface it is only possible to measure adhesion if the bond is the weakest of the three near the interface. An adhesion test based on breaking the joint indicates only which of the three is weakest. For practical purposes any epitaxial joint will have a strength more than adequate for service conditions. 

Transition joints are used to join dissimilar metals where flanged, screwed, or threaded connections are not practical. They are used when fusion welding of two dissimilar metals forms interfaces that are deficient in mechanical strength and the ability to keep the system leak-tight. Transition joints consist of a bimetallic composite, a stainless steel, and a particular kind of aluminum bonded together by some proprietary process. Some of the types in use throughout the cryogenic industry are friction- or inertia-welded bond, roll-bonded joint, explosion-bonded joint, and braze-bonded joint. 

Wilkinson s catalyst after its discoverer, G. Wilkinson. In 1973, the Nobel Prize in chemistry was awarded jointly to Wilkinson and E. O. H. Fischer for their respective contributions to the field of organometallic chemistry. As you will see in this and later chapters, compounds with carbon-metal bonds (organometallic compounds) are extremely useful reagents, reactive intermediates, or catalysts in organic reactions. To a very large extent, the work of Fischer and Wilkinson created the current interest and developments in the field of transition-metal organic chemistry, which will be discussed in Chapter 31. 

When a corrosive medium contacts the edge of a bonded joint and finds an extremely active surface, such as that produced by a fresh acid treatment of the metal substrate to improve adhesion, corrosion at the metal-adhesives interface can occur. This initial corrosion and the subsequent penetration can take several forms. 

Some primers will improve the durability of the joint by protecting the substrate surface area from hydration and corrosion. These primers suppress the formation of weak boundary layers that could develop during exposure to wet environments. Primers that contain film-forming resins are sometimes considered interfacial water barriers. They keep water out of the joint interface area and prevent corrosion of the metal surfaces. By establishing a strong, moisture-resistant bond, the primer protects the adhesive-adherend interface and lengthens the service life of the bonded joint. 

Olson, W. Z., et al., Resistance of Adhesive Bonded Metal Lap Joints to Environmental Exposure, Report No. NADC TR59-564, October 1962. Also in DeLollis, N. J., Durability of Structural Adhesive Bonds A Review, Adhesives Age, September 1977. 

In selecting a pretreatment process for aluminum or any other substrate, both the initial strength and the permanence in a specific operating environment must be considered. Mechanical abrasion is a useful pretreatment in that it removes the oxide and exposes bare aluminum. When this is done, however, many of the benefits of the protective oxide layer are lost. For example, if bare abraded aluminum is bonded, the reactive metal at the joint interface can potentially become hydrolyzed and oxidized, which will displace the adhesive. Hence, this bonded joint may initially be much stronger than one made with unabraded metal, but it will deteriorate rapidly when exposed to a harsh environment such... 

Parallel to the development of the chemistry of dinuclear transition metal complexes with metal-metal bonds, studies related to bimetallic complexes, hetero- and homonuclear, without metal-metal bonds are appearing in the literature at ever increasing frequency (10-13). In these complexes, two or more metals are separated by a common ligand which acts as a bridge between them. Two metal centers acting in a joint fashion could enhance the activation of an organic substrate considerably and in... 

The adhesive components can also be produced as layers on films or textiles or without carrier materials if they form solid films at room temperature. For this purpose, they are applied warm, in thin layers, or in solute form, to foils and textiles and the carrying materials (preferably made of glass fibers, carbon fibers, or metal netting) are integrated into the bonding joint after the thermosetting process. The adhesive films frequently contain fillers made of A1 oxide, Cu, Ag, Al, etc. 

Double-sided bonding This refers to the actual contact bonding that has to be applied whenever solvent-impermeable or dense materials (metals, glass, plastics) are to be bonded or when the bonded joint is required to show very high initial strength. Absorptive materials may require a second adhesive application. 

In contrast to the young adhesives on synthetic bases, adhesives deriving from natural products have partly been known for millennia. The essential differences compared to reactive adhesives are the partially poor ageing stabilities in humid atmosphere as well as the low bonding strengths. They are not used for highly strained bonded joints of metals, plastics, glasses and similar. However, they record... 

Bonding of metal materials In the case of bonding with hot-melt adhesives, metal materials require preheating to the melt temperature to develop sufficient adhesive bonded joints, due to the high thermal conductivity. 

Most metals show a low deformability in comparison to nonmetal materials. For bonded joints this property means that adhesive layers, when exposed to mechanical stress (tension, shear, pressure, bending, torsion), are subjected to deformation stress only to the same extent. 

For the production of bonded joints with metal materials, appropriate surface pretreatment is of priority. In the technical literature, various formulations of pickling solutions are to be found, their application, however, is limited for reasons of occupational safety and due to the disposal problem. Therefore, we refrain from describing them here. 

Base metal, that is, if stored, layers of different chemical compositions (oxides, hydroxide, oxydhydrates, carbonates) cover the surfaces their adhesion to the base metal does not then guarantee sufficient strength for a bonded joint. Mechanical surface pretreatment is also required. 

The diffusion bonding method described in Section 9.2.5 cannot be applied to the bonding of plastics to metals, since the metal surfaces are not swellable by means of organic solvents. Bonded joints are only possible after an adequate surface treatment with the known reactive adhesives. 

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