Multilayer, particularly bilayer, metal articles are used where no single metal meets the physical, chemical, or economic requirements of an application. Examples of such articles are containers that must be corrosion resistant or chemically inert, such as tubes for heat exchangers in corrosive service. Metals meeting these requirements, such as copper, gold, or platinum may lack the strength or be too expensive to be used alone. Combining these metals with a layer of stronger or less expensive metal such as steel is a way of providing strength or reducing cost. Various methods of joining such layers have been developed. Their suitability depends upon the use to which the article will be put. Extended high temperature use is especially severe in its demands on interlayer bonds.
Among the methods for combining metal layers, one of the more general is the use of loose liners, that is linings or inserts that are not bonded to the substrate metal. However, the absence of interlayer bonding adversely affects the efficiency of heat transfer through the layers of the article. Thus, loose liners are unsuitable for applications that require good heat transfer between the metal layers. Furthermore, because the loose liner is not bonded to the substrate metal, it is not supported against collapse, which may occur, particularly at high temperature or high flow conditions.
Where bonding between layers is desired, various ways of bonding are available. One such way is adhesive bonding using organic or inorganic adhesives. Such bonding techniques are limited by the temperature tolerance of the adhesive. In addition, the adhesive layer generally has poorer thermal conductivity than the metal layers it joins, and it thereby interferes with heat transfer through the joined layers.
Explosive cladding (Gold Bulletin, vol. 10, no. 2, pp. 34-37, Apr. 2, 1977) gives metal to metal bonding. In this method, an explosive is coated on one of the metal layers. On detonation, the explosive force drives the coated metal layer against the second metal layer and bonding is achieved. However, the bond is not always uniform in strength or coverage because the shock wave characteristic of explosions causes a variation in the impact pressure on the metals. For the same reason, the bonded interface may have a waviness and therefore a nonuniform thickness in the metal layer. Another drawback of this method is that the blast force can cause work hardening of the metal, which is not always desirable. Furthermore, explosive cladding is unsuitable when one of the metals lacks the strength to withstand the explosive force necessary to get an acceptable bond between the layers. Also, by its nature, this method imposes safety requirements in its application, and can be difficult to control.
Making bonded layered metal articles by rolling or pressing the layers together is known, but is suitable only for flat articles such as sheets. For articles that are not flat, rolling is often not possible, and pressing can be done only with tools that match the shape of the article, that is, dies. For larger articles and where a variety of sizes and lengths are to be made, this can be prohibitively expensive.
Electroplating is practical for the application of thin layers only, and not all metals can be electroplated. Weld overlay is limited to articles of shapes and sizes that permit access of the welding equipment. Coextrusion can be used only with metals for which the rheological properties are closely matched at the extrusion temperature. This need for matching rheological properties limits the combinations of materials that can be coextruded. Gas pressure bonding or hot isostatic pressing is used to bond metals. It is done in an autoclave, and temperatures of 1100-1700° C., pressures of 10,000-15,000 psi (70-100 MPa) are typical. This method is not well suited to applying a liner to the inner surface of a vessel or tube.
There is a need for non-flat uniformly metallurgically bonded layered articles of metals having dissimilar properties, and for a process for making such articles.