A solid-state welding process that produces a weld by the application of pressure at elevated temperature with no macroscopic deformation or relative motion of the workpieces. A solid filler metal may be inserted between the faying surfaces take up gaps or facilitate the diffusion process.
This is a process in which two absolutely clean, perfectly matched, metal (or ceramic or intermetallic) surfaces are placed in contact and heated, but not to the
melting point. As a result of the heating, the diffusion of atoms in each direction across the interface will interlock the two atomic structures so that they become one, eliminating the interface.
Temperature is a very important factor in diffusion welding. Pressure may be of secondary importance, as long as intimate contact is maintained throughout the solid state diffusion process.
One of the difficulties in diffusion welding is that the surfaces to be joined are seldom, if ever, perfectly clean or perfectly matched. All metal and many inter- metallic surfaces, no matter how carefully finished, have surface irregularities and become covered with an oxide film or other tarnish layers when exposed to air. Ceramic surfaces are normally free of hindering tarnish layers. Other surface materials may also be present, such as oil, grinding compounds or cleaning chemicals. A weakened or defective bond is sometimes the result.
Applications
A wide variety of similar and dissimilar metal combinations may be successfully joined by diffusion welding and brazing. Most applications involve titanium, nickel, and aluminum alloys, as well as several dissimilar metal combinations. The mechanical properties of the joint depend on the characteristics of the base metals. For example, the relatively low creep strength and the solubility of oxygen at elevated temperatures contribute to the excellent properties of titanium alloy diffusion weldments.
Several industries use the diffusion welding process to advantage, particularly the aerospace industry. The engine mount of the space shuttle vehicle was designed to have 28 diffusion welded titanium parts, ranging from large frames to interconnecting box
tubes. This structure is capable of withstanding three million pounds of thrust. Tubes 203 mm (8 in.) square were fabricated with diffusion welding in lengths up to 457 cm (180 in.). The gas turbine industry has used diffusion welding to produce a Ti-6%A1-4%V component for an advanced high-thrust engine. This application marked the first production use of diffusion welding in a rotating engine component.
Diffusion Welding Principles
Metal surfaces have several general characteristics:
(1) roughness, (2) an oxidized or chemically reacted and adherent layer, (3) other randomly distributed solid or liquid (oil, grease, and dirt), and (4) adsorbed gas or moisture, or both. Two necessary conditions must be met for a satisfactory diffusion weld:
(1) Mechanical intimacy of faying surfaces
(2) The disruption and dispersion of interfering surface contaminants to permit metallic bonding.
A diffusion weld is formed in three stages. In the first stage, deformation of the contacting surface roughness occurs primarily by yielding and by creep deformation mechanisms which produce intimate contact over a large fraction of the interfacial area. At the end of this stage, the joint is essentially a grain boundary at the areas of contact with voids between these areas. During the second stage, diffusion becomes more important than deformation, and many of the voids disappear as grain boundary diffusion of atoms continues. Simultaneously, the interfacial grain boundary migrates to an equilibrium configuration away from the original weld interface, leaving many of the
remaining voids within the grains. In the third stage, the remaining voids are eliminated by volume diffusion of atoms to the void surface (equivalent to diffusion of vacancies away from the void). The stages overlap, and mechanisms that may dominate one stage
also operate to some extent during the other stages.
This description is consistent with several experimentally observed trends:
(1) Temperature is the most influential variable, since it, together with pressure, determines the extent of contact area during stage one, and it alone determines the rate of diffusion that governs void elimination during the second and third stages of welding.
(2) Pressure is necessary only during the first stage of welding to produce a large area of contact at the welding temperature. Removal of pressure after this stage does not significantly affect joint formation. However, removal of pressure before completion of
the first stage is detrimental to the process.
(3) Rough initial surface finishes generally adversely affect welding by impeding the first stage and leaving large voids that must be eliminated during the later stages of welding.
(4) The time required to form a joint depends on the temperature and pressure used; time is not an independent variable. This description of diffusion welding is not applicable to diffusion brazing or hot pressure welding processes where intimate contact is achieved through the use of molten filler metal and bulk deformation, respectively.
Advantages and Limitations
Diffusion welding and brazing have a number of advantages over the more commonly used welding and brazing processes, as well as a number of distinct
limitations on their applications. Following are advantages of diffusion welding and brazing:
(1) Joints can be produced with properties and microstructure very similar to those of the base metal. This is particularly important for lightweight fabrications.
(2) Components can be joined with minimum distortion and without subsequent machining or forming.
(3) Dissimilar alloys can be joined that are not weldable by fusion processes or by processes requiring axial symmetry, such as friction welding.
(4) A number of joints in an assembly can be made
simultaneously
(5) Members with limited access can be joined.
(6) Large joint members of base metals that would require extensive preheat for fusion welding can be more readily joined. An example is thick copper.
(7) Defects normally associated with fusion welding are not encountered. Among the disadvantages of diffusion welding and brazing are the following:
(1) The thermal cycle is normally longer than that of conventional welding and brazing processes.
(2) Equipment costs are usually high, and this can limit the maximum size of components that can be produced.
(3) The processes are not adaptable to a high production rate, although a number of assemblies may be joined simultaneously.
(4)Adequate nondestructive inspection techniques for quality assurance are not available, particularly those that assure design properties in the joint.
(5) Suitable filler metals and procedures have not yet been developed for all structural alloys.
(6) The faying surfaces and the fit of joint members generally require greater care in preparation than for conventional hot pressure welding or brazing processes. Surface smoothness may be an important factor in quality control in the case of diffusion brazing.
(7) The need to apply heat and a high compressive force simultaneously in the restrictive environment of a vacuum or protective atmosphere requires specialized equipment.
Gas Pressure Bonding. This process is a type of diffusion welding. In gas pressure bonding, the workpieces to be joined are finished to final size, cleaned to an acceptable surface condition, and assembled inside a container. The container may be an expendable sheet metal box, or it may be made from the parts themselves, by fusion welding around the edges. After the container holding the parts is made pressure-tight, it is evacuated and then placed in an autoclave containing an inert gas at high pressure, usually around 10000 psi. Under this extreme pressure the matched surfaces are pressed into intimate contact, regardless of the surface contour. After only a few hours the joints are diffusion welded.
In addition to gas pressure, fusion welding can be achieved by pressing the workpieces together between dies after heating by resistance heating. This system works well for small parts, but is not appropriate when the pieces to be joined are large, since it is difficult to keep the dies hot. See COLD WELDING, FORGE WELDING, and HOT PRESSURE WELDING. Reference: American Welding Society, Welding Handbook, 8th Edition, Vol 2, Welding Processes. Miami Florida: American Welding Society, 1992.