Machinery's Handbook, 31st Edition
Superplastic Forming and Diffusion Bonding 1437 b) Products that are usually formed in several parts separately can be integrally formed as a single part through superplastic forming. The single piece forming minimizes the number of parts and joints, and thus leads to weight savings. c) Since the forming needs only the female die, the investment cost for the die is reduced. d) Little or no residual stress develops in the formed parts. e) Superior transferability of the die surface to the workpiece is provided. f) Material waste is minimized. Disadvantages are these: a) There is low productivity because of low strain rates, typically 10 −4 to 10 −2 /s b) Material costs are high. Although the process is increasingly being applied in the aerospace and automotive industries as a way of manufacturing very complex geometries at a fraction of the cost of conventional stamping, some practical problems are still of concern, the main ones being predicting the final thickness distribution of the formed parts, determining the optimum pressure cycles, and learning more about the microstructure of superplastic material and how it changes during such dramatic elongation. Diffusion Bonding.— The International Institute of Welding (IIW) has accepted the defi nition of solid state diffusion bonding proposed by Kazakov. This definition is: “Diffusion bonding of materials in the solid state is a process for making a monolithic joint through the formation of bonds at atomic level, as a result of closure of plastic deformation at elevated temperature, which aids interdiffusion at the surface layers of the materials being joined.” The process is dependent on a number of parameters, such as time, applied pressure, bonding temperature, and the method of heat application. The process allows bonding of homogeneous or heterogeneous materials. Hence, structures can be manufactured from two or three metal sheets. Diffusion bonding generally occurs in three stages: a) The deformation process results in the surfaces to be joined coming into intimate con tact, but not enough to produce gross deformation. b) Bonds are formed by diffusion-controlled mechanisms where the diffusion grain boundary predominates. At this stage, pores are eliminated and the grain boundary arrangement finally ensues. c) In the third stage, volume diffusion dominates and the joining process is completed. The mechanism of diffusion bonding involves holding together sheet metal components under moderate pressure, about 10 MPa (1450 psi), at an elevated temperature of (0.5– 0.8) T m (where T m is the melting temperature in K), usually in a protective atmosphere or vacuum to protect oxidation during bonding. The length of time the materials are held at this temperature depends upon the materials being bonded, the joint properties required, and the remaining bonding parameters. The aim in diffusion bonding is to bring the surfaces of two or more pieces being joined sufficiently close so that interdiffusion can result in bond formation. To form a high quality bond, surface roughness must be limited to minimum values (Ra < 4 microns); cleanliness must be absolute; and flat surfaces’ waviness must be held to less than 400 microns. A minimum of deformation and an almost complete lack of residual stresses are charac teristics of the process, except possibly when two different metals being diffusion-bonded together have large differences in their coefficients of thermal expansion (CTE). This can cause strains to develop at the interface, which can cause premature failure of the bond. The process is most commonly used for titanium in the aerospace industry, and some times it is combined with superplastic forming. Titanium is the easiest of all common engineering materials to join by diffusion bonding, due to its ability to dissolve its own oxide at bonding temperatures (bonding of Ti alloys takes place at 925ºC). The more conventional form of diffusion bonding usually takes place in a uniaxial load ing press. Pressure and heat can be applied by different means. More complex geometries than are possible by the uniaxial process can be handled by hot isocratic pressing, which
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