Fabrication diffusion bonding

Fig. 4. Diffusion bonding process (a) apply metal foil and cut to shape, (b) lay up desired pHes, (c) vacuum encapsulate and heat to fabrication temperature, (d) apply pressure and hold for consoHdation cycle, and (e) cool, remove, and clean part.

The solution for a diffusion couple in which two semi-infinite ternary alloys are bonded initially at a planar interface is worked out in Exercise 6.1 by the same basic method. Because each component has step-function initial conditions, the solution is a sum of error-function solutions (see Section 4.2.2). Such diffusion couples are used widely in experimental studies of ternary diffusion. In Fig. 6.2 the diffusion profiles of Ni and Co are shown for a ternary diffusion couple fabricated by bonding together two Fe-Ni-Co alloys of differing compositions. The Ni, which was initially uniform throughout the couple, develops transient concentration gradients. This example of uphill diffusion results from interactions with the other components in the alloy. Coupling of the concentration profiles during diffusion in this ternary case illustrates the complexities that are present in multicomponent diffusion but absent from the binary case. 

Find the diffusion profile solutions for ternary diffusion in a diffusion couple fabricated by bonding two semi-infinite alloy blocks face to face along a planar interface. Assume constant diffusivities. 

Fabrication is effected by precision milling of PMMA laminates which are sealed by diffusion bonding [149],... 

An early application of a combined steam reformer/catalytic combustor on the meso scale was realized by Polman et al. [101]. They fabricated a reactor similar to an automotive metallic monolith with channel dimensions in the millimeter range (Figure 2.65). The plates were connected by diffusion bonding and the catalyst was introduced by wash coating. The reactor was operated at temperatures between 550 and 700 °C 99.98% conversion was achieved for the combustion reaction and 97% for the steam reforming side. A volume of < 1.5 dm3 per kW electrical power output of the reformer alone was regarded as feasible at that time, but not yet realized. 

The use of photoetched stainless steel foil diffusion bonded together was described as one fabrication method currently under study for developing miniature heat exchangers. Very uniform gap spacing is required to maintain uniform flow distribution and high effectiveness in the heat exchanger. Measured effectiveness was lower than the calculated value, which indicates a possible problem with non-uniform flow. 

Kim and Dimiduk, 1991 Fujitsuna etal., 1991 Tokizane et al., 1991 Wurzwallner etal., 1993). The diffusion bonding process has been studied in particular detail (Yan and Wallach, 1993). With respect to turbine engine applications, component fabrication techniques have been demonstrated for airfoils and compressor cases (Lipsitt, 1985 a Chesnutt, 1990 Yamaguchi and Umakoshi, 1990 Kim and Dimiduk, 1991 Bondarev et al., 1991). However, much more development work is necessary to overcome the problems of ductility and toughness, corrosion resistance, strength and costs. 

Recently, a cellular, structural biomaterial comprised of 15 to 25% tantalum (75 to 85% porous) has been developed. The average pore size is about 550 p,m, and the pores are fully interconnected. The porous tantalum is a bulk material (i.e., not a coating) and is fabricated via a proprietary chemical vapor infiltration process in which pure tantalum is uniformly precipitated onto a reticulated vitreous carbon skeleton. The porous tantalum possesses sufficient compressive strength for most physiological loads, and tantalum exhibits excellent biocompatibility [Black, 1994]. This porous tantalum can be mechanically attached or diffusion bonded to substrate materials such as Ti alloy. Current commercial applications included polyethylene-porous tantalum acetabular components for total hip joint replacement and repair of defects in the acetabulum. 

Further methods have been developed for Al-SiC and Ti-SiC MMCs which make use of diffusion bonding/solid-state processing of stacks of plasma sprayed foils or stacks of alternating metal foils and fiber fabrics (foil-fiber-foil). 

One candidate is a titanium alioy that is reinforced with iarge diameter SiC/C filaments (see Chapter 4) and is fabricated by superplastic forming/ diffusion bonding. This MMC is suited to the fabrication of bladed compressor rings, shafts, ducks, fan components or structural rods for jet engines. Their use for parts submitted to still higher temperatures is limited by tiber/matrix reaction and environmental considerations. Titanium aluminide TisAI (or y-TiAl) matrices could permit an increase in the service temperature of the composites. 

Boron filaments (BFs) consist of polyciystalline boron with a core of tungsten and tungsten borides, which form during processing [/4]. B/Al MMCs are usually fabricated by diffusion bonding BFs between aluminum foils [/5]. Aluminum borides (i.e., AIB12 and AIB2) have been found at BF-matrix interfaces [/6],... 

B/Al and Gr/Al MMCs have been fabricated using techniques where aluminum foils, filaments, and precursor wires have been consoUdated by diffusion bonding. The diffusion bonds, if of low integrity or if contaminated, may cause severe corrosion damage. 

The heat exchanger can be fabricated in any material which can be diffusion bonded - stainless steel, titanium, higher alloy steels and nickel. As an alternative, brazing can be used, which allows metals such as aluminium to be considered. This potentially could lead to capital cost reductions compared to exchangers fabricated using more bespoke procedures. 

A heat exchange unit is fabricated by hot isostatic pressing (HIP) process HIP is one of the diffusion bonding technologies and a process that uniquely combines pressure and temperature to produce materials and parts with substantially better properties than achievable by other methods. Rectangular tubes are bent into a plate-like case formed by the outer plates the inner plates envelop the side of tubes. Hundreds of units of plate type heat exchangers may be azimuthally installed instead of an intermediate heat exchanger (MX). Further R D is needed to define how to assemble the parts for the HIP and how to perform pre-service inspection. 

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