DIFFUSION BONDING - AN ADVANCED MATERIAL PROCESS FOR AEROSPACE TECHNOLOGY
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Diffusion bonding is a method of joining metallic or non-metallic materials. This bonding technique is based on the atomic diffusion of elements at the joining interface. Recently, diffusion bonding has become a viable process in the fabrication of structural hardware or fluid and gas flow devices for aerospace and electronic industries. This paper presents the fundamentals of the diffusion bonding process and its applications in aerospace technology including thermal management devices for missiles, liquid rocket engines, and millimeter wave hybrid horns for land and space surveillance.
Low-cost and high reliability material processes have been playing a key role in the development of advanced technologies for the next century. Among the considered material processes1,2,3,4, such as casting, plasma spray, powder metallurgy, bulge forming and others, diffusion bonding recently became a viable process in the fabrication of structural hardware and fluid and gas flow devices for aerospace and electronic industries. The following section presents the fundamentals of diffusion bonding process and its application in aerospace technology including thermal management devices for missiles, liquid rocket engines, and millimeter wave hybrid horns for land and space surveillance.
Fundamentals of Diffusion bonding
Diffusion bonding is a method of joining metallic or non-metallic materials. This bonding technique is based on the atomic diffusion of elements at the joining interface.
Diffusion process actually is the transport of mass in form of atom movement or diffusion through the lattice of a crystalline solid. Diffusion of atoms proceeds by many mechanisms, such as exchange of places between adjacent atoms, motion of interstitial atoms or motion of vacancies in a crystalline lattice structure5. The latest is the preferable mechanism due to low activation energy required for atom movement. Vacancy is referred to an unoccupied site in a lattice structure.
Diffusion of atoms is a thermodynamic process where temperature and diffusibility of the material are considerable parameters. In general, the diffusion rate, in term of diffusion coefficient D, is defined as D = Do exp(-Q/RT) , where Do is the frequency factor depending on the type of lattice and the oscillation frequency of the diffusing atom. Q is the activation energy, R is the gas constant and T is the temperature in kelvins.
The activation energy for atomic diffusion at the surface, interface and grain boundaries is relatively low compared to the bulk diffusion due to a looser bond of the atoms and higher oscillation frequency of the diffusing atom6,7 . This enhances the atomic diffusion, and thus eases the diffusion bonding of two metal pieces assuming that a perfect interface contact exists.
The interface contact can be optimized by a treatment of the surface to be bonded through a number of processes, such as mechanical machining and polishing, etching, cleaning, coating, and material creeping under high temperature and loading. Creep mechanism allows a material flow to produce full intimate contact at the joint interface as required for diffusion bonding. Therefore, surface treatment and selection of bonding temperature and loading are basically important factors of the diffusion bonding process. Other factors such as thermal conductivity, thermal expansion, and bonding environment also effect the bonding process, particularly at high bonding temperature.
Figure 1 presents a typical microstructure of a diffusion bonded joint. The initial joint interface is still visible in some locations. The tensile strength of the bond joint is comparable to that of the parent metal.
Fig. 1 Typical microstructure of a diffusion bonded joint
Since diffusion bonding is driven by the diffusion of atoms, diffusion bonding process can be used to bond dissimilar materials that are difficult to weld, such as, steel and copper alloys.
In the following section, diffusion bonding of metallic platelets and their applications are presented.
Platelet Diffusion Bonding and Applications
Platelet diffusion bonding process8,9, as shown in Fig. 2, involves precise photo-etching or laser-cutting of thin platelets to the designed channel configurations. Subsequently, the etched platelets are arranged and stacked together and diffusion bonded at elevated temperature. The diffusion of the elements occurs at the platelet interface and results in a metallurgical bond joint. The bonded platelet panels are then formed and/or machined to the final hardware configuration.
Platelet diffusion bonding process has been successfully applied to a wide range of engineering materials, such as stainless steels, copper, aluminum and titanium alloys, and refractory materials9,10,11,12. This process offers a significant cost reduction in the production of fluid or gas flow devices with extremely small flow channels, particularly for aerospace and electronic applications.
Fig. 2 Platelet diffusion bonding concept
Figure 3 shows the fabrication of a copper liner for a liquid rocket combustion chamber which requires extensive cooling during operation in a severe hot gas environment13,14. Flat panels were fabricated by diffusion bonding of thin copper platelets with etched channels, and then formed to the chamber configuration. In the assembly process, hot isostatic diffusion bonding or brazing could be used to join the platelet panels to each other and to the structural support jacket of a high strength cast alloy (Fig. 3 ). Internal channels in the liner are designed to allow the flow of liquid hydrogen for cooling the combustion chamber.
Fig. 3 Fabrication of platelet liner of a liquid rocket combustion chamber
Similar process was used to fabricate a stainless steel window frame in the forebody of a land based missile. Cooling of the sapphire window is required to protect the electronic sensor underneath due to severe hypersonic flight environments. The temperature of the sapphire window must be uniformly controlled, because any temperature gradients in the window can cause a shift in the apparent target location and can blur or distort the target signal. Platelet diffusion bonding technology offers a unique design and fabrication process producing extremely small and complicated cooling channels as shown in Fig. 4, assuring an uniform temperature as required.
Fig. 4 Fabrication of platelet window frame
Another application of platelet diffusion bonding is the fabrication of platelet horn arrays for antenna systems used for land and space surveillance as shown in Fig. 5.
The platelet horn is designed similar to a conventional corrugated horn which is an excellent feed because of its symmetrical Gaussian beam, low side lobes and low cross polarization15,16. The fabrication of such platelet horns is simple and economic compared to conventional techniques such as machining or electroforming of individual horns15,16.
Figure 5 shows the fabrication process in which thin platelets containing patterns of etched holes, were sandwiched together in a stack of many layers and then diffusion bonded to produce arrays of horns for millimeter wave frequencies up to 1000 GHZ offering a relatively high performance15.
Fig. 5 Fabrication of platelet horn arrays
Diffusion bonding is an advanced material process for joining materials. Thin platelets of various metals can be diffusion bonded to produce cooling devices with extremely small and complicated flow channels for liquid rocket combustion chambers and missile sensor windows.
Platelet diffusion bonding was also used to fabricate arrays of corrugated horns for millimeter wave frequencies in antenna systems. This fabrication technique has been proven as a simple and accurate process to produce low-cost and high performance engineering devices, which is a basic requirement for manufacturing technologies in the 21st century.
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