The Welding Technique of Diffusion Bonding - Term Paper Example

Diffusion Bonding YourFirst YourLast Diffusion Bonding Introduction Diffusion Bonding is a method applied in welding (SolidState) metals and other related materials. Diffusion Bonding is able to join similar metals as well as metals that are not proportionate to each other. This method of metal joinery works under the principle of materials science of solid-state diffusion. In this principle, different atoms belonging to two or more metallic solids are blended together over time under high temperatures. The Diffusion Bonding technique is normally put into practice by putting into effect-elevated temperatures and at the same time elevated pressures to the matter intended on being joined or welded. Analysis Heatric initially developed this technique around 1980’s. Diffusion Bonding attaches together all the flow plates that are found in a Heatric Stack through the application of extremely high temperatures and pressures. The structural model of this technique does not allow any melting channels or deformation and therefore ensures that the models are completely bonded in the stack. In the Diffusion Bonding process, there is no brazing flux or metallic filers are applied to fill in the gaps between the plates. The resultant outcome of this process is high-intensity solid blocks of the original metallic sheets that contain the internal flow channels passing through the core of the metallic blocks. The Diffusion Bonding technique is applied in a number of industrial productions. This process of bonding provides numerous advantages, the main advantage being that it provides stronger bonding lines than other techniques. Sometimes, engineers equate the bonding line strength of this method to base metals. The Microstructure of the bonding line has the same atomic properties as the parent metals. Conversely, the Diffusion Bonding technique requires a strictly confined environment to operate. The technique demands that surfaces be cleaned thoroughly and be smoothened completely to free them from oxides and other impurities. It also needs very high levels of temperatures to facilitate the process of diffusion. In this technique, the strength of the bonded materials is achieved by way of applying pressure during the process of bonding. Strength is additionally attained through the application of extremely high temperatures and the period allowed for contact. Diffusion is the main contributor of the strength rather than any deformation of plastic materials. The fusion segment of the process additionally deals with high temperature flow characteristics and refined grain sizes. In attaining a tight contact of two or more non-attached surfaces, the process of diffusion plays a less significant function in mass atomic transportation, the most part being gotten through plastic deformation. Therefore, the small flow stresses linked with refined grain dimensions are attractive for the bonding process, similar to the formation of Superplastics. Additionally, internal grain-growth processes can negatively affect the process of Diffusion Bonding and therefore are required to be of special regard for metallic bonds to be attained in the cycle process. Characteristics of Diffusion Bonding The process involves no filler metals or fusion liquids No additional weights are put to the total Joint is strong enough to withstand base metal pressures as well as temperature variations of the base metals Materials are not susceptible to plastic deformation Process does not involve the introduction of residual stress Process is free from contamination of any kind Process can be performed on similar or dissimilar metallic sizes and thickness. Process is relatively expensive when compared to other techniques Metallic Combinations Numerous metallic combinations can be joined using Diffusion Bonding. Identical metals can be attached directly to develop a solid-state weld product. For identical pieces, temperatures and surface preparation are vital if forming a strong bond. The characteristics of the metals to be joined are additionally vital. Identically sized metals can be attached together with a thin metallic sheet of different qualities placed between them. In this situation, the metallic sheet placed between the metals may assist in speeding up the diffusion process or may allow for greater micro-deformation along the joints to offer extra complete contact within the two metallic surfaces. The in-between metallic sheet may be diffused to form part of the base metals through the application of appropriate heat treatment up to that point it diffuses and is no longer considered a separate part. Two different metals may be attached directly through the application of the diffusion process to form a strong bond. Different metals may be attached with a block metal between the two, so as to assist in enhancement of the weld formation. The metallic block may help in acceleration of the diffusion process or may also help in allowing complete first contact in the same manner as applied when using metal sheets. There are two fundamental requirements that need to be fulfilled prior to having a successful diffusion weld process. They are- (a) MI (Mechanical Intimacy) between the two metals must be present. (b) All the impurities present must be removed to facilitate metallic bonding The two requirements are fundamental in ensuring the right standards are met in the process of Diffusion Bonding. One element of a suitably ready surface is the nature of smoothness and levelness. In order to attain a standardized uniformity when joining the metals, a particular minimum level of smoothness and flatness is needed (Ting & Kennedy, 1989). It is important to protect the refined surfaces to avoid oxidization. Oxidized surfaces tend to undergo the process of crystallization, which in turn increases the rate of diffusion in the regions that have been welded. Oxidized surfaces in many cases prevent metal contact. The weld formation can be described in a three stages mechanical model. The initial level involves the deformation of the metals in contact. In this level, creep deformation mechanisms are applied. The first process renders the joint between the two elements a grain boundary, with visible spaces between at the point of contact. The second stage involves the diffusion of grain atoms at the boundary. The diffusion process at this stage is essential since it ensures that all the spaces are filled. The third level involves the elimination of the remaining spaces through the process of volume diffusion. Atoms are diffused between the surfaces therefore eliminating all the voids in the process. The processes discussed above are highly mechanized and are mostly used in high-end industrial production. As noted earlier, the diffusion bonding process is very expensive to run because of the working environment and the equipment needed for facilitating this process. Nonetheless, there have been investigations into the development of low-cost diffusion bonding processes. These investigations have yielded two methods in this regard. They are: Thin Sheet Diffusion bonding Massive Diffusion Bonding The latter process is applied in the development and manufacture of huge weighty structures using plate metal sheets and materials. Massive Diffusion Bonding process is preferred to the machining from solid process because it is able to develop heavy structures with minimal wastages. Thin Sheet Bonding is commonly used in the manufacture of frames used in air transport (Lee, Yoon & Yoo, 2011). It is widely preferred because of the enormous area bonds related with structures made using this method. In addition, this method is preferred because the metallic sheets are normally at SPF (Superplastic Forming) temperatures. This method uses the application of inert gases pressures, which are applied as an attachment or bonding element. The thin sheet diffusion bonding process allows for uniform applicability of pressures on the whole area being worked on. Any mismatch during the process of bonding is overcome by the Superplastic Forming properties of the raw materials. Variables of Diffusion Bonding There are four major variables of diffusion bonding. 1) High temperatures are required because they determine the rates of diffusion (movement of atoms between the surfaces) 2) High pressures are vital during the initial stages of welding so as to create a large contact area at the recommended temperature levels 3) Adequately prepared surfaces are essential to this process. The surfaces should be well polished to remove impurities, ensure flatness and eliminate oxidants that may affect the process. 4) The Amount of time needed to create a strong joint is dependent on the levels of temperatures applied and pressures. Time is a dependent variable. References Fabrication of Tungsten-Vanadium Hybrid Material with Sufficient Toughness for High-Temperature Applications by Diffusion Bonding. (2014). Fusion Sci. Technol. doi:10.13182/fst13-773 Lee, H., Yoon, J., & Yoo, J. (2011). Manufacturing of Aerospace Parts with Diffusion Bonding Technology. AMM, 87, 182-185. doi:10.4028/www.scientific.net/amm.87.182 Ting, E., & Kennedy, J. (1989). Superplastic forming and diffusion bonding of rapidly solidified, dispersion strengthened aluminum alloys for elevated temperature structural applications. Hampton, Va.: National Aeronautics and Space Administration, Langley Research Center. Read More