Mühendislik Malzemelerinin Sert Lehimlenebilme Karakteristikleri

Abstract

DIN 8505 'e göre lehimleme "Metal malzemelerin erimiş bir ilave metal (lehim) yardımıyla, gerektiğinde bir dekapan ve/veya koruyucu gaz kullanarak birleştirilmesi yöntemidir." Lehimleme de ana metaller erimediğinden birleşme olayı ana metal ile ilave metal (dolgu metali) arasındaki difüzyon olayı sonucu oluşur. Bu olay da sert lehimlemeyi kaynaktan ayıran en önemli özelliktir. Ana metalin erimemesi demek kaynağa göre çok daha az ısı girdisi ve ana metalde mekanik ve kimyasal karakteristik değişmelerinin en az seviyede olması demektir. Sert lehimleme operasyonunun diğer imal usullerine tercih edilmesinin birçok sebepleri vardır. Bu sebeplerden bazıları şunlardır. a)Güçlü, sünek, şok ve titreşimlere dayanıklı malzemeler eldesi, b)Kolay ve hızlı yapılabilmesi, c)Benzer olmayan metallerin ideal birleştirilmesi, d)Temiz bir yüzey elde edilmesi, e)Ekonomikliği, f)Otomatik metodlara uygulanabilirliği'dir. Sert lehimleme basit fakat yapılması şart olan altı temel adımda gerçekleştirilir. Bunlar sırasıyla; 1)Parçalarda uygun bir boşluğun sağlanması, 2)Temizleme, 3)Dekapanlama, 4)Monte etme, 5)Sert lehimleme, 6)fşlem sonrası temizliktir. Bu çalışma ana hatlarıyla mühendislik malzemelerinin sert lehimlenebilme karakteristiklerini inceleyen bir literatür araştırmasından oluşmaktadır. Malzemelerin sert lehimlenebilme yetenekleri, metal ve metal dışı malzemeler olarak iki sınıfa ayrılarak incelenmiştir. Üçüncü bölümde malzemeler kabaca tanıtıldıktan sonra sert lehimlenmelerinde hangi dolgu metallerinin ve hangi sert lehimleme yöntemlerinin kullanılacağı yeri geldikçe belirtilmiştir. Bunların dışında çalışma, sert lehimleme öncesi ve sonrasında yapılması gereken işlemler ve lehimlemenin kısaca tarihi, tanımı ve sınıflandırılmasından oluşan bölümlere de sahiptir. v

Brazing is defined in AWS A3.0 as "A group of welding processes where in coalescence is produced by heating to suitable temperatures above 427 °C and by using a non-ferrous filler metal having a melting point below that of the base metals. The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction." It should be noted that the AWS Committee on Brazing and soldering has proposed the following modifications to the standart definition. 1. The filler metal should not to be limited to non-ferrous alloys since some filler metals do contain iron. 2. The temperature requirment should be based on the fact that the filler metal must have a liquidus tempareture above 427 °C but below that of the base metal. The brazing definition is composed of three parts. These are: a)The coalescence, joining or uniting of an assembly of two or more parts into one structure is is achived by heating the assembly or the region of the parts to be joined to a temperature of 427 °C or above, b)The use of a filler metal with a melting range below that of the base metals, c)The qualification that the filler metal must wet the base metal surfaces. The flow of filler metal in the joints by capillary attraction is predicated on this assumption. Brazing is probably the most versatile method of metal joining to day, for a number of reasons. Brazed joints are strong. On non-ferrous metals and steels, the tensile strength of a properly made will often exceed that of the metals joined. On stainless steels, it is possible to develop a joint whose tensile strength is 90 kg/mm2. IX Brazed joints are ductile, able to withstand considerable shock and vibration. Brazed joints are generally easy and rapid to make, and operator skill readily acquired. Brazing is ideally suited to the joining of dissimilar metals. You can easily join asemmblies that combine ferrous with non-ferrous metals, and metals with widely differing melting points. Brazing essentially a one-operation proces. There is seldom any need for grinding, filling or mechanical finishing after the joint is completed. Brazing is performed at relatively low temperatures, reducing the possibility of warping, over heating or diluting the metals being joined. Brazing is economical. The cost per-joint compares favorably with joints made by other metal joining methods. Brazing is highly adaptable to automated methods. The flexibility of the brazing process enables to match production techniques very closely to product requirements. However, with all its advantages, brazing is still only one of the ways in which you can join metals. Attributes of the braze joining process can be listed as follows: 1. Economical fabrication of complex and multicomponent assemblies. 2. Excellent stress distribution and heat transfer. 3. Joint temperature capability approaching that of base metal. 4.Abi 5.Abi 6.Abi 7.Abi 8.Abi 9.Abi ty to preserve protective metal coating or cladding. ty to join cast materials to wrought metals. ty to join non-metals to metals. ty to join widely different metal thickness. ty to join dissimilar metals. ty to join porous metal component. 10. Ability to preserve special metallurgical characteristics of metals. 11. Ability to join fiber and dispersion strengthened composites. 12.Cabability for precision production tolerance. 13. Reproducibility and reliable quality control techniques available. A brazed joint makes itself in the sense that capillary action, more than operator skill, insures the distribution of the filler metal into the joint. The real skill lies in the design and engineering of the joint. But even a properly designed joint can turn out imperfectly unless correct brazing procedures are followed. These procedures boil down to six basic steps. They are generally simple to perform (some of them may take only a few seconds), but none of them sholud be omitted from brazing operation. STEP 1 : Good fit and proper clearances. Brazing utilizes the principle of capillary action to distribute the molten filler metal between the surfaces of the base metals. Therefore, during the brazing operation, a clearance between the base metals must be maintained. It is necessary for capillary action to work its best. However, there is a special factor. It must be considered carefully in planning joint clearances. Brazed joints are made at brazing temperature (over 427 °C) not at room temperature. So it must be taken account the "coefficient of expansion" of the metals being joined. This is particularly true of tubular assemblies in which dissimilar metals are joined. STEP 2 : Cleaning the metals. Capillarity action will work properly only when the surfaces of the metals are clean. If they are "contaminated" -coated with oil, grease, rust, scale or just plain dirt- those contaminants have to be removed. If they remain, they will form a barrier between the base metal surfaces and the brazing materials. Oil and grease will carbonize when heated, forming a film over which the filler metal will not flow. And brazing filler metal will not bond to rusty surface. XI Cleaning the metal parts is seldom a complicated job, but it has to be done in the right sequence. Once the parts are throughly clean, it is good idea to flux and braze as soon as possible. That way, there is the least chance for recontamination of surfaces by factory dust or body oils deposited through handling. STEP 3 : Fluxing the parts. Flux is a chemical compound applied to the joint surfaces before brazing. Its use is essential in the brazing process (with a few exceptions). Heating a metal surface accelerates the formation of oxides, the result of chemical combination between the hot metal and oxygen in the air. These oxides have to go or they would prevent the brazing filler metal from wetting and bonding to the surfaces. A coating of flux on the joint area However, will shield the surfaces from the air, preventing oxide formation. And the flux will also dissolve and absorb any oxides that form during heating or that were not completely removed in the cleaning process. STEP 4 : Assembly for brazing. The parts of the asembly are cleaned and fluxed. Now they have to be hold in position for brazing. And operator must be sure that they remain in correct alignment during the heating and cooling cycles, so that capillary action can do its job. The simplest way to hold parts together is by gravity, if the shape and weight of the parts permit. Or it can be given gravity a helping hand by additional weight. If there is a few number of parts, any clamping or supporting device can be used. These devices must hold the parts together long enough to complete the brazing cycle. If there is a number of assemblies to braze and their configuration is too complex for self-support or clamping, it may be a good idea to rig up a brazing support fixture. In planning such a fixture, it must be designed for the least possible mass, and the least contact with the parts of the assembly. STEP 5 : Brazing the assembly. The first four steps -good fit and proper clearance, cleaning the metals, fluxing, asembly- were preparatory steps in the brazing process. The fifth step is actual accomplishment of the brazed Xll joint. It involves heating the assembly to brazing temperature, and flowing the filler metal through the joint. First the heating process. In brazing, heat is applied broadly to the base metal. If small assemblies are brazed heat may be applied the entire assembly to flow point of the brazing filler metal. If large assemblies are brazed heat will be applied a broad area around the joint. Heat must be apply to base metals, not to filler metals (direct flame on filler metal causes over heating and fuming). STEP 6 : Cleaning the brazed joint. After the assembly have been brazed, it must be clean. And cleaning is usually a two steps operation. First-removal of the flux residues. Second-pickling to remove any oxide scale formed during the brazing process. If an atmosphere or unobjectionable flux is used, no further cleaning may be necessary. ANALYSIS of the brazebility of engineering materials requires the following considerations: a)Joining process characteristics (type and characteristics of heat source). b)Chemical composition of the base metal. c)Cleaning preparation and after-process cleaning. d)Chemical composion of filler metal. e)Joint protection against oxidation (flux, bath composition, protective atmospheres). f)Joint tempareture and time. g)Joint design (joint geometry, joint clearance, joining position) Each of these topics influences wetting and spreading behavior, joint mechanical properties, corrosion resistance and residual stress levels.