Otomotiv endüstrisinde kullanılan çelik ve alüminyum alaşımı saçların nokta kaynağı ve yapıştırma ile kombinasyon bağlantıları
Spot welding and weldbonding
- Tez No: 66715
- Danışmanlar: PROF. DR. ADNAN DİKİCİOĞLU
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: İmalat Bilim Dalı
- Sayfa Sayısı: 161
Özet
ÖZET Bu çalışmada özellikle otomotiv endüstrisinde yaygın olarak kullanılan nokta kaynağı ve bu sanayide yeni yeni kullanılmaya başlanan yapıştırma-nokta kaynağı kombinasyon bağlantılarının temel esasları, çelik ve alüminyum sac parçaları için uygulamada dikkat edilmesi gereken önemli noktalar ve örnekler ele alınmıştır. Nokta kaynağının en önemli avantajlarından birisi, yüksek üretim hızlarına erişebilme ve otomasyona uygun bir yöntem olmasıdır. Nokta kaynağının esası; düşük voltajlı yüksek akımın çok kısa bir zaman diliminde elektrotlar araşma sıkıştırılmış iki parça arasından geçirilmesi ile, Joule kanununa göre oluşan ısının parça temas yüzeylerinde metali ergitmesine dayanır. Ortaya çıkan ısı, içinden akım geçirilen parçaların direnci ile orantılıdır. Günümüzde özellikle enerji fiyatlarındaki sürekli artış, üretimde ekonomikliğin ön plana çıkmasına neden olmuştur. Taşıtlar, petrolün tüketildiği en önemli alanlardan bir tanesidir. Taşıt ağırlığının azalması ile yakıt tüketiminde önemli azalmalar olacak ve çevre temizliğine de katkı sağlanacaktır. Bu nedenle çelik malzemeden daha hafif alternatif malzeme ve birleştirme teknikleri arayışları başlamıştır. Bunların arasında başta gelenler düşük alaşımlı yüksek mukavemetli çelikler, alüminyum alaşımları ve yapıştırma-nokta kaynağı kombinasyon teknolojisi olarak gösterilebilir. Taşıtlarda ağırlık azaltılması, ince kesitli ve yeterli dayanımdaki çelik türlerinin veya alüminyum alaşımlarının kullanılması ile mümkün olabilmektedir. Uygulamada maliyetten doğan bazı sınırlamalar olsa bile bu tür malzemelerin taşıtlarda kullanım aram gittikçe artmaktadır. Otomotiv sanayinde alüminyuma olan yoğun ilgi nedeniyle alüminyum alaşımların nokta kaynağı özellikleri önem kazanmaktadır. Günümüzde otomotiv endüstrisinde çelik sacların birleştirilmesinde kullanılan nokta kaynağı makinaları, alüminyum alaşımı sac birleştirmelerinde de kullanılabilmektedir. Otomobil fabrikalarının çoğunda üretim teknolojisi, nokta kaynağı esaslarına göre kurulmuştur. Bu nedenle otomobil üretimine yeni giren alüminyum alaşımı sacların kaynağında en ekonomik çözüm nokta kaynağı olmaktadır. Alüminyum alaşımlarının fiziksel özelliklerine bağlı olarak nokta kaynağında bazı önemli değişikliklerin gözönüne alınması gerekmektedir. Bundan yaklaşık 45 yıl önce yapıştırma ve nokta kaynağı, Sovyetler birliğinde ilk defa uçak yapımında beraber uygulanmıştır. Otomotiv endüstrisinde de bu yöntemin kullanılmasıyla ağırlıktan ve taşıma gücünden dolayısıyla yakıttan tasarruf ve korozyona dayanım sağlanmıştır. Alüminyum alaşımlarının otomotiv endüstrisinde hafiflik nedeniyle kullanılmaya başlaması ile, yapıştırma-nokta kaynağı kombinasyon uygulamalarında artış görülmüştür. Yapıştırma-nokta kaynağı kombinasyon işlemi, yapıştırılan metallere nokta kaynağı uygulanması işlemidir. Görünüş itibariyle, normal elektrik direnç nokta kaynağına benzemektedir. Aynı tip makinalar kullanılmaktadır. Elektrodlar aynıdır. Önemli olan doğru yapıştırıcıyı seçmektir. Kombinasyon bağlantılarının ses, titreşim sönümleme, mukavemet ve dinamik özellikleri sadece nokta kaynağı uygulanmış sac birleştirmelerine göre oldukça üstün olmaktadır.
Özet (Çeviri)
SUMMARY In this study, the general principles, application criterians, types and quality control methods of the spot welding and weldbonding method for steel and aluminum alloys which are used in industry and especially in automotive industry, has been explained. The main advantage of spot welding from a production viewpoint is the high level of productivity that can be achieved and the suitability of the process for automation. Also, resistance welding is a very inexpensive joining process. It is very low cost and a very efficient way of joining metals together. Spot welding has been used successfully for many years to join mild steel components. There won't be any major changes in resistance spot welding on the most of automotive factory for the next several years. The spot welding relies on Joule heating of the compenents by passing a large current of short duration between the sheets to be joined. The contacting surfaces in the region of current concentration are heated by a short time pule of low-voltage, high amperage current to form a fused nugget of weld metal. When the flow of current ceases, the electrode force is maintained while the weld metal rapidly cools and solidifies. The electrodes are retracted after each weld, which usually is completed in a fraction of a second. The size and shape of the individually formed welds are limited primarily by the size and contour of the electrode faces. The weld nugget forms at the faying surfaces, but doesn't extend comletely to the outer surfaces. The spots should be at a sufficient distance from the edge of the workpiece so that there is enough base metal to withstand the electrode force and to ensure that the local distortion during welding doesn't allow expulsion of metal from the weld. Also, spacing between adjacent spot welds or rows of spot welds must be enough to prevent shunting. Shunting occurs when a second spot weld is made so close to the first one that the welding current can flow either through the metal between the electrodes at the point of the second weld. The welding current flows in inverse proportion to the resistance of the two paths, tf the distance to the first spot weld is short, a significant fraction of the current is shunted through the first spot weld. Many assemblies of two or more sheet metal stampings that don't require gastight or liquid-tight joints can be more economically joined by high speed resistance spot welding than by mechanical methods. This process is also used to join metals that are dissimilar in thickness or composition, and to join steel coated with another metal. In making a spot weld between dissimilar metals, a heat balance must be achieved that compensates for the differing properties of the two metals and results in the production of a weld nugget having approximately the same thickness on each side of the interface. More heat must be provided to the more thermally and electrically Xlllconductive metal, which generates less resistive heat and has greater loss of heat by conduction. The resistance spot welding cycle is divided into four major time segments: Squeeze, weld, hold and off. Squeeze time is an interval of delay between closing of the initiating switch and application of the welding current. This time should be sufficient to ensure that the parts have maintained intimate contact. Weld time is the interval during which the welding current flows through the circuit. Hold time is the interval during which, after the welding current is off, the electrode force is held on the workpiece until the metal of the spot weld has solidified. Off time is the interval from the end of the hold time until the beginning of the squeeze time for the next cycle. During the welding operation, the electrodes are subject to great compressive stresses at elevated temperature and must be frequently dressed and periodically replaced. Because the current conducted to the workpieces must remain localized with in a fixed area, the electrodes must resist these stresses without excessive deformation. The electrode force, in addition to forging the heated workpieces together, influences the passage of current to the localized area. The shape, dimensions and surface condition of the electrode tips or contact surfaces are important for consistent weld quality in resistance spot welding. Shape and dimensions of electrode tips are affected by mechanical wear and deformation, or mushrooming, at a rate depending on tip material and design, operating temperatures, rates of heating and cooling, and welding force. Alloying between the electrode tip and the work metal can greatly increase the rate of deterioration of the electrode tip. Careful attention to electrode tip condition is needed to avoid such defects as week or missed welds, irregularly shaped welds, erratic indentation, burning or discoloration of the work surface, surface melting, and electrode deposits on the work surface. Materials for spot welding electrodes, should have sufficiently high thermal and electrical conductivities, and sufficiently low contact resistance to prevent burning of the workpiece surface or alloying of the electrode face with it and should have adequate strength to resist deformation at operating pressures and temperatures. The probability of pickup, alloying and deformation is a major consideration in electrode design. In minimizing pickup or alloying of the work metal and electrode material, the affinity of one for the other is important. Dimensions of the electrode face are governed by thickness of the work metal, desired size of the weld nugget, and shape and size of assembly. For consistent production of spot welds of the highest quality, the resistance at the workpiece surfaces that contact the electrodes must be kept to a minumum. This can be done by having smooth, clean work-metal surfaces and by controlling the electrode force. If the workpiece surfaces that contact the electrodes have too high a contact resistance, the temperature rise at these surfaces is almost as fast as at the faying xivsurfaces. Also, removal of foreign substances from workpiece surfaces reduces electrode pickup and consequently increases electrode life. ".* The equipment needed for resistance spot welding may be simple and inexpensive, or complex and costly, depending on the degree of automation. Machines for direct energy welding generally are composed of these principal elements: Electrical circuit, Control equipment and Mechanical system. Electrical controls for direct energy resistance welding machines perform three principal functions: (1) initiating and terminating the flow of current to the welding transformer, (2) controlling the magnitude of the current, (3) timing and cotrolling the mechanical operations of the welding machine. The choice between single-phase and three-phase direct energy machines for resistance spot welding is based mainly on machine capability and on initial, operating, and maintenance costs. Power factor and load balance among the three phases of the power power supply should also be considered, particularly when a high-capacity machine is needed in a plant where the supply of electric power is limited. Single-phase machines are more widely used than three-phase machines, because they are simpler to operate and cost less to buy, install, and maintain. When equipped with suitable controls, however, a single-phase machine has the same performance capabilities as a three-phase machine of the same size and rating. The quality of spot welds generally is checked -by visual inspection and by destructive testing. On the surface of a resistance spot welded assembly, the weld spot should be uniform in shape and relatively smooth, and it should be free of surface fusion, deep electrode indentations, electrode deposits, pits, cracks, sheet seperation, abnormal discoloration around the weld, or other conditions indicating improper maintenance of electrodes or functioning of equipment. However, surface appearance is not always a reliable indicator of spot weld quality, because shunting and other causes of insufficient heating or inadequate penetration usually leave no visible effects on the work piece. Destructive testing, can be performed on the actual workpiece or on test specimens. For small, inexpensive parts, actual production samples from each machine are taken at random or at prescribed regular intervals for destructive testing. Test specimens are used, where production parts are large or costly and, with experience in interpretation of results, can give valuable information on the quality of production welds. Strength of the weld is usually determined by the tension-shear and tension tests. The lazer welding is one of the important contender against resistance spot welding. But the real key to lazer welding is that very good fitup and that's very difficult to do in the entire structure of a car. A number of experts claim that filler metal addition will be needed for the laser welding. This can increase the complexity of the process. On the other side, resistance welding is a little more forgiving ; it cures the fit up problem. Pressures are clamping down on the industry to produce cars that are free from emissions and much more fuel efficient. For this reason, the next generation of cars will have to be made out of a much lighter weight material than today's vehicle. xvPresent automobiles are steel unibodies, but competition is coming from aluminum unibodies, steel space frames and aluminum space frames. There is the possibility that manyftof tomorrow's cars will be built aluminum. If not that, it does appear that there will be much more aluminum in the average automobile. Lighter weight steels are also being developed. A recent addition to the family of spot weldable steels is the high-strength low alloy (HSLA) steels. Aluminum has a number of advantages for vehicle applications when compared to steel, particularly with regard to reducing fuel consumption and the consequent reduction of CO2 The density of aluminum is one-third that of steel. Reductions in mass of body-in-white up to 50% can be achieved while equaling or exceeding the strength and stiffness of comparable steel bodies. In the lead at present in the manufacture of its aluminum automobiles is Audi of Germany. İt is making relatively large numbers of these aluminum automobiles on the production line. Overall vehicle mass could be reduced by at least 10% and that would reduce fuel consumption by a similar amount. The increase in interest in the use of aluminum in the construction of automobiles has led to a need to characterize the properties of the spot weld in aluminum alloys. The spot weld is currently the joining method of choice in the automobile industry for economic reasons. The welding practices used for steel must be altered significantly for aluminum due to their differences in physical properties. Aluminum is a very good electrical conductor with a bulk resistivity one third that of steel. Joule heating is proportional to resistance for a given current. It is understandable that a significant increase in welding current will be required to join aluminum, compared to an equivalent gauge of steel. Further, aluminum has a high thermal conductivity and the localized heat generated by the welding current will be conducted away rapidly. It is therefore necessary to use short weld times, typically in the range 0,04 to 0,16 s. Aluminum alloys can be resistance welded with single-phase direct energy, three- phase direct energy, and stored energy machines. The quality of resistance welds in aluminum alloys is more dependent on the contour, surface finish, and cleanness of the electrode face than the quality of welds made in other metals and alloys. In the spot welding of aluminum alloys, electrode life is determined by metal pickup on the electrode face, not by deformation, as in the welding of steel. The copper-aluminum alloy formed on the electrode face by metal pickup has low electrical conductivity. Thus, if welding is continued, the electrodes will stick to the work metal and the surface of the work metal will melt. Aluminum must be removed from the electrode face by periodic hand dressing to avoid marking the surfaces of succeeding welds and to maintain the original condition of the face. Although aluminum spot welds for some purposes can be made satisfactorily without any preweld surface preparation; welds that are free of cracks, porosity, and sheet separation, and that have the most uniform strength and symmetry, are obtained only with correct procedures for cleaning and reduction or removal of oxide film. In addition, adequate surface preparation reduces electrode contamination. Oxide can be removed by mechanical or chemical methods. XVIThe cost of aluminum is crucial to the future of the automobile. Because of the variable cost of using aluminum, the future automobile will still have a lot of steel in it. Aluminum wiil be there at least in the hang-ons (fenders, hoods, decks and doors). About 45 years ago, a joining process that had been used in the Soviet Union for the manufacture of certain aluminum panels in aircraft. The process involved making resistance spot welds through epoxy adhesives. The process known as weldbonding, is being researched now in the automotive industry for the joining of steel and aluminum. Weldbonding has been and is also being used on certain joints in steels with in the automotive industry. For example, weldbonding is used in the front wheelhouse on the Chevrolet Camaro and the Pontiac Firebird automobiles. Compared the conventional resistance spot welding, weldbonding or the technique of making a spot weld through an adhesive demonstrated stiffness and durability improvements with no weight increase and a 54 % spot weld reduction. Tests indicated that the fatique strength of a weld-bonded joint was superior not only to a resistance spot welded joint in aluminum, but one in steel as well. For sound-deadening and vibration-resistance reasons, weldbonding is used there as well. The spot welds may also act as peel stoppers under impact loading. Weldbonding can significantly increase the fatique strength and stiffness of structures compared with just spot welded sheet. In making that joint, the adhesive is first applied in the middle, between the two sheets to be welded.- The resistance spot weld is then made through the adhesive. The process combines adhesive bonding and spot welding. Weldbonding looks almost the same as conventional resistance spot welding. The same type machine is used. The electrodes are the same. The magic is in picking the right adhesive. The adhesive is a single-part epoxy modified with certain materials like carbon black or other proprietary fillers that help to conduct electricity. The epoxy has to meet two requirements. First, it has to have weld through properties. A severe test has been established to evaluate those properties. Also the adhesive tends to cure on the tip of the resistance welding electrode. This epoxy adhesive has a good balance of impact and creep properties over the temperature range of -40 to 120 °C. The adhesives introduce additional variables to the process due to modification of contact resistance. It is necessary to ensure that adhesive is moved from the weld zone before welding, either mechanically during the presqueeze and squeeze cycles or by using a very low current preweld. xvii
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