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|Title:||Elektron Işın Kaynağı Ve Otomotiv Endüstrisindeki Uygulamaları|
|Other Titles:||Electron Beam Vvelding And Its Applications İn The Automotive Industry|
|Keywords:||Elektron ışın kaynağı|
Electron beam welding
|Publisher:||Fen Bilimleri Enstitüsü|
Institute of Science and Technology
|Abstract:||Bu çalışmada eritme kaynağı yöntemlerinden biri olan elektron ışın kaynağı ve bu kaynak yönteminin özellikle otomotiv endüstrisindeki uygulamaları üzerinde durulmuştur. Yöntemin anlaşılması bakımından ışın üretilmesindeki fiziksel prensipler ve işlemin en önemli noktasını oluşturan elektron ışın tabancası anlatılmıştır. Hala çok çeşitli araştırmalara konu olan ışının malzeme üzerindeki davranışı çeşitli örneklerle anlatılmış ve kaynak yerinde ışının ve malzemenin etkileşim fenomeni açıklanmaya çalışılmıştır. Diğer bir üzerinde çalışılan konuda kaynak parametrelerinin birbiriyle etkileşimleri ve nasıl bir optimizasyona gidileceğidir. Etkili parametrelerden ivmelendirme voltajı, ışın akımı, mercek akımı, odaklama pozisyonu, kaynak hızı, ışın sapması ve çalışma basıncı gibi olanlar açıklanmıştır. Yöntemin üstünlüğünün belirginleşmesi açısından zengin örneklerle malzemelerin kaynak kabiliyeti tartışılmış, çelik, alüminyum ve titanyumun geniş kullanım sahası nedeniyle üzerinde daha fazla durulmuştur. Ayrıca farklı malzemelerin kombinasyonları irdelenmiştir. Tezin diğer önemli konusu olan otomobil endüstrisindeki uygulamalar öncelikle yüksek vakum altında yapılan çalışmalar ve sonra atmosferik basınç altında yapılan tatbikatlar detaylarıyla incelenmiştir. Son bölüm, bizzat üzerinde çalışılan bir elektron ışın kaynağı makinasının spesifik özelliklerini tanıtmakta ve bu makinayla kaynak edilen parça üzerinde yapılan kimyasal analizleri, sertlik ölçümlerini ve mikroyapı incelemelerini içermektedir.|
Electron beam welding (EBW), the most powerful fusion welding process, has been developed for many years and is increasingly implemented in various sectors such as aircraft manufacture, aerospace industry, nuclear engineering, metal-processing industry and automotive industry. in this study, the principles of the process, its advantages and applications of in automotive industry are described. in addition, EB weldability of metallic materials is explained with wide range of example. At the time of its inception in 1950s, electron beam welding was strictly a high vacuum process. Today the process has a much wider scope, being also practiced at a medium vacuum and at atmospheric pressure. The electron beam is always generated in a high vacuum but the vvorkpiece may be at the same pressure as the beam generator ör at a higher pressure. The heart of the process is the electron gün. The operation of this equipment may be either automatic ör semiautomatic. in the electron gün, electrons are emitted from a cathode, with the speed and direction imparted to these electrons by their attraction to a positively charged anode. Usually, the electrons are generated from heated filament, and a precisely shaped cathode electrode surrounding the filament electrostatically shapes the electrons into a converging beam. This converging beam of electrons, accelerated to speeds in the range of 50 000 to 200 000 km/s, passes through a small hole in the center of the anode and continues tovvards the vvorkpiece. After the beam leaves the anode, the mutual repulsion of the electrons causes the beam to diverge as it travels. To counteract this effect, an electromagnetic lens system is used to re-converge the beam and thus control the size of the beam focal spot at the vvorkpiece. VVhen this highly accelerated electron hits the base metal, it will penetrate slightly below the surface and at that point release the bulk of its kinetic energy that turns to heat energy. The addition of heat brings about a substantial temperature increase at the point of impact. The succession of electrons striking the same place causes melting and then evaporation of the base metal. This creates metal vapors but the electron beam travels through the vapor much easier than solid metal. This causes the beam to penetrate deeper into the base metal. The vvidth of penetration is extremely narrovv. This operation is progressive until the beam finally emerges from the base of the work, leaving a hole filled with central column of vapor surrounded by a liquid. With a vapor/liquid hole established in the material, a weld can be made by translating the hole along a predetermined joint line. This is done by moving either the beam ör the vvorkpiece beneath the beam. Material behind the advancing hole solidifies as the heat source is removed. XVIII Heat input for vvelding is controlled by four basic parameters: the number of electrons per second impinging on the vvorkpiece (beam current) the speed of the electrons (accelerating voltage), the diameter of the electron beam at the surface of ör within the vvorkpiece (beam spot size) and the speed with which the vvorkpiece ör electron gün is being translated (vvelding speed). A fundamental characteristic of an electron beam, vvhich can help to define its vvelding capability, is the electron beam's povver density. At the vvorkpiece; this factor, obviously, is dependent upon the basic gün parameters of beam povver (current x voltage) and beam spot size. Povver densities more than 108 W/cm2 can be obtained by concentrating high kinetic energy onto a small work area. Such a high povver density cannot be achieved with any other knovvn continuos beams, including laser beams. This characteristic has made it possible for EBW to produce deep, naırovv, and almost parallel-sided vvelds vvith relatively low total heat input and narrovv heat-affected zones in a wide variety of common and exotic metals. Electron beam must function in a vacuum to prevent the dispersal of the focused electrons by collision vvith air ör other gaseous ör physical molecules. The required vacuum chamber, though a limiting restraint on vvorkpiece accommodation does simplify shielding of personnel from the harmful X-rays, by lead lining the chamber. The behavior of the electron beam on penetrating a metal is öne of the most important and difficult parts of the electron beam vvelding process to understand. Numerous investigations have dealt vvith the phenomenon by vvhich the beam is able to vveld metal several centimeters thick in spite of the fact that the electrons themselves penetrate only a few hundredths of a millimeter into the surface of the material. Even today, some 30 years after the first trials involving vvelding of greater material thicknesses although the physical mechanism of the electron beam process are knovvn in principle, the individual processes taking part are so complex that the stili cannot be quantitatively described. The difficulty in accurately and completely describing them lies in part in the fact that the object of investigation, the vvelding process, to a large extent cannot be studied directly by experiment This, theoretical observations can only be proven by examination of the resulting vveld produced. Nevertheless, vvelding vvith contrasting materials, radiography during vvelding, frequency measurements of the penetrating current, as vvell as calculations of the depth of vveld and of the energy processes involved have ali served to increase our knovvledge of the deep penetration vvelding effect considerably. The characteristic narrovv, parallel sided, single pass, autogenous vvelds produced by the EB process, are made by the "keyholing" mechanism. This vvorks by creating a hole through the material being vvelded and then running this "keyhole" along the joint. Molten metal flovvs around the sides of the keyhole and solidifies behind, so forming the vveld. The keyhole can only be formed if a sufficient vapour pressure exists vvithin it, to counteract surface tension, gravity and other forces vvhich tend to favour its closure. This means that the material in and around the keyhole sees peak temperatures and temperature gradients in excess of those found in other non-keyholing fusion vvelding processes. The liquid metal in the keyhole is literally boiled. This, in combination vvith the vacuum environment, encourages preferential vaporization of some alloying elements in certain materials. The chemistry of the vveld is usually altered to some extent, and occasionally a material is found to be altogether unvveldable because of this effect. Hovvever, in the vast majority of cases, losses of XIX ._,? this type are very small, with EB weld metal analyses remaining vvithin the parent material's compositional specification. Another effect of the intense heat and potentially rapid thermal cycle offered by EB is that materials that have very high melting points and/or very high thermal conductivities are readily EB weldable vvithout preheat. Moreover, when EB welding dissimilar materials together, factors such as differences in melting point and thermal conductivity betvveen the two materials become almost insignificant in determining the feasibility of the joint. The weld penetration of an electron beam is a function of the following parameters: the beam's own characteristic in the plane of the weld, such as its diameter, convergence angle, current, and accelerating voltage; the physical properties of the vvelded material, such as its thermal conductivity and melting point; the vvelding speed; the vvelding environment. Many attempts have been made to correlate the above variables, vvith some success. A universal relationship is unlikely to be found, hovvever, since our present understanding of the mechanism of deep penetration is not complete. There is also the question of variation of beam current profile from öne experiment to another, and such variations are even greater from öne design of electron guns to another. Even beam diameter is not always recorded, although it is a critical parameter since it decides the power density of the beam and hence its penetration capabilities. Nevertheless, attempts at correlating vvelding parameters are useful since the may reveal discrepancies due to inadequate theory. Some significant vvelding parameters are accelerating voltage, beam current, lens current, focal position, vvelding speed, beam deflection, beam pulsing, positional vvelding and vvorking pressure. Determining the optimum parameters before starting a new vvelding operation can involve a very wide range of considerations. Not only must a weld of suitable thickness be produced to fulfill the functions and loadings expected of the component, but it must also be free of both extemal and intemal defects. The settings required to achieve this are thus best determined in two stages if previous parameters for similar types of vvelding are not available. in general, the accelerating voltage is first set to UBmax and thereafter kept constant. it is also recommended that initially a lens current İL is chosen vvhich produces a slight under-focusing. Apart from this the beam current ls, depending on the as yet stili unknovvn vvelding speed vs, must also be determined. Each of these parameters has a very complicated effect on the vvelding process and thus on the weld itself. For example, in some cases the optimum beam current betvveen being "under" and "över" vvelded may lie within very close limits. The features of the electron beam facilitate the joining of refractory metals and, to a certain extent, materials ör material combinations that can hardly be handled by other vvelding processes it is impossible to make a general statement about the weldability of ali materials. Handbooks have good discussions on the EB vveldability of most materials. in fact, materials that are readily arc-welded are readily EB vvelded. The qualifications on this statement are that some materials must be vvelded vvith little ör no restraint, others vvith pre-heat and others vvith post-heat. Additional qualifications XX._,? are that some materials require careful cleaning, others careful selection of the base metal chemistry. Öne of the more interesting materials considerations appearing in recent years is that of vvelding dissimilar metals. Joining of dissimilar metals leads to special problems in both solidification and processing. in the area of solidification, attention must be paid to the chemistry of both metals as well as to the resulting fusion zone. Without this attention, cracking is likely occurring. in stainless steels, cracking usually results from too little ferrite in the fusion zone. Low-alloy ör hardenable steels usually crack as a result of the formation of martensite, although they too can crack as a result of low ferrite content. Joining of dissimilar Al alloys can result in joints with cracks as a result of insufficient alloy content. Melting and vaporization of a material at the start of the vvelding process is measured in milliseconds. This rapid heating process has a mostly indirect effect on the behavior of the material. The microstructural changes occurring in the solid material, as a result of the extremely rapid temperature cycle, also mean that no coarsening of the structure takes place. in ali evaluations of the fusion and heat affected zones, the microstructure and strength changes that occur are limited to a very narrow band of materials in comparison with other vvelding processes. Bctremely rapid and localized heating by the electron beam leaves the material adjacent to the weld almost unaffected thermally. The extremely rapid solidification can act very detrimentally on the closing of the "keyhole" and degassing of the melt. in particular, when vvelding seams that do no penetrate through the full thickness of the material, unstable processes take place at the base of the keyhole. Most iron and steel materials are very suitable for vvelding. Case hardening and nitriding steels, can be successfully electron beam welded prior to surface treatment of after machining away the surface treated regions in the area of the weld. As the carbon content increases the tendency to hardening also increases. As a result of the high rates of cooling experienced during electron beam vvelding hardening becomes significant at about 0.2% carbon content. Even high alloy steels are generally suitable for vvelding. The decrease in the tendency to höt cracking during electron beam vvelding has been confirmed by observations made during the vvelding of hardenable steels. it must be mentioned that, in the case of high alloy CrNi-steels, in many instances the vveldability is dependent both on the oxygen and nitrogen contents of the metal as well as on the stabilising elements niobium and titanium. Thus certain alloy steels, and in particular high temperature steels, vvill only meet the stringent requirements for successful electron beam vvelding if they have been vacuum melted. The high thermal conductivity of copper causes the molten metal to solidify very rapidly vvhich thus impedes closing of the keyhole and the escape of gaseous elements. OF - coppers are quite suitable for vvelding. Püre nickel and nickel materials with low alloy contents are suitable for vvelding. Electron beam vvelding is now an extremely important process in fabrication of high strength nickel alloys for engine and türbine construction. XXI in general in vvelding of aluminum the tenacious oxide skin, the high melting point difference betvveen the oxide and the parent metal and the high affinity of the weld pool for atmospheric gases, in particular for hydrogen. However with electron beam vvelding no additional steps need be taken. The electron beam easily breaks through the oxide skin, melting in the process, whilst the vacuum keeps any harmful gases away from the weld pool. Electron beam vvelding is thus excellently suited to vvelding of aluminum, and as such only a few comments on the metallurgical processes which take place are necessary. in electron beam vvelding of aluminum, zinc is regarded as an especially unvvelcome alloying element, it increases the sensitivity to höt cracking, as well as causing considerable formation of porosity the melt because of it high vapour pressure. A large proportion of cast aluminum materials is also suitable for vvelding. VVork hardened and hardenable aluminum loses a certain amount of their hardness in the narrovv fusion zone. With few exceptions, ali the titanium alloys are very suitable for electron beam vvelding. The only exceptions are the p-alloys which tend to become brittle in their heat affected zones because of metallurgical effects. The low thermal conductivity of titanium causes marked grain grovvth in the fusion and heat affected zones such that during electron beam vvelding high speeds are preferred. Amongst the special metals are materials such as beryllium, molybdenum, niobium, tantalum, vanadium, tungsten and zirconium, ali of vvhich like titanium are of particular importance in certain branches of industry, but compared with normal materials such as steel, aluminum, ete, are not often vvelded. Ali of them have a marked tendency to become embrittled as a result of absorption of atmospheric gases even at relatively low temperatures belovv about 300° C. These materials are highly suitable for electron beam vvelding in contrast to other vvelding methods. An other important and interesting situation is the relation betvveen beam deflection and material. Electron beams are affected by both electrical and magnetic fields, vvhich in particular instances can result in both unexpected and disruptive beam deflection. Thus, for example damping arrangements and other vvelding aids to be used close to the position of vvelding should either be made from non-magnetic materials of in the case of ferritic steels should be demagnetised before use. Although electron beam vvelding under vacuum is a long-established process. Less knovvn in industry is the electron beam vvelding of component under atmospheric pressure (non-vacuum electron beam vvelding, NV-EBVV). it has been used successfully for more than 25 years, mainly in the automobile industry and in other forms of mass production. Apart from the robust nature of the process, the superior advantage of non-vacuum vvelding machines is the very short cycle time achievable due to the omission of evacuation time for the vvorkpiece chamber. Enclosing the vvorkpiece in a vacuum chamber imposes limitations on the size of the vvorkpiece and on high-speed, high -production automation of the process unless large ör complex chambers and vacuum systems are provided. The use of portable vacuum chambers also tends to be complex and cumbersome. Thus the development of a practical system for electron-beam vvelding with the vvorkpiece at atmospheric pressure vvould significantly increase the scope of application of this vvelding technique. Tvvo basic technical problems must be solved in order to produce a practical nonvacuum electron-beam vvelding system. First, a technique must be developed XXII that permits the electron beam to escape from the vacuum environment in which it is formed and controlled into a region of gas at atmospheric pressure vvithout significant loss of power. Second, the electron beam must be öne that provides a reasonable vvorking distance extemal of the gün structure before scattering reduces its power density to levels too low for metalworking. Non-vacuum welding is increased application in the automotive industry where production requirements are high. in the automotive industry, advantage is taken of the relatively simple tooling required, the high speeds obtainable, and the reliability and reproducibility of the process. VVelding of torque converters, exhaust gas converters, steering column jackets, car frame parts are realized and proven examples. The future non-vacuum electron beams as well as ali other types of electron beams are especially promising. it is now up to the user to select carefully the equipment and tool the component in order to realize the full potential of electron beam vvelding. in this study, it is vvorked on a high-vacuum electron beam vvelding machine that has an accelerating voltage of 45 KV. it is electron beam vvelded gearbox components (forvvard and back gear transmission components). After giving considerable knowledge about working range, parameters and important parts of the machine, it is examined a few specimen from the point of view of metallurgical and mechanical features. Each specimen polished, etched and examined under a microscope. it is applied dynamic loading because of vvorking condition of specimens and taken good results. Especially, a microstructure of vvelding area, heat affected zone and parent metal is depicted with a wide range of examples. An evaluation of weld quality has been carried out based on the results of microstructure and hamess distribution of vvelding joint, as well as chemical composition analysis of vvelding joint Specimens that are subjected to chemical analysis have carbon rates 0,363 %; 0,368% and 0,385 %. Because medium carbon steel has a carbon rage of 0,30 to 0,50 %, results fit to that type of steel. Although carbon content is not the only factor affecting vveldability, it is generally thought that as carbon content increases, vveldability decreases. When carbon levels reach the 0,30 to 0,35 % range, special precautions, such as preheating, controlling heat input and postvveld heat treating, are normally required. As a result of rapid cooling in electron beam vvelds some fusion zone hardening is encountered. Also, with very fast cooling rates, martensite forms. Recent developments in EB vvelding have been in the areas of computerized control, data acquisition, filler metal additions, video monitoring, seam tracking and very high voltage. Continued success in production depends on rigid control. XXIII Although already accepted by some as an important joining process, electron beam vvelding stili is a process in its infancy with substantial growth and development indicated. As worldwide fundamental and technological research continues on an ever-increasing scale it is expected that ELECTRON BEAM VVELDING will experience further and continued expansion.
|Description:||Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1998|
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 1998
|Appears in Collections:||Makine Mühendisliği Lisansüstü Programı - Yüksek Lisans|
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