Elektron ışın kaynağı yöntemi ile kaynatılan ınconel 718 malzemesi üzerinde seçili değişkenlerin etkisinin incelenmesi
Investigation of the effect of selected variables on the inconel 718 material welded by electron beam welding
Yazar: BARIŞ BÖYÜKER
Danışman: PROF. DR. MURAT VURAL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Elektron ışın kaynağı = Electron beam welding ; Makine mühendisliği = Mechanical engineering

Özet:
Kaynaklı imalat yönteminin insanlık tarihinde metal malzemelerinin işlenmesi ile neredeyse aynı tarihten itibaren kullanıldığı bilinen bir durumdur. Çok çok eskiden kaynaklı imalat yönteminin karanlık bir sanat yada kaba bir inşaat tekniği olarak kabul edilmesi ise ilginç bir durumdur. Özellikle keşiflerin hızlanması, elektriğin yaygınlaşması gibi gelişmelerin ışığında, on dokuzuncu yüzyılda yeni tip kaynak yöntemlerinin geliştirilmesinin önü açıldı.
Mukavemet ve maliyet açısından bakıldığında kaynaklı imalat yöntemi hava araçlarının parçalarında perçinler ile yer değiştirmiştir. Bu kaynaklı imalat yöntemlerinden Elektron Işın Kaynağı yöntemi askeri hava araçlarının titanyum parçalarının kaynatılmasında sıklıkla tercih ediliyordu. Elektron Işın Kaynağı düşük ısı girdisi ve malzemenin kaynak sonrası yapısal dayanımındaki durumdan ötürü popüler olmaya başlamıştı.İlk olarak 1950'li yılların sonlarına doğru nükleer enerji ve havacılık sektörlerinde tercih ediliyordu. Diğer kaynak türlerine göre yüksek güvenilirlik ve kalite sunması önemli bir etkendi. Ayrıca imalat maliyetlerini de azaltmış, imalatı yapılan parça sayısı ise artmıştı.
Elektron Işın Kaynağı yönteminde elektronlar yüksek vakumlu ortamda hızlandırılıp, iş parçasına doğru hareket ettirilir. İş parçasına temas eden elektronların sahip olduğu kinetik enerji ısı enerjisine dönüşür ve iş parçasının ergimesi gerçekleşir. Elektronlar iş parçası üzerinde çok ufak bir noktada yoğunlaştığından ötürü bu bölgedeki enerji yoğunluğu çok yüksek seviyelere çıkar.
Çalışmamızda Inconel 718 malzeme kullanılmıştır. Inconel 718 malzeme en yaygın kullanılan ve bulunan nikel alaşım malzemelerden biridir. Kaynaklanabilirliği çok yüksek olan bu malzeme savuma ve havacılık sektörlerinde sıklıkla kullanılır.
Inconel 718 malzemeden oluşan 0,080" (2,032 mm) kalınlığında sac malzemelerin Elektron Işın Kaynağı tezgahında farklı parametreler ile kaynatılıp bu parametrelerin etkileri bu çalışmada ele alınmıştır. Bu inceleme için üç farklı parametre seçilip her bir parametre için de üç farklı değer atanmıştır. Bu üç farklı parametrei voltaj, akım ve kaynak hızıdır. Değerler ise voltaj için 30,35,40 kV; akım içim 30,33,36 A; hız için ise 12,16,20 mm/sn olarak seçilmiştir.
Toplamda 27 adet numune hazırlanarak kaynak denemesi tamamlanmıştır. Kaynak denemesi sonrasında numuneler metal laboratuvarında metalografik incelemeye tabi tutulmuşlardır. Bu inceleme sonrasında görülmüştür ki, voltajın veya akımın artışı kaynak nüfuziyetini arttırmaktadır ayrıca kaynak hızının düşüşü de nüfuziyeti arttırmaktadır.

Abstract:
It is known that welded manufacturing method has been used in the history of mankind since almost the same date as the processing of metal materials. It is interesting to note that the very old welded manufacturing method is regarded as a dark art or a rough construction technique. Especially in the light of developments such as the acceleration of discoveries and the spread of electricity, the nineteenth century paved the way for the development of new types of welding methods.
In terms of strength and cost, the welded manufacturing method was replaced by rivets in the parts of aircraft. Of these welded manufacturing methods, the Electron Beam Welding method was often preferred for welding titanium parts of military aircraft. Electron Beam Welding was becoming popular due to the low heat input and the post-weld structural strength of the material. The high reliability and quality compared to other types of welds was an important factor. It also reduced manufacturing costs and increased the number of manufactured parts.
In Electron Beam Welding method, electrons are accelerated in high vacuum environment and moved to the workpiece. The kinetic energy of the electrons contacting the workpiece is transformed into heat energy and the workpiece melts. Because the electrons are concentrated at a very small point on the workpiece, the energy density in this region increases to very high levels.
Electron welding or electron beam welding (EBW) is a welding method that is applied by means of heat which is formed by a high concentration of electron beam that strikes the material surface rapidly. The electron gun operates with high voltage (10-150 kV) to accelerate the electrons, while the current values are significantly low (at milliampere). What is essential in EBW is not the power, but the power density. The electrons focus on the material at a very small point, resulting in a high energy density.
In addition to all metals that can be welded by the arc welding, EBW can weld some metals which cannot be welded by the arc welding and metals which are very difficult to weld. The materials to be welded are varied from thin foils to thick sheets. This welding method is mostly used in automotive, aerospace industry, nuclear power industry. The fields of application of the method in automotive industry are welding joining of aluminum manifolds, transmission parts, steel torque converters, catalytic converters. The high quality of the welds in the electron welds, the deep and narrow welds are very narrow ITAB and a low rate of thermal expansion. Welding speeds in this method are faster than other welding methods. There is no need for filler metal, dust or gas to cause slag. The flaws of this welding method include high cost, precise and demanding welding preparation and difficulties with vacuum.
Output variables in electron beam welding are weld penetration dimensions and the level of welding errors. Like other welding methods, welding defects can occur in electron beam welding, such as pores, cracks, combustion chutes. Resources to be quality tested must be within the limits defined by the standards. Input variables must be strictly controlled to ensure that the source complies with these standards.
Voltage and current input parameters determine the power intensity of the beam. In most electron beam welding machines, the voltage is kept constant and the required power density is obtained by changing the current. The welding speed is the speed of the workpiece in the welding direction relative to the beam. The speed affects the heating and cooling speed and the depth of penetration. The focal current is the input variable of the magnetic lens that determines the focal distance. This distance may be below the surface of the workpiece or above the surface of the workpiece.
For special welding applications, the beam variables are controlled by the computer system during the process. However, due to interference elements or other variables, electron beam of various properties is obtained, which causes changes in penetration sizes and error types. Another variable is the human effect that decides the sharp focus setting. Because the beam itself is visible, the sharp focus is usually adjusted by the operator by observing the brightness of the beam on the workpiece, when the low-current beam is positioned on the workpiece. Consistency in adjusting the sharp focus is a big question mark because different operators may see different luminescence due to metal dust in optics. As a result, even if welding variables are set consistently, machine setup and operation can have different results in welding performance.
Simply the electron beam is formed by the electron gun containing the cathode, the source to heat the electrons, the grid, the electrode and the anode specially shaped according to the hot cathode emitter. The hot cathode emitter is made of high radiation material, for example tungsten or tantalum. This spreading material is usually available as wire or sheet metal and can be manufactured to the desired shape.
The electrons emitted from the cathode emitter are accelerated to high velocity and become a beam aligned by the electrostatic field geometry provided by the cathode / grid / anode. Thus, constant flow electrons flow towards the anode is provided.
When electrons leave the anode, they receive the highest energy input by the application voltage applied to the electron gun. The electrons descend towards the electron beam column assembly and reach the magnetic lens field. This focusing lens reduces the diameter of the electron beam. The beam continues to flow and the electron beam with a much smaller cross-sectional area is focused on the workpiece. The decrease in beam diameter increases the energy density and creates a very small, high intensity beam on the workpiece. In addition, the electromagnetic deflection coil can be used to bend the beam, which provides flexibility in moving the focused beam.
The electron gun is generally kept separate from the welding section. In some cases, the gun can be operated in a vacuum (13 mPa / 1 x 10-4 torr) environment, which generally applies to situations where the workpiece is large and difficult to access. This level of vacuum in the gun is necessary to keep the gun parts clean, to prevent oxidation and to prevent the gun from making arc. A similar amount of vacuum during welding is necessary for both the electron gun and the welding chamber, because if there is no vacuum medium during the welding, the electron beam can strike and deviate from the air molecules along the path it receives. Such an interaction produces a wider electron beam point, which leads to a reduction in energy density. Generally, electron guns are operated in the voltage range from 30 to 200 kV and in the current range from 0.5 to 1500 mA. Electron beam welding equipment is common up to 30 kW, but some units up to 200 kW are commercially available. Generally, high-vacuum electron beam sources can be formed with diameters between 0.25 and 1.3 mm (0.010 to 0.050 in.) And a power density of 107 W / cm2 (106 W / in2). The high level beam point produces a temperature of about 14000 ° C (25000 ° F) and is sufficient to evaporate almost any metal, which provides a steam hole leading to the depth of the material.
Electron beam welding is usually only done under high vacuum (13 mPa / 1 x 10-4 torr) because vacuum medium is necessary to produce the beam. Boiling the material in a clean atmosphere is useful enough to consider. However, it has been found that the amount of vacuum in the welding chamber need not be as high as that of the electron gun, since more part manufacturing volume is desired. In addition, it has been revealed for some applications that there is no problem in welding the workpiece without any vacuum.
Inconel 718 material was used in our study. Inconel 718 material is one of the most widely used and found nickel alloy materials. This material has high weldability and is frequently used in the defense and aerospace industries.
Inconel 718 and its other commonly used name, alloy 718 is one of the most commonly used materials for nickel alloy materials. Inconel 718 material is a nickel - chromium - molybdenum composition similar to the inconel 625 material. Inconel 718, which corresponds to 2,466 according to DIN Standard, also corresponds to the UNS N07718 standard. Alloy 718, which has a very high corrosion resistance, has a chemical expansion of NiCr19Fe19Nb5Mo3.
Inconel 718, which can be hardened by aging, can maintain its mechanical properties even at different temperatures. The tensile strength of this alloy material is very high. In addition, the breaking resistance and fracture resistance are quite high. This material, which is also resistant to high temperatures, has a much harder structure when it is deposited by aging.
Inconel 718 or Alloy 718 is often welded using additional metal in the industry and subjected to post-weld heat treatment. It is highly resistant to stress cracks that may occur after welding and is a superalloy with high welding ability. Precipitation hardening Nickel alloys are normally subjected to deformation aging cracking after welding. Our material, inconel 718, is exposed to ITAB cracks, because large grains facilitate these cracks. These cracks are actually intergranular disintegration in the ITAB region and these cracks can occur in Nb-Ni-containing alloys in various welding methods. It is explained that the tendency of crack formation in the ITAB region during welding is again due to a liquid phase that occurs between the beads in the ITAB during welding.
Inconel 718 materials consisting of 0,080 "(2,032 mm) thick sheet materials are welded with different parameters in the Electron Beam Welding machine and the effects of these parameters are discussed in this study. Three different parameters are selected for this study and three different values are assigned for each parameter. These parameters are voltage, current and welding speed. So, values are 30.35.40 kV for voltage, 30.33.36 A for current and 12.16.20 mm / s for speed.
A total of 27 samples were prepared and the welding experiment was completed. After the welding test, the samples were subjected to metallographic examination in the metal laboratory.