1　Introduction

Conventional welding produces a weld by conducting heat from the weld torch producing a semicircular weld or conduction mode weld. Keyhole welding produces a deep narrow weld by first drilling a hole then conducting heat outward from the vapor column. EB welding machines can produce both keyhole and conduction mode welds. In electron beam welding, the high velocity of electrons hits the metals to be welded. During this process heat transfer is accomplished by a phase change in the conducting medium due to the absorption or release of heat in the active zone.

2　The role of input parameters on the weld

Input parameters like beam current, focus current, voltage, welding speed, work distance, vacuum etc. has important role in producing a quality weld. Hence these parameters have to be optimized for the corresponding material.

Benjamin et al.[1] studied the mechanical and metallurgical properties of electron beam welded 316L(N) austenitic stainless steel by varying beam power and welding speed. They found that due to the differencein cooling rate, the hardness and toughness of the material decreases with increases in Q/V ratio. They also identified that the grain growth is high in the fusion zone. Agilan et al.[2] investigated the effect of heat input on microstructure and mechanical properties of electron beam welded plates of Inconel-718 by varying the welding current from 18 mA to 90 mA. The welding current is considered to be the important input parameter and it decides the heat generated during the weld operation. It was observed that good results were recorded in the lowest heat input. Ananth et al[3] have successfully welded AISI 304L to AISI 446 using electron beam welding and optimized the parameters like accelerating voltage, beam current, welding speed, focus current, vacuum level to obtain the best quality of weld built on Taguchi method. Based on the values of the depth of penetration characterized using optical microscope, the optimum parameters are described. This research concludes with the statement thatthe beam current has maximum influence in determining the depth of penetration

3　Generation of heat source

Peter et al.[4] studied the energy transfer mechanism from an electron beam to a metal target, and found that the nature of the heat source in a weld pool is non-stationary and the dynamic processes occurring in both the welding pool and the plasma cavity play a dominant role in the formation of the welding seam during electron beam welding of metals. The main causes for the non-stationary nature of the heat source are connected with the processes of dissipation of the electron beam in evaporated metal as well as with the penetrating mass transport of liquid metal in the welding pool.Tian et al.[5] describes a finite element model to simulate the temperature field in Al alloy structure during electron beam welding. Several process parameters including input energy and welding speed were studied. A complex heat source with a modified Gaussian distribution heat flux and a uniform heat flux was assumed in this analysis. The results of this research shows that the heat input was the core factor affecting the degree of distortion of the welded structure. With the increase of heat input, the distortion of welded structure was augmented.

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Based on the laws of heat conduction and energy conservation Luo[6] developed a model for the joint in magnesium alloy using electron beam welding. The main metal elements produced vaporization in the keyhole will be Mg element. The calculation results showed that, the strong evaporation effect of Mg elements will occur within several milliseconds. In addition to the boiling point, the vaporizing time is the need of thermal diffusivity. The heat transfer effects of welding process is having a great change as the beam spot is altered. So that, the transforming of focus coil current will affect the actual heat input, evaporation of alloying elements, weld microstructures and other quality factors.

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Joining of dissimilar metal joints is serious issue that aerospace industry is facing today since the dissimilar materials to be jointed possess differences in physical and metallurgical properties. Some of the truly used dissimilar metal joining in aerospace industry are as follows. Two different titanium alloys are welded together in engine parts, two differentaluminum alloys are welded together in fuselage and two different ultra high strength stainless steels are welded together for landing gears. Very few researchers have focused on dissimilar metal joints using electron beam welding.

Wang et al.[14] done experiments in electron beam welding of titanium alloy to stainless steel with different filler metals, such as Ni, V, and Cu. Microstructures and mechanical strength of the welded joints were studied. Influences of filler metals on microstructures and mechanical properties were also discussed. Direct electron beam welding of Ti/Fe joint failed and succeed by using the intermittent layers. The microhardness value order of the joint was V, Ni, Cu from high to low order but the highest tensile strength was obtained in the joint welded with copper filler metal.

Lacki et al.[9] have done the thermo-mechanical analysis of Inconel 706 tube using electron beam welding EBW. Process simulation was performed using finite element method. The results calculated by partial least square method PLS model were used to build FEM model. FZ based on welding process heat input cannot be found out using FEM alone. PLS technique helps to detect the reliance between welding process parameters and dimensions of FZ. Chen et al.[10] welded hard alloy to steel by electron beam. Finite element analysis was carried out to find the maximum residual stress distributed at base metal where the macroscopic cracks are generated. It was found that the weldability of hard alloy and steel by electron beam welding is reduced, and cracks easily occur because of the presence of martensite and carbides in the joint. Therefore, it is suggested that an intermittent layer is required in welds. Ni-Fe alloy was used as an intermediate, and microhardness of the weld was found reduced considerably and the welding stress is relieved.

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Kolevaa et al.[11] studied the oscillations situated in the plasma over the welding zone during electron beam welding of chrome-molybdenum steel. For beams situated at the central part of the keyhole, the collected signal was minimal. From the observations of spikes in the weld root, at EBW with deflection oscillations, it was concluded that self-focusing of the beam in the bottom part of the keyhole in the cases of the most focusing positions was occurring.

4　Electron beam welding for titanium

One of the main considerations in any aerospace design is to select the suitable material which will give maximum strength without compromising the required requirements. In aerospace industry welded joints are preferred considering the advantages of minimum weight and maximum strength. Titanium and its alloys present in a wide range of high strength materials with good fatigue and corrosion resistance. The welding of Titanium and its alloy are complicated due to the fact that considerable contamination of Oxygen, Nitrogen and Hydrogen can occur above 650 ℃ resulting in embrittlement and the metal rapidly dissolves these gases.

Qi et al.[12] studied the microstructures, properties and technical parameters of 0.5 mm thick sheets of commercial purity titanium using electron beam welding, CO2 laser beam welding (LBW) and gas tungsten arc welding (TIG). Micro hardness has been observed in all the welds. They determined that the size of grains is the finest by LBW, EBW, and largest by TIG. The electron beam welding method is suitable for titanium sheets welding, since the welded seam are attained without defects.

Tosto and Nenci[24] studied the possibility ofattaining a defect free Electron Beam welded joint in 2219-T6 A1 base alloy for aerospace application. Post-weld treatments also have been carried out on EB welded joints of the alloy. The results show that suitable post-weld EB heat treatments modify the microstructure of the molten zone and produces better weld.

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Wu, Li and Tang[15] investigated the electron beam (EB) welding of Ti casting plates. The microstructure and the tensile properties of joint was studied. The results show that the double-side molding in electron beam welding of can be achieved by adjusting the welding speed and welding current. In the case of thick plates, it is difficult to achieve a double-sided weld shape, so an excess material should be set aside in the direction of weld penetration to ensure the structural quality. The microstructure of welded TC4 alloy, is composed of α laths and β phases, the weld consists of acicular martensite, and HAZ consists of thin acicular martensites, α laths and β phase. The tensile strength of ZTC4 joints is higher than that of the base metal, so the tensile strength of ZTC4 EB-welds can be improved by optimizing the composition and the microstructure of the base metal.

Adamus et al.[16] have done numerical analysis for welding of a titanium sheet using electron beam welding technology. In this work the thermo-mechanical simulation of the EBW process was investigated. The higher number of linear element layers in the thickness direction contributes to the higher degree of transverse bending. The comparison of linear and quadratic elements, with respect to calculation time and accuracy, showed that similar results are obtained for cases with either 2 quadratic or 6 linear element layers. It was found that by increasing the number of element layers above these values better results are obtained for linear elements because of lower calculations time.

5　Dissimilar metal joints

Luo et al.[7] derived the governing equations from the laws of mass and momentum conservation for the molten pool by electron beam welding. The results showed that the gravity is an important factor to drive the convection of the bubble flow in the deep penetration molten pool during electron beam welding. Due to the gravity effect cavity-type defects tend to distribute at the bottom of weld. They concluded that the gas convection velocity in molten pool is comparatively low under the weightless or microgravity circumstances.Wang et al.[8] developed a heat source model for electron beam welding of titanium alloy by numerical simulation. The combined point-linear heat source model was brought forward and to simulate the welding temperature fields of EBW. With the heat source model, the weld shapes predicted from temperature fields are conformable to those of the actual weld, and four weld shapes are gained including nail pattern, bell pattern, funnel pattern and chock pattern.

Albert et al.[17] studied the microstructure and toughness of dissimilar joints between 1.0 wt.% W RAFM steel and AISI 316L (N) SS using EBW process. This dissimilar welds experiences fast cooling rate and does not provide sufficient time for mixing of both the fused base metals in the fusion zone. As a result, com-positional and microstructural heterogeneities exist in the fusion zone, which in turn results in difference in properties. A detailed study of the mechanical properties, including long term properties like creep between RAFM steel and 316LNusing EB welding is reported as future study.

Karppi[18] made a survey to indicate that there is still a considerable need to further examine existing and new combinations. He identified that the use of the EBW process to join dissimilar metals is still at the development stage. Very limited amount of information is available concerning the joining of different dissimilar combinations. This will stimulate many future studies.

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Rajkumar et al.[19] studied the tensile strength in the laser welded dissimilar joints of stainless steel and copper. They optimized the process parameters such as welding speed, laser power and pulse duration to improve the weld strength. This same combination has to be compared with Electron Beam Welding and the test results have to be compared.

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Dasa et al.[22] studied the microstructure and mechanical properties of the 9Cr-1W welded joint using electron beam welding. They reported that the width of HAZ is smaller weldment prepared by EB process compared with TIG weldments. The joint is also found to be stronger than that of the base metal.

Paradiso et al.[21] studied the dissimilar metal joints of Magnesium and Aluminium alloys by Friction stir welding. The microstructure and mechanical characterization has been done to analyze the strength of the weld. The formation of an intermetallic compound was investigated in this research which in turn reduces the weld strength. Electron beam welding has to be carried out in the same metal combination and the results can be compared.

6　Mechanical strength and microstructure

Gupta et al.[20] studied the Microhardness, mechanical properties and fracture toughness of similar and dissimilar metals of Fe-31Ni-5Co alloy and Co-20Cr-15W-10Ni. The weld efficiency is found to be the maximum in the case of both similar metal combination, but it was found that in the case of dissimilar metal joints the failure takes place in the base metal Fe-31Ni-5Co alloy which is of less strength when compared with Co-20Cr-15W-10Ni.

Tu et al.[23] done experiments to investigate the crack propagation on S355NL steel welded using electron beam welding. Mechanical properties are mined from the base material, the fusion zone, and heat affected zone respectively. Numerical investigations also been presented for the same and are compared with the experiments They found that the void initiation, and coalescence on the base material are the main reasons for ductile fracture.

Chih Jen Tsai and Le Min Wang[13] investigated the effect of post weld annealing treatments on Ti-6Al-4V alloy using electron beam welding (EBW). They identified that the mechanical properties of annealed Ti-6Al-4V weldments are noticeably developed by the process of cooling and the intermetallic compounds detected retain the characteristics of Ti-Al based alloys.

Zhao and Liu[25] studied the fatigue, tensile, and microstructural properties for the square butt welded joints of 06Cr19Ni10 austenitic stainless steel sheet using EBW. They identified that the fatigue properties of EBW joint has a great implication to the change of fatigue standard and the choice of welding methods. The study of fatigue performance of EBW joint will play a huge role in promoting its application in aeronautics.

Saresh et al.[26] made experiments to weld 17.5 mm thick joint in a spherical titanium (Ti-6Al-4V) gas bottle using EBW. In this study a two-pass double side welding is adopted for welding of 17.5 mm thick titanium joints. The mechanical properties are found to be higher and consistent throughout the weld cross section. The micro hardness is also uniform across the parent metal, HAZ and the weld fusion zone. The microstructure is comparable with that of a normal full penetration EB weld.

7　Conclusion

(1) From the above study it was crystal clear that the process parameters like beam current, focus current, voltage, welding speed, work distance, vacuum level take the key role in determining the quality weld.

(2) It was observed that no heat loss is found in EB welding and hence the microstructure shows a high quality weld with good mechanical strength.

(3) Considerable research has to be focused in the area of titanium welding since welding of titanium is more complicated with other existing methods.

(4) The above review evidently shows that research have been carried out only in very few dissimilar metal joints and hence there is a lack in research in the area of dissimilar metal joints.

(5) Therefore, future researches in the area of Electron Beam Welding have to be focused mainly with titanium and dissimilar metal joints.

农村水电增效扩容改造与世行贷款项目群管理比较研究…………………………………………… 付自龙，陈晓健(22.42)

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