Control of the electron beam active zone position in electron beam welding processes

It is known that charged particles emitted from the region of electron beam (EB) interaction with the material being processed, are an important source of information for the understanding of EB welding processes. Measurements for the three largest groups of charged particles, namely, backscattered electrons, true secondary electrons and ions are presented here. It was estimated that only the signals of the direct component amplitude of these particles’ currents, processed by neural networks, could be used to effectively control the EB welding process. Computer simulations of various models of neural networks are described. The best result was obtained for a network that determines an optimal value of focusing current for the weld being made, based on the amplitude of signals measured with a moderately defocused EB.     

A new generation of power supplies for electron beam welding machines

Widely used electron beam welding machines are equipped with heavy power supplies, located in a special oil tank and connected with an electron gun to a high-voltage cable. A special system detects electric discharges in the electron gun space, which may arise during welding and then it tries to switch off the high voltage to interrupt an electric arc. Such disadvantages have been eliminated with the novel power supply described here. The Q of resonance circuit of this supply is stabilized and as a result circulating power appears. During an electric discharge in the gun, power is not sent to the electron gun but circulates between the electronic parts of the resonance circuit without losses and “waits” for the break in the short circuit. The power supply is much smaller and lighter than supplies of similar rating used nowadays. It is connected directly to the electron gun chamber without a high-voltage cable. The first construction of the supply was designed for electron beam welding machine of 5 kW power and 60 kV accelerating voltage. Tests of the new power supply in laboratory and industrial conditions have shown its usefulness for electron beam welding.     

Design optimization of a hairpin electron source for electron beam welding

We report the optimization of the design and performance of recently reported hairpin tungsten electron source as a cathode in diode type geometry of the gun. The temperature maximum has been shifted close to the crown of the source. Focusing of the beam has been achieved up to 1 mm in diameter with Gaussian profile of the beam at the target. The perveance and power density measured are 10⁻⁵ A V^−3/2 and 10⁶ W cm⁻², respectively.     

New vacuum electron beam processing method based on temperature closed-loop control

This is a novel combination of vacuum electron beam (EB) processing and automatic control theory. The concept of full EB processing method was presented. The digital control system for temperature closed-loop control in EB processing based on scanning track control was realized. The method of controlling electron beam current and heating power by real-time adjusting grid bias voltage is feasible. EB processing methods consist of two stages: heating-up stage and temperature maintaining stage. In the former stage, a fuzzy controller was used to control the temperature rising velocity, the sensitivity of the control was evaluated; in the latter stage, an integral separation PID controller was used to keep the workpiece's temperature constant, ideal static and dynamic performances were obtained. The temperature rising velocity control and the maintaining control compose the full EB processing method, the transition of the two stages is bumpless.     

Beam quality test technology and devices of electron beam welding

The electron beam four-dimensional quality test system was developed targeted at the 5-100 mA electron beam current of a high-voltage electron beam welding machine. The system includes the control module, sensor module, driver module, and analysis software, which is based on the complex programmable logic device (CPLD). The quality test system obtains the data of quasi-instantaneous power density distribution (QIPDD) of the electron beam by controlling the beam periodically, scanning the surface of a Faraday cup. Fixing the Faraday cup at an X- and Y-axis coordinate position under constant conditions, the three-dimensional graphic of QIPDD of the beam about the data of a single cross-section was showed by using computer reconstruction technology. Several QIPDD of the electron beam cross-section at different working distance in axial direction reconstructed the four-dimensional graphics of the beam.     

The electron beam gun with thermionic hairpin-like cathode for welding and surface modifications

An axial thermionic electron beam emitter assembly with a special geometry of the cathode along with particular spacing of the electrodes has been used to produce a stable, sharp and high power density image at an acceleration voltage of 10 kV only. A hairpin-like tungsten wire, with diameter of 0.7 mm having semi-spherical emitting area at the crown with an angle of 45 degree at the vertex was used as a cathode. A direct heating method was used to heat the cathode. The emission current of the gun is in accordance with the Langmuir relation. An electromagnetic coil was used for focusing the beam at the target. A two dimensional programmable movement was applied to control the work site in the x-y direction. Focusing of the beam has been achieved up to 1 mm in diameter at an acceleration voltage of 10 kV.Thermionic efficiency of the gun is 4 mA W⁻¹ and the power density measured is ∼10⁵ W cm⁻².The gun was used for welding and surface modification of different materials including refractory metals.     

Analytical solution of the spherical cavity expansion process

The dynamic spherical cavity expansion is treated through a complete analytical solution of the equations of motion for an elasto-plastic solid obeying von-Mises yield criterion. The solutions for various metals, with different elastic and plastic properties, are fitted with a third order polynomial which relates the normalized pressure inside the cavity with the normalized velocity of the cavity wall. An extensive search for material similarities is conducted in order to highlight the roles of the elastic properties of the solid, as well as its strength and equation of state parameters. Using the simple terms we derive, for the coefficients in the third order polynomial, one can easily calculate the relation between pressures inside the cavity and their wall velocities for any solid to within 1%.     

Model-based approach for quality improvement of electron beam welding applications in mass production

A model-based approach is presented to obtain a definite geometry of the seam as well as to find the regimes where the results will repeat with less deviation from the desired values in the electron beam welding. Using the response surface methodology, polynomial regression models for the behaviour of weld depth and the weld width are found. In order to improve the quality of the process in mass production, by a decrease of the deviation from a target value of the performance characteristic, parameters in two models describing the mean value and the variance for the weld depth and for the weld width in mass production are estimated. Using these models quality improvement, defined as an optimisation problem to produce reproducible welds while keeping the mean value of weld depth or/and width constant, is discussed. This approach is applied to the electron beam welding of stainless steel, for beam powers in the region of 4.2-4.8 kW and welding speeds of 3.333-13.333 mm/s.     

Relational analysis between parameters and defects for electron beam welding of AZ-series magnesium alloys

In the last half century, lightweight magnesium alloy has gradually shifted from military applications to civil applications. More noteworthy is that its low melting point, high thermal conductivity, and superior fluidity are good for weld pool flow and welding parameter research. This paper presents a novel approach to these characteristics, which analyzes the influences of electron beam welding parameters on weldment strength and defect formation by linking Taguchi's method with the grey relational analysis. Not only are the parameter contribution and the defect weight individually quantified, but also the relationship between welding parameters and defect dimensions can also be obtained this way.     

Microstructure evolution and fracture behaviour for electron beam welding of Ti-6Al-4V

The effect of microstructural characteristics on fracture behaviour mechanism for electron beam welding of Ti-6Al-4V was investigated. The results indicated that the welded microstructure composed of coarse needle α + β phases presenting disordered and multidirectional short needle morphology to make fracture mechanism complex. The coarse grains in weld seam with microhardness 536 HV were easy to be fractured in the region where welding heat input was ≥ 68.8 kJ/m. There exists flat curves of Ti, Al and V, Fe concentration distribution fluctuation to cause microstructural amplitude-modulated decomposition to increase the joint ductility and cleavage strength. The uneven distribution of the partial micropores located at the interior of the specimen acting as crack initiation sites lead to non-linear branch propagating path. The α + β interlaced structure results in the fracture location near α/β interface. The existence of stacking fault structure caused pile-up of dislocation to produce micropores to be new fracture initiation sites.     

An analytical model and tomographic calculation of vacuum electron beam welding heat source

According to the analysis to the characteristics of welding heat source and thermal effect, a mathematic model of rotary Gaussian body heat source with incremental power-density-distribution was developed, which was in line with the characteristics of heat source during vacuum electron beam welding. The affecting radius of source model decreases progressively with the law of Gaussian function and the power density varies gradually with the law of exponential function in depth direction. The evaluation of peak-power-density coefficient β and the tomographic calculation of the source model in different focused conditions were discussed. The results showed that the focused conditions, which were the deviation of depth of field in electron beam, were dependent on the coefficient β in the source model. Simulation of thermal effect and the analysis of weld formation in vacuum electron beam welding validated the feasibility of the model.     

Influence of the filler metal on the properties of Ti-6Al-4V electron beam weldments. Part I: Welding procedures and microstructural characterization

The use of different procedures for electron beam welding of 17 mm thick Ti-6Al-4V plate and the difficulties found in this process are analysed. When this alloy was welded autogeneously the presence of significant amounts of α martensite was observed, recommending looking for another solution. In the early trials a V joint design was used but distortions and defects were detected in the welds when multi-pass procedures were considered. Consequently, for the remaining weldments K or I joint configurations were selected. Initially, Ti-6Al-4V wire was preferred in order to match mechanical properties with base material but no significant improvement was found leading to consideration of using a less alloyed filler metal. Different commercially pure titanium filler metals have been employed to optimise the performance of the fusion zone of electron beam weldments. In a second paper [1] the influence of the welding procedure on the mechanical properties of the various joints will be discussed.     

Interfacial characteristics of electron beam welding joints of SiCp/Al composites

Abstract

The electron beam welding of SiCp/101Al composites has been carried out. The influence of welding parameters on weldability and mechanical properties of the welded joints was discussed. The welding parameters were therefore optimised under the current experimental circumstance. Results show that only weak interfacial reaction between SiC particle and liquid aluminium occurred. Minute quantity brittle Al₄C₃ compounds and single phase Si were generated in the welded joint. The interfacial reaction between SiC particles and Al matrix could be greatly suppressed by adopting appropriate technique measures such as high welding speed and low heat input. The content of Al₄C₃ can be therefore greatly decreased in the welded joint. Moreover, modification welding and electron beam scanning could further improve the appearance of weld, and the welded joint with better quality could be obtained.     

Research on fatigue behavior of electron beam welding joint of 06Cr19Ni10 austenitic stainless steel sheet

06Cr19Ni10 austenitic stainless steel sheet square butt joints without filler metal addition were fabricated using EBW (electron beam welding) processes. Fatigue properties, tensile properties and microstructure of the welded joints were studied. It was found that the yield strength, tensile strength, elongation and Vickers hardness of EBW joint can reach the level of base material performance. Fusion zones consist of coarse columnar dendritic grains, which are perpendicular to the weld pool boundary. The hardness of heat affected zone is lower than weld centreline. The reason is that the grain of heat affected zone under the action of electron beam heat source happened recovery and recrystallization. The present work suggests that S–N curve slope of welding joint should be revised to m = 10 under the condition of high stress levels, and S–N curve slope is still m = 3.0 under the condition of low stress levels. Fatigue cracks did not extend along the minimum thickness of the section. On the contrary, fatigue cracks extended along the maximum height of the section.     

Analytical solution for bi-material beam with graded intermediate layer

Using the Airy stress function, an analytical solution is obtained for a bi-material beam with a graded intermediate layer, which is subjected to a uniform load on the upper surface and has different boundary conditions at the two ends. Young’s modulus of the graded intermediate layer is assumed to be an arbitrary function of the thickness coordinate and its Poisson’s ratio is kept a constant. The solution can easily degenerate into the ones of the tri-material beam, the bi-material beam, the homogeneous beam, and the graded beam, and some of them coincide with the available solutions. The analytical and numerical (finite-element-based) results are in agreement with each other for several examples. The influence on the stress distribution for the cantilever beam is discussed when Young’s modulus of the graded intermediate layer takes different functions.     

Direct evidence for electron beam irradiation-induced phenomena in nanowired ZnO thin films

In-situ electron beam induced microstructural transformation experiments, leading to porosity in nanowires of ZnO, have been performed under a TEM operated at an electron accelerating voltage of 200 kV. For this purpose, nanowired (diameter: 20 to 80 nm) films of ZnO with thickness ~ 100 to 120 nm, were grown via metal-catalyst free-vapor phase mechanism. The evolved porosity (pore size about 2 to 20 nm) in nanowires, under electron beam irradiation, has been attributed to different bond-breaking phenomena at molecular Zn-O. Such nanoporous objects of ZnO are beneficial for various optical and sensing devices.     

Simulation on welding thermal effect of AZ61 magnesium alloy based on three-dimensional modeling of vacuum electron beam welding heat source

The deep-penetration thermal effect of keyhole and surface thermal effect of high-temperature metal vapor by the direct-acting mechanism during vacuum electron beam welding were analyzed. According to the thermal effect, a composite source model working for magnesium alloy welding was developed. This model was composed of Gaussian surface source and conical heat source. By the welding experiments on AZ61 magnesium alloy, it can be obtained those two key factors, which were welding heat input and focus coil current (I_f), affected the keyhole thermal effect and weld shape significantly. In order to simulate the shape of keyhole and weld, the varying of focus conditions in model were realized by the power coefficient of composite welding heat source. It can be seen that there was a good consistency between the calculation results and experimental results.     

电子束曝光机的偏转系统

电子束曝光机的偏转系统控制电子束偏转扫描。像差低、偏转灵敏度高、扫描速度快是它的基本要求。对各种偏转器、偏转方式进行分析、比较,从偏转器空间场的数值计算方法、偏转系统的优化、像差校正、偏转器制作工艺、电气参数等方面阐述设计过程和工程实现上一些值得注意的问题。综合考虑偏转器和偏放电路的设计可以得到最优性能的系统。     

Bubble flow and the formation of cavity defect in weld pool of vacuum electron beam welding

Seen from gas-liquid two-phase-flow system, the gas phase and liquid phase of bubble flow in weld pool are studied by means of isolated phase based on the conservation of mass and momentum. The two-dimensional fractional flow model of bubble flow in weld pool of vacuum electron beam welding is developed. And the gas distribution and the phenomenon of bubble flow in weld pool of AZ91D magnesium alloy are simulated to analyze the formation and distribution of cavity defects. The results show that the possibility of gas escape in fully penetrated weld pool is much greater than non-penetrated weld. It appears that the probability of cavity defects is lower than non-penetrated weld to some extent. The formation of typical cavity defects is closely related to the flow pattern and flow characteristics of the bubble flow in deep penetration weld pool of vacuum electron beam welding. Higher liquid flow rate is more conducive to the escape of gas in molten metal, so that the final porosity in weld is low.     

EBW-真空抽气系统设计

大抽速的真空抽气系统是大型电子束焊接设备(electron beam welding,EBW)中必不可少的组成部分。本文介绍了一个大型电子束焊设备中真空抽气系统的组成及其设计思路、方法,并对此真空抽气系统所能得到的本底真空度、工况真空度以及各阶段的抽气时间进行了详细的计算。为大型真空设备的研制提供一个有益的思路和便捷的方法。