New electron beam gun for welding in space

Abstract

A new design of electron beam gun for welding in open space has been developed. The gun design is based on the application of triode emission system with improved quality of electron beam formation. This provides increased specific power of the beam and, as a result, the high capability to increase the penetration depth of welded joints. To reduce the dimensions of the gun and increase its service reliability, the high-voltage insulation has been provided applying metal-ceramic heat-resistant welded-brazed vacuum-tight components. The original technologies for their manufacture have been developed. The performed tests of the new gun design have proved the possibility of its application under open space conditions.

Keywords

Electron-beam gun welding triode emission system stacked structure welded-brazed components ‘sandwich’ structure welded joints metal-ceramic components

Introduction

Electron beam welding (EBW) has a series of indisputable advantages for application under space environment conditions. They are its suitability for the operation in vacuum, virtually reference quality of the joints from the most important aerospace materials, namely alloys of titanium [1], aluminium [2] and stainless steel [3], including dissimilar and difficult-to-weld materials with high thermal conductance [4] and even natural metallic materials of space origin [5]. This method provides the maximum penetration depth independent on material type and its surface condition. It has excellent energy efficiency and can be used also in the processes of melting, brazing, cutting, coating deposition and additive technologies [6]. At last, EBW passed successfully multiple tests under real space conditions and the aspects of its safe operation were studied in details during ground tests [7].

The 1973 experiment on SkyLab demonstrated that an electron beam can be used in zero gravity for cutting, welding, or melting of metal alloys. The first manual tool for electron beam welding in space was successfully tested under open space conditions on the space station ‘Salyut-7’ in the 1980th [8]. The performed experiment showed a real possibility of welding and application of related technologies in space. From a modern point of view, it had a series of disadvantages, the main of which is low power (up to 1 kW) of the electron beam gun and connected with this impossibility of welding of different materials of more than 1 mm thickness. At the same time the thickness of structural elements, which are used in the apparatus construction and can be subjected to welding during the repair of the manned spacecraft, reaches 4 … 6 mm. For example, a calculated thickness of the rigid shell of International Space Station (ISS) Columbus module from aluminium equals 2.57 mm [9]. To solve the problem of welding of various materials of such thickness it is necessary to increase welding modes and quality of electron beam formation. On the other hand, an increase in welding modes complicates the equipment, results in more rigid requirements to its electrical insulation and rapidly increases the integral intensity of a continuous spectrum of X-ray radiation, which appears in deceleration of beam electrons. Thus, the authors’ attention was focused on providing the technological capabilities for an increase in the effective power of electron beam gun with simultaneous maintenance of its ergonomics, reliability, and safety.

Materials and methods

One of the most important constituents of the developed tool is a new small-size gun with an electron beam total power up to 2.5 kW, i.e. 2.5 times more than the power of the previous gun design. The new gun contains a triode emission system in contrast to diode emission systems earlier used in the guns with inertia control of a beam current by means of variation of cathode filament current. In the new gun, the control of the beam current is kept virtually without inertia by means of the change of control potential on a focusing electrode. Furthermore, it becomes possible to use massive washers from lanthanum hexaboride as reliable and effective cathodes and their economical heating by electron bombardment. In this case, the necessary power of cathode heating reduces to 30 … 40 W (as compared to 100 … 120 W for straight channel cathodes), operation life of the cathode rises up to 40 … 50 hours in comparison with 1 hour for straight channel cathode.

Accelerating voltage U_acc of the gun should not exceed 10 kV, which is due to safety measures in the operator –cosmonaut work, since the low power of a dose of continuous X-ray radiation at indicated parameters makes unnecessary the development of additional biological protection [8]. Higher penetrating power of the beam was obtained by the increase in the quality of beam formation in the emission system in addition to the increase in total beam power. The main quality criterion of the formed electron beam in the electron optics is its phase characteristic, which is a dependence of inclination angle α_i of electron trajectories on their radial coordinate r_i [10] made for any beam intersection. In the case of the ideal converging laminar beam, its minimum intersection (crossover) is transformed into a point and each beam trajectory has the larger inclination, the further it is located from a beam axis. Moreover, a phase characteristic of the beam is a straight line, which comes through an origin of coordinates in r_i–α_i plane. The measure of the quality of the real electron beam is a deviation of its phase characteristic from the straight line, which is, mainly, a consequence of spherical aberration of the emission system.

The well-approved procedure of synthesis of triode emission systems using an apparatus for analysis of trajectory with further experimental correction of the calculations [10] was used in this development in order to get high beam quality. Following the procedure, the system of electrodes of emission system is constructed with the initial application of its required geometric and electric parameters. Thus, the new emission system was calculated for a maximum accelerating voltage of 10 kV and a maximum beam current of 250 mA (Figure 1). A so-called tablet of lanthanum hexaboride with a flat emitting surface with a diameter d_c = 4.2 mm (1) was used as a cathode, which is fixed in the molybdenum holder (2). The diameter d_e = 6.5 mm of the central hole in the focusing electrode (3) was chosen to maintain the electrical strength of the annular gap between this electrode and the holder (2) throughout the range of the control voltage of the gun. Basing on the required working distance of the gun and the diameter d_a = 3.2 mm in the anode (4), the angle α of the coincidence of the outermost trajectories of the electron beam (5) in the crossover (6) was also given.

In most of the cases, two optimising criteria are enough for the process of step-by-step approximation of emission system optimum geometry. A requirement for the formation of the electron beam with a set angle α of its convergence before the crossover is taken as the first criterion. When estimating the convergence angle on the outermost trajectory, for example, 10^th, this criterion is presented by the following formula: (1) Another criterion is a requirement for beam formation with a linear phase characteristic. In order to estimate the deviation of a phase characteristic of the beam from the straight line it is enough to determine an inclination of two trajectories for selected beam crossing (in space behind the anode), for example, the outermost 10th, and one of the internal trajectories, for example 7th using trajectory analysis, as well as radii of location of these trajectories r₍₁₀₎ and r₍₇₎. Then the second optimising criterion is formulated as: (2) In the calculation process, the emission system approaches the zero values of the optimisation criteria and finally we determine the angular dimensions in the details of the electrodes (β, ϕ, θ) and their linear distance from each other (h_e). As a result, designed in such a way the triode emission system with set energy and geometry parameters forms a virtually laminar beam (with close to linear phase characteristic) and with the maximum possible specific power in the crossover.

The synthesised triode emission system was designed in two modifications, namely, long-focus and short-focus one. Figure 1(a) schematically represents the long-focus emission system, which is designed for the application in the gun, which is a constituent of a manual electron beam tool. In this case, the emission system forms the beam that is focused directly in a welding zone. Taking into account that in welding of small thickness metals in most of the cases a focal plane of the beam is located on a sample surface, the beam-specific power in the welding zone, and, as a consequence, its penetration ability completely depend on the intersection plane of the beam in its focal plane, i.e. on the beam diameter in the crossover. The long-focus emission system, due to high laminarity of the beam, forms the crossover of 0.6 mm diameter at set working distance L_wor = 100 mm and maximum beam current 250 mA. In this case, the maximum specific power of the beam in the crossover reaches 9 kW mm^-2. The long-focus emission system does not have an additional electromagnetic focusing lens, and the sharp focus setting is realised by changing the working distance to the object and changing the modulating voltage on the focusing electrode.

The emission system of the second modification (short-focus) is designed for an electron-optical system with combined focusing, in which the beam is focused by the emission system in the primary crossover at a small distance from the cathode (approximately at the output from the anode channel, Figure 1(b)). In this case, the beam diameter does not exceed 0.2 … 0.3 mm at beam current 250 mA due to an increase of its convergence angle in the primary crossover. Final beam focusing, in this case, takes place due to the additional focusing electromagnetic lens. It represents the primary crossover as an object in the welding zone on the focal plane (second crossover) with magnification or demagnification depending on this lens location relative to the primary and secondary crossovers. The working distance L_wor (from gun to welding sample) in this system can be varied from 50 to 150 mm. The crossover diameter in the focal plane does not exceed 0.4 mm at working distance L_wor = 100 mm and beam current 250 mA. In this case, the specific power of the beam reaches approximately16 kW mm^-2.

The short-focus triode emission system is designed for application as a constituent of the electron beam tool in automated electron beam technological complexes. This system also can be used as a part of the small-size manual tool for heating of materials with a defocused electron beam.

The power supply of the electron beam gun is provided by an inverter high-voltage power source with a power of 5 kW, an accelerating voltage of 10 kV and a maximum beam current of up to 250 mA. Accelerating voltage deviations and beam current deviations is ±2% (peak-to-peak value) with accuracy of stabilisation and reproducibility of ±1%. The power source is equipped with a spiral glow unit and a cathode electron bombardment block, which provide heating of the cathode for producing electron beam emission. The instability of the bombardment current is ±3%. Adjustment of the beam current is carried out by supplying a negative voltage to the control electrode from the modulator unit, which is also included in the power supply. The necessary accuracy of positioning the focused beam on the surface of the welded object for the hand tool is determined by the characteristics of the support or guide tool, which is selected individually for the specific welding task.

Obtained values of specific beam power in the welding zone allow welding in space of aluminium alloys of up to 6 mm thickness, titanium alloys as well as stainless steels of up to 4 mm thickness. This virtually covers all the materials and their thicknesses, which potentially can be subjected to welding in the repair of manned spacecraft.

The development of new design of the gun (Figure 2) takes into account the needs for providing the minimum weight of the gun and high operation reliability. During the welding process, the outer surface of the anode unit (1), the body (2) and the cover (7) should not be heated above 50°C with regard to the safety of the astronaut operator; the temperature of the components of the changed cathode component (3) can reach 1800°C. Further, the electrical insulation of the contact socket of the changed cathode component (5), the high-voltage insulator (4) and the high-voltage input (6), which are in contact with the heated node (3), should withstand applied working accelerating voltage 10 kV. Besides, control voltage up to 4 kV is also applied in the triode emission system between the cathode and the focusing electrode. In such a case, the gun during operation in space vacuum should provide reliable functioning without the application of the traditional fluid cooling system. Moreover, the gun outside surface can be additionally heated by intensive solar emission under conditions of operation in open space.

Therefore, significant attention during the development of new gun design was given to the creation of heat-resistant and hermetic components of high-voltage isolation having minimum dimensions and weight as well as to the problems of joining these components between each other and gun body constituents.

Aluminium oxide Al₂O₃ (corundum) with a minimum amount of additives [11] was selected as a basic ceramic material for isolation components of the gun. Such ceramics provide the required strength (300 MPa) of the parts of high-voltage insulation and their high resistance to voltage breakdown (to 45 kV mm^-1). Taking into account that these ceramics have low thermal expansion coefficient (TEC = 8 … 9 × 10⁻⁶ × °C^-1), metallic parts that have to contact with ceramic ones were selected in such a way as to be close to Al₂O₃ TEC values. These are alloys 29 NK (kovar Fe-29Ni-17Co or 4J29) and 46N (fernico family alloy Fe-46Ni-17Co) [12, 13].

Developed metal-ceramic components have mainly stacked structure, therefore, metallic and ceramic parts of different shapes, sizes and physical–chemical properties (in total more than 30 designations) shall be strongly and tightly joined between each other with the accuracy of mutual alignment not worse than 0.1 mm with high indices of component operating parameters. In this connection, precision, vacuum, multi-layer technology of brazing of ceramic parts with metallic elements without application of traditional preliminary metalising of surfaces of ceramic parts having contact with metal [14, 15] was developed. Conducted investigations made a basis for selection of the optimum brazing filler compositions, namely 72% Ag, 28% Cu + 4 wt-% Ti. Moreover, the brazing filler metal in form of the three-sheet structure of ‘sandwich’ type was proposed, i.e. basis (Ag, Cu)/titanium/basis (Ag, Cu). The thicknesses of brazing filler metal layers were selected with the general requirement, namely 4–5 wt-% of titanium. The following thicknesses were used in practice: 10 microns for titanium and 120 microns for basis (Figure 3). The ‘sandwiches’ were produced using the developed method. It is the solid-phase backing of sheet layers of the brazing filler metal under pressure in 2 × 10⁻³ Pa vacuum at 750°C temperature. In this case, the required high level of ceramic material wetting with brazing filler metal is reached.

Developed and proven technology of vacuum brazing of metal-ceramic components in one vacuum-thermal cycle allows producing a stacked block that contains 8 … 10 edge circumferential metal-ceramic joints brazed with molten brazing filler of the next diameters, namely 15 … 60 mm and more (Figure 4).

Specially developed devices of various modifications were used for assembly of stacked blocks of different designation with a necessary accuracy of mutual positioning of their constituent parts (ceramic rings, ‘sandwiches’, embedded metallic parts) and their further brazing. Each device should provide the necessary alignment of indicated above elements from specific block structure at the preliminary assembly as well as in process of heating and brazing of the block. In addition, preliminary diffusion vacuum baking of ‘sandwiches’ to corresponding metallic parts under pressure was used in order to simplify the process of blocks’ assembly in the devices.

The samples of all metal-ceramic components were produced using the developed brazing technology for new gun furnishing.

The whole complex of main high-voltage insulation of a new gun was made dismountable in order to simplify its assembly and maintenance in process of operation. For reduction of the dismountable component dimensions and providing their sufficient electric strength, it was proposed to set the inserts from an elastic insulation material, i.e. silicon rubber between the split solid dielectrics of this insulation. The newly developed gun for application as a part of manual electron beam tool is shown in Figure 5.

Results and discussion

A complex of investigations of brazed metal-ceramic components was carried out to set a correspondence to operational requirements of a new gun. The measured bending strength of the brazed joints equals 200 MPa. Also, the components were heated and cooled from +500°C to –75°C without violation of their strength and tightness. Heat cycling resistance was checked at 500-20-500°C mode in the vacuum and in the air (500°C is the maximum temperature, to which brazed metal-ceramic components of the gun are heated).

The results of vacuum tightness tests showed that all welded-brazed joints allowed getting and keeping the vacuum up to 1 × 10⁻⁵ Pa (at sufficient 1 × 10⁻³ Pa).

High-voltage testing in 5 × 10⁻³ Pa vacuum showed that produced metal-ceramic components can withstand voltages, which is 1.5 higher than operating ones, namely accelerating voltage to 17 kV (operating 10 kV); voltage of electron cathode bombardment to 2 kV (operating 1 kV); voltage on focusing electrode to 6 kV (operating up to 3.8 kV).

Electric resistance between the electrodes of metal-ceramic assemblies was also investigated. The investigations showed that resistance between the electrodes makes from 1 to 1.5 GOhm at allowable 0.5 GOhm. In this case vacuum tightness and electric power were completely preserved. With the indicated resistance values of the insulating elements, the additional current load associated with the leakage currents does not exceed 10⁻² W and is insignificant.

The comparative characteristics of the main parameters of the electron beam equipment used in the experiments on ‘SkyLab’ [16], ‘Salyut-7’ [8] as well as the new gun are given in Table 1. It should be noted that Metals Melting Experiment on ‘SkyLab’ was the investigation of the effect of zero gravity on molten metals (aluminium 2219, stainless steel AISI 304, pure tantalum), stationary welding equipment was located inside the spacecraft and communicated with the vacuum of outer space environment by an adjustable channel. The experiment on ‘Salyut-7’ demonstrated the practical possibility of electron beam welding of stainless steel and titanium samples with thicknesses of 1 mm outside the space station with the Versatile Hand Tool.

Table 1

Comparative characteristics of electron beam equipment on SkyLab, Salyut-7 and the electron beam gun of the new design.

	SkyLab electron beam heat source	Salyut-7 Versatile Hand Tool	New gun
Type of equipment	Stationary	Manual	Manual
Accelerating voltage/kV	20	5	10
Beam current/mA	80	100	250
Beam focusing method	Focusing electromagnetic lens	Changing the working distance	Changing the working distance and changing of the modulating voltage on the focusing electrode
Dimensions, (L/W/H)/mm	Integrated into a spherical vacuum chamber with a diameter of 400 mm	290/135/230	220/80/290
Weight/kg	–	3.5	1.8

The electron beam gun of the new design was used for welding of aluminium alloys, which are the most widespread structural materials in the area of space apparatus construction. Figure 6 shows macro sections of different typical joints of aluminium alloys made without the use of filler material. Welding in the manual mode of the butt joint of the alloy 2219 with a thickness of 4 mm (Figure 6(a)) and the halving joint of the alloy 5456 with the thickness of 5 mm (Figure 6(b)) was performed in both cases on the accelerating voltage 10 kV at a working distance of 100 mm. For connecting (Figure 6(a)), the welding current was I_P = 142 mA at the welding speed V_P = 20 m h⁻¹. Accordingly, the connection (Figure 6(b)) was obtained at parameters I_P = 152 mA, V_P = 25 m h⁻¹. When welding these specimens having the length of 200 mm with an electron beam, the seams were obtained without pores and without cracks. According to geometrical parameters, the welding of the seams approximated to the quality of the plasma welding method. It is worth noting that the weld aspect ratio in this case is explained by the low speed of the process and it significantly changes when the gun is used in the automated technological complexes. In addition, Figure 6(c) shows the result of a ground-based experiment to reconstruct a fragment of a typical grid stiffened structure of an ISS module shell by electron beam welding. In this case, the sealing plate (RP) is welded to vertical stringers or bulkheads (SB), which form rectangular cells of the shell of aluminium alloy 5456 damaged by micrometeorites. For the joint (Figure 6(c)), the welding current was I_P = 114 mA at the welding speed V_P = 32 m h^-1.

Chemical composition and mechanical properties of base metal samples and welded joints (a) and (b) are given in Table 2. Chemical composition and analysis of the distribution of alloying elements were determined using the method of micro X-ray spectral analysis on Camebax SX-50 microanalyzer under the following modes of investigation: probe diameter ∼1 µm, accelerating voltage 20 kV, probe current 20 nA. Failure tests of the received samples were conducted on testing system MTS^® 318.25 in accordance with the requirements of ISO 4136. Performed examinations of the structure of welded joints demonstrated their high quality; all welded joints in mechanical tests were broken on the base metal. Reliable operation of the gun under 5 × 10⁻³ Pa vacuum without forced cooling in process of sample welding at electron beam power up to 2.5 kW was proved.

Table 2

As-received materials chemical compositions and mechanical properties used in this study.

	Base metal samples				Welded joints
	Alloy 2219		Alloy 5456		Alloy 2219		Alloy 5456
Tensile strength/MPa	370		320		340		315
Chemical composition/wt-%	Mg	–	Mg	6.2	Mg	–	Mg	6.0
	Cu	6.2	Cu	0.1	Cu	6.2	Cu	0.1
	Al	base	Al	base	Al	base	Al	base
	Mn	0.3	Mn	0.6	Mn	0.3	Mn	0.5
	Zn	–	Zn	0.2	Zn	–	Zn	0.18
	Fe	0.1	Fe	0.4	Fe	0.1	Fe	0.4

Conclusions

The electron beam gun of new design having 2.5 kW power and accelerating voltage 10 kV was developed for welding under open space conditions. With the indicated parameters the welding operator has the possibility to use the gun without additional means for biological protection.

The triode emission systems were calculated, produced and tested. They form an electron beam of a high quality sufficient for electron beam welding of different metals of 4 … 6 mm thickness.

The technologies for electron beam welding of metallic components of high-voltage electrodes were developed. It allowed providing necessary vacuum tightness of dissimilar joints. Optimum composition and form of brazing filler metal of ‘sandwich’ type were developed for joining ceramic and metallic parts in isolating components of the gun. Also, brazing technology of stacked metal-ceramic isolating components without ceramic pre-metalising was created. Applied technologies allowed getting the minimum weight of the gun and high reliability of electric insulation of its components under conditions of heating of cathode assembly in operating condition up to 1800°C. Tests of electron beam gun of the new design on land during the welding of samples proved its suitability for operation under space conditions.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

References

Wang

, Chen

, Li

, Microstructure and mechanical properties of electron beam welded titanium alloy Ti–6246. J Mater Sci Technol. 2018; 34:570–576. doi: 10.1016/j.jmst.2016.10.007

Mastanaiah

, Sharma

, Reddy

GM.

Process parameters weld bead geometry interactions and their influence on mechanical properties: a case of dissimilar aluminium alloy electron beam welds. Def Technol. 2018; 14(2):137–150. doi: 10.1016/j.dt.2018.01.003

Hajitabar

, Naffakh-Moosavy

Electron beam welding of difficult-to-weld austenitic stainless steel/Nb-based alloy dissimilar joints without interlayer. Vacuum. 2017; 146:170–178. doi: 10.1016/j.vacuum.2017.09.046

Enzinger

, Loidolt

, Wiednig

, Electron beam welding of thick-walled copper components. Sci Technol Weld Join. 2017; 22(2):127–132. doi: 10.1080/13621718.2016.1204516

Elmer

, Evans

, Embree

, Electron beam weldability of a group IAB iron meteorite. Sci Technol Weld Join. 2014; 19(4):295–301. doi: 10.1179/1362171813Y.0000000188

Wglowski

, Błacha

, Phillips

Electron beam welding – techniques and trends – review. Vacuum. 2016; 130:72–92. doi: 10.1016/j.vacuum.2016.05.004

Fragomeni

, Nunes

Jr . A study of the effects of welding parameters on electron beam welding in the space environment. Aerosp Sci Technol. 2003; 7(5):373–384. doi: 10.1016/S1270-9638(03)00031-2

Paton

, editor. Space: technologies, materials, structures. New York (NY): Taylor and Francis Inc.; 2003.

Martinez

, Kingston

Space radiation analysis: radiation effects and particle interaction outside the earth's magnetosphere using GRAS and GEANT4. Acta Astronaut. 2012; 72:156–164. doi: 10.1016/j.actaastro.2011.09.001

10.

Akop'yants

, Zubchenko

Method of calculation of electron-optical system of welding gun. Avtomatich Svarka. 1979; 11:33–36.

11.

Degtyareva

ÉV

, Skorodumova

, Kabakova II , Properties of highly refractory corundum ceramics containing glass phase, and their influence on the surface roughness after mechanical treatment. Refractories. 1985; 26:61–66. doi: 10.1007/BF01398618

12.

ASM metals handbook . Properties and selection nonferrous alloys and special purpose materials. Materials Park (OH): ASM International; 1990.

13.

Messler

Jr . Joining of materials and structures: from pragmatic process to enabling technology. Burlington (MA): Butterworth-Heinemann; 2004.

14.

Naidich

YuV

, Krasovskyy

, Durov

, Sydorenko

TV.

Wettability and brazing of iono–ionocovalent ceramic materials by metal alloys containing electronegative elements. In: Proceedings of 6th international conference on brazing and soldering; 2015 Apr 19–22; Long Beach (CA): American Welding Society; 2015. p. 40–48.

15.

Zhuravlev

VS.

About the possibility of brazing of niobium to alumina ВК94-1 ceramics by copper and copper-nickel brazing fillers without applying pressure. Adhesion of Melts and Brazing of Materials. 2015; 48:94–102.

16.

Poorman

RM.

Skylab M551 metals melting experiment. NASA Marshall Space Flight Center, Huntsville (US); 1975. NASA-TM-X-64960. Available from: https://ntrs.nasa.gov/search.jsp?R=19760003076 (cited 2018 Sep 26)