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êEvgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems ofELW Technology.
UDC 621.791.722
BASIC PROVISIONS AND PROBLEMS OF ELW TECHNOLOGY FOR THE MANUFACTURE OF ALUMINUM-MAGNESIUM ALLOYS CONSTRUCTIONS
Evgenii I PRYAKHIN1, Nikolai I SHARONOV2
1 Saint-Petersburg Mining University, Saint-Petersburg, Russia
2 Central Research Institute of Materials, Saint-Petersburg, Russia
Existing problems of electron beam welding of aluminum alloy constructions are considered. For research purposes, the aluminum-magnesium alloy of grade 1561 up to 60 mm thick was used.
The thermal field in the heat influence zone is studied experimentally and analytically on the basis of the finite element method (the «Ansys» program). The effect of electron beam movement (scanning) influence on the quality of welded connections and the surface of the welded parts was studied. On the basis of metallographic studies and mechanical tests of welded metal, it is proved that high quality of welded joints is ensured when the beam moves along a curve of the «compressed brackets» shape. A special generator is designed to control the electron beam, which allows to implement a new type of scanning (compressed brackets). The fundamentals of welding technology for alloy 1561 up to 60 mm thick are outlined. Specific recommendations are given, and two new methods are proposed that will allow the successful use of the developed technology in factories in the production of new products and in the repair processes.
Examples and analysis of thermal cycles obtained by calculation and experimental method are given. The patterns of heat distribution along the trajectory of the beam movement for different types of scanning are established. The main types of defects in the formation of the welded joints and those formed in the metal during crystallization are considered. Their interrelation with the parameters of the welding mode is shown.
Key words: electron beam welding, thermal influence, electron beam, welded joints, alloy 1561, aluminum alloy
How to cite this article: Pryakhin E.I., Sharonov N.I. Basic Provisions and Problems of ELW Technology for the Manufacture of Aluminum-magnesium Alloys Constructions. Zapiski Gornogo instituta. 2018. Vol. 229, p. 84-91. DOI: 10.25515/PMI.2018.1.84
Introduction. Electron-beam welding (ELW) of aluminum alloys differs from arc welding methods by significantly lower heat input. At the same time welded connections have narrow seams and small sizes of the heat affected zone (HAZ), in which there exists the possibility of structural and phase transformations of the metal, leading to the weakening the welded joints [7]. In this connection, the temperature and time conditions of the ELW are of considerable interest, which are evaluated and analyzed using the example of welding alloy 1561, according to the results of experimental studies performed by the contact method (St.PSMTU, 2011).
Methodology. The welding of the plates was performed on the ELU-20A unit. The welding mode for the main process parameters is given below:
Parameter Uacc, kW Ibc, mA If, mA Ib, mA Fwel, m/h f, Hz mm) lg.p, mm
Welded connections with thickness of 20 mm 60 80 757 32-33 21 440 (3.5) 180
Thermocouples were located in the main metal at a depth equal to half of its thickness at a distance of 3, 5, 12, 20, 30 and 60 mm from the axis of the heat source displacement (Fig. 1). Thermal
electrodes were welded to the bottom of the holes by a pulse-capacitive discharge. To monitor the position of the connections relative to the axis of the holes, after welding the samples were cut in the appropriate sections and measured on an MBS-2 microscope with an accuracy of 0.01 mm.
Results and discussion. Figure 1 shows thermal cycles studied at unequally distant points from the weld axis (3, 5 and 12 mm). The analysis of the maximum temperatures distribution curves (Fig.2) of ELW of the aluminum alloy having 20 mm thickness shows that the width of the heat affected zone limited by half of the isotherm with a temperature of 300330 °C is 5-10 mm and corresponds to the area established by the results of metallographic analysis.
Fig. 1. Thermal cycles at points located from the seam axis at a distance 3 mm (1); 5 mm (2) and 12 mm (3)
êEvgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems ofELW Technology.
The maximum temperatures are an important characteristic of thermal input in the weld region of the welded metal, since the final, post-weld structure of the HAZ metal and, consequently, the properties of the welded connection as a whole depend on it [3, 6].
In addition to the experimental method for determining the temperatures in the heat affected zone at the ELW of alloy 1561 with the help of thermocouples for beam sweeps «circular» and «compressed bracket» a numerical study was made of the effect of the nature of the electron beam trajectory on the parameters of the transient thermal field under ELW in vacuum using the finite element method (FEM) and the program «Ansys» [2, 10]. The welding mode data and the properties of alloy 1561 as a function of temperature were used as finite element data for the calculation, they are shown below:
T, ° c 600
500
400
300
200
100
0
20
40
60
y, mm
Fig.2. Distribution curve of maximum temperatures for ELW of alloy 1561 with thickness of 20 mm
T, °c 20 100 200 300 400 500 600
X, W/(m-K) 117.3 121.5 125.7 129.9 138.3 146.7 155
o0,2, MPa 180 160 150 90 20 10 10
E, MPa 72000 70000 6500 5600 4500 3000 1000
a, K -10-6 23.1 24.1 25 26 27 28 28.5
Cp, J/(kg-K) 0.84 0.92 1.01 1.05 1.09 1.11 1.14
Properties of the alloy: X is the coefficient of thermal conductivity; o0.2 is the conditional yield limit; E is the normal modulus of elasticity; a is the coefficient of linear expansion; cp is the heat capacity.
With the indicated finite element data of the model, the coincidence of the calculation results and the experimental work on the determination of the temperature fields of the heat-affected zone was found to be satisfactory (Fig.3).
With respect to a particular technology, an important point in the research is the distribution of the electron beam energy (EB) along the chosen trajectory. It is established that the change in the sign of the velocity at the ends of the «compressed brackets» of the beam sweep and the overlapping of «traces» of the beam on each other leads to the fact that the input of EL energy into the metal at the joint edges is higher than in the middle of the seam (Fig.4).
The present intensification of heating initiates an increase in the molten metal residence time of the weld pool in the liquid state, which allows the gas bubbles to come to the surface. In addition, the
t, °c 1000
900
700
500
300
100
T, °c 1000
900
700
500
300
100
T, °c 1000
900
700
500
300
100
2 6 10 12 16 18 t, s
2 6 10 12 16 18 t, s
2 6 10 12 16 18 t, s
Fig.3. Temperature cycles at control points (blue - calculated). Distances from the seam axis: 3 mm (a); 5 mm (b); 12 mm (c)
b
a
c
Evgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems of ELW Technology.
applied beam sweep scan provides a smooth transition from the weld to the base metal, which is an important point in the operation of welded structures with the remaining convexity of the welded seam.
To implement the ELW technology with the use of the «compressed brackets» beam sweep, a special beam sweeping control generator (BSCG) was developed. The prolonged use of the «compressed brack-Fig.4. Beam energy distribution in the melting zone at etS>> beam Sweep in the production COndi-
sMmmry beam (a); with a beam sweep «compressed tad^s» (b) tions showed the absence of pores in the
weldment zone, which, for example, was observed with the application of the most widespread and easily implemented «circular» type of beam sweep.
In the electron beam control device for welding of BSCG, the principle of obtaining sweeps of various shapes is implemented by feeding sinusoidal voltages shifted in phase by 90° to the electromagnetic deflecting coils of the electron gun. By creating different combinations of sinusoidal and rectified voltages in each coil, it is possible to obtain a variety of beam sweep shapes, for example, an ellipse, an arc with a return along the diameter, etc.
The electron beam sweep relative to the joint along different trajectories is used for welding seams of different configurations and for materials with various thicknesses [1, 4, 5, 8, 11, 12]. It allows to control the parameters of welded connections, the rate of transfer and crystallization of the molten metal of the weld pool within a wide range, to increase the density of seams, to reduce or eliminate the possibility of root defects (Fig.5), and to reduce the requirements for accuracy of parts assembly and manufacturing of tooling.
The BSCG device is designed for sweeping the beam along four trajectories:
• linear (alone one of the coordinate axes);
• ellipse with independent adjustment along both axes (circle is a special case);
• semi-ellipse with a return along the same trajectory («bracket»);
• semi-ellipse with a return along one of the coordinate axes, it allows adjusting the sweep frequency and amplitude along the X, Y axes, rotate the image by an angle multiple of 90°.
The device is based on a quadrature harmonic oscillator generating three sinusoidal voltages with a phase shift of 0, 90, 180°. The scheme of «smooth» amplitude-voltage limiting provides stabilization of the generator at a level of ±7.5 V and a negligible nonlinear harmonic distortion coefficient of signals. From the generator output the voltages go to the beam sweep shape switch, and the voltages with a phase shift of 0 and 180° go through the full-wave rectifier.
Fig.5. The macrosection of the longitudinal section of the welded joint (a) made by the ELW with a stationary beam with incomplete penetration on an aluminum alloy 1561 with a root «sawtooth» defect; the surface of the welded joint made with a «circular» beam sweep, and the macrosection of its longitudinal section (b); surface of the welded joint made with a «compressed brackets» beam sweep, and macrosection of its longitudinal section (c)
éEvgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems ofELW Technology.
Due to the low cutoff voltage of germanium diodes and the high level of the generator output signal, the distortion of the signal during polarity reversal is almost imperceptible, which allows not to use the complex precision rectifier.
From the beam sweep shape switch, the signals on two channels are fed to the X and Y level controllers, and from them through the 90° sweep image turn switch go to the current amplifiers X and Y. The output stage of the current amplifiers is made in a common emitter (OE) circuit and covered by local negative feedback (NF), allows fuller use of the power source voltage and obtain an amplitude of the output voltage exceeding the amplitude of the signal at the output of the operational amplifier.
The device circuit, developed on a modern element base, provides a smooth change in amplitude and frequency. The beam sweep shape and its parameters are visually monitored on the oscilloscope screen. The control switches are placed on the front panel of the device. Structurally the device is executed either in the desktop version or, if necessary, it is built into the control cabinet of electron beam equipment of the type U250A or ELA60/60. It can be used in combination with any electron beam equipment for welding various metals.
The reliability of the device is confirmed by the successful operation in production conditions for welding products made of aluminum alloys, homogeneous and dissimilar materials up to 60 mm thick.
The industrial technology of ELW should ensure the simulation of the welded joints parameters, respectively, their mechanical properties, which is an important point for assessing the performance of the welded structure. The most preferred form of the seams for through penetration is the welded joints with parallel walls, which significantly reduces the angular deformations (Fig. 6). When calculating the mode parameters for seams with incomplete penetration, the depth of the required penetration is used.
The list of main technological characteristics of the electron-beam welding process includes: Uacc - accelerating voltage, kW; Ibc - beam current, mA; If - focusing lens current, mA; Ib - bombardment current, mA; Vwel - welding speed, m/h; lg-p - working fistance «gun - part», mm; f -sweep frequency, Hz.
As additional parameters of the ELW the following characteristics are used: Ip - passing beam current, mA; If minQ - focusing lens current - minimal point (the smallest beam diameter) of the welded metal surface, mA; Sbs - beam sweep shape; Ay - amplitude along Y axis; Ax - amplitude along X axis.
The main provisions of the technology, which must be observed, are as follows: preparation of metal using the ELW; selection of the position of the weld pool in space; choice of ELW modes; determination of the method of beginning and closing of the seam during the implementation of the ELW process (including consideration of the peculiarities of annular seams welding); and determination of repair methods for defective ELW seams.
The preparation of the metal for welding is the critical factor for obtaining high-quality welded joints of aluminum alloys [9, 13]. First of all, it is necessary to ensure the removal of the oxide film Al2O3 (melting point 2060 °C), which makes it difficult to form a common weld pool and, when it enters the seam, reduces the static and cyclic strength of the welded joint. Immediately before loading into the cell, the welded edges are scraped until removing the traces of machining.
A very important operation is the removal of the plating, which when present in the seam also reduces the static and cyclic strength of the welded joint and the formation of a common weld pool. Plating is removed by scraping or deep etching followed by scraping.
Fig.6. Macrosections of welded joints of aluminum alloys with full (a) and incomplete (b) penetration, thickness 40 mm
êEvgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems ofELW Technology.
EB
B
EB
I
Fig.7. Vertical beam welding patterns: - S < 20 mm (a); b - any thickness of welded joints
One of the most important conditions for the successful implementation of ELW technology is the choice of the weld pool position. Depending on the metal welding thickness, the position of the welding pool is determined, it could be placed vertically or horizontally.
With a vertical beam (vertical position of the welding pool), high-quality welds can be easily obtained up to a thickness of 20 mm, inclusive, without a technological lining (Fig.7). The retention of the metal «by weight» is due to a rigid thermal cycle and a high rate of crystallization. It should be borne in mind that an increase in welding speed above 50-60 m/h can lead to the appearance of pores in the welded joints and at the weld fusion boundary. The presence of a technological lining (welding without its penetration) simplifies the technique of making a welded seam.
The materials with thicknesses exceeding 20 mm are preferably welded in a «horizontal beam on a vertical wall» pattern (Fig.8) with through penetration (if there is lining, for example, a «lock» joint, the lining is melted for guaranteed elimination of root defects). The control is done by the passing beam current. Five main butt-welded joints for the ELW have been developed (Fig.9).
In the course of the research, two computational and graphic methods have been developed that allow obtaining with practically 95-98 % certainty the welded connections with practically parallel welded penetration walls due to the constancy of the ELW mode parameters, regardless of whether an old or new cathode is used for welding.
With the help of the first technique, the required and sufficient beam current is set for welding material with a specified thickness value, while the thermal load on the cathode is optimized, pro-
EB
b
EB
Fig.8. Horizontal beam welding patterns: S < 20 mm; b - any thickness of welded joints
a b
g
b a g S
b
c/2
n_
■
rr b
■U a
■
b
bi ±
bi
a
S
c
c
S
e
b
b
a
Fig.9. Structural design of butt-welded joints of the ELW: a - on the lining to be removed; b - on the remaining lining «lock» joint; c - with an enlarged section of the seam due to the ledges on the edges of the base metal of the root part of the joint; d - standard butt-welded joint; e - butt-welded joint with an enlarged cross-section at the input and output of the electron beam
êEvgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems ofELW Technology.
Electron beam
Fig.10. Refocusing of electron beam +A7f(F1)
AIf, mA 40 "
30 -
20 -
10 -
0 10 20 30 40 50 60 70 S, mm
Fig. 11. Diagram of dependency of beam refocusing value from thickness of welded metal
Electron beam
longing its service life. The first method is based on the determination of the optimal temperature regime for the cathode operation in order to obtain the minimum required electron beam current for a particular thickness of the metal to be welded,
N > kSV,
where N - minimal calculated power of electron beam, kW; S - calculated cross-section of welding seam, mm2; V - welding speed, mm/s; k - performance factor, for aluminum and its alloys k = 0.014-0.016 kW (s/mm3).
When obtaining a numerical value of the minimum current, bombardment should be increased by 20-25 % to avoid poisoning of the cathodes with metal vapors, this is the rule of welding aluminum and its alloys. At the same time, the welding current (reserve) increases. This technique increases the period of its efficiency (resource). The procedure is repeated before each welding, regardless of the use of the old or new cathode.
The second technique allows to establish a focal plane (focus) in the middle value of the welded metal thickness, which is the main condition for obtaining welding seams with parallel walls. In the second method, the parameters of the «minimum point» If min(-) are used for a given «gun-detail» distance, equal to 180 mm (Fig.10). After determining the quantity If min(-), the refocusing value +AIf is found on the welded metal surface using the previously constructed graph (Fig.11). Then a check test piece is welded, which is the basis for the further use of this or that welding mode. Depending on the position of the focus, the shape of the seam changes (Fig.12).
With the application of these techniques weld control connections with thicknesses of 20, 40 and 60 mm were welded for their complex research.
The success of the ELW technology application depends on how well the «beginning and closing» of the welding method is carried out (Fig.13). When welding annular seams, one should consider how well the weld closure is done. The tried-and-true method of annular seam welding involves the use of a number of techniques that are included in the technology as a «be-ginning-closing» block of welding activities.
I
mmm / mmm
Fig. 12. Changes of seam shape depending from focus position F
êEvgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems ofELW Technology.
Fig.13. The «beginning-closing» of welding block of equipment ELA-60 ^) according to «input-output» pattern of electron beam (b)
Conclusions
1. The preferred type of beam sweep for welding aluminum alloys is a «compressed bracket» shape. The symmetry of the beam sweeping during electron beam scanning during the application of «compressed bracket» shape ensures a more uniform melting and greater parallelism of the walls of the welded seam. The use of the «compressed bracket» version due to the specificity of its trajectory (higher heat input at the edges and a more gradual temperature decrease rmax) almost completely excludes pore formation on the weld-fusion line and in the seam itself. To implement the beam sweep shape «compressed bracket» there have been developed a generator for controlling the beam sweep (BSSG).
2. Variants of different types of welded joints for ELW of aluminum alloy structures are proposed.
To obtain high-quality welded joints, thorough preparation of the welded metal is required in the part of mechanical removing the oxide film and the cladding layer from the welded zone, which is done by chemical etching.
Immediately before loading the metal piece into the working cell, the welded edges must be scraped for removing traces of previous processing.
3. The determining factor for ELW with through penetration is the position of the focus in the thickness of the welded metal - the welded seam with parallel walls - the focus should be in the middle of the thickness. The starting point for implementing this technology requirement is the value of the focusing lens current at the minimum point (the minimum diameter of the electron beam on the metal surface). Two computational and graphical techniques have been developed to facilitate the production of welded connections with high-quality through-penetration.
4. The developed technology of EL welding of alloy 1561 provides high strength characteristics of the welded connection (0.96-1.00 of the actual strength of the base metal). The technology is implemented in the construction of frame structures - foil systems of «SPK Olympia».
REFERENCES
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êEvgenii I. Pryakhin, Nikolai I. Sharonov
Basic Provisions and Problems ofELW Technology.
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Authors: Evgenii I. Pryakhin, Doctor of Engineering Sciences, Professor, mthi@spmi.ru (Saint-Petersburg Mining University, Saint-Petersburg, Russia), Nikolai I Sharonov, Head of the Electron Beam Welding Laboratory, mthi@shmi.ru (Central Research Institute of Materials, Saint-Petersburg, Russia).
The paper was accepted for publication on 20 November, 2016.
