METHOD AND DEVICE FOR THE ADDITIVE MANUFACTURING OF COMPONENTS

Abstract

The invention relates to the field of the additive manufacturing of components, which are formed by the direct deposition of a substance, in the form of granules of a metal or non-metal, which passes from a reservoir into a melt bath, produced by the thermal energy of a laser or electron beam, and subsequently crystallizes. The granules enter the melt bath without the intervention of a gas stream, the path and rate of travel of said granules changing while they are in flight under the effect of an electromagnetic field. The granules travel within a chamber, falling into the melt bath from above from a reservoir, from which they are fed at a set speed by the rotation of an adjustable screw feed, and passing through a system of electromagnetic devices, which control the path of the granules by means of electromagnetic fields. The coordinates of this path are tracked by sensors, which transmit a signal to a computer, wherein the flight path of the granules is adjusted by control via the electronic devices and the delivery speed and volume of the substance is adjusted by adjusting the rotation of the screw feed. The invention increases the efficiency of the production cycle, reduces the dimensions of the equipment and increases the accuracy and speed with which material is delivered for the manufacture of a component, while enabling adjustment of the amount, temperature, path and fraction of said material and increasing the strength of the component.

Claims

1.-25. (canceled)

26. An additive manufacturing method of components in accordance with the set program, comprising: forming a body of component by direct deposition of a substance in a form of granules of metal or non-metal, wherein the substance comes from a reservoir under the influence of gravity and electromagnetic forces and is directed to a bath melt, produced by heat energy of at least one laser or electron beam, and subsequently crystallizes; adjusting path and rate of said substance to reach the desired bath melt location by acting on neutral and/or charged said granules with by electromagnetic field.

27. The method according to claim 26, wherein the granules move in vacuum and they may be heated or melted over a surface at their approach to the bath melt.

28. The method according to claim 26, wherein the granules at their approach to the bath melt may be remelted so that when the granule is deposited in the bath melt the outer part of said granule can be crystallized and the inner part of said granule can remain molten or vice versa.

29. The method according to claim 26, wherein the substance path and rate are adjusted with by alternating or direct electromagnetic field to reach the desired bath melt location.

30. The method according to claim 26, wherein the substance path and rate are adjusted in accordance with the set program to reach the desired bath melt location.

31. The method according to claim. 26, wherein the granules can be of different chemical composition and diameter.

32. An additive manufacturing device by direct deposition of a substance in a form of granules of metal or non-metal, comprising: a support table, which is made with a possibility of movement and rotation, installed in a chamber to fix and form a body of component, at least one laser or electron beam heating source to generate a bath melt in the body of component, at least one reservoir for holding the substance in the form of granules, a means for regulating the volume of the substance located in the reservoir, a dispenser to direct the substance into the melt bath connected with the reservoir, at least two electromagnetic devices, which control the path of the granules into the dispenser by means of electromagnetic fields, wherein said devices attached to the dispenser, at least two sensors to monitor at a set frequency path coordinates of the substance and/or laser or electron beam heating source, wherein the sensors transmit a signal to a computer, which controls the means for regulating the volume of the substance and the electromagnetic devices to control the substance path.

33. The device according to claim 32, further comprising an at least one inductor in the form of an electromagnetic funnel installed at a certain distance from the support table, wherein the electromagnetic funnel directs the granules in electromagnetic field into the bath melt and is moved with by a mechanism in accordance with the set program.

34. The device according to claim 33, wherein the electromagnetic funnel is for control the granule flow section area.

35. The device according to claim 33, further comprising an electromagnetic deflecting contour located after the electromagnetic funnel, wherein said contour is configured to change an incidence angle of the substance into the bath melt and to change the flight path geometry to a predetermined radius with any direction change including reorientation to the starting position.

36. The device according to claim 32, wherein the sensors are configured to perform the function of a radar, scanner, pyrometer and spectrometer.

37. The device according to claim 32, further comprising an electromagnetic deflecting contour that allows granules to be placed in the bath melt by a certain electromagnetic field in a certain space changing their path along a predetermined curve with the possibility reorientation to the starting position.

38. The device according to claim 35, wherein the electromagnetic funnel and the electromagnetic deflecting contour are moved by the mechanism that in accordance with the set program moves them to the desired set points in the solid component construction space.

39. The device according to claim 35, further comprising the electromagnetic funnel and the deflecting contour through which an electron beam melting the bath melt is passed, which are moved by the mechanism at predetermined set points, allowing precise coordination of the melting and the fall of the substance.

40. The device according to claim 32, wherein the granule falling and the bath melting are possible straight along the rotation radius with the body of component manufacturing in desired volume where the granules are fed normal to the rotation plane and the radiation is at an angle what allows constraint of the component formation area in a certain window.

41. The device according to claim 32, wherein the granules can be of different chemical composition and diameter.

42. The device according to claim 32, wherein at least one laser or electron-beam heating source is realized with the possible focusing control at the desired height, where the body of component is formed through variation of the granule incident path and variation of the beam incident path under the influence of electromagnetic field with no mechanisms used.

43. The device according to claim 32, wherein the dispenser for the substance direction into the bath melt is configured to perform the function of a vibratory chute, sand glass with magnetic lock, hopper tilting and other similar mechanisms.

44. The device according to claim 32, wherein the electromagnetic devices are realized in the form of electromagnetic coil with various winding to stabilize granule flow along the desired paths in the dispenser volume for the substance orientation.

45. The device according to claim 32, further comprising a focusing and a deflecting inductor installed after the dispenser, wherein the focusing inductor may focus, slow-down, speed-up or change temperature the granule flow and the deflecting inductor may direct the required granule flow into the bath melt or to the reservoir again, wherein before and after the unductors sensors are installed, reading all the parameters characterizing the movement and thermal state of the granules.

46. The device according to claim 32, wherein an information of the granule flow and the beam is registered by the sensors with set frequency from the moment of falling from the reservoir to the moment of deposit in the bath melt, is processed in accordance with the set program with the same frequency and after that corrections are introduced for control of all units of the additive manufacturing device.

47. The device according to claim 45, further comprising focusing, deflecting, decelerating, accelerating and sorting electromagnetic devices installed between the focusing and the deflecting inductor, which may include an electromagnetic splitter, circumferential electromagnetic funnels, a collecting and accelerating-decelerating inductor, where each electromagnetic device is equipped with monitoring sensors.

48. The device according to claim 32, wherein the granules may obtain a certain positive or negative charge through friction, radiation or direct connection to the electron or proton substance.

49. The device according to claim 32, wherein charged granule is controlled by the electromagnetic field through vertical and/or horizontal condenser plates assembled into a grid structure located in rows with various intensity created between the two plates.

Description

DETAILED DESCRIPTION OF THE NON-LIMITING EMBODIMENTS

[0033] The declared additive component fabrication method and the device for its implementation are interrelated by a common invention design and are illustrated by the following examples of specific application.

[0034] FIG. 2 shows major proposed manufacturing method of components 4 with direct granule 1 deposition into the bath melt 3 created by beam 2, where the substance falling path is formed under electromagnetic field and monitored throughout the movement path by sensors 5, which, when required, may function as a radar, scanner, pyrometer, spectrometer, etc.

[0035] FIG. 2a shows the process employing one beam 2 creating the bath melt 3 in the body of component 4 formed by granules 1 due to falling from a certain height under gravity force and controlled by electromagnetic fields. Sensors 5 are installed along the granule movement path; they determine the granule coordinates at certain moment of time, its speed, direction, temperature and other required parameters with certain frequency as well as the location of the granule attachment to the body of component.

[0036] Body of component formation in 3D space is ensured by its rotation and moving down as the layers are formed. Due to this the granules are falling straight along the rotation radius with the movement direction changing from the rotation center to periphery and back. For ex. granules are fed in X direction along the rotation radius, the solid component is moved in Y direction due to turning around the rotation axis to certain angle and the solid component is raised and lowered to a certain value along the rotation axis in Z direction. Thus a mechanism of the component formation working as per the 3D printer principle is created. Such mechanism is used in machining equipment when creating components where, for example, transverse feeding is used in X direction, the component is turned in Y direction and longitudinal feeding is used in Z direction.

[0037] In the proposed invention the process of the body of component formation allows granule direction along the straight line normal to the rotation plane and the radiation from a certain site and at a certain angle, what allows constraint of the body of component formation area by a certain window. The window relative to the overall component section may comprise only several per cents. This feature allows high accuracy component formation.

[0038] In FIG. 2b no movement of the formed body of component is used in the proposed device design. Several (two, three, four and more) beams 2 are used for quality and thorough melting of bath melt and they may control the focusing at height H, where the body of component is formed.

[0039] The process is different in that the component is formed in space by direct substance deposition changing the granule falling path and the electron beam focusing and path under electromagnetic field with no mechanisms used.

[0040] In FIG. 2c the component forming process is similar to the previous one, however additional reheating or surface melting of granular powder or granules by a separate beam or beams at the substance approach to the bath melt may be used.

[0041] The process allows use of cheaper granular powder which conglobates after surface melting. The process allows the substance temperature control at the approach to the bath melt. The granule falls into the bath melt and acquires the set temperature, at that its surface may be melted while the central portion may be not melted and vice versa.

[0042] In FIG. 2d for the initial try-out of the proposed process the use of an electromagnetic funnel 6 is possible, it entraps the granules and directs them along its axis into the bath melt, i.e. adjusts the granule falling path more precisely through the path adjustment in the funnel electromagnetic field.

[0043] To adjust the movement path of the granules with a positive or negative charge direct electromagnetic field may be used. In the alternating field the path and heating may be adjusted for the granules having no charge but conducting current. Through variation in the electromagnetic field intensity focusing or unfocusing is possible for the flow of granules along the electromagnetic funnel axis. The process may be used in the modification where the granules may be directed into the bath melt by an electromagnetic deflecting contour 7 installed directly after the funnel and allowing granule laying into the bath melt at a certain radius from the funnel axis. To create special components consisting of closed honeycomb cavities the deflecting contour allows changing of the granule flow where the falling path on the solid component may be horizontal or follow the curve with reorientation to the starting position, i.e., the contour may allow the substance deposition into the bath melt at any fall angle. Consistent use of the funnel and the contour increases the granule laying accuracy and area and therefore the component formation rate. The deflecting contour allows more dynamic effect on the granules, thus improving the process rate.

[0044] When an electromagnetic funnel is used, FIG. 2d, it is expedient to use the process similar to that shown in FIG. 2a, where the body of component rotates about its axis and moves down as builds up and the funnel with the contour moves along the straight line, i.e. along the rotating radius. The funnel and the contour movement is ensured by the mechanism which according to the program moves them along the straight line and positions at set points. When the proposed mechanism is used the pool melting and the substance deposition in the course of the component formation take place in a certain angular sector.

[0045] FIG. 2e suggests a device design where the body of component rotation is not used, at that the mechanism of the body of component movement up or down along the vertical axis as the layers are smelted is used.

[0046] To ensure quality pool fusing several beams 2 may be used, at that the electromagnetic funnel 6 and the electromagnetic deflecting contour 7 are moving in the same plane parallel to the upper layer formation plane along X and Y axes through the use of a 2D printer. The area of the circle where the granules are deposited through the use of a deflecting contour exceeds that of the granule falling, where only one electromagnetic funnel is used, thus the funnel and the contour movement along the component cross section may be smoother and to a shorter distance.

[0047] In FIG. 2f the process used is similar to that shown in FIG. 2a where rotation with the component moving down as the layers are built up is also used.

[0048] To ensure the more accurate setting to the beam pool melting location the second electromagnetic funnel 8 is used; it has the second electromagnetic deflecting contour 9 attached to it in the bottom part. The second electromagnetic funnel 8 focuses beam 2 and directs it along the internal axis and the second electromagnetic contour 9 directs the beam to the required location of the bath melt. This allows electron beam direction along a more complicated path relative to the solid componentboth tangentially and along the curved path including the path reorientation to the beam origin location. This feature allows manufacture of honeycomb hollow components with closed cavities; to do it the beam path shall be bent as well as that of the substance falling at a set angle. At that the bath melt may be melted not only on the top of the body of component, but also from the side and from the bottom, i.e. the pool may be melted on the body of component horizontally from the top, vertically form the side and horizontally from the bottom. Closed cavities with the shape close to the ball shell require the pool melting and the substance deposition from various directions in space. The method capability to direct the substance and the beam path into the required location in space, despite the gravity forces, under the electromagnetic fields without any mechanisms provides significant process advantages.

[0049] The second electromagnetic funnel 8 with the second electromagnetic contour 9 are fixed to the electromagnetic funnel 6 and the electromagnetic contour 7 and move for a stationary distance. This feature allows very precise location coordination for the bath melt by the beam and the path of granules falling into it.

[0050] Additional beam 2 directed into the electromagnetic funnel 6 or to the substance movement path may be used for reheating of granules 1 and pool 3 heating up. When passing through the substance path and the funnel axis the beam reheats the granule surface and the bath melt. Such device is particularly efficient in case of construction of new additive equipment using the method of simultaneous beam and substance control by electromagnetic field.

[0051] If required the component tilting in space or tilting of the component rotation axis may be used. Major deposition processes shown in FIG. 2 may be grouped in different combinations for construction of various equipment modifications.

[0052] To implement the claimed additive component fabrication method based on electrodynamics the device shown in FIG. 3 is proposed. The device contains a vacuum chamber 10 with an electron beam gun 11 having a focusing electromagnetic lens 12 and a deflecting electromagnetic lens 13 of beam 2, which melts the bath melt 3 at the set point on the body of component 4 top face in the construction zone and the powder in the form of spherical granules 1 gets into it falling with certain speed from a loading hopper 14. The granules are pushed out of the hopper with a feed screw 15, which may rotate with a preset speed. By controlling the rotation speed a certain volume of granule feed per second may be ensured. The granule feed may be ensured by various mechanisms, i.e. their feed from the hopper through a vibratory chute, as in sand glass where a magnetic lock may be installed in the narrow and it may slow down or speed up the substance flow, also other feeding systems may be used. The granules sent from the reservoir get into the dispenser 16 in the form of a tube. Induction electromagnetic coils 17, 18, 19 are installed on the dispenser to get the powder feed from the hopper in certain amount and with certain rate. The coils have various winding patterns, i.e. coil 17 winding is about the tube axis what allows the granule focusing in the tube center. The second coil 18 has transverse winding allowing granule deflection along the pipe cross section. The third coil 19 winding is at an angle to the pipe axis, what allows the granule distribution at an angle to the pipe section. Due to interaction of the induced current in the granule metal with the coil field causing it mechanical forces exerting pressure on the granule metal emerge and distribute them among the desired paths in the piping volume.

[0053] Granules getting into the induction coil coverage causes variation of the field intensity in the electromagnetic contour registered by the instruments; through it an approximate amount and rate of the fed powder may be determined. The coil field intensity and the feed screw drive rotation rate may be computer controlled with a certain software via a feedback switch box. When a signal is taken from the induction coil on the certain substance amount getting into it, it gets to the switch box and then to the computer which determines the amount correspondence of the fed in powder as per the software. After comparing the signal gets via a switch box to the feed screw rotation and coil field intensity control device and feeding of the predetermined substance amount into the dispenser is achieved. After that the rate, direction and quantity stabilized granule flow is established as per the software which gets into the focusing inductor 20 and deflecting inductor 21 coverage area.

[0054] Sensors 5 are installed at the dispenser 16 output, they register all parameters describing the granule movement and thermal state more accurately, the gathered information via the switch box gets to the computer, is processed and the control signal again via the switch box is set to the focusing inductor 20 and deflecting inductor 21. Under the control signal the focusing inductor 20 may speed up or slow down the granules, focus the path along the axis and change the temperature, while the deflecting inductor 21 directs the granules into the electromagnetic funnel 6 or electromagnetic trap 22, from which the granules may get into hopper 14 again. This design allows exact control of amount, rate, path, temperature and other parameters of granules getting into the electromagnetic funnel 6. The mechanism moves the funnel on X axis at the certain straight line section and at a certain moment gets a computer signal on the coordinates. Upstream and downstream the electromagnetic funnel 6 sensors 5 are installed, they register the granule coordinates in space up to their deposition into the molten pool. Information on the granules is registered by sensors 5 with certain frequency per second from the location of falling from the hopper to the location of deposition into the bath melt. Information is processed by software, introducing corrections with the same frequency through the control signals to all devices effecting the component additive fabrication process. Beam 2 is controlled as per the software according to the similar design via a focusing electromagnetic lens 12 and a deflecting electromagnetic lens 13. Turning of table 23 change the coordinates on Y axis as per the software. The table is installed on the screw 24, which may be lowered or raised as per the control software thus changing the Z axis coordinates. The substance deposition is in vacuum created via nozzle 25. If required the substance deposition is possible in any gas.

[0055] For higher determination accuracy and granule control use of more focusing, deflecting, slowing down, accelerating and sorting devices may be resorted for the substance. FIG. 4 shows an example of a similar device where devices allowing more accurate impact on the path and other granule 1 parameters are installed at the area between the focusing inductor 20 and the deflecting inductor 21. This set of devices determines various granule parameters more accurately. The required parameters of granules getting into and out of the focusing inductor 20 are registered by instruments 5 and then directed to the splitting inductor 26 which divides the flow into several granule flows trapped by circumferential funnels 27 installed around the splitter. Prior to the circumferential funnels and after them the instruments determine the parameters of each granule flow allowing more accurate monitoring of certain flow parameters. Then each flow gets into the second deflecting inductors 28, which sort the granules and send one portion to the hopper and another to the bath melt. The more circumferential funnels with deflecting inductors are installed the higher is the process control accuracy of the substance feeding for the solid component formation. After the deflecting inductors the desired amount of granules gets into the collecting inductor 29, after which the third deflecting inductor 30 is installed; its function is more accurate sorting of granule quantity fed for the component fabrication.

[0056] The final sorted granule flow gets into the accelerating-decelerating inductor 31, where the granule acquires the desired flying speed and gets to the deflecting inductor 21 at the output.

[0057] This example shows that the system assembled from the focusing and the deflecting inductors 20 and 21 may be more complicated due to additional similar device introduction. However such complication allows multiple accuracy increase for the substance monitoring and movement in electromagnetic field. FIG. 4 shows that each device is equipped with sensors 5, the number of which at this section is increased from two to ten. The number of control inductors and monitoring instruments for a particular additive machine is selected based on economic considerations and the current task with regard to the component fabrication accuracy. These devices allow the granule flow transformation in the course of falling into the bath melt both into a solid line of required section similar to a solid wire or a cut line of granules where the area and the length of continuous areas consisting of granules are preset as well as distances between the granule areas.

[0058] The above control circuits are intended for metals and nonmetals, where currents may be induced in granules with electromagnetic field generated around them. To control the flow of nonmetal granules, where it is difficult to accomplish, the design allowing the control of their path in electromagnetic field shall be used. For that end a neutral granule has to acquire a positive or a negative charge. A nonmetal particle in the form of a granule may be charged due to friction, radiation or direct connection to an electron or proton particle.

[0059] To control a charged particle a device shown in FIG. 5 may be used. In FIG. 5a granules 1 fall between vertical plates of deflecting condensers 32, which may be represented by two vertical planes of plates assembled into a grid structure, and be located in rows creating various intensity between the two planes. Eventually a device is constructed consisting of condenser plates in the form of a grid covering all the area through which the granules fall. The granule passing through the plates as affected by the electromagnetic field may descend horizontally its path to the set point. At the input and output of the condenser row the charged granule falling path parameters are monitored with the instruments. At the output from a line-type vertical condenser system functioning as a sorting and path deflecting device, the granules in certain amount are sent aside or for deposition into the bath melt. After the vertical deflecting condensers the granule flow collected all over the area gets into the acceleratingdecelerating row of horizontal condensers 33 located in the central part. The horizontal condenser 33 is assembled of several ring plates with a hole, which plane is normal to the granule falling path. Between every two plates various field intensities may be created by their connection to various fields, therefore the granule falling process may be accelerated or decelerated. Then a granule flow stabilized with regard to a granule falling rate gets through a a deflecting coil 34, which sends the granules to the set point.

[0060] FIG. 5b shows a design using only rows of the deflecting condensers 32 consisting of vertical plates alternating with rows of decelerating-accelerating horizontal condensers 33 consisting of horizontal plates for the granule path direction and falling rate change.

[0061] Thus the movement path and speed may be impacted by the use of both alternating and direct electromagnetic field.

[0062] Device designs shown in FIG. 4 and FIG. 5, may be of different configurations due to the use of various proposed design combinations.

[0063] To extend the direct material deposition method capabilities in electromagnetic field the equipment may be provided with several hoppers from which the material of the same or various fractions and chemistry is fed. Use of several sources of material feed into the bath melt allows increased weighing accuracy, feed rate, but above all it allows creation of composite components by combining feeding of various materials.

[0064] FIG. 6 shows the device design for conventional or composite component manufacture. The device design is similar to that shown in FIG. 3, however it allows using the substance of different materials with simultaneous or consistent deposition of various fraction and chemistry granules possible. The device includes several electron beam guns 11, several loading hoppers 14 with an independent feed control system for the granules which at the moment and in the quantity as required get to the body of component 4 set point. In FIG. 6 the focusing inductor 20 and the deflecting inductor 21 system is used as an example, the granules get there and then to the electromagnetic funnel 6 with the deflecting electromagnetic contour 7 and to the bath melt 3. The bath melt may be melted by one or several beams depending on the component complexity, deposit material rate and weight. An additional beam (or beams) allow the granule fusing or reheating to the set temperature at the approach to the bath melt. The additional beam allows melting or fusing of the granules deposited on the component surface thus enhancing the equipment capabilities.

[0065] The additive fabrication rate on the device shown in FIG. 6 may be improved if a component is formed at the substance deposit on several radii simultaneously, i.e. in different angle sectors of the circle area.

[0066] All impacts on the substance and the beam are caused by electromagnetic fields what allows monitoring of the component formation process with the maximum possible rate. Device designs shown in FIG. 16 reveal the method mechanism and demonstrate extensive capabilities of various modification equipment creation for the component fabrication.

[0067] The presented device allows high rate additive component fabrication. For example, if the granules fall from the height of two meters into the bath melt, they develop the rate of 6.26 m/sec under the gravity force. Suppose that considering the granule slowdown and flight path curving in the electromagnetic field the falling rate will reduce to 4.5 m/sec. Let's assume that the area of material deposition into the bath melt is 1 mm.sup.2 with the material falling in a solid stream, so in one second the solid component volume will be increased by 4.5 cm.sup.3. To deposit this amount the corresponding amount of the bath melt needs to be generated in a second. Assume that the bath melt volume will comprise half of the deposit substance volume, i.e. 2.25 cm.sup.3. If titanium is used as the substance, it is known, that when melting with a 1 MW electron beam the amount of material melted in one minute will comprise 12 kg. Thus, creation of a bath melt with the volume of 2.25 cm.sup.3 per second will require the beam power of 50.6 kW.

[0068] The titanium component mass growth rate will comprise 20.25 grams per second, 1215 grams per minute and 72.9 kg per hour. Such rate of additive manufacturing method of components is quite high and exceeds the rate of component fabrication by machining.

[0069] Currently the state-of-the-art technologies allow computer processing of a signal passing through a switch box with the frequency of 10000 Hz. This allows very accurate flight monitoring of a granule released from the two-meter height and moving with the average rate of 2.25 m/sec. The flight time comprises 0.9 sec., therefore the flight will be monitored with the time frequency of 8.8.Math.10.sup.5 sec, at that the granule flight path may be registered at an average length section of 0.18 mm. For a modern computer equipment the granule flight will be practically frozen in time, what allows very accurate aiming and path adjustment and therefore more accurate and rapid additive component formation with the substance deposition under the magnetic field control.

[0070] The control method of the substance and the beam in vacuum or gas medium by electromagnetic fields opens significant prospects for additive process development which are not available for the prototypes.

REFERENCES

[0071] [1]. V. M. Dovbysh, P. M. Zabednov, M. A. ZelenkoArticle. Additive Processes and Metal Products, [electronic resource]
http://nami.ru/uploads/docs/centr_technology_docs/55a62fc89524bAT_metall.pdf/
[2]. A. M. Kulish, 4-th year student, Laser Processes in Engineering, Moscow State Technical University named after N. E. BaumanArticle. Use of Additive Processes for Engineering Component Manufacture, All-Russia R&D Student Conference Studencheskay Nauchnay Vesna 2015: Engineering

[3]. A. G. Grigoriants, A. I. MisurovCapabilities and Prospects of Laser Clad Deposit. Engineering, 2005, No. 10 p. 52-56.

[0072] [4]. A. A. FogelInduction Method of Liquid Metal Suspension/edited by A. N. ShamovEd. 2, amendedL.: Mashinostroenie, Leningrad Division, 1989. (page 167; 174-181).
[5]. O. F. Kabardin O. .Physics: Alloy. Materials: Student Text Book.Ed. 4.M.: Prosveshenie: AO Training Literature, 1996. (p. 3; 5-11)