rus
TS_pub
technospheramag
technospheramag
ТЕХНОСФЕРА_РИЦ
Issue #6/2022
E. V. Zemlyakov, N. R. Alymov, A. M. Vildanov, K. D. Babkin, S. Yu. Ivanov, N. G. Kislov, D. S. Tarasov, A. S. Myatlev, A. A. Ivanovsky
Application of Laser and Additive Technologies in the Manufacturing of Advanced Industrial Gas Turbine Units
Application of Laser and Additive Technologies in the Manufacturing of Advanced Industrial Gas Turbine Units
DOI: 10.22184/1993-7296.FRos.2022.16.6.436.452
To ensure the competitiveness of high-tech industries, the introduction of new technologies of materials processing is required. The technological capabilities of advanced laser and additive technologies are shown by the example of manufacturing the gas collector of the combustion chamber of the GTE‑65.1 gas turbine unit. The main stages of preparation for the production of high-precision blanks by direct laser deposition from heat-resistant nickel alloy and stainless steel, and their subsequent processing are described. The results of metallographic studies, mechanical tests and geometry control are presented, confirming the high level of quality of the products obtained. The article demonstrates possibility of combining additive technologies, laser welding and cutting technologies, and technologies of thermal, mechanical and electroerosion processing in the manufacture of technically complex assemblies and parts.
To ensure the competitiveness of high-tech industries, the introduction of new technologies of materials processing is required. The technological capabilities of advanced laser and additive technologies are shown by the example of manufacturing the gas collector of the combustion chamber of the GTE‑65.1 gas turbine unit. The main stages of preparation for the production of high-precision blanks by direct laser deposition from heat-resistant nickel alloy and stainless steel, and their subsequent processing are described. The results of metallographic studies, mechanical tests and geometry control are presented, confirming the high level of quality of the products obtained. The article demonstrates possibility of combining additive technologies, laser welding and cutting technologies, and technologies of thermal, mechanical and electroerosion processing in the manufacture of technically complex assemblies and parts.
Теги: additive technologies direct laser deposition gas turbine units heat-resistant nickel alloys laser technologies stainless steels аддитивные технологии газотурбинная установка жаропрочные никелевые сплавы лазерные технологии нержавеющие стали прямое лазерное выращивание
Application of Laser and Additive Technologies in the Manufacturing of Advanced Industrial Gas Turbine Units
E. V. Zemlyakov 1, N. R. Alymov 1, A. M. Vildanov 1,
K. D. Babkin 1, S. Yu. Ivanov 1, N. G. Kislov 1, D. S. Tarasov 2, A. S. Myatlev 2, A. A. Ivanovsky 2
Institute of Laser and Welding Technologies
of St. Petersburg State Marine Technical University
(ILWT SPbSMTU), St. Petersburg, Russia
Power Machines JSC, St. Petersburg, Russia
To ensure the competitiveness of high-tech industries, the introduction of new technologies of materials processing is required. The technological capabilities of advanced laser and additive technologies are shown by the example of manufacturing the gas collector of the combustion chamber of the GTE‑65.1 gas turbine unit. The main stages of preparation for the production of high-precision blanks by direct laser deposition from heat-resistant nickel alloy and stainless steel, and their subsequent processing are described. The results of metallographic studies, mechanical tests and geometry control are presented, confirming the high level of quality of the products obtained. The article demonstrates possibility of combining additive technologies, laser welding and cutting technologies, and technologies of thermal, mechanical and electroerosion processing in the manufacture of technically complex assemblies and parts.
Key words: laser technologies, additive technologies, direct laser deposition, gas turbine units, heat-resistant nickel alloys, stainless steels
The article is received: 03.08.2022
The article is accepted:09.09.2022
1. Introduction
Among the laser technologies of material processing, the most prominent are laser welding technologies and related technologies, which include laser cutting, cladding and thermal hardening technologies. The high energy density in the laser exposure zone ensures high welding speeds and low heat input into the materials of the parts being welded compared to traditional welding technologies. The ability to minimize heat input is especially important when welding complex alloyed superalloys prone to cracking during welding [1].
In the field of materials processing technologies, additive technologies (AT) are distinguished as a separate class. Their application in high-tech industries is deposition along with the expansion of technological capabilities of additive technologies. The National Standard of the Russian Federation (GOST R 57558–2017) [2], as well as the standards of the International Organization for Standardization and the American Society for Testing and Materials (ISO / ASTM 52900:2015) [2], defines seven types of additive manufacturing processes. The most common additive processes for producing metal workpieces are processes based on direct energy deposition and powder bed fusion.
Rational use of the AT ensures obtaining unique operational and mass-dimensional characteristics of parts due to the introduction of new materials, topologically optimized structures, the possibility of obtaining calculated internal structures and channels, combining and enlarging assembly units and reducing the number of subsequent welding and assembly operations [3]. The technological capabilities of the AT can significantly reduce the material consumption of production, as well as time costs, which is especially important in the production of test samples and pilot runs. At the same time, it should be noted that with the help of additive methods, as a rule, high-precision workpiece blanks are obtained that require a certain amount of subsequent processing. The most effective technological processes for manufacturing of technically complex and critical assemblies and parts are based on a combination of additive and traditional technological operations [4].
The main consumers of the AT, both in Russia and around the world, are aviation and rocket engine building, thermal and nuclear power industries [5].
The article demonstrates the possibilities of domestic laser technologies by the example of manufacturing the gas collector of the combustion chamber of the GTE‑65.1 gas turbine unit, currently being developed by Power Machines JSC in cooperation with leading research institutes and enterprises of the country.
When creating advanced industrial gas turbine units (GTU), the development of a low-emission combustion chamber is a prerequisite for meeting both national requirements and the requirements of the international standards to achieve the environmental characteristics of GTU NOx ≤ 25 ppm. The experience of the world’s leading manufacturers of power equipment (GE, Siemens, Alstom, Mitsubishi, etc.) has shown that the use of technology for burning poor pre-mixed fuel-air mixtures can reduce NOx emission levels by more than ten times. Strict requirements on the level of NOx emissions led to the need to switch to low-emission fuel combustion technology.
For the combustion chamber of GTU‑65.1, the traditional scheme for the combustion chambers of stationary GTU is adopted: a cannular combustion chamber with identical, reverse-flow heat pipes, burner devices, housings and gas collectors. The number of individual burner units is 6 pcs. In the combustion chamber, a method has been implemented to ensure low emission of nitrogen oxides by burning a previously prepared poor fuel-air mixture.
The combustion chamber gas collector considered in the article is designed to supply combustion products from the heat pipe and organize a smooth transition from six individual burner assemblies to the common annulus of the nozzle diaphragm of the turbine first stage. The gas collector also generates a height-specified temperature profile at the inlet to the nozzle diaphragm blades.
The use of additive technology of direct laser deposition along with laser welding and cutting technologies and advanced 3D laser scanning methods for geometry control makes it possible to significantly reduce the production time, labor intensity and material costs compared to the traditional manufacturing method by stamping, machining and subsequent arc welding.
1. Theory and calculations
The gas collector of the combustion chamber GTE‑65.1 has a two-wall construction (Fig.1). The outer shell is made of austenitic stainless steel, the remaining elements of the gas collector are made of nickel heat-resistant alloy Haynes‑230. The gas collector has the following dimensions: 670 × 389 × 634 mm. The total mass of the gas collector is 66 kg.
The production sequence of the gas collector includes the following steps:
production of workpiece blanks of gas collector elements by direct laser deposition;
heat treatment of workpiece blanks;
machining of workpiece blanks;
electroerrosive burning of holes in the frame;
laser welding of the inlet ring, the inner shell and the frame;
laser cutting of the outer shell: cutting holes and dividing of the shell into the “left” one and the “right” one.
The technology of direct laser deposition (DLD) makes it possible to produce high-precision workpiece blanks of figurine-shaped large-sized products from a wide range of materials, including nickel, titanium, cobalt alloys, steels, bronzes, as well as their combinations [6, 7]. The main criteria for the quality of the obtained workpiece blanks are the level of mechanical properties and the magnitude of maximum deviations from a given geometry (geometric accuracy).
The required mechanical properties for the most materials are ensured by the defect-free internal structure obtained in the DLD process with correctly selected process parameters and strategies, as well as due to subsequent optimized heat treatment.
During the DLD process, a relatively small volume of the manufactured product undergoes local and short-term heating to high temperatures. As the heat source moves, more and more volumes of metal are heated, and in previously heated places the temperature is equalized. An uneven temperature field with a large temperature gradient in the area of local heating is formed in the build part. This causes nonuniform volume changes in the adjacent sections of the manufactured parts, which lead to the appearance of the internal forces in the metal and the formation of a stress and deformation field. As the heat spreads and the temperature equalizes, there is a continuous change in the fields of deformations and stresses. Unlike the temperature field, which disappears after the complete cooling of the structure, the stress field does not disappear, since the process of its formation is irreversible. Therefore, after complete cooling, there are residual deformations and stresses in the product.
The accuracy of DLD manufactured parts is largely determined by the possibility of predicting residual deformations, their accounting and compensation in the preparation of technological 3D CAD models, generation of tool trajectories and preparation of control programs.
The nature of the deformations depends on the size and geometry of the build part. In the case of massive, rigid or axisymmetric products, such as the inlet ring and the frame (Fig. 1. pos. 1, 3), deformations manifest themselves in the form of uniform shrinkage in plane perpendicular to the build-up direction. The amount of shrinkage primarily depends on the material and persists when the dimensions of the product change. This makes it possible to compensate the deformations with scaling the trajectory by a predetermined factor.
If the rigidity of the build part is insufficient for the balanced distribution of stresses, both in the inner and the outer shells (Fig. 1. pos. 2, 4), then they manifest themselves in the form of large-scale deformations. The nature of such deformations is preserved with a small change in the geometry of the build part (reverse bending), which allows an iterative approach to increase the manufacturing accuracy. The approach is to change the geometry based on the results of the trial deposition in order to balance the detected deformations. This makes it possible to increase the accuracy of the build part up to the required values with each iteration [8]. It should be noted that due to the high cost of the iterative method, in practice it is combined with mathematical modeling of the stress and strain state of the build parts and with the determination of reasonable allowances in order to minimize the number of attempts to obtain a suitable workpiece blank [9, 10].
Also on products with low rigidity, such as the outer shell (Fig. 1. pos. 4), deformations manifest themselves in the form of warping, that is, loss of stability of the surface with the formation of waves of various lengths and amplitudes. On large-sized products, the amount of deformation can reach 3 to 5 cm. In this case, to increase the accuracy of the build part, the reverse bending method is not suitable, it only redistributes deformations. Such products require modification of the structure in order to increase its rigidity, for example, by adding stiffeners (stringers), which will prevent warping and allow the use of the reverse bending method. These elements are technological and, as a rule, are subject to subsequent removal.
The process of computational determination of the stress and strain state of the product in the DLD process consists in the sequential solution of related problems of thermal conductivity and thermo-elastoplasticity [11]. All time intervals between passes are divided into time steps. Thus, the whole kinetics of temperature changes, stresses and deformations is traced. The process of sequential deposition of the product was taken into account using the following artificial technique: in those finite elements in which there is currently no deposited metal, the thermal conductivity, enthalpy and elastic modulus were set 104 times lower. The solution took into account the temperature dependences of the thermophysical and mechanical properties of the material.
An example of the simulated deformation field of the outer shell workpiece blank with stringers is shown in Fig. 2. The use of numerical modeling of the stress and strain state of the workpiece blank allowed optimizing the geometry and location of U-shaped stringers, as well as compensating for possible deformations in order to obtain the smallest deviations from the specified geometry.
The solid-state model obtained in this way was used to develop a technological model, generate trajectories and create control programs.
When preparing technological models of fabricated workpiece blanks, it is necessary to take into account possible deformations of the workpiece blanks in subsequent processes of their thermal and mechanical processing.
2. Equipment and materials
The outer shell was made of a metal-powder composition (MPC) of 316L steel. The frame, the inner shell, and the inlet ring were made of heat-resistant alloy H23X-A (Haynes‑230). The chemical composition of the materials used in the work is given in Table 1.
The incoming inspection of the MPCs used was carried out in accordance with GOST R 59035-2020 [14], sampling for incoming inspection is carried out in accordance with GOST 23148-98 [15].
The chemical composition of the MPC and the morphology of the powder particles were studied using a Tescan Mira3 scanning electron microscope with the Aztec Live Advanced Ultim Max 65 energy dispersive microanalysis system using ImageJ and OriginPro8 software. Metallographic studies were carried out using an inverted metallographic microscope Leica DMi8.
Mechanical tests were carried out on a universal electromechanical testing machine SHIMADZU AGS‑100Knx. The geometry of the workpieces was monitored using a measuring pair consisting of an optical 3D scanner and a Metroscan Elite 750 tracker. The production of workpiece blanks of gas collector elements was carried out on the direct laser deposition unit ILIST-L (Fig. 3). The technical characteristics of the ILIST-L unit are given in Table 2.
3D laser cutting (cutting workpiece blanks from substrates, cutting holes and cutting the outer shell) was performed on a universal robotic technological complex for laser processing of large-sized parts (Fig. 4), created as part of the implementation of the event to equip the laboratory of laser and ATs of the world-class scientific center “The Advanced Digital Technologies” with the financial support of Russian Ministry of Education and Science (Agreement No. 075–15–2020–903 dated November 16, 2020). The universal technological complex is being built on the basis of a high-precision robot Fanuc M‑800iA / 60, a two-axis tilt-rotary positioner and a fiber laser with a power of 3 kW.
Laser welding of the workpiece blanks of the gas collector elements was carried out on the ILIST‑2XL unit. The technical characteristics of the ILIST‑2XL unit are shown in Table. 3. Trajectories and control programs for robotic laser processing of workpiece blanks of gas collector elements were created in the Autodesk PowerMill software package.
3. Process design
The first stage of the DLD process design is the production of technological samples to determine the ranges of the DLD process parameters, ensuring a defect-free deposited material [16]. The production of technological samples is also an additional operation of the MPC incoming inspection.
During the production of technological samples, the following DLD process parameters vary: laser power, the working tool movement speed, the offset between the passes (dx), the step between the layers (dz), the pause between the passes. The Tables 4 and 5 show the ranges of changes in the DLD operating parameters in the manufacture of technological samples from the MPCs of the 316L steel and the H23X-A heat-resistant nickel alloy, respectively.
The choice of optimal process parameters was carried out on the basis of the analysis of the results of metallographic studies of the manufactured technological samples (Fig. 5, 6).
At powers of 1 600 and 1 800 W, lacks of fusion are observed in samples made of 316L steel. Lacks of fusion are formed in the overlap zone of adjacent passes due to a lack of supplied energy. This type of defects has an irregular shape in contrast to gas porosity and is characterized by a periodic arrangement in the volume of the material. At capacities over 2000 W, no defects were detected.
A similar pattern was observed on samples from the MPC of the heat-resistant nickel alloy H23X-A.
At capacities of 1 200 and 1 400 watts and a linear velocity of the working tool relative to the sample of 25 mm / s, lacks of fusion are observed in the samples. Increasing of the power also makes in possible to get rid of lacks of fusion, but, unlike stainless steel, hot cracks were observed in samples made of heat-resistant H23X-A nickel alloy already at a power of 1 800 watts [17]. In order to expand the working ranges of the process parameters that ensure a defect-free internal structure, an additional series of experiments on the production of samples with lower productivity was carried out. At the same time, the power, the pause between passes, the step between layers, the deposition speed, the step between the passes varied.
The results of additional studies have shown that a decrease in the productivity of the DLD process favorably affects the quality of deposited samples (Fig. 7). At a power of 1100 W and a speed of 15 mm / s, no visible defects are observed, while there is a power reserve, because at 1300 W, an almost defect-free structure was also achieved.
As a result of the experiments carried out, the operating process parameters of the DLD from the MPCs of 316L steel and heat-resistant nickel alloy H23X-A were determined (Table 6).
To determine the mechanical properties of the deposited material, witness samples were made in the selected parameters [18], from which standard tensile test samples were cut (Type IV GOST 1497-84 (ISO 6892-84) [19]). Mechanical tests were carried out both on samples without subsequent heat treatment and with the heat treatment.
The Tables 7 and 8 present the results of mechanical tests for uniaxial tension of samples made of H23X-A ally and 316L steel, respectively. The samples made of H23X-A alloy without heat treatment after testing have a ductile fracture in the longitudinal direction (X) and a layered fracture in the transverse direction (Z), which indicates anisotropy of the properties. The fracture pattern of heat-treated samples is ductile both in the longitudinal and transverse directions, which indicates the elimination of anisotropy of mechanical properties with the help of properly selected heat treatment. After testing of the samples without heat treatment made of 316L steel, the fracture pattern is ductile fracture in both the longitudinal (X) and transverse (Z) directions. After heat treatment of the 316L alloy, the fracture pattern does not change, there is an increase in percentage elongation and a partial decrease in yield stress and tensile strength due to incomplete recrystallization of the alloy and removal of internal stresses.
4. Direct laser deposition of workpiece blanks
Based on the results of the preliminary finite element simulation and the process design, the control programs were generated for the direct laser deposition units ILIST-L for the manufacture of workpiece blanks of gas collector elements.
The process of direct laser deposition of the workpiece blanks of the frame, the inlet ring, the inner and the outer shells of the gas collector is shown in Fig. 8.
After deposition, the geometry of the obtained workpiece blanks was monitored, heat treatment was performed to relieve internal stresses for the 316 L steel workpiece blank and to achieve the required strength properties for the H23X-A alloy workpiece blanks. After the heat treatment, the geometry was re-checked.
5. Discussion of the results
The results of the geometry control of the deposited blanks are shown in Fig. 9. The geometry control showed that the deviations of the geometry of the inlet ring and the frame workpiece blanks lie within acceptable limits and make it possible to obtain suitable parts from the deposited workpiece blanks during subsequent processing.
Unacceptable geometry deviations were detected on the workpiece blanks of the inner and outer shells – up to 1.5 mm on the inner shell and up to 3.5 mm on the outer shell.
To eliminate the detected geometry deviations, the technological models of the workpiece blanks were changed. With the help of reverse bending of the sections with maximum deviations, the detected deformations were compensated. The results of the geometry control of the workpiece blanks obtained during repeated direct laser deposition according to the modified technological models are shown in Fig. 10. The changes effected made it possible to obtain workpiece blanks suitable for subsequent processing.
For the manufacture of the gas collector, the workpiece blanks obtained by the direct laser deposition were processed mechanically. Cutting holes in the outer shell and dividing it into two parts was carried out using robotic 3D laser cutting (Fig. 11).
Combining the frame, the inlet ring and the inner shell into one assembly unit was carried out using laser welding (Fig. 12). Previously, an electroerosive burning of holes was carried out in the frame.
After the finishing operations of laser cutting and laser welding with the help of 3D laser scanning, the final control of the geometry of the manufactured GTE‑65.1 combustion chamber gas collector was carried out, confirming its compliance with the requirements of the design documentation.
Conclusions
The technological capabilities of advanced laser and additive technologies can significantly reduce material and time costs in the development and manufacture of complex-shaped parts for the needs of high-tech industries.
At the same time, the process of obtaining high-precision workpiece blanks using additive methods is multi-stage and requires preliminary numerical and real experiments.
When developing technologies for the additive production of large-sized blanks particular attention should be paid to the issues of forecasting, accounting and compensation of possible thermal deformations that occur both during the additive process itself and during subsequent processing.
The high level of strength characteristics obtained and the possibility of combining the additive and traditional technological operations ensure high efficiency of the manufacturing process of technically complex and critical components and parts.
Credits
The article presents the results of research obtained during the performance of work under the contract with Power Machines JSC dated October 01, 2021 No. 0000000002019RMO0002 / 76071 and the implementation of the strategic academic leadership program “Priority 2030” (Agreement on the provision of grants from the federal budget in the form of subsidies dated September 30, 2021 No. 075-15-2021-1206, strategic project “Digital Industrial Technologies”).
AUTHORS
Zemlyakov E. V., Institute of Laser and Welding Technologies of St. Petersburg State Marine Technical University (ILWT SPbSMTU), ilwt@ilwt.smtu.ru, St. Petersburg, Russia.
ORCID 0000-0001-9594-2831,
Alymov N. R., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0003-1066-1446
Vildanov A. M., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0002-7319-0605
Babkin K. D., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0003-1098-1319
Ivanov S. Yu., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0002-0077-2313
Kislov N. G., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0002-1103-5802
Tarasov D. S., Power Machines JSC, St. Petersburg, Russia.
ORCID 0000-0001-8673-7254
Myatlev A. S., Power Machines JSC, St. Petersburg, Russia.
ORCID 0000-0002-5300-836X
Ivanovsky A. A., Power Machines JSC, St. Petersburg, Russia.
E. V. Zemlyakov 1, N. R. Alymov 1, A. M. Vildanov 1,
K. D. Babkin 1, S. Yu. Ivanov 1, N. G. Kislov 1, D. S. Tarasov 2, A. S. Myatlev 2, A. A. Ivanovsky 2
Institute of Laser and Welding Technologies
of St. Petersburg State Marine Technical University
(ILWT SPbSMTU), St. Petersburg, Russia
Power Machines JSC, St. Petersburg, Russia
To ensure the competitiveness of high-tech industries, the introduction of new technologies of materials processing is required. The technological capabilities of advanced laser and additive technologies are shown by the example of manufacturing the gas collector of the combustion chamber of the GTE‑65.1 gas turbine unit. The main stages of preparation for the production of high-precision blanks by direct laser deposition from heat-resistant nickel alloy and stainless steel, and their subsequent processing are described. The results of metallographic studies, mechanical tests and geometry control are presented, confirming the high level of quality of the products obtained. The article demonstrates possibility of combining additive technologies, laser welding and cutting technologies, and technologies of thermal, mechanical and electroerosion processing in the manufacture of technically complex assemblies and parts.
Key words: laser technologies, additive technologies, direct laser deposition, gas turbine units, heat-resistant nickel alloys, stainless steels
The article is received: 03.08.2022
The article is accepted:09.09.2022
1. Introduction
Among the laser technologies of material processing, the most prominent are laser welding technologies and related technologies, which include laser cutting, cladding and thermal hardening technologies. The high energy density in the laser exposure zone ensures high welding speeds and low heat input into the materials of the parts being welded compared to traditional welding technologies. The ability to minimize heat input is especially important when welding complex alloyed superalloys prone to cracking during welding [1].
In the field of materials processing technologies, additive technologies (AT) are distinguished as a separate class. Their application in high-tech industries is deposition along with the expansion of technological capabilities of additive technologies. The National Standard of the Russian Federation (GOST R 57558–2017) [2], as well as the standards of the International Organization for Standardization and the American Society for Testing and Materials (ISO / ASTM 52900:2015) [2], defines seven types of additive manufacturing processes. The most common additive processes for producing metal workpieces are processes based on direct energy deposition and powder bed fusion.
Rational use of the AT ensures obtaining unique operational and mass-dimensional characteristics of parts due to the introduction of new materials, topologically optimized structures, the possibility of obtaining calculated internal structures and channels, combining and enlarging assembly units and reducing the number of subsequent welding and assembly operations [3]. The technological capabilities of the AT can significantly reduce the material consumption of production, as well as time costs, which is especially important in the production of test samples and pilot runs. At the same time, it should be noted that with the help of additive methods, as a rule, high-precision workpiece blanks are obtained that require a certain amount of subsequent processing. The most effective technological processes for manufacturing of technically complex and critical assemblies and parts are based on a combination of additive and traditional technological operations [4].
The main consumers of the AT, both in Russia and around the world, are aviation and rocket engine building, thermal and nuclear power industries [5].
The article demonstrates the possibilities of domestic laser technologies by the example of manufacturing the gas collector of the combustion chamber of the GTE‑65.1 gas turbine unit, currently being developed by Power Machines JSC in cooperation with leading research institutes and enterprises of the country.
When creating advanced industrial gas turbine units (GTU), the development of a low-emission combustion chamber is a prerequisite for meeting both national requirements and the requirements of the international standards to achieve the environmental characteristics of GTU NOx ≤ 25 ppm. The experience of the world’s leading manufacturers of power equipment (GE, Siemens, Alstom, Mitsubishi, etc.) has shown that the use of technology for burning poor pre-mixed fuel-air mixtures can reduce NOx emission levels by more than ten times. Strict requirements on the level of NOx emissions led to the need to switch to low-emission fuel combustion technology.
For the combustion chamber of GTU‑65.1, the traditional scheme for the combustion chambers of stationary GTU is adopted: a cannular combustion chamber with identical, reverse-flow heat pipes, burner devices, housings and gas collectors. The number of individual burner units is 6 pcs. In the combustion chamber, a method has been implemented to ensure low emission of nitrogen oxides by burning a previously prepared poor fuel-air mixture.
The combustion chamber gas collector considered in the article is designed to supply combustion products from the heat pipe and organize a smooth transition from six individual burner assemblies to the common annulus of the nozzle diaphragm of the turbine first stage. The gas collector also generates a height-specified temperature profile at the inlet to the nozzle diaphragm blades.
The use of additive technology of direct laser deposition along with laser welding and cutting technologies and advanced 3D laser scanning methods for geometry control makes it possible to significantly reduce the production time, labor intensity and material costs compared to the traditional manufacturing method by stamping, machining and subsequent arc welding.
1. Theory and calculations
The gas collector of the combustion chamber GTE‑65.1 has a two-wall construction (Fig.1). The outer shell is made of austenitic stainless steel, the remaining elements of the gas collector are made of nickel heat-resistant alloy Haynes‑230. The gas collector has the following dimensions: 670 × 389 × 634 mm. The total mass of the gas collector is 66 kg.
The production sequence of the gas collector includes the following steps:
production of workpiece blanks of gas collector elements by direct laser deposition;
heat treatment of workpiece blanks;
machining of workpiece blanks;
electroerrosive burning of holes in the frame;
laser welding of the inlet ring, the inner shell and the frame;
laser cutting of the outer shell: cutting holes and dividing of the shell into the “left” one and the “right” one.
The technology of direct laser deposition (DLD) makes it possible to produce high-precision workpiece blanks of figurine-shaped large-sized products from a wide range of materials, including nickel, titanium, cobalt alloys, steels, bronzes, as well as their combinations [6, 7]. The main criteria for the quality of the obtained workpiece blanks are the level of mechanical properties and the magnitude of maximum deviations from a given geometry (geometric accuracy).
The required mechanical properties for the most materials are ensured by the defect-free internal structure obtained in the DLD process with correctly selected process parameters and strategies, as well as due to subsequent optimized heat treatment.
During the DLD process, a relatively small volume of the manufactured product undergoes local and short-term heating to high temperatures. As the heat source moves, more and more volumes of metal are heated, and in previously heated places the temperature is equalized. An uneven temperature field with a large temperature gradient in the area of local heating is formed in the build part. This causes nonuniform volume changes in the adjacent sections of the manufactured parts, which lead to the appearance of the internal forces in the metal and the formation of a stress and deformation field. As the heat spreads and the temperature equalizes, there is a continuous change in the fields of deformations and stresses. Unlike the temperature field, which disappears after the complete cooling of the structure, the stress field does not disappear, since the process of its formation is irreversible. Therefore, after complete cooling, there are residual deformations and stresses in the product.
The accuracy of DLD manufactured parts is largely determined by the possibility of predicting residual deformations, their accounting and compensation in the preparation of technological 3D CAD models, generation of tool trajectories and preparation of control programs.
The nature of the deformations depends on the size and geometry of the build part. In the case of massive, rigid or axisymmetric products, such as the inlet ring and the frame (Fig. 1. pos. 1, 3), deformations manifest themselves in the form of uniform shrinkage in plane perpendicular to the build-up direction. The amount of shrinkage primarily depends on the material and persists when the dimensions of the product change. This makes it possible to compensate the deformations with scaling the trajectory by a predetermined factor.
If the rigidity of the build part is insufficient for the balanced distribution of stresses, both in the inner and the outer shells (Fig. 1. pos. 2, 4), then they manifest themselves in the form of large-scale deformations. The nature of such deformations is preserved with a small change in the geometry of the build part (reverse bending), which allows an iterative approach to increase the manufacturing accuracy. The approach is to change the geometry based on the results of the trial deposition in order to balance the detected deformations. This makes it possible to increase the accuracy of the build part up to the required values with each iteration [8]. It should be noted that due to the high cost of the iterative method, in practice it is combined with mathematical modeling of the stress and strain state of the build parts and with the determination of reasonable allowances in order to minimize the number of attempts to obtain a suitable workpiece blank [9, 10].
Also on products with low rigidity, such as the outer shell (Fig. 1. pos. 4), deformations manifest themselves in the form of warping, that is, loss of stability of the surface with the formation of waves of various lengths and amplitudes. On large-sized products, the amount of deformation can reach 3 to 5 cm. In this case, to increase the accuracy of the build part, the reverse bending method is not suitable, it only redistributes deformations. Such products require modification of the structure in order to increase its rigidity, for example, by adding stiffeners (stringers), which will prevent warping and allow the use of the reverse bending method. These elements are technological and, as a rule, are subject to subsequent removal.
The process of computational determination of the stress and strain state of the product in the DLD process consists in the sequential solution of related problems of thermal conductivity and thermo-elastoplasticity [11]. All time intervals between passes are divided into time steps. Thus, the whole kinetics of temperature changes, stresses and deformations is traced. The process of sequential deposition of the product was taken into account using the following artificial technique: in those finite elements in which there is currently no deposited metal, the thermal conductivity, enthalpy and elastic modulus were set 104 times lower. The solution took into account the temperature dependences of the thermophysical and mechanical properties of the material.
An example of the simulated deformation field of the outer shell workpiece blank with stringers is shown in Fig. 2. The use of numerical modeling of the stress and strain state of the workpiece blank allowed optimizing the geometry and location of U-shaped stringers, as well as compensating for possible deformations in order to obtain the smallest deviations from the specified geometry.
The solid-state model obtained in this way was used to develop a technological model, generate trajectories and create control programs.
When preparing technological models of fabricated workpiece blanks, it is necessary to take into account possible deformations of the workpiece blanks in subsequent processes of their thermal and mechanical processing.
2. Equipment and materials
The outer shell was made of a metal-powder composition (MPC) of 316L steel. The frame, the inner shell, and the inlet ring were made of heat-resistant alloy H23X-A (Haynes‑230). The chemical composition of the materials used in the work is given in Table 1.
The incoming inspection of the MPCs used was carried out in accordance with GOST R 59035-2020 [14], sampling for incoming inspection is carried out in accordance with GOST 23148-98 [15].
The chemical composition of the MPC and the morphology of the powder particles were studied using a Tescan Mira3 scanning electron microscope with the Aztec Live Advanced Ultim Max 65 energy dispersive microanalysis system using ImageJ and OriginPro8 software. Metallographic studies were carried out using an inverted metallographic microscope Leica DMi8.
Mechanical tests were carried out on a universal electromechanical testing machine SHIMADZU AGS‑100Knx. The geometry of the workpieces was monitored using a measuring pair consisting of an optical 3D scanner and a Metroscan Elite 750 tracker. The production of workpiece blanks of gas collector elements was carried out on the direct laser deposition unit ILIST-L (Fig. 3). The technical characteristics of the ILIST-L unit are given in Table 2.
3D laser cutting (cutting workpiece blanks from substrates, cutting holes and cutting the outer shell) was performed on a universal robotic technological complex for laser processing of large-sized parts (Fig. 4), created as part of the implementation of the event to equip the laboratory of laser and ATs of the world-class scientific center “The Advanced Digital Technologies” with the financial support of Russian Ministry of Education and Science (Agreement No. 075–15–2020–903 dated November 16, 2020). The universal technological complex is being built on the basis of a high-precision robot Fanuc M‑800iA / 60, a two-axis tilt-rotary positioner and a fiber laser with a power of 3 kW.
Laser welding of the workpiece blanks of the gas collector elements was carried out on the ILIST‑2XL unit. The technical characteristics of the ILIST‑2XL unit are shown in Table. 3. Trajectories and control programs for robotic laser processing of workpiece blanks of gas collector elements were created in the Autodesk PowerMill software package.
3. Process design
The first stage of the DLD process design is the production of technological samples to determine the ranges of the DLD process parameters, ensuring a defect-free deposited material [16]. The production of technological samples is also an additional operation of the MPC incoming inspection.
During the production of technological samples, the following DLD process parameters vary: laser power, the working tool movement speed, the offset between the passes (dx), the step between the layers (dz), the pause between the passes. The Tables 4 and 5 show the ranges of changes in the DLD operating parameters in the manufacture of technological samples from the MPCs of the 316L steel and the H23X-A heat-resistant nickel alloy, respectively.
The choice of optimal process parameters was carried out on the basis of the analysis of the results of metallographic studies of the manufactured technological samples (Fig. 5, 6).
At powers of 1 600 and 1 800 W, lacks of fusion are observed in samples made of 316L steel. Lacks of fusion are formed in the overlap zone of adjacent passes due to a lack of supplied energy. This type of defects has an irregular shape in contrast to gas porosity and is characterized by a periodic arrangement in the volume of the material. At capacities over 2000 W, no defects were detected.
A similar pattern was observed on samples from the MPC of the heat-resistant nickel alloy H23X-A.
At capacities of 1 200 and 1 400 watts and a linear velocity of the working tool relative to the sample of 25 mm / s, lacks of fusion are observed in the samples. Increasing of the power also makes in possible to get rid of lacks of fusion, but, unlike stainless steel, hot cracks were observed in samples made of heat-resistant H23X-A nickel alloy already at a power of 1 800 watts [17]. In order to expand the working ranges of the process parameters that ensure a defect-free internal structure, an additional series of experiments on the production of samples with lower productivity was carried out. At the same time, the power, the pause between passes, the step between layers, the deposition speed, the step between the passes varied.
The results of additional studies have shown that a decrease in the productivity of the DLD process favorably affects the quality of deposited samples (Fig. 7). At a power of 1100 W and a speed of 15 mm / s, no visible defects are observed, while there is a power reserve, because at 1300 W, an almost defect-free structure was also achieved.
As a result of the experiments carried out, the operating process parameters of the DLD from the MPCs of 316L steel and heat-resistant nickel alloy H23X-A were determined (Table 6).
To determine the mechanical properties of the deposited material, witness samples were made in the selected parameters [18], from which standard tensile test samples were cut (Type IV GOST 1497-84 (ISO 6892-84) [19]). Mechanical tests were carried out both on samples without subsequent heat treatment and with the heat treatment.
The Tables 7 and 8 present the results of mechanical tests for uniaxial tension of samples made of H23X-A ally and 316L steel, respectively. The samples made of H23X-A alloy without heat treatment after testing have a ductile fracture in the longitudinal direction (X) and a layered fracture in the transverse direction (Z), which indicates anisotropy of the properties. The fracture pattern of heat-treated samples is ductile both in the longitudinal and transverse directions, which indicates the elimination of anisotropy of mechanical properties with the help of properly selected heat treatment. After testing of the samples without heat treatment made of 316L steel, the fracture pattern is ductile fracture in both the longitudinal (X) and transverse (Z) directions. After heat treatment of the 316L alloy, the fracture pattern does not change, there is an increase in percentage elongation and a partial decrease in yield stress and tensile strength due to incomplete recrystallization of the alloy and removal of internal stresses.
4. Direct laser deposition of workpiece blanks
Based on the results of the preliminary finite element simulation and the process design, the control programs were generated for the direct laser deposition units ILIST-L for the manufacture of workpiece blanks of gas collector elements.
The process of direct laser deposition of the workpiece blanks of the frame, the inlet ring, the inner and the outer shells of the gas collector is shown in Fig. 8.
After deposition, the geometry of the obtained workpiece blanks was monitored, heat treatment was performed to relieve internal stresses for the 316 L steel workpiece blank and to achieve the required strength properties for the H23X-A alloy workpiece blanks. After the heat treatment, the geometry was re-checked.
5. Discussion of the results
The results of the geometry control of the deposited blanks are shown in Fig. 9. The geometry control showed that the deviations of the geometry of the inlet ring and the frame workpiece blanks lie within acceptable limits and make it possible to obtain suitable parts from the deposited workpiece blanks during subsequent processing.
Unacceptable geometry deviations were detected on the workpiece blanks of the inner and outer shells – up to 1.5 mm on the inner shell and up to 3.5 mm on the outer shell.
To eliminate the detected geometry deviations, the technological models of the workpiece blanks were changed. With the help of reverse bending of the sections with maximum deviations, the detected deformations were compensated. The results of the geometry control of the workpiece blanks obtained during repeated direct laser deposition according to the modified technological models are shown in Fig. 10. The changes effected made it possible to obtain workpiece blanks suitable for subsequent processing.
For the manufacture of the gas collector, the workpiece blanks obtained by the direct laser deposition were processed mechanically. Cutting holes in the outer shell and dividing it into two parts was carried out using robotic 3D laser cutting (Fig. 11).
Combining the frame, the inlet ring and the inner shell into one assembly unit was carried out using laser welding (Fig. 12). Previously, an electroerosive burning of holes was carried out in the frame.
After the finishing operations of laser cutting and laser welding with the help of 3D laser scanning, the final control of the geometry of the manufactured GTE‑65.1 combustion chamber gas collector was carried out, confirming its compliance with the requirements of the design documentation.
Conclusions
The technological capabilities of advanced laser and additive technologies can significantly reduce material and time costs in the development and manufacture of complex-shaped parts for the needs of high-tech industries.
At the same time, the process of obtaining high-precision workpiece blanks using additive methods is multi-stage and requires preliminary numerical and real experiments.
When developing technologies for the additive production of large-sized blanks particular attention should be paid to the issues of forecasting, accounting and compensation of possible thermal deformations that occur both during the additive process itself and during subsequent processing.
The high level of strength characteristics obtained and the possibility of combining the additive and traditional technological operations ensure high efficiency of the manufacturing process of technically complex and critical components and parts.
Credits
The article presents the results of research obtained during the performance of work under the contract with Power Machines JSC dated October 01, 2021 No. 0000000002019RMO0002 / 76071 and the implementation of the strategic academic leadership program “Priority 2030” (Agreement on the provision of grants from the federal budget in the form of subsidies dated September 30, 2021 No. 075-15-2021-1206, strategic project “Digital Industrial Technologies”).
AUTHORS
Zemlyakov E. V., Institute of Laser and Welding Technologies of St. Petersburg State Marine Technical University (ILWT SPbSMTU), ilwt@ilwt.smtu.ru, St. Petersburg, Russia.
ORCID 0000-0001-9594-2831,
Alymov N. R., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0003-1066-1446
Vildanov A. M., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0002-7319-0605
Babkin K. D., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0003-1098-1319
Ivanov S. Yu., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0002-0077-2313
Kislov N. G., ILWT SPbSMTU, St. Petersburg, Russia.
ORCID 0000-0002-1103-5802
Tarasov D. S., Power Machines JSC, St. Petersburg, Russia.
ORCID 0000-0001-8673-7254
Myatlev A. S., Power Machines JSC, St. Petersburg, Russia.
ORCID 0000-0002-5300-836X
Ivanovsky A. A., Power Machines JSC, St. Petersburg, Russia.
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