rus
Issue #5/2024
A. Yu. Kovchik, A. M. Vildanov, N. R. Alymov, S. Yu. Ivanov, R. V. Mendagaliyev
Application of Residual Deformation Compensation Methods in Direct Laser Deposition of Large-Sized Products
Application of Residual Deformation Compensation Methods in Direct Laser Deposition of Large-Sized Products
DOI: 10.22184/1993-7296.FRos.2024.18.5.406.418
Direct laser deposition is widely used in high-tech industries due to the possibility of creating complex parts, which manufacturing is impossible using traditional production methods. However, the production process is complicating by the formation of residual stresses and deformations in the part, which negatively affect its quality. The field of distribution of stresses and deformations in the part is associated with its geometry. This article presents the main compensation methods of part distortion and describes the types of typical part deformations for DLD. The deformation compensation results are presented on example of four different parts made of stainless steel grade 12Ch18Ni10Ti (analogue AISI 304) and heat-resistant alloy VZh159.
Direct laser deposition is widely used in high-tech industries due to the possibility of creating complex parts, which manufacturing is impossible using traditional production methods. However, the production process is complicating by the formation of residual stresses and deformations in the part, which negatively affect its quality. The field of distribution of stresses and deformations in the part is associated with its geometry. This article presents the main compensation methods of part distortion and describes the types of typical part deformations for DLD. The deformation compensation results are presented on example of four different parts made of stainless steel grade 12Ch18Ni10Ti (analogue AISI 304) and heat-resistant alloy VZh159.
Теги: аддитивные технологии газотурбинная установка компенсация деформаций остаточные напряжения и деформации прямое лазерное выращивание
Application of Residual Deformation Compensation Methods in Direct Laser Deposition of Large-Sized Products
A. Yu. Kovchik, A. M. Vildanov, N. R. Alymov, S. Yu. Ivanov, R. V. Mendagaliyev
Saint Petersburg State Marine Technical University Institute of Laser and Welding Technologies (SMTU ILWT), St. Petersburg, Russia
Direct laser deposition is widely used in high-tech industries due to the possibility of creating complex parts, which manufacturing is impossible using traditional production methods. However, the production process is complicating by the formation of residual stresses and deformations in the part, which negatively affect its quality. The field of distribution of stresses and deformations in the part is associated with its geometry. This article presents the main compensation methods of part distortion and describes the types of typical part deformations for DLD. The deformation compensation results are presented on example of four different parts made of stainless steel grade 12Ch18Ni10Ti (analogue AISI 304) and heat-resistant alloy VZh159.
Key words: Additive technologies, Direct laser deposition (DLD), Residual stresses and deformations, Turbines, Deformations compensation.
Article received: 18.06.2024
Article accepted: 11.07.2024
Introduction
Direct laser deposition is an additive technology method based on layer-by-layer deposition of metal powder onto a substrate using of highly concentrated laser radiation. Each deposited layer becomes the basis for the next one, thus the material using is aiming directly at forming the part. The technology allows utilize wide range of materials, such as titanium, aluminum, heat-resistant nickel alloys, and stainless steels [1].
This production method has changed the approach to product design, it is able to produce parts with strict design geometry in a shorter time in opposite of traditional production technologies. Thereby, direct laser deposition is increasingly using in such knowledge-intensive industries as, for example: aircraft and rocket engine industries, thermal and nuclear power engineering [2].
These industries require the production of high-quality products corresponding all the requirements, with minimal costs. Direct laser deposition of any product is accompanying by the formation of residual stresses and plastic deformations directly during the deposition process. Thus, the DLD production cycle is a sequence of stages shown in Fig. 1.
Plastic deformations forms in the part during thermal expansion of the material in consequence of local influence of highly concentrated laser radiation [3]. Residual stresses are forming after plastic deformations during crystallization and cooling of the material. They reduce the strength characteristics of the material and staying after completion of deposition, therefore the part is subjecting to heat treatment. Residual stresses are the main forces unevenly distributed in the part, they are accompanying by the elastic deformations, and when they relax, a part geometry is change [4]. Plastic deformation can be eliminating by changing the trajectory of the deposition so as to the product takes the desired shape during deformation. The difficulty is that to obtain reliable data of the part deformations, it is necessary to make a tested part deposition. In addition, after changing the deposing trajectory, the distribution of deformations may also change, which makes the process iterative.
The deposition trajectory is determining by the part geometry, so the distribution of residual stresses and deformations becomes unique. However, several parts, which have similar shape and size, may have similar deformation patterns, which means that the solution for deformations compensating may also be common. Have determined a connection between the part geometry and the deformations patterns, it will be possible to solve the problem of iterations numbers in DLD process and increase its efficiency.
Deformations types and their compensation methods.
Before describing the deformations compensation methods, it is necessary to classify the deformations types. In point of the welding deformations and stresses theory [5], deformations are dividing on two groups: “General”, which cause distortions of the shape and size of the entire element; and “local”, which spread to individual elements of the part design. Such effects like thermal shrinkage can be attributing to general deformations. Meaning of thermal shrinkage is the volume changing of the deposited material during its cooling is proportional to the temperature changing, as a result the linear dimensions of the part decrease. Local deformations includes several types: saddle-shaped, distributed deformations, warping.
The degree of part thermal shrinkage is determining by material characteristics. Compensation for this effect achieved by scaling on shrinkage coefficient, which can be determinate by the ratio of the linear dimensions depositing part to the dimensions of etalon part. The determining method of the shrinkage coefficient presented in the source [6]. Shrinkage compensation uses to every part. However, for axis-symmetrical part for example: rings, cylinders, etc., the scaling is only necessary compensation method.
The substrate is the base where each part begins its deposition, so some characteristic features should be noted. Сconsider two options for fixing the substrate: the first is when the substrate is rigidly fixed to the table (Fig. 2a), the position is limited on all sides, the second one is when the substrate is fixed in the center (Fig. 2b), the edges of the substrate are free. The first method should be used if the substrate consider some elements of part geometry, or the depositing part is massive, i. e. the area of the layer deposition is large. Full fixation is accompanying a high stress concentration in the part material. Therefore this fixation method using for plastic metals deposition with a high relative elongation index. However is also necessary to take into account the part geometry. For low-plasticity materials or products with low construction rigidity, such as thin-walled shells, this can cause destruction (Fig. 2a). Using by the second fixing method, the formation of a saddle-shaped deformation of the substrate compensate residual stresses partially. Unfixed edges rise in the direction of deposition, which allows maintaining part integrity. However, the main program does not take into account the substrate changing, the surfacing continues along the specified trajectory. The free ends of the substrate rise faster than the dispositioning occurs, so a fragment of part geometry is lost (Fig. 2b). Therefore, it use increased compensation layer in deformable substrate case, leveling out the influence of substrate deformations (Fig. 2c).
Compensation layer – a segment preceding the growth of the main product, which adds in each part to simplify the process of cutting from the substrate.
Distributed deformations are large areas with a low gradient of deviations from the required geometry of the product [7]. This type of deformation encounters in parts, with the exception of massive design with high rigidity. Compensation carry out by preliminary changing the part geometry in the direction opposite to the part deformations (Fig. 3). When deposition process repeats the part takes desire shape. This type of deformation becomes the cause of additional iterations. The problem is especially relevant in the production of large-sized thin-walled shells. For these parts minor changes in geometry lead to significant changes in the deformations patterns due to the low construction rigidity Therefore, compensation of deformations for these products accompanied by computer modeling of the stress-strain state.
Warping or loss of stability are wave-like deformations of the parts wall with a high gradient of deviations from the desire geometry. This defect forms as result of insufficient part construction rigidity it is typical for thin-walled shells [8].
The compensation method applicable to distributed deformations is not effective in this case. The solution to this problem is to increase the rigidity of the shell. This achieved in several ways: by increasing the thickness of the shell wall or by adding stiffening elements (stringers) to the part surface. Stringers transform warping into distributed deformation, which compensation was describe earlier. Stringers are technological elements, after production they are removing from the part surface.
The use of compensation methods allows solving the problem of deformations, however, it does not lead to relaxation of residual stresses, therefore, after completion of the technological process, and a part subjected to heat treatment.
An application of deformation compensation methods shown on example of parts included in the gas collector of the combustion chamber GTE‑65.1 gas turbine unit. The complete production process of the set of products is presenting in the source [9]. The gas collector includes four elements with different types of geometry, which made of different materials (Fig. 4). The inner shell of the gas collector, the frame, and the ring are made of heat-resistant nickel-based alloy VZh159, the outer shell of the gas collector is made of stainless steel grade 12Ch18Ni10Ti (analogue AISI 304L). The inner and outer shells (1, 4) are thin-walled large-sized parts and deformed easily due to the low rigidity of the construction. It is difficult to predict the pattern and degree of deformation without using of computational methods for modeling the DLD process or trial production. The frame (2) is a thick-walled product with mirror symmetry, which unexpected large-scale deformations. The ring (3) is a rigid part consisting of two axisymmetric shells connected by truss elements, in addition, it has reinforcements on the outer wall, and the change in the geometry of the ring is expecting mainly due to thermal shrinkage.
Equipment and materials
The production of gas collector elements carried out using the ILIST-L direct laser deposition complex manufactured by SMTU (Fig. 5).
The technical characteristics of the complex ILIST-L presents in Table 2. The trajectories and control programs for robotic laser processing of gas collector element created in the Autodesk PowerMill software package. The geometry of the workpieces controlled using a measuring pair included an optical 3D scanner Metroscan Elite 750 and c-track Black Elite. The reverse deformation of the part 3D-models carried out using the Matlab application package for technical calculations based on the geometry deviation field data obtained in the Geomagic Control X program.
Technical preparing
The first preparatory stage is the transformation of product models into blank models. Compensation layers added to the part models, scale coefficients applied, allowances for mechanical processing added to the frame and ring. Stringers with a wall thickness of 2 mm added on surface of outer shell in order to increase the part rigidity, while the geometry of the shell did not change at the first stage.
To deformations compensation of the outer shell and the inner shell of the gas collector, it was necessary to simulate the stress-strain state using the finite element method. The modeling process consisted of solving thermal conductivity problem and the elastic-plastic problem, taking into account the temperature dependences of the thermal and mechanical properties, the growth mode and the sequence of layer deposition. To save the estimated time, the material added in every layer at once. As practice shows, the results of this solution coincide well with the results of calculating the movement of the heat source, while the number of calculation steps is significantly reduced [10]. On Fig. 6 and Fig. 7 shown results of modeling the stress-strain state of part. The maximum deviation of the outer shell geometry exceeds 10 mm, taking into account the use of stringers (Fig. 6a). The result showed a deviation several times greater than the wall thickness of the product, hence it follows that the product does not have sufficient rigidity for the application of reverse deformation. The necessary rigidity of the part achieved by increasing the wall thickness of the stringers to 4 mm, and the stringers also grouped in the area of the greatest deformations. In the current configuration, the process modeling showed (Fig. 6b) deviations not exceeding 2.8 mm from stringer surface, and not more than 2 mm from the product surface, which is a satisfactory result.
Adding stringers to the inner shell was not required since its own rigidity was sufficient to avoid warping due to the greater wall thickness. According to the simulation results, the maximum deviations in the geometry of the inner casing, taking into account the compensation of distributed deformations, do not exceed 1 mm (Fig. 7). Based on the obtained data, the geometry of the outer shell and inner shell 3d-models changed, after which control programs for deposition created.
Heat treatment modes for 12Ch18Ni10Ti and VZh159 steel were developed. The heat treatment of the outer casing required special attention. The treatment made without separation from the metal substrate. Otherwise, a redistribution of stress would occur, which would lead to unpredictable deformations of the part. A special frame made from 12Ch18Ni10Ti stainless steel to keep the ring shape undeformed after heat treatment (Fig. 8a). The installation principle shown in the figure (Fig. 8b).
Experiment
The next stage was direct laser deposition of part in the DLD complex ILIST-L.
The frame is the only product among those presented whose deformation nature was determined experimentally. Modeling of the frame DLD process potentially required significantly more time than the deposition process itself. The frame manufactured and scanned. According to the geometry control data, the product model was reversely deformed, and a new control program created, after the entire frame was re-deposited.
The remaining parts manufactured and subjected to geometry control. Then each of the products subjected to heat treatment in accordance with the developed modes. After the heat treatment, the part separated from the substrates and each of its were re-scanned.
Results and discussions
Figures 9 and 10 show the results of geometry control after heat treatment maintenance of the frame and ring, respectively, the deviation scale limits are ±2 mm. Both products demonstrated a generally satisfactory result. Maximum deviations of the frame reach 1.8 mm, however, they are localized in the areas of allowance for mechanical processing, deviations of principal dimensions do not exceed 1 mm. Deviations of ring geometry do not exceed 0.6 mm, which is also a satisfactory result.
The geometry control of the gas collector inner shell after production and heat treatment shown in Fig. 11. The limits of the deviation scale are ±2 mm. After separation from the substrate, the maximum deviations do not exceed 1.2 mm, in the places of assembly with other parts, the deviations do not exceed 0.5 mm.
After production, the gas collector outer shell subjected to heat treatment, next stringers cut off and the substrate removed. Upon completion of each technological stage, from the end DLD to substrate separation, the part scanned. The control results are presented in Figure 12, the deviation scale limits are ± 5 mm. In the segment of the heat treatment frame installation, the deviations do not exceed 1.2 mm, which meets the tolerance requirements of ±2 mm, an undesirable deviation of 2.8 mm observed at the bottom of the part, but this area is in the compensation layer and separated from the product. After removing the stringers and separating from the substrate, the geometry of the shell has undergone changes in comparison with the control results after heat treatment, which means incomplete relaxation of residual stresses. The maximum deviations of the shell geometry after separation from the substrate were 3 mm, which exceeds the form tolerance requirement of ±2 mm.
It should be noted that modeling the stress-strain state takes into account deformations of products only during the deposition process, therefore the values of deviations of the final product geometry and the calculated data is partially different. However, the deviations pattern of the calculated geometry relative to the final data and does not exceed ±2 mm. It indicates the viability of the calculation model, and positively characterizes the use of computer modeling for predicting and compensating for deformations of products during direct laser deposition.
Conclusions
Depending on the rigidity of the product structure, the necessary and sufficient volume of preliminary preparation and compensation method changes. The ring, which is an example of a product with high constructional rigidity, requires only compensation for thermal shrinkage. The outer shell is an easily deformable thin-walled part, requiring complex preparation using computer modeling.
Using the computer modeling of the stress-strain state of a part is advisable for easily deformable products, such as thin-walled shells which deviation pattern and deformation degree is difficult to predict. In combination with deformation compensation methods, computer modeling allows reducing the number of iterations of the direct laser deposition process for easily deformable products.
The technological process must necessarily provide for heat treatment of the part. However, it is important to select the parameters for maintenance correctly; otherwise, the relaxation of residual stresses will not occur fully, which will cause additional deformations after separation of the product from the substrate.
Funding
The reported study was funded by RFBR, according to the research project No 20-38-90206.
AUTHORS
Kovchik A. Yu., Saint Petersburg State Marine Technical University Institute of Laser and Welding Technologies (SMTU ILWT), St. Petersburg, Russia.
ORCID 0000-0001-5494-2405
Vildanov A. M., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0002-7319-0605
Alymov N. R., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0003-1066-1446
Ivanov S.Yu., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0002-0077-2313
Mendagaliyev R. V., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0003-4358-1995
CONFLICT OF INTEREST
The authors state that they have no conflict of interest. All the authors took part in writing the article and supplemented the manuscript in part of their work.
CONTRIBUTION OF THE MEMBERS OF THE AUTHOR’S TEAM
The article is based on the work of all members of the author’s team.
A. Yu. Kovchik, A. M. Vildanov, N. R. Alymov, S. Yu. Ivanov, R. V. Mendagaliyev
Saint Petersburg State Marine Technical University Institute of Laser and Welding Technologies (SMTU ILWT), St. Petersburg, Russia
Direct laser deposition is widely used in high-tech industries due to the possibility of creating complex parts, which manufacturing is impossible using traditional production methods. However, the production process is complicating by the formation of residual stresses and deformations in the part, which negatively affect its quality. The field of distribution of stresses and deformations in the part is associated with its geometry. This article presents the main compensation methods of part distortion and describes the types of typical part deformations for DLD. The deformation compensation results are presented on example of four different parts made of stainless steel grade 12Ch18Ni10Ti (analogue AISI 304) and heat-resistant alloy VZh159.
Key words: Additive technologies, Direct laser deposition (DLD), Residual stresses and deformations, Turbines, Deformations compensation.
Article received: 18.06.2024
Article accepted: 11.07.2024
Introduction
Direct laser deposition is an additive technology method based on layer-by-layer deposition of metal powder onto a substrate using of highly concentrated laser radiation. Each deposited layer becomes the basis for the next one, thus the material using is aiming directly at forming the part. The technology allows utilize wide range of materials, such as titanium, aluminum, heat-resistant nickel alloys, and stainless steels [1].
This production method has changed the approach to product design, it is able to produce parts with strict design geometry in a shorter time in opposite of traditional production technologies. Thereby, direct laser deposition is increasingly using in such knowledge-intensive industries as, for example: aircraft and rocket engine industries, thermal and nuclear power engineering [2].
These industries require the production of high-quality products corresponding all the requirements, with minimal costs. Direct laser deposition of any product is accompanying by the formation of residual stresses and plastic deformations directly during the deposition process. Thus, the DLD production cycle is a sequence of stages shown in Fig. 1.
Plastic deformations forms in the part during thermal expansion of the material in consequence of local influence of highly concentrated laser radiation [3]. Residual stresses are forming after plastic deformations during crystallization and cooling of the material. They reduce the strength characteristics of the material and staying after completion of deposition, therefore the part is subjecting to heat treatment. Residual stresses are the main forces unevenly distributed in the part, they are accompanying by the elastic deformations, and when they relax, a part geometry is change [4]. Plastic deformation can be eliminating by changing the trajectory of the deposition so as to the product takes the desired shape during deformation. The difficulty is that to obtain reliable data of the part deformations, it is necessary to make a tested part deposition. In addition, after changing the deposing trajectory, the distribution of deformations may also change, which makes the process iterative.
The deposition trajectory is determining by the part geometry, so the distribution of residual stresses and deformations becomes unique. However, several parts, which have similar shape and size, may have similar deformation patterns, which means that the solution for deformations compensating may also be common. Have determined a connection between the part geometry and the deformations patterns, it will be possible to solve the problem of iterations numbers in DLD process and increase its efficiency.
Deformations types and their compensation methods.
Before describing the deformations compensation methods, it is necessary to classify the deformations types. In point of the welding deformations and stresses theory [5], deformations are dividing on two groups: “General”, which cause distortions of the shape and size of the entire element; and “local”, which spread to individual elements of the part design. Such effects like thermal shrinkage can be attributing to general deformations. Meaning of thermal shrinkage is the volume changing of the deposited material during its cooling is proportional to the temperature changing, as a result the linear dimensions of the part decrease. Local deformations includes several types: saddle-shaped, distributed deformations, warping.
The degree of part thermal shrinkage is determining by material characteristics. Compensation for this effect achieved by scaling on shrinkage coefficient, which can be determinate by the ratio of the linear dimensions depositing part to the dimensions of etalon part. The determining method of the shrinkage coefficient presented in the source [6]. Shrinkage compensation uses to every part. However, for axis-symmetrical part for example: rings, cylinders, etc., the scaling is only necessary compensation method.
The substrate is the base where each part begins its deposition, so some characteristic features should be noted. Сconsider two options for fixing the substrate: the first is when the substrate is rigidly fixed to the table (Fig. 2a), the position is limited on all sides, the second one is when the substrate is fixed in the center (Fig. 2b), the edges of the substrate are free. The first method should be used if the substrate consider some elements of part geometry, or the depositing part is massive, i. e. the area of the layer deposition is large. Full fixation is accompanying a high stress concentration in the part material. Therefore this fixation method using for plastic metals deposition with a high relative elongation index. However is also necessary to take into account the part geometry. For low-plasticity materials or products with low construction rigidity, such as thin-walled shells, this can cause destruction (Fig. 2a). Using by the second fixing method, the formation of a saddle-shaped deformation of the substrate compensate residual stresses partially. Unfixed edges rise in the direction of deposition, which allows maintaining part integrity. However, the main program does not take into account the substrate changing, the surfacing continues along the specified trajectory. The free ends of the substrate rise faster than the dispositioning occurs, so a fragment of part geometry is lost (Fig. 2b). Therefore, it use increased compensation layer in deformable substrate case, leveling out the influence of substrate deformations (Fig. 2c).
Compensation layer – a segment preceding the growth of the main product, which adds in each part to simplify the process of cutting from the substrate.
Distributed deformations are large areas with a low gradient of deviations from the required geometry of the product [7]. This type of deformation encounters in parts, with the exception of massive design with high rigidity. Compensation carry out by preliminary changing the part geometry in the direction opposite to the part deformations (Fig. 3). When deposition process repeats the part takes desire shape. This type of deformation becomes the cause of additional iterations. The problem is especially relevant in the production of large-sized thin-walled shells. For these parts minor changes in geometry lead to significant changes in the deformations patterns due to the low construction rigidity Therefore, compensation of deformations for these products accompanied by computer modeling of the stress-strain state.
Warping or loss of stability are wave-like deformations of the parts wall with a high gradient of deviations from the desire geometry. This defect forms as result of insufficient part construction rigidity it is typical for thin-walled shells [8].
The compensation method applicable to distributed deformations is not effective in this case. The solution to this problem is to increase the rigidity of the shell. This achieved in several ways: by increasing the thickness of the shell wall or by adding stiffening elements (stringers) to the part surface. Stringers transform warping into distributed deformation, which compensation was describe earlier. Stringers are technological elements, after production they are removing from the part surface.
The use of compensation methods allows solving the problem of deformations, however, it does not lead to relaxation of residual stresses, therefore, after completion of the technological process, and a part subjected to heat treatment.
An application of deformation compensation methods shown on example of parts included in the gas collector of the combustion chamber GTE‑65.1 gas turbine unit. The complete production process of the set of products is presenting in the source [9]. The gas collector includes four elements with different types of geometry, which made of different materials (Fig. 4). The inner shell of the gas collector, the frame, and the ring are made of heat-resistant nickel-based alloy VZh159, the outer shell of the gas collector is made of stainless steel grade 12Ch18Ni10Ti (analogue AISI 304L). The inner and outer shells (1, 4) are thin-walled large-sized parts and deformed easily due to the low rigidity of the construction. It is difficult to predict the pattern and degree of deformation without using of computational methods for modeling the DLD process or trial production. The frame (2) is a thick-walled product with mirror symmetry, which unexpected large-scale deformations. The ring (3) is a rigid part consisting of two axisymmetric shells connected by truss elements, in addition, it has reinforcements on the outer wall, and the change in the geometry of the ring is expecting mainly due to thermal shrinkage.
Equipment and materials
The production of gas collector elements carried out using the ILIST-L direct laser deposition complex manufactured by SMTU (Fig. 5).
The technical characteristics of the complex ILIST-L presents in Table 2. The trajectories and control programs for robotic laser processing of gas collector element created in the Autodesk PowerMill software package. The geometry of the workpieces controlled using a measuring pair included an optical 3D scanner Metroscan Elite 750 and c-track Black Elite. The reverse deformation of the part 3D-models carried out using the Matlab application package for technical calculations based on the geometry deviation field data obtained in the Geomagic Control X program.
Technical preparing
The first preparatory stage is the transformation of product models into blank models. Compensation layers added to the part models, scale coefficients applied, allowances for mechanical processing added to the frame and ring. Stringers with a wall thickness of 2 mm added on surface of outer shell in order to increase the part rigidity, while the geometry of the shell did not change at the first stage.
To deformations compensation of the outer shell and the inner shell of the gas collector, it was necessary to simulate the stress-strain state using the finite element method. The modeling process consisted of solving thermal conductivity problem and the elastic-plastic problem, taking into account the temperature dependences of the thermal and mechanical properties, the growth mode and the sequence of layer deposition. To save the estimated time, the material added in every layer at once. As practice shows, the results of this solution coincide well with the results of calculating the movement of the heat source, while the number of calculation steps is significantly reduced [10]. On Fig. 6 and Fig. 7 shown results of modeling the stress-strain state of part. The maximum deviation of the outer shell geometry exceeds 10 mm, taking into account the use of stringers (Fig. 6a). The result showed a deviation several times greater than the wall thickness of the product, hence it follows that the product does not have sufficient rigidity for the application of reverse deformation. The necessary rigidity of the part achieved by increasing the wall thickness of the stringers to 4 mm, and the stringers also grouped in the area of the greatest deformations. In the current configuration, the process modeling showed (Fig. 6b) deviations not exceeding 2.8 mm from stringer surface, and not more than 2 mm from the product surface, which is a satisfactory result.
Adding stringers to the inner shell was not required since its own rigidity was sufficient to avoid warping due to the greater wall thickness. According to the simulation results, the maximum deviations in the geometry of the inner casing, taking into account the compensation of distributed deformations, do not exceed 1 mm (Fig. 7). Based on the obtained data, the geometry of the outer shell and inner shell 3d-models changed, after which control programs for deposition created.
Heat treatment modes for 12Ch18Ni10Ti and VZh159 steel were developed. The heat treatment of the outer casing required special attention. The treatment made without separation from the metal substrate. Otherwise, a redistribution of stress would occur, which would lead to unpredictable deformations of the part. A special frame made from 12Ch18Ni10Ti stainless steel to keep the ring shape undeformed after heat treatment (Fig. 8a). The installation principle shown in the figure (Fig. 8b).
Experiment
The next stage was direct laser deposition of part in the DLD complex ILIST-L.
The frame is the only product among those presented whose deformation nature was determined experimentally. Modeling of the frame DLD process potentially required significantly more time than the deposition process itself. The frame manufactured and scanned. According to the geometry control data, the product model was reversely deformed, and a new control program created, after the entire frame was re-deposited.
The remaining parts manufactured and subjected to geometry control. Then each of the products subjected to heat treatment in accordance with the developed modes. After the heat treatment, the part separated from the substrates and each of its were re-scanned.
Results and discussions
Figures 9 and 10 show the results of geometry control after heat treatment maintenance of the frame and ring, respectively, the deviation scale limits are ±2 mm. Both products demonstrated a generally satisfactory result. Maximum deviations of the frame reach 1.8 mm, however, they are localized in the areas of allowance for mechanical processing, deviations of principal dimensions do not exceed 1 mm. Deviations of ring geometry do not exceed 0.6 mm, which is also a satisfactory result.
The geometry control of the gas collector inner shell after production and heat treatment shown in Fig. 11. The limits of the deviation scale are ±2 mm. After separation from the substrate, the maximum deviations do not exceed 1.2 mm, in the places of assembly with other parts, the deviations do not exceed 0.5 mm.
After production, the gas collector outer shell subjected to heat treatment, next stringers cut off and the substrate removed. Upon completion of each technological stage, from the end DLD to substrate separation, the part scanned. The control results are presented in Figure 12, the deviation scale limits are ± 5 mm. In the segment of the heat treatment frame installation, the deviations do not exceed 1.2 mm, which meets the tolerance requirements of ±2 mm, an undesirable deviation of 2.8 mm observed at the bottom of the part, but this area is in the compensation layer and separated from the product. After removing the stringers and separating from the substrate, the geometry of the shell has undergone changes in comparison with the control results after heat treatment, which means incomplete relaxation of residual stresses. The maximum deviations of the shell geometry after separation from the substrate were 3 mm, which exceeds the form tolerance requirement of ±2 mm.
It should be noted that modeling the stress-strain state takes into account deformations of products only during the deposition process, therefore the values of deviations of the final product geometry and the calculated data is partially different. However, the deviations pattern of the calculated geometry relative to the final data and does not exceed ±2 mm. It indicates the viability of the calculation model, and positively characterizes the use of computer modeling for predicting and compensating for deformations of products during direct laser deposition.
Conclusions
Depending on the rigidity of the product structure, the necessary and sufficient volume of preliminary preparation and compensation method changes. The ring, which is an example of a product with high constructional rigidity, requires only compensation for thermal shrinkage. The outer shell is an easily deformable thin-walled part, requiring complex preparation using computer modeling.
Using the computer modeling of the stress-strain state of a part is advisable for easily deformable products, such as thin-walled shells which deviation pattern and deformation degree is difficult to predict. In combination with deformation compensation methods, computer modeling allows reducing the number of iterations of the direct laser deposition process for easily deformable products.
The technological process must necessarily provide for heat treatment of the part. However, it is important to select the parameters for maintenance correctly; otherwise, the relaxation of residual stresses will not occur fully, which will cause additional deformations after separation of the product from the substrate.
Funding
The reported study was funded by RFBR, according to the research project No 20-38-90206.
AUTHORS
Kovchik A. Yu., Saint Petersburg State Marine Technical University Institute of Laser and Welding Technologies (SMTU ILWT), St. Petersburg, Russia.
ORCID 0000-0001-5494-2405
Vildanov A. M., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0002-7319-0605
Alymov N. R., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0003-1066-1446
Ivanov S.Yu., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0002-0077-2313
Mendagaliyev R. V., SMTU ILWT, St. Petersburg, Russia.
ORCID 0000-0003-4358-1995
CONFLICT OF INTEREST
The authors state that they have no conflict of interest. All the authors took part in writing the article and supplemented the manuscript in part of their work.
CONTRIBUTION OF THE MEMBERS OF THE AUTHOR’S TEAM
The article is based on the work of all members of the author’s team.
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