rus
Issue #2/2017
M.A.Melnikova, D.M.Melnikov, D.S.Kolchanov
Selective Laser Melting: Application and Formation Features of Three-Dimensional Structural Engineering Elements
Selective Laser Melting: Application and Formation Features of Three-Dimensional Structural Engineering Elements
A number of exploratory works in the field of selective laser melting technology including features of supporting piece growing that necessary for successful create solid continuous products are presented in the article.
The group of the additive technologies or rapid prototyping techniques is one of the most intensively developing branches of the modern mechanical engineering. Methods of selective laser sintering and melting are included among other in this group. The selective laser melting is among the most important fields as it allows receiving practically finished products by sintering of metal powders. The feature of this technology usage is bound, as a rule, to manufacture of composite geometry products, or products from materials badly machinable.
The selective laser melting is one of methods allowing growing details from various metal powders, layer by layer forming a unique configuration of a product by local impact of a laser radiation on powder with remelting of each layer. For each type of powder differing by the size and form of fraction, chemical composition there are particular features at cultivation, however the common principle is identical for all types. The main feature of selective laser melting is a possibility of irregular shapes parts creation, including those with internal channels and cavities [1] having the given mechanical and physical properties. This technology gradually penetrates into space and automobile branches, instrumental production of composite parts and other fields where it will shortly become widely used [2]. Selective laser melting is already widely used in the jewellery industry and medicine (biomedical dentures, cutter heads and other –Fig.1, [3, 4]).
Various problems arising when developing selective laser melting technology demand deep study at the following different levels: theoretical, i. e. creation of detailed process model, and practical, i. e. conducting the experimental research at the choice of optimum parameters for cultivation of products from particular powders. The complexity of the experimental research consists in multifactorial experiment statement for comprehension of formation process both of separate fusion zones, and a product in general, and the subsequent exception of such negative effects as spheroidizing of melted grains of powder, porosity, thermal deformation, cracks, etc. [5].
One of the most complex but important stages at parts growing is a creation of supporting piece which holds the acting and hanging elements of a part at manufacture. The feature of these elements is defined by fine periodic structure and tiny elements existence at layers remelting. It should be noted that today hollow cellular products are produced rather easily by selective laser melting method, but parts growing having 100% density is still complicated. One of components of this problem is the supporting pieces optimum structure receiving which would guarantee a possibility of growing the dense product and its subsequent separation from supporting piece.
At MT‑12 Department of Bauman Moscow State Technical University laser additive machines and technologies, including those actively developed for selective laser melting. Recently developed complex (Fig. 2) allows conducting the efficient researches of various aspects of this technology. The complex meets requirements imposed by the modern industry and is not inferior to Western counterparts.
The key diagram of selective laser melting process is shown in Fig. 3. The pressure-tight chamber filled with noble gas with a possibility of atmosphere heating includes a growing platform which has vertical movement system, and also two hoppers for powder. The powder moves from the hopper feeder by vertical movement of platform on surface. By means of a knife or a roller the necessary volume of powder is transferred to a platform for growing and is settled on it with even layer of the given thickness. Excess powder is poured to the second hopper. Further there is an impact of a laser radiation on powder, its alloy age at the first stage with a substrate, and further with the previous grown layer. As soon as the layer is grown, the growing platform with a detail goes down, and the process of new layer creation is repeated according to the same scheme.
At creation of experimental studies model and the choice of optimum conditions of carrying out selective laser melting it is very important to consider developing the process repeatability with the given accuracy [8]. All varied parameters of selective laser melting process can be divided into three basic groups [6, 7]:
• depending on powder characteristics,
• depending on laser radiation characteristics,
• depending on scannings strategy.
It is necessary to emphasize repeatability ensuring at products growing with tiny geometry. It is possible to provide repeatability with the right choice of modes and precise creation of supporting piece that guarantees lack of next layer distortions which can be collected and be resulted in serious defects at a large number of layers.
Researches of optimum supporting piece growing on stainless steel were conducted. Owing to high degree of alloyability and to a wide spreading in the industry the powder of stainless steel 316L is chosen for a research. Its analogues are stainless steel of 1.4429 and 03H17N14M3 grades. The powder composition is presented in the table (globular homogeneous powder), its morphology is shown Fig. 4. The chemical composition of powder after X-ray inspection is presented in Fig. 5. Proceeding from the obtained data (after monitoring) and the declared composition of powder have slight distinctions. For growing the square, specifically prepared substrate from steel 316L was chosen.
Researches were conducted by means of the SLM 110 complex, and also specifically designed laboratory equipment [9]; the complex has the following content: system of substrate movement with a possibility of layers drawing 60 microns thick; a knife for layers drawing; the fiber laser with the maximal power of 100 W; a scanner with the maximum speed of beam movement of 10000 mm/sec with spot diameter in focus of about 60 microns and processing area of 100Ч100 mm; the chamber with noble gas (argon) supply. A number of experiments on development of the simple alloyed paths and selfcontained figures formation were conducted on this inventory (Fig.6).
Results of growing the forms of a simple layer having 100% density are shown in Fig. 7. The parameters received according to the experimental study allow avoiding almost complete porosity when using of the installation developed by us. However geometrical characteristics are uneven and comparable with those which are received by means of weld deposit. It is possible to lower essential warpings at selective laser melting process, having reduced a heat quantity in alloyage zone. For this purpose it is necessary to refuse a continuous substrate and to use reticulate three-dimensional structures, i. e. supporting piece.
The example of simple three-dimensional parts growing on the example of thin walls is shown in Fig. 8. We managed to receive the stable walls sizes that are important for formation of supporting pieces smooth geometry. The thickness of walls is 200 microns, and the height is 5 mm with a dimensional defect of no more than 20 microns though there were some edge defect (the reason of which was an imperfection of movement system connection and a control system of laser radiation supply).
For identification of fine defects the analysis of sample microstructure (Fig. 9) was carried out. It showed that the structure of material is homogeneous and compact-grained that corresponds to a steel grade. The fine grained structure is defined by cooling high speed reached at carrying out the process of selective laser melting in the optimum modes. According to Hall–Petch [10], it leads to high hardness of samples which is about 250 BH (hardness of 316L sheet steel samples reaches 170 BH. A large pores is absent in the samples received by us.
The received results were used for optimization of supporting piece growing modes with three-dimensional structure. The form of supporting piece should satisfy to a number of conditions among which there is durability necessary for reliable deduction of grown product elements; the thickness of a simple segment defining ease of supporting piece separation; a step between the bearing elements of supporting pieces. We grew up a series of supporting pieces (Fig. 10) with the structure, having body-centered cubic lattice form [11].
Such form is convenient for actual products growing as it is capable to satisfy to the above described signs. We received sufficient repeatability of results without serious defects on 15 pieces series.
CONCLUSIONS
Application of selective laser melting technology which relates to high developing additive technologies has larger perspectives not only from the point of view of hollow structures manufacture the main application of which is in medical area, but also from the point of view of products manufacture having 100% density in various branches. The thin-walled structures filled with metal gauze with particular structure allow receiving light and rather firm structures, but the main machine-building areas, such as parts of aircraft equipment demand completely continuous geometry that meets a number of difficulties at use of selective laser melting technology. Nevertheless, the exact approach to creation of the bearing supporting piece is one of success components of this technique. The verified geometry and space errors lack at growing the supporting pieces props are necessary for high-quality creation of a final part.
At MT‑12 Department of Bauman Moscow State Technical University laser additive machines and technologies including selective laser melting are actively developed. Recently the modern complex which is meeting all requirements of high-quality selective laser melting process performing and not conceding to foreign analogues was created. This complex was used for selective laser melting modes development including supporting pieces growing. We received the structures of steel powder with a good accuracy and repeatability. At geometry growing of 100% density also it was succeeded to avoid serious defects.
The use of the basis of fine reticulate structure at growing of products by selective laser melting method helps to lower significantly a quantity of heat in alloyage area that positively affects the total quality of the received products. Further carrying out further works on definition of the factors allowing operating distribution of heat at selective laser melting, in particular a research of transfer of radiation and heat in alloyed particles group that should allow to fulfil the qualitative technology of receiving completely continued products by selective laser melting method is planned.
The selective laser melting is one of methods allowing growing details from various metal powders, layer by layer forming a unique configuration of a product by local impact of a laser radiation on powder with remelting of each layer. For each type of powder differing by the size and form of fraction, chemical composition there are particular features at cultivation, however the common principle is identical for all types. The main feature of selective laser melting is a possibility of irregular shapes parts creation, including those with internal channels and cavities [1] having the given mechanical and physical properties. This technology gradually penetrates into space and automobile branches, instrumental production of composite parts and other fields where it will shortly become widely used [2]. Selective laser melting is already widely used in the jewellery industry and medicine (biomedical dentures, cutter heads and other –Fig.1, [3, 4]).
Various problems arising when developing selective laser melting technology demand deep study at the following different levels: theoretical, i. e. creation of detailed process model, and practical, i. e. conducting the experimental research at the choice of optimum parameters for cultivation of products from particular powders. The complexity of the experimental research consists in multifactorial experiment statement for comprehension of formation process both of separate fusion zones, and a product in general, and the subsequent exception of such negative effects as spheroidizing of melted grains of powder, porosity, thermal deformation, cracks, etc. [5].
One of the most complex but important stages at parts growing is a creation of supporting piece which holds the acting and hanging elements of a part at manufacture. The feature of these elements is defined by fine periodic structure and tiny elements existence at layers remelting. It should be noted that today hollow cellular products are produced rather easily by selective laser melting method, but parts growing having 100% density is still complicated. One of components of this problem is the supporting pieces optimum structure receiving which would guarantee a possibility of growing the dense product and its subsequent separation from supporting piece.
At MT‑12 Department of Bauman Moscow State Technical University laser additive machines and technologies, including those actively developed for selective laser melting. Recently developed complex (Fig. 2) allows conducting the efficient researches of various aspects of this technology. The complex meets requirements imposed by the modern industry and is not inferior to Western counterparts.
The key diagram of selective laser melting process is shown in Fig. 3. The pressure-tight chamber filled with noble gas with a possibility of atmosphere heating includes a growing platform which has vertical movement system, and also two hoppers for powder. The powder moves from the hopper feeder by vertical movement of platform on surface. By means of a knife or a roller the necessary volume of powder is transferred to a platform for growing and is settled on it with even layer of the given thickness. Excess powder is poured to the second hopper. Further there is an impact of a laser radiation on powder, its alloy age at the first stage with a substrate, and further with the previous grown layer. As soon as the layer is grown, the growing platform with a detail goes down, and the process of new layer creation is repeated according to the same scheme.
At creation of experimental studies model and the choice of optimum conditions of carrying out selective laser melting it is very important to consider developing the process repeatability with the given accuracy [8]. All varied parameters of selective laser melting process can be divided into three basic groups [6, 7]:
• depending on powder characteristics,
• depending on laser radiation characteristics,
• depending on scannings strategy.
It is necessary to emphasize repeatability ensuring at products growing with tiny geometry. It is possible to provide repeatability with the right choice of modes and precise creation of supporting piece that guarantees lack of next layer distortions which can be collected and be resulted in serious defects at a large number of layers.
Researches of optimum supporting piece growing on stainless steel were conducted. Owing to high degree of alloyability and to a wide spreading in the industry the powder of stainless steel 316L is chosen for a research. Its analogues are stainless steel of 1.4429 and 03H17N14M3 grades. The powder composition is presented in the table (globular homogeneous powder), its morphology is shown Fig. 4. The chemical composition of powder after X-ray inspection is presented in Fig. 5. Proceeding from the obtained data (after monitoring) and the declared composition of powder have slight distinctions. For growing the square, specifically prepared substrate from steel 316L was chosen.
Researches were conducted by means of the SLM 110 complex, and also specifically designed laboratory equipment [9]; the complex has the following content: system of substrate movement with a possibility of layers drawing 60 microns thick; a knife for layers drawing; the fiber laser with the maximal power of 100 W; a scanner with the maximum speed of beam movement of 10000 mm/sec with spot diameter in focus of about 60 microns and processing area of 100Ч100 mm; the chamber with noble gas (argon) supply. A number of experiments on development of the simple alloyed paths and selfcontained figures formation were conducted on this inventory (Fig.6).
Results of growing the forms of a simple layer having 100% density are shown in Fig. 7. The parameters received according to the experimental study allow avoiding almost complete porosity when using of the installation developed by us. However geometrical characteristics are uneven and comparable with those which are received by means of weld deposit. It is possible to lower essential warpings at selective laser melting process, having reduced a heat quantity in alloyage zone. For this purpose it is necessary to refuse a continuous substrate and to use reticulate three-dimensional structures, i. e. supporting piece.
The example of simple three-dimensional parts growing on the example of thin walls is shown in Fig. 8. We managed to receive the stable walls sizes that are important for formation of supporting pieces smooth geometry. The thickness of walls is 200 microns, and the height is 5 mm with a dimensional defect of no more than 20 microns though there were some edge defect (the reason of which was an imperfection of movement system connection and a control system of laser radiation supply).
For identification of fine defects the analysis of sample microstructure (Fig. 9) was carried out. It showed that the structure of material is homogeneous and compact-grained that corresponds to a steel grade. The fine grained structure is defined by cooling high speed reached at carrying out the process of selective laser melting in the optimum modes. According to Hall–Petch [10], it leads to high hardness of samples which is about 250 BH (hardness of 316L sheet steel samples reaches 170 BH. A large pores is absent in the samples received by us.
The received results were used for optimization of supporting piece growing modes with three-dimensional structure. The form of supporting piece should satisfy to a number of conditions among which there is durability necessary for reliable deduction of grown product elements; the thickness of a simple segment defining ease of supporting piece separation; a step between the bearing elements of supporting pieces. We grew up a series of supporting pieces (Fig. 10) with the structure, having body-centered cubic lattice form [11].
Such form is convenient for actual products growing as it is capable to satisfy to the above described signs. We received sufficient repeatability of results without serious defects on 15 pieces series.
CONCLUSIONS
Application of selective laser melting technology which relates to high developing additive technologies has larger perspectives not only from the point of view of hollow structures manufacture the main application of which is in medical area, but also from the point of view of products manufacture having 100% density in various branches. The thin-walled structures filled with metal gauze with particular structure allow receiving light and rather firm structures, but the main machine-building areas, such as parts of aircraft equipment demand completely continuous geometry that meets a number of difficulties at use of selective laser melting technology. Nevertheless, the exact approach to creation of the bearing supporting piece is one of success components of this technique. The verified geometry and space errors lack at growing the supporting pieces props are necessary for high-quality creation of a final part.
At MT‑12 Department of Bauman Moscow State Technical University laser additive machines and technologies including selective laser melting are actively developed. Recently the modern complex which is meeting all requirements of high-quality selective laser melting process performing and not conceding to foreign analogues was created. This complex was used for selective laser melting modes development including supporting pieces growing. We received the structures of steel powder with a good accuracy and repeatability. At geometry growing of 100% density also it was succeeded to avoid serious defects.
The use of the basis of fine reticulate structure at growing of products by selective laser melting method helps to lower significantly a quantity of heat in alloyage area that positively affects the total quality of the received products. Further carrying out further works on definition of the factors allowing operating distribution of heat at selective laser melting, in particular a research of transfer of radiation and heat in alloyed particles group that should allow to fulfil the qualitative technology of receiving completely continued products by selective laser melting method is planned.
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