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Determination of the possibility of manufacturing mesh structures using additive technologies

https://doi.org/10.38013/2542-0542-2020-2-74-82

Abstract

In comparison with conventional technologies, additive technologies are based on fundamentally different physical phenomena. One research direction is the determination of the marginal possibilities of complicating the shape of manufactured products. This article discusses the possibility of introducing mesh structures into the volume of a manufactured part, the possibility of manufacturing such structures without additional supporting elements and boundary conditions, i.e. the minimum thickness of the mesh and the largest length of the hanging surface. Another objective was to develop and test a methodology for designing and creating G-codes when developing mesh structures.

For citation:


Gorbatov I.V., Pilshchikov A.A., Orlov Yu.A., Antyufeev V.A., Antyufeeva S.A., Orlova N.Yu., Karpov D.Yu. Determination of the possibility of manufacturing mesh structures using additive technologies. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(2):74-82. https://doi.org/10.38013/2542-0542-2020-2-74-82

Development of a new product is a complicated and complex task, its successful and optimal solution depending on the cooperation between the designers and manufacturing engineers, as well as on the design methods and manufacturing facilities applied by them. The capabilities of engineering methods applied for products manufacture may have a vital and in certain cases even a paramount importance for both the design of product as a whole and the geometry of its components.

Today, one of the most important problems consists in decreasing the weight of products without sacrificing the functionality, and in certain cases, in the course of modernisation of existing mechanisms the task gets even more complicated and consists in decreasing the weight of the product without sacrificing the volume and performance. The above can be addressed as follows:

  • either by replacement with a material having lower density, but in this case, there is a possibility of mismatch between the material performance properties, including the range of operating temperatures,
  • or by applying an innovative approach to the design, i. e. introducing mesh structures into the product (Fig. 1), which will allow to significantly decrease the product weight. Based on the design experience, introduction of such structures allows to decrease the product weight by 20-40 % depending on the mesh structure parameters.

Fig. 1. Example of mesh structures introduction into the part design

It is impossible to manufacture such structures using standard technologies; therefore, it is necessary to draw the focus toward additive technologies and consider the capabilities of manufacture and constraints.

The analysis of capabilities and constraints related to these manufacturing technologies will allow to determine the acceptable degrees of freedom when designing the product as a whole.

The essence of additive technologies constitutes in the layerwise building-up of objects of the base material (powder, thread, liquid resin) based on a CAD model.

By their physics, additive technologies are fundamentally different from the traditional ones and, consequently, their capabilities are different as well.

Manufacturing of objects with the use of additive technologies also entails certain constraints, which may vary depending on the technology used (SLA, SLS, SLM, FDM, etc.), but their basic principle is the same - manufacturing/building-up of a single layer. Therefore, when ascertaining the part design producibility (feasibility) for manufacturing with the use of additive technologies, two constraints shall be taken into account:

  • capability of single layer manufacturing;
  • constraints related to the differences between the subsequent layer and the previous one.

A single layer may be viewed as a 2D model, the formulation of which determines the movement trajectory of a tool (laser beam, printing head, etc.). When building-up a single layer, the following factors shall be taken into account:

 the tool (laser, nozzle) movement trajectory cannot allow repeated passage through a single point on a plane - otherwise layer thickness in this point will be different from the overall layer thickness;

  • high differential temperature through the layer cannot be allowed, since significant temperature difference may cause high internal stresses in the layer, hence its distortion;
  • cooling of the layer or its part down to certain temperatures and lower is not allowable - otherwise there won’t be adhesion between the layers, i. e. the product will flake off.

Standard programs for G-code generation during designing the process of complex 3D objects manufacture, for example, with introduction of mesh structures, report an error: impossibility to create the tool (laser, nozzle) movement trajectory at the stage of control program design. Several options were considered to eliminate this error:

  • completely manual development of the entire control program, but this is associated with high labour intensity of the process and the risk of subjective errors;
  • usage of standard programs for G-code generation with subsequent correction of errors in the manual mode, but due to a large number of trajectory crossings this method of program correction leads to a significant increase in labour intensity of control program preparation process.

The most effective option for error elimination is a revolutionary approach based on the technology of building-up/modelling of a solid 3D-model used for G-code generation. When building-up a solid 3D-model, crossing of volumes, planes is not allowable. For example, when building-up a mesh structure, it shall be modelled in the form of openings (pockets) of certain shape rather than in the form of elements crossover; at that, the thickness of elements (materials) cannot be less that two single passes of a laser (SLM technology) / nozzle (FDM technology) (Fig. 2).

Fig. 2. Examples of building up 3D-models of mesh structures for manufacturing with the use of additive technologies: а - correct, b - incorrect

 

This factor may be omitted for parts manufacturing using SLA technology (stereolithog-raphy), since in this technology the entire layer is manufactured simultaneously.

The second influencing factor - significant temperature difference through the layer - may be eliminated in two ways.

First way. Additional heating of product building area. If a product has sufficiently large dimensions of a single layer, this approach will not prove effective enough.

Second way. The trajectory of a single layer building is formed in a way ensuring maximum heat distribution throughout the entire single layer field. For example, the trajectory will first pass along the outline of a single layer, then it will complete a part (10 %) in the bottom right corner of a single layer, move to the upper left corner, then to the middle part, etc. With such trajectory of single layer building, the temperature difference will be neither significant nor expressed.

The optimal result is achieved by combining the first and the second ways.

Unless the third factor is taken into account, the obtained product will flake off. When using FDM and SLM technologies, this issue can be solved by a combination of certain practices: increasing of nozzle/laser travelling speed during building of a single layer, distribution of building areas between different sections of a single layer, coverage with subsequent layer by at least 50 %, which will ensure sufficient heat delivery for good adhesion between the layers. The latter imposes constraints on the spatial geometry of the entire product and/or requires additional (supporting) elements.

These factors may be omitted for parts manufacturing using SLA technology (stereoli-thography), since it implies simultaneous manufacture of the entire layer.

The next factor to be considered during design with additive technologies is interfacing between the layers. Each subsequent layer shall rest against the previous one. When printing the subsequent layer, its connection with the previous layer shall be ensured. This can be achieved only when the temperature is sufficient. The required temperature values are ensured as follows:

  • maintaining sufficient temperature of the previous layer by external heating: in SLM technology - heating of the work platform, in FDM technology - heating of the work platform and the entire building chamber;
  • sufficient contact (not less than 50 %) between the previous and subsequent layers; thus, when building non-vertical surfaces, a shift between the layers shall not exceed a single pass width by more than 0.5;
  • the less is layer thickness, the better it is heated and the higher is adhesion, but this leads to a significant increase of printing time.

Based on the above, side surface inclination cannot exceed 45° without the use of additional supporting elements. Inclined surfaces along z axis will have an expressed stepwise appearance (Fig. 3). The height of each step represents the layer thickness in this section of a part.

 

Fig. 3. Model of building up inclined surfaces with the use of additive technologies

 

A decrease of layer height will lead to an increase of the number of part layers and its manufacturing time but it will also reduce the deviation of the obtained surface from that shown in the drawing.

Higher accuracy of surface curves can be achieved through their orientation in the working area of a 3D-printer. For example, the inclined surfaces can be moved closer to the horizontal and/ or vertical relative to the 3D-printer working tool.

These factors shall be considered when optimizing the location of a 3D-model and selecting printing modes, in particular model slicing thickness.

The procedure of control programs preparation for additive processes uses the concept of suspended elements. Within the framework of additive technologies, elements not resting against the previous layer are considered suspended. If an angle of side face exceeds 45°, then overlapping of the subsequent layer is less than 50 %, consequently, this layer looses stability and additional supporting elements (supports) are required. When the process of mesh structures manufacturing is considered, their supplementing with supporting elements is not allowable, therefore the 3D-model and the manufacturing process itself shall be designed in such a way as to avoid supporting elements even for suspended building elements. For this purpose, several experiments were carried out, which revealed that the size of suspended elements depends on the technology, base material, product location in the working area of a 3D-printer. For FDM technology - on hot-end temperature and location of a suspended element in the working area of a 3D-printer. For SLM technology (Fig. 4), the largest size of a suspended element, i. e. manufactured without a support, may be up to 4 mm, but at the same time this leads to some surface distortions, which will have impact on the strength of the mesh-structure product.

Fig. 4. Samples manufactured with the use of SLM technology, with different rounding radii, for identification of producibility of suspended planes without supporting elements

Considering the above, single elements of mesh structures were developed (Fig. 5); these elements allow to avoid supporting elements on suspension bridges.

Fig. 5. Single elements of mesh structures

The next stage includes evaluation of strength properties of the developed structures. This is required to assess performance characteristics of the products, the design of which implements mesh structures. The obtained surfaces of mesh structures have very complicated shape, therefore, modern CAE systems cannot perform its quantitative assessment.

Let’s perform an analytical assessment of possible structure strength loss based on the physics of the processes employed in the course of SLM-based manufacturing. SLM-based manufacturing process is based on successive melting of elemental areas with a laser to obtain mini-melted areas having an interface surface. The interface tension [1] in each point of melt surface will be determined by a degree of curve in this point, and it can be evaluated by Laplace formula. The product is built layerwise. The sharper the transition of metal volumes in the lattice, the higher the temperature difference. These points will be characterized by increased internal stresses, which may lead to different consequences depending on their value: separation of supporting elements from the building platform, deformation of the entire product after its printing, lattice ruptures in points being under maximum stress. The analysis of the developed structures demonstrates that the maximum stress may appear in a cubic lattice (Fig. 5а), the minimum stress - in the lattice shown in Figure 5d. Based on the analysis of theoretical considerations, the lattice shown in Figure 5 a will have lower strength properties, and it is possible that during its manufacture mesh rib cross-section integrity may be disturbed. Based on the single structures, the macrostructures were developed (Fig. 6).

Fig. 6. Mesh structures for manufacturing with the use of SLM technology

The developed structures were successfully manufactured (Fig. 7) on a 3D-printer using SLM technology. The base material is Russian-made stainless steel 12Х18Н10Т. The application of a revolutionary mesh structure design technology with standard programs for G-code generation in manufacturing eliminated the need to use either internal or external additional supporting elements; besides, this technology allowed keeping integrity of all lattice rib cross-sections.

Fig. 7. Part manufactured with the use of SLM technology, with introduced mesh structures

Manufacture of similar structures using FDM technology revealed certain difficulties. In accordance with previously developed and justified recommendations, the lattice rib thickness shall not be less than two single nozzle passages, which is at least 0.6 mm. It has been experimentally proven that for FDM technology, PLA material, the size of suspended element (bridge) without a deflection doesn’t exceed 1.2 mm, and with larger sizes a deflection appears. In accordance with this, modified structures were developed, and these structures allowed to decrease the part weight by a less significant value.

The 3D-model of a part with incorporated mesh structure was developed (Fig. 5b) and manufactured (Fig. 7) of stainless steel using SLM technology.

Thus, the following results were obtained based on several theoretical and practical studies:

  • a fundamentally new design method was developed for solid 3D-models with mesh structure, which at the design stage ensures G-code generation using standard slicing programs with the laser movement trajectories corresponding to the parameters of building-up of a high-quality single layer;
  • several mesh structures with different cell forms were developed and successfully manufactured of Russian-made stainless steel 12Х18Н10Т without additional supporting elements;
  • maximum sizes of suspended (unsupported) sections were determined for printing using the considered technologies;
  • the obtained structures were analysed and the results demonstrated that the developed structures have different strength properties and internal stresses depending on the form of cells and ribs. The method of mesh structures digital design was developed: it allows to generate G-codes of control programs without additional correction in the manual mode;
  • these structures were experientially manufactured, which confirmed the capability of their manufacture without defects;
  • engineering capabilities of SLM (Selective Laser Melding) technology allow to introduce mesh structures into the body of a metal part and manufacture it as a single piece, which ensures higher strength properties without increasing labour intensity and with significant weight reduction;
  • FDM technology allows to introduce mesh structures into the part body, but with smaller pore size.

Usage of such structures during parts design and manufacture will allow to significantly decrease total weight of the product, which is very important for the aircraft and rocket production; at that, the strength of such products is high enough, while the labour intensity doesn’t increase, in some cases even lowering. Introduction of additive technologies into the manufacturing process is a very complicated and comprehensive task, but its completion will allow to get significant advantages, which are impossible when manufacturing parts with the use of “subtracting” technologies, even the most advanced ones.

References

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2. Зленко М. А., Нагайцев М. В., Довбыш В. М. Аддитивные технологии в машиностроении: Пособие для инженеров. М.: ГНЦ РФ ФГУП «НАМИ», 2015. 220 с.

3. Шишковский И. В. Основы аддитивных технологий высокого разрешения. СПб.: Питер, 2016. 400 с.

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About the Authors

I. V. Gorbatov
Snezhinsk Physics and Technology Institute – Branch of National Research Nuclear University “MEPhI”
Russian Federation

Gorbatov Igor Viktorovich –Postgraduate student, Senior Lecturer

Research interests: additive technologies, cutting using CNC machines.



A. A. Pilshchikov
Snezhinsk Physics and Technology Institute – Branch of National Research Nuclear University “MEPhI”
Russian Federation

Pilshchikov Aleksandr Aleksandrovich – Senior Researcher

Research interests: additive technologies, CAD systems.



Yu. A. Orlov
Snezhinsk Physics and Technology Institute – Branch of National Research Nuclear University “MEPhI”
Russian Federation

Orlov Yury Aleksandrovich – Postgraduate student, Senior Lecturer, Department of Physics and Technology

Research interests: additive technologies, materials science of metals.



V. A. Antyufeev
Snezhinsk Physics and Technology Institute – Branch of National Research Nuclear University “MEPhI”
Russian Federation

Antyufeev Vladimir Aleksandrovich – Postgraduate student

Research interests: additive technologies, IT.



S. A. Antyufeeva
Snezhinsk Physics and Technology Institute – Branch of National Research Nuclear University “MEPhI”
Russian Federation

Antyufeeva Svetlana Aleksandrovna – Postgraduate student

Research interests: additive technologies, IT



N. Yu. Orlova
Snezhinsk Physics and Technology Institute – Branch of National Research Nuclear University “MEPhI”
Russian Federation

Orlova Natalia Yurievna – Cand. Sci. (Engineering), Departmental Head, Department of Engineering Technology

Research interests: additive technologies, materials science.



D. Yu. Karpov
Snezhinsk Physics and Technology Institute – Branch of National Research Nuclear University “MEPhI”
Russian Federation

Karpov Dmitry Yuryevich – Senior Lecturer 

Research interests: optimization design methods based on CAD systems; innovative, including additive, technologies



For citation:


Gorbatov I.V., Pilshchikov A.A., Orlov Yu.A., Antyufeev V.A., Antyufeeva S.A., Orlova N.Yu., Karpov D.Yu. Determination of the possibility of manufacturing mesh structures using additive technologies. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(2):74-82. https://doi.org/10.38013/2542-0542-2020-2-74-82

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ISSN 2542-0542 (Print)