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Mechanical behaviour of additive manufactured 316L f2ccz lattice structure under static and cyclic loading
Journal Pre-proofs Mechanical Behaviour of Additive Manufactured 316L f2ccz Lattice Structure Under Static and Cyclic Loading D. Ashouri, M. Voshage, K. Burkamp, J. Kunz, A. Bezold, J.H. Schleifenbaum, C. Broeckmann PII: DOI: Reference:
S0142-1123(20)30034-7 https://doi.org/10.1016/j.ijfatigue.2020.105503 JIJF 105503
To appear in:
International Journal of Fatigue
Received Date: Revised Date: Accepted Date:
15 July 2019 2 December 2019 18 January 2020
Please cite this article as: Ashouri, D., Voshage, M., Burkamp, K., Kunz, J., Bezold, A., Schleifenbaum, J.H., Broeckmann, C., Mechanical Behaviour of Additive Manufactured 316L f2ccz Lattice Structure Under Static and Cyclic Loading, International Journal of Fatigue (2020), doi: https://doi.org/10.1016/j.ijfatigue.2020.105503
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Elsevier Ltd. All rights reserved.
Mechanical Behaviour of Additive Manufactured 316L f2 ccz Lattice Structure Under Static and Cyclic Loading D. Ashouria,∗, M. Voshageb , K. Burkampa , J. Kunza , A. Bezolda , J.H. Schleifenbaumb,c , C. Broeckmanna a Chair
and Institute for Materials Applications in Mechanical Engineering IWM, RWTH University of Aachen, Germany b Digital Additive Production DAP, RWTH University of Aachen, Germany c Fraunhofer Institute for Laser Technology ILT, Aachen, Germany
Abstract Additive Manufacturing (AM) provides new freedom of constructive possibilities for design engineers. Many structures with features like cavities or thin beams are difficult to be manufactured using conventional methods such as casting or machining while AM can ease this difficulty. Nevertheless, the various parameters that can influence the fatigue and structural behavior of the AM metals should not be neglected. So far, there are only a few studies on the influence of process parameters on the failure of lattice structures under fatigue loading. For this perspective, this article presents the manufacturing of lattice structures, and investigates basic failure mechanisms at the meso and macro levels. Within this framework, the process parameters of Laser-Powder Bed Fusion (L-PBF) are initially varied in order to influence the structure and to set it in a targeted manner. The varied process parameters are three different laser powers with adjusted speeds, which are applied to the additively manufactured samples. Static bending and cyclic axial loading are performed to investigate the mechanical strength of the specimens. Keywords: Additive Manufacturing (AM), Laser-powder bed fusion (L-PBF), Lattice structure (LS), Cyclic loading, Fatigue.
1. Introduction Due to the increasing demand for fabricating the components with complex geometries as well as high mechanical quality, Additive Manufacturing (AM) has become of great importance over the past few decades (1; 2). Moreover, the manufacturing costs for AM are independent of the complexity of the components, so that the production of geometrical complex structures can be more economical using AM (3). According to ASTM definition, AM is a process that converts data from a 3D model to assembled material usually layer over layer (4). Unlike the conventional, subtractive production methods, AM makes it possible to produce parts vertically- upward in layer-wise pattern with a combination of material deposition and energy delivery (5). Basically, AM methods can be classified according to the
∗ Corresponding
author Email address: [email protected] (D. Ashouri) URL: www.iwm.rwth-aachen.de (D. Ashouri) 1 this is the first author Preprint submitted to International Journal of Fatigue
type and extent state of the feedstock and also the binding mechanism between the layers (6). To additively manufacture nearly any geometry, the raw material in terms of powder or wire in a certain class of AM process is fully melted by the laser energy input and transformed layer over layer on a solid platform (7). Additive manufacturing can be divided into several types based on the manufacturing technique and the energy source. According to Frazier (8), there are three main groups of powder feed, wire feed and powder bed methods, which can be used for manufacturing and maintenance purposes with specific limitations (9). Since the focus of this work is on the production of lattice structures (LS) with a strut diameter < 500 µm, a high detail resolution is required, which is only achieved with the powder bed based process. Therefore, the present paper will investigate only this manufacturing process. The most common powder bed AM methods for metals can be named as Laser-Powder Bed Fusion (L-PBF) (10). However, there are other criteria regarding the rapid melting and solidification in L-PBF method that can limit the material selection (11; 12). The procedure in L-PBF method is spreading a layer of powDecember 2, 2019
(a) specimen A
define and produce the targeted modification of the grid geometry with a constant lattice type by means of LPBF. Different process parameters are used to study the influence of the varying laser powers on the microstructures, porosity, and the mechanical behavior of the LS.
(b) specimen B
Figure 1: Geometries that have been used to produce LS specimens for mechanical testings with specimen A for static loading and specimen B for cyclic loading
2. Materials and Methods 2.1. L-PBF sample fabrication In this work, 316L powder with elemental chemical composition shown in table 1 is used for additively manufactured LS. The powder has been produced by gas atomization using Ar as atomizing gas.
der on the platform and then melt it selectively with the laser according to the geometry of the 3D design dataset. This process will be repeated layer by layer to build up the component. To this day, various studies have been done regarding the significant advances in the field of L-PBF method, for instance, the mechanical behavior of the parts under different applied loads (13), microstructural transformation during the process (14) and L-PBF process for high- performance materials (15). Despite the fact that L-PBF is a very convenient process due to its capability of manufacturing the final part without any need for further processing, it can face problems in terms of surface and microstructure quality. Continuous melting and solidification layer by layer can lead to weak spots like porosity. Moreover, this process requires certain heat input levels and beam quality in order to melt the feedstock powder(16; 12). The powder must be spread as a thin homogeneous layer and a good flowability and depending on the particular AM process, the powder particles must have a certain particle size, density, and roundness. L-PBF is mostly used when high mechanical strength and fatigue strength are required as well as for complex near-net shapes like lattice structures (LS)(3). LS can be considered as the epitome of the solution to solve the shortcoming of the crystalline materials due to the fact that its elemental composition, micro and macrostructure changes or repeats gradually within the component as a function of position (17). As well as functionally graded materials, LS can be described as a periodical arrangement of unit cells that is repeated in space to create a continuum. Accordingly, besides the microscopic grading, the structural elements have their own structure, which offers them more freedom in improving and adjusting properties compared to conventional homogeneous materials (17). For heat- transfer purposes, the empty space in LS can be filled with fluid, which is one of the advantages of this structure (18; 19). However, in order to achieve a successful consolidation in critical structures, studying the mechanical behavior and predictability of LS under loading is an essential consideration. Following this review, the focus of the present work is to
Figure 2: Schematic representation of the zones in the web of each strut
In the production process, an oxygen level of <100 ppm was ensured. The L-PBF experiments were carTable 1: The 316L elemental chemical composition of the powder used for L-PBF
Element in mass %
C Cr Mn Mo Ni Si 0.03 16.8 1.75 2.4 10.4 0.9
ried out on a system designed by Aconity3D (Aconity MINI), which is specially developed for laboratory use. The beam source was a single mode fiber laser (wave length 1070 nm) with up to 400 W power output. Three different parameter combinations were applied to produce the lattice specimens as the variable process parameters. These parameters are introduced as down-skin (DS), in-skin (IS) and the combination Figure 3: The favorite strucof DS and IS (DI). The apture chosen to fabricate the plied hatching power and specimens speed for IS is 155 W and 825 mm/s respectively while these values for DS is 75 2
Figure 4: (a) EBSD figure map and (b) Phase analysis for L-PBF processed 316L full specimen at the reference state
W and 500 mm/s. The described parameter combinations (DS, IS, ID) are used as shown in Fig. 2. In order to investigate the mechanical behavior of LS, static and cyclic loading tests were conducted. Fig. 1 shows the samples that have been used for mechanical testing. The geometry (specimen A) used for static loading test has a cross section of 3.2×10 mm2 at the full part and 2.3×4.3 mm2 at the LS where the sum of the cross sections of six nodes is considered. The specimens for the cyclic loading (specimen B) have geometry of 12.3 × 12.3 mm2 with 54 mm length. To design the structure of the components, criteria such as periodical structure, open cells to remove the powder and classification based on classification for crystals had to be followed. Therefore, considering the mechanical properties of LS, anisotropy and the building time, the f2 ccz structure shown in Fig. 3 was chosen, where the load is applied along the Z axis. The f2 ccz unit cell structure is the structure used to fabricate all the specimens. This LS was chosen by Rheme et. al. for being best suitable for the manufacturing by L-PBF (20).
tested under monitoring loading. The data from each test contains the recorded force for necking, crack initiation and complete breakthrough among which, the maximum force was used to report the results. In this exper-
Figure 5: Schematic view of the specimen type A with LS in the middle part under 4-point bending test
iment, the data are shown as a bar chart representative of specimens, with their production conditions and the maximum applied force. Fig. 5 shows a schematic view of the specimen type A with a LS under 4-point bending experiment. The fatigue tests were performed with specimen type B under sinusoidal loading using the conventional high frequency pulsator (sinusoidal axial loading) equipment. The stress ratio of R = −1 and a test frequency of f = 80 Hz were used for the experiments. The investigation has been performed for 30 specimens at the reference state with different applied load amplitudes starting from Fa = 700 N using the staircase method for six specimens from each production condition. The run out limit was assigned as N = 107 . All the experiments were performed at room temperature.
2.2. Experimental details For characterization of the microstructure, optical microscopy (Olympus SZX12), as well as SEM technique (LEO 1450VP) operating at 15 kV equipped with EBSD unit were used. Due to the indication of the chemical characterization in the inner area of the fractured surface, phase analysis has been carried out. Fig. 4 illustrates the microstructure characterization of a full specimen at reference state in which the applied laser parameter is not modified. 4-point flexural experiments were conducted in the scanning electron microscope (SEM) by means of in-situ equipment with the capability of the maximum load of 10 kN. All tests were carried out in force control setup with 2 N/s load increment rate. For each condition, two specimens with type A have been
3. Results and discussion 3.1. Variation of the fabrication process parameters Structure of the specimens exclusively using the IS parameter in which the laser power is relatively large 3
(a) In-skin parameter
(b) Down-skin parameter
(c) Combination of DS and IS parameters
Figure 6: Lattice structure manufactured using three different laser powers as process parameters
provides low porosity. However, due to the large melting depth in overhanging areas, shown in Fig. 6a, these structures show low dimensional stability. This means that a proper web diameter of 300 µm cannot be guaranteed. The DS parameter, on the other hand, ensures dimensional stability with the desired web width of 300 µm. Although, due to the low laser power, in this case, the melting depth and the amount of sintered powder grains is reduced. In other words, the energy input in the down-skin area is low, which causes high porosity. Fig. 6b shows the LS produced by DS laser power. The third manufacturing process was introduced as a combination of DS and IS parameters. As it is illustrated in Fig. 6c, this type of laser power provides low porosity and ensures a web width of 300 µm also in overhanging areas.
Figure 8: Crack propagation steps from initiation until complete breakthrough
Figure 7: SEM in-situ crack observation on lattice structure during static bending Figure 9: Result of the 4-point bending test on two specimens for each applied laser power
3.2. Static loading (In-situ bending tests) To investigate the fracture behaviour of a single lattice cell, static bending tests with continuously increasing loads were performed in-situ in SEM. Fig. 7 shows detailed damage development at a node of the lattice structure. Notch effect at the marked node causes a crack to develop from the inside of the structure towards
the outside. The complete visualization of the crack propagation is captured in SEM, as it is shown in Fig. 8. Considering the different applied process parameters as IS, DS, and DI, Fig. 9 shows the results of the 4point bending test conducted on two specimens for each 4
(a) Webs of LS broken at the nodes
(b) Crack initiation spots at surface particles
Figure 10: Fracture planes of fatigue failure for LS
applied laser power. Specimens produced using DS pa-
parameter, show higher bending strength due to the high level of input energy. Combination of DS and IS parameters leads to relative equal mechanical behavior as IS condition. 3.3. Cyclic loading (High Cycle Fatigue Tests) Force controlled HCF experiments were conducted on the specimens manufactured with different building parameters such as the reference state, DS, IS, and DI. During fatigue testing, LS provides excellent emergency running properties before failure. If one node
(a) Close up of fracture origin
(b) Striations on fracture surface
Figure 12: S-N curve of 30 specimens at the reference state at R = −1
Figure 11: SEM micrograph from the fracture surface of the LS
fails in the first place, the overall stiffness of the structure drops, the entire structure is still able to withstand further load cycles. reference Referring to Fig. 10a, fracture planes of fatigue failure show that the LS fails only at the nodes and not at the web area. Fig. 10b shows the existence of multiple spots as crack initiation,
rameter fail under less applied force in comparison with the other specimens. This can be addressed to the high porosity of the LS processed with DS parameter regarding the low energy input in this case as it can be seen in Fig. 6b. Specimens manufactured under IS process 5
Figure 13: Comparison of fatigue strength for different process parameters in terms of scatter of fracture probability density with respect to number of the cycles
which has been found at surface particles. The SEM micrograph in Fig. 11a shows that crack initiation occurs from so called satellite powder particles on the surface, which causes a notch effect at the surface. Striations of crack propagation can be seen in Fig. 11b. To study the fatigue behavior, 30 specimens at the reference state were tested with an alternating axial load amplitude at R = −1. The S-N curve representative of this experi-
with DS parameter, where the applied energy input is low, have a rather large scatter in the presented data. It is necessary to improve the fabrication process in order to solve the current problems that may cause defects in the additively manufactured specimens. In metallography, it is not possible to hit the correct lattice level with diagonal grinding due to tilting during the sample preparation. Moreover, identifying the dimensional accuracy is only possible when meeting the correct level. During the fabrication process, the per-
Figure 14: Decreased filter power leads to porosity formation at top levels of the LS
ment is shown in Fig. 12. The result of HCF testing for three different process parameters is shown in terms of scattering of fracture probability density with respect to the number of cycles. According to Fig. 13, the result corresponding to the specimens manufactured with IS parameter where the input energy is higher, shows higher fatigue strength with a narrower scatter. Data representative of the exclusively DI parameter, however, shows a larger scatter of failure probability distribution with failure at earlier load cycles. Specimens produced
Figure 15: Defects in connection area between In-skin and Down-skin area
formance of the filter decreases, so that as shown in Fig. 14, the filter settles during the process with larger amounts of melted particles. In other words, the filter loses power that leads to the formation of porosity. As 6
it is shown in Fig. 15 weaknesses in the connection area between in-skin and down-skin parts can be the possible cause of the occurrence of porosity and voids on the webs of the LS. Heat dissipation is another source in the L-PBF process for defects and possible different structure in the LS. The heat from the melting of the upper levels of the structure stands by the top area while it should be distributed towards the lattice struts.
[5] e. a. A. Yadollahi, Effects of process time interval and heat treatment on the mechanical and microstructural properties of direct laser deposited 316l stainless steel, Materials Science Engineering A (2015) 171–183 (2015). doi:http://dx.doi.org/10.1016/j.msea.2015.07.056. [6] A. Gebhardt, Rapid Prototyping. 2, Hanser, 2000 (2000). [7] e. a. I. Tolosa, Study of mechanical properties of aisi 316 stainless steel processed by “selective laser melting”, following different manufacturing strategies, International Journal of Advanced Manufacturing Technology 51 (2010) 639–647 (2010). doi:https://doi.org/10.1007/s00170-010-2631-5. [8] W. Frazier, Metal additive manufacturing: a review, Material Eng and Perform (2014) 1917–28 (2014). doi:https://doi.org/10.1007/s11665-014-0958-z. [9] E. Herderick, Additive manufacturing of metals: A review, ASM International, EWI (2011). [10] W. J. Sames, The metallurgy and processing science of metal additive manufacturing, International Materials Reviews (2016) 315–360 (2016). doi:https://doi.org/10.1080/09506608.2015.1116649. [11] J. Edgar, Additive manufacturing technologies: 3d printing, rapid prototyping, and direct digital manufacturing, Johnson Matthey Technology Review 59 193–8. [12] T. S. T.S. Srivatsan, Additive Manufacturing Innovations, Advances, and Applications, CRC Press, 2015 (2015). doi:https://doi.org/10.1201/b19360. [13] e. a. S. Leuders, On the mechanical behaviour of titanium alloy tial6v4 manufactured by selective laser melting: Fatigue resistance and crack growth performance, International Journal of Fatigue 48 300–307. [14] e. a. B. Vrancken, Heat treatment of ti6al4v produced by selective laser melting: Microstructure and mechanical properties, Journal of Alloys and Compounds 541 177–85. [15] e. a. T. Niendorf, Processing of new materials by additive manufacturing: Iron-based alloys containing silver for biomedical applications, Metallurgical and Materials Transactions A 46 2829– 33. [16] D. Gu, Laser Additive Manufacturing of High-Performance Materials, Springer, 2015 (2015). doi:https://doi.org/10.1007/9783-662-46089-4. [17] e. a. K. Hazeli, Microstructure-topology relationship effects on the quasi-static and dynamic behavior of additively manufactured lattice structures, Materials Design (2019). doi:https://doi.org/10.1016/j.matdes.2019.107826. [18] K. V. C. Tien, Convective and radiative heat transfer in porous media, Advances in Applied Mechanics 27 (1989) 225–281 (1989). doi:https://doi.org/10.1016/S0065-2156(08)70197-2. [19] e. a. T.J.Lua, Heat transfer in open-cell metal foams, Acta Materialia 46 (1998) 3619–3635 (1998). doi:https://doi.org/10.1016/S1359-6454(98)00031-7. [20] O. Rehme, Cellular Design for Laser Freeform Fabrication, Technische Universit¨at Hamburg-Harburg, 2009 (2009).
4. Conclusion The primary goal of this study was to evaluate the influence of different building parameters in L-PBF process on the mechanical behavior of LS of alloy 316L. Manufacturing LS using three different process parameters showed that the combination of DS and IS parameters provides the best compromise between surface roughness and dimensional stability. From mechanical testing, it was discovered that damage only occurs at the nodes of the LS and not at the web area. Moreover, it was noticed that LS show excellent failure safety behavior so that when a node ever fails, the total stiffness of the structure falls off while the entire structure can endure further loading. Performing the HCF test with different L-PBF process parameters showed that the IS process parameter offers the most fatigue strength with narrowest scatter of data. DI and DS show lower fatigue strength and large distribution scatter respectively. Moreover, it was shown from the static testing that the suggested process parameter as combination of DS and IS did improve the mechanical behavior of the specimens. Future research should be extended in order to study the root causes of the production problems, which were noticed in the present work.
References [1] K. U. Zerbst, Damage development and damage tolerance of structures manufactured by selective laser melting – a review, International Symposium on Fatigue Design and Material Defects (2017) 19–22 (2017). [2] S. S.Beretta, A comparison of fatigue strength sensitivity to defects for materials manufactured by am or traditional processes, International Journal of Fatigue 94 (2017) 178–191 (2017). doi:https://doi.org/10.1016/j.ijfatigue.2016.06.020. [3] M. Schneider, Laser additive manufacturing of metals, Hagener Symposium (2018). [4] e. a. J. Alcisto, Tensile properties and microstructures of laserformed ti-6al-4v, Journal of Materials Engineering and Performance 20(2) (2011) 203–212 (2011).
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S0142-1123(20)30034-7 https://doi.org/10.1016/j.ijfatigue.2020.105503 JIJF 105503
To appear in:
International Journal of Fatigue
Received Date: Revised Date: Accepted Date:
15 July 2019 2 December 2019 18 January 2020
Please cite this article as: Ashouri, D., Voshage, M., Burkamp, K., Kunz, J., Bezold, A., Schleifenbaum, J.H., Broeckmann, C., Mechanical Behaviour of Additive Manufactured 316L f2ccz Lattice Structure Under Static and Cyclic Loading, International Journal of Fatigue (2020), doi: https://doi.org/10.1016/j.ijfatigue.2020.105503
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Elsevier Ltd. All rights reserved.
Mechanical Behaviour of Additive Manufactured 316L f2 ccz Lattice Structure Under Static and Cyclic Loading D. Ashouria,∗, M. Voshageb , K. Burkampa , J. Kunza , A. Bezolda , J.H. Schleifenbaumb,c , C. Broeckmanna a Chair
and Institute for Materials Applications in Mechanical Engineering IWM, RWTH University of Aachen, Germany b Digital Additive Production DAP, RWTH University of Aachen, Germany c Fraunhofer Institute for Laser Technology ILT, Aachen, Germany
Abstract Additive Manufacturing (AM) provides new freedom of constructive possibilities for design engineers. Many structures with features like cavities or thin beams are difficult to be manufactured using conventional methods such as casting or machining while AM can ease this difficulty. Nevertheless, the various parameters that can influence the fatigue and structural behavior of the AM metals should not be neglected. So far, there are only a few studies on the influence of process parameters on the failure of lattice structures under fatigue loading. For this perspective, this article presents the manufacturing of lattice structures, and investigates basic failure mechanisms at the meso and macro levels. Within this framework, the process parameters of Laser-Powder Bed Fusion (L-PBF) are initially varied in order to influence the structure and to set it in a targeted manner. The varied process parameters are three different laser powers with adjusted speeds, which are applied to the additively manufactured samples. Static bending and cyclic axial loading are performed to investigate the mechanical strength of the specimens. Keywords: Additive Manufacturing (AM), Laser-powder bed fusion (L-PBF), Lattice structure (LS), Cyclic loading, Fatigue.
1. Introduction Due to the increasing demand for fabricating the components with complex geometries as well as high mechanical quality, Additive Manufacturing (AM) has become of great importance over the past few decades (1; 2). Moreover, the manufacturing costs for AM are independent of the complexity of the components, so that the production of geometrical complex structures can be more economical using AM (3). According to ASTM definition, AM is a process that converts data from a 3D model to assembled material usually layer over layer (4). Unlike the conventional, subtractive production methods, AM makes it possible to produce parts vertically- upward in layer-wise pattern with a combination of material deposition and energy delivery (5). Basically, AM methods can be classified according to the
∗ Corresponding
author Email address: [email protected] (D. Ashouri) URL: www.iwm.rwth-aachen.de (D. Ashouri) 1 this is the first author Preprint submitted to International Journal of Fatigue
type and extent state of the feedstock and also the binding mechanism between the layers (6). To additively manufacture nearly any geometry, the raw material in terms of powder or wire in a certain class of AM process is fully melted by the laser energy input and transformed layer over layer on a solid platform (7). Additive manufacturing can be divided into several types based on the manufacturing technique and the energy source. According to Frazier (8), there are three main groups of powder feed, wire feed and powder bed methods, which can be used for manufacturing and maintenance purposes with specific limitations (9). Since the focus of this work is on the production of lattice structures (LS) with a strut diameter < 500 µm, a high detail resolution is required, which is only achieved with the powder bed based process. Therefore, the present paper will investigate only this manufacturing process. The most common powder bed AM methods for metals can be named as Laser-Powder Bed Fusion (L-PBF) (10). However, there are other criteria regarding the rapid melting and solidification in L-PBF method that can limit the material selection (11; 12). The procedure in L-PBF method is spreading a layer of powDecember 2, 2019
(a) specimen A
define and produce the targeted modification of the grid geometry with a constant lattice type by means of LPBF. Different process parameters are used to study the influence of the varying laser powers on the microstructures, porosity, and the mechanical behavior of the LS.
(b) specimen B
Figure 1: Geometries that have been used to produce LS specimens for mechanical testings with specimen A for static loading and specimen B for cyclic loading
2. Materials and Methods 2.1. L-PBF sample fabrication In this work, 316L powder with elemental chemical composition shown in table 1 is used for additively manufactured LS. The powder has been produced by gas atomization using Ar as atomizing gas.
der on the platform and then melt it selectively with the laser according to the geometry of the 3D design dataset. This process will be repeated layer by layer to build up the component. To this day, various studies have been done regarding the significant advances in the field of L-PBF method, for instance, the mechanical behavior of the parts under different applied loads (13), microstructural transformation during the process (14) and L-PBF process for high- performance materials (15). Despite the fact that L-PBF is a very convenient process due to its capability of manufacturing the final part without any need for further processing, it can face problems in terms of surface and microstructure quality. Continuous melting and solidification layer by layer can lead to weak spots like porosity. Moreover, this process requires certain heat input levels and beam quality in order to melt the feedstock powder(16; 12). The powder must be spread as a thin homogeneous layer and a good flowability and depending on the particular AM process, the powder particles must have a certain particle size, density, and roundness. L-PBF is mostly used when high mechanical strength and fatigue strength are required as well as for complex near-net shapes like lattice structures (LS)(3). LS can be considered as the epitome of the solution to solve the shortcoming of the crystalline materials due to the fact that its elemental composition, micro and macrostructure changes or repeats gradually within the component as a function of position (17). As well as functionally graded materials, LS can be described as a periodical arrangement of unit cells that is repeated in space to create a continuum. Accordingly, besides the microscopic grading, the structural elements have their own structure, which offers them more freedom in improving and adjusting properties compared to conventional homogeneous materials (17). For heat- transfer purposes, the empty space in LS can be filled with fluid, which is one of the advantages of this structure (18; 19). However, in order to achieve a successful consolidation in critical structures, studying the mechanical behavior and predictability of LS under loading is an essential consideration. Following this review, the focus of the present work is to
Figure 2: Schematic representation of the zones in the web of each strut
In the production process, an oxygen level of <100 ppm was ensured. The L-PBF experiments were carTable 1: The 316L elemental chemical composition of the powder used for L-PBF
Element in mass %
C Cr Mn Mo Ni Si 0.03 16.8 1.75 2.4 10.4 0.9
ried out on a system designed by Aconity3D (Aconity MINI), which is specially developed for laboratory use. The beam source was a single mode fiber laser (wave length 1070 nm) with up to 400 W power output. Three different parameter combinations were applied to produce the lattice specimens as the variable process parameters. These parameters are introduced as down-skin (DS), in-skin (IS) and the combination Figure 3: The favorite strucof DS and IS (DI). The apture chosen to fabricate the plied hatching power and specimens speed for IS is 155 W and 825 mm/s respectively while these values for DS is 75 2
Figure 4: (a) EBSD figure map and (b) Phase analysis for L-PBF processed 316L full specimen at the reference state
W and 500 mm/s. The described parameter combinations (DS, IS, ID) are used as shown in Fig. 2. In order to investigate the mechanical behavior of LS, static and cyclic loading tests were conducted. Fig. 1 shows the samples that have been used for mechanical testing. The geometry (specimen A) used for static loading test has a cross section of 3.2×10 mm2 at the full part and 2.3×4.3 mm2 at the LS where the sum of the cross sections of six nodes is considered. The specimens for the cyclic loading (specimen B) have geometry of 12.3 × 12.3 mm2 with 54 mm length. To design the structure of the components, criteria such as periodical structure, open cells to remove the powder and classification based on classification for crystals had to be followed. Therefore, considering the mechanical properties of LS, anisotropy and the building time, the f2 ccz structure shown in Fig. 3 was chosen, where the load is applied along the Z axis. The f2 ccz unit cell structure is the structure used to fabricate all the specimens. This LS was chosen by Rheme et. al. for being best suitable for the manufacturing by L-PBF (20).
tested under monitoring loading. The data from each test contains the recorded force for necking, crack initiation and complete breakthrough among which, the maximum force was used to report the results. In this exper-
Figure 5: Schematic view of the specimen type A with LS in the middle part under 4-point bending test
iment, the data are shown as a bar chart representative of specimens, with their production conditions and the maximum applied force. Fig. 5 shows a schematic view of the specimen type A with a LS under 4-point bending experiment. The fatigue tests were performed with specimen type B under sinusoidal loading using the conventional high frequency pulsator (sinusoidal axial loading) equipment. The stress ratio of R = −1 and a test frequency of f = 80 Hz were used for the experiments. The investigation has been performed for 30 specimens at the reference state with different applied load amplitudes starting from Fa = 700 N using the staircase method for six specimens from each production condition. The run out limit was assigned as N = 107 . All the experiments were performed at room temperature.
2.2. Experimental details For characterization of the microstructure, optical microscopy (Olympus SZX12), as well as SEM technique (LEO 1450VP) operating at 15 kV equipped with EBSD unit were used. Due to the indication of the chemical characterization in the inner area of the fractured surface, phase analysis has been carried out. Fig. 4 illustrates the microstructure characterization of a full specimen at reference state in which the applied laser parameter is not modified. 4-point flexural experiments were conducted in the scanning electron microscope (SEM) by means of in-situ equipment with the capability of the maximum load of 10 kN. All tests were carried out in force control setup with 2 N/s load increment rate. For each condition, two specimens with type A have been
3. Results and discussion 3.1. Variation of the fabrication process parameters Structure of the specimens exclusively using the IS parameter in which the laser power is relatively large 3
(a) In-skin parameter
(b) Down-skin parameter
(c) Combination of DS and IS parameters
Figure 6: Lattice structure manufactured using three different laser powers as process parameters
provides low porosity. However, due to the large melting depth in overhanging areas, shown in Fig. 6a, these structures show low dimensional stability. This means that a proper web diameter of 300 µm cannot be guaranteed. The DS parameter, on the other hand, ensures dimensional stability with the desired web width of 300 µm. Although, due to the low laser power, in this case, the melting depth and the amount of sintered powder grains is reduced. In other words, the energy input in the down-skin area is low, which causes high porosity. Fig. 6b shows the LS produced by DS laser power. The third manufacturing process was introduced as a combination of DS and IS parameters. As it is illustrated in Fig. 6c, this type of laser power provides low porosity and ensures a web width of 300 µm also in overhanging areas.
Figure 8: Crack propagation steps from initiation until complete breakthrough
Figure 7: SEM in-situ crack observation on lattice structure during static bending Figure 9: Result of the 4-point bending test on two specimens for each applied laser power
3.2. Static loading (In-situ bending tests) To investigate the fracture behaviour of a single lattice cell, static bending tests with continuously increasing loads were performed in-situ in SEM. Fig. 7 shows detailed damage development at a node of the lattice structure. Notch effect at the marked node causes a crack to develop from the inside of the structure towards
the outside. The complete visualization of the crack propagation is captured in SEM, as it is shown in Fig. 8. Considering the different applied process parameters as IS, DS, and DI, Fig. 9 shows the results of the 4point bending test conducted on two specimens for each 4
(a) Webs of LS broken at the nodes
(b) Crack initiation spots at surface particles
Figure 10: Fracture planes of fatigue failure for LS
applied laser power. Specimens produced using DS pa-
parameter, show higher bending strength due to the high level of input energy. Combination of DS and IS parameters leads to relative equal mechanical behavior as IS condition. 3.3. Cyclic loading (High Cycle Fatigue Tests) Force controlled HCF experiments were conducted on the specimens manufactured with different building parameters such as the reference state, DS, IS, and DI. During fatigue testing, LS provides excellent emergency running properties before failure. If one node
(a) Close up of fracture origin
(b) Striations on fracture surface
Figure 12: S-N curve of 30 specimens at the reference state at R = −1
Figure 11: SEM micrograph from the fracture surface of the LS
fails in the first place, the overall stiffness of the structure drops, the entire structure is still able to withstand further load cycles. reference Referring to Fig. 10a, fracture planes of fatigue failure show that the LS fails only at the nodes and not at the web area. Fig. 10b shows the existence of multiple spots as crack initiation,
rameter fail under less applied force in comparison with the other specimens. This can be addressed to the high porosity of the LS processed with DS parameter regarding the low energy input in this case as it can be seen in Fig. 6b. Specimens manufactured under IS process 5
Figure 13: Comparison of fatigue strength for different process parameters in terms of scatter of fracture probability density with respect to number of the cycles
which has been found at surface particles. The SEM micrograph in Fig. 11a shows that crack initiation occurs from so called satellite powder particles on the surface, which causes a notch effect at the surface. Striations of crack propagation can be seen in Fig. 11b. To study the fatigue behavior, 30 specimens at the reference state were tested with an alternating axial load amplitude at R = −1. The S-N curve representative of this experi-
with DS parameter, where the applied energy input is low, have a rather large scatter in the presented data. It is necessary to improve the fabrication process in order to solve the current problems that may cause defects in the additively manufactured specimens. In metallography, it is not possible to hit the correct lattice level with diagonal grinding due to tilting during the sample preparation. Moreover, identifying the dimensional accuracy is only possible when meeting the correct level. During the fabrication process, the per-
Figure 14: Decreased filter power leads to porosity formation at top levels of the LS
ment is shown in Fig. 12. The result of HCF testing for three different process parameters is shown in terms of scattering of fracture probability density with respect to the number of cycles. According to Fig. 13, the result corresponding to the specimens manufactured with IS parameter where the input energy is higher, shows higher fatigue strength with a narrower scatter. Data representative of the exclusively DI parameter, however, shows a larger scatter of failure probability distribution with failure at earlier load cycles. Specimens produced
Figure 15: Defects in connection area between In-skin and Down-skin area
formance of the filter decreases, so that as shown in Fig. 14, the filter settles during the process with larger amounts of melted particles. In other words, the filter loses power that leads to the formation of porosity. As 6
it is shown in Fig. 15 weaknesses in the connection area between in-skin and down-skin parts can be the possible cause of the occurrence of porosity and voids on the webs of the LS. Heat dissipation is another source in the L-PBF process for defects and possible different structure in the LS. The heat from the melting of the upper levels of the structure stands by the top area while it should be distributed towards the lattice struts.
[5] e. a. A. Yadollahi, Effects of process time interval and heat treatment on the mechanical and microstructural properties of direct laser deposited 316l stainless steel, Materials Science Engineering A (2015) 171–183 (2015). doi:http://dx.doi.org/10.1016/j.msea.2015.07.056. [6] A. Gebhardt, Rapid Prototyping. 2, Hanser, 2000 (2000). [7] e. a. I. Tolosa, Study of mechanical properties of aisi 316 stainless steel processed by “selective laser melting”, following different manufacturing strategies, International Journal of Advanced Manufacturing Technology 51 (2010) 639–647 (2010). doi:https://doi.org/10.1007/s00170-010-2631-5. [8] W. Frazier, Metal additive manufacturing: a review, Material Eng and Perform (2014) 1917–28 (2014). doi:https://doi.org/10.1007/s11665-014-0958-z. [9] E. Herderick, Additive manufacturing of metals: A review, ASM International, EWI (2011). [10] W. J. Sames, The metallurgy and processing science of metal additive manufacturing, International Materials Reviews (2016) 315–360 (2016). doi:https://doi.org/10.1080/09506608.2015.1116649. [11] J. Edgar, Additive manufacturing technologies: 3d printing, rapid prototyping, and direct digital manufacturing, Johnson Matthey Technology Review 59 193–8. [12] T. S. T.S. Srivatsan, Additive Manufacturing Innovations, Advances, and Applications, CRC Press, 2015 (2015). doi:https://doi.org/10.1201/b19360. [13] e. a. S. Leuders, On the mechanical behaviour of titanium alloy tial6v4 manufactured by selective laser melting: Fatigue resistance and crack growth performance, International Journal of Fatigue 48 300–307. [14] e. a. B. Vrancken, Heat treatment of ti6al4v produced by selective laser melting: Microstructure and mechanical properties, Journal of Alloys and Compounds 541 177–85. [15] e. a. T. Niendorf, Processing of new materials by additive manufacturing: Iron-based alloys containing silver for biomedical applications, Metallurgical and Materials Transactions A 46 2829– 33. [16] D. Gu, Laser Additive Manufacturing of High-Performance Materials, Springer, 2015 (2015). doi:https://doi.org/10.1007/9783-662-46089-4. [17] e. a. K. Hazeli, Microstructure-topology relationship effects on the quasi-static and dynamic behavior of additively manufactured lattice structures, Materials Design (2019). doi:https://doi.org/10.1016/j.matdes.2019.107826. [18] K. V. C. Tien, Convective and radiative heat transfer in porous media, Advances in Applied Mechanics 27 (1989) 225–281 (1989). doi:https://doi.org/10.1016/S0065-2156(08)70197-2. [19] e. a. T.J.Lua, Heat transfer in open-cell metal foams, Acta Materialia 46 (1998) 3619–3635 (1998). doi:https://doi.org/10.1016/S1359-6454(98)00031-7. [20] O. Rehme, Cellular Design for Laser Freeform Fabrication, Technische Universit¨at Hamburg-Harburg, 2009 (2009).
4. Conclusion The primary goal of this study was to evaluate the influence of different building parameters in L-PBF process on the mechanical behavior of LS of alloy 316L. Manufacturing LS using three different process parameters showed that the combination of DS and IS parameters provides the best compromise between surface roughness and dimensional stability. From mechanical testing, it was discovered that damage only occurs at the nodes of the LS and not at the web area. Moreover, it was noticed that LS show excellent failure safety behavior so that when a node ever fails, the total stiffness of the structure falls off while the entire structure can endure further loading. Performing the HCF test with different L-PBF process parameters showed that the IS process parameter offers the most fatigue strength with narrowest scatter of data. DI and DS show lower fatigue strength and large distribution scatter respectively. Moreover, it was shown from the static testing that the suggested process parameter as combination of DS and IS did improve the mechanical behavior of the specimens. Future research should be extended in order to study the root causes of the production problems, which were noticed in the present work.
References [1] K. U. Zerbst, Damage development and damage tolerance of structures manufactured by selective laser melting – a review, International Symposium on Fatigue Design and Material Defects (2017) 19–22 (2017). [2] S. S.Beretta, A comparison of fatigue strength sensitivity to defects for materials manufactured by am or traditional processes, International Journal of Fatigue 94 (2017) 178–191 (2017). doi:https://doi.org/10.1016/j.ijfatigue.2016.06.020. [3] M. Schneider, Laser additive manufacturing of metals, Hagener Symposium (2018). [4] e. a. J. Alcisto, Tensile properties and microstructures of laserformed ti-6al-4v, Journal of Materials Engineering and Performance 20(2) (2011) 203–212 (2011).
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