Experimental evaluation of 3D printing equipment for the production of large-scale metal parts through LPBF
technology
Pedro de Oliveira Pombinha
Thesis to obtain the Master of Science Degree in
Mechanical Engineering
Supervisors: Prof. Carlos Manuel Alves da Silva Eng. João Pedro da Fonseca Matos Pragana
Examination committee
Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista Supervisor: Eng. João Pedro da Fonseca Matos Pragana
Members of the committee: Prof. Inês da Fonseca Pestana Ascenso Pires
Prof. Ivo Manuel Ferreira de Bragança
Acknowledgments
Em primeiro lugar, quero agradecer à Adira – Metal Forming Solutions, pela oportunidade de participar num projeto tão estimulante como o SLM-XL.
O meu agradecimento ao prof essor Carlos Silva e Eng. João Pragana pelo apoio e orientação inestimáveis ao longo da realização desta tese.
Obrigado mãe e avó, os meus alicerces.
E, por último, quem nunca me deixou desistir, apoiou incondicionalmente e, sobretudo, acreditou em mim. Obrigado, Iolanda.
Resumo
A crescente popularidade das tecnologias de f abrico aditivo deve-se ao f acto de possibilitarem a produção de peças com geometrias complexas, impossíveis de obter através de f abrico convencional.
No campo do f abrico aditivo de metais, uma das tecnologias mais utilizada é o Laser Powder Bed Fusion (LPBF).
Esta dissertação tem como objetivo principal a avaliação experimental de um equipamento de LPBF desenvolvido especif icamente para o f abrico de peças metálicas de grandes dimensões. O trabalho consistiu na análise da inf luência dos parâmetros de f abrico (vector size, hatch spacing e atmosf era protetora) nas peças produzidas através deste equipamento. Para tal, f oi ef etuada uma revisão bibliográf ica em torno da deposição de aço inoxidável AISI 316L através de LPBF, o que permitiu def inir um intervalo de parâmetros para o f abrico de peças neste material com densidades relativas superiores a 99%.
Após esta análise, f oram f abricadas amostras com parâmetros de f abrico dentro deste intervalo e ef etuada a medição da sua densidade através de duas técnicas: análise micrográf ica e método de Arquimedes. Os resultados indicam que um aumento do vector size inf luencia negativamente a densidade. Além disso, uma atmosf era protetora de Azoto permite obter uma densidade relativa de 99.87%.
Após este estudo, os parâmetros ótimos obtidos f oram utilizados para o f abrico de uma peça de grandes dimensões, obtendo-se uma densidade relativa média de 99%. A integridade desta peça f oi avaliada através de ensaios não destrutivos, destacando -se a técnica de ultrassons como a mais adequada para a avaliação de peças obtidas por LPBF.
Palavras-chave
LPBF, parâmetros de processamento, densidade, aço -inoxidável 316L, peças de grandes dimensões
Abstract
Additive manufacturing technologies’ growing popularity is due to their ability to produce parts with complex geometries not achievable by conventional manuf acturing. In metal additive manuf acturing, one of the most widely used technologies is Laser Powder Bed Fusion (LPBF).
This thesis f ocuses on the experimental evaluation of a LPBF machine specif ically developed to produce large dimension metal parts. The developed worked consisted on the analysis of the inf luence of processing parameters (vector size, hatch spacing and gas atmosphere) on the parts produced by this equipment. For that purpose, a literature review was conducted on LPBF manuf acturin g of 316L stainless steel, which allowed def ining a processing parameter range to achieve parts with relative densities above 99%.
Following this review, samples were manuf actured with processing parameters within the determined range and their density was measured through two dif f erent techniques: micrograph analysis and Archimedes method. The results show that an increase in vector size negatively inf luences part density.
Moreover, a Nitrogen atmosphere allowed producing specimens with a relative density of 99.87%.
Finally, the determined optimal processing parameters were used to manuf acture a large dimension part, achieving an average relative density of 99%. The integrity of this part was evaluated through NDTs, with the ultrasound technique proving to be the most appropriate to evaluate LPBF produced parts.
Keywords
LPBF, processing parameters, density, 316L stainless steel, large-dimension parts
Index
Acknowledgments ... iii
Resumo ... iv
Abstract...v
List of tables ... viii
List of f igures ... ix
Acronyms ... xii
Symbols ... xiv
1. INTROD UCTION... 15
1.1. Thesis structure ... 15
2. STA TE-OF-THE -ART ... 16
2.1. Additive Manuf acturing ... 16
2.2. Metal Additive Manuf acturing ... 18
2.2.1. Binder Jetting ... 19
2.2.2. Sheet Lamination ... 21
2.2.3. Direct Energy Deposition ... 23
2.2.4. Material Extrusion ... 26
2.2.5. Material Jetting ... 27
2.3. Powder Bed Fusion ... 28
2.3.1. Laser Powder Bed Fusion... 29
2.4. LPBF on 316L Stainless Steel ... 34
2.5. Large build volume LPBF equipment ... 38
3. EXPERIME NTAL WORK ... 39
3.1. Equipment ... 39
3.2. Material ... 40
3.3. Processing parameters variation study ... 41
3.3.1. Density measurements ... 43
3.3.2. Microhardness ... 44
3.3.3. Electrical conductivity ... 45
3.4. Large dimension part ... 45
3.4.1. Dye penetrant inspection ... 46
3.4.2. Inf rared Thermography ... 46
3.4.3. Ultrasonic inspection ... 46
4. RESULTS AND DISCUSSION ... 48
4.1. Inf luence of processing parameters on part density ... 48
4.1.1. Hatch spacing... 48
4.1.2. Vector size ... 50
4.1.3. Energy density ... 51
4.1.4. Argon and Nitrogen atmosphere ... 52
4.1.5. Microhardness measurements ... 53
4.1.6. Electrical Conductivity ... 54
4.2. Large dimension part ... 54
4.2.1. Visual inspection ... 55
4.2.2. Electrical conductivity ... 55
4.2.3. Dye penetrant inspection ... 56
4.2.4. Inf rared Thermography ... 57
4.2.5. Ultrasonic inspection ... 58
4.2.6. Density measurements ... 58
4.2.7. World record for largest metal LPBF produced part ... 59
5. CONCLUS IONS ... 61
6. REFERE NCES ... 62
List of tables
Table 2.1 – AM families: terminology, brief description and typical materials. Adapted from [7]... 17 Table 2.2 – Recommended type of bonding according to intended application of the part [17] ... 22 Table 2.3 - Selected parameters variation and resulting part density in previous work with 316L SS in LPBF... 37 Table 3.1 – Add Creator 100 specifications ... 39 Table 3.2 - Chemical composition of the AISI 316L powder (wt. %). (LPW Technology, 2018) ... 40 Table 3.3 - Mechanical properties of the as-built material with AISI 316L powder. (LPW Technology, 2018) ... 41 Table 3.4 – Sample parameters for hatch space, vector size, energy density and atmosphere influence studies. ... 42
List of figures
Figure 2.1 – Types of slicing on a 3D CAD model of a part [4] ... 16
Figure 2.2 - Timeline of signif icant events in metal AM development [9] ... 19
Figure 2.3 – Schematic of a typical Binder Jetting machine [1] ... 20
Figure 2.4 – Green part, sintering schedule and thermo -gravimetric analysis [13] ... 20
Figure 2.5 – Post sintering part, porosity before and after sintering [13] ... 21
Figure 2.6 - Example of a SL produced metal part: Laminated die [17]... 21
Figure 2.7 – Form-then-bond and bond-then-form approaches [18] ... 22
Figure 2.8 - Schematic of ultrasonic consolidation [1] ... 22
Figure 2.9 - Schematic illustrations of the microstructural evolution during the UAM process: 1 through 3 f ormation of microasperities on the top tape surf ace, 4 through 6 bonding process between the top two tapes, and 7 and 8 accumulative process of additional tapes ... 23
Figure 2.10 - LENS system with thermal monitoring [25] ... 24
Figure 2.11 - Schematic repres entation of EBF3 system [26] ... 24
Figure 2.12 – DED produced blade. (a) Damaged; (b) DED repaired. [27]... 25
Figure 2.13 – Typical WAAM equipment [28] ... 25
Figure 2.14 – WAAM produced parts [28] ... 25
Figure 2.15 - Schematic of extrusion-based systems [1] ... 26
Figure 2.16 – Metal FDM part [29] ... 26
Figure 2.17 - Schematic of a drop-on-demand printing system [1] ... 27
Figure 2.18 - Direct writing of liquid metal 3D structures. (a) Thin wire. (b) Fibres suspended over a gap despite being composed of liquid. (c) Free-standing liquid metal arch. (d) Tower of liquid metal droplets. (e) 3D cubic array of stacked droplets. (f ) A liquid metal wire and an arch (g) Array of in-plane lines of f ree-standing liquid metal. Scale bars represent 500 μ m [32]... 27
Figure 2.19 - Example of a micro 3D object manuf actured through Material jetting by Yamaguchi [33] ... 28
Figure 2.20 – Classification of LPBF technologies according to binding mechanism [3] ... 29
Figure 2.21 – LPBF machine [33] ... 30
Figure 2.22 – Laser-material interaction [36] ... 30
Figure 2.23 - Eff ect of laser power and scan speed on substrate appearance of line scanning [43] .... 31
Figure 2.24 - Ef f ect of scan speed on the relative density f or AISI 316L stainless steel processed on a Concept Laser M3 Linear SLM machine [49] ... 32
Figure 2.25 - Material density as a f unction of energy density for variation in hatch spacing [45]. ... 32
Figure 2.26 - Stripe hatch with (a) continuous wiring mode and (b) pulse mode; (c) meander hatch; (d) chess board hatch. [53] ... 33
Figure 2.27 - Path planned by the combinations of three laser path and three laser power modes. Image shows the scan on a stainless-steel plate [55] ... 33
Figure 2.28 – Schematic view of hatch spacing, vector size and spot size. ... 35
Figure 2.29– Results for part density as a function of energy density available in literature. ... 36
Figure 3.1– A dira Add Creat or 100 large-dimension LPBF machine ... 39
Figure 3.2 – Tiled Laser Melting (TLM) mobile processing chamber ... 40
Figure 3.3 - (a) LPBF produced sample. Illustration of used scan strategies: (b) Stripes and (c) Chess. ... 42
Figure 3.4 – Schematic representation of cross-sections for optical porosity measurements: (a) Vertical and (b) Horizontal. ... 43
Figure 3.5 Schematic representation of the Archimedes method: (a) Mass in air (dry mass); (b) Mass in the f luid (immerse mass). ... 44
Figure 3.6 – Schematic representation of the indentations perf ormed in the cubic samples f or Vickers microhardness measurements. ... 44
Figure 3.7 – Experimental setup of the electrical conductivity measurements: (a) Eddy current def ect detector Olympus Nortec 500C; (b) XY moving table. ... 45
Figure 3.8 – IRSX-I336 infrared camera [71] ... 46
Figure 3.9 – General Electric Krautkramer USM 36 flaw detector [72] ... 47
Figure 4.1 - Relative density obtained with. Archimedes and micrograph analysis as a f unction of hatch spacing (vector size of 15 mm)... 48
Figure 4.2 – Influence of hatch spacing on the overlapping of consecutive scan tracks. (a) overlapping f or 0.05 mm of hatch spacing (HS1); (b) coincident edges of beam diameter f or hatch spacing of 0.08 mm (HS2); (c) area wit hout direct heat input f rom the laser f or hatch spacing of 0.1 mm (HS3). ... 49
Figure 4.3 – Porosity with hatch spacing variation. (a) Hatch spacing 0.05mm (Sample HS1). (b) Hatch spacing 0.08mm (Sample HS2). (c) Hatch spacing 0.1mm (Sample HS3). ... 49
Figure 4.4 – Relative density obtained with. Archimedes and micrograph analysis as a function of vector size (hatch spacing of 0.1 mm) ... 50
Figure 4.5 – Porosity with vector size variation: (a) Vector size of 1 mm (Sample VS1). (b) Vector size of 5 mm (Sample VS2). (c) Vector size of 15 mm (Sample VS4)... 51
Figure 4.6 – Relative density variation with energy density increase. Archimedes and micrograp h analysis (Constant vector size of 15 mm with variable laser power and hatch spacing) ... 51
Figure 4.7 - Cross sections of Sample ED1: (a) Porosity 2.5%; (b) Porosity 0.008% ... 52
Figure 4.8 – (a) Relative density vs Vector size for Argon and Nitrogen (Hatch spacing of 0.1 mm). (b) Relative density vs Vector size f or Argon and Nitrogen (Hatch spacing of 0.05 mm). All density measurements by Archimedes’ method ... 53
Figure 4.9 – Hardness measurements at different points of the samples with different energy densities ... 53
Figure 4.10 – Electrical conductivity profiles. (a) Energy density 75.56 J/mm3; (b) Energy density 135.00 J/mm3... 54
Figure 4.11 – Large-dimension part produced in the AC100 machine. Overall dimensions: 530x150x25mm ... 55
Figure 4.12 – Lattice defect (a) and major structural defect (b) ... 55
Figure 4.13 - Eddy current test result. ... 56
Figure 4.14 – Result of level 4 dye penetrant test ... 56
Figure 4.15 – Result of level 1 dye penetrant test: a) macrograph; b) micrograph of cross -section A-A;
c) detail of the previous showing the cracks previously identif ied at high magnif ication ... 57
Figure 4.16 – Result of Thermography test at a current intensity of 50 A. Zone A – no defect detected; zone B – defected ... 57
Figure 4.17 - Ultrasonic C-scan of tested part; b) micrograph of the section B-B... 58
Figure 4.18 – Areas where samples were extracted for density analysis ... 58
Figure 4.19 – Relative density at different points of the part ... 59
Figure 4.20 – Largest metal LPBF produced part in the world ... 59
Figure 4.21 – View of the cross section of the world record LPBF part ... 60
Acronyms
3D CAD – three-dimensional Computer Aided Design 3DP - 3D Printing
3SP - Scan, Spin, and Selective Photocure AC100 – Adira Add Creator 100
AISI – American Iron and Steel Institute AM – Additive Manufacturing
ASTM - American Society f or Testing and Materials CLIP – Continuous Liquid Interface Production DED – Direct Energy Deposition
DLP - Digital Light Processing
DMD - Direct Metal Deposition (DM3D) DMLS - Direct Metal Laser Sintering DPI – Dye Penetrant Inspection EB – Electron Beam
EBF3 – Electron Beam Freeform Fabrication EBM - Electron Beam Melting
FDM – Fused Deposition Modelling FFF - Fused Filament Fabrication
INEGI - Instituto de Ciência e Inovação em Eng enharia Mecânica e Engenharia Industrial i.e. – id est – “that is”
IR – Infrared
IRT – Infrared Testing
ISO – International Standards Organization LENS - Laser Engineered Net Shaping LMD - Laser Metal Deposition
LOM - Laminated Object Manuf acture
LPBF – Laser Powder Bed Fusion MCG – Manuel da Conceição Graça, Lda MJF - Multi-Jet Fusion
MJM – Multi-Jet Modelling
MOD - Metal-Organic-Decomposition NDT – Non-Destructive-Test
NETD – Noise Equivalent Temperature Dif ference PBF – Powder Bed Fusion
RP – Rapid Prototyping
SCP – Smooth Curvatures Printing SDL - Selective Deposition Lamination SHS - Selective Heat Sintering
SL – Sheet Lamination
SLA - Stereolithography Apparatus SLM – Selective Laser Melting SLS - Selective Laser Sintering SM – Subtractive Manufacturing SS – Stainless Steel
SSS – Solid State Sintering TLM – Tiled Laser Melting
UAM - Ultrasonic Additive Manuf acturing WAAM – Wire Arc Additive Manufacturing
Symbols
µm – micrometres A – Ampere Ar - Argon Bi – Bismuth C – Carbon Cd – Cadmium 𝑑 - Hatch spacing
Ga – Gallium ℎ - Layer thickness
In – Indium J – Joule
J/mm3 – Joule per cubic millimetre m/s - Meters per second
mm – millimetres
mm/s - millimetres per second N - Nitrogen
O2 –Oxygen 𝑃 - Laser power
Pb – Led Si – Silicon Sn – Tin
𝑣 - Scan speed W - Watt
1. INTRODUCTION
Additive Manuf acturing (AM) is, nowadays, regarded as a viable technology f or producing f unctional parts rather than simple prototypes, as per its original def inition: Rapid Prototyping (RP). It is precisely this evolution in part quality that led to the nomenclature change f rom RP to AM [1]. AM comprises dif f erent types of technologies which allow the production of parts f rom dif f erent materials, such as metals, polymers or ceramics, but all are based on the same layer-by-layer building principle that allows parts to have complex geometries which would be very dif f icult, if not impossible, to obtain by Subtractive Manuf acturing (SM) technologies. Nonetheless, AM has its drawbacks, namely, geometric and dimensional tolerancing, surf ace f inish or part dimension [2].
In the f ield of metal AM, one of the technologies available is Laser Powder Bed Fusion (LPBF), in which a bed of powder metal is selectively melted with a laser, according to the geometry of the layer of the part being produced. Then, another layer of powder is added, and the process is repeated until the f inal part is obtained. LPBF, being powder-based, produces parts with lower density than parts produced by SM f rom bulk material, due to pores that appear within the part [3]. The way to control part porosity is by adjusting the processing parameters.
This work f ocused on evaluating a customized LPBF equipment developed f or manuf actur ing large- dimension metal components (work volume of 1020x1020x520 mm). The evaluation was perf ormed by analysing the inf luence of the processing parameters on part density, thus f inding the optimal processing parameter range. Additionally, as a proof -of-concept, a large-dimension part was manuf actured, and its density was analysed.
1.1. Thesis structure
This thesis is divided in f ive chapters: Introduction, State-of -the-art, Experimental Work, Results and Discussion and Conclusions.
The introduction (present chapter) contains a preview of the work perf ormed and the motive behind it.
A brief introduction to AM and LPBF is perf ormed and the process f or evaluating the large -dimension LPBF machine is introduced.
In the State-of -the-art chapter a deeper analysis of AM and, specif ically, LPBF technology will be perf ormed, namely, the phenomena behind it and its pros and cons.
The Experimental work chapter will contain all the procedures perf ormed regarding the evaluation of the parts produced in the customized LPBF machine, i.e., determining the optimal processing parameters, part density and Non-Destructive Tests.
In Results and discussion, the results obtained during the experimental procedures will be analysed, discussed and compared with the available literature on the subject.
Finally, Conclusions will present a short summary of the results and what can be extracted f rom them.
2. STATE-OF-THE-ART
2.1. Additive Manufacturing
Additive Manuf acturing (AM), or 3D printing, first appeared in the 1980’s as a process for Rapid Prototyping (RP). As the term RP states, the goal, originally, was not to produce a f unctional component, but rather to f abricate a model in a quick and cost -ef f ective manner, in order to have a physical representation of the product to show to a client or design team. As the technology progressed, the quality of the produced parts increased to the point where naming the technology Rapid Prototyping became obsolete. So, as the parts became, in many cases, f unctional, and considering that the term RP overlooks the additive nature of the process, ASTM committee F42 agreed that the nomenclature of the technology should be changed f rom Rapid Prototyping to Additive Manuf acturing [1].
Additive Manuf acturing begins with a three-dimensional Computer Aided Design (3D CAD) model of the part that is going to be manuf actured. This model can be obtained by designing the part in a commercial 3D CAD sof tware or by acquiring its geometry though a 3D scanner. To proceed with the 3D printing of the part, the model needs to be converted to the STL (stereolithography) f ormat and then processed by a slicing sof tware. Here, the 3D model of the part is sliced into two-dimensional layers, with a chosen layer thickness, as show in Figure 2.1. The thickness of the layer determines the part’s resolution: the thinner the layer, the higher will be the accuracy. The slicing sof tware also determines the support structures, if needed, f or overhang f eatures. Finally, the part is ready to be printed, layer-by-layer, until the f inal geometry is obtained.
Figure 2.1 – Types of slicing on a 3D CAD model of a part [4]
AM technologies can be divided into seven groups: VAT Photopolymerization, Powder Bed Fusion (PBF), Binder Jetting, Material Jetting, Sheet Lamination, Material Extrusion and Direct Energy Deposition (DED), as f irst def ined in the ASTM F2792 standard [5]. Table 2.1 presents a summary of these technologies. There are also hybrid technologies that incorporate additive and subtractive
methods into one machine. In 2015, the ASTM F2792 was replaced by the ISO/ASTM 52900 [6], but the nomenclature f or the printing f amilies was maintained.
Though the f irst RP machines used polymers, waxes, or paper-based laminates, nowadays, with the dif f erent AM technologies available, it is also possible to manuf acture parts in ceramics or metals.
Hencef orth, the f ocus of this dissertation will be on metal AM.
Table 2.1 – AM families: terminology, brief description and typical materials. Adapted from [7]
Technology Variations Brief description Typical materials Vat
photopolymerization
SLA - Stereolithography
Apparatus DLP - Digital Light
Processing 3SP - Scan, Spin, and
Selective Photocure CLIP – Continuous
Liquid Interf ace Production
Vat of liquid photopolymer resin selectively exposed to
light, that starts the polymerization process, converting the exposed areas to
solid state.
UV-Curable photopolymer resins
Powder Bed Fusion SLS - Selective Laser Sintering DMLS - Direct Metal
Laser Sintering SLM – Selective Laser
Melting EBM- Electron Beam
Melting SHS - Selective Heat
Sintering MJF - Multi-Jet Fusion
Powdered material is selectively melted using a heat source, f or instance, laser or
electron beam.
Plastic, metallic and ceramic powders.
Sand
Binder Jetting 3DP- 3D Printing ExOne Voxeljet
Liquid bonding agents are selectively applied
onto powdered material. Metal or ceramic parts are typically f ired in a f urnace af ter printing.
Plastic, metallic and ceramic powders.
Glass and Sand.
Technology Variations Brief description Typical materials Material Jetting Polyjet
SCP – Smooth Curvatures Printing
MJM – Multi-Jet Modelling
Projet
Droplets of material are selectively deposited to f orm the layers. Alternatively, a photocurable resin
may be jetted and cured with UV light.
Photopolymers, polymers, waxes and metals (experimental)
Sheet Lamination LOM - Laminated Object Manuf acture
SDL - Selective Deposition Lamination
UAM - Ultrasonic Additive Manuf acturing
Sheets of material are stacked and laminated together. The layers are cut, one by one, until the f inal geometry
is obtained.
Paper, plastic sheets, and metal f oils/tapes
Material Extrusion FFF - Fused Filament Fabrication FDM – Fused Deposition Modelling
Material is extruded through a nozzle in beads, which are then
combined into multi- layer models.
Thermoplastic f ilaments and pellets.
Metal and ceramics (experimental)
Direct Energy Deposition
LMD - Laser Metal Deposition LENS - Laser Engineered Net
Shaping DMD - Direct Metal Deposition (DM3D)
Powder or wire is f ed into a melt pool which has been generated on the surf ace of the part, by using an energy source (laser
or electron beam).
Metal powder and wire, with, or without,
ceramics
2.2. Metal Additive Manufacturing
What is, arguably, considered the first AM metal equipment was reported in Deckard’s work [8] in 1986, where he began developing what would later become known as Selective Laser Sintering (SLS). From then on, metal AM has known great advances. Sames [9] presents a timeline of the evolution of metal AM, up to 2012, that is shown in Figure 2.2.
Figure 2.2 - Timeline of signif icant events in metal AM development [9]
Even though metal AM started as SLS technology, nowadays, several dif ferent approaches can be used to produce metal parts. For instance, the f eedstock may be in powder, wire or sheet f orm, with f usion of the f eedstock or of a binder agent [9], [10]. The AM f amilies, as def ined by the ISO/ASTM 52900 standard, that are typically used to produce metal parts are: PBF, Binder Jetting, Sheet Lamination, DED. Though experiments with metals have been perf ormed on Material Jetting and Material Extrusion, there is still no commercial equipment available so, f or the moment, these are not viable metal AM techniques [11]. An overview of these technologies will be presented below, exception being made to LPBF that, being the f ocus of this thesis, will have a more detailed analysis.
2.2.1. Binder Jetting
In Binder Jetting, droplets of a binder agent (typically polymeric, f or metals) are selectively released onto a powder bed, in order to f orm the desired geometry of the layer. Like in all other AM technologies, this process is repeated, layer by layer, until the f inal part is obtained. The as -built part is called a “green part” and is, at this point, very frail. So, the part needs to be infiltrated or go through a sintering process to gain structural integrity [1]. A depiction of the typical arrangement of a Binder Jetting machine is shown in Figure 2.3.
Though the typical printing head contains several nozzles, this technology enables the addition of extra nozzles with relative ease, making it possible to obtain low cost high build rate m achines. Sachs [12]
describes, in 1990, a linear printing speed of 20 m/s.
Figure 2.3 – Schematic of a typical Binder Jetting machine [1]
For metal Binder Jetting, the parts need to be subjected to three f urnace cycles in order to ga in structural integrity.
• 1st cycle: Perf ormed at low temperature f or long periods of time, to burn of f the polymeric binder agent;
• 2nd cycle: High temperature to perf orm sintering of the metal particles. This stage requires special attention as to not let the metal f use, at the risk of losing geometric and dimensional accuracy.
• 3rd cycle (optional): Bronze inf iltration in order to f ill the part’s pores, thus achieving parts with densities >90% [1].
Extracted from Rishmawi’s work [13], Figure 2.4 shows the heat cycle and thermo-gravimetric analysis f or a green part, and Figure 2.5 shows the variation in porosity with the sintering cycle.
Figure 2.4 – Green part, sintering schedule and thermo-gravimetric analysis [13]
Figure 2.5 – Post sintering part, porosity before and after sintering [13]
Some variants of the standard Binder Jetting process include the use of metal nanoparticle ink as a binder precursor to replace the polymer adhesives. Bai and Williams show this concept with Binder Jetting of copper using Metal-Organic-Decomposition (MOD) ink [14]. The authors also state that the produced parts have a denser core but a less dense shell, when compared to polymer adhesive manuf actured parts.
Copper seems to be a material of particular interest f or this technology as the ability to manuf acture complex copper geometries would allow great developments in thermal management systems and structural electronics [15]. Nonetheless, the Binder Jetting produced parts present high porosity values that are detrimental to the mechanical properties [16].
2.2.2. Sheet Lamination
In Sheet Lamination (SL) manuf acturing, the layers, instead of being obtained through extrusion or melt of the base material, are comprised of sheets of material which go through a cutting and bonding process, according to the geometry of the cross section of the desired part, as shown in Figure 2.6.
Figure 2.6 - Example of a SL produced metal part: Laminated die [17]
The sheets that f orm the cross sections of the part may be cut to shape bef ore or af ter the joining process - f orm-then-bond or bond-then-f orm (Figure 2.7). There are also dif f erent methods to bond the layers which dif f er, namely, according to the material that is going to be joined. For metal base material, the lamination can be achieved through chemical or polymeric adhesives [18], brazing [19], welding, ultrasound (dif f usion) or clamping [20].
Figure 2.7 – Form-then-bond and bond-then-form approaches [18]
Nowotny [17] provides the recommended type of bonding according to the intended application f or the part (Table 2.2).
Table 2.2 – Recommended type of bonding according to intended application of the part [17]
Laser welding Diffusion welding
Bonding by adhesives
Screws, anchors
Application range
Metal sheet f orming tools,
core boxes
Injection moulding tools,
pressure die casting tools
Metal sheet f orming tools
Metal sheet f orming tools,
core boxes
Out of the af orementioned bonding techniques, the most commonly used is dif f usion welding by ultrasound [11], or UAM (Ultrasonic Additive Manuf acturing). This bonding process is achieved by applying pressure and an ultrasound wave to the metal sheets, as depicted in Figure 2.8.
Figure 2.8 - Schematic of ultrasonic consolidation [1]
A more detailed analysis of the dif fusion process between the layers is presented by Fujii [21]. According to the author, microasperities are f ormed on the top surf ace of the top layer, due to contact with the sonotrode. Then, af ter a process of dynamic rec rystallization, an additional layer is ultrasonically bonded to the previous one. The interf ace region is f ormed at the def ormed microasperity zones. Finally, the interf ace region expands by static recrystallization during the addition of extra layers. Thi s process is depicted in Figure 2.9.
Figure 2.9 - Schematic illustrations of the microstructural evolution during the UAM process: 1 through 3 f ormation of microasperities on the top tape surf ace, 4 through 6 bonding process between the top
two tapes, and 7 and 8 accumulative process of additional tapes
One of the major problems of this building process is that the parts show an anisotropic behaviour, as they of ten have a much lower yield strength in the layer stacking direction, comparatively to the planar directions. Shimizu mentions this limitation in his work with Al alloy 6061 [22].
2.2.3. Direct Energy Deposition
Direct Energy Deposition (DED) is an AM process that produces parts by melting the material, in powder or wire f orm, as it is being deposited. The heat sources used in DED equipment are laser, electron beam or plasma arc.
This technology works by f ocusing the heat source into a narrow area, melting the f eedstock material, as well as the substrate below, to build up the parts. Even though several materials can be processed using this principle, the most commonly used f eedstock is metal.
Several combinations of f eedstock-power source can be used, namely, powder-laser, wire-electron beam, or wire-plasma arc. The most common variants of DED are the LENS (Laser Engineered Net Shaping) [23] and EBF3 (Electron Beam Freef orm Fabrication) [24] technologies.
The LENS process occurs in an inert gas f looded chamber, where a nozzle is used to converge the powder f eedstock and the laser beam into a f ocal plane, as shown in Figure 2.10, melting the f eedstock into the melt pool created in the substrate below.
.
Figure 2.10 - LENS system with thermal monitoring [25]
The principle of EBF3 is similar to LENS, but, as the f eedstock is in wire f orm, there is no need f or a convergent nozzle. The Electron Beam (EB) melts the wire directly into the melt pool on the substrate (Figure 2.11).
Figure 2.11 - Schematic representation of EBF3 system [26]
Materials processed by DED include stainless steel, tool steels, nickel alloys, cobalt alloys, titanium alloys or aluminium alloys.
DED may be used not only f or building, but also f or repairing parts. Wilson [27], shows the repairing of titanium turbine blades with this technique (Figure 2.12).
(a) (b)
Figure 2.12 – DED produced blade. (a) Damaged; (b) DED repaired. [27]
Another common type of DED is Wire Arc Additive Manuf acturing (WAAM) were the heat source is a plasma arc and the f eedstock is in wire f orm. This technique acquired its popularity due to the f act that it typically uses of f the shelf welding equipment, such as power source, torches and wire f eeding system and the dynamic part of the equipment can be provided either by robotic systems or computer numerical controlled gantries [28]. Typical WAAM equipment and manuf actured parts are shown in Figure 2.13 and Figure 2.14, respectively.
Figure 2.13 – Typical WAAM equipment [28]
Figure 2.14 – WAAM produced parts [28]
One of the drawbacks of this technology is that, like mo st metal AM technologies, DED shows anisotropy and density issues [11].
2.2.4. Material Extrusion
Material Extrusion is the most widespread AM process due to the relatively cheap equipment available.
The most common type is Fused Deposition Modelling (FDM) where a polymer f ilament is melted and extruded through a nozzle on the printing head, once again, according to the geometry of the part. The schematic of a typical extrusion head is depicted in Figure 2.15.
Figure 2.15 - Schematic of extrusion-based systems [1]
Though FDM is mostly a polymer orientated process, there are still some experiments with thermoplastic-metal composites [29], [30]. For metal FDM, the f eedstock f ilament is usually replaced by pellets, and the parts, like in Binder Jetting, need to go through a sintering process in order to obtain structural integrity.
One of the most important parameters in metal FDM is the thermoplastic bonding agent to metal powder ratio. A higher percentage of thermoplastic will make the extrusion easier, but will negatively af f ect the dimensional accuracy and sintering process [30].
Despite still not being a commercially viable metal AM technique, Lieberwirth [29] was able to manuf acture and sinter (small scale) parts by Material Extrusion. Figure 2.16 shows one of these parts, made f rom thermoplastic binder (45% weight ratio) and 1.4542 stainless -steel powder, where it is possible to see the shrinking problem associated to this technology.
Figure 2.16 – Metal FDM part [29]
2.2.5. Material Jetting
Material Jetting’s process is very similar to the 2D ink-jet printing of any document performed in every household or of f ice: a liquid is deposited in droplet f orm to achieve the desired shape. The major dif f erence, asides f rom the material used, lies on t he use of the third dimension in order to deposit additional layers and achieve a 3D object. An example of a droplet deposition system is shown in Figure 2.17.
Figure 2.17 - Schematic of a drop-on-demand printing system [1]
Though Material Jetting is not yet a viable process to produce metal AM parts, it has some applications in the f ield of electronics (soldering) [31]. Additionally, there is some experimental work on 3D microstructures such as cylinders, droplet arrays or wires. Ladd [32], shows in his work examples of this type of structures (Figure 2.18) made f rom a binary eutectic alloy of gallium and indium (EGaIn, 75%
Ga 25% In by weight) at room temperature.
Figure 2.18 - Direct writing of liquid metal 3D structures. (a) Thin wire. (b) Fibres suspended over a gap despite being composed of liquid. (c) Free-standing liquid metal arch. (d) Tower of liquid metal droplets. (e) 3D cubic array of stacked droplets. (f ) A liquid metal wire and an arch (g) Array of in-plane
lines of f ree-standing liquid metal. Scale bars represent 500 μ m [32].
Yamaguchi [33] also describes micro 3D metal objects manuf actured with a Bi–Pb–Sn–Cd–In alloy (Figure 2.19). The author also claims to achieve relative densities up to 98% in the manuf actured parts.
Figure 2.19 - Example of a micro 3D object manuf actured through Material jetting by Yamaguc hi [33]
2.3. Powder Bed Fusion
In Powder Bed Fusion (PBF), a bed of powdered material is selectively melted with a heat source in order to obtain the desired geometry. The powdered material may be a mix of f eedstock and binding agent or only f eedstock, depending on the binding mechanism that is going to be used.
According to Kruth [34], there are f our dif f erent types of binding mechanisms in PBF: solid state sintering (SSS), chemically induced binding , liquid phase sintering (partial melting) and f ull melting.
• Solid state sintering: thermal process that promotes dif f usion between the powder particles and occurs at a temperature between the melting point and half of the melting point of the material. Preheating of the powder is required.
• Chemically induced binding: binding occurs by chemical reaction, without the addition of any binding agents. For instance, Kruth [34] describes SiC PBF production with chemically induced binding: “When heating the SiC particles to a very high temperature, partial disintegration of the SiC into Si and C occurs. The free Si forms SiO2, which acts as a binder between the SiC particles. The parts are thus composed of a mixture of SiC and SiO2. Afterwards an infiltration step using Si yields full dense parts.”
• Liquid phase sintering – partial melting: the most common variation of this process is when the binding material is liquef ied while the structural material remains solid. Nonetheless, in some cases, the solid and the liquid phases may result f rom the same material.
• Full melting: f ull melting of the f eedstock without the need of binding material. This binding process provides the best mechanical properties.
Hitzler [3] presents a detailed schematic of the nomenclatures of the LPBF processes f ound in related literature according to the binding mechanism, as shown in Figure 2.20.
Figure 2.20 – Classification of LPBF technologies according to binding mechanism [3]
For the scope of this thesis, the parts will be manuf actured with f ull melting of metal powder and wi thout binding agent. The most common term f or this type of PBF process is Selective Laser Melting (SLM).
Nonetheless, in 2013, SLM Technologies f iled SLM as a trademark [35] and, f rom then on, unless a SLM Technologies machine is used, the term should not be applied. In order to avoid the trademark claim, the more generic term LPBF was adopted to describe this technology.
2.3.1. Laser Powder Bed Fusion
A typical LPBF equipment is comprised of a build chamber, powder f eed chamber, scraper, laser system and gas system, as shown in Figure 2.21.
The build chamber is the working area, i.e., where the powder is melted by the laser. Af ter a layer is f inished, the build chamber is lowered in order to allow the deposition of powder f or the next layer. Then, the powder f eed chamber is raised, and the scraper sweeps the new layer of powder into the build chamber, according to the predetermined layer thickness. The process takes place in a controlled inert gas atmosphere, leading to the need of a gas inlet and outlet.
Figure 2.21 – LPBF machine [33]
When the laser beam hits the metal powder, the f irst step in the f usion process is a densif ication by sintering. As more energy is provided by the laser, the sintered powder melts, loosing additional volume and drags more powder f rom the surroundings due to surf ace tension. As the build moves to the adjacent scan track, strong convection currents are created due to surf ace tension gradients in the melt pool.
During this process, shown in Figure 2.22, melting of the surf ace of the previous layer is also achieved.
Finally, the melt pool solidifies by nucleation of epitaxial grains [36].
The melting and solidif ication process is strongly dependent on the powder material properties [36] as well as on the pre-determined processing parameters [3], [37], [38].
Figure 2.22 – Laser-material interaction [36]
2.3.1.1. Processing parameters
Yadroitsev [39] established that there are approximately 130 parameters that inf luence LPBF AM.
Nonetheless, the author has narrowed down the most inf luential ones to eight: laser power, scan speed, hatch spacing, layer thickness, scanning strategy, wo rking atmosphere, powder f eedstock characteristics and powder bed temperature. Though each of them af f ects the characteristics of the part
individually, f our of these parameters are of ten grouped into a single parameter: laser power, scan speed, hatch spacing and layer thickness are usually represented as energy density [37], [40]–[42].
• Energy density (Laser power, scan speed, layer thickness and hatch spacing)
Energy density (equation 1) strongly inf luences the outcome of the manuf acturing process as too much or too little of it will provoke excessive porosity in the part [37], though f or dif ferent reasons.
𝐸𝑑 = 𝑃
𝑑 ℎ 𝑣 [ 𝐽
𝑚𝑚3] (1)
Where 𝑃 is the laser power [W], 𝑑 the hatch spacing [mm], ℎ layer thickness [mm] and 𝑣 the scan speed [mm/s].
As it can be seen in equation 1, these parameters inf luence energy density in a directly or inversely proportional manner: if 𝑃 increases, 𝐸𝑑 increases, if 𝑑, ℎ or 𝑣 increase, 𝐸𝑑 decreases. Though some authors research the inf luence of laser power [37], [43], [44], hatch spacing [45]–[47], layer thickness [48], [49] and scan speed [50]–[52] individually, the explanation for their influence on the part’s properties usually traces back to their inf luence on the energy density applied to the powder.
For instance, Laohaprapanon [43] shows the inf luence of varying laser power and scan speed on the quality of LPBF on 316L SS (Figure 2.23).
Figure 2.23 - Ef f ect of laser power and scan speed on substrate appearance of line scanning [43]
As it can be seen in the image above, the areas corresponding to too high or too low energy density values provide the worst results. To obtain the best part characteristics, energy density should be adjusted to an intermediate optimal value.
Kruth [49], f or instance, while evaluating the inf luence of processing parameters on the porosity of LPBF produced parts, shows that an increase in layer thickness becomes detrimental to part density, though f or low scan speeds this inf luence is not noticeable, as shown in Figure 2.24.
Figure 2.24 - Ef f ect of scan speed on the relative density f or AISI 316L stainless steel processed on a Concept Laser M3 Linear SLM machine [49]
Morgan [45] studied the inf luence of hatch spacing – the spacing between consecutive scan tracks - on part density. The author claims that there is a trend of increasing material density as hatch spacing increases, shown in Figure 2.25, which happens because increasing hatch space reduces excessive overlapping of consecutive scan tracks. Nonetheless, the hatch spacing cannot be indef initely increased as unmolten areas will appear and increase the porosity [47].
Figure 2.25 - Material density as a f unction of energy density f or variation in hatch spacing [45].
• Scan strategy
Scan strategy is the pattern drawn by the laser as it sweeps through each layer of the part. There are several dif f erent ways to do that, but the most common are the stripes, meander and chess board, shown in Figure 2.26.
Figure 2.26 - Stripe hatch with (a) continuous wiring mode and (b) pulse mode; (c) meander hatch; (d) chess board hatch. [53]
The dif f erent scan strategies inf luence surf ace f inish, porosity or residual stress [54]. Some recent studies are also evaluating an advanced control of scan paths by tempering with the G-Code of the LPBF equipment. Yeung [55] shows three variations f or laser vector strategy and laser power:
Laser vector strategies
1) Exact stop – complete laser stop at the end of each move.
2) Constant build speed – constant scan speed.
3) Continuous – match the end and start velocity of two moves.
Laser power modes
1) Constant power – constant laser power during each scan.
2) Constant power density – power/speed ratio constant.
3) Thermal adjusted – adjust power as per predefined thermal properties or feedback from real-time monitoring
The author claims that these strategies allow f or a better control of the melt pool and, consequently, of the part’s properties. A result of the different advanced scan strategies can be seen in Figure 2.27.
Figure 2.27 - Path planned by the combinations of three laser path and three laser power modes.
• Working atmosphere
The atmosphere where the melting process of the powder takes place plays a very important part in the success of the build. If oxidation of the metal being processed could not be avoided, or at least reduced, metal LPBF would probably not be possible [56]. So, the Oxygen content of the working atmosphere must be reduced either by f looding the chamber with an inert gas or by working in vacuum [57]. Some common inert gases used f or LPBF include nitrogen, argon or helium and the oxygen content should be, ideally, below 0.1%. Wang [58], f or instance, studied the ef f ect of changing the inert gas atmosphere while manuf acturing an Al-12Si alloy, and concluded that changing the inert gas does not af f ect considerably the density (variation of 0.7%) or microstructure of the material, but the parts produced with nitrogen and argon reveal better mechanical properties.
For stainless steel, though not related to AM, Elmer [59] and Katayama [60] describe the inf luence of changing the inert gas atmosphere o n the density of laser welds in AISI 304 SS, where both authors mention a reduction of porosity in the presence of a Nitrogen atmosphere, comparatively to using an Argon atmosphere.
• Powder feedstock characteristics
The most important characteristics of the powder f eedstock f or LPBF are f lowability, powder particle size distribution and morphology [61]. Though there is plenty of work regarding powder characterization, most of the research does not study the inf luence of varying these parameters on the manuf actured part’s quality [62]. However, some authors correlate powder particle size with final part properties [37], [42], [63]. For instance, Levy [63] states that - though it’s clear that particle size influences mechanical properties, surf ace roughness or density - def ining that a f iner or coarser powder is better or worse f or the properties of the part is not straightf orward. Fine particles are easier to melt and, as such, will produce higher density parts and high mechanical strength can be expected, but coarser p articles will produce parts with higher breaking elongations (close to bulk material values). So, when choosing a powder particle size, these contrary ef f ects must be considered.
• Powder bed temperature
Pre-heating the powder bed plays an important role in reducing residual stress and distortion as well as preventing cracking of the part [64]. Kempen [65] achieved relative densities of 99.8% with LPBF, while using M2 high speed steel, pre-heating the powder bed to 200°C.
2.4. LPBF on 316L Stainless Steel
Several authors have published work on LPBF produced 316L SS parts, while f ocusing on the ef f ect of varying processing parameters on part density. A review of some of this work was perf ormed, serving two purposes: benchmarking the achievable densities with this specif ic material and technology, and determining the optimal processing parameters to obtain those density values.
Though most of the reviewed work f ocuses on the parameters previously discussed, some authors mention two additional ones which haven’t been approached in previous chapters, namely, spot size, which is the laser beam diameter, and vector size which correlates to the scan strategy as it is the length of the scan tracks perf ormed by the laser. A depiction of these parameters is shown below, in Figure 2.28.
Figure 2.28 – Schematic view of hatch spacing, vector size and spot size.
Published work regarding the inf luence of LPBF processing parameters on 316L stainless steel parts’
density was initially perf ormed without achieving f ull melting of the metal powder, due to the experiments using low laser power [45], [66]. The low laser power explains why the obtained relative densities were f ar f rom the ideal 100% value (maximum relative densities of 60% [66] and 87% [45]). These results point that there is the need to achieve f ull melting of the metal powder, in order to achieve high densities.
The f irst near f ully dense parts – relative density of 100% - were described in work performed by Meier [38] and Spierings [42] (maximum relative densities of 99% [38] and 99.5% [42]). Both studies showed the ef f ect of energy density in part density. However, Spierings [42] also presented the individual ef f ect of scan speed and powder particle size. Both studies used dif f erent density measurement techniques:
micrograph analysis [38] and the Archimedes method [42].
Kruth [49] and Yasa [47] studied the ef f ect of the scan strategy on 316L part density, surf ace quality, microstructure and residual stress. Kruth [49] also showed that post-processing by laser re-melting can signif icantly increase the density of the part (maximum relative density of 99.9%). Yasa [47] built specimens using dif f erent vector size values. However, the individual inf luence of this processing parameter on part density was not analysed.
Yasa [67] also continued the work perf ormed by Kruth [49] on the ef f ect of laser re-melting on part properties such as density, microstructure and surf ace quality, resulting in the highest recorded LPBF 316L part density: 99.968%. Meanwhile, Levy [63] f urther elaborated the work perf ormed by Spierings [42], detailing the inf luence of the particle size distribution of powder materials on part properties, such as density, surf ace quality and mechanical properties.
Kamath [50] used high laser power (up to 400W) in order to achieve f ull melting with high scan speeds (up to 2250 mm/s), concluding that with higher power values, high density parts can be obtained over a wider range of scan speed values. These results indicate that the utilization of higher laser power could provide greater f lexibility in choosing process parameters that optimize the properties of the built part.
Sun [52] also analysed the part density using high build rates, achieving near f ully dense parts f or scan speeds f rom 625 mm/s to 3000 mm/s, f or a laser power of 380W.
Cherry [37] investigated the ef f ect of energy density on the shape and quantity of pores. This work stated that the total porosity is strongly inf luenced by laser energy density, where the minimum porosity value (part density of 99.62%) was obtained at 104.52 J/mm3. Moreover, this work also presents the negative inf luence of porosity on the measured material hardness. However, this inf luence is only signif icantly visible in cases of high porosity (over 2%).
A summary of the reviewed literature on 316L SS LPBF is shown in Table 2.3, which shows the parameters mentioned by the authors, and the respective relative density achieved. Unvaried parameters show as the corresponding numerical value, and variable parameters are s hown in range form: “minimum value” – “maximum value”.
Af ter f urther analysing Table 2.3, it is possible to see that energy density is the most varied parameter, whether it is directly mentioned by the author or inf erred by verif ying that one, or more, of the parameters that def ine it (scan speed, hatch spacing, layer thickness and laser power), as per equation 1, is varied.
A selection of the analysed studies that present relative densities above 98% is shown in Figure 2.29.
Figure 2.29– Results for part density as a function of energy density available in literature.
Meier [38]
Yasa1 [47]
Yasa2 [67]
Sun [52]
Cherry [37]
Cloots [40]
Sperings [48]
Kruth [49]
Table 2.3 - Selected parameters variation and resulting part density in previous work with 316L SS in LPBF.
Laser power [W]
Energy Density [J/mm3]
Gas atm.
Hatch spacing
[mm]
Layer thickn ess [µm]
Scan speed [mm/s]
Spot size [µm]
Vector size [mm]
Powder particle size [µm]
Relative density range
[%]
Ref.
15-30 - - 0.080-
0.240
75 900-
1500
- - 40; 2-3% 50-60 [66]
80 - N 0.025-
0.075
100 50-500 100 - 1-50 30-87 [45]
30-90 40-90 Ar - 50-75 40-640 - - 22 69-99 [38]
104 33.330- 106.670
- 0.130 30 250-800 200 - D10=6.300-
15.640 D50=15.050
-37 D90=30.790
-59.690
99.530 99.330
[42]
85-105 - - 0.020-
0.125
20-40 ≈150- 510
200 - - ≈94.100
-99.968
[49]
100 - - 0.081-
0.126
30 300 180 1-10 - 98.400-
98.800
[47]
85-105 - - 0.125 30 50-380 200 - - 99.230-
99.968
[67]
104 ≈33-101 - 0.130 30-45 175-800 - 5 D10=7.120-
15.260 D50=15.120
-37.700 D90=24.170
-55.540
≈94.300 -99.500
[63]
150- 400
- Ar 0.150 30 500-
2250
54 5 - ≈89-
99.500
[50]
380 98.700- 108.570
Ar 0.025- 0.120
50 625-
3000
80 - 20-63 >99 [52]
180 41.810- 209.030
Ar 0.124 50 200-
1000
70 - 15-45 91.160-
99.620
[37]
50-100 31.700- 79.900
- 0.070-
0.150
30 417-
1250
87 - D50≈35 ≈52-
99.700
[44]
175 - Ar -1 60 700 - - - 93.800–
97.500
[68]
200 - - 0.050-
0.110
30 1000-
2500
90 - D10=7.120 D90=24.170
86.900- 98.900
[40]
104 34.300- 97.200
- 0.120 30 300-850 200 - D10=6.300
D90=30.800
90.200- 99.300
[48]
75-175 63-547 N 0.040-
0.060
100 80-200 - - 36.600 80.400-
98.510
[43]
100 - - 0.126 30 175-380 180 1-10 - 95.830-
98.880
[51]
2.5. Large build volume LPBF equipment
The premise to develop the machine that will be characterized during the experimental work of this thesis was to achieve a high perf ormance, large build volume LPBF equipment, as the build volume is one the restrictions to AM equipment [2]. While reviewing the currently available large build volume LPBF equipment [69], the biggest competitor appears to be the SLM 800, by SLM solutions, shown in Figure 2.30.
Figure 2.30 – The SLM 800 machine [70]
The SLM 800 has a build volume of 500 x 280 x 850 mm, f our 400 W or 700 W lasers and a build rate of up to 171cm3/h. So, the goal is to increase the available build envelope while maintaining part quality.
3. EXPERIMENTAL WORK
To conduct the experimental evaluation of the new LPBF machine, the workplan was divided into two major steps:
1. Processing parameter variation study to determine the optimal values f or maximum relative density;
2. Build and analyse a “proof -of-concept" large dimension part.
These steps will be f urther developed in the sub -sections of the present Chapter.
3.1. Equipment
The machine being evaluated during the experiments was the Adira Add Creator 100 (AC100), shown in Figure 3.1, developed by manuf acturer Adira, alongside Instituto Superior Técnico, NOVA School of Science and Technology, INEGI and MCG as part of the SLM -XL project. The Add Creator’s specif ications are depicted in Table 3.1.
Figure 3.1– Adira Add Creator 100 large-dimension LPBF machine
Table 3.1 – Add Creator 100 specifications Laser 400 W fibre laser Minimum layer thickness 25 µm
Maximum scan speed 9 m/s Laser beam diameter 60 – 600 µm
Build volume 1020x1020x520 mm
The AC100 has undergone several upgrades and testing stages since the initial prototype, in order to achieve the current conf iguration. One of the major changes f rom the previous version is the depth of the powder bed that was increased f rom 200 to 520 mm.
The Add Creator has a closed f rame structure with a mobile platf orm on which the powder bed sits, as well as a mobile processing chamber. The powder cycle is f ully automated, which is a major advantage f rom an economic point of view, as it reduces wasted material and saves time by avoid ing having to replenish the powder system between consecutive operations .
The machine is prepared to operate with an Argon or Nitrogen protective atmosphere. The key f eature that enables the production of large-dimension parts is the Tiled Laser Melting (TLM) technology, which consists of a mobile processing chamber, shown in Figure 3.2.
Figure 3.2 – Tiled Laser Melting (TLM) mobile processing chamber
The mobile processing chamber splits the working area into “tiles” of 250x250 mm that are sequentially processed, thus reducing the volume of controlled atmosphere that needs to be maintained. The modular nature of this technology also allows, eventually, the expansion of the working area to larger dimensions.
3.2. Material
For both the samples and large-dimension part production, a gas atomized AISI 316L stainless steel powder, supplied by LPW Technologies, was used. The powder had a particle size distribution of D90=46 µm. The chemical composition and mechanical properties of the as -built material, specif ied by the material supplier, are depicted in Table 3.2 and
Table 3.3, respectively.
The choice f or 316L SS was due to its wide range of applications, namely in the f ield of medicine, which is, arguably, one of the most interesting markets f or AM produced parts [3], [37], [38], [66].
Table 3.2 - Chemical composition of the AISI 316L powder (wt. %). (LPW Technology, 2018) Weight Percent
AISI 316L C Cr Ni Mo Cu Mn Others Fe
0.03 18.00 13.00 2.50 0.50 2.00 <1 Balance
Table 3.3 - Mechanical properties of the as-built material with AISI 316L powder. (LPW Technology, 2018)
AISI 316L powder
Tensile Strength Horizontal Direction (XY) 590-690 MPa Vertical Direction (Z) 489-595 MPa Yield Strength Horizontal Direction (XY) 470-590 MPa Vertical Direction (Z) 380-560 MPa Young’s Modulus Horizontal Direction (XY) 159-175 GPa Vertical Direction (Z) 117-151 GPa Elongation Horizontal Direction (XY) 25-55%
Vertical Direction (Z) 30-70%
Hardness Horizontal Direction (XY) 210-214 HV0.5 Vertical Direction (Z) 114-226 HV0.5
3.3. Processing parameters variation study
To analyse the inf luence of processing parameters on part density, samples with 10 mm edge cubic geometry (Figure 3.3 (a)) were produced. The specimens were manuf actured varying one processing parameter while maintaining the others constant, so that a cause-ef f ect relation between the parameter and the part’s density could be established. The processing parameters under evaluation were hatch space, vector size, energy density and gas atmosphere, and the values used f or each study are presented in Table 3.4. For repeatability purposes, f ive sets of samples were produced f or each study.
The processing parameters not mentioned in Table 3.4 were kept constant, namely, spot size at 80 µm, layer thickness at 50 µm and scan speed at 800 mm/s f or the hatch, and 600 mm/s f or the contour. This choice was made in order to maintain the energy density values within the optimal range f or obtaining
≈99% relative densities, according to the results extracted from the literature review shown in Figure 2.29.
One of the conclusions extracted f rom the literature review on 316L SS LPBF was that inf ormation on the inf luence of varying the gas atmosphere and vector size on part density is scarce. So, to f urther complement the literature, a study on the inf luence of the inert gas atmosphere and vector size was perf ormed. For that purpose, the vector length was varied between 1 and 15 mm (dimension of the largest vector represented in Figure 3.3 (b)) and parts were built using either Argon or Nitrogen atmospheres.
The specimens were also produced using two dif f erent scan strategies: stripes (Figure 3.3 (b)) and chessboard (Figure 3.3 (c)). In both scan strategies the entire surf ace scan orientatio n was rotated by
(a) (b) (c)
Figure 3.3 - (a) LPBF produced sample. Illustration of used scan strategies: (b) Stripes and (c) Chess.
Table 3.4 – Sample parameters for hatch space, vector size, energy density and atmosphere influence studies.
Sample no.
Power Hatch (W)
Power Contour (W)
Gas atm.
Hatch spacing (mm)
Scan strategy
Vector size (mm)
Energy
Density - Hatch (J/mm3) Hatch spacing influence study
HS1 270 200 Argon 0.05 Stripes 15 135.00
HS2 270 200 Argon 0.08 Stripes 15 84.38
HS3 270 200 Argon 0.10 Stripes 15 67.50
Vector size influence study
VS1 270 200 Argon 0.10 Chess 1 67.50
VS2 270 200 Argon 0.10 Chess 5 67.50
VS3 270 200 Argon 0.10 Stripes 10 67.50
VS4 270 200 Argon 0.10 Stripes 15 67.50
Energy density influence study
ED1 270 200 Argon 0.10 Stripes 15 67.50
ED2 245 245 Argon 0.08 Stripes 15 76.56
ED3 270 200 Argon 0.08 Stripes 15 84.38
ED4 270 200 Argon 0.05 Stripes 15 135.00
Gas atmosphere influence study
Ar1 270 200 Argon 0.10 Chess 1 67.50
N1 270 200 Nitrogen 0.10 Chess 1 67.50
Ar2 270 200 Argon 0.10 Chess 5 67.50
N2 270 200 Nitrogen 0.10 Chess 5 67.50
Ar3 270 200 Argon 0.05 Stripes 10 135.00
N3 270 200 Nitrogen 0.05 Stripes 10 135.00
Ar4 270 200 Argon 0.05 Stripes 15 135.00
N4 270 200 Nitrogen 0.05 Stripes 15 135.00
3.3.1. Density measurements
Density measurements were perf ormed by the Archimedean buoyancy principle and micrograph analysis. The measured densities were compared with a ref erence value f or bulk 316L stainless steel of 8 g/cm3 and presented in relative density f orm.
3.3.1.1. Micrograph analysis
In order to evaluate the relative density of each specimen, there were, in total, f our cross -sections prepared f or micrographs: three horizontal cross -sections, with a minimum distance of 1 mm between them, and one vertical cross-section (Figure 3.4). Ten micrographs of each cross-section (2.5 x 2.5 mm2) were taken with 10x magnif ication and f urther processed in ImageJ sof tware (Image Processing and Analysis in Java, www.imagej.net). The number, as well as the area percentage, of pores in each image was identif ied by adjusting the threshold values f or the black -and-white micrographs. Five micrograph analysis were perf ormed f or each cross -section and the average value was considered.
Prior to taking the micrographs, the specimens were cleaned and grinded with abrasive paper up to 2000 grit and later polished with 6 µm and 1 µm diamond suspensions . No chemical etching was perf ormed.
(a) (b)
Figure 3.4 – Schematic representation of cross-sections for optical porosity measurements: (a) Vertical and (b) Horizontal.
3.3.1.2. Archimedes method
The Archimedes method consists in comparing the dry and immerse weight of the specimen to obtain its density, using an apparatus similar to the one depicted in Figure 3.5, and then resorting to equation (2):
𝜌𝑠 = 𝑚𝑑
𝑚𝑑 − 𝑚𝑖𝜌𝑓 (2)
Where 𝜌𝑠 is the sample density, 𝑚𝑑 is the dry mass of the sample, 𝑚𝑖 is the immerse mass of the sample and 𝜌𝑓 is the f luid density.
(a) (b)
Figure 3.5 Schematic representation of the Archimedes method: (a) Mass in air (dry mass); (b) Mass in the f luid (immerse mass).
The measurements were perf ormed in a Sartorius 410 BP 410S scale with maximum weight of 410 g and a resolution of 0,001 g, with an Archimedes measuring kit supplied by the manuf acturer. All measurements were done using deionized water. The specimens were polished bef ore the measurements in order to avoid air bubble f ormation in rough edges .
3.3.2. Microhardness
Vickers microhardness measurements were perf ormed along a line in the centre of the sample’s top horizontal f ace with a displacement of 0.5 mm between consecutive indentations , as per Figure 3.6, in order to estimate the mechanical strength of all prepared samples. The equipment used was a Mitutoyo HM-112 Vickers hardness tester with an applied load of 4.905 N f or 10 seconds.
Figure 3.6 – Schematic representation of the indentations perf ormed in the cubic samples f or Vickers microhardness measurements.
