NTNU Norwegian University of Science and Technology Faculty of Natural Sciences Department of Materials Science and Engineering
Selma Lund
Selma Lund
Additive Manufacturing with
Corrosion Resistant Alloys (CRA) - Microstructure and Mechanical Properties of Inconel 625
Master’s thesis in Material Science and Engineering Supervisor: Ida Westermann
Co-supervisor: Roy Johnsen June 2021
Master ’s thesis
Selma Lund
Additive Manufacturing with Corrosion Resistant Alloys (CRA) - Microstructure and Mechanical Properties of Inconel 625
Master’s thesis in Material Science and Engineering Supervisor: Ida Westermann
Co-supervisor: Roy Johnsen June 2021
Norwegian University of Science and Technology Faculty of Natural Sciences
Department of Materials Science and Engineering
Preface
The work of this master thesis has been carried out in the spring semester of 2021 at the Department of Materials Science and Engineering (IMA) at the Norwegian University of Science and Technology (NTNU). It is submitted as a part of TMT4905 - Materials Technology, Master’s Thesis. The work is a continuation of the research conducted in the course TMT4500 Materials Technology, Specialization Project in the fall of 2020 [1].
This thesis is conducted as part of a parallel master thesis that focused on the corrosion properties of the same materials. The author has conducted the work presented in this thesis. However, some of the porosity measurements presented in 4.2.4 are performed by Kristina E. Kindem [2].
This thesis could not be performed without contributions from other people who deserve acknow- ledgment. First of all, I would like to thank my supervisors Professor Ida Westermann and Professor Roy Johnsen, for their guidance and encouragement. Thanks for always answering questions and being supportive. This thesis could not have been done without the laboratory work conducted.
Therefore, I would like to thank Berit Vinje Kramer and P˚al Christian Skaret for their technical assistance and introduction to the testing equipment. I would also direct a special thanks to Senior Engineer Yingda Yu for his help with operating the TEM. Thanks for your patience and exciting conversations.
The thesis has been conducted in collaboration with the industrial partners Equinor, Fieldmade, and Frank Mohn (Framo). The test material was supplied by Framo Flatøy, Fieldmade, EOS GmbH and DMG Mori. I would like to thank Gisle Rørvik and Hans Huseby from Equinor, Svein Hjelmtveit from Fieldmade, and Linn Cecilie Gjelseng from Framo, for their contribution of materials, guidance, and sharing of knowledge.
Finally, I will thank my fellow students for five amazing years and Adrian Langseth for his great support and proofreading.
Abstract
This thesis aims to contribute to an increased understanding of how Additive Manufacturing (AM) technologies such as Selective Laser Melting (SLM) affect the microstructure and mechanical properties of Inconel 625 (IN625), compared to traditional manufacturing methods. The effect of various heat treatments is analyzed to see how the microstructure is affected regarding grain growth and precipitation kinetics, as well as the mechanical properties such as tensile strength and hardness. Two SLM-produced IN625 samples from different vendors are analyzed to see if the same properties are obtained in the samples before and after heat treatment. The purpose is to apply AM as a production method in industries where the requirements of the material properties are strict and reliable materials are essential. Since Inconel 625 has high strength, it is challenging to process. However, by using AM technologies, more complex geometries can be produced, which helps reduce the weight of the component, as well as time and cost of production.
The technology is relatively new and rapidly growing, so there are still many uncertainties of how this production method affects the mechanical- and corrosion properties, since a whole new set of process parameters are used. It is also observed that heat treatment affects AM samples in a whole new way compared to Traditional Manufacturing (TM). More knowledge of the various heat treatment is also needed.
Two SLM-produced IN625 materials were characterized in SEM and TEM after heat treatment between 850°C and 1100°C. The microstructural observations were compared to a TM sample of IN625. Tensile tests were conducted for the three different build directions (0°, 45°, 90°) to see if the mechanical properties are comparable with the TM sample, both before and after heat treatments.
The results of the microstructural characterization showed that the AM samples had a dendritic structure with elongated grains in the build direction. From the tensile tests it was found that the AM samples had better mechanical properties than the TM material. However, anisotropy in different directions was observed. After Stress Relief (SR) at 850°C and 900°C precipitates, assumed to be δ phase, formed in both AM materials. Recrystallization was almost complete at 1100°C, where twins and equiaxed grains were formed, resulting in a structure similar to the TM sample. Moreover, it was observed that the tensile strength reduced with an increasing heat treatment, and the properties were isotropic after annealing at 1100°C.
Sammendrag
Denne masteroppgaven har som m˚al ˚a bidra til økt forst˚aelse av hvordan additive tilvirknings (AM) teknologier som selektiv lasersmelting (SLM) p˚avirker mikrostruktur og mekaniske egenskaper av Inconel 625 (IN625), sammenlignet med tradisjonelle produksjonsmetoder. Effekten av ulike varme- behandlinger analyseres for ˚a se hvordan mikrostrukturen p˚avirkes n˚ar det gjelder kornvekst og presipitat-kinetikk, i tillegg til mekaniske egenskapene som strekkfasthet og hardhet. To SLM- produserte IN625-prøver fra forskjellige leverandører analyseres for ˚a se om de samme egenskapene er oppn˚add b˚ade før og etter varmebehandling. Form˚alet er ˚a anvende AM som produksjonsmetode i industrier der kravene til materialegenskapene er strenge og p˚alitelige materialer er avgjørende.
Siden Inconel 625 har høy styrke, er det utfordrende ˚a forme. Ved hjelp av additiv produks- jonsteknologi kan imidlertid mer komplekse geometrier produseres, noe som bidrar til ˚a redusere vekten av komponenten, samt redusere b˚ade tid og kostnader for produksjon. Teknologien er relat- ivt ny og raskt voksende, s˚a det er fortsatt mye usikkerhet om hvordan denne produksjonsmetoden p˚avirker de mekaniske egenskapene samt korrosjonsmotstanden, siden et helt nytt sett med prosess- parametere benyttes. Det observeres ogs˚a at varmebehandling p˚avirker AM-prøver p˚a en helt ny m˚ate sammenlignet med trandisjonelt produserte (TM) materialer. Mer kunnskap om de ulike varmebehandlingene er derfor ogs˚a nødvendig.
To SLM-produserte IN625-materialer ble karakterisert i SEM og TEM etter varmebehandling mel- lom 850°C og 1100°C. De mikrostrukturelle observasjonene ble sammenlignet med en TM IN625 prøve. Strekktesting ble utført for de tre forskjellige byggeretningene (0°, 45°, 90°) for ˚a se om de mekaniske egenskapene er sammenlignbare med TM-prøven, b˚ade før og etter varmebehandlinger.
Resultatene av den mikrostrukturelle karakteriseringen viste at AM-prøvene hadde en dendrittisk struktur med langstrakte korn i byggeretningen. Strekktesten viste at AM hadde bedre mekaniske egenskaper enn TM. Imidlertid ble anisotropi i forskjellige retninger observert. Etter spenning- sreduksjon (SR) ved 850°C og 900°C ble utfellinger, antatt ˚a være δ fase, dannet i begge AM materialene. Rekrystallisering var nesten fullført ved 1100°C, hvor tvillinger og equiaxed korn ble dannet, noe som resulterte i en struktur som ligner den for TM-prøven. Strekkfastheten ble redusert med en økende varmebehandling, og egenskapene ble observert ˚a være isotrope etter varmebehandling ved 1100°C.
Abbreviations
AB As-Built.
AM Additive Manufacturing.
BF Brightfield.
BSE Back Scatter Electrons.
CAD Computer Aided Data.
DED Direct Energy Deposition.
DF Darkfield.
EBSD Electron Backscatter Diffraction.
EDS Energy Dispersive X-Ray Spectroscopy.
EPBF Electron Beam Powder Bed Fusion.
FEM Field Emission Gun.
IN625 Inconel 625.
IPF Inverse Pole Figure.
IQ Image Quality.
LPBF Laser-Based Powder Bed Fusion.
MDS Material Data Sheet.
PBF Powder Bed Fusion.
PSD Particle Size Distribution.
SA Soft Annealing.
SAED Selected Area Electron Diffraction.
SEI Secondary Electron Image.
SEM Scanning Electron Microscope.
SLM Selective Laser Melting.
SR Stress Relief.
TEM Transmission Electron Microscope.
TM Traditional Manufacturing.
TTT Time-Temperature-Transformation.
XRD X-Ray Diffraction.
Contents
Preface i
Abstract ii
Sammendrag iii
Abbreviations iv
List of Figures viii
List of Tables x
1 Introduction 1
1.1 Motivation and Background . . . 1
1.2 Aim and Scope of the Work . . . 2
2 Theoretical Background 3 2.1 Additive Manufacturing . . . 3
2.1.1 Additive Manufacturing Technologies . . . 3
2.1.2 Selective Laser Melting . . . 4
2.1.3 Important Parameters for Selective Laser Melting . . . 5
2.1.4 Defects and Residual Stress . . . 9
2.2 Inconel 625 . . . 12
2.2.1 Microstructure . . . 12
2.2.2 Mechanical Properties . . . 16
2.2.3 Heat Treatment . . . 18
3 Experimental Procedure 22 3.1 Materials and Methods . . . 22
3.1.1 Material Selection . . . 22
3.1.2 Building Parameters . . . 24
3.1.3 Heat Treatment . . . 24
3.2 Metallographic Characterization . . . 25
3.2.1 Sample Preparation . . . 25
3.2.2 Particle Size Distribution . . . 26
3.2.3 Porosity . . . 27
3.2.4 SEM . . . 28
3.2.5 TEM . . . 30
3.3 Mechanical Testing . . . 32
3.3.1 Tensile Tests . . . 32
3.3.2 Hardness Measurement . . . 33
4 Results 34 4.1 Powder Analysis . . . 34
4.2 Microstructure . . . 35
4.2.1 SEM Characterization . . . 36
4.2.2 TEM Characterization . . . 45
4.2.3 Grain Size and Texture . . . 48
4.2.4 Porosity . . . 52
4.3 Mechanical Properties . . . 54
4.3.1 Tensile Tests . . . 54
4.3.2 Fracture Surface . . . 60
4.3.3 Hardness Measurements . . . 62
5 Discussion 65 5.1 Microstructural Observations . . . 65
5.1.1 Comparison of AM and TM . . . 65
5.1.2 Effect of Heat Treatment . . . 66
5.1.3 Porosity . . . 70
5.2 Mechanical Properties . . . 70
5.2.1 Comparison of AM and TM . . . 70
5.2.2 Effect of Heat Treatment . . . 71
5.2.3 Hardness . . . 73
5.2.4 Fractography . . . 74
5.3 Further Work . . . 75
6 Conclusion 76
Bibliography 78
Appendix 86
A Material Data Sheet for TM IN625 (A) 86
B Metal Powder Certificate of Nickel Superalloy 625 from Vendor B 88
C Material Data Sheet of NickelAlloy IN625 from Vendor C 89
D EDS of TM sample (A) 92
E Hardness Measurement from Specialization Project 94
F Microstructure of AB AM Samples in XY-Plane 95
G Microstructure of B1 and C Samples at 500x 96
H Microstructure of Sample C in XY-plane at900°C 97
I Fracture Surface 98
List of Figures
1 AM Processing Methods . . . 4
2 SLM Process Illustration . . . 5
3 Illustration of Melting of Powder in SLM . . . 6
4 Illustration of Hatch Spacing and Point Distance . . . 7
5 Scanning Strategies . . . 8
6 Contour-Hatch Scanning Strategy . . . 8
7 Type of Porosity . . . 10
8 Microstructure of TM and AM IN625 . . . 14
9 Time-Temperature-Transformation Diagram of IN625 . . . 15
10 Time-Temperature-Transformation Diagram for Precipitation ofδPhase . . . 18
11 Isopleth Diagram of Mass Fraction Nb in IN625 . . . 20
12 Flow Chart of Experimental Work . . . 22
13 Materials Tested . . . 23
14 Cutting Strategy . . . 26
15 PSD Analysis with Horiba LA-960 . . . 27
16 SEM . . . 29
17 Illustration of TEM and Preparation Methods . . . 31
18 Illustration of Brightfield and Darkfield in TEM . . . 32
19 Dimension of the Tensile Specimens . . . 33
20 Hardness Measurement Strategy . . . 33
21 SEM Image of Powder Morphology . . . 34
22 PSD Analysis of Powder . . . 35
23 Microstructure of TM * . . . 36
24 EDS Spot Analysis of Dark Particles in TM IN625 . . . 37
25 EDS Spot Analysis of Bright Particles in TM IN625 . . . 38
26 Microstructure of As-Bilt AM samples . . . 39
27 Microstructure of Heat-Treated B1 Samples . . . 41
28 Microstructure of Heat-Treated C Samples* . . . 42
29 Microstructure of Heat-Treated B2 Samples . . . 43
30 Higher Magnification of Microstructure of Heat-Treated B2 Samples . . . 43
31 EDS Point Analysis ofB190/1000 . . . 44
32 EDS Point Analysis of Black Spots in AM Samples . . . 45
33 TEM Images of As-Built AM Sample . . . 45
34 TEM Images of SR AM Sample . . . 47
35 TEM Images of SA AM Samples . . . 48
36 IPF of TM and AM samples . . . 50
37 IQ map of TM and AM samples . . . 51
38 Grain Size Obtained from EBSD . . . 52
39 Porosity Measurement of B2 Samples . . . 53
40 Porosity Measurement of C Samples . . . 53
41 Tensile Test of As-Built Samples . . . 55
42 Tensile Test of SR Samples at 850°C . . . 56
43 Tensile test of SR Samples at 900°C . . . 57
44 Tensile Test of SA Samples at 1000°C . . . 58
45 Tensile Test of Annealed Samples at 1100°C . . . 59
46 Fracture Surface of B1 Samples . . . 61
47 Fracture Surface of Heat-Treated B1 Samples . . . 62
48 Hardness of B2 Samples . . . 63
49 Hardness at Different Annealing Temperatures . . . 64
50 Tensile Test Comparison of Build Directions at Various Temperatures . . . 73
51 EDS spot analysis of dark particles in TM sample A . . . 92
52 EDS spot analysis of bright particles in TM sample A . . . 93
53 Hardness Measurement Conducted in the Specialization Project . . . 94
54 Microstructure of AM Samples . . . 95
55 Microstructure of AM in XY-plane . . . 96
56 Microstructure of Heat-Treated Sample A . . . 96
57 Microstructure of C Showing Precipitates . . . 97
58 Fracture surface of C0/AB showing ductile fracture. . . 98
List of Tables
1 Explanation of Different AM Technologies . . . 4
2 Process Parameters for SLM . . . 6
3 Effect of Process Parameters from Literature . . . 9
4 Chemical Composition of IN625 . . . 12
5 Phases in IN625 . . . 13
6 Tensile Properties of IN625 from Literature . . . 17
7 Overview of at which Temperatures Different Phases are Present . . . 21
8 Chemical Composition of Test Materials . . . 23
9 Explanation of Sample Names . . . 24
10 Process Parameters for B Samples . . . 24
11 Overview of Samples Characterized in SEM . . . 28
12 SEM Settings . . . 30
13 Tensile Properties Summarized . . . 59
14 Hardness Measurements Conducted in the Specialization Project. . . 94
1 Introduction
1.1 Motivation and Background
Additive Manufacturing (AM) is an advanced manufacturing method where three-dimensional parts are made by adding layer upon layer. This technology makes it possible to produce near-net shape components with unique geometries requiring fewer process steps and without the use of expensive tools. In addition, by optimizing the design of already existing components, the weight of the manufactured part can be reduced, leading to a decrease in energy consumption. This will, together with a reduction in production time, result in significant cost savings [3].
Metal-based additive manufacturing is primarily applied to certain materials like stainless steel, titanium, aluminum, and nickel alloys due to their desirable properties, such as weldability and availability of feedstock [4]. A material commonly used for AM is Inconel 625 (IN625), also known as UNS N06625, but it has not gained much attention until recent years, making it popular for fur- ther research. IN625 is a nickel-based superalloy with an attractive combination of properties with high strength at elevated temperatures and excellent corrosion resistance. However, manufacturing complex shapes with this material is difficult due to its high hardness and poor machinability [5]
[6]. Production with AM technologies is therefore highly interesting for IN625.
Methods like Selective Laser Melting (SLM) have gained much attention and demonstrate prom- ising results for many materials. Alloys produced with SLM experience locally high heating- and cooling rates, leading to a different microstructure than what is observed in Traditional Manufac- turing (TM). Post-processing heat treatment is often necessary to release residual stress and obtain desired properties. Whether IN625 is designed for corrosion resistance or fatigue, controlling the microstructure is essential to obtain the required material properties. Due to a different micro- structure resulting from AM, heat-treatment affects the material properties differently than what is observed for TM materials. Therefore, new recommended heat treatments are developed for AM materials. However, further research is needed to increase the understanding of how various temperatures affect the microstructure.
Until recently, AM was mainly utilized for prototyping but is today applied in industries like aerospace and medicine, and currently, other industries are also adopting this technology. For example, Equinor is a company in the energy industry interested in utilizing this technology due to the many advantages it creates. Instead of having a large physical inventory for materials that may be needed someday, a digital inventory can be established where the specific parts exist as a 3D model and can be produced on-demand. A digital inventory eliminates the storage cost and reduces the production time from months to weeks [7], which is a considerable advantage on oil platforms, where transportation of materials is challenging. A further advantage is that AM makes it easier to repair parts instead of replacing them. These advantages are favorable for reducing cost and saving time but further helps increase the safety, performance, and lifetime of the components.
AM also leads to reduced carbon emissions due to using less raw material, producing less waste, and requiring less transportation. In industries like this, there exist strict requirements and standards that need to be met. Reliable materials with suitable properties and minor defects are therefore necessary.
1.2 Aim and Scope of the Work
This master thesis is an extension of previous work conducted in the specialization project where selected parts of the theory still applies, and some of the results have been further investigated and included in the present work to complement the experimental results. The thesis aims to increase knowledge of how additive manufacturing affects the microstructure and mechanical properties of IN625. AM materials are rarely used in their As-Built (AB) condition, and post-processing is therefore necessary. The goal is also to increase the understanding of how various heat treatments affect the microstructure regarding precipitation kinetics and grain size. The microstructure is fur- ther used to analyze how the mechanical properties vary with different heat treatments. Moreover, the material properties are compared to a traditionally manufactured material to see if AM is just as good or better.
In cooperation with Fieldmade, SLM produced samples of Inconel 625 from two different vendors were studied and compared to a forged reference sample. The AM samples were analyzed in three different build directions (0°, 45°, and 90°), and exposed to various temperatures between 850°C and 1100°C. Scanning Electron Microscope (SEM) was used to characterize the microstructure with techniques such as Back Scatter Electrons (BSE), Energy Dispersive X-Ray Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD). Transmission Electron Microscope (TEM) were used for further analysis of the microstructure. Mechanical properties such as tensile strength and hardness were measured, where the results were analyzed focusing on the effect of microstructure.
The results were compared with previous research found in the literature.
This thesis is a parallel activity and supplementary to another master thesis where the same materials are used, where the main focus is corrosion properties of additive manufacturing with IN625. The results of the microstructure are therefore essential for analyzing the findings from the corrosion tests [2].
2 Theoretical Background
This chapter gives an introduction to additive manufacturing technology, where information of Inconel 625 is presented with a focus on microstructure and mechanical properties. This is, as mentioned, based on the specialization project [1], however, changes are made.
2.1 Additive Manufacturing
Additive Manufacturing (AM) is a technology which has grown over the last couple of years due to the many advantages it can offer, as well as an increased knowledge in the field. Three-dimensional parts can be produced from a digital model, often a Computer Aided Data (CAD) model, by adding layer upon layer, making it possible to produce parts with fewer process steps compared to more conventional production methods. Further advantages of AM is the high productivity and the possibility to produce parts with more complex design and new functionalities, which has previously not been achievable. As opposed to more traditional ways of producing materials like subtracting or formative manufacturing, AM uses less raw materials and is often more efficient as the desired shape is not obtained by selectively removing material or using expensive tools or forms, such as die or casting mold, but can instead be built from scratch [8].
AM technology was developed more than 30 years ago, and until recently, it was mainly used for prototyping, due to the high production cost. Today, AM is often used when producing high- performance components in the aerospace, medical, energy, and automobile applications. Boeing has, for example, installed ten thousand AM parts in several of their commercial and military aircrafts [9]. AM technology has been rapidly evolving, and more industries are interested in using this new technology. However, the potential is not fully utilized, and it still faces unique processing and materials development issues compared to conventional process methods [10]. The process conditions for AM are different from the conventional methods, such as casting, forging, or welding, and the prior research is limited. Understanding the limitations and possibilities is essential to optimize the process and achieve material properties that can compete with those of conventional manufacturing methods.
A wide spectrum of materials can be used for AM technology. The most popular metal-based materials include stainless steel, titanium alloy Ti-6Al-4V, and nickel-based superalloys due to their attractive properties, such as weldability, availability of feedstock, and strength [4]. Depending on the type of application and industry, many different processes and materials can be used.
2.1.1 Additive Manufacturing Technologies
According to ISO 17296 [11], there exists seven different process categories of additive manufac- turing technologies. The methods have the same production principles, with the deposition of material on a building platform built up in successive layers. However, they have different binding mechanisms, source of activation, feedstock, and secondary processing requirements. An overview of the different process mechanisms is shown in Figure 1. Time, cost, surface finish, and mechanical properties are crucial factors when deciding which method to use. This report focuses on Powder Bed Fusion (PBF), specifically Selective Laser Melting (SLM). The methods are briefly explained in Table 1.
Figure 1: The seven different AM processing methods including typical feedstock and binding mechanism. Figure inspired by [12].
Table 1: Short explanation of the seven different AM technologies [11].
Process Description
Powder Bed Fusion (PBF)
Thermal energy selectively melts areas of powder on a bed. Technologies such as Electron Beam Powder Bed Fusion (EPBF) and Laser-Based Powder Bed Fusion (LPBF) Binder Jetting
(BJ)
Powder is joined by spraying binder agent to create bonding by chemical reaction.
The manufactured part is further sintered. Works for various materials.
Direct Energy Deposition (DED)
Powder or wire is pushed through a nozzle and melted by thermal energy while deposited on a building platform.
Material Jetting (MJ)
Droplets are selectively deposited on a building platform. Binding by chemical reaction or adhesion. Possible to print various materials in one part.
Material Extrusion (ME)
Material dispensed through a nozzle bonded together either by thermal or chemical bonding. The feedstock is thermoplastics or ceramics.
Vat Polymerization (VP)
Material is bonded together by a chemical reaction. Layer-on-layer of
photopolymer resin is solidified by curing. Technologies such as Sterolithography Sheet Lamination
(SL)
Thin sheets bonded together with a binder, either by ultrasonic waves or adhesive coating. The sheet are heated and pressed together.
2.1.2 Selective Laser Melting
Selective Laser Melting (SLM), also know as Laser-Based Powder Bed Fusion (LPBF), is one of the main AM technologies used to produce nickel-based superalloys [13]. SLM is a technology based on the PFB process and is utilized to create all the AM samples tested in this master thesis.
Materials are built by a laser selectively melting regions of metal powder on a preheated bed to
cause rapid consolidation of the powder. The building platform is lowered in accordance with the layer thickness, and a new layer of powder is added and fused with the previous one [14]. The process is repeated until the desired shape is obtained. A schematic illustration of the SLM process is given in Figure 2.
SLM emits short-wavelength light from a high-energy-density fiber laser to melt thin layers of metal powder [15]. The use of laser as a heat source gives controlled heating and solidification and leads to dimensional accuracy [8]. An inert atmosphere, typically argon, is used in the build chamber to prevent oxidation of the material [16]. A fine powder distribution and a small layer thickness are mainly used, which results in a fine surface finish [10] [17]. By optimizing the process parameters, SLM can produce near fully dense components [18]. The parameters affecting the material properties are further discussed in Section 2.1.3.
Powder-based processes such as SLM often produce shapes and features that require little finishing to achieve a functional form [8]. However, they often require post-processing to release residual stress and adjust the material properties [19] [20]. Post-processing may include subtractive manu- facturing, surface finish, thermal processing, or other operations according to ISO/ASTM 52910.
Figure 2: Illustration of the SLM process and equipment [14].
2.1.3 Important Parameters for Selective Laser Melting
Controlling the different process parameters is essential to achieve a consistent and reliable part.
However, when producing parts with AM, a whole new set of process parameters must be evaluated, and the industry experience is limited. The process parameters result in several uncertainties regarding material properties of the manufactured part compared to production with traditional methods like shaping, cutting, and joining, where there is a lot more knowledge. There are many different process parameters for SLM, where they all vary a lot from conventional methods. In return, they result in a higher degree of freedom and the possibility to easily be adjusted to get the desired microstructure with minimal defects. The parameters are constantly optimized to improve the densification, microstructure, and mechanical properties of the produced part [15]. Some of the essential parameters further explained in this section are shown in Table 2.
Table 2: Explanation of some of the essential process parameters for SLM.
Parameter Definition Unit
Laser Power (P) The energy output from the laser W
Scanning Speed (v) The velocity at which the laser beam moves over the powder m/s Layer Thickness (t) Depth of single powder re-coat layer mm
Hatch Spacing (h) The space between the laser tracks mm
Energy Density (E) Amount of energy power per unit volume J/mm3 Exposure Time Time the laser scans at one point in pulse mode µs Point Distance Distance between scanning points in pulse mode µm
Laser Parameters
Some critical factors influencing the material properties are the laser parameters, such as scanning speed, hatching space, layer thickness, and laser power. These influence the heat input or volu- metric energy density (E), which is given in J/mm3 and is a crucial factor in SLM processes as it makes it possible to heat up and melt the powder. A schematic illustration of the melting of powder in SLM are given in Figure 3. The energy density is given by the equation,
E= P
v·h·t (1)
where P is the laser power [W], v is the scanning speed [m/s], h is the hatch distance [mm], and t is the layer thickness [mm]. A low energy input, resulting from low laser power and a relatively high scanning speed, can cause the powder to not melt entirely and lead to a discontinuous melt pool and the formation of fusion defects [21]. It is crucial to control the energy density since both too high and too low values can lead to defects in the material [22]. Defects are further explained in Section 2.1.4.
Figure 3: Illustration of melting of powder in SLM [14].
The process parameters influence the structure and material properties differently, and it is essential
to know how each of them affects the manufactured part. Caiazzo et al. [23] analyzed a material of Inconel 718, and found that the energy density affected the surface roughness. For the laser power, it was found in a study by Choo et al. [24] on 316 stainless steel that it affects the porosity, where a decrease in laser power leads to an increase in porosity. They also discovered that the texture went from a strong orientation to a random orientation and that the cellular spacing was refined.
The hatch spacing is defined by Xia et al. [25] as the distance between the center lines of two neighboring scanning paths, illustrated in Figure 4. They found that the hatch spacing influenced the surface quality of the SLM-processed components. The same result was also found by Marchese et al. [26], where a reduction in hatch spacing improved both the densification levels and hardness.
It was further found that an increase in the scan speed would suppress this effect. A high speed would lead to more shear stress in the liquid phase, generating higher surface tension and leading to a discontinuity in the laser scan track. Control over the layer thickness is also a necessity, as an overly thin layer leads to overheating, whilst a too thick layer can result in insufficient bonding with the previous layer [14]. With so many new parameters, a great deal of research has been done to optimize these for better results. Table 3 summarizes some significant results from the literature of how various process parameters affect the material quality of IN625.
Figure 4: Scan pattern with a pulse mode laser, showing hatch spacing and point distance [27].
Scanning Strategy
In addition to the laser parameters, the use of different scanning patterns and laser types are also essential factors to consider as it can result in different microstructures and properties (e.g., density, hardness, residual stress) [28]. Mostafa et al. [29] referred to the scanning strategy as the path a laser beam follows when scanning a layer and the angle between the layers. Li et al.
[30] found that due to the difference in thermal energy, the scanning strategy had an effect on defects such as porosity and inclusions, as well as the surface roughness. Two common types of scanning build strategy are pulsed mode and continuous scanning [8] [17]. For a pulse mode laser, parameters like exposure time and point distance must be considered, as shown in Figure 4. Here, the laser is held at a fixed point for a short amount of time before it turns off and rapidly moves a given distance to the next point, referred to as point distance. The process is repeated along the scan track. A smaller melt-pool and a higher cooling rate can be achieved by a pulsing laser, which results in a finer microstructure [27] [28].
Figure 5: Different scanning strategies. a) unidirectional, b) bi-directional, c) island scanning, d) spot melting, e) spot contours with bi-directional fill, and f) line contour with bi-directional fill [10].
There exist many different scanning strategies, some of which are shown in Figure 5. One of the most common strategies is the Contour-Hatch strategy, where a laser traces the contours of the scan while the area enclosed by the contours are filled with hatch scans. This is illustrated in Figure 6. Scanning parameters like laser power, spot size, and scanning speed, can be different for hatching and contouring. Ghouse et al. [28] analyzed how scanning strategies affected the mechanical performance. They found that the contour strategies gave the highest strength vs.
weight ratio, but that it was also material-dependent.
Figure 6: Illustration of the contour-hatch scanning strategy [28].
Powder Properties
Powder properties such as particle size, morphology, thermal conductivity, and liquid surface ten- sion will influence the final microstructure of the AM produced part. For SLM, the typical particle sizes are in the range of 10–60 mm [8]. Large particles can result in low resolution, while small
particles tend to agglomerate together due to Van der Waals forces [22]. Fine powder particles with uniform size distribution are desired because they promote homogenous melting, good interlayer bonding, structure, mechanical properties, and surface finish [8]. For PBF processes, it is possible to reuse the metal powder to reduce the cost, but as a result, the final part can end up with an irregular shape, and a poor surface finish [8].
Table 3: The effect of some process parameters on IN625 found in the literature.
Process Variables Process Parameters Experimental Value Result Source Surface Roughness Laser Power 60Wand 80W A lower laser power results in a higher
balling effect and rougher surface [30]
50-80Wand 85-95W
Surface roughness decreased from 3- 7µmfor a low laser power to 2-3µmfor a higher laser power
[31]
Hatch spacing <200µm An increase in hatch spacing led to a
rougher surface [30]
Scanning speed 500-2500mm/min Surface roughness is independent of
scanning speed [31]
Cell structure Laser Power 65Wto 95W
Primary arm spacing increased from 0.6µmto 1.6 µm with an increase in laser power
[31]
Heat Treatment 870◦C, 980◦C, and 1150◦C
870◦C and 980◦C had still dendritic structure, while 1150◦C was fully re- crystallized with random grain growth
[30]
Scanning speed 0.1m/s, 0.2m/s, 0.3m/s Longer columnar grains when decreas-
ing the laser speed [32]
Texture Laser power 400Wand 1000W A higher laser power results in a
stronger texture [33]
Hatch length 10:1 A tenfold increase in hatch length re-
duced the texture by a factor of two [34]
2.1.4 Defects and Residual Stress
Defects are present in all materials and can adversely affect the performance of the manufactured parts. They tend to occur during manufacturing, and the production methods have a significant impact on the outcome. For AM of metal parts, it is critical to control the parameters described in Section 2.1.3 to obtain dense material with few defects. This can be achieved by fully melting the powder, so it fuses with the previous layer, as well as the neighboring scanning tracks [35].
In general, by using a preheated bed, optimized process parameters, and performing stress relief annealing, defects in SLM parts will be reduced effectively [36]. Typical defects like porosity, surface roughness, residual stress, and texture will be explained in this section.
Porosity
As in traditional processing methods, porosity is a common defect in AM. According to Milewski [35], porosity is often resulting from either gas entrapped in the melt pool, which is released as bubbles upon cooling and solidification, or due to voids formed by other mechanisms such as lack of fusion. Variables like the size and depth of the melt pool are affecting the porosity. Therefore, parameters influencing this, such as laser power, scanning speed, and layer thickness, are essential to control and optimize [35]. Poulin et al. [37] found that the relation between scanning speeds and porosities is not linear, and the porosities increased when the scanning speed exceeded a particular value. An inert atmosphere is used to avoid contamination of the melt pool and entrapment of gases, but even then, argon, which is a gas used to attain an inert atmosphere, can be trapped in
the melt pool [8].
Figure 7 illustrates different types of porosity common in SLM. Milewski [35] found that since porosities are formed by different mechanisms, it is important to separate the porosity types to assure control. A lack of fusion voids is due to insufficient melting with neighboring tracks, inconsistent scan track, or inadequate penetration to the previous layer or substrate [8]. The lack of fusion voids can result from a low laser power and a large hatch spacing or deviation in scan speed [14]. Lack of fusion voids can be minimized using a large melt pool and small layer thickness [8]. The keyhole effect, which results from an excessive energy density (e.g., high laser power and low scan speed), is often desired in welding, but it can often create porosity in AM [22]. If the keyhole collapses, it will leave voids inside the metal where the vapor is trapped [35]. The balling effect is also a form of porosity where voids occur as a result of unmelted powder, and appears as irregular beads where the melt pool elongates and separates into small spherical balls. This is an undesirable effect. The balling effect can occur if the scanning speed is too high, the laser power is too low, or due to a combination of large layer thickness and insufficient penetration [8]
[38]. A study of the Ti6Al4V alloy produced with SLM showed that an increased scanning speed decreased the balling effect and spattering. However, this resulted in an increased micro-cracking due to shrinkage and gas trapped in the melt during solidification [21].
Figure 7: Four different types of porosity characteristic in SLM, due to (A) entrapped gas, (B) incomplete melting-induced, (C) lack of fusion with unmelted particles inside large irregular pores, and (D) cracks [14].
Surface Roughness
Surface roughness,Ra, has a significant influence on the tensile and fatigue properties according to Witkin et al. [39]. They found that a finer surface lead to improvements in the mechanical properties. Production with SLM gives a relatively lowRacompared to many other AM methods but still requires post-processing like machining or heat treatment. This tends to be both time- consuming and expensive but can, to a large extend, be avoided by improving laser processing [8]. Surface roughness is affected by parameters like hatch spacing, scanning speed, laser power, scanning strategy, and geometry [30]. According to Li et al. [30] porosity and balling spheres are responsible for the poor surface finish for As-Built (AB) IN625 samples. It was further found that a low laser power (60W) was more likely to generate a balling effect, which leads to a rougher surface finish. Moreover, a decrease in hatch spacing led to a smooth surface when the laser power and scan speed were constant.
Residual Stress
Residual stress is defined by Milewski et al. [35] as mechanical forces locked up within the structure of a part due to expansion and shrinkage. High residual stress results from a combination of high cooling rates and significant temperature gradients, which is unique for SLM-produced parts [40]. This effect includes rapid heating above melt temperature followed by rapid cooling and solidification when the heat source moves away. In the layer-by-layer fusion process, this is followed by re-heating and re-cooling [3]. In combination with the thermal gradient between the melt pool and the relatively cold substrate, or previously melted layer, the material is exposed to a high level of residual stress [36]. The high temperature gradient will affect the microstructure in a new way, which will be more explained in Section 2.2. It can also lead to micro-cracks, delamination, and distortion of the final part [41]. Residual stress affects the mechanical properties negatively, and can lead to geometrical distortions [10]. Residual stress can be controlled through the design of the part, process optimization, and post-processing. Poulin et al. [37] found that residual stress decreased with the increase of the substrates preheat temperature due to a lower temperature gradient. The laser heat input can be significantly reduced when the substrate is preheated, resulting in a reduced temperature gradient [36].
Texture
Process parameters are reported to influence the texture of the samples produced by AM. It is crucial to control the texture as it affects the mechanical properties [36]. Typical for SLM is a columnar microstructure with a strong texture with a [100] orientation parallel to the build direction [30] [36]. Kreitcberg et al. [42] found that this is generally the most favorable since the heat flow is more significant in this direction. Wei et al. [43] found that a high scanning speed, high power density, and low laser power are beneficial for columnar grain growth. However, the scanning strategy will also affect the texture [36].
DebRoy et al. [8] referred to studies where it is shown that components can have different textures even if the size and shape of the parts are the same due to different processing parameters. It was found by Li et al. [33] that laser energy input has a strong effect on the texture. A stronger texture was achieved using a laser power of 1000W instead of 400W. Another study by Wang et al. [44] also found that a higher laser power (of 200W, 2 kW, and 6 kW) resulted in a stronger texture.
2.2 Inconel 625
Inconel 625 (IN625), also known as Alloy 625 or UNS N06625 [45], is a nickel-based superalloy. It is often used in applications exposed to challenging environments, typically in marine, petrochemical, and aerospace industries [46] due to its high strength, corrosion resistance, and metallurgical stability at high temperatures [3]. Important alloying elements contributing to these properties are chromium (Cr), niobium (Nb), and molybdenum (Mo). The nominal composition of IN625 is given in Table 4. The alloy is challenging to manufacture by conventional methods due to its low workability and high hardness [36], but because of its excellent weldability [8], this material can be suited for production with AM technologies like PBF. However, there are some uncertainties how production with AM affects the microstructure and the mechanical properties of IN625, and how these vary with different heat treatments. Therefore, a deeper understanding is necessary. In Section 2.2.1, the microstructure, material properties, and different heat treatments are explained further to get an idea of how AM technologies vary from traditional processing methods and what to expect from the manufactured parts.
Table 4: Nominal chemical composition (wt%) of IN625 [47]. The amount given represents the maximum unless anything else is specified.
Ni Cr Fe Mo Nb + Ta C Mn Si Al Ti S P Co
Min.
58.00
20.00 -
23.00 5.00 8.00 - 10.00
3.15 -
4.15 0.10 0.50 0.50 0.40 0.40 0.015 0.015 1.0
2.2.1 Microstructure
Traditional Inconel 625
Inconel 625 is mainly designed as a solid solution strengthening alloy, where most of its strength comes from the high levels of Cr and Mo, which also provides corrosion resistance [48]. Elements like Nb and Fe provide further solution strengthening. However, due to containing a sufficient amount of alloying elements, the alloy can also experience precipitation hardening when heated [49]. This occurs after exposure to high temperature for an extended period of time, either in the form of intermetallic phases or carbides. There are three different types of carbides that can precipitate in IN625, which areM C, M6C, or M23C6 [48]. Typical intermetallic phases are the metastable body-centered tetragonal (bct)D022 N i3N b γ00 phase, the orthorhombicD0a N i3N b δ phase, and the hexagonal close packed (hcp) A2B N i2(Cr, M o) Laves phase [3] [49] [50]. An overview of the different phases is provided in Table 5.
The carbides in IN625 are either in the form of primary carbides such asM Cor secondary carbides in the form ofM6CandM23C6. The carbides tend to precipitate at the grain boundary [49], where M Ccarbides are often in the form ofN bC, either with a blocky shape or a dendritic morphology [48]. The secondary carbides can be challenging to distinguish from each other, but it is possible due to the differing Mo and Si content inM6CandM23C6[50]. The type to precipitate depends on the temperature, but as seen in the Time-Temperature-Transformation (TTT) diagram in Figure 9, they can form in less than 10 minutes. This is further explained in Section 2.2.3.
If an adequate amount of Ti, Nb, and Al is present,γ00can form as fine dispersoids. Sinceγ00has a bct-structure, it forms a mismatch with the fcc-structured Ni-matrix, resulting in a plate or disc- shaped form due to high coherency strains [48] [49]. According to Floreen et al. [48], precipitation
ofγ00can be significantly affected by minor variations in the chemical composition. Suave et al. [50]
observed thatγ00 were likely to nucleate at sub-grain boundaries, twin boundaries, or dislocation segments. The authors also noticed a change in the morphology during thermal aging where the phase transformed from spheres to ellipsoids.
While γ00 leads to strength contribution and enhanced corrosion resistance, Laves phase and δ phase tend to have a detrimental effect on corrosion resistance, and should therefore be avoided [3] [48]. δ is the equilibrium phase and given enough time, the metastable γ00 will transform to incoherentδprecipitates. According to Floreen et al. [48],δphase has a acicular morphology with a characteristic needle-like form and is therefore easy to identify [51]. Laves phase is harder to characterize due to the morphology being very similar to the block formed and irregularM6Cand M23C6 carbides [48]. Verdi et al. [51] observed that Laves phase contains a high amount of Nb, Mo, and Si, which are some of the elements promoting the formation of this phase. Due to the atomic structure, Laves phase is not plastically deformable and high hardness at room temperature [49]. Both δand Laves phases are dependent on the Nb-diffusion, and the precipitation kinetics are approximately the same, as can be observed in the TTT diagram shown in Figure 9 [48].
Table 5: Phases that typically precipitate in IN625.
Phases Structure Shape Composition Effect
γ00 bct Plate/disc or
spheres/ellipsoids N i3N b Increase strength and corrosion resistance
δ orthorombic Needle-like N i3N b Brittle
Laves hcp Irregular N i2(Cr, M o) Brittle
M C,M6C,M23C6 cubic Block shaped Complex
Twin boundary is often present in IN625 and is a phenomenon that can work as a strengthening mechanism, as well as increasing ductility. In the case of IN625, twin boundary is typically in the form of deformation twinning or annealing twins in the (111) plane [52]. Twin deformation can reduce the local stress concentration in a material by altering unfavorable crystal orientation.
The alteration is done by promoting interaction between twinning and slip to improve material placidity [53]. Figure 8 a) shows a typical microstructure of TM IN625 where arrows points at twins.
During welding of conventional produced IN625, segregation of solute elements during solidification is found to increase the chance of precipitating intermetallic phases, such asγ00,δ, and Laves phase [4]. Wrought IN625 typically experiences high-temperature deformation processes, such as forging or rolling, which breaks up dendritic solidification structure and accelerates recrystallization. This leads to the material typically having a homogeneous composition [3]. Traditionally wrought IN625 often exhibits an equiaxed grain structure [4].
AM IN625
Microstructure depends on chemical composition as well as thermal history. During production with SLM, the material is rapidly heated and cooled, and this will influence the material in a new way not yet thoroughly explored. Although welding and additive manufacturing are similar, both featuring melting and solidification of the material, the microstructure of these is expected to be different due to the difference in the thermal gradient and cooling rate. Where welded material
Figure 8: Microstructure of TM and AM IN625 samples taken in light microscope. a) shows the TM material where number 1 points at twins, while b) shows the melt pool boundary and cellular structure in a SLM produced sample [54] [55].
experiences hundreds ofK/s, LPBF, in contrast, goes through hundreds of thousands K/s [42].
Due to this rapid heating and cooling, AM parts tend to experience micro-segregation during solidification [56]. This affects the microstructure of as-built sample, and as illustrated in Figure 8 b), SLM-produced IN625 has a dendritic microstucture with both a cellular and columnar structure [57]. Mostafa et al. [29] found that the columnar structure is parallel to the build direction with a texture along [100] due to heat flow in negative Z-direction. Moreover, it was concluded that the AM materials differ from wrought parts by exhibiting anisotropic properties as a result of the layer-wise building method and the elongated dendritic substructure. There is typically no visible twin boundary in AM built parts due to the new way of forming material with rapid heating and cooling, and little deformation during manufacturing. Nevertheless, twins can form during annealing of the additive manufactured part [36].
Precipitates discovered in the as-built material are mostly nano-size and often found along the cell- or dendritic boundaries, but rarely determined [30]. Keller et al. [56] found that it is difficult to confirm the different precipitates in the as-built state due to high dislocation density and residual stress, which makes it difficult to index the phases in diffraction patterns using a Transmission Electron Microscope (TEM). There are, however, in multiple occasions observed segregation where the interdendritic regions are enriched in heavy elements such as Mo and Nb while the dendritic areas consist of Ni and Cr [3] [58] [13]. Along the melt pool boundaries, Amato et al. [59]
discovered precipitates enriched in Nb that were concluded to beγ00. However, even though this was investigated in TEM, Energy Dispersive X-Ray Spectroscopy (EDS), Selected Area Electron Diffraction (SAED) and X-Ray Diffraction (XRD) analysis, Dubiel and Sieniawski [58] were not convinced and found that the possibility for it to beδphase could not be eliminated.
Figure 9: TTT diagram for conventionally produced IN625 showing at what temperature and time the different phases will precipitate [48].
Grain Size
Grain size is one important parameter for the material properties because it affects the hardness, yield strength (YS), ultimate tensile strength (UTS), fatigue strength, and impacts the strength of materials. For a recrystallized alloy of IN625, Ferrer et al. [60] found that grain size was the main factor influencing the mechanical properties, and that second phases like γ00 had no significant effect. Similarly, the small amount and size of intergranular carbides andδ precipitates do not determine the final grain size of forged products. Moreover, they also found that the grain size affects the corrosion properties of the material.
The high tensile strength reported for the as-built specimen is closely related to the fine dendritic structure, often below 1µm, as well as the high dislocation density [13] [61]. Small grain size can result in higher strength and toughness of the alloy. This is evident in the Hall-Petch relation where the yield strength,σy increases with a decrease in grain size, D. The Hall-Petch equation is given as,
σy=σ0+ k
√D, (2)
whereσ0is the resistance to dislocation motion through a crystal [62], andkis a material-dependent constant, often referred to as Hall-Petch constant [63]. The mechanical properties are strengthened due to increased resistance to dislocation motion as a result of finer grains [64]. However, grain growth leads to higher ductility [13]. There are other factors contributing to the Hall-Petch equa- tion, and Harrison et al. [65] showed that an assumption of other parameters should be included when calculating the grain size for superalloys, where contribution from solid solution and temper-
ature are important.
2.2.2 Mechanical Properties
Traditional IN625
Inconel 625 is generally known for its high strength, toughness and corrosion resistance. This comes from a combination of solid solution and precipitation strengthening, which depends on the chemical composition as well as manufacturing method. Research has shown that various precip- itates and phases influence mechanical properties differently. Research shows that the formation of precipitates increases the strength and reduces the ductility of wrought IN625 [42] [45] [52].
According to Suave et al. [50], hardness and tensile strength are affected by precipitation ofγ00 andN i2(Cr, M o) phase, while toughness and ductility are highly influenced by the combination of secondary carbides such asM6CandM23C6, andN i2(Cr, M o) intermetallic phases.
Li et al. [33] found that precipitation of δ phase increases the hardness due to the incoherence with the matrix. Formation ofδphase leads to reduction in ductility and fracture toughness. This is considered a undesirable phase [4] [3]. The same is said about Laves phase andN bC carbides, which is also said to have a negative effect on the ductility. According to Wickens et al. [49], precipitates of Laves phase is said to have a more significant effect on the reduction of ductility thanN bC. Laves phase, however, can be eliminated by solution annealing. This is not the case for N bCcarbides, which are more stable and impossible to eliminate by conventional procedures once they are formed. Grain boundary carbides are said to have a detrimental effect on the mechanical properties, as well as on the corrosion resistance. Precipitation of the secondary carbidesM6C andM23C6 leads to a reduction in ductility and toughness. These can be eliminated by solution annealing [49]. However, according to Floreen et al. [48], some grain boundary carbides have a beneficial effect, like increasing the resistance to stress corrosion.
AM IN625
The mechanical properties of metals are strongly dependent on the microstructure. Since AM- produced IN625 has a different structure than traditionally produced IN625, it is expected that the mechanical properties will also be different. Some cases have reported that the AM part has similar or better mechanical properties as the conventionally produced part [26]. Thomas and Tait et al. [46] found that the yield stress and ultimate tensile strength for AM IN625 are comparable to wrought IN625, while elongation to failure is less than half of the wrought sample. It also showed high orientation dependency, so the build direction must be taken into consideration.
Texture in the material leads to anisotropy of tensile strength and other mechanical properties [36]. Kreitcberg et al. [42] found that due to this effect, the vertical specimen has the highest elongation but the lowest strength compared to the other orientations. Furthermore, they showed that the vertical specimens exhibit almost 4 times higher elongation to failure than their horizontal and 45°direction. In contrast, the 45°build direction has shown to have the highest UTS and YS of the as-built specimens. According to NORSOK M-630 for MDS-N130, tensile strength should be tested in XY, Z, and 45-direction, where the Z-direction should have an UTS≥690 MPa. Table 6 shows results from previous research of mechanical properties for IN625.
Table 6: Tensile properties of IN625 from literature.
IN625 UTS [MPa] YS [MPa] Elongation [%] Source
Wrought 955 482 41 [66]
Wrought 827 414 30 [67]
SLM as-built 925 652 32 [67]
SLM 900
◦C/1h 869 567 38 [67]
SLM 1100
◦C/1h 886 409 56 [67]
LPBF (XY-direction) 906 396 62 [20]
LPBF (Z-direction) 842 349 56 [20]
Li et al. [33] found that the hardness of as-built SLM part was 343 HV, while in comparison a forged sample had typically a value of 305 HV. It was observed that the hardness decreased at 700◦C due to the release of residual stress. However, after annealing temperature at 800◦C and 900◦C, it increased due to formation of δ precipitates. After annealing at 1000◦C the hardness reduced due to dissolution of the precipitates.
2.2.3 Heat Treatment
TM IN625
Precipitates in IN625 have displayed to have a complex behavior where they depend on the manu- facture method. Gola et al. [68] showed that for all production methods, both intermetallic phases and carbides tend to be present in the temperature range of 550◦C to 900◦C [68]. Figure 9 shows a model prediction of which temperatures and times these phases typically precipitate. Table 7 shows an overview from the literature of when the different phases are observed in AM and TM IN625.
As mentioned, three types of carbides can form in IN625, where the type to precipitate depends on the temperature during heat treatment. At higher temperatures, in the range of 870◦C and 1037◦C, carbides can be bothM6CorM C in form ofN bC. At temperatures between 705◦Cand 915◦C mostly M23C6 are present, where M is almost only Cr. All three carbides can be found after heat treatment at an intermediate temperature [48].
If exposed to elevated temperatures for a long period, conventional produced IN625 is known to precipitate both γ00 and δ phase [4]. Suave et al. [50] observed that γ00 were more present at 700◦Cthan at 650◦C, while at 750◦Cmostlyδphase was present. δis generally discovered in the temperature range of 650-900◦C, while γ00 precipitates between 550-750◦C. For wrought IN625, δphase does not precipitate before an extended time at elevated temperature. This is illustrated in a experimentally obtained Time-Temperature-Transformation (TTT) diagram shown in Figure 10.
Figure 10: Experimentally obtained TTT diagram for precipitation of δ phase in IN625 in AM and TM. Red lines referres to manufactureing by LPBF, while the black lines represent wrought IN625 [69].
AM IN625
The as-built condition of LPBF IN625 alloy is not suitable for practical use due to residual stress distortion, material inconsistency, and heterogeneous microstructure [40]. Therefore, a form of post-processing heat treatment is necessary to reduce these adverse effects and to achieve desired mechanical properties [70]. Good control and knowledge of the process is essential as heat treatment can also lead to undesired effects, such as unexpected intermetallic phases and carbides, such as for TM IN625 [3]. As explained earlier, theδphase, which negatively affects the mechanical properties, tends to precipitate in interdendritic phases during annealing. Figure 10 shows a TTT diagram of whenδphase has been observed for LPBF compared to when the same phase is seen in wrought IN625.
There exists recommended post-processing heat treatment for traditionally produced IN625. How- ever, as the heating and cooling cycles for LPBF IN625 results in a more segregated microstructure, there is a need to find another heat treatment for AM parts to get the desired microstructure [69].
For example, for conventionally produced IN625, annealing in the temperature range of 900–980
◦C results in complete recrystallization. In contrast, only a small amount of recrystallization is observed in the LPBF IN625 after annealing at 980◦C [34] [40]. Complete recrystallization of a SLM sample were observed by Li et al. [30] after annealing at 1150◦C. The authors observed that there was also grain growth in a random direction at this annealing temperature. The formation of nearly equiaxed grains in addition to twins were also observe. Moreover, a high level of residual stress was found to be released due to the formation of annealing twins.
For conventionally produced IN625, stress-relief is completely achieved when heated to 870◦C, and solution treatment occurs between 1093 ◦C and 1204 ◦C [47]. For SLM IN625, there exist different heat treatment recommendations. Stress relief annealing is performed to reduce residual stress and decrease distortion without significantly affecting the grain structure, where the stress relief temperatures range from 650°C to 870°C (ASME grade 1). Lass et al. [4] found that an abundant amount of δ phase precipitated when following the recommended stress-relief heating treatment of 1 h at 870°C. Zhang et al. [3] also found that δ phase formed at the same SR of 870°C at 1h. Moreover, they discovered that the precipitation kinetics depended on the chemical composition, where elements such as Nb were significant. An equilibrium isopleth phase diagram was calculated, showing the mass fraction of niobium between 3-10% against various temperatures.
This is illustrated in Figure 11, where it is seen that at lower SR temperatures, more phases tend to form in the material.
Figure 11: Equilibrium isopleth diagram of IN625 showing the mass fraction of Nb in % against temperature. The dashed lined shows an average composition of 3.75 % Nb. [3].
Recrystallization annealing (soft annealing) aims to reduce anisotropy by producing a recrystallized grain structure with a temperature from 930◦Cto 1040◦C(ASME grade 2) [42]. Thomas and Tait [46] found that after Stress Relief (SR) and Soft Annealing (SA), the preferred texture for IN625 became weaker. This was also confirmed by Kreitcberg et al. [42] where it was discovered that a higher temperature heat treatment resulted in larger grains and lower anisotropy. However, they also found that a higher temperature led to higher elongation, but lower strength.
Table 7: Overview of at which temperatures different phases are present at in AM and TM.
Phase Temperatures discovered Source
AM TM
γ00 700-850◦C 550-750◦C [33] [4] [50]
δ 800-1000◦C
870◦C from 0.5 to 8h
750-980◦C 700◦C for 3000h 650-900◦C
760◦C, 871◦C for 48h - 1093◦C for 24h
[33] [4]
[54]
[3] [50]
[48]
Laves 760◦C, 871◦C for 48h - 1093◦C for 24h [48]
Carbides 1000◦C
still present at 1150◦C
Dissolved at 1093◦C for 1h
(Grain boundary carbides) [33] [48]
M C, M6C 870-1037◦C [48]
M23C6 705-915◦C [48]
3 Experimental Procedure
This chapter contains information of the materials used in this thesis, including an explanation of the experimental work conducted and details of the different processes used to produce the test samples. Figure 12 shows a flow chart which gives an overview of the experimental work. Some of the procedures are also conducted in the specialization project [1], therefore some information is similar. However, new material is used, and new tests are performed.
Figure 12: A flow chart showing the order of the experimental work conducted in this thesis. The upper figure refers to the AM metal parts, while the lower figure represents the tests performed on the powders.
3.1 Materials and Methods
3.1.1 Material Selection
In this master thesis, three different types of IN625 materials were studied. One traditional man- ufactured reference sample supplied by Frank Mohn (Framo), and two AM materials from DMG MORI and EOS GmbH, where both AM samples are produced with SLM technology, also called LPBF. However, in this report SLM will be referred to. The chemical composition of the materials is presented in Table 8, and Figure 13 shows a picture of the different samples tested.
Table 8: The chemical composition of the samples given in wt%. Framo shows the actual amount while DMG and EOS are based on the DIN EN 10095 standard. For EOS and DMG, the values in the table represent the maximum amount.
Ni C Cr Mn Mo Si Co Ti Al P S Fe Ta Nb
Framo 61.4 0.01 22.10 0.13 8.34 0.27 0.008 0.28 0.22 0.008 0.001 3.70 0.0063 Cb(Nb): 3.440 Nb+Ta: 3.446 DMG bal. 0.05 20.0-23.0 0.5 8.0-10.0 0.5 0.1 0.4 0.4 0.03 0.015 5.0 0.05 3.15-4.15 EOS 58.0-bal. 0.1 20.0-23.0 0.5 8.0-10.0 0.5 1.0 0.4 0.4 0.015 0.015 5.0 0.05 3.15-4.15
The AM samples received in this thesis are in the shape of small cylinders as seen in Figure 13 b) and c). These were machined to tensile specimens, while the blocks and big cylinders in the same pictures are used in a another master thesis focusing on corrosion [2]. The AM samples were supplied by Fieldmade.
The TM sample is a forged cylinder produced by Framo. The material has been annealed at 1020°C for 1.5hand water cooled according to the Material Data Sheet (MDS) (Appendix A. Figure 13 a) shows the TM sample where both the tensile- and Charpy specimens were machined from the big cylinder. These samples were tested in the specialization project, but the material were further used for characterization in this thesis.
Figure 13: The materials tested in this thesis, where a) is TM material from Framo, b) is AM samples from DMG, and c) is AM samples from EOS. The thin cylinders in b) and c) are tested in this thesis, as well as the tensile- and Charpy specimens in a).
To make it easier to separate the different samples, they are given new names by which they will be referred to in the rest of the thesis. The naming is illustrated by the following equation,
