Direct Laser Metal Deposition (DLMD) Additive Manufacturing (AM)of Inconel 718 Superalloy: Elemental, Microstructural and Physical Properties Evaluation

In this study direct laser metal deposition (DLMD) technique is adopted for the additive manufacturing (AM) of Inconel 718 Superalloy. To conduct the experiments, a 1 kW fiber laser with a coaxial nozzle head is used. The effects of scanning speed (for two values of 2.5 and 5 mm/s) as well as powder feed rate (for two values of 17.94 and 28.52 g/min) on the process were investigated. Characteristics of the 3D printed wall specimens such as the geometrical dimensions (width and height), microstructure observations, and the microhardness were obtained. In order to study the stability of the 3D manufactured walls, the height stability was considered for the investigation. Optical microscopy (OM), field emission electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and mapping analysis were performed to derive the microstructural features of the additive manufactured samples. The Vickers microhardness test is used to evaluate the hardness distributions of additively manufactured parts. Catchment concept of the powder in DLMD process is used for explaining different trends of the process. Results indicated that, by decreasing the scanning speed, the width and height of the deposited layer increase. The average width of the additively manufactured samples directly depends on the scanning speed and the powder feed rate. Scanning speed has a reverse effect on the height stability; that is, the lower the scanning speed, the larger the stability. Microstructural results showed that because of the solidification process, the alloying elements will be accumulated in the grain boundaries. The non-uniform cooling rate and non-steady solidification rates of molten area in additive manufacturing process, the microhardness values of the additively manufactured samples following a fluctuated trend.


Introduction
The processes for additive manufacturing (AM), also referred to as 3D printing, have impressive improvements in recent years and companies and institutions have made a fairly competition for expanding the useful aspects of this technology, such as cost, complex geometries, replacement and maintenance [1][2][3][4]. With the development of specialized facilities in AM, and providing superb conditions for the manufacturing the high accuracy and affordable products, it has attracted the attention of many scholars and industries [5][6][7]. The AM process is also referred to layer production [8][9][10]. This method is one of the fastest and most reliable manufacturing processes, which is used to create a high quality and accuracy new sample [11]. 3D printing has many advantages over traditional production methods [12]. The production of complex geometries by traditional manufacturing methods is very sophisticated and is not affordable in many cases.
A wide range of materials are applied for AM approaches such as plastics, metals, ceramics, and composites [13][14][15][16]. The AM technology is divided into different categories based on their approach of printing, equipment or printer devices, materials and etc [17][18][19][20]. The laser device is one of the most useful equipment in the industrial applications that have a significant effect on properties of additively manufactured parts [21][22][23][24].
The laser AM method is a modern and evolving process, which is capable of many complex components by using a variety of powders, including metals, nonmetallics and composites [25][26][27][28]. In AM process by laser, the layers are produced by various input parameters [29][30]. Recently, extensive research has been done in this field of science. Kong et al. [31] investigated the effect of deposition created by laser AM process on non-traditional machining via a high-throughput dual-feed system. Wolff et al. [32] evaluated laser AM by twin feeders where Inconel 718 was used as powder. The goal of this study was that how the cooling rate after AM process can be affected on additively manufactured additively manufactured sample's structure. An experimental study from the AM process of Inconel 718 which was shown the sidewall nonuniformity and deposit bulge was performed by Lee et al. [33]. The quality and dimensions accuracy of additively manufactured specimens were studied in references [34][35][36][37][38]. The results showed that this experiment provided some additively manufactured samples with a suitable surface quality. A similiar research about the quality and microhardness trend was conducted by Liu et al. [39]. It is worth mentioning that the Inconel 718 material is one of the most used materials in the manufacturing industry.
Shang et al. [40] joined two parts of multi-material objects by the laser AM process. Due to the joining parts of AM process some parts were joined very well.
Caiazzo et al. [41] generated some layers by changing nozzle scanning speed and laser power in the Directed Metal Deposition (DMD) method. These parameters have a critical role in AM process because by controlling these inputs the samples were generated very well. Liu et al. [42] investigated Laser Powder Deposition (LPD) method by using the AlSi10Mg alloy and Taguchi approach for the optimization of AM process. Wang et al. [43] studied microstructure of the deposited layer of additively manufactured samples. DLAM method in the laser AM process for biomaterials based titanium and molibden (Ti-15Mo) was studied by Bhardwaj et al. [44]. With this approach, they printed some samples which are usable in human surgery parts. Momenzadeh et al. [45] investigated the simulation and analysis of specific implant, which was printed by the laser AM process. AISI 304L is a very popular stainless steel in the manufacturing process.
This material was joined to a Ti-6Al-4V with the laser AM process by Reichardt et al. [46]. The deposition layer from the laser AM process via high power diode laser by AISI 316L powder was investigated in Guo et al. [47]. The microstructure of additively manufactured parts shown that the stability between interfaces of layers was a superb joint together.
As the above review shows, the relation between direct laser metal deposition and microstructure effects after AM of super alloys have been addressed.
However, there are still many areas which have not been considered. By conducting some research, it is clear that, the behavior of the AM process on the microstructure of Inconel 718 is very sophisticated because the cooling rate and controlling the input parameters for generating a high-quality structure is very complicated. Inconel 718 is one of the much usable superalloys which its behavior from the procedures for material processing is very important in industry applications. In the present study, the DLMD process is applied for AM of Inconel 718 nickel-base super-alloy deposited wall on the AISI 4130 alloy steel substrate using a 1kW fiber laser. The effects of powder feed rate and scanning speed on geometrical aspects (i.e. height, average of width), microstructural observations and microhardness trend of additively manufactured samples in different layers are investigated. The results are explained by powder catchment ratio concept in DLMD process. Considering the powder catchment ratio concept and studying the geometrical stability of the additively manufactured walls, are relatively new accompaniments to this field.
The organisation of the remainder of this paper is as follows. In Section 2, experimental study of DLMD process and some parameters are discussed such as catchment, stability rate, which are considered as outputs. In Section 3 and 4, results from measurements and conclusions are discussed, respectively.

Experiment materials, apparatus and configuration
Commercial Inconel 718 powder was used. The particle size of the powder was imaged with a field emission electron microscopy (FE-SEM) (MIRA lll; TE-SCAN) and is shown in Figure 1. As can be seen, the particle size of the powder is in the range of 45 to90 μm. Due to previous studies and research the size of powder must be in this range. When the size of a particle smaller or larger than 40 to120 μm, the quality of additively manufactured additively manufactured samples may be decreased. The chemical compositions for the Inconel 718 powder and the AISI 4130 steel are given in based on regulated CNC profram, the powder is deposited by the laser beam, and the additively manufactured sample is constructed as shown in Figure 1. The powder supply on the AM process is a twin powder feeder system. The powders are carried out by gas pressure transfers into the feeder and powder inside the vessel. Ar gas was used as a shielding gas as well as the powder carrier gas. In all experiments, axial gas pressure was3 l/min, powder carrier gas pressure was1.5 l/min and 250 W laser power were considered as constant parameters. The focal plane position (FPP), the number of deposition layers (5 layers), the dwell time of the deposition layers and the continuous laser wave are also considered as fixed parameters. In addition, two parameters of this process, the powder feed rate and CNC table scanning speed are variable. Table 2 presents the values of process input and output parameters.

Sample analysis techniques
After the AM process, the specimens were cut by the wire cut, and their crosssections were etched according to ASTM E 407: 07 by Glyceregia electrochemistry for the metallographic process [49]. The Ws in Table 2, show the width of additively manufactured additively manufactured samples in three areas, top, middle, and bottom of additively manufactured additively manufactured samples, respectively.  Figure 3 and Figure 4 show, respectively, the additively manufactured samples and their geometrical dimensions which are made by the DLMD method.
In Figure 3, some parts of additively manufactured additively manufactured are divided for understanding the concept of the height of samples. Also, in Figure 4, the concept of width in their zones for four samples are illustrated.   Figure 4). The stability is a modern approach, and it is a suitable criterion for the height quality of deposited layers. The length of the deposited layer is divided into three sections. The maximum and minimum heights are measured, and the differences of the heights are calculated. The maximum difference is defined as stability value. A lower stability rate can be more suitable for this process because the lower amount of stability means that the differences between minimum and maximum heights are less, as is evedient from Figure 3.
The equations related to the calaution of stability value are shown in followings: The microstructure images in this study were captured by using and optical microscope (Device model: RADICAL model RMM-2) and FE-SEM. ImageJ software was used to analyse the geometric dimensions of the specimens. The Vickers microhardnes for samples were evaluated via BUEHLER models. The specimens were tested with a loading rate of 100 g for 30 seconds on the microhardness test. Powders feed focus point, spray bandwidth, and powder flow feed rate measurements were examined to characterize the powder feed conditions. To determine the powder focus point, the axial gas feed rate, powder carrier gas feed, and powder feeder rotation speed must be adjusted. This is vital because the powder particles fed from the four outlet channels converge at one particular point. The parameters of scanning speed, distance of laser head to the substrate and the rotation speed of powder feeder were changed to evaluate the effect of parameters. The spray bandwidth is measured by three times along the path. The mass feed rate is defined as the feed rate of the powder passing through the powder outlet channels of the twin powder feeder. Powder mass feed rate, gas pressure, and disk powder velocity were varied to measure the mass feed rate.
The lens of the fiber laser device has a ±4 mm displacement, which is related to sample position. The focal point position is at the highest point on -4 mode. Also, at the zero points the focal point is on the substrate [15].

Catchment
The concept of catchment in AM has an applied aspect because it shows the absorption of powder components. According to equation (5), catchment is dependent on the powder feed rate and laser scanning speed. It is expected that the catchment factor increases by increasing the powder feed rate, but the powder feed rate has a turning point. It means that the powder feed rate increases to a certain extent, but suddenly it makes a situation for absorbing more powder, and in this stage, the amount of catchment declines [50].
Generally, the samples' geometry and height are dependent on three input factors; namely, powder feed rate; laser scanning speed; and laser power. The melt pools can absorb more powder by increasing the powder feed rate; however, the height of samples sometimes does not change by increasing the powder feed rate because the laser power is low and the situation for absorbing more powder is not provided in the melted pond. In the specific circumstances, the melt pool, which is created by laser irradiation, can receive and absorb more powder by increasing the powder feed rate. When the powder feed rate and the scanning speed are regulated on a constant amount, generated energy can attend to absorb a limited amount. This phenomenon means that, when the powder feed rate is in the highest rate, it cannot add any more powder to the melt pool. In addition, the uncompleted melting of powder affects samples' height because the input energy is low. Also, Cong et al. [51] showed that when the powder feed rate changed from 0.1 gr/min to 0.4 gr/min, the melt pool caught more powder and finally the height of samples increased. However, the samples' height may still remain constant until the powder feed rate goes to a proper rate. This is because the laser power and scanning speed are regulated on the constant amount and the rate of powder which is absorbed by energy absorption has been limited. The focus on issues of final surface is one of the most critical limitations in the DMD process.
The difference between the roughness and waviness as two specific factors in modifications of surface is proposed. Gharbi et al. [52] conducteded a research which showed that tiny additively manufactured layer and bier melt pool improved the quality of surface. Also, it was observed that by increasing the interaction between the powder feed and laser beam, more powder would be absorbed that is one of the most beneficial effects for the final surface. When the amount of catchment is at a high rate, the interaction between powder and laser beam increases, and finally, it makes a surface with improved quality.

Effect of catchment parameters on the wall height
According to Equation (5) Figure 5 shows the effects of powder feed rate and scanning speed on the additively manufactured wall height. It is clear that by decreasing the scanning speed when the power rate is considered as a constant parameter, the height of the samples increased. This phenomenon completely depended on the time for the additive process which more powder melted and the hight increased.

Effect of catchment parameter on the wall width
The maximum additively manufactured wall width is 1086 μm, which was observed at minimum scanning speed of 2.5 mm/min. It was found out that the lower the scanning speed the higher would be the wall width of additively manufactured samples. At a low scanning speed, the powder particles that are interacted and melted by laser beam would have more time to wider deposition, causing an increase in the wall width. The minimum wall width was 820 μm that was observed at minimum scanning speed of 2.5 mm/min. With increasing in the powder feed rate, much more powder particles feed coming out from nuzzles and it caused that, more powder interacted with the laser beam and melted. Therefore, the wall width increased. The maximum wall width of additively manufactured samples was observed at powder feed rate of 28.52 g/min.

Elemental and microstructural characterization
The ϒ phase is the base phase in Inconel 718 superalloy called ϒ Matrix. In this superalloy, some of other phases such as ϒ ′ , ϒ ′′ , δ , Metallic Carbides (MC) and Laves phases are also generated. The ϒ ′ and ϒ ′′ phases are the main reinforcement phases and those are coherent with ϒ phase. The ϒ ′ phase composition is Ni 3 (Al-Nb-Ti) and the ϒ ′′ phase composition is Ni 3 Nb [53]. The Laves phases are not desirable in terms of mechanical properties, but the heat treatment can dissolve Laves phases into the matrix. According to Figure 6, the ϒ phase is dispersed uniformly in all over the sample microstructure and it is the gray phase. Also, from Figure 6, it is found out that the most part of the additively manufactured sample microstructure is the ϒ phase. The white areas in Figure 6a are the Laves phases that are in irregular shapes and dispersed non-uniformly.
The Laves phases are precipitated into the grains and in the grain boundaries. The dark-gray areas in Figure 6-b & c are the ϒ ′ phase.   The Map analysis provides the frequent distribution of elements in an image.
In any area where the color intensity is higher, it indicates that the percentage of the element in that area is higher. Figure 8 shows the Map analysis of sample  The grain morphology of Inconel 718 is diverse in different regions.
Columnar dendrites, cellular dendrites, cells, and equiaxial dendrites with Laves phases are observed in the interdendritic areas. Generally, the grain growth was epitaxial, but in some areas, the dendritic growth was observed. The dendrites were grown along of the deposition layer direction. The Microstructure of Inconel 718 is columnar dendrites growing epitaxially along the deposition direction. Figure 9 shows the FE-SEM images for samples number 1 to 4. The columnar growth was observed, and in some areas, the grains were more extensive and more stretched. The orientation of grains changed in the areas near to the substrate. The substrate acts as a heat source for the first layers of deposited wall, and large columnar dendrites were grown. When laser energy melted the new layer, it simultaneously affected an earlier layer and caused grain growth in previous layer. The grain growth would occur due to increase in temperature and reheating the layer. Figure 10 showed the growth in grain size in the overlap zone of two layers. The heat input of the laser beam from the upper layer is affected by the interfaces of layers, and the grains are grown. Figure 11 shows the microstructure of samples number 1 and 4. The laser power was 250 W. Thus, the heat input changed due to the alteration of scanning speeds. When adding melted upper layer to the hardened layer or lower layer, the solidified layer was affected by heat input caused from the upper layer. Therefore, grains of lower layers were grown. A high laser power leads to a deeper melt pool, whereas a low scanning speed causes much more powder particles to be deposited in the melt pool. Therefore, at low scanning speed, strong joints between layers are created, and it will avoid cracking and separating in the interfaces. In Figure 12, the equiaxed dendritic microstructures at the surface of deposition were illustrated. With increasing in the additively manufactured wall height, the grains orientation in top section of samples is differed from the parts, which are near to the substrate. The columnar microstructure with equiaxed dendrites showed that changes in the microstructure is caused by the changing solidification conditions at top of the deposition. In the top sections of samples, the columnar grains and equiaxed dendrites were created due to low-temperature gradients and high solidification speed [54][55][56].
The top of the workpiece after the deposition of the melted substrate could be quenched by air; however, the bottom of the melted substrate is quenched by thermal transformation to the lower layer, which is addetived before.  are the usual defects that may happen during the AM process. In Figure 13, it is shown that the closed and open pores are the black spots. The porosity of the additively manufactured samples is due to the powder quality and excessive pressure of shielding gas flow [57]. The powder particles quality depends on powder production methods. The powders produced by PREP and GA methods are better powders than the other powder production methods for reducing the porosity in the additively manufactured samples. In this research, Inconel 718 powder particles were gas atomized and made lower porosity in the additively manufactured samples. during the AM process. It is due to the heat input from upper layers that affected the lower layer, and changed the microstructure, grain size as well as microhardness. In other words, the heat input from the upper additively manufactured layer led to an increase in microhardness of lower additively manufactured layers. It is worth mentioning that the morphology and concentration of the Laves phase were found to be the most critical factors in the microstructure of Inconel 718 alloy. On the other hand, as mentioned in section 3.2, the Laves phase is not a hard phase. The Laves phase is the Nb-rich phase in the ϒ matrix (see Figure 6 and Figure 8). Accordingly, the precipitation of the Laves phase reduced the microhardness of the additively manufactured samples.

Conclusions
In this study, the influence of two effective parameters on the direct laser metal 3. Height stability is an important parameter in measuring the quality of the additively manufactured samples. A low ΔH value means that the difference between the highest and the lowest heights of the sample is lower. The lowest ΔH, which has proper stability, is obtained in the less scanning speed. 4. Change of microhardness in the samples does not follow any specific trend.
The microstructural changes along with the height of a sample by DLMD.
The heat input from the upper layers that affected the lower layer and the laser heat input acts as a heat treatment. 5. Various phases with different grain growth morphology in different parts of additively manufactured samples are generated which cause fluctuiation in microhardness regims. 6. In DLMD of Inconel 718 superalloy, during the solidification process, the alloying elements such as: Nb and Mo, will be accumulated in the grain boundaries. The Laves phases are Nb and Mo rich phases, thus the Laves phases will be precipitated in the grain boundaries. The Laves phases are not desirable in terms of echanical properties.

Conflict of Interest
Potential conflict of interest exists: We wish to draw the attention of the Editor to the following facts, which may be considered as potential conflicts of interest, and to significant financial contributions to this work: The nature of potential conflict of interest is described below: No conflict of interest exists.
We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Funding
Funding was received for this work.
All of the sources of funding for the work described in this publication are acknowledged below: No funding was received for this work.

Intellectual Property
We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

Research Ethics
We further confirm that any aspect of the work covered in this manuscript that has involved human patients has been conducted with the ethical approval of all relevant bodies and that such approvals are acknowledged within the manuscript.
IRB approval was obtained (required for studies and series of 3 or more cases) Written consent to publish potentially identifying information, such as details or the case and photographs, was obtained from the patient(s) or their legal guardian(s).

Authorship
The International Committee of Medical Journal Editors (ICMJE) recommends that authorship be based on the following four criteria:

Contact with the Editorial Office
The Corresponding Author declared on the title page of the manuscript is: This author submitted this manuscript using his/her account in EVISE.
We understand that this Corresponding Author is the sole contact for the Editorial process (including EVISE and direct communications with the office). He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs.
We confirm that the email address shown below is accessible by the Corresponding Author, is the address to which Corresponding Author's EVISE account is linked, and has been configured to accept email from the editorial office of American Journal of Ophthalmology Case Reports: We understand that this author is the sole contact for the Editorial process (including EVISE and direct communications with the office). He/she is responsible for communicating with the other authors, including the Corresponding Author, about progress, submissions of revisions and final approval of proofs.
We the undersigned agree with all of the above.  Schematic diagram showing the DLMD con guration [48]. The macro size images of additively manufactured samples Figure 5 The effect of powder feed rate and scanning speed on wall height. Inconle 718 addetived structure a) γ phase b) γ' phase c) Laves phases The EDS analysis in 5 layers (L1-L5) of addetived sample #3  FE-SEM of samples number 1 to 4.

Figure 10
The common boundary between the deposit layers.

Figure 11
The common boundary layer of samples number 1 and 4.

Figure 12
Top section microstructure of the additively manufactured samples for samples #1 to #4 Figure 13 Defects in the microstructure of the addetived samples