Effect of energy density and scanning strategy on densification, microstructure and mechanical properties of 316L stainless steel processed via selective laser melting

Journal Pre-proof Effect of energy density and scanning strategy on densification, microstructure and mechanical properties of 316L stainless steel processed via selective laser melting Taban Larimian, Manigandan Kannan, Dariusz Grzesiak, Bandar AlMangour, Tushar Borkar PII:

S0921-5093(19)31241-9

DOI:

https://doi.org/10.1016/j.msea.2019.138455

Reference:

MSA 138455

To appear in:

Materials Science & Engineering A

Received Date: 6 June 2019 Revised Date:

24 September 2019

Accepted Date: 25 September 2019

Please cite this article as: T. Larimian, M. Kannan, D. Grzesiak, B. AlMangour, T. Borkar, Effect of energy density and scanning strategy on densification, microstructure and mechanical properties of 316L stainless steel processed via selective laser melting, Materials Science & Engineering A (2019), doi: https://doi.org/10.1016/j.msea.2019.138455. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Effect of energy density and scanning strategy on densification, microstructure and mechanical properties of 316L stainless steel processed via selective laser melting Taban Larimian1, Manigandan Kannan2, Dariusz Grzesiak3, Bandar AlMangour4, Tushar Borkar1* 1

Mechanical Engineering Department, Cleveland State University, Cleveland, Ohio, 44115 2 Mechanical Engineering Department, The University of Akron, Akron, Ohio, 44325 3 Department of Mechanical Engineering and Mechatronics, West Pomeranian University of Technology, Aleja Piastow 17, Szczecin, Poland 4 Saudi Arabia Basic Industries Corporation, P.O. Box, 11669, Jubail 31961, Saudi Arabia

* Corresponding authors: Tushar Borkar: E-mail address: [email protected] Tel.: +1 216 687 2568; Fax: +1 216 687 5375

1

Abstract Laser-based additive manufacturing opens up a new horizon in terms of processing novel alloys that are difficult to process using conventional techniques. Selective laser melting (SLM) is a powder bed fusion type additive manufacturing (AM) process, for fabricating metallic parts where powder particles are fused using a high energy laser beam as a thermal source. Although SLM is widely used for manufacturing end-use metal tools and components, it requires careful tailoring of a range of parameters (e.g. layer thickness, laser spot size, laser power, hatch spacing, scanning strategy, etc.) to achieve the required densification, microstructures, and mechanical properties. Therefore, there is a critical need to systematically investigate the effect of these processing parameters on densification, microstructures and mechanical properties of materials. In this research work, 16 samples fabricated with SLM process by varying processing parameters have been investigated. We have studied the effect of scanning speed, scanning strategy, and energy density on microstructure and mechanical properties of these samples by performing microhardness tests, tensile tests, and a scanning electron microscopy (SEM) analysis. We have concluded that samples fabricated with alternate hatches and single pass of a laser beam exhibited highest densification and most refined microstructure. Furthermore, samples processed at higher scanning speeds had better densification, as well as excellent mechanical properties. We have also observed an increase in the width of dendrites as a result of decreasing the scanning speed primarily due to decrease in cooling rate.

Keywords: Additive Manufacturing (AM), Selective Laser Melting (SLM), 316L Stainless steel, Microstructure, Mechanical Properties.

2

1.

Introduction Additive manufacturing (AM) is a process in which 3-dimentional (3D) objects are

created by adding and joining material together layer by layer using a computer aided design (CAD) file of the model to be printed. Additive manufacturing processes have been around for nearly three decades and are capable of building 3D parts using variety of material. Selective laser melting (SLM) is an additive manufacturing process, which uses a laser beam as a thermal energy source to selectively melt metallic powder that have been spread in the form of thin layers (50 to 100 µm) on a metallic substrate [1][2]. Laser beams scan the powder bed and melt selected portions of the powder, based on the CAD data, and melted powder particles fuse together and then quickly solidify. After completion of one layer, the base platform goes down by one-layer thickness and a new layer of metal powder is spread and the laser beam scans the next cross section of the part to be built. This process continues until all the layers are fabricated and the complete 3D model is built. The surrounding powder (un-fused powder) which act as a support material for subsequent cross-sections is then removed, part is taken out, and further postprocessing steps are performed if required [3] [4]. Due to significant development in additive manufacturing, AM parts are no longer used only as prototypes, but the SLM-processed parts are now used as a functional end-use product, with little or no post processing, which are almost fully dense and have complex shaped geometries [5][6][7]. The SLM processing parameters (such as layer thickness, laser spot size, laser power, hatch space, scanning strategy, scanning speed, energy density, etc.) play an important role in tailoring microstructure as well as mechanical properties of 3D printed parts. The 316 stainless steel exhibits excellent corrosion resistance as compared to other steels, primarily due to a passive layer of chromium oxide, which prevents the material from further oxidation [8]. Stainless steel is widely used in industries such as aerospace, automotive, biomedical, etc. [5]. SLM-processed 316 stainless steel is specifically used in dental caps, ultralight structures, chirurgical devices, etc. due to its excellent corrosion resistance as well as biocompatibility, which is also the reason for its wide use in patient specific customized implants [1][5]. In order to have better mechanical properties we need to optimize the processing parameters. In the past, some research works have been performed regarding the effect of

3

processing parameters on selective laser sintering (SLS) as well as selective laser melting (SLM) processed parts. Hao et al. [9] focused on the effect of scanning speed, energy density, scanning strategy, and laser power on mechanical behavior of 316L stainless steel processed via SLS. From their work it was found that scanning speed, scanning strategy, and laser power are the most effective processing parameters for fabrication of a part. It was also discovered that height of the sintered layers can be highly affected by the scanning speed and laser power. For instance, with the same laser power, the height of each sintered layer would increase with decreasing of the scanning speed; with the same scanning speed, height of each layer increases as the laser power rises. Hao et al. [9] also discovered that particle fusion increased with increase of laser power which resulted in decreasing of porosity. Kong et al. [10] have investigated the effect of laser power on SLM-fabricated 316L stainless steel and observed improvement in bio compatibility and corrosion resistance as well as elongation with increasing laser power. Liu et al. [11] observed that the number of sintered layers can also affect the SLM-fabricated part. By doing numerical and experimental investigations Liu at al. [11] discovered decreased in both heating as well as cooling rates of the molten pools by increasing the number of layers. Moreover, the dimension of the molten pool increases with increasing the number of layers which leads to formation of coarser grains. Simchi and Pohl [6] have investigated effects of processing parameters for microstructure and density of iron-based alloys fabricated by SLS. From this research it was revealed that there was an optimum value for which the densification rate of the sintered parts would be maximized. Meaning that the densification increased with increasing the energy input until it reached an optimum value, after which the densification rate started to decrease. Furthermore, it was observed that scanning pattern can significantly affect the density of the final product, as increasing the scan space while keeping the other parameters constant can reduce the density. Klocke et al. [12] also stated that decreasing the energy density reduces the balling defect in SLS-processed parts, causing more dense products. However too low energy density values will result in delamination of the fabricated parts from the platform due to unfavorable wetting characterization[13]. Yakout et al. [14] showed that there was an optimum value for energy density in SLM process below which balling defect would cause void formation and above which vaporization will occur. Choong et al [13] also discovered an optimum energy density for obtaining highest density as well as lowest surface roughness by manipulating the amount of energy density. Simmons et al. [15] found a critical scanning speed

4

for fabrication of 316L stainless steel by SLM above which the porosity would increase drastically as a result of incomplete fusion. Hao et al [5] have compared microstructure and mechanical properties of 316L stainless steel and stainless steel/hydroxyapatite composite processed via SLM. From this work it was indicated that although higher laser power may result in poor surface quality, by increasing it and consequently increasing the melting level, density and mechanical properties would improve. It was also found that there is an optimum value for scanning speed for which the tensile strength is the highest (meaning that reduction of scanning speed increases the tensile strength to a certain point and then it deteriorates it). Ahmadi et al [16] concluded that, if the scanning speed is the only altering parameter in the SLM process, the tensile strength decreases with increasing the scanning speed. This was attributed to higher energy density and enhancement of metallurgical bonding between the particles. Moreover, this study showed that, laser power is a more affective parameter than scanning strategy in defining the mechanical properties of the part. It was shown by zhu et al. [17] that by increasing the energy density (by increasing the laser power or decreasing the scanning speed) thermal shrinkage of the fabricated parts increased, which lead to residual thermal stress in the part. The reason for thermal stress in SLM-built parts is the shrinkage of the top layer as it solidifies on the previously fabricated layers. Shrinkage caused by the transformation of molten material to solid state rapidly causes residual stress and consequently microcracks that will deteriorate the mechanical properties of the fabricated part. According to previous studies on SLM processing parameters, energy density is the key factor in defining the cooling rate of the process. Manipulating the laser energy density can significantly affect the densification of the SLM-built parts. Energy density decreases with increasing the scanning speed and lowering the laser power. Wang et al [18] stated that higher energy density will lead to increase in grain size as well as primary dendrite spacing. Moreover, this study linked the low densification levels, higher porosity and rough surface quality with insufficient energy density. Grain coarsening which is caused by increasing the energy density, will deteriorates the microhardness values. By increasing the energy density to a certain value the microhardness increases. However after that optimal value for energy density (for which the microhardness is maximum) the microhardness values begin to decrease again [18]. In another study done by Song et al. [19] regarding the SLM process, high strength of the fabricated parts was linked to refinement of grains. Low energy density/high scanning speed/ low laser power

5

results in insufficient melting and higher viscosity and limited densification. Miranda et al [20] stated that increasing the laser power in SLM results in progressive elimination of porosity and consequently higher density, as well as higher hardness and shear straight values. This is due to the fact that decreasing the laser powder equals to less energy density, which will lead to consolidation defect. On the other hand, the opposite effect was observed for scanning speed, where increasing the velocity resulted in deterioration of the mechanical properties. Lastly, it was discovered from this research work that increasing the scan spacing disintegrates the density and mechanical properties of the fabricated part, as well as coarsens the microstructure [20]. Li et al. [21] also reported the behavior of 316L stainless steel during SLM. They stated that increasing the laser power as well as decreasing the scanning speed, alongside lowering the layer thickness and decreasing the hatch space, results in higher density values. It was shown that reduction in wetting angles with increasing the laser power and therefore there will be better wetting characterization and more depth of penetration for the melt. Li et al. [21] have also compared the gas and water atomized powder and came to the conclusion that gas atomized powder produces denser parts due to less oxygen content. Moreover, gas atomized samples had less porosity and had finer microstructure in comparison to water atomized samples. Simchi et al. [22] stated that lower energy density in the selective laser sintering process has a negative effect on mechanical properties of the part as it will lead to smaller melt pools, un- melted powder, and more porosity which reduces the part density. However, using extremely high energy leads to delamination, which is the result of thermal stress and gas porosity as well as material shrinkage [22]. The microstructures as well as mechanical properties of 316L stainless steel can be improved by properly tailoring SLM processing parameters. Therefore, the objective of this research article is to investigate the effect of processing parameters such as, scanning strategies, scanning speeds, and energy densities, on microstructures and mechanical properties of 316L stainless steel process via SLM. To the best of authors’ knowledge, the effect of scanning strategy complimenting with energy density on microstructural and mechanical behavior of 316L stainless steel have never been studied in detail. This research work also gives a more thorough comparison between SLM-fabricated 316L stainless steel samples with different scanning speeds and energy densities in terms of microhardness and tensile strength. This forms the basis for

6

selection of optimized processing parameters for future researchers in order to obtain desired microstructures and mechanical properties of SLM processed 316L stainless steel.

2.

Material and experimental procedure.

2.1.

Alloy and SLM processing

Gas-atomized 316L stainless steel powder (powder supplier: MTT Technologies GmbH) was used as the matrix material. The 316L powder had the particle size distributions of d10 = 30.24 µm, d50 = 40.80 µm, and d90 = 56.25 µm. The SLM system consisted of a fiber laser, automatic powder-layering apparatus, argon gas protection system to avoid material oxidation under hightemperature processing, and computer-based process control panel. The layering system deposited a powder layer with a thickness of 50 µm (d) on a carbon steel substrate. The SLM processing condition for 316L stainless steel samples are listed in Table 1. For this study, six batches of blocks were fabricated, with the same layer thickness of 0.05 mm, laser spot size of 0.2mm, laser power of 100 W, and hatch spacing of 0.114 mm. Each batch (except for batch 2) contained three samples built with the same scanning speed, energy density, and different scanning strategies (A, B, or C) as shown in Figure 1. Each batch of samples has a specific scanning speed (0.25 mm/s, 0.175 mm/s, 0.12 mm/s, 0.239 mm/s, 0.167 mm/s, and 0.111 mm/s) and energy density (70 J/mm3, 100 J/mm3, and 150 J/mm3). Since building direction affects the mechanical properties of the built part [1], it should be mentioned that all the blocks were built horizontally on the substrate. For this experiment, we have fabricated 16 blocks of 316L stainless steel processed via SLM, with dimensions of 80mm x 10mm x 6mm. These blocks were then cut using EDM for tensile testing, as well as cut into horizontal and vertical sections for microstructural and microhardness characterization.

2.2 Tensile Testing Uniaxial tensile testing experiments were conducted at room temperature to determine the stressstrain curves at quasi-static rates (0.02 mm/min) in an INSTRON mechanical testing machine with a 50 N load cell and tensile specimens, as shown in Figure 2. Specimens were prepared as per ASTM E8M standard and tests were carried out to failure. In total, three tensile samples of each condition were tested.

7

2.3

Microhardness

The microhardness of SLM processed 316L stainless steel samples were measured using a standard Vickers microhardness tester (Wilson) under a load of 10 N for 15 s. The microhardness samples were cut from the same block as tensile specimens. One horizontal and one vertical cuts were made on each block in order to study effect of build direction, and these cuts were mounted and polished before doing the microhardness test. The average of 20 readings were taken into account, 10 readings from the horizontal section and 10 readings from the vertical section and the mean values of the readings are presented in Table 2.

2.4

Microstructure, density and grain size calculation

SLM processed 316L stainless steel samples were characterized in FEI-INSPECT 50 SEM. Same samples used for microhardness testing were used for SEM characterizations as well. The samples were polished using different grades of polishing cloths from 400 to 1200 and then polished for 30 minutes on a micro cloth using colloidal silica. SEM characterization was done on horizontal as well as on vertical sections of each sample in order to investigate the effect of SLM processing parameters on the build directions of these samples. Both secondary as well as backscattered electron imaging was done with various magnifications ranging from 100x to 10,000x. Images were taken from the top, middle and bottom part of the horizontal and vertical sections of each sample. Density/porosity calculation was then done with the same magnification SEM images using the ImageJ software and the results are presented in Table 1. In order to obtain the relative density of the samples 10 SEM images were uploaded to the ImageJ software. Using ImageJ software’s thresholding feature in addition to contrast difference, porosities are highlighted and separated from remaining matrix. The images were then compared and analyzed relative density values for SLM processed samples. Moreover, the grain size measurements analysis was performed on ImageJ data analysis software. At least 10 SEM images of each samples were taken at same magnifications.

8

3

Results and discussion

3.1

Tensile properties

The microstructures and mechanical properties of SLM processed materials are highly affected by temperature profile and the amount of energy induced to the powder during processing; this energy can be altered by changing the laser power, hatch spacing and scanning speed[23][24]. However enhancement of the mechanical properties does not necessarily have a linear correlation with increasing or decreasing the processing parameters [25]. To investigate the effect of these processing parameters on mechanical properties of SLM processed 316L stainless steel samples, tensile stress-strain plots were plotted (Fig. 3-7) for varying parameters (scanning speed, scanning strategy, and energy density). The first group of graphs compared the stress vs strain values for samples built with the same energy density and same scanning speed under scanning strategies A, B, and C, in order to reveal the role of scanning strategy on ultimate tensile strength, yield strength, and elongation (Fig. 3 and 4). The second group of graphs depicted the role of scanning speed on mechanical properties by comparing stress/strain values for samples fabricated with the same energy density and scanning strategy but different scanning speed (Fig. 5-7). The tensile yield strength, ultimate tensile strength, and elongation of the SLM processed 316L stainless samples for the various processing conditions are listed in Table 2. Fig. 3 and 4 show stress-strain plots for SLM processed 316L stainless samples processed for 100 W laser power and 0.2 mm spot size at different scanning speeds, whereas Fig. 5, 6, and 7 show stress-strain plots for 316L stainless steel samples processed at different energy densities and for different scanning strategies. In case of Fig. 3 and 4 each graph represents a sample that is built with the same energy density and scanning speed under different scanning strategies (A, B and C). The corresponding engineering stress-strain curves for SLM processed 316L stainless samples processed for laser power 100 W, laser spot size of 0.2 mm, and laser scanning speeds at 0.25, 0.239, and 0.12 mm/s are also shown in Fig. 3. 316L stainless steel samples processed using scanning strategy A (i.e. alternate hatches, single pass of laser beam) exhibit higher yield strength, ultimate tensile strength, and elongation as compared to samples processed via scanning

9

strategy B (alternate hatches, multiple passes of laser beam) and C (cross hatches, single pass of laser beam) for different scanning speeds varying from 0.25 to 0.12 mm/s (Fig. 3). This is primarily due to overheating that occurs during multiple/alternate crossing of the laser beam in scanning strategies B and C (Fig. 1). This overheating resulted in balling defect possibly caused by splashing of the molten material leading towards deterioration of the mechanical properties. In some cases it might also be due to evaporation of the liquid as a result of very high temperature that results in lower density. The mechanical properties (ultimate strength, yield strength, and elongation) of SLM processed 316L stainless steels using scanning strategy C are relatively better, as compared to those of processed via scanning strategy B. Again, this can be attributed to overheating of particles in strategy B mainly due to multiple passes of the laser beam, whereas in strategy A and C, particles have had some time to cool down from the previous laser scan due to the single pass of the laser beam. However, the above-mentioned trend did not observe in SLM processed 316L stainless steel samples processed using scanning speed 0.167 mm/s and energy density 100 J/mm3 (Fig. 4(a)), as the order of values for yield strength, ultimate strength and elongation are: A crystallographic texture as appose to usual <001> texture. According to this study [26], an increase in the <011> can enhance the tensile straight and ductility of the fabricated part. Increasing the scanning speed results in an increase in cooling rate which leads to a refined microstructure (Equiaxed fine grains is attributed to high cooling rates) and consequently better mechanical properties[27]. A similar trend has also been observed for samples processed at 100 J/mm3 energy density (Fig. 6). However, higher scanning speeds can also have a deteriorating influence on mechanical

10

properties. Ni et al. [28] showed that high scanning speeds results in more oxidation in the powder and creation of more voids and in some cases un melted particles. As laser scanning speed decreases, bigger melt pools form as compared to higher scanning speed, which results in a strong bond formation between the particles resulting into better mechanical properties. This might be reason in the case of 316L stainless steel samples processed at energy density 150 J/ mm3, where mechanical properties (i.e. yield strength, ultimate strength, and elongation) of samples decrease with increasing scanning speed. Hao et al. [5] have investigated the effect of processing parameters on mechanical properties where it was hypothesized that optimal processing parameters prevent grain growth, which mainly resulting from higher scanning speed while maintaining adequate bonding between the melted particles. It has also been observed that SLM processed 316 stainless steel samples fabricated using same scanning strategy, samples processed at higher energy density (150 J/mm3) exhibits better mechanical properties as compared to that of processed at lower energy density (70 and 100 J/mm3). This is mainly due to formation of bigger and deeper melt pools that are more stable at high temperatures, as well as more particle fusion and better bonding between powders particles in samples processed at higher energy densities. Furthermore, higher energy density results in better and more even melting of powder and consequently less holes or balling defect [29]. Altering the energy density can affect the size of melt pools; lower energy density reduces melt pool size, resulting in to smaller melt pools and consequently a more porous part containing more horizontal cracks that have lower density, resulting into lower microhardness as well as mechanical properties. Ductility values for all samples are listed in Table 2. In all the cases, samples with the same scanning speed and energy density, scanning strategy A showed the highest values of ductility compared to strategies B and C. This is primarily due to higher densification of samples processed with scanning strategy A and with higher scanning speed, which is discussed in detail in section 3.4.

3.2

Microhardness

The microhardness values of SLM processed 316L stainless samples fabricated via different scanning strategies, scanning speed, and energy densities are listed in Table 2. SLM processed 316L stainless steel samples processed using the same energy density as well as scanning speed, samples fabricated using scanning strategy A exhibits higher microhardness as compared to that

11

of processed via scanning strategy B and C. Furthermore, 316L stainless steel samples processed using scanning strategy A but with varying scanning speed and energy density, sample fabricated with lowest scanning speed of 0.111 mm/s and highest energy density of 150 J/ mm3 exhibits highest microhardness (224.7 HV) as compared to other samples. Lower laser scanning and therefore lower cooling rate which results in more coarsen microstructure, along with high energy density, leads to fully melted particles with strong bonding resulting into lower porosity with denser samples and therefore a high hardness value. There is not much difference in microhardness values for 316L stainless steel samples processed at same energy density and scanning strategies. However, in some cases, samples with higher scanning speed exhibited higher microhardness values. In these cases, high microhardness values have been mainly attributed to formation of smaller melt pools leads to smaller grain size and therefore a more refined microstructure resulting in enhanced mechanical properties [30].

3.3

Microstructure

Fig. 8 and 9 shows the SEM images taken along horizontal section (perpendicular to build direction) of SLM processed 316L stainless steel samples fabricated with 70 J/mm3 and 150 J/mm3 for scanning strategies A, B, and C with the scanning speed varying from 0.25 mm/s to 0.111 mm/s respectively. The stainless samples processed with scanning strategy A show refined microstructure with smaller grains as compared to that of processed with scanning strategies B and C in addition to similar laser energy densities and scanning speed. Also, samples processed with higher scanning speed (0.25 mm/s) exhibits much more refined microstructure as compared to that of processed with lower scanning speed with similar energy densities. Both these observations clearly suggest that cooling rate plays an important role in the refinement of microstructures. 316L stainless steel samples processed with higher cooling rates and scanning strategy A shows very refined microstructure as compared to that of those samples processed using lower scanning speed and scanning strategies B and C for similar energy densities. Also, Cherry et al [23] have shown that lower scanning speed will result in formation of larger melt pools and consequently resulting in to coarser grains. Fig. 10 and 11 shows the SEM images taken along the vertical section (parallel to build direction) of SLM processed 316L stainless steel samples fabricated with 70 J/mm3 and 150 J/mm3 for scanning strategies A, B, and C, with the scanning speed varying from 0.25 mm/s to 0.111 mm/s respectively. The columnar-dendritic

12

grains growing along build direction are clearly seen in all SEM images. Casati et al. [31] associated the formation of columnar dendrites to fast solidification and high thermal gradient Also, as scanning speed reduces the width of these dendrites increases gradually due to decrease in cooling rate. Kong et al. [32] stated that formation of these columnar grains along the build direction is as a result of severe thermal gradient due to the heat sink of the previous layer. According to this study [32] when the thermal gradient is too high it encourages the planar grain growth (in this case along the build direction). Growth of columnar grains along the build direction can also be attributed to formation of wider and shallower melt pools as a result of high laser power and decrease in cooling rate [33]. Fig. 12 shows SEM images of 316L stainless steel samples processed using scanning strategy A and laser scanning speed varying from 0.239 to 0.111 mm/s. SLM processed 316L stainless steel sample fabricated with higher scanning speed (0.239 mm/s) exhibits very refined microstructure with average grain size varying of 4-6 µm, whereas sample processed with lower scanning speed (0.111 mm/s) exhibits coarser grains (1012 µm). This is due to an increase in the applied energy density which results in coarser grains. Fig. 13 shows SEM images of SLM processed 316L stainless steel samples using scanning strategies A, B, and C with laser energy densities varying from 70 to 150 J/mm3. As laser energy densities increases from 70 to 150 J/mm3, coarsening in the microstructure has been observed. Grain sizes were measured for samples fabricated with scanning strategy A under different energy densities of 70, 100 and 150 J/mm3. The grain size for parts fabricated with 70 J/mm3 varied from 4-6 µm when the grain size for 100 J/mm3 fabricated samples was in the range of 910 µm and samples fabricated with 150 J/mm3 had the grain size range of 10-12 µm. The same trend was observed for scanning strategies B and C. For scanning strategy B grain size for 70 J/mm3 was in the range of 7-9 µm. For energy density 100 J/mm3 the grain size range was 16-20 µm and for 150 J/mm3 the range was 17-21 µm. In case of samples fabricated with scanning strategy C for energy density 70 J/mm3 the grain size varied from 10-13 µm, for energy density 100 J/mm3 the range was 10-16 µm and for energy density energy density 150 J/mm3 the range was 15-20 µm. As it is obvious from these results in all cases of samples fabricated with the same energy density samples fabricated with strategy B and C have higher grain sizes than samples fabricated with strategy A. Furthermore, these results are in exact correlation with microhardness values from the previous section. In all the cases the samples with lower average grain size had higher microhardness values. 316L stainless steel samples processed with

13

scanning strategy A and 70 J/mm3 energy density exhibits refined microstructure, as compared to that of processed with scanning strategies B and C with higher energy densities of 100 and 150 J/mm3.

3.4

Density

Table 1 shows relative densities of SLM processed 316L stainless steels samples processed for different scanning strategies, scanning speeds, and energy densities. All stainless steel samples exhibit more that 92% densification. However, the main objective of this study is to investigate the effect of SLM processing parameters on densification of 316L stainless steel samples, which will help researchers as well as form basis for selecting appropriate processing parameters to achieve desired densification of samples. Porosity, balling, and cracking defects are the main factors contributing to decrease in relative density. Balling defects leads to creation of more porosity in the sample as it prevents the uniform deposition of new powder on previously built layers; therefore, the new layer will not properly bind with the previous layers. Balling can be caused because of the particles’ tendency to reduce the surface tension by merging into bigger droplets [34]. In general when the surface tension is too high, it disables the molten material from wetting the previous layer and causes the liquid to spheroids [35]. Moreover, high viscosity of the liquid, as well as insufficient melting of powder particles caused by low energy density can also result in more balling defect [36]. Almangour et al. [34] linked balling and pore chain defect to high or low energy density values. According to this study [34], low-viscosity which is associated with lower energy density, can cause disordered liquid solidification and consequently leads to balling defect and creation of pores inside the sample. Therefore, samples built under lower energy densities have more pores and are less dense in comparison to samples built under higher energy densities. Gu et al. [29] correlated the increase of the energy density (by increasing the laser power, decreasing the scanning speed or decreasing the powder layer thickness) with decreasing of the balling tendency. Moreover, samples built with lower energy density are exposed to higher cooling rates, and as a result they will have more refined microstructure that effects the mechanical properties such as compressive yield strength. Simchi et al. [22], have shown the relationship between laser power (P), laser beam spot size (d), scan rate (v), and η coupling efficiency with laser energy density via this formula: Q = πηP/4d

14

Since laser beam spot size and laser power were kept constant for all the samples in this experiment, it can be seen that scanning speed has an inverse effect on densification of SLM processed 316L stainless steel samples. When all the other parameters are kept constant, increasing the scanning speed results in creation of melt pools that are not sufficiently sized, thus causing more porosity and decreasing the density. Furthermore, increasing the scanning speed and therefore decreasing the energy density results in inadequate re-melting of the previous layer and consequently poor bonding between the layers. Also, high cooling rates that follows the rapid heating of the specimen causes internal thermal stress and high thermal gradient which leads to creation of solid-state micro and macro cracks that lower the final density of the built part [33]. Too much increase in the laser power also deteriorates the density as it leads to intense heat transfer that results in partial evaporation of the powder and more porosity [37]. Hatch spacing can affect the surface quality of each fabricated layer and therefore affect the porosity level and density of the part. Surface quality is important since, if there are existing gaps on the surface, the new laid powder will end in filling the gaps of the previous powder, which results in more porosity and will create a chain effect that leads to rougher and more porous layers[21]. According to Li et al. [21], hatch spacing is one of the factors that defines the shape and size of the pores in a SLM-built sample. In this study, for samples built with the same energy density of 70 J/mm3, mean relative density is 95.63 for samples fabricated with hatch space of 0.114 mm, which is lower than the mean relative density for samples with hatch space of 0.12mm. However, for the case of energy density 150 J/mm3 ,the mean relative density for samples with smaller hatch space of 0.111 mm that is 95.71, which is lower than mean relative density of samples fabricated with hatch space of 0.12mm that is 98.25.

4.

Conclusion

The effect of SLM processing parameters (i.e. scanning strategy, scanning speed, and energy density) on densification, microstructure, and mechanical properties of 316L stainless steel samples have been investigated. Here, we have seen scanning strategy, scanning speed, and energy density significantly affect densification, microstructure as mechanical properties of 316L stainless steel samples. This study will help future researchers in selecting as well as optimizing SLM processing parameters to obtain desired microstructure and mechanical properties.

15

•

The 316L stainless steel samples processed using scanning strategy A (alternate hatches

and single pass of laser beam) exhibited highest densification and refined microstructure as compared to samples processed with scanning strategy B (alternate hatches and multiple passes of laser beam) and C (cross hatches and single pass of laser beam), primarily due to highest cooling rate. •

The better mechanical properties (yield strength, ultimate tensile strength, and elongation

as well as microhardness) have been observed in SLM processed samples with scanning strategy A than that of processed with scanning strategy B and C. •

The SLM samples processed at higher scanning speed exhibited better densification,

refined microstructure, and excellent mechanical properties as compared to samples processed with lower scanning speed. This is primarily due to higher densification and refined microstructure due to higher cooling rate obtained in samples processed with higher scanning speeds. •

The columnar dendritic microstructure has been observed in all samples along building

direction. Also, as laser scanning speed decreases, width of dendrites increases, mainly due to lower cooling rate which gives enough time for dendrite coarsening.

16

References [1]

I. Tolosa, F. Garciandía, and F. Zubiri, “Study of mechanical properties of AISI 316 stainless steel processed by ‘ selective laser melting ’ , following different manufacturing strategies,” pp. 639–647, 2010.

[2]

R. Li, J. Liu, Y. Shi, and L. Wang, “Balling behavior of stainless steel and nickel powder during selective laser melting process,” pp. 1025–1035, 2012.

[3]

K. A. Mumtaz, P. Erasenthiran, and N. Hopkinson, “High density selective laser melting of Waspaloy ®,” vol. 5, pp. 77–87, 2007.

[4]

F. Verhaeghe, T. Craeghs, J. Heulens, and L. Pandelaers, “A pragmatic model for selective laser melting with evaporation,” Acta Mater., vol. 57, no. 20, pp. 6006–6012, 2009.

[5]

L. Hao, S. Dadbakhsh, O. Seaman, and M. Felstead, “Journal of Materials Processing Technology Selective laser melting of a stainless steel and hydroxyapatite composite for load-bearing implant development,” vol. 209, pp. 5793–5801, 2009.

[6]

A. Simchi and H. Pohl, “Effects of laser sintering processing parameters on the microstructure and densification of iron powder,” vol. 359, 2003.

[7]

E. Yasa and J. Kruth, “Procedia Engineering Microstructural investigation of Selective Laser Melting 316L stainless steel parts exposed to laser re-melting,” Procedia Eng., vol. 19, pp. 389–395, 2012.

[8]

L. Angeles, Powder metallurgy of stainless steel : State-of- the art , challenges , and development, no. January 2015. 2016.

[9]

P. Service, “Selective laser sintering of hydroxyapatite reinforced polyethylene 17

composites for bioactive implants and tissue scaffold development,” vol. 220, no. February, pp. 521–531, 2008. [10]

D. Kong et al., “Bio-functional and anti-corrosive 3D printing 316L stainless steel fabricated by selective laser melting,” Mater. Des., vol. 152, pp. 88–101, 2018.

[11]

Y. Liu, J. Zhang, and Z. Pang, “Numerical and experimental investigation into the subsequent thermal cycling during selective laser melting of multi-layer 316L stainless steel,” Opt. Laser Technol., vol. 98, pp. 23–32, 2018.

[12]

C. Wagnec, “Coalescence Behaviour of Two Metallic Particles as Base Mechanism of Selective Laser Sintering 1,” pp. 4–7, 2000.

[13]

C. Y. Y. Choong, G. K. H. Chua, and C. H. Wong, “Investigation on the integral effects of process parameters on properties of selective laser melted stainless steel parts,” Proc. 3rd Int. Conf. Prog. Addit. Manuf., 2018.

[14]

M. Yakout, M. A. Elbestawi, and S. C. Veldhuis, “A study of thermal expansion coefficients and microstructure during selective laser melting of Invar 36 and stainless steel 316L,” Addit. Manuf., vol. 24, no. September, pp. 405–418, 2018.

[15]

J. Simmons, M. Daeumer, A. Azizi, and S. N. Schiffres, “Local Thermal Conductivity Mapping of Selective Laser Melted 316L Stainless Steel,” pp. 1694–1707, 2018.

[16]

A. Ahmadi, R. Mirzaeifar, N. Shayesteh, A. Sadi, H. E. Karaca, and M. Elahinia, “Effect of manufacturing parameters on mechanical properties of 316L stainless steel parts fabricated by selective laser melting : A computational framework,” vol. 112, pp. 328– 338, 2016.

[17]

J. F. HH Zhu, L Lu, “Study on shrinkage behaviour of direct laser sintering metallic powder.” Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 220(2), pp. 183–190, 2006.

[18]

D. Wang, C. Song, Y. Yang, and Y. Bai, “Investigation of crystal growth mechanism during selective laser melting and mechanical property characterization of 316L stainless steel parts,” vol. 100, pp. 291–299, 2016.

[19]

B. Song, S. Dong, S. Deng, H. Liao, and C. Coddet, “Optics & Laser Technology Microstructure and tensile properties of iron parts fabricated by selective laser melting,” Opt. Laser Technol., vol. 56, pp. 451–460, 2014.

[20]

G. Miranda et al., “Materials Science & Engineering A Predictive models for physical and

18

mechanical properties of 316L stainless steel produced by selective laser melting,” vol. 657, pp. 43–56, 2016. [21]

R. Li, Y. Shi, Z. Wang, L. Wang, J. Liu, and W. Jiang, “Applied Surface Science Densification behavior of gas and water atomized 316L stainless steel powder during selective laser melting,” Appl. Surf. Sci., vol. 256, no. 13, pp. 4350–4356, 2010.

[22]

A. Simchi, “Direct laser sintering of metal powders : Mechanism , kinetics and microstructural features,” vol. 428, pp. 148–158, 2006.

[23]

J. A. Cherry, H. M. Davies, S. Mehmood, and N. P. Lavery, “Investigation into the effect of process parameters on microstructural and physical properties of 316L stainless steel parts by selective laser melting,” pp. 869–879, 2015.

[24]

B. S. Das, “Physical Aspects of Process Control in Selective Laser Sintering of Metals,” vol. 2125, no. 10, pp. 701–711, 2003.

[25]

Y. Zhu, T. Peng, G. Jia, H. Zhang, S. Xu, and H. Yang, “Electrical energy consumption and mechanical properties of selective- laser-melting-produced 316L stainless steel samples using various processing parameters,” J. Clean. Prod., vol. 208, pp. 77–85, 2019.

[26]

Z. Sun and X. Tan, “Simultaneously enhanced strength and ductility for 3D-printed stainless steel 316L by selective laser melting,” NPG Asia Mater., pp. 127–136, 2018.

[27]

A. S. Amador, P. Kucita, and S. C. Wang, “Process Optimization and Microstructure Characterization of Ti6Al4V Manufactured by Selective Laser Melting Process Optimization and Microstructure Characterization of Ti6Al4V Manufactured by Selective Laser Melting,” pp. 2–8, 2017.

[28]

X. Ni et al., “Corrosion Behavior of 316L Stainless Steel Fabricated by Selective Laser Melting Under Different Scanning Speeds,” J. Mater. Eng. Perform., vol. 27, no. 7, pp. 3667–3677, 2018.

[29]

D. Gu and Y. Shen, “Balling phenomena in direct laser sintering of stainless steel powder : Metallurgical mechanisms and control methods,” Mater. Des., vol. 30, no. 8, pp. 2903– 2910, 2009.

[30]

D. D. Gu, W. Meiners, K. Wissenbach, and R. Poprawe, “Laser additive manufacturing of metallic components: materials, processes and mechanisms,” Int. Mater. Rev., vol. 57, no. 3, pp. 133–164, 2012.

[31]

M. V. R.Casati, J.Lemke, “Microstructure and Fracture Behavior of 316L Austenitic

19

Stainless Steel Produced by Selective Laser Melting,” J. Mater. Sci. Technol., vol. 32, no. 8, pp. 738–744, 2016. [32]

D. Kong, X. Ni, C. Dong, L. Zhang, C. Man, and X. Cheng, “Anisotropy in the microstructure and mechanical property for the bulk and porous 316L stainless steel fabricated via selective laser melting,” Mater. Lett., vol. 235, pp. 1–5, 2019.

[33]

M. L. Montero-sistiaga, M. Godino-martinez, K. Boschmans, J. Kruth, J. Van Humbeeck, and K. Vanmeensel, “Microstructure evolution of 316L produced by HP-SLM ( high power selective laser melting ),” Addit. Manuf., vol. 23, no. May, pp. 402–410, 2018.

[34]

B. Almangour, D. Grzesiak, T. Borkar, and J. Yang, “Densi fi cation behavior , microstructural evolution , and mechanical properties of TiC / 316L stainless steel nanocomposites fabricated by selective laser melting,” vol. 138, pp. 119–128, 2018.

[35]

J. P. Kruth, L. Froyen, J. Van Vaerenbergh, P. Mercelis, M. Rombouts, and B. Lauwers, “Selective laser melting of iron-based powder,” vol. 149, pp. 616–622, 2004.

[36]

A. Simchi and H. Pohl, “Direct laser sintering of iron – graphite powder mixture,” vol. 383, pp. 191–200, 2004.

[37]

C. Y. Ying, G. K. Hong, and C. How, “This document is downloaded from DR-NTU , Nanyang Technological,” 2018.

20

Figure Captions: Fig. 1: Scanning strategies: (a)-alternate hatches, single pass of laser beam; (b)- alternate hatches, multiple passes of laser beam; (c)- cross hatches, single pass of laser beam. Fig. 2: Tensile test sample dimensions. Fig. 3: Engineering stress-strain curves of SLM processed 316L stainless steel samples at laser spot size 0.2 mm, laser power: 100 W, and laser scanning speeds and energy densities (a) 0.25 mm/s and 70 J/mm3; (b) 0.239 mm/s and 70 J/mm3; and (c) 0.12 mm/s and 150 J/mm3, for scanning strategies (SS) A, B and C obtained from tensile test. Fig. 4: Engineering stress-strain curves of SLM processed 316L stainless steel samples at laser spot size 0.2 mm, laser power: 100 W, and laser scanning speeds and energy densities (a) 0.167 mm/s and 100 J/mm3,and (b) 0.111 mm/s and 150 J/mm3, for scanning strategies (SS) A,B and C, obtained from tensile test. Fig. 5: Engineering stress-strain curves of SLM processed 316L stainless steel samples at laser spot size 0.2 mm, laser power: 100 W, energy density: 70 J/mm3, scanning strategies (a) a; (b) b; and (c) c, for scanning speeds of 0.25 and 0.239 mm/s, obtained from tensile test. Fig. 6: Engineering stress-strain curves of SLM processed 316L stainless steel samples at laser spot size 0.2 mm, laser power: 100 W, energy density: 100 J/mm3, scanning strategies a, and for scanning speeds of 0.175 and 0.167 mm/s, obtained from tensile test. Fig. 7: Engineering stress-strain curves of SLM processed 316L stainless steel samples at laser spot size 0.2 mm, laser power: 100 W, energy density: 150 J/mm3, scanning strategies (a) a; (b) b; and (c) c, for scanning speeds of 0.12 and 0.111 mm/s, obtained from tensile test.

21

Fig. 8: Microstructure of samples (horizontal cross-section, perpendicular to build direction) built with energy density 70J/mm3. . (a) and (b) with scanning strategy A, (c) and (d) with scanning strategy B, (e) and (f) with scanning strategy C; and scanning speeds 0.25 mm/s and 0.239 mm/s respectively. Fig. 9: Microstructure of samples (horizontal- cross-section, perpendicular to build direction) built with energy density 150J/mm3. . (a) and (b) with scanning strategy A, (c) and (d) with scanning strategy B, (e) and (f) with scanning strategy C; and scanning speeds 0.12 mm/s and 0.111 mm/s respectively. Fig. 10: Microstructure of samples (vertical-cross-section, parallel to build direction) built with energy density 70J/mm3. . (a) and (b) with scanning strategy A, (c) and (d) with scanning strategy B, (e) and (f) with scanning strategy C; and scanning speeds 0.25 mm/s and 0.239 mm/s respectively. Fig. 11: Microstructure of samples (vertical-cross-section, parallel to build direction) built with energy density 150J/mm3. . (a) and (b) with scanning strategy A, (c) and (d) with scanning strategy B, (e) and (f) with scanning strategy C; and scanning speeds 0.12 mm/s and 0.111 mm/s respectively. Fig. 12: Microstructure of samples built with scanning strategy A and (a) 0.25 (b) 0.239, (c) 0.175, (d) 0.167, (e) 0.12, and (f) 0.111 mm/s laser scanning speed. Fig 13: Microstructure of samples built with energy density 70 J/mm3 and scanning speed 0.239 mm/s. (a) with scanning strategy A, (b) with scanning strategy B and (c) with scanning strategy C. Energy density 100 J/mm3 and scanning speed 0.167 mm/s, (d) with scanning strategy A, (e) with scanning strategy B and (f) with scanning strategy C. Energy density 150 J/mm3 and scanning speed 0.111mm/s. (g) with scanning strategy A, (h) with scanning strategy B and (i) with scanning strategy C.

22

Table 1: SLM processing parameters along with relative densities.

23

24

Table 2: Mechanical properties (yield strength, ultimate tensile strength, and elongation) of SLM

25

processed samples along with microhardness values.

26