Smit 2016

Effect of layer thickness on Inconel 718 parts manufactured with selective laser melting S TEFAN S MIT * University of Twente – Drienerlolaan 5, 7522 NB Enschede, The Netherlands *Corresponding author s.a.smit@student.utwente.nl February 22, 2016

ABSTRACT: This research aims to investigate the effect of the layer thickness on several properties. A higher layer thickness decreases the build time and thus the cost of a product, but could have worse properties as a result. The density, porosity, microstructure and hardness of Inconel 718 samples manufactured with selective laser melting (SLM) were determined. The samples were manufactured with a layer thickness of 30 µm and with a layer thickness of 50 µm. The density and the porosity are for both layer thicknesses roughly the same, but the microstructure and the hardness do show some differences.

Key words: selective laser melting, SLM, additive manufacturing, layer thickness, Inconel 718 1

important factors, that can be changed, for the build rate of the SLM process. The process related build rate is calculated according to equation 1[2]. Wherein V˙ process is the process related build rate, Ds is the layer thickness, vs is the scanning velocity and ∆ys is the scanline spacing.

V˙ process = Ds ∗ vs ∗ ∆ys (1)

INTRODUCTION

Selective laser melting, or SLM, is an additive manufacturing method. By cutting a CAD model in layers, laying down a layer of powder and melting the powder with a laser an object is being build. The first layer will be melted on a support structure, the following layers will be melted on the previous layer. The laser melts the powder including a part of the previous layer to create a solid object. This process is being illustrated in figure 1.

Kurzynowski et al.[1] did research into the effect of the various parameters on the porosity of the printed part.

The result of the research was that the density of the part manufactured with a layer thickness of 50 µm was higher than that of the part with a layer thickness of 75 µm. However the conclusion was that the layer thickness of 75 µm would be better since the difference in the porosity was very small.

Schleifenbaum et al.[3] created a prototype SLM machine with two lasers, but also collected parameters from previous studies for different materials. The collected parameters include the layer thickness for different materials. Inc718, the material used in this research, has an advised layer thickness of between 50 µm and 100 µm. These parameters were all optimised to create the quickest build rate.

Chlebus et al.[4] researched the effect of heat treat-

Figure 1: Working of SLM [1]

The layer thickness of the powder, together with the scanning velocity and the scanline spacing, are the most 1

ment on specimens produced with selective laser melting. The tested specimens were manufactured in four different print orientations and were tested as-built and after heat treatment. They looked at the microstructure and mechanical properties. They concluded that the asbuild parts had fine columnar grains across several layers. The columnar grains were characterized by dendrite cells. They also concluded that the heat treatment is almost necessary.

In the research here, partly executed at the NLR, layer thicknesses of 30 µm and 50 µm were used. NLR uses a layer thickness of 50 µm for most of their SLM processes. Depending on the requirements a layer thickness of 30 µm is also sometimes used. A layer thickness of 50 µm is preferred since the build time, and thus the costs, reduces greatly.

2 2.1

2.2

Experimental procedure

In order to acquire accurate test results, each sample was used for the same tests. Out of each sample the information that was obtained is: • Density • Porosity • Microstructure • Hardness To extract all the information, the samples were cut in the middle to split the parts printed with 30 µm layer thickness and 50 µm layer thickness. The density of the samples was determined using the Archimedes principle. The samples were weighted dry and then weighted completely submerged in ethanol to determine the volume of the samples. The upward buoyant force exerted on the sample is equal to the weight of the fluid that the sample displaces, so the difference in measured weight is the weight of the displaced fluid. The density was calculated using equation 2. Ethanol was used to prevent corrosion of the samples.

METHODOLOGY Test samples

There were 5 samples manufactured with a SLM 280 HL machine. The material used is Inconel 718, a nickel superalloy. The dimensions of the samples are 6 mm x 6 mm x 12 mm. The bottom 6 mm were printed with a layer thickness of 50 µm, the top 6 mm were printed with 30 µm, see figure 2. Figure 2 also shows the building direction with an arrow. The other parameters, such as scanning velocity or laser power, were changed to the optimal parameters, these parameters are confidential and as such not published. The samples were tested as-built without any post manufactured treatment, like stress relieving treatment.

Density =

Weight dry ∗ Density ethanol Weight dry − Weight submerged

(2)

The samples were then embedded in a polymer to make the grinding of the samples possible. The samples were ground with increasingly smooth abrasive grinding paper and polished using an oxide polishing suspension (OPS) to smoothen the cut surface and to remove the scratches on the surface. After the polishing pictures were taken using a microscope. The pictures were analysed by breaking each pixel of the picture down to the red, green and blue values and counting the white pixels, including some less white pixels at the corner of a pore. After the determination of the porosity the samples were treated with Kalling’s reagent no. 2 to expose the microstructure. Kalling’s reagent no. 2 consists of 5 g CuCl2 , 100 mL HCl and 100 mL ethyl alcohol. Pictures were taken of the microstructure to analyse. The hardness was tested using a Vickers hardness tester. Since the samples were already polished, the Vickers hardness test did not need additional smoothing of the samples. Of each sample several hardness measurements were taken.

Figure 2: Sample

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3

RESULTS

The result of the density measurements do show that the density of the samples printed with a layer thickness of 30 µm do not differ much from the samples printed with a layer thickness of 50 µm. This means that the average porosity does not deviate too much between the samples.

The research distinguishes between the test samples in which the melt pools and the building direction are visible, the xz-plane, and the test samples in which the scan lines are visible, the xy-plane. To show the dispersion in test results, all the test results are plotted in box plots, The data contained within the box is 50% of the total data, the line within the box is the median of the results and the diamond in the box is the average of all the results.

3.1

3.2

Porosity

As shown in figure 5, 6, 7 and 8, the porosity appears as white regions in the photographs. The measured porosity is shown in figure 4 and table 2. The half printed with a layer thickness of 30 µm shows more small porosities, while the total average porosity remains the same. This means that there are either less big pores or the big pores are smaller in size. Figure 5 and 6 also show the scanlines, from bottom right to top left. The scanlines are clearer to see at the 50 µm layer thickness than at the 30 µm layer thickness. There is no apparent connection between the places of the porosities and the distance from the scanlines. There is a big dispersion of porosity between the different samples and even within the samples. The dispersion within the samples is clearly seen in figure 5 to 8. The dispersion in the porosity means that there is also a dispersion in the mechanical properties because material failure often starts at pores in the material.

Density

Using the Archimedes method the density of each of the test samples was determined, see figure 3 and table 1.

The density of the SLM printed samples is higher than the density of the conventionally produced Inconel 718.

Even the sample with the lowest density still has a density of 0.8 percent higher than standard Inc718. This means that the composition of the Inconel 718 powder used for the printing of the samples had a higher density than the average, this is only 1 percent and so probably due to for example the nickel percentage. Nickel has a variation of 5 percent in the composition of Inconel 718.

Figure 3: Density of 30 µm and 50 µm layer thickness Figure 4: Porosity of 30 µm and 50 µm layer thickness

Table 1: Density

Source

Density

Table 2: Porosity

kg/m3

Standard Inc718[5] 8220 Average 30 µm 8301 kg/m3 Average 50 µm 8298 kg/m3 3

Source

Porosity

Average 30 µm Average 50 µm

0.29% 0.30%

Figure 5: 30 µm in xy-plane

Figure 8: 50 µm in xz-plane

3.3

Microstructure

Figure 9 shows a sample printed with a layer thickness of 30 µm and the scanlines are not recognisable, as opposed to figure 10 of a sample printed with a 50 µm layer thickness where the scanlines are clearly visible.

This is supported by figure 5 and figure 6 of the porosity, where the scanlines of the 30 µm layer thickness are less visible than with the 50 µm layer thickness.

The particle surrounding the scanlines are more spread and thus not visible on the same scale. The particles in figure 12 of the 50 µm layer thickness are more equally spread than the particles in figure 11 of the 30 µm layer thickness. On this scale the scanlines are no longer visible.

Figure 6: 50 µm in xy-plane

Figure 7: 30 µm in xz-plane

Figure 9: 30 µm in xy-plane

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As seen in figure 13 and figure 14, the melt pools are clearly visible. The building direction is indicated by an arrow in the upper left corner. The melt pools of the sample with a layer thickness of 50 µm are bigger than the 30 µm layer thickness. The difference in size of the melt pools is because of the layer thickness, as a higher layer thickness creates bigger melt pools. At the border of the melt pools somewhat darker particles gather. These darker particles are probably segregated Nb an Mo, since these are susceptible to segregation.

The border of the melt pool of the 50 µm layer thickness is thinner, probably because the particles of the 50 µm layer thickness are more equally spread as discussed before. Both figure 13 and figure 14 show a big porosity, the pores were recognisable by changing the focus of the microscope.

Figure 10: 50 µm in xy-plane

Figure 11: 30 µm in xy-plane

Figure 13: 30 µm in xz-plane

Figure 12: 50 µm in xy-plane

Figure 14: 50 µm in xz-plane

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places a higher local hardness.

Figure 15: 30 µm in xz-plane Figure 17: Hardness of 30 µm and 50 µm layer thickness Table 3: Hardness

Source Standard Inc718[6] Average 30 µm Average 50 µm

278 - 385 296 286

The SLM manufactured samples show a hardness at the lower end of the hardness range compared to normal Inconel 718, the lower end of the hardness is shown as a line in figure 17 at a hardness of 278. Some of the test results from the samples manufactured with a layer thickness of 50 µm even have a lower hardness than 278, of all the test results are roughly 75% above the 278 line. The low hardness could be because there was no stress relieving treatment. As with the porosity, the hardness also has dispersion between the samples and within the samples. The reason for the dispersion is the porosity and the segregation of Nb and Mo. The segregation of Nb and Mo has a lower or higher hardness as a result.

Figure 16: 50 µm in xz-plane

Figure 15 and figure 16 show the samples in the xzplane on a different scale, in these figures the difference in border of the melt pools is more clearly visible. The sample with a layer thickness of 30 µm shows a thicker border, this means there was more segregation of Nb and Mo. 3.4

Hardness Vickers

Hardness

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The results of the hardness test is shown are figure 17 and table 3. There is a difference in hardness between the different samples and within the samples. A layer thickness of 30 µm results in a higher average hardness than a layer thickness of 50 µm. The samples manufactured with a layer thickness of 50 µm do have more spread in the values for hardness, and have at some

4.1

DISCUSSION Limitations

The limitations of the research were mostly cost and time. The geometry and amount of samples was limited by NLR because of the high cost associated to the manufacturing. Because of this only the hardness was tested as a mechanical property. Because of the limited 6

50 µm layer thickness. There are however visible difference in the distribution of the porosity. The samples manufactured with a layer thickness of 30 µm show more small pores. The microstructure shows that there is more segregation of Nb and Mo with a layer thickness of 30 µm. The hardness of the samples manufactured with a layer thickness of 30 µm show a higher average hardness than with a layer thickness of 50 µm.

25% of the test results from the samples manufactured with a layer thickness of 50 µm show a hardness lower than the hardness of standard Inconel 718. If hardness is a requirement for the manufactured product, it is recommended to manufacture with a layer thickness of 30 µm. In all other cases it is recommended to manufacture with a layer thickness of 50 µm since a higher layer thickness greatly reduces the building time and also the costs.

amount of samples it is not possible to say whether the results obtained from the samples are valid. As shown in the results there is variation of density, porosity and hardness between the samples, there is also dispersion of porosity and hardness within the samples. Because of the dispersion of test results it is possible that the tested samples are of higher or lower quality than the average. Time was also a limiting factor, since it takes a lot of time to manufacture, test and make appointments.

4.2

Polishing scratches

After the grinding the samples were polished with 1 µm diamond spray, but this resulted in scratches on the surface. Diamond spray would normally result in a scratch free surface since the particles are very small and not noticeable with a microscope. This could be due to the manufacturing technique. The diamond spray probably resulted in loose particles of the Inconel to scratch the surface of the samples. OPS was used afterwards for polishing and did not leave any scratches, this is because the particles in OPS are smaller than the particles in the diamond spray.

4.3

ACKNOWLEDGEMENTS The author would like to thank Marc de Smit from NLR for his support during the research and supplying the test samples. The author would also like to thank Prof. dr. ir. T. Tinga from the University of Twente for his support during the research and Laura Córdova González for her help during the research and the testing.

Recommendations REFERENCES

All the samples that were tested were as-built, whereas products manufactured with selective laser melting usually undergo stress relieving treatment. The stress relieving treatment could change the mechanical properties. It is also recommended to test the tensile strength and the elongation at break because the porosity has a big influence at tensile testing. The pores will be the start of tears with tensile testing. Tensile testing will also show the anisotropy of the samples.

[1]

[2] [3]

[4]

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CONCLUSION [5]

From the results of the experiments it is not possible to draw a final conclusion of which layer thickness is better. According to the results, the porosity and the density are the same for 30 µm layer thickness as for

[6]

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Kurzynowski, T., Chelbus, E., Ku´znicka, B., et al., (2012) Parameters in Selective Laser Melting for processing metallic powders, Wroclaw University of Technology, Centre for Advanced Manufacturing Technologies.

Buchbinder, D., Schleifenbaum, H., Heidrich, S., et al., (2001) High Power Selective Laser Melting (HP SLM) of Aluminimun Parts, In: Physics Procedia 12, 271–278 Schleifenbaum, H., Meiners, W., Wissenbach, K., et al., (2013) Individualized production by means of high power Selective Laser Melting, In: CIRP Journal of Manufacturing Science and Technology 2, 161–169 Chlebus, E., Gruber, K., Ku´znicka, B., et al., (2015) Effect of heat treatment on the microstructure and mechanical properties of Inconel 718 processed by selective laser melting, In: Materials Science & Engineering A 639, 647–655 Inconel 718 Technical data, (2016, January 20), Retrieved from http://www.hightempmetals.com/techdata/hitempInconel718data.php Inconel 718 Technical data, (2016, February 12), Retrieved from http://www.specialmetals.com/documents/Inconel%20alloy%20718.pdf