INVESTIGATION INTO THE EFFECT OF BUILD PARAMETERS …

African Journal of Pure and Applied Science Education Volume 18, Number 1, pp 190 – 204, July 2020 www.ajopase.com; [email protected] ISSN 11187670

Umar, I, Ohwofasa, O & Janga, A. A AJOPASE, Vol. 18, No. 1, July 2020 pg. 190

INVESTIGATION INTO THE EFFECT OF BUILD PARAMETERS AND BUILD ANGLE ON THE DOWN SKIN SURFACES FOR STAINLESS STEEL LASER MELTED PARTS

1Umar, I, 2Ohwofasa, O, 3Janga, A. A 1Mechananical Engineering Department, Niger State Polytechnic, Zungeru, Niger State 2Electrical Electronics Department, Federal College of Education Technical Akoka lagos

3Mechananical Engineering Department, Federal Polytechnic, Damaturu, Yobe State [email protected] [email protected] [email protected]

Abstract In the additive manufacturing [AM] technologies, several powder fusion technologies for obtaining metal parts can be adopted. The selective laser sintering technology was used in this experiment to investigate the effect of build parameters and build angle on the down skin surface for stainless steel laser melted parts. Some input parameters, such as laser power, scan speed, and hatching distance were selected for the investigation. The result shows that the surface roughness and surface parameters Ra average value at 45o comparing it with all the other values, at the standard parameters settings, the minimum angle values for overhanging surfaces that can be considered acceptable for SLM process without the use of supports were ranging between 39⁰-45⁰.However, for overhanging surfaces with an angle below 36⁰ will require support structures are needed. The orientation of the part must be considered in the design stage in order to reduce the number of supports and avoid damaging the quality of the surface after their removal. Keywords: Selective Laser Sintering [SLM], Stainless Steel, Surface Roughness [Ra], Additive Manufacturing [AM]

Introduction In the Additive Manufacturing (AM) industry several technologies have been developed, powder bed

fusion being the leading technology for obtaining metal parts. Some of powder bed fusion processes are

alternately known as selective laser sintering, selective laser melting, direct metal laser sintering, direct

metal laser melting, and electron beam melting. All powder bed fusion processes have a similar basic

operating principle. The main differences between processes are in the way layers are deposited to

create parts and in the materials that are used (King et al., 2015). The design stage of the process starts

by creating 3D CAD model of the desired object. Using a pre-defined slicing program, the 3D CAD file is

converted into a series of thin parallel layers that fully describe the geometry of the desired object

creating a 2D image of each layer (Direct Laser Metal Sintering, 2015). The data obtained is transferred

to a computer controlled laser device. Depending on the equipment and the method used, thin layers

of powder are spread with a recoating system onto a platform and a laser or electron beam is used to

http://www.ajopase.com/

mailto:[email protected]

mailto:[email protected]



fuse the powder at locations specified by the model of desired geometry. When one layer is completed,

a new layer of powder is applied and the process is repeated until a 3D part is produced (Bose, Ke,

Sahasrabudhe & Bandyopadhyay, 2017). The applications of rapid prototyping are vast. Over the last

years, AM technologies are continuously expanding in industrial sectors like architectural, medical,

dental, automotive, furniture and jewellery, and many others (Metal, 2014).

Theory and Background

In AM processes, laser sintering is a technique that uses the directed energy generated by a laser to

melt particles of metal powder, layer by layer forming a solid structure. For the experiment discussed

in this paper, the laser sintering method applied is Selective Laser Melting (SLM). SLM is a process based

on powder bed fusion where metal powder is completely melted in order to obtain dense parts

(Jhabvala, Boillat, Antignac & Glardon, 2010). SLM method involves a number of steps that move from

the virtual CAD description to the physical resultant part. The first step is to create a 3D CAD and then

convert the CAD file onto an STL file format because nearly every SLM machine accepts this type of file

format. After completing these steps, using computer software the STL file will be mathematically sliced

it in 2D cross sections, representing the layers that will form the solid structure of the part. The data

obtained is converted into a readable file for the SLM equipment and transferred to the machine that

will be used to create the physical object. The machine must be properly set up prior to the build

process. Such settings would relate to the build parameters. The primary process parameters for SLM

that are related to the quality of the part in terms of surface finish are the scanning speed, laser power,

layer thickness, hatch distance and beam offset (Tian, Tomus, Rometsch, & Wu, 2017), shown in Figure

1.




Figure 1- Process parameters

The build parameters can be standard depending on the machine used, or either modified by entering

them manually by a trained operator (Gibson, Rosen & Stucker, 2015). Laser power is the energy brought

by the laser beam to the powder bed and influences the melting temperature. (Hanzl, Zetek, Bakša &

Kroupa, 2015) Scanning speed (speed of beam over the powder bed) has an important role in laser

sintering because it can affect the mechanical proprieties of the final part. When increasing the

scanning speed, a better quality of the surface finish can be obtained (Taimisto, 2009). Layer thickness

usually influences the building time and represents the depth of each successive addition of metal

powder to the building platform. A lower thickness of layers can decrease surface roughness (Dadbakhsh

& Hao, 2014). Therefore, all build parameters need to be carefully chosen to avoid creating a defective

part.

Experimentation

In the following experiment two sets of parts was manufactured and an investigation into the effects of

the build angle and downskin condition on laser melting Stainless Steel powdered material, has been

carried out. The SLM equipment used in this work is an EOS M270 3 machine which has a Yb-fibre laser

with a variable focus diameter 100 μm - 500 μm and a maximum power output of 200 W (3RSystems,




2014). Following the generic steps for the SLM process, first of all, the 3D CAD model for both sets of

test samples was created using Solidworks 2017 software. In Figure 2 is illustrated the 3D CAD model

and its dimensions.

Figure 2- 3D CAD model of test samples

In Table 1 the angle dimensions (α) for all 11 A parts and B parts are presented. Table 1- Built Angle

parts “A” and “B”

Built Angle (α) for parts “A” and “B”

1 2 3 4 5 6 7 8 9 10 11

18 21 24 27 30 33 36 37 39 42 45

After designing the 3D CAD models for both sets of test samples, the CAD files were converted onto STL

files. Using Magics Software both sets of parts were fixed onto a virtual platform, without the aid of

support structures, although for overhanging structures inclined from 0 to 45 degrees support

structures are needed (Kajima et.al. 2017). For the next step, the STL files for all test samples and the

support files, are sliced and converted using EOS RP TOOLS software in the SLI format, which in the EOS

language represents the part layer by layer. The thickness of each layer was set at 0.02 mm. After




completing this step, the files were transferred to the machine, where settings for the build parameters

and exposure strategies are made as shown in Table 2:

Table 2- Built parameters test samples

Build parameters for set “A” Build parameters for set “B”

Pre-contour Post-contour Pre-contour 1

Pre-contour 2

Post-contour1

Post-contour 2

Power =

40W

Power = 40W Power = 40W Power = 40W Power = 40W Power = 40W

Scan speed =

700 mm/s

Scan speed =

700 mm/s

Scan speed =

800 mm/s

Scan speed =

1200 mm/s

Scan speed =

1600 mm/s

Scan speed =

1800 mm/s

Offset =

0.020 μm

Offset = 0.00 μm

Offset = 0.03 μm Offset = 0.02 μm

Offset = 0.01 μm Offset = 0.00 μm

In the exposure strategy, pre-contours and post-contours can be given by the standard settings of the

machine. Pre-contours are used to define sections to be melted and post contour are used to define the

final size of the component. Contours are an important aspect because the quality of surface finish is

influenced by them. (Calignano, Manfredi, Ambrosio, Iuliano, and Fino, 2012) in figure 3 below, the

pre-contour is illustrated as “contour without beam offset”, and post –contour is presented as “contour

with beam offset”




Figure 3- Contour strategy

In this experiment, the contouring values for group “B” of test samples were modified in order to

investigate if the surface finish of the downskin surfaces will be improved when using two pre-contours

and two post-contours with different scanning speed. Once the build contours and other parameters

ware set on the EOS M270 machine, the build platform was fixed and orientated inside the build

chamber. The build chamber was prefilled with argon gas to protect the parts from the effects of

oxidation. After the completion of the SLM process, the test samples were cut off from the build

platform using WireEDM process. In Figure 4 both sets of test samples obtained by SLM are presented.

Figure 4 – test samples obtained by SLM




Using Guyson Euroblast 4SF dry blasting machine, all parts were blasted with aluminium oxide and after

this process, for each test sample, the downward facing surface was analysed using the confocal laser

microscope Olympus LEXT TS 150 (Figure 5).

Figure 5- Analysing surface finish

RESULTS AND ANALYSIS

Surface roughness is characterized by the deviations in the direction of the normal vector of a real

surface from its ideal form (Strano, Hao, Everson & Evans, 2013). In Table 3 the average values obtained

by analyzing the surface roughness using the confocal microscope are presented, where Ra is the

arithmetical mean deviation of the roughness profile. Ra can be calculated by the formula shown in

Figure 6, where Z(x) is the deviation of surface height at x from the mean height over the profile.




Figure 6– Ra representation

For this experiment, all values for the average surface roughness have been measured by the confocal

laser microscope. Knowing that the parts were designed with varying angles of the overhang surface to

determine the quality of the downward-facing surface when changing the build parameters, and knowing

that for Stainless Steel the angle of 45⁰ is used as the minimum build angle for the process without support

structures, the surface roughness values obtained for set A and set B will be compared and discussed.

Table 3– Ra average values for downskin surfaces.

Part

Number

Overhanging

Angle (α)

Ra for Set A Ra for Set B

1 18 30.9338 29.8409

2 21 21.8308 22.4608

3 24 24.5165 27.3188

4 27 36.7436 25.0643

5 30 24.5165 29.126

6 33 22.1805 30.0908

7 36 14.2019 18.4023

8 37 30.1682 10.0956




9 39 9.2447 13.4115

10 42 6.4263 13.7339

11 45 14.9983 11.5549

For part number A1 and B1, Ra values are approximately the same and the aspect of the overhang

surface built at 18⁰ is poorly. See images from Figure 7 below:

Figure 7– Downskin surface Part A1 and B1

In the chart below, Figure 8 a profile of the surface roughness for part A1 and B1 is drowned.

Figure 8- Profile of surface roughness for part A1 and B1

Looking at Figure 4, a slight improvement can be observed regarding the quality of surface roughness

when increasing the overhang angle. However, even when modifying the build parameters for contours,

PART A1 PART B1




for the overhang angles that are lower than 30 degrees, the quality of surface finish is still unsatisfying.

An image of parts A5 and B5 with the overhanging angle of 30 degrees is presented in Figure 9.

PART A1 PART B1

Figure 9 – Surface finish part A5 and B5

In the chart from Figure 10, a profile of the surface roughness for part A5 and B5 is showing that there

is a there is a small difference between the profiles created. For set B, even if the average Ra values are

higher (see Table 3), the quality of the surface seems to improve.

Figure 10- Profile of surface finish for part A5 and B5

Looking at Table 3, the Ra average value for part A8 is considerably increased compared to Ra average

value for B8 with the overhanging build angle at 37 degrees. In figure 11 the profile of the surface for




both parts is highlighting the fact that for the prescribed dimensions of sample A8 and B8, set B has a

better quality of the downskin surface where two pre-contours and two post contours ware applied.

Figure 11-Profile of surface finish for part A8 and B8

In fig 12, the quality of the surface finish can be compared by simply visualising the images showed. The

differences between set A and set B are clearly visible.

Fig. 12 - Images of set A8 to A 11 and B 8 to B 11




Comparing the Ra average values for sample A11 and B11 (fig 12), the difference between them is

insignificant. In Figure 13 the profile of surface finish shows that the quality of downskin surface for

both sets is almost similar with small differences of Ra values.

Figure 13- Profile defining surface finish for part A11 and B11

Therefore, it is revealed that surface roughness by SLM can be varied by modifying the processing

parameters.

Discussion

Analysis of Ra average values for both set of test samples in relation with build angle (α) of the

downward facing surfaces, can be seen in Figure 14, which shows a clear dependence of surface angle

on Ra. As α increases, the value of Ra decreases




Figure14- Ra in relation with α

The variable parameters for contouring as the offset of the beam, and the scanning for set A. Looking

at Figure 14, for set B when the SLM process was optimised by changing the contouring parameters, the

average values for Ra are variating between 10μm-30μm, and for set A Ra values are between 7μm-

37μm. Even though the difference between both sets of parts is not enormous in terms of surface finish,

modifying the exposure type by adding one more pre-contour and a post-contour, seems favourable

when increasing the scanning speed and laser beam offset. A study made by Caligano (2014) reveals the

importance of using support structures for overhanging surfaces in order to avoid ”staircase effect” of

angled walls and surfaces. It was discovered that for surfaces with an angle smaller than 30⁰, the

staircase effect tended to increase. The minimum orientation of the overhang surface that provided an

acceptable quality of surface finish was the orientation at 45⁰.The experiment was carried out on two

types of material AlSi10Mg and Ti6Al4V alloys. Referring to Figure 14, the minimum value of Ra is

obtained at the 36⁰ angle for set B, and for set A at 39⁰. For both sets of test samples, surface roughness

seems to remain stabilized. A good quality of the downward-facing surfaces is difficult to obtain by SLM

process without the aid of supports. Introducing support structures for the overhanging surfaces will

make the process less cost-effective, however the surface that has the lowest requirement for surface

finish can be considered as the bottom surface when orientating and fixing the part in the design stage

of the process, since the surface attached to the support structure will have an increased roughness

after removal of the support structure (Zeng, 2015).




Conclusion

An investigation into the effect of build angle and downskin condition obtained in SLM using Stainless

Steel has been presented to further the understanding of the relationship between surface roughness

and process parameters. It was found that for Stainless Steel, considering ideal the Ra average value

obtained at 45⁰, and comparing it with all the other values, at the standard parameters settings, the

minimum angle values for overhanging surfaces that can be considered acceptable for SLM process

without the use of supports are ranging between 39⁰- 45⁰. When the process parameters ware

modified, the minimum angle values for overhanging surfaces are ranging between 36⁰-45⁰.To

conclude, for overhanging surfaces with an angle below 36⁰ support structures are needed. The

orientation of the part must be considered in the design stage in order to reduce the number of supports

and avoid damaging the quality of the surface after their removal.

References Bose, S., Ke, D., Sahasrabudhe, H & Bandyopadhyay, A (2017). Additive Manufacturing of Biomaterials.

Journal of Progress in Materials Science. Vol. 92, pp 45-111 Calignano, F., Manfredi, D., Ambrosio, E. P., Iuliano, L & Fino, P. (2012). Influence of process parameters

on surface roughness of aluminium parts produced by DMLS. Retrieved at http://porto.polito.it/2505584/2/2505584.pdf>

Dadbakhsh, S & Hao, L (2014). Effect of Layer Thickness in Selective Laser Melting on Microstructure of Al/5 wt. % Fe2O3 Powder Consolidated Parts. The Scientific World Journal. Vol. 28, pp 91-117

Direct Laser Metal Sintering (2015) Selective laser melting. Retrieved at https://en.wikipedia.org/wiki/Selective_laser_melting

Gibson, I., Rosen, D & Stucker, B. (2015). 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing, Second Edition. Additive Manufacturing Technologies. ISBN 978-1-4939- 21133, pp 3-5

Hanzl, P., Zetek, M., Bakša, T & Kroupa, T (2015). The Influence of Processing Parameters on the Mechanical Properties of SLM Parts. Journal of Procedia Engineering. Vol. 100, pp.1405-1413

Kajima, Y, Takaichi, A, Nakamoto, T, Kimura, T, Kittikundecha, N, Tsutsumi, Y, Nomura, N, Kawasaki, A, Takahashi, H & Hanawa. T (2017). Effect of adding support structures for overhanging part on fatigue strength in selective laser melting. Journal of the Mechanical Behaviour of Biomedical Materials. Vol. 78, pp. 1-9

King, W. E, Anderson, A. T, Ferencz, R. M, Hodge, N. E, Kamath, C, Khairallah, S. A. & Rubenchik, A. M. (2015). Laser powder bed fusion additive manufacturing of metals; physics, computational, and materials challenges. Applied Physics Reviews. Vol. 2, pp 1-3

Metal, A.M (2014), Applications for metal Additive Manufacturing technology. Retrieved at http://www.metal-am.com/introduction

Strano, G., Hao, L., Everson, R.M. & Evans, K.E. (2013). Surface roughness analysis, modelling and prediction in selective laser melting. Journal of Materials Processing Technology. Vol. 213, pp. 589-597

Taimisto, L (2009). Process parameters in laser sintering process. (Bachelor’s thesis). Retrieved at https://www.doria.fi/bitstream/h&le/10024/70831/nbnfife201109082361.pdf?sequence=3


http://porto.polito.it/2505584/2/2505584.pdf

http://porto.polito.it/2505584/2/2505584.pdf

https://en.wikipedia.org/wiki/Selective_laser_melting

http://www.metal-am.com/introduction



Tian, Y., Tomus, D., Rometsch, P & Wu, X (2017). Influences of processing parameters on surface roughness of Hastelloy X produced by selective laser melting. Journal of Additive Manufacturing. Vol. 13, pp 103-112

Zeng, K (2015). Optimization of support structures for selective laser melting. Retrieved at https://ir.library.louisville.edu/cgi/viewcontent.cgi?article=3250&context=etd


Documents

INVESTIGATION INTO THE EFFECT OF BUILD PARAMETERS …