CHARACTERIZATION OF SURFACE QUALITY FOR INCONEL 625

COMPONENTS MANUFACTURED BY

SELECTIVE LASER MELTING

Benjamin Schiller1, André Edelmann2, Ralf Hellmann3 1 B. Eng., Master student

2 Ph. D., Head of Centre of Additive Manufacturing 3 Prof. Dr., Head of Applied Laser and Photonics Group

1,2,3 Applied laser and photonics group, University of Applied Sciences

Aschaffenburg, 63743 Aschaffenburg, Germany

ABSTRACT

The surface quality of components manufactured by Selective Laser Melting is

strongly affected by various process parameters. In this study, we focus on the

influence of laser power and scanning speed on the surface roughness of test

components made of the nickel-based alloy Inconel 625. We find that a superior

surface roughness RZ of about 155 µm can be achieved with a laser power level of

250 W, an exposure time of 50 µs and a point distance of 40 µm. Even small

deviations from these optimized parameters may significantly alter the surface quality

in the form an increased surface roughness and the appearance of a balling effect and

splatter.

INTRODUCTION

Additive Manufacturing (AM) technologies become increasingly important for

industrial applications, especially in medical engineering, automotive and

aerospace [1]. AM allows the fabrication of three-dimensional components with

highly complex and filigree structures due to the underlying layer wise process [2].

This opens a new scope of applications such as individualized prosthesis or

lightweight structures [1]. Despite the high potential and the rapid development of

these technologies, there remains a high need for basic and applied research in order

to produce marketable, economic and fully functional products. One of these aspects

is the qualification of materials through the optimization of process parameters [3].

For this purpose, it is mandatory to identify the most influencing parameters on

process efficiency, component properties and device performance, as well as their

interactions. Selective Laser Melting (SLM) enables the production of metal

components from various metals, such as, e.g., tool-steel, aluminium or titanium. The

processability of different metals, however, depends sensitively on the thermo-

physical, metallurgical and optical properties, such as enthalpy of fusion, thermal

conductivity, or reflection coefficient [4]. This paper addresses the SLM process of

the nickel-based alloy Inconel 625. In particular, we focus on the influence of the

applied laser power and scanning speed on the surface roughness.

MultiScience - XXXI. microCAD International Multidisciplinary Scientific ConferenceUniversity of Miskolc, Hungary, 20-21 April 2017

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DOI: 10.26649/musci.2017.022

SELECTIVE LASER MELTING

Selective Laser Melting is a powder bed based approach generating metal

components in a layer-wise process. Firstly, a wiper applies a thin layer of metal

powder with a thickness between 20 and 100 µm on a building platform.

Subsequently, a laser beam, which is focussed and controlled by a galvo-scanner

system mounted over the building platform, locally melts the powder along a

trajectory that represents the geometry of the component to be manufactured [1].

While the laser scans across the powder bed, the melted powder solidifies. After

completion of the entire geometry within one layer, the building platform is lowered

by one pre-set layer thickness in z-direction and the cycle repeats with the next layer.

The process iterates as long as the component is completely built. Finally, the excess

powder, which was not molten can be removed and the generated component can be

separated from the building platform [5]. To prevent oxidation, the building space is

flooded with an inert gas, like argon. Further, the building platform is heated to reduce

warpage [6].

Fig. 1

Process principle of SLM: The wiper applies a layer of powder on the building

platform (left) before a laser beam melts the powder (right)

The metallurgical, geometrical and mechanical properties of thus fabricated

components depend on various influencing factors, such as user defined conditions

as pre-set layer thickness, component orientation and selected material, or process-

related parameters as, e.g., applied laser power and scanning speed, scanning strategy

or powder quality [7, 8]. The relevant process parameters are described in Table 1

and illustrated in Figure 2. The scanning strategy results by a well-defined

combination of the scanning parameters (Table 1). For an increased homogeneity of

the material, the scanning directions can be shifted by 90° and alternate after each

layer [4, 5, 9]. In particular, the scanning strategy defines the energy input into the

powder layer. Please note that not only the top layer but also material beneath is

melted during the process. Thus, a complete connection of both layers without lack

of fusion and a homogenous microstructure can be ensured [5]. However, this leads

to an anisotropic material behaviour, because the connection on x-y-level (layer level)

is usually higher than in z-direction [10].

Table 1

Description of process parameters of the SLM-Process

Process parameter Description

Laser power PL [W] A measure for power of the laser beam

Point distance Δxpoint [µm] Distance between two melting points

Exposure time tex [µs] Residence time of the laser beam in a point

Hatch Movement or scanning vectors of laser beam

Hatch distance Δxhatch [mm] Distance of two parallel scan vectors

Inner and outer Boundary Delimitation to the Hatch

Scanning speed vs [mm/s] Quotient of point distance and exposure time

Fig. 2

Energy input process (left) and exposure parameters of a single layer (right) [5, 7]

As during the SLM process the powder is merely smoothed by a wiper and not

compressed, the flowability and the bulk density of the powder are central aspects for

powder quality consideration with the objective to instantly achieve maximum

powder density after depositing the next layer by the wiper system [7]. The

flowability in turn depends on the adhesive forces of the single powder particles

among each other, their form and size as well as on the relative humidity. In general,

the grain shape should be spherical and without any adhesions. Therefore, for a high

stability of the SLM-process it is necessary to control and to persistently examine the

powder quality [7].

EXPERIMENTAL

Our experimental study is conducted using a selective laser melting machine

equipped with a single-mode fibre laser (SLM 300i, Realizer GmbH Germany) and

being loaded by the nickel based alloy Inconel 625. To optimize the surface roughness

we vary the laser power PL (210 W to 290 W) and scan speed vs, which is being

defined by the ratio between the point distance Δxpoint and exposure time tex). To vary

the scanning speed only the exposure time was changed (30, 40 and 50 µs), while the

point distance was kept constant at 40 µm. Other influencing parameters such as hatch

distance, laser spot size or component orientation remain unchanged (see table 2).

For a statistically based analysis, five specimens (a cube with an edge length of

10mm) for each parameter set were produced (see figure 3). Based on the following

parameter table, 75 specimens with 15 different parameter settings have been

generated.

Table 2

Parameters of SLM-Process

Process parameter Values for Hatch Values for Boundary

Laser power PL [W] 210, 230, 250, 270, 290 137

Exposure time tex [µs] 30, 40, 50 20

Point distance Δxpoint [µm] 40 10

Scanning speed vs [mm/s] 1333, 1000, 800 500

Hatch distance Δxhatch [mm] 0,08 -

Hatch Offset xoffset [mm] 0,05 -

Layer thickness Δzlayer [µm] 50

Platform heated [°C] 100

Atmosphere argon

Fig. 3

Test series during (left) and after (right) the manufacturing process

RESULTS AND DISCUSSION

Figure 4 summarizes microscope pictures of the generated specimens for different

laser power levels and exposure times with the measured surface roughness Rz (3D-

profilometer, Keyence VR 3200) given as an inset. Apparently, the surface quality

varies significantly for different parameter sets. For example, employing the lowest

laser power of 210 W and the shortest exposure time of 30 µs results in an incomplete

melted powder layer. Although the powder is melted by the laser beam, there is no

homogenous and consistent melting track. This can be associated to the so-called

balling effect, defined by the appearance of balls with diameters larger than the layer

thickness. As a result, on the surface randomly distributed balls can be observed

instead of a complete and uniformly molten layer or track [4]. Using, on the contrary,

the higher laser power of 290 W and the longer exposure time 50 µs, leads also in a

poor surface quality that can be ascribed to splatters during the SLM process.

Fig. 4

Microscope images of the specimen surfaces (10 mm x 10 mm) produced with

different SLM parameters

In between these extreme combinations of the laser power and exposure time, the

visual surface quality and measured surface roughness vary significantly, yet showing

an optimum in a power range about 250 W and an exposure time around 50 µs. In

this parameter regime, the surface is characterized by a homogenous melting track

without any splatters and lowest Rz. The results of the surface roughness

measurements are shown in Figure 5, confirming lowest surface roughness at a laser

power level of 250W and an exposure time of 50 µs. In addition, it appears that small

variations of the laser power may alter the surface roughness considerably. This in

turn, implies a profound need for stable process conduction.

Fig. 5

Average surface roughness RZ as function of laser power PL for three different

exposure times

CONCLUSION

We have demonstrated the influence of laser power and exposure time on the

surface quality of selective laser melted Inconel 625 components. As a result of a

comprehensive parameter study, we identify an optimized process window to achieve

high visual surface quality as being characterized by a homogeneous melting track

with vanishing balling effect and lowest surface roughness. In addition, we find the

surface quality being very sensitive to process parameter variations, which in turn

necessitates heedful process control.

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