www.elsevier.com/locate/apsusc

Applied Surface Science 253 (2007) 8064–8069

Parametric analysis of the selective laser melting process

I. Yadroitsev, Ph. Bertrand, I. Smurov *

Ecole Nationale d’Ingenieurs de Saint-Etienne (ENISE), DIPI Laboratory, 58 rue Jean Parot, 42023 Saint-Etienne Cedex 2, France

Available online 4 March 2007

Abstract

Selective laser sintering/melting (SLS/SLM) technology is used for manufacturing net-shaped objects from commercial Inox 904L powder with

�20 mm particle size. Experiments were carried out on PHENIX-PM100 machine equipped with a 50 W cw fiber laser. Powder is layered by a

roller over the surface of a 100 mm-diameter build cylinder. Optimal parameters of layer thickness and power input per unit speed for SLM were

determined. It was shown that the greater the value of P/V ratio is, the larger is the remelted line (called as ‘‘vector’’). Influence of the shifting of

consecutive single vectors on the process of forming the first layer was studied. Different strategies for forming objects with less than 1 mm-sized

inner structures were tested, as, for example, forming a 20 mm � 20 mm � 5 mm box with 140 mm-thick inner compartment walls.

# 2007 Elsevier B.V. All rights reserved.

PACS : 42.62.–b; 64.70.Dv; 28.52.–s

Keywords: Rapid manufacturing; Selective laser melting; Powder

1. Introduction

Nowadays, SLS/SLM technologies are widely used in

various industries, medicine and research offering a range of

advantages compared to conventional manufacturing techni-

ques: shorter time to market, use of inexpensive materials,

higher production rate, versatility, high part accuracy, ability to

produce more functionality in the parts with unique design and

intrinsic engineered features [1].

Consolidation of loose powder by local laser heating is

becoming a promising manufacturing technique because of the

easy control over both powder deposition and laser radiation.

SLS/SLM technology makes it possible to create fully

functional parts directly from metals, ceramics, plastics without

using any intermediate binders or any additional processing

steps after the laser sintering operation [2,3].

SLM process begins with a completely defined CAD model

of the part to be made. Divided into cross-sections by a special

software, the model is then directly involved in the process.

The essential operation is the laser beam scanning over the

surface of a thin powder layer previously deposited on a

* Corresponding author. Tel.: +33 4 7791 0161; fax: +33 4 7774 3497.

E-mail address: smurov@enise.fr (I. Smurov).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.02.088

substrate. The forming process goes along the scanning

direction of the laser beam. Each cross-section (layer) of the

part is sequentially filled with elongated lines (vectors) of

molten powder. The quality of a part produced by this

technology depends strongly on the quality of each single

vector and each single layer. Identification of the optimal

parameters of laser power and scanning speed is a crucial task

because these parameters happen to be the most influential on

the part’s characteristics (porosity, hardness and mechanical

properties).

Powder binding mechanisms, such as melting and solid-state

or liquid-phase sintering, depend on temperature, thus local

temperature fields are important for process stability and

quality of the fabricated objects [4–6].

The following index of process variables for SLS/SLM

technologies can be established:

(a) P

owder: composition, size distribution, shape, optical and

heat transfer properties, thickness of deposited layer for

each cycle of fabrication.

(b) L

aser: power, spot size, beam spatial distribution, scanning

velocity and application of protective gas atmosphere.

(c) S

trategy of manufacturing: decomposition of each plane to

be sintered on a number of elementary elements (vectors),

definition of orientation and distance between them,

I. Yadroitsev et al. / Applied Surface Science 253 (2007) 8064–8069 8065

definition of relative positions of elementary elements in

two consecutive planes.

Fig. 1. Width of a laser sintered line (so-called ‘‘vector’’) from Inox 904L

powder on steel substrate (layer thickness 50 mm, P = 50 W) versus beam

velocity and of a remelted track on the same substrate formed without powder.

Fig. 2. Laser sintered vectors from Inox 904L powder on steel substrate.

Thickness of the deposited powder layer is 50 mm, effective power input per

unit speed P1/V1 = P2/V2 is constant for the corresponding vectors in (a) and (b):

(a) P1 = 50 W, variation of scanning speed in the range V1 = 0.06–0.24 m/s; (b)

P2 = 25 W, V2 = 0.03–0.12 m/s.

A fairly evident trend for miniaturization of devices and

components in multiple applications makes it important to

develop definable, repeatable, predictable processes of fabri-

cating objects with less than 1 mm-sized inner structures. Using

SLM technology in this purpose shows considerable promise

and therefore requires research effort to maximize process

stability and accuracy in respect to the desired dimensions of

the final product.

2. Experimental procedure

2.1. Experimental equipment

In this study the PM 100 machine of Phenix Systems with

the typical for this kind of equipment design was used [7,8].

This system allows to manufacture parts from metallic and

ceramic powders within a cylindrical volume of 100 mm

diameter and up to 100 mm in height. The source of radiation is

a YLR-50 cw fiber laser manufactured by IPG Photonics with a

maximum power P = 50 W, the wavelength l = 1075 nm and

the laser spot size d � 70 mm.

Powder is layered by a roller. The minimum layer thickness

is 5 mm (for finer grained powders than those used in this

study). After the laser beam scanning of each cross-section, the

manufacturing plate is lowered by the thickness of the layer to

be deposited thus keeping the surface in the focal plane.

2.2. Materials

The difference of PHENIX strategy in comparison to other

manufacturers is the use of a wide range of commercially

available powder. In the present experiments Inox 904L powder

was used that is an austenitic stainless steel with low carbon level.

The composition is the following: Fe (Bal.), Ni (23–28%), Cr

(19–23%), Mo (4–5%), Cu (1–2%), Mn (2% max), Si (1% max),

C (0.02% max), P (0.045% max), S (0.035% max). The powder

size distribution was studied using a granulo-morphometer

ALPAGA 500NANO (OCCHIO s.a.). Powder particles were

mainly spherical and 95% of them had a size less than 20 mm.

3. Results and discussion

3.1. Influence of the substrate on forming single vectors of

the first layer

It is evident that the conditions of powder melting close to the

substrate and far from it are different because of the heat losses.

An initial experiment in forming single vectors on the substrate

(stainless steel 304L) without powder was carried out. Tracks on

the substrate surfacewere formed by laser beam with the power of

50 W at different scanning speeds ranging from 0.06 to 0.24 m/s

with a step of 0.02 m/s. Thewidth of a track was considered as the

width of the molten zone and measured. Then a 50 mm thick layer

of Inox 904L powder was deposited onto the steel substrate, and

single vectors were formed under the same laser irradiation

conditions.Fig. 1 plots thewidth of the singlevectors formed with

and without powder versus scanning speed of the laser beam.

The single vector formed with powder is wider than the

molten zone on the steel substrate. Powder has a smaller

thermal conductivity and a higher absorptivity compared to the

substrate material. It’s worth to mention the effect of

denudation of the substrate owing to the fact that not only

the powder in the laser irradiation zone is involved in the single

vector forming process but also the powder from adjacent areas.

Fig. 2 shows photographs of single vectors of a 50 mm thickness

layer of Inox 904L powder formed on the substrate, a light halo

around the sintered vectors stands for the effect of denudation

of the substrate.

3.2. Influence of the energy input on forming single vectors

of the first layer

The next step of experiments consisted in the comparison of

the results obtained for the single vectors formation at a

Fig. 3. Laser sintered vectors from Inox 904L powder on steel substrate

(thickness of each powder layer is 50 mm) for V = 0.14 m/s. Variation of power

in the range of P = 5–50 W with a step of 5 W: (a) one layer, (b) two layers and

(c) 10 layers of powder.

Fig. 4. Width of laser sintered vectors of the first, second and 10th layer from

Inox 904L powder on steel substrate for scanning speed V = 0.08 m/s and

V = 0.18 m/s; thickness of each powder layer is 50 mm; variation of power is in

the range of P = 5–50 W with a step of 5 W.

I. Yadroitsev et al. / Applied Surface Science 253 (2007) 8064–80698066

constant power per unit speed P/V = const, where P – laser

radiation power and V – scanning speed (Fig. 2). One may note

that this is equivalent to energy input per unit length. For the

maximum laser power of 50 W the scanning speed ranged from

0.06 to 0.24 m/s with a step of 0.02 m/s, for the laser power of

25 W—from 0.03 to 0.12 m/s with an step of 0.01 m/s.

At a constant power per unit speed the corresponding single

vectors are equal in width within the measurement error. The

existence of stability and instability zones must be noted: the

former ones are characterized by the formation of a stable

molten pool and continuous vectors, the latter ones are

presented by non-continuous vectors and the drops formation.

As it is seen in Fig. 2, stable zones of single vectors are formed

at a scanning speed from 0.06 to 0.09 m/s at the power of 25 W

and from 0.06 to 0.18 m/s at the power of 50 W. At the laser

power of 50 W non-continuous single vectors occurs at the

scanning speed of 0.20 m/s and higher; at that of 25 W broken

single vectors are formed at scanning speeds under 0.06 m/s,

while at speeds above 0.09 m/s the drops formation starts

abruptly.

It is evident that for the given laser power the temperature

and the volume of the molten powder is higher at lower

scanning speeds. It is also known that surface tension

coefficient as well as melt viscosity decrease with temperature

[9]. Therefore when the effective power (i.e. power per unit

speed) is insufficient, surface tension brakes the melting vector

on a series of individual droplets (see the right part of Fig. 2).

With the increase of energy input, continuous melted vectors of

regular width are formed (central part in Fig. 2). With the

further increase of effective power that is accompanied by an

increase of the melt volume and a decrease of the melt viscosity,

melt hydrodynamics (driven by Marangony effect) becomes

more important. With the decrease of the scanning speed, the

heat affected zone becomes larger involving more powder from

the vector boundaries, the molten vectors become irregular. The

formation of individual melt droplets outside the ‘‘vectors’’ is

seen in the left part of Fig. 2.

It was found that the same effective energy input into the

powder material at different laser power leads to the formation

of different patterns of the single vectors. In the range of the

studied parameters of laser irradiation, good quality single

vectors from Inox 904L may be formed by choosing P = 50 W,

V = 0.1–0.18 m/s and P = 25 W, V = 0.06–0.09 m/s, P/

V � 270–420 W s/m or 270–420 J/m.

3.3. Influence of the laser irradiation parameters on the

process of forming multilayer structures

The actual tendency in SLS/SLM is that the treated powder

layers become thinner (down to few monolayers of particles)

and their thickness becomes comparable with the penetration

depth of laser radiation [10].

A 50 mm thick powder layer was deposited onto the steel

substrate and sintered at a given scanning speed and with the laser

power varying from 5 to 50 W with a step of 5 W. Values of the

scanning speed were chosen taking into account the results of the

previous experiments in forming a continuous single vector that

is from 0.06 to 0.18 m/s with a step of 0.02 m/s. The second

powder layer of the same thickness was then sintered at the same

parameters of laser irradiation. By this way, 10-layer structures

were built. A comparative analysis of the experimental results for

the first, second and 10th layer was carried out.

Fig. 3 represents photographs of the structures obtained at a

scanning speed of 0.14 m/s. Fig. 4 plots the width of the single

vectors for the different scanning speed values and for the

different numbers of deposited layers.

By analyzing Fig. 3, the conclusion comes that the formation

of continuous single vectors starts at lower laser powers with

the increase of the number of layers because of the weakening

of the influence of the substrate. The thermal conductivity

coefficient of the bulk material of the substrate is much higher

than that of the powder, in consequence, when forming the first

layer, much energy goes to heating the substrate thereby single

I. Yadroitsev et al. / Applied Surface Science 253 (2007) 8064–8069 8067

vectors are formed non-continuous when laser power is

insufficient. With the increase of the number of layers, heat

exchange with the substrate and thus heat transfer losses

diminish, a greater amount of laser energy absorbed by the

powder goes to melting it, i.e. the energy input in the powder

increases. This may cause shifting from the instability zone to

the steady one and vice versa (see Fig. 2). For example, at the

power of 30 W single vectors of the first layer are non-

continuous, the second layer demonstrates a better quality of

the molten zone while the 10th-layer single vector is practically

continuous.

It is known that SLS/SLM technique suffers from various

instabilities like reported in Refs. [11,12] which increase the

surface roughness and volumetric porosity of the fabricated

parts. Identifying the optimal parameters of energy input and

the heat transfer phenomena allows to stabilize thermal

processes and avoid the ‘‘balling effect’’.

Fig. 4 shows that with the increase of the number of layers

the difference in width of single vectors formed at different

scanning speeds becomes smaller. It is explained by the fact

that with the diminishing of the heat transfer losses into the

substrate, the influence of the laser spot size and the optical/

thermophysical properties of the powder on the single vector

width becomes predominating.

3.4. Influence of the single vector shifting on the process of

forming the first layer

The next step in studying the conditions for the formation

of a 3D object was experiments in the single consecutive

Fig. 5. Surfaces of the first layer obtained at different vector shifting distances. Thick

speed is V = 0.14 m/s, vector shifting distances are (a) 60 mm, (b) 100 mm, (c) 12

vectors shifting by a given value in the plane of the beam

scanning.

Based on the results of the previous experiments, the laser

irradiation parameters were fixed as follows: laser power

P = 50 W, scanning speed V = 0.14 m/s. The value of the

consecutive vectors shifting varied from 60 to 280 mm with a

step of 20 mm. Having in mind the effect of denudation of the

substrate, 60 mm is the minimal initial shifting distance for the

given powder.

Fig. 5 represents characteristic images of the surfaces of the

first layer obtained at different vector shifting distances

(thickness of the powder layer on the substrate is 50 mm). A

gap between single vectors appears at shifting distances greater

than 120 mm which means that the quantity of powder is

insufficient. Therefore in order to obtain a good quality part the

value of the maximum shifting distance should not exceed

120 mm under the given conditions.

3.5. Strategy of the thin wall formation

At the first stage of this experiment, a series of thin

walls was fabricated from CAD data: the widths are 100,

200 and 400 mm, the length is 20 mm, the height is 5 mm.

Three strategies of single vectors forming were applied:

in lengthwise (08), transversal (908) and angular (458)direction. The laser irradiation power is 50 W, the scanning

speed is 0.14 m/s, the powder layer thickness is 50 mm,

the single vector shifting distance is 60 mm. Objects

fabricated by the above mentioned strategies are presented

in Fig. 6.

ness of the powder layer on the steel substrate is 50 mm, power is 50 W, scanning

0 mm and (d) 140 mm.

Fig. 6. Top view of thin walls fabricated by different strategies for forming

process: in lengthwise (08), transversal (908) and angular (458) direction.

Specified CAD data are (a) 400 mm, (b) 200 mm and (c) 100 mm.

Fig. 7. Example of thin-walled object fabricated by SLM technology from Inox

904L powder on PM100 machine of Phenix Systems: (a) general view and (b)

top view of the thin wall. Powder layer thickness on steel substrate is 50 mm,

power is 50 W, scanning speed is V = 0.14 m/s.

I. Yadroitsev et al. / Applied Surface Science 253 (2007) 8064–80698068

As it is seen in Fig. 6, the minimal possible thickness of a

wall from the applied powder is 140 mm (and not 100 mm as in

CAD data) as it is defined by the width of the single vectors

which form the wall. The optimal strategy for forming thin

walls is found to be the lengthwise one.

The above experiment shows that in order to fabricate

objects with less than 1 mm-sized inner structures directly from

CAD data, correction factors must be applied.

The concern of the second phase of the experiment was to

fabricate a 20 mm � 20 mm � 5 mm box with inner compart-

ment walls (Fig. 7). The parameters of laser irradiation were the

following: power is 50 W, scanning speed is 0.14 m/s; powder

layer thickness is 50 mm. A good quality object with the inner

walls of a thickness of 140 mm was obtained. Long-term

stability of SLM process was controlled by fabrication of other

thin-walled matchbox-shaped objects during 36 h. Finally the

developed technology was applied for fabrication of parts for

mechanical (mini pomp), medical (dental prosthesis, implants),

bio-medical (diagnostic equipment) applications.

4. Conclusion

By parametric analysis of SLM process for Inox 904L

powder with 95% of particles having size less than 20 mm, the

optimal parameters of powder layer thickness and power input

per unit speed were determined as 50 mm and P/V = 270–

420 W s/m, respectively. It was shown that the greater the value

of P/V ratio is, the larger is the remelted vector, besides with the

increase of the number of layers the difference in width of

single vectors formed at different scanning speeds lessens.

Analysis of the influence of the single vector shifting on

I. Yadroitsev et al. / Applied Surface Science 253 (2007) 8064–8069 8069

forming the first layer showed that the value of the maximum

shifting distance should not exceed 120 mm. Strategy for

forming objects with less than 1 mm-sized inner structures was

elaborated: the optimal method for forming thin walls is to do it

in lengthwise direction necessarily applying a correction factor

to make the resulting dimensions of the part conform to those

specified in CAD. The results of the parametric analysis were

successfully implemented in manufacturing functional proto-

types for mechanical and bio-medical applications.

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