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Experimental design approach to optimize selective laser melting of martensitic 17-4 PH powder: part I – single laser tracks and first layer M. Averyanova Ecole Nationale d’Ingeńieurs de Saint-Etienne (ENISE), Universite´ de Lyon, Saint-Etienne, France

E. Cicala Mechanical Engineering Faculty, “Politehnica” University of Timisoara, Timisoara, Romania

Ph. Bertrand Ecole Nationale d’Ingeńieurs de Saint-Etienne (ENISE), Universite´ de Lyon, Saint-Etienne, France, and

Dominique Grevey Laboratoire Interdisciplinaire Carnot de Bourgogne, CNRS-Universite´ de Bourgogne, Le Creusot, France Abstract Purpose – The purpose of this paper is to investigate the effect of main process parameters of selective laser melting (SLM) technology on single lines and single layers manufactured from 17-4 PH martensitic powder using the experimental design approach. Design/methodology/approach – A fractional factorial approach has been applied to vary and to identify the optimal set of process parameters using three different powder particle size distributions for 17-4 PH steel. This paper assesses the impact of influence factors such as process and material parameters on objective factors such as dimension of single lines and single layers, as well as surface roughness. Findings – The influence of process parameters and materials properties on single line and single layer manufacture is shown and proved statistically. The effect of each process parameter and their interactions on single layer and single line stability and quality has been investigated, and a complex objective function analyzing geometrical stability of single lines has been proposed. The findings indicate the most appropriate 17-4 PH powder particle size distribution. Originality/value – The research provides a systematic scientific approach using fractional factorial experiment design to identify the influence of process parameters, materials parameters and their combinations on essential martensitic steels (17-4 PH steel) single lines and single layers characteristics such as geometrical stability and surface roughness. This approach will be extended to 3D parts fabrication and reported in a later paper. Keywords Manufacturing systems, Steels, Sintering, Selective laser melting, SLM, Experimental design, Martensitic steel Paper type Research paper

structure, functional-graded materials, functional-graded coatings, etc. for aerospace, nuclear, chemical and petrochemical uses, among other applications (Kruth et al., 2004a, b; Emmelmann et al., 2009; Stamp et al., 2009; Yadroitsev et al., 2007a; Hollander et al., 2003; Hao et al., 2009). Other applications can be found in the medical field (implants, tissue engineering scaffolds), porous structure, dental restoration – creation of dental crowns, etc. (Gibson, 2005; Bartolo et al., 2009). However, the list of materials available to SLM is still limited. The most investigated materials are iron powders, alloy Inconel 625, stainless steel powders 316L, M2, tool steels, and some maraging steels (Kruth et al., 2004a, b; Badrossamay and Childs, 2007; Osakada and Shiomi 2006). The SLM is defined as a complex, non-equilibrium process characterized by several fast-occurring physical mechanisms such as melting, evaporation and resolidification (Kruth et al., 2007; Santos et al., 2006; Fox et al., 2008). The successful

Introduction Selective laser melting (SLM) is a powder-based additive manufacturing technology that makes it possible to manufacture complex high geometrical accuracy parts directly from its CAD data. This is a layered fabrication process that can create functional parts with different porous or approximately 100 percent dense internal structure by melting powder particles using a laser (Meiners et al., 2005). These functional parts are difficult to create using conventional manufacturing methods. The fields of application are varied and include the fabrication of internal cooling channels, complex weight-light The current issue and full text archive of this journal is available at www.emeraldinsight.com/1355-2546.htm

Rapid Prototyping Journal 18/1 (2012) 28– 37 q Emerald Group Publishing Limited [ISSN 1355-2546] [DOI 10.1108/13552541211193476]

This research work was supported by CETIM (French Technical Centre for Mechanical Industry).

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Selective laser melting of martensitic 17-4 PH powder: part I

Rapid Prototyping Journal

M. Averyanova, E. Cicala, Ph. Bertrand and Dominique Grevey

Volume 18 · Number 1 · 2012 · 28 –37

manufacturing of functional parts depends on the process and materials parameters. Currently, the effect of process parameters on densification mechanism and parts properties is more studied (Abe et al., 2001; Karapatis, 2001; Kruth et al., 2004a, b) than the powder properties. The SLM explores a systematic approach in simulation, optical diagnostics and process engineering. The technology poses a certain number of input-controlled and noncontrolled process and materials parameters that make it possible to manufacture complex geometry parts with desirable mechanical properties and microstructure. The SLM produced parts are characterized by anisotropic microstructure, high residual stresses, high surface roughness, presence of low porosity (about 2 percent) (Mercelis and Kruth 2006; Bacchewar et al., 2007). The reproducibility of parts with the desired properties was shown in Rehme and Emmelmann (2005). Actually, neither the process nor manufactured parts meet any standards, which also accounts for the limitation of SLM industrial dissemination.

through shrinkage modelling in the SLS process is obtained by using the Taguchi method (Raghunath and Pandey, 2007). It shows that the quality of each manufactured single layer strongly depends on energy density. Consequently, in order to obtain parts with the necessary properties, it is important to investigate the influence of each process parameter (Mumtaz and Hopkinson, 2009; Chatterjee et al., 2003). Finally, the literature review shows that for several types of material with the proper choice of input parameters, the SLM technology is able to produce parts with mechanical properties closely resembling parts manufactured by conventional methods. Using experimental design approach, it is important to conduct as few experiments as possible and to obtain as much information as possible. Based on our laboratory research experience and the SLS/SLM literature review, four process parameters (input parameters) namely, laser power, scanning speed, layer thickness and hatch distance were identified as being the most important ones to be analyzed.

Objectives Literature review

The objective of the present paper is to use a scientific approach to better understand and characterize the physical processes managing the SLM process. One may analyze the effect of main process and materials input parameters, and their interaction on 1D (single track) and 2D (single layer) objects fabrication via fractional factorial design of experiments (DOE) method (Montgomery, 1991). The strategy that has been applied by the authors consists of a three-stage method. First, the preliminary tests will be performed in order to classify and analyze the significance of input parameters like laser power, scanning speed, layer thickness and hatch distance on stability, dimensional accuracy of tracks and roughness of single layers. Second, a number of experiments will be conducted to determine an optimal domain of input factors. Finally, 3D parts with necessary density, hardness and microstructure will be manufactured (reported in a latter paper). The choice of 17-4 PH martensitic precipitation hardening stainless steel powder has been industrially driven. This type of material is widely used in aerospace, petrochemical and chemical applications, as well as other fields, because the mechanical properties present an appropriate balance and also good corrosion resistance (Antony et al., 1963).

In literature review the laser power, the scanning speed and interaction between them (energy density) is presented as the most significant input parameter that affects the quality of produced parts (Simchi, 2006; Mumtaz and Hopkinson 2009; Kruth et al., 2004a, b). The enhancement of energy density up to an optimal value changes the viscosity and surface tension of the melted material that finally results in a part with higher density (Simchi and Pohl, 2003). It is estimated that the layer density and how homogenous it is, influences the final density of the part. Spherical, non-porous particles provide a denser layer (Karapatis, 2001; Rombouts et al., 2006). It is reported that an optimal layer thickness that depends on the type of material, the powder particles size and the building time, should be determined (Dai and Shaw, 2006). Before the fabrication of parts, a search for an appropriate “parameters window” is necessary. This “parameters window” differs according to the type of materials and depends on materials thermo-physical properties. Three different types of process behaviour can be observed: lack of melting with irregular unstable tracks, melting with stable tracks and the balling effect (Klocke and Wagnec, 2003). Balling is defined as the breakup of the melt pool into spheres due to the high surface tension forces (Gusarov et al., 2007; Kruth et al., 2007). The effect of the main process parameters depends on the combinations of their values. Experiment design should be applied to determine the significance of the parameters, the effect of their interactions, to better understand the process and optimize the “parameters window”. It should be noted that contrary to the SLM process a considerable number of investigations of the selective laser sintering (SLS) process has already been done and reported (Kumar, 2003). An overview of the published studies about the application of experiment design to SLS/SLM process shows that researchers often use a Taguchi method to find out the significance of the impact of process factors on the mainly characterized properties manufactures samples such as density, porosity and hardness (Liao et al., 2007; Dingal et al., 2008; Jailani et al., 2009). One distinctive feature of the laser melted process is shrinkage due to density changes. Improving accuracy

Experimental methodology and equipment Process parameters Experiments were carried out on PHENIX System PM 100 machine. The PM 100 is equipped with IPG Photonics fiber laser with a Gaussian profile, delivering a maximum power P ¼ 50 W, at l ¼ 1,075 nm and having a laser spot size B , 70 mm. The parts are manufactured in a 100-mm diameter 304L steel substrate under Argon protective atmosphere. A single layer of powder is deposited on a solid substrate from 304L. Then, according to CAD data, the necessary section of a powder is scanned by laser. The powder is melted and solidified. Single tracks or single layers are manufactured and analyzed. Materials parameters Table I gives the chemical composition of investigated 17-4 PH powder that meets standard specification A 693-06. 29




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The reproducibility of the results is also different. For the same energy density one should adjust exactly the layer thickness according to the powder particle size distribution. When the powder layer is too thick, or the process parameters (laser power, scanning speed) are not optimized, the complete melting will not occur, and pores will form. The results show that the deposition of a homogeneous fine powder layer with a layer thickness less than 50 mm of a big Powder3 (D90 , 50 mm) using Phenix PM100 is impossible. Its optimal domain using input parameters (listed before) has not been found. Those results clearly demonstrate that the powder layer thickness should be chosen according to the powder particle size. The full analyses have been performed for Powder1 (D90 , 15 mm) and Powder2 (D90 , 25 mm). It has been shown in Dingal et al. (2008) that higher particle size provides a lower porosity in manufactured parts. The present analyses give an opposite conclusion. It should be noted that the conclusions can be given only for a definite type of SLM machine and a definite type of powder material. For the moment the types of lasers, their power, the system of powder deposition, etc. differ according to the type of SLM machines. The comparison of results can be done only for the same machine systems. From the preliminary experiments the following inferences can be shown (Figure 2): . The effect of layer thickness (Ep) on the height of a single track (h1) was significant for both powders. . The height of dilution zone h2 is affected by laser power P. The factor of influence for Powder2 is more important. . The width w1 and w2 of the tracks depend on laser power P, scanning speed V and interactions between these parameters (energy density) for both powders. The effect of layer thickness Ep on w1 and w2 is non-significant, contrary of its effect on the contact angle A.

Table I Chemical composition of 17-4 PH powder Cr (%)

Ni (%) Cu (%) Mn (%) Si (%) C (%) P (%) S (%) Fe (%)

15.0-17.5 3.0-5.0 3.0-5.0

1.00

1.00

0.07

0.04

0.03

Bal.

The main powder characteristics are: the particle size and particle size distribution which are determined by using laser diffraction technology (Mastersizer, 2000), and powder morphology which is analyzed with a scanning electron microscope (JCM-6400). The powder used for this research is a spherical gas atomised powder (Figure 1) with a particle size normally distributed D90 # 15 mm (Powder1), D90 # 25 mm (Powder2) and D90 # 50 mm (Powder3). The microstructure, stability, and geometrical characteristics of samples etched in oxalic acid reagent were analyzed by optical microscopy (Olympus BH-2).

Experimental design It was envisaged to reduce the number of experiments by applying the fractional factorial analyses with an orthogonal array. The choice of an appropriate orthogonal array suitable for a research task is one of the difficulties of experiment design (Taguchi et al., 2004). Four factors with four levels have been chosen for the experiment. The fractional factorial design reduced the number of experiments (in our case to 16) and took into account interactions between parameters (Taguchi et al., 2005; Roy, 2001). The levels of the main SLM parameters selected in the present research for an orthogonal array are shown in Table II. The orthogonal array used in the present work is presented in Table III. The output parameters are geometrical characteristics, stability and also the surface layer roughness for a single layer (2D object). The parameters are presented in Table IV. Five single tracks and one single layer were fabricated for each set of parameters. The obtained results have made it possible to assess the effect of each influence factor on geometrical characteristics.

It should be noted that for a thin layer thickness Ep (less than 30 mm) different dependences, sometimes non-monotonic, of output parameters (h1, h2, w1, etc.) from input parameters (P, V, Ep) for 17-4 PH powders can be found. Our scientific experience shows that applied powder particles size distribution for such thin layer thickness (less than 30 mm) is not adopted. For this reason, different dependences for powders could be obtained. It could be concluded that the domain of process parameters for layer thickness of 20-30 mm is not optimized. It has been demonstrated that track dimensional characteristics depend on different process parameters and

Results and discussion Single-line analyses Owing to a large process parameters domain analyzed at a first step of experimental design method, single tracks of various shapes, dimensional characteristics, stabilities are obtained. Figure 1 Morphology of 17-4 PH powders

(a)

(b)

Notes: (a) Powder1; (b) Powder2 and (c) Powder3 30

(c)




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Table II Levels of each of the investigated SLM process parameters tgM ¼ tg

Level of each parameter Level 1 Level 2 Level 3 Level 4

No.

Parameter

1 2 3 4

Laser power (W) Scanning speed (mm/s) Layer thickness (mm) Hatch distance (mm)

35 50 20 100

40 80 30 120

45 120 40 140

Table III Orthogonal array. Main SLM process parameters with their levels

50 80 120 150 50 80 120 150 50 80 120 150 50 80 120 150

20 30 40 50 30 20 50 40 40 50 20 30 50 40 30 20

100 120 140 160 140 160 100 120 160 140 120 100 120 100 160 140

interaction between them. In order to choose rapidly the optimal set of parameters a special function should be proposed. All dimensional characteristics explaining the physical phenomenon during manufacturing of a track should be taken into account. At the present time, it has been decided to develop a complex objective function (F) without units which will describe single track parameters and identify their stability: F ¼ tgD þ tgP þ tgM

ð1Þ

where the first parameter presents the factor related to the shape of the upper part of the fused track: tgD ¼

h1 2h1 ¼ w1 =2 w1

ð2Þ

where the h1 – height of a track and w1 – width of a track. The second parameter presents the factor related to the shape of the penetration depth of the fused track: tgP ¼

h2 2h2 ¼ w2 =2 w2

2ðh1 þ h2 Þ þ tgðaÞ w

ð5Þ

where h1 – height of a track, h2 – height of a dilution zone, w – width, and a – angle of contact. The obtained function F determines the physical shape of the track. If F is small (F , 1), the track is spread. If F is too big (F . 2), the balling phenomenon is observed. The notion of optimal shape enables the optimized process parameters window. The possible tracks shapes obtained with different values of F are shown in Figure 3. A number of experiments were conducted to determine an optimal domain of input parameters (a second stage of the method). The effect of Powder1 and Powder2 on output parameters is approximately the same. Besides, the mean square standard deviation for Powder1 is less than for Powder2. The repeatability of single tracks manufacturing is better for Powder1. Generally, at the highest laser powers (50 W), medium scanning speed (70-130 mm/s) and layer thickness (30-50 mm) the balling phenomenon has not been observed. Once this optimum process parameters domain for the best powder (Powder1) is defined the second step of the present work is to study more in detail the influence of input factors using the following DOE: EFCP23 and central point (Figure 4). For this, eight experiments will be conducted using the maximum and minimum optimal values and one experiment – using the parameters that provide a medium optimal value. The previously obtained results show that in order to manufacture regular single tracks and single layers the laser power should be fixed at maximum – 50 W. During the second step of DOE scanning speed, layer thickness and hatch distance will be varied and laser power will be kept constant. It is considered that the effect of laser power in this new domain is less pronounced than in previous domain of first stage of DOE method. As a result, it is proved statistically that the width of a scan (w) depends on laser power, scanning speed, interaction between them and on layer thickness (Table V). Using little value of scanning speeds and big value of layer thickness, unstable irregular tracks are manufactured. The height of the track h1 is particularly influenced by layer thickness, scanning speed, and also by interaction between them (Figure 5). Based on single-track analyses obtained by the second stage of experimental design method, the optimum domain has been specified: maximum laser power (50 W), medium scanning speed (130 mm/s), fine layer thickness (30 mm) using Powder1 (particle size normally distributed D90 # 15 mm).

Experiment Laser Scanning Layer Hatch no. power (W) speed (mm/s) thickness (mm) distance (mm) 35 35 35 35 40 40 40 40 45 45 45 45 50 50 50 50

ð4Þ

where a1 and a2 are the contact angles of a solidified track. Based on our proper experience one should say that the empirical results give that w1 ¼ w2 and a1 ¼ a2 ¼ a. Finally, the objective function F can be defined as follows:

50 150 50 160

F¼

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

a1 þ a2 ¼ tg a 2

ð3Þ

where the h2 – height of a dilution zone of a track and w2 is width of a dilution zone of a track. The third parameter presents the angle of contact of the fused track:

Single-layer analyses Applying 23 factorial design and central point plan for a single layer fabrication has allowed to analyze the influence of the two main manufacturing strategies (“1 zone” and “2 zone” 31

Layer (2D)

Track (1D)

Sample

h1 – height

w1 – width

h2 – height of dilutionzone

w2 – width of dilution zone

h3 – height after 2D laser passage

h1 – height

h4 – height of dilution zone after 2D laser passage

h2 – height of dilution zone

Output factors

L1 ¼ Ec hatch distance; Ec/two- half of hatch distance

a1 – contact angle

Roughness h1

a1

h2

Visualization

Table IV Output parameters for a single track (1D object) and for a single layer (2D object) manufactured from 17-4PH martensitic powder by SLM technology

32

Ec/2

w1

w2

h2

h1

h3

h4

Ec/2

Selective laser melting of martensitic 17-4 PH powder: part I Rapid Prototyping Journal

M. Averyanova, E. Cicala, Ph. Bertrand and Dominique Grevey Volume 18 · Number 1 · 2012 · 28 –37




Volume 18 · Number 1 · 2012 · 28 –37

Figure 2 Main effect plots

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Notes: (a) Effect of process parameters on the mean value of height (h1) of a track with mean square deviation S = 5.6 mm2 manufactured from Powder1 and (b) effect of process parameters on the mean value of the height (h1) of a track with mean square deviation S = 5.6 mm2 manufactured from Powder2, where P is laser power, V is scanning speed, and Ep is layer thickness; effect of process parameters on the mean value of the height h2 of a track with mean square deviation S (c) S = 3.6 mm2 manufactured from Powder1 and (d) S = 2.6 mm2 manufactured from Powder2, where P is laser power, V is scanning speed, and Ep is layer thickness; effect of process parameters on the mean value of the width w1 of a track with mean square deviation S (e) S = 7.0 mm2manufactured from Powder1 and (f) S = 6.8 mm2 manufactured from Powder2, where P is laser power, is scanning speed, and Ep is layer thickness; effect of process parameters on the mean value of the width w2 of a track with mean square deviation S (j) S = 6 mm2 manufactured from Powder1 and (h) S = 6.4 mm2 manufactured from Powder2, where P is laser power, V is scanning speed, and Ep is layer thickness

33




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Figure 3 Possible single track shapes manufactured from 17-4 PH Powder1 using different F-values F2

Conclusions Applying the experimental design approach, the present work investigates the effect of main process parameters on single lines and single layers manufactured from 17-4 PH martensitic powder using SLM technology. This statistical approach aimed to identify the impact of influence factors, such as laser power, scanning speed, layer thickness and hatch distance, on objective functions for a single fused track (1D object), such as width, height of a track and a dilution zone, contact angle, and for a single fused layer (2D objects) such as roughness and geometrical characteristics. The authors propose the development of a complex objective function taking into account all physical and geometrical parameters. This function describes the shape of a fused track and enables to determine rapidly an optimum process parameters window. The methodology applied to explore SLM process is the following: on the first step of the experimental design approach, among a big number of process parameters values the optimal set of parameters can be quickly determined by the complex objective function. Once the first parameter window is determined, the second step of the statistical approach leads to study more in detail

techniques) applied in the DIPI laboratory (Yadroitsev et al., 2007b) (Figure 6). Using the first manufacturing strategy, “1 zone” technique, each powder layer is processed by one laser beam pass with a constant hatch distance. In order to ensure necessary overlapping the laser beam melts simultaneously the previous track as well as the powder layer (Figure 6(b)). The socalled “2 zones” technique is processed in two steps: first, the powder layer is processed with a hatch distance equal the average width of a single track. Then, once the full section of the layer is fused, the laser beam passes between the previously melted tracks of the same layer (blue track in the scheme) – (Figure 6(c)). The full geometrical analysis has been performed for single layers manufactured from Powder1 and Powder2 (Figure 7). The results show the significant influence of input parameters and their interactions. However, their influence is different and depends on which scanning strategy is applied (Figures 8 and 9). Sometimes, the effect of input parameters 34




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Figure 5 Estimated effect and estimated response surface of process parameters on the height of the track h1 manufactured from Powder1, where (1) CV – scanning speed, (2) CEp – layer thickness, and 1*2 – interaction between scanning speed and layer thickness

2.09

(2)CEp

16 h1 [µm]

14 –0.94

1*2

12 10 8 6

–0.37

40

Ep

0 15 0 14 0 13 0 12 0 11 0 10 90 80 70 30

50

(1)CV

[µ m ]

m V[

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 Estimated effect (µm)

m/

14.864 14 13 12 11 10 9 8

s]

Figure 6 Scheme of scanning strategies of SLM process 60 µm

120 µm 120 µm

Laser beam

60 µm

60 µm

Powder

50 µm

50 µm

Substrate

100 µm

100 µm

100 µm

180 µm

180 µm

(a)

(b)

120 µm

(c)

Notes: (a) general view of laser beam/powder interaction; (b) “1 zone” scanning strategy using 60 mm hatch distance; (c) “2 zones” scanning strategy using 120 mm hatch distance

Figure 7 Example of geometrical analysis of single layer manufactured from Powder1 applying 23 factorial design þ central point plan with different layer thickness Layer thickness 30 µm

Layer thickness 40 µm

the influence of input factors using the following “EFCP23 and central point” DOE. Regarding the impact of powder materials properties on objective functions, it was observed that for SLM technology, the specific surface of the powder is more important than its volumetric properties (volume of particles). The obtained results show clearly that in order to manufacture stable, regular single tracks and dense single layers the finest 17-4 PH powder – D90 , 15 mm (Powder1) should be applied. This powder possesses the most important specific area that mainly depends on powder shape and particle size distribution. The greater surface area of fine particles leads to high melting activity and, by consequence, to a higher melting rate. As a result, one can

Layer thickness 50 µm

conclude that the use of finer powder is favourable for the specific PHENIX SYSTEM SLM machine. The layer thickness has the most significant effect on the properties of the fused section of the powder bed (density, roughness). The robustness of manufacturing strategies was studied. It has been proved that applying the “2 zones” technique leads to better characteristics of the manufactured layer. Based on the analyses and conclusions shown before, additional investigations concerning manufactured parts (3D objects) quality in terms of hardness, microhardness, porosity, mechanical properties, heat treatment, etc. will be conducted and proposed in the part II of this present paper. 35




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Figure 8 Estimated response of process parameters on the height h1 of a single manufactured layer produced from Powder1 using (a) “1 zone” technique and (b) “2 zones” technique, where (1) CV – scanning speed; (2) CEp – layer thickness; (3) CEc – hatch distance, 2*3 is interaction between input parameters (2) and (3), respectively, 1*3 is interaction between input parameters (1) and (3), and 1*2 is interaction between input parameters (1) and (2) 8.9

(2)CEp 6.85

2*3

1*3

0

0.5 2

1.725 –1.425

2*3

–1.8

1

2.125

(2)CEp

4.95

(1)CV 1*2

1*3

–5.55

(3)CEc

5.075

(3)CEc

3 4 5 6 7 Estimated effect [µm] (a)

8

9

1*2

0.875

(1)CV

–0.625

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Estimated effect [µm] (b)

10

30 28 26 24 22 20 18 16 14 12 10 8

h1 [µm]

h1 [µm]

Figure 9 Estimated response surface of process parameters of the height h1 of a single manufactured layer produced from Powder1 using (a) “1 zone” technique and (b) “2 zones” technique

50

50

(a)

80 70

60 m] [µ Ec

/s]

mm

V[

0 15 0 14 0 13 0 12 0 11 0 10 90

70

60 m] [µ Ec

0 15 0 14 0 13 0 12 0 11 0 10 90 80 70

70

26 24 22 20 18 16

32 30 28 26 24 22 20 18 16 14

28 26 24 22 20 18

/s]

mm

V[

(b)

References

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