Experimental investigation on densification behavior and surface roughness of AlSi10Mg powders produced by selective laser melting

Optics and Laser Technology 96 (2017) 88–96

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Experimental investigation on densification behavior and surface roughness of AlSi10Mg powders produced by selective laser melting Lin-zhi Wang ⇑, Sen Wang, Jiao-jiao Wu Chongqing Key Laboratory of Additive Manufacturing Technology and Systems, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, PR China

a r t i c l e

i n f o

Article history: Received 2 February 2017 Received in revised form 5 April 2017 Accepted 9 May 2017

Keywords: Laser energy density Selective laser melting Densification behavior Surface roughness

a b s t r a c t Effects of laser energy density (LED) on densities and surface roughness of AlSi10Mg samples processed by selective laser melting were studied. The densification behaviors of the SLM manufactured AlSi10Mg samples at different LEDs were characterized by a solid densitometer, an industrial X-ray and CT detection system. A field emission scanning electron microscope, an automatic optical measuring system, and a surface profiler were used for measurements of surface roughness. The results show that relatively high density can be obtained with the point distance of 80–105 lm and the exposure time of 140–160 ls. The LED has an important influence on the surface morphology of the forming part, too high LED may lead to balling effect, while too low LED tends to produce defects, such as porosity and microcrack, and then affect surface roughness and porosities of the parts finally. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Aluminum alloys have excellent properties such as specific strength, stiffness, electrical and thermal conductivity. It has been the second widely used metal material in the world [1], whose application potential is only less than steels. With the fast development in aerospace, military engineering, energy, automobile and rail transportation industries, the applications of aluminum alloys with lightweight and high performance are increasing in those areas, whose aims to satisfy the requirements of functional, reliable, economic and lightweight equipment [2,3]. Currently, most of aluminum alloy parts are manufactured by casting, forging, extrusion and powder metallurgy. Those traditional processes have long production period and high cost. Moreover, parts with the complex shapes are difficult to be prepared by them, which restrict a further development and application of aluminum alloys [4]. Therefore, manufacturing aluminum alloy parts with complex shape in a more efficient and convenient way has been one of the hottest topics in the field of advanced manufacturing technology. Nowadays, Selective Laser Melting (SLM) is the most successful forming process for complex metal structures. Actually, this type of process has advantages of high degree of flexibility, complex shape designing and material saving [5–7]. Moreover, the part manufactured by SLM generally has much higher density ⇑ Corresponding author. E-mail address: [email protected] (L.-z. Wang). http://dx.doi.org/10.1016/j.optlastec.2017.05.006 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.

and more excellent surface quality, which dismiss the shortcut existing in traditional aluminum alloy parts forming methods. Therefore, SLM technology has a promising market application prospect and high research value in aluminum alloy complex structure forming industry. In fact, the heat source during SLM process has significant difference with traditional forming methods, which mainly performs in two aspects: (1) SLM forming equipment adopts a fiber laser device with high power density, and the laser spot diameter is always less than 0.1 mm, which melts solid powders in a rapid velocity [8]. (2) The SLM manufactured parts is formed by partition zones scanning, in addition, metal powders are molten point by point then consolidate together in layers. Local excessive accumulated heat in powder bed can dissipate through thermal conduction of substrate, therefore, the solidification rate of molten pools are extremely high [9,10]. As a result, those characteristics of SLM process mentioned above lead to the difference of density and surface roughness evolution mechanisms between SLM with traditional forming process. For aluminum alloys, due to the low melting point and high thermal conductivity, the SLM forming process has a larger heating and cooling rate. Therefore, it is necessary to carry out further study on the inner relationship between SLM process parameters and manufactured parts’ density and surface roughness. Gu et al. [11] researched densification behavior of TiC nanoparticle reinforced AlSi10Mg bulk-form nanocomposites prepared by SLM. They found that using an insufficient laser energy input of

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particle size analyzer (Fritsch Gmbh, Analysette22 MicroTec plus). The results show that the particle size of the AlSi10Mg powder is in the range of 5–50 lm (as seen in Fig. 1(b)).

250 J/m lowered the SLM densification due to the balling effect and the formation of residual pores. The highest densification level (>98% theoretical density) was achieved for SLM-processed parts processed at the aser energy input of 700 J/m. Dalgarno et al. [12] reported densification mechanism in selective laser sintering of Al-12Si powders, they pointed out that the degree of porosity and its orientation as well as the densification of the interagglomerates of Al-12Si parts made by direct SLS were controlled by the choice of processing parameters. Iuliano et al. [13] studied influence of process parameters on surface roughness of AlSi10Mg parts produced by direct metal laser sintering, it was found that scan speed had the greatest influence on the surface roughness. The best results were obtained with a scan speed of 900 mm/s, a laser power of 120 W and a hatching distance of 100 lm. Above investigations reported single process parameter on density and surface morphologies of the SLM processed AlSi10Mg samples with continuous exposure scan pattern. Actually, there are two different laser scan pattern including point exposure scan and continuous exposure scan in SLM. However, comprehensive influence of process parameters on densification behavior and surface roughness of the SLM processed samples with point exposure scan are very limited. In this paper, laser energy density (LED) under point exposure scan pattern was introduced. Moreover, the influence of LEDs on density and surface roughness were investigated in details. The results provide theoretical basis for the aluminum alloy samples with high performance and high precision fabricated by SLM process.

2.2. SLM process The SLM process is carried out by a Renishaw AM-250 powderbed machine. The powders are melted by a diode pumped Nd: YAG laser with the maximum power of 400 W. The adjustable parameters are point distance and exposure time. As illustrated in Fig. 2(a), point distance is the distance between adjacent fusion points along laser scanning direction, and exposure time is the duration time of the laser remaining on each fusion point. In order to minimize part residual stress, a checkerboard scanning strategy was selected for all trials. For one specific powder layer, the checkerboard is scanning in a linear way, and the linear scanning direction rotates with 67° for the next processing powder layer, as shown in Fig. 2(b). Other parameters are set as constants and their effects on part’s properties are not discussed here. Those parameters are listed in Table 2. In the SLM experiments, an aluminum substrate was leveled and fixed on the building platform. The platform temperature was preheated and maintained at 150 °C in order to reduce residual stresses and distortion of components. The rectangular specimens with dimensions of 10 mm
10 mm
10 mm were built layer-by-layer until the whole shape completely formed. The LED expressed by Eq. (1) describes the energy input per volume during SLM [14,15], and the LED is a combination of effects of the SLM process parameters.

2. Experimental procedure

LED ¼ P=ðv hdÞ

ð1Þ

v ¼ Pd =T e

ð2Þ

where P (W) denotes the laser power, v (mm/s) is the scanning velocity, h (mm) is the hatching space and d (mm) is the layer thickness. Further, the scanning velocity v is determined by:

2.1. Powder preparation Hypoeutectic AlSi10Mg alloy has been used widely due to its good melt flowability, mechanical properties and corrosion resistance. Chemical composition of the AlSi10Mg powder used in this study is shown in Table 1. The AlSi10Mg powders are with spherical shape, and the spheroidization ratio of AlSi10Mg powders is almost 100% (as seen in Fig. 1(a)). In addition, Granulometric parameters of the AlSi10Mg powders were evaluated by a laser

where Pd (lm) is the point distance and T e (ls) is the exposure time. Therefore, Eq. (1) can be drawn in another way as:

LED ¼ PT e =ðPd hdÞ

ð3Þ

Table 1 Chemical composition of AlSi10Mg. Element

Cu

Mg

Fe

Si

Mn

Zn

Other

Al

Composition (wt%)

0.06

0.623

0.08

9.628

0.01

0.03

<0.2

Balance

(b) 100

Frequency distribution (%)

10 Spherical powder

8

6

D[10] D[50] D[90] Raw powder

Raw (µm) Spherical (µm) 80 27 33.9 52.6 59.5 µm92.8 96.8 60

4

40

2

20

0 0

20

40

60

80

100

120

140

160

180

Cumulative distribution (%)

(a)

0 200

Particle size (µm) Fig. 1. Typical particle morphology and particle size distribution curve of the starting AlSi10Mg powder: (a) SEM image of AlSi10Mg powder and (b) particle size distribution curve.

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Fig. 2. Diagram of the basic process parameters of the SLM: (a) process parameters and (b) scanning strategy.

AlSi10Mg parts were measured by a surface profiler (Alpha-step IQ) with a scan length of 4 mm, a scan speed of 50 lm/s and a sampling rate of 200 Hz.

Table 2 Experiments parameters of SLM process for AlSi10Mg. Parameter

Value

Laser power Radius of laser beam Layer thickness Hatching space Exposure time Point distance

400 W 67.5 lm 25 lm 130 lm 100, 120, 140, 160, 180 ls 80, 105, 130, 155, 180 lm

In this study, Consequently,

P = 400 W,

h = 0.14 mm,

LED ¼ 8
105 T e =ð7Pd Þ

3. Results and discussion 3.1. Effects of process parameters on densities of the samples

d = 0.025 mm.

ð4Þ

Effects of laser scanning speed and point distance on the densities and surface morphologies of AlSi10Mg produced by SLM at the constant exposure time of 140 ls are shown in Figs. 4 and 5, respectively. At the same scanning speed, the parts obtain a relatively high density when the point distance is 80 lm and 105 lm, and the maximum relative density reaches 99%. As shown

According to Eq. (4), LEDs in the SLM process are calculated and displayed in Fig. 3. Typical LEDs of 89, 109, 131, 174 and 200 J/mm3 are selected to analyze in experiments.

2.64 2.61

A solid densitometer (Beyong, DE-120M) was selected for density measurement. At least three test results were collected for every type of specimen and the average values were adopted as results. Porosities of the SLM manufactured AlSi10Mg samples at different LEDs were tested by an industrial X-ray and CT detection system (Nikon Metrology, XT H225). The surface morphologies of the SLM manufactured AlSi10Mg samples perpendicular to building directions at different LEDs were characterized by a field emission scanning electron microscope (JEOL, JSM-7800F, FESEM). Moreover, 2D and 3D morphologies of the surfaces were characterized using an automatic optical measuring system (Talysurf CCI Lite – Non-contact 3D Profiler). In addition, surface roughness of top surfaces of SLM manufactured

Density(g/cm3 )

2.3. Determination of density and surface roughness

2.58 2.55 2.52

Point Distance(µm) 80 105 130 155 180

2.49 2.46 240

260

280

300

320

340

Scanning Speed (mm/s) Fig. 4. Effects of laser scanning speed on the densities of AlSi10Mg produced by SLM.

Fig. 3. (a) 3D plot and (b) limit diagram showing relationship between laser energy density, point distance and exposure time of the as-SLMed AlSi10Mg samples.

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91

Fig. 5. Effects of laser scanning speed on surface morphologies of AlSi10Mg produced by SLM.

2.62

Density (g/cm3)

2.60 2.58 2.56 2.54 Exposure Time(µs) 100 120 140 160 180

2.52 2.50 240

260

280

300

320

340

Scanning Speed (mm/s) Fig. 6. Effects of laser scanning speed on the densities of AlSi10Mg produced by SLM.

in Fig. 5(a), (d) and (g), the specimen surfaces are dense and uniform with no obvious holes and inclusions. When the point distance continues to increase, densities show a downward trend, and some semi-melt powder particles were entrained in the interior of the specimen as shown in Fig. 5(c), (f) and (i). As we all known, the overlapping area of laser spot is large with a small scanning point distance. Therefore, the laser energy density is sufficient and distributed homogeneously. In this case, the AlSi10Mg can obtain excellent surface quality and high density due to full melting of the powders. On contrary, when the point

distance is greater than the laser spot diameter (130 lm), the laser energy is insufficient caused by relatively small overlapping area of the laser spots. In this case, some inner particles would not be melt and leave pores within the part, which results in poor surface quality and low density of the AlSi10Mg parts. Effects of laser scanning speed and exposure time on the densities and surface morphologies of AlSi10Mg produced by SLM at the constant point distance of 130 lm are shown in Figs. 6 and 7, respectively. When the scanning speed is 290–310 mm/s and the exposure time equals to 140–160 ls, the maximum relative density can reach 98%. Moreover, the specimen surfaces are dense and smooth as shown in Fig. 7(b), (e) and (h). This is because the powder particles can be melted sufficiently and good interlayer metallurgical bonding can be obtained due to deep penetrations of the molten pools. In addition, the molten liquid can be sufficiently filled the pores of the solid phase, so that the specimen can obtain higher density. When the exposure time is 100–120 ls, the powder particles cannot be melted completely caused by inadequate energy input, which result in relatively low density of the specimens. When the exposure time is 180 ls, interaction time between laser and powder is relatively long, leading to local excessive liquid and balling effects as shown in Fig. 7(c), (f) and (i). On the other hand, too long exposure time may result in the large sized molten pool. In this case, powder particles around the molten pool are sucked into the pool, resulting in insufficient powder in original position. Consequently, the specimen shows low density and poor surface quality with obvious forming pits and holes. Typical laser energy density values and relationships between laser energy density and the density of the as-SLMed AlSi10Mg

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Fig. 7. Effects of exposure time on surface morphologies of AlSi10Mg produced by SLM.

2.62

3

Density (g/cm )

2.61

2.60

2.59

2.58

2.57 80

100

120

140

160

180

200

3

Laser energy density (J/mm ) Fig. 8. (a) Typical laser energy density values and (b) relationship between laser energy density and the density of the as-SLMed AlSi10Mg samples.

samples are shown in Fig. 8. The density is proportional to the laser energy density. When the laser energy density is less than 180 J/mm3, the relative density is about 97%. However, when the laser energy density is greater than 180 J/mm3, the relative density is more than 99%. In general, the density of the specimen is mainly affected by metallurgical bonding forces between the melt tracks and layers. Meanwhile, laser energy density has an important impact on the metallurgical bonding ability. In the SLM process, the dynamic viscosity of the liquid melt is actually temperature dependent, the higher the pool temperature, the lower the viscosity of the liq-

uid and the higher instability of the capillary [16]. A large number of liquids with low viscosity promote liquid diffusion and improve metallurgical bonding capability between adjacent layers. It can be found that metallurgical bonding capability change significantly at different laser energy densities. When with low laser energy density, a small amount of liquids with high viscosity form in molten pool caused by relatively low temperature and short liquid lifetime. In this case, AlSi10Mg liquids are difficult to spread smooth between adjacent tracks. Further, width of the molten pool is smaller than the hatch space and even depth of the molten pool is smaller than the layer thickness. Therefore, large-sized pores

L.-z. Wang et al. / Optics and Laser Technology 96 (2017) 88–96

Fig. 9. Porosity versus laser energy density for AlSi10Mg samples, data points are measured from optical microscopic images on top faces and build directions.

form in interlayer and then result in poor metallurgical bonding. With increase of the laser energy density, the amounts of liquids with low viscosity increase caused by higher temperature, and more liquid filling and larger depth and width of the molten pool can be obtained. In this case, width and depth of the molten pool are larger than the hatch space and the layer thickness, respectively. Hence, good overlap and metallurgical bonding between the adjacent tracks can be obtained. In addition, the reduction of large sized pores ensures good metallurgical bonding ability between the adjacent hatches and adjacent layers, which has a significant enhancement of the density of the as-SLMed AlSi10Mg samples. Simchi et al. [17] studied the effect of laser energy density on densification of the iron-based powder fabricated by SLM, they found that with the increase of the laser energy density, viscosity of the liquid metal is reduced, and the liquid metal is easier to flow and fill the pores, thereby increasing the density of the part. Porosity is the most common defect and can be controlled by adjusting the SLM parameters. Maskery et al. [18] analyzed distribution and evolution of the pores in SLMed AlSi10Mg parts using X-ray computed tomography method, they found that porosity of the sample was less than 0.1%, but the diameters of some pores

(a) Partially melted powder particles

(b)

(c)

Dense and smooth surface

(d)

93

(e)

Balling effects

Fig. 10. Surface morphologies at different LEDs perpendicular to building directions: (a) 89 J/mm3; (b) 109 J/mm3; (c) 131 J/mm3; (d) 174 J/mm3 and (e) 200 J/mm3.

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Fig. 11. Surface morphologies perpendicular to building directions at different LEDs and test results for surface roughness: (a) 89 J/mm3; (b) 109 J/mm3; (c) 131 J/mm3; (d) 174 J/mm3; (e) 200 J/mm3 and (f) test results for surface roughness.

were up to 260 lm along the deposition direction, and the maximum pores exhibited highly anisotropic. In this study, the effects of LEDs on the porosity were determined for two different sides of the cubes (as seen in Fig. 9). Squares represent the porosity along the deposition direction, and circles represent the porosity perpendicular to the deposition direction. One can see similar variation trends of the porosity for top and side face, indicating that the porosity is distributed uniformly throughout the build. The porosity reaches a maximum (7.55%) when with a low LED (89 J/mm3). Moreover, irregular pores, cracks and partially melted powder particles can be observed (as seen in Fig. 9(a)). This is because sizes of the molten pool (including width and depth) are relatively small and powder particles cannot be melted completely due to low LED. Therefore, there is insufficient liquid melt to fill the

pores between powder particles and ensure sufficient bonding between the layers [19]. The porosity decreases to 1.24% and 0.25% when the LED increases to 109 J/mm3 and 131 J/mm3. Although the pores still exist in the SLMed part, the sizes of the pores are very small (less than 20 lm), and pores are mostly in spherical shape. In addition, the porosity is localized showing a porosity-free area (as seen in Fig. 9(b)). Aboulkhair et al. [20] studied the windows of parameters required to produce high density parts from AlSi10Mg alloy using selective laser melting, they found that the metallurgical pores were not significant with increasing laser energy density. With further increase of the laser energy density, the porosity increases from 3.45% to 6.47% and the sizes of the pores are increase synchronously (as seen in Fig. 9(c)). This excessive LED

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(d)

(e)

3

3

E: 89 J/mm , Ra=5.547 µm

(f) 100

E: 109 J/mm , Ra=4.897 µm

3

E: 131 J/mm , Ra=4.127 µm

80

3

E: 174 J/mm , Ra=6.546 µm

3

E: 200 J/mm , Ra=8.096 µm

Roughness (µm)

60 40 20 0 -20 -40 -60 0

500

1000

1500 2000 2500 Scan length (µm)

3000

3500

4000

Fig. 11 (continued)

may cause vaporization of low melting elements (such as Al and Mg), and the evaporated gas is entrapped inside the molten pool and forms pores due to extremely fast solidification rate [21]. Taha et al. [22] investigated the porosity of the ultrahigh carbon steel at higher laser energy condition during SLM process, they found both the quantities and sizes of the pores were increased. 3.2. Effects of laser energy density on surface roughness of the samples Surface morphologies at different LEDs perpendicular to building directions are shown in Fig. 10. When the laser energy density is 89 J/mm3, partially melted powder particles can be observed

clearly from the surface of the SLMed part, and the powder particles are ellipsoidal and spherical in shapes with the sizes of about 50–100 lm (as shown in Fig. 10(a)). Relatively high viscosity of the liquid melt results in poor fluidity and wettability caused by low laser energy density. In this case, there is not enough liquid to infiltrate the powders, which results in the poor metallurgical bonding and formation of some semi-melted powder particles [23]. When the laser energy density increase to 109 J/mm3 and 131 J/mm3, the amount of liquid melt increase due to the increased energy absorbed by the powder particles. In addition, viscosity and the surface tension gradient of the melt are reduced, thus increasing the depth and width of the molten pool. Conse-

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quently, smooth and dense surface of the SLMed parts can be obtained (as shown in Fig. 10(b) and (c)). Gu et al. [24] reported that the higher laser energy density promotes higher temperature of the melt and better surface tension and wettability, which is beneficial to the formation of stable molten pool and smooth track surface. With further increase of the laser energy density, the tracks are discrete and accompanied by an increase in balling (100 lm) (as shown in Fig. 10(d) and (e)). A steep temperature gradient is developed between the center and edge of the pool across the surface caused by excessive laser energy input, giving rise to surface tension gradients and resultant Marangoni convention. Due to the action of Marangoni flow, the melt tends to flow radially inward towards the melt pool center, instead of spreading outward on the underlying surface. Consequently, the instable melt track breaks up into several spherical agglomerates to achieve the equilibrium state, which is termed as ‘‘balling” effect. Previous studies [25] have shown that a higher laser energy density creates a melt pool with a long liquid lifetime and high superheat. This provides enough input energy and time for molten metal to split into droplets as described by Khan and Dickens [26]. 2D and 3D morphologies of the surfaces perpendicular to building directions at different LEDs and test results for surface roughness are shown in Fig. 11. This enables each method of changing the energy input to be assessed independently. As previously seen with the porosity, the surface roughness follows the same trend, where a reduction in surface roughness is observed with increased LED until an optimum level is obtained, when further increases are detrimental to the surface finish. The lowest surface roughness (Ra = 4.127 lm) was measured at an energy input of 131 J/mm3, obtained at a 80 lm point distance and 100 ls exposure time. The highest surface roughness (Ra = 8.096 lm) was obtained at 200 J/mm3, obtained at a 180 lm point distance and 160 ls exposure time. This indicates the requirement to assess surface finish by each method of varying energy input. 4. Conclusions The effects of laser energy density (LED) on densities and surface roughness of AlSi10Mg samples processed by selective laser melting were studied. The main conclusions are summarized as follows: (1) The densities of the AlSi10Mg parts fabricated by SLM are relatively high (the maximum relative density is 99%) when the point distance is 80 lm and 105 lm. Moreover, the densities decrease gradually with increase of the point distance. When the exposure time is 140–160 ls, the density is up to a maximum, too long or too short exposure time leads to the reduction in density. (2) The densities of AlSi10Mg parts produced by SLM increase significantly and then decrease slowly with the increasing laser energy density. In addition, the porosities and surface roughnesses are decrease obviously and then increase slowly with the increase of laser energy density. (3) The laser energy density has an important influence on the surface morphology of the parts produced by SLM, too high or too low laser energy density leads to the formation of defects (such as porosity and microcrack). Acknowledgements The study is supported by the National Natural Science Foundation of China (NSFC, China) under Grant Number of 51405467,

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