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Effects of laser scanning speeds on different states of the molten pool during selective laser melting: Simulation and experiment
Journal Pre-proof Effects of laser scanning speeds on different states of the molten pool during selective laser melting: Simulation and experiment
Weihao Yuan, Hui Chen, Tan Cheng, Qingsong Wei PII:
S0264-1275(20)30075-7
DOI:
https://doi.org/10.1016/j.matdes.2020.108542
Reference:
JMADE 108542
To appear in:
Materials & Design
Received date:
17 December 2019
Revised date:
22 January 2020
Accepted date:
1 February 2020
Please cite this article as: W. Yuan, H. Chen, T. Cheng, et al., Effects of laser scanning speeds on different states of the molten pool during selective laser melting: Simulation and experiment, Materials & Design(2020), https://doi.org/10.1016/j.matdes.2020.108542
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© 2020 Published by Elsevier.
Journal Pre-proof Effects of Laser Scanning Speeds on Different States of the Molten Pool During Selective Laser Melting: Simulation and Experiment Weihao Yuana, Hui Chena,b,*, Tan Chenga, Qingsong Weia,* a
State Key Lab of Materials Forming and Die&Mould Technology, School of
Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b
Department of Mechanical Engineering, National University of Singapore,
Singapore 117575, Singapore Abstract
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Using numerical simulations and experimental tests, the temperature and velocity
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fields of the molten pool during selective laser melting (SLM) were investigated, where the laser scanning speed ranging from 2.5 m/s to 0.3 m/s was employed.
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Experiments for single tracks and part samples were conducted for verification. Three kinds of molten pool states were identified and investigated: unstable state, transition
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state and stable state. The unstable state is characterized by numerous balling defects,
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where the bulk density is severely deteriorated. The transition state is featured by the transition region where the melt velocity is relatively lower, and the molten pool is vulnerable to the necking defect. The molten pool with a depression region is
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identified as the stable state. A small depression is favorable for improving the surface quality of single track and the bulk density. However, exorbitant energy input will
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convert the depression into a keyhole. Additionally, a threshold of the scanning speed was found, where the bulk density peaked. Over the threshold, the density decreased continuously with the speed increasing. However, the density slightly decreased by 1.5% when the speed was below the threshold; this anomaly was ascribed to the residual pores induced by the recoil pressure. Keywords: Scanning speed; Selective laser melting; Temperature and velocity fields; Molten pool states; Threshold
*
Corresponding authors. Email address: [email protected] (Q. Wei) and [email protected] (H. Chen). 1 / 25
Journal Pre-proof 1. Introduction Selective laser melting (SLM) is one of the state-of-the-art additive manufacturing (AM) technologies for rapid production of geometrically complex metal parts [1]. In the past few years, SLM has become a novel processing route to a number of industry applications from the aerospace [2, 3] to the biomedicine [4]. However, further promotion of SLM is needed for widespread adoption of metal components manufacturing [5], as this kind of AM process can be susceptible to many defects. For instance, the pores formed during SLM is one of the most notable defects that would deteriorate final component properties such as bulk density. According to the recent
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studies of metal AM [6, 7], the surface morphology of metal particle can influence the
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fusion behaviors and metallurgy bonding between neighboring particles. The ambient gas may also be trapped into the particles to form some micro-pores, and a more
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unstable molten pool may increase the chances of this kind of gas entrapment [8]. Since it is commonly believed that the continuous and uniform melted single tracks
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would lead to a relatively dense final component [9], some experiments were conducted for the purpose of parameters optimization [10-12]. However, relying on
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trial-error method, the experimental tool is too inefficient and costly. It is appealed to
physical perspective.
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comprehend the underlining mechanism of the defects formation from a mathematical
Numerical simulation is a powerful method to investigate the heat transfer and fluid
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flow during SLM, and it is also vital to understand the formed defects. Carolin Koner et al. [13, 14] proposed 2D and also 3D numerical programs based on the LBM (Lattice Boltzmann Method) method to study the fusion process of metal powder bed. With FVM (Finite Volume Method), Qiu et al. [15, 16] researched the influence of processing parameters on the pore evolution and surface morphology, and the resultant microstructure was also discussed [5]. The conclusions were arrived that the increased laser scanning speed and layer thickness was mainly responsible for the porosity and surface defect. Similar work was undertaken by Gu et al. [17], focusing on the evolution mechanism of the molten pool and porosity in the nickel-based alloy, however, the recoil pressure due to evaporation is excluded. Wayne King et al. [18] of Lawrence Lab focused on the complex physical phenomena including melting, capillary flow and evaporation in SLM process based on the home-made code. They discussed the potential defects such as pores, spatters, and denudation as well as their 2 / 25
Journal Pre-proof forming mechanisms. Nevertheless, the effects of the variable processing parameters are not studied in their work. Recently, some studies proposed the combined model of DEM (discrete element method) and CFD (Computational Fluid Dynamic) to simulate the complete process of powder-based additive manufacturing, such as Lee [19], Yan [20] and Tang [21]. The main focus of these literatures are the mesoscopic scale defects and the molten pool evolution. Tang emphasized dominant role of the recoil pressure and pointed out that this phenomenon might produce pore defects with different shapes. Besides, Wu et al. [23] found that the presence or absence of the recoil pressure significantly changed the patterns of heat transfer and fluid flow within
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the molten pool. Thus, the recoil pressure should be paid special attentions, but many
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researches did not [17, 20, 21]. Additionally, what extent would the molten pool behaviors and the potential defects affect the part quality is another key point that is
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worthwhile to be disclosed.
The objectives of this study are to research the effects of laser scanning speed on the
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molten pool behaviors and the forming quality in SLM through computational and experimental methods. The molten pool states are classified, and then the relationship
2. Numerical modeling 2.1. Physical model
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comprehensively revealed.
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between the bulk densities and the molten pool states under recoil pressure is
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The physical model for the selective laser melting (SLM) process is shown in Fig. 1. The whole computational domain in the Cartesian coordinate system is 0.8×0.2×0.28 mm3, where the height of baseplate and gas region are set 200μm and 80μm (Z-axis direction), respectively. A fixed monitoring point P (0.4mm, 0.1mm, 0.2mm) is set to approximately measure the molten pool temperature during laser scanning. In order to accelerate the calculation process and enhance the accuracy, the regular hexahedron with as small a mesh size as possible is desirable. However, too small mesh size results in plenty of computational time. Thus, the mesh size of 2.5 μm was adopted with reference to the literatures [21, 22], and the number of entire grids is approximately 3.0 million. The 316L stainless steel particles with the mean size of 30 μm are paved on the baseplate to form a 30 μm thickness of powder bed. The powder spreading process is simulated using discrete element method (DEM) model, which has been built in one of our previous studies [1, 26]. 3 / 25
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Fig. 1. Physical model for the powder bed fusion process.
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The simulation is conducted through the commercial software Ansys-Fluent 18.0, and several assumptions are adopted to simplify the numerical model: (1) The fluid flow
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of molten pool is incompressible and laminar; (2) The latent heat of metal powder varies linearly with temperature and all the thermo-physical properties are functions
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of temperature only; (3) The plasma effect and the Fresnel absorption in SLM process
2.2 Governing equations
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are ignored.
The governing equations, i.e. the continuity, momentum and energy equations showed ( u) 0
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as eq. (1), (2), and (3) are solved [20]:
(1) (2)
( H ) ( uH ) kT Sh t
(3)
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( u) ( uu) P g (u) Sm t
where u and g denote the velocity and the gravitational acceleration, respectively.
, , P , T , k refer to the density, viscosity, pressure, temperature, and thermal conductivity, respectively. H is the enthalpy of the material considering the heat transfer due to the melting and solidification, defined by H c dT H , in p
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which H and l denote the latent heat of melting and the liquid volume fraction, respectively. l is assumed to be linear interpolation of the solid temperature Ts and liquid temperature Tl [20]:
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T Ts Ts T Tl
(4)
T Tl
Additionally, the Volume of Fluid (VOF) method is used to capture the free surface, which solves the equation
F t
( Fv) 0 to get the metal phase friction F in each
cell. Sm and S h in eq. (2) and (3) are the source terms. In the momentum conservation equation, four kinds of body forces are incorporated, and the source term
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Sm can be expressed as follows. (5)
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Sm Sdam f sur f mag prec
where the first term S dam denotes the Darcy force responsible for dampening the
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velocity to zero when the temperature is lower than the melting temperature [16]. The
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second term f sur denotes one of the components of the surface tension whose direction is vertical to the gas-liquid interface. The third term f mag denotes the
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tangential component of the surface tension, which would be exerted onto the metal-gas interface. This term is also known as Marangoni forces [24]. Continuum
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surface stress method that considers the effect of variable surface tension is used [25]. It should be noticed that the recoil pressure which is caused by the evaporation is
pr p0 exp[
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considered in the model [27]. mv Lv
(
1 1 )] Tb T
(6)
where pr , p0 , m v , , Lv and Tb refer to the recoil pressure, operation pressure, molecular mass, boltzmann constant, latent heat of evaporation and ambient temperature. Since evaporation can only occurs on the gas-metal interface, a multiplier F1
2 1 +2
is added to the eq. (6), where F1 refers to the volume
fraction gradient of metallic phase, and
2 1 + 2
is used to smear out the effect of very
different density phases [16, 17]. 1 and 2 denote the density of metal and gas, while denotes the volume-average density, defined by the sum of the product of density and corresponding volume fraction ( 1F1 2 F2 ). Therefore, the forth term prec in eq. (5) can be expressed as [16]: 5 / 25
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mv Lv
(
1 1 2 )] F1 Tb T 1 2
(7)
In the energy conservation equation (eq. 3), the source term S h consists of two parts: the absorption and releasing of the latent heat of melting, which has been united within eq. (3). The laser heat input QL is expressed by eq. (8). The adopted laser source model is a surface heat source with a Gaussian distribution. Similarly, an extra multiplier, 1
2 cp 1c p1 2c p 2
, is used to transfer the surface heat source into a volumetric
distribution [24].
2
exp[
2r 2
2
] 1
2 c p 1c p1 c p 2
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2 AP
(8)
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QL
The model does not consider extra thermal boundary conditions at the metal-gas
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interface. Actually, the total quantity of the heat dissipation from the other conditions such as convection and radiation is far smaller compared with the laser heat source,
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and they have little effect on the molten pool behaviors and states.
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Fig. 2 shows the temperature-dependent physical properties including thermal conductivity, specific heat and liquid viscosity. Other properties of the material are listed in table 1 in detail [22]. Table 2 shows the processing parameters for
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simulations.
Fig. 2. Temperature-dependent physical properties: (a) Thermal conductivity, (b) specific heat, and (c) liquid viscosity. Table 1 Thermophysical properties of the 316L stainless steel. Parameters, symbol
Value
Density,
7500 kg/m3
Operating pressure, p 0
0.1 MPa
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1658 K
Liquid temperature, Tl
1723 K
Latent heat, H
270000 J/kg
Surface tension, f
1.8 N/m
Surface tension gradient,
d f
-0.0003 N/m-k
dT
Radiation emissivity, ε
0.4
Metal evaporation heat, Lv
7.45e6 J/kg
Table 2
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Processing parameters for simulations. Name, symbol
Value
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Beam radius,
35 μm
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Absorption, A Power, P
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Scanning speed, v
3. Experiments
200 W 0.3, 0.8, 1.3, 2.0, 2.5 m/s 30 μm
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Powder thickness
0.38
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In order to validate the numerical model, corresponding experiments were performed for the fabrication of single tracks and part specimens. The near-spherical 316L stainless steel powder was used as the raw material for SLM experiments, as shown in
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Fig. 3. The powder size distribution was tested, and the Dv10, Dv50 and Dv90 were 18.5 μm, 30.9 μm and 49.3 μm, respectively. The powder was put in a drying oven at 90 ℃ for at least 10 hours prior to the SLM. A self-developed SLM system containing a SPI-400 fiber laser device with the maximum laser power of 400 W and the spot size of 70 μm was employed. Apart from the processing parameters adopted in the simulation process (Table 2), three more groups of experiments with scanning speed of 0.5 m/s, 1.0 m/s and 1.6 m/s were added to further investigate the tendency, and two samples were prepared at each laser scanning speed. The experiments were performed under a high-purity argon gas protection atmosphere with a concentration of 99.9%. The operation pressure was kept approximately 0.1 Mpa. Two specimens were fabricated under each scanning speed. The optimized hatch spacing was 65 μm. The dimension of the formed part was 10 mm × 10 mm × 2mm. After that, the ultra-depth of field optical microscope (VHX-1000C, Japan) was used to observe the 7 / 25
Journal Pre-proof surface morphology of single tracks. The fully automatic true density analyzer (AccuPyc1330, USA) was used to measure the relative density after specimens being cut down from the substrate and side walls being ground and polished. The values were obtained from the average of six measurements (each sample for three tests). The cross sections of the specimens were then corroded with aqua regia consisting of
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HCl (15 ml) and HNO3 (5 ml) for about 25 seconds to expose the internal features.
Fig. 3. Morphology of the 316L stainless steel powder with the tested powder size
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distribution.
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4. Results 4.1 Morphologies of single tracks
The surface morphologies of the melted tracks resulted from simulations and
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experiments are shown in Fig. 4. The results of the simulation agree well with those of the experiments. As laser scanning speed decreases from 2.5 m/s to 0.3 m/s, i.e., the
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energy density increases, the melted tracks can be divided into three categories: unstable state, transition state, and stable state based on the flow behaviors of molten pool, which will be discussed in the section 5.2. When a higher scanning speed (2.0 m/s and 2.5 m/s) was employed, the molten pool entered the unstable state. The uniformity and continuity of the melted tracks were completely destroyed by the balling defects. As the laser scanning speed decreased to 1.3 m/s, the molten pool was in the transition state. Although a necking defect occurred, the surface quality of the melted track was improved and most part of the track kept continuous. When the laser scanning speed continued to decrease to the range of 0.8 m/s - 0.3 m/s, the molten pool reached the stable state. Under this circumstance, a highly uniform and continuous track was obtained. So, a more uniform and continuous track could be achieved by decreasing the laser scanning speed appropriately. The similar conclusion can be found in a previous research [28]. 8 / 25
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Fig. 4. Surface morphologies of the melted tracks from simulations (left) and
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corresponding experiments (right) with different laser scanning speeds: (a) - (b) 2.5 m/s and 2.0 m/s (unstable states), (c) 1.3 m/s (transition state), and (d) - (e) 0.8 m/s
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and 0.3 m/s (stable states). 4.2 Bulk density of specimens
Generally, a uniform and continuous melted track in SLM process is beneficial to
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produce the part with a relatively higher bulk density [29]. As presented in Fig. 4, the melted track transforms from the unstable state into the stable state gradually when the laser scanning speed decreases from 2.5 m/s to 0.3 m/s. Meanwhile, the uniformity and continuity of melted tracks improves obviously. Fig. 5(a) displays the experimental specimens, a total of 16. Fig. 5(b) shows the bulk density of the specimens. Table 3 list the correspondence between the sample number and the scanning speed. From the errors bars, it is clearly that at the scanning speed of 0.3 m/s, 0.5 m/s and 0.8 m/s, the molten pools stay in the stable state. The transition states appear at 1.0 m/s and 1.3 m/s, and 2.0 m/s and 2.5 m/s are for unstable states. Besides, two opposite trends are observed. On the one hand, the density reaches top when the scanning speed is 0.5 m/s. As the scanning speed gradually decreased to 0.5 m/s, the bulk density increases continuously. On the other hand, the bulk density is slightly lower than that at both 0.5 m/s and 0.8 m/s when the scanning speed reduces to 0.3 9 / 25
Journal Pre-proof m/s, although the surface quality of the melted track is improved further. The decrease in the density is attributed to the keyhole evolution in the molten pool, which will be discussed in section 5.2.3. The two opposite trend indicates an existence of the threshold of the scanning speed. Namely, the speed of 0.5 m/s is not necessarily the threshold, but it does demonstrate an existence. Whether the speed is over or below
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the threshold, the bulk density would decrease.
Fig. 5. (a) The fabricated samples; (b) The relative densities of the samples under
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scanning speeds of 0.3 m/s, 0.5 m/s, 0.8 m/s, 1.3 m/s, 1.6 m/s, 2.0 m/s and 2.5 m/s, respectively.
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Table 3 The correspondence between the sample number and scanning speed.
0.3
3, 4
0.5
5, 6 7, 8 9, 10 11, 12
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1, 2
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Sample number Scanning speed (m/s)
0.8 1.0 1.3 1.6
13, 14
2.0
15, 16
2.5
5. Discussions 5.1. Temperature evolutions of molten pools The temperature evolutions of the molten pools (point P, as described in Fig. 1) are plotted in Fig. 6. For the five laser scanning speeds employed, the resultant temperatures reached the melting point. Roughly the lower the scanning speed, the longer the melting time. When the molten pool stayed in the unstable state (2.5 m/s 10 / 25
Journal Pre-proof and 2.0 m/s), the peak temperature of the molten pool did not reach the boiling point of the material. When the scanning speed decreased to 1.3 m/s (the transition state), the highest temperature in the molten pool was just above the boiling point. As the scanning speed further decreased to 0.8 m/s and 0.3 m/s, where the molten pool was in the stable state, the highest temperature exceeded the boiling point. It was worth noticing that when the temperature exceeded the boiling point, it did not continue to rise, but fluctuated around the boiling point; this was remarkably different from the
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previous research [17] where the recoil pressure in molten pools was not considered.
Fig. 6. Temperature evolutions of the monitoring point P in different states of the
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molten pools.
Fig. 7 shows the temperature fields of the powder beds when the laser scans to the
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monitoring point. As the scanning speed decreased from 2.5 m/s to 2.0 m/s, the peak temperature increased because the laser energy density became relatively stronger. However, the heat did not accumulate enough to completely fuse the powder bed, resulting in a discontinuous melted track and some balling defects. When the scanning speed decreased to 1.3 m/s, the accumulated heat increased, and thus more powder particles were fused and the balling defects were eliminated. As the scanning speed decreased to 0.8 m/s and 0.3 m/s, the peak temperature exceeded the boiling point of the metal. At this stage, part of the liquid was subject to the evaporation, and then the recoil pressure was exerted onto the surface of the molten pool. Consequently, the molten pool surface sunk down, resulting in a depression region. Assuming the heat loss caused by the evaporation could be neglected, the temperature of the monitoring point P derived from the natural convection and conduction between liquid metal and gas. As the point P was located within several numerical grids distance to the molten 11 / 25
Journal Pre-proof pool region, its temperature could represent the molten pool temperature approximately. Noticeably, the temperature of the molten pool did not keep rising but fluctuate near the boiling point (Fig. 6). This conclusion is different from those of the literatures [17, 30] which pointed out the peak temperature of molten pools increased continually with the laser scanning speeds decreasing. Whether or not to consider the
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effect of the recoil pressure is the predominant reason for this difference.
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Fig. 7. Morphologies and temperature fields of the powder beds when the laser scans to the monitoring point P (where, the laser scanning speeds are 2.5 m/s, 2.0 m/s, 1.3 m/s, 0.8 m/s, 0.3 m/s, respectively, and the laser power is 200 W.). 5.2. Molten pool states and defects 13 / 25
Journal Pre-proof The formation and characteristics of three states of the molten pool that are described in Fig. 4 can be classified by their velocity fields. 5.2.1 Unstable state Fig. 8 illustrates the velocity field of the molten pool under the scanning speed of 2.0 m/s and 2.5 m/s, respectively. In the unstable state, the molten pool was composed of a heating zone where the laser directly struck the powder bed and some island regions. In general, a moving laser beam with a Gaussian distribution tends to drive the fluid melt to flow backwards under the Marangoni force. However, the laser heat input in this case was too weak to penetrate deeply and fuse the powder bed completely, as the
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scanning speed was too fast. The flow behavior of the liquid metal within the molten
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pool was totally determined by the surface tension. On the other hand, as the powder particles were randomly distributed and the surface tension of the metal was
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negatively correlated with the temperature, an absolutely irregular flow in the heating zone was produced. Only a small portion of the liquid metal flowing backwards could
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be observed in the heating zone (see the third column of Fig. 8). In addition, the high scanning speed was inclined to break up the continuity of the molten pool owing to
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the Plateau–Rayleigh instability [28], thus giving birth to the isolated islands (Fig. 8b). Due to the fact that the surface tension acted to minimize the surface energy, the melt
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in the island regions flowed outward. However, the melt flow in the island regions were too weak to connect the isolated islands, which was the vital cause for the
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balling defects. Consequently, the uniformity and continuity of the solidified track would be seriously deteriorated. During the successive scanning and forming process, a wealth of pores defects would occur inside the specimens [8], drastically reducing the bulk density (Fig. 5b).
Apparently, the unstable state of the molten pool is undesirable for its numerous balling defects. In this case, the best remedy is to increase the energy density to prevent the isolated islands, which will be discussed in the following sections.
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Fig. 8. Velocity fields of the molten pools in an unstable state: (a) with the scanning
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speed of 2.0 m/s and (b) with the scanning speed of 2.5 m/s. 5.2.2 Transition state
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For the molten pool in the transition state, the balling defects are eliminated to a large
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scale, as Fig. 4c shows. Nonetheless, the occurrence of the necking defects may lead to a decrease in the surface quality of the melted track, which also reduces the bulk
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density (Fig. 5). Fig. 9 shows the snapshots of the velocity contours and vector plots of the molten pool. Compared to the molten pool in the unstable state (Fig. 8), the
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main difference is that the island region is not completely separated from one another, owing to the decrease of scanning speed (i.e., the increase of laser energy density).
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Instead, the island regions were gradually connected by the transition region during the scanning process. From Fig. 9(a), the island regions were prone to be dragged out
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of the molten pool due to the Plateau–Rayleigh instability (t = 183 μs), which is similar to Fig. 8. However, as indicated by the velocity vectors (Fig. 9b), most of the liquid metal flowed backwards intensely from the heating zone, preventing the breakup of the molten pool and forming a transition region. The melt in the transition region could flow smoothly with a relatively smaller velocity from t = 183 μs to t = 230 μs. When t = 268 μs, there were no obvious islands or transition regions, and the molten pool was an integral whole. Notwithstanding, in order to minimize the surface energy, the surface tension induced a lateral flow in the edge of the molten pool. Thus, some unfavorable defects such as a necking might occur because of the inadequate reflux, although the balling defects were eliminated to a great extent. Based on the above discussion, the remedy to alleviate the forming quality could be obtained. The transition state of the molten pool is acceptable, but sufficient backward flow from the heating zone should be guaranteed to fill the transition region, and the 15 / 25
Journal Pre-proof lateral flow should be prevented for the necking defect would occur. So, reducing the
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scanning speed slightly is advantageous.
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Fig. 9. Velocity fields of the molten pools in a transition state (with the scanning
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speed of 1.3 m/s): (a) velocity contours, and (b) the corresponding velocity vector plots.
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5.2.3 Stable state
The velocity field of the molten pool at the scanning speed of 0.8 m/s are shown in
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Fig. 10. In this case, the main feature of the molten pool is the disappearance of the transition region. In Fig. 10(a), some small dispersed islands like that in Fig. 9 could
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be found in the tail of the molten pool, which is circled by the red line (t = 306 μs and 414 μs). As laser scanned forward, these small islands might be merged with the
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strong backward melt flow from the heating zone. When t = 517 μs, the molten pool maintained as a whole, and it would finally give birth to a continuous and uniform melt track after the solidification. Moreover, the depression region that was induced by the recoil pressure could be seen apparently. The surface tension drove the metal fluid within the molten pool to flow outwards from the depression region; conversely, the recoil pressure forced the melt liquid to flow towards the depression region. These two behaviors competed with each other to achieve a mechanical equilibrium, and thus resulting in a counter-clockwise melt circulation in the molten pool (Fig. 10). Fig. 10(b) shows the velocity fields of the molten pool from a longitudinal view at different moments. It could be found that the shape of the molten pool was almost not affected by the melt circulation. In consequence, the molten pool kept a nearly fixed shape in the wake of the laser beam. It is desirable to obtain this kind of molten pool, as the deposited energy is appropriate, 16 / 25
Journal Pre-proof resulting in a fine surface quality and a relatively higher density. A small depression region does favor to balance the surface tension. In this case, to further improve the forming quality, attention should be paid to other factors such as powder packing
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quality and hatch spacing, but it is not within the scope of the present study.
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Fig. 10. Velocity fields of the molten pools in the stable state (with the scanning speed
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of 0.8 m/s): (a) top views and (b) longitudinal sections. As shown in Fig. 4 and Fig. 5, although the surface morphology of the molten pool at
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the laser scanning speed of 0.3 m/s is almost the same as that at 0.8 m/s, the density of the samples formed at 0.3 m/s was slightly lower. The pore defects that are resulted from the evolution of the keyhole is to blame for this abnormal phenomenon [31, 32].
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Fig. 11 displays the process of pores formation in the molten pool because of the keyhole. In this case, the shape of the keyhole was unstable and changeable, owing to the unbalance between the surface tension and the recoil pressure. The extraordinarily high laser energy density fused enormous metallic material. Then, a prominent melt like a hump occurred on one side of the keyhole (Fig. 11a). As the melts flowed within the molten pool, the hump-like melt might collapse and connect the other side of the keyhole (Fig. 11b), then a lapping occurred. A small amount of the gas was trapped into the molten pool, forming some moving gas pores (Fig. 11c). In the same time, as Fig. 11(d) shows, two melt circulations with opposite flow directions were produced in the molten pool; this was different from the flow at the scanning speed of 0.8 m/s, where there was only one circulation (Fig. 10b). In addition, with laser marching forward, the micro-scale molten pool moved rapidly with an ultra-high solidification rate. Part of the moving gas pores might escape from the molten pool 17 / 25
Journal Pre-proof due to the melt circulation, whereas others might be confined in the solidified metal and were converted into the residual pores (Fig. 11d). With laser scanning, the molten pool and the keyhole experienced the above evolution process repeatedly. In consequence, after removing the laser beam (Fig. 11e), the whole keyhole collapsed. A large amount of the gas might be trapped, which would also become a residual pore defect in the solidification area (Fig. 11f). Notably, the pores formation is keyhole-related. The best solution to diminish the pores is to increase the scanning
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speed gradually with a small spacing so as to reduce the degree of the depression.
Fig. 11. The pores formation within the molten pool in a stable state with the laser
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scanning speed of 0.3 m/s.
Additionally, to validate the simulation, Fig 12 shows the cross-sectional morphology
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of the specimens fabricated at 0.3 m/s and 0.8 m/s. When employing 0.3 m/s (Fig. 12a), there are residual pores at the bottom of the molten pool, which is consistent with the above simulation prediction. Some experimental studies [33, 34] have also found the similar results by employing the micro-CT to characterize the pores after the SLM process.
Fig. 12. Cross-sectional morphologies of the specimens fabricated at the scanning speeds of (a) 0.3 m/s and (b) 0.8 m/s, respectively. Additionally, as Fig. 5 shows, this kind of residual pores caused by the keyhole 18 / 25
Journal Pre-proof slightly reduce the bulk density although the reduction is not obvious. Many researches [35-37] have pointed out that when the laser energy is further increased, the resultant melt volume would be fairly enormous. It may lead to a melt accumulation and greatly reduce the flatness of the molten pool surface, thus causing a host of porosity in the fabricated parts. The combination of the above two factors is the reason why the scanning speed is low but the density decreases. 6. Conclusions Through the powder-scale numerical simulation and the corresponding experiments, the different states of the molten pool were defined and discussed, and the defects
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formation were also revealed to be closely related to the melt flow. The relationship
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between the molten pool states, defects and bulk densities were revealed. Some major findings are highlighted as follows:
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(1) As the laser scanning speed decreased, the effects of the recoil pressure became increasingly apparent. When the scanning speed dropped to the range of 0.8 m/s - 0.3
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m/s, a depression region was produced. The existing keyhole restrained the temperature of the molten pool near the boiling point of the metal, which is
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completely different with the temperature evolutions without the recoil pressure. (2) The laser scanning speeds could result in three states of the molten pools: unstable
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state (over 1.3 m/s), transition state (0.8 m/s - 1.3 m/s) and stable state (0.3 m/s - 0.8 m/s). The unstable state of the molten pool was undesirable as it severely deteriorated
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the surface quality and bulk density owing to the balling defect. It was acceptable that the transition region occurred in the transition state of the molten pool, but sufficient backflow must be produced and lateral flow should be avoided in case of the necking defect. A small depression region induced by the recoil pressure was beneficial to increase the bulk density. However, too low laser scanning speed might lead to a decrease in the bulk density owing to the keyhole evolution, wherein residual pores might be produced.
Acknowledgements This work is sponsored by the National Key R&D Program of China (grant number: 2018YFB1105301), the National Natural Science Foundation of China (grant bumber: 51705170), the Academic Frontier Youth Team at Huazhong University of Science 19 / 25
Journal Pre-proof and Technology, the Wuhan Science and Technology Plan Project (grant number: 2018010401011281), and the China Postdoctoral Science Foundation (grant number: 2018T110756). The authors are also grateful for the State Key Laboratory of Materials Processing and Die & Mould Technology for the measurements. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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Authorship contributions Weihao Yuan: Software, data visualization & analysis, writing - original draft. Hui Chen: Conceptualization, methodology, writing - review & editing. Tan Cheng: Validation. Qingsong Wei: Writing - review & editing, supervision.
Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal
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relationships that could have appeared to influence the work reported in this paper.
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Highlights
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Graphical abstract
A molten pool has three different states due to different laser scanning speeds
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Recoil pressure confines the molten pool temperature around the boiling point There existing a threshold in the scanning speed where the bulk density peaks Residual pores can be caused by the evolution of the molten pool with a keyhole
25 / 25
Weihao Yuan, Hui Chen, Tan Cheng, Qingsong Wei PII:
S0264-1275(20)30075-7
DOI:
https://doi.org/10.1016/j.matdes.2020.108542
Reference:
JMADE 108542
To appear in:
Materials & Design
Received date:
17 December 2019
Revised date:
22 January 2020
Accepted date:
1 February 2020
Please cite this article as: W. Yuan, H. Chen, T. Cheng, et al., Effects of laser scanning speeds on different states of the molten pool during selective laser melting: Simulation and experiment, Materials & Design(2020), https://doi.org/10.1016/j.matdes.2020.108542
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier.
Journal Pre-proof Effects of Laser Scanning Speeds on Different States of the Molten Pool During Selective Laser Melting: Simulation and Experiment Weihao Yuana, Hui Chena,b,*, Tan Chenga, Qingsong Weia,* a
State Key Lab of Materials Forming and Die&Mould Technology, School of
Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b
Department of Mechanical Engineering, National University of Singapore,
Singapore 117575, Singapore Abstract
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Using numerical simulations and experimental tests, the temperature and velocity
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fields of the molten pool during selective laser melting (SLM) were investigated, where the laser scanning speed ranging from 2.5 m/s to 0.3 m/s was employed.
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Experiments for single tracks and part samples were conducted for verification. Three kinds of molten pool states were identified and investigated: unstable state, transition
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state and stable state. The unstable state is characterized by numerous balling defects,
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where the bulk density is severely deteriorated. The transition state is featured by the transition region where the melt velocity is relatively lower, and the molten pool is vulnerable to the necking defect. The molten pool with a depression region is
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identified as the stable state. A small depression is favorable for improving the surface quality of single track and the bulk density. However, exorbitant energy input will
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convert the depression into a keyhole. Additionally, a threshold of the scanning speed was found, where the bulk density peaked. Over the threshold, the density decreased continuously with the speed increasing. However, the density slightly decreased by 1.5% when the speed was below the threshold; this anomaly was ascribed to the residual pores induced by the recoil pressure. Keywords: Scanning speed; Selective laser melting; Temperature and velocity fields; Molten pool states; Threshold
*
Corresponding authors. Email address: [email protected] (Q. Wei) and [email protected] (H. Chen). 1 / 25
Journal Pre-proof 1. Introduction Selective laser melting (SLM) is one of the state-of-the-art additive manufacturing (AM) technologies for rapid production of geometrically complex metal parts [1]. In the past few years, SLM has become a novel processing route to a number of industry applications from the aerospace [2, 3] to the biomedicine [4]. However, further promotion of SLM is needed for widespread adoption of metal components manufacturing [5], as this kind of AM process can be susceptible to many defects. For instance, the pores formed during SLM is one of the most notable defects that would deteriorate final component properties such as bulk density. According to the recent
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studies of metal AM [6, 7], the surface morphology of metal particle can influence the
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fusion behaviors and metallurgy bonding between neighboring particles. The ambient gas may also be trapped into the particles to form some micro-pores, and a more
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unstable molten pool may increase the chances of this kind of gas entrapment [8]. Since it is commonly believed that the continuous and uniform melted single tracks
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would lead to a relatively dense final component [9], some experiments were conducted for the purpose of parameters optimization [10-12]. However, relying on
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trial-error method, the experimental tool is too inefficient and costly. It is appealed to
physical perspective.
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comprehend the underlining mechanism of the defects formation from a mathematical
Numerical simulation is a powerful method to investigate the heat transfer and fluid
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flow during SLM, and it is also vital to understand the formed defects. Carolin Koner et al. [13, 14] proposed 2D and also 3D numerical programs based on the LBM (Lattice Boltzmann Method) method to study the fusion process of metal powder bed. With FVM (Finite Volume Method), Qiu et al. [15, 16] researched the influence of processing parameters on the pore evolution and surface morphology, and the resultant microstructure was also discussed [5]. The conclusions were arrived that the increased laser scanning speed and layer thickness was mainly responsible for the porosity and surface defect. Similar work was undertaken by Gu et al. [17], focusing on the evolution mechanism of the molten pool and porosity in the nickel-based alloy, however, the recoil pressure due to evaporation is excluded. Wayne King et al. [18] of Lawrence Lab focused on the complex physical phenomena including melting, capillary flow and evaporation in SLM process based on the home-made code. They discussed the potential defects such as pores, spatters, and denudation as well as their 2 / 25
Journal Pre-proof forming mechanisms. Nevertheless, the effects of the variable processing parameters are not studied in their work. Recently, some studies proposed the combined model of DEM (discrete element method) and CFD (Computational Fluid Dynamic) to simulate the complete process of powder-based additive manufacturing, such as Lee [19], Yan [20] and Tang [21]. The main focus of these literatures are the mesoscopic scale defects and the molten pool evolution. Tang emphasized dominant role of the recoil pressure and pointed out that this phenomenon might produce pore defects with different shapes. Besides, Wu et al. [23] found that the presence or absence of the recoil pressure significantly changed the patterns of heat transfer and fluid flow within
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the molten pool. Thus, the recoil pressure should be paid special attentions, but many
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researches did not [17, 20, 21]. Additionally, what extent would the molten pool behaviors and the potential defects affect the part quality is another key point that is
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worthwhile to be disclosed.
The objectives of this study are to research the effects of laser scanning speed on the
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molten pool behaviors and the forming quality in SLM through computational and experimental methods. The molten pool states are classified, and then the relationship
2. Numerical modeling 2.1. Physical model
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comprehensively revealed.
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between the bulk densities and the molten pool states under recoil pressure is
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The physical model for the selective laser melting (SLM) process is shown in Fig. 1. The whole computational domain in the Cartesian coordinate system is 0.8×0.2×0.28 mm3, where the height of baseplate and gas region are set 200μm and 80μm (Z-axis direction), respectively. A fixed monitoring point P (0.4mm, 0.1mm, 0.2mm) is set to approximately measure the molten pool temperature during laser scanning. In order to accelerate the calculation process and enhance the accuracy, the regular hexahedron with as small a mesh size as possible is desirable. However, too small mesh size results in plenty of computational time. Thus, the mesh size of 2.5 μm was adopted with reference to the literatures [21, 22], and the number of entire grids is approximately 3.0 million. The 316L stainless steel particles with the mean size of 30 μm are paved on the baseplate to form a 30 μm thickness of powder bed. The powder spreading process is simulated using discrete element method (DEM) model, which has been built in one of our previous studies [1, 26]. 3 / 25
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Fig. 1. Physical model for the powder bed fusion process.
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The simulation is conducted through the commercial software Ansys-Fluent 18.0, and several assumptions are adopted to simplify the numerical model: (1) The fluid flow
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of molten pool is incompressible and laminar; (2) The latent heat of metal powder varies linearly with temperature and all the thermo-physical properties are functions
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of temperature only; (3) The plasma effect and the Fresnel absorption in SLM process
2.2 Governing equations
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are ignored.
The governing equations, i.e. the continuity, momentum and energy equations showed ( u) 0
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as eq. (1), (2), and (3) are solved [20]:
(1) (2)
( H ) ( uH ) kT Sh t
(3)
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( u) ( uu) P g (u) Sm t
where u and g denote the velocity and the gravitational acceleration, respectively.
, , P , T , k refer to the density, viscosity, pressure, temperature, and thermal conductivity, respectively. H is the enthalpy of the material considering the heat transfer due to the melting and solidification, defined by H c dT H , in p
l
which H and l denote the latent heat of melting and the liquid volume fraction, respectively. l is assumed to be linear interpolation of the solid temperature Ts and liquid temperature Tl [20]:
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Journal Pre-proof 0 T Ts l Tl Ts 1
T Ts Ts T Tl
(4)
T Tl
Additionally, the Volume of Fluid (VOF) method is used to capture the free surface, which solves the equation
F t
( Fv) 0 to get the metal phase friction F in each
cell. Sm and S h in eq. (2) and (3) are the source terms. In the momentum conservation equation, four kinds of body forces are incorporated, and the source term
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Sm can be expressed as follows. (5)
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Sm Sdam f sur f mag prec
where the first term S dam denotes the Darcy force responsible for dampening the
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velocity to zero when the temperature is lower than the melting temperature [16]. The
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second term f sur denotes one of the components of the surface tension whose direction is vertical to the gas-liquid interface. The third term f mag denotes the
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tangential component of the surface tension, which would be exerted onto the metal-gas interface. This term is also known as Marangoni forces [24]. Continuum
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surface stress method that considers the effect of variable surface tension is used [25]. It should be noticed that the recoil pressure which is caused by the evaporation is
pr p0 exp[
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considered in the model [27]. mv Lv
(
1 1 )] Tb T
(6)
where pr , p0 , m v , , Lv and Tb refer to the recoil pressure, operation pressure, molecular mass, boltzmann constant, latent heat of evaporation and ambient temperature. Since evaporation can only occurs on the gas-metal interface, a multiplier F1
2 1 +2
is added to the eq. (6), where F1 refers to the volume
fraction gradient of metallic phase, and
2 1 + 2
is used to smear out the effect of very
different density phases [16, 17]. 1 and 2 denote the density of metal and gas, while denotes the volume-average density, defined by the sum of the product of density and corresponding volume fraction ( 1F1 2 F2 ). Therefore, the forth term prec in eq. (5) can be expressed as [16]: 5 / 25
Journal Pre-proof prec p0 exp[
mv Lv
(
1 1 2 )] F1 Tb T 1 2
(7)
In the energy conservation equation (eq. 3), the source term S h consists of two parts: the absorption and releasing of the latent heat of melting, which has been united within eq. (3). The laser heat input QL is expressed by eq. (8). The adopted laser source model is a surface heat source with a Gaussian distribution. Similarly, an extra multiplier, 1
2 cp 1c p1 2c p 2
, is used to transfer the surface heat source into a volumetric
distribution [24].
2
exp[
2r 2
2
] 1
2 c p 1c p1 c p 2
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2 AP
(8)
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QL
The model does not consider extra thermal boundary conditions at the metal-gas
-p
interface. Actually, the total quantity of the heat dissipation from the other conditions such as convection and radiation is far smaller compared with the laser heat source,
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and they have little effect on the molten pool behaviors and states.
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Fig. 2 shows the temperature-dependent physical properties including thermal conductivity, specific heat and liquid viscosity. Other properties of the material are listed in table 1 in detail [22]. Table 2 shows the processing parameters for
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simulations.
Fig. 2. Temperature-dependent physical properties: (a) Thermal conductivity, (b) specific heat, and (c) liquid viscosity. Table 1 Thermophysical properties of the 316L stainless steel. Parameters, symbol
Value
Density,
7500 kg/m3
Operating pressure, p 0
0.1 MPa
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Journal Pre-proof Solidus temperature, Ts
1658 K
Liquid temperature, Tl
1723 K
Latent heat, H
270000 J/kg
Surface tension, f
1.8 N/m
Surface tension gradient,
d f
-0.0003 N/m-k
dT
Radiation emissivity, ε
0.4
Metal evaporation heat, Lv
7.45e6 J/kg
Table 2
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Processing parameters for simulations. Name, symbol
Value
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Beam radius,
35 μm
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Absorption, A Power, P
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Scanning speed, v
3. Experiments
200 W 0.3, 0.8, 1.3, 2.0, 2.5 m/s 30 μm
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Powder thickness
0.38
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In order to validate the numerical model, corresponding experiments were performed for the fabrication of single tracks and part specimens. The near-spherical 316L stainless steel powder was used as the raw material for SLM experiments, as shown in
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Fig. 3. The powder size distribution was tested, and the Dv10, Dv50 and Dv90 were 18.5 μm, 30.9 μm and 49.3 μm, respectively. The powder was put in a drying oven at 90 ℃ for at least 10 hours prior to the SLM. A self-developed SLM system containing a SPI-400 fiber laser device with the maximum laser power of 400 W and the spot size of 70 μm was employed. Apart from the processing parameters adopted in the simulation process (Table 2), three more groups of experiments with scanning speed of 0.5 m/s, 1.0 m/s and 1.6 m/s were added to further investigate the tendency, and two samples were prepared at each laser scanning speed. The experiments were performed under a high-purity argon gas protection atmosphere with a concentration of 99.9%. The operation pressure was kept approximately 0.1 Mpa. Two specimens were fabricated under each scanning speed. The optimized hatch spacing was 65 μm. The dimension of the formed part was 10 mm × 10 mm × 2mm. After that, the ultra-depth of field optical microscope (VHX-1000C, Japan) was used to observe the 7 / 25
Journal Pre-proof surface morphology of single tracks. The fully automatic true density analyzer (AccuPyc1330, USA) was used to measure the relative density after specimens being cut down from the substrate and side walls being ground and polished. The values were obtained from the average of six measurements (each sample for three tests). The cross sections of the specimens were then corroded with aqua regia consisting of
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HCl (15 ml) and HNO3 (5 ml) for about 25 seconds to expose the internal features.
Fig. 3. Morphology of the 316L stainless steel powder with the tested powder size
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distribution.
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4. Results 4.1 Morphologies of single tracks
The surface morphologies of the melted tracks resulted from simulations and
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experiments are shown in Fig. 4. The results of the simulation agree well with those of the experiments. As laser scanning speed decreases from 2.5 m/s to 0.3 m/s, i.e., the
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energy density increases, the melted tracks can be divided into three categories: unstable state, transition state, and stable state based on the flow behaviors of molten pool, which will be discussed in the section 5.2. When a higher scanning speed (2.0 m/s and 2.5 m/s) was employed, the molten pool entered the unstable state. The uniformity and continuity of the melted tracks were completely destroyed by the balling defects. As the laser scanning speed decreased to 1.3 m/s, the molten pool was in the transition state. Although a necking defect occurred, the surface quality of the melted track was improved and most part of the track kept continuous. When the laser scanning speed continued to decrease to the range of 0.8 m/s - 0.3 m/s, the molten pool reached the stable state. Under this circumstance, a highly uniform and continuous track was obtained. So, a more uniform and continuous track could be achieved by decreasing the laser scanning speed appropriately. The similar conclusion can be found in a previous research [28]. 8 / 25
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Fig. 4. Surface morphologies of the melted tracks from simulations (left) and
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corresponding experiments (right) with different laser scanning speeds: (a) - (b) 2.5 m/s and 2.0 m/s (unstable states), (c) 1.3 m/s (transition state), and (d) - (e) 0.8 m/s
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and 0.3 m/s (stable states). 4.2 Bulk density of specimens
Generally, a uniform and continuous melted track in SLM process is beneficial to
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produce the part with a relatively higher bulk density [29]. As presented in Fig. 4, the melted track transforms from the unstable state into the stable state gradually when the laser scanning speed decreases from 2.5 m/s to 0.3 m/s. Meanwhile, the uniformity and continuity of melted tracks improves obviously. Fig. 5(a) displays the experimental specimens, a total of 16. Fig. 5(b) shows the bulk density of the specimens. Table 3 list the correspondence between the sample number and the scanning speed. From the errors bars, it is clearly that at the scanning speed of 0.3 m/s, 0.5 m/s and 0.8 m/s, the molten pools stay in the stable state. The transition states appear at 1.0 m/s and 1.3 m/s, and 2.0 m/s and 2.5 m/s are for unstable states. Besides, two opposite trends are observed. On the one hand, the density reaches top when the scanning speed is 0.5 m/s. As the scanning speed gradually decreased to 0.5 m/s, the bulk density increases continuously. On the other hand, the bulk density is slightly lower than that at both 0.5 m/s and 0.8 m/s when the scanning speed reduces to 0.3 9 / 25
Journal Pre-proof m/s, although the surface quality of the melted track is improved further. The decrease in the density is attributed to the keyhole evolution in the molten pool, which will be discussed in section 5.2.3. The two opposite trend indicates an existence of the threshold of the scanning speed. Namely, the speed of 0.5 m/s is not necessarily the threshold, but it does demonstrate an existence. Whether the speed is over or below
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the threshold, the bulk density would decrease.
Fig. 5. (a) The fabricated samples; (b) The relative densities of the samples under
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scanning speeds of 0.3 m/s, 0.5 m/s, 0.8 m/s, 1.3 m/s, 1.6 m/s, 2.0 m/s and 2.5 m/s, respectively.
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Table 3 The correspondence between the sample number and scanning speed.
0.3
3, 4
0.5
5, 6 7, 8 9, 10 11, 12
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1, 2
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Sample number Scanning speed (m/s)
0.8 1.0 1.3 1.6
13, 14
2.0
15, 16
2.5
5. Discussions 5.1. Temperature evolutions of molten pools The temperature evolutions of the molten pools (point P, as described in Fig. 1) are plotted in Fig. 6. For the five laser scanning speeds employed, the resultant temperatures reached the melting point. Roughly the lower the scanning speed, the longer the melting time. When the molten pool stayed in the unstable state (2.5 m/s 10 / 25
Journal Pre-proof and 2.0 m/s), the peak temperature of the molten pool did not reach the boiling point of the material. When the scanning speed decreased to 1.3 m/s (the transition state), the highest temperature in the molten pool was just above the boiling point. As the scanning speed further decreased to 0.8 m/s and 0.3 m/s, where the molten pool was in the stable state, the highest temperature exceeded the boiling point. It was worth noticing that when the temperature exceeded the boiling point, it did not continue to rise, but fluctuated around the boiling point; this was remarkably different from the
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previous research [17] where the recoil pressure in molten pools was not considered.
Fig. 6. Temperature evolutions of the monitoring point P in different states of the
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molten pools.
Fig. 7 shows the temperature fields of the powder beds when the laser scans to the
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monitoring point. As the scanning speed decreased from 2.5 m/s to 2.0 m/s, the peak temperature increased because the laser energy density became relatively stronger. However, the heat did not accumulate enough to completely fuse the powder bed, resulting in a discontinuous melted track and some balling defects. When the scanning speed decreased to 1.3 m/s, the accumulated heat increased, and thus more powder particles were fused and the balling defects were eliminated. As the scanning speed decreased to 0.8 m/s and 0.3 m/s, the peak temperature exceeded the boiling point of the metal. At this stage, part of the liquid was subject to the evaporation, and then the recoil pressure was exerted onto the surface of the molten pool. Consequently, the molten pool surface sunk down, resulting in a depression region. Assuming the heat loss caused by the evaporation could be neglected, the temperature of the monitoring point P derived from the natural convection and conduction between liquid metal and gas. As the point P was located within several numerical grids distance to the molten 11 / 25
Journal Pre-proof pool region, its temperature could represent the molten pool temperature approximately. Noticeably, the temperature of the molten pool did not keep rising but fluctuate near the boiling point (Fig. 6). This conclusion is different from those of the literatures [17, 30] which pointed out the peak temperature of molten pools increased continually with the laser scanning speeds decreasing. Whether or not to consider the
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effect of the recoil pressure is the predominant reason for this difference.
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Fig. 7. Morphologies and temperature fields of the powder beds when the laser scans to the monitoring point P (where, the laser scanning speeds are 2.5 m/s, 2.0 m/s, 1.3 m/s, 0.8 m/s, 0.3 m/s, respectively, and the laser power is 200 W.). 5.2. Molten pool states and defects 13 / 25
Journal Pre-proof The formation and characteristics of three states of the molten pool that are described in Fig. 4 can be classified by their velocity fields. 5.2.1 Unstable state Fig. 8 illustrates the velocity field of the molten pool under the scanning speed of 2.0 m/s and 2.5 m/s, respectively. In the unstable state, the molten pool was composed of a heating zone where the laser directly struck the powder bed and some island regions. In general, a moving laser beam with a Gaussian distribution tends to drive the fluid melt to flow backwards under the Marangoni force. However, the laser heat input in this case was too weak to penetrate deeply and fuse the powder bed completely, as the
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scanning speed was too fast. The flow behavior of the liquid metal within the molten
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pool was totally determined by the surface tension. On the other hand, as the powder particles were randomly distributed and the surface tension of the metal was
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negatively correlated with the temperature, an absolutely irregular flow in the heating zone was produced. Only a small portion of the liquid metal flowing backwards could
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be observed in the heating zone (see the third column of Fig. 8). In addition, the high scanning speed was inclined to break up the continuity of the molten pool owing to
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the Plateau–Rayleigh instability [28], thus giving birth to the isolated islands (Fig. 8b). Due to the fact that the surface tension acted to minimize the surface energy, the melt
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in the island regions flowed outward. However, the melt flow in the island regions were too weak to connect the isolated islands, which was the vital cause for the
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balling defects. Consequently, the uniformity and continuity of the solidified track would be seriously deteriorated. During the successive scanning and forming process, a wealth of pores defects would occur inside the specimens [8], drastically reducing the bulk density (Fig. 5b).
Apparently, the unstable state of the molten pool is undesirable for its numerous balling defects. In this case, the best remedy is to increase the energy density to prevent the isolated islands, which will be discussed in the following sections.
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Fig. 8. Velocity fields of the molten pools in an unstable state: (a) with the scanning
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speed of 2.0 m/s and (b) with the scanning speed of 2.5 m/s. 5.2.2 Transition state
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For the molten pool in the transition state, the balling defects are eliminated to a large
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scale, as Fig. 4c shows. Nonetheless, the occurrence of the necking defects may lead to a decrease in the surface quality of the melted track, which also reduces the bulk
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density (Fig. 5). Fig. 9 shows the snapshots of the velocity contours and vector plots of the molten pool. Compared to the molten pool in the unstable state (Fig. 8), the
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main difference is that the island region is not completely separated from one another, owing to the decrease of scanning speed (i.e., the increase of laser energy density).
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Instead, the island regions were gradually connected by the transition region during the scanning process. From Fig. 9(a), the island regions were prone to be dragged out
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of the molten pool due to the Plateau–Rayleigh instability (t = 183 μs), which is similar to Fig. 8. However, as indicated by the velocity vectors (Fig. 9b), most of the liquid metal flowed backwards intensely from the heating zone, preventing the breakup of the molten pool and forming a transition region. The melt in the transition region could flow smoothly with a relatively smaller velocity from t = 183 μs to t = 230 μs. When t = 268 μs, there were no obvious islands or transition regions, and the molten pool was an integral whole. Notwithstanding, in order to minimize the surface energy, the surface tension induced a lateral flow in the edge of the molten pool. Thus, some unfavorable defects such as a necking might occur because of the inadequate reflux, although the balling defects were eliminated to a great extent. Based on the above discussion, the remedy to alleviate the forming quality could be obtained. The transition state of the molten pool is acceptable, but sufficient backward flow from the heating zone should be guaranteed to fill the transition region, and the 15 / 25
Journal Pre-proof lateral flow should be prevented for the necking defect would occur. So, reducing the
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scanning speed slightly is advantageous.
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Fig. 9. Velocity fields of the molten pools in a transition state (with the scanning
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speed of 1.3 m/s): (a) velocity contours, and (b) the corresponding velocity vector plots.
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5.2.3 Stable state
The velocity field of the molten pool at the scanning speed of 0.8 m/s are shown in
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Fig. 10. In this case, the main feature of the molten pool is the disappearance of the transition region. In Fig. 10(a), some small dispersed islands like that in Fig. 9 could
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be found in the tail of the molten pool, which is circled by the red line (t = 306 μs and 414 μs). As laser scanned forward, these small islands might be merged with the
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strong backward melt flow from the heating zone. When t = 517 μs, the molten pool maintained as a whole, and it would finally give birth to a continuous and uniform melt track after the solidification. Moreover, the depression region that was induced by the recoil pressure could be seen apparently. The surface tension drove the metal fluid within the molten pool to flow outwards from the depression region; conversely, the recoil pressure forced the melt liquid to flow towards the depression region. These two behaviors competed with each other to achieve a mechanical equilibrium, and thus resulting in a counter-clockwise melt circulation in the molten pool (Fig. 10). Fig. 10(b) shows the velocity fields of the molten pool from a longitudinal view at different moments. It could be found that the shape of the molten pool was almost not affected by the melt circulation. In consequence, the molten pool kept a nearly fixed shape in the wake of the laser beam. It is desirable to obtain this kind of molten pool, as the deposited energy is appropriate, 16 / 25
Journal Pre-proof resulting in a fine surface quality and a relatively higher density. A small depression region does favor to balance the surface tension. In this case, to further improve the forming quality, attention should be paid to other factors such as powder packing
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quality and hatch spacing, but it is not within the scope of the present study.
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Fig. 10. Velocity fields of the molten pools in the stable state (with the scanning speed
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of 0.8 m/s): (a) top views and (b) longitudinal sections. As shown in Fig. 4 and Fig. 5, although the surface morphology of the molten pool at
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the laser scanning speed of 0.3 m/s is almost the same as that at 0.8 m/s, the density of the samples formed at 0.3 m/s was slightly lower. The pore defects that are resulted from the evolution of the keyhole is to blame for this abnormal phenomenon [31, 32].
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Fig. 11 displays the process of pores formation in the molten pool because of the keyhole. In this case, the shape of the keyhole was unstable and changeable, owing to the unbalance between the surface tension and the recoil pressure. The extraordinarily high laser energy density fused enormous metallic material. Then, a prominent melt like a hump occurred on one side of the keyhole (Fig. 11a). As the melts flowed within the molten pool, the hump-like melt might collapse and connect the other side of the keyhole (Fig. 11b), then a lapping occurred. A small amount of the gas was trapped into the molten pool, forming some moving gas pores (Fig. 11c). In the same time, as Fig. 11(d) shows, two melt circulations with opposite flow directions were produced in the molten pool; this was different from the flow at the scanning speed of 0.8 m/s, where there was only one circulation (Fig. 10b). In addition, with laser marching forward, the micro-scale molten pool moved rapidly with an ultra-high solidification rate. Part of the moving gas pores might escape from the molten pool 17 / 25
Journal Pre-proof due to the melt circulation, whereas others might be confined in the solidified metal and were converted into the residual pores (Fig. 11d). With laser scanning, the molten pool and the keyhole experienced the above evolution process repeatedly. In consequence, after removing the laser beam (Fig. 11e), the whole keyhole collapsed. A large amount of the gas might be trapped, which would also become a residual pore defect in the solidification area (Fig. 11f). Notably, the pores formation is keyhole-related. The best solution to diminish the pores is to increase the scanning
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speed gradually with a small spacing so as to reduce the degree of the depression.
Fig. 11. The pores formation within the molten pool in a stable state with the laser
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scanning speed of 0.3 m/s.
Additionally, to validate the simulation, Fig 12 shows the cross-sectional morphology
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of the specimens fabricated at 0.3 m/s and 0.8 m/s. When employing 0.3 m/s (Fig. 12a), there are residual pores at the bottom of the molten pool, which is consistent with the above simulation prediction. Some experimental studies [33, 34] have also found the similar results by employing the micro-CT to characterize the pores after the SLM process.
Fig. 12. Cross-sectional morphologies of the specimens fabricated at the scanning speeds of (a) 0.3 m/s and (b) 0.8 m/s, respectively. Additionally, as Fig. 5 shows, this kind of residual pores caused by the keyhole 18 / 25
Journal Pre-proof slightly reduce the bulk density although the reduction is not obvious. Many researches [35-37] have pointed out that when the laser energy is further increased, the resultant melt volume would be fairly enormous. It may lead to a melt accumulation and greatly reduce the flatness of the molten pool surface, thus causing a host of porosity in the fabricated parts. The combination of the above two factors is the reason why the scanning speed is low but the density decreases. 6. Conclusions Through the powder-scale numerical simulation and the corresponding experiments, the different states of the molten pool were defined and discussed, and the defects
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formation were also revealed to be closely related to the melt flow. The relationship
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between the molten pool states, defects and bulk densities were revealed. Some major findings are highlighted as follows:
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(1) As the laser scanning speed decreased, the effects of the recoil pressure became increasingly apparent. When the scanning speed dropped to the range of 0.8 m/s - 0.3
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m/s, a depression region was produced. The existing keyhole restrained the temperature of the molten pool near the boiling point of the metal, which is
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completely different with the temperature evolutions without the recoil pressure. (2) The laser scanning speeds could result in three states of the molten pools: unstable
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state (over 1.3 m/s), transition state (0.8 m/s - 1.3 m/s) and stable state (0.3 m/s - 0.8 m/s). The unstable state of the molten pool was undesirable as it severely deteriorated
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the surface quality and bulk density owing to the balling defect. It was acceptable that the transition region occurred in the transition state of the molten pool, but sufficient backflow must be produced and lateral flow should be avoided in case of the necking defect. A small depression region induced by the recoil pressure was beneficial to increase the bulk density. However, too low laser scanning speed might lead to a decrease in the bulk density owing to the keyhole evolution, wherein residual pores might be produced.
Acknowledgements This work is sponsored by the National Key R&D Program of China (grant number: 2018YFB1105301), the National Natural Science Foundation of China (grant bumber: 51705170), the Academic Frontier Youth Team at Huazhong University of Science 19 / 25
Journal Pre-proof and Technology, the Wuhan Science and Technology Plan Project (grant number: 2018010401011281), and the China Postdoctoral Science Foundation (grant number: 2018T110756). The authors are also grateful for the State Key Laboratory of Materials Processing and Die & Mould Technology for the measurements. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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Authorship contributions Weihao Yuan: Software, data visualization & analysis, writing - original draft. Hui Chen: Conceptualization, methodology, writing - review & editing. Tan Cheng: Validation. Qingsong Wei: Writing - review & editing, supervision.
Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal
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relationships that could have appeared to influence the work reported in this paper.
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Highlights
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Graphical abstract
A molten pool has three different states due to different laser scanning speeds
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Recoil pressure confines the molten pool temperature around the boiling point There existing a threshold in the scanning speed where the bulk density peaks Residual pores can be caused by the evolution of the molten pool with a keyhole
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