Influence of hatch spacing on heat and mass transfer, thermodynamics and laser processability during additive manufacturing of Inconel 718 alloy

International Journal of Machine Tools & Manufacture 109 (2016) 147–157

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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool

Influence of hatch spacing on heat and mass transfer, thermodynamics and laser processability during additive manufacturing of Inconel 718 alloy Mujian Xia a,b, Dongdong Gu a,b,n, Guanqun Yu a,b, Donghua Dai a,b, Hongyu Chen a,b, Qimin Shi a,b a b

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, PR China Institute of Additive Manufacturing (3D Printing), Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 30 May 2016 Received in revised form 22 July 2016 Accepted 26 July 2016 Available online 27 July 2016

A transient three-dimensional powder-scale model has been established for investigating the thermodynamics, heat and mass transfer and surface quality within the molten pool during selective laser melting (SLM) Inconel 718 alloy by finite volume method (FVM), considering the powder-solid transition, variation of thermo-physical properties, and surface tension. The influences of hatch spacing (H) on the thermodynamics, heat and mass transfer, and resultant surface quality of molten pool have been discussed in detail. The results revealed that the H had a significant influence on determining the terminally solidified surface quality of the SLM-processed components. As a relatively lower H of 40 μm was used, a considerable amount of molten liquid migrated towards the previous as-fabricated tracks with a higher velocity, resulting in a stacking of molten liquid and the attendant formation of a poor surface quality with a large average surface roughness of 12.72 μm. As an appropriate H of 60 μm was settled, a reasonable temperature gradient and the resultant surface tension tended to spread the molten liquid with a steady velocity, favoring the formation of a flat surface of the component and an attendant low average surface roughness of 2.23 μm. Both the surface morphologies and average surface roughness were experimentally obtained, which were in a full accordance with the results calculated by simulation. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Additive manufacturing Surface quality Heat and mass transfer Thermodynamics Hatch spacing Laser processability

1. Introduction Selective Laser Melting (SLM), known as a newly developed Additive Manufacturing (AM) technique, enables the quick manufacturing of complex-shaped components with complete densification directly from metallic powder. SLM fabricates components in a layer-by-layer manner by selective fusion and solidification of thin layers of loose metallic powder under a high-energy laser beam [1]. SLM technology is greatly applicable for producing geometry complex components directly from the user-defined computer-aided design (CAD) data files without acquiring any subsequent postprocessing, which accordingly allows for a soundly expanded design freedom with minimal feedstock waste [2]. In case of SLM, it provides considerably possibilities to overcome geometry complex limitations and furnish an almost unchallenged freedom of design n Corresponding author at: College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, PR China. E-mail address: [email protected] (D. Gu).

http://dx.doi.org/10.1016/j.ijmachtools.2016.07.010 0890-6955/& 2016 Elsevier Ltd. All rights reserved.

without special-purpose tooling in comparison to the conventional manufacturing processes. Meanwhile, SLM also affords series of advantages, such like reduction of production processes, high flexibility and efficiency of material consumption, indicating that the conventional manufacturing techniques (e.g. forging and casting) cannot keep pace with easily [3]. Recent research efforts have demonstrated that SLM is regarded as a potential candidate process for the net-shape fabrication of high-performance composite components, owing to its flexibility in feedstock and net-shaping [2,3]. Nevertheless, SLM is based on the metallurgical principle of full melting-solidification and, accordingly, refers to multiple aspects of heat and mass transfer caused by a localized laser scanning [4]. In this situation, the surface quality of the SLM-processed components tends to exhibit with fluctuations due to the transfer of heat and mass [5]. Thus, it is reasonable to conclude that the uncontrollability and/or unpredictability of the surface quality of the SLMprocessed components remain a major challenge. Previous researches have reported that the relatively poor surface quality is still one of the major limitations in the ultimate SLM-processed components [6,7].

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Generally, the surface quality of parts plays a significant role in many industrial applications, since an excellent surface quality favors to improving the mechanical and/or other properties. The subsequently time-consuming surface treatment can cover up the original advantages of SLM process, such as surface laser polishing [6], coating deposited on its surface [7] to obtain the desired surface properties. Commonly, the above mentioned post-treatments do not aim at strengthening the SLM-processed components. Furthermore, the post-treatments are also eventually time-consume and cost-waste. Since the thermodynamic characterization (heat and mass transfer) may directly influence the surface quality of the SLM-processed components, it is, therefore, highly essential to be able to understand thoroughly and control during SLM. However, the complex processing of SLM is generally unbelievable to be obtained by experiments. Fortunately, the established numerical models considering thermal behavior, heat and mass transfer and melt flow are provided a unique insight into predicting the surface quality of the SLM-processed components. Recently, the considerable efforts have focused on tailoring the surface quality by numerous approaches. For instance, Dai et al. [8] have established a numerical model to investigate the effect of different shield gas on the surface quality of SLM-processed aluminum alloy as well as consider the evaporation during SLM, revealing that the shield gas plays a key role in the thermodynamics behavior of the molten pool and the surface morphology. Qiu et al. [9] recently have developed a melt flow dynamics numerical model only for one signal scanning track to explain the variation of surface features under different processing conditions. Meanwhile, compared to the corresponding simulation results, the validity of the developed model was verified and the influence of laser scanning speed and powder layer thickness parameter on the melt flow and surface quality was analyzed, as well. These researches are briefly demonstrated that the melt flow and SLM processing parameters play a crucial role in the surface quality. As is known to all, during SLM, each layer of the as-built component is continuously filled with elongated tracks of molten powder, i.e. the surface of SLM-processed component is composed of numerous tracks [10]. Since the SLM-processed components are built in a layer-by-layer fashion, and to importantly each layer is always fabricated in a track-by-track manner, the track morphology substantially determines the ultimate surface quality. The attempt concerning the laser assisted re-melting of scanning tracks is successfully performed to improve the surface quality of Ti6Al4V alloy during SLM [11]. It also indicates that a promising method to enhance the surface quality of SLM-processed parts paying a considerable operating time. On the other hand, it is noted that the building rate of the current SLM process is commonly low [9]. As for a signal building layer, the building rate could be enhanced by increasing hatch spacing (H) or decreasing overlapping level. In general, a higher building rate is also obtained by increasing H. Nevertheless, when the H increases, low level of defects (such as insufficient melting and porosity) also need to be guaranteed to ensure the required surface integrity and properties. It is, therefore, quite necessary to give an overall investigation into the influence of H on surface quality. Previously, Su et al. [12] have discussed the effect of overlapping patterns on relative density of 316 L stainless steel and have demonstrated that a high relative density is acquired by applying the inter-layer overlapping regime. Indeed, during SLM, numerous simultaneously occurring physical behaviors (for instance, heat and mass transfer, phase transitions, melt flow, and metallurgical reactions) influence the properties of the as-built components with various applied processing parameters [13], which is generally responsible for the surface quality of the SLM-processed parts. Due to the complex processing aspects, it is unbelievable that the individual experiments are sufficient to thoroughly understand the influence of physical behaviors

within the neighboring tracks on the fluctuation of surface quality. Thus, it is reasonable to conclude that investigating the physical aspects behaviors under various processing parameters is favorable to control the surface quality. So far, few published models have entirely studied the heat and mass transfer and melt flow within the neighboring tracks together with the H on the evolution of surface quality during SLM. Therefore, there still lacks of a comparatively comprehensive and clear comprehending in the influence of thermodynamic behavior within the neighboring tracks on the surface quality during SLM. Inconel 718 alloy, widely known as a type of the extensively used nickel-based superalloys, is recognized as a potential candidate material used for high temperatures. Accordingly, it is commonly used in many industrial fields, e.g., aircraft turbine engines, blades or nuclear industry, due to its high ability to retain mechanical properties at elevated temperature and excellent resistance to oxidation [14]. It has been developed and usually produced in wrought, cast, and powder metallurgy techniques and also has demonstrated the reasonable microstructure and desire mechanical properties [2]. However, the segregation of high-concentration refractory elements (such as Nb and Mo elements), and the high hardness and low thermal conductivity characteristics make it difficult to use conventional processing techniques owing to the severe damage of tool wear and poor part surface integrity [15–18]. In literature, because of its low content of aluminum and titanium, Inconel 718 alloy is well known for its good weldability and ideally appropriated for the SLM process [1]. In this study, we established a novel powder-scale model to analyze the influence of the H on heat and mass transfer, melt flow and surface quality evolution within the neighboring tracks during SLM of Inconel 718 alloy, using a commercial Fluent finite volume method (FVM) software. The melt flow driven by surface tension gradient is taken in account in this numerical model, resulting in the heat and mass transfer within the molten pool. The temperature profiles, temperature gradient and top surface morphology were simulated, as well. Moreover, the experimental characterizations of top surface morphology of SLM-processed components were compared with those obtained by numerical simulations, aiming at validating the accuracy of the developed physical model.

2. Modeling approach and numerical simulation 2.1. Description of physical model SLM of metallic powder is generally known as a non-equilibrium physical and chemical metallurgical process, involving multiple modes of heat, mass, momentum transfer, etc [13]. Regarding the complicated physical and chemical metallurgical aspects between the moving laser beam and powder particles, a schematic of SLM physical model is depicted in Fig. 1a, including melting and solidification, phase transformation and interactions between laser beam and powder particles. Indeed, the powder particles irradiated by laser beam are successively subjected to melting, collapse, solidification, etc, which significantly affects the surface quality of the solidification parts. On the other hand, the fluctuation of the free surface within the molten pool is extremely influenced by the rippling effect resulted from the surface tension, which plays a crucial role in obtaining a high-quality surface of the ultimate components. Therefore, the interface between the argon protecting gas and the free surface of pool is necessary to be considered in the simulation model, taking into account the heat and mass transfer in the interface as being irradiated by the moving laser beam. Moreover, in order to make a further accordance with the real operating conditions, a novel scaled-down

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Fig. 1. Schematic of the SLM physical model (a); the established three-dimensional simulation model (b); the X–Y view (Z¼ 15 μm) and the multi-track scan strategy during the SLM process(c); the X–Z view (Y ¼ 0) and the size of the applied powder of the established model (d).

powder-scale model was established with the three dimensions of 400
300
50 μm3, as shown in Fig. 1b. The hatch spacing (H), which was used in the physical model, was defined as the distance between the central lines of two neighboring scanning paths (i.e., the central lines of laser beam), as seen in Fig. 1c. The multi-track scan strategy was considered to investigate the influence of H on the surface quality of the parts. The Inconel 718 alloy phase defined in the present numerical model was nearly consistent with the practicable sphere-like powder particles used in SLM, as shown in Fig. 1d. Expect for the metal phase, the residual region within the powder-bed was filled with the argon protecting gas. In order to make the complicated problem mathematically tractable, several simplifying assumptions are proposed as follows. Except viscosity, thermal conductivity and surface tension, the other thermal physical constants are taken to be temperature-independent. The coefficient of heat conduction between powderbed and argon protecting gas is considered as a constant. The powder of Inconel 718 alloy is closely packed in regular forms. Considering the influence of the length of scanning path on the temperature evolution, the scanning strategy was settled as follows: (i) in the first scanning path, the laser beam irradiated the powder and moved along X-axis ranging from 0 to 280 μm at a constant velocity of 400 mm/s, and then remained at X¼280 μm for the residual time without irradiating the powder according to the experiments; (ii) in the second scanning path, the laser beam irradiated the powder and moved along the negative direction of X-axis ranging from 280 to 0 μm at a constant velocity of 400 mm/ s. 2.2. Governing equations In general, the motions of melt flow mainly are followed by the three basic physical conservation laws, i.e., the conservation of mass, momentum and energy. Based on the above assumptions, the continuity equation for mass, momentum and energy conservation can be summarized in Ref. [13].

2.3. Boundary conditions Boundary condition on the free surface can be described as following [19]:

K

⎛ x 2 + y2 ⎞ ⎛ z⎞ W ∂T ⎟exp⎜ − ⎟ − hA(T − T∞) exp⎜ − = 2 ∂n ⎝ ze ⎠ ⎝ 2πzeσ 2σ 2 ⎠ 4 − σsεr (T 4 − T∞ ) − qvap

(1)

where n denotes the normal component. W is the total beam power. K is defined as the standard deviation of the Gaussian function (equals to FWHM/2.355) and the variable ze signifies the amount of laser penetration. In Eq. (1), the second and third terms were defined for the heat losses due to the ambient convection and heat radiation. hA represents the convection coefficient and the T1 is the ambient temperature setting for 293 K. ss, εr and qvap represent the Stefan-Boltzmann constant, emissivity and heat loss caused by vaporization, respectively. Calculated the heat flux from Eq. (1) rapidly melts and evaporates the powder-bed. Then the vaporization acts as a repulsive force on the molten pool surface referred to as recoil pressure, which is considered to be acted as an important role in affecting the surface quality of the SLM-processing components, such like stainless steel [20] and aluminum alloy [8]. The mathematical formula defined for the pressure boundary condition for the free surface including the surface tension in flow was estimated in Ref [19]: It is well known that the molten liquid material is generally dominated by the thermodynamics convection caused by the variable temperature and the attendant surface tension gradient. The surface tension, γ, is regarded in the present simulation and can be written as [21]:

−μ

∂u ∂γ ∂T = ∂z ∂T ∂x

(2)

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−μ

∂v ∂γ ∂T = ∂z ∂T ∂y

−μ

∂w ∂γ ∂T = ∂z ∂T ∂z

(3)

(4)

The heat transfer mechanisms of the other lateral surfaces of the powder-bed include heat conduction and convection. The bottom of the powder-bed is connected to the metal substrate. Thus, it is assumed to be conductive. 2.4. Physical properties and numerical simulation Thermal conductivity is considered as one of the most important physical properties during SLM. Rombouts et al. [22] have found that the effective thermal conductivity of the powder-bed strongly depends on the processing parameters, such as laser wavelength, powder particle size and morphology, and as well as the packing density of powder. In the present simulation, the powder particles are assumed to be sphere-like and packed in regular forms. Therefore, the effective conductivity of powder-bed, keff, can be estimated as [23]:

k eff kf

⎛ ϕk r ⎞ ⎟⎟ = 1− 1−ϕ ⎜⎜1 + kf ⎠ ⎝

(

)

⎛ ⎜ ⎜ 2 + 1−ϕ ⎜ k ⎜ 1− f ⎝ ks

⎞ ⎛ ⎞ ⎟ ⎛ ⎞ ⎜ 1 ⎟ kr ⎟ ks ⎜ ⎟ ln − + 1 ⎜ kf ⎜ ⎟ ⎟ ⎟ ⎜ 1− ⎝ kf ⎠ ⎟ kf ⎟ ⎝ ks ⎠ ⎠

(5)

where ϕ is the fractional porosity of the powder-bed. kf denotes the thermal conductivity of the ambient gas in the charmer, i.e., Argon gas. ks presents the thermal conductivity of the solid. kr indicates the thermal conductivity portion of the powder-bed owing to radiation among powder particles, and it is defined as:

k r = 4FσsT 3x r where ss is the Stefan-Boltzmann constant. xr represents the average diameter of the powder particles. T indicates the temperature of powder particles, and F is a view factor that is approximately settled as 1/3. The commercial FLUENT finite volume method package was used to simulate the thermodynamic behavior, heat and mass transfer, velocity field and resultant surface integrity. Based on our previous research, the SLM processing parameters are optimized and given in Table 1. In our present study, during the multi-track SLM processing, the center of laser beam source moves along X-axis (Y¼ 0) for the first scanning, and thereafter it still moves along X-axis but shifts toward to the Y-axis positive direction with an offset of H. In order to investigate the effect of H on heat and mass transfer and the resultant surface quality, the temperature evolution, temperature gradient and velocity plots are obtained in quantity along Y-axis direction when the laser beam scans at X ¼ 100 μm (Z¼15 μm) at the second scanning track with variable H. 2.5. Experimental procedures The gas atomized pre-alloy Inconel 718 powder (a purity of 99.7%) with an approximately sphere-like shape and an average diameter of 30 μm were used in this study. The samples were prepared by the SLM system (SLM-150 Guangzhou, China), which primarily contains a YLR-500-SM Ytterbium fiber laser (IPG Laser GmbH, Germany) with a maximum power of 500 W and a spot

Table 1 The as-used material properties and SLM processing parameters. Parameters

Value

Density, ρ The Stefan-Boltzmann constant, s Radiation emissivity, ε Ambient temperature, T0 Powder layer thickness, d Radius of laser beam, D Hatch spacing, s Laser power, P Scanning speed, v

8200 kg/m3 5.67
10  8 W/(m2 K4) 0.36 293 K 30 μm 35 μm 40, 50, 60, 70 μm 110 W 400 mm/s

size of 70 μm. An automatic metallic powder layering apparatus, an inert argon gas protecting system and a microcomputer system for process control are employed as the auxiliary apparatus, as well. The maximum of the fabricated dimension is 150
150
150 mm3.The detailed processing procedures of SLM have been described thoroughly in Ref [24]. Each sample was fabricated in a three-dimensional size of 5
5
5 mm3. The experimental processing parameters were properly settled, which has a full accordance with those of numerical simulation. The three-dimensional top surface morphologies and roughness of the SLM-processing components were characterized by the MicroXAM-3D topographic profiler. Samples were cut, ground, and polished according to standard preparation procedures. And subsequently the polished samples were etched with a solution consisting of H2O2 (3 ml) and HCl (10 ml) for 10 s. High-magnitude observations of the surface morphologies of the SLM-processed specimens were acquired by using a Quanta FEG 250 field emission scanning electron microscope (FE-SEM).

3. Results and discussion 3.1. Thermodynamic behavior of molten pool The calculated temperature profiles and temperature gradient versus locations along Y-axis direction (X ¼100 μm, Z¼15 μm) at the second scanning track under different H are depicted in Fig. 2. According to the temperature distribution curves, it was apparent that the temperature attained its maximum at the center underneath the laser beam and decreased rapidly outward due to the applied laser beam with Gaussian energy density distribution. As the H successively increased from 40 μm to 70 μm, the attendant peak temperature decreased from 2909 K to 2421 K. Although the calculated peak temperature tended to reduce with increasing the H, there was a significant distinction in the magnitude. For a given laser power and scanning speed, an increasing in H implied less energy penetration into per volume of powder-bed and a relatively lower temperature would be obtained. Acquiring a relatively higher temperature tended to result in a sufficient melting of the irradiated powder particles and a considerable amount of melting flow generated within the molten pool, revealing an efficient metallurgical bonding between the neighboring tracks. Nevertheless, it was worth noting that an excessively high temperature was also easy to give rise to the instability of melt flow. From Fig. 2, it was obvious that the temperature gradient appeared to be reduced from 6.05
107 K/m at 40 μm successively to the minimum value of 4.41
107 K/m at 70 μm. This was primarily attributed to that for a given laser power and scanning speed, increasing H would weaken the effect of thermal accumulation and decreased the obtained energy of per unit area. Thus, the attendant temperature gradient of molten flow correspondingly reduced. Interestingly, it is clearly observed that all the locations of peak

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Fig. 2. Temperature distribution profiles and temperature gradient versus locations along Y-axis direction where X ¼ 100 μm and Z ¼15 μm at the second scan track under different hatch spacing: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

temperature were not consistent with those of the temperature gradient, but shifted slightly towards the negative direction of Yaxis with a lagging. It was associated with the transformation of physical properties from powder particles to solid on both sides of the molten pool [25]. That is to say, perpendicular to the laser moving direction, one side of the molten pool is the prefabricated tracks, while the other is the un-melted powder particles. Commonly, the powder particles with a considerable high absorptivity

and a relative low thermal conductivity tended to absorb more irradiated energy and dissipated slowly in comparison to the previous fabricated tracks [26]. Moreover, it was noted that the region above the liquid line of temperature curves signified the molten area and accordingly the width of the molten pool could be acquired. It was evident that the width of molten pool was obviously reduced from 64 μm to 48 μm as the H increased. This was ascribed to the fact that decreasing

Fig. 3. Simulated geometry profiles and velocity vectors plots of the molten pool in the Y–Z cross-section view (X ¼ 100 μm) of the second track at various hatch spacing: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

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the H tended to result in a more significant thermal accumulation, leading to a higher operating temperature and sufficient liquid spreading to wet the surrounding powder particles. Meanwhile, it accordingly enlarged the width of molten pool. On the other hand, a higher working temperature was also favorable to enlarge the width of molten pool and spread the melt to the prefabricated tracks, which was benefit for the formation of a sufficient metallurgical bonding between the neighboring tracks and alleviated the generation of balling and porosity [27]. However, a considerably larger dimension of molten pool accompanying with an exceeding amount of liquid formation at a reduced value of H (such as 40 μm or 50 μm) was unfavorable to control the vigorous melt flow convection and it was not expected as well [13]. Therefore, care should be taken to choose an appropriate value of H, which can efficiently obtain a reasonable melt flow and dimension of molten pool. The geometry profiles and velocity vectors plots of molten pool in the X–Z cross-section view of second track at different H are shown in Fig. 3. At a comparatively lower H of 40 μm, the velocity vectors pointed towards the prefabricated track with a certain angle to Y-axis, implying a considerable amount of molten liquid material flowed towards the previous tracks and a significant fluctuation of surface roughness obviously presented with a maximum height of 39.7 μm (Fig. 3a). As the H increased to 50 μm, a certain number of molten liquid material migrated towards the previous fabricated track and the resultant fluctuation of surface roughness was alleviated with a maximum height of 33.5 μm (Fig. 3b). At an evaluated H of 60 μm, the velocity vectors were nearly paralleled to Y-axis, indicating the formation of approximate laminar flow convection. It was beneficial to alleviate the heat and mass transfer within the molten pool and, accordingly, the geometry profiles tended to present relatively flatten free of the apparent humping with a height of 22.7 μm (Fig. 3c). At an even higher H of 70 μm, only a small amount of molten liquid material flowed in the local regions. Meanwhile, some insufficient melting powders existed between the neighboring tracks. Correspondingly, the evident fluctuation of surface roughness emerged with a maximum height of 30.2 μm (Fig. 3d). Thus, it was reasonable to infer that the change of H had a significant influence on the heat and mass transfer between the neighboring tracks. In this situation, when the powder was irradiated by the laser beam, the localized powder melting occurs and, in the present study, the laser beam with Gaussian energy density distribution was used to irradiate the powder-bed. This laser beam was known as a characterization that a higher energy density was obtained in the center than that located in the edge. When the laser beam with Gaussian energy density distribution scans over the powderbed, a higher temperature was obtained in the central region of the molten pool than that located in the edge of molten pool. That is to say, the temperature gradients will be formed within the molten pool, and the attendant surface tension gradients will be generated, as well [28]. Normally, under the combined influence of the higher surface tension near the edge of the molten pool and the thermal capillary force induced by the temperature gradient, the molten liquid material tends to flow away from the center of pool, where the surface tension is considerably lower due to the higher working temperature. The applied lower H (such as H ¼40 μm and 50 μm), which indicates considerably higher operating temperature and temperature gradients (Fig. 2), and the resultant surface tension gradient is correspondingly generated. In this case, under the action of the higher surface tension gradient, the molten liquid material tends to be dragged from the center to the edge within the molten pool. That is to say, it is favorable for the molten liquid material to migrate towards the edge of pool (i.e. the previously fabricated tracks), as shown in Fig. 3(a) and (b). When a reasonable H of 60 μm is used, the temperature gradient is

Fig. 4. Velocity magnitude profiles of the melt flow on the top surface of the molten region along Y-axis direction under various hatch spacing: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

low and the resultant surface tension gradient is relevantly decreased. Under this simulation, the dragging effect is not observed compared to that of the lower H. However, when the H increases to 70 μm, the discontinuous molten tracks are generated and prevent the molten liquid material from spreading and wetting the previous fabricated track. It can be concluded that the migration of the molten liquid material, i.e., the heat and mass transfer within the molten pool, significantly influences the surface roughness and geometry profiles. The velocity evolutions of melt flow on the top surface of molten region along Y-axis direction under various H are provided in Fig. 4. The peak velocity of the molten liquid reduced sharply from 0.54 m/s to 0.13 m/s as the H varied from 40 μm to 70 μm. It was apparent that the peak velocity of molten liquid attained the maximum at the center of the pool, where the high-energy laser beam located. During SLM, the dynamic viscosity, μ, of a molten pool composing of liquid is primarily depended on temperature (T) and can be defined by [28]:

μ=c

m γ kT

where m is the atomic mass and k is the Boltzmann constant. c is the constant and γ is the surface tension of liquid. For Ni-Cr liquid system, the surface tension (γ) is dependent with the working temperature (T), γ can be further assessed by [29]:

γ = 1816 − 0.417 × (T − 1717) As a considerably low H is applied for SLM, the thermal accumulation of the pool is enhanced, thereby significantly enhancing the operating temperature. Accordingly, the attendant dynamic viscosity and surface tension of the molten liquid within the pool decrease. The combined effects of the low dynamic viscosity and surface tension tend to resulting in the migration of the molten liquid towards the previous fabricated tracks. The higher operating temperature is applied, the more molten liquid is migrated. Since the dynamic viscosity and surface tension are extremely relied on the applied H, the response for the resultant variation of the velocity magnitude correspondingly is noticeable, which further reveals that the migration of the heat and mass transfer of the molten liquid material indeed becomes more vigorous with decreasing the H.

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Fig. 5. Simulated surface morphologies on top surface at different laser processing parameters: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

3.2. Surface morphologies Typical simulated surface morphologies obtained with variable H are shown in Fig. 5. It was evident that the surface morphology of SLM part was particularly sensitive to the applied H. The similar observation was observed in the SLM-processed stainless steel

components [5]. For instance, at a relatively lower H of 40 μm, the molten liquid migrated towards the prefabricated tracks under the action of considerably larger surface tension, and the resultant surface quality combined with a severe fluctuation and material stacking was obtained. It eventually resulted in a seriously rough morphology, showing a humping with a larger height (Fig. 5a). As

Fig. 6. Schematic for describing the melt flow movement within the molten pool and solidification profiles on increasing hatch spacing: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

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for the H of 50 μm, the reduced temperature gradient favored slowing down the mass migration of molten liquid and alleviated the effect of the fluctuation and material stacking. Thus, the surface exhibited a rough morphology with a moderate height (Fig. 5b). As the H increased to 60 μm, under a reasonable temperature gradient, the laminar molten liquid convection was contributed for forming a flatten surface presented no apparent humping (Fig. 5c). The sinking region was observed from the top surface without an apparent characterization due to the existing voidage in powders, which also slightly influence the surface roughness. As the H further increased to 70 μm, the surface presented with discontinuous tracks between the neighboring tracks where a certain amount of insufficient melting of the powder particles existed. Moreover, due to the larger H applied and the attendant interlayer defects, a considerably poor inter-track metallurgy bonding between the neighboring tracks was commonly formed, contributing to the generation of the poor surface quality and/or other properties (Fig. 5d). The schematic for illustrating the movement of molten liquid and solidification profiles under variable H are revealed in Fig. 6. At a comparatively lower H of 40 μm, a higher temperature and resultant lower viscosity were obtained within the molten pool. Accordingly, a powerful surface tension reduced by a larger temperature gradient near the edge of molten pool was formed, which

was the driving force for the migration of molten liquid. In this case, the molten liquid tended to migrate towards the edge of the pool. On the other hand, under the same processing parameters (e.g., laser power and scanning speed, etc.), lowering the H revealed that a larger portion of the previous as-fabricated tracks were re-melted. As a result, a considerable amount of molten liquid speedily migrated towards the edge of the pool and subsequently solidified with a substantially high height (Fig. 6a). On increasing H to 50 μm, the operating temperature were somewhat reduced and the resultant viscosity and surface tension increased to a certain value. The migrated speed of molten liquid flowing away the center of the pool was slowed down slightly. Thus, the attendant solidification was presented with a comparatively lowaltitude profile (Fig. 6b). As H increased to 60 μm, due to the relatively lower working temperature, both the viscosity and surface tension were further enhanced and, accordingly, the migration of molten liquid was evidently alleviated. A certain amount of molten liquid only filled the regions, where it was located within the neighboring tracks. Thus, the surface presented more even free of larger fluctuation (Fig. 6c). On further increasing H to 70 μm, the discontinuous tracks were unfavorable for molten liquid to spread and migrate. Moreover, the limiting molten liquid could not spread and wet the neighboring tracks fully. In this instance, the surface exhibited with a larger surface roughness and some defects (e.g.,

Fig. 7. FE-SEM images displaying the typical morphologies of the top surface of SLM-processed Inconel 718 parts under various laser processing conditions: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

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Fig. 8. Three-dimensional morphologies on top surface at different laser processing parameters: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

insufficient melting powder and porosity, etc.). Thus, it has considerable possibilities to obtain a high surface quality by applying an appropriate H. 3.3. Experimental investigations Fig. 7 illustrates the typical surface morphologies of the top surface of SLM-processed Inconel 718 components under various laser processing conditions. It was apparent that the formation of imperfections in SLM-processed components, e.g. micro-humping and interior porosity, was evidently influenced by the applied H. At a relatively lower H of 40 μm, a considerable amount of long lathshaped micro-humping, which was paralleled to the laser scanning direction, was presented on the top surface of SLM-processed components (Fig. 7a). As shown in Fig. 7b, the top surface exhibited a certain amount of lath-shaped micro-humping and interior porosity as the H increased to 50 μm. In contrast, the dense, flatten, and coherent metallurgical bonded surface without any apparent micro-humping and porosity were generated as the H further increased to 60 μm (Fig. 7c). However, at an even higher H of 70 μm, it was worth noting that some insufficient melting powders were presented between the neighboring tracks, leading to a poor metallurgical bonding of the SLM-processed components. Meanwhile, the discontinuous tracks presented intermittently. Generally, the localized powder particles start to melt during the laser beam with a Gaussian energy distribution scanning, yielding a large temperature gradient and resultant surface tension. At a relatively lower H of 40 μm, a considerable thermal accumulating also results in a higher operating temperature and temperature gradient (6.05
107 K/m) within the molten pool. Accordingly, the resultant surface tension gradient within the pool is beneficial for molten liquid material to migrate towards the edge

Fig. 9. Average roughness of the top surface of ultimate SLM-processed parts at different hatch spacing: (a) H ¼ 40 μm; (b) H ¼ 50 μm; (c) H ¼ 60 μm; (d) H ¼ 70 μm.

of the pool, leading to an obvious stacking of molten liquid and resultant micro-humping after solidification (Fig. 7a). When the H increases to 50 μm, a reduced temperature gradient of 5.8
107 K/ m decreases the surface tension to a certain value. Correspondingly, the migration of molten pool is alleviated and a modest number of micro-humping is presented on the top surface of the SLM-processed components (Fig. 7b). When the applied H is 60 μm, a reasonable temperature gradient and surface tension are obtained, giving rise to an excellent surface quality free of apparent defects (Fig. 7c). As the H further increases to 70 μm, the low thermal accumulation produces a lower peak temperature and temperature gradient, thereby generating a limiting amount of

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molten liquid and some residual insufficient melting powder (Fig. 7d). Fig. 8 depicts the three-dimensional morphologies on top surface of SLM-processed Inconel 718 components at different laser processing parameters. Detailed surface roughness of the corresponding surface is shown in Fig. 9. It was apparent that the applied H for various Inconel 718 components produced a pronounced influence upon the obtained surface morphologies and attendant surface roughness. At a relatively lower H of 40 μm, the top surface primarily consisted of a larger number of parallel and high micro-humps representing a rough surface (Fig. 8a), resulting in a considerably evaluated average surface roughness of 12.71 μm (Fig. 9). This was ascribed to that a considerable amount of molten liquid material moved towards the previously fabricated track and the resultant thickness of molten liquid material within the current track was decreased. On the other hand, using a relatively low H revealed that a higher operating temperature also was obtained and resulted in a larger transition between the powder and the solid. In this case, the molten region was solidified and the attendant shrinkage was generated correspondingly. Thus, a poor surface quality and a larger surface roughness were observed evidently. On increasing H to 50 μm, the top surface showed a certain amount of medium-high micro-humps (Fig. 8b), thereby reducing the average surface roughness to a comparatively lower value of 7.45 μm (Fig. 9). Since the H was increased to 50 μm, to some extent, the migration of molten liquid material was decreased, and the resultant thickness of molten liquid material within the current track was decreased slightly. Interestingly, applying a reasonable H of 60 μm, the top surface of SLM-processed component exhibited rather smooth, free of any apparent microhumps (Fig. 8c). The lowest average surface roughness of 2.23 μm was accordingly obtained (Fig. 9). As a result of the rather lower thermal accumulation and temperature gradient, only little amount of the molten liquid material migrated to the previously fabricated track and correspondingly the thickness of current molten liquid material was presented without obvious fluctuations. For SLM-processed Inconel 718 components fabricated at an even higher H of 70 μm, although the top surface presented without any obvious large-height micro-humps in comparison to those of the SLM-processed component produced at 40 μm, a considerable amount of discontinuous tracks and insufficient melting powder were still observed on the top surface (Fig. 8d). The measured average surface roughness of 7.01 μm showed a considerably rough surface in this case (Fig. 9). It is well known that the surface roughness can influence the distribution of the metallic powder within the powder-bed. A larger amount of powder is filled in the low-lying region of the previously fabricated layer, which will increase the real thickness of the powder-bed. On the contrary, the real thickness of the powder-bed will decrease in the humping region of the previously fabricated layer. Moreover, it can also influence the surface emissivity and consequently radiative heat transfer [30]. During SLM, under the irradiated of a constant laser, the powder-bed with a larger thickness tends to generate some insufficient melting powder and residual porosity in the newly fabricated layer, in comparison to the powder-bed with a relatively small thickness. Thus, it is reasonable to conclude that a fabricated layer with a better surface quality is favorable to the processing of the subsequent layers.

4. Conclusions In conclusion, the numerical simulation concerning the influence of H on the pool thermodynamics, heat and mass transfer, and surface quality of SLM-processed Inconel 718 powder system

is performed. Accordingly, the main conclusions are drawn as follows: (1) Both the peak temperature and temperature gradient of the molten liquid reduced as a higher H was applied due to the decrease of thermal accumulation. (2) The heat and mass transfer within the pool of SLM Inconel 718 powder system occurred due to the formation of surface tension. Normally, a higher surface tension near the edge of the pool tended to drag the molten liquid away from the center of the melt pool. As a relatively lower H was used, considerable molten liquid migrated to the previous as-fabricated tracks with a higher velocity, resulting in the stacking of molten liquid. (3) H played a crucial role in determining the surface quality of SLM-processed Inconel 718 components. As an appropriate H of 60 μm was settled, a reasonable surface tension tended to spread the molten liquid with a steady velocity, leading to the formation of a flat surface of the component. (4) Both the surface morphologies and average surface roughness were experimentally obtained, which were in a good accordance with the results calculated by simulation. A considerably flat surface morphology and the resultant lower average surface roughness of 2.23 μm were obtained.

Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 51575267 and 51322509), the National Key Research and Development Program “Additive Manufacturing and Laser Manufacturing” (No. 2016YFB1100101), the Top-Notch Young Talents Program of China, the NSFC-DFG Sino-German Research Project (No. GZ 1217), the Outstanding Youth Foundation of Jiangsu Province of China (No. BK20130035), the Program for New Century Excellent Talents in University (No. NCET-13-0854), the Science and Technology Support Program (The Industrial Part), Jiangsu Provincial Department of Science and Technology of China (No. BE2014009-2), the 333 Project (No. BRA2015368), the Aeronautical Science Foundation of China (No. 2015ZE52051), the Shanghai Aerospace Science and Technology Innovation Fund (No. SAST2015053), the Fundamental Research Funds for the Central Universities (Nos. NE2013103, NP2015206 and NZ2016108), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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