materials

Article Thermo-Fluid-Dynamic Modeling of the Melt Pool during Selective Melting for AZ91D Magnesium Alloy

Hongyao Shen 1,2,* , Jinwen Yan 1,2 and Xiaomiao Niu 1,2

1 The State Key Laboratory of Fluid Power and Mechatronic Systems, College of Mechanical Engineering, Zhejiang University, Hangzhou 310027, China; [email protected] (J.Y.); [email protected] (X.N.) 2 Key Laboratory of 3D Printing Process and Equipment of Zhejiang Province, College of Mechanical Engineering, Zhejiang University, Hangzhou 310027, China * Correspondence: [email protected]

 Received: 29 July 2020; Accepted: 15 September 2020; Published: 18 September 2020 

Abstract: A three dimensional finite element model (FEM) was established to simulate the temperature distribution, flow activity, and deformation of the melt pool of selective laser melting (SLM) AZ91D magnesium alloy powder. The latent heat in phase transition, Marangoni effect, and the movement of laser beam power with a Gaussian energy distribution were taken into account. The influence of the applied linear laser power on temperature distribution, flow field, and the melt-pool dimensions and shape, as well as resultant densification activity, was investigated and is discussed in this paper. Large temperature gradients and high cooling rates were observed during the process. A violent flow occurred in the melt pool, and the divergent flow makes the melt pool wider and longer but shallower. With the increase of laser power, the melt pool’s size increases, but the shape becomes longer and narrower. The width of the melt pool in single-scan experiment is acquired, which is in good agreement with the results predicted by the simulation (with error of 1.49%). This FE model provides an intuitive understanding of the complex physical phenomena that occur during SLM process of AZ91D magnesium alloy. It can help to select the optimal parameters to improve the quality of final parts and reduce the cost of experimental research.

Keywords: numerical simulation; selective laser melting; AZ91D; melt pool; Marangoni flow

1. Introduction There is a growing interest in the use of magnesium alloys in automotive and aerospace industries, a reason behind which is their very low density compared to aluminum alloys (2/3 of density at room temperature) and steel (2/9 of density at room temperature) [1]. The low density gives them high specific strength and modulus, coupled with their excellent thermal and damping characteristics, making them a potential candidate for low-temperature structural applications in the aerospace and automotive industries [2–4]. However, they also have defects such as low creep resistance and easy corrosion, which limit their application in industrial and medical fields. Therefore, alloy elements such as Nd and Zn are added to make them suitable for these applications [5,6]. Magnesium AZ91A, B, C, D, and E have the same nominal compositions but differ in ranges and/or specified impurity limits. This high-purity alloy has excellent corrosion resistance. It is the most commonly used magnesium die-casting alloy. The compositions of impurities in the AZ91D magnesium alloy used in this work are listed in Table1.

Materials 2020, 13, 4157; doi:10.3390/ma13184157 www.mdpi.com/journal/materials Materials 2020, 13, 4157 2 of 19 Materials 2020, 13, x FOR PEER REVIEW 2 of 19

TableTable 1. Compositions of impurities in AZ91D magnesium alloy.

ChemicalChemical Composition Composition Al Al Zn Zn Mn Fe, Fe,Cu, Cu, etc. etc. Mg Mg ContentContent (wt%) (wt%) 9.08 9.08 0.650.65 0.23 ≤0.00510.0051 others others ≤ Casting and forming manufacturing are traditional methods for manufacturing magnesium Casting and forming manufacturing are traditional methods for manufacturing magnesium alloys. alloys. However, these two methods both suffer from some common disadvantages. Forming However, these two methods both suffer from some common disadvantages. Forming provides better provides better mechanical properties than casting, but it suffers from oxidation and induced mechanical properties than casting, but it suffers from oxidation and induced anisotropy. Casting anisotropy. Casting is prone to defects such as porosity, which reduces the mechanical properties of is prone to defects such as porosity, which reduces the mechanical properties of the manufactured the manufactured part [7]. This drives the research of new manufacture methods for magnesium part [7]. This drives the research of new manufacture methods for magnesium alloys, and Addictive alloys, and Addictive Manufacture (AM) is a potential choice. Manufacture (AM) is a potential choice. In the international standard ISO/ASTM 52,900 [8], Additive Manufacturing (AM) is defined as In the international standard ISO/ASTM 52,900 [8], Additive Manufacturing (AM) is defined a “process of joining materials to make parts from 3D model data, usually layer upon layer”. It allows as a “process of joining materials to make parts from 3D model data, usually layer upon layer”. It building companies to produce geometrically complex structures, to vary materials within a componentallows building according companies to its functions, to produce and geometrically to automate complexthe construction structures, process to vary starting materials from withina digital a modelcomponent [9]. Selective according laser to its melting functions, (SLM) and tois automatea specific theAM construction technique which process utilizes starting a fromhigh-power- a digital densitymodel [ 9laser]. Selective to fully laser melt melting and fuse (SLM) metallic is a specific powders AM layer technique by layer which to produce utilizes ahigh-freedom-shape high-power-density partslaser towith fully near-full melt and density fuse metallic [10–12] powders. With additive layer by support layer to producestructur high-freedom-shapees and unirradiated partspowders, with SLMnear-full technology density [has10– 12the]. ability With additive to manufacture support structuresmetal parts and of unirradiated complex shape powders, that would SLM technologyotherwise nothas be the produced ability to manufactureby using conv metalentional parts manufacturing of complex shape methods that would[13–16] otherwise. Figure 1 notshows be produced a schematic by ofusing SLM conventional process. manufacturing methods [13–16]. Figure1 shows a schematic of SLM process.

Figure 1. SchematicSchematic of selective laser melting ( (SLM)SLM) process.

Lots of experimental researchesresearches on on magnesium magnesium alloy alloy SLM SLM forming forming have have been been carried carried out, out, most most of whichof which are concentratedare concentrated on the on parts, the parts, e.g., the e.g., compactness the compactness of the parts, of the and parts, the mechanical and the mechanical properties, properties,such as the yieldsuch as strength the yiel ofd the strength material of [17the– 20material]. Mahesh [17–20] et al.. studiedMahesh theet al. eff ectstudied of preheating the effect and of preheatinglayer thickness and onlayer magnesium thickness on SLM. magnesium When the SLM. layer When thickness the layer is 0.15–0.20 thickness mm is and0.15–0.20 0.25–0.30 mm mm,and 0.25–0.30the elastic mm, modulus the elastic is 31.88–34.28 modulus GPais 31.88–34 and 28.43–31.47.28 GPa and GPa, 28.43–31.47 respectively GPa, [13 respectively]. Kaiwen et [13]. al. indicateKaiwen etthat al. laserindicate energy that input laser playsenergy a input significant plays rolea signif in determiningicant role in formationdetermining qualities formation of the qualities SLM-treated of the SLM-treatedsamples. High-density samples. High-density samples without samples obvious withou macro-defectst obvious macro-defects can be obtained can be obtained between between 83 and 16783 and J/mm 1673 [ 21J/mm]. However,3 [21]. However, SLM involves SLM involves complex complex thermophysical thermophysical phenomena, phenomena, such as laser such scattering as laser scatteringand absorption, and absorption, rapid heat rapid conduction, heat conduction, metal melting metal and melting solidification and solidification in a short time, in a andshort intense time, andevaporation. intense evaporation. Moreover, all Moreover, of these physical all of these phenomena physical occur phenomena in the melt occur pool in with the amelt size ofpool hundreds with a sizeof microns, of hundreds making of itmicrons, very diffi makingcult to collectit very characteristic difficult to collect data ofcharacteristic the SLM process, data of such the as SLM temperature process, suchrevolution as temperature profile, velocity revolution of the profile, melt-pool velocity flow, of and the melt-pool melt-pool deformation flow, and melt-pool history [22deformation–24]. As a historytypical quantitative[22–24].As analysisa typical method, quantitative numerical analysis simulation method, has numerical gradually becomesimulation a powerful has gradually tool for becomestudying a thepowerful SLM process tool for [ 25studying]. the SLM process [25]. Due to the dominant position of laser heating in SLM processing, it is very important to consider the influence of thermo-fluid dynamics as fully as possible when modeling. Qiang Chen has divided the dominating thermal factors of SLM process into three categories, the laser as a heat source,

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Due to the dominant position of laser heating in SLM processing, it is very important to consider the influence of thermo-fluid dynamics as fully as possible when modeling. Qiang Chen has divided the dominating thermal factors of SLM process into three categories, the laser as a heat source, evaporation that affects heat dissipation, and the violent flow in the melt pool caused by the Marangoni effect [26]. The interaction between laser and powder plays an important role for the reason that the laser is the driving force of the SLM process. Gusarov [27] described a model for assimilating powder into a uniform absorbing and scattering medium and found that the absorptance of a semi-infinite powder bed of opaque particles is a universal function of the absorptivity of the solid phase being independent of the specific surface and the porosity. Vaporization reduces the maximum temperature by taking away large amounts of energy, but it only occurs when the maximum temperature exceeds the boiling temperature [28]. The Marangoni effect is the tangential stress gradient caused by the temperature gradient on the surface of the molten pool. Dongyun Zhang [29] found that the Marangoni includes convective and conductive heat flux, and both of them have effects on molten pool shape, but the effect of convective heat flux is dominant. Trong-NhanLe [30] investigated the effects of Marangoni convection on the melt-pool formation during the selective laser melting of SS316 powder. The factors that promote the formation of the molten pool are summarized into three modes: the conduction mode, in which the melt-pool formation is dominated by thermal conduction; the keyhole mode, in which the melt-pool formation is determined mainly by the recoil pressure; and an additional transition mode that exists between these two modes, in which the melt-pool formation is driven mainly by the Marangoni convection effect. The author found that the Marangoni effect played a dominating role among the three models. In summary, the Marangoni effect is considered to have a significant influence on the flow generation in the melt pool [31]. At present, the numerical simulation research on the SLM process can be divided into two aspects. The first one is based on particle scale, focusing on investigating the complex reflection and absorption of laser between metal particles by building a numerical model [22–24,32,33]. The other is based on the scale of the part, treating the powder as a whole made of uniform material, and analyzing the temperature field and stress field during SLM process by numerical simulating [34,35]. However, due to the high thermal conductivity and low melting point of magnesium alloys, the current numerical simulation research on magnesium alloys processed by SLM is not very common. Mishra established a finite element model of SLM processing for AZ91D, studied the temperature change and the size of the melt pool during the processing, but did not conduct an in-depth study of the flow field in the melt pool [1]. The study of laser power on temperature distribution, melt-pool flow, and melt-pool morphology is not systematic enough. Therefore, it is of great importance and necessity to find a feasible method to reveal the relationship between the densification behavior and process parameters. In this work, a numerical simulation model is presented, to investigate the thermophysical phenomena and their influence on the forming process of AZ91D alloy powder SLM, and the model is verified by experiments. Surface stress caused by the Marangoni effect and gravity in flow generation, the phase change latent heat, and Gaussian distributed laser beam were considered in building the model. The influence of temperature gradient and high cooling rate on the formability of final part is investigated and discussed. The convection flow in the melt pool and the resultant influence on melt pool dimensions are presented. The relation between melt-pool dimensions and shape with laser power is revealed.

2. Modeling The Fluid Heat Transfer module and Laminar Flow module in COMSOL were involved in building the model. To simplify the model, the following assumptions are made in this study: (1) The powder material is considered to be a homogeneous whole, and the porosity of the powder material is indicated by mathematical methods. (2) The flow in the melt pool is considered to be incompressible laminar flow. (3) The evaporation of AZ91D magnesium alloy is ignored. Materials 2020, 13, 4157 4 of 19 Materials 2020, 13, x FOR PEER REVIEW 4 of 19 2.1. Physical Model 2.1. Physical Model The X–Z plane is set as the symmetry plane in the model as shown in Figure2. Thus, simulation can The X–Z plane is set as the symmetry plane in the model as shown in Figure 2. Thus, simulation be performed on half of the model, in a symmetrical way, to reduce the calculation time. The calculation can be performed on half of the model, in a symmetrical way, to reduce the calculation time. The domain is divided into two layers. The upper layer represents the AZ91D powder material with a calculation domain is divided into two layers. The upper layer represents the AZ91D powder thickness of 0.04 mm, and the lower layer represents the formed part with a thickness of 0.12 mm. material with a thickness of 0.04 mm, and the lower layer represents the formed part with a thickness The size of the entire calculation domain is 0.2 0.6 0.16 mm. The laser heat source is defined as the of 0.12 mm. The size of the entire calculation ×domain× is 0.2 × 0.6 × 0.16 mm. The laser heat source commonly used Gaussian distribution surface heat source, perpendicularly incident from the upper is defined as the commonly used Gaussian distribution surface heat source, perpendicularly incident surface of the model, and moving at a constant speed 1000 mm/s along the X axis direction. At the from the upper surface of the model, and moving at a constant speed 1000 mm/s− along the −X axis same time, convective heat dissipation with nitrogen and heat radiation are also introduced on the direction. At the same time, convective heat dissipation with nitrogen and heat radiation are also upper surface of the model. The initial temperature of the domain is set as the room temperature. introduced on the upper surface of the model. The initial temperature of the domain is set as the room The parameters of the SLM process, such as laser power and scanning speed, are shown in Table2, temperature. The parameters of the SLM process, such as laser power and scanning speed, are shown and the material properties of AZ91D are shown in Table3. in Table 2, and the material properties of AZ91D are shown in Table 3.

Figure 2. Single scan model geometry. Table 2. Process parameters for simulation. Table 2. Process parameters for simulation. Computational domain dimensions (mm) 0.6 0.2 0.16 Computational domain dimensions (mm) 0.6× ×× 0.2 × 0.16 Layer thickness (mm) 0.04 Layer thickness (mm) 0.04 Laser spot size (mm) 0.08 Laser spot size (mm) 0.08 Laser power (W) 100, 125, 150, 175, 200 Laser power (W) 100, 125, 150, 175, 200 Scan velocity (mm/s) 1000 Scan velocity (mm/s) 1000 Porosity (ϕ) 0.475 Porosity (φ) 0.475 Table 3. Properties of AZ91D alloy (Friedrich and Mordike, 2006) [36]. Table 3. Properties of AZ91D alloy (Friedrich and Mordike, 2006) [36].

Solidus temperature (TS) 743 K Solidus temperature () 743 K Liquidus temperature (TL) 868 K Liquidus temperature () 868 K Specific heat capacity CP,S 1014 J/kg-K (solid, 293 K) Specific heat capacity , 1014 J/kg-K (solid, 293 K) CP,L 1230 J/kg-K (liquid) , 1230 J/kg-K (liquid) 3 Thermal conductivity kS (Solid) 72 10 W/K 3 Thermal conductivity (Solid) × 72 × 10 W/K 3 kL(Liquid) 82.9 10 W/K 3 (Liquid) × 82.9 × 10 W/K LatentLatent heat heat of fusionfusionL 373 kJ373/kg kJ/kg Dynamic µ 3 10 3 Pa s Dynamic viscosity μ × − 3 × 10· −3 Pa·s Thermal expansion coefficient 2.6 10 5 K 1 Thermal expansion coefficient × 2.6− × −10−5 K−1 5 1 Volumetric thermal expansion coefficient (βT) 9.541 10− K− −5 −1 Volumetric thermal expansion coefficient () × 9.541 × 10 K 4 Temperature coefficient for (∂γ/∂T ) 2.13 10 N/m-K−4 Temperature coefficient for surface tension (/) − −×2.13− × 10 N/m-K EmissivityEmissivity (ε()) 0.18 0.18

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2.2. Powder Bed Properties The powder material is regarded as a whole in the numerical model, without considering the surface morphology and internal voids. Therefore, void parameters are introduced to characterize the effect of voids on thermal conductivity and density.

ρ = ρ (1 ϕ) (1) powder s −

k = ks(1 ϕ) (2) powder − 3 where ρpowder is density of powder valued at 0.95 g/cm (see Section3 for its testing). Thus, the value of ϕ can be calculated by Equation (1). The value of ϕ is 0.475. kpowder is the dynamic viscosity of powder, and ϕ is the porosity of the powder.

2.3. Governing Transporting Equations (1) Momentum conservation equation:

∂→u     ρ + ρ →µ →u = p→I + →k + →F + ρ→g (3) ∂t · ∇ ∇ · −   ρ →u = 0 (4) ∇ ·

  T →k = µ →u + →u (5) ∇ ∇ In Equation (3), ρ, →u, µ, and p represent density, velocity field, dynamic viscosity, and dynamic pressure, respectively. →I is the three-dimensional unity tensor, and ρ→g is the gravity force. →F is a source term representing the sum of other body force, such as the buoyancy forces and the mushy-region flow resistance. The Marangoni effect can be modeled as a shear stress on the upper surface of the melt pool, as in the following equation:

  T  →   2  → p I + µ →u + →u u →u I →n = γ tT (6) − ∇ ∇ − 3 ∇· ∇ where γ is surface tension.

(2) Energy conservation equation:

∂T ρCp + ρCp→u T + →q = Q + Qp + Q (7) ∂t · ∇ ∇ · vd

→q = k T (8) − ∇ Heat transfer in the process is described by Equations (5) and (6), in which the latent heat of phase change is also considered as follows: ρ = θSρS + θLρL (9)

S ∂αm CP = (θSρSCP,S + θLρLCP,L) + LS L (10) ρ → ∂t

S θLρL θSρS αm = − (11) L θSρS + θLρL

k = θSkS + θLkL (12) Materials 2020, 13, 4157 6 of 19

θS + θL = S (13) where θS and θL are the mass ratio of solid phase and liquid phase, respectively.

(3) Heat source modeling

A Gaussian power distribution model is used as the laser heat source, which moves at a constant speed, v, along the X-axis. −   2   2 (x vt) + y2  2εP  −  Qlaser = exp  (14) πR2 − R2 

Qlaser is the input heat flux of the laser, ε is the emissivity of the powder, σ is Stefan–Boltzmann constant, P is the laser power, R is the Gaussian laser spot radius, v is the scanning speed, and t is the scanning time.

(4) Boundary conditions

The side and bottom surfaces are set as thermal insulation, and the Y–Z plane is set as a symmetrical surface to reduce the amount of calculation:

→n →q = 0 (15) − · The boundary conditions on the top surface include laser beam deposition, forced convection heat dissipation, and heat radiation. The control equation is as follows:   →n →q = Q + h(Text T) + εσ Text4 T4 (16) − · − laser − −

 0.3387P1/3R1/2  k r el 5  2 1/4 i f Rel 5 10  l   2/3    0.0468   ≤ ·   +   h =  1    (17)   Pr   1  4   k  2 P 3 0.37R 5 871 i f R > 5 105 l r el − el · where Text is the set ambient temperature valued at 293.15 K.

3. Results and Discussion

3.1. Temperature Distribution Figure3 shows the change of temperature field over time, when the scanning speed is v = 1000 mm/s and laser power is P = 100 W. The color map represents the temperature field, and the black line is the melt point isotherm of the AZ91D alloy which shows the boundary of the melt pool representatively. The temperature starts to rise as soon as the laser irradiates the upper surface of the powder, and the melt pool is generated immediately once the temperature exceeds the solidus temperature of the AZ91D alloy. With continuous laser irradiation, the volume of the melt pool increases rapidly and reaches a steady state after a short time. It has been found that the dimension of the melt pool stays almost constant after 300 µs (results are presented in the following section), which also shows that the SLM process has reached a stable state. Therefore, it seems that this model of 0.6 0.2 0.16 mm × × calculation domain can provide adequate information about the temperature distribution, flow in the melt pool, and melt-pool size. Materials 2020, 13, 4157 7 of 19 Materials 2020, 13, x FOR PEER REVIEW 7 of 19

FigureFigure 3. 3.Temperature Temperature evolution evolution with with time time at at P =P 100= 100 W. W.

FigureFigure4 shows 4 shows the maximumthe maximum temperature temperature in the calculationin the calculation domain withdomain time with when time laser when power laser is P power= 100 W, is whichP = 100 gives W, which a sight gives of temperature a sight of temper changeature history change in the history domain. in The the maximum domain. The temperature maximum rises rapidly to 1600 K in 10 µs, which contributes to a spectacularly high growth rate (1.6 108 K/s temperature rises rapidly to 1600 K in 10 μs, which contributes to a spectacularly high growth× rate on(1.6 average). × 10 / However, on average). the rise rateHowever, slows down, the rise and rate the slows maximum down, temperature and the maximum gradually stabilizestemperature at aroundgradually 1651 stabilizes K after 300 at µarounds. In order 1651 to K reduceafter 300 the μ amounts. In order of calculation,to reduce the the amount computational of calculation, domain the shouldcomputational be as small domain as possible. should By be calculating as small withas possible. other conditions By calculating unchanged, with the other model conditions with a scanunchanged, path length the of model 1 mm with found a thatscan thepath highest length temperature of 1 mm found of the that computational the highest domaintemperature gradually of the stabilizedcomputational at 1651 domain K after 300 graduallyµs. Therefore, stabilized the currentlyat 1651 K selectedafter 300 computational μs. Therefore, domain the currently size meets selected the simulationcomputational requirements. domain Sincesize meets the fluctuation the simulation range ofrequirements. the maximum Since temperature the fluctuation does not range exceed of 1% the aftermaximum 300 µs, ittemperature can be considered does not that exceed a stable 1% stateafter has300 been μs, it reached. can be considered This phenomenon that a stable in which state the has maximumbeen reached. temperature This phenomenon increases rapidly in which and the reaches maximum a steady temperature state in a short increases time canrapidly be attributed and reaches to thea steady high thermal state in conductivity a short time ofcan magnesium be attributed alloys. to the The high high thermal thermal conductivity conductivity of of magnesium AZ91D leads alloys. to strongThe high heat conduction,thermal conductivity and, as a result,of AZ91D large leads amounts to strong of heat heat are quicklyconduction, transferred and, as to a theresult, molded large partamounts and substrate. of heat are quickly transferred to the molded part and substrate.

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Materials 2020, 13, 4157 8 of 19 Materials 2020, 13, x FOR PEER REVIEW 8 of 19

Figure 4. Maximum temperature of domain vs. time at 100 W. FigureFigure 4. 4.Maximum Maximum temperaturetemperature of domain vs. vs. time time at at 100 100 W. W. The spatial temperature gradient is an important factor related to residual stress and TheThe spatial spatial temperature temperature gradient gradient is an importantis an important factor related factor to related residual to stress residual and microstructure stress and microstructure of final parts. Therefore, it is important to study the temperature distribution along of finalmicrostructure parts. Therefore, of final itparts. is important Therefore, to it study is important the temperature to study distributionthe temperature along distribution the scan direction along the scan direction and the temperature–time history of a significant point. andthe the scan temperature–time direction and the history temperature–time of a significant history point. of a significant point. Figure 5 shows the temperature distribution along the scan path, at different laser power, when FigureFigure5 shows 5 shows the the temperature temperature distribution distribution alongalong thethe scanscan path, at at different di fferent laser laser power, power, when when the laser incident center is at point A (0,0,0.16). It is obvious that there is an enhancement in maximum thethe laser laser incident incident center center is is at at point point A A (0,0,0.16). (0,0,0.16). ItIt is obvious obvious that that there there is is an an enhancement enhancement in inmaximum maximum temperature with the increase of laser power. The ratio of the temperature drop to the corresponding temperaturetemperature with with the the increase increase of of laser laser power. power. TheThe ratio of the the temperature temperature drop drop to to the the corresponding corresponding distance is used to define the average temperature gradient. As laser power rises from 100 to 200 W, distancedistance is usedis used to to define define the the average average temperaturetemperature gradient. As As laser laser power power rises rises from from 100 100 to to200 200 W, W, the maximum temperature rises from 1651 to 2739 K, as well. Due to high thermal conductivity of thethe maximum maximum temperature temperature rises rises from from 1651 1651 to to 2739 2739 K, K, as as well. well. Due Due to to high high thermal thermal conductivity conductivity of of the thethe AZ91D AZ91D magnesium magnesium alloy, alloy, the the temperature temperature decrease decreasess along the scanningscanning path path at at a avery very high high speed, speed, AZ91D magnesium alloy, the temperature decreases along the scanning path at a very high speed, which reaches the highest value of 7.727.72 × × 10 10 K/m at P = 200 W. The great temperature gradient whichwhich reaches reaches the the highest highest value value of 7.72 of 106 K/m at K/m P = at200 P W.= 200 The W. great The temperature great temperature gradient gradient may lead maymay lead lead to tohigh high stress stress field, field, and and defects defects× such such asas crcracksacks that are causedcaused byby stress stress relief relief are are likely likely to to to high stress field, and defects such as cracks that are caused by stress relief are likely to occur in final occuroccur in finalin final parts parts accordingly accordingly [37,38]. [37,38]. It It is is wort worthh notingnoting that the highesthighest temperature temperature point point does does not not parts accordingly [37,38]. It is worth noting that the highest temperature point does not coincide with coincidecoincide with with laser laser incident incident center, center, butbut slightlyslightly behindbehind it, oppositeopposite toto scanscan direction.direction. This This laser incident center, but slightly behind it, opposite to scan direction. This phenomenon is due to the phenomenonphenomenon is dueis due to to the the combination combination of of the the therma thermall accumulationaccumulation effecteffect and and the the change change of of thermal thermal combination of the thermal accumulation effect and the change of thermal conductivity caused by phase conductivityconductivity caused caused by by phase phase transition transition [39]. [39]. It It ca cann bebe seenseen from FigureFigure 5 5 that that the the temperature temperature curve curve transition [39]. It can be seen from Figure5 that the temperature curve is not axisymmetric, and the is notis not axisymmetric, axisymmetric, and and the the temperature temperature drop drop rate rate isis delayeddelayed between 0.12 0.12 and and 0.18 0.18 mm. mm. This This results results temperature drop rate is delayed between 0.12 and 0.18 mm. This results from the latent heat released fromfrom the the latent latent heat heat released released during during solidification solidification ofof moltenmolten AZ91DAZ91D alloy,alloy, when when the the temperature temperature during solidification of molten AZ91D alloy, when the temperature comes to solidus temperature. comescomes to solidusto solidus temperature. temperature.

Figure 5. Temperature along the scan path. FigureFigure 5. 5. TemperatureTemperature along along the scan path.

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The cooling rate is another significantsignificant factor determining the microstructure and mechanical properties of finalfinal parts. Figure 66 showsshows thethe temperaturetemperature changechange curvecurve ofof thethe fixedfixed pointpoint BB (0.1,0,0.16)(0.1,0,0.16) on the scanning path, at didifferentfferent laserlaser powerpower levels.levels. The cooling rate is presented by the average temperature drop rate of point B. It can be seen that the temperature of point B falls rapidly with a 4.31 × 10 7.37 × 10 significantlysignificantly high high cooling cooling rate, rate, which which varies varies from from 4.31 106 K/sK/ sat at P P= =100100 W W to to 7.37 106K/sK /ats atP × × P= 200= 200 W. W However,. However, due due to tothe the latent latent heat heat of of solidification, solidification, there there is is an an obvious obvious delay delay of of the the cooling cooling rate when the temperature drops to the phase-transition temperature, as well. The cooling rate can affect affect the growthgrowth of of magnesium magnesium alloy alloy grains grains directly; directly; a high coolinga high rate cooling will refine rate solidified will refine microstructure solidified obviously.microstructure There obviously. are researches There clarifying are researches that a high clarifying cooling ratethat can a high make cooling the eutectic rate distributecan make more the homogeneouslyeutectic distribute and more its volumehomogeneously fraction and decrease its volume in Mg-Gd-Y-Zr fraction decrease alloy [40 in]. Mg-Gd-Y-Zr In addition, duealloy to [40]. the In addition, due to the high cooling rate of the melt pool during the SLM process, the formed material high cooling rate of the melt pool during the SLM process, the formed material undergoes repeated undergoes repeated remelting, and some elements in the powder material may be burned during the remelting, and some elements in the powder material may be burned during the process, so the parts process, so the parts are prone to microsegregation and coarse crystals. The layer-by-layer melting are prone to microsegregation and coarse crystals. The layer-by-layer melting and sintering of the and sintering of the powder bed may also lead to anisotropy of the mechanical properties of final powder bed may also lead to anisotropy of the mechanical properties of final parts [41–43]. parts [41–43].

Figure 6. TemperatureTemperature of point B with time.

Thus, comparedcompared with with traditional traditional manufacturing manufacturing methods, methods, SLM processingSLM processing technology technology can produce can partsproduce with parts finer with grains finer and grains better and tensile better yield tensile strength, yield which strength, is attributed which is to attributed the high coolingto the high rate duringcooling process.rate during On theprocess. other hand,On the great other temperature hand, great gradient temperature due to gradient high thermal due conductivityto high thermal can resultconductivity in a high can stress result field, in a andhigh defects stress field, such and as cracks defects are such likely as tocracks occur are in likely the final to occur parts. in Therefore, the final extraparts. high-input Therefore, laserextra energy high-input should laser be avoidedenergy should in the SLM be avoided process, in and the substrate SLM process, preheating and substrate can help topreheating reduce thermal can help stress. to reduce Further thermal research stress. in this Furt fieldher requires research experimental in this field verification requires experimental and will not beverification discussed and in thiswill article. not be However,discussed thein this FE modelarticle. can However, provide the a deeper FE model understand can provide of the a processdeeper andunderstand help to selectof the suitableprocess parametersand help to withselect lower suitable cost. parameters with lower cost. 3.2. Flow in the Melt Pool 3.2. Flow in the Melt Pool Thermocapillary flow (Marangoni convection) induced by surface tension variations along free Thermocapillary flow (Marangoni convection) induced by surface tension variations along free surface is an important phenomenon in the SLM process [44], because it plays a crucial role in surface is an important phenomenon in the SLM process [44], because it plays a crucial role in determining the dimension and stability of melt pool [41,42]. Figure7 shows the shape of melt pool and determining the dimension and stability of melt pool [41,42]. Figure 7 shows the shape of melt pool flow in it at different laser power levels, when the melt-pool shape gets stable after 300 µs. The color and flow in it at different laser power levels, when the melt-pool shape gets stable after 300 μs. The map shows the temperature gradient, and the arrows represent the velocity field. It can be found that color map shows the temperature gradient, and the arrows represent the velocity field. It can be the dimensions of the melt pool, maximum temperature, and maximum speed of flow all increase found that the dimensions of the melt pool, maximum temperature, and maximum speed of flow all with the enhancement of the laser power. For a relatively lower power of 100 and 125 W there is no increase with the enhancement of the laser power. For a relatively lower power of 100 and 125 W remelting in the previously formed parts. Under this condition, the insufficient energy input may result there is no remelting in the previously formed parts. Under this condition, the insufficient energy input may result in poor metallurgical bonding ability between the current processed layer and the

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Materials 2020, 13, x FOR PEER REVIEW 10 of 19 in poor metallurgical bonding ability between the current processed layer and the previous processed one [38]. While at a considerably high laser power of 200 W, obvious remelting is observed in the previous processed one [38]. While at a considerably high laser power of 200 W, obvious remelting previously formed layer, the maximum temperature is as high as 2738.53 K, and the maximum velocity is observed in the previously formed layer, the maximum temperature is as high as 2738.53 K, and in the melt pool is up to 10.14 m/s. In such a situation, overheating in the local area causes the capillary the maximum velocity in the melt pool is up to 10.14 m/s. In such a situation, overheating in the local of the melt pool to be highly unstable, and the melt is liable to splash. Thus the spheroidization effect area causes the capillary of the melt pool to be highly unstable, and the melt is liable to splash. Thus is likely to occur, which is a typical metallurgical defect in the SLM process. Thereby, the densification the spheroidization effect is likely to occur, which is a typical metallurgical defect in the SLM process. level of final part may be reduced [45]. Meanwhile, arrows show a radial flow on the upper surface of Thereby, the densification level of final part may be reduced [45]. Meanwhile, arrows show a radial melt pool, diverging from the highest temperature in the center to the boundary of melt pool, which is flow on the upper surface of melt pool, diverging from the highest temperature in the center to the consistent with the Marangoni effect. boundary of melt pool, which is consistent with the Marangoni effect.

FigureFigure 7.7. Melt-pool comparison for didifferentfferent laserlaser powerpower levelslevels ((tt == 300 µμs).s).

FigureFigure8 8 shows shows thethe maximummaximum velocity in the melt pool pool over over time, time, at at P P = =100100 W. W. The The velocity velocity in inthe the melt melt pool pool increases increases rapidly rapidly in in 150 150 μµs,s, accompanied accompanied by by the the conspicuous conspicuous rise rise of of temperature. Moreover,Moreover, thethe maximummaximum speed reaches a basically stable state around 4.67 4.67 mm/s/s,, from from 300 300 to to 400 400 μµs. TheThe high-speedhigh-speed flowflow inin thethe meltmelt poolpool is caused by the tangential stress on the melt-pool surface which isis contributedcontributed by by the the significant significant temperature temperature gradient. gradient Therefore,. Therefore, flow flow rate rate is positively is positively correlated correlated with temperature.with temperature. Maximum Maximum velocity velocity increases increase and stabilizes,s and accompaniedstabilizes, accompanied with maximum with temperature, maximum astemperature, can be seen as from can Figuresbe seen3 from and8 Figures. 3 and 8.

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FigureFigure 8. 8.Maximum Maximum speedspeed in the molten poll, poll, at at P P == 100100 W. W. Figure 8. Maximum speed in the molten poll, at P = 100 W. TheThe maximum maximum velocity velocity in in the the melt melt pool pool varies varies from from 4.67 4.67 m m/s/s at at P P= =100 100 W W to to 10.14 10.14 m m/s/s at atP P= =200 200 W. SuchW. violentSuchThe violentmaximum flow flow will velocity greatlywill greatly in enhance the enhance melt the pool the convective varies convective from heat 4.67 heat transfer m/s transfer at P in = in 100 SLM, SLM, W makingto making 10.14 m/s it itsignificant significant at P = 200 to studyW.to study Such the flow violentthe flow in theflow in melt the will melt pool. greatly pool. Figure enhance Figure9 shows the9 shows convective the velocitythe velocity heat vector transfer vector of the inof SLM, convectivethe convective making flow it flowsignificant in thein the melt pool,tomelt study at pool, P =the at100 Pflow = W. 100 in The W.the Thecolor melt color pool. map map representsFigure represents 9 shows di differentfferent the velocity speeds, speeds, vector andand theof the arrows arrows convective represent represent flow the thein flow the flow direction.meltdirection. pool, The Theat maximumP maximum= 100 W. flowThe flow color velocity velocity map occurs representsoccurs between between different the the laser laserspeeds, incident incident and the centercenter arrows andand represent thethe meltingmelting the frontflow of thedirection.of melt the pool,melt The whichpool, maximum iswhich accompanied flow is accompanied velocity by theoccurs mostby between the significant most the significant laser temperature incident temperature center gradient. and Agradient. the high melting temperature A fronthigh gradientoftemperature the ismelt the pool,gradient root causewhich is the ofis flow.rootaccompanied cause A great of flow.by surface the A mostgreat tension significantsurface gradient tension temperature is causedgradient by gradient.is thecaused temperature A by high the gradient;temperature furthermore, gradient;gradient the isfurthermore, the generated root cause tangentialthe generatedof flow. force A tangential great pushes surface meltedforce tension pushes magnesium gradient meltedalloy magnesiumis caused to flow by alloy at the high speedtemperatureto flow (the at maximum high gradient; speed velocity (the furthermore, maximum reaches the velocity 4.67 generated m/ reachess on thetangential 4.67 upper m/s surface).force on thepushes upper melted surface). magnesium alloy to flow at high speed (the maximum velocity reaches 4.67 m/s on the upper surface).

Figure 9. The velocity vector plots of convective flow in the melt pool, at P = 100 W. (a) shows the flow FigureFigureon the 9. upper9.The The velocity velocitysurfacevector ofvector melt plots plots pool, ofof (b convectiveconvective) shows the flowflow flow in onthe the melt melt Y-Z pool, pool, section at at P P =through =100100 W. W. (thea) ( ashows )laser shows theincident the flow flow oncenter,on the the upper upper(c) shows surface surface theof flowof melt melt on pool, thepool, X-Z ((bb) )section showsshows trough the flow flow the on on laser the the incidentY-Z Y-Z section section center. through through (d) shows the the laser the laser shapeincident incident of center,center,the melt (c) ( showsc pool) shows and the the the flow flow flow on on in the theit X-Zat X-Z P section= 100section W trough and trough t = the 300 the laser μ . incident incident center. center. (d ()d shows) shows the the shape shape of of the meltthe pool meltand pool the and flow the inflow it atin Pit =at100 P = W100 and W and t = 300t = 300µs. μs.

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Molten alloy flows from the center to the edge on the upper surface of the melt pool (Figure9a), and then it flows from the upper surface to the bottom, along the boundary of the melt pool (Figure9b). The flow converges at the center of the melt pool bottom and rises from the bottom to the upper surface (Figure9c). In general, the shape of the melt pool is determined by two forms of heat transfer in the melt pool: conductive heat transfer and convective heat flux. Moreover, the latter plays a decisive role in SLM [29,46]. The radial flow takes a large amount of heat from the laser incident center to the melting boundary and solidification boundary, which promotes the boundary expansion. The width and length of the melt pool are expended as a result. Meanwhile, high-speed flow on the melt-pool surface can greatly accelerate the heat loss to atmosphere, resulting in a reduction of heat conducted to the bottom of the melt pool, contributing to the decrease of melt-pool depth. The melt pool becomes larger but shallower in the end. Thus, metallurgical bonding between the current processed layer and previous processed one may be weakened, and porosity and other defects are more likely to occur. Since the model does not take evaporation into account, which can greatly promote heat conduction of the melt pool, the flow velocity should be slower, and the shape of the melt pool will be narrower and deeper when it comes to the experiment. Nevertheless, the simulation model still can help to see the flow phenomenon in the melt pool. Moreover, the influence of the high-speed flow on heat dissipation and evolution of melt-pool morphology can be investigated by the FE model.

3.3. Melt-Pool Size and Shape In the current SLM process simulation, the size of the melt pool was mostly determined on the basis that the temperature is higher than the melting point of the powder material [47]. Powder begins to melt when the temperature exceeds the solidus temperature (743 K), and it completely melts when the temperature exceeds liquidus temperature (868 K), at which the semi-melted parts exist between them will adhere to the final part. Therefore, the actual boundary of melt pool will be beyond the boundary of the melting point. In order to verify the model through experiments better, a phase-transition model is introduced. Governing equations of phase transition are Equations (9)–(11) in the former section, and ω is introduced in to describe the state of the AZ91D alloy as shown in Table4.

Table 4. State of AZ91D magnesium powder represented by ω.

ω State of Powder ω = 1 Liquid (completely melted) 0 < ω < 1 Semi-melted ω = 0. Solid (unmelted)

Different colors in Figure 10 indicate different states of powder: The red region indicates that the powder is totally melted (where ω = 1), the blue region represents the part that has not begun to melt (ω = 0), and the color-transition zone shows the semi-melted part. The simulation was conducted with laser power at P = 100 W. MaterialsMaterials 20202020,, 1313,, 4157x FOR PEER REVIEW 1313 of 1919

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FigureFigure 10.10. Schematic diagram of melt-pool sizesize atat 400400 µμss andand laserlaser powerpower atat PP == 100 W. Figure 10. Schematic diagram of melt-pool size at 400 μs and laser power at P = 100 W. FigureFigure 1111 showsshows thethe melt-poolmelt-pool sizesize withwith timetime atat PP == 100 W. ItIt can bebe seen thatthat thethe size ofof thethe meltmelt poolpoolFigure (length,(length, 11 width, width,shows andandthe melt-pool depth)depth) increasesin creasessize with rapidlyrapidly time at withwith P = timetime100 W. inin theItthe can initialinitial be seen period,period, that andandthe size thethe growthgrowthof the melt raterate poolslowsslows (length, downdown afterwidth,after50 50 andµ sμs, until,depth) until it reachesitin reachescreases a steadyrapidlya steady state. with state. Thetime Th depth ein depth the of initial melt of melt poolperiod, p stabilizesool and stabilizes the at growth 40 atµ m,40 rate first,μm, slowsandfirst, then anddown thethen after width the 50 width stabilizes μs, until stabilizes it at reaches 171.4 at 171.4µ m.a steady Theμm. lengthThe state. length finallyThe depthfinally reaches of reaches melt a stable p aool stable value stabilizes value of 220 atofµ 40m220 μ after m,μm first,300afterµ and s300. This then μs is. the becauseThis width is thebecause stabilizes residual the at temperature residual171.4 μm. temperature The and length latent heatfinallyand oflatent solidusreaches heat reducea stableof solidus heat value dissipation reduce of 220 μheatm in afterthedissipation rear 300 end μs in. of Thisthe the rear meltis becauseend pool of along the the melt theresidual pool scan along direction. temperature the scan direction.and latent heat of solidus reduce heat dissipation in the rear end of the melt pool along the scan direction. 250 length 250 lengthwidth 200 widthdepth 200 depth

150 150

100 dimention(mm) 100 dimention(mm) 50 50

0 0 100 200 300 400 500 0 time(μs) 0 100 200 300 400 500 time(μs) FigureFigure 11.11. Melt-pool sizesize withwith timetime atat PP == 100 W.

Figure 12 shows the diffFigureerent 11. melt-pool Melt-pool dimensions size with time at diatff Perent = 100 laserW. power levels. It is obvious Figure 12 shows the different melt-pool dimensions at different laser power levels. It is obvious that the three dimensions (length, width, and depth) all increase with the laser power, as a result of the thatFigure the three 12 showsdimensions the different (length, melt-pool width, and dimensions depth) all atincrease different with laser the power laser power,levels. Itas is a obviousresult of increased input energy of the laser, as discussed in the former section. However, it is worth noting thatthe increasedthe three dimensionsinput energy (length, of the laser, width, as anddiscussed depth) in all the increase former withsection. the However, laser power, it is as worth a result noting of that the length of the melt pool increases faster with the increase of laser power, compared to the thethat increased the length input of theenergy melt of pool the laser, increases as discussed faster with in the the former increase section. of laser However, power, it compared is worth noting to the width and depth. The tail of the melt pool is surrounded by a compact formed part with high thermal thatwidth the and length depth. of theThe melttail of pool the meltincreases pool isfaster surrounded with the by increase a compact of laser formed power, part withcompared high thermal to the conductivity, while the melting front and two sides of the melt pool are adjacent to the unmelted widthconductivity, and depth. while The the tail meltingof the melt front pool and is surroundedtwo sides of by the a compactmelt pool formed are adjacent part with to highthe unmelted thermal powder. With the increase of laser power, large amounts of heat are transferred to the solidification conductivity,powder. With while the increase the melting of laser front power, and twolarge sides amou ofnts the of melt heat pool are transferred are adjacent to tothe the solidification unmelted powder. With the increase of laser power, large amounts of heat are transferred to the solidification

Materials 2020, 13, x FOR PEER REVIEW 14 of 19 Materials 2020, 13, 4157 14 of 19 Materials 2020, 13, x FOR PEER REVIEW 14 of 19 front of the melt pool through the formed part with higher thermal conductivity, resulting in a higher frontascentfront of ofspeed the the melt melt in the pool pool length through through of the the the melt formed formed pool. part part with with higher higher thermal thermal conductivity, conductivity, resulting resulting in in a a higher higher ascentascent speed speed in in the the length length of of the the melt melt pool. pool. 400 400 length 350 widthlength 350 300 depthwidth 300 depth 250 250 200 200 150 150 Melt pool diensions(μm) 100

Melt pool diensions(μm) 100 50 50 0 75 100 125 150 175 200 225 0 75 100 125Laser 150power(W) 175 200 225 Laser power(W)

Figure 12. Melt-pool dimensions at different laser power levels. Figure 12. Melt-pool dimensions at different laser power levels. Figure 12. Melt-pool dimensions at different laser power levels. IncreasedIncreased laserlaser powerpower cancan alsoalso aaffectffect thethe shapeshape ofof thethe meltmelt pool.pool. The variable ww/l/l (width(width/length/length ratio)ratio) isIncreasedis introduced,introduced, laser toto power characterizecharacterize can also thethe affect shapeshape the ofof shape thethe moltenmolten of the poolmeltpool alongpool.along The thethe scanningvariablescanning w/l direction,direction, (width/length whilewhile thetheratio) variablevariable is introduced, dd/w/w (depth(depth/width to /characterizewidth ratio)ratio) the isis usedused shape toto of describedescribe the molten thethe shapeshapepool along inin thethe the depthdepth scanning direction.direction. direction, FigureFigure while 1313 showsshowsthe variable thethe twotwo d/w variables (depth/width changingchanging ratio) trendstrends is withusedwith didifferenttoff describeerent laserlaser the powerpower shape levels.levels. in the It Itdepth cancan bebe direction. seenseen that Figure the two 13 variablesvariablesshows the developdevelop two variables inin oppositeopposite changing trendstrends trends asas thethe with laserlaser different powerpower increases. increases.laser power When levels. the It laser can be power seen isisthat lessless the thanthan two 175175variables W,W, ww/l/l develop continuescontinues in tooppositeto decrease.decrease. trends However, as the laser it risesrise powers slightly increases. when theWhen laser the power power laser powercomes comes isto to less 200 200 thanW. W. On175 thethe W, contrary,contrary, w/l continues dd/l/l continuescontinues to decrease. toto increaseincrease However, withwith it thetherise increasesincrease slightly ofof when laserlaser the power,power, laser butbutpower thethe ratecomesrate ofof to increaseincrease 200 W. graduallyOn the contrary, dropsdrops down,down, d/l continues andand itit isis almostalmostto increase stablestable with whenwhen the thethe increase laserlaser powerpower of laser increasesincreases power, fromfrombut the 175175 rate toto 200 200of W.increaseW. gradually drops down, and it is almost stable when the laser power increases from 175 to 200 W. 1 w/l 0.91 d/ww/l 0.80.9 d/w 0.70.8 0.60.7 0.50.6 0.40.5 0.30.4 0.20.3 0.10.2 0.10 75 100 125 150 175 200 225 0 Laser power(W) 75 100 125 150 175 200 225 Laser power(W) Figure 13. Ratio of width/length and depth/width at different laser power levels. Figure 13. Ratio of width/length and depth/width at different laser power levels. Figure 13. Ratio of width/length and depth/width at different laser power levels.

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In summary, the melt pool became narrow and deep as the laser power increased, which can lead Into summary,poor stability the meltof the pool melt-pool became shape. narrow Co andupled deep with as the the laser violent power evaporation increased, caused which canby high lead laserto poor power, stability it is ofeasy the to melt-pool cause defects shape. such Coupled as pores with in the violentfinal part. evaporation On the other caused hand, by it high has laserbeen power,analyzed it isbefore easy tothat cause lower defects laser such power as poreswill lead in the to an final insufficient part. On the melt-pool other hand, size, it which has been can analyzedresult in inadequatebefore that lowerbonding laser quantity power willbetween lead tothe an forming insufficient layers. melt-pool Therefore, size, laser which power can resultof 100 in W inadequate and layer thicknessbonding quantity of 0.04 mm between are the the most forming reasonable layers. pr Therefore,ocessing laserparameters power for of 100such W an and experiment. layer thickness of 0.04 mm are the most reasonable processing parameters for such an experiment. 3.4. Experiment Verification 3.4. Experiment Verification The width of the laser track, which is strongly related to the scanning patch, is an important factorThe affecting width the of thefinal laser material. track, Therefore, which is stronglyit is considered related to to be the an scanning effective patch,verification is an method important to comparefactor aff ectingthe simulation the final material.width with Therefore, experiments it is considered[48]. In order to to be verify an eff ectivethe credibility verification of this method model, to twocompare different the simulationexperiments width were with conducted experiments to observ [48].e Inthe order width to of verify the melt the credibilitychannel. Then of this the model, melt- pooltwo di widthfferent in experiments the simulation were model conducted was compar to observeed with the width the width of the measured melt channel. in experiments. Then the melt-pool widthGas in theatomized simulation AZ91D model magnesium was compared alloy with powder the width (Supplier: measured Tangshan in experiments. Weihao Magnesium PowderGas Co., atomized Ltd. Tangshan, AZ91D magnesium China) with alloy a mean powder particle (Supplier: size of Tangshan 53 μm was Weihao used Magnesiumin the experiments. Powder AnCo., Nd:YAG Ltd. Tangshan, laser with China) 1064 with nm a meanwave particle length sizeand of80 53 μµmm spot was usedsize was in the used experiments. to scan and An Nd:YAGmelt the powder.laser with The 1064 process nm wave was length protected and 80byµ argon,m spot and size the was oxygen used to concentration scan and melt in the the powder. building The chamber process was protectedbelow 0.1%. by argon, and the oxygen concentration in the building chamber was below 0.1%. Experiment I is a multi-track scheme. Square areas 10 mm 10 mm were melted in the beds’ Experiment I is a multi-track scheme. Square areas 10 mm ×× 10 mm were melted in the beds’ surface, withwith laserlaser powerpower 100100 W W and and scanning scanning speed speed 1000 1000 mm mm/s./s. The The laser laser scanned scanned parallel parallel to to a sidea side of ofthe the square square area, area, with with scan scan spacing spacing of 0.2 of mm, 0.2 mm, and theand thickness the thickness of each of layer each waslayer 0.04 was mm. 0.04 A mm. typical A typicalresult of result this experimentof this experiment is shown is inshown Figure in 14 Figure. Twenty 14. dataTwenty of melt data channel of melt widthchannel were width collected were collectedfrom the surfacefrom the of surface final part of byfinal AXIO part Scopeby AXIO A1 ofScope Carl A1 Zeiss of Carl (Produced Zeiss (Produced by Carl Zeiss, by Carl Germany). Zeiss, TheGermany). average The is considered average is toconsidered be the experimental to be the expe widthrimental of the singlewidth meltof the channel. single melt The experimentalchannel. The widthexperimental result is width 153.43 resultµm, as is shown 153.43 in μ Tablem, as5, whichshown is in slightly Table lower5, which than is the slightly simulation lower of than the melt the simulationchannel width of the of 171.40melt channelµm, with width deviation of 171.40 of 10.65%.μm, with deviation of 10.65%.

Figure 14. Multi-channel scan experiment at P = 100100 W. W.

Table 5. Comparison between average width in experimentexperiment I and the model melt-pool width.

AverageAverage width width of of melt channelchannel in in experiment experiment I (µ Im) (μm) 153.14153.14 AverageAverage width width of of melt melt poolpool in in simulation simulation model model (µm) (μm) 171.40171.40 ErrorError 10.65%10.65%

The evaporation temperature of the AZ91D alloy was 1373.15 K, while the maximum The evaporation temperature of the AZ91D alloy was 1373.15 K, while the maximum temperature temperature of the molten pool was 1660 K in the simulation, which is much higher than the of the molten pool was 1660 K in the simulation, which is much higher than the evaporation temperature. evaporation temperature. Therefore, violent evaporation will occur on the surface of the melt pool, Therefore, violent evaporation will occur on the surface of the melt pool, which can accelerate the loss which can accelerate the loss of heat into the gas environment. The heat transferred to the powder

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material is reduced as a result, so the width of the melt pool in the experiment is smaller than the of heat into the gas environment. The heat transferred to the powder material is reduced as a result, so result obtained by simulation. the width of the melt pool in the experiment is smaller than the result obtained by simulation. Experiment II was carried out in a more intuitive way. A straight line of 5 mm was melted with Experiment II was carried out in a more intuitive way. A straight line of 5 mm was melted with laser power 100 W and scanning speed 1000 mm/s. The laser scanned six layers of AZ91D alloy laser power 100 W and scanning speed 1000 mm/s. The laser scanned six layers of AZ91D alloy powder, and the thickness of each layer was 0.04 mm. The experimental results are shown in Figure powder, and the thickness of each layer was 0.04 mm. The experimental results are shown in Figure 15. 15. Twenty data are collected from the surface of final part by AXIO Scope A1 of Carl Zeiss. The Twenty data are collected from the surface of final part by AXIO Scope A1 of Carl Zeiss. The average is average is considered as the experimental width of the single melt channel. The average width of the considered as the experimental width of the single melt channel. The average width of the single melt single melt channel in this experiment is 173.95 μm, which is slightly larger than the simulation result channel in this experiment is 173.95 µm, which is slightly larger than the simulation result of 171.40 µm of 171.40 μm with an error of 1.49%. The results are listed in Table 6. with an error of 1.49%. The results are listed in Table6.

FigureFigure 15.15. Single-channel scan experiment at P = 100 W. Table 6. Comparison between average width in experiment II and the model melt-pool width. Table 6. Comparison between average width in experiment II and the model melt-pool width. Average width of melt channel in experiment II (µm) 173.95 Average width of melt channel in experiment II (μm) 173.95 µ AverageAverage width width of melt pool pool in in simulation simulation model model ( m) (μm)171.40 171.40 ErrorError 1.49%1.49%

The result of experiment II is closer to the FE model. However, because the semi-melted powder particlesparticles adheredadhered to to the the melt melt pool pool (reflective (reflective particles particles in Figure in Figure 15), the 15), experimental the experimental results areresults slightly are largerslightly than larger the than simulation the simulation results. However,results. Howe the experimentver, the experiment results are results in good are agreementin good agreement with the simulationwith the simulation results, with results, an error with ofan 1.49%, error of which 1.49%, proves which the proves reliability the reliability of the model of the directly. model directly.

4. Conclusions A three-dimensionalthree-dimensional model model of selectiveof selective laser meltinglaser melting of AZ91D of magnesiumAZ91D magnesium alloy was established,alloy was inestablished, which the in phase which change the phase latent change heat andlatent Marangoni heat and Marangon floe was considered.i floe was considered. The model The was model used towas investigate used to investigate the temperature the temperature profile of theprofile domain, of the flow domain, behavior flow of behavior the melt of pool the andmelt melt-pool pool and dimensionsmelt-pool dimensions and shape and under shape diff undererent laser different power. laser The power. model The can model help tocan investigate help to investigate the complex the physicalcomplex phenomenonphysical phenomenon in SLM and in selectSLM properand select processing proper parametersprocessing withparameters lower cost. with Conclusions lower cost. canConclusions be drawn can as follows:be drawn as follows:

(1)(1) TheThe temperature of the domain increased rapidly when the laser started to irradiate, and the maximummaximum temperature showed a strong correlation with input laser energy. Severe temperature gradientgradient andand high high cooling cooling rate rate were were observed, observed, which iswhich attributed is attributed to the high to thermal the high conductivity thermal conductivityof AZ91D magnesium of AZ91D alloy.magnesium alloy. (2)(2) ViolentViolent flow flow caused by the Marangoni effect effect waswas observed in the melt pool, and the maximummaximum velocity was found to increase when the laser power would rise. The high-speed flowflow enhanced convectionconvection heat heat transfer transfer and made the melt pool wider and longer but shallower. (3) The dimensions of the melt pool stabilized rapidly., accordingly with the temperature and flow velocity. The size of the melt pool increased as the laser power rose, but the length of the melt

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(3) The dimensions of the melt pool stabilized rapidly., accordingly with the temperature and flow velocity. The size of the melt pool increased as the laser power rose, but the length of the melt pool increased faster than its width and depth. The shape of the melt pool became narrower and deeper as the laser power increased, which may lead to poor stability of the melt pool’s shape. (4) The width of the melt pool was acquired by single scan melt experiment, which was in good agreement with the results predicted by the FE model (with an average error of 1.49%).

Author Contributions: H.S. and J.Y. conceived and designed the study. J.Y. conducted simulation and data analysis, X.N. and J.Y. conducted experimental verification. H.S. and J.Y. wrote the original draft. H.S., J.Y. and X.N. edited and reviewed the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by the National Nature Science Foundation of China (No. 51975518), the Science Fund for Creative Research Groups of National Natural Science Foundation of China (No. 51821093), the Key Research and Development Plan of Zhejiang Province (No 2018C01073), the Ningbo Science and Technology Plan Project (2019B10072), and the Fundamental Research Funds for the Central Universities (No. 2019QNA4004). Conflicts of Interest: The authors declare no conflict of interest.

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