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Molecular Dynamics Simulation Study of Liquid-Assisted Laser Beam Micromachining Process Vivek Anand Menon and Sagil James *

ID

Department of Mechanical Engineering, California State University Fullerton, Fullerton, CA 92831, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-657-278-3337 Received: 29 June 2018; Accepted: 6 August 2018; Published: 9 August 2018

Abstract: Liquid Assisted Laser Beam Micromachining (LA-LBMM) process is an advanced machining process that can overcome the limitations of traditional laser beam machining processes. This research involves the use of a Molecular Dynamics (MD) simulation technique to investigate the complex and dynamic mechanisms involved in the LA-LBMM process both in static and dynamic mode. The results of the MD simulation are compared with those of Laser Beam Micromachining (LBMM) performed in air. The study revealed that machining during LA-LBMM process showed higher removal compared with LBMM process. The LA-LBMM process in dynamic mode showed lesser material removal compared with the static mode as the flowing water carrying the heat away from the machining zone. Investigation of the material removal mechanism revealed the presence of a thermal blanket and a bubble formation in the LA-LBMM process, aiding in higher material removal. The findings of this study provide further insights to strengthen the knowledge base of laser beam micromachining technology. Keywords: laser beam machining; molecular dynamics; liquid-assisted

1. Introduction The Laser Beam Micromachining (LBMM) process is a non-traditional technique that is capable of machining a wide range of materials with ultraprecision in a short period [1]. The LBMM process offers several advantages including high resolution, minimum wastage, ease-of-control, repeatability and reproducibility [1]. This process finds application in several fields from micromechanics to microfluidics [2,3]. The LBMM process uses focused thermal energy with high temperatures to remove material from the substrates through melting and vaporization [4]. The LBMM process that is performed in the air results in the formation of Heat Affected Zones (HAZ), which is one of the significant limitations of the process [5]. Also, LBMM process causes other undesired effects including tapered kerf formation, high surface roughness, micro-crack formation, and recast and re-deposition of molten material [5,6]. Additionally, there are possibilities for the amorphization of supercooled liquid along with the recrystallization of the amorphous phase during the LBMM process [7,8]. Performing the LBMM process in a liquid medium is a potential approach to overcome these limitations of the LBMM process [9]. The process known as Liquid-Assisted Laser Beam Micromachining (LA-LBMM) is capable of micromachining materials with features ranging from 100 to 500 µm with reduced thermal damage, with relatively narrow kerf width, and a reduced re-deposition of debris [10,11]. During the LA-LBMM process, the substrate to be machined is submerged entirely in a liquid medium having a thickness of 2–3 mm [12]. The schematic of the LA-LBMM process is shown in Figure 1. The LA-LBMM process can be performed both in static mode (still water) and dynamic mode (flowing water) [13]. Studies have reported that the liquid layer helps in cooling the workpiece, which minimizes the HAZ formation during the machining process [10,11,13,14]. During static mode, J. Manuf. Mater. Process. 2018, 2, 51; doi:10.3390/jmmp2030051

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which minimizes the HAZ formation during the machining process [10,11,13,14]. During static mode, the the molten molten debris debris re-deposit re-deposit on on the the machined machined area, area, which which piles piles up up in in the the center center and and is is removed removed by by subsequent laser energy [13]. During dynamic mode, the flowing water helps to flush the debris away subsequent laser energy [13]. During dynamic mode, the flowing water helps to flush the debris away from zone, preventing re-casting and and re-deposition, resulting in a smooth surface surface [13,15]. fromthe themachining machining zone, preventing re-casting re-deposition, resulting in a smooth Several have been used during LA-LBMM process including water, ethylene [13,15]. liquid Severalmediums liquid mediums have been usedthe during the LA-LBMM process including water, glycol, methanol, and propanol [16,17]. However, the most preferred liquid medium is pure water, ethylene glycol, methanol, and propanol [16,17]. However, the most preferred liquid medium is pure as it is environmentally friendlyfriendly and a low-cost option [12]. It has reported that thethat LA-LBMM water, as it is environmentally and a low-cost option [12].been It has been reported the LAprocess can provide higher material removalremoval rates owing shockwaves and cavitation that helps LBMM process can provide higher material rates to owing to shockwaves and cavitation that faster ejection [18]. [18]. helps material faster material ejection

Figure 1. 1. Schematic Schematic of of Liquid-Assisted Liquid-Assisted Laser Laser Beam Beam Machining Machining Process. Process. Figure

It is evident that LA-LBMM is an improved process that addresses some of the critical issues of It is evident that LA-LBMM is an improved process that addresses some of the critical issues of the LBMM process. However, it is critical to understand the role of liquid medium and the material the LBMM process. However, it is critical to understand the role of liquid medium and the material removal mechanisms involved in the LA-LBMM process to make this process commercially relevant. removal mechanisms involved in the LA-LBMM process to make this process commercially relevant. Experimental studies fail to provide a clear understanding of the material removal mechanisms Experimental studies fail to provide a clear understanding of the material removal mechanisms involved in the LA-LBMM process considering the complexities and dynamic nature of the process involved in the LA-LBMM process considering complexities and dynamic nature of the process at the micron scale. In the past, the finite elementthe analysis (FEA) technique is used to understand the at the micron scale. In the past, the finite element analysis (FEA) technique is used to understand mechanisms involved in the LA-LBMM process [10,19]. One study used FEA tool ANSYS and found the involvedsignificantly in the LA-LBMM process [10,19]. study used FEA ANSYS and thatmechanisms the liquid medium reduced the HAZ forOne sub-millimeter thicktool water film [10]. found that the liquid medium significantly reduced the HAZ for sub-millimeter thick water film [10]. Another study used the finite element modeling technique and Smooth Particle Hydrodynamic Another study used the finitetoelement modeling technique and Smooth Particle Hydrodynamic (SPH) (SPH) modeling technique study CO 2 laser underwater machining and reported that the liquid modeling technique to study CO2 laser underwater andand reported that the liquid medium medium helped to reduce surface defects includingmachining recast layer heat damages [19]. The finite helped to reduce surface defects including recast layer and heat damages [19]. The finite difference difference method (FDM) was used to study the hybrid laser-water jet micro-grooving process on the method (FDM) was used studyshowed the hybrid micro-grooving process on thethe silicon silicon substrates [20]. Thetostudy thatlaser-water introducingjethigh-pressure waterjets during laser substrates [20]. The study showed that introducing high-pressure waterjets during the laser machining machining process helps remove materials in the soft-solid form below its melting temperature. process remove process materials in the soft-solid form belowbetween its melting Thehelps LA-LBMM involves several interactions thetemperature. laser beam, substrate, water The LA-LBMM between the lasertechniques beam, substrate, water molecules and the process debris involves particles.several While interactions finite element simulation can provide molecules and the debris particles. While finite element simulation techniques can provide information information on the temperature distribution and substrate deformation, further investigations are on the temperature distribution andof substrate deformation, further are neededand to needed to understand the exact role water medium along with theinvestigations underlying complexities understand the exact role of water medium along with the underlying complexities andsimulation dynamic dynamic interactions involved in the LA-LBMM process. Molecular Dynamics (MD) interactions involved in the LA-LBMM process. Molecular Dynamics (MD) simulation technique is technique is used as a useful tool in the past to understand the molecular level interaction during used a usefulprocesses tool in the[21–23]. past to understand the molecular interactionlaser during laser ablation laser as ablation The MD simulation studylevel of ultrashort ablation process processes [21–23]. The MD simulation study of ultrashort laser ablation process revealed the revealed that the material removal happens due to thermo-elastic stress developed through that thermal material removal happens due to thermo-elastic stress developed through thermal heating. It results heating. It results in ablation of the substrate materials resulting in the formation of clusters of varying in ablation thesimulation substrate materials resulting the formation of clusters of varying [21]. MD sizes [21]. of MD technique is also in used to study the effect of water onsizes nanoparticle simulation technique is also used to study the effect of water on nanoparticle generation during the laser generation during the laser ablation of metal foils [24]. The study found that nanoparticle formation ablation of metal foils [24]. The study found that nanoparticle formation happens due to nucleation happens due to nucleation and growth of molten metal by rapid cooling in the metal-water mixing and growth of The molten metal rapid cooling in the metal-water The10study 12 K/s environment. study alsobyreported an extremely high coolingmixing rate ofenvironment. approximately 12 K/s during the interaction of the also reported an extremely high cooling rate of approximately 10 during the interaction of the growing nanoparticles and water [24]. MD simulation study was used to understand the ablation process of silicon by water-jet-guided laser [25]. The study showed that

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growing nanoparticles and water [24]. MD simulation study was used to understand the ablation process of silicon by in water-jet-guided [25]. The study showed thatsubstrate water cooling helps in water cooling helps minimizing thelaser thermal-affected zones and the maintained its minimizing the thermal-affected zones and the substrate maintained its original structure. original structure. The The present present study study uses uses the the MD MD simulation simulation technique technique to to investigate investigate the the underlying underlying process process mechanisms involved in the LA-LBMM process in both static and dynamic mode. The study mechanisms involved in the LA-LBMM process in both static and dynamic mode. The study also also investigates of of laser heatheat fluxflux on the and size the machined cavity forcavity two different investigatesthe theeffects effects laser onquality the quality andofsize of the machined for two substrate and Siliconand Carbide. The study evaluates machined dimensions different materials—Copper substrate materials—Copper Silicon Carbide. The studythe evaluates thecavity machined cavity and amount and of material during the LBMM andprocess LA-LBMM process (both static(both and dimensions amountremoved of material removed during process the LBMM and LA-LBMM process dynamic static andmode). dynamic mode). 2. 2. Molecular Molecular Dynamics Dynamics Simulation Simulation In In this this study, study, the the MD MD simulation simulation of of the the LBMM LBMM and and LA-LBMM LA-LBMM process process is is performed performed using using aa “Large-scale atomic/molecular massively parallel simulator” (LAMMPS) [26]. For the LA-LBMM “Large-scale atomic/molecular massively parallel simulator” (LAMMPS) [26]. For the LA-LBMM process, process, water water molecules molecules are are added added above above the the substrate substrate surface. surface. For For the the LBMM LBMM process, process, no no water water molecules ofof a substrate having a size of 80 Å moleculesare areadded addedtotothe thesystem. system.The Thesimulation simulationsystem systemconsists consists a substrate having a size of 80 × 82 Å × 30 Å and is filled with Newtonian atoms. The copper (Cu) substrate consists of approximately Å × 82 Å × 30 Å and is filled with Newtonian atoms. The copper (Cu) substrate consists of 40,000 Cu atoms40,000 initially in a face-centered lattice structure a lattice constant approximately Cuarranged atoms initially arranged incubic a face-centered cubicwith lattice structure withofa 3.61 Å. The Silicon Carbide (SiC) substrate consists of approximately 9000 Si and 9000 carbon lattice constant of 3.61 Å. The Silicon Carbide (SiC) substrate consists of approximately 9000 atoms Si and initially arranged a diamond latticeinstructure with a lattice constant The substrates 9000 carbon atomsininitially arranged a diamond lattice structure withofa 4.3596 lattice Å. constant of 4.3596 are initially given aare temperature of 293 K. A fixed layer a thickness of 3 Å aenvelopes Å. The substrates initially given a temperature of 293having K. A fixed layer having thickness all of the 3Å sides of the substrate except the top surface to prevent any undesired movement of the substrate. envelopes all the sides of the substrate except the top surface to prevent any undesired movement of A thin layer (2 A Å)thin of thermostat is provided between the Newtonian atoms and the fixed the substrate. layer (2 Å)atoms of thermostat atoms is provided between the Newtonian atomslayer. and The are kept at 293 K and used ensure consistent conduction away from the thermostat fixed layer.atoms The thermostat atoms areare kept at to 293 K and are usedheat to ensure consistent heat the laser heataway affected conduction fromregion. the laser heat affected region. For For the the LA-LBMM LA-LBMM process, process, the the substrate substrate is is placed placed below below an an equilibrated equilibrated system system of of water water molecules. For this study, 9000 water molecules are considered including 18,000 hydrogen atoms molecules. For this study, 9000 water molecules are considered including 18,000 hydrogen atoms and and 9000 having a thickness of 30 boundary conditions are 9000oxygen oxygenatoms atomsininthe theform formofofa block a block having a thickness of Å. 30 Periodic Å. Periodic boundary conditions considered in this considering the extremely small small size ofsize the of simulation modelmodel compared to the are considered in study this study considering the extremely the simulation compared experimental conditions. The periodic boundary conditions are maintained on all the atoms along the X to the experimental conditions. The periodic boundary conditions are maintained on all the atoms and Y directions so that the simulation box is replicated throughout the space to form an infinite lattice along the X and Y directions so that the simulation box is replicated throughout the space to form an that effectively eliminates the eliminates spurious size of size the isolated The oxidation phenomena infinite lattice that effectively the effects spurious effects ofsystem. the isolated system. The oxidation during the laser micromachining process is not considered in the present MD study. The schematic phenomena during the laser micromachining process is not considered in the present MD study. The representation of the MD simulation models used to study LBMM processes for copper schematic representation of the MD simulation models usedand to LA-LBMM study LBMM and LA-LBMM substrates are shown in Figures 2 and 3, respectively. Figure 4 shows the MD simulation models processes for copper substrates are shown in Figures 2 and 3, respectively. Figure 4 shows theused MD to study LBMM process siliconLBMM carbideprocess substrate. simulation models usedfor to study for silicon carbide substrate.

(a)

(b)

Figure 2. 2. Molecular Molecular Dynamics Dynamics Simulation Simulation Model Model of of the the Laser Laser Beam Beam Micromachining Micromachining (LBMM) (LBMM) Process Process Figure on Copper Substrate (a) Sectional view (b) 3-Dimensional view. on Copper Substrate (a) Sectional view (b) 3-Dimensional view.

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Figure 3. Molecular Dynamics Simulation Model of the Liquid-Assisted Laser Beam Micromachining Figure3.3.Molecular MolecularDynamics DynamicsSimulation SimulationModel Modelofofthe theLiquid-Assisted Liquid-AssistedLaser LaserBeam BeamMicromachining Micromachining Figure (LA-LBMM) Process (LA-LBMM) Processon onCopper CopperSubstrate Substrate(a) (a)Sectional Sectionalview view(b) (b)3-Dimensional 3-Dimensionalview. view. (LA-LBMM) Process on Copper Substrate (a) Sectional view (b) 3-Dimensional view.

(a) (a)

(b) (b)

Figure 4. Molecular Dynamics Simulation Model of the LBMM Process on Silicon Carbide Substrate Figure 4.4.Molecular Dynamics Model of the LBMM Process on Silicon Carbide Substrate Dynamics Simulation Simulation (a)Figure SectionalMolecular view (b) 3-Dimensional view. Model of the LBMM Process on Silicon Carbide Substrate (a) (a) Sectional view (b) 3-Dimensional view. Sectional view (b) 3-Dimensional view.

The laser beam is simulated using a heat source within a spherical region at the top surface of The laser beam is simulated using a heat source within a spherical region at the top surface of the substrate having a radius of 8 Å. using Atomistic temperature distribution in the substrate is calculated The laser beam is simulated a heat source within a spherical region at the top surface the substrate having a radius of 8 Å. Atomistic temperature distribution in the substrate is calculated using thesubstrate equationhaving below to understand theAtomistic variation in temperature during theinLBMM and LA-is of the a radius of 8 Å. temperature distribution the substrate using the equation below to understand the variation in temperature during the LBMM and LALBMM processes, calculated using the equation below to understand the variation in temperature during the LBMM and LBMM processes, LA-LBMM processes, 2 (1) = 2T = 2Ke (1)(1) =3 3NK 3 B the Boltzmann constant and and Ke where the atomistic temperature, N is thethe number of atoms; KB is whereTTTisisis the atomistic temperature, number atoms; is the Boltzmann constant isBthe Boltzmann constant and Ke where the atomistic temperature, NN is is the number of of atoms; KB K isKthe total kinetic energy of the group of atoms. The LA-LBMM and LBMM process simulation is kinetic energy of the group of atoms. LA-LBMM LBMM process simulation e is the is the totaltotal kinetic energy of the group of atoms. TheThe LA-LBMM andand LBMM process simulation is conducted for aforduration of 1ofpicosecond (ps). TheThe simulation model assumes that aa single is conducted a duration 1 picosecond (ps). simulation model assumes that singlelaser laser conducted for a duration of 1 picosecond (ps). The simulation model assumes that a single laser heating the material heatingevent eventisisisaaagood goodunderstanding understandingof thecomplex complexphenomenon phenomenonof materialremoval removalduring duringthe the heating event good understanding ofofthe complex phenomenon ofofmaterial removal during the laser beam machining process. The process parameter used for the MD simulation study is laser heat laser beam machining process. The process parameter used for the MD simulation study is laser heat laser beam machining process. The process parameter used for the MD simulation study is laser heat flux (or(or laser heat intensity), which is the raterate of heat addition. The unit of laser heat flux used this flux laser heat intensity), which is the of heat addition. of laser flux in used flux (or laser heat intensity), which is the rate of heat addition. The The unitunit of laser heat heat flux used in thisin study is in energy/time units—Kcal/mol/fs. The effect of percussive laser beam heating is this study in energy/time units—Kcal/mol/fs. The effect of percussive laser beam heatingisisnot not study is in isenergy/time units—Kcal/mol/fs. The effect of percussive laser beam heating not considered ininthis study. Additionally, it itisisassumed that the substrates have uniform and constant considered this study. Additionally, assumed that the substrates have uniform and constant considered in this study. Additionally, it is assumed that the substrates have uniform and constant thermal aspect ofof laser wavelength onon the material removal is isnot this thermalproperties. properties.The The aspect laser wavelength the material removal notconsidered consideredin this thermal properties. The aspect of laser wavelength on the material removal is not considered ininthis study. Also, quantitative calculation of surface roughness is not performed in this study as the lengths study. Also, quantitative calculation of surface roughness is not performed in this study as the lengths study. Also, quantitative calculation of surface roughness is not performed in this study as the lengths scales MD scalesof MDsimulation simulationand andexperiments experimentsare aresignificantly significantlydifferent. different. scales ofofMD simulation and experiments are significantly different. The interatomic forces between the Cu-Cu atoms in Theinteratomic interatomicforces forcesbetween betweenthe theCu-Cu Cu-Cuatoms atomsininthe theCu Cusubstrate substrateare arecalculated calculatedusing using The the Cu substrate are calculated using Embedded Atom Method (EAM) model function [27]. The potential energy of an atom using EAM Embedded Atom Method (EAM) model function [27]. The potential energy of an atom using EAM Embedded Atom Method (EAM) model function [27]. The potential energy of an atom using EAM model, is given by model, Ei is is given given by by model, ! 1 Ei = Fα ∑ ρ β rij1 + ∑ Φαβ rij (2) 2 j 6 =i (2) = + 1 j 6 =i (2) = +2 2 where i and j (i 6= j) label the atoms in the solid, rij is the distance between atoms i and j, and ρ β is the electron density position of atom i due to all other in between the solid.atoms It is supposed that this where and ( ≠at )the label the atoms in the solid, is the atoms distance and , and where and ( ≠ ) label the atoms in the solid, is the distance between atoms and , and is the electron density at the position of atom due to all other atoms in the solid. It is supposed that is the electron density at the position of atom due to all other atoms in the solid. It is supposed that this density can be given as a sum of individual atomic densities ( ) where is the potential this density can be given as a sum of individual atomic densities ( ) where is the potential


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density can be given as a sum of individual atomic densities f rij where Φαβ is the potential function, ρ β is the electron charge density from atom j of type β at the location of atom i and F is an embedding function that represents the energy required to place atom i of type α into the electron cloud. ρi =

∑ f (rij )

(3)

j

The interatomic forces between the Si-C, Si-Si and C-C atoms in the SiC substrate are calculated using Tersoff many-body potential, a suitable potential for the simulations of covalent bonding materials like silicon and carbon [28]. The energy E, between any two neighboring atoms i and j, is given by 1 E= (4) ∑ Vij 2 ∑ i j 6 =i Vij = f c rij ( f C (r ) = f ( x ) =

h

i f R rij + bij f A (rij)

: 1 π r−R 1 1 − sin : 2 2 2 D 0 :

(5)

r < R−D R−D < r < R+D r > R+D

f R (r ) = A exp(−λ1 r ) f A (r ) = − B exp(−λ2 r ) − 1 2n 1 + βn ζ ij n h m i f C (rik ) g θijk exp λ3 m rij − rik bij =

ζ ij =

∑

k 6=i, j

 g (θ ) = γijk

 2 2 c c 1 + i − h d2 d2 + (cosθ − cos θ0 )2

The extended simple point charge (SPC/E) model of liquid water is used to describe the water molecules. The water molecule is modeled as a rigid isosceles triangle, having charges situated on each of the three atoms—a positive charge on two hydrogen atoms and an excess negative charge on one oxygen atom. The water molecules interact via the standard Lennard–Jones (LJ) potential [29]. The potential energy in the LJ potential function is calculated as σ 12 σ 6 Vij = 4ε − r r

(6)

where σ is the distance at which the two particles are at equilibrium, ε is the strength of the interaction, and r is the distance between the particles. The parameters have different constant values for different interacting particles. The LJ potential is applied to describe the Cu-O and the Cu-H potential energy for water-copper interactions. The Si-O and the C-O potential energy for water-silicon and water-carbon interactions are also described using the LJ potential. The cutoff distances used are 9.8 Å for O-O interactions, 5 Å for Cu-O and Cu-H interactions, 7 Å for C-O interactions, 10 Å for Si-O interactions and 10 Å for all other interactions. The detailed parameters and values for all LJ interaction pairs are listed in Table 1. The Velocity–Verlet algorithm is employed to calculate the position and velocity of the atoms. The conditions used for simulation of LBMM and LA-LBMM processes are shown in Table 2. Validation of the MD simulation model through experimentation is beyond the scope of this study.


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Table 1. LJ potential parameters for O-O, Cu-O, Si-O, and C-O atom pairs. J. Manuf. Mater. Process. 2018, 2, xParameter FOR PEER REVIEW

O-O

Cu-O

Si-O

C-O

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3.166 2.644 3.629 2.744 Equilibrium distance (σ, Å) 3 , eV) The Velocity–Verlet is−employed to calculate43the position of the atoms. 6.736 231.9 and velocity 62.0 Cohesivealgorithm energy (ε, 10 9.8 LA-LBMM 5 10.0 distance (Å) The conditions usedCut-off for simulation of LBMM and processes are7.0shown in Table 2.

Validation of the MD simulation model through experimentation is beyond the scope of this study. Table 2. Simulation Conditions Used in the MD Simulation of LBMM and LA-LBMM Process. Table 2. Simulation Conditions Used in the MD Simulation of LBMM and LA-LBMM Process. Materials Materials

Operating Operating Conditions Conditions

Substrates Substrates Water Water InitialTemperature Temperature Initial Laser Heat Flux Laser Heat Flux Potential Used Potential Used Duration of Simulation Duration of Simulation

•• ••

Cu Block (80 Å × 82 Å × 30 Å), 40,000 Atoms Cu Block (80 Å × 82 Å × 30 Å), 40,000 Atoms SiC Block (80 SiC Block (80ÅÅ×× 82 82 Å Å×× 30 30 Å), Å),19,000 19,000Atoms Atoms H O Block 30 Å Thick, 9000 Molecules 2 H2O Block 30 Å Thick, 9000 Molecules 293 K 293 K 3000 Kcal/mol/fs (Low)–9000 Kcal/mol/fs (High) 3000 Kcal/mol/fs (Low)–9000 Kcal/mol/fs (High) EAM, Tersoff, Lennard-Jones (LJ) EAM, Tersoff, Lennard-Jones (LJ) 1 picosecond (ps) 1 picosecond (ps)

3. Results and Discussion 3. Results and Discussion Figure 5 shows a representative atomic configuration of the Cu substrates machined during the Figure 5 shows a representative atomic configuration of the Cu substrates machined during the LBMM and LA-LBMM processes using a laser heat flux value of 3000 kcal/mol/fs for a simulation LBMM and LA-LBMM processes using a laser heat flux value of 3000 kcal/mol/fs for a simulation duration of 1 ps. In this case, the LA-LBMM is performed in static mode. During LBMM and duration of 1 ps. In this case, the LA-LBMM is performed in static mode. During LBMM and LALA-LBMM processes, substrate material is removed through melting vaporization. LBMM processes, thethe CuCu substrate material is removed through bothboth melting and and vaporization. In Inthis thisstudy, study,the themelting meltingand andvaporization vaporizationpoints pointsofofCu Cumaterial materialisisconsidered consideredasas1358 1358KKand and2835 2835K,K, respectively. the LA-LBMM LA-LBMMprocess processresults resultsininlarger larger respectively.From Fromthe thefigure, figure,ititisisseen seenthat that machining machining during during the cavity cavitycompared comparedtotothat thatofofthe theLBMM LBMMprocess. process.

(a)

(b)

Figure5.5.Molecular Molecular Dynamics Cavity Machined on on Cu Cu Substrate during (a) Figure Dynamics Simulation SimulationSnapshot Snapshotofof Cavity Machined Substrate during and (b) LA-LBMM Process (Static Mode). (a)LBMM LBMMProcess Process and (b) LA-LBMM Process (Static Mode).

Figure 6 shows a representative atomic configuration of the SiC substrates machined during the Figure shows a representative atomic configuration of heat the SiC machined during LBMM and6 LA-LBMM processes (static mode) using a laser fluxsubstrates value of 3000 kcal/mol/fs forthe a LBMM and LA-LBMM processes (static mode) using a laser heat flux value of 3000 kcal/mol/fs for simulation duration of 1 ps. The SiC material is primarily removed through the ablation process, and a the simulation duration of 1 ps. SiC material is primarily removed through ablationasprocess, threshold temperature for The ablation is considered as 2973 K. A similar trend isthe observed in the and the threshold temperature for ablation is considered as 2973 K. A similar trend is observed case of Cu machining where the LA-LBMM process machined larger cavity compared to thatasofin the case of Cu machining where the LA-LBMM process machined larger cavity compared to that of LBMM process. LBMM process.

(a)

(b)

Figure 6. Molecular Dynamics Simulation Snapshot of Cavity Machined on SiC Substrate during (a) the LBMM Process and (b) LA-LBMM Process (Static Mode).

Figure 6 shows a representative atomic configuration of the SiC substrates machined during the LBMM and LA-LBMM processes (static mode) using a laser heat flux value of 3000 kcal/mol/fs for a simulation duration of 1 ps. The SiC material is primarily removed through the ablation process, and the threshold temperature for ablation is considered as 2973 K. A similar trend is observed as in the case of Cu machining where the LA-LBMM process machined larger cavity compared to that of J. Manuf. Mater. Process. 2018, 2, 51 7 of 15 LBMM process.

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Figure 6. Molecular Dynamics Simulation Snapshot of Cavity Machined on SiC Substrate during (a) Figure 6. Molecular Dynamics Simulation Snapshot of Cavity Machined on SiC Substrate during (a) the

the LBMMPower Processon and (b) LA-LBMM Process (Staticand Mode). 3.1. EffectLBMM of Laser Cavity Size Process during LBMM Process and (b) LA-LBMM (Static Mode). LA-LBMM Process

Figure 7 shows the variation in cavity depth with respect to different heat flux for both the 3.1. Effect of Laser Power on Cavity Size during LBMM and LA-LBMM Process LBMM and LA-LBMM process in static mode. It is seen that the depth of the cavities machined shows the flux variation in cavity depth with respect different heat flux forprocess both theisLBMM increasesFigure as the7laser heat is increased. The depth of the to cavity in LA-LBMM relatively and LA-LBMM process in static mode. It is seen that the depth of the cavities machined increases as the that larger than that in the air for the corresponding laser heat flux. It can be explained by the fact laser heat flux is increased. The depth of static the cavity in LA-LBMM process is relatively larger in during the LA-LBMM machining under water conditions, the thermal energy ofthan the that vaporized the air for the corresponding laser heat flux. It can be explained by the fact that during the LA-LBMM atoms remains in the machining zone for a longer duration causing more material to be removed. machining under static water conditions, the thermal energy of the vaporized atoms remains in the The vaporized molecules move slowly due to the presence of water molecules above the substrate. machining zone for a longer duration causing more material to be removed. The vaporized molecules Figure 8 shows the MD simulation snapshot of the cavity machined on Cu substrate using the LAmove slowly due to the presence of water molecules above the substrate. Figure 8 shows the MD LBMM process.snapshot In the figure, the red coloredon atoms represent thethe molten and vaporized Cufigure, substrate, simulation of the cavity machined Cu substrate using LA-LBMM process. In the and the thered black colored atoms represent the and superheated vapor and molecules. the case of the colored atoms represent the molten vaporized water Cu substrate, the blackIn colored atoms LBMM process, the copper water atomsvapor are molecules. vaporizedInand fromprocess, the machining represent the superheated the move case of away the LBMM the copper zone atomswith higher velocity. Moreover, the molten materials tend to redeposit on the surface during the LBMM are vaporized and move away from the machining zone with higher velocity. Moreover, the molten tendLA-LBMM to redepositprocess, on the surface during the LBMM case. During the LA-LBMM case.materials During the the molten material cools when it comes in contactprocess, with water the molten material cools when it comes in contact with water causing solidification, which prevents causing solidification, which prevents its re-deposition. its re-deposition.

Figure 7. Variation ininCavity Depthwith with respect to Heat Flux during the Process LBMMand Process and LAFigure 7. Variation Cavity Depth respect to Heat Flux during the LBMM LA-LBMM LBMM Process (Static Mode) Cu Substrate. Process (Static Mode) on Cuon Substrate.

Figure 7. Variation in Cavity Depth with respect to Heat Flux during the LBMM Process and LA8 of 15 LBMM Process (Static Mode) on Cu Substrate.


Figure 8. MD Simulation Snapshot of Cavity Machined during LA-LBMM Process (Static Mode).

8. MD 2018, Simulation Snapshot of Cavity Machined during LA-LBMM Process (Static Mode). 8 of 15 J. Manuf.Figure Mater. Process. 2, x FOR PEER REVIEW

The variation in cavity depth with respect to the varying heat flux for the LBMM and LA-LBMM process in static mode on a SiC workpiece is shown in Figure 9. It is seen that the machining depth is higher in in the thecase caseofofthe the LA-LBMM process compared of LBMM process. However, the LA-LBMM process compared withwith that that of LBMM process. However, the depth depth of machining SiC is relatively lower than thatCu of substrate. the Cu substrate. can be explained by of machining for SiCfor is relatively lower than that of the It can beItexplained by the fact the the SiC material underwent in the of sublimation at a relatively thatfact the that SiC material underwent materialmaterial removalremoval in the form ofform sublimation at a relatively higher higher temperate compared to the Cu material. temperate compared to the Cu material.

Figure 9. Variation VariationininCavity Cavity Depth with respect to Heat during the LBMM Process and LAFigure 9. Depth with respect to Heat FluxFlux during the LBMM Process and LA-LBMM LBMM (Static on Mode) on SiC Substrate. Process Process (Static Mode) SiC Substrate.

3.2. Effect of Heat Flux on Number of Atoms Removed during the LBMM and LA-LBMM Process 3.2. Effect of Heat Flux on Number of Atoms Removed during the LBMM and LA-LBMM Process In this study, the number of atoms removed is considered as the atoms whose temperature have In this study, the number of atoms removed is considered as the atoms whose temperature have exceeded the melting point of the substrate material. Figures 10 and 11 show the variation in the exceeded the melting point of the substrate material. Figures 10 and 11 show the variation in the number of atoms removed with respect to the laser heat flux during the LBMM process and LAnumber of atoms removed with respect to the laser heat flux during the LBMM process and LA-LBMM LBMM process in static mode for copper and silicon carbide substrates respectively. It is seen that process in static mode for copper and silicon carbide substrates respectively. It is seen that the larger the larger number of atoms are removed in the case of the LA-LBMM process compared with the number of atoms are removed in the case of the LA-LBMM process compared with the LBMM process. LBMM process. The increased removal during LA-LBMM can be attributed to the formation of The increased removal during LA-LBMM can be attributed to the formation of bubbles near the bubbles near the machining zone immediately after the application of laser heat. The bubble machining zone immediately after the application of laser heat. The bubble formation pushes the formation pushes the liquid away from the machining zone along with the debris. It helps increase the removal of more atoms from the substrate. Moreover, shockwaves are observed during the simulation towards the water layer and also towards the bulk of the substrate. The shockwave, which is moving towards the substrate, causes cracks in the periphery of the cavity. This shockwave is caused due to the rapid heating and cooling of the copper atoms in extremely short duration.

exceeded the melting point of the substrate material. Figures 10 and 11 show the variation in the number of atoms removed with respect to the laser heat flux during the LBMM process and LALBMM process in static mode for copper and silicon carbide substrates respectively. It is seen that the larger number of atoms are removed in the case of the LA-LBMM process compared with the J. Manuf. Mater. Process. 2018, 2, 51 9 of of 15 LBMM process. The increased removal during LA-LBMM can be attributed to the formation bubbles near the machining zone immediately after the application of laser heat. The bubble formation pushes the liquid away from the machining zone along with the debris. It helps increase liquid away from the machining zone along with the debris. It helps increase the removal of more the removal of more atoms from the substrate. Moreover, shockwaves are observed during the atoms from the substrate. Moreover, shockwaves are observed during the simulation towards the simulation towards the water layer and also towards the bulk of the substrate. The shockwave, which water layer and also towards the bulk of the substrate. The shockwave, which is moving towards is moving towards the substrate, causes cracks in the periphery of the cavity. This shockwave is the substrate, causes cracks in the periphery of the cavity. This shockwave is caused due to the rapid caused due to the rapid heating and cooling of the copper atoms in extremely short duration. heating and cooling of the copper atoms in extremely short duration.

Figure 10. Variation in Number of Atoms Removed with respect to Heat Flux during LBMM and LAFigure 10. Variation in Number of Atoms Removed with respect to Heat Flux during LBMM and 9 of 15 LA-LBMM Process (Static Mode) on Cu Substrate.

J. Manuf. Mater.Process Process. 2018, 2, xMode) FOR PEER REVIEW LBMM (Static on Cu Substrate.

Figure inthe theNumber Numberofof Atoms Removed respect the Heat Flux during the Figure 11. 11. Variation Variation in Atoms Removed withwith respect to thetoHeat Flux during the LBMM LBMM and LA-LBMM (Static Mode) on the SiC Substrate. and LA-LBMM ProcessProcess (Static Mode) on the SiC Substrate.

3.3. 3.3.Comparison Comparisonbetween betweenthe theLA-LBMM LA-LBMMProcess ProcessStatic Static and and Dynamic Dynamic Mode Mode The TheMD MDsimulation simulationstudy studyisisperformed performedto tounderstand understandthe theeffect effectof ofthe the motion motionof of water water above above the surface. For this study, a flow velocity of 1000 m/s is provided to the layer of water the surface. For this study, a flow velocity of 1000 m/s is provided to the layer of water molecules molecules above abovethe thesubstrate substratesurface. surface.The Theflow flowvelocity velocityconsidered consideredin inthis thisstudy studyisissignificantly significantlylarger largerthan thanthe the typical values (of the order of 10 m/s) used during the LA-LBMM experiments. The increased value typical values (of the order of 10 m/s) used during the LA-LBMM experiments. The increased value of of flow velocity can be justified by the fact that the mass flow rate of water during the MD simulation flow velocity can be justified by the fact that the mass flow rate of water during the MD simulation isisconsiderably considerablylow lowin inthe theatomistic atomisticscales. scales.The Theincreased increasedvelocity velocitycould couldcompensate compensatefor forthe thereduced reduced kinetic energy of the water molecules. Figure 12 shows the snapshot of the MD simulation during the kinetic energy of the water molecules. Figure 12 shows the snapshot of the MD simulation during LA-LBMM process for the dynamic mode. In the figure, the red atoms in the figure represent the the LA-LBMM process for the dynamic mode. In the figure, the red atoms in the figure represent the molten moltenand andvaporized vaporizedCu Cusubstrate substrate atoms atoms while while the the black black atoms atoms represent represent superheated superheated steam. steam.

typical values (of the order of 10 m/s) used during the LA-LBMM experiments. The increased value of flow velocity can be justified by the fact that the mass flow rate of water during the MD simulation is considerably low in the atomistic scales. The increased velocity could compensate for the reduced kinetic energy of the water molecules. Figure 12 shows the snapshot of the MD simulation during the LA-LBMM process for the dynamic mode. In the figure, the red atoms in the figure represent the J. Manuf. Mater. Process. 2018, 2, 51 10 of 15 molten and vaporized Cu substrate atoms while the black atoms represent superheated steam.

Figure Mode). Figure 12. 12. MD MD Simulation Simulation Snapshot Snapshot of of Cavity Cavity Machined Machined during during the the LA-LBMM LA-LBMM Process Process (Dynamic (Dynamic Mode).

The comparison between variations in the number of atoms removed during LA-LBMM for both The comparison between variations in the number of atoms removed during LA-LBMM for both static and dynamic modes are shown in Figure 13. It is seen that the number of atoms removed from static and dynamic modes are shown in Figure 13. It is seen that the number of atoms removed from the substrate is relatively less in the dynamic mode compared to static mode. It can be explained the substrate is relatively less in the dynamic mode compared to static mode. It can be explained because the flowing water carries the heat away from the machining zone, resulting in lesser material because the flowing water theREVIEW heat away from the machining zone, resulting in lesser material J.removal. Manuf. Mater. Process. 2018, x carries FOR PEER 10 of 15 Moreover, the2,cavities have reduced re-deposition during the dynamic mode. removal. Moreover, the cavities have reduced re-deposition during the dynamic mode.

Figure 13. Variation in the Number of Atoms Removed with respect to Heat Flux during the LAFigure 13. Variation in the Number of Atoms Removed with respect to Heat Flux during the LA-LBMM LBMM forMode Staticand Mode and Dynamic ProcessProcess for Static Dynamic Mode. Mode.

3.4. Process Mechanisms Involved in LA-LBMM Process 3.4. Process Mechanisms Involved in LA-LBMM Process The MD simulation study revealed that various mechanisms are involved in the LA-LBMM The MD simulation study revealed that various mechanisms are involved in the LA-LBMM affecting the material removal process. These mechanisms include (1) Effect of Thermal Blanket (2) affecting the material removal process. These mechanisms include (1) Effect of Thermal Blanket Effect of Cavity and Bubble Formation and (3) Effect of Flowing Water Removing Debris. The effect (2) Effect of Cavity and Bubble Formation and (3) Effect of Flowing Water Removing Debris. The effect of individual process mechanisms is explained below. of individual process mechanisms is explained below. During LA-LBMM in static mode, the water molecules form a barrier to the motion of the During LA-LBMM in static mode, the water molecules form a barrier to the motion of the vaporized atoms causing their solidification in the liquid. On the other hand, the water molecules vaporized atoms causing their solidification in the liquid. On the other hand, the water molecules carry carry the debris along with it during the LA-LBMM process in dynamic mode. Both static and the debris along with it during the LA-LBMM process in dynamic mode. Both static and dynamic dynamic modes showed shockwave propagation through the bulk of the substrate resulting in nanoscale cracking. The water molecules played an essential role in the formation of the bubbles between the cavity surface and water layer. 3.4.1. Effect of Thermal Blanket


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modes showed shockwave propagation through the bulk of the substrate resulting in nanoscale cracking. The water molecules played an essential role in the formation of the bubbles between the cavity surface and water layer. 3.4.1. Effect of Thermal Blanket During the LA-LBMM process in static mode, the laser heat flux converts the water molecules in the vicinity to superheat steam. The presence of the superheated steam along with the molten and vaporized substrate molecules form a thermal blanket above the machined cavity as shown in Figure 14 (black atoms). The thermal blanket ensures that the machined region remains hot and aids in subsequent material removal. As the time progresses, the superheated steam gradually disperses into the block of water molecules. During the LBMM process, the ablated material is removed from the surface as the laser heat is applied and there is no thermal blanket formation. The presence of the thermal blanket above the machined cavity can be attributed as one of the reasons for larger material removal during the LA-LBMM process as compared to the LBMM process. However, during the LA-LBMM process in dynamic mode the thermal blanket is displaced away from the machining region by the flowing water as shown in Figure 15. This prevents the region to stay warm unlike the machining during static mode. It could provide an explanation for the reduced material removal during LA-LBMM process dynamic mode compared to static mode. J. Manuf. the Mater. Process. 2018, 2, x FORin PEER REVIEW 11 of 15


(a) (a)

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(b) (b)

Figure 14. 14. MD MD Simulation Simulation Snapshot Snapshot Showing Showing Thermal Thermal Blanket Blanket Formation Formation during during LA-LBMM LA-LBMM Process Process Figure Figure 14. MD Simulation Snapshot Showing Thermal Blanket Formation during LA-LBMM Process in Static Mode (a) Initial Stage (b) Final Stage. in Static Static Mode Mode (a) (a) Initial Initial Stage Stage (b) (b) Final Final Stage. Stage. in

(a) (a)

(b) (b)

Figure 15. MD Simulation Snapshot Showing Thermal Blanket Formation during LA-LBMM Process Figure 15. Simulation Snapshot Showing Thermal Figure 15. MD MD Simulation Snapshot Thermal Blanket Blanket Formation Formation during during LA-LBMM LA-LBMM Process Process in Dynamic Mode (a) Initial Stage (b)Showing Final Stage. in Dynamic Mode (a) Initial Stage (b) Final Stage. in Dynamic Mode (a) Initial Stage (b) Final Stage.

3.4.2. Effect of Cavity and Bubble Formation 3.4.2. Effect of Cavity and Bubble Formation During the LA-LBMM process in both static and dynamic modes, the removal of atoms from the During the LA-LBMM process in both static and dynamic modes, the removal of atoms from the substrate surface results in the formation of cavities in the water in vicinity of the machined region as substrate surface results in the formation of cavities in the water in vicinity of the machined region as shown in Figure 16. The cavities lead to the formation of bubbles of varying sizes. The bursting of the shown in Figure 16. The cavities lead to the formation of bubbles of varying sizes. The bursting of the bubbles leads to cavitation and shockwave propagation thorough the substrate resulting in an increased bubbles leads to cavitation and shockwave propagation thorough the substrate resulting in an increased material removal. The bubble formation during LA-LBMM is also witnessed during the experimental material removal. The bubble formation during LA-LBMM is also witnessed during the experimental studies. Figure 17 shows the presence of bubbles during the LA-LBMM process during the machining studies. Figure 17 shows the presence of bubbles during the LA-LBMM process during the machining

(a) J. Manuf. Mater. Process. 2018, 2, 51

(b) 12 of 15

Figure 15. MD Simulation Snapshot Showing Thermal Blanket Formation during LA-LBMM Process in Dynamic Mode (a) Initial Stage (b) Final Stage.

3.4.2. Effect of Cavity and Bubble Formation 3.4.2. Effect of Cavity and Bubble Formation During the LA-LBMM process in both static and dynamic modes, the removal of atoms from the substrate surface results in process the formation cavities the water in vicinity of the of machined region During the LA-LBMM in bothofstatic and in dynamic modes, the removal atoms from the as shownsurface in Figure 16. The cavities lead of to cavities the formation of bubbles of varying The region bursting substrate results in the formation in the water in vicinity of the sizes. machined as of the bubbles leads to cavitation andto shockwave propagation thorough the sizes. substrate inthe an shown in Figure 16. The cavities lead the formation of bubbles of varying The resulting bursting of increased material removal. bubble formation during LA-LBMM is also witnessed during the bubbles leads to cavitation andThe shockwave propagation thorough the substrate resulting in an increased experimental studies. shows the presence of bubbles during the LA-LBMM process during material removal. The Figure bubble17 formation during LA-LBMM is also witnessed during the experimental the machining substrates in static mode. The presence of cavitiesprocess and bubbles inthe themachining machined studies. Figure of 17SiC shows the presence of bubbles during the LA-LBMM during region thus playsina static critical role in higher material removal LA-LBMM processregion compared with of SiC substrates mode. The presence of cavities andduring bubbles in the machined thus plays the LBMM process. a critical role in higher material removal during LA-LBMM process compared with the LBMM process.

Figure 16. 16. MD to Bubble Figure MD Simulation Simulation Snapshot Snapshot Showing Showing the the Formation Formation of of Cavities Cavities Leading Leading to Bubble Formation Formation during LA-LBMM LA-LBMM Process Process in in Static Static Mode. Mode. during


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Figure 17. Bubble Formation during LA-LBMM Process Experimentation (Right side image shows the Figure 17. Bubble Formation during LA-LBMM Process Experimentation (Right side image shows the magnified view of the circled region). magnified view of the circled region).

3.4.3. Effect of Flowing Water Removing Debris 3.4.3. Effect of Flowing Water Removing Debris During the LA-LBMM process in dynamic mode, it is observed that the debris particles of the During the LA-LBMM process in dynamic mode, it is observed that the debris particles of the substrates are removed from the machining area, as shown in Figure 18. The removal of debris plays substrates are removed from the machining area, as shown in Figure 18. The removal of debris a critical role during the LA-LBMM process. The removal of debris also helps in achieving a smoother plays a critical role during the LA-LBMM process. The removal of debris also helps in achieving a surface after the machining process. However, the debris removal does not happen during the LAsmoother surface after the machining process. However, the debris removal does not happen during LBMM process in static mode as well as the LBMM process. This means that the LA-LBMM process the LA-LBMM process in static mode as well as the LBMM process. This means that the LA-LBMM produces a surface that has better finish compared to the other two processes. process produces a surface that has better finish compared to the other two processes.

During the LA-LBMM process in dynamic mode, it is observed that the debris particles of the substrates are removed from the machining area, as shown in Figure 18. The removal of debris plays a critical role during the LA-LBMM process. The removal of debris also helps in achieving a smoother surface after the machining process. However, the debris removal does not happen during the LALBMM process in static mode as well as the LBMM process. This means that the LA-LBMM process J. Manuf. Mater. Process. 2018, 2, 51 13 of 15 produces a surface that has better finish compared to the other two processes.

Figure 18. MD Bubble Formation Formation Figure 18. MD Simulation Simulation Snapshot Snapshot Showing Showing Formation Formation of of Cavities Cavities Leading Leading to to Bubble during LA-LBMM Process in Dynamic Mode. during LA-LBMM Process in Dynamic Mode.

3.5. 3.5. Validation Validation of of MD MD Simulation Simulation Results Results with with Experimentation Experimentation Figure Figure 19 19 shows shows the the variation variation in in cavity cavity depth depth during during LBMM LBMM and and LA-LBMM LA-LBMM process process performed performed experimentally on borosilicate glass substrates (size 25 mm × 75 mm and 1.2 mm thickness). CO2 laser experimentally on borosilicate glass substrates (size 25 mm × 75 mm and 1.2 mm thickness). CO 2 laser machine manufactured by Guang Zhou Amonstar Trade Co., Ltd. (Guang Zhou, China) is machine manufactured by Guang Zhou Amonstar Trade Co., Ltd. (Guang Zhou, China) is used used for for 2 and 150 W/mm2 2. For the 2 the experimental studies. The intensities of the laser used are 50 W/mm the experimental studies. The intensities of the laser used are 50 W/mm and 150 W/mm . For the experimental resultsfor formachining machiningperformed performedunder under (LBMM process), maximum cavity depths experimental results airair (LBMM process), maximum cavity depths are are seen to be 177 μm and 248 μm for laser power of 10 W and 30 W respectively. The cavity depth seen to be 177 µm and 248 µm for laser power of 10 W and 30 W respectively. The cavity depth values values for experimental results for machining performed under film of(LA-LBMM water (LA-LBMM for experimental results for machining performed under thin filmthin of water process process in static in static mode with water layer thickness of 0.5 mm) are 205 μm and 393 μm, respectively. This mode with water layer thickness of 0.5 mm) are 205 µm and 393 µm, respectively. This experimental experimental study thus finds that the depth of machining increases with increases in laser power. study thus finds that the depth of machining increases with increases in laser power. Additionally, Additionally, the study experimental study found is that thefor depth is more forprocess the LA-LBMM process the experimental also found thatalso the depth more the LA-LBMM compared to the compared to the LBMM process. These results are in agreement with the finding of the current MD LBMM process. These in agreement with the finding of the current MD simulation study. J. Manuf. Mater. Process. 2018,results 2, x FORare PEER REVIEW 13 of 15 simulation study.

Figure 19. Figure 19. Variation Variation in in Cavity Cavity Depth Depth with with Laser Laser Intensity Intensity obtained obtained through through the the LBMM LBMM and and LA-LBMM LA-LBMM (Static Mode) Experimentations. (Static Mode) Experimentations.

4. Conclusions 4. Conclusions In this this study, MD simulation simulation is is performed performed to to understand understand the the process process mechanism mechanism involved involved in in the the In study, MD LA-LBMM process. The effect of laser heat flux on the depth of the machined cavity and the number LA-LBMM process. The effect of laser heat flux on the depth of the machined cavity and the number of atoms comparison between thethe machining results obtained during the LAof atoms removed removedare arestudied. studied.AA comparison between machining results obtained during the LBMM process andand LBMM process is presented. The effect LA-LBMM process LBMM process is presented. The effectofofwater watermotion motionon onthe the cavity cavity formation formation is studied modes. AnAn explanation of is studied by by comparing comparingthe theLA-LBMM LA-LBMMprocess processduring duringstatic staticand anddynamic dynamic modes. explanation material removal mechanisms during the LA-LBMM process is presented. The major conclusions derived from this study are as follows: •

The MD simulation study revealed that the cavity machined during the LA-LBMM process is having more depth than that of LBMM process. It is attributed to the fact that the thermal energy is entrapped in the machining zone. The velocity of the vaporized atoms is lower during the LA-


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of material removal mechanisms during the LA-LBMM process is presented. The major conclusions derived from this study are as follows:

•

•

•

•

•

The MD simulation study revealed that the cavity machined during the LA-LBMM process is having more depth than that of LBMM process. It is attributed to the fact that the thermal energy is entrapped in the machining zone. The velocity of the vaporized atoms is lower during the LA-LBMM process due to the presence of a layer of water molecules above the substrate; The number of atoms removed during LA-LBMM process is found to be significantly higher than that during LBMM process. The LA-LBMM process in dynamic mode showed lesser material removal compared with that of static mode; A comparison between the LA-LBMM processes in static and dynamic modes showed the material removal in higher in the case of static mode compared with dynamic mode. However, the surface finish obtained in dynamic mode is better than static mode because of the removal of machining debris; The MD simulation study revealed various mechanisms involved in the LA-LBMM process including the formation of a thermal blanket and the formation of cavities and bubbles in the vicinity of the machined region. The LA-LBMM process in dynamic mode suggested the removal of debris from the machining region, leading to reduced re-deposition of molten material on the cavity surface; The results of the MD simulation study are consistent with findings of experimental results of both the LBMM and LA-LBMM processes.

Author Contributions: S.J. and V.A.M. conceived and designed the Molecular Dynamics (MD) simulation model. V.A.M. performed the simulations and S.J. interpreted and analyzed the simulation output. Both V.A.M. and S.J. prepared the manuscript together. Funding: This research received no external funding. Acknowledgments: Authors would like to thank the College of Engineering and Computer Science at California State University Fullerton for the financial support. The authors would also like to thank Sandia National Laboratories for providing LAMMPS software for this research. Conflicts of Interest: The authors declare no conflict of interest.

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