Numerical simulation of particle tracks in the cold gas dynamic spraying process

Baosteel Technical ResearchCGDS technologiesVolume 5. Number 1, March 2011, Page 5PrefaceSenior Scientist of AdvancedDr. ZHANG JuTechnology Division, ResearchunDo Institute Baoshan ironSteel oLtdn the mid-1980s, scientist A. Papyrin and his colleagues from the Institute ofTheoretical and Applied Mechanics, the Russian Academic of Science in Novosibirskdiscovered some metal particles on the tunnel wall, when they conducted wind tunnelexperiments. The adhesion was not simple mechanical riveting, but they containedmetallurgical bonding to some degree, and the bonding strength was very high. Theyoriginally defined this new phenomenon as a cold gas dynamic spraying CGDSprocess. From then on, the development of this process has been very rapidExperiments and research have found that metal particles will directly adhere to thesubstrate and form a coating if their impact velocity is higher than a certain criticalvalue. The concept of the CGDs technology was first proposed at the 1998International Thermal Spray Conference held in America, and a considerable amountof interest from both the academia and the industry was immediately focused on theprocess. In 2000, some CGDs equipment was developed by the Cold Gas TechnologyCGT)Company in Germany.In the Cgds process, small metal particles are accelerated to a very high velocityby high-pressure and high-speed gas, which is usually higher than the sonic velocityand the particles impact the substrate at a relatively low temperature. The temperatureof the impacting metal particles is usually lower than half of their melting point, whichis a low-temperature forming technology. Therefore, in order to more quicklyunderstand and promote the technology, technical researchers have also called theprocess "cold spray", compared with the thermal sprat中国煤化工cellentperformance due to its special process characteristics, such as ult sInian abull of heatdelivered to the coated part, the avoidance of in-flight oxidation and other chemicalBaosteel Technical Research. Vol 5. No I Mar 2011reactions, the ability of having thermal and oxygen-sensitive depositing materialssprayed and the ability of having the nano phase, intermetallic compound andamorphous materials, which is microstructure instable formed. So CGDs has soonbecome a perfect technology for cold-state forming, nondestructive repairing andsurface modifying. At present, many developed countries have been actively engagedin the development of CGDs, mostly majoring in the cutting-edge technologyespecially in the aviation and astronavigation field. However, most of theachievements are still unavailable to the publicBaosteel started to import key equipment from the CGT Company in 2001, as one ofthe earliest companies that have been engaged in the industrial application of CGDSWe have conducted fundamental theoretical research on cgds as well as basicresearch on application and potential industry application, and analyzed dynamicmodes, such as on-site characteristics of the gas-particle two-phase flow field and thedistribution of the solid powder velocity and angle. We have also observed theinfluence of various processing parameters on the coating deposition effect andinvestigated the microstructure and performance of the coating and its post treatmentWe have also explored its potential industrial applications. Many kinds of freeoxidation, high density thick coatings were successfully developed and fabricated byBaosteel in 2005. The maximum thickness of the coating was more than 40 mmwhich laid a firm foundation for industrializationThis issue publishes seven articles regarding computer simulation of the CGDsprocess, heating technology for stainless steel particles and substrates, microstructurecharacteristics of stainless steel and superalloy, introduction of copper alloy and nickelcoating fabrication and the mechanism of change in application. These results includenot only Baosteels achievements, but also the achievements made by some othercolleges and institutes. However, these articles only cover a small part of BaosteelsCGDS research work. I would like to give my thanks to the editors and writers fortheir outstanding work, efforts and cooperation, which makes it possible to showtechnological innovation, provide reference to researchers and accelerate the CGDsdevelopment中国煤化工CNMHGBaosteel Technical ResearchVolume 5, Number I, March 2011, Page 7Numerical simulation of particle tracks in the cold gas dynamic spraying processZHANG Yujun, LIANG Yongli and ZHANG JunbaoAdvanced Technology Division, Research Institute, Baoshan Iron Steel Co, Ltd, Shanghai 201900, ChinaAbstract: In this study, the distribution behavior of the particle flow field in cold gas dynamic spraying( CGDs )wassimulated through the computational fluid dynamics( CFD)method. Traces of the particles with different diameters in thegas flow field were analyzed, and effects of flat and sphere substrates on the particle tracks were also compared Simulationresults indicate that different escaping directions of particles flow with the two substrates. These investigations gaveinstructions on how to design the powder recovery and dusting machines in a CGDS systenKey words: cold gas dynamic spraying( CGDS); particle tracks; numerical simulationdoi:10.3969/ J. Issn.1674-3458.2011.01.002typical nozzle, both the mass loading rate and volume1 Introductionloading rate are very low; meanwhile, the averagedistance among the particles is so large that theCold gas dynamic spraying( CGDS )has a broad particles can be treated as isolated ones in simulationapplication prospect. It is used to repair the continuous That means that collisions among the particles can becasting mold and to prepare the copper coating atignored in computation. With this assumption, EulerBaosteel In CGDS, most particles are deposited on the Lagrange model for DPM was adopted in this studysubstrate, while a few escape into the ambient space. For The gas phase was treated as a continuous medium andthe protection of the environment, humans and devicesthe Navier Stokes equation was set and solved directlythe powder recovery and dusting machine usually needs he track of the particles as the solid phase was able toto be designed and amounted after the grasp of the track be solved independently.2)of the powder after impacting the substrate in the CGDssystem. Furthermore, powder recovery will benefit the2.2 Basic assumptions of the two-phase flow inengineering of the surface nano-crystallization SNC)Therefore. the tracks of thecles(1) Nitrogen as the camier was treated as the ideimpacting different substrates must be researchedgas and it obeys adiabatic flow equations.The key technology of CGDs is the nozzle design2)Diameters and densities of the particles wereParticles obtain a high kinetic energy after their speedsconsidered as uniform, their shapes as spherical andare accelerated through the nozzle from the subsonictheir surfaces as smooth(3)The motion of the particles had no impact onsubstrate, some of the particles arent deposited on thethe gas flow fieldsubstrate will impact twice or more times before their(4) There were no collisions among the particlesescape, because of the influence of the drag force and i. e. the interactions among the particles were ignoredthe gas flow field There is no literature discussing this(5)Only the steady-state aerodynamic drag forcewas taken into account. Other forces. like Saffman liftThe essence of the CGDs process is a particle-gasforce, Basset historical force and gravity force, weretWo-phase flow. Besides, the properties of the gas ignoredturbulent flow field, the interaction of the particle-gas(6) The frictional force between the particles andflow field should be taken into account for the volumethe wall was not consideredfraction of particle to gas is far less than 10%. A discrete 2.3 Geometric modeling and meshingphase model DPM) in the computational fluidThe geometric model of the CGDs nozzle wasdynamics(CFD ) software is selected in this study to established as shown in Figsimulate the track of the particles in the gas flow fieldThe flat substrate andboth inside and outside a typical CGDS nozzlesphere substrate were selected, respectively in simulationwhere the roller treated with CGDs was simplified as aflat substrate, because its diameter in real production is2 Computational methodnormally within the size from 600 mm to 1800 mmwhich is much中国煤化工nozk2.1 Selection of a discrete phase model(o7 mm). In thiThrough calculation, it is known that around a only theCNMHGelucidatee inlierthe gas flowCorrespondingauthor:ZHANGYujun;E-mail:zhangyujun@baosteel.comBaosteel Technical Research, Vol 5. No. I. Mar. 2011field and particle tracks, so that the simplified spherePressure- inlet boundary is used. The pressure is set toshaped substrate with two-dimensional axis-symmetric 2.3 MPa and temperature is set to 753 K. Themodel was adopted instead of a cylinder-shaped substrate turbulence intensity is set to 1% and the hydrauliewith a three-dimensional model. For discretization, diameter is set according to the size of the nozzle inletquadrilateral meshes were used and their number was(2)Pressure-outletwithin 10 for every computational examplePressure-outlet boundary condition is used at thenozzle exit. The pressure is set to 101325 Pa and theFirst convergent sectiortemperature is set to 300 KFlow field outside the nozzl(3)WallThe standard wall function is used in computation(4)Initial condition for particlesCopper is selected as the material of solid particlesThe diameters of most copper particles are betweenCarrier gas(nitrogen) inletI um and 50 um. The sizes of the particles, such asNozzle outletand rosin-ramler distributionetc, are selected in simulation. The initial flowPowder injectordirections of the particles are in parallel to the nozzleFig. 1 Computational domain and geometric modeling axisinside and outside the nozzle(5)Mass flow inlet2. 4 Governing equationsMass flow inlet boundary is applied in simulationIn simulation, goverming equations of the steadycompressible flow in the Cartesian coordinate system 3 Computational results and analysiswere applied, which include the continuity equationmomentum equation, energy equation, Standard kAs shown in Fig. 2, the CFD computation demonturbulence equation and particle motion equationsstrated that strong wall-attachment effect existed in thegas flow field. That means, the gas flow scattered along2.5 Initial and boundary conditionsthe direction close to the substrate surface, after the(1)Pressure-inletimpact of the carrier gas against the flat substrateGas flow3(c)2 um(d)3 um中国煤化工CNMHGZHANG Yujun, ef al. Numerical simulation of particle tracks in the cold gas dynamic spraying process(g)6 um(h)7 um15m()15μmRosin-rammlerFig 2 Analysis of the particle tracks with different particle diameters and fat substrateFig 2(b)indicates that those particles with a to escape. Some particles escaped into the ambientdiameter of I um impacted the flat substrate at a high space directly after they impacted the substrate threespeed following a rebound from the substrate to a small times. Others returned to the substrate surface afterheight and then impacted the substrate again with a their second impact on the substrate and escapedreduced velocity Based on the assumption of elastic approximately along the parallel direction of thecollision, the particle velocity attenuated continuously substrate surface, because theirwere so lowafter repeated collision till they were reduced to a (<30 m/s) that the motion ofarticles wasspeed lower than 200 m/s, which was small enough for controlled from the gas flowparticle escaping close to the substrate surface, as Fig. 2(1)indicates the motion of the particles withhown in the enlarged picture of Fig. 2 (b)a rosin-rammler distribution, its minimum diameter isWith the increase of particle diameters from I um to 1 um, the maximum diameter 50 um and the average2-8 um, due to the augment of mass and inertia, the diameter 22 um. Particles with small diametersvelocities of the particles at their first impact decreased(<8 um)escaped along the parallel direction of theand their spring back height increased accordingly. substrate surface while large particles(> 15 um)Although the wall-attachment effect still existed, yet escaped basically along the direction, the angle betweenthe width of the particle tracks increased with the which and the substrate was larger than 45. This resultamplification of the particle diameter. Particles flow is will help to design a system of powder recovery andescaped with the gas flow directly and didpact screening in the CGDs processthe substrate any more as their speed wasAbove were the results with a flat substrate. For30mpherical substrates, as shown in Fig 3, it also showsThose particles with a diameter of 15 um had notable wall-attachment effects in the gas flow fieldimpacted the substrate three times before they escapedSimilar to thV凵中国煤化工1-lat substrateinto the ambient space approximately along the normalparticles of Idirection of the substrate. At that moment, the wall- substrate withCN MH Gded accordingattachment effect no longer existed. However, those to the rule of elastic collision from the substrate withinparticles with 22 um diameter had different directions a very small height influenced by the gas flow field10Baosteel Technical Research, Vol 5. No I, Mar. 2011time at a reduced speed, before they rebounded again surface no more and passed the spherical substrateThen the particles impacted the substrate at the second substrateith the gas flow after two times offrom the substrate with further reduced velocities. bombardAfter the particles came across theBecause the inertia of 1 um particles at speeds lower spherical surface, they escaped along the parallelthan 200 ms was so small that they impacted the direction to the nozzle axisGas flow(a Gas flow2(e)4m(f5 um7(g)6μm(h)7μm中国煤化工CNMHGZHANG Yujun, et al. Numerical simulation of particle tracks in the cold gas dynamic spraying processRosin-rammler(k)22 um(1 Rosin-rammlerFig 3 Analysis of the particle tracks with different particle diameters and spherical substratesWith the increase of particle diameters from I um to(2)For spherical substrates, escaping directions of2-5 um, due to the augment of mass and inertia, the the particle flows with small diameters(<7 um)werefirst impact velocities of particles decreased and their almost in parallel to the axis of the nozzle, while thosespring back height increased accordingly. Although the of the particle flows with large diameters(>8 Hm)wall-attachment effect was still observed, the width of are approximately normal to the substrate. Escapingparticle track increased with the amplification of directions were mainly along the forward and backwardparticle diameters. Particles flow may escape with the directions of the nozzle axisgas flow directly and impact the substrate no more as(3)For both flat and spherical substrates, with thetheir speed was lower than 30m/sincrease of particle diameters from l um to 7 um, thedirectly along the normal direction of the substrate after their spring back height increa decreased andFor 6-7 Am particles flow, some particles escaped first impact velocities of the particlesthe second bombardment on the spherical substrate, Although the wall-attachment effect still existed, yetwhile most escaped still along the horizontal direction the width of the particle tracks increased also with thewith the gas flow fieldincrease of the particle diameters. Particles may escapeescaped along articles flow, almost all the particles with the gas flow directly and not impact the substratehe normal direction of the spherical any more as their speed was lower thanms. Insurface after their second impact on the substrate. Aladdition, as particle diameters were larger than 15the moment the wall-attachment effect did not exist. In the wall-attachment effect did not exist and under thisaddition, for 22 Hm particles flow, some of these condition, the gas flow field had little influence on theparticles escaped along the tangential direction of the final particle tracesspherical surface and the rest of them escaped along theResults mentioned above may give instruction onnormal direction after their third bombardment on the how to design the powder recovery device and dustingsubstrate due to their relatively high mass and inertia. machines in a CGDs systenParticles with the diameter of rosin-rammler distribution were also considered in simulation. It shows Referencesfrom Fig. 3 that the final runaway track of smallparticles(< 7 um)was approximately along the[1 Karami M, Fartaj A, Rankin G, ef aL. Numericalsimulation of the cold gas dynamic spray process[J]parallel direction to the nozzle axis. The final escapeJournal of Thermal Spray Technology, 2006, 15 (4): 518-track of large particles(>8 um) was approximatelalong the normal direction of the spherical surface. This [2] Li Wenya, Liao Hanlin, Wang Hongtao, et aL. Optimalresult gave directions on how to design the powderdesign of a convergent-barrel cold spray nozzle byrecovery and dusting machine.numerical method J]. Applied Surface Science, 2006n general, for spherical substrates, the escaping253:708-713direction was mainly along the axial forward and [3] Jen Tien Chien, Li Longjian, Cui Wenzhi, et al.Numericalinvestigations on cold gas dynamic spray process withbackward directions of the nozzlenano and microsize particles[J]. Intermational Journal ofHeat and Mass Transfer, 2005, 48: 4384-4396ConclusionsFor particles whose initial flow directions are inparallel to the nozzle axis, following conclusions weredrawn through the computational simulation of flowfield characteristics in the CGDs nozzles(1)For flat substrates, escaping directions ofparticle flows with small diameters (<8 Hm) were中国煤化almost in parallel to the substrate, while those ofCNMHparticle flows with large diameters(>15 um) formedZHANG YuiunLIANCIongZHANG JunbaoSubstrate