A Non-thermal Approach to Additive Manufacturing

Transcription

1 A Non-thermal Approach to Additive Manufacturing Rahul Kumar PG Student, Department of Mechanical Engineering Manipal University Jaipur Jaipur, India Ashish Goyal Assistant Professor, Department of Mechanical Engineering Manipal University Jaipur Jaipur, India Abstract Conventional additive manufacturing techniques (such as, SLS and DMD) are thermal (laser) based. They utilize heat energy to melt the material to be deposited. The high temperature processing employed in the course has certain pernicious effects over the deposit material in the form of poor mechanical and metallurgical properties. Moreover, processing of low melting point, speedily oxidized and materials with different coefficient of thermal expansion is challenging with present additive manufacturing methods. Non-thermal additive manufacturing (NTAM) is a new material deposition technique under the canopy of additive manufacturing. NTAM phenomena is based on high velocity (supersonic) particle/substrate impingement. In the process powder particle cum carrier gas (helium, nitrogen or argon) are accelerated to supersonic speeds in specifically fabricated CD de-laval nozzle and impinged on substrate causing material adherence layer upon layer. This paper presents the state of art of the process and various scientific and technological aspects to NTAM such as nozzle geometry, powder preheating, powder injection and track geometry. Main trend applications to the process are also discussed. Keywords additive manufacturing, non-thermal additive manufacturing, solid-state additive manufacturing I. INTRODUCTION Product manufacturing sector is facing two important challenging phases: (1) substantial reduction of product development time; and (2) improvement on flexibility for manufacturing complex shapes [1]. The on-trend manufacturing technique which is subtractive in nature and removes material to form a desired shape is limited to not very complex shape products. Further, pre and post manufacturing processes carried on the part makes the manufacturing time taking. In today s exponentially increasing industrial demand, additive manufacturing technologies have already gained the pace in optimal man, material and time driven manufacturing. Additive manufacturing (AM) is a highly competitive and escalating field under the umbrella of manufacturing processes. AM techniques employ addition of material, layer by layer from a generated 3D CAD model of the part to develop the final product. Moreover, AM techniques being less time taking and very much capable of producing the desired shape irrespective of its complexity are highly researched [2]. Notation A nozzle cross sectional area C D drag coefficient m mass mass flow rate P pressure R gas constant T temperature V velocity x axial distance from the nozzle throat M Mach number specific heat ratio density Subscripts e nozzleexit quantity s shock related quantity o stagnation related quantity p particle quantity Superscripts * nozzle throat quantity Present AM techniques such as SLS (selective laser sintering), DMD (direct metal deposition) and few more are laser based i.e. requires thermal energy to deposit the material onto the substrate. The encountered high temperature processing (thermal energy) during the process has certain detrimental effects in the form of poor mechanical and metallurgical properties over the final product [1]. Phase change, deposit oxidation and induced thermal residual stresses are few

2 undesirable phenomena in conventional AM techniques. Moreover, materials in low melting point range (Zn, Al), speedily oxidized (Ti) and with different coefficient of thermal expansion (dissimilar materials) are difficult to deposit and limits the thermal based AM techniques [1], [3], [4]. Fig. 1below shows classification of thermal deposition process with respect to NTAM deposition in accordance with particle velocity and temperature processed. Fig. 1. Thermal deposition vs NTAM deposition Intensifying global research is being focused on to the development of a non-thermal additive manufacturing (NTAM), a solid-state approach to surmount the limitations associated with thermal processes [5], [6].Non-thermal additive manufacturing (NTAM) is a breakthrough in the field of additive manufacturing. Therefore, we undertook the feasibility study of NTAM process. The process is based on the phenomena of high velocity (supersonic) gasparticle/substrate impact using a converging-diverging de- Laval nozzle. In the process, small sized powder particles (preferably below 45 µm) are accelerated with the preheated carrier gas (helium, nitrogen or argon) to supersonic speeds in specifically fabricated CD de-laval nozzle and impinged on substrate causing massive plastic deformation resulting in material adherence layer upon layer. There are two types of NTAM process. The two processes differ in initial gas pressure and powder injection point. If the stagnation pressure of gas is above 20 bar and depositing material is injected axially it is designated as high pressure NTAM process and below 20 bar and material is injected radially is designated as low pressure NTAM process. It is also seen that not at any random velocity, the deposition occurs; moreover, there exists this minimum critical velocity of the powders at which deposition is evident [3]. The critical velocity is defined as the minimum velocity of the particle in order to attain plastic deformation and material adherence. It is always desirable to maximize the particle velocity to deposit a broad range of materials [5, 6]. The requisite exit particle and gas velocities are obtained by flowing high pressure and preheated gas in Converging Diverging (CD) nozzle. Therefore, the pressurized gas is preheated in order to achieve high flow velocity into the nozzle. Due to aerodynamic properties, inert gases are preferred for NTAM process however dry air or mixture of either may also be utilized as per economy. The parameters that affect particle velocity in supersonic NTAM deposition are nozzle geometry, type of carrier gas and powder properties. The profile of CD nozzle geometry is one of the prime aspect for successful deposition with NTAM process [1]. The work insight a detailed analysis of the effect of carrier gas (diatomic and monatomic), preheat temperatures, particle size of feed powder particles and geometry of CD nozzle. The optimal profile of convergingdiverging length of the CD nozzle for various exit velocities are estimated, which most of the earlier researches did not address. An analytical nozzle design assuming one dimensional isentropic two phase flow (suspended particlecarrier gas) is proposed with varying exit Mach number (M), nozzle expansion ratio and optimal nozzle diverging length. Fig. 2 below shows the process schematic. Fig. 2. Schematic of the NTAM process II. PARTICLE-SUBSTRATE BONDING PHENOMENA The physical nature of the forces responsible for the bonding mechanism in cold spray is not completely understood. It has been studied that two surfaces brought back together into contact, the whole process being performed under high vacuum, a cohesive bond may be formed with strength equal to that of the parent material. However, to achieve a strong bond between particles of metal powders under atmospheric conditions is an entirely different matter. Metal surfaces that

3 have been exposed to air are covered by a native oxide film and some other organic contaminants [7]. In order for strong, metallurgical bonds to form between two metal bodies, the metal lattices must be brought into intimate contact and the intervening layers removed. A common feature of NTAM impacts that results in formation of strong bonds is connected with intense localization of strain at the interface, called as adiabatic shear instability taking place at a high deformation rate in the near-surface zone of particle during high velocity impact with a substrate under certain conditions [7]. The initiation of adiabatic shear instability is usually described by thermal softening [7]. During work hardening, the distortion of grain structure and the generation and glide of dislocations occur. The rest of the plastic work, which can be as much as 90% of the total, is dissipated as heat. Here in adiabatic process heat generated by high velocity impact softens the material. At a certain point, thermal softening dominates over work hardening such that eventually stress falls with increasing strain [7]. As a result, the material becomes locally unstable and additional imposed strain tends to accumulate in a narrow band [7]. Consequently, an interfacial jet composed of the highly deformed material is formed. III. CRITICAL VELOCITY OF IMPACT The NTAM process is characterized by an impact critical velocity below which no particle adhesion to the substrate is possible. The particles start to stick on the surface when their impact velocity is higher than the critical velocity, because sufficient kinetic energy must be available to plastically deform the solid material. If particles impinge on the surface with a velocity lower than this velocity, they rebound on the surface. The critical velocity depends on spray material and the powder quality, particle and substrate temperature and particle size. Since the original powder exhibits a particle size range, the particle spray jet exhibits a velocity range and, at low gas pressure, only the finest and therefore lightest particles reach the critical velocity. When the gas velocity is increased, a larger number of particles reach this velocity and the deposition efficiency increases as the number of particles that rebound on the surface decreases. In NTAM, successful bonding of an impacting particle requires localized deformation and adiabatic shear instabilities, which occur at sufficiently high impact velocities, the so called critical velocity. Numerical analyses are able to indicate shear instabilities and thus provide a basis for the calculation of the critical velocity in terms of materials properties and process parameters. Assadi et al. have used numerical simulation to work out the effect of various material properties on the critical velocity in cold spraying. They summarized these effects into a simple expression for the critical velocity (here presented in SI units) as follows: Critical velocity Density of material Melting temperature of deposit material UTS Carrier gas inlet temperature The parameters defined in aforementioned equation can be used to estimate the influence of only small changes in material and process parameters on the critical velocity. IV. NOZZLE DESIGN The two processes occur when gas and powder particle mixture propel through converging diverging nozzle. These two process are conversion of carrier gas thermal energy into kinetic energy and the transfer of momentum and heat from the gas to the powder particles. In the following sections, model equations are introduced. The isentropic gas flow model is considered based on typical geometry of a CD de- Laval nozzle. Secondly, the particle flow model is considering for different nozzle shapes which results in optimum particle acceleration. A. Isentropic Gas Flow Model The resulting particle velocity in NTAM is limited by the gas velocity. Using higher gas pressure and gas flow, long diverging nozzle and smaller particle size results in attaining particle velocity comparable with gas velocity; further, the gas velocity could be increased by lower molecular weight gases, higher gas temperatures and large expansion ratio nozzles. Nevertheless, the limitations exist and it is desirable to design a CD nozzle which produces adequate particle velocity. The model considers the geometry of a simple CD de- Laval nozzle, shown schematically in Fig. 2. with following assumptions: i The gas flow is one- dimensional. ii The gas flow is isentropic (adiabatic and frictionless). iii The gas is assumed to be perfect gas with constant specific heats. The one- dimensional flow ignores the process gas flow boundary layer formation along the walls of the nozzle. Thus, the analytically calculated gas velocity through the model is higher than actual measured.

4 As stated, the total temperature of the carrier gas is higher than the ambient temperature in NTAM process [7], which is required to increase the exit gas velocity. The particle gets heated up as they encounter the flowing high temperature gas. However, as the kinetic energy of the gas is increased, the temperature decreases. Thus, the powder particles must get accelerated quickly in order to experience the higher gas temperature [7]. Previous studies and findings predicts that the gas-particle interaction shall be at the region of origin of gas expansion due to maximize the gain of heat transfer which results in net higher temperature of deposition particle at the impact [7]. The carrier gas flow is assumed to originate from a large chamber/reservoir where the initial gas velocity is zero. The pressure (stagnation pressure) P o, the temperature (stagnation temperature) T o and mass flow rate are user settable. As per basic dynamics and thermodynamics relations for the compressible fluid flow, the gas temperature T * (all quantities with * are throat or sonic conditions) at the nozzle throat area (smallest area in the nozzle) of the CD where the Mach number (the velocity of the gas divided by the local velocity of sound) is unity, is derived by [15]: And velocity at throat is calculated as: Where is the ratio of C p (constant pressure) and C v (constant volume) specific heats. for monoatomic gas is 1.66 and 1.4 for diatomic gases), R is the specific gas constant (ratios of universal gas constant and gas molecular weight). Eq. (2) enlightens the preference of monoatomic gas due to higher value of Universal constant and.both of these gas properties favors in raising gas velocity at throat. Helium is a preferred carrier gas than the gas with high molecular weight such as nitrogen, oxygen or air. Therefore, for the same inlet temperature monoatomic gases can achieve higher exit velocity. Using Equation 1 & 2 the gas density at throat is calculated as: (1) (2) (3) Where, is gas mass flow rate and is user settable and A * is the known/ user calculated cross-sectional area of the nozzle throat. Using the perfect gas law, the pressure at throat is estimated as: The stagnation pressure P o is calculated using the following isentropic relation: For maintaining the sonic conditions (M =1) at throat, the throat pressure must be above ambient pressure. After estimation of inlet and throat conditions of gas, nozzle exit conditions need are defined along the diverging section of the nozzle. It is assumed that the Mach number is specified and the following equation is used to calculate the exit area: Once the Mach number is known, the gas conditions at exit are obtained from following isentropic relations: (10) Eqs. (7)- (10) is used to determine P, T, and at nozzle exit, if the given cross-sectional area A e is substituted for A in Eq. (6). However, these values give the true conditions of the gas only if a normal shock doesn t occur in the nozzle (since, exit pressure calculated is less than the ambient pressure). Therefore, the shock pressure ( is calculated using below relation: (4) (5) (6) (7) (8) (9) (11)

5 If the shock pressure (P s) is equal to the ambient pressure, a shock occurs at the nozzle exit. If the pressure is less than the ambient pressure, a shock occurs somewhere inside the nozzle. For normal operating condition the exit pressure has to be less than the ambient pressure and shock pressure greater than the ambient pressure. As the gas leaves the nozzle, it slows down as the gas pressure tries to adjust to the ambient pressure. Nevertheless, because of short nozzle-exit, the gas deceleration is not such significant [16]. At the same time, the exit gas pressure is generally lower than the ambient pressure to maximize the exit gas/particle velocity [8], [16]. B. Particle Acceleration Model Dykhuizen and Smith [17] analyzed the interaction of the carrier gas with the powder particles under the two phase flow (gas and particles) and assumed that it is dilute enough so that above equations hold. It is the particle velocity that determine the efficiency of the process rather than the gas velocity. The acceleration of the particle velocity can be computed by following differential equation: (12) Where m pand A p are the average mass and the crosssectional area of the particles, respectively, C D is the drag coefficient, t the time and x the axial distance traveled by the particle measured from the nozzle throat (diverging length of the nozzle). From Eq. (12), it is noted that the ultimate deposition particle velocity is equal to the gas velocity. As per Eqs. (6), (8) and (9) it is stated that the gas velocity within the nozzle is the function of inlet temperature and the nozzle geometry only. Hence, the gas pressure does not affect gas velocity. But, the use of a CD de-laval nozzle does not guarantee for a supersonic flow in the diverging part of the nozzle; for it to occur, the pressure acting on the gas must be sufficient enough for it to reach Mach 1 at the throat, calling it to be chocked flow to allow the particle to approach the gas velocity in the short distance on the diverging section of the nozzle (initial drag on particle is linearly dependent on stagnation pressure only) [Eq. (9), (10) and (12)]. Eq. (12) is integrated under the conditions of constant gas velocity, density and the drag coefficient. (13) For the above equation may not be generally valid for both the gas density and drag coefficient varies over the entire length of the nozzle [7], [8]. For lower values of particles Eq. (13) can be simplified as: (14) The equation yields that the deposition particle velocity is proportional to square root of the distance travelled over the particle diameter. It also shows the essentiality of stagnation temperature and large gas density. V. AXIAL POWDER INJECTION VS. RADIAL POWDER INJECTION AND POWDER PREHEATING The powder injection in NTAM nozzle could be performed in the subsonic part (prechamber) or the supersonic part (divergent) of the nozzle. The main advantage of injection to the prechamber is the possibility of increasing the particle impact temperature due to the intensive heat exchange interaction with the working gas in the prechamber [19]. The main technological disadvantage of prechamber injection is the necessity to keep the stagnation pressure of the carrier gas flow higher than the static pressure of the working gas in the nozzle prechamber, therefore specially developed highpressure powder feeders have to be applied [8,9]. In some cases, the powder is injected in the supersonic zone of the nozzle [10]. Generally, it is done in order to take advantage of the low static pressure in the diverging nozzle section that allows powder feeding at significantly lower carrier gas pressure than in the case of prechamber injection. The main disadvantage of supersonic injection is the lower particle impact temperature due to the low intensity of gas particle heat exchange in the supersonic zone Fig. 3(a) represents the distribution of pressure along diverging path of nozzle; which shall facilitate to outline the location of powder feed. Huang et.al. has discussed the effect of powder feeding at different part of nozzle under DYMET 413 [18]. The powder feeding point cannot be too close to the throat because the syphonage effect will be weak enough. The powder feeding point cannot be too far from the throat because the length of the nozzle is not sufficient enough for required particle acceleration [18]. For an injection point at diverging section in the CD nozzle, the location strongly influences the pressure difference between the injection point and the ambient, subsequently the mass flow rate in the powder feeding pipe [18]. Usually, to get the maximum mass

6 flow rate for powder feeding, the pipe must be located at the furthermost negative pressure point in the nozzle[18]. Fig. 3. (a) Pressure distribution inside proposed nozzle; (b) Temperature distribution inside proposed nozzle; (c) Velocity distribution inside proposed nozzle; (d) Effect of gas type on particle velocity. As plotted in fig. 3; (a), (b) and (c) the pressure decreases through a converging diverging de-laval nozzle that results in an increase in gas velocity which is also accompanied by a higher velocity using helium as a carrier gas with same temperature and pressure as that of air. Helium bears a lower molecular weight; thus, a higher specific gas constant. And temperature decrease. The increase of kinetic energy is hence, has higher specific heat ratio than diatomic gases. accomplished at the expense of gas enthalpy, and therefore Using helium as carrier gas yields to a higher gas velocity in pressure and temperature with the increase of gas exit comparison to nitrogen or air at same temperature and velocity. Due to exponential decrease in gas temperature in the pressure. Since, helium is costlier than air or nitrogen; in some diverging section along the length of CD de-laval nozzle, the of the applications mixture of helium and nitrogen is used as a particles melting temperature are potentially lower than room carrier gas; Nitrogen being a diatomic gas increases the enthalpy of the carrier gas for greater heat transfer values, but temperature. This enlightens how the melting, oxidation and also decreases its velocity resulting in quality of deposition. other chemical reactions at the surface of the particles may be Therefore, a selection of the gas has significant affect in circumvented in their flight within the nozzle. It also explains NTAM process. why particle preheating may be beneficial as otherwise the particles may cool down to temperatures that could affect their VII. CONCLUSION ductility, thus ability to deform upon impact with the substrate. VI. EFFECT OF GAS TYPE ON PARTICLE ACCELERATION Though air is used for the analytical calculations for the present work, but brittle and hard materials are difficult to deposit using air or nitrogen gas, because of requisite high exit gas velocity. Fig. 3 (d) shows particles are accelerated to This work studies the feasibility of the NTAM, a solid state approach to AM.The analytical model equations presented could be used to calculate the gas dynamics and particle velocity in NTAM process. It also establishes a relationship between particle velocity and nature of particle; type of gas used. The analytical results state the successful deposition of particles under conditions considered. The particle velocity achieved is found to be sufficiently higher than the critical velocity. Pressure, temperature and velocity distribution inside

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