Spattering is an unavoidable phenomenon in the selective laser melting (SLM) process, which can cause various printing defects and harmful powder recycling. Since the size of powder spattering is too small at the micron level, it is difficult to investigate the entire dynamic spattering process experimentally. The comprehensive understanding of the intricate dynamics of powder spattering during the SLM process remains incomplete. Therefore, we develop a new multiphase flow model to study the transient dynamic behaviors of the gas phase and powder spattering, which agrees well with the experimental observation result. It is the first time that the whole transient dynamic process of powder motion from starting to move induced by the vapor jet to falling to the substrate wall and stopping completely was observed. Powder spattering motion dynamics induced by metal vapor jet and argon gas flow, as a function of time, laser parameters, and location, are presented. The moving speed, total amount, and dropping distribution on the substrate of powder spattering that varies with laser parameters are quantified.

Knowledge of thermal interactions between the substrate and deposited particles during cold spraying can shed light on coating formation and bonding mechanisms. In this study, a mathematical model based on the differential quadrature method was used to solve the hyperbolic heat conduction problem to predict the transient thermal evolution associated with the impact of a single particle. In addition, a 2D finite element model was developed to simulate the thermal and dynamic behavior of particle impact. The two models showed good agreement in predicting the maximum temperature at the particle-substrate interface. It was concluded that the proposed mathematical model could be used to predict the transient temperature of metallic and nonmetallic particle-substrate interfaces during cold spray deposition.

A typical feature of cold spray process is that a deposit can be formed without change of the original structure and compositions of spray materials. Only particles which reach the velocity higher than the critical velocity can be deposited on a substrate in cold spraying. When the spray particle impacts on the substrate at an off-normal angle, the normal component of particle impact velocity will change with the approaching angle of spray particle to substrate. In the present study, copper and titanium powders are used to deposit coating using cold spray process at different impact angles with regard to substrate. The deposition characteristics of spray materials are examined. The results show that the impact angle has a significant influence on the deposition characteristics. The relative deposition efficiency changes with the spray angle. It has been found that there is a critical impact angle at certain particle conditions below which no deposition occurs. The relation between spray angle and relative deposition efficiency can be divided into three spray angle ranges: maximum deposition angle range, transient angle range and no deposition angle range. In the transient angle range, the relative deposition efficiency increases with an increase in spray angle from zero at the critical spray angle to 100%. The transient angle range depends on the particle velocity distribution. A model is proposed to explain the relation between the spray angle and the relative deposition efficiency.

Cold Spraying, as a novel Thermal Spraying Technology can deposit a wide range of ductile materials on different substrates; a problem arises when the objective is to use this technology to deposit brittle materials such as ceramics in applications where melting must be avoided. One example is when spraying Bio-Ceramics, in which particle´s melting often leads to changes in physical and chemical properties and subsequently resulting in unknown variations in the coating´s biocompatibility. While Non-Ductile Materials can only be deposited if they are co-cold-sprayed with a ductile (matrix) material a Metal-Ceramic Powder Composite must be used for these objectives. The goal of this paper is to show a transient Non-Linear Dynamics Computational Analysis of a Metal- Ceramic Powders Composite impacting a metallic substrate by the means of Cold Spraying Technology; all this in order to understand the formation of cold sprayed composite coatings while comparing them with experimental results and using ANSYS-AUTODYN© to assess the suitability of this spraying technology for coating special application substrates.

Three-dimensional, transient simulations of the plasma flow inside different plasma spray torches have been performed using a local thermodynamic equilibrium model solved by a multiscale finite element method. The model describes the dynamics of the arc without any further assumption on the reattachment process except for the use of an artificially high electrical conductivity near the electrodes. Simulations of an F4-MB torch from Sulzer-Metco and two configurations of the SG-100 torch from Praxair are presented. The simulations show that, when straight or swirl injection is used, the arc is dragged by the flow and then jumps to form a new attachment, preferably at the opposite side of the original attachment, as has been observed experimentally. Although the predicted reattachment frequencies are at present higher than the experimental ones, the model is suitable as a design tool.

This paper presents a simulation of the simultaneous spraying of a metal and a ceramic powder with different configurations for the injection of the powder into the plasma jet. The plasma jet and the behavior of the injected particles were modeled with a commercially available computational model of the dynamics of liquid bodies. The particles are modeled as discrete Lagrangian objects. Three series of numerical tests were carried out: simultaneous spraying of the powder in a three-dimensional plasma jet in a stable state; simulation of the 3-D plasma flow, assuming that it fluctuates at the same frequency as the arc voltage; and simulation of the effect of the current fluctuation on particle behavior. A pre-calculation with an analytical model made it possible to determine the suitable gas flow rate so that the "average" trajectories of the metal or ceramic powders coincide at the same point on the surface. Paper includes a German-language abstract.

This paper is devoted to the investigation of the transient pressure occurring at the impact of a molten particle onto a flat substrate surface under different thermal spray conditions. In this paper, the mathematical model developed is based on the following assumptions: laminar, viscous and incompressible fluid; the mixed velocity-pressure model is employed to construct the finite element model. The choice of pressure penalty coefficient, which depends on the Reynolds number, is crucial for the results. This is discussed in this paper. The numerical results show that the pressure at the particle-substrate interface is strongly affected by the processing parameters, especially by the particle density and the impact velocity. The influence of spray parameters and material properties are also discussed.

When spraying ceramic particles with a low thermal conductivity such as zirconia using Ar-H 2 direct-current (d.c.) plasma jets where the heat transfer is important, heat propagation phenomena take place with the propagation of melting, evaporation or even solidification fronts. Most models neglect these heat propagation phenomena assuming the particle as a lumped media. This work is aimed at developing a model coupling the effect of heat propagation with the particle dynamic within plasma jets. It uses an adaptative grid in which the coordinates of the phase change fronts are fixed. It allows minimizing the calculation costs (approximately 10 seconds on PC under windows XP against 1hour with an enthalpy model). Such calculations are illustrated for dense and porous agglomerated zirconia as well as iron particles which evaporation in an Ar- H 2 (25 vol %) plasma is important.

Since cold spray is widely considered as an additive manufacturing and damage repair technology, it is crucial to understand the coating build-up process and the temperature evolution. In this work, a 3D numerical model was developed to simulate the transient coating build-up process as well as the heat transfer in cold spray. By coupling the heat transfer with the ALE (Arbitrary Lagrangian–Eulerian) moving mesh and coating thickness model, this 3D model is able to investigate the temperature evolution of a coating which simultaneously grows according to the nozzle trajectory. The nozzle trajectory that represents the heat source and mass flux of particle impact is generated and simulated in the offline programming software RobotStudio. By assigning the results of coating thickness distribution, the simultaneous build-up of coating computational domain is achieved by ALE moving mesh method. The validation of the FEA (finite element analysis) model was carried out by measuring the coating surface temperature via an infrared imaging camera. With the proposed model, it is able to study the actual coating build-up process as well as the heat transfer phenomena, which may provide more insights for the application in additive manufacturing and damage repair.

This paper describes the development of a numerical model and explains how it is used to investigate arc-cathode interactions in a plasma arc torch. The model is based on magnetohydrodynamic (MHD) theory and couples Navier-Stokes equations for a nonisothermal fluid with Maxwell’s equations for electromagnetic fields. The equations account for the internal geometry of the torch as well as arc current and gas type and flow rate. They are solved using CFD code and relevant boundary conditions and are shown to provide insight on arc dynamics and the effect of cathode shape on arc behavior.

In this study, the temperature distribution of the surfaces of several substrates under an impinging gas jet from a cold spray nozzle was determined. A low-pressure cold-gas dynamic spraying unit was used to generate a jet of hot compressed nitrogen that impinged upon flat substrates. Computer codes based on a finite differences method were used to solve a simplified 2-D temperature distribution equation for the substrate to produce non-dimensional relationships between the surface temperature and the radius of the impinging fluid jet, the substrate thickness, and the heating time. It was found that a single profile of the transient non-dimensional maximum surface temperature could be used to estimate the dimensional maximum surface temperature, regardless of the value of the compressed gas temperature. It was found further that as the thermal conductance of the substrate increased, the maximum surface temperature of the substrate beneath the gas jet decreased. The close agreement of the numerical results with the experimental results suggests that the non-dimensionalized results may be applied to a wide range of conditions and materials.

The study of heat transfers in a substrate exposed to an impinging plasma jet is proposed using two different software products. Thermal exchanges between the plasma jet and the substrate were first calculated using the PHOENICS CFD software in which a two-layer extension to the Chen-Kim k-s model was implemented in order to consider both the turbulent nature of the plasma jet and heat transfer phenomena through the viscous sub-layer formed at the surface of the substrate. The model is supposed to provide accurate predictions of thermal exchanges. However this preliminary step is not described since it is part of some previous studies. In a second step, two different commercial software products are used to perform three dimensional transient calculations of the heat conduction inside the substrate. The first approach consists in the use of the finite element based SYSWELD software whereas the second one consists in the use of the finite volume based PHOENICS software. Numerical results are presented and compared for the case of an impinging plasma jet displacing linearly on the substrate. Additionally, the influence of different parameters such as the substrate sample thickness, the stand-off distance, the displacement velocity or the nature of the substrate is also discussed. The results show a good accordance between numerical predictions obtained using the two methods concerning the maximum temperature observed. These results are useful since the substrate temperature is known to have an important influence on the coating adhesion and properties.

The aim of this paper is to develop a model that describes the heating of porous aluminum oxide particles in the plasma jet of the APS process. The model is then used to analyze sintering effects and thermal gradients in particles of varying porosity in-flight. Paper includes a German-language abstract.

This work deals with a 3-D transient simulation of the air plasma spraying of ceramic powders using a C.F.D. commercial code ESTET v3.4 that has been adapted to thermal plasma conditions. The mathematical model computes the distribution of particle velocity, temperature, molten state and size at impact and predicts the heat transfer to the substrate by plasma jet and particles. It incorporates the conversion from electrical to thermal energy in the torch nozzle as well as coating formation on the substrate. It makes it possible to predict the shape of the coating footprint when the torch and the substrate are fixed. The projections of the model are compared with experimental results that involve flow characteristics, time-dependant particle behavior in the flow and heat flux to the substrate.

The present work deals with numerical simulations based on a computational heat transfer model for spherical composite particles typically used under plasma conditions. Results describe heat transfer in mono and two layers steel/alumina particles immersed in an uniform infinite plasma. Time dependent behaviours interacting with phase change occurrence and taking into account the contact quality between the two layers are considered.

Plasma spraying using liquid feedstock makes it possible to produce thin coatings (< 100 µm) with more refined microstructures than in conventional plasma spraying. However, the low density of the feedstock droplets makes them very sensitive to the instantaneous characteristics of the fluctuating plasma jet at the location where they are injected. In this study the interactions between the fluctuating plasma jet and droplets are explored by using numerical simulations. The computations are based on a three-dimensional and time-dependent model of the plasma jet that couples the dynamic behavior of the arc inside the torch and the plasma jet issuing from the plasma torch. The turbulence that develops in the jet flow issuing in air is modeled by a Large Eddy Simulation model that computes the largest structures of the flow which carry most of the energy and momentum.

In this study, three kinds of engineering metals, which are aluminum (1100-H12), commercially pure titanium and mild steel were combined as particle/substrate and classified into four cases, i.e., soft/soft, hard/hard, hard/soft and soft/hard, according to their physical and mechanical properties respectively. Based on finite element modeling, impacting interface elements of four cases were analyzed and impact behaviors were numerically characterized. For soft/soft and hard/hard cases, the maximum temperature at the substrate side, which approached melting point, is higher than that of particle side when the shear instabilities occur. In particular, the different size of thermal boost-up zone was numerically estimated and theoretically discussed for these two cases. Meanwhile, for soft/hard and hard/soft cases, the specific aspect of shear instability, which has very high heat-up rate, was always observed at the relatively soft impact counterpart, and a thin molten layer was expected as well. Thus, the successful bonding of the above mentioned four cases can be predicted as a result of the synergistic effect of localized shear instability with interfacial melting.

It has been well accepted that thermal sprayings present a typical lamellar structure with limited lamellar interface bonding. It has been a great challenge to deposit a fully dense coating with fully bonded lamellae. In this report, three different novel approaches are introduced to deposit fully dense ceramic coatings and metal alloy coatings. With the deposition of a specific ceramic coating, it was found that there exists an intrinsic bonding temperature corresponding to the glass transient temperature of spray material. A chemical bonding is formed at the interface upon splatting of a molten ceramic droplet, as far as the maximum interface temperature between the spreading splat and the solid splat reaches over the intrinsic bonding temperature. Moreover, it will be presented that a simple critical bonding temperature in a linear relation with the melting point of coating materials can be utilized to deposit fully dense ceramic coatings by controlling the deposition temperature. Furthermore, with metal alloy coatings, a self-bonding mechanism is proposed utilizing the ultrahigh temperature molten droplet for dense coating with fully bonded lamellae. Using specially designed core-shell structured powders, the investigators demonstrated that a bulk-like metal coating is deposited by creating ultra-high temperature molten droplet. It will be found that such coatings present excellent properties and performance comparable to bulk materials. Moreover, it will be shown that, for ductile metal alloys, the solution-impermeable dense metal coatings can be deposited by using the novel in-situ shot-peening assisted cold spraying.

A three-dimensional transient heat conduction model was developed to evaluate analytically the surface temperature profile of substrates that were exposed to the impingement of a moving time-dependent heat source such as a cold spray nozzle or thermal spray torch. The estimated surface temperature profile was compared to the experimental results of the surface temperature of an aluminium substrate under impingement of a moving air jet generated by a low-pressure cold spray system. Close agreement between the analytical model and the experimental data was found. It was also found that the velocity of the moving heat source significantly affected both the profile and maximum temperature of the substrate. A non-dimensional characteristic velocity was defined and considered to take into account the effect of the velocity of the moving heat source, to broaden the application of the model to a wide range of materials and conditions. It was found that as the non-dimensional characteristic velocity increased, the thermal energy that was conducted into the substrate decreased. It was concluded that, with knowledge of the characteristic velocity, the analytical model was capable of predicting the spatial and temporal surface temperature of the substrate exposed to a moving heat source produced by spray nozzles and torches.

The temperature distribution of glass fiber-reinforced epoxy flat plates coated with a thin oxy-acetylene flame-sprayed aluminum-12silicon coating was determined experimentally. The composite plates were fabricated by filament winding. Following winding, but prior to and during curing, garnet sand was uniformly distributed on the glass fiber-reinforced epoxy plate surface. The sand roughened the surface such that there was adhesion of the aluminum-12silicon particles to the surface. A resistive heating wire was attached to the coated surface. Thermocouples were attached to the composite and coating surfaces to measure transient and spatial surface temperature distributions. The spatial temperature of the coating and polymer surfaces decayed uniformly throughout the coating-composite ensemble from the heating wire. It was also observed that the coating served to increase the surface temperature of the coating-polymer system compared to uncoated samples. This was attributed to the large thermal conductivity of the metal coating and the low thickness of the samples.