R. A. Neiser

Impact of high velocity cold spray particles

This article presents experimental data and a computational model of the cold spray solid particle impact process. Copper particles impacting onto a polished stainless steel substrate were examined in this study. The high velocity impact causes significant plastic deformation of both the particle and the substrate, but no melting was observed. The plastic deformation exposes clean surfaces that, under the high impact pressures, result in significant bond strengths between the particle and substrate. Experimental measurements of the splat and crater sizes compare well with the numerical calculations. It was shown that the crater depth is significant and increases with impact velocity. However, the splat diameter is much less sensitive to the impact velocity. It was also shown that the geometric lengths of the splat and crater scale linearly with the diameter of the impacting particle. The results presented will allow a better understanding of the bonding process during cold spray.

Particle Velocity and Deposition Efficiency in the Cold Spray Process

Copper powder was sprayed by the cold gas-dynamic method. In-flight particle velocities were measured with a laser two-focus system as a function of process parameters such as gas temperature, gas pressure, and powder feed rate. Mean particle velocities were uniform in a relatively large volume within the plume and agreed with theoretical predictions. The presence of a substrate was found to have no significant effect on in-flight particle velocities prior to impact. Cold-spray deposition efficiencies were measured on aluminum substrates as a function of particle velocity and incident angle of the plume. Deposition efficiencies of up to 95% were achieved. The critical velocity for deposition was determined to be about 640 m/s for the system studied.

In situ measurement of residual stresses and elastic moduli in thermal sprayed coatings. Part 2, processing effects on properties of Mo coatings.

Abstract Residual stresses and mechanical properties of plasma sprayed coatings play a critical role in their functionality and lifetime. These can be influenced by modification of spraying parameters. Among the most important parameters are the deposition temperature, particle temperature and velocity and deposition rate. Using the in situ thermo-elastic curvature method described in Part 1 of this paper, their effects on the stresses and moduli of plasma sprayed molybdenum were investigated. The results for quenching, thermal, and residual stresses as well as Young’s moduli are presented and discussed with respect to the spraying parameters and material properties. The most influential factors are determined and means for property modification using this technique are described. The development of the in situ approach along with the ability to measure elastic modulus and residual stresses enables capturing the quality and integrity of the deposit. This will allow for future downstream process control based on materials characteristics.

The effect of a simple annealing heat treatment on the mechanical properties of cold-sprayed aluminum

Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray-forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. This work investigates the mechanical properties of cold-sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately 1 cm thick were prepared using three different feedstock powders: Valimet H-10: Valimet H-20: and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed; and after a 300°C, 22h air anneal. The as-sprayed material showed high ultimate strength and low ductility, with <1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, with up to 10% elongation. The annealed samples exhibited mechanical properties that were similar to those of wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed materials. These results indicate good potential for cold spray as a bulkforming process.

Oxidation in wire HVOF-sprayed steel

It is widely held that most oxidation in thermally sprayed coatings occurs on the surface of the droplet after it has flattened. Evidence in this paper suggests that, for the conditions studied here, oxidation of the top surface of flattened droplets is not the dominant oxidation mechanism. In this study, a mild steel wire (AISI 1025) was sprayed using a high-velocity oxy-fuel (HVOF) torch onto copper and aluminum substrates. Ion milling and Auger spectroscopy were used to examine the distribution of oxides within individual splats. Conventional metallographic analysis was also used to study oxide distributions within coatings that were sprayed under the same conditions. An analytical model for oxidation of the exposed surface of a splat is presented. Based on literature data, the model assumes that diffusion of iron through a solid FeO layer is the rate limiting factor in forming the oxide on the top surface of a splat. An FeO layer only a few nanometers thick is predicted to form on the splat surface as it cools. However, experimental evidence shows that the oxide layers are typically 100× thicker than the predicted value. These thick oxide layers are not always observed on the top surface of a splat. Indeed, in some instances the oxide layer is on the bottom, and the metal is on the top. The observed oxide distributions are more consistently explained if most of the oxide forms before the droplets impact the substrate.

Development of process maps for plasma spray : case study for molybdenum.

Abstract A schematic representation referred to as “process maps” examines the role of process variables on the properties of plasma-sprayed coatings. Process maps have been developed for air plasma spraying of molybdenum. Experimental work was done to investigate the importance of such spray parameters as gun current, primary gas flow, auxiliary gas flow, and powder carrier gas flow. In-flight particle temperatures and velocities were measured and diameters estimated in various areas of the spray plume. Empirical models were developed relating the input parameters to the in-flight particle characteristics. Molybdenum splats and coatings were produced at three distinct process conditions identified from the first-order process map experiments. In addition, substrate surface temperature during deposition was treated as a variable. Within the tested range, modulus, hardness and thermal conductivity increases with particle velocity, while oxygen content and porosity decreases. Increasing substrate deposition temperature resulted in dramatic improvement in coating thermal conductivity and modulus, while simultaneously increasing coating oxide content. Indentation reveals improved fracture resistance for the coatings prepared at higher substrate temperature. Residual stress was significantly affected by substrate temperature, although not to a great extent by particle conditions within the investigated parameter range. Coatings prepared at high substrate temperature with high-energy particles suffered considerably less damage in a wear test. The mechanisms behind these changes are discussed within the context relational maps, which have been proposed.

Current Problems in Plasma Spray Processing

This article summarizes eight contributions from a thermal spray conference that was held in late 1991 at Brookhaven National Laboratory, Upton, Long Island, New York. Plasma spray processing is discussed in terms of plasma-particle interactions, deposit formation dynamics, thermal properties of thermal barrier coatings, mechanical properties of coatings, feedstock materials, porosity, manufacture of intermetallic coatings, and synchrotron X-ray microtomographic methods for thermal spray materials. Each section is intended to present a concise statement of a specific practical and/or scientific problem. It then describes current work that is being performed to investigate this area, and finally suggests areas of research that may be fertile for future activity.

Surface roughness of thermal spray coatings made with off-normal spray angles

The formation of a thermal spray coating using an off-normal direction angle for the spray has been analyzed to identify the causes of the large surface roughness of the coating. In the analysis, the string method was used for modeling the formation of the coating. The method uses a string of equally spaced node points to define the shape of the coating surface and to track the change in this shape as the thermal spray mass is deposited. The method allows for the calculation of arbitrary shapes for the coating surface that may be very complex. The model simulates the stochastic deposition of a large number of thermal spray droplets. Experiments were carried out to obtain the data used in the model for the mass flux distribution on the target surface. The data show that when the thermal spray mass impinges on the target surface a large fraction of it, called overspray, splashes off the target and is redeposited with a small direction angle. This component of the deposited mass results in a large coating roughness.

Computational fluid dynamics analysis of a wire-feed, high-velocity oxygen fuel (HVOF) thermal spray torch

The fluid and particle dynamics of a high-velocity oxygen fuel (HVOF) thermal spray torch are analyzed using computational and experimental techniques. Three-dimensional computational fluid dynamics (CFD) results are presented for a curved aircap used for coating interior surfaces such as engine cylinder bores. The device analyzed is similar to the Metco diamond jet rotating wire (DJRW) torch. The feed gases are injected through an axisymmetric nozzle into the curved aircap. Premixed propylene and oxygen are introduced from an annulus in the nozzle, while cooling air is injected between the nozzle and the interior wall of the aircap. The combustion process is modeled using a single-step, finite-rate chemistry model with a total of nine gas species which includes dissociation of combustion products. A continually fed steel wire passes through the center of the nozzle, and melting occurs at a conical tip near the exit of the aircap. Wire melting is simulated computationally by injecting liquid steel particles into the flow field near the tip of the wire. Experimental particle velocity measurements during wire feed were also taken using a laser two-focus (L2F) velocimeter system. Flow fields inside and outside the aircap are presented, and particle velocity predictions are compared with experimental measurements outside of the aircap.

Evaluation of plasma-sprayed tungsten for fusion reactors

Tungsten coatings are being considered for a variety of uses inside fusion reactors. Measurements of several properties of vacuum plasma- sprayed tungsten important for fusion energy applications are reported. Of vital concern is the thermal conductivity of the sprayed tungsten. Over the temperature range 25 to 1500 ‡, the thermal conductivity was approximately 60% of the value for high- purity tungsten. Of greater importance to reactor safety is the reactivity of tungsten with steam. It was found that the volatilization and reaction rates of plasma-sprayed tungsten from 800 to 1200 ‡ are similar to hot rolled tungsten. Several other useful properties have also been reported. The elastic constants of the sprayed tungsten were measured ultrasonically. The deposits were anisotropic; for example, the Young’s modulus measured in a direction parallel to the substrate was greater than that measured in the direction perpendicular to it, and their elastic moduli were approximately 30% lower than tabulated values for bulk tungsten. The average bulk density of the sprayed tungsten was approximately 17.4 g/cm , which is 90% of the theoretical density.