Ronald C. Dykhuizen

Gas dynamic principles of cold spray

This paper presents an analytical model of the cold-spray process. By assuming a one-dimensional isentropic flow and constant gas properties, analytical equations are solved to predict the spray particle velocities. The solutions demonstrate the interaction between the numerous geometric and material properties. The analytical results allow determination of an optimal design for a cold-spray nozzle. The spray particle velocity is determined to be a strong function of the gas properties, particle material density, and size. It is also shown that the system performance is sensitive to the nozzle length, but not sensitive to the nozzle shape. Thus, it is often possible to use one nozzle design for a variety of operational conditions. Many of the results obtained in this article are also directly applicable to other thermal spray processes.

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.

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.

An analysis of solute diffusion in rocks

The diffusion of unreactive solutes through a fine-grained rock (Culebra Dolomite) was studied experimentally and theoretically. The measured diffusive flux is less than half that predicted from independent knowledge of the porosity and reasonable estimates of tortuosity. This low measured flux led to a review of the relationship between solute diffusion and pore geometry in rocks and sediments. We therefore examined solute transport in hypothetical pore networks where the effect of pore geometry on the solute flux is directly calculable. A conventional interpretation of pore tortuosity, as a normalized length of diffusion through a pore, must be expanded for cases where pores intersect in networks. Some important variables affecting the tortuosity are: (i) the distribution of pore sizes, (ii) the distribution of pore lengths, (iii) the number of pores which intersect at a node, and (iv) the pore shape between nodes. Furthermore, in porous materials with a preferential distribution of pore sizes and orientation, tortuosity is a tensor. For the Culebra Dolomite, the wide range of pore sizes causes the diffusive flux to vary considerably from that predicted from conventional theory. Solute diffusion through the Culebra Dolomite greatly resembles diffusion through an isotropic network of pores, once this network is assigned pore sizes similar to the rock.

Transport of solutes through unsaturated fractured media

Abstract A numerical model is presented to represent the transport of solutes through a highly fractured, unsaturated, porous medium. To accomplish this, the solute is tracked separately in two flow systems, a matrix pore flow system and a fracture network, with interaction terms. Compatible hydraulic equations for such a dual system are also presented to enable solution of the solute transport. The hydraulic equations chosen use the equivalent porous media concept. These equations can also be applied to a saturated medium without modification; however, many of the transport terms will be negligible for such an application. A brief sample calculation illustrates the method.

Effect of chamber pressure on particle velocities in low pressure plasma spray deposition

Abstract A laser velocimeter has been used to measure spray particle velocities in a low pressure plasma spray system at chamber pressures ranging from 6.7 to 80 kPa (50 to 600 Torr). For Al2O3 spray powder with a mean diameter of 44 μm, peak particle velocities were of the order of 200 – 400 m s-1. The measured velocity distributions were strongly dependent upon spray chamber pressure, with the highest particle velocities at intermediate pressures of about 40 kPa (300 Torr). Particle velocities predicted with a simple analytical model are in reasonable agreement with experimental results close to the spray gun, where drag due to chamber gases can be neglected. This simple model also correctly predicts a particle velocity maximum at 45 kPa (340 Torr).

Investigations into the plasma spray process

Abstract We have previously described a simple analytical model which was developed to examine momentum and thermal transfer from the plasma jet to the particles in a low pressure plasma spray deposition process. This model correctly predicted the experimentally observed maximum in particle acceleration at intermediate chamber pressures. The calculated results were in good agreement with experimentally measured particle velocities. In the present study, this model has been used to investigate the effects of particle size and mass density on particle velocity. The model has also been used to explore the effect of variations in selected process parameters on the thermal response of the particles. The analytical and experimental results for particle velocity are again in good agreement, with substantially lower acceleration rates and lower peak velocities for larger, more massive particles. The results of the thermal calculations indicate that particle melting is influenced by many parameters. Better particle melting is achieved at higher chamber pressures. Detailed thermal data which would verify the thermal model are not yet available; however, the predictions of the thermal model appear to be in qualitative agreement with empirically developed spray conditions for good particle melting. The model indicates that difficulties in melting refractory materials at very low chamber pressures are related to decreased plasma temperatures and plasma densities at low chamber pressures.

Process-based quality for thermal spray via feedback control

Quality control of a thermal spray system manufacturing process is difficult due to the many input variables that need to be controlled. Great care must be taken to ensure that the process remains constant to obtain a consistent quality of the parts. Control is greatly complicated by the fact that measurement of particle velocities and temperatures is a noisy stochastic process. This article illustrates the application of quality control concepts to a wire flame spray process. A central feature of the real-time control system is an automatic feedback control scheme that provides fine adjustments to ensure that uncontrolled variations are accommodated. It is shown how the control vectors can be constructed from simple process maps to independently control particle velocity and temperature. This control scheme is shown to perform well in a real production environment. We also demonstrate that slight variations in the feed wire curvature can greatly influence the process. Finally, the geometry of the spray system and sensor must remain constant for the best reproducibility.

Load relaxation of helical extension springs in transient thermal environments

The load relaxation behavior of small Elgiloy helical extension springs has been evaluated by a combined experimental and modeling approach. Isothermal, continuous heating, and interrupted heating relaxation tests of a specific spring design were conducted. Spring constants also were measured and compared with predictions using common spring formulas. For the constant heating rate relaxation tests, it was found that the springs retained their strength to higher temperatures at higher heating rates. A model, which describes the relaxation behavior, was developed and calibrated with the isothermal load relaxation tests. The model incorporates both time-independent deformation mechanisms, such as thermal expansion and shear modulus changes, as well as time-dependent mechanisms such as primary and steady state creep. The model was shown to accurately predict the load relaxation behavior for the continuous heating tests, as well as for a complex stepwise heating thermal cycle. The model can be used to determine the relaxation behavior for any arbitrary thermal cycle. An extension of the model to other spring designs is discussed.