Progress in Powder Metallurgy

Abstract: A new mathematical simulation technique for physico-mechanical properties of multicomponent powder materials is proposed in this paper. The main advantage of the technique is that finite elements representing different components are placed into a common mesh and may exchange their properties. The input data are properties of components and specified value of porosity. The output data are properties of material after sintering. The technique allows us to investigate the influence of each component of a material on the properties and distribution of properties inside the sample. The comparative analysis of materials with different compositions is based on simulation results that are well concordant with the results of the laboratory experiments.

Abstract: Fine and coarse alumina-titania composite particles were overlaid by allowing the particles to be deposited for a short time without moving a plasmatron [spot spraying bead]. Both the deposition efficiency and maximum deposition rate were measured at the different plasma gas composition. Considering the normalized maximum deposition rate [(maximum deposition rate)x(deposition efficiency)-1], effects of particle size and plasma gas composition on the particle segregation within a cross-section of mass flux could be estimated. Also, particle melting state according to the position within a mass flux at the moment of impact could be also estimated through the investigations of microstructure and phase composition of the spot spraying bead.

Abstract: In order to enhance understandings of the coating formation in the plasma spraying, an empirical method known as the spot spraying bead formation, was designed. This study is focused on the applicability of the proposed empirical method to explain the microstructural inhomogeneity during the process. Based on the in-flight particle diagnostics using DPV-2000, two different plasma gas compositions were chosen to evaluate the plasma gas effect on the microstructural evolution of Al2O3-TiO2 composite feedstock. A blended NiCr-Cr2O3-Ag-Ca/BaF2 feedstock was sprayed in order to examine the effects of particle parameters on the particle trajectory and the coating microstructures. Through the empirical approach, impacting particle properties and particle trajectory depending on process parameters could be directly shown and coating microstructures could be deduced from the characteristics of the spot spraying bead.

Abstract: Correlations between in-flight particle, splat and coating microstructure of thermally sprayed Ni20Cr were investigated. Flame spray and arc spray systems were employed for spraying Ni20Cr powder and Ni20Cr wire, respectively. The results showed that the arc spray process produced a broader size distribution for both in-flight particles and splats compared to flame process. Flower-like splat morphology was obtained from the arc spray whereas a pancake-like splat was obtained by flame spray. Ni20Cr coating sprayed by arc process had a denser microstructure, lower porosity and better adhesion at the interface. This could be due to the higher temperature and velocity from the arc spray process enhancing the melting and adhering for coating formation.

Abstract: In Spray Forming, specific enthalpy is a key parameter in the deposition process as it influences the thermal condition of the impinging droplets as well as that of the deposit surface. An empirical model for the distribution of specific enthalpy in the spray cone was developed as an easy to handle alternative to numerical models with which the descriptive partial differential equations are solved numerically. The model results were compared with the experimental data to validate its applicability.

Abstract: Ti(Al,O)-Al2O3 composite powders were produced by high energy mechanical milling of a mixture of Al and TiO2 powders followed by combustion reaction. The powders were then thermally sprayed on H13 steel substrates. Microstructural examination was conducted on the composite powders and thermally sprayed coatings using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The performance of the coatings was evaluated in terms of microhardness and thermal fatigue. The thermally sprayed coatings showed fairly good performance in the preliminary thermal fatigue tests and did not display any wetting tendency to molten aluminum.

Abstract: Tungsten coatings with different interlayers onto the oxygen-free copper substrates were fabricated by atmosphere plasma spraying. The effects of different interlayers of NiCrAl, NiAl and W/Cu on bonding strength were studied. SEM, EDS and XRD were used to investigate the photographs and compositions of these coatings. The tungsten coatings with different initial particle sizes resulted in different microstructures. Oxidation was not detected in the tungsten coating, but in the interlayer, it was found by both XRD and EDS. The tungsten coating deposited directly onto the copper substrate presented higher bonding strength than those with different interlayers.

Abstract: Porosity in spray-formed materials is an important issue, but the formation of porosity is not completely understood. Many experimental results and some theoretical models have been presented in the past. Nevertheless, the prediction of porosity in a deposit is still not possible today. The paper will give some examples picked from literature, which show some general correlations between process parameters and porosity. These correlations can be helpful to form a basic understanding of the process. Finally it is necessary to know more about the conditions of the droplets and the deposit at the point of impingement. These impacting conditions have to be correlated to the porosity to improve the understanding of the process and to make a prediction possible. Determining the impact conditions is a challenge because usually they are not constant with time and some values are difficult to measure. Our experiments show a strong correlation between the surface temperature of the deposit and the porosity. For IN718 and U720 as-sprayed porosities below 1 vol.% were achieved if the deposit surface temperature is app. 1250 °C. The average impact angle weighted by the local particle mass flux is also an important parameter. The probability of low as-sprayed porosity is high if the average weighted impact angle is below 25° but decreases dramatically for higher impact angles.

Abstract: Cold spraying is a fairly new coating technique, which within the last decade attracted serious attention of research groups and spray companies. As compared to thermal spraying, the low process temperatures in cold spraying result in unique coating properties, which promise new applications. Since particles impact with high kinetic energy in the solid state, new concepts to describe coating formation are requested to enable the full potential of this new technology. The present contribution gives a brief review of current models concerning bonding, supplying a description of the most influential spray parameters and consequences for new developments. With respect to spray forming by cold cold spraying, microstructures and thick, further machineable structures are presented.