Correlations between electrode phenomena and coating properties in wire arc spraying

doi:10.1016/S0040-6090(98)00409-X

Thin Solid Films

Volume 316, Issues 1–2, 21 March 1998, Pages 169-173

https://doi.org/10.1016/S0040-6090(98)00409-X Get rights and content

Abstract

Electric arc spraying with dual wires is an economical coating process finding diverse applications. Electrode phenomena, such as oxidation and turbulence have strong effects on the droplet formation, therefore on the coating properties. Low turbulence intensity of an atomizing gas flow can be related to higher frequency and smaller amplitude of the arc voltage fluctuations. Nitrogen reduces oxidation and turbulence at the electrode region. A converging–diverging nozzle provides higher gas velocity with less turbulence. These operations lead to coatings with lower oxidation and lower porosity.

Introduction

Electric arc spraying with dual wires is an economical coating process finding diverse applications [1]. Arc spraying has been used widely to coat engineering structures to protect them against corrosion and wear. The material to be deposited is introduced into the arc in the form of two wires serving as consumable arc electrodes. A cold gas jet across the arc drives the molten atomized droplets from the electrode tips.

Arc fluctuations due to periodic removal of molten droplets from the electrode tips have strong effects on the droplet formation, therefore on the coating properties, such as porosity, microstructure and oxide content. We have investigated the existence of correlations between the electrical fluctuations, the droplet formation and the coating properties [2]. The purpose of this study is to correlate the coating properties and the arcing phenomena which are controlled by the atomizing gas or by the nozzle design. The gas flow-droplet interaction, such as turbulence [2]and oxidation [3]are considered important parameters for coating quality control. Finding correlations between these phenomena and the coating properties will raise the possibility of using the voltage fluctuation analysis as process diagnostics for coating quality control.

Section snippets

Operating conditions

The Miller arc spray system used consists of a power supply (Miller Thermal, Inc., Maxtron 450), a control unit (PF400R) and an arc spray gun (BP400). Aluminum has been sprayed with air or nitrogen used as an atomizing gas. Deposits have been obtained with atomizing gas pressures from 276 kPa to 414 kPa, with arc currents from 100 A to 200 A and with the arc voltage set at 34 V. Before spraying, steel substrates are treated by grit blasting, acetone degreasing and ultrasonic cleaning. The

Characteristics in electrode phenomena

The arc image of electrode tips during spraying is shown in Fig. 3. The electrode tips and the produced molten droplets were illuminated by a laser and recorded with a CCD camera. Asymmetric melting behavior of the cathode (top wire) and the anode (bottom wire) is observed in the photographs. The cathode wire melting is more localized and the smaller molten droplets are immediately blown away by an atomizing gas. In contrast, the anode wire tip melts more slowly and unevenly, resulting in

Conclusions

Aluminum wire has been sprayed with air or nitrogen using a standard nozzle, a HV cap and a c-d nozzle. The existence of correlations between the arc voltage fluctuations and the coating properties has been investigated. Arc voltage fluctuations can be used as the indication for the coating properties which have strong relationships with turbulence and velocity of the atomizing gas. Turbulence can be estimated from the frequency and the amplitude of the voltage fluctuations. The c-d nozzle

Acknowledgements

Partial support for this work by the NSF through the ERC for Plasma-Aided Manufacturing, grant No. EEC-8721545 is gratefully acknowledged.

References (4)

E.R. Sampson, Proceedings of the 5th National Thermal Spray Conference, 1993, p....
T. Watanabe et al., Proceedings of the 9th National Thermal Spray Conference, 1996, p....

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