Metallic coating deposited by Cold Gas Spray onto Light alhoys

Abstract

This thesis focuses on the use of Cold Gas Spray technology (CGS) to spray different nature powders onto light alloys with the aim of increasing their wear resistance.

The growing industrial interest for costs reduction (fuel consumption, machinery lifetime, or personal security) has emphasized the necessity to investigate the potential applications that light alloys can offer.

Weight reduction is a reason why light metals and its alloys have been associated with strong industries as transport or aerospace.

Nevertheless, weight saving aspect should not opaque the fact that light metals possess other “relevant technological” properties like the high corrosion resistance and high electrical and thermal conductivity of aluminium, the machinability of magnesium and the extreme corrosion resistance of titanium.

It must be taken into account that despite these technological importance properties, due to their density, among some other physical properties, aluminium and magnesium have a low hardness and low wear resistance, which provokes the need of improving its surface properties.

The main objective has been the obtention and optimization of Ti6Al4V and 316L stainless steel coatings onto light alloys, by means of Cold Gas Spray technology, a thermal spray technique which provides a dense and with no oxidation traces coatings, keeping the original microstructure of the spraying powders.

Ti6Al4V and SS316L coatings have been produced onto Al-7075-T6 and AZ31 substrates and Ti6Al4V coatings have been also produced onto Ti6Al4V substrate.

Actually industrial machinery made of titanium alloy Ti6Al4V, that due to use has been damaged, needs repair and it will be easily repairable through this technique leading to a reduction in costs and repair times.

In the present thesis, coatings based on Ti6Al4V and Stainless Steel 316L have been successfully obtained by means of Cold Gas Spray Technologies onto three different light alloys (Ti6Al4V, Al-7075-T6 and AZ31) improving their properties with respect to wear and corrosion resistance.

Two different powder morphologies (spherical and irregular) of each spraying material have been tested onto the three substrates.

Coatings have been obtained under the conditions of two different design of experiments, each one developed for the different used cold gas spray guns (KINETIC 8000 and KINETIC 4000)

From the obtained coatings and its properties it can be said that:

Irregular powders reach higher in flight velocities than the spherical ones due to drag coefficient, and despite both powders have the same hardness, the deposition efficiencies of the spherical powders are higher. Irregular powders are more oxidized due to production process (gas atomization vs. hydride-dehydride and water atomization) and this reduces the DE. Comparing both systems, spherical Ti6Al4V and irregular Ti6Al4V spraying powders, higher deposition efficiencies are obtained with spherical powder, arriving to 140% higher on the optimal spraying conditions.

As the spraying temperature and pressure increases, deposition efficiency does it also because more particles are reaching its critical velocity. In addition, porosity reduces because particles are more ductile due to the temperature and they are able to deform more closing the existing pores of the coating. For structural applications where the light alloy structures would undergo to wear and corrosion processes, coatings must be thick and dense. Fully dense coatings have been obtained using He as propellant, gas protecting the substrates against corrosion, but He is more than 4 times expensive than N2.

When spraying powder on substrate with the same or similar nature, as it happened with Ti6Al4V spraying powder and Titanium or Ti6Al4V substrate, microfusions are produced due to impact, making strong the bond between particles and substrate, but when spraying powder and substrate has different nature and properties (hardness, ductility, thermal conductivity, crystal structure, deformation mechanism), as it has been seen with Ti6Al4V as spraying powder and Al-7075-T6 and AZ31 as substrates, mechanical bonding is produced by anchoring. No fusion zones are founded.

The obtained results of cavitation test, have helped to identify experimentally CGS bonding mechanisms as a function of particle/substrate nature. The order in bonding strength is Ti, Ti6Al4V, Al-7075-T6 and finally AZ31. These results are in accordance to those obtained with adhesion tests.

Concerning to corrosion resistance, the obtained Ti6Al4V coatings onto the three substrates, using He as propellant gas, are fully resistant to corrosion. After 24 hours testing, no electrolyte has reached substrate surface showing fully resistance and ensuring the accomplishment of ASTM D-1411 standard. SS316L coatings onto Al-7075-T6 and AZ31 substrates, using N2 as propellant gas, are fully resistant to corrosion, following the same standard as before.

Esta tesis se centra en el uso de la tecnología de Cold Gas Spray para proyectar polvos micrométricos sobre diferentes aleaciones ligeras con el fin de obtener un recubrimiento que aumente la resistencia al desgaste y a la corrosión.

El creciente interés por parte de la industria en reducir al máximo los costes (consumo de combustible, duración maquinaria, o la seguridad personal) ha fomentado la necesidad de investigar las posibles aplicaciones que las aleaciones ligeras pueden ofrecer.

La reducción de peso es una razón por la cual los metales ligeros y sus aleaciones se han asociado con fuertes industrias como el transporte o la industria aeroespacial.

Sin embargo, los aspectos de ahorro de peso no debe ocultar el hecho de que los metales ligeros poseen otras propiedades "tecnológicas" como la alta resistencia a la corrosión y alta conductividad eléctrica y térmica del aluminio, la mecanizabilidad de magnesio y la resistencia a la corrosión extrema de titanio.

Hay que tener en cuenta que a pesar de estas propiedades de importancia tecnológica, debido a su densidad el aluminio y magnesio tienen una baja dureza y una baja resistencia al desgaste, lo que provoca la necesidad de mejorar sus propiedades superficiales.

El objetivo principal ha sido la obtención y optimización de recubrimientos de Ti6Al4V y de acero inoxidable 316L sobre aleaciones ligeras a través del uso de técnicas de proyección térmica, concretamente la proyección fría (Cold Gas Spray), que permite obtener recubrimientos densos y sin rastros de oxidación manteniendo la microestructura original de los materiales de partida, tanto los sustratos como los polvos de proyección.

Se han obtenido satisfactoriamente recubrimientos de Ti6Al4V y SS316L sobre sustratos de Al-7075-T6, AZ31 y Ti6Al4V con eficiencias cercanas al 100% y porosidades menores al 0,1%.

Se ha mejorado la resistencia al desgaste en un 300% con respecto a los sustratos de partida, y la resistencia a la corrosión de los sustratos empleados debido a la formación de recubrimientos completamente densos.

También se han obtenido de forma experimental indicios que confirman las teorías de enlace establecidas hasta el momento para el proceso de proyección fría, entre las partículas y el sustrato así como entre las propias partículas depositadas.