EP0818274A1 - Surface treatment of aluminium alloys by projection of abrasive powder under pressure - Google Patents

Abstract

The surfaces of components made of aluminium or their alloys are treated by sand blasting, using an abrasive powder projected against the surfaces in a jet of compressed air. The powder is prepared by electro-melting a mix of corundum and 20-45% by weight of zirconia.

Description

Field of the invention

The invention relates to a method for treating aluminum alloy surfaces, such as sheets, bars, tubes, profiles or molded parts or machined, by pressure spraying of an abrasive powder, designated operation usually under the name of sandblasting.

State of the art

When sanding an aluminum alloy surface, it is advisable to obtain a fast action, to use a hard abrasive whose grains have edges lively; thus the bite of the product allows rapid removal of the metal. So that the effect is long-lasting, the abrasive grains must both be tough enough not to not quickly break into finer particles, and yet exhibit a fragile fracture phenomenon to form new sharp and cutting edges. The material chosen must therefore have mechanical characteristics which are a compromise between hardness and tenacity, which are in general contradictory.

The material usually used is ground and sieved electroconducted white corundum. It is this abrasive that is recommended by general treaties on aluminum such as "Aluminum" by the Engineers of the Pechiney Group, Editions Eyrolles, Paris, 1964 (volume 2) or "Aluminum" by Kent Van Horn, American Society of Metals, 1967 (volume 3), or works on surface treatments, such as for example "The surface treatment and finishing of aluminum and its alloys" by S. Wernick and R. Pinner, edited by Robert Draper Ltd, 1964.

Subject of the invention

The subject of the invention is a method for treating aluminum or alloy surfaces aluminum by spraying an abrasive powder under pressure, in which uses an fused corundum-zirconia powder as an abrasive powder containing from 20% to 45% by weight of zirconia.

Description of the invention

The invention is based on the finding made by the applicant of an improvement spectacular results obtained when sanding aluminum alloy surfaces by replacing the white corundum with corundum-zirconia, in particular as regards the polishing time and the duration of use of the abrasive powder.

Corundum-zirconia alloys are obtained by melting a mixture of alumina and zirconia and are known as abrasives, but their use as a material for sandblasting has never been considered.

The sandblasting operation is carried out under the same conditions as with the use of a corundum powder, i.e. using compressed air at a pressure between 0.05 and 0.2 MPa for alloys wrought in low or medium thickness and from 0.15 to 0.3 MPa for solid parts, such as foundry parts. The nozzle of projection is located between 10 and 50 cm from the surface to be treated and is directed approximately perpendicular to this surface. The size of the abrasive powder depends on the condition surface area sought. It can range from 20 to 60 (depending on the FEPA designation) for a coarse polishing, from 40 to 80 for a medium grain, from 100 to 200 for a fine grain and beyond 200 for a very fine grain. It is possible, especially for fine grains where the operation gives off a lot of dust, to make a wet sandblasting.

Examples

1) une première série de disques a été sablée au moyen de corindon blanc neuf en grains de calibre 60 selon la désignation de la FEPA (Fédération Européenne des Producteurs d'Abrasifs), sous une pression d'air comprimé de 2 bars (0,2 MPa) à 20 cm du disque à polir. L'état de surface désiré a été obtenu après un temps moyen de polissage de 2 mn 50 s. Après cette première utilisation, l'abrasif qui, à l'état neuf, contenait environ 2% de grains de calibre 90, en contenait 7%. Pour réutiliser cet abrasif en vue d'obtenir le même état de surface que précédemment sur un autre disque, il faut alors un temps de polissage de 3 mn 5 s. Après cette seconde opération, le taux de grains de calibre 90 dans l'abrasif atteint 15% et, pour continuer le polissage, avec ce même abrasif, à état de surface constant sur un autre disque, il faut alors 3 mn 25 s.

2) La même opération de polissage sur les mêmes disques bruts que dans l'exemple précédent, a été effectuée avec un abrasif de même calibre 60 en corindon-zircone électrofondu à 25% en poids de zircone. Avec le produit neuf, le temps de polissage est de 1 mn 35 s. Après cette première utilisation, l'abrasif qui, à l'état neuf, contenait 2% de grains de calibre 90, en contient 3,5% A la seconde utilisation, le temps de polissage était de 2 mn et le taux de grains de calibre 90 était de 6%. A la troisième utilisation, l'abrasif permet encore d'obtenir le même état de surface après un temps de polissage de 2 mn 15 s.

From a sheet of aluminum alloy 3003 (according to standard NF-EN 573) with a thickness of 4 mm, a series of discs with a diameter of 300 mm was prepared.

1) a first series of discs was sandblasted with new white corundum in grain size 60 according to the designation of the FEPA (European Federation of Producers of Abrasives), under a compressed air pressure of 2 bars (0, 2 MPa) at 20 cm from the polishing disc. The desired surface finish was obtained after an average polishing time of 2 min 50 s. After this first use, the abrasive which, in new condition, contained approximately 2% of 90-gauge grains, contained 7%. To reuse this abrasive in order to obtain the same surface condition as previously on another disc, a polishing time of 3 min 5 s is then required. After this second operation, the rate of 90-caliber grains in the abrasive reaches 15% and, to continue polishing, with this same abrasive, at a constant surface state on another disc, it then takes 3 min 25 s.

2) The same polishing operation on the same raw discs as in the previous example, was carried out with an abrasive of the same caliber 60 in corundum-zirconia electrofused to 25% by weight of zirconia. With the new product, the polishing time is 1 min 35 s. After this first use, the abrasive which, in new condition, contained 2% of 90-gauge grains, contains 3.5%. At the second use, the polishing time was 2 min and the grain content of caliber 90 was 6%. On the third use, the abrasive still makes it possible to obtain the same surface condition after a polishing time of 2 min 15 s.

• la dureté, mesurée par indentation d'un petit diamant sous microscope optique, sur des échantillons enrobés et polis. Il existe une mesure Vickers à partir d'une empreinte pyramidale à base carrée, et une mesure KNOOP à partir d'une empreinte pyramidale à base losange, prenant mieux en compte l'anisotropie.
• la tenacité, c'est à dire la résistance à la fissuration, mesurée par le facteur critique d'intensité de contrainte en déformation plane K_1c, exprimé en MPa√m.
• la solidité, qui mesure la tenue des grains sous l'effet d'une contrainte mécanique. Le test de mesure consiste à effectuer un recalibrage des grains au calibre choisi pour la mesure, puis un broyage pendant un temps équivalent à un broyage de 15 mn d'un grain de calibre 16 de même solidité, puis un tamisage pour mesurer le refus.

The mechanical characteristics of the two types of grain were measured, namely:

• hardness, measured by indenting a small diamond under an optical microscope, on coated and polished samples. There is a Vickers measurement from a pyramidal footprint with a square base, and a KNOOP measurement from a pyramidal footprint with a diamond base, taking better account of the anisotropy.
• the toughness, ie the resistance to cracking, measured by the critical stress intensity factor in plane strain K _1c , expressed in MPa√m.
• solidity, which measures the resistance of the grains under the effect of mechanical stress. The measurement test consists in recalibrating the grains to the size chosen for the measurement, then grinding for a time equivalent to a grinding of 15 minutes of a 16-gauge grain of the same strength, then sieving to measure the refusal.

The results of these 4 measurements are given in the following table: corundum corundum-zirconia KNOOP hardness (kg / mm ² ) 2050 1610 VICKERS hardness (kg / mm ² ) 1940 1870 tenacity (MPa√m) 1.9 3.7 solidity (%) 85 93

Claims (1)

Process for treating aluminum alloy surfaces by spraying under pressure of an abrasive powder, characterized in that the abrasive powder is in fused corundum-zirconia containing (by weight) from 20 to 45% of zirconia.