Electrolysis: processes – compositions used therein – and methods – Electrolytic erosion of a workpiece for shape or surface... – With programmed – cyclic – or time responsive control
Reexamination Certificate
2000-10-23
2003-09-30
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic erosion of a workpiece for shape or surface...
With programmed, cyclic, or time responsive control
C205S662000, C205S664000, C205S668000, C205S684000, C205S685000, C204S297010, C204S297050
Reexamination Certificate
active
06627064
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to an electrolytic passivation method for removing a hard metal layer, applied on a hard metal workpiece. Furthermore, it relates to a method for the renewed preparation of a hard metal workpiece with the help of the aforementioned coating-removal method, as well as to a holding device.
To improve the surface properties of hard metal workpieces, such as the service life and/or the effectiveness of tools, especially of throwaway carbide indexable inserts, drills, milling cutters, metal forming tools and stamping tools, it is customary at the present time to coat such workpieces with a layer of hard material, such as titanium nitride or titanium carbonitride. As coating methods, preferably the widespread vacuum-coating methods are used. Since hard metal workpieces, especially those of complex shape, such as those of the above-named tools, are expensive, an attempt is made to rescue the workpiece, if the coating is faulty or to restore the shape and to coat the workpiece once again, when the latter has become worn through use. For this purpose, the coating of the workpieces in question must first be removed. Removing the coating from workpieces of the type mentioned is a difficult and challenging technical problem. Whether this problem is brought to a solution that can be used economically is, in the final analysis, of decisive importance for the question of whether workpieces mentioned can be reworked or disposed of economically. Even if the coating procedure is carried out carefully, process defects or process interruptions, which can lead to a defective coating of the hard metal workpieces with the hard material coating, occur time and again. In commission coating plants, customer requirements with respect to the type and/or thickness of the coating can be mixed up. The cost of rejects resulting therefrom is particularly high in the case of hard metal workpieces. The cost of the uncoated workpiece, the basic body, can easily amount to 3 to 10 times the value of the coating.
For removing coatings from steel workpieces, a method has been known for some time from DE-41 01 843, which permits the stripping of the layer of hard material. This method uses a solution of hydrogen peroxide, which dissolves titanium compounds and other hard material compounds. The intent here is not to attack and damage the workpiece itself in an impermissible manner. The use of acids and alkalis, for example, would damage the steel workpieces in an impermissible manner. This method is completely unsuitable for hard metal, which consists predominantly of the hard material tungsten carbide and the chemically less stable cobalt, because the basic hard metal body of the workpieces would be destroyed at a rate faster than that at which the layer of hard material is dissolved.
For this reason, mechanical methods are used largely at the present time for removing the hard material layer from defectively coated hard metal workpieces or for such workpieces, which are to be coated once again. The layer of hard material is ground or polished off. The costs of this procedure are high. For this reason, such methods are used extremely infrequently and, instead, defective workpieces are disposed of.
From the “Research Disclosure” of April 1996, a method has now become known under the category No. 38447 for removing a layer of hard material, applied on a hard metal workpiece, for which the layer is removed by electrolytic passivation. For this method, a tungsten oxide layer is formed between the hard material layer and the hard metal basic body. This passivation of the tungsten carbide is accomplished by the anodic polarization of the workpiece in a suitable electrolyte. Because of the transformation of the tungsten carbide into tungsten oxide, the hard material layer loses its adhesion to the basic body and flakes off.
The conditions, which must be selected for removing the coating, are extremely critical. If the wrong parameters are selected, either the layer is not removed or the workpiece is damaged irreparably.
The state of the art also involves the re-coating of tools without a prior removal of layers. For this purpose, the tools, preferably shaft tools of a hard metal, such as milling tools, drills and hobbers, are reground at the cutting surfaces and subsequently coated once again. This method has the disadvantage that only the cutting surfaces are freed from the coating by regrinding; however, the remaining regions of the tool remain coated. Therefore, during the subsequent over-coating, the thickness of the layer of hard material steadily increases in the regions that have not been reground. As the thickness of the layer increases, the inherent stresses in the layer of hard material increase and lead to a reduction in the service life of the tool in comparison to a tool which has only been coated once. When a coated tool is reground, the inherent stresses frequently lead to portions breaking off in the region of the transition from the coated to the uncoated part and decrease the operating efficiency of the tool.
It is therefore an object of the present invention to eliminate the disadvantages of the state of the art and to realize a more economic coating-removal process. This is accomplished according to the present invention owing to the fact that, at the start of the coating-removal process, a current density maximum is brought about at the workpiece, which is at least 0.01 A/cm
2
and preferably even 0.1 A/cm
2
. By these means, the above-mentioned problem is solved and service lives are attained which are at least almost equivalent to those attained with new tools.
It has namely turned out that, when these conditions are maintained, the necessary time span during which the workpiece is exposed to the electrolysis until complete removal of the coating is achieved can be reduced drastically and the time of action and the depth of action of the electrolysis on the hard metal can also be reduced drastically. The economic efficiency of the inventive method is simultaneously increased from two points of view, namely, owing to the fact that, on the one hand, the time necessary for the removal of the coating is reduced significantly, which obviously increases the throughput of the decoating process significantly. On the other, a subsequent mechanical treatment is significantly shorter, because the surface erosion necessary for a good adhesion of the layer of hard material that is to be applied once again is reduced significantly.
Surprisingly, it has turned out that, by adhering to the current conditions mentioned, the layer of hard material is blasted off practically all of a sudden and, with that, there is hardly any more time left over for the electrolysis to damage the basic hard metal body than is required for the above-mentioned blasting-off.
As far as is known at the present time, the method proposed pursuant to the invention is suitable for removing all conductive hard material coatings, which are customarily used, such as hard material coatings of nitrates and carbides, carbonitrides of metals or metal compounds, such as TiAlN, TiAlNC, WC, WCN, etc., but also of chromium-containing hard material coatings, such as those of Cr, CrN, CrC, CrNC, as well as combinations of these hard material layers or also multi-layer arrangements.
Contrary to previous teachings, the electrolyte selected does not play a significant role, provided that it is in the acidic range and that its conductivity, together with the voltage applied, permits the above-mentioned current conditions to be maintained. At the same time, the workpiece is rapidly brought to an electrical potential, at which the tungsten of the hard metal is rapidly brought into a passive state.
Furthermore, with respect to the cathode, a voltage is preferably applied to the workpiece, which amounts to at least 1 V and especially preferably at least 6 V and particularly at least 15 V. Preferably, this voltage applied is kept constant over the treatment time and is, for example, controlled and
Unaxis Balzers Aktiengesellschaft
Valentine Donald R.
LandOfFree
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