EP0017139B1 - Process for the simultaneous electroplating and mechanical honing of surfaces of a workpiece made of light metal - Google Patents

Description

The invention relates to a method for the simultaneous galvanic coating and mechanical honing of surfaces of a light metal workpiece according to the preamble of claim 1, as is known for example from an article in the magazine "Products Finishing", 1974, pp. 48-57.

The simultaneous galvanic coating and mechanical honing of mostly hollow cylindrical surfaces is generally referred to in the professional world as honing molds. The advantages of this method lie in a layered structure of the layer to be applied and in a faster structure of this layer compared to a purely galvanic coating, because the surface is continuously ground and honed with honing stones. In this respect, reference is made to DE-PS 2 029 646 and DE-OS 2 237 834 in relation to the prior art. When using the known method on light metal workpieces, in particular made of aluminum, however, a thin base layer must first be applied in an independent upstream process step before the simultaneous galvanic coating and mechanical honing, to which the actual layer can then only be applied by honing. A zincate precoating is proposed in the above-mentioned literature reference in order to achieve good adhesion for the actual layer to be applied in the honing process.

Our own tests have confirmed the need for an intermediate layer. It has been shown that cleaning the surface to be coated, e.g. B. with aluminum as the base material is not sufficient. If a cleaned surface of an aluminum workpiece is honed and galvanically coated at the same time, the relatively soft aluminum surface is damaged by the honing stones in the initial phase and there is no perfect adhesion of the layer to be applied, e.g. B. Chrome on the aluminum base material. The intermediate layer previously provided for this purpose - a zincate layer - had to be applied in a separate device and with a liquid other than that used in honing. This made the coating of light metal workpieces by the honing process particularly cumbersome.

The object of the invention is to demonstrate a method which makes it possible to produce the required base layer with as little effort as possible.

According to the invention, this object is achieved by the characterizing features of claim 1. Thereafter, the application of the required base layer is combined with the process step of simultaneous galvanic coating and mechanical honing in such a way that the base layer is applied in the honing machine itself and with the same electrolyte liquid, so that pre-coating in an additional device separate from the honing machine is unnecessary.

• Fig. 1 einen schematischen Verfahrensaufbau zur Durchführung des Beschichtungsverfahrens gemäß der Erfindung,
• Fig.2 2 einen Teilausschnitt aus der den Arbeitsspalt bildenden Elektrode bzw. Werkstückoberfläche als Gegenelektrode und
• Fig. einen Querschnitt durch eine fertige nach der Erfindung aufgebrachte Beschichtung in stark vergrößerter Darstellung.

Further refinements and advantages of the invention result from the subclaims or from the following description of an exemplary embodiment schematically illustrated; shows

• 1 shows a schematic process setup for carrying out the coating process according to the invention,
• 2 shows a partial section of the electrode or workpiece surface forming the working gap as counter electrode and
• Fig. A cross section through a finished coating applied according to the invention in a greatly enlarged view.

1, a honing machine 5 is indicated with an arrangement for the defined accommodation of a light metal workpiece 1 and a spindle head, which carries a rotatable and up and down movable honing stick 18, at the lower end of which a cylindrical electrode 6 with several is arranged from surface lines of the electrode cylinder honing stones 3. The honing stones are adjustable in the radial direction, for which purpose a pull rod 17 is arranged axially movably in the interior of the honing stick and the electrode. In the simplified representation here, a pressure cone 16 is attached to spread or press the honing stones at the lower end of the pull rod, which cone interacts with an inclined surface on the underside of the honing stones. The honing stones, which are also bevelled on the upper side, work in a spreading sense with a cone that is fixed inside the electrode. When the pull rod 17 is relieved, the honing stones can be contracted inwards, for example, by the force of a spring, not shown. The electrode and the honing stones can oscillate within certain limits to compensate for small position inaccuracies, for which purpose at least one joint is attached in the honing stick, which is not shown here, however. This entire arrangement is known per se, which is why this is shown here only schematically in simplified form.

The light metal workpiece 1 is attached concentrically around the electrode with the honing stones, the surface of which is to be coated in the illustrated embodiment is a hollow cylinder. The light metal workpiece is not only held in a defined manner in relation to the electrode within the honing machine, but it is also liquid-tight in a closed circuit 4 for electrolyte liquid speed 19 included. For this reason, a liquid space is formed below and above the workpiece, sealing off from the outside, into and through which the honing stick and the electrode can be moved axially. The annular working gap 14 formed between the electrode 6 and the surface 2 to be coated is arranged vertically in the honing machine. The liquid circuit 4 - as indicated by flow arrows - flows through in such a way that the working gap 14 is flowed through in a falling manner over its entire length (flow arrow 15). The electrolyte liquid emerging at the bottom of the working gap is collected in a liquid space and returned via a line to a collecting container in which particles mechanically torn off by the honing stones can settle. The forced circulation of the liquid in the specified direction of circulation is ensured by a circulation pump 13. The pump output can be regulated in order to be able to adjust the throughput speed of the electrolyte liquid through the working gap to optimal speed values. In order to set and monitor an optimal electrolyte temperature, a thermometer 11 is arranged in the collecting container receiving the electrolyte liquid, which thermometer acts via a temperature monitoring device 20 on a heating source 12 fed by electrical current. If the temperature of the electrolyte liquid drops below a set value that can be set on the temperature monitoring device 20, the electrolyte liquid is heated up again to the required set value via the heating source 12 and then automatically switched off again.

A DC voltage source 7 is also arranged in the overall arrangement, the positive pole of which is connected to the electrode 6 and the negative pole of which is connected to the light metal workpiece 1. It is therefore necessary that at least the electrode 6 is arranged within the honing machine in an electrically insulated manner. To monitor the current density within the working gap 14, an ammeter 10 is switched on in the circuit.

In one embodiment of the coating method according to the invention, the bore of an aluminum workpiece was provided with a chrome layer. The aluminum material was called AL si 12 Cu Mg Ni; the chromium was deposited from an electrolyte liquid of the usual composition. Before inserting the light metal workpiece. In the honing machine, the surface to be coated was pretreated in detergent, lye and acid to remove grease and oxide layers. The light metal workpiece pretreated in this way was then inserted into a honing machine known per se and shown schematically in FIG. 1. During the first precoating step, the honing stones 3 are lifted off the surface to be coated, but the honing spindle and the electrode 6 rotate and are moved up and down in order not to obtain any shadow images of the honing stones on the surface to be coated. The precoating step is naturally carried out with the same electrolyte liquid 19 as the subsequent main coating during honing. The other parameters of the process design during the precoating step are largely the same as compared to the subsequent honing. This initially applies to the gap width a of the working gap 14 between the electrode 6 and the surface 2 to be coated, which is optimally in the range of approximately 1 to 2.5 mm. This measure takes into account the desire for the smallest possible gap. For a given current, the curvature of the surfaces to be coated increases the current density with decreasing gap width, which has a favorable effect on the deposition rate. On the other hand, as the gap width becomes smaller, the budiness of the electrodeposited layer increases, which is undesirable. The given value of 1 to 2.5 mm represents a usable compromise value between the two opposing demands for the highest possible current density and the lowest possible budiness. Like the gap width, the flow geometry of the electrolyte forced through the working gap is the same as in the pre-coating step subsequent honing molds. Instead of a flow flowing along the entire length, an ascending flow or a simultaneously falling and rising flow when the electrolyte liquid is fed in or out at a medium height of the working gap would also be conceivable. It turns out, however, that the smallest roundness and cylindricity errors can be achieved with a flow falling over the entire length of the working gap.

Taking into account the cross section of the working gap, the delivery rate of the circulating pump 13 within the liquid circuit 4 is set such that a flow rate of electrolyte liquid through the working gap of approximately 2.5 to 3 m at least during the precoating step, but preferably also during honing / sec results. At high flow velocities, a more uniform layer growth can be achieved at high current densities, especially if the electrolyte temperatures are relatively low, which will be discussed further below. An increase in the flow rate above 2.5 to 3 m / sec does not increase the deposition speed, but the formation of the layer surface is more uniform at higher speeds.

With regard to the fact that both for the precoating step and for the actual Liche honing forms the same electrolyte liquid is used, it is obvious to use the same liquid circuit for both process steps and thus to carry out both process steps at the same electrolyte temperature. Electrolyte temperatures in the range of about 55 ° C are common. It has been shown that at temperatures in the range from approximately 30 to 40 ° C., in particular approximately 35 ° C., a lower roughness of the applied layer, a better current efficiency and faster layer growth can be achieved. For this reason, a temperature control for the electrolyte liquid is provided in the schematic structure shown in FIG. 1. Should the process flow of the method result in such a large amount of heat being introduced into the electrolyte liquid that it heats up automatically beyond the optimal temperature range, then the electrolyte liquid must be artificially and automatically cooled down by the temperature control by means of a cooling coil 21, which is indicated by dash-dotted lines in the collecting container will. In the event that the precoating step is to be carried out at a different electrolyte temperature than the actual honing, two separate collecting tanks for electrolyte liquid with separate temperature controls would have to be provided. The return line from the honing machine and the suction port of the circulating pump would each have to be equipped with a changeover valve and with a branch line into each of the collecting tanks.

Thanks to the constant mechanical processing of the surface of the electrodeposited layer during honing, the surface is - as I said - constantly electrically activated and smoothed. Because of this, high current densities can be permitted without the risk of wild bud growth, which cause rapid layer growth. In view of the fact that mechanical smoothing of the first layer begins after the precoating step, high current densities in the range of approximately 250 to 750 A / dm ^{2 can also be} permitted in the same way during the precoating step. In this way, 20 gm / min of chromium can be deposited with optimal coordination of the process sequence.

The thickness b of the - first - layer 8 applied during the precoating step should be dimensioned so strongly that after the honing stones have been placed on this first layer, they cannot reach the underlying material with certainty. It has been shown in the honing of surfaces on light metal workpieces that the honing stones roughen the surface, which is initially still uncoated, very strongly, which is an obstacle to an orderly and smooth layer structure. Such roughening of the base material by the honing stones is to be avoided by the first layer. A layer thickness of approximately 40 μm for the first layer 8 will certainly be sufficient for this. This layer is thus also significantly smaller than the second layer 9 with the dimension a, which is applied during the honing. In the absence of mechanical honing during the precoating step, 1 buds form on the surface 2 of the workpiece, which grow essentially perpendicularly away from the surface to be coated into the room. The buds also grow in the width direction and more and more buds form until the entire surface is completely overgrown with buds. In the absence of constant mechanical abrasion of the surface formed, the layer essentially has a column structure, which is indicated in FIG. 1 by hatching perpendicular to the surface 2. The uniform layer thickness of the first layer shown in FIG. 1 results only from the subsequent honing of this layer. In reality, the first layer before the start of honing is relatively uneven. The specified layer thickness of about 40 11 m is also chosen with a view to the fact that there is a layer thickness that is certainly bearing even at the thinnest points. In view of the layer thickness growth rates of about 20 pm / min that can be achieved with optimal process setting, a precoating time of about 2 minutes may be sufficient.

Following the precoating step, the honing stones 3 are turned on by means of the pull rod 17. The honing stick 18 performs an axial stroke H which is adjustable during the rotation. It has been shown that the macro errors on the surface to be coated are smallest after coating if the honing stone lift H is as large as possible. The contact pressure for the honing stones should also be set optimally. The layer thickness growth is higher at low contact pressures of the honing stones; the achievable surface is also smoother. At higher contact pressures, however, smaller macro errors and better adhesion of the layer can be achieved. Values that can be determined empirically can represent a good compromise between the opposing demands.

It goes without saying that a nickel layer could also be applied instead of a chromium layer, which of course presupposes the use of a different electrolyte liquid. With a view to a different position of the metal nickel within the voltage series than chromium, it may be necessary, however, that a zincate layer must be applied in a separate process step before the application of a nickel layer in a precoating step in order to achieve sufficient adhesion. However, even after the application of such a buffer layer, the precoating in the honing machine is without applied honing stones make sense because the zincate layer can also be removed relatively quickly with honing stones applied immediately.

Claims (9)

1. Process for the simultaneous galvanic coating and mechanical honing of surfaces of a light metal workpiece in a hone-shaping machine with electrolyte circulation, in which a first layer is applied galvanically before the simultaneous galvanic coating and mechanical honing, characterised in that the pre-coating step is likewise carried out in the hone-shaping machine, but without the intervention of the honing stones, and the same electrolyte is used for the precipitation of both coatings.

2. Process according to Claim 1, characterised in that the thickness (b) of the first coating is adjusted to less than or approximately equal to the thickness (c) of the second coating.

3. Process according to one of Claims 1 to 2, characterised in that the current density is adjusted during the pre-coating step to approximately the same magnitude as in the simultaneous galvanic coating and mechanical honing.

4. Process according to one of Claims 1 to 3, characterised in that the temperature of the electrolytic liquid is adjusted to 30 to 40°C, preferably about 35° C.

5. Process according to one of Claims 1 to 4, characterised in that the gap through which electrolyte flows, formed between electrode and light metal workpiece surface to be coated, is adjusted to 1 to 2.5 mm.

6. Process according to one of Claims 1 to 5, characterised in that the speed of the electrolyte in the gap, at least during the pre-coating step but preferably also during the simultaneous galvanic coating and mechanical honing, is adjusted to at least 2.5 to 3 m./s., preferably 5 to 6 m./s.

7. Process according to one of Claims 1 to 6, characterised in that the current density is adjusted to 250 to 750 A/sq.dm., at least during the pre-coating step but preferably also during the simultaneous galvanic coating and mechanical honing.

8. Process according to one of Claims 1 to 7, characterised in that the first coating is applied in a thickness of 20 to 50, preferably about 40 µn₁.

9. Process according to one of Claims 1 to 8, characterised in that the pre-coating is carried out for about 2 minutes long.

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