Patent Grant
7105472
The present invention relates to a coating solution to be applied onto the surface of metals or metal alloys, a process for cold forming of metals and metal alloys using said coating solution, and a coated blank made of a metal or a metal alloy.
In cold forming, no heat is applied to the metallic workpiece before forming. However, its temperature can increase during the forming process, as the energy expanded in forming is almost completely converted into heat. The main advantages of cold forming are the saving of materials and working time, the use of lower strength steels, the improved quality of workpieces, lower machine costs, better size accuracy, high surface quality and an extended tool life. In order to achieve these advantages, a protective film must be formed between the tool and the work piece, which must remain intact during the working process. Such a protective film is mainly made up of molybdenum disulfide dispersions or graphite as lubricants. Hitherto, in order to obtain such a protective lubricating film in cold forming processes, lubricant carriers are applied onto the surface of the workpiece, which can absorb about five times the quantity of lubricants compared to the untreated metal surface. However, the application of such lubricant carriers onto the surface of workpieces is carried out by applying aggressive acids and metallic soaps in huge amounts, which is not desirable from an ecological standpoint.
It is an object of the present invention is to provide a new system for cold forming of metals and metal alloys, which should avoid the treatment of the working piece with aggressive acids and/or metallic soaps prior to the cold forming process, but should exhibit at least the same results compared to a cold forming process using workpieces treated with aggressive acids and/or metallic soaps.
In particular, there is provided a coating solution to be applied onto the surface of a metal or a metal alloy, comprising a polyphosphate and a dithiocarbamate.
In a preferred embodiment of the present invention, the polyphosphate is selected from the group consisting of ammonium polyphosphate and alkali metal polyphosphates, such as sodium polyphosphate, or mixtures thereof. The amount of said polyphosphate is preferably in the range of from about 0.1% (w/v) to about 80% (w/v).
The dithiocarbamate is preferably a dialkyl dithiocarbamate, wherein the alkyl groups are the same or different and can have 1 to 12 carbon atoms. In a preferred embodiment of the present invention the dithiocarbamate is a heavy metal dithiocarbamate, wherein the heavy metal is for example molybdenum. In a more preferred embodiment of the present invention the dithiocarbamate is a molybdenum dibutyldithiocarbamate. The amount of said dithiocarbamate is preferably in the range of from about 0.1% (w/v) to about 80% (w/v).
The coating solution of the present invention can further include graphite and/or molybdenum disulfide, each of which can be present preferably in an amount of from about 0.1% (w/v) to about 80% (w/v).
In a preferred embodiment of the present invention the coating solution may further contain at least one component selected from the group consisting of alcohols in an amount of from 0% to about 100% (v/v); waxes such as polyalkylene waxes, e.g. polyethylene waxes, microcrystalline waxes, and Montan ester waxes, in an amount of from 0% to about 80 (v/v); emulsifying agents such as alkyloxyethylates and alkylaryloxyethylates, in an amount of from 0% to about 50% (v/v); pH-modulating agents such as buffers, e.g. phosphorous acid and esters thereof, in an amount of from 0% to about 50% (v/v), polymeric compounds such as polyethylene glycols, preferably having a weight average molecular weight of from 500 to 1,000,000, polysaccharides and methylcellulose, in an amount of from 0% to about 50% (v/v); oxides such as silicon dioxide, in an amount of from 0% to about 50% (v/v); friction-reducing agents such as sodium stearate, in an amount of from 0% to about 80% (v/v); defoaming agents such as fatty alcohol polyglycol ethers, in an amount of from 0% to about 50% (v/v); carboxylic acids and derivatives thereof such as 2-phosphonobutanetricarboxylic acid, in an amount of from 0% to about 50% (v/v); organophosphorous compounds such as alkylphosphates, in an amount of from 0% to about 5% (v/v); and phosphates such as boron phosphate, sodium phosphate, P2O5 etc., in an amount of from 0% to about 80% (v/v); and mixtures thereof containing two or more of said components.
In a preferred embodiment of the present invention, the coating solution has a pH-value ranging from about 3 to about 4, adjusted by the above-defined pH-modulating agent.
The solvent used for the coating solution of the present invention is H2O in an amount of from 0% to 100% (v/v), alcohols, preferably alcohols having up to 10 carbon atoms such as methanol, ethanol, etc., or a mixture of different alcohols, in an amount of from 0% to 100% (v/v), or a H2O/alcohol-mixture in an amount of from 0% to 100% (v/v).
Further, the present invention relates to a process for cold forming of metals or metal alloys, comprising the steps of:
In the preferred embodiment, the blank can be sand-blasted prior to the above process step (a).
Further, there is provided a blank made of a metal or a metal alloy such as steel, having a coating obtainable by the above-defined process steps (a) and (b).
When using the above-defined coating solution for the treatment of blanks (“workpieces”) to be processed in cold forming, there can be observed that on the one hand the coating solution of the present invention does not exhibit the severe drawbacks with respect to environmental pollution, and on the other hand the blanks having a coating based on the coating solution of the present invention show at least the same results for the processed workpiece and the tool when compared with blanks having a coating known in the prior art. Moreover, the cold forming process using blanks treated with the coating solution of the present invention is superior from an economical standpoint, i.e. the weight of the coating on the blank is much less as compared to the weight of a coating known in the art. Additionally, in comparison with a coating procedure known in the art, wherein about eleven steps for applying the coating on the blank are necessary, only two steps—(i) immersing the blank in the coating soluting according to the pursuant invention and then (ii) drying the blank—are necessary resulting in a drastically saving of time.
The present invention will now be further illustrated in the following examples, without being limited thereto.
25 cylindrically shaped steel blanks (examples 1 to 25) having a length of 500 mm are immersed in a coating solution containing 2–8% (w/v) polyphosphate, 2–5% (w/v) dithiocarbamate, 5–12% (w/v) graphite and 5–12% (w/v) molybdenum disulfid for a time period of about 1 min. Then, the blanks having the coating solution on their surfaces are transferred to a drying stove wherein they are dried at a temperature of about 60° C. for about 30 min. For comparison, 5 cylindrically shaped steel blanks (Comp. Examples 1 to 5) having a length of 500 mm are coated using the agents and procedure known in the art. The results are summarised in Table 1.
As can be taken from Table 1, the coated blanks according to the present invention exhibit a drastically reduced coating weight, but the coating reveals at least the same properties for the blank to be cold formed as the coating of blanks obtained according to the prior art, upon evalution of the processed coated blanks during and after their cold forming (+=good results of the formed blanks after 3 operations in a cold forming process).
This application claims priority under 35 U.S.C. 119 to U.S. Provisional Application No. 60/369,561 entitled COATING SOLUTION FOR METALS AND METAL ALLOYS and filed on Apr. 4, 2002, the entire content of which is hereby incorporated by reference.
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