CA1084391A

CA1084391A - Preparation of aluminum for cold working

Info

Publication number: CA1084391A
Application number: CA245,863A
Authority: CA
Inventors: Wolfgang Ernst; Heinrich Fleischhauer; Han Y. Oei; Werner Rausch
Original assignee: Oxy Metal Industries Corp
Current assignee: Oxy Metal Industries Corp
Priority date: 1975-02-17
Filing date: 1976-02-16
Publication date: 1980-08-26
Also published as: AU1077276A; SE416962B; JPS51106676A; DE2506665C2; CH598881A5; ATA833175A; SE7601724L; NL7513442A; IT1055223B; ZA76491B; BR7600476A; AT336367B; DE2506665A1; AU501557B2; GB1501842A

Abstract

ABSTRACT OF THE DISCLOSURE

PREPARATION OF ALUMINUM FOR COLD WORKING

Disclosed is a process for providing an aluminum surface with a coating prior to cold forming. The surface is treated with an alkaline aqueous composition of at least one alkaline earth metal compound and thereafter an organic lubricant composition.

Description

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B.Z\CXGROUI`ID OF Tl~ . INVE:MTION
The inve~n~ioll is concerned with an lmproved method for the preparation of aluminum and alununum alloys for cold working by the application of a carrier layer and a superimposed organic - 5 lubricant lay~r.
Workpieces of aluminum and aluminum alloys which are to undergo cold working are usually provided with a lubricant film.
The purpose of the application of a lubricant film is to prevent cold flow between the aluminum and the die, and to achieve high . ~ .
quality o the workpieces, as well as high die life. Metal stearates or other soaps are often used as lubricants. For heavy deformation, solid lubricants such as graphite or molyb~denum disulfide-containing lubricants on a mineral oil basis are also used~ The lubricants alone are not sufficient for the efficient removal of the workpiece from the die after certain deformations.
According to a known process, a phosphate layer is applied on the aluminum workpieces as a preparation for cold working; this layer firmly adheres to the surfaces and serves as an adhesive ground or carrier for the lubricant, resulting in increased efficiency ..
~o as a separating layer tog~ther with the lubricant. The preparation of suitable phosphate layers on aluminum workpieces, however, is difficult, because it requires correct control and replenishing of the phosphatizing bath, as well as the maintenance of spe~ific process parameters, Deformation tests conducted under the most 3 severe conditions often show an insufficient separation or carrier layer.
Therefore, it is necessary to propose a method with which it is possible to apply a lubricant carrier layer on work-~ pieces made of aluminum and aluminum alloys in a simple process, :, '~' ' ;
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and to achieve a layer which satisfies even the increased re-quiremerlts posed during severe cold formirlg.
SUMMARY OF 1'HE INVE~rION
According to the present invention, an aqueous solu-tion containing an alkaline-eart.h metal compound, preferably a ~ calcium salt, and having a pH value over 8, preferably in a - range of 10 - 13, is used for the application of the lubricant : carrier layer on the workpiece surfaces.
In accordance with a broad aspect of the invention, there is provided an improvement in a process for preparing an aluminum surface for cold forming wherein an inorganic layer i5 formed on the surface followed by application of a lubricant.
The carrier layer is formed by contacting the surface with an aqueous composition containing a~ alkaline-earth metal compound adjusted to a pH value in excess of 8. ~ .
The invention is also concerned with an aluminum article coated by this process. :
. The coating solution according to the present inven- ~
: tion is placed in contact with the workpieces to be deformed : ~.
at a temperature range of 50-100C, possibly after preliminary cleaning, to obtain a tenacious coating. Treatment of the work-pieces i5 also possible by the dipping method~ The duration of the treatment is usually between 2 and 30 min. A layer weight between 3 and 8 g/m proved to be preferable for the treated pieces.
By means of the coating solution used according to the present invention, it is possible to achieve tenacious layers which in connection with lubricants bring about a very .-~ good separation of the workpiece from the die even under high ~30 severity cold forming conditions.
:.: DETAILED DESCRIPTION OF THE IN~ENTION

~ The method according to the present invention can be ;~
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g~ i employed in practice in a simple and expedient manner in tha-t a solid mixture containing an al]caline-earth metal hydroxide and/or alkaline-earth metal oxicle (equivalents for the purposes of the present invention) as wel.l as a water soluble alkaline-earth metal salt is used for the preparation of the treating solution. In this mixture, the alkaline-earth metal hydroxide and/or alkaline-ear-th metal oxide content is preferably chosen so that the aqueous solution will have tha desired pH value over 8, ''' .~

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pre~erably within a range oE 10 to 13. At too low pH values, the alkalinity of the treating solution may threaten to drop too rapidly, which results in the layer-~ormlng reaction becoming too slow. Too high pH values (excess of alkaline-earth metal hydroxide and/or alkaline-earth metal oxide) are not harm~ul in principle but would lead to the presence in the bath of components that are no longer soluble. Such a phenomenon should be avoided in order to prevent any unnecessary loading of the treatment solution with insoluble components from the very beginning.
The use of the above-mentioned mixtures for the pre~
paration of the treating solutions to be used according to the present invention offers the advantage in practice that starting products that are fluid and consequently easy to handle can be used.
~-15 Calcium compounds are the preferred alkaline-earth metal compounds used fvr the method according to the present invention, N`itrates, chlorides, bromides, borates, or acetates or mixtures thereof can be used, for example, as wat~r-soluble salts of the alkaline--earth metals.
Examples of suitable mixtures for preparation of the coating baths are mixtures composed of 78~ by weight CaC12 and 22% by weight Ca(OH)2, or 73~ by weight Ca(NO3)2 and 27~ by weight Ca(OH)2.
The treatment solution is prepared in such a manner ~hat the mixtura composed of alkaline-earth metal hydroxide and/or alkaline-earth metal oxide and other alkaline-earth m~tal salt is - slowly dissolved in water under steady stirring. To expedite dissolution, the bath temperature can be increased to between 50 and 100C. A slight turbidity of the bath, caused by incompletely dissolv~d alkaline-earth metal hydroxide and/or alkaline-earth metal oxide, does not cause detrimental effectsO

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The overall concen-tration of the dissolved alkaline-earth metal ions in the coating bath shall preferably be at least 5 g/liter so that the bath is not exhausted too rapidly. The concentration range of 5 to ~0 g/liter is preferable. Too high a concentration is not harmful in principle; however, such concen-trations are avoided since they would cause increased losses due to drag-out, and the bath operation would be uneconomical.
The workpieces to undergo cold~working are treated in the above-described preferred bath, possibly after preliminary ~10 cleaning, at a temperature between 50 and 100C for 2 to 30 minutes. The temperatures and dipping times to be applied are determined by the given heating capacity, cycle times, the desired layer thickness, and the kind and wéight of the workpieces to be .~, - .
deformed. A temperature of between 60 and 80C and a dipping time between 5 to 15 minutes have proved to be normally satisfactory in practice.
The kind of cleaning pretreatment of the workpieces is determined by the surface finish; greasy workpieces can be cleaned in solvents or in aqueous alkaline solutions, while grease-free workpeices can be deoxidized in acids and subsequently rinsed with water.
~- After the workpieces are treated in the bath used according to the present invention, they are thoroughly rins~d -`~, wlth water. Then, the workpieces can be dried in an oven and then wetted with a lubricant at a later date on the cold-working ma~hine, or they may be transferred without preliminary drying into an aqueous lubricant bath which preferabIy has a temperature ~igher than 60C so that the workpieces can be dried in air by t~eir own heat following soap coating.

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Suitable aqueous lubrican-ts can be based, for example, on an al~ali soap or an alkal ne-earth soap suspension. When using alkali soaps as lubricants, c~oncentrations between 2% and 6% are best; alkaline-earth soap suspellsions shall preferably have a concentration of between 5~ and 25~ solids.
During the passage of the workpieces through the carrier bath, the bath gradually loses its alkalinity and its alkaline-earth metal ion concentration. Therefore, the bath needs to be controlled and replenished. It was found possible to maintain the efficiency of the bath used according to the present invention in a simple manner through the addition of a mixture comprising an alkaline~earth metal hydroxide and/or alkaline-earth oxide and alkaline-earth metal salt, with the molar ratio of the alkaline-- earth metal hydroxide and/or alkaline-earth metal oxide to the alkaline-earth metal salt being 1:(12-0.05), and preferably 1:(5-0.5)~
For the control of the bath concentration, 10 ml bath ~ samples may be titrated with 0.1 N HCl solution against bromcresol - green as an indicator. The titration value thus determined is a measure of the bath concen~ration and of the necessary quantity of substance to be added.
~ EX~MPLE 1 -~ Disks of AlMg 3 grade aluminum alloy were treated in the following manner as preparation for two-stage cold impact forming into a complicated cup:
The workpieces were first dip-cleaned in a solution based on sodium pyrophosphate, sodium silicate, and a surfactant ~concentration 30 g/liter) at 80C for 10 minutes, and were - subsequently rinsed with water. Then, they were pickled at ambient ;30 temperature for 5 minutes in a solution based on phosphoric acid .. . .
and hydrofluoric acid, after which they were again rinsed with water.

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The disks were the~n dip--treated in a coating so]ution pr~pared by dissolving 40 g/liter of a solid mixture of 78% hy weiyht CaC12 and 22~ by weight Ca(OH)2 in water (Ca content 16 g/liter) at 70C for a period of 5 minutes. The solution pH was 12.5. The specimens were then rinsed with water. The carrier layer prepared was solidly intergrown with the surface and was white in color. The layer weight was 6 g/m2.
The disks were then dipped into a 5~ sodium stearate - solution for 5 minutes at 80C, after which they were dried in air.The cold forming of the disks to form cups was satis-factory without any scoring in either step.
For comparison, identical disks were treated in the same manner but without the alkaline treatment to form the carrier layer. Rough scoring was observed during the pressing of these I5 disks even already after the first cold forming step~
~or further comparison, identical disks were cleaned in the same manner, after which they were dipped into a phosphatizing solution based on zinc phosphate as commonly used for aluminum workpieces at 70C for 10 minutes, and were subsequently rinsed with water. The layer weight of the phosphate layer thus produced `~ was 6 g/m2.
- The coated disks, rinsed with water, were then dipped into 5% sodium stearate solution for 5 minutes at 80C, and were subsequently dried in air. The cold forming was satisfactory in the first step. Rough scores were found, however, after the second step.

A coating solution was prepared by dissolving a 30 g/llter mixture compos d of 73~ by weight Ca(NO3)2 and 27~ by weight Ca(OH)2 in water, The pH value was 12.5. The titration It` ` ' ' ~.. `.' , ` ' . .
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of a 10 ml bath sample with 0~1 N ~IC]. solution against bromcresol green revealed a consump,ion of 20 ml (20 points).
Pre~clealled aluminum plates (Al~g 3) were dipped into the coatin~ solution at 70C for 5 minutes. The overall through~
put amounted to 4 m2/liter solution.
~- Each time after the passage of 1 m surface per liter, the bath concentration was tested by titration of a 10 ml sample.
When a reduction of the point value to 12.4 was observed, the bath - ~ was then replenished with a mixture composed of 67% by weight ~10 Ca(NO3)2 and 33% b~ weight Ca(OH)2 (molar ratio of Ca(OH)2 to Ca(NO3)2 = 1:0.9). In order to restore the point value to 20, ~ 8.5 g/liter bath solu~ion were added.
;~ During the entire treatment process, no differences in the layer weight (5 to 7 g/m2), or in the appearance and adhesive strength of the coatings was observed.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for preparing an aluminum surface for cold forming wherein an inorganic carrier layer is formed on the surface followed by application of a lubricant the improvement comprising forming the carrier layer by contacting the surface with an aqueous composition containing an alkaline-earth metal compound adjusted to a pH value in excess of 8.
2. The process of Claim 1, wherein the aqueous compo-sition used to apply the carrier layer is prepared by adding to water an alkaline-earth metal hydroxide and a water-soluble alkaline-earth metal salt.
3. The process of Claim 1, wherein the the composition used to apply the carrier layer is supplemented with an alkaline-earth metal hydroxide and a water-soluble alkaline-earth metal salt, with the molar ratio of the alkaline-earth metal hydroxide to the alkaline-earth metal salt being in a range of l:(12-0.05).
4. The process of Claim 1, wherein the alkaline-earth metal is calcium.
5. The process of Claim 1, wherein the pH value of the composition for applying the carrier layer is between 10 and 13.
6. The process of Claim 1, wherein the carrier layer composition is maintained at a temperature of 50 to 100°C.
7. The process of Claim 1, wherein the alkaline-earth metal content of the bath is at least 5 g/l.
8. An aluminum article when coated by the process of

Claim 1.