EP2077949B1

EP2077949B1 - Manufacturing process to produce litho sheet

Info

Publication number: EP2077949B1
Application number: EP07759824.1A
Authority: EP
Inventors: Juan Francisco Del Rio Martin; Jose Alberola Fuster; Gemma Mora Azuar
Original assignee: Aludium Transformacion de Productos SL
Current assignee: Acr Ii Aluminium Group Cooperatief Ua
Priority date: 2006-03-31
Filing date: 2007-03-30
Publication date: 2015-09-30
Anticipated expiration: 2027-03-30
Also published as: WO2007115167A3; KR101152169B1; EP2077949A2; BRPI0709691A2; CN101484322A; WO2007115167A2; US20080035488A1; KR20080109058A

Description

Field of the Invention

This invention in one embodiment relates to an A1 alloy and yet in another embodiment relates to a process suitable for producing lithographic sheet having increased strength and improved electro-graining response.

Background of the Invention

Lithographie sheet manufacturing places high requirements on purity and uniformity of litho strip surfaces. Lithographie sheet manufacturing typically includes a roughening process step. It is standard practice to perform electrochemical (EC) roughening, also referred to as electrograining. It is desirable for electro-graining of the lithographic sheet to result in a plate that is rough across its entire surface and exhibits a very uniform non-directional appearance (no streakiness effects).
A finished printing plate formed of lithographic sheet is inserted into the printing machine, wherein the exact clamping of the plate on the printing cylinder so that no play will result during the printing process. When the printing plate is not perfectly secured and is thus cyclically subjected to bending or torsional loads during printing, plate cracking occurs according to practical experience in the fast running rotary offset printing machines. The reason for plate cracking is fatigue fracture, and the result is an immediate interruption of the printing process. Therefore, Al-materials for offset printing plates must exhibit sufficiently high fatigue strength or reversed bending fatigue strength so that plate cracking can be prevented.
EP-A-1 172 228 discloses a support for a lithographic printing plate and a presensitized plate that can be processed into a lithographic printing plate. The support for a lithographic printing plate contains 0.2 - 0.5 wt% Fe, 0.04 - 0.11 wt% Si, 0.003 - 0.04 wt% Cu, 0.010 - 0.040 wt% Ti, 0.002 - 0.02 Zn and 0.05 - 0.5 wt% Mg, the balance being Al and incidental impurities. It is treated wit a graining treatment and an anodizing treatment.
JP 2005002429 A discloses an aluminum alloy material for a planographic printing plate having a composition containing 0.10 - 1.00 wt% Fe, 0.01 - 0.5 wt% Si, 0.001 - 0.05 wt% Cu, 0.005 to 0.03% wt% Ti, 0.06 to 0.8 wt% Mg and 0.001 to 0.2 wt% Ni, and the balance Al with inevitable impurities.

Summary of the Invention

In one embodiment, the present invention provides a lithographic sheet comprising an alloy that provides increased strength and suitable graining response Performance.
In one embodiment, the aluminum alloy consists of:

0.05 wt % to 0.25 wt % Si;
0.25 wt % to 0.4 wt % Fe;
less than or equal to 0.04 wt % Cu;
0.01 wt % to 0.25 wt % Mn;
0.31 wt % to about 0.40 wt % Mg;
less than or equal to 0.03 wt % Zn; and
less than or equal to 0.03 wt % Ti; the balance being aluminum and incidental impurities, the incidental impurities no greater than 0.05 wt% each and 0.15 wt% in combination.
In another embodiment, the aluminum alloy includes:
0.05 wt % to 0.12 wt % Si;
0.28 wt % to 0.32 wt % Fe;
less than or equal to 0.007 wt % Cu;
less than or equal to 0.02 wt % Mn;
0.31 wt % to 0.35 wt % Mg;
less than or equal to 0.03 wt % Zn; and
less than or equal to 0.014 wt % Ti.

In another aspect of the present invention, a method is provided for forming a lithographic sheet including a electrolytic pre-etching step. In one embodiment, the method for producing a lithographic sheet includes: the steps according to claim 10.
The current having a non-sinusoidal wave form with substantially constant peaks may be obtained from a thyristor power supply which conducts in either one or both directions to provide a desired current density applied to the aluminum sheet by controlling the phase angle of the switching point of the power supply.

Brief Description of the Drawings

The following detailed description, given by way of example and not intended to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, wherein like reference numerals denote like elements and parts, in which:

Figure 1 shows the sinusoidal wave form of current used in the prior art and the non-sinusoidal wave form with constant peak values here disclosed, in accordance with the present invention.
Figures 2a-2c represent micrographs of a lithographic sheet surface formed using the alloy and process of the invention, wherein the sheet was treated to an electrograining treatment with 8% HNO₃ acid and current densities of 10A/dm² for a period of 90 seconds.
Figures 3a-3c represent micrographs of a lithographic sheet surface formed of an alloy outside the scope of the present invention, which includes 0.2 wt % Mg, wherein the sheet was treated to an electro-graining treatment with 8% HNO₃ acid and current densities of 10A/dm² for a period of 90 seconds.
Figures 5a-5c represent micrographs of a lithographic sheet surface formed using an alloy outside the scope of the present invetion and processed in accordance with the present invention, wherein the sheet was treated to an electrograining treatment with 8% HCl acid and current densities of 15A/dm² for a period of 20 seconds.

Detailed Description of Preferred Embodiments

Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely illustrative of the invention that may be embodied in various forms. In addition, each of the examples given in connection with the various embodiments of the invention are intended to be illustrative, a.nd not restrictive. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.
In accordance with one embodiment of the present invention, an aluminum alloy is provided for forming lithographic sheet that provides increased strength and suitable electro-graining performance. Lithographie sheet is used in printing applications to provide a printing plate. As used herein the term aluminum alloy means an aluminum metal with soluble alloying elements either in the aluminum lattice or in a phase with aluminum. All component percentages herein are by weight percent unless otherwise indicated. When referring to any numerical range of values, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 5-15 wt. % Si, for example, would expressly include all intermediate values of about 5.1, 5.2, 5.3 and 5.5%, all the way up to and including 14.5, 14.7 and 14.9% Si. The same applies to each other numerical property, relative thickness and/or elemental range set forth herein.
In one embodiment, the lithographic sheet of the present invention includes an aluminum alloy as defined in claim 1.
In one embodiment, the Mg content may range from 0.31 wt % to 0.35 wt %. In one embodiment, the Si content may range from 0.05 wt % to 0.25 wt %. In yet another embodiment, the Si content may range up to 0.12 wt %. Si in solution may alter the reactivity of the lithographic sheet during electro-graining. If the Si content is too low, a low pitting density may disadvantageously occur during electro-graining, which may render the surface not suitable for lithographic sheet. Low pitting density may be have a surface characterized as including flat areas, which may be referred to as plateaus, that may be detected by scanning electron microscopy (SEM), wherein in one embodiment a low pitting density may have a negative skewness (S) value, with absolute values being higher than 0.4. If the Si content is too great, too few pits may form during electro-graining, in which the size of the individual pits may be too large. In one embodiment, excess pitting may be characterized as a surfacing having at least two pits with a diameter greater than 10 lim, as observed in a 100x60 p.m scanning electron microscopy (SEM) image.
In one embodiment, the Fe content may range from 0.25 wt % to 0.4 wt % Fe. In yet another embodiment, the Fe content may range from 0.28 wt % to 0.32 wt %. Similar to Si, Fe in solution may alter the reactivity of the lithographic sheet during electro-graining, wherein excess pitting may occur if the Fe content is too low or insufficient pitting may occur when the Fe content is too great. Additionally, increasing the Fe content above the specified range may result in increased intermetallic phases present as particles within the sheet, which are detrimental to the sheet's electro-graining performance.
In one embodiment, Mg present ranging from 0.31 wt % to 0.40 wt %, in a.ccordance with the present invention, provides for an electrograining response that may provide a topography of round pits having a diameter of less than five microns when processed with acids, such as HNO₃, HCl or combinations thereof, and mixtures further including additives selected from the group including but not limited to acetic acid and boric acid. In one embodiment, the Mg content may range from 0.31 wt % to 0.35 wt %. Mg is one element in the alloy that may provide for strengthening in work hardening. The Mg content of the present alloy surprisingly helped attained improved mechanical strength, while maintaining electro-graining perfonnance. Prior to the present invention, a suitable electrograining response was not achievable in alloys having the Mg content of the present alloy with commercial electrograining acids, such as HNO₃ or HCl.
In one embodiment, the term increased mechanical strength means that a lithographic sheet formed from the inventive alloy and work hardened to H18 temper has a greater ultimate tensile strength (UTS) and yield strength (YS) being at least about 20 MPa higher than similarly prepared lithographic sheets of AA 1050.
In one embodiment, the lithographic sheet formed in accordance with the present invention and work hardened to H18, may have an ultimate tensile strength greater than about 165 MPa, in another embodiment being greater than about 175 MPa, and a yield strength greater than about 155 MPa, in another embodiment being greater than about 160 MPa. Additionally, the inventive aluminum alloy has a higher ultimate tensile strength a.nd yield strength than AA 1050 when heat treated following working. The H18 designation means that the material was bold rolled at a temperature not exceeding about 50°C for significant periods of time to a reduction of about 74% or more as the last processing step, thereby producing a hard material. For the purposes of this disclosure a hard material devotes a Brinell hardness greater than about 50.
In one embodiment, Zn ma.y be present in less than or equal to 0.03 wt %. In another embodiment, the Zn content may range from 0.01 wt % to 0.03 wt %. In some embodiments, Zn is advantageous for electro-graining in nitric acid. In one embodiment, Zn is electrochemically anodic with respect to aluminum and functions as the initiator for pit formation during electrograining.
In one embodiment, Ti may be present in less than or equal to 0.03 wt %, preferably being less than 0.014 wt %. In one embodiment, a lower Ti content favors graining in producing a homogeneous finish, in which 100x60 ium SEM micrographs do not include isolated pits having a diameter greater than about 10 µm in diameter or flat areas (plateaus) having a topography with a surface area greater than about 25 lan. Grain refiner, may or may not be present. Ti combined with B is not detrimental to graining.
In some embodiments, Mn may have a strengthening effect. In one embodiment, Mn may be present within a range of 0.01 wt % to 0.25 wt %. In one embodiment, Mn may be present from 0.05 wt % to 0.25 wt % to take advantage of Mn's presence in solid solution or intermetallic particles.
Cu may be present in up to 0.04 %, and in one embodiment of the present invention is limited to 0.007 wt % or less.
The tenn "incidental impurities" refers to elements that are not purposeful additions to the alloy, but that due to impurities and/or leaching from contact with manufacturing equipment, trace quantities of such elements being no greater than about 0.05 wt % each and in combination no greater than about 0.15 wt % of the final alloy, which may nevertheless find their way into the final alloy product.
In one embodiment, the alloy includes 0.05 wt % to 0.12 wt % Si; 0.28 wt % to 0.32 wt % Fe; less than or equal to 0.007 wt % Cu; less than or equal to 0.02 wt % Mn; 0.31 wt % to 0.35 wt % Mg; less than or equal to 0.03 wt % Zn; less than or equal to 0.0 14 wt % Ti; and a balance of Al and incidental impurities.
In another aspect of the invention, a method is provided for processing an aluminum alloy, such as the alloy described-above, for producing a lithographic sheet.
The lithographic sheet forming process begins with providing a direct cast ingot preferably in accordance with the above compositions. In one embodiment, titanium boride may be employed as a grain refiner. The ingot is scalped in a machining step to remove the non-uniformities from the ingot's surface that are typically formed during the casting process.
Following preparation of the ingot as described above, the ingot is treated by a pre-heat step. The pre-heat step prepares the ingot for hot rolling and provides for a uniform microstructure throughout ingot. In one embodiment, the pre-heat step is conducted in a gas/electric furnace at a temperature between 500°C to 600°C. The pre-heat time may range from 2-20 hours depending on the heat up cycle of the furnace.
The ingot is then hot rolled to a thickness ranging from about 7.5 mm to about 10 mm. The hot rolling apparatus may be single stand or multi-stand hot mill. Following hot rolling, the strip is then coiled, in which the coiling temperature is maintained between about about 320°C to about about 360°C to obtain a grain recrystallized structure (fine grain structure). The coiling temperature is maintained between about 320°C and about 360°C by cooling sprays. If the temperature drops below about 320°C undesirable cold working effects may be observed. In one embodiment, temperatures drops to below about 320°C adversely effect recrystallization of the structure, which may result in streaking during electrograining. If the temperature is greater than 360°C the sheet may experience surface defects including but not limited to welding laps, damages or pick ups that may result in physical defects on the lithographic sheet product.
In a next series of process steps, in one embodiment, the strip is cold rolled to a thickness ranging from about 1.0 mm to about 5.0 mm, in yet another embodiment to a thickness ranging from about 1.5 mm to about 3.0 mm, and then annealed for approximately 2 to approximately 6 hours at a temperature ranging from about 280°C to about 500°C, in which the annealing atmosphere may or may not be an inert atmosphere. The strip is then cold rolled to a final gauge, i.e. ranging from about 0.1 mm to about 0.5 mm, with a minimal reduction of about 74%. Once cold rolled to its final gauge, the strip is then trimmed and tension leveled.
The aluminum strip is then treated with an electrolytic pre-etching or degreasing step, hereafter referred to as an electrolytic pre-etching step, including a combination of chemical and electrical treatments that produce an anodized coating an the sheet's surface, which provides for greater graining response. The electro-graining response provided by the present invention is characterized as a topography having fine round pits of a diameter of less than about 5 microns. In one embodiment, the anodized coating may be an aluminum oxide having a thickness of about 100 nm or less, and in yet another embodiment may be a thickness ranging from about 1 nm to about 30 mn. It is noted that other thickness for the anodized coating have been contemplated and are within the scope of the present invention, so long as the thickness of the anodized coating should provide protection from oxidation, yet be thin enough to be easily removed in subsequent operations.
In one embodiment, the electrolytic pre-etching step includes passing the aluminum strip through a mineral acid bath (electrolyte), and applying a current density ranging from 4A/dm² to 12A/dm' for dwell times of 0.5 to 3.0 seconds using silicon controller rectifier (SCR) pulse waves. In one embodiment, the charge density is about 3000 Qm^-2. In one embodiment, the electrolytic pre-etching step is a continuous in-line process, wherein the aluminum strip enters the mineral acid bath, a current is applied and the aluminum strip is removed with an anodized coating.
In one embodiment, the mineral acid bath (electrolyte) ma.y include any mineral acid in a concentration of less than 35%, and in another embodiment the mineral acid is in a concentration of 5% to 35%, and yet in an even further embodiment the mineral acid bath may be 15% to 25%. In one embodiment, the mineral acid includes sulfuric, phosphoric, or sulfuric-phosphoric mixtures. In one embodiment, the aluminum content of the electrolyte should be kept below 15 g/l (of Al ion) in phosphoric acid electrolytes, and below 20 g/l in sulphuric acid, wherein higher levels may decrease conductivity. In one embodiment, the mineral acid bath includes phosphoric acid ranging from 10% to 30%, and in yet another embodiment approximately 20% phosphoric acid, and containing 2 g/l to 15 g/l aluminum, wherein the aluminum concentration may be equal to approximately 0 g/l during start up operations. In one embodiment, the temperature of the mineral acid bath may range from 40°C to 100°C, and in another embodiment may range from 50°C to 80°C. Alternatively, it has also been contemplated that the mineral acid bath may include chromic, boric, and tartaric acids and combinations thereof.
Figure 1 shows the non-sinusoidal wave form 10 of the current generated by a thyristor power supply which is used during pre-etching when practicing this invention as compared to the sinusoidal wave form 5 generated by a prior art AC autotransformer. The operating frequency of the thyristor power supply is at least several cycles per second and is preferably at the commercial frequency. The wave form of the current here disclosed is non-sinusoidal with constant peak voltage up to 60 volts, can be symmetrical or asymmetrical and provide a selected charge density up to 30,000 Qcm. to the minus 2 which depends upon the strip width or final product requirements. As depicted in Figure 1, in counter distinction to prior AC autotransformers which provide current having sinusoidal wave form 5, current with non-sinusoidal wave form 10 here disclosed can be generated by a thyristor power supply where the conduction angle is selected for the exact current density applied to the aluminum sheet. In one embodiment, the peak voltage ranges from 35 to 60 volts.
The thyristor power supply maintains a constant peak voltage. Degreasing of the aluminum sheet requires cathodic and anodic current. Cathodic current provides mechanical cleaning of oil, debris, and fines from the aluminum sheet. Anodic current provides the generation of thin aluminum oxide coating (anodized coating). Operating with a current having a wave form here disclosed provides increase cathodic current and anodic current. Peak current is related to peak voltage. By maintaining a constant peak voltage and employing a current having a non-sinusoidal wave form 10, uniformity to the cathodic and anodic current is obtained. Therefore, by providing uniformity to the cathodic and anodic current, the current having a non-sinusoidal wave form 10, provides uniformity to mechanical cleaning of the aluminum strip through gas generation and uniformity to the formation of the anodized coating, resulting in a more reactive degreasing step than is possible with a current having a sinusoidal wave form 5 from an AC autotransformer.
Following the pre-etch step the aluminum strip may be roughened by electrograining and may be treated by similar processes used to provide lithographic sheet and plates. Suitable electrograining response may be achieved with the alloy and method of the invention using Hydrochloric or Nitric acid.
The present alloy and processing method provides a lithographic sheet having higher mechanical properties than AA1050, better fatigue behavior, and allows for longer press runs.
In accordance with the principles of the invention, there is disclosed a method of processing a lithographic sheet, in which the current has a non-sinusoidal wave form 10 which can be asymmetrical or symmetrical and has a constant peak voltage
By changing the switching point of the thyristor power supply, the exact current density desired an the aluminum sheet can be obtained.
Although the invention has basis described generally above, the following examples are provided to further illustrate the present invention and demonstrate some advantages that arise therefrom. It is not intended that the invention be limited to the specific examples disclosed.

EXAMPLES

Table 1 below shows the composition of an alloy outside the scope of the claimed invention, designated "ALLOY", and an alloy representative of Aluminum Associations (AA) 1050, which is hereafter referred to as the comparative example. Table 1

Si Fe Cu Mn Mg Cr Ni Zn Ti

ALLOY 0.093 0.32 0.001 0.006 0.32 0.001 0.004 0.004 0.004

AA 1050 0.082 0.4 0.001 0.004 0.2 0.001 0.002 0.015 0.015
Lithographic sheets were formed using the "ALLOY" and the alloy representative of AA 1050. Each sheet was prepared from a DC cast ingot, pre-heat treated, hot rolled, coiled, cold rolled with intermediate anneal steps to a final gauge, and trimmed. In accordance with the present invention, the sheet formed of the "ALLOY" is degreased with a pre-etching step. The pre-etching step included a sulphuric acid bath and a current having a non-sinusoidal wave form with constant peak voltage to provide a current density ranging from 4A/dm' to 12A/dm' for dwell times of 0.5 to 3.0 seconds. The comparison sheet formed from AA 1050 was not treated with the pre-etching step and was processed with a prior art sinusoidal AC wave form current from an AC autotransformer.
The "ALLOY" sheet and comparison sheet were then tested for ultimate tensile strength (UTS), yield strength (YS), and Elongation (%) alter being worked to H18 temper. Samples were also tested for ultimate tensile strength (UTS), yield strength (YS), and Elongation (%) following a heat treatment at a temperature of 280°C for a period of 4 minutes. Table 2

TEMPER H18 BAKING 280C/4 MINUTES

COMP UTS (MPa) YS (MPa) e1. (%) UTS (MPa YS (MPa) e1. (%)

ALLOY >165 >155 >1.0 >115 >115 >1.5

AA 1050 >145 >135 >1.5 >105 >100 >1.5
Table 2 shows the mechanical strength advantages of the "ALLOY" having increased Mg content and processed with the inventive pre-etching step, when compared to a conventionally processed AA 1050 sheet. Specifically, the sheets comprising the "ALLOY" displayed greater than a 10% increase in ultimate tensile strength (UTS) and yield strength (YS) when compared to similarly prepared AA 1050, wherein the samples had been worked to 1-118 temper. Similar results were observed in the samples that had been heat treated. Specifically, after a heat treatment of 280°C for 4 minutes, (baking test) sheets prepared in accordance with the present invention displayed greater than an 8% increase in ultimate tensile strength and greater than a 13% increase in yield strength when compared to similarly prepared AA 1050.
Lithographie sheet prepared with the " ALLOY" and processed in accordance with the present invention and comparative examples formed from compositions similar to AA 1050, were then tested for electro-graining behavior. An electro-graining step was conducted using about 8% HNO₃ acid with current densities of about 10 A/dm² for a time period of about 90 seconds.
Figures 2a-2c represent micrographs of electrograin roughened lithographic sheet surface formed using the ALLOY and a process in accordance with the present invention, as designated in Table 1.
Figures 3a-3c represent micrographs of a comparative example of an electrograin roughened lithographic sheet surface formell from an alloy composition similar to AA 1050, which included about 0.2 wt % Mg. Specifically, the comparative example depicted in Figures 3a-3c was formed from an alloy composition including 0.082 wt % Si, 0.40 wt % Fe, 0.00 1 wt % Cu, 0.004 wt % Mn, 0.2 wt % Mg, 0.00 1 wt % Cr, 0.002 wt % Ni, 0.015 wt % Zn, and 0.015 wt % Ti.
Figures 5a-5c represent micrographs of a lithographic sheet surface formed using an alloy outside the scope of the present invention and the process steps in accordance with the present invention, wherein the sheet was treated to an electrograining treatment with about 8% HCl acid and current density of about 15A/dm' for a period of 20 seconds. Specifically, the alloy was composed of 0.096 wt % Si, 0.33 wt % Fe, 0.002 wt % Cu, 0.005 wt % Mn, 0.34 wt % Mg, 0.001 wt % Cr, 0.005 wt % Ni, 0.002 wt % Zn and 0.015 wt % Ti.
The electro-graining aspect for the lithographic sheet formed in accordance with the process of the present invention was equal to the comparative examples of AA 1050.

Claims

A lithographic sheet comprising an aluminum alloy, wherein the aluminum alloy consists of:
0.05 wt % to 0.25 wt % Si;

0.25 wt % to 0.4 wt % Fe;

less than or equal to 0.04 wt % Cu;

0,01 wt % to 0.25 wt % Mn;

0.31 wt % to 0.40 wt % Mg;

less than or equal to 0.03 wt % Zn; and

less than or equal to 0.03 wt % Ti;

the balance aluminum and incidental impurities, the incidental impurities no greater than 0.05 wt % each and 0.15 wt % in combination.
The lithographic sheet of Claim 1, wherein the aluminum alloy comprises Si up to 0.12 wt %.
The lithographic sheet of Claim 1, wherein the aluminum alloy comprises Fe ranging from 0.28 wt % to 0.32 wt %.
The lithographic sheet of Claim 1, wherein the aluminum alloy comprises Zn ranging from 0.01 wt % to 0.03 wt %.
The lithographic sheet of Claim 1, wherein the aluminum alloy comprises Ti in less than or equal to 0.014 wt %.
The lithographic sheet of Claim 1, wherein the aluminum alloy comprises Mg ranging from 0.31 wt% to 0.35 wt. %.
The lithographic sheet of Claim 1, wherein the aluminum alloy comprises less than or equal to 0.007 wt % Cu.
The lithographic sheet of Claim 1, wherein the aluminum alloy comprises:
0.05 wt % to 0.12 wt % Si;

0.28 wt % to 0.32 wt % Fe;

less than or equal to 0.007 wt % Cu;

less than or equal to 0.02 wt % Mn;

0.31 wt % to 0.35 wt % Mg;

less than or equal to 0.03 wt % Zn; and

less than or equal to 0.014 wt % Ti;
The lithographic sheet of Claim 1 or 7, wherein the aluminum alloy comprises Mn ranging from 0.05 to 0.25 wt %.
A method for producing a lithographic sheet comprising the steps of:
providing an aluminum sheet made from the aluminum alloy according to claim 1; degreasing the aluminum sheet by an electrolytic pre-etching step, wherein

the electrolytic pre-etching step comprises:
contacting the aluminum sheet with an electrolyte bath; and

applying a current having a non-sinusoidal wave form to the electrolyte bath at a constant peak voltage; and

roughening the aluminum sheet by electrograining after degreasing the aluminum sheet.
The method of Claim 10, wherein the non-sinusoidal wave form is either symmetrical or asymmetrical and is generated by a thyristor power supply having a switching point to provide a desired current density to the aluminum sheet by moving the switching point of the thyristor power supply.
The method of Claim 10, wherein the constant peak voltage ranges from 35 to 60 volts.
The method of Claim 11, wherein the method further comprises the step of applying the current with the desired current density ranging from 4 to 12 A/dm².
The method of Claim 11, wherein the method further comprises the step of applying the desired current density for dwell times ranging from 0.5 to 3.0 seconds.
The method of Claim 10, wherein the electrolyte bath comprises a mineral acid in a concentration of less than 35%.
The method of Claim 15, wherein the mineral acid comprises sulfuric, phosphoric, or sulfuric-phosphoric mixtures.
The method of Claim 16, wherein an aluminum content of the electrolyte bath being less than 20 g/l.
The method of Claim 10, wherein a temperature of the electrolyte bath ranges from 40°C to 100°C.
The method of Claim 10, wherein the method further comprises the step of using a thyristor power supply having a switching point to generate the current, wherein the thyristor power supply is configured to provide the current with a desired current density applied to the aluminum sheet by moving the switching point of the thyristor power supply.
The method of Claim 19, wherein the electrolyte bath comprises sulphuric acid and the desired current density ranges from 4-12 A/dm² for dwell times of 0.5 to 3.0 seconds.