EP2077949A2

EP2077949A2 - Manufacturing process to produce litho sheet

Info

Publication number: EP2077949A2
Application number: EP07759824A
Authority: EP
Inventors: Juan Francisco Del Rio Martin; Jose Alberola Fuster; Gemma Mora Azuar
Original assignee: Alcoa Inc
Current assignee: Acr Ii Aluminium Group Cooperatief Ua
Priority date: 2006-03-31
Filing date: 2007-03-30
Publication date: 2009-07-15
Anticipated expiration: 2027-03-30
Also published as: KR20080109058A; EP2077949B1; US20080035488A1; WO2007115167A3; CN101484322A; KR101152169B1; BRPI0709691A2; WO2007115167A2

Abstract

The present invention provides an aluminum alloy for lithographic sheet including about 0.05 wt % to about 0.25 wt % Si; about 0.25 wt % to about 0.4 wt % Fe; less than or equal to about 0.04 wt % Cu; less than or equal to about 0.25 wt % Mn; 0.31 wt % to 0.35 wt % Mg; less than or equal to about 0.03 wt % Zn; less than or equal to about 0.03 wt % Ti; and incidental impurities. Another aspect of the invention is a method of processing a lithographic sheet including the steps of providing an aluminum sheet; contacting the aluminum sheet with an electrolyte bath; and applying a current having a non-sinusoidal wave form with a constant peak voltage to said electrolyte bath.

Description

MANUFACTURING PROCESS TO PRODUCE LITHO SHEET

[0001] This application claims the benefit of U.S. Provisional Application No.

60/787,826 filed March 31, 2006.

Field of the Invention [0002] This invention in one embodiment relates to an Al 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 [0003] Lithographic sheet manufacturing places high requirements on purity and

uniformity of litho strip surfaces. Lithographic 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).

[0004] 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.

Summary of the Invention [0005] In one embodiment, the present invention provides an alloy suitable for

lithographic sheet applications that provides increased strength and suitable graining

response performance.

[0006] In one embodiment, the aluminum alloy includes:

[0007] about 0.05 wt % to about 0.25 wt % Si;

[0008] about 0.25 wt % to about 0.4 wt % Fe;

[0009] less than or equal to about 0.04 wt % Cu;

[0010] less than or equal to about 0.25 wt % Mn;

[0011] 0.31 wt % to about 0.40 wt % Mg;

[0012] less than or equal to about 0.03 wt % Zn; and

[0013] less than or equal to about 0.03 wt % Ti;

[0014] In another embodiment, the aluminum alloy includes:

[0015] about 0.8 wt % to about 0.12 wt % Si;

[0016] about 0.28 wt % to about 0.32 wt % Fe;

[0017] less than or equal to about 0.007 wt % Cu;

[0018] less than or equal to about 0.02 wt % Mn;

[0019] 0.31 wt % to about 0.35 wt % Mg;

[0020] less than or equal to about 0.03 wt % Zn; and

[0021] less than or equal to about 0.014 wt % Ti. [0022] 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:

[0023] providing an aluminum sheet;

[0024] contacting the aluminum sheet with an electrolyte bath; and

[0025] applying a current having a non-sinusoidal wave form and substantially

constant peak values to said electrolyte bath.

[0026] 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 [0027] 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:

[0028] 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.

[0029] 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 electro graining treatment with 8% HNO₃ acid and current densities of 10A/dm² for a

period of 90 seconds.

[0030] 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 10 A/dm² for a period of 90 seconds.

[0031] Figures 4a-4c represent micrographs of a lithographic sheet surface formed

an alloy outside the scope of the present invention, which includes 0.2 wt % Mg and 0.07

wt % Mn, wherein the sheet was treated to an electro-graining step with 8% HNO₃ acid

and current densities of 10 A/dm² for a period of 90 seconds.

[0032] Figures 5a-5c represent micrographs of a lithographic sheet surface formed

using the alloy and process of the present invention, wherein the sheet was treated to an

electrograining treatment with 8% HCl acid and current densities of 15 A/dm^" for a period

of 20 seconds.

Detailed Description of Preferred Embodiments [0033] 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, and 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.

[0034] 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. Lithographic 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.

[0035] In one embodiment, the alloy of the present invention includes:

[0036] about 0.05 wt % to about 0.25 wt % Si;

[0037] about 0.25 wt % to about 0.4 wt % Fe;

[0038] less than or equal to about 0.04 wt % Cu;

[0039] less than or equal to about 0.25 wt % Mn;

[0040] 0.31 wt % to about 0.40 wt% Mg;

[0041] less than or equal to about 0.03 wt % Zn; [0042] less than or equal to about 0.03 wt % Ti; and

[0043] a balance of Al and incidental impurities.

[0044] In one embodiment, the Mg content may range from 0.31 wt % to about

0.35 wt %. In one embodiment, the Si content may range from about 0.05 wt % to about

0.25 wt %. In yet another embodiment, the Si content may range from about 0.8 wt % to

about 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 μm,

as observed in a 100x60 μm scanning electron microscopy (SEM) image.

[0045] In one embodiment, the Fe content may range from about 0.25 wt % to

about 0.4 wt % Fe. In yet another embodiment, the Fe content may range from about

0.28 wt % to about 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.

[0046] In one embodiment, Mg present ranging from 0.31 wt % to approximately

0.40 wt %, in accordance 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 approximately 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

performance. 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.

[0047] In one embodiment, the temi increased mechanical strength means that a

lithographic sheet formed from the inventive alloy and work hardened to Hl 8 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.

[0048] In one embodiment, the lithographic sheet formed in accordance with the

present invention and work hardened to H 18, 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 and yield strength than AA 1050 when heat treated following working.

The H 18 designation means that the material was cold rolled at a temperature not

exceeding about 5O⁰C 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 denotes a Brinell hardness greater than about 50.

[0049] In one embodiment, Zn may be present in less than or equal to about 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.

[0050] In one embodiment, Ti may be present in less than or equal to about 0.03

wt %, preferably being less than about 0.014 wt %. In one embodiment, a lower Ti

content favors graining in producing a homogeneous finish, in which 100x60 μm 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 μmf. Grain refiner, such as TiB₂, may or may not be present. Ti combined with

B is not detrimental to graining.

[0051] In one embodiment, Mn is present in less than about 0.25 wt %, preferably

being less that 0.02 wt%. In some embodiments, Mn may have a strengthening effect. In

one embodiment, Mn may be present within a range of about 0.01 wt % to about 0.25 wt %. In one embodiment, Mn may be present from about 0.05 wt % to about 0.25 wt % to

take advantage of Mn's presence in solid solution or intermetallic particles.

[0052] Cu may be present in up to about 0.04 %, and in one embodiment of the

present invention is limited to about 0.007 wt % or less.

[0053J The term "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.

[0054] In one embodiment, the alloy includes about 0.8 wt % to about 0.12 wt %

Si; about 0.28 wt % to about 0.32 wt % Fe; less than or equal to about 0.007 wt % Cu;

less than or equal to about 0.02 wt % Mn; 0.31 wt % to 0.35 wt % Mg; less than or equal

to about 0.03 wt % Zn; less than or equal to about 0.0 14 wt % Ti; and a balance of Al

and incidental impurities.

[0055] 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.

[0056] 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. [0057] 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 500⁰C to 600⁰C. The pre¬

heat time may range from 2-20 hours depending on the heat up cycle of the furnace.

[0058] 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 320⁰C to about about 360⁰C to obtain a grain re-

crystallized structure (fine grain structure). The coiling temperature is maintained

between about 320⁰C and about 36O⁰C by cooling sprays. If the temperature drops below

about 320⁰C undesirable cold working effects may be observed. In one embodiment,

temperatures drops to below about 32O⁰C adversely effect recrystallization of the

structure, which may result in streaking during electro graining. If the temperature is

greater than 360⁰C 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.

[0059] 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 280⁰C to

about 500⁰C, 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.

[0060] 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 on

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 nm. 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.

[0061] 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 about 4 A/dm² to about 12 A/dm² for dwell times of about 0.5 to about 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. [0062] In one embodiment, the mineral acid bath (electrolyte) may include any

mineral acid in a concentration of less than about 35%, and in another embodiment the

mineral acid is in a concentration of about 5% to about 35%, and yet in an even further

embodiment the mineral acid bath may be about 15% to about 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 about 15 g/1

(of Al ion) in phosphoric acid electrolytes, and below about 20 g/1 in sulphuric acid,

wherein higher levels may decrease conductivity. In one embodiment, the mineral acid

bath includes phosphoric acid ranging from about 10% to about 30%, and in yet another

embodiment approximately 20% phosphoric acid, and containing about 2 g/1 to about 15

g/1 aluminum, wherein the aluminum concentration may be equal to approximately 0 g/1

during start up operations. In one embodiment, the temperature of the mineral acid bath

may range from about 40⁰C to about 100⁰C, and in another embodiment may range from

about 50⁰C to about 80⁰C. Alternatively, it has also been contemplated that the mineral

acid bath may include chromic, boric, and tartaric acids and combinations thereof.

[0063] 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

autotrans former. 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 about 60 volts,

can be symmetrical or asymmetrical and provide a selected charge density up to about 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 autotrans formers

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 about 35 to about 60 volts.

[0064] 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.

[0065] 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.

[0066] The present alloy and processing method provides a lithographic sheet

having higher mechanical properties than AA 1050, better fatigue behavior, and allows

for longer press runs.

[0067] 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.

[0068] By changing the switching point of the thyristor power supply, the exact

current density desired on the aluminum sheet can be obtained.

[0069] Although the invention has been 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

[0070] Table 1 below shows the composition of an alloy within the scope of the

present invention, designated "ALLOY", which is hereafter referred to as the inventive

alloy, and an alloy representative of Aluminum Associations (AA) 1050, which is

hereafter referred to as the comparative example. Table 1

[0074] Lithographic sheets were formed using the alloy representative of the

invention 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 inventive 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 about 4A/dm^~ to about 12 A/dm^" for

dwell times of about 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 auto trans former.

[0075J The inventive alloy sheet and comparison sheet were then tested for

ultimate tensile strength (UTS), yield strength (YS), and Elongation (%) after being

worked to Hl 8 temper. Samples were also tested for ultimate tensile strength (UTS),

yield strength (YS), and Elongation (%) following a heat treatment at a temperature of

280⁰C for a period of 4 minutes.

Table 2

[0076] Table 2 shows the mechanical strength advantages of the inventive 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 inventive 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 H 18 temper. Similar results were observed in

the samples that had been heat treated. Specifically, after a heat treatment of 280⁰C 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.

[0077] Lithographic sheet prepared in accordance with the present invention and

comparative examples formed from compositions similar to AA 1050, where 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.

[0078] Figures 2a-2c represent micrographs of electrogram roughened lithographic

sheet surface formed using the alloy and process in accordance with the present

invention, as designated in Table 1.

[0079] Figures 3a-3c represent micrographs of a comparative example of an

electrogram roughened lithographic sheet surface formed 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, 0.02 wt % Mg, 0.00 1 wt

% Cr, 0.002 wt % Ni, 0.015 wt % Zn, and 0.015 wt % Ti.

[0080] Figures 4a-4c represent micrographs of a comparative example of an

electrograin roughened lithographic sheet surface formed from an alloy composition

similar to AA 1050, which included about 0.2 wt % Mg and about 0.07 wt % Mn.

Specifically, the comparative example depicted in Figures 4a-4c was formed from an

alloy composition including 0.090 wt % Si, 0.34 wt % Fe, 0.004 wt % Cu, 0.07 1 wt %

Mn, 0.018 wt % Mg, 0.001 wt % Cr, 0.002 wt % Ni, 0.013 wt % Zn, and 0.013 wt % Ti.

[0081J Figures 5a-5c represent micrographs of a lithographic sheet surface formed

using the alloy and process 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/dm² for a period of 20 seconds. Specifically, the alloy was composed of .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.

[0082J The electro-graining aspect for the lithographic sheet formed in accordance

with the present invention was equal to the comparative examples of AA 1050.

[0083] It will be readily appreciated by those skilled in the art that modifications

may be made to the invention without departing from the concepts disclosed in the

foregoing description. Such modifications are to be considered as included within the

following claims unless the claims, by their language, expressly state otherwise.

Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of

the appended claims and any and all equivalents thereof.

Claims

What is claimed is:

1. An aluminum alloy comprising:

about 0.05 wt % to about 0.25 wt % Si;

about 0.25 wt % to about 0.4 wt % Fe;

less than or equal to about 0.04 wt % Cu;

less than or equal to about 0.25 wt % Mn;

0.31 wt % to about 0.40 wt % Mg;

less than or equal to about 0.03 wt % Zn; and

less than or equal to about 0.03 wt % Ti;

2. The alloy of Claim 1 comprising Si ranging from about 0.8 wt % to about 0.12 wt

%.

3. The alloy of Claim 1 comprising Fe ranging from about 0.28 wt % to about 0.32

wt %.

4. The alloy of Claim 1 comprising Zn ranging from .01 wt % to .03 wt %.

5. The alloy of Claim 1 comprising Ti in less than or equal to about 0.0 14 wt %.

6. The alloy of Claim 1 comprising Mg ranging from 0.31 wt% to about 0.35 wt. %.

7. The alloy of Claim 1 comprising less than or equal to 0.007 wt % Cu.

8. An aluminum alloy comprising:

about 0.8 wt % to about 0.12 wt % Si;

about 0.28 wt % to about 0.32 wt % Fe;

less than or equal to about 0.007 wt % Cu;

less than or equal to about 0.02 wt % Mn;

0.31 wt % to about 0.35 wt % Mg;

less than or equal to about 0.03 wt % Zn; and

less than or equal to about 0.0 14 wt % Ti;

9. A method of producing a lithographic sheet comprising:

providing an aluminum sheet;

contacting the aluminum sheet with an electrolyte bath; and

applying a current having a non-sinusoidal wave form to said electrolyte bath at a

constant peak voltage.

10. The method of Claim 9 wherein the wave form of the non-sinusoidal current is

either symmetrical or asymmetrical and is generated by a thyristor power supply to

provide a desired current density to the aluminum sheet by moving the switching point of

the thyristor power supply.

1 1. The method of Claim 10 wherein the constant peak voltage ranges from about 35

to about 60 volts.

12. The method of Claim 10 further comprising applying the non-sinusoidal wave

form current with a current density ranging from about 4 to about 12 A/dnr.

13. The method of Claim 12 comprising applying the non-sinusoidal wave form

current for dwell times ranging from about 0.5 to about 3.0 seconds.

14. The method of Claim 10 wherein the electrolyte bath comprises a mineral acid in a

concentration of less than about 35%.

15. The method of Claim 13 wherein the electrolyte bath comprises sulfuric,

phosphoric, or sulfuric-phosphoric mixtures.

16. The method of Claim 15 wherein the electrolyte bath comprises less than 20 g/1.

17. The method of Claim 16 wherein the electrolyte bath comprises a temperature

ranging from about 40⁰C to about 100⁰C.

18. The method of Claim 9 wherein providing the aluminum sheet comprises an alloy

comprising:

about 0.05 wt % to about 0.25 wt % Si;

about 0.25 wt % to about 0.4 wt % Fe;

less than or equal to about 0.04 wt % Cu;

less than or equal to about 0.25 wt % Mn;

0.31 wt % to about 0.40 wt % Mg;

less than or equal to about 0.03 wt % Zn; and

less than or equal to about 0.03 wt % Ti;

19. The method of Claim 18 further comprising using a thyristor power supply to

generate the current having a non-sinusoidal wave form, wherein the power supply is

configured to provide a current with a desired current density applied to the aluminum

sheet by moving the switching point of the thyristor power supply.

20. The method of Claim 19 wherein the electrolyte bath comprises sulphuric acid and

then applying the pulse wave current comprises a current density ranging from about 4-

12 A/dm^" for dwell times of 0.5 to 3.0 seconds.