WO2009144108A1

WO2009144108A1 - Composite aluminium lithographic sheet

Info

Publication number: WO2009144108A1
Application number: PCT/EP2009/055195
Authority: WO
Inventors: Jeremy Brown; Robert Minor; David Wright
Original assignee: Novelis Inc.
Priority date: 2008-05-28
Filing date: 2009-04-29
Publication date: 2009-12-03

Abstract

The invention relates to an aluminium lithographic sheet product in which a clad layer is applied to at least one side of a core material, in particular the invention relates to an aluminium lithographic sheet product comprising a composite structure having a core layer and at least one dad layer wherein the core alloy is selected from the 1XXX, 3XXX or 5XXX series alloys and the clad layer is either a 1XXX or 3XXX series alloy.

Description

C^QMPQSiTE ALUMINSUy LITHOGRAPHIC SHEET

The present invention relates to an aluminium lithographic sheet product in which at least one clad layer is applied to at least one side of a core layer material.

Composite materials or clad materials of this kind are known within the aluminium industry and find uses in applications in aerospace or as brazing sheet. In brazing sheet the core alloy is typically an alloy from the 3XXX series of alloys and, typically, the clad layer is a 4XXX series alloy. For an understanding of the number designation system most commonly used in naming and identifying aluminium and its alloys see "international Alloy Designations and Chemicai Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys", published by The Aluminum Association, revised January 2001. The main alloying element in a 3XXX series alloy is manganese and in a 4XXX series alloy the main alloying element is silicon. The lower melting point of the 4XXX series alloy allows the brazing sheet to be brazed to other components such as fins or tubes. In aerospace, combinations of 2XXX series alloys as the core alloy (with copper as the main alloying element) with a 1XXX series as the clad layer are widely used. Combinations of 6XXX series alloys in the core layer with 7072 or are also known for use as fuselage sheet.

Alioys of the 1XXX type or 3XXX type are used as monolithic lithographic sheet because they are particularly suited to provide the kind of surface finish required by lithographic sheet users. They also provide enough mechanical strength to allow print runs of required durations. The fiat sheet is usually roughened, or "grained", then anodized, (to provide a hard, durable oxide layer), and then coated with an oleophilic layer prior to use in the printing operation. The hydrophobic layer is often a polymer which is subjected to a curing or "stoving" treatment at temperatures between 150 and 300⁰C for 3 to 10 minutes.

Alloys based on 1XXX or 3XXX are preferred because they are adaptable to different roughening techniques and can be readily anodized.

The 1XXX series of alloys covers aluminium compositions where the aluminium content is >99.00% by weight. The 1XXX series is normally considered to fall into two categories. One category relates to wrought unalloyed aluminium having natural impurity limits. Common alioys include compositions known as 1050 or 1050A but this group also includes super-pure compositions such as 1090 and 1098 where the aluminium content is at least 99.9 weight %. The second category covers alloys where there is special control of one or more impurities. For this category the alloy designation includes a second numeral that is not zero, such as 1100, 1145, and so on.

Alloys of 1050 or 1050A are the main 1XXX series alloys used in lithographic sheet as unclad monolithic sheet materials. Alternatively, alloys based on the 1XXX series but with small additions of elements such as magnesium, manganese, iron or silicon may be used. Other elements that have been added include vanadium and zinc. The addition of controlled quantities of these and other elements, alone or in combination, has usually been made with a view to enhancing a particular property such as yield strength after stoving, fatigue resistance, or to make the surface more responsive to the various treatment steps.

Classification of alloy compositions is not completely precise, of course, and there are a number of compositions mentioned in prior art publications which do not conveniently fall within a particular class. Although 1XXX series alloys are generally considered to have >99.00 weight % aluminium, for the purpose of this invention compositions described by the foilowing patent specifications are also considered as 1XXX series alloys: US6,447,982, WO07/093605, WO07/045676, US20080035488, EP1341942, EP0589996, and EP1425430. Most of these compositions have not been registered with the Aluminum Association but are known to those in the lithographic sheet industry. Of the 3XXX series alloys, the most common alloy for use as lithographic sheet is the alloy 3103, although the alloy 3003 may also be used. Again, various other 3XXX series type alloys have been developed with special alloying additions or combinations, essentially for the same reasons as mentioned above, and the definition of 3XXX series alioys according to this invention is intended to cover alloys which, by virtue of their Mn content would be considered as a 3XXX series alloy. In contrast to the 1XXX series alloys the mechanical properties of 3XXX series alloys are higher but there are often problems during surface treatment operations due to the presence of Mn or Mg rich intermetaliic phases at or near the surface.

Other alioy series such as 2XXX (where the main alloying element is copper), 5XXX (main alloying element is magnesium), 6XXX (main alloying elements Mg and Si), 7XXX (main alloying element Zn) and 8XXX (main alloying element is other elements) are known but have not been generally used for lithographic sheet.

Users of aluminium lithographic sheet experience various problems which the scope of this invention is intended to address. The lithographic sheet or plate may break during printing and this is generally caused by two factors. One factor is that sheet is rarely perfectly flat. Off-flatness waviness can cause fatigue such that an increase in fatigue resistance is desirable. The other factor that can cause premature breakage is that the sheet is wrapped a single time around a drum, the drum having a narrow gap into which the ends of the sheet are bent and placed. The wrapped sheet then remains in place on the drum during the print run. During use, the rotation of the drum may cause fatigue cracking to develop in the vicinity of the gap when the sheet is not tight to the drum. The bending that takes place as the ends of the sheet are bent into the gap may cause cracking if the bendabiϋty of the sheet is poor. In order to maximize available printing space, it is desirable that the gap be as narrow as possible and narrowing the gap requires that the sheet have improved bending properties,

Another problem is the difficulty in handling very wide sheets. There has been a trend in the printing industry to increase the width of sheets used, beyond widths of 1.7m. During handling the sheet is susceptible to damage through scratches or dents and is also susceptible to creasing or folding. In order to improve handling it is desirable that the sheet have higher stiffness and dent or scratch resistance.

Another problem experienced by lithographic sheet users is corrosion of the sheet due to the aggressive, highly alkaline, printing environment. As mentioned above, the lithographic sheet is wrapped around a cylindrical drum and bent into a gap in the drum to fix the sheet in place. The corrosive attack that occurs in use takes place predominantly on the reverse side of the sheet, that is, on the back surface of the sheet opposite from the printing surface. The invention described herein mitigates this effect by providing a highly corrosion resistant iayer to serve as the back surface of the sheet whiist providing an upper printing surface which provides performance consistent with that required for printing purposes. A further problem concerns the graining response of the sheet, especially during electrolytic graining processes. Lithographic sheet users require a roughened sheet that displays uniformity of colour and a uniform and fine pit size. The 1XXX series alloys are generally better in graining response than the 3XXX series alloys but even so, problems occasionally arise with graining of 1XXX series alloys. Super-pure or carbide-free alloys can be used to provide excellent electrolytic graining responses but the cost of obtaining large quantities of such metals for use as monolithic sheet is prohibitive. The mechanical properties of super-pure aluminium are also insufficient to provide long print runs, A fine grain at the sheet surface will provide an excellent graining response. A fine grain size at the surface usually means a fine grain size through the thickness, In an alloy like AA1050, however, recrystaltisation and significant growth of the grains may take place during the stoving treatment and this can have a deleterious effect on mechanical properties, most notably a drop in strength. As a monolithic the grain size could be controlled to mitigate this effect somewhat but then the graining response wouid be poorer.

Trace elements within an alloy composition can have a significant effect on the graining response because they are often electrochemicalty active. It is, therefore, usually necessary when making monolithic lithographic sheet products to control their content or take steps to mitigate their effect. Another reason for careful control of molten metal quality prior to ingot casting is to avoid excessive inclusions or particulates which wouid also cause a deterioration of the surface quality.

The object of this invention, therefore, is to obtain a lithographic sheet product which provides enhanced mechanical properties whilst maintaining surface treatment characteristics. An alternative object is to provide a product having enhanced surface treatment or surface performance characteristics whilst maintaining mechanical properties. A further object is to provide a product having the desired or improved surface treatment or surface performance characteristics which is abie to tolerate the presence of excessive trace eiements or excessive alioying additions that would otherwise be damaging. A preferred objective is to provide a product having improvements in surface treatment or surface performance characteristics combined with enhanced mechanical properties.

These objectives are met by the present invention in which an aiuminium lithographic sheet product comprises a composite structure having a core layer and at least one clad layer wherein the core layer is an alloy selected from the group of alloys consisting of the 1XXX, 3XXX and 5XXX series alloys and the at least one clad layer is selected from the group of alloys consisting of the 1XXX or 3XXX series alloys.

In one embodiment the composite structure comprises three layers with a core layer positioned between two clad layers. In a further embodiment the two clad layers are of the same composition. Within the scope of this invention is envisaged a composite product comprising just two layers. In the normal use of the terms within the industry, the clad layer is usually the term given to that layer which dictates surface characteristics such as corrosion resistance or graining response. The clad layer is usually, but may not be, thinner than the core layer. The primary purpose of the core layer is to influence the bulk mechanical properties of the overall sheet product.

A lithographic sheet according to the invention provides improved fatigue properties when the core layer is selected from the groups of alloys comprising the 3XXX or 5XXX series of alloys and the clad layer is selected from the group of alloys within the 1XXX or 3XXX series of alloys. The alloys of the core layer are selected to be inherently stronger than the alloys used in the clad layer. The 3XXX series alloys suitable for use as a core layer may be selected from the group consisting of AA3004, AA3104 and AA3105. The preferred 3XXX series core layer alloy is AA3004. The 5XXX series alloys suitable for use as a core layer may be selected from the group consisting of AA5052, AA5056, AA5083, AA5383, AA5086, AA5186, AA5154, AA5254, AA5754, AA5456 and AA5652. The preferred 5XXX series core layer alloys are those selected from the group consisting of AA5056, AA5083 and AA5383. The most preferred 5XXX series core layer aϋoy is AA5083. For the core layer the 5XXX series alloys are preferred over the 3XXX series alloys.

For the clad layer alloy, alloys from the 1XXX series of alloys are preferred because these softer alloys reduce crack initiation, thereby also improving fatigue performance, The 1XXX series alloy for use as the clad layer alloy may be a super-pure aluminium layer or it may be selected from the group containing AA1050, AA1050A, AA1200 and the compositions described by US6,447,982, WO07/093605, WO07/045676, US20080035488, EP1341942 and EP1425430. The preferred 1XXX series clad layer alloys are those selected from the group consisting of AA1050A and the compositions described by US6,447,982, WO07/045676 and EP1341942. The most preferred 1XXX series clad layer alloy is that described by EP1341942.

In terms of fatigue resistance, the most preferred combination of core and clad layers is a core layer of AA5083 with a clad layer according to the alloy disclosed in EP1341942. Although a product with a single clad layer is in accordance with the invention it is normal to have a core layer positioned between two clad layers and, further, to have both clad layers of the same composition. A three-layer product like this means the users of the sheet do not have to consider its orientation.

The alloy combinations in accordance with the invention that improve the sheet for handling purposes are also 3XXX or 5XXX series alloys in the core layer with either 1XXX or 3XXX series alloys in the one or more clad layers. An alternative possibility, to mitigate dent resistance in particular, is to incorporate a 1XXX series alloy in the core with a 3XXX series alloy as the clad layer. As above, the 3XXX series alloys suitable for use as a core layer may be selected from the group consisting of AA3004, AA3104 and AA3105. The preferred 3XXX series core layer alloy is AA3004. The 5XXX series alloys suitable for use as a core layer may be selected from the group consisting of AA5052, AA5056, AA5083, AA5383, AA5086, AA5186, AA5154, AA5254, AA5754, AA5456 and AA5652, The preferred 5XXX series core layer alloys are those selected from the group consisting of AA5056, AA5083 and AA5383. The most preferred 5XXX series core layer alloy is AA5083. For the core layer the 5XXX series alloys are preferred over the 3XXX series alloys.

When a 1XXX series alloy is used in the core layer in combination with a

3XXX series alloy in the clad iayer, the 1XXX series alloy may be selected from the group containing AA1050, AA1050A, AA1200 and the compositions described by 1156,447,982, WO07/093605, WO07/045676, US20G80035488, EP1341942 and EP1425430.

The preferred 1XXX series clad layer alloys are those selected from the group consisting of AA1050A and the compositions described by US6,447,982, WO07/045676 and EP1341942. The most preferred 1XXX series clad layer alloy is that described by EP1341942.

A further version of the invention comprises a product where the core layer is a "non-conforming" composition. It is normal within the industry to cast alloys according to the known registered compositions and these always specify the range of main alloying elements as well as a maximum content of impurities or "other" elements. Producers analyze the composition for the main alloying elements and may analyze for trace elements but analysis of trace elements is not necessary. However, in situations where the content of an "other" element exceeds the limit shown {usually 0.03 or 0.05 weight %), then the composition is considered to be non-conforming to the registered composition, in other words, one might try to cast AA5083 but if the analyzed composition contains 0.07% vanadium, this would make the composition non-conforming and it should not be described as AA5083. Likewise, if the composition contained a slight excess of one of the major alloying elements, the composition would also be non- conforming. By way of example, if for AA5083 the Mn content were analyzed at 1.2 weight %, it would be non-conforming and, again, should not be described as AA5083. The scope of this invention makes it possible for the producers of lithographic sheet to take less care of the control of alloying elements when casting the core layer because the core layer has at least one clad layer upon it which means the core layer has no impact on surface critical properties. Although compositions have been mentioned above in the 3XXX and 5XXX series alloys as suitable for use in the core layer, the invention is also intended to cover compositions which are similar but non-conforming by virtue of excessive trace efement content or excessive main alloying element content. It is not possible to define specific compositions along such lines because the number of possible permutations is so targe but the skilled person wil! readily understand the scope of the invention. This aspect of the invention makes it possible for the lithographic sheet producer to mix recycled metal from various sources without the need for complicated composition controls. Therefore, within the meaning of this invention the broad terms 1XXX, 3XXX or 5XXX series alloys are intended to include compositions which do not conform to the registered composition ranges but which, by virtue of their main alloying elements, would otherwise be considered as 1XXX₁ 3XXX or 5XXX series alloys.

Alloy combinations in accordance with the invention that improve the corrosion performance of the sheet adjacent the drum may have a different combination of layers compared with those described above, in this situation the corrosion performance is improved if the composition on the reverse side (non-printing side) of the sheet is a 1XXX series aitoy. The corrosion performance is even better if the composition is a super-pure composition within the 1XXX series, super-pure alloys according to this aspect of the invention being alloys with an aluminium content equal to or greater than 99.9 weight %. This leads to various possibilities within the scope of the invention. In one instance there is provided a composite sheet wherein the core layer is a 1XXX series alloy or 3XXX series alloy and there is a single clad layer of a 1XXX series alloy or super-pure composition. In this instance the core alloy provides the printing surface and this is in contrast to above descriptions of the invention where the dad layer provided the printing surface. The reason for the distinction is that, with regard to corrosion performance, the surface critical surface is the reverse side of the sheet, the nonprinting side. For example, when the core layer is a 1XXX series alloy which is not super-pure, then the clad layer may be super-pure, if the core layer is a 3XXX series alloy the ciad layer may be a 1XXX series alloy or a super-pure composition, in a second instance the composite sheet may comprise two clad layers where a first clad layer provides the corrosion protection and the second clad layer provides the printing surface, In this second instance the core layer may be selected for other reasons such as to obtain superior mechanical properties, in this second instance the first and second clad layers may be of different compositions. For example, to provide corrosion resistance the first dad layer may be a 1XXX series alloy or a super-pure composition, the second clad layer providing the printing surface may be a 1XXX series alloy or a 3XXX series alloy, and the core layer may be a 3XXX series alloy or 5XXX series alloy. In such cases the core alloys may be the same as those mentioned above. In an embodiment according to this second instance a product is provided having a super-pure first clad layer, a core layer of AA5083, and a second clad layer from the 1XXX series alloys, such as AA1050 or one of the other 1XXX series alloys mentioned above.

The invention addresses the problems that would be caused by using a fine grain sized product, a super-pure aluminium sheet, or a carbide-free sheet in monolithic form for optimised electrolytic graining response. Namely, clad layers possessing such characteristics may be used in combination with a core layer of different composition yet possessing suitable mechanical property characteristics. The solution according to the invention means significantly less super-pure or carbide-free material can be used, thereby realizing significant cost savings. The invention also allows the use of a ciad layer having a very fine grain size to optimize graining response whilst the deterioration in mechanical properties due to recrystallization is compensated for by use of a core layer having better mechanical properties or resistance to recrystailization.

When the clad layer is super-pure aluminium or a carbide-free aluminium alloy or one with a very fine grain size, the core layer is selected from the groups of alloys comprising the 3XXX or 5XXX series of alloys. The alloys of the core layer are inherently stronger than the compositions used in the clad layer or can be selected to resist stoving treatments, for example. As above, the 3XXX series alloys suitable for use as a core layer may be selected from the group consisting of AA3004, AA3104 and AA3105. The preferred 3XXX series core layer alloy is AA3004. The 5XXX series alloys suitable for use as a core layer may be selected from the group consisting of AA5052, AA5Q56, AA5083, AA5383, AA5086, AA5186, AA5154, AA5254, AA5754, AA5456 and AA5652. The preferred 5XXX series core layer alloys are those selected from the group consisting of AA5056, AA5083 and AA5383. The most preferred 5XXX series core layer alloy is AA5083. For the core layer the 5XXX series alloys are preferred over the 3XXX series alloys. An optimised product according to this invention is a composite structure having a clad layer of super-pure aiuminium and a core layer of a strong 5XXX series alloy such as AA5083.

The product according to this invention can be fabricated by conventional methods known to those in the aluminium industry. For example, the product can be made by a traditional roll bonding approach where the layers are initially cast as separate ingots, homogenized and hot rolled to an intermediate thickness, then hot or cold rolled together to form the composite structure, followed by further rolling as necessary. As is known to the skilled person, various heat treatment steps may be incorporated within this process if necessary, such as but not limited to intermediate anneals or solution heat treatment.

An alternative method of manufacture involves casting the two or more layers at the same time or in the same casting operation to form a single ingot having distinct compositional layers. Such methods are also well known in the aluminium industry and are described by patents such as WO04/112992, WO98/24571 , or WO03/035305, incorporated herein by reference. The process according to WO04/112992 is better suited to manufacture of the products according to this invention because there is no need for an interlayer or permanent divider during casting. Once the composite ingot has been cast it can be processed in the conventional manner and process steps may include homogenization, hot and cold rolling, together with other standard manufacturing steps and heat treatments as deemed necessary by the skilled person.

Another advantage of the process according to WO04/112992 is that the outermost sump is of such a size that the chance of floating crystals forming is reduced and there is less space for the crystal to move if they do form. Both factors reduce the possible size of floating crystals in the at least one clad layer thereby helping to improve or maintain surface quality Exampies of products according to this invention are shown in Table 1 along with an aiioy according to the monolithic prior art, sample 1. The temper designations O, H19 and H321 will be readily understood by the skilled person.

Table 1 :

The mechanical properties possible with these combinations depends on the thickness of the clad iayer relative to the overall product thickness and will depend on the hardness temper of each layer. It can be predicted that these examples will have the following mechanical properties shown in Tables 2.1 to 2.3.

Table 2.1:

Table 2.2:

Table 2.3:

Of these examples, sample 2 is preferred. Although its mechanical properties are slightly lower than samples 3 and 4, they are stiil above those obtained with AA1050 H19. In addition the corrosion resistance will be superior and the graining response will be excellent. To provide the best balance of mechanical properties and surface performance the above tables suggest that the preferred clad thickness will be 5% and below.

Claims

1. An aluminium lithographic sheet product comprising a composite structure having a core layer and at least one cfad layer characterised in that the core layer is an alloy selected from the group of alloys consisting of the 1XXX, 3XXX and 5XXX series alloys and the at least one clad layer is selected from the group of alloys consisting of the 1XXX or 3XXX series alloys.

2. A product as claimed in claim 1 characterised in that the composite structure comprises two clad layers with one clad layer on each side of the core layer.

3. A product as claimed in claim 1 characterised in that the clad layer is an alloy selected from the group consisting of a super-pure 1XXX series composition, AA1050, AA1050A, AA1200 and the compositions described by US6,447,982, WO07/093605, WO07/045676, US20080035488, EP1341942 and EP1425430.

4. A product as claimed in claim 3 characterised in that the two clad layers are of the same composition.

5. A product as claimed in claim 4 characterised in that the two clad layers are of a super-pure 1XXX series composition.

6. A product as claimed in claim 3 characterised in that the two clad layers are of different compositions.

7. A product as claimed in claim 6 characterised in that a first clad layer is a super-pure alioy and a second clad layer is AA1050.

8. A product as claimed in claim 3 characterised in that the thickness of the two clad layers is 5% or iess of the total product thickness.

9. A product as claimed in claim 1 characterised in that the core layer is a 3XXX series alloy.

10. A product as claimed in claim 7 characterised in that the core layer comprises an alioy selected from the group consisting of 3002, 3102, 3003, 3103 and 3004.

11. A product as claimed in claim 1 characterised in that the core layer is a 5XXX series alloy.

12. A product as claimed in claim 11 characterised in that the core Sayer comprises an alloy selected from the group consisting of AA5083, AA5383,

AA5059, AA5051 , AA5182, AA5454, AA5754, AA5086, AA5186, AA5056 and AA5456.

13. A product as claimed in claim 12 characterised in that the core layer is AA5083.