Patent Publication Number: US-2017349989-A1

Title: Multipurpose heat treatable aluminum alloys and related processes and uses

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/078,027, filed Nov. 11, 2014, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the fields of material science, material chemistry, metallurgy, aluminum alloys, aluminum fabrication, and related fields. 
     BACKGROUND 
     Aluminum alloys used for various applications must achieve certain properties. For instance, aluminum alloys are used for fabrication of inner and outer panels of transportation machinery. Aluminum alloys are useful for this application due to a combination of their light weight, which leads to increased fuel efficiency, strength, and other properties. Among other things, the aluminum alloys used for fabrication of inner and outer panels of transportation machinery should possess good formability, paint or other finish quality, dent resistance and immunity to natural aging. It is also desirable for the alloys used in the fabrication of transportation machinery to be recyclable. New and improved metal alloys with desirable characteristics suitable for fabrication of transportation machinery panels can expand the range of alloys available for these applications, lower the material costs, increase the aluminum recycling rates, decrease the capacity limits on the production of such alloys, and decrease the environmental impact of aluminum production and use. 
     SUMMARY 
     The terms “invention,” “the invention,” “this invention” and “the present invention” used herein are intended to refer broadly to all of the subject matter of this patent application and the claims below. Statements containing these terms should be understood not to limit the subject matter described herein or to limit the meaning or scope of the patent claims below. Covered embodiments of the invention are defined by the claims, not this summary. This summary is a high-level overview of various aspects of the invention and introduces some of the concepts that are further described in the Detailed Description section below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification, any or all drawings and each claim. 
     The present invention provides improved heat treatable aluminum alloys containing higher amounts of Mg than conventionally considered suitable for heat treatment and can exhibit age hardening if solutionized in continuous solution heat treatment lines. The improved aluminum alloys provided herein can be produced as sheet alloys and can be more suitable for recycling processes than conventional alloys. Some embodiments of the present invention are improved aluminum alloys suitable for fabricating automotive and other transportation machinery panels. Some other embodiments of the present invention are innovative new uses and applications of the aluminum alloys, improved innovative processes for making, fabricating or manufacturing aluminum alloys, processes for fabricating aluminum alloy forms, objects and parts, such as stamped sheet forms, the panels for transportation machinery. Aluminum alloy objects, parts and forms that are fabricated from the improved aluminum alloys and/or according to the innovative processes provided herein are also provided among the embodiments of the present invention. 
     One embodiment of the present invention provided herein is an aluminum alloy comprising ≧1.5% Mg by weight produced by a process comprising heat treatment. The heat treatment process can comprise T4 temper. The aluminum alloy can further comprise 0.2 to 0.4% Si by weight. The aluminum alloy can undergo age hardening. The aluminum alloy can be a sheet aluminum alloy. Another embodiment of the present invention provided herein is a stamped sheet form fabricated from the above sheet aluminum alloy. The stamped sheet form can be an automotive panel. One embodiment of the present invention provided herein is a process for fabricating a sheet aluminum alloy comprising ≧1.5% Mg and 0.2 to 0.4% Si by weight, comprising heat treatment. The process can comprise T4 temper. The resulting sheet aluminum alloy can exhibit age hardening. One more embodiment of the present invention described herein is a process for fabricating a stamped sheet form, comprising stamping the above sheet aluminum alloys. The stamped sheet form can be an automotive panel. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic diagram illustrating process steps used for producing sheet aluminum alloys. 
         FIG. 2  is a schematic illustration of various sheet stampings used in automobile production. 
         FIG. 3  is a bar graph showing DIN tensile properties of an alloy in O temper and paint bake. 
         FIG. 4  is a bar graph showing tensile properties of an alloy in the T4, 2% stretch, and 2% stretch followed by 20 min at 185° C. 
         FIG. 5  is a bar graph showing tensile properties of an alloy in the T4 temper and after paint bake simulation (60 min at 180° C.). 
         FIG. 6  is a line plot illustrating age hardening of AA5251-T4 alloy. 
     
    
    
     DETAILED DESCRIPTION 
     In this description, reference is made to alloys identified by AA numbers and other related designations, such as “series.” For an understanding of the number designation system most commonly used in naming and identifying aluminum and its alloys, see “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys,” published by The Aluminum Association. 6xxx series aluminum alloys, such as AA6111, AA6016 and AA6022, are typically used for producing automotive outer skin panels. 
     In general terms, 6xxx series alloys contain relatively high levels of Si and low levels of Mg, are heat treatable, and exhibit age hardening, which confers on these alloys the strength parameters suitable for fabrication of the outer panels for transportation machinery, such as automobiles. 5xxx series aluminum alloys in O temper, such as AA5182-O or AA5754-O, are often preferred for inner panel fabrication in automotive and related industries due to their formability properties. 5xxx series aluminum alloys have very little tolerance to retain Si in solid solution. If Si is added to 5xxx series aluminum alloys, it tends to combine with Mg to form coarse Mg 2 Si particles during casting. These particles are difficult to solutionize to produce super saturated solid solution of Mg and Si during solutionizing and fast cooling on the continuous annealing lines. For this reason, 5xxx series aluminum alloys contain relatively low Si levels and relatively high Mg levels, and are considered to be non-heat treatable due to their high Mg content. The presence of coarse Mg 2 Si is potentially detrimental to formability. 
     Currently, 6xxx and 5xxx aluminum alloys cannot be easily combined and recycled for fabrication of automotive and related panels, because the resulting recycled aluminum alloys may contain undesirably high levels of both Si (as compared to 5xxx series aluminum alloys) and Mg (as compared to 6xxx series alloys), and thus be neither suitable for heat treatment, due to high Mg levels, nor possess the formability of 5xxx series alloys, due to a combination of relatively high Si and Mg levels. In addition, the presence of other metals, such as Cu, Mn, Fe or Zn, or combinations thereof, present in the alloys recycled from the combination of 5xxx and 6xxx alloys can lead to undesirable properties of the recycled aluminum alloys. For example, an undesirable combination of properties can leave a recycled aluminum alloy unsuitable for fabrication of either inner or outer panels for transportation machinery. 
     The inventors discovered that alloys that contain relatively high levels of Mg, such as ≧1.5% Mg, are heat treatable and exhibit age hardening, if appropriate amounts of Si and/or Cu are present in such alloy. This property makes aluminum alloys with relatively high magnesium content, as compared to traditional 6xxx alloys, unexpectedly and advantageously suitable for applications where age hardening is desirable. For instance, the inventors discovered that some aluminum alloys containing higher amounts of Mg than conventionally considered suitable for heat treatment, but lower amounts of Mg and higher amounts of Si in comparison to 5xxx series aluminum alloys traditionally used for inner automotive panel fabrication, such as AA5754 or AA5182 alloys, can exhibit age hardening if solutionized in continuous solution heat treatment lines. 
     The inventors&#39; discoveries are embodied in the improved aluminum alloys described herein. The improved aluminum alloys described herein can be produced as sheets, in which case they can be referred to as “sheet aluminum alloys,” “aluminum sheets,” “sheet alloys” or by other related terms, in singular or plural. The term “aluminum alloy” and similar terms used herein are broader in scope than “sheet aluminum alloy” and similar terms. In other words, sheet aluminum alloys are a subset of aluminum alloys. Sheet aluminum alloys can possess the same or similar composition but, in some instances, different properties than the same alloy not in a sheet form. Some of these properties may be conferred by the manufacturing or fabrication processes used in the production of sheet aluminum alloys. 
     The improved aluminum alloys that embody applicants&#39; discoveries exhibit age hardening similarly to 6xxx series alloys. They can also exhibit formability properties similar to those of 5xxx series aluminum alloys. The improved aluminum alloys are heat treatable. The improved aluminum alloys can be suitable for fabricating automotive and other transportation machinery panels, and, more generally, in the applications where high-Mg 5xxx series alloys are traditionally used. Increased content of Si and/or Cu in the improved aluminum alloys according to some embodiments of the present invention is beneficial in the applications where age hardening is desirable, because Si and/or Cu are capable of conferring hardening on solutionized alloys due to precipitation of Mg 2 Si and Al 2 CuMg particles during natural or artificial ageing. In addition to Si and/or Cu, some other elements can be present in the improved aluminum alloys described herein in higher amounts than in some 5xxx series aluminum alloys conventionally used for fabrication of automotive panels. The presence of such elements can confer advantageous properties on the improved aluminum alloys described herein. For example, increased levels of Mn may promote formation of dispersoids, which can help to disperse slip, thus improving formability. The inventors also discovered that improved aluminum alloys described herein are more suitable for recycling processes than conventional alloys, because the improved aluminum alloys are tolerant to relatively higher amounts of Si, Cu, Fe or Mn, as compared to 5xxx series aluminum alloys conventionally used for automotive panel manufacturing, such as AA5754 and AA5182 alloys. Accordingly, improved recycling processes embody some of the inventors&#39; discoveries. 
     In addition to the improved aluminum alloys, the inventors&#39; discoveries are embodied in innovative new uses and applications of the aluminum alloys, in improved innovative processes for making, fabricating or manufacturing aluminum alloys, in the processes for fabricating aluminum alloy forms, objects and parts, such as stamped sheet forms, the panels for transportation machinery. Aluminum alloy objects, parts and forms that are fabricated from the improved aluminum alloys and/or according to the innovative processes described herein also embody the inventors&#39; discoveries. 
     Alloys 
     The improved aluminum alloys according to the embodiments of the present invention differ from the conventional alloys used in automotive applications in that they contain higher levels of one or more of Si, Cu, Fe, Mn, or Zn and lower levels of Mg, than at least some of 5xxx series alloys and/or higher levels of Mg than at least some 6xxx series alloys The composition of the improved aluminum alloys is illustrated in Table 1, below. The content of the listed element can fall within the ranges delimited by a lower range limit and an upper range limit shown in Table 1. A lower range limit can be delineated by expressions “equal to or more than” (≧ sign) or “more than” (&gt; sign), or other related signs and expression, such as “from . . . ,” “higher than” etc. An upper range limits can be delineated by expressions “equal to or less than” (≦ sign), “less than” (&lt; sign) or other related signs and expressions, such as “to,” “less than,” etc. Other types of expressions can also be used to delineate the ranges, such as “between,” “in the range of,” etc. When a range is delineated by only the upper range limit, it is to be understood that, in some examples, an element in question may not be present, may not be present in detectable quantities, or may be present in such low quantities that they are conventionally not recognized as meaningful in the field of aluminum alloys. It is also to be understood that some other additives and/or elements can be present in the aluminum alloys described herein, which are not necessarily listed in the tables below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Composition of improved aluminum 
               
               
                 alloys (element content in wt %) 
               
            
           
           
               
               
               
               
            
               
                   
                 Examples of 
                 Examples of 
                   
               
               
                 Ele- 
                 lower range 
                 upper range 
                 Range examples 
               
            
           
           
               
               
               
               
               
               
            
               
                 ment 
                 limit 
                 limit 
                 Range 1 
                 Range 2 
                 Range 3 
               
               
                   
               
               
                 Mg 
                 1.5; 1.55; 
                 1.8; 1.85; 1.9 
                 1.6 to 2   
                 1.65 to 1.9 
                   
               
               
                   
                 1.6; 1.61; 
               
               
                   
                 1.62; 1.63; 
               
               
                   
                 1.64; 1.65 
               
               
                 Cu 
                   
                 0.3; 0.35; 0.4; 
                 ≦0.8 
                 ≦0.5 
                 ≦0.3  
               
               
                   
                   
                 0.45; 0.5; 0.55; 
               
               
                   
                   
                 0.6; 0.65; 0.7; 
               
               
                   
                   
                 0.75; 0.8 
               
               
                 Fe 
                   
                 0.35; 0.4; 0.45; 
                 ≦0.5 
                 ≦0.4 
                 ≦0.35 
               
               
                   
                   
                 0.5 
               
               
                 Mn 
                   
                 0.4 
                 ≦0.4 
               
               
                 Si 
                 0.2 
                 0.4 
                 0.2 to 0.4 
               
               
                 Zn 
                   
                 0.25; 0.3; 0.35; 
                 ≦0.5 
                 ≦0.3 
                 ≦0.25 
               
               
                   
                   
                 0.4; 0.45; 0.5 
               
               
                 Cr 
                   
                   
                  ≦0.25 
                  ≦0.20 
                 ≦0.15 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Exemplary composition of conventional 5xxx series alloys used in 
               
               
                 automotive applications (element content is expressed in wt %) 
               
            
           
           
               
               
               
               
            
               
                   
                 ELEMENT 
                 AA5182 
                 AA5754 
               
               
                   
                   
               
               
                   
                 Mg 
                 4 to 5 
                 2.6 to 3.6 
               
               
                   
                 Cu 
                 ≦0.15 
                 ≦0.10 
               
               
                   
                 Fe 
                 ≦0.35 
                 ≦0.40 
               
               
                   
                 Mn 
                 0.2 to 0.5 
                 ≦0.50 
               
               
                   
                 Cr 
                 ≦0.10 
                 ≦0.30 
               
               
                   
                 Si 
                 ≦0.20 
                 ≦0.40 
               
               
                   
                 Zn 
                 ≦0.20 
                 ≦0.20 
               
               
                   
                   
               
            
           
         
       
     
     Properties and Advantages 
     Improved aluminum alloys described herein, including sheet aluminum alloys, possess one or more properties that make them suitable for the use in automotive applications, such as fabrication of automotive panels or, more generally, panels for various types of transportation machinery, or, even more generally, stamped sheet forms. Some of these properties are formability, yield strength and age hardening. Improved aluminum alloys also possess advantageous recycling compatibility with 6xxx series aluminum alloys, such as AA6111, AA6022 or AA6016. The expression “recycling compatibility” and related terms are used herein to describe a notion that improved aluminum alloys according to some embodiments of the present invention can be combined with 6xxx series alloys (and, optionally, other alloys or elements) during metallurgical processes to fabricate commercially and technologically useful aluminum alloys, which can be characterized as “recycled.” 
     Formability and Paint Bake Response 
     Formability properties of the aluminum alloys described herein can be influenced by a number of variables. Formability properties include, but are not limited to, deep drawability and stretchability. One variable affecting formability properties is the composition of an aluminum alloy. For example, formability, including castability, is influenced by the amounts of Mg, Cu and Si in an aluminum alloy. High combined amounts of Mg, Si and/or Cu generally make it more difficult to cast and hot roll an aluminum alloy. Accordingly, the content of one or more of these elements can be varied to arrive at the desired formability properties. Other variables that can affect formability are fabrication process variations and conditions, such as, but not limited to, aluminum sheet processing steps and conditions, surface texturing process steps and conditions and lubrication process steps and conditions. One or more of the above variables can be adjusted to achieve desired formability properties. Another important property that can be varied by one or more of the variables discussed above is paint bake response of an aluminum alloy, which refers to change in strength during the paint cure process. Paint bake response is usually tested in the laboratory by ageing the deformed or nondeformed material in the T4 temper at elevated temperature. The exact simulation conditions determine the paint bake response vary from one car company to the other. For example, the paint bake response can be defined as change in strength by ageing an aluminum alloy at 180° C. 
     Strength 
     The improved aluminum alloys according to the embodiments of the present invention can exhibit 80 to 160 MPa yield strength (YS), which can be similar or equivalent to that of AA5754 or AA5182 in a typical finished and painted part required for automotive application. In some embodiments, strength of an improved aluminum alloy is influenced by increasing an amount of Cu in the aluminum alloy, as compared to Cu content of the alloys conventionally used for fabrication of panels for automobiles and other transportation machinery. 
     Hardness 
     Certain embodiments of the improved aluminum alloys described herein are heat treatable and exhibit age-related hardening, while exhibiting formability comparable to typical 5xxx aluminum alloys conventionally used in automotive applications. 5xxx aluminum alloys were previously not known to be heat treatable or exhibit age related hardening upon heat treatment. Improved aluminum alloys according to some embodiments of the present invention contain higher levels of Mg than the aluminum alloys conventionally recognized as heat treatable. Some examples of the improved aluminum alloys of the present invention contain ≧1.5% of Mg and are heat treatable. The presence of appropriate amounts of Si and/or Cu confers heat treatability and age hardening properties on an improved aluminum alloy containing ≧1.5% of Mg. This allows some improved aluminum alloys according to the embodiments of the present invention to achieve an unexpectedly advantageous combination of formability (conferred by higher Mg levels than those conventionally present in heat-treatable alloys) and age hardening upon heat treatment such as T4 temper (conferred by higher Si levels than those conventionally present in 5xxx series alloys). 
     In comparison to some of the 5xxx aluminum alloys, such as those conventionally employed for manufacturing of inner automotive panels, in some embodiments improved aluminum alloys of the present invention contain reduced amount of Mg. Reduced levels of Mg can result in lower cost of the improved aluminum alloys described herein, as well as in the lower costs of the forms the objects manufactured from such alloys, since less Mg is required for production. Reduced levels of Mg in the improved aluminum alloys described herein can also result in improved solubility of Si in aluminum during solutionizing, which advantageously affects the properties of the alloys. Both Si and Cu are capable of improving hardening of solutionized the improved aluminum alloys described herein due to precipitation of Mg 2 Si and Al 2 CuMg or Q (AlMgSiCu) containing particles during ageing. 
     Recyclability 
     The improved aluminum alloys of this invention possess a tolerance for higher amounts of Si than conventional 5xxx series alloys used for manufacturing of automotive panels. This higher tolerance for Si and/or the ability of the improved aluminum alloys described herein to exhibit paint bake response makes them suitable and compatible with 6xxx alloys for recycling. 
     In summary, the improved aluminum alloys of the present invention have an advantageous combination of properties that allows these improved alloys to be used in place of conventional high-Mg aluminum alloys for various applications. The improved aluminum alloys described herein can expand the range of alloys available for a variety of applications, one of which is manufacturing of stamped sheet forms, such as panels for automobiles and other transportation machinery, increase aluminum recycling rates, lower the costs of aluminum alloy manufacturing, and decrease the environmental impact of aluminum production 
     Fabrication Processes 
     The processes for making or fabricating the improved aluminum alloys are also included within the scope of the present invention. Improved aluminum described herein can be fabricated by the processes that include at least some of the technological steps described below. At least some of these technological steps can confer advantageous properties on the improved aluminum alloys. It is therefore important, in some cases, to include process steps when describing the improved aluminum alloys. For example, one exemplary embodiment of an improved aluminum alloy described herein is AA5251 alloy. Prior to the inventors&#39; discovery, AA5251 alloy, which contains &gt;1.5% Mg, was not known to be suitable for heat treatment, and to exhibit age hardening, when in the T4 temper. Accordingly, an exemplary embodiment of improved aluminum alloys described herein is AA5251 alloy in T4 temper, which can be referred to as AA5251-T4. 
     The processes of making or fabricating the improved aluminum alloys can involve heat treating in order to alter the physical and/or chemical properties of the improved aluminum alloys. Heat treatments involve the use of heating and/or chilling, of an aluminum alloy to achieve a desired result, such as hardening. An embodiment of the processes described herein employs T4 or T4P temper, which involves solution heat treatment and natural aging of an aluminum alloy to a substantially stable condition. T4P temper refers to special thermal heat treatment included following solutionizing. This treatment can be implemented either by controlled cooling from solutionizing temperature or be reheating to a temperature ranging from 50 to 110° C. within an hour of solutionizing. In some other embodiments, T6 and T8 tempers can also be used. 
     It is to be understood that descriptions and illustrations of the processes described herein are non-limiting. The process steps described herein can be combined and modified in various ways and suitably employed for fabricating the improved aluminum alloys or forms and objects from such alloys. Process steps and conditions that are not explicitly described herein, yet commonly employed in the areas of metallurgy and aluminum processing and fabrication, can also be incorporated into the processes described herein. 
     One exemplary process is schematically illustrated in  FIG. 1 . It is to be understood that one or more of the process steps illustrated in  FIG. 1  can be incorporated into the processes for making improved aluminum alloys. 
     Another example of a process that incorporates one or more steps that can be combined in various ways and suitably employed for fabricating the improved aluminum alloys is described in this paragraph. An improved sheet aluminum alloy is produced from a direct chilled (DC) ingot. However, the hot rolling stock may also be produced from a continuous cast slab. The DC cast ingots are scalped to remove near surface segregation layer on both sides of the ingot and homogenized at a temperature between 500 and 575° C. for time periods between 1 to 48 hours before being subjected to hot and cold rolling to the final gauge. Improved sheet aluminum alloy can also be subjected to special surface texturing, such as, but not limited to, electro discharge texturing, in order to improve formability of the final sheet. The cold rolled strip is solutionized by heating at &gt;3° C./s in a continuous annealing line to a temperature between 500 and 575° C., followed by fast cooling and natural ageing to produce sheet in the T4 temper. Solution heat treatment can re-dissolve soluble particles, such as Mg 2 Si or other particles back into the matrix, depending on the alloy composition. Fast quenching is used to produce a super saturated solid solution, in terms of both solutes and excess vacancies. The fast cooling from the solutionizing temperature can be carried out in forced air, water mist, or combination of both water mist and forced air. Coiling is performed at a temperature between 50 to 110° C., followed by coil cooling at a rate ≦10° C./hour. The coil can be reheated in the strip form to ensure the coiling temperature between 50 to 110° C. It is possible to subject the solutionized sheet alloy to either acidic or alkaline cleaning, followed by pre-treatment with special chemicals and lubricants, oils or waxes before coiling at a temperature between 50 and 110° C. The coil can be blanked and used for stamping inner panels, such as those illustrated in  FIG. 2 . 
     Yet another example of a process that incorporates one or more steps that can be combined in various ways and suitably employed for fabricating the improved aluminum alloys is described in this paragraph. A direct chilled cast alloy ingot is homogenized above 500° C. for ≧2 hours, hot rolled to an intermediate gauge with coiling temperature between 280 to 400° C., cold rolled to the final gauge in one or more passes with either mill or optimized finished texture and solutionized in the strip form at temperatures above 480° C. in a continuous annealing line, fast cooled and coiled between 50° C. and 120° C. The hot coiling step is optional and is used to improve the paint bake response of the alloy. In some situations, the solutionized coil may also be cleaned, pretreated and lubricated prior to stamping. 
     The following discussion is included to illustrate the advantageous properties that fabrication process steps can confer onto the improved aluminum alloys described herein. Traditionally, AA5754 or AA5182 alloys are supplied for manufacturing of automotive panels in the soft O temper, so that a part can be formed from these alloys and then subjected to paint cure operation. AA5754 or AA5182 in O temper exhibit softening due to recovery during paint bake. The improved aluminum alloys according to some embodiments of the present invention are not subject to such softening or are not subject to it to the same extent as AA5754 or AA5182 in O temper. The improved aluminum alloys described herein can maintain strength closer to AA5754 and AA5182 after forming and paint cure. For example, the strength properties on the final part manufactured from the improved aluminum alloys of the present invention can be similar or equivalent to AA5754 alloy. 
     Uses and Applications 
     Uses and applications of the improved aluminum alloys described herein are included within the scope of the present invention, as are objects, forms, apparatuses and similar things fabricated from or comprising the improved alloys described herein. The processes for fabricating, producing or manufacturing such objects, forms, apparatuses and similar things are also included within the scope of the present invention. For example, some embodiments of the improved aluminum alloys described herein are suitable for manufacturing of automotive panels. Various automotive panels, including inner and outer automotive panels, are therefore included within the scope of the present invention. They are described, for example, in U.S. Patent Publication No. 2010/0279143, and are also illustrated in  FIG. 2 . 
     It is to be understood, however, that the uses and applications of the improved aluminum alloys and objects that are manufactured from such alloys are not limited to automobile panels. Other objects can be suitably manufactured from the improved aluminum alloys described herein. One example is the panels generally incorporated into various transportation vehicles and other moving machinery, which can be termed “transportation panels” or “machinery panels.” For instance, the panels used for transport trucks can be advantageously manufactured from the improved aluminum alloys described herein. Transport trucks with aluminum cabs are traditionally produced from AA5052 alloy. This alloy has a tendency to exhibit stretch bands or yield point elongation during forming, causing objectionable surface appearance. Improved aluminum alloys according to some embodiments of the present invention do not exhibit yield point elongation and can be used to advantageously replace AA5052 alloy for manufacturing of panels used in transport trucks. 
     More generally, some embodiments of the improved aluminum alloys described herein, in comparison to conventional 5xxx alloys, show less tendency to display Liiders bands, also known as “slip bands” or “stretcher-strain marks,” which are localized bands of plastic deformation in metals experiencing tensile stresses. Accordingly, the improved aluminum alloys described herein can be advantageously employed in the manufacturing of parts or objects where Liiders bands are objectionable, such as outer panels for automobiles and other transportation vehicles and moving machinery. 
     Some embodiments of the alloys described herein are suitable for complex electronic applications. One example of such application is aluminum TV frames. More generally, various sheet stamping, stamped sheet forms, stamped panels, or related objects fabricated from the improved aluminum alloys described herein are included within the scope of the embodiments of the present invention. 
     The following examples will serve to further illustrate the present invention without, at the same time, however, constituting any limitation thereof. On the contrary, it is to be clearly understood that resort may be had to various embodiments, modifications and equivalents thereof which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the invention. During the studies described in the following examples, conventional procedures were followed, unless otherwise stated. Some of the procedures are described below for illustrative purpose. 
     Example 1 
     Testing of Tensile Properties of AA5251 Alloy in O Temper 
     An aluminum ingot containing 1.85% Mg, 0.3% Fe, 0.28% Mn and 0.29% Si was homogenized at 540° C. for &gt;5 hours, hot rolled to 3.2 mm gauge, cold rolled to the final 1.3 mm gauge and batch annealed for 1 hour at 340° C. to obtain O temper. The transverse tensile properties of the annealed sheets were determined using DIN specimens.  FIG. 3  shows the DIN tensile properties of the alloy in both O and paint bake (5% stretch plus 20 min at 185° C.). The alloy exhibited 70 MPa yield strength (YS), 164 MPa Ultimate Tensile Strength (UTS) and 23% total elongation in the O temper and showed no hardening after ageing for 20 min at 185° C. The higher YS in the paint bake temper (5% stretch plus 20 minutes at 185° C.) is the net result of work hardening due to stretching and recovery due to ageing. 
     Example 2 
     The Effect of Solutionizing on the Tensile Properties of AA5251 Aluminum Alloy 
     This example shows the effects of solutionizing on the tensile properties of an aluminum alloy. An aluminum ingot containing 1.85% Mg, 0.3% Fe, 0.28% Mn and 0.29% Si was homogenized at 540° C. for &gt;5 hours, hot rolled to 3.2 mm gauge and cold rolled to the final 1.3 mm gauge. The cold rolled 1.3 mm gauge sheets were solutionized for 2 min at 560° C., cooled and immediately pre-aged for 8 h at 85° C. The transverse ASTM properties of the solutionized alloy were determined after 24 hours of natural ageing.  FIG. 4  shows comparative tensile properties of the alloy in the T4 temper, 2% stretch and 2% stretch plus 20 min at 185° C. tempers. The aluminum alloy in the T4 temper was stronger in comparison to its O temper counterpart, as illustrated by the comparison of  FIGS. 3 and 4 . The aluminum alloy in T4 temper exhibited a significant increase in YS due to 2% stretch and after subjecting the stretch sample to ageing at 185° C. for 20 min. The tensile properties of the aluminum alloy in the T4 temper were close to the conventional AA5754 alloy. The yield strength of the aluminum alloy was close to the expected strength of AA5182 or AA5754 alloy, after subjecting it to similar paint bake treatment. 
     Example 3 
     The Role of Cu Addition to an Alloy 
     An aluminum ingot containing 1.75% Mg, 0.78% Cu, 0.23% Fe, 0.11% Mn and 0.38% Si was homogenized at 560° C. for &gt;18 hours, then hot rolled and cold rolled to the final 1.6 mm gauge and solutionized in a continuous annealing line at 540° C., cooled and pre-aged. The transverse tensile properties of the 1.6 mm gauge sheets were determined using ASTM specimens. 
       FIG. 5  shows the tensile properties of the alloy in both T4 and paint bake (60 min at 180° C.). This alloy, which contains higher levels of copper than AA5251 alloy discussed in Examples 1 and 2, was significantly stronger in comparison to the AA5251 alloy. The alloy tested in this example exhibited 143 MPa YS, 284 MPa UTS and 28% total elongation in the T4 temper, and showed significant hardening after ageing for 60 min at 180° C. due to precipitation of CuMgAl 2  and Mg 2 Si particles. 
     Example 4 
     Comparative Testing of AA5754 in O Temper and AA5251 in O and T Tempers 
     The aluminum ingots of AA5754 and AA5251 alloy having the composition shown in Table 3 were homogenized at 540° C. for &gt;5 hours, hot rolled and cold rolled to the final 1 and 1.3 mm gauges, respectively, in separate trials. Coils of AA5754 and AA5251 were solutionized on the continuous annealing line at 500 and 560° C., respectively. 
     The tensile test results from the trial coils are shown in Table 4. It can be seen that the yield strength and ultimate tensile strength of the conventional AA5754 sheet in O temper in the 0°, 45° and 90° with respect to the rolling direction is close to 100 MPa and within the 219 to 231 MPa range, respectively. AA5251 alloy in O temper exhibits lower values compared to the AA5754, except for the strain hardening exponent (n) value. AA5251 alloy in T temper exhibits significant improvement in strength properties, such as yield strength and ultimate tensile strength, compared to AA5251 O temper alloy. In terms of strength, AA5251 T temper alloy falls between AA5754 and AA5251-O temper. AA5251 T temper alloy exhibits paint bake response typically not observed in the AA5251 and AA5754-O temper alloys. The detected improvements in AA5251 T temper alloy offer a possibility of using it as a substitute for AA5754 and possibly AA5182 alloys. Marginally inferior forming characteristics of AA5251 T temper alloy, indicated by lower elongation, UTS and n values can be compensated by variety of techniques including optimizing alloy and process composition, using preferred sheet surface texture, or choice of lubricant during forming. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Aluminum alloy composition 
               
            
           
           
               
               
            
               
                   
                 Composition wt % 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Alloy 
                 Cu 
                 Fe 
                 Mg 
                 Mn 
                 Si 
                 Cr 
                 Ti 
               
               
                   
               
               
                 AA5754 
                 0.02 
                 0.20 
                 3.10 
                 0.22 
                 0.06 
                   
                 0.05 
               
               
                 AA5251 
                 0.01 
                 0.30 
                 1.83 
                 0.30 
                 0.29 
                 0.03 
                 0.01 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Comparative testing results of AA5754 in O temper and AA5251 in O and T tempers 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Yield 
                 Tensile 
                   
                   
               
               
                   
                 Gauge 
                 Strength 
                 Strength 
                 Total 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Temper 
                 Dir 
                 mm 
                 ksi 
                 MPa 
                 ksi 
                 MPa 
                 Elongation % 
                 n 
                 R 
               
               
                   
               
            
           
           
               
            
               
                 CASH Annealed AA5754 Coil#2271809 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 O 
                 0 
                 1.0 
                 14.6 
                 101 
                 33.4 
                 230 
                 26 
                 0.30 
                 0.87 
               
               
                   
                 45 
                   
                 14.5 
                 100 
                 32.3 
                 223 
                 26 
                 0.31 
                 0.57 
               
               
                   
                 90 
                   
                 14.4 
                 99 
                 31.9 
                 220 
                 24 
                 0.31 
                 0.64 
               
            
           
           
               
            
               
                 Batch Annealed AA5251 Coil#L55203R1 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 O 
                 0 
                 1.3 
                 9.7 
                 67 
                 24.9 
                 172 
                 23 
                 0.34 
                 0.67 
               
               
                   
                 45 
                   
                 9.6 
                 66 
                 24.2 
                 166 
                 26 
                 0.32 
                 0.59 
               
               
                   
                 90 
                   
                 9.6 
                 66 
                 23.7 
                 163 
                 21 
                 0.32 
                 0.55 
               
            
           
           
               
            
               
                 CASH Solutionizd AA5251 Coil#L55203R2 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 T4 
                 0 
                   
                 14.1 
                 97 
                 28.4 
                 196 
                 26 
                 0.26 
                 0.81 
               
               
                   
                 45 
                   
                 13.9 
                 96 
                 27.8 
                 192 
                 26 
                 0.26 
                 0.55 
               
               
                   
                 90 
                   
                 13.8 
                 95 
                 27.4 
                 189 
                 24 
                 0.26 
                 0.61 
               
               
                 2% + 20 min @ 
                 0 
                   
                 20.1 
                 139 
                 31.5 
                 217 
                 21 
                 0.21 
                 0.83 
               
               
                 185° C. 
                 45 
                   
                 20.0 
                 138 
                 31.0 
                 214 
                 20 
                 0.21 
                 0.52 
               
               
                   
                 90 
                   
                 19.8 
                 137 
                 30.5 
                 210 
                 20 
                 0.21 
                 0.60 
               
               
                   
               
            
           
         
       
     
     Example 5 
     Age Hardening of AA5251 T4 Temper Alloy at 185° C. 
     Age hardening studies of AA5251 T4 temper alloy were performed by placing tensile samples of the alloy in a furnace set at 180° C. The samples were taken out of the furnace after different ageing times.  FIG. 6  shows the ageing hardening behavior of the alloy at 180° C. The alloy exhibited about 70% and 20% increase in YS and UTS, respectively, after about 8 h of ageing. The results illustrated in  FIG. 6  support a conclusion that the alloy underwent age hardening. 
     All patents, publications and abstracts cited above are incorporated herein by reference in their entirety. Different arrangements and combinations of the elements and the features described herein are possible. Similarly, some features and subcombinations are useful and may be employed without reference to other features and subcombinations. Various embodiments of the invention have been described in fulfillment of the various objectives of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention.