Patent Publication Number: US-2019194779-A1

Title: Aluminium alloy and aluminium alloy strip for pedestrian impact protection

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application is a continuation of PCT/EP2017/070676, filed Aug. 15, 2017, which claims priority to European Application No. 16184151.5, filed Aug. 15, 2016, the entire teachings and disclosure of which are incorporated herein by reference thereto. 
    
    
     FIELD 
     The invention relates to an aluminium alloy for vehicle applications, an aluminium alloy strip manufactured from an aluminium alloy and a sheet metal body part of a motor vehicle manufactured from the aluminium alloy strip according to the invention. 
     BACKGROUND 
     Aluminium alloys of the type AA6xxx are used in automotive engineering due to their advantageous combination of good formability and their ability to increase strength by hardening after heat treatment. In contrast to the commonly used aluminium alloy sheet metals consisting of an AA6xxx aluminium alloy, which can achieve particularly high yield strengths through hardening, for sheet metals, which are relevant for pedestrian impact protection, it is required that the lowest possible hardening is carried out, for example after a heat treatment due to paintwork. On the one hand, a sheet metal for pedestrian impact protection must have sufficient energy absorption capacity and convert the impact energy into deformation energy in a targeted manner so that it must have moderate yield strengths. On the other hand, the properties of a sheet metal relevant for pedestrian impact protection should not or only negligibly change over time. In addition to more highly-alloyed aluminium alloys of the type AA6xxx, in the case of which the hardening properties are reduced by specific manufacturing methods, aluminium alloys of the type AA6xxx with very low Mg and/or Si contents can also be used in order to achieve this goal. However, in the case of more highly-alloyed aluminium alloys, the manufacturing method and here in particular the control of the solution annealing conditions is very complex and expensive. Moreover, aluminium alloys with very low magnesium contents of about 0.2 wt % provide only very low yield strengths and tensile strengths. They are therefore too soft to be used at a motor vehicle. A third possibility is to use soft AA5xxx aluminium alloys. However, these are partly prone to flow lines so that they cannot be used for the visible bodywork region. In addition, they prevent a uniform alloy concept, for example based on AA6xxx aluminium alloys, and therefore make recycling concepts more difficult. 
     From the European patent application EP 1 533 394 A1 a bodywork component is known which can absorb a large proportion of the kinetic energy in the case of an impact through plastic deformation and is therefore suitable for pedestrian impact protection. The European patent application proposes that the elements magnesium and silicone required in solid solution are present in the form of precipitated Mg 2 Si and/or Si particles to prevent artificial ageing and therefore, this only results in low hardening effects after a further heat treatment, for example by drying paintwork. In order to achieve the precipitation state, the European patent application proposes to carry out no homogenisation annealing of the cast ingot, only a partial solution annealing at lower temperatures or a partial heterogenisation annealing at end thickness of the rolled sheet metals in the coil in a batch furnace. These measures either lead to notably higher costs during the production of the aluminium alloy strips or have negative effects on their workability. Homogenisation that was not carried out may, for example, influence the quality of the manufactured sheet metals. 
     Based on this, the present invention has the object to provide an aluminium alloy for vehicle applications which can be processed into a strip using conventional method steps such that the manufactured strip, with a moderate level of strength, exhibits only a very low tendency to hardening from the T4 condition and a use in the area of pedestrian impact protection is possible. In addition, a corresponding aluminium alloy strip and a sheet metal body part for a motor vehicle will be proposed. 
     BRIEF SUMMARY 
     According to a first teaching, the indicated object for an aluminium alloy strip having an aluminium alloy for vehicle applications is achieved in that the aluminium alloy has the following alloy constituents in percent by weight: 
       0.40 wt %≤Si≤0.55 wt %,
 
       0.15 wt %≤Fe≤0.25 wt %,
 
       Cu≤0.06 wt %,
 
       0.15 wt %≤Mn≤0.40 wt %,
 
       0.33 wt %≤Mg≤0.40 wt %,
 
       Cr≤0.03 wt %,
 
       0.005 wt %≤Ti≤0.10 wt %,
 
     the remainder being Al and unavoidable impurities, individually at most 0.05 wt %, in total at most 0.15 wt %. 
     It has been found that the aluminium alloy strip according to the invention, in addition to a level of tensile strength of at least 130 MPa required for the field of application in automotive engineering, exhibits a notably weakened hardening so that the increase in the yield strength R P0.2  compared to the known AA6xxx aluminium alloys is lower. In particular in the case of an application of the aluminium alloy strip in heat-stressed regions, the aluminium alloy strip according to the invention exhibits a notably lower tendency to hardening than aluminium alloy strips made of comparable alloys. Silicone and magnesium proportions of the aluminium alloy are generally responsible for the hardening of the aluminium alloy composing the aluminium alloy strip according to the invention. The closely-balanced contents of silicone and magnesium, here 0.40 wt % to 0.55 wt % for silicone and 0.33 wt % to 0.40 wt % for magnesium, ensure, on the one hand, a sufficient level of strength of the aluminium alloy strip according to the invention. On the other hand, excess silicone is bonded by the formation of Al—Fe—Mn—Si phases due to the content of manganese of 0.15 wt % to 0.40 wt %, preferably 0.2 wt % to 0.4 wt % or 0.20 wt % to 0.30 wt % in combination with the iron content of 0.15 wt % to 0.25 wt % so that less silicone is available for hardening by precipitation of silicone-containing particles. As a result, the hardening of the aluminium alloy of the aluminium alloy strip according to the invention can be reduced. Preferably, the silicone content can also be limited to at most 0.50 wt % to reduce hardening. Copper is generally added to increase the strength of the aluminium alloy. Due to the balanced contents of silicone, iron, manganese and magnesium, this is, however, not required according to the invention. Copper may also impair the corrosion properties. The aluminium alloy of the aluminium alloy strip according to the invention is therefore almost copper-free and has at most 0.06 wt % copper. Since copper often also represents an undesired impurity in the case of recycling, not only is the corrosion resistance of the aluminium alloy strip improved as a result, but the recycling of the aluminium alloy strip is also made easier. In order to improve the formability of the aluminium alloy strip, the chromium content is limited to at most 0.03 wt % and the titanium content to 0.005 wt % to 0.10 wt %. Titanium improves the grain refinement during casting of the aluminium alloy strip and is therefore contained with at least 0.005 wt % in the aluminium alloy composing the aluminium alloy strip according to the invention. For grain refinement, up to 0.10 wt % titanium is generally contained. The use of at most 0.03 wt % titanium allows the titanium content to be minimised in the case of good grain refinement. It has been found that in the narrowly limited range of the alloy composition, particular hardening properties of the aluminium alloy composing the aluminium alloy strip according to the invention are present, which are characterised in particular by a low long-term hardening in the case of heat stress. The aluminium alloy strip according to the invention, due to the lower hardening, is therefore outstandingly suitable to be used in automotive engineering for sheet metals, which are used for pedestrian impact protection due to their defined energy absorption capacity. 
     According to the invention, the aluminium alloy strip according to a first embodiment has a manganese content of 0.25 wt % to 0.35 wt %. This manganese content allows the hardening of the aluminium alloy composing the aluminium alloy strip according to the invention to be reduced even further since even more manganese is available to bind excess silicone by forming Al—Fe—Mn—Si phases. At the same time, the further limitation of manganese counteracts an increase in strength in the T4 condition. 
     The corrosion behaviour of the aluminium alloy strip according to the invention can, according to a further embodiment, be improved in that the copper content is reduced to below 0.05 wt %, preferably at most 0.01 wt %. 
     According to a further embodiment of the aluminium alloy strip according to the invention, the aluminium alloy has a silicone content of 0.40 wt % to 0.48 wt %. This specific reduction of the upper limit of the silicone content allows the hardening of the aluminium alloy to be reduced further by reducing the silicone atoms available for precipitation. 
     According to the next embodiment of the aluminium alloy strip according to the invention, the aluminium alloy has a magnesium content of 0.35 wt % to 0.40 wt %. This magnesium content allows the aluminium alloy, with identical hardening properties, to have slightly increased tensile strengths and improved forming properties owing to the higher Mg contents. 
     According to a preferred embodiment of the aluminium alloy strip, it has, in the T4 condition, a yield strength R p0.2  of 55 MPa to 70 MPa and a tensile strength R m  of 130 MPa to 160 MPa measured transverse to the rolling direction. The preferred combination of yield strength values and tensile strength values of this embodiment of the aluminium alloy strip allows a preferred use for manufacturing bodywork components, which are suitable for pedestrian impact protection. Due to the moderate increase in strength or due to the reduced hardening, corresponding sheet metals continue to have good properties for pedestrian impact protection, even after extended use, even with sustained heat stress. 
     According to a further embodiment of the aluminium alloy strip according to the invention, it has, in the T6x condition, a yield strength R p0.2  of less than 100 MPa measured transverse to the rolling direction. As the T6x condition, a particularly practical heat treatment is designated in the present patent application. This includes solution annealing at 530° C. for 5 minutes with subsequent quenching to room temperature, natural aging for 7 days at room temperature, heating to 205° C. for 30 minutes and heating to 80° C. for 500 hours. The first heat treatment steps to achieve the T6x condition correspond to normal conditions used to achieve the T4 condition, namely the known sequence of heat treatments consisting of solution annealing, quenching and natural aging. Heating for 30 minutes to 205° C. follows which is supposed to simulate the heat treatment of the sheet metal in the case of painting and drying of the paint (paint bake). Subsequently, the continuous use of the aluminium alloy sheet metal at high temperature, for example when used in the proximity of the engine, is simulated by further heat treatment. To this end, the sheet metal is heated to 80° C. for 500 hours. The heat stress generally promotes the hardening of the aluminium alloy. The aluminium alloy strip according to the invention, however, exhibits notably reduced hardening in spite of the increased heat stress and can provide yield strengths R p0.2  of less than 100 MPa. 
     The above-mentioned object is achieved according to a further teaching of the present invention by a method for manufacturing an aluminium alloy strip according to the invention from an aluminium alloy, the method comprising the steps of casting a rolling ingot or casting a cast strip, homogenising the rolling ingot, hot rolling the rolling ingot or the cast strip and optionally cold rolling with or without intermediate annealing to end thickness. Unlike the teaching of the European patent application EP 1 533 394 A1, the method according to the invention comprises conventional method steps and, due to the narrowly specified aluminium alloy, also ensures a sufficient level of tensile strength with reduced hardening, in particular in the case of a long-lasting heat stress, for example in the case of a bonnet of a motor vehicle. Using the method according to the invention, aluminium alloy strips for the production of metal sheets for pedestrian impact protection can be manufactured cost-effectively and with high quality. 
     The rolling ingot is preferably homogenised at temperatures of 450° C. to 580° C., preferably 500° C. to 570° C. for more than 1 hour. Hot rolling is preferably carried out at temperatures of 280° C. to 550° C. Hot strip production is also conceivable with quenching of the aluminium alloy strip at the end of the hot rolling, the hot strip temperature being reduced with the last hot rolling pass to at most 230° C. and the strip being subsequently wound up. These hot rolling methods allow a hot strip to be manufactured economically. Optionally, the hot strip is subjected to cold rolling, in the case of which the cold rolling is carried out with or without intermediate annealing dependent upon the starting thickness of the hot strip and the end thickness of the strip to be achieved. Preferably, strips for the production of sheet metal body parts with a thickness of 0.8 mm to 2.5 mm are manufactured with or without intermediate annealing. The intermediate annealing can be carried out at temperatures of 280° C. to 430° C. in a batch furnace for at least 30 minutes or in a continuous furnace. Subsequently, solution annealing is carried out in a continuous furnace with subsequent quenching, for example to room temperature, followed by natural aging for about 3 to 7 days so that sheet metals and strips can be made available in a stable manner in the T4 condition for further processing. The T4 condition constitutes the preferred starting condition of the sheet metals since, in the T4 condition, maximum forming degrees can generally be made available for AA6xxx alloys. The aluminium strip according to the invention can be manufactured cost-effectively due to the conventional production thereof and still provides reduced hardening. 
     The temperature of the aluminium strip during solution annealing is preferably at least 480° C., preferably at least 500° C. for at least 20 seconds. In the case of these solution annealing temperatures, the strip according to the invention is insensitive to fluctuations or change in temperature and duration during solution annealing so that an aluminium alloy strip provided with a constant solution state can be provided. 
     According to a further teaching of the present invention, the stated object is achieved by a sheet metal body part of a motor vehicle manufactured from an aluminium alloy strip according to the invention. The sheet metal body part can be made available particularly cost-effectively since, due to the alloy composition, conventional method steps for manufacturing the aluminium alloy strip are sufficient in order to provide a sheet metal with reduced hardening tendency and moderate strength properties. The sheet metal body part, due to the reduced hardening tendency, exhibits only a small increase in the yield strength R p0.2  during continuous use. Furthermore, a corresponding aluminium alloy strip enables a uniform alloy concept made of AA6xxx alloys with alloy constituents that are positive in terms of recycling. To this end, reference is in particular made to the low proportions of copper, chromium and titanium of the aluminium alloy according to the invention. 
     Preferably, the sheet metal body part is a sheet metal provided for pedestrian impact protection, preferably a part of a wing, a part of a bonnet or a vehicle roof, a roof frame or a tailgate. Sheet metal body parts provided for the pedestrian impact protection must have permanently moderate yield strengths R p0.2  in order to absorb the impact energy by deformation in the event of an impact and to cushion the impact. The hardening properties of the aluminium alloy according to the invention are advantageous here since the increase in the yield strength R p0.2  remains moderate over the lifetime of the sheet metal body part. Furthermore, a sufficient level of strength of the sheet metal is also provided so that sheet metal parts for wings, bonnet, tailgate, vehicle roof or roof frame can be easily handled. 
     According to a further teaching, the stated object for an aluminium alloy for vehicle applications is achieved in that the aluminium alloy has the following alloy constituents in percent by weight: 
       0.40 wt %≤Si≤0.55 wt %,
 
       0.15 wt %≤Fe≤0.25 wt %,
 
       Cu≤0.06 wt %,
 
       0.20 wt %≤Mn≤0.40 wt %, preferably 0.25 wt %≤Mn≤0.40 wt %,
 
       0.33 wt %≤Mg≤0.40 wt %,
 
       Cr≤0.03 wt %,
 
       0.005 wt %≤Ti≤0.10 wt %,
 
     the remainder being Al and unavoidable impurities, individually at most 0.05 wt %, in total at most 0.15 wt %. 
     It has been found that the aluminium alloy according to the invention, in addition to a level of tensile strength of at least 130 MPa required for the field of application in automotive engineering, exhibits a notably weakened hardening so that the increase in the yield strength R p0.2  compared to the known AA6xxx aluminium alloys is lower. In particular in the case of an application of the aluminium alloy in heat-stressed regions, the aluminium alloy according to the invention exhibits a notably lower tendency to hardening than comparable alloys. The silicone and magnesium proportions of the aluminium alloy are generally responsible for the hardening of the aluminium alloy according to the invention. The closely-balanced contents of silicone and magnesium according to the invention, here 0.40 wt % to 0.55 wt % for silicone and 0.33 wt % to 0.40 wt % for magnesium, ensure, on the one hand, a sufficient level of strength of the aluminium alloy according to the invention. On the other hand, excess silicone is bonded by the formation of Al—Fe—Mn—Si phases due to the content of manganese of 0.2 wt % to 0.40 wt %, 0.25 wt % to 0.4 wt % or 0.20 wt % to 0.30 wt % in combination with the iron content of 0.15 wt % to 0.25 wt % so that less silicone is available for hardening by precipitation of silicone-containing particles. As a result, unlike the aluminium alloys previously known, the hardening of the aluminium alloy according to the invention can be reduced. Preferably, the silicone content can also be limited to at most 0.50 wt % to reduce hardening. Copper is generally added to increase the strength of the aluminium alloy. Due to the balanced contents of silicone, iron, manganese and magnesium, this is, however, not required according to the invention. Copper may also impair the corrosion properties. The aluminium alloy according to the invention is therefore almost copper-free and has at most 0.06 wt % copper. Since copper often also represents an undesired impurity in the case of recycling, not only is the corrosion resistance of the aluminium alloy improved as a result, but the recycling of the aluminium alloy is also made easier. In order to improve the formability of the aluminium alloy, the chromium content is limited to at most 0.03 wt % and the titanium content to 0.005 wt % to 0.10 wt %. Titanium improves the grain refinement during casting of the aluminium alloy and is therefore contained with at least 0.005 wt % in the aluminium alloy. For grain refinement, up to 0.10 wt % titanium is generally contained. The use of at most 0.03 wt % titanium allows the titanium content to be minimised in the case of good grain refinement. It has been found that in the narrowly limited range of the alloy composition, particular hardening properties of the aluminium alloy are present, which are characterised in particular by a low long-term hardening in the case of heat stress. The aluminium alloy according to the invention, due to the lower hardening, is therefore outstandingly suitable to be used in automotive engineering for sheet metals, which are used for pedestrian impact protection due to their defined energy absorption capacity. 
     According to the invention, the aluminium alloy according to a first embodiment has a manganese content of 0.25 wt % to 0.35 wt %. This manganese content allows the hardening of the aluminium alloy to be reduced even further since even more manganese is available to bind excess silicone by forming Al—Fe—Mn—Si phases. At the same time, the further limitation of manganese counteracts an increase in strength in the T4 condition. 
     The corrosion behaviour of the aluminium alloy according to the invention can, according to a further embodiment, be improved in that the copper content is reduced to below 0.05 wt %, preferably at most 0.01 wt %. 
     According to a further embodiment of the aluminium alloy according to the invention, the aluminium alloy has a silicone content of 0.40 wt % to 0.48 wt %. This specific reduction of the upper limit of the silicone content allows the hardening of the aluminium alloy to be reduced further by reducing the silicone atoms available for precipitation. 
     According to the following embodiment of the aluminium alloy according to the invention, the aluminium alloy has a magnesium content of 0.35 wt % to 0.40 wt %. This magnesium content allows the aluminium alloy, with identical hardening properties, to have slightly increased tensile strengths and improved forming properties due to the higher Mg contents. 
     Furthermore, the invention will be explained in greater detail on the basis of exemplary embodiments in connection with the drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the Drawings: 
         FIG. 1  shows the process steps for manufacturing an aluminium alloy strip according to the invention; 
         FIG. 2  shows in a diagram the change of the yield strength values after different heat treatments starting from the T4 condition; and 
         FIG. 3  shows, in a schematic, perspective representation a motor vehicle with sheet metal body parts relevant for pedestrian impact protection. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows, in a very schematic representation, the method sequence with regard to an exemplary embodiment of a manufacturing method for aluminium alloy strips according to the invention. 
     In step 1, a rolling ingot is initially cast with an aluminium alloy according to the invention with the following alloy constituents in percent by weight: 
       0.40 wt %≤Si≤0.55 wt %, preferably 0.50 wt %, more preferably 0.48 wt %,
 
       0.15 wt %≤Fe≤0.25 wt %,
 
       Cu≤0.06 wt %,
 
       0.15 wt %≤Mn≤0.40 wt %,
 
       0.33 wt %≤Mg≤0.40 wt %,
 
       Cr≤0.03 wt %,
 
       0.005 wt %≤Ti≤0.10 wt %,
 
     the remainder being Al and unavoidable impurities, individually at most 0.05 wt %, in total at most 0.15 wt %. 
     Subsequently, the aluminium alloy ingot is homogenised according to step 2 at temperatures of 450° C. to 580° C. Homogenisation is carried out at least for a period of one hour. Alternatively to manufacturing a rolling ingot according to step 1, according to step 3, a cast strip can also be cast directly from the aluminium alloy according to the invention. 
     According to step 4, the rolling ingot or the cast strip is hot rolled. Hot rolling takes place at a temperature of 280° C. to 550° C. The hot strip is then wound up. However, the hot strip can also, for example, be manufactured with quenching in the last two hot rolling passes to a temperature below 230° C. and then wound up. A hot strip manufactured in this manner can, according to step 5, also be subjected to solution annealing, quenching and subsequent natural aging in order to provide a hot strip consisting of the aluminium alloy according to the invention in the T4 condition. 
     The hot strips can have thicknesses of about 2 mm to 12 mm. According to step 5, the hot strips are transferred into the T4 condition in order to provide maximum forming properties for the production of sheet metal parts from the strips. The T4 condition can, for example, be achieved by solution annealing at 530° C. for 5 minutes, quenching to room temperature and subsequent natural aging at room temperature for 7 days. In this case, it has been shown that, due to the relatively low Mg and Si contents, the aluminium alloy strip according to the invention is relatively insensitive to the solution annealing parameters, especially the solution annealing temperature, provided the temperature is at least 480° C., preferably at least 500° C. 
     Optionally, the hot strip can firstly be subjected to cold rolling  6 , which is followed by an intermediate annealing  7 , the intermediate annealing  7  preferably taking place on the coil in a temperature range of 300° C. to 450° C. for at least 30 minutes in a batch furnace. The intermediate annealing can, however, also take place in a continuous furnace. The subsequent cold rolling to end thickness according to step 8 is carried out, insofar as intermediate annealing is necessary. Otherwise, also right after cold rolling  6 , the strip can be supplied for solution annealing, quenching to room temperature and natural aging according to step 9. The aluminium alloy strips manufactured in this manner in the T4 condition have end thicknesses of typically 0.8 mm to 2.5 mm and are preferably used in the bodywork construction of motor vehicles. 
     Six different aluminium alloys have now been processed by a method according to  FIG. 1 . From these six different aluminium alloys, according to step 1, rolling ingots were cast in the continuous casting method, which were subsequently homogenised according to step 2 for 2 hours at temperatures of 550° C. The homogenised rolling ingots were then hot rolled according to step 4 at temperatures of 280° C. to 550° C. to a hot strip thickness of 8 mm and subsequently cooled to room temperature. The resulting hot strips were cold rolled according to step 6, 7, and 8 with an intermediate annealing at an intermediate thickness of 3.5 mm to end thicknesses of 1.0 to 1.5 mm. The intermediate annealing was carried out at a maximum temperature of 350° C. for one hour in a batch furnace. 
     The different aluminium alloys with their alloy constituents are shown in Table 1. For all alloys in Table 1, the values are given in percent by weight. The following applies for all alloys: the remainder being Al and unavoidable impurities, individually at most 0.05 wt %, in total at most 0.15 wt %. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Al loy 
                 Si [%] 
                 Fe [%] 
                 Cu [%] 
                 Mn [%] 
                 Mg [%] 
                 Cr [%] 
                 Ti [%] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 A 
                 0.44 
                 0.2 
                 0.0012 
                 0.081 
                 0.25 
                 &lt;0.0005 
                 0.0165 
               
               
                 Comparative 
                 B 
                 0.43 
                 0.18 
                 0.003 
                 0.049 
                 0.37 
                 0.0021 
                 0.0063 
               
               
                 Comparative 
                 C 
                 0.46 
                 0.21 
                 0.0017 
                 0.141 
                 0.36 
                 0.0011 
                 0.0068 
               
               
                 Invention 
                 D 
                 0.48 
                 0.21 
                 0.0016 
                 0.25 
                 0.37 
                 0.0011 
                 0.0065 
               
               
                 Comparative 
                 E 
                 0.4 
                 0.19 
                 0.037 
                 0.064 
                 0.37 
                 0.021 
                 0.016 
               
               
                 Comparative 
                 F 
                 0.59 
                 0.19 
                 0.056 
                 0.086 
                 0.38 
                 0.0041 
                 0.0252 
               
               
                   
               
            
           
         
       
     
     All comparative alloys A, B, C, E and F have excessively low Mn contents compared to the alloy according to the invention. It is assumed that only the Mn content according to the invention in combination with the contents of Si, Fe and Mg causes the reduction in the hardening of the aluminium alloy in the T6 or T6x condition. Alloy A also has an excessively low proportion of magnesium. Alloy F, with 0.59 wt %, contains too much silicone. The manufactured aluminium alloy strips were initially brought to the T4 condition by solution annealing at 530° C. for 5 minutes with subsequent natural aging for one week at room temperature and both, the yield strength R p0.2  and the tensile strength R m , were measured transverse to the rolling direction according to the international standard ISO 6892-1:2009. Notably excessively low measurement values for the yield strength R p0.2  and the tensile strength R m  resulted for the comparative alloy A. A corresponding aluminium sheet metal would be too soft to absorb the impact energy of a pedestrian. The comparative alloy F, in contrast, already has notably excessively high yield strength values R p0.2  in the T4 condition and is therefore not optimally suitable for sheet metals used in the area of pedestrian impact protection. 
     The exemplary embodiment according to the invention of the aluminium alloy D is, along with the comparative alloys B and C, in the preferred strength range in the T4 condition of a yield strength R p0.2  of about 55 MPa to 70 MPa with tensile strengths R m  of 130 MPa to 160 MPa measured transverse to the rolling direction. The comparative alloy E falls slightly behind the alloys B, C and D with regard to the tensile strengths R m  and the yield strength R p0.2 . The measurement values of the exemplary embodiment according to the invention and the comparative examples in the T4 condition are shown in Table 2. The excessively high measurement values for the yield strength and the tensile strength of the comparative example F are traced back to the increased Si content and the notably reduced manganese content compared to the exemplary embodiment according to the invention. The overall excessively low level with regard to the yield strength or tensile strength of the comparative example A is traced back to the reduced magnesium content of 0.25 wt %. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 R p0.2   
                 R m   
               
               
                 Alloy 
                 Heat treatment: T4 
                 [MPa] 
                 [MPa] 
               
               
                   
               
             
            
               
                 A 
                 5 mins at 530° C. + 7 d room temperature 
                 46 
                 118 
               
               
                 B 
                 5 mins at 530° C. + 7 d room temperature 
                 55 
                 132 
               
               
                 C 
                 5 mins at 530° C. + 7 d room temperature 
                 56 
                 129 
               
               
                 D 
                 5 mins at 530° C. + 7 d room temperature 
                 58 
                 134 
               
               
                 (inven- 
               
               
                 tion) 
               
               
                 E 
                 5 mins at 530° C. + 7 d room temperature 
                 50 
                 124 
               
               
                 F 
                 5 mins at 530° C. + 7 d room temperature 
                 77 
                 167 
               
               
                   
               
            
           
         
       
     
     The measurement values for the comparative examples and the exemplary embodiment of the present invention are now shown in Table 3 for the heat treatment state T6. The T6 heat treatment simulates the effect of painting and baking the paint by heating to 205° C. for 30 minutes, after solution annealing, quenching and natural aging. Table 3 does indeed show that, with regard to the absolute and relative increase in the yield strength and tensile strength, in particular in comparison to the comparative example of the alloy C, the exemplary embodiment according to the invention does not have the lowest increase in these values, but the increases in the tensile strength R m  and the yield strength R p0.2  of the exemplary embodiment of the aluminium alloy D according to the invention remain low and notably below 10 MPa. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 R p0.2   
                 R m   
                 Δ R p0.2 (abs)   
                   
               
               
                 Alloy 
                 T6 heat treatment 
                 [MPa] 
                 [MPa] 
                 [MPa] 
                 Δ R p0.2 (rel)   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 A 
                 5 mins at 530° C. + 7 d room temperature + 
                 51 
                 117 
                 5.0 
                 11% 
               
               
                   
                 30 mins at 205° C. 
               
               
                 B 
                 5 mins at 530° C. + 7 d room temperature + 
                 63 
                 127 
                 7.5 
                 14% 
               
               
                   
                 30 mins at 205° C. 
               
               
                 C 
                 5 mins at 530° C. + 7 d room temperature + 
                 61 
                 125 
                 4.5 
                  8% 
               
               
                   
                 30 mins at 205° C. 
               
               
                 D 
                 5 mins at 530° C. + 7 d room temperature + 
                 65 
                 131 
                 7.0 
                 12% 
               
               
                 (inven- 
                 30 mins at 205° C. 
               
               
                 tion) 
               
               
                 E 
                 5 mins at 530° C. + 7 d room temperature + 
                 59 
                 124 
                 9.0 
                 18% 
               
               
                   
                 30 mins at 205° C. 
               
               
                 F 
                 5 mins at 530° C. + 7 d room temperature + 
                 119 
                 174 
                 42.0 
                 55% 
               
               
                   
                 30 mins at 205° C. 
               
               
                   
               
            
           
         
       
     
     The exemplary embodiment D according to the invention exhibits notably reduced hardening, insofar as a heat treatment is examined, which reflects long-term heat stress of a component. The long-term behaviour was determined via the T6x heat treatment condition. The T6x condition is achieved starting from the T6 condition, as was already stated above, by subsequently carrying out artificial aging at 80° C. for 500 hours. The artificial aging at 80° C. for 500 hours simulates the practical use of the aluminium alloy sheet metals in the application, for example in a motor vehicle, in the case of heat stress. Since heat increases the hardening effects in AA6xxx alloys, the values for the yield strength generally increase relatively sharply. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 R p0.2   
                 R m   
                 Δ R p0.2 (abs)   
                   
               
               
                 Alloy 
                 T6x heat treatment 
                 [MPa] 
                 [MPa] 
                 [MPa] 
                 Δ R p0.2 (rel)   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 A 
                 5 mins at 530° C. + 7 d room temperature + 
                 68 
                 135 
                 22.0 
                 48% 
               
               
                   
                 30 mins at 205° C. + 500 h 80° C. 
               
               
                 B 
                 5 mins at 530° C. + 7 d room temperature + 
                 89 
                 152 
                 34.0 
                 62% 
               
               
                   
                 30 mins at 205° C. + 500 h 80° C. 
               
               
                 C 
                 5 mins at 530° C. + 7 d room temperature + 
                 80 
                 145 
                 23.5 
                 42% 
               
               
                   
                 30 mins at 205° C. + 500 h 80° C. 
               
               
                 D 
                 5 mins at 530° C. + 7 d room temperature + 
                 77 
                 147 
                 19.5 
                 34% 
               
               
                 (inven- 
                 30 mins at 205° C. + 500 h 80° C. 
               
               
                 tion) 
               
               
                 E 
                 5 mins at 530° C. + 7 d room temperature + 
                 76 
                 142 
                 26.0 
                 52% 
               
               
                   
                 30 mins at 205° C. + 500 h 80° C. 
               
               
                 F 
                 5 mins at 530° C. + 7 d room temperature + 
                 140 
                 197 
                 63.5 
                 83% 
               
               
                   
                 30 mins at 205° C. + 500 h 80° C. 
               
               
                   
               
            
           
         
       
     
     Unlike the other comparative alloys, the exemplary embodiment D according to the invention exhibits a notably reduced increase in the yield strength after aging for 500 hours at 80° C. Not only is the absolute increase in yield strength by 19.5 MPa notably lower, but also the relative increase in yield strength of only 34 wt % is notably lower than the relative or absolute increase in the yield strengths of all other aluminium alloys. With regard to the long-term behaviour of the aluminium alloy, an unanticipated reduction in hardening was observed which is traced back to the formation of Al—Fe—Mn—Si phases. It is assumed that these Al—Fe—Mn—Si phases do not contribute to the precipitation hardening of the aluminium alloy and therefore reduce the effect of silicone precipitations. 
     In Tables 5 and 6, the aluminium alloy strips were measured in the T6 condition with 2% or 5% cold forming. The T6 condition (2%) and T6 condition (5%) are supposed to simulate deformation of the sheet metal part with subsequent painting. To this end, the sheet metal is subjected to a heat treatment with a duration of 20 minutes at a temperature of 185° C. 
     The exemplary embodiment of the aluminium alloy D according to the invention was also measured in the case of these heat treatments simulating the application-oriented processing of the sheet metals in the area of automotive engineering likewise with the lowest values with regard to the absolute or relative increase in yield strength R p0.2  in the T6 condition with 2% cold forming or 5% cold forming. 
     The measurement values of the relative increase in yield strength starting from the T4 condition are again shown in the diagram of  FIG. 2 . The positive hardening behaviour of the aluminium alloy according to the invention can be read on the basis of the exemplary embodiment D in comparison to the remaining variants in particular from the comparison in the T6x condition. 
       FIG. 3  schematically shows in a perspective view sheet metal body parts of a motor vehicle, which are provided for pedestrian impact protection. The bonnet  10 , the wing  11 , the vehicle roof or the roof frame  12  and the indicated tailgate  13  of a motor vehicle are in principle sheet metal body parts which must be designed for pedestrian impact protection. They must therefore have a specific energy absorption behaviour which is in particular still present even with prolonged heat stress. If parts of these sheet metals provided for pedestrian impact protection are manufactured from an aluminium alloy according to the invention, the long-term hardening of the sheet metals can also be reduced in heat-stressed regions. As is discernible based on the exemplary embodiment according to the invention of alloy D, it is advantageous to manufacture corresponding sheet metal body parts of a vehicle from an aluminium alloy according to the invention since they have a particularly advantageous hardening behaviour with moderate yield strength values and tensile strength values. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                 R p0.2   
                 R m   
                 Δ R p0.2 (abs)   
                   
               
               
                 Alloy 
                 T6 (2%) heat treatment 
                 [MPa] 
                 [MPa] 
                 [MPa] 
                 Δ R p0.2 (rel)   
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 A 
                 5 mins at 530° C. + 7 d room temperature + 
                 77 
                 130 
                 30.5 
                 66% 
               
               
                   
                 2 wt % + 20 mins at 185° C. 
               
               
                 B 
                 5 mins at 530° C. + 7 d room temperature + 
                 91 
                 147 
                 36.0 
                 65% 
               
               
                   
                 2 wt % + 20 mins at 185° C. 
               
               
                 C 
                 5 mins at 530° C. + 7 d room temperature + 
                 87 
                 141 
                 31.0 
                 55% 
               
               
                   
                 2 wt % + 20 mins at 185° C. 
               
               
                 D 
                 5 mins at 530° C. + 7 d room temperature + 
                 88 
                 143 
                 30.5 
                 53% 
               
               
                 (inven- 
                 2 wt % + 20 mins at 185° C. 
               
               
                 tion) 
               
               
                 E 
                 5 mins at 530° C. + 7 d room temperature + 
                 83 
                 137 
                 33.0 
                 66% 
               
               
                   
                 2 wt % + 20 mins at 185° C. 
               
               
                 F 
                 5 mins at 530° C. + 7 d room temperature + 
                 113 
                 175 
                 36.5 
                 48% 
               
               
                   
                 2 wt % + 20 mins at 185° C. 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                 R p0.2   
                 R m   
                 Δ R p0.2 (abs)   
                   
               
               
                 Alloy 
                 T6 (5%) heat treatment 
                 [MPa] 
                 [MPa] 
                 [MPa] 
                 Δ R p0.2 (rel)   
               
               
                   
               
             
            
               
                 A 
                 5 mins at 530° C. + 7 d room temperature + 
                 100 
                 144 
                 53.5 
                 116%  
               
               
                   
                 5 wt % + 20 mins at 185° C. 
               
               
                 B 
                 5 mins at 530° C. + 7 d room temperature + 
                 117 
                 162 
                 61.5 
                 112%  
               
               
                   
                 5 wt % + 20 mins at 185° C. 
               
               
                 C 
                 5 mins at 530° C. + 7 d room temperature + 
                 110 
                 154 
                 54.0 
                 96% 
               
               
                   
                 5 wt % + 20 mins at 185° C. 
               
               
                 D 
                 5 mins at 530° C. + 7 d room temperature + 
                 111 
                 155 
                 53.0 
                 92% 
               
               
                 (inven- 
                 5 wt % + 20 mins at 185° C. 
               
               
                 tion) 
               
               
                 E 
                 5 mins at 530° C. + 7 d room temperature + 
                 108 
                 152 
                 57.5 
                 115%  
               
               
                   
                 5 wt % + 20 mins at 185° C. 
               
               
                 F 
                 5 mins at 530° C. + 7 d room temperature + 
                 143 
                 191 
                 66.0 
                 86% 
               
               
                   
                 5 wt % + 20 mins at 185° C. 
               
               
                   
               
            
           
         
       
     
     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.