Abstract:
A component, in particular a safety, structural or ground connection component for a motor vehicle, cast under pressure, and made of an aluminum alloy with the following composition (in wt. %): Mg: 1.0-4.5; Si: 0.2-1.3; Cu&lt;0.3; Zn&lt;0.1; Fe&lt;0.3, Ti&lt;0.3 and at least one element for reducing adherence to the mold which is Mn (0.3-1), Cr (0.1-0.4), Co (0.1-0.4), V (0.1-0.4) and Mo (0.1-0.4), other elements&lt;0.05 each and 0.15 in total, the remainder being aluminum.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the field of Al—Mg—Si alloys intended for the manufacture of relatively thin die-cast components, particularly of structural and safety components of a motor vehicle.  
       STATE OF THE ART  
       [0002]     The need to reduce vehicle weight has encouraged an increase in the development of complex-shaped cast safety components in which several functions are integrated and that have to satisfy the different deformation tests used by automobile manufacturers such as the “crash test” or the “body block”, and requirements of different assembly methods.  
         [0003]     With known alloys such as the Al—Si9Cu3Mg alloy, it is possible to obtain a tensile strength R m  in the non-treated temper F equal to at least 300 MPa and a yield strength R p0.2  equal to at least 230 MPa. However, the elongation at rupture A does not exceed 2%. But structural and safety components of a motor vehicle require sufficient ductility to absorb energy and prevent rupture in case of shock, and to adapt to different assembly modes.  
         [0004]     Moreover, the high content of copper may have a favourable influence for the mechanical strength, but it makes the alloy sensitive to corrosion. Yet good resistance to corrosion is necessary to avoid deterioration of the component in a corrosive environment such as de-icing salt, to prevent deterioration of the component.  
         [0005]     Various alloy formulations have been proposed to satisfy these requirements. Patent EP 0792380 filed by Aluminium Rheinfelden in 1994 describes the use for casting of an alloy with the following composition (% by weight) in the semi-solid state:  
         [0006]     Mg 3.0-6.0; Si 1.4-3.5; Mn 0.5-2.0; Fe&lt;0.15; Ti 0.2.  
         [0007]     The divisional application EP 0853133 derived from the previous patent application applies to an alloy for die casting with the following composition:  
         [0008]     Mg 4.6-5.8; Si 1.8-2.5; Mn 0.5-0.9; Fe&lt;0.15; Ti&lt;0.2  
         [0009]     for which heat treatment of the cast components is not essential to obtain good mechanical properties.  
         [0010]     U.S. Pat. No. 5,667,602 filed by Alcoa in 1995, describes an alloy for die casting, intended particularly for structural nodes of a “space frame” type automobile bodywork structure with composition Si&lt;0.30; Fe&lt;1.00; Mg 2.00-5.00; Mn 0.20-1.60; Zr 0.10-0.30. Good mechanical properties are obtained without heat treatment of cast components. U.S. Pat. No. 6,132,531, also filed by Alcoa in 1997, applies to an alloy of the same type, for the same application, with composition Si&lt;0.20; Fe&lt;0.20; Mg 2.80-3.60; Mn 1.10-1.40; Ti&lt;0.15; Be 0.0005-0.0015.  
         [0011]     AlMgMn alloys were also used for making die-cast vehicle steering wheels. Patent DE 3827794 (Toyoda Gosei) applies to the use firstly of an alloy with composition Mg 1.5-2.5; Si 0.2-0.4; Fe 0.4-1.0; Mn 0.4-0.6, and secondly an alloy with composition Mg 1.5-2.4 Si&lt;1.0; Fe 0.3-0.8; Mn 0.2-0.4, for a special shaped steering wheel.  
         [0012]     Patent EP 0412605 (Kolbenschmidt) describes the use of an alloy with composition Mg 2.5-3.5; Si 0.10-0.30; Fe 0.40-0.60; Mn 0.25-0.45; Cu 0.015-0.05; Sn 0.035-0.065, for the same application.  
         [0013]     Patent application EP 0911420 (Aluminium Rheinfelden) describes a casting alloy with composition Mg 2.0-3.5; Si 0.15-0.35; Mn 0.2-1.2; Fe&lt;0.40; Cu&lt;0.10; Zn&lt;0.10; Cr&lt;0.05; Co&lt;0.60; Ce&lt;0.80.  
         [0014]     Patent application EP 1138794 (Corus Aluminium) describes an alloy for die casting with the following composition Mg 2.7-6.0; Si&lt;1.4; Mn 0.4-1.4; Zn 0.10-1.5; Fe&lt;1.0; Sr&lt;0.3; V&lt;0.3; Sc&lt;0.3.  
       PURPOSE AND SUBJECT OF THE INVENTION  
       [0015]     The purpose of the invention is to provide die-cast structural, ground connection and safety components of a motor vehicle, with good mechanical strength, high ductility and a good energy absorption capacity, very resistant to corrosion and with good castability that can be manufactured in large series under acceptable economic conditions.  
         [0016]     The subject of the invention is a die-cast component, particularly a safety, structural or suspension component of a motor vehicle, made of an aluminium alloy with the following composition (% by weight):  
         [0017]     Mg 1.0-4.5; Si 0.2-1.3; Cu&lt;0.3; Zn&lt;0.1; Fe&lt;0.3; Ti&lt;0.3, and at least one element for reducing adherence to the mould: Mn (0.3-1); Cr (0.1-0.4); Co (0.1-0.4); V (0.1-0.4) and Mo (0.1-0.4), other elements&lt;0.05 each and 0.15 in total, the remainder being aluminium.  
         [0018]     The preferred contents are Mg: 2.0-4.0; Si 0.4-1.0; Cu&lt;0.1; Fe&lt;0.2; Ti: 0.005-0.25.  
         [0019]     The alloy may possibly contain Be to a content of not more than 50 ppm.  
         [0020]     Another subject of the invention is a die-cast component with the indicated composition, with a yield strength in the T5 temper (aged at a temperature of between 150 and 200° C.), equal to more than 100 MPa and elongation at rupture equal to more than 15%. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0021]     The invention is based on the applicant&#39;s observation that by reducing the content of magnesium and silicon compared with alloys according to prior art to be used for the manufacture of die-cast safety components, a useful compromise can be obtained between usage properties, particularly between the yield strength in the F temper (as-cast structure) and ductility, while maintaining an acceptable castability. The magnesium content is at least 4.5% so that the alloy remains castable and must not exceed 4.5% if a high ductility is required even in the T5 temper, in other words in the aged temper at a temperature of between 100 and 200° C. corresponding to the baking temperature of a paint for a motor vehicle. The preferred content is between 2 and 4%.  
         [0022]     The silicon content is equal to at least 0.2% to obtain a sufficient tensile strength and yield strength, while maintaining high ductility. It is preferably between 0.4 and 1%. Magnesium and silicon combine together to form Mg 2 Si which is in the form of a very fine eutectic at die casting solidification rates. The presence of a eutectic fraction, in non-negligible quantities, contributes to improving the usage properties while reducing the formation of cracks when hot. Furthermore, a certain quantity of magnesium remains in the solid aluminium solution due to the fast cooling rate in the mould, and the aluminium solution is thus supersaturated.  
         [0023]     The applicant has observed that elongation at rupture for a given content of silicon within the limits according to the invention, increases quickly when the magnesium content reduces. Conversely, for a given content of magnesium, a reduction in the silicon content increases elongation at rupture, while keeping the yield strength above 100 MPa.  
         [0024]     Copper must be kept below 0.3%, and preferably below 0.1%, to obtain sufficient resistance to atmospheric corrosion and stress corrosion.  
         [0025]     Iron, manganese, chromium, cobalt, molybdenum and nickel form intermetallic compounds either individually or in combination with aluminium, that cause embrittlement in the case of sand casting or permanent mould gravity casting. However, in die-casting, considering cooling rates in the mould, these compounds are small and their morphology is such that the mechanical strength is not affected. On the other hand, the presence of these elements provides a means of limiting adherence of components in the mould, by reducing the chemical potential of the alloy with respect to steel in the mould. Since the iron content has a bad influence on the elongation and must be limited to 0.5% and preferably to 0.2%, the presence of at least one of the other elements mentioned above in addition to iron is essential.  
         [0026]     Thorough refining of primary grains in the solid aluminium solution by a master alloy or a salt containing titanium or boron, for example a mix of fluoborate and potassium fluotitanate, provides a means of almost completely eliminating the tendency to the formation of cracks when hot, and contributes to reducing the size of microporosities resulting from contraction during solidification, which improves the compactness of components. The refined alloy then contains between 0.05 and 0.25% of titanium and between 10 and 30 ppm of residual boron.  
         [0027]     Alkaline elements such as sodium, calcium and strontium, must be kept at a very low content, preferably below 10 ppm, since they have an unfavourable influence on the mechanical properties. An addition of beryllium, limited to 50 and preferably 30 ppm, is advantageous to limit the tendency of alloys to oxidise in the liquid state.  
         [0028]     Die-cast components, with or without vacuum assistance, are preferably used in the as-cast structure state, to prevent deformations in quenching and subsequent extensive straightening. They are not very sensitive to ageing resulting from the baking of paint performed in the automobile industry. The typical duration of this treatment is between fifteen minutes and one hour at a temperature of between 150 and 200° C.  
         [0029]     Components according to the invention have a yield strength that is always more than 100 MPa, or even 120 MPa, in the F temper or the T5 temper, with an elongation of more than 15%, which improves energy absorption capacity under shock and makes it possible to use assembly techniques requiring good ductility, such as hemming or riveting.  
         [0030]     This reduction in the magnesium content compared with alloys according to prior art intended for use in similar components improves the suitability for TIG, MIG or laser welding, and achieves excellent compatibility with aluminium alloys in the 6000 series used for bodywork plates.  
       EXAMPLES  
     Example 1  
       [0031]     The influence of the composition and the Mg/Si ratio on static mechanical properties of die-cast components was studied. Test plates made of 9 different alloys A to I with the composition given in table 1 were produced by vacuum assisted die-casting (residual pressure in the mould 80 hPa). The size of the plates was 120×220 mm and their thickness was 2.5 mm. Casting was done on a press with a closing force of 3200 kN at a piston injection velocity of 0.7 m/s. The metal temperature in the furnace was 780° C.  
                                                                         TABLE 1                                   Alloy   Si   Fe   Mg   Ti   Mg/Si                                        A   0.3   0.12   3   0.15   10           B   0.3   0.12   4   0.15   13.3           C   0.3   0.12   5   0.15   16.2           D   0.8   0.12   3   0.15   3.7           E   0.8   0.12   4   0.15   5           F   0.8   0.12   5   0.15   6.2           G   1.4   0.12   3   0.15   2.1           H   1.4   0.12   4   0.15   2.8           I   1.4   0.12   5   0.15   3.6                      
 
         [0032]     Tensile test specimens, taken from areas of these plates that had not received any heat treatment and in which there are no faults visible in radiography, were machined, and the tensile strength R m  (in MPa), the conventional yield strength at 0.2% elongation Rp0.2 (in MPa) and the elongation at rupture A (in %) were all measured. The results (averages of 10 test specimens) are included in table 2.  
                                                                 TABLE 2                                   Alloy   R m     R p0.2     A average   A max                                        A   240   124   20.9   25.3           B   266   140   21.2   26.4           C   270   154   9.6   15.7           D   242   128   15.8   20.7           E   269   143   15.3   20.3           F   277   158   9.3   16.1           G   263   163   7.8   12.9           H   273   146   12.6   20.3           I   287   158   9.8   15.6                      
 
         [0033]     The results show that the elongation for alloys with 3 and 4% Mg increases significantly when the Si content reduces from 1.4 to 0.3%. This tendency is not continued for alloys with 5% Mg, for which the elongation remains close to 10%. For a Si content equal to 1.4%, alloys with 3, 4 or 5% Mg have similar values for R m , R p0.2  and A. For Si contents of less than 1%, alloys with 3 and 4% Mg have a yield strength of more than 120 MPa and an elongation of more than 15%.  
       Example 2  
       [0034]     The effect of a paint baking treatment on the hardness of components was studied in comparison with the as-cast temper naturally aged at ambient temperature. The hardnesses Hv 5/30 in the naturally aged temper and in the T5 temper were measured after 20 minutes ageing at 190° C. on tensile test specimens similar to those in example 1, for alloys B, D, E and H. The results are given in table 3.  
                               TABLE 3                                   Alloy   Aged as cast temper   T5 temper                           B   80.8   80.8           D   80.8   82.4           E   84.0   84.4           H   88.4   89.6                      
 
         [0035]     It can be seen that the T5 treatment gives a slightly higher hardness for alloys for which the Mg/Si ratio is low, and the hardness can be kept constant for alloys with a higher Mg/Si ratio.  
         [0036]     The effect of the treatment duration on the mechanical properties of alloy A with 0.5% Mn was studied by comparing them with the F temper and with two T5 tempers, one with 20 minutes ageing, the other 60 minutes ageing at 190° C. The results are given in table 4:  
                                   TABLE 4                                   State   R m  (MPa)   R p0.2  (MPa)   A (%)                           F   246   129   22.2           20 min. at 190° C.   244   125   20.2           60 min. at 190° C.   249   141   20.4                      
 
         [0037]     It can be seen that this type of treatment hardly affects the mechanical properties, which is an advantage since the energy absorption properties for safety component with a high deformation capacity are maintained in assemblies. Furthermore, there is no risk that machined dimensions of components will be modified by this treatment.