Patent Application
20230008838
The invention relates to the field of motor vehicle structure parts or components, also referred to as “body in white”, manufactured in particular by stamping aluminium alloy sheets, more particularly alloys in the AA6xxx series in accordance with the designation of the Aluminium Association, intended to absorb energy irreversibly at the time of an impact, and having excellent compromise between high mechanical strength and good behaviour in a crash, such as in particular impact absorbers or “crashboxes”, reinforcement parts, linings, or other bodywork structure parts.
More precisely, the invention relates to the manufacture of such components by stamping in a solution-hardened, quenched and naturally aged temper state followed by hardening by on-part ageing and a treatment of baking the paint or “bake hardening”.
Aluminium alloys are increasingly used in automobile construction in order to reduce the weight of the vehicles and thus reduce fuel consumption and discharges of greenhouse gases.
Aluminium alloy sheets are used in particular for manufacturing many parts of the “body in white”, among which there are bodywork skin parts (or external bodywork panels) such as the front wings, roofs, bonnet, boot or door skins, and the lining parts or bodywork structure components such as for example door, bonnet, tailgate or roof linings or reinforcements, or spars, bulkheads, load-bearing floors, tunnels and front, middle and rear pillars, and finally the impact absorbers or “crashboxes”.
If numerous skin parts are already produced from aluminium alloy sheets, the transposition of steel to aluminium of lining or structure parts having complex geometries proves to be trickier. Firstly, because of the less good formability of aluminium alloys compared with steels and secondly because of the mechanical properties that are in general inferior to those of steels used for this type of part.
This is because this type of application requires a set of properties, sometimes conflicting, such as:
There do however now exist mass-produced motor vehicles having a body in white consisting mainly of aluminium alloys. For example, the Ford F-150 model 2014 version consists of AA6111 structure alloy. This alloy was developed by the Alcan group in the years 1980-1990. Two references describe this development work:
The main property remains high mechanical strength, even if it is initially designed to withstand indentation for applications of the skin type: “A yield-strength of 280 MPa is achieved after 2% pre-strain and 30 min at 177° C.”.
Moreover, other alloys in the AA6xxx family with high mechanical characteristics have been developed for aeronautical or automobile applications. Thus the alloy of the type AA6056, the development of which dates from the 1980s at Pechiney, has been the subject of many works and numerous publications, either to optimise the mechanical properties or to improve the resistance to intergranular corrosion. This was the subject of a patent application (WO 2004/113579 A1).
Alloys of the AA6013 type have also been the subject of numerous works. For example, at Alcoa, in the application US 2002/039664 published in 2002, an alloy comprising 0.6-1.15% Si; 0.6-1% Cu; 0.8-1.2% Mg; 0.55-0.86% Zn; less than 0.1% Mn; 0.2-0.3% Cr and approximately 0.2% Fe, used in the T6 temper, combines good resistance to intergranular corrosion and an Rp0.2 of 380 MPa.
At Aleris, an application published in 2003, WO 03006697, relates to an alloy in the AA6xxx series with 0.2% to 0.45% Cu. The object of the invention is to propose an alloy of the AA6013 type with a reduced Cu level, targeting 355 MPa of Rm in the T6 temper and good resistance to intergranular corrosion. The composition claimed is as follows: 0.8-1.3% Si, 0.2-0.45% Cu; 0.5-1.1% Mn; 0.45-0.1% Mg.
Structural parts for an automobile application made from a 7xxx alloy as described for example in the application EP 2 581 218 are also known.
Furthermore, for producing parts with a complex geometry from aluminium alloy, such as for example a door lining, which cannot be achieved by conventional stamping with the aforementioned alloys, various solutions have been envisaged and/or implemented in the past:
US20180119261 described 6xxx series aluminum alloys with unexpected properties and novel methods of producing such aluminum alloys. The aluminum alloys are highly formable and exhibit high strength. The alloys are produced by continuous casting and can be hot rolled to a final gauge and/or a final temper. The alloys can be used in automotive, transportation, industrial, and electronics applications, just to name a few.
US20180171452 disclosed high-strength, highly deformable aluminum alloys and methods of making and processing such alloys. More particularly, disclosed is a heat treatable aluminum alloy exhibiting improved mechanical strength and formability. The processing method includes casting, homogenizing, hot rolling, solutionizing, pre-ageing and in some cases pre-straining. In some cases, the processing steps can further include cold rolling and/or heat treating.
Having regard to the increasing development of the use of aluminium sheets for automobile bodywork components and mass production, there still exists a demand for further improved grades making it possible to reduce thicknesses without impairing the other properties so as always to increase lightening.
The invention aims to obtain an excellent compromise between formability in T4 temper and high mechanical strength as well as good behaviour of the finished component under riveting and in a crash, by proposing a method for manufacturing such components including forming in T4 temper after natural ageing at ambient temperature, followed optionally by age hardening on the formed part and baking of the paints or bake hardening. One problem is also to achieve a short and economically advantageous method and to improve compared to a product made of alloy AA 6111.
These components must also have very good corrosion resistance and good behaviour in the various assembly processes such as spot welding, laser welding, adhesive bonding, clinching or riveting.
Another object of the invention is a rolled product obtainable by the method of the invention.
Another object of the invention is a part obtainable by the method of the invention.
Another object of the invention is the use of the part in in a car as bodywork skin parts (or external bodywork panels) such as the front wings, roofs, bonnet, boot or door skins, and the lining parts or bodywork structure components such as for example door, bonnet, tailgate or roof linings or reinforcements, or spars, bulkheads, load-bearing floors, tunnels and front, middle and rear pillars, and finally the impact absorbers or “crashboxes”.
Unless defined otherwise within this description, the general terms are defined is the NF EN 12258-1. A sheet is a flat rolled product of rectangular cross-section with uniform thickness between 0.20 mm and 6 mm.
All aluminium alloys in question hereinafter are, unless indicated to the contrary, designated by the designations defined by the Aluminium Association in the Registration Record Series that it publishes regularly.
All the indications relating to the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy.
The definitions of the metallurgical temper are indicated in the European standard EN 515 unless defined otherwise herein.
The static tensile mechanical characteristics, in other words the ultimate tensile strength Rm, the tensile yield strength at 0.2% elongation Rp0.2, and the elongation at break A %, are determined by a tensile test in accordance with NF EN ISO 6892-1.
The bending angles are determined by a three-point bending test in accordance with NF EN ISO 7438 and the procedures VDA 238-100 and VDA 239-200.
The bendability is also measured with the norm ASTM E290-97a.
The inventors selected a set of composition of aluminium alloys in conjunction with suitable methods which offer to car manufacturer interesting properties to produce parts.
The subject of the invention is a method for manufacturing a rolled product for automobile bodywork or body structure, also referred to as “body in white”, from aluminium alloy, comprising the following steps. The casting of an ingot with the following composition: (% by weight):
The casting can be made with various casting process. Continuous casting, which is usually a horizontal casting, is possible. It is also preferred to use a vertical semi continuous casting, which is also known under the name of direct chill casting. The vertical semi continuous casting is preferred because it more homogenous through the thickness of the sheet.
The ingot is homogenised, hot rolled and cold rolled into a sheet. The sheet is solution heat treated and quenched. Preferably the homogenization treatment of the ingot is at a temperature from 520 to 560° C. during preferably from 2 to 8 hours. Preferably the hot rolling rolls the ingot to a rolled intermediate product having a thickness from 3 to 10 mm. Preferably the cold rolling rolls the rolled intermediate product into a sheet having a thickness from 1 to 4 mm. The sheet is then solution heat treated typically at a temperature beyond the solvus temperature of the alloy while avoiding incipient melting. Preferably the solution heat treatment temperature is from 530° C., preferably 540° C. to 580° C. during preferably from is to 5 minutes. Quenching is then applied to the sheet. Water quenching is suitable with a temperature about 15 to 60° C., preferably 15° C. to 40° C. A pre ageing is applied during preferably at least 8 hours with preferably a temperature from 50 to 120° C. Natural ageing is then applied. Natural ageing is defined in NF EN 12258-1 and room temperature is defined in NF EN ISO 6892-1. Preferably the duration of the natural ageing is from 72 hours to 6 months.
The pre ageing step is preferably achieved by coiling of the sheet at a coiling temperature and cooling it in open air at the room temperature.
A convenient continuous annealing line device to realise the pre ageing is described by
Preferably, the pre ageing is obtained by coiling the sheet at a coiling temperature from 50 to 120° C., preferably from 60 to 120° C., followed by cooling the coiled sheet in open air, and its duration is 8 hours at least.
The rolled product of the invention comprises the product obtainable with the above method from casting to natural ageing. The temper of the rolled product after natural ageing is T4.
T4 temper rolled product tensile yield strength varies less than 5 MPa, preferably 3 MPa between the tensile yield strength in the transverse and 45° directions within the same rolled product. The same sheet is defined a rolled product made from the same ingot, same homogenization, same hot and cold rolling, same solution heat treatment, same quenching, same pre aging, same natural aging and the tensile testing samples are cut off from the rolled product as close as possible. This is a useful property for part stamping.
The rolled product in T4 temper can be characterized in 6 others specific tempers, T8A, T8C, T8D, T6B, T6C and T8D, which estimate the material properties of the part.
The T8A, T8C and T8D tempers are achieved by applying on the T4 rolled product a 2% strain followed each by a specific heat treatment. T8A temper uses a bake hardening heat treatment of 20 minutes at a temperature of 180° C. T8C temper uses a light and short bake hardening heat treatment of 5 minutes at a temperature of 160° C. T8D temper uses a light and long bake hardening heat treatment of 20 minutes at a temperature of 160° C.
The T6B, T6C and T6D tempers are achieved by applying on the T4 rolled product a specific heat treatment. T6B temper uses a heat treatment at a temperature of 225° C. during 30 minutes. T6C temper uses a light and short bake hardening heat treatment of 5 minutes at a temperature of 160° C. T6D temper uses a light and long bake hardening heat treatment of 20 minutes at a temperature of 160° C.
The T4 rolled product can then be formed, in particular by press stamping, in order to obtain a shape. Optionally, the shape is aged. The shape may be painted and bake hardened into a part at a temperature from 150 to 190° C., and preferably from 170 to 190° C., during from 5 to 30 minutes, preferably from 15 to 30 minutes.
An object of the invention is a part obtainable with the above method with the rolled product of the invention. The part can be used in a car as bodywork skin parts (or external bodywork panels) such as the front wings, roofs, bonnet, boot or door skins, and the lining parts or bodywork structure components such as for example door, bonnet, tailgate or roof linings or reinforcements, or, preferably, spars, bulkheads, load-bearing floors, tunnels and front, middle and rear pillars, and finally the impact absorbers or “crashboxes”.
In a first embodiment the coiling temperature is from 50° C. to 95° C., 95° C. being excluded, preferably from 60 to 95° C., 95° C. being excluded. The T4 temper rolled product of this first embodiment is characterized by a tensile yield strength lower than 165 MPa, which can be useful for customer formability at press stamping. The T6B temper rolled product of this first embodiment, as described formally, has a minimum tensile yield strength of 345 MPa and preferably a minimum tensile yield strength of 350 MPa.
A preferred composition for the method according to the first embodiment is
With this preferred composition and with a coiling temperature from 50° C. to 95° C., 95° C. being excluded, preferably 60 to 95° C., 95° C. being excluded, the bendability of the T4 rolled product of the first embodiment is 0.19 maximum. This is advantageous in part forming.
A still more preferred composition of the first embodiment is
With this still more preferred composition, in conjunction with a coiling temperature from 50° C. to 70° C., preferably from 60 to 70° C., the VDA angle of the T4 temper rolled product is greater than 125°. The bendability of the T4 rolled product is still smaller than 0.19. This can be useful in some press stamping application.
In another preferred method of the first embodiment the coiling temperature is between 70° C. and 95° C. With this method, the T8A temper rolled product has a minimum tensile yield strength of 275 MPa. In a more preferred method of this embodiment, the T8A temper rolled product has a minimum tensile yield strength of 280 MPa with a coiling temperature between 70° C. and 95° C. and with a composition of
In a second embodiment of the invention the coiling temperature is from 95° C. to 120° C. and preferably from 95° C. to 105° C. with preferably the composition: Si: 0.75-1.10 and more preferably less 0.90%;
The advantage of this second embodiment is in particular the low sensitivity of the yield strength of the part to a variation of the bake hardening treatment. The bake hardening conditions are dependent on the location inside the car body assembly, parts having a low sensitivity to bake hardening conditions are thus favourable because the car manufacturer has more flexibility. This low sensitivity can be assessed by comparing properties in T6C temper to those in T6D temper and/or properties in T8C temper to those in T8D temper which are obtained from the same T4 temper rolled product.
With rolled product obtained with the method of the second embodiment, the tensile yield strength of the rolled product in T8C and T8D tempers and made from the same rolled product in T4 temper, differ by less than 5 MPa. The T8C and T8D rolled product samples differs only by the duration of the bake hardening, the temperature of which is 160° C.
The T6C and T6D rolled product samples differs only by the duration of the bake hardening the temperature of which is 160° C. With rolled product obtained with the method of the second embodiment, the tensile yield strength of the rolled product in T6C and T6D tempers and made from the same rolled product in T4 temper, differ by less than 5 MPa.
More generally, the rolled product can be heat treated with a temperature from 150 to 190° C., and preferably from 170 to 190° C., during from 5 to 30 minutes, preferably from 15 to 30 minutes. The yield strength of the rolled product, heat treated at a given temperature in the above temperature ranges, during any duration in the above duration ranges, varies by less than 15 MPa, preferably 10 MPa and more preferably 5 MPa.
More generally, the 2% strained rolled product can be heat treated with a temperature from 150 to 190° C., and preferably from 170 to 190° C., during from 5 to 30 minutes, preferably from 15 to 30 minutes. The yield strength of the 2% strained rolled product, heat treated at a given temperature in the above temperature ranges, during any duration in the above duration ranges, varies by less than 15 MPa, preferably 10 MPa and more preferably 5 MPa.
With the second embodiment, the T4 temper rolled product has a maximum tensile yield strength of 190 MPa. With the second embodiment, the T6B temper rolled product has a minimum tensile yield strength of 340 MPa. With the second embodiment, the T8A temper rolled product has a minimum tensile yield strength of 280 MPa, preferably of 290 MPa.
Recyclability of any alloy is an important technical and economical parameter. Reducing the range any element is useful in order to strengthen recycling process as it gives predictability of the future melt. Reducing the maximum of the addition element is also advantageous as they can be more expensive than aluminium. Reducing Si content is advantageous for recycling because in many alloys, this element is not only an impurity but also detrimental to aluminium product properties. Therefore, an advantageous embodiment of the invention is to reduce the Si content to maximum of 0.95%. It is also an advantageous embodiment to reduce Fe maximum to 0.30% and/or to increase the Fe minimum to 0.15%. Another advantageous embodiment is to reduce the Cu maximum to 0.70% and preferably to 0.65% and/or to increase the Cu minimum to 0.55%. Another advantageous embodiment is to reduce the Mn maximum content to 0.35% and more preferably to 0.30% and/or to increase its minimum content to 0.15% and more preferably to 0.25%. Another embodiment is also to reduce the Ti maximum content to 0.05% and/or to increase the minimum content to 0.01%. Another embodiment is to classify the V as an impurity with a maximum of 0.05%
All those combinations of alloys composition and coiling temperature of the invention gives many possibilities for the car manufacturer with different forming properties. The car manufacturer can also optimize its processing and the design of its part. The shape ageing allows a high strength part but it requires a specific heat treatment of the shape ageing. High strength alloys are useful to lightweight part. If the part does not require high strength material, the car manufacturer can avoid the shape ageing, which is advantageous to simplify the production. Hence, the invention gives flexibility to car manufacturer.
Preamble
Table 1 summarises the chemical compositions (% by weight) of the alloys used during tests. The proportion of the others inevitable elements and impurities were lower than 0.05%, the total lower is than 0.15%, and the remainder is aluminium. Alloy G is an exemplary AA6111 alloy and alloy H is an exemplary of a modified AA6056.
The rolling ingots of these various alloys were obtained by vertical semi-continuous casting. After scalping, these various ingots underwent homogenisation heat treatment at 540° C. during about 4 hours directly followed by the hot rolling to a 5 mm intermediate rolled product. The 5 mm intermediate rolled product was cold rolled to obtain sheets with a thickness of 2 mm.
The rolling steps were followed by a solution heat treatment followed by quenching. The solution heat treatment was at a temperature beyond the solvus temperature of the alloy while avoiding incipient melting. In this non limitating example the solutionizing temperature was 570° C. The solutionized sheet was then water quenched in a 20° C. water. The sheet samples were coiled with 3 coiling temperatures of 100° C., 80° C. and 60° C. for a pre ageing of 8 hours followed by a natural ageing. Two natural ageing were used: 7 days and 30 days at room temperature to obtain T4 temper rolled products.
The T4 rolled products were transformed into a T8A temper with a 2% strain and then heat treatment with a typical bake hardening heat treatment of 180° C. during 20 minutes. T8A samples were then characterized.
The T4 rolled product were also heat treated into a T6B temper with a heat treatment of 225° C. during 30 minutes. T6B samples were then characterized.
Tests Results
Tensile tests at ambient temperature were carried out in accordance with NF EN ISO 6892-1 with non-proportional test pieces, with a geometry widely used for sheets, and corresponding to the type of test piece 2 in table B.1 of Appendix B of said standard. These test pieces in particular have a width of 20 mm and a calibrated length of 120 mm. Tensile tests were done on rolled product in T4, T8A and T6B temper. The results obtained with a coiling temperature of 80° C. and 30 days of naturel ageing are presented in Table 2. The results obtained with a coiling temperature of 60° C. and 30 days of naturel ageing are presented in Table 3. The results obtained with a coiling temperature of 60° C., 80° C. and 100° C. and 7 days of naturel ageing are presented in Table 4.
The coiling temperature is an important parameter for T4 temper tensile yield strength. At 60 and 80° C. it allows to limit the T4 tensile yield strength below 165 MPa which can be advantageous for car manufacturer if it is needed to maintain stamping easiness.
Example alloys B, D, E and F, have a tensile yield strength minimum of 350 MPa in T8B temper. Those example alloys have a tensile yield strength minimum of 275 MPa in T8A temper.
Reducing the range of Ti to maximum 0.05%, the V to an impurity of 0.05% maximum and reducing Cu to less than 0.65% is also advantageous as exemplified by alloy E and D because it reduces the bendability to 0.15, which eases the manufacturability of the component independently of the coiling temperature.
In addition to the above reduced range of V, Ti and Cu, the optimized range of Mn from 0.25 to 0.35% offers with the 60° C. coiling temperature a very advantageous 3 points bending test with a high VDA angle which is good for formability. This is exemplified by alloy E with coiling temperature of 60° C.
Rolled products manufactured with alloy E, with coiling temperatures 80° C. and 100° C. and after 7 days of natural ageing were used for others trials. Samples at both coiling temperature were split in 2 groups: in the first group a strain of 2% was applied and the second group there was not any strain. Then a bake hardening temperature of 160° C. was applied, with two different durations of 5 and 20 minutes.
Those results, provided in Table 5 for a coiling temperature of 80° C. and Table 6 for a coiling temperature of 100° C., show another advantageous embodiment: with a coiling temperature of 100° C., the rolled product tensile yield strength is nearly independent from bake hardening duration. This is an advantageous behaviour for parts which can be installed in the car body assembly either at the surface or deep inside a multiple parts assembly because their yield strength remains similar. This offer flexibility for part design for car manufacturer.
An ingot of the following composition was cast
An ingot with the chemical composition in table 7 (% by weight) was cast using a vertical semi continuous casting. The proportion of the others inevitable elements and impurities were lower than 0.05%, and the total is lower than 0.15%, the remainder is aluminium.
The rolling ingot were heated at 554° C. during 4 hours. The ingot was directly hot rolled. The temperature of the ingot just before the start of hot rolling was 540° C. The thickness at the end of hot rolling was 5 mm. The thickness at the end of cold rolling was 2 mm. The sheet was split in three in order to solutionize at three different temperatures, 535° C., 544° C. and with each a different duration above 525° C.: 20s, 45s and 68s. The sheets were quenched in 22° C. water. The sheets were pre aged by coiling the sheets at a temperature of 96° C. and cooling in open air followed by a natural ageing at room temperature about 20° C. during 3 days to obtain T4 temper rolled products.
The T4 rolled products were transformed into a T8A temper with a 2% strain and then heat treatment with a typical bake hardening heat treatment of 180° C. during 20 minutes. T8A samples were then characterized.
The T4 rolled product were also heat treated into a T6B temper with a heat treatment of 225° C. during 30 minutes. T6B samples were then characterized.
Tensile tests were done in the rolling direction (L), in the transverse direction to the rolling direction (T) and direction at 45° the rolling direction (45′).
Table 8 shows the solution heat treatment is reliable to process variation about temperature or duration to obtain the mechanical properties.
T4 temper tensile yield strength shows an anisotropy of less than 3 MPa between the tensile yield strength in the T and 45° directions within the same rolled product as it can be seen in table 8.
Bending radius was also measured on T6B temper to check the crash behaviour of the rolled product. Results are disclosed in table 9.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19306659.4 | Dec 2019 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/086256 | 12/15/2020 | WO |
