EP0896637B2 - Use of aluminium-silicon-magnesium alloy for manufacturing of motor vehicle body parts and method for manufacturing the same. - Google Patents

Description

Technical field of the invention

The invention relates to the field of aluminum alloys of the AlSiMg type, or alloys of the 6000 series according to the classification of the Aluminum Association, intended in particular for the manufacture of automobile body parts, for which the final income of the alloy is done during the cooking operation of the paint.

State of the art

AlSiMg alloy plates have been used for many years automotive bodywork because they have good formability before curing, an absence of Lüders lines for stamping and high mechanical strength after a heat treatment comprising dissolution, quenching, ripening and tempering. This hardening income can be advantageously during the baking operation of the paint applied to the bodywork element, this which saves a specific treatment. This is usually referred to as a hardening effect cooking, or in English "bake hardening".

Many alloys have been developed for this application. US Patent 4082578 to ALCOA discloses automotive body composition alloys (% by weight): Si: 0.4 - 1.2 Mg: 0.4 - 1.1 Cu: 0.1 - 0.6 Fe: 0.05 - 0.35 with addition of one or more of Mn, Cr or Zr. Alloys of this type have been registered at Aluminum Association under the designations 6009 and 6010. The hardening income of these alloys is effected in a manner between 190 and 205 ° C.

US Patent 4589932, also from ALCOA, relates to an alloy, subsequently registered under the designation AA 6013, of composition:
If: 0.4 - 1.2 Mg: 0.5-1.3 Cu: 0.6-1.1 Mn: 0.1- 1.0 and advocates an income of 2 to 15 h at a temperature greater than 182 ° C. This alloy has better mechanical strength and indentation resistance than 6009 or 6010, as well as a more stable cure response.

US Pat. No. 4,614,552 to ALCAN relates to the AA 6111 alloy of composition:
If: 0.6 - 1.0 Mg: 0.62 - 0.82 Cu: 0.65 - 0.79 Mn: 0.1 - 0.5 Fe <0.4

This alloy has a mechanical strength greater than 6009 and a formability greater than 6010, as well as a good response to the curing of the paint, which the patent recommends to carry out during about 1 h at 177-204 ° C. FIG. 2 of the patent compares, for alloys 6009, 6010 and 6111, the curves of the elastic limit as a function of the curing temperature at different rates of deformation, and clearly shows at 150 ° C, hardening has not yet started in any of the cases shown and is not starting to significant than above 180 ° C.

US Pat. No. 4,840,852 to ALCOA relates to an alloy with a higher copper content, which explains its designation. AA 2008 in the category of alloys Al-Cu of the series 2000. It has for composition: Si: 0,5 - 0,85 Mg: 0.25 - 0.55 Cu: 0.75 - 1.10 Fe: 0.05 - 0.4. The preferred range for income is 177 - 218 ° C, and more specifically, the range 190 - 204 ° C. The only example is an income of 204 ° C.

The international application WO 95/14113 of the Applicant relates to a range of alloys of composition: Si: 0.5-1.3 Mg: 0.25-0.8 Cu <0.9 Mn: 0.1-0.8 Fe <0.5, with fine manganese precipitates of type Al (Mn, Fe) Si and advocates a precondition between quenching and maturation. The examples all indicate a 30 minutes at 180 ° C.

Problem

In recent years, there has been a clear downward trend in the paint's cooking temperature in the automotive industry, which went from 180 ° C to about 150 ° C. This decrease in cooking temperature Consequently, the hardening of the alloys during this operation tends to decrease sharply. It is therefore become necessary, for exterior body panels, to propose alloys that are sufficiently hard at low cooking temperatures, while keeping a good formability after dissolution, quenching and ripening at room temperature.

Moreover, the requirements of cost and ease of manufacture require to choose alloys easy to put in solution, so as to avoid undissolved coarse phases detrimental to the formability.

The object of the present invention is to select, in Al-Si-Mg-Cu alloys, a narrow domain of composition which allows both to achieve sufficient mechanical strength after baking at 150 - 155 ° C and to put the alloy in solution easily.

Object of the invention

The subject of the invention is the use of an aluminum alloy for the manufacture of exterior parts of automobile bodywork, of composition (% by weight):
If: 0.5 -0 8 Mg: between 0.50 and 0.60 Cu: between 0.60 and 0.70 Mn and / or Cr: 0.1 - 0.3 remains aluminum with unavoidable impurities, with: 1.6 <Si + Mg + Cu <2

Description of the invention

The composition range of the alloys according to the invention is, for Si and Mg, within the domain 6009/6010, but with a higher copper content. Compared to the 6111 domain, it is less busy in magnesium, whereas on the contrary it is more compared to 2008. But its main characteristic is to have, for compared to recent high strength bodywork alloys and good formability, a total amount of of addition, represented by the sum Si + Mg + Cu, lower. This feature allows for homogenization and solution dissolution under easier conditions, but in the narrow composition field of the invention, this reduction does not cause, as one would expect, a decrease in mechanical strength on the finished part after a short dissolution in solution and an income practiced during a paint baking at 150 ° C.

The alloy according to the invention is cast in the form of plates, homogenized between 500 and 580 ° C., hot rolled to a thickness of between 10 and 3 mm, then cold rolled to the final thickness, generally included, for automotive bodywork applications, between 0.8 and 1.5 mm. An annealing of 1 h to 10 h between 300 and 500 ° C can be applied to the strip after hot rolling or during cold rolling.

The dissolution of the strip is between 500 and 560 ° C for a period of between 20 seconds and 2 minutes. It is followed by quenching and maturing at room temperature for more than a week. The body parts, generally shaped by stamping and covered with paint, are subjected to a paint-baking operation at a temperature below 170 ° C, most often around 150 ° C, and the resulting income by this cooking makes it possible to reach the piece with an elastic limit R _0.2 > 170 MPa.

The compositions according to the invention thus lead to a complete dissolution, favorable to the formability sheet metal, in a time compatible with the industrial requirements of productivity. Unexpectedly, the mechanical resistance, after this short dissolution and a revenue associated with baking paints to 150 at 155 ° C., is of the same order, and sometimes even better, than that of alloys of the prior art which are nevertheless more in addition elements.

Example

30 × 180 × 250 mm plates of 6 alloys were cast, the composition of which is shown in Table 1 (in% by weight). Alloy Yes mg Cu mn Cr AT 0.53 0.55 0.63 0.22 0.04 B 0.74 0.54 0.63 0.22 0.05 VS 0.97 0.66 0.64 0.01 0.05 D 0.63 0.75 0.64 0.21 0.05 E 0.99 0.55 0.65 0.21 0.06 F 0.51 0.79 0.62 0.21 0.05

The alloys A and B are in accordance with the invention, the sum Si + Mg + Cu being respectively 1.71 and 1.91.

Alloy C, of the type AA 6056, is more charged, with Si> 0.8, the sum Si + Mg + Cu = 2.27, and the contents of Mn and Cr below those of the alloys of the invention. Alloy D is a conventional AA 6111 with Mg> 0.65 and Si + Mg + Cu = 2.02. The alloy E has an Si> 0.8 and a sum Si + Mg + Cu = 2.19. Alloy F, of the AA 6013 type, has a Mg content> 0.65.

The plates were homogenized for 24 hours at a temperature of 555 ° C for alloy C and 570 ° C for the others. The effectiveness of this homogenization can be appreciated, in a manner known per se, by a differential enthalpy analysis (AED) on the homogenized plates. Indeed, if homogenization is well done, this analysis leads to peak peak areas (measured in J / g) and high peak start temperatures. The results obtained were as follows (Table 2): Alloy Peak area (J / g) Temperature (° C) AT - 0.015 580 B - 0.088 568 VS - 1.52 563 D - 1.04 575 E - 0.075 563 F - 0.33 580

It is found that the 2 alloys according to the invention have a lower peak area than the other 4, which indicates that their homogenization was more complete.

The plates then followed a hot rolling simulation annealing by holding for 9 h at 500 ° C and cooling at a rate of 25 ° C / h. Cold rolling was then carried out to the thickness of 1 mm, then to a solution in a salt bath, partly at 550 ° C., and partly at 525 ° C., for respective periods of time. 30, 60, 300 and 1800 s, and finally to a quench. As for the homogenization, one can appreciate the quality of the solution solution by differential enthalpic analysis of the samples put in solution. The results of this analysis are as follows for a dissolution of 30 s at 550 ° C (Table 3): Alloy Peak area (J / g) Temperature (° C) AT 0 566 B - 1.31 572 VS -4.52 565 D - 6.57 574 E - 1.47 566 F - 5.28 578

It is found that the 2 alloys according to the invention have a peak area less than the other 4, which shows a better solution in solution.

The dissolved samples were then matured at room temperature for 3 weeks, a traction at 2% of deformation and a return in oil bath of 20 mn to 155 ° C.

The elastic limits R _0.2 (in MPa) at the T4 state obtained for the 4 alloys A, B, C and D as a function of the temperature and the dissolution time are given in Table 4: Alloy Temp. MS 30s 60s 300s 1800s AT 525 ° C 122 131 120 123 AT 550 ° C 129 127 123 129 B 525 ° C 147 151 149 152 B 550 ° C 154 160 157 151 VS 525 ° C 151 153 168 173 VS 550 ° C 145 153 173 185 D 525 ° C 133 132 147 162 D 550 ° C 126 118 134 154

The following conclusions can be drawn from these results: for the alloys A and B according to the invention, the increase the dissolution time does not lead to a significant increase in mechanical strength. The maximum elastic limit is reached for a treatment of 30 s at 525 ° C for alloy A, and for a treatment from 30 s to 550 ° C for alloy B. On the other hand, dissolution is not complete after 1800 s at 550 ° C for alloys C and D. This incomplete dissolution is detrimental to the formability.

Moreover, after a short dissolution (30 s), the elastic limit of alloy A is of the same order as that of D, and that of B is greater than that of C and D, while the alloys A and B are less loaded in elements of addition that the alloys C and D (sum Si + Mg + Cu lower).

The elasticity limits obtained after being dissolved for 1 min at 550 ° C., quenching, deformation of 2% and baking for 20 minutes at 155 ° C. are given in Table 5: Alloy AT B VS D E F R _0.2 MPa 180 203 170 183 190 179

It is found that the alloy A has the same level of elastic limit as the alloys C, D and F, whereas is significantly less charged with hardening additives, and surprisingly, alloy B has a limit elasticity greater than that of alloys C to F.

Claims (3)

1. Use of an alloy with composition (% by weight):
   Si = 0.5 - 0.8; Mg between 0.50 and 0.60; Cu between 0.60 and 0.70; Mn and / or Cr = 0.1 - 0.3; remainder aluminium with inevitable impurities and 1.6 < Si + Mg + Cu < 2 for manufacturing of external automobile bodywork parts.
2. Method for manufacturing a painted bodywork part made from an alloy comprising:

   casting of an alloy plate with the following composition (% by weight):
      Si = 0.5 - 0.8; Mg between 0.50 and 0.60; Cu between 0.60 and 0.70; Mn and / or Cr = 0.1 - 0.3; remainder aluminium with inevitable impurities and 1.6 < Si + Mg + Cu < 2.

   this plate is homogenised at a temperature of between 500°C and 580°C;

   the plate is hot rolled in a strip with a thickness of between 10 mm and 3 mm, followed by cold rolling to a thickness of between 0.8 and 1.5 mm;

   solution heat treatment of the rolled strip at a temperature between 500 and 560°C for a duration of between 20 s and 2 min, followed by quenching and natural aging for more than a week at ambient temperature;

   a part is manufactured from this strip and coated with liquid paint;

   this paint is baked at a temperature between 150°C and 170°C.
3. Method according to claim 2 in which, after hot rolling or during cold rolling, artificial aging is applied to the strip for 1 h to 10 h at a temperature between 300°C and 500°C.