EP0896637B1 - Aluminium-silicon-magnesium alloy for motor vehicle body - 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 especially to the manufacture of automobile body parts, for which the final income of the alloy is done during the paint curing operation.

State of the art

AlSiMg alloy sheets have been used for many years for the manufacture of exterior elements of automobile bodywork because they have a good formability before hardening, absence of Lüders lines at deep drawing and high mechanical strength after heat treatment comprising dissolution, quenching, maturation and tempering hardening. This hardening income can advantageously be obtained during the paint curing operation applied to the body element, which saves a specific treatment. This is what we usually call by hardening effect during cooking, or in English "bake hardening".

Many alloys have been developed for this application. The US patent 4082578 of ALCOA describes alloys for automobile bodywork of composition (% by weight): Si: 0.4 - 1.2 Mg: 0.4 - 1.1 Cu: 0.1 - 0.6 Fe: 0.05 - 0.35 with the addition of one or more of the elements Mn, Cr or Zr. Alloys of this type have been registered with the Aluminum Association under the designations 6009 and 6010. The hardening income of these alloys is carried out appropriately 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 recommends an income of 2 to 15 h at a temperature above 182 ° C. This alloy has better mechanical strength and better indentation resistance than 6009 or 6010, as well as a more stable response to hardening.

US Patent 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 resistance greater than 6009 and a formability greater than 6010, as well as a good response to the curing by baking of the painting, which the patent recommends to carry out for approximately 1 h between 177 and 204 ° C. FIG. 2 of the patent compares, for the alloys 6009, 6010 and 6111, the curves of the yield strength as a function of the curing temperature at different rates of deformation, and clearly shows that at 150 ° C, hardening has not yet started in any of the cases shown and it doesn't start to be significant only above 180 ° C.

ALCOA patent US 4,840,852 relates to an alloy with a higher copper content, this which explains its designation AA 2008 in the category of Al-Cu alloys of the series 2000. Its composition is: Si: 0.5 - 0.85 Mg: 0.25 - 0.55 Cu: 0.75 - 1.10 Fe: 0.05 - 0.4. The preferred range for tempering is 177 - 218 ° C, and above specifically, the range 190 - 204 ° C. The only example indicates an income at 204 ° C.

The applicant's international application WO 95/14113 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 Al type (Mn, Fe) If and recommends a pre-income between hardening and maturation. Examples show all an income of 30 min at 180 ° C.

Problem

There has been a clear downward trend in the temperature of baking of paints in the automotive industry, which went from 180 ° C to around 150 ° C. This drop in cooking temperature means that the hardening of alloys during this operation tends to decrease strongly. It therefore became necessary, for the exterior body panels, to offer alloys which are sufficiently hard at baking temperatures low, while maintaining good formability after dissolution, quenching and maturing at room temperature.

In addition, the imperatives of cost and ease of manufacture make it necessary to choose alloys easy to put in solution, so as to avoid coarse phases not dissolved harmful to formability.

The aim of the present invention is to select, in Al-Si-Mg-Cu alloys, a narrow range of composition which allows both to achieve resistance sufficient mechanical after firing around 150 - 155 ° C and put the alloy in easy solution.

Subject of the invention

The subject of the invention is an aluminum alloy capable of being used for the manufacture of exterior parts of automobile bodywork, of composition (% by weight):
If: 0.5 -0 8 Mg: 0.45 - 0.65 Cu: 0.55 - 0.75 Mn and / or Cr: 0.1 - 0.3 aluminum remains with the inevitable impurities, with: 1, 6 <Si + Mg + Cu <2
Preferably, Mg is between 0.50 and 0.60% and Cu between 0.60 and 0.70%.

Description of the invention

The range of composition of the alloys according to the invention is, for Si and Mg, at inside the range 6009/6010, but with a higher copper content. Through compared to the 6111 domain, it is less loaded with magnesium, whereas on the contrary it is more so than in 2008. But its main characteristic is to have, for compared to recent body alloys with high resistance and good formability, a total quantity of addition elements, represented by the sum Si + Mg + Cu, plus low. This feature allows homogenization and implementation solution under easier conditions, but, in the area of narrow composition of the invention, this reduction does not lead, as might have been expected, to a reduction in mechanical resistance on the finished part after dissolving short and an income practiced during a paint curing around 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 between 10 and 3 mm, then rolled to cold to final thickness, generally included, for bodywork applications automotive, between 0.8 and 1.5 mm. Annealing from 1 h to 10 h between 300 and 500 ° C can be applied to the strip after hot rolling or during cold rolling.

The strip is dissolved between 500 and 560 ° C for a period of between 20 s and 2 min. It is followed by quenching and maturing at room temperature for more than a week. The bodywork parts, generally shaped by stamping and covered with paint, are subjected to an operation of baking the paint at a temperature below 170 ° C, most often around 150 ° C, and the income caused by this cooking, an elastic limit R _0.2 > 170 MPa can be reached on the part.

The compositions according to the invention thus lead to complete dissolution, favorable to the formability of the sheet, within a period compatible with the requirements industrial productivity. Unexpectedly, the mechanical strength, after this short dissolution and an income associated with cooking the paintings towards 150 to 155 ° C, is of the same order, and even sometimes better, than that of alloys of the prior art, however, more loaded with addition elements.

Example

Plates were cast in format 30 x 180 x 250 mm of 6 alloys, the composition of which is indicated 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 1.71 and 1.91 respectively.

Alloy C, 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 classic AA 6111 with Mg> 0.65 and Si + Mg + Cu = 2.02. The alloy E has a Si> 0.8 and a sum Si + Mg + Cu = 2.19. Alloy F, type AA 6013, has an Mg content> 0.65.

The plates were homogenized for 24 h at a temperature of 555 ° C for the alloy C and 570 ° C for the other 5. The efficiency of this homogenization can be assessed, in a manner known per se, by a differential enthalpy analysis (AED) on the homogenized plates. Indeed, if the homogenization is well done, this analysis leads to low peak areas (measured in J / g) and to 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 noted that the 2 alloys according to the invention have a lower peak area than the other 4, which indicates that their homogenization has been more complete.

The plates then followed a simulated annealing of the hot rolling by maintaining for 9 h at 500 ° C and cooling at a speed of 25 ° C / h. Then cold rolling to the thickness of 1 mm was carried out, followed by dissolution in a salt bath, partly at 550 ° C, and partly at 525 ° C, for respective durations of 30, 60, 300 and 1800 s, and finally to a hardening. As with homogenization, the quality of the solution can be assessed by differential enthalpy analysis of the samples 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 can be seen that the 2 alloys according to the invention have a peak area less than 4 others, which shows a better solution.

The samples in solution were then matured at temperature ambient for 3 weeks, traction at 2% deformation and an income in the bath oil for 20 min at 155 ° C.

The elastic limits R _0.2 (in MPa) in the T4 state obtained for the 4 alloys A, B, C and D as a function of the temperature and the duration of dissolution, are indicated in table 4: Alloy Temp. MS 30 s 60s 300 s 1800 s 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 alloys A and B according to the invention, the increase in the dissolution time does not lead to a significant increase in mechanical strength. We reach the maximum of elastic limit for a treatment of 30 s at 525 ° C for alloy A, and for a treatment of 30 s at 550 ° C for alloy B. On the contrary, the dissolution is not complete after 1800 s at 550 ° C for alloys C and D. This dissolution incomplete is detrimental to formability.

Furthermore, after a short solution time (30 s), the elastic limit of the alloy A is of the same order as that of D, and that of B is greater than that of C and D, then that alloys A and B are less loaded with additives than alloys C and D (lower Si + Mg + Cu sum).

The elastic limits obtained after dissolving for 1 min at 550 ° C, quenching, deformation of 2% and baking for 20 min 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 alloy A has the same level of elastic limit as the alloys C, D and F, while it is significantly less loaded with addition elements hardening, and that, surprisingly, alloy B has an elastic limit higher than that of alloys C to F.

Claims (5)

1. Aluminum alloy particularly intended for the manufacture of external automobile bodywork components, of composition (% by weight)
   Si: 0.5 - 0.8 Mg: 0.45 - 0.65 Cu: 0.55 - 0.75
   Mn and/or Cr: 0.1 - 0.3, the remainder being aluminum with the inevitable impurities
   and with 1.6 < Si + Mg + Cu < 2.0.
2. Aluminum alloy according to Claim 1, characterized in that Mg is between 0.50 and 0.60%.
3. Aluminum alloy according to one of Claims 1 and 2, characterized in that Cu is between 0.60 and 0.70%.
4. Method of manufacturing a painted bodywork component made of an alloy according to one of Claims 1 to 3, comprising:

   casting a plate of alloy with the composition given,

   homogenization of this plate at a temperature between 500 and 580°C

   hot rolling of the plate into 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 period of between 20 seconds and 2 minutes, followed by quenching and natural aging for more than one week at ambient temperature,

   manufacture of a component from this strip and coating it with liquid paint,

   curing of this paint at a temperature of between 150 and 170°C.
5. Method according to Claim 4 in which, after hot rolling or during the cold rolling, the strip is subjected to annealing for a period of from 1 hour to 10 hours at between 300 and 500°C.