Improvement of product strength and formability in stamping of Al–Mg–Si alloy sheets having bake hardenability by resistance heat and artificial aging treatments

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Abstract

Resistance heat and artificial aging treatments were introduced into a stamping operation to improve the product strength and formability of Al–Mg–Si alloy sheets having bake hardenability used for automobile body panels. In this treatment, the sheets undergo re-solution by resistance heat treatment, composed of resistance heating and water quenching just before the stamping. Stamped sheets are artificially aged just after the stamping to increase product strength. In the experiment, Al–0.60% Si–0.74% Mg alloy sheets were chosen as an example of Al–Mg–Si alloy sheets having bake hardenability. The re-solution solution treatment of the sheets was sufficiently accomplished by rapid resistance heat treatment, and formability of the sheets was improved. Hardness of the formed products was increased by artificial aging. It was found that the present process is effective in improving the product strength and formability of Al–Mg–Si alloy sheets having bake hardenability due to the compactness and rapidness.

Introduction

To reduce the weight of cars, the use of aluminum alloy sheets tends to increase because of their high specific strength. Especially, 6000-type Al–Mg–Si alloy sheets having bake hardenability are targeted as materials for body panels [1]. In these sheets, the flow stress in the stamping is low, and then strength of the stamped products is increased by paint baking. In the conventional production process, however, the increase in strength is not very large due to the growth in precipitation by natural aging. In general, the time from solution heat treatment in an aluminum making factory to baking in a forming factory is about 2 weeks, and thus the precipitation by natural aging considerably grows. Although the pre-aging treatment [2], [3] has been reported to prevent the natural aging, the recovery of paint bake hardenability is not enough. It is desirable in the automobile industry to develop heat treatments suitable for the stamping of aluminum alloy sheets.

To increase the final strength of products stamped from the aluminum alloy sheets having bake hardenability, not only the production processes of the sheets but also the stamping processes are taken into consideration. An introduction of a re-solution heat treatment into the stamping process is effective in preventing the precipitation by natural aging. The re-solution heat treatment of the sheets is installed just before stamping, and the stamped products are artificially aged just after stamping. High formability and low flow stress in stamping are attained by the re-solution heat treatment, and the strength of the products is increased by artificial aging. Since an apparatus used for the conventional solution heat treatment is large and the treating time is long, it is not easy to introduce this treatment into the stamping operation. Ishiguro et al. [4] have developed a resistance heat treatment in order to refine grains in aluminum alloy sheets. Because the sheet is directly heated by means of electrical resistance during electrifying, the so-called Joule heat, the resistance heat treatment is compact enough for introduction into the stamping operation. They have applied resistance heating to powder forming [5], semi solid forging [6], [7] and warm and hot stamping [8].

In the present study, resistance heat and artificial aging treatments were introduced into the stamping operation to improve the product strength and formability of Al–Mg–Si alloy sheets having bake hardenability. For the present treatments of Al–0.60% Si–0.74% Mg alloy sheets, the degree of re-solution, the improvement in formability and the increase in product strength were examined.

Section snippets

Introduction of resistance heat and artificial aging treatments into stamping

The sequences of the old, conventional and present processes for the production of products stamped from Al–Mg–Si alloy sheets are compared in Fig. 1. In the old process, the sheets are produced by a sequence of casing, hot rolling, cold rolling and solution heat treatment in an aluminum making factory, then the sheets are stamped after about 1 week in a forming factory, and the stamped products are baked for the painting about 1 week later. Since the precipitation grows due to natural aging

Conditions for resistance heat treatment

Since the resistance heating is very rapid, about 2 s for the heating at 540 °C necessary for the re-solution heat treatment of the aluminum alloy sheets, the degree of re-solution for this rapid treatment was evaluated. The resistance heat treatment is not a complete treatment because of compactness and rapidness applicable to the stamping operation.

Al–0.60% Si–0.74% Mg alloy sheets having the chemical composition shown in Table 1 were employed as an example of the heat treatment. The length,

Sequences of processes

The sequence of the present process using resistance heat and artificial aging treatments is illustrated in Fig. 5. Before the present process, the sheets underwent solution heat treatment at 540 °C for 4 h and cold rolling of a total reduction of 90 % as mentioned in Section 3. Stamping and paint bake hardening were replaced with cold rolling and heat treatment at 175 °C for 2 h, respectively. In rolling, the strain in the sheet becomes uniform, unlike as in stamping. The sequence of the present

Conclusions

Resistance heat and artificial aging treatments were introduced into the stamping operation to improve the product strength and formability of Al–Mg–Si alloy sheets having bake hardenability. The resistance heat treatment is compact enough for the introduction. The following results were obtained from the experiment using Al–0.60% Si–0.74% Mg alloy sheets:

  • (1)

    The re-solution was attained by the rapid resistance heat treatment.

  • (2)

    The decrease in flow stress and the increase in n-value were obtained by

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