EP3279350B1

EP3279350B1 - Method for producing an object made from a hardenable aluminium alloy

Info

Publication number: EP3279350B1
Application number: EP16182951.0A
Authority: EP
Inventors: Manoj Kumar
Original assignee: LKR Leichtmetallkompetenzzentrum Ranshofen GmbH
Current assignee: LKR Leichtmetallkompetenzzentrum Ranshofen GmbH
Priority date: 2016-08-05
Filing date: 2016-08-05
Publication date: 2020-01-08
Anticipated expiration: 2036-08-05
Also published as: ES2783599T3; EP3279350A1

Description

The invention concerns a method for producing an object made from a hardenable aluminium alloy.
Most automotive structural and body parts are made from steels and are therefore inherently heavy. Replacing steel with a lightweight material would provide a direct and simple solution for an efficient weight reduction. Lightweight materials such as aluminium alloys are playing an important role in this consideration about weight reduction due to a higher specific strength and stiffness compared to steel. For example, heat treatable aluminium alloys including alloys of the series AW-2xxx, AW-6xxx and AW-7xxx can increase the strength to weight ratio of the parts that is required to enhance the safety for passengers while reducing the weight.
However, one of the major drawbacks of using heat treatable (hardenable) aluminium alloys is the limited formability at room temperature. This represents a major challenge, in particular as regards forming such materials in a series production chain.
Up to date, for the forming of aluminium alloys the state of the art forming processes were mainly developed by adapting processes from forming of steel grades. The adapted sheet metal forming approaches are warm forming (P. J. Bolt, N. A. P. M. Lamboo, P. J. C. M. Rozier, Feasibility of warm drawing of aluminium products, J. Mater. Proc. Techn. 115, 2001, 118), hot forming (X. Fan, Z. He, S. Yuan, K. Zheng, Experimental investigation on hot forming-quenching integrated process of 6A02 aluminum alloy sheet, Mater. Sci. Eng. A. 573, 2013, 154, doi: 10.1016/j.msea.2013.02.058) and superplastic forming (P. A. Friedman, S. G. Luckey, W. B. Copple, R. Allor, C. E. Miller, C. Young, Overview of superplastic forming research at ford motor company, J. Mater. Eng. Perform. 13, 2004, 670, doi: 10.1361/10599490421277). These approaches may form sheet metal aluminium parts by improving the ductility and the forming limits as compared to room temperature but can result in excessive thinning and necking due to lack of adequate work hardening capability. It was also observed that strength of the parts was getting reduced after forming. These effects are attributed to mechanisms of recovery, recrystallization and precipitation. The mentioned effects are the main reasons why the structural elements in car industry are still preferably made of steel grades.
JP06081066 A discloses cryogenic forming of Al-Mg-Si alloys.
However, there has been made progress in forming aluminium alloys so that crack-free structural parts for automotive applications can be produced. In order to increase the work hardening behavior and strength of aluminium alloys, a new approach of cryogenic forming has been introduced ( R. J. Selines, J. S. van den Sype, US 4,159,217 ; G. Falkinger, F. Grabner, G. Schmid, R. Schneider, R. J. Grant, Improved formability of AA5182 aluminium alloy sheet at cryogenic temperatures, Mater. Today Proc. 2, 2015, 113, doi: 10.1016/j.matpr.2015.05.027). Aluminium alloys show the characteristic that the uniform elongation, work hardening, yields and tensile strength significantly increase with decreasing temperature. To demonstrate this improved formability under cryogenic conditions, a process for cryogenic aluminium sheet metal forming was developed by the inventors. The inventors observed that a sheet of a non-heat treatable alloy AW-5182-O formed at very low temperature led to a crack-free complex part. However, the inventors also observed that under same conditions a heat treatable alloy AW-6016-T4 sheet did not lead to a crack-free part when formed under identical conditions. Therefore, a need exits which allows forming crack-free parts made from a heat treatable aluminium alloy.
It is therefore an object of the instant invention to provide a method which allows to form a crack-free object from a half-finished product of a heat treatable aluminium alloy.
According to the invention a method for producing an object made from a heat treatable aluminium alloy is provided, the method comprising:

providing a raw product of the hardenable aluminium alloy;
heating the raw product of the hardenable aluminium alloy to an elevated temperature in order to dissolve precipitates;
optionally holding the raw product of the aluminium alloy at the elevated temperature for a predetermined time;
rapidly quenching the raw product of the hardenable aluminium alloy to a temperature lower than the elevated temperature in order to maintain at least partly dissolved precipitates within the aluminium alloy;
forming the raw product of the hardenable aluminium alloy below a temperature of 0 °C, in particular below-150 °C to produce the object.

A method according to the invention allows for the production of crack-free objects from a hardenable heat treatable aluminium alloy. The object may be a structured component for use in the automotive industry. However, the method of the invention is also applicable to other parts which receive a certain shape by a forming process.
Within the context of the instant invention hardenable aluminium alloys are such aluminium alloys which are heat treatable. Heat treatable aluminium alloys can be hardened by a first heat treatment at a higher temperature to dissolve precipitates within the matrix and a second heat treatment at lower temperature whereby new precipitates form.
The invention is based on the following considerations: The work hardening behavior of heat treatable aluminium alloys can be significantly affected by the nature of precipitates. The effect of shearable precipitates on work hardening has generally been considered in terms of the possibility of flow localization on a glide plane as the precipitate strength is decreased by the dislocation shearing process. On the other hand, for the case where a low volume fraction of non-shearable particles is present, very high initial hardening rates are observed (i. e., dispersion hardening systems). This has been attributed to two fundamental mechanisms, namely, the storage of additional so-called geometrically necessary dislocations and the storage of elastic energy in the second-phase particles (L. M. Cheng, W. J. Poole, J. D. Embury, D. J. Lloyd, The influence of precipitation on the work-hardening behavior of the aluminum alloys AA6111 and AA7030, Metall. Mater. Trans. A. 34, 2003, 2473, doi: 10.1007/s11661-003-0007-2).
In non-heat treatable aluminium alloys, the solute elements form a solid solution that is known to improve the work hardening behavior primarily by making dynamic recovery a more difficult process. This may arise from multiple mechanisms, namely, changes in stacking fault energy due to alloying, solute drag effects on dislocation and so on (L. M. Cheng, W. J. Poole, J. D. Embury, D. J. Lloyd, The influence of precipitation on the work-hardening behavior of the aluminum alloys AA6111 and AA7030, Metall. Mater. Trans. A. 34, 2003, 2473, doi: 10.1007/s11661-003-0007-2). It is the concept of the invention to achieve a similar solid solution state in a heat treatable, hardenable aluminium alloy by dissolving the hardening precipitates by applying a suitable heat treatment directly before the forming process at low temperature.
The half-finished product of the heat treatable aluminium alloy is usually heated to an elevated temperature of 250 °C to 600 °C, in particular 300 °C to 450 °C. A very fast heating up to the elevated temperature is preferred. In particular, the half-finish product of the hardenable aluminium alloy is heated to an elevated temperature with a heating rate of at least 10 K/s.
In order to dissolve precipitates completely or at list predominantly, the half-finished product of the hardenable aluminium alloy is held at the elevated temperature for a time up to 60 s, in particular up to 40 s.
After the heating to the elevated temperature and optionally holding at this temperature, the half-finished product of hardenable aluminium alloy is advantageously rapidly quenched to room temperature, preferably by cooling rates of more than 5 K/s, more preferably more than 10 K/s, in particular more than 20 K/s. However, it is also possibly to quench the half-finished product to other temperatures as long as it is ensured that the former precipitates remain resolved during and after the quenching process.
For the quenching step, the half-finished product of the hardenable aluminium alloy can be rapidly quenched by contact with a gas like air or a liquid. For liquids, water or oils can be used.
For the forming step, the half-finished product of the hardenable aluminium alloy can be formed that the temperature below -190 °C. In particular, liquid nitrogen can be used in order to cool directly or indirectly the half-finished product to a temperature of about -196 °C before the forming step is performed.
After the forming process, a hardening step by a heat treatment can be applied. This hardening can be performed during a paint bake cycle, in particular when producing an automotive part.
The method according to the invention is applied to sheets. Advantageously, the method is applied to form an automotive component.
Further features and advantages of the invention will become evident from the following examples. In the drawings show:

Fig. 1a and 1b flow curves of two aluminium alloys at different temperatures;
Fig. 2a and 2b parts formed at room temperature and cryogenic temperature;
Fig. 3 a schematic diagram of a method according to the invention;
Fig. 4a and 4b diagrams showing formabilities of heat treatable aluminium alloys treated according to the invention.

In Fig. 1a and 1b flow curves of two sheets made from a non-heat treatable aluminium alloy AW-5182-O (Fig. 1a) and a heat treatable aluminium alloy AW-6016-T4 (Fig. 1b) are shown. As indicated, the sheets of the two different materials were formed at room temperature as well as low temperatures. For the sheet of the non-heat treatable alloy AW-5182-A, it was possible to obtain crack free automotive components, namely a mini-B-pillar part, when formed at a temperature of -196 °C (Fig. 2a) whereas at room temperature cracks were observed (Fig. 2b). For the heat treatable, hardenable alloy AW-6016-T4 crack-free parts could neither be produced at room temperature nor at low temperatures.
Fig. 3 depicts schematically a process according to the invention for forming objects from a half-finished product like a sheet or a tube wherein the method is applicable to hardenable aluminium alloys in order to produce crack-free objects. In a first step, a half-finished product like a sheet, tube or another shaped product is provided. The sheet is than exposed to a shock heat treatment as a second step. In this second step, the sheet material is heated with a heating rate of more than 10 K/s to reach a shock heat treatment temperature between 250 °C and 600 °C. This fast heating can be performed by contact, laser, plasma, infrared, resistance and/or induction heating technologies. Afterwards, the sheet is usually held for a predetermined holding time at the elevated temperature. Typical holding times are in the range up to 60 s, preferably 20 s to 40 s. Afterwards, the half-finished product is quenched to room temperature by applying a quenching medium like a gas or a liquid. Typical quenching media are gas or water. Afterwards, the quenched half-finished product is transferred to a tool which is cooled to a low temperature and allows for low temperature forming of the half-finished product to a structured component. For example, the tool can be cooled with liquid nitrogen in order to provide temperatures of about -196 °C for the forming process. After this step, a structured component, for example for use in automotive industry, is obtained. After storage, the structural component can be exposed to a usual paint baking process.
During the shock heat treatment, the dissolution of hardening precipitates occurs mainly during the rapid heating step and further completes during the optional holding time. This process enriches the aluminium matrix with solutes which are trapped inside the matrix upon quenching down to room temperature. During forming, enhanced work hardening behavior is achieved and in result the formability limit extended to higher levels as compared to the initial starting condition of the alloy. This is depicted in Fig. 4a for a sheet of an alloy AW-6016-T4 and in Fig. 4b for a sheet of an alloy AW-7075-T6. In both cases the shock heat treatment was performed by heating the sheets to a temperature of 400 °C and by applying a holding time of 30 s.
Main benefits of the combination of a shock heat treatment with cryogenic metal forming are the extension of the forming limit of the as-received sheet (or any other half-finished product) to a much higher level. The application of a shock heat treatment leads to the dissolution of the majority of the hardening precipitates and retaining adequate solute in solution such that a strengthening response is still achievable during downstream head treatments such as a paint bake cycle.
Moreover, applying the shock heat treatment to the half-finished product of the heat treatable aluminium alloy within a limited time before the forming operation is the fact that it is not necessary to take into account the effect of shelf life on the formability.
Further, if some precipitates are stable enough not to be dissolved back into solution, this precipitates will remain in the microstructure to facilitate the nucleation and/or growth of the strengthening precipitates during the paint bake cycle.
Still further, the instant method can be applied to variets products made from sheet metal, in particular automotive componence like your inner panels, door outer panels, side panels, inner hoods, outer hoods and/or trunk lid panels as well as A-pillars, B-pillars and C-pillars and other automotive parts.

Claims

A method for producing an object (1) made from a hardenable aluminium alloy, the method comprising:
providing a raw product of the hardenable aluminium alloy, wherein the raw product is a sheet;

heating the raw product of the hardenable aluminium alloy to an elevated temperature in order to dissolve precipitates, wherein the raw product of the hardenable aluminium alloy is heated to an elevated temperature with a heating rate of at least 10 K/s;

optionally holding the raw product of the hardenable aluminium alloy at the elevated temperature for a predetermined time;

rapidly quenching the raw product of the hardenable aluminium alloy to a temperature lower than the elevated temperature in order to maintain at least partly dissolved precipitates within the aluminium alloy;

forming the raw product of the hardenable aluminium alloy below a temperature of 0 °C, in particular below -150 °C, to produce the object (1).
The method according to claim 1, wherein the raw product of the hardenable aluminium alloy is heated to an elevated temperature of 250 °C to 600 °C, in particular 300 °C to 450 °C.
The method according to one of claims 1 or 2, wherein the raw product of the hardenable aluminium alloy is held at the elevated temperature for a time of up to 60 s, in particular up to 40 s.
The method according to one of claims 1 to 3, wherein the raw product of the hardenable aluminium alloy is rapidly quenched to room temperature.
The method according to one of claims 1 to 4, wherein the raw product of the hardenable aluminium alloy is rapidly quenched by contact with a gas or a liquid.
The method according to one of claims 1 to 5, wherein the raw product of the hardenable aluminium alloy is formed at a temperature below -190 °C.
The method according to one of claims 1 to 6, wherein the object (1) is an automotive component.