WO2020178076A1 - Method of manufacturing an automotive part from a 7xxx-series aluminium alloy - Google Patents

Abstract

The invention relates to a method of manufacturing a three-dimensionally formed aluminium alloy automotive part for a motor vehicle, the method comprising the sub-sequent steps of, providing a rolled aluminium alloy bare sheet or composite sheet product having a gauge in a range of 0.5-5 mm, wherein the sheet product comprises at least one layer made from a 7XXX-series aluminium alloy comprising at least(in wt.%): Zn 3.8%-6.8%, Mg 0.5%-3.0%, and Cu up to 2.2%, and is in an F-temper or an O-temper; and forming the sheet product in a forming operation to obtain said three-dimensionally formed aluminium alloy automotive part, and wherein the rolled aluminium alloy bare sheet or composite sheet product in the F-temper or O-temper only prior to forming is heat-treated at a temperature of 370-545°C, and wherein the time-delay between the end of said heat-treatment and the start of the forming operation is less than 8 hours.

Description

METHOD OF MANUFACTURING AN AUTOMOTIVE PART FROM A 7XXX-SERIES ALUMINIUM ALLOY

FIELD OF THE INVENTION

The invention relates to a method of manufacturing a formed automotive part or body-in-white part out of an 7XXX-series aluminium alloy bare sheet or composite sheet material comprising at least one layer made from a 7XXX-series aluminium alloy, the 7XXX-series aluminium alloy comprising at least (in wt.%): Zn 3.8% to 6.8%, Mg 0.5% to 3.0%, and Cu up to 2.2%.

BACKGROUND OF THE INVENTION

Body-in-white consists of the structural components of the automobile or motor vehicle, not including closures and moving parts (e.g. door panels, hood panels, trunk lid panels). Other automotive structural parts are for example an A-pillar, B- pillar, C-pillar, tunnel section, side impact crash beam, crash box or engine bulk head, all of which may be manufactured by the method according to this invention. During the last decade high strength steel grades were developed and different pro duction, forming and joining techniques were created and setup which allowed car manufactures to use these grades to produce automotive structural parts and body- in-white ("BIW") parts. Using these grades allows the car manufactures to achieve high passenger safety in case of crash while adding (much) less weight as would be the case when traditional steel grades and also AA5XXX and AA6XXX-series aluminium alloys would be used for this purpose. To be able to achieve even higher weight savings, there is a demand for the use of high strength aluminium alloys, in particular for formed structural and BIW parts, which are formable and having in particular increased strength after being subjected to a paint bake cycle. In addition, the material properties normally required to produce such parts include a high form- ability for the forming operation (typically by means of stamping, deep drawing, or roll forming), high mechanical strength after paint baking so as to enable down gaug ing and thus minimising the weight of the part, good behaviour in the various as sembly methods used in motor vehicle manufacturing such as spot welding, laser welding, laser brazing, clinching or riveting, and an acceptable cost for mass pro duction. 7XXX-series aluminium alloys in the form of a sheet product are able to deliver the required strength after being subjected to a paint bake cycle. However, formability of these sheet alloys is not optimal, in particular because 7XXX-series alloys naturally age very fast after solution heat treatment followed by quenching (together referred to herein as "SHT"), leading to a significant strength increase by about 80% within one week after solution heat treatment and quenching. This would bring the sheet product to a T4 temper. It is known in the art that 7XXX-series sheet products can be provided also in a T6 or T7x temper having a higher strength but less ductility than a T4 temper. For that reasons the forming or shaping operations are often performed at elevated temperatures, often referred to as hot-forming or half-warm forming, as at elevated temperature the formability characteristics of these alloys are improved. To produce functional motor vehicle parts meeting the required strength specifications, the formed parts are typically heat treated post production and subsequently age hardened, in particular by subjecting the formed parts to a paint-bake cycle.

Patent document AT-1 1744-U1 (AMAG Rolling) discloses a method of manu facturing a formed component from a 7XXX-series aluminium alloy sheet (for exam ple an 7025 alloy), wherein the 7XXX-series aluminium alloy sheet is provided in a T6 (peak-aged) or T7 (over-aged) condition, subsequently heated to a temperature in a range of 150°C to 250°C, preferably at about 200°C, then formed and cooled to room temperature.

Patent document WO-2014/040939-A1 (Aleris) discloses a method of manu facturing a formed aluminium alloy automotive structural part or a body-in-white (BIW) part of a motor vehicle, the method comprising at least the steps of: (a) provid ing a rolled AA7xxx-series aluminium alloy sheet product having a gauge in a range of 0.5 mm to 4 mm, the sheet product having been subjected to solution heat treat ment and quenching followed by a period of natural ageing of at least 1 day; (b) subjecting the naturally aged sheet product to a reversion annealing heat treatment at a temperature between 100-350°C for 0.1 to 60 seconds; (c) optionally subjecting the heated sheet product to a forced cooling operation; (d) within 2 hours, preferably within 30 minutes, from the reversion annealing heat treatment, forming the sheet product to obtain a three-dimensionally formed automotive structural part or body- in-white (BIW) part.

Patent document WO-2010/142579-A1 (Aleris) discloses a method of manu facturing a formed aluminium alloy body-in-white (BIW) part of a motor vehicle, the BIW part having a yield strength of more than 500 MPa after being subjected to a paint-bake cycle, the method comprising: (a) providing a rolled aluminium sheet product having a gauge in a range of 0.5 to 4 mm and being subjected to a solution heat treatment (SHT) and having been quenched following said SHT, and wherein the SHT and quenched aluminium sheet product has a substantially recrystallised microstructure, and a chemical composition of, in wt.%, Zn 6.9-8.0, Mg 1.2-2.4, Cu 1.3-2.4, Mn <0.3, either 0.05-0.25 of Cr or Zr, Si <0.3, Fe <0.35, Ti <0.1 , impurities and balance aluminium; (b) forming the aluminium alloy sheet to obtain a formed BIW part, (c) assembling the formed BIW part with one or more other metal parts to form an assembly forming a motor vehicle component; (d) subjecting said motor vehicle component to a paint-bake cycle and wherein the aluminium alloy sheet in the formed BIW part has a yield strength of more than 500 MPa.

Patent document WO-2017/062398-A1 (Novelis) discloses a process of shap ing an article made of an age-hardenable, heat treatable aluminium, such as 2XXX, 6XXX and 7XXX aluminium alloys, comprising the step of heating the article to a temperature of about 100°C to 600°C at a heat rate of about 3°C/second to about 90°C/second and shaping the article, and wherein the article is in T4 temper before the heating step.

DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated, alumin ium alloy and temper designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2018 and are well known to the persons skilled in the art. The temper designations are laid down in European standard EN515.

For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.

The term“up to” and“up to about”, as employed herein, explicitly includes, but is not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.10% Mn may include an alloy having no Mn.

It is an object of the invention to provide a method of manufacturing a formed automotive part or body-in-white part out of an 7XXX-series aluminium alloy bare sheet or composite sheet material comprising at least one layer made from a 7XXX- series aluminium alloy, which method results in formed products having a good bal ance of mechanical properties, corrosion resistance and crash performance..

This and other objects and further advantages are met or exceeded by the present invention providing a method of manufacturing a three-dimensionally formed aluminium alloy automotive part, in particular a structural part, or a body-in- white (BIW) part for a motor vehicle, the method comprising the subsequent steps of,

providing a rolled aluminium alloy bare sheet product or composite sheet prod uct having a gauge in a range of 0.5 mm to 5 mm, wherein the sheet product com prises at least one layer made from a 7XXX-series aluminium alloy and is in an F- temper or an O-temper, wherein the 7XXX-series aluminium alloy comprising at least, in wt.%., Zn 3.8% to 6.8%, Mg 0.5% to 3.0%, and Cu up to 2.2%; and

forming the sheet product in a forming operation to obtain the three-dimension- ally formed aluminium alloy automotive part, in particular a structural part or a body- in-white part, and wherein the rolled aluminium alloy bare sheet or composite sheet product in the F-temper or O-temper only just prior to forming in the forming opera tion is heat-treated at a temperature of 370°C to 545°C, and wherein the time-delay between the end of said heat-treatment at a temperature of 370°C to 545°C and the start of the forming operation is less than 8 hours.

It is an important feature of this invention that prior to the forming operation the 7XXX-series sheet product employed is provided in an F-temper or in an O-temper.

“F-temper” means that the 7XXX-series aluminium alloy bare sheet product or composite sheet product comprising at least one layer made from a 7XXX-series aluminium alloy is as-fabricated, optionally incorporating a small stretching opera tion of up to about 1 % to improve product flatness, and there are no mechanical properties specified. In the case at hand this means that the 7XXX-series aluminium alloy bare sheet or composite sheet material comprising at least one layer made from a 7XXX-series aluminium alloy sheet material has been cast into a rolling ingot, pre-heated and/or homogenised, hot-rolled, and cold-rolled to final gauge as is reg ular in the art but without or devoid of any further purposive annealing, solution heat- treatment or artificial ageing following the cold rolling to final gauge.

“O-temper” means that the 7xxx-series starting plate product has been an nealed to obtain lowest strength temper having more stable mechanical properties. In the case at hand this means that the plate material has been cast into a rolling ingot, pre-heated and/or homogenised, hot-rolled, and cold-rolled to final gauge as is regular in the art, optionally incorporating a small stretching operation of up to about 1 % to improve product flatness. As is known in the art, a recommended an nealing to obtain lowest strength temper typically comprises soaking for about 2 to 3 hours at about 405°C, cooling to about 205°C or lower, reheat to about 232°C, and soak for about 4 hours, followed by cooling to ambient temperature and whereby the cooling rate to ambient temperature is not critical.

In accordance with the invention it has been found that by the method accord ing to the invention 7XXX-alloy sheet products are provided that enable the produc tion of automotive parts or components, in particular structural parts or BIW compo nents, having a good crash performance. The structural automotive or BIW parts may have a yield strength of more than 400 MPa, and typically less than 500 MPa for the embodiments having a very low Cu content, after being subjected to a paint- bake cycle customary in the art. The automotive parts have after being subjected to a paint-bake cycle a good corrosion resistance or at least similar of the same 7XXX- series alloy provided in a T4, T6 or T7x-temper prior to forming in the forming oper ation. These engineering properties are achieved while the 7XXX-alluminium alloy sheet product is employed in an F-temper or O-temper instead of a T4, T6 or T7x temper. This enables a much more cost and time effective production of the 7XXX- series sheet product to be used as the processing steps of solution heat-treatment, quenching, and optionally stretching as commonly used at least for the T7x temper, and natural ageing and/or artificial ageing steps and all the logistic handling of the materials accompanying these production steps are avoided. The method accord ance to this invention achieves a good formability of the sheet product and final mechanical and corrosion resistance at least similar as those achieved by using 7XXX-series sheet material of the same alloy composition but otherwise provided in a T4, T6 or T7x-temper. In the best examples the crash performance is even better than those obtained from sheet products the same alloy composition provided in a T6 or T7x-temper.

In a preferred embodiment of the method according to this invention, for those forming operations which require significant or strong deformation of the rolled alu minium alloy bare sheet or composite sheet product comprising at least one layer made from a 7XXX-series aluminium alloy, for example by means of drawing or stamping, it is preferred that after storage of the rolled product in an F-temper or 0- temper and only just prior to the forming operation the rolled aluminium alloy bare sheet or composite sheet product is subjected to a heat treatment wherein it is soaked for a period of 5 sec. to 15 min, preferably at least 30 sec, and preferably less than 10 min., at a temperature in a range of about 370°C to 545°C, and then rapidly cooled or quenched, preferably to below 100°C, and more preferably to am bient temperature, for example by means of water such as water quenching or water spray quenching or by an air jet or by direct contact cooling. The cooling rate to below 100°C is preferably more than 10 °C/sec, and more preferably more than 50 °C/sec. The quenching can also be performed via contact between two cold contact plates. A preferred minimum temperature for the heat-treatment is about 400°C, more preferably about 430°C, and most preferably about 450°C. A more preferred mini mum temperature is about 480°C. It has been found that higher heat-treatment tem peratures just prior to the forming operation lead to a better crash performance and slightly higher mechanical properties in the final automotive part. A preferred maxi mum temperature is about 530°C. For example, the heat-treatment is performed at a temperature of about 465°C for about 2 minutes or at a temperature of about 515°C for about 2 minutes.

It has been found that such a very short heat treatment facilitates the forming of the rolled product during a forming operation into a formed or shaped product. The time-delay between the end of the heat-treatment or completion of the heat- treatment at a temperature of 370-545°C and the start of the forming is less than 8 hours, preferably less than 2 hours, and more preferably less than 1 hour, to avoid an increase in mechanical properties due to natural ageing. Natural ageing for more than one day, typically more than four days, would lead to an undesirable stable T4 condition. In a more preferred embodiment this time-delay is less than 15 minutes and more preferably less than 10 minutes. This facilitates process economy and leads to consistent mechanical properties in the final products.

This heat treatment can be carried out in or near the press shop on the coiled rolled material provided in the F-temper or O-temper and then re-coiled and typically cut to size into blanks for forming or shaping. In a preferred embodiment, this heat treatment is carried out in or near the press shop and preferably performed on blanks cut from the coil or strip of the rolled material in F-temper or O-temper and then heat treated and subsequently formed in a forming operation, which may con sist of several forming steps. Thus, the heating step(s), the quenching step and the forming step(s) are carried out in an in-line configuration. Optionally further trimming and/or perforation operations can be carried out during the forming process or also after the forming process.

The heating of the rolled sheet products can be done in various ways, in par ticular the heating is selected from the group consisting of infrared, radiant-tube, gas-fired furnace, direct resistance, induction heating, and combinations thereof. Another way of heating is by contact heating wherein the rolled sheet product is held between two heated contact plates. This method is sometimes referred to as“Waf- feleisen”, and in this case, upon abutting contact, the heat of the contact plate is passed to the sheet product by thermal conduction. Either way, it is preferred that the heating or heating up is performed very fast. This means that the sheet product is heated to a temperature between 370°C and 545°C at a heating rate of more than 10 °C/sec, and more preferably of more than 15 °C/sec, in a period of preferably less than 20 seconds, but at least in a few seconds. As soon as the target temper ature has been reached by means of the heating process, the sheet product should have homogeneously this temperature across its surface and across its wall thick ness. To ensure a homogenous temperature and the development of the required microstructure, the sheet product is preferably kept at the target temperature for some time as herein described and claimed.

Prior to the forming operation, the rolled sheet product may be coated with a lubricant, oil or dry lubricant, suitable for the forming operation, the assembly and the surface treatment of the structural part to be produced. The lubricant application is carried out, in particular, by means of spraying, doctoring, rolling on; and, as an alternative, the lubrication may take place in the forming step itself. The forming dies of the forming tool are supplied with a lubricant. The advantage of cold forming is that any lubricant can be used; and it does not have to be able to withstand thermal stress.

The rolled sheet product may also be treated prior or after the forming opera tion to apply a surface passivation layer to enhance subsequent adhesive bonding performance.

The forming or shaping operation is preferably a cold forming operation, i.e. it is performed at ambient (room) temperature, typically at about 15°C to 45°C. It can be any forming operation used to shape three-dimensional motor vehicle components, and includes in particular operations like stamping, deep drawing, pressing, press forming, and roll forming, or combinations thereof. It has been found that the 7XXX- series alloy processed in accordance with the invention can be formed for example at an OEM using existing tools, lubrication practices and presses. The forming or shaping into the three-dimensionally formed part can be done in one or more se quential steps. Optionally further trimming and/or perforation operations can be car ried out during the forming process or also after the forming process.

Alternatively, a hot forming operation can be done, for example the heat-treated aluminium sheet at target temperature is rapidly transferred from the heat-treatment section to a forming tool, e.g. a cold forming tool, and pressed. The forming and the quenching of the aluminium sheet can be performed simultaneously. Or the forming operation is done at a temperature in the range of about 150°C to 250°C, followed by rapid cooling to below 100°C, and preferably to ambient temperature. A series of tests have demonstrated that 7XXX-series sheet products processed in accordance with this invention and formed in a cold forming operation have a slightly better crash performance than those formed in a half-warm or warm-forming operation.

Following the forming or shaping operation(s) the 7XXX aluminium alloy of the formed part or component is in a T4 temper. Preferably the formed or shaped prod uct it kept and stored at ambient temperature for at least 4 days, preferably for at least 7 days, to reach a stable T4 condition and to obtain a sufficient strength in crease following a e-coat or a paint-bake heat-treatment commonly applied to auto motive parts.

In order to obtain a good crash performance in the formed part it is preferred that the 7XXX aluminium alloy sheet product after the heat-treatment with subse quent rapid cooling and prior to the forming operation has a non-recrystallized mi crostructure. A non-recrystallized metallographic microstructure means that in the bulk of the sheet product less than 50%, preferably less than 40%, and more pref erably less than 30% of the grains in this condition are recrystallized. The degree of recrystallization can be determined by any suitable method known in the art. For example, in a micrograph, such as a scanning electron micrograph or an optical micrograph. The skilled person is familiar with the required processing to arrive at a sheet product having such a microstructure. This non-recrystallized microstructure is retained during the forming operation, in particular during a cold forming opera tion, and enhances the crash performance of the formed part or product. By per forming hot and cold rolling steps for manufacturing the aluminium sheet product in an F-temper or an O-temper it has a metallographic structure composed of fibrous structures or grains. It is to be avoided that as a result of the heat-treatment the microstructure is fully recrystallized often resulting in a more equiaxed grain struc ture. For an improved crash performance substantial parts of the fibrous structure should be retained.

The crash performance can be tested in a dynamic crash test using a hollow crash box manufactured from components manufactured in accordance with the present invention and testing conditions as described in patent document WO- 2016/037922-A1 (Aleris) and incorporated herein by reference.

Following the forming or shaping operation the formed part is made part of an assembly of other metal components as regular in the art for manufacturing motor vehicle components and subjected to a paint bake operation to cure any paint or lacquer layer or e-coating applied. The paint bake operation or cycle comprises one or more sequential short heat treatment in the range of 140°C to 190°C for a period of 10 to less than 40 minutes, and typically of less than 30 minutes. A typical paint bake cycle would comprise a first heat treatment of 180°C@20 minutes, cooling to ambient temperature, then 160°C@20 minutes and cooling to ambient temperature. In dependence of the OEM such a paint bake cycle may comprise of 2 to 5 sequen tial steps and includes drying steps, but either way the cumulated time at elevated temperature (100°C to 190°C) of the aluminium alloy product is less than 120 minutes.

For testing the product or material behaviour following a paint-bake cycle it is practice in the art, for example as reflected in the product specification of an OEM, samples are subjected to a simulated paint-bake cycle, which consists of soaking at 185°C for 20 minutes and resulting in the material being in a T6x condition.

The 7XXX-series aluminium alloy processed in accordance with the invention may show a yield strength of more than 400 MPa, and preferably more than 410 MPa, and typically less than 500 MPa for the alloy embodiment having up to 0.45% Cu, after being subjected to a paint-bake cycle.

The rolled aluminium alloy bare sheet or composite sheet product having a gauge in a range of 0.5 mm to 5 mm, and wherein the sheet product comprises at least one layer made from the 7XXX-series aluminium alloy and is in an F-temper or O-temper is produced as in known in the art, and comprises the steps, in that order, of: (i) casting stock of an ingot of the 7XXX-series aluminium alloy according to this invention and comprising at least, Zn 3.8% to 6.8%, Mg 0.5% to 3.0%, and Cu up to 2.2%;; (ii) preheating and/or homogenizing the cast stock; (iii) hot rolled the stock; and (iv) cold rolling of the hot rolled stock to a final gauge of 0.5 mm to 5 mm. The process is thus devoid of any solution heat-treatment or artificial ageing following the cold rolling to final gauge, that would otherwise lead to an aluminium alloy sheet product in a T4, T6 or T7x temper.

The aluminium alloy can be provided as an ingot or slab or billet for fabrication into a suitable wrought product by casting techniques regular in the art for cast prod ucts, e.g. Direct-Chill (DC)-casting, Electro-Magnetic-Casting (EMC)-casting, Elec tro-Magnetic-Stirring (EMS)-casting. Slabs resulting from continuous casting, e.g. belt casters or roll casters, also may be used, which in particular may be advanta geous when producing thinner gauge end products. Grain refiners such as those containing titanium and boron, or titanium and carbon, may also be used as is well- known in the art. The Ti-content in the aluminium alloy is up to about 0.2%, and preferably up to 0.15%, and more preferably in a range of 0.01 % to 0.1 %. As is known in the art 7XXX-series alloy products may optionally further comprise at most 0.02% Ca, at most 0.015% Sr, and/or at most 0.004% Be, which fall each within the regular ranges for impurity elements. Traditionally, beryllium additions have served as a deoxidizer/ingot cracking deterrent and may be used in the aluminium alloy product according to this invention. Though for environmental, health and safety reasons, more preferred embodiments of this invention are substantially Be-free. Minor amounts of Ca and Sr alone or in combination can be added to the aluminium alloy product for the same purposes as Be. Preferred addition of Ca is in a range of about 10 to 100 ppm. Optionally a cast ingot can be stress relieved, for example by holding it at a temperature in a range of about 350°C to 450°C followed by slow cooling to ambient temperature. After casting the alloy stock, an ingot is commonly scalped to remove segregation zones near the as-cast surface of the ingot.

The purpose of a homogenisation heat treatment has at least the following objectives: (i) to dissolve as much as possible coarse soluble phases formed during solidification, and (ii) to reduce concentration gradients to facilitate the dissolution step. A preheat treatment achieves also some of these objectives.

Commonly a pre-heat refers to the heating of an ingot to a set temperature and soaking at this temperature for a set time followed by the start of the hot rolling at about that temperature. Homogenisation refers to a heating, soaking and cooling cycle with one or more soaking steps, applied to a rolling ingot in which the final temperature after homogenisation is ambient temperature.

A typical pre-heat treatment for the AA7XXX-series alloy used in the method according to this invention would be a temperature of 370°C to 545°C with a soaking time in the range of 2 to 50 hours, more typically for 2 to 20 hours.

Firstly, the soluble eutectic phases and/or intermetallic phases such as, if any, the S-phase, T-phase, and M-phase, in the alloy stock are dissolved using regular industry practice. This is typically carried out by heating the stock to a temperature of less than 500°C, typically in a range of 450°C to 485°C, as S-phase (A MgCu- phase) has a melting temperature of about 489°C in AA7XXX-series alloys and the M-phase (MgZn2-phase) has a melting point of about 478°C. This can be achieved by a homogenisation treatment in said temperature range and allowed to cool to the hot rolling temperature, or after homogenisation the stock is subsequently cooled and reheated before hot rolling. The homogenisation process can also be done in two or more steps if desired, and which are typically carried out in a temperature range of 430°C to 490°C for the AA7XXX-series alloy. The soaking time at the ho mogenisation temperature is in the range of 1 to 50 hours, and more typically for 2 to 20 hours. The heat-up rates that can be applied are those which are regular in the art.

Following the preheat and/or homogenisation practice the stock is hot worked by means of hot rolling to a hot rolled gauge. In an embodiment the plate material is hot rolled in a first hot rolling step to an intermediate hot rolled gauge, followed by an intermediate annealing step and then hot rolled in a second hot rolling step to final hot rolled gauge.

Thereafter, the hot rolled stock at final hot rolled gauge is cold rolled to a final gauge. An intermediate anneal may be used before or during the cold rolling oper ation. The 7XXX-series aluminium alloy forming the bare sheet or the core of the composite sheet, has a composition comprising as its main alloying constituents, at least, in wt.%.,

Zn 3.8% to 6.8%, preferably 4.0%-6.8%, more preferably 5.1 %-6.8%;

Mg 0.5% to 3.0%, preferably 1.0%-2.2%; and

Cu up to 2.2%.

In an embodiment the 7XXX-series aluminium alloy forming the bare sheet or the core of the composite sheet has a Cu-content of up to 0.45%, and preferably of up to 0.25%.

In another embodiment the 7XXX-series aluminium alloy forming the bare sheet or the core of the composite sheet has a Cu-content of 0.8% to 2.2%, and preferably in a range of 1.1 % to 2%, and encompasses for example the AA7075- series alloy and near compositional variations thereof. In this embodiment the 7XXX-series aluminium alloy processed in accordance with the invention may show a higher yield strength levels after being subjected to a paint-bake cycle.

In an embodiment the 7XXX-series aluminium alloy forming the bare sheet or the core of the composite sheet, has a composition comprising of, in wt.%.,

Zn 3.8% to 6.8%, preferably 4.0%-6.8%, more preferably 5.1 -6.8%,

Mg 0.5% to 3.0%, preferably 1 0%-2.2%,

Cu up to 0.45%, preferably up to 0.25%,

optionally one or more elements selected from the from consisting of:

(Zr up to 0.30%, preferably 0.08%-0.20% Zr, Cr up to 0.35%, preferably up to 0.10% Cr or 0.10%-0.35% Cr, Mn up to 0.5%, preferably up to 0.1 % Mn or 0.05%-0.5% Mn),

Ti up to 0.2%,

Fe up to 0.4%, preferably up to 0.35%, more preferably 0.10%-0.30%,

Si up to 0.4%, preferably up to 0.35%, more preferably 0.06%-0.30%, impurities each <0.05%, total <0.15%, balance aluminium.

In an embodiment the 7XXX-series aluminium alloy forming the bare sheet or core of the composite sheet, has a composition consisting of, in wt.%., Zn 3.8%- 6.8%, Mg 0.5%-3.0%, Cu up to 0.45%, optionally one or more elements selected from the from consisting of: (Zr up to 0.30%, Cr up to 0.35%, Mn up to 0.5%), Ti up to 0.2%, Fe up to 0.4%, Si up to 0.4%, impurities each <0.05%, total <0.15%, bal ance aluminium, and with preferred compositional ranges as herein described and claimed.

In another embodiment the 7XXX-series aluminium alloy forming the bare sheet or the core of the composite sheet, has a composition comprising of, in wt.%.,

Zn 3.8% to 6.8%, preferably 4.0% to 6.8%, more preferably 5.1 % to 6.4%, Mg 0.5% to 3.0%, preferably 1.5% to 3.0%, more preferably 1.8% to 3.0%, Cu 0.8% to 2.2%, preferably 1.1 % to 2%,

optionally one or more elements selected from the from consisting of:

(Zr up to 0.30%, preferably 0.08%-0.20% Zr, Cr up to 0.35%, preferably up to 0.10% Cr or 0.10%-0.35% Cr, Mn up to 0.5%, preferably up to 0.1 % Mn or 0.05%-0.5% Mn),

Ti up to 0.2%,

Fe up to 0.4%, preferably up to 0.35%, more preferably 0.10%-0.30%,

Si up to 0.4%, preferably up to 0.35%, more preferably 0.06%-0.30%, impurities each <0.05%, total <0.15%, balance aluminium.

In an embodiment the 7XXX-series aluminium alloy forming the bare sheet or core of the composite sheet, has a composition consisting of, in wt.%., Zn 3.8%- 6.8%, Mg 0.5%-3.0%, Cu 0.8% to 2.2%, optionally one or more elements selected from the from consisting of: (Zr up to 0.30%, Cr up to 0.35%, Mn up to 0.5%), Ti up to 0.2%, Fe up to 0.4%, Si up to 0.4%, impurities each <0.05%, total <0.15%, bal ance aluminium, and with preferred compositional ranges as herein described and claimed.

In the case of a composite product, the core layer is made from an aluminium alloy of the 7XXX-series clad on one or both faces with another aluminium alloy. The one or several clad layers are preferably made from an 1XXX-series, 3XXX- series, 4XXX-series, 5XXX-series, 6XXX-series alloy or a different7XXX-series alu minium alloy compared to the core alloy, preferably a lean 7XXX-series alloy such as an AA7072 alloy. The clad layer material can have a chemical composition within the ranges of for example AA3003, AA3004, AA3005, AA6016, AA6016A, AA6005, AA6005A, AA5005, AA5005A, AA5754, AA5051A, AA5052, AA5252, AA5352, AA5018, and modifications thereof.

Although the dimensions of the aluminium bare or composite sheet products can be varied in many ways for use in automotive structural parts in accordance with this invention, its total thickness (viz. the core and all clad layers taken together in case of a composite material) is in a range of about 0.5 mm to about 5 mm, prefer ably about 0.7 mm or 1.0 mm to about 4 mm, and more preferably of about 1.5 to about 3.5 mm. In case a composite material is used, the clad layer or clad layers are usually much thinner than the core sheet, and each clad layer constituting about 2% to about 20% of the total composite sheet thickness. A clad layer more typically constitutes around about 2% to about 15% of the total composite sheet thickness. In case of a bare sheet product, it is wholly composed of the 7XXX-series aluminium alloy, in the case of a composite sheet product, at least the core layer is made of a 7XXX series alloy as herein described and claimed.

In an embodiment of the invention the structural automotive component or body-in-white part is highly suitable for safety-relevant components or parts, in par ticular selected from the group: crash box, A-pillar, B-pillar, C-pillar, side impact beam structure, roof structure, tunnel section, side support(s) of a battery case for an electrical vehicle.

In another aspect of the invention it relates to a motor vehicle incorporating a formed aluminium alloy structural part or body-in-white part obtained by the method according to this invention.

Furthermore, the present invention relates to the use of the 7XXX-series al uminium alloy sheet product as herein described and claimed for producing an au tomotive vehicle component.

In the following, the invention will be explained by the following non-limitative example. Example.

On an industrial scale of manufacturing a rolling ingot has been DC-cast of a 7xxx-series alloy having a composition of, in wt.%, 6.4% Zn, 1.9% Mg, 0.14% Cu, 0.09% Zr, 0.02% Si, 0.12% Fe, 0.01 % Mn, 0.02% Ti, balance aluminium and una- voidable impurities. The ingot has been homogenised, hot rolled and cold rolled to a final gauge of 2.5 mm according to regular industry practice.

One coil (Coil A in accordance with this invention) was kept in the as-cold rolled condition (F-temper) without any further thermal heat treatments. Whereas another coil was solution heat-treated in a continuous annealing furnace at a temperature of about 465°C for about 2 minutes and quenched to ambient temperature, recoiled and next natural aged for 7 days to reach a stable T4 condition (Coil B, comparative).

Blanks have been cut from each of Coil A and Coil B for cold forming in a forming die into U-shape sections for a hollow crash box. Only prior to forming the blanks were heat treated by applying a very fast heat-up to a temperature of 465°C and soaked at this temperature for about 3 minutes and quenched to ambient tem perature. For each blank the time delay between quenching and the start of the cold forming operation was less than about 3 minutes. After cold forming the formed parts were stored at ambient temperature for 7 days to allow natural ageing to occur and to reach a T4 condition.

Tensile properties have been measured after a simulated paint-bake cycle of

185°C for 20 minutes as is regular in the art. The mechanical properties have been measured in accordance with DIN-EN-ISO 6892-1 (2016). The mechanical proper ties (average over three measurements) are listed in Table 1. Table 1 Mechanical properties of the blanks after a simulated paint-bake cycle

From the results of Table 1 it can be seen that the 7XXX sheet material when in an F-condition (Coil A) prior to the forming operation provides mechanical prop erties in the final product at least similar to the same 7XXX sheet material in a T4 condition (Coil B).

Furthermore, the cold formed components have been assembled into a hollow crash box and subjected to a dynamic crash test.

Fig. 1 shows a drawing of a typical axial folding crash box configuration as known in the art for use in a dynamic crash test.

The hollow crash box is made of the 2.5 mm gauge aluminium sheet cold formed in a U-shape having a length of 400 mm and a flat back cover sheet of 2.5 mm made from the same material. Both are joined by means of riveting using 13 rivets on either side of the U-shape and distanced 30 mm from each other. The height of the U-shape is 90 mm and the width of the flat top of the U-shape is 64 mm; there is an 87° angle between the flat back cover sheet and the 90 mm standing web of the crash box. Two flat cover plates (120x140 mm provided with a centre hole of 40 mm diameter) made from 6016 aluminium sheet material are welded to the box at the bottom and the top. The whole box is subjected to a simulated paint- cycle of 185°C@20 min as is regular in the art. After that the crash box is placed in a drop tower test bench, where a guided drop weight of 250 kg is released from a height of 4.9 meters, resulting in impacting the crash box at a speed of about 35 km/h. During the impact the crash box absorbs the kinetic energy and deforms plas tically by folding. Failure of the crash box is amongst others detected by recording the moment of the formation of the first crack by using high-speed camera film.

Each of the crash boxes made from Coil A and Coil B had the same time-to- first-crack in the axial direction. The crash boxes showed a similar folding behaviour. Also the intrusion depth was about the same for all crash boxes. These results illus trate also that the 7XXX sheet material when in an F-condition (Coil A) prior to the forming operation provides a crash performance in the final product at least similar to the same 7XXX sheet material in a T4 condition (Coil B) prior to the forming op eration. The method according to this invention enables a much more cost and time effective production of the 7XXX-series sheet product to be used as the processing steps of solution heat-treatment and natural ageing and all the logistic handling of the materials accompanying these production steps are avoided. The method ac- cordance to this invention achieves a good formability of the sheet product and final mechanical and corrosion resistance at least similar as those achieved by using 7XXX-series sheet material of the same alloy composition but otherwise provided in a T4, T6 or T7x-temper. The invention is not limited to the embodiments described before, and which may be varied widely within the scope of the invention as defined by the appending claims.

Claims

Claims

1. A method of manufacturing a three-dimensionally formed aluminium alloy au tomotive part, in particular a structural automotive part, for a motor vehicle, the method comprising the steps of: providing a rolled aluminium alloy bare or composite sheet product having a gauge in a range of 0.5 mm to 5 mm and is in an F-temper or an O-temper; wherein the sheet product comprises at least one layer made from a 7XXX-series aluminium alloy, the 7XXX-series alumin ium alloy comprising at least, in wt.%., Zn 3.8% to 6.8%, Mg 0.5% to 3.0%, and Cu up to 2.2%; forming the sheet product in a forming operation to obtain the three-dimensionally formed aluminium alloy automotive part, and wherein the sheet product in the F-temper or O-temper only prior to forming in a forming operation is heat-treated at a temperature of 370°C to 545°C, and wherein the time-delay between the end of said heat-treatment and the start of the forming operation is less than 8 hours.

2. The method according to claim 1 , wherein the sheet product in the F-temper or O-temper prior to forming is heat-treated at a temperature of 370°C to 545°C for a period of 5 seconds to 15 minutes.

3. The method according to claims 1 or 2, wherein the time-delay between the end of the heat-treatment at a temperature of 370-545°C and the start of the forming operation is less than 2 hours, and preferably less than 1 hour.

4. The method according to any one of claims 1 to 3, wherein the sheet product is cut into blanks before the forming operation.

5. The method according to any one of claims 1 to 4, wherein the sheet product in an F-temper or an O-temper is cut into blanks before said heat-treatment at a temperature of 370-545°C, preferably of 400-530°C, and more preferably of 450-530°C.

6. The method according to any one of claims 1 to 5, wherein forming the sheet product is in a cold forming operation.

7. The method according to any one of claims 1 to 5, wherein forming the sheet product during the forming operation is at a temperature in the range of 150°C to 250°C, followed by cooling to ambient temperature.

8. The method according to any one of claims 1 to 7, wherein the bare sheet or core of the composite sheet is made from a 7XXX-series alloy comprising at least, in wt.%.,

Zn 4.0% to 6.8%,

Mg 1.0% to 2.2%,

Cu up to 2.2%.

9. The method according to any one of claims 1 to 8, wherein the bare sheet or core of the composite sheet is made from a 7XXX-series alloy having a com position, in wt.%.,

Zn 3.8% to 6.8%, preferably 4.0% to 6.8%,

Mg 0.5% to 3.0%, preferably 1.0% to 2.2%,

Cu up to 0.45%, preferably up to 0.25%,

Zr up to 0.30%,

Cr up to 0.35%,

Mn up to 0.5%,

Ti up to 0.2%,

Fe up to 0.4%,

Si up to 0.4%,

balance aluminium and impurities.

10. The method according to any one of claims 1 to 8, wherein the bare sheet or core of the composite sheet is made from a 7XXX-series alloy having a com position, in wt.%.,

Zn 3.8% to 6.8%, Mg 0.5% to 3.0%,

Cu 0.8% to 2.2%,

Zr up to 0.30%,

Cr up to 0.35%,

Mn up to 0.5%,

Ti up to 0.2%,

Fe up to 0.4%,

Si up to 0.4%,

balance aluminium and impurities.

1 1 . The method according to any one of claims 1 to 10, wherein the rolled sheet product is a composite sheet product having a core layer made from the 7XXX- series aluminium alloy and an aluminium alloy clad layer on at least one side thereof.

12. The method according to any one of claims 1 to 1 1 , wherein the three-dimen- sionally formed aluminium alloy automotive part or a body-in-white part is a selected from the group of a B-pillar, crash box, side impact beam, roof struc ture, side support of a battery case for an electrical vehicle.

13. An automotive part of a motor vehicle manufactured by the method according to any one of claims 1 to 12.

14. A motor vehicle incorporating a formed aluminium alloy part or body-in-white part obtained by the method according to any one of claims 1 to 12.