EP0132650A1

EP0132650A1 - Aluminium alloy for structural shaped elements of vehicles and process for manufacturing said elements

Info

Publication number: EP0132650A1
Application number: EP84107852A
Authority: EP
Inventors: Domenico Morri
Original assignee: Aluminia SpA
Current assignee: Aluminia SpA
Priority date: 1983-07-19
Filing date: 1984-07-05
Publication date: 1985-02-13
Also published as: IT1163803B; IT8322119A0

Abstract

Aluminium alloy for shaped elements of vehicles and the like, which comprises from 0.25 to 0.35% of Si, from 0.20 to 0.30% of Fe; from 0.85 to 1.1% of Mg, from 0.10 to 0.25% of Cu; other elements, as impurities, each below 0.1%, in the aggregate below 0.2%; the balance to 100% being Al.

A process for manufacturing shaped elements of vehicles starting from a cast body of said alloy, which is homogenized at 540-580°C and then worked to obtain sheets (or the like), which are then subjected to a solution heat-treatment at 525-535°C followed by hardening and natural ageing to state T4; said sheets are then formed by means of dies to shaped elements, which, at last, are subjected to artificial ageing at 180-220°C.

Description

BACKGROUND OF THE INVENTION

This invention relates to an aluminium alloy and to a process, which is particularly useful for manufacturing, with said alloy, structural shaped elements of vehicles, in particular simple shaped elements or box-type composite shaped elements for motor vehicles, such as hoods for motors and for baggage compartments, doors, mudguards, roofs, panelings and the like.

DESCRIPTION OF THE PRIOR ART

As is known, the present tendency is to manufacture lighter and lighter motor vehicles in order to obtain several advantages, known to the technicians of this branch, among which, in particular, that of achieving substantial savings in the energy required in the running of said motor vehicles.
In the frame of this tendency, considerable efforts have been made and are being made with a view to preparing and improving light alloys based on aluminium and susceptible of being utilized in the manufacture of structural shaped elements for vehicles in general and for motor vehicles in particular, said light alloys assuring, besides a weight saving, also an excellent corrosion resistance and good properties of energy absorption, such properties resulting in advantages in terms of safety in case of vehicle crash. In order that the aluminium alloy may be suited to the above--mentioned use, it is indispensable for said alloy to possess, at the same time, a high mechanical resistance (stable in the long run) and a high cold-formability, said formability being meant also as a good capability of retaining the shape after pressing. The physical-mechanical properties of the alloy, moreover, must be such as to assure a good resistance to dent, the possibility of carrying out, without defects, seaming operations with "block" folding, and, finally, of avoiding, during forming, the formation of "Lueder bands" which, as is known, may appear on a surface of a metal when such metal is deformed beyond its elastic limit (with creep of the metal itself), and which involve successive surface treatment operations.
The abovesaid properties cannot be imparted all together to a single light alloy to the extent in which, conversely, they can be imparted to a normal mild steel for deep pressing, especially in relation to the severe requirements connected with the forming of the panelings and relevant inner reinforcing frameworks for vehicles and for automobiles in particular.
As a consequence thereof, the current technology is prevailingly directed to utilize two different alloy families, namely : a first family characterized by the optimization of the mechanical characteristics (to the prejudice of the formability which results to be by far lower than the one of mild steel) for the use in the outer panelings, and a second family characterized by the optimization of the formability (to the prejudice of the mechanical resistance) for the use in the inner frameworks, which require very marked shapings.
In the limits of the said first alloy family, the prevailing trend of the users . is directed towards the utilizatinn of alloy 6009-T4 (designation according to Aluminium Association), while in the limits of the second family, alloy 5182-0 is predominantly used. The typical compositions of such alloys are those known and indicated hereinbelow (% by weight) :
Obviously,the use of two alloys different from each other involves problems in the storage of the stocks and above all in the scrap recovery and recycling operations, with consequent substantial technical and economical disadvantages.
Furthermore, the aluminium alloys of said first family are not susceptible of being shaped in conventional dies as are utilized for the shaping of mild steel, wherefore it is practical impossible to use the existing pressing lines of the automotive industry, what entails, of course, disadvantages of both technical and economical nature.

SUMMAR Y OF THE INVENTION

Thus, it is an object of the present invention to provide a light alloy based on aluminium which exhibits, jointly, in a balanced manner, a good mechanical resistance, a good formability and a good capability of retaining the shape, in order to be suitable for being utilized for the manufacturing of both outer and inner shaped elements of vehicle components, said mouldability and capability of retaining the shape being such as to permit the use, without substantial modifications, of the dies and pressing processes in use for steel and similar materials, for said manufacture.
Another object of the present invention resides in a process for manufacturing shaped elements of vehicles starting from cast plates and billets of said alloy, in particular for manufacturing box-type composite shaped elements for motor vehicles.
These and still other objects, which will become apparent from the following description, are achieved, according to the present invention, by :

A) an aluminium alloy having the following composition (in % by weight) :
- - Si : from 0.25 to 0.35%
- - Fe : from 0.20 to 0.30%
- - Mg : from 0.85 to 1.1%
- - Cu : from 0.10 to 0.25%
- - Other elements representing impurities: each below 0.1%, in the aggregate below 0.2%
- - Al : the balance . to 100%, said Mg and Si amounts being selected in order that Mg results in a stoichiometric excess with respect to the compound Mg₂ Si,said excess being comprised between 0.25 and 0.67%; and by :
- B) a process for working said alloy which comprises the following operations :
  - (a) casting a body for plastic working consisting of the alloy as per point A) hereinbefore;
  - (b) homogenizing said Al-alloy body at a temperature ranging from 540 to 580°C, followed by quenching;
  - (c) working said homogenized body, in the form of semifinished products, such as sheets, structural shapes and the like;
  - (d) solution heat-treatment of said semifinished products attemperaturesranging from 525 to 535°C for 5-10 minutes, followed by hardening and ageing at room temperature, said semifinished.products assuming a yield strengt (Rp 0.2) and a modulus of elasticity
  - (E) such that the Rp 0.2/E ratio ranges from 1.2×10^-3 to 1.7×10^-3, thus obtaining semifinished products in state T4, which are formable in an easy and stable manner;
  - (e) forming of said semifinished products T4 in form of shaped elements;
  - (f) artificial ageing of said shaped elements at a temperature ranging from 180 to 220°C for a time from 30 to 90 minutes, in order to raise their machanical resistance to a degree of substantial stability, with a yield strength (Rp 0.2) ranging from 165 to 200 N/mm .

By means of said alloy and said process for transforming it into shaped elements, it becomes possible (as will be demonstrated later on) to utilize one alloy only, namely the alloy according to the invention, for both the outer panelings and the inner frameworks of vehicles, said panelings and frameworks being obtainable, according to the invention, by shaping sheets and the like composed of said alloy (in the state T4) by means of dies and pressing processes in use for steel.

DETAILED DESCRIPTION OF THE INVENTION

The characteristics of the alloy according to the invention and the corresponding process for transforming it into shaped elements for vehicle components are described in detail hereinafter (as a preferred but not exclusive embodiment), making reference to the annexed drawings, wherein :

- figure 1 shows, in an exploded perspective view, an outer panel of a motor hood with relevant inner element (reinforcing framework);
- figure 2 shows a sectional (partial and enlarged) view in elevation of the hood and relevant reinforcing inner element, after assembling, shown in figure 1.

The alloy according to the invention is obtained by casting into water a body having the above-indicated composition, in the form of a rolling plate or of an extrusion billet. (Hereinafter, for the sake of simplicity, reference will be made to said plate, being understood that the corresponding metallurgical process of successive working is transferable to said billet). Said plate is homogen ized at a temperature in the range of from 540 to 580°C for a time of maximum 24 hours under steady state (usually 8-12 hours), whereupon it is quenched to room temperature, preferably by means of water rain. (In the case of a continuous working cycle, said cooling can be stopped at the rough-working temperature, as explained hereinafter).
The so homogenized alloy plate is then roughly worked, by subjecting it to strong runs, starting from a pre--heating temperature ranging from 460° to 500°C.
(For a fully cooled plate after the aforesaid homogenizing operation, the preheating time period for rough-working shall be as short as possible). The rough-worked plate is then directly transformed,by rolling, into semifinished products in the form of sheets having final thicknesses,already suitable for use, in the range of from 0.7 to 1.5 mm.
Said semifinished products are then subjected to a solution heat-treatment by means of heating to 525°-535°C (5-10 minutes under steady state) followed by hardening by means of water quenching. During said solution heat--treatment/hardening step, Mg₂Si forms in a supersaturated solid solution at room temperature, while free Mg remains dissolved in aluminium since the amounts of Mg and Si in the alloy, according to the invention, are reciprocally proportioned in such amounts as to have a stoichiometric excess of Mg in respect of said Mg₂Si compound, to the specific purpose of attaining an optimization of the alloy's formability characteristics, in particular of its drawability.
On the other hand, said Mg excess shall not exceed amounts of the order of 0.6-0.7% by weight, because a high- erMgexcess would reduce the solubility of said compound Mg₂Si in Al and, consequently, the hardening effect by precipitation of same during the successive artificial ageing step after forming (as better illustrated later), which causes an increase in the mechanical resistance of the alloy.
Said hardened sheets are flattened and then allowed to naturally age at room temperature (T4 state): after 15 days, the yielding values are substantially stabilized at low values, and the sheets are ready for being formed into shah ed elements in an easy and stable manner. In fact, according to the invention, the alloy composition and relevant metallurgical working cycle (described hereinabove) are combined in such way as to impart to the semifinished products (sheets, structural shapes) in the T4 state, ready for the forming operations,a yield strength Rp 0.2 and an elastic modulus (E) such that the Rp 0.2/E ratio ranges from 1.2 x x 10^-3 to 1.7×10^-3 (very close to the one of steels for deep pressing), so that the semifinished products are particularly suited to be easily shaped in a stable manner (i.e. with an exact shape retaining after shaping) and free from defects, also by means of dies and with conventional pressing process used for steel, without bringing substantial modifications thereto.
The semifinished products in state T4 mentioned above are then transformed, by cold-pressing, into shaped elements for vehicles, such elements, after pressing, being subjected to an artificial ageing operation to bring about hardening (precipitation of Mg₂Si) with consequent raising of the mechanical characteristics to a substantial stability level, with a yield strength ranging from 165 to 200 N/ mm.
Said artificial ageing is accomplished by heating said shaped elements to a temperature from 180° to 220°C for a time between- 30 and 90 minutes.
Before said ageing step, said shaped elements can be provided with a surface coating (painting) susceptible of hot-hardening, so that, in a single successive heating step at 180-220°C, the abovesaid artificial ageing of the alloy as well as the stable hardening of the paint take place.
The abovesaid shaped elements can be utilized for outer panelings as well as for inner frameworks of vehicle components, in particular box-type composite components of motor vehicles, such as hoods for motors and for baggage compartments,doors,mudguards,roofs and the like,since, as is mentioned hereinabove, the semifinished parts (sheets, structural shapes) obtained from the alloy and by the process according to the invention are, in state T4, easily shapable without defects (in particular: without Lueder bands) and, after shaping, they can be hardened till assuming, permanently, a high mechanical resistance.
In particular, the formability-drawability and shape stability of the alloy according to the invention are better than those (already good) of alloy 5182-0, and very close to those (excellent) of deep pressing steel FeP04 (UNI 8092, according to Italian Standards UNI), while the deformation resistance characteristics (after artificial ageing) are always higher than those of alloy 5182-0 and only slightly lower than those of alloy 6009-T4, in respect of which, however, the alloy of the invention has the considerable advantage of improving both formability and shape retaining (as will be proved later on, in the examples).
The formability of the alloy sheets according to the invention (state T4) enables to seam the sheets' edges by "block" folding, as shows position 2 in figure 2, while the mechanical strength of said sheets after shaping and artificial ageing secures a good resistance to dent to shaped elements.
Finally, the alloy and the relevant process for transforming same (which are described hereinabove) permit to obtain a statistically disoriented and substantially free from rough precipitates, fine grain recrystallization structure of the alloy, said structure being suitable for ensuring, besides excellent formability properties, also an excellent corrosion resistance.
In relation to what is described hereinbefore it is therefore apparent that a composite component for vehicles (for example a motor hood represented in figure 1, to which reference is made here) which has to be composed of an outer shaped paneling 1a (endowed with a good shape stability with surface flatness, as well as with a good dent resistance) and of an inner reinforcing framework 1b shaped to deep and complex forms, can be advantageously manufactured by using a single alloy, namely the alloy of the invention, which thus replaces both alloy 5182-0 in said inner framework, and alloy 6009-T4 in said outer paneling.
The following examples, which are only for an illustrative but not limitative purpose, will provide more detailed data relating to the invention, always making reference to the annexed drawings already briefly described herein.

Example 1

An alloy having the composition given hereinbelow (% by weight) was cast in the form of a plate :

Si = 0.25%; Fe = 0.20%; Mg = 0.85%; Cu = 0.12%; other elements, such as impurities: Mn = 0.015%; Cr =
= 0.001%; Zn = 0.013%;
Al = the balance to 100% (the free Mg,once the quenching product Mg₂Si has formed, resulted to be of 0.42%).

Said plate was homogenized by means of a 12-hour heating at 560°C (followed by water-rain cooling) and then rough-shaped after heating to 475°C. The rough-shaped product so obtained was then rolled, by a conventional method, till obtaining a sheet with a thickness of 0.9 mm. This sheet was subjected to a solution heat-treatment-hardening by heating to 530°C (8 minutes) followed by cooling with water-rain. The so hardened sheet was allowed to age at room temperature for 15 days, thus obtaining said alloy in the T4 state, the characteristics of which are indicated in Tables I and II (in such Tables, the alloy is designated as AI-02/1). Said sheet in the T4 state was then caused to artificial age for 1 hour at 200°C: the characteristics, after such ageing, are indicated too in said Tables I and II, in which, by way of comparison, also analogous characteristics of alloys 5182 and 6009, as well as of mild steel Fe P04 are reported.
In said Tables, the symbols of the various characteristics therein indicated have the following meanings :

- Rm = unitary ultimate tensile strength;
- Rp 0.2 = unitary deviation from proportionality strength (yielding strength);
- A5% = percentage elongation at break;
- HB = Brinell hardness (2.5-mm ball, load : 613 Newton; time: 15 seconds);
- Dent = static load in N necessary to produce,on a 0.9 mm thick, hydraulically preformed (with a 2% deformation) cap, a permanent 1 mm impression (dent resistance parameter);
- Rp 0.2/E = ratio between unitary yielding strength and elastic modulus;
- Bendab. = bendability, expressed as % work-hardening degree by which a 0.9 mm thick specimen breaks due to bending around a spindle having a 0.5 mm radius;
- Formability, expressed by :
- "r" index = normal plastic anisotropy coefficient, according to known standard ASTM-E 517;
_- "n" index ₌ work-hardening index,according to known standard ASTM-E 646;
- Erichsen deflection = deep drawing index according to known standard ISO/R 149.
- State of the material :
- T4 : after hardening and natural ageing (ready for forming);
- T6 : after artificial ageing;
- 0 : annealed, ready for forming;
- ttv: after heat treatment simulating paint baking (180°C for 0.8 hours, plus 140°C for 1 hour);
- Spot weldability: evaluated according to the known US standard MIL-W-6858.

Example 2

In the form of a plate, an alloy having the following composition (% by weight) was cast :

Si = 0.35%; Fe = 0.30%; Mg = 1.1%; Cu = 0.20%;
other elements, such as impurities: Mn = 0.013%; Cr = = 0.002%; Zn = 0.013%; Al = the balance to 100%.
(The free Mg, once the quenching product Mg₂ Si has formed, resulted to be of 0.50%).

Said plate was homogenized, rough-shaped, rolled, hardened and aged by processes and relevant parameters (temperatures, times) substantially like those of Example 1.
The characteristics of the abovesaid alloy (designated as AI-02/2) are indicated in Tables I and II.

Example 3

Using 0.9 mm thick sheets in the T4 state obtained in Example 2, there were formed (employing conventional forming dies and cycles as are utilized for mild steel) two shaped elements, respectively an outer element and an inner element for motor-vehicle motor-hood (indicated with 1a and 1b in figures 1 and 2).
Said two shaped elements were then assembled (to obtain said motor-hood) by means of seamings involving block foldings, as indicated at 2 in figure 2. All the above--mentioned forming and assembling operations were readily accomplished without problems, obtaining at last a motor-hood endowed with a perfect flatness and shape correspondence to the dies, such features being retained even after an ageing heat-treatment of the hood at 220°C for 1 hour.
Said motor hood so obtained according to the invention exhibited, as compared with an analogous hood produced (with equal sheet thicknesses) respectively from alloy 6009 (outer element) and from alloy 5182 (inner element), the following advantages :

- possibility of utilizing,.without process drawbacks and product defects, conventional forming dies and cycles as are used for steel;
- a lower alloy cost;
- economical storing of the sheets intended for forming;
- economical and simplified technique for the _Bcrap recovery;

Example 4

Specimens obtained from the sheets produced as per Examples 1 and 2 (both in the T4 state and after artificial ageing) were subjected to corrosion tests in neutral saline mist according to standard UNI 5687/65: the obtained results proved, for both short times and long times (400 hours) of tests, an excellent corrosion resistance of the alloy according to the invention.

Example 5

This examples proves the criticity of the Si and Mg content of the alloy.
Following the process of Example 1, there were prepared two sheets having a thickness of 0.9 mm (up to the artificial ageing state for 1 hour at 220°C), respectively made of an alloy AI/3 and of an alloy AI/4, both comprising Fe, Al and other elements, such as impurities, in amounts substantially equal to those of the alloy according to the invention of example 2, while Si and Mg were in different amounts from those of the alby according to the invention, namely :
In alloy AI/3, in relation to the too low Si content, Mg₂Si did not form in a sufficient amount to cause the desired hardening, while in the AI/4 alloy the too high free Mg content above the stoichiometric composition Mg Si(1.3%) brought about an inhibiting effect on the solid solubility of Mg₂Si in Al during the solution heat-treatment,with consequent substantial decrease of hardening (due to insufficient precipitation of said Mg₂Si during artificial ageing), as is proved by the data recorded in Table III, compared with the ones of alloy AI/02 of examples 1 and 2 (all the alloys are in state T6, after a 1-hour artificial ageing at 200°C)..
The aluminium alloy according to the invention is particularly suited to be used in shaped elements for vehicles, but, as is evident, it can find a useful utilization in all those appliances which require an Al-based light alloy endowed with balanced characteristics of formability, shape stability and mechanical stregth.
Furthermore, as is apparent, the alloy according to the invention is employable, in a few cases, in composite shaped elements comprising one or more shaped elements consisting of said alloy and one or more shaped elements consisting of another material.

Claims

1. Aluminium alloy for shaped elements of vehicles and the like, in particular shaped elements for composite components of motor vehicles, characterized by the following composition (% by weight) :

- Si : from 0.25 to 0.35%

- Fe : from 0.20 to 0.30%

- Mg : from 0.85 to 1.1%

- Cu : from 0.10 to 0.25%

- Other elements constituting impurities: each below 0.1%, in total below 0.2%

- Al : the balance to 100%,

said amounts of Mg and Si being present in such a ratio that Mg is in a stoichiometric excess in respect of the compound Mg₂Si, said excess ranging from 0.25 to 0.67%.

2. A process for manufacturing shaped elements of vehicles, in particular simple or composite-box-type shaped elements for motor vehicles, such as hoods for motors and for baggage compartments,doors,mudguards,roofs and the like, characterized in that it comprises the following operations :

(a) casting a body for plastic working in Al alloy, in the form of a plate or the like, having the following composition (% by weight) :

- Si : from 0.25 to 0.35%

- Fe : from 0.20 to 0.30%

- Mg : from 0.85 to 1.1%

- Cu : from 0.10 to 0.25%

- Elements representing impurities: each below 0.1%, in total below 0.2%.

- Al : the balance to 100%,

said Mg and Si aounts being selected in such way that Mg is in a stoichiometric excess with respect to the compound Mg₂Si, said excess being in the range of from 0.25 to 0.67%;

(b) homogenizing of said Al-alloy body at a temperature ranging from 540 to 580°C, for a time of from 8 to 24 hours, followed by quenching;

(c) working said body in Al alloy, thus homogenized, to the form of semifinished products such as sheets, structural shapes and the like;

(d) solution heat-treatment of said semifinished products at a temperature in the range from 525 to 535°C for a period of time from 5 to 10 minutes, and successive hard _ening of the semifinished products, which are so caused to assume, after 15 days of natural ageing.at room temperature, a yielding strength (RP 0.2) and a modulus of elasticity (E), the values of which are in a ratio to each other (RP0.2/E) that ranges from 1.2×10^-3 to 1.7×10^-3;

(e)forming of said semifinished products to shaped elements;

(f) artificial ageing of said shaped elements to increase their mechanical strength to a level of substantial stability, with a yielding strength Rp0.2 ranging from 165 to 200 N/mm².

3. The process according to claim 2, characterized in that said working (c) includes a rolling which imparts a shape of sheet to said body.

4. The process according to claim 2, characterized in that said working (c) comprises an extrusion which imparts the form of a structural shape to said body.

5. The process according to claim 2, characterized in that said artificial ageing (f) is accomplished at a temperature from 180 to 220°C for a time ranging from 30 to 90 minutes.

6. The process according to claim 2, characterized in that said shaped elements are endowed with a surface coating susceptible of hot hardening, said hardening being accomplished during said artificial ageing (f).

7. Composite-box-type components of vehicles and the like, in particular composite-box-type components of motor vehicles, such as motor hoods, baggage compartments hoods, doors,mudguards,roofs and the like,characterized in that they comprise at least two shaped elements obtained by the process according to claims 2 to 6.

8. The Al alloy as claimed in claim 1,' characterized in that it is particularly suitable for manufacturing semifinished products such as sheets and the like, easily and stably shapable in conventional dies as are used for the shaping of steel semifinished articles and the like.