EP2072628A1

EP2072628A1 - High strength crash resistant aluminium alloy

Info

Publication number: EP2072628A1
Application number: EP08171736A
Authority: EP
Inventors: Luisa Maria Marzoli; Jörgen VAN DE LANGKRUIS; Johan Boezewinkel
Original assignee: Aleris Aluminum Bonn GmbH
Current assignee: ST Extruded Products Germany GmbH
Priority date: 2007-12-19
Filing date: 2008-12-16
Publication date: 2009-06-24
Anticipated expiration: 2028-12-16
Also published as: EP2072628B1

Abstract

The present invention relates to an AlMgSi-type aluminium alloy extruded or forged product having a high impact resistance, the aluminium alloy comprising, in wt.%: Si 0.5 to 0.95, Mg 0.6 to 0.95, Mn 0.1 to 0.3, V 0.05 to 0.25, Ni 0.05 to 0.25, Cu maximum 0.3,optionally one or two element(s) selected from the group consisting of: (Cr 0.05 to 0.2, and Zr 0.05 to 0.2), Zn< 0.2, Fe < 0.5, Ti < 0.1, inevitable impurities and balance aluminium. The invention further relates to a crash box made from such aluminium alloy.

Description

FIELD OF THE INVENTION

The invention relates to an AlMgSi-alloy suitable for manufacturing components having increased strength and retaining a good crash behaviour and to a method for the manufacture of such a new aluminium alloy.

BACKGROUND TO THE INVENTION

As will be appreciated herein below, except as otherwise indicated, alloy designations 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 2007.
For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.
In the automotive industry, environmental legislation and transportation costs provide a strong driver for weight reduction in cars. This promotes the usage of aluminium alloys as compared to steels, for their superior specific strength and stiffness. The restrictions on CO₂ exhaust and other gases urge for ever higher weight reductions, which can only be achieved through improved materials, such as improved aluminium alloys with a further improved balance of strength, formability, corrosion resistance and crash worthiness. A part of the weight of the car consists of the bumper systems. With crash parts that are used for such a systems an increasing strength leads to a decrease in crash worthiness of the alloy by making the alloy more susceptible to tearing during crash. This limits the desired further weight savings through this system.
AlMgSi-type aluminium alloys such as AA6xxx series aluminium alloys are widely used and favoured for their moderately high yield and tensile strengths, low quench sensitivity, good corrosion resistance and favourable forming characteristics. AA6xxx series alloys are increasingly attractive to industries such as transportation because of these well-known properties.
Such aluminium alloys are known from the prior art, for example EP-0902842-B2 disclosing a component made from an aluminium alloy containing, in wt.%: 0.4 to 0.8 Si, 0.4 to 0.7 Mg, max. 0.30 Fe, max. 0.20 Cu, max. 0.15 Mn, 0.05 to 0.20 V, max. 0.10 Cr, max. 0.10 Ti, max. 0.10 Zn, others each < 0.05 and total <0.15, balance aluminium. This aluminium alloy has been processed to an overaged T72 condition via a heat treatment of 1 to 5 hours at 190 to 230°C.
EP-0936278-B1 discloses an AlMgSi-alloy suitable for manufacturing components having a high ductility, wherein the alloy contains, in wt.%: 0.3 to 1.0 Mg, 0.3 to 1.2 Si, max. 0.35 Fe, > 0.15 to 0.4 Mn, 0.05 to 0.20 V, max. 0.2 Cu, max. 0.2 Cr, max. 0.2 Zn, max. 0.1 Ti, and whereby the Mn/Fe ratio is in a range of 0.67 to 1.0, impurities max. 0.05, total max. 0.15, balance aluminium.
However, there is a need for AlMgSi-type alloys suitable for crash box applications and having a medium to high yield strength, meaning a yield strength (Rp0.2) of at least 280 MPa while having an elongation at fracture (A5) of at least 7%.

DESCRIPTION OF THE INVENTION

It is an object of the invention to provide an aluminium alloy suitable for the manufacture of an extruded or forged products having an increased strength in comparison with the components of the prior art, thereby retaining a good crash behaviour in comparison with crash resistant alloys such as AA6008 and related alloys.
It is a further object of the invention to provide a method for the manufacture of a product of the aluminium alloy.
These and other objects and further advantages are met or exceeded by the present invention concerning an aluminium alloy comprising the alloying elements, in wt.%:

Si 0.5 to 0.95
Mg 0.6 to 0.95
Mn 0.1 to 0.3
V 0.05 to 0.25, preferably 0.06 to 0.2,
Ni 0.05 to 0.25
Cu maximum 0.3,
optionally one or two element(s) selected from the group consisting of: (Cr 0.05 to 0.2, and Zr 0.05 to 0.2)
Zn <0.2, preferably <0.1
Fe <0.5, preferably <0.3
Ti < 0.1,

The aluminium alloy according to the invention has an increased strength, low quench sensitivity, good corrosion resistance, favourable forming characteristics and good crash resistance behaviour combined with a high thermal stability.
The Si content is in a range of 0.5% to 0.95%, preferably in a range of 0.75% to 0.95%, and more preferably in a range of 0.80% to 0.95%. In this range the strength is optimized, in particular when used in combination with the Mg content in a range of 0.6% to 0.95%, preferably in a range of 0.70% to 0.95%, and more preferably in a range of 0.75% to 0.90%. These ranges allow for a balanced Mg/Si ratio which is favourable for a more optimal crash performance than for example an alloy having excess Si. At too high Si and Mg levels the aluminium alloy becomes too quench sensitive and it becomes more difficult to retain all Mg and Si into solid solution after press quenching.
In another embodiment of the alloy in accordance with the invention the Si content is in a range of 0.8% to 0.95% and the Mg content is in a range of 0.75% to 0.90%. In such a particular embodiment of the alloy the mechanical properties are enhanced.
With a Mn content in the range of 0.1% to 0.3%, preferably in the range of 0.1% to 0.25%, and more preferably in the range of 0.16% to 0.25%, the aluminium alloy in accordance with the invention is less sensitive for hot-cracking during welding and after extrusion and heat-treatments and provides a fine-grained recrystallized microstructure. Moreover with a Mn content in the above mentioned range an optimum in mechanical properties and extrudability is obtained by the beneficial effect of Mn on the ductility and on the formation of alpha-type Fe-containing intermetallics.
V is added to enhance recrystallisation and is present in a range of 0.05% to 0.25%, and preferably in a range of 0.06% to 0.2%.
Ni is important for the aluminium alloy in accordance with the invention as Ni levels above 0.05% increases both the yield strength and the tensile strength and significantly improvse the thermal stability of the aluminium alloy products. Ni should be in a range of 0.05% to 0.25%, preferably in a range of 0.08% to 0.2%, and more preferably in a range of 0.09% to 0.18%.
Cu can be present in the aluminium alloy according to the invention up to 0.3%. In a preferred embodiment the Cu is present at 0.2% maximum, and more preferably at 0.05% maximum.
The optional addition of Cr and/or Zr are used to control the grain structure. Therefore, one or both of Cr and Zr can be added in a range of 0.05% to 0.2% Cr and/or in a range of 0.05% to 0.2% Zr. In one particular embodiment there is no Zr added other then present, if any, at impurity level, whereas Cr is added is a range of 0.08% to 0.2%.
Zn is considered to be an impurity element and can be tolerated up to 0.2%, but is preferably less than 0.1 %.
Although iron provides a slight increase in strength, it should be present in an amount not more than 0.5%, preferably less than 0.3% to reduce the adverse formation of intermetallic particles which could initiate fracture during a crash of the final component.
Ti may serve as a grain refiner during solidification of both ingots and welded joints produced using the aluminium alloy of the invention. The preferred range for Ti is not more than 0.1%, and a typical Ti content for grain refiner purposes is in a range of about 0.01% to 0.04%.
The balance is aluminium and inevitable impurities. Typically each impurity element is present at 0.05 wt.% maximum and the total of impurities is 0.15 wt.% maximum.
In addition, the invention provides a method for manufacturing a product of the aluminium alloy in accordance with the invention, wherein in method comprising the steps of:

(a) casting the aluminium alloy into ingots;
(b) homogenizing the cast ingot;
(c) cooling;
(d) optionally preheating;
(e) extruding or forging;
(f) quenching after extrusion; and
(g) ageing of the product.

In this way, it is achieved that the aluminium alloy product acquires the desired properties.
For casting the aluminium alloy into extrusion billets semi-continuous casting processes can be used. Preferably, semi-continuous Direct Chill (DC)-casting is used.
After casting, the aluminium alloy is homogenised. The aim of the homogenising treatment is amongst other things, to homogenise the microstructure, to dissolve Mg and Si, to level off possible residual stresses resulting from the casting process, to form dispersoid type particles for controlling the extruded grain structure, and to spheroidize sharp or needle shaped intermetallic compounds formed during solidification of the aluminium alloy. A high homogenisation temperature is favoured against a low homogenisation temperature.
After homogenisation, the alloy can be cooled, for example by means of air cooling. Further the alloy can be preheated, preferably to a temperature in the range of about 480°C and extruded. It is possible to use direct or indirect extrusion. Using an extrusion process the aluminium alloy in accordance with the invention can be processed into e.g. two-hole crash boxes having a cross section with a width of about 40 to 50 mm and a wall thickness of about 1 to 3 mm. In an alternative embodiment the aluminium alloy can be processed by means of forging.
After extrusion the aluminium alloy of the invention is quenched, ideally press-quenched, for example by means of water, water spray, forced air, other cooling liquid or by means of nitrogen.
In a following step, the material is aged to desired level of mechanical and physical properties. Preferably, the alloy of the present invention is artificially aged to a desired temper, which would ideally be an overaged temper such as T7, in particular when used for applications requiring a high capacity for absorbing kinetic energy by plastic deformation. Alternatively the aluminium alloy can be aged to a T6 condition for higher strength or to an underaged condition, or subjected to a stabilisation anneal at a temperature in a range of 50 to 120°C to improve on cold formability and/or paint bake response.
After the complete treatment cycle, the material can be processed into products of many kinds. The aluminium alloy is preferably suitable for application to components which, amongst other things require a high capacity for absorbing kinetic energy by plastic deformation, such as components suitable for application in automotive and railway vehicles, such as bumpers. Although the aluminium alloy according to the invention is preferably processed via extrusion, it is also suitable to be applied in forged constructions for example as a suspension part in a car, for which application the formation of coarse recrystallised grains is avoided as this has an adverse effect on the fatigue performance of the component.
The invention is now illustrated by some examples, which do not limit the scope of the invention.

BRIEF DESCRIPTION OF THE FIGURE

The appended Fig. 1 shows an extruded crash box according to an embodiment of the invention.

EXAMPLE 1

Table 1 lists the chemical compositions in weight percent of some comparative materials (alloys 3-4) and alloys which fall within the scope of the present invention (alloys 1-2). All these aluminium alloys were processed by the steps of:

(i) DC-casting of ingots having about 260 mm diameter;
(ii) Homogenizing by holding at 590°C for several hours;
(iii) air cooling;
(iv) preheating to about 480°C;
(v) extruding with a single hole die into a crash box;
(vi) press-quenching with water;
(vii) ageing with different practices.

Table 2 shows the mechanical properties of the alloys in T7 (210°C for 4.5h) and after a subsequent thermal stability ("TS") treatment of 225h at 170°C. This thermal stability treatment is an important requirement also for the specific application in a crash box, as the extruded product will be installed in a car near or close to the engine. To simulate this, the products in their final temper were subjected to a further cycle (thermal stability treatment) of 225h at 170°C. "Rm" is the ultimate tensile strength, "Rp0.2" is the 0.2% yield strength and "A" is the elongation at fracture ("A5" in accordance with German standards).

Table 1. Chemical compositions of alloys 1 to 4, all in wt.%, balance aluminium and unavoidable impurities.

Alloy	Invention	Mg	Si	Ti	V	Mn	Fe	Cu	Ni	Zr
1	Yes	0.95	0.90	0.04	0.12	0.21	0.29	<0.01	0.13	<0.01
2	Yes	0.88	0.82	0.04	0.11	0.18	0.27	<0.01	0.13	<0.01
3	No	0.86	0.81	0.03	0.11	0.18	0.28	<0.01	<0.01	0.11
4	No	0.86	0.79	0.04	0.11	0.16	0.26	<0.01	<0.01	0.11

Table 2. Mechanical properties of the extruded aluminium alloy products after ageing and after ageing combined with a subsequent thermal stability treatment.

Alloy	Rp T7 (MPa)	Rm T7 (MPa)	A5 T7 (%)	Rp after TS (MPa)	Rm after TS (MPa)	A5 after TS (%)	Rp(TS)/Rp(T7)
1	300	308	9.2	249	271	8.2	0.83
2	296	303	9.3	262	279	9.1	0.89
3	270	270	10.8	213	246	10.0	0.79
4	278	293	10.8	223	253	12.2	0.80

From the results of Table 2 it can be seen that alloys 1 and 2 in accordance with the present invention having Ni as an alloying element show an improved strength after T7 ageing. Moreover, these alloys show an improved strength after thermal stability treatment.

Claims

An AlMgSi-type aluminium alloy extruded or forged product having a high impact resistance, the aluminium alloy comprising, in wt.%:
Si 0.5 to 0.95

Mg 0.6 to 0.95

Mn 0.1 to 0.3

V 0.05 to 0.25

Ni 0.05 to 0.25

Cu maximum 0.3,

optionally one or two element(s) selected from the group consisting of:
Cr 0.05 to 0.2, and Zr 0.05 to 0.2,

Zn < 0.2, preferably < 0.1

Fe < 0.5, preferably < 0.3

Ti < 0.1,
others and inevitable impurities each <0.05, total <0.15, and balance aluminium.
An aluminium alloy product according to claim 1, wherein the Si content is in a range of 0.75% to 0.95%, and preferably 0.80% to 0.95%.
An aluminium alloy product according to claim 1 or 2, wherein the Mg content is in a range of 0.70% to 0.95%, and preferably 0.75% to 0.9%.
An aluminium alloy product according to any one of claims 1 to 3, wherein the Mn content is in a range of 0.1 % to 0.25%, and preferably 0.16% to 0.25%.
An aluminium alloy product according to any one of claims 1 to 4, wherein the alloy is devoid of Zr and has Cr in a range of 0.08% to 0.2%.
An aluminium alloy product according to any one of claims 1 to 5, wherein the Ni content is in a range of 0.08% to 0.2%, and preferably 0.09% to 0.18%.
An aluminium alloy product according to any one of claims 1 to 6, wherein the alloy product has in the over-aged condition a 0.2% Yield Strength (Rp0.2) of at least 280 MPa, and preferably at least 295 MPa.
Method of manufacturing an extruded aluminium alloy product according to any one of claims 1 to 7, wherein the method comprises the steps of:
(a) casting the alloy into ingots;

(b) homogenizing the cast ingot;

(c) cooling;

(d) optionally preheating;

(e) extruding;

(f) quenching; and

(g) ageing of the product.
Method according to claim 8, wherein the aluminium alloy product is in the form of a crash box.
Crash box made from the aluminium alloy extruded product according to any one of claims 1 to 7 or by the method according to claim 8.