EP1479786A1

EP1479786A1 - Wrought aluminium alloy

Info

Publication number: EP1479786A1
Application number: EP04076340A
Authority: EP
Inventors: Marc-Jan De Haas; Jean Pierre Jules Baekelandt; Peter De Smet; Erik Smet; Linzhong Zhuang
Original assignee: Corus Aluminium NV
Current assignee: Aleris Aluminum Duffell BVBA
Priority date: 2003-05-20
Filing date: 2004-05-04
Publication date: 2004-11-24
Anticipated expiration: 2024-05-04
Also published as: ES2286556T3; DE602004005529T2; EP1479786B1; ATE358190T1; EP1479786B8; DE602004005529D1

Abstract

A wrought alloy of composition in weight percent:

Mg 3.0 - 4.0

Si 0.6 - 1.0

Cu up to 0.3

Fe up to 0.4

Mn up to 0.3

Zn up to 0.8

impurities each up to 0.05 and total up to 0.25, aluminium balance.

Description

This invention relates to a new wrought aluminium alloy containing Mg and Si, which is suitable for sheet and in particular for automotive body sheet and/or structural sheet. The invention further relates to a method of making the wrought aluminium alloy.
Automotive body sheet in particular is typically made from series AA5xxx or AA6xxx alloys. Series AA5xxx alloys comprise Mg as their most important alloying element and Mg is generally present in a concentration above 2 wt % whilst Si levels are usually low. Series AA5xxx alloys are typically used for automotive inner panels. Series AA6xxx alloys generally comprise both Mg and Si. Si and Mg are generally present up to a concentration of 1.5 wt % each. Series AA6xxx alloys are typically used for automotive outer panels.
Some disclosures found in the prior art include:
US-6,248,193 discloses a process for the production of aluminium alloy sheet having enhanced resistance to stress corrosion cracking under stress and improved shape fixability. The process includes continuous casting, rolling and a stabilization treatment with a specific working window. The process applies for AlMg-alloys of the AA5xxx-series.
US-5,908,518 discloses an AlMgMn-alloy for welded mechanical constructions and having a microstructure comprising Mg₂Si-particles in a defined size and density range.
US-6,086,690 discloses a process of producing aluminium alloy sheet products using a twin belt caster and a series of rolling steps without intermediate coiling or full annealing of the intermediate sheet article. The process applies for non-heat treatable aluminium alloys of the AA5xxx-series.
US-2001/0006606-A1 discloses an aluminium base alloy for production of casting alloys and wherein it contains at least 50% scrap metal of a primary aluminium base and as the residue primary aluminium and/or scrap metal on a secondary aluminium base. The scrap metal on a primary base is mainly metal obtained from foodstuff or animal feed packing. The alloy is for production of engine blocks, cylinder heads and oil pumps, by means of sand casting, chilled casting, die-casting, thixocasting and thixoforging.
It is an object of the invention to obtain a wrought alloy composition that can be used for sheet applications, e.g. automotive body sheet, structural sheet, as it has a balance of sufficient strength as well as good formability and corrosion resistance.
According to the invention the object is achieved by a wrought alloy of composition in wt % of:
Mg 3.0-4.0
Si 0.6 - 1.0
Cu up to 0.3
Fe up to 0.4
Mn up to 0.3
Zn up to 0.8
impurities each up to 0.05 and total up to 0.25
balance aluminium.
The wrought alloy of the invention is tolerant to high levels of both Mg and Si, whilst providing a desirable balance of properties in terms of formability, strength and corrosion resistance.
An advantage of the wrought alloy of the present invention is its good scrap absorbing capabilities whilst still being particularly suitable for sheet e.g. automotive body sheet and/or structural sheet as it has sufficient formability, strength and corrosion resistance whilst production costs are kept low as the alloy can be made from a majority, preferably more than 50%, of scrap material and a minority of smelter-grade material.
Recycling scrap material to produce an alloy is cost effective in comparison with producing smelter-grade material. However, due to the presence of relatively high levels of elements such as Si and/or Mg and/or Zn and/or Cu and/or Mn and/or Fe in scrap, it is usually necessary to also use significant amounts of smelter-grade material as well as scrap material in order to obtain desirable mechanical properties. It has been found that the wrought alloy of the present invention requires the addition of less smelter-grade material as it can absorb relatively large amounts of Mg, Si, Zn, Fe, Mn and Zn without having an adverse effect on its mechanical properties. The wrought alloy of the present invention thus has considerable cost advantages over aluminium alloys that do require the addition of substantial amounts of smelter-grade material.
Some of the reasons for the control of the alloying elements of the wrought aluminium alloy according to the invention are described below. All compositions are by weight.
Mg is the principal solid solution strengthening addition in the wrought alloy and the relatively high Mg content of the present invention of between 3.0% and 4.0% results in increased strength and formability. The strength and formability is not sufficient if the Mg level is below 3.0 wt%. However if the Mg level is raised above 4.0% it has been found that the strength level becomes too high. Sheet production therefore becomes difficult. Mg can be present preferably in the range 3.2 % - 4.0% and more preferably 3.4 % - 3.8%. The Mg level at between 3.0 wt% and 4.0 wt % provides good strength and formability but surprisingly despite this relatively high Mg level the alloy of the invention has a high resistance to intergranular or grain boundary corrosion.
Si is an element that improves strength and in order to optimise the strength of the wrought alloy of the present invention at least 0.6% is present.
It has been found that levels of Mg above 4.0 wt % combined with levels of Si below 0.6 wt % lead to poor corrosion performance because the Mg and Al form Mg₅Al₈ phase at the grain boundaries. This phase is very anodic with respect to the matrix and leads to localised corrosion at the grain boundaries. When Si is present above 0.6 wt.% it is available to combine with Mg and form Mg₂Si. As a result there is less Mg available to form Mg₅Al₈ and the material is less susceptible to grain boundary corrosion. The Mg₂ Si also contributes in enhancing the mechanical strength after a baking operation. However, a Si content above 1.0% gives rise to reduced ductility and formability as the Mg₂Si constituents have a detrimental effect on the formability at higher concentrations. The Si content may be 0.65% - 1.0% and preferably 0.65% - 0.9%.
Cu can be present up to 0.3%. Cu up to this concentration enhances the strength and bending property of the wrought alloy. The strength enhanced by Cu is retained after a paint bake cycle. It has been found that Cu present in concentrations above 0.3% leads to increased pitting and filiform corrosion. Cu may be present preferably up to 0.25 % and more preferably between 0.1 % and 0.2%.
Fe can be present up to 0.4%. Fe contributes to dispersion strengthening and grain refinement but lowers formability at concentrations above 0.4%. Fe may be present up to 0.3 % and more preferably between 0.15 % and 0.23 %.
Mn effectively refines the recrystallised grains and reunifies the structure of the wrought alloy. When Mn is present at a content exceeding 0.3% the formability is impaired as coarse intermetallic compounds are formed during casting. Preferably up to 0.2% Mn may be present or more preferably 0.1-0.2 wt.%.
Zn may be present up to 0.8% and preferably up to 0.5% and more preferably up to 0.3%. It may be present in the scrap materials from which the present wrought alloy is produced and it may also be added to the alloy. In this range the Zn further improves the intergranular corrosion resistance of the wrought alloy.
Other impurities such as Zr, Ti and Cr may be present in the wrought alloy each in a concentration of up to 0.05% with a total of up to 0.25%. Although at impurity level Ti is present as a grain refinement element in the casting operation. The total impurity level may preferably be up to 0.15% with each impurity present in a concentration of up to 0.05%.
Aluminium makes up the balance of the wrought alloy composition.
It has been found that the Mg range of 3.0 - 4.0 wt % and the Si range of 0.6 - 1.0 wt % in particular means that scrap material such as a combination of AA6xxx and AA5xxx type alloys in different ratios can comprise the majority (more than 50%) of the material from which the wrought alloy is made. The scrap metal, which comprises the majority of the material from which the wrought alloy is made, is preferably scrap wrought metal. The scrap AA6xxx and AA5xxx type alloys may originate from non-separated production scrap or non-separated End of Life Vehicle scrap such as for example mixed body sheet scrap. The non-separated End of Life Vehicle scrap may be in the form of both inner and outer automotive body sheets and may comprise shredded hoods, roofs, lids etc. The non-separated End of Life Vehicle scrap may preferably comprise two or more of the alloys AA6016, AA6111 and AA 5182.
The wrought alloy composition of the present invention in the soft annealed condition has the following mechanical properties: yield strength of at least 100 MPa and preferably 100-115 MPa, and ultimate tensile strength of at least 220 MPa and preferably 220-230 Mpa, and elongation A50 of at least 17% and preferably in the range of 17-25% measured according to Euronorm. Standard alloy AA5754 in the soft annealed condition has typical properties of yield strength of at least 80 MPa, ultimate tensile strength of 190-240 MPa and elongation A50 of at least 14%, measured according to Euronorm. As well as its scrap absorbing capabilities the wrought alloy of the present invention thus also has mechanical properties at least comparable to those of AA 5754, an alloy that is typically used for automotive applications.
The wrought alloy of the present invention is suitable for semi-continuous direct chill casting (DC-casting) rather than requiring continuous casting.
In a further aspect the invention relates to a method of manufacturing the wrought alloy according to this invention comprising the steps of:
i) direct chill casting of a rolling ingot having a chemical composition as defined in the present description and preferably composed of scrap material as herein defined,
ii) preheating or homogenisation,
iii) hot-rolling,
iv) cold-rolling,
v) recrystallisation annealing in a temperature range of 330°C - 480°C or solution annealing in a temperature range of 480°C-570°C, preferably 520°C-570°C,
vi) quenching,
vii) optionally pre-ageing or stabilisation up to 200°C.
Recrystallisation annealing is generally used for non-heat treatable alloys such as the AA5xxx-series whilst solution annealing is used for heat-treatable alloys such as AA6xxx series.
However, it has been found that solution annealing of the wrought alloy according to the present invention results in additional bake hardening due to the high levels of both Mg and Si. When the wrought alloy of the present invention is bake hardened after solution annealing the yield strength increases by 60-85 MPa and possibly even more. The ultimate tensile strength also increases by 15-45 MPa. Such additional bake hardening does not occur in the regular AA5xxx-series alloys which are typically used for automotive inner panels. Bake hardening can be used as an additional optional processing step after steps vi) or vii) above. Solution annealing can thus be performed when additional strength is required.
The invention will now be illustrated by means of a non-limitative example.

Example

Wrought alloy composition in weight percent:

Chemical composition(wt%)	Mg	Si	Cu	Fe	Mn	Zn	Al + impurities
Sample 1 according to invention	3.65	0.76	0.15	0.21	0.19	0.07	balance
Typical 5754 sample	2.7	0.15	0.02	0.30	0.25	0.02	balance
Typical 5454 sample	2.9	0.15	0.05	0.35	0.80	0.02	balance

As can be seen the wrought alloy of the present invention has relatively high levels of Mg and Si and Cu in comparison with standard AA5754 and AA5454.

The following mechanical properties were obtained for sample 1 according to the invention, having the composition shown in Table 1. After DC-casting, the sample according to the invention was homogenised at 560°C for 5 hours. The material was subsequently hot-rolled to a thickness of 4mm, soft annealed at 360°C and cold-rolled to a thickness of 1mm. The material was subsequently soft-annealed at 420°C for 1 sec or solution annealed, or heat-treated, at 560°C for 10 secs. The solution-annealed material was also subjected to a simulated baking operation using an oil bath. The material was 2% prestretched and baked at 185°C for 20 mins or annealed at 205°C for 30 mins without pre-stretching.

Mechanical properties:	Rp (MPa)	Rm (MPa)	A50 (%)
Sample 1 soft annealed condition	108	224	22
5754 soft annealed condition	105	222	21
5454 soft annealed condition	115	245	20
Sample 1 after solution annealing	84	224	23
Sample 1 after 2% + 185°C/20min bake hardening	156	249	19.2
Sample 1 after 205°C/30min bake hardening	160	259	14.2

Sample 1 in the soft annealed condition thus has properties, which are at least comparable to those of AA5754, and has better formability than both AA5454 and AA5754. As can be seen from Table 2 by bake hardening the sample 1 after solution annealing the yield strength of sample 1 increases considerably by 72 to 76 MPa whilst the ultimate tensile strength increases by 25-35 MPa.
Intergranular corrosion or grain boundary corrosion tests were also done on5754 in the soft annealed condition, 5454 in the soft annealed condition, sample 1 in the soft annealed condition and sample 1 in the solution annealed condition. The soft annealed composition was reached by homogenising at 560°C for 5 hours, subsequently hot-rolling to a thickness of 4mm, soft annealing at 360°C and cold-rolling to a thickness of 1mm before soft-annealing at 420°C for 1 sec.
The solution annealed condition was obtained by homogenising the sample at 560°C for 5 hours, subsequently hot-rolling to a thickness of 4mm, soft annealing at 360°C and cold-rolling to a thickness of 1 mm before being solution annealed (or heat-treated) at 560°C for 10 secs.
The soft annealed and solution annealed samples were first "sensitised" by annealing for 100 hours at 100°C or annealing for 20 days at 100°C. This sensitising process makes the samples more sensitive to intergranular corrosion and enables a reasonably accurate prediction to be made regarding the intergranular corrosion properties of the material over longer periods of time.
The intergranular corrosion tests were done following ASTM G67 according to which the test method consists of immersing test specimens in concentrated nitric acid at 30°C for 24 hours and determining the mass lost per unit area as a measure of susceptibility to intergranular corrosion.

The results as shown in Table 3 were obtained.

	Sensitising treatment	Weight loss after intergranular corrosion test
Sample 1 soft annealed	100°C/100 hours	2 mg/cm²
Sample 1 soft annealed	100°C/20 days	2 mg/cm²
Sample 1 solution annealed	100°C/100 hours	2 mg/cm²
AA5754 soft annealed	100°C/100 hours	4 mg/cm²
AA5754 soft annealed	100°C/20 days	12 mg/cm²
AA5454 soft annealed	100°C/100 hours	3 mg/cm²
AA5454 soft annealed	100°C/20 days	22 mg/cm²

These results indicate that the alloy of the present invention has a higher resistance to intergranular corrosion than standard alloys AA5754 and AA5454.

Claims

A wrought alloy of composition, in weight percent:

Mg 3.0 - 4.0,

Si 0.6 - 1.0,

Cu up to 0.3,

Fe up to 0.4,

Mn up to 0.3,

Zn up to 0.8,

impurities each up to 0.05 and total up to 0.25,

Aluminium balance.
The wrought alloy of claim 1, wherein the Cu content is up to 0.2 wt %, and preferably in the range of 0.1 - 0.2 wt %.
The wrought alloy of any preceding claim, wherein the Fe-content is up to 0.3 wt %, and preferably in the range of 0.15 to 0.23 wt.%.
The wrought alloy of any preceding claim, wherein the Mn-content is up to 0.2 wt %, and preferably in the range of 0.1-0.2 wt.%.
The wrought alloy of any preceding claim, wherein the Si-content is in the range of 0.65-1.0 wt.%, and preferably in the range of 0.65-0.9 wt %.
The wrought alloy of any preceding claim, wherein the Mg-content is in the range of 3.2 - 4.0 wt %, and preferably in the range of 3.4-3.8 wt.%.
The wrought alloy of any preceding claim wherein the wrought alloy is a rolled product.
The wrought alloy of any preceding claim wherein the wrought alloy comprises scrap metal, and preferably comprises automotive scrap.
The wrought alloy of any preceding claim comprising recycled scrap of both AA6xxx and AA5xxx alloys, and preferably comprising more than 50% recycled scrap AA6xxx and AA5xxx alloys.
A method of manufacturing the wrought alloy according to any one of claims 1 to 9 comprising the steps of:

i) direct chill casting,

ii) preheating or homogenisation,

iii) hot-rolling,

iv) cold-rolling,

v) recrystallisation annealing in a temperature range of 330°C - 480°C or solution annealing in a temperature range of 480°C-570°C, preferably 520°C-570°C,

vi) quenching,

vii) optionally pre-ageing or stabilisation up to 200°C.
A method according to claim 10 additionally comprising a bake-hardening step after step vi) or vii).
Part of an automotive body and/or structural sheet made from the wrought alloy of any preceding claims 1 to 9 or obtained by the method according to claims 10 or 1.