Metal treatment – Stock – Age or precipitation hardened or strengthened
Reexamination Certificate
1999-10-12
2002-04-02
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Age or precipitation hardened or strengthened
C148S550000, C420S544000, C420S546000
Reexamination Certificate
active
06364969
ABSTRACT:
The present invention relates to aluminium alloys of the 6XXX series, to methods of processing such alloys and to a method for designing such alloys.
The 6XXX series aluminium alloys are aluminium based alloys that include magnesium (Mg) and silicon (Si), with the Mg and Si each generally being present in the range of 0.2 to 1.5% by weight.
The 6XXX series alloys are widely used in applications which require medium-high strength with good formability, weldability and extrudability. The applications include a wide range of architectual/structural/electrical applications. Typically, the 6XXX alloys are cast as billets and then extruded to form small round bars or other profiled shapes or forged (from extrusions or billets) into larger components.
Conventional theories of precipitation hardening in 6XXX series alloys state that hardening occurs via the precipitation and growth of Mg
2
Si in accordance with the following sequence:
i) Si atom clusters form during delay before ageing;
ii) GPI zones form during heat up to ageing temperature,
iii) GPII zones form-precipitation of &bgr;″ Mg
2
Si;
iv) &bgr;′ precipitate forms via transformation from &bgr;″ and grows with the amount of &bgr;′ depending upon the temperature and time; and
v) if overageing occurs, &bgr; Mg
2
Si precipitate forms.
As a consequence of the conventional theories that the ratio of Mg to Si in the precipitates that form in 6XXX alloys is approximately 2 (on an atomic weight basis), in order to produce alloys that are “balanced” with respect to Mg and Si, the standard practice has been to calculate the relative amounts of Mg and Si to add to 6XXX alloys so that the alloys include atomic weight ratios of Mg:Si of 2:1.
In some instances, instead of forming balanced alloys, it is known to design 6XXX alloys to contain excess Si to increase the strength thereof. In this instance any Si that does not precipitate as Mg
2
Si or does not form intermetallics is free to form other phases, such as precipitates with other elements, which have an added strengthening effect. The level of excess Si is varied to produce the desired strengthening effect—with the limit of Si addition often being determined by factors such as the effect of Si addition on extrudability.
Other alloying element additions and heat treatment sequences of the 6XXX alloys are also predicated on the precipitation of Mg
2
Si. For example, maganese (Mn) can be added to alloys to produce a distribution of Mn which acts as heterogenous nucleation sites and increases the chance of forming &bgr;′ Mg
2
Si rods. This significantly increases the flow stress for extrusion, but also increases the level of pinning of grain boundaries, and thus reduces or even prevents recrystallisation and course grain band formation.
There are a wide range of different options for processing cast billets of 6XXX alloys to manufacture final extruded or forged products.
By way of example, it is known to homogenise 6XXX series billets to dissolve the maximum possible amount of Mg and Si present as intermetallics at grain boundaries in the as-cast billets, producing a supersaturated solid solution which, upon cooling, produces uniform precipitation of intermetallics and Mg
2
Si. It also breaks up the cast structure and transforms AlFeSi intermetallics. This leads to greater uniformity of flow stress and final properties of the extrusions and allows the development of full mechanical properties. Typically, slow cooling rates, such as 100-200° C./hour, are used.
Moreover, it is known to use induction heating to heat billets quickly to required temperatures before extrusion. Typically, gas heating is used to bring the billets to approximately 300° C. and induction heating is used to complete heating billets to the extrusion temperatures. The rapid heat-up rate with induction heating does not allow sufficient time for &bgr;′ Mg
2
Si precipitates to grow, and thus provides a fine dispersion for extrusion. Flow stresses are thus considerably reduced. Similarly, it is possible to maintain the same properties whilst using a substantially lower billet temperature, also allowing faster extrusion speeds to be used.
Furthermore, it is known to vary post-extrusion quenching rates depending on the alloy being extruded. A desirable feature of an alloy is that it has a low quench sensitivity, i.e. it can reach full properties with slow cooling. The benefits of this are that distortion can be minimised, properties are more uniform, and quenching equipment is not required.
There is a range of known practices for alloy selection, homogenisation, billet heating and quenching, and these are largely empirical optimisations within the boundaries of commonly used alloy systems. By way of example, practices, such as step cooling, slow cooling and fast cooling, are recommended after homogenisation.
Typical alloy specifications are provided in Table 1 for several alloys of the 6XXX series:
TABLE 1
Alloy specifications for several 6XXX series aluminium alloys.
From “Aluminium Standards, Data and Design Wrought Products”,
the Aluminium Council of Australia.
Composition (wt %)
Alloy
Si
Fc
Cu
Mn
Mg
Cr
Zn
Ti
6060
.3-.6
.1-.3
.1
.1
.35-.6
.05
.15
.1
6063
.2-.6
.35
.1
.1
.45-.9
.1
.1
.1
6061
.4-.8
.7
.15-.4
.15
.8-1.2
.04-.35
.25
.15
6082
.7-1.3
.5
.1
.4-.1
.6-1.2
.25
.2
.1
6101
.3-.7
.5
.1
.03
.35-.8
.03
.1
—
6262
.4-.8
.7
.15-.4
.15
.8-1.2
.04-.14
.25
.15
6351
.7-1.3
.5
.1
.4-.8
.4-.8
—
.2
.2
In the above table, unless ranges are stated, the amounts stated are maximum concentrations.
It has been discovered recently that age hardening of 6XXX series alloys does not occur by precipitation of Mg
2
Si—as has been previously accepted throughout the industry—but rather occurs via the precipitation of MgSi.
The discovered MgSi precipitation mechanism involves the nucleation and growth of &bgr;′ MgSi precipitate with an Mg:Si ratio of 1 (atomic weight basis), and not 2 as previously believed, and comprises the following sequence:
i) formation of separate clusters of Mg and Si atoms;
ii) co-clustering of Mg and Si atoms, with the Mg:Si ratio increasing during low temperature ageing and eventually reaching 1;
iii) formation of small precipitates of unknown structure with a Mg:Si ratio close to 1;
iv) transformation of these precipitates to &bgr;″ MgSi, with the ratio being 1; and
formation of &bgr;′ and B′ in the next stage of ageing, with the ratio of Mg and Si being 1.
One consequence of the above discovery is that current commercial 6XXX alloys that have been produced in accordance with conventional theories on the basis that they are balanced with respect to Mg and Si, i.e with Mg and Si precipitating as Mg
2
Si, in fact are not balanced.
Moreover, significantly, the applicant has found that better properties can be obtained with 6XXX alloys that are balanced with respect to Mg and Si, as this is now understood by the applicant. The properties of interest include, by way of example, extrudability, forgeability, conductivity, strength, and machinability.
According to the present invention there is provided a 6XXX series aluminum alloy containing Mg and Si which is characterised in that the Mg and Si that is available to form MgSi precipitates is present in amounts such that the ratio of Mg:Si, on an atomic weight basis, is between 0.8:1 and 1.2:1.
It is understood that for any given 6XXX series aluminium alloy the amount of Mg and Si that will be available to form Mg/Si precipitates will be less than the total amount of these elements added to the alloy composition. The reason for this is that there will always be a proportion (typically, relatively small) of the Mg and Si that remains in solution and a proportion of the Mg and Si that precipitate with other elements, such as iron (Fe) and copper (Cu), added to the alloys.
It is also understood herein that a 6XXX series aluminium alloy having Mg and Si that is available to form MgS
Combs-Morillo Janelle
Kerins John C.
Miles & Stockbridge P.C.
Wyszomierski George
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