T 1195/00 24-05-2004

1. The appeal is admissible.

2. Amendments

Article 123(2), (3) EPC

The new claims are fairly supported by the application as originally filed and are restricted in scope as compared with the claims as granted. The description has been amended for consistency with the claimed invention so that the amended application is allowable. The respondent did not object to the amended patent on formal grounds.

3. Clarity

The last part of claims 1 and 3 contain a desideratum, namely that the extruded section should have, after ageing, an ultimate tensile strength of at least 240 MPa. This is allowable in the present case for the reasons given below in point 5.6.

4. Novelty has not been an issue during the opposition or appeal procedure. The Board agrees with the parties in this respect.

Inventive step

The broadest claim is claim 3, but the following considerations apply equally to claims 1 and 3 since these claims have substantially the same scope. The respondent has set out two lines of attack, one starting from PD3 as the closest prior art and the other from PD1, and each of these is considered in turn below.

5. Starting from PD3 as the closest prior art

The patent in suit concerns an extruded section of an excess silicon intermediate strength Al-Mg-Si alloy whose ultimate tensile strength after ageing is at least 240 MPa. Thus, the alloy is initially ductile enough to be extrudable, and is subsequently heat treated to deposit intermetallic compounds in the matrix in order to harden it. A high strength is imparted to the alloy by further heat treatment.

5.1. PD3 discloses an intermediate strength Al-Mg-Si alloy whose ultimate tensile strength after ageing is high. The document teaches (see page 3) the beneficial effect of Mn on refining the recrystallisation structure and improving the extrudability, and the effect of Cu on improving the mechanical properties on heating. Sample 2 of Table 1, with 0.60% Si, 0.18% Fe, 0.15% Cu, 0.10% Mn, 0.42% Mg and the balance Al after extrusion and artificial ageing, presents a good surface quality and a tensile strength of 274 Mpa.

5.2. Claim 1 of the patent in suit defines a composition which differs from the alloy composition of PD3 in that the maximum impurity level for any one element, including Cu, is 0.05%, as against 0.10-0.30% in PD3. The claim also defines an important property, that the extruded section, after ageing, has an ultimate tensile strength of at least 240 Mpa.

It remains, therefore, to be determined whether it would be obvious to reduce the Cu content of the alloy of PD3 to below 0.05% and at the same time expect the ultimate tensile strength to be at least 240 Mpa after ageing.

5.3. It is not disputed that the prior art teaches a certain level of Cu to be indispensable for attaining a satisfactory tensile strength value. Thus, PD3 states (on page 3) that Cu is the constituent which acts to improve mechanical properties and that its effects are small if less than 0.10% is present. HD2 states (page 28, right column) that the strengthening mechanism of these alloys (ie Mg and Si) could be further enhanced by Cu additions such as the levels added to 6061 (which is 0.15%-0.40% according to AL6). HD9 says (page 7, right column) that tensile properties are the first requirement to be met and that Cu is extremely beneficial in increasing the T6 temper strength.

Similarly, AL2 teaches (page 3, last paragraph) the addition of 0.2% Cu as a strengthening element to the Al-Mg-Si alloy under discussion, and AL3 says (column 3) that below the minimum values given above for the main elements Si, Mg, and Cu (ie 0.10%-0.50% Cu) the desired mechanical characteristics in the treated state are not attained.

5.4. At the same time, however, Cu can have a deleterious effect on the corrosion resistance of Al-Mg-Si alloys having a high Si excess, and the prior art warns against adding too much Cu. PD3 says that adding more than 0.30% Cu is inappropriate because this can be linked to reduced corrosion resistance of the material, and HD2 states that additions of Cu to alloys with high Si excesses reduces corrosion resistance. HD9 reports that an addition of Cu to the excess Si alloys adversely affects the corrosion resistance, especially under severe conditions, and AL2 discusses (page 4, last paragraph) corrosion aspects of Cu additions and says that this subject was open to dispute.

5.5. Therefore, there is consensus in the prior art that the Cu level must be maintained between certain levels in the interest of balancing the mechanical and corrosion properties of the alloys, the minimum being 0.1%. The question is whether the prior art additionally suggests reducing the Cu level to below 0.1%.

The cited paragraph on page 3 of PD3 indicates that corrosion becomes a problem only at Cu levels above 0.3%. HD9 does not state at which level of Cu corrosion becomes a problem, but on page 11, left column it is stated that AA-6070 is preferred over AA-6066 because of its better corrosion resistance, and the former has Cu in the range 0.16%-0.40% according to AL6, which means that at these levels of Cu corrosion is not a problem. Although the corrosion problem was disputed in AL2, according to page 4 Mr Greutert recommends increasing the Cu content to values which have proved good in practice, which appears to be approximately 0.2%.

The statement in HD2, that additions of Cu to alloys with high Si excesses is reported to reduce corrosion resistance, but the limit for these alloys allows only up to 0.10% Cu, is not a recommendation to reduce the Cu level to below 0.10%, it is merely a statement to the effect that the Cu content of a 6351 or 6082 alloy must be limited to this level if the alloy is conform to the nomenclature of the registration record of international alloy designations and chemical composition limits for wrought aluminum and wrought aluminum alloys of the Aluminum Association (AL6). The impugned decision states that HD2 sets an upper limit of 0.10% for Cu because of the negative effect on the corrosion resistance, but this is not derivable from HD2.

Therefore, the prior art does not teach reducing the Cu level to below 0.10% and at the same time expect a high value of tensile strength to be maintained.

5.6. Despite the fact that the alloy of the patent is the same as that of PD3 except that it has Cu as a trace element only, the alloy, nevertheless, has an ultimate tensile strength of at least 240 Mpa. The difference between the subject-matter of claim 1 of the patent in suit and PD3, therefore, corresponds to the desideratum that a high Si excess Al-Mg-Si alloy with only trace amounts of Cu be produced but without adversely affecting the high value of tensile strength attained in prior art alloys with Cu in significant amounts.

T 595/90 (OJ 1994, 695) concerns a product which could be envisaged as such with all characteristics determining its identity including its properties in use, but for which no known method of manufacture existed. Such an otherwise obvious entity might become non-obvious and claimable as such if there was no known way or applicable method in the art for making it and the method for its preparation was therefore the first to achieve this and do so in an inventive manner. The decision T 233/93 (unpublished) arrived at the same conclusion.

A high Si excess Al-Mg-Si extrusion alloy having no Cu but a high tensile strength could be envisaged as such, at least since Cu is expensive and it affects the extrusion pressure. This alloy would be an obvious entity, but for the fact that the prior art does not disclose a method of preparing such an entity. This is achieved for the first time in the patent in suit by the processing steps used, in particular the ageing process, which depends significantly on the rate of heating. A slow heating rate of 10 - 100°C is employed in the patent (page 3, lines 42 to 46). An effect equivalent to slow heating can be achieved by a two-stage heating schedule, with a hold temperature typically in the range of 80 - 140°C for a time sufficient to give an overall heating rate within the above range. When aged to peak strength, extrusions are typically found to have an ultimate tensile strength of at least 240 MPa with acceptable toughness.

The respondent's argument that the slow heating rate of 10. - 100°C is conventional is not accepted in the absence of documentary or other evidence of this. The prior art discloses homogenisation heating rates of this order of magnitude, but the heating rate for homogenisation is not the same as the heating rate used for ageing, as the respondent argues, since the former concerns thick blocks and the latter thinner extruded sections. Moreover, heating is normally done as rapidly as possible for economic reasons, so that a relatively low heating rate for aging is unconventional.

5.7. The disclosed method of preparing a high Si excess Al-Mg-Si alloy extrusion alloy having no Cu but a reliably high tensile strength of a minimum of 240 MPa was not known in the prior art.

Starting from PD3 it is found that the extruded article involves an inventive step, accordingly. Therefore, the claimed extruded article is patentable.

6. Starting from PD1 as the closest prior art

The alloy of Sample 1 of document PD1 contains no Mn and the respondent argues that it would be obvious to add Mn for the sake of improving its surface qualities. The parties are agreed that Mn has little effect on the tensile strength.

The alloy, according to Table 2 of PD1, has a tensile strength of about 215 MPa after a T5 temper treatment. The document HD9 indicates that after peak ageing using a T6 temper treatment instead, a higher tensile strength would be achieved. However, the improvement in tensile strength between a T5 treatment and a T6 treatment would amount only to about 5% according to HD9, which is not sufficient to raise the tensile strength of the alloy of Sample 1 to 240 MPa.

As shown in point 5.6 above, the required tensile strength is achieved in the patent in suit by the special heating rate used in the ageing process, which is not suggested in the prior art. Moreover, a further advantage is that improved surface quality is obtained by the addition of Mn. Therefore, starting from PD1 also, it is found that the claimed extruded article involves an inventive step.