Title of Invention

"A PROCESS FOR TREATING ALUMINIUM ALLOY CONTAINING MAGNESIUM AND SILICON"

Abstract

Process of treating an aluminium alloy consisting of 0.5 -2.5 % by weight of an alloying mixture of magnesium and silicon, the molar ratio of Mg/Si lying between 0.70 and 1.25, an additional amount of Si equal to 1/3 of the amount of Fe, Mn and Cr in the alloy, as expressed by % by weight, other alloying elements and unavoidable impurities, and the remaining being made up of aluminium, wherein said alloy after cooling has been submitted to homogenising, preheating before extrusion and ageing, wherein said ageing takes place after extrusion as a dual step ageing operation to a final hold temperature between 160°C and 220°C, characterized in that the ageing includes a first stage wherein the extrusion is heated with a heating rate above 100°C/hour to a temperature between 100 to 170°C, a second stage in which the extrusion is heated with a heating rate between 5 and 50°C/hour to the final hold temperature, and in that the total ageing cycle is performed in a time between 3 and 24 hours.

Full Text

FORM 2 THE PATENTS ACT 1970 [39 OF 1970] COMPLETE SPECIFICATION (See Section 10) "A PROCESS FOR THREATING ALUMINIUM ALLOY CONTAINING MAGNESIUM AND SILICON" NORSK HYDRO ASA, a Norwegian Company, of Bygdoy Alle 2, N-0240, Oslo, Norway The following specification particularly describes the nature of the invention and the manner in which it is to be performed :- The invention relates to aluminium alloy containing 0,5 - 2,5 % by weight of an alloying mixture of magnesium and silicon, the molar ratio of Mg / Si lying between 0,70 and 1,25, an additional amount of Si equal to approximately 1/3 of the amount of Fe, Mn and Cr present in the alloy in % by weight and the rest being made up of aluminium, unavoidable impurities and other alloying agents, which alloy after cooling has been submitted to homogenising, preheating before extrusion, extrusion and ageing, which ageing takes place at final temperatures between 160 and 220°C. An alloy of this type has been described in WO 95.06759. According to this publication the ageing is performed at a temperature between 150 and 200°C, and the rate of heating is between 10 - 100°C / hour preferably 10 - 70°C / hour. An alternative two-step heating schedule is proposed, wherein a hold temperature in the range of 80 - 140°C is suggested in order to obtain an overall heating rate within the above specified range. It is generally known that higher total amounts of Mg and Si will have a positive effect on the mechanical properties of the final product, whereas this has a negative effect on the extrudability of the aluminium alloy. It has previously been anticipated that the hardening phase in the Al-Mg-Si alloys had a composition close to Mg2Si. However, it was also known that an excess of Si produced higher mechanical properties. Later experiments have shown that the precipitation sequence is quite complex and that except for the equilibrium phase, the phases involved do not have the stoichiometric ratio Mg2Si. In a publication of S.J. Andersen, et. al, Acta mater, Vol. 46 No. 9 p. 3283-3298 of 1998 it has been suggested that one of the hardening phases in Al-Mg-Si alloys has a composition close to Mg5Si6. It is therefore an object of the invention to provide an aluminium alloy which has better mechanical properties and a better extrudability, which alloy has the minimum amount of alloying agents and a general composition which is as close as possible to the traditional aluminium alloys. This and other objects are obtained in that the ageing after cooling of the extruded product is performed as a dual rate ageing operation, including a first stage in which the extrusion is heated with a heating rate above 30°C/hour to a temperature between 100 - 170°C, a second stage in which the extrusion is heated with a heating rate between 5 and 50"C/hour to the final hold temperature between 160 and 220°C and in that the total ageing cycle is performed in a time between 3 and 24 hours. The optimum Mg/Si ratio is the one where all the available Mg and Si is transformed into Mg5Si6 phases. This combination of Mg and Si gives the highest mechanical strength with the minimum use of the alloying elements Mg and Si. It has been found that the maximum extrusion speed is almost independent of the Mg/Si ratio. Therefore, with the optimum Mg/Si ratio the sum of Mg and Si is minimised for a certain strength requirement, and this alloy will thus also provide the best extrudability. By using the composition according to the invention combined with the dual rate ageing procedure according to the invention, it has been obtained that the strength and extrudability are maximised with a minimum total ageing time. In addition to the Mg5Si6 phase there is also another hardening phase which contains more Mg than the Mg5Si6 phase. However, this phase is not as effective, and does not contribute so much to the mechanical strength as the Mg5Si6 phase. On the Si rich side of the MgsSie phase there is most probably no hardening phase, and lower Mg/Si ratios than 5/6 will not be beneficial. The positive effect on the mechanical strength of the dual rate ageing procedure can be explained by the fact that a prolonged time at low temperature generally enhances the formation of a higher density of precipitates of Mg-Si. If the entire ageing operation is performed at such temperature, the total ageing time will be beyond practical limits and the throughput in the ageing ovens will be too low. By a slow increase of the temperature to the final ageing temperature, the high number of precipitates nucleated at the low temperature will continue to grow. The result will be a high number of precipitates and mechanical strength values associated with low temperature ageing but with a considerably shorter total ageing time. A two step ageing also give improvements in the mechanical strength, but with a fast heating from the first hold temperature to the second hold temperature there is substantial chance of reversion of the smallest precipitates, with a lower number of hardening precipitates and thus a lower mechanical strength as a result. Another benefit of the dual rate ageing procedure as compared to normal ageing and also two step ageing, is that a slow heating rate will ensure a better temperature distribution in the load. The temperature history of the extrusions in the load will be almost independent of the size of the load, the packing density and the wall thickness' of the extrusions. The result will be more consistent mechanical properties than with other types of ageing procedures. As compared to the ageing procedure described in WO 95.06759 where the slow heating rate is started from the room temperature, the dual rate ageing procedure will reduce the total ageing time by applying a fast heating rate from room temperature to temperatures between 100 and 170°C. The resulting strength will be almost equally good when the slow heating is started at an intermediate temperature as if the slow heating is started at room temperature. Dependent upon the class of strength envisaged different compositions are possible within the general scope of the invention. So it is possible to have an aluminium alloy with a tensile strength in the class F19 - F22, the amount of alloying mixture of magnesium of silicon being between 0,60 and 1,10 % by weight. For an alloy with a tensile strength in the class F25 - F27, it is possible to use an aluminium alloy containing between 0,80 and 1,40 by weight of an alloying mixture of magnesium and silicon and for an alloy with a tensile strength in the class F29 * F31, it is possible to use an aluminium alloy containing between 1,10 and 1,80 % by weight of the alloying mixture of magnesium and silicon. Preferably and according to the invention a tensile strength in the class F19 (185-220 MPa) is obtained by an alloy containing between 0,60 and 0,80 % by weight of the alloying mixture, a tensile strength in the class F22 (215-250 MPa) by an alloy containing between 0,70 and 0,90 % by weight of the alloying mixture, a tensile strength in the class F25 (245-270 MPa) by an alloy containing between 0,85 and 1,15 % by weight of the alloying mixture, a tensile strength in the class F27 (265-290 MPa) by an alloy containing between 0,95 and 1,25 % by weight of the alloying mixture, a tensile strength in the class F29 (285-310 MPa) by an alloy containing between 1,10 and 1,40 % by weight of the alloying mixture, and a tensile strength in the class F31 (305-330 MPa) by an alloy containing between 1,20 and 1,55 % by weight of the alloying mixture. With additions of Cu, which as a rule of thumb increases the mechanical strength by 10 MPa per 0.10 wt.% Cu, the total amount of Mg and Si can be reduced and still match a strength class higher than the Mg and Si additions alone would give. For the reason described above it is preferred that the molar ratio Mg / Si lies between 0.75 and 1.25 and more preferably between 0.8 and 1.0. In a preferred embodiment of the invention the final ageing temperature is at least 165°C and more preferably the ageing temperature is at most 205°C. When using these preferred temperatures it has been found that the mechanical strength is maximised while the total ageing time remains within reasonable limits. In order to reduce the total ageing time in the dual rate ageing operation it is preferred to perform the first heating stage at the highest possible heating rate available, while as a rule is dependent upon the equipment available. Therefore, it is preferred to use in the first heating stage a heating rate of at least 100"C / hour. In the second heating stage the heating rate must be optimised in view of the total efficiency in time and the ultimate quality of the alloy. For that reason the second heating rate is preferably at least 7°C / hour and at most 30°C / hour. At lower heating rates than 7°C / hour the total ageing time will be long with a low throughput in the ageing ovens as a result, and at higher heating rates than 30°C / hour the mechanical properties will be lower than ideal. Preferably, the first heating stage will end up at 130-160"C and at these temperatures there is a sufficient precipitation of the Mg5Si6 phase to obtain a high mechanical strength of the alloy. A lower end temperature of the first stage will generally lead to an increased total ageing time. Preferably the total ageing time is at most 12 hours. In order to have an extruded product with almost all the Mg and Si in solid solution before the ageing operation, it is important to control the parameters during extrusion and cooling after extrusion. With the right parameters this can be obtained by normal preheating. However, by using a so-called overheating process described in EP 0302623, which is a preheating operation where the alloy is heated to a temperature between 510 and 560°C during the preheating operation before extrusion, after which the billets are cooled to normal extrusion temperatures, this will ensure that all the Mg and Si added to the alloy are dissolved. By proper cooling of the extruded product the Mg and Si are maintained solved and available for forming hardening precipitates during the ageing operation. For low alloy compositions the solutionising of Mg and Si can be obtained during the extrusion operation without overheating if the extrusion parameters are correct. However, with higher alloy compositions normal preheating conditions are not always enough to get all Mg and Si into solid solution. In such cases overheating will make the extrusion process more robust and always ensure that the all the Mg and Si are in solid solution when the profile comes out of the press. Other characteristics and advantages will be clear from the following description of a number of tests done with alloys according to the invention. Example 1 Eight different alloys with the composition given in Table 1 were cast as 095 mm billets with standard casting conditions for 6060 alloys. The billets were homogenised with a heating rate of approximately 250°C / hour, the holding period was 2 hours and 15 minutes at 575°C, and the cooling rate after homogenisation was approximately 350°C / hour. The logs were finally cut into 200 mm long billets. Table 1 Alloy Si Mg Fe Total Si+Mg 1 0,34 0,40 0,20 0.74 2 0,37 0,36 0,19 0.73 3 0,43 0,31 0.19 0,74 4 0,48 0.25 0,20 0,73 5 0,37 0,50 0,18 0,87 6 0,41 0,47 0,19 0,88 7 0.47 0,41 0,20 0,88 8 0,51 0.36 0,19 0,87 The extrusion trial was performed in an 800 ton press equipped with a 0100 mm container, and an induction furnace to heat the billets before extrusion. The die used for the extrudability experiments produced a cylindrical rod with a diameter of 7 mm with two ribs of 0.5 mm width and 1 mm height, located 180° apart. In order to get good measurements of the mechanical properties of the profiles, a separate trial was run with a die which gave a 2 * 25 mm2 bar. The billets were preheated to approximately 500°C before extrusion. After extrusion the profiles were cooled in still air giving a cooling time of approximately 2 min down to temperatures below 250°C. After extrusion the profiles were stretched 0.5 %. The storage time at room temperature were controlled before ageing. Mechanical properties were obtained by means of tensile testing. The complete results of the extrudability tests for these alloys are shown in table 2 and 3. Table 2 Extrusion tests for alloys 1-4 Alloy no. Ram Speed mm/sec. Billet Temperature °C Remarks 16 502 OK 17 503 OK 18 502 Tearing 17 499 OK 19 475 OK 20 473 OK 21 470 Tearing 2 16 504 OK 2 17 503 Small Tearing 2 18 500 Tearing 2 20 474 OK 2 19 473 OK 2 18 470 OK 2 21 469 Small Tearing 3 17 503 Tearing 3 16 505 OK 3 15 504 OK 3 19 All OK 3 18 ATI OK 3 20 All OK 3 21 470 Tearing 4 17 504 OK 4 4 18 16 505 502 Tearing OK 4 19 477 OK 4 20 478 OK 4 4 20 21 480 474 Small Tearing Tearing For alloys 1-4, which have approximately the same sum of Mg and Si but different Mg/Si ratios, the maximum extrusion speed before tearing is approximately the same at comparable billet temperatures. Table 3 Extrusion tests for alloys 5-8 Alloy no. Ram Speed Billet Temperature Remarks mm/sec. °C 5 14 495 OK 5 14,5 500 Tearing 5 15 500 Tearing 5 14 500 Small Tearing 5 17 476 Tearing 5 16,5 475 OK 5 16,8 476 Small Tearing 5 17 475 Tearing 6 14 501 Small Tearing 6 13,5 503 OK 6 14 505 Tearing 6 14,5 500 Tearing 6 17 473 Tearing 6 16,8 473 Tearing 6 16,5 473 OK 6 16,3 473 OK 7 14 504 Tearing 7 13,5 506 Small Tearing 7 13,5 500 OK 7 13,8 503 Small Tearing 7 17 472 Small Tearing 7 16,8 476 Tearing 7 16,6 473 OK 7 17 475 Tearing 8 13,5 505 OK 8 13,8 505 Tearing 8 13,6 504 OK 8 14 505 Tearing 8 17 473 Small Tearing 8 17,2 474 Small Tearing 8 17,5 471 Tearing 8 16,8 473 OK For alloys 5-8, which have approximately the same sum of Mg and Si but different Mg/Si ratios, the maximum extrusion speed before tearing is approximately the same at comparable billet temperatures. However, by comparing alloys 1-4 which have a lower sum of Mg and Si with alloys 5-8, the maximum extrusion speed is generally higher for alloys 1-4. The mechanical properties of the different alloy aged at different ageing cycles are shown in tables 4-11. As an explanation to these tables, reference is made to Fig. 1 in which different ageing cycles are shown graphically and identified by a letter. In Fig. 1 there is shown the total ageing time on the x-axis, and the temperature used is along the y-axis. Furthermore the different columns have the following meaning : Total time = Total ageing time for the ageing cycle. Rm = ultimate tensile strength ; RPO2 = yield strength ; AB = elongation to fracture ; Au = uniform elongation . All these data has been obtained by means of standard tensile testing and the numbers shown are the average of two parallel samples of the extruded profile. Alloy 1 - 0.40Mg + 0.34Si Time[hrs] Rm Rp02 AB Au 3 143,6 74.0 16,8 8,1 4 160,6 122,3 12.9 6,9 5 170,0 137,2 12,6 5.6 6 178,1 144.5 12.3 5,6 7 180,3 150,3 12,3 5,2 3,5 166,8 125,6 12,9 6,6 4 173,9 135,7 11,9 6,1 4,5 181,1 146.7 12,0 5,4 5 188,3 160,8 12,2 5,1 6 196.0 170,3 11,9 4,7 4 156,9 113,8 12,6 7,5 5 171.9 134,7 13.2 6,9 6 189.4 154,9 12.0 6,2 7 195,0 168,6 11,9 5.8 8 199,2 172,4 12.3 5,4 7 8,5 10 11,5 13 8 10 12 14 16 185,1 196,5 201.8 206,4 211,7 190,5 200,3 207.1 211,2 213.9 140,8 159.0 171,6 177,5 184.0 152,9 168,3 176,7 185,3 188,8 12.9 13.0 13.3 12,9 12,5 12,8 12,1 12,3 12,4 12,3 6,4 6,2 6.0 6.1 5.4 6,5 6,0 6,0 5.9 6,6 Table 5 Alloy2-( 136Mg + 0.37Si Total Time fhrsl Rm Rp02 AB AM A 3 150.1 105.7 13,4 7,5 A 4 164,4 126,1 13,6 6.6 A 5 174,5 139,2 12,9 6,1 A 6 183,1 154,4 12,4 4,9 A 7 185,4 157,8 12,0 5,4 B 3,5 175,0 135,0 12,3 6,3 B 4 181,7 146,6 12,1 6,0 B 4.5 190,7 158,9 11,7 5,5 B 5 195,5 169,9 12,5 5.2 B 6 202,0 175.7 12,3 5,4 C 4 161,3 114,1 14,0 7,2 C 5 185,7 145,9 12,1 '6.1 c 6 197,4 167,6 11,6 5,9 c 7 203,9 176,0 12,6 6,0 c 8 205,3 178,9 12,0 5,5 D 7 195,1 151,2 12,6 6,6 D 8,5 208,9 180,4 12,5 5,9 D 10 210,4 181,1 12,8 6,3 D 11,5 215,2 187,4 13,7 6.1 D 13 219,4 189,3 12,4 5,8 E 8 195,6 158,0 12,9 6.7 E 10 205,9 176,2 13,1 6,0 E 12 214,8 185,3 12,1 5,8 E 14 216,9 192,5 12,3 5,4 E 16 221,5 196,9 12,1 5,4 Table 6 1 Alloy 3 - ( Total Time Thrs] Rm A 3 154,3 A 4 172,6 A 5 180.6 A 6 189,7 A 7 192,5 B 3,5 187,4 B 4 193,0 B 4,5 197,7 B 5 203,2 B 6 205,1 C 4 170,1 C 5 193,3 C 6 207,3 c 7 212,2 c 8 212,0 D 7 205,6 D 8,5 218,7 D 10 219,6 D 11.5 222,5 D 13 226,0 E 8 216,6 E 10 217,2 E 12 221,6 m 14 225,7 E 16 224,4 Rp02 AB 111,0 15,0 138,0 13,0 148,9 13,0 160,0 12,2 164,7 12,6 148,9 12,3 160,3 11,5 168,3 11,6 177,1 12.4 180,6 11,7 127,4 14,3 158,2 13,4 179,2 12,6 185,3 12,9 188,7 12,3 157,5 13,2 190,4 12,7 191,1 12,9 197,5 13,1 195,7 12,2 183,5 12,6 190,4 12,6 193,9 12,4 200,6 12,4 197,8 12,1 Au 8,2 6,5 5,7 5,5 5,3 6,3 5,9 5,1 5,5 5,4 7,5 6,2 6.4 5,7 5,6 6,7 6,0 6,7 5,9 6,1 6,8 6,9 6,6 6.0 5.9 Table 7 Alloy 4 - 0 .25Mg + 0.48Si Total Time Fhrsl Rm Rp02 AB Au A 3 140,2 98,3 14,5 8,6 A 4 152,8 114,6 14,5 7.2 A 5 166,2 134,9 12,7 5,9 A 6 173,5 141,7 12,8 5,7 A 7 178,1 147,6 12,3 5,2 B 3,5 165,1 123,5 13,3 6,4 B 4 172,2 136,4 11,8 5,7 B 4,5 180,7 150,2 12,1 5,2 B 5 187,2 159,5 12,0 5,6 B 6 192,8 164,6 12,1 5,0 C 4 153,9 108,6 13,6 7,7 C 5 177,2 141,8 12,0 6,5 C 6 190,2 159,7 11,9 5,9 C 7 197,3 168,6 12,3 6,1 8 197,9 170,6 12,5 5,6 D 7 189,5 145,6 12,3 6.4 D 8,5 202,2 171,6 12,6 6.1 D 10 207,9 178,8 12,9 6,0 D 11,5 210,7 180,9 12.7 5.6 D 13 213,3 177,7 12,4 6,0 E 8 195,1 161,5 12,8 5,9 E 10 205,2 174,1 12,5 6,4 E 12 208,3 177,3 12.8 5,6 E 14 211,6 185,9 12.5 6,3 E 16 217,6 190.0 12,4 6,2 Table 8 Alloy 5 - D.SOMg + 0.37Si Total Time [hrsl Rm Rp02 AB Au A 3 180,6 138,8 13,9 7,1 A 4 194,2 155,9 13,2 6,6 A 5 203,3 176,5 12.8 5,6 A 6 210,0 183,6 12,2 5,7 A 7 211,7 185,9 12,1 5,8 B 3.5 202,4 161.7 12,8 6,6 B 4 204,2 170,4 12.5 6,1 B 4,5 217,4 186,7 12,1 5,6 B 5 218,9 191,5 12,1 5,5 B 6 222,4 198,2 12,3 6,0 C 4 188,6 136,4 15,1 10,0 C 5 206,2 171,2 13,4 7,1 c 6 219,2 191,2 12,9 6,2 c 7 221,4 194,4 12,1 6,1 c 8 224,4 202,8 11,8 6,0 D 7 213,2 161,5 14,0 7,5 D 8.5 221,5 186,1 12,6 6,7 D 10 229,9 200,8 12,1 5,7 D 11,5 228,2 200,0 12,3 6,3 D 13 233,2 198,1 11,4 6,2 E 8 221,3 187,7 13,5 7,4 E 10 226,8 196,7 12,6 6,7 E 12 227,8 195,9 12,8 6,6 E 14 230,6 200,5 12,2 5,6 E 16 235,7 207,9 11,7 6,4 Alloy6-0.47Mg + 0.41Si Time[hrsl Rm Rp02 AB Au 3 189,1 144,5 13,7 7,5 4 205,6 170,5 13,2 6,6 5 212,0 182,4 13,0 5,8 6 216,0 187,0 12,3 5,6 7 216,4 188.8 11.9 5,5 3,5 4 4,5 5 6 4 5 6 7 8 7 8,5 10 11,5 13 8 10 12 14 16 208,2 213,0 219,6 225,5 225,8 195,3 214,1 227,3 229,4 228,2 222,9 230,7 236,6 236,7 239,6 229,4 233,5 237,0 236,0 240,3 172,3 175,5 190,5 199,4 202,2 148,7 178,6 198,7 203,7 200,7 185,0 194,0 205,7 208,0 207,1 196,8 199,5 206,9 206.5 214,4 12,8 12,1 12,0 11,9 11,9 14,1 13,8 13,2 12,3 12,1 12,6 13,0 13,0 12,4 11,5 12,7 13,0 12,3 12,0 12,4 6,7 6,3 6,0 5,6 5,8 8.1 6.8 6,3 6,6 6,1 7,8 6,8 6,6 6,6 5.7 6.4 7,1 6,7 6,2 6,8 Table 1Q Alloy 7 - 0.41 Mg + 0.47Si Total Time Jhrsl Rm Rp02 AB Au A 3 195,9 155,9 13,5 6,6 A 4 208,9 170,0 13,3 6,4 A 5 216,2 188,6 12,5 6,2 A 6 220,4 195,1 12,5 5,5 A 7 222,0 196,1 11,5 5,4 B 3,5 216,0 179,5 12,2 6,4 B 4 219,1 184.4 12,2 6,1 B 4,5 228,0 200,0 11,9 5,8 B 5 230,2 205,9 11.4 6.1 B 6 231,1 211,1 11,8 5,5 4 205,5 157,7 15,0 T,S C 5 225,2 190,8 13,1 6,8 C 6 230,4 203,3 12,0 6,5 7 234,5 208,9 12,1 6,2 8 235,4 213,4 11,8 5,9 D 7 231,1 190,6 13,6 7,6 D 8,5 240,3 208,7 11,4 6,3 D 10 241,6 212,0 12,5 7,3 D 11,5 244,3 218,2 11,9 6,3 D 13 246,3 204,2 11,3 6,3 E 8 233,5 197,2 12,9 7,6 1 E 10 241,1 205,8 12,8 7,2 E 12 244,6 214,7 11,9 6.5 E 14 246,7 220,2 11,8 6,3 E 16 247,5 221,6 11,2 5,8 Table 11 Alloy8-0.36Mg + 0.51Si Total Time fhrsl Rm Rp02 AB Au A 3 200,1 161,8 13,0 7,0 A 4 212,5 178,5 12,6 6,2 A 5 221,9 195,6 12,6 5,7 A 6 222,5 195,7 12,0 6,0 A 7 224,6 196,0 12,4 5,9 222,2 186,9 224,5 188,8 230,9 203,4 231,1 211,7 232,3 208,8 215,3 168,5 228,9 194,9 234,1 206,4 239,4 213,3 239,1 212,5 236,7 195,9 244,4 209,6 247,1 220,4 246,8 217,8 249,4 223,7 243,0 207,7 244,8 215,3 247,6 219,6 249,3 222,5 250,1 220,8 B 3,5 222,2 186,9 12,6 6,6 B 4 224,5 188,8 12,1 6,1 B 4,5 230,9 203,4 12,2 6,6 B 5 231,1 211,7 11,9 6,6 B 6 232,3 208,8 11,4 5,6 C 4 215,3 168,5 14,5 8,3 C 5 228,9 194,9 13,6 7,5 C 6 234,1 206,4 12.6 7,1 C 7 239,4 213,3 11,9 6,4 C 8 239,1 212,5 11,9 5,9 D 7 236,7 195,9 13,1 7,9 D 8.5 244,4 209,6 12,2 7,0 D 10 247.1 220,4 11,8 6,7 D 11,5 246,8 217,8 12,1 7,2 D 13 249,4 223,7 11,4 6,6 E 8 243,0 207,7 12,8 7,6 E 10 244,8 215,3 12,4 7.4 E 12 247,6 219,6 12.0 6,9 E 14 249,3 222,5 12,5 7,1 E 16 250,1 220,8 11.5 7,0 Based upon these results the following comments apply. The ultimate tensile strength (UTS) of alloy no. 1 is slightly below 180 MPa after ageing with the A - cycle and 6 hours total time. With the dual rate ageing cycles the UTS values are higher, but still not more than 190 MPA after a 5 hours B - cycle, and 195 MPa after a 7 hours C - cycle. With the D - cycle the UTS values reaches 210 MPa but not before a total ageing time of 13 hours. The ultimate tensile strength (UTS) of alloy no. 2 is slightly above 180 MPa after the A - cycle and 6 hours total time. The UTS values are 195 MPa after a 5 hours B - cycle, and 205 MPa after a 7 hours C - cycle. With the D - cycle the UTS values reaches approximately 210 MPa after 9 hours and 215 MPa after 12 hours. Alloy no. 3 which is closest to the Mg5Si6 line on the Mg rich side, shows the highest mechanical properties of alloys 1 - 4. After the A - cycle the UTS is 190 MPa after 6 hours total time. With a 5 hours B - cycle the UTS is close to 205 MPa, and slightly above 210 MPa after a 7 hours C - cycle. With the D - ageing cycle of 9 hours the UTS is close to 220 MPa. Alloy no. 4 shows lower mechanical properties than alloys 2 and 3. After the A - cycle with 6 hours total time the UTS is not more than 175 MPa. With the D - ageing cycle of 10 hours the UTS is close to 210 MPa. These results clearly demonstrate that the optimum composition for obtaining the best mechanical properties with the lowest sum of Mg and Si, is close to the Mg5Si6 line on the Mg rich side. Another important aspect with the Mg / Si ratio is that a low ratio seem to give shorter ageing times to obtain the maximum strength. Alloys 5 - 8 have a constant sum of Mg and Si that is higher than for alloys 1-4. As compared to the Mg5Si6 line, all alloys 5 - 8 are located on the Mg rich side of MgsSi6>. Alloy no. 5 which is farthest away from the Mg5Si6 line shows the lowest mechanical properties of four different alloys 5 - 8. With the A - cycle alloy no. 5 has a UTS value of approximately 210 MPa after 6 hours total time. Alloy no. 8 has an UTS value of 220 MPa after the same cycle. With the C - cycle of 7 hours total time the UTS values for alloys 5 and 8 are 220 and 240 MPa, respectively. With the D - cycle of 9 hours the UTS values are approximately 225 and 245 MPa. Again, this shows that the highest mechanical properties are obtained with alloys closest to the Mg5Si6 line. As for alloys 1 - 4, the benefits of the dual rate ageing cycles seem to be highest for alloys closest to the Mg5Si6 line. The ageing times to maximum strength seem to be shorter for alloys 5 - 8 than for alloys 1 - 4. This is as expected because the ageing times are reduced with increased alloy content. Also, for alloys 5 - 8 the ageing times seem to be somewhat shorter for alloy 8 than for alloy 5. The total elongation values seem to be almost independent of the ageing cycle. At peak strength the total elongation values, AB, are around 12%, even though the strength values are higher for the dual rate ageing cycles. Example 2 Example 2 shows the ultimate tensile strength of profiles from directly and overheated billets of a 6061 alloy. The directly heated billets were heated to the temperature shown in the table and extruded at extrusion speeds below the maximum speed before deterioration of the profile surface. The overheated billets were preheated in a gas fired furnace to a temperature above the solvus temperature for the alloy and then cooled down to a normal extrusion temperature shown in table 12. After extrusion the profiles were water cooled and aged by a standard ageing cycle to peak strength. Table 12 Ultimate tensile strength (UTS) in different positions of profiles from directly heated and overheated billets of a AA6061 alloy . Preheating Billet temperature °C UTS (front) MPa UTS (middle) MPa UTS (rear) MPa Dir. Heated 470 287,7 292,6 293.3 Dir. Heated 472 295.3 293.9 296,0 Dir. Heated 471 300.8 309,1 301.5 Dir. Heated 470 310.5 318,1 315.3 Dir. Heated 482 324.3 312.6 313.3 Dir. Heated 476 327,1 334,0 331.9 Dir. Heated 476 325,7 325.0 319,5 Dir. Heated 475 320.2 319,0 318,8 Dir. Heated 476 316.0 306,4 316,0 Dir. Heated 485 329.1 329,8 317.4 Dir. Heated 501 334.7 324,3 331,2 Dir: Heated 499 332.6 327,8 322,9 Dir. Heated 500 327,8 329,8 318,8 Dir. Heated 505 322.9 322,2 318.1 Dir. Heated 502 325,7 329,1 334,7 Dir. Heated 506 336,0 323.6 311,2 Dir. Heated 500 329.1 293,9 345.0 Dir. Heated 502 331,2 332,6 335,3 Dir. Heated 496 318,8 347,8 294,6 Average UTS and standard deviation for directly heated billets 320,8 /13,1 319,6/14,5 317,6/13,9 Overheated 506 333.3 325,7 331,3 Overheated 495 334,0 331,9 335,3 Overheated 493 343,6 345,0 333.3 Overheated 495 343.6 338.8 333,3 Overheated 490 339.5 332,6 327.1 Overheated 499 346,4 332,6 331,2 Overheated 496 332.6 335,3 331.9 Overheated 495 330,5 331,2 322.9 Overheated 493 332,6 334,7 333,3 Overheated 494 331,2 334,0 328,4 Overheated 494 329.1 338.8 337,4 Overheated 459 345.7 337,4 344.3 Overheated 467 340.2 338,1 330,5 Overheated 462 344,3 342,9 331,9 Overheated 459 334.0 329.8 326,4 Overheated 461 331,9 326.4 324,3 Average UTS and standard deviation for overheated billets 337 / 5,9 334,7 / 5,2 331,4/5,0 By utilising the overheating process the mechanical properties will generally be higher and also more consistent than without overheating. Also, with overheating the mechanical properties are practically independent of the billet temperature prior to extrusion. This makes the extrusion process more robust with respect to providing high and consistent mechanical properties, making it possible to operate at lower alloy compositions with lower safety margins down to the requirements for mechanical properties. We claim: 1. Process of treating an aluminium alloy consisting of 0.5 -2.5 % by weight of an alloying mixture of magnesium and silicon, the molar ratio of Mg/Si lying between 0.70 and 1.25, an additional amount of Si equal to 1/3 of the amount of Fe, Mn and Cr in the alloy, as expressed by % by weight, other alloying elements and unavoidable impurities, and the remaining being made up of aluminium, wherein said alloy after cooling has been submitted to homogenising, preheating before extrusion and ageing, wherein said ageing takes place after extrusion as a dual step ageing operation to a final hold temperature between 160°C and 220°C, characterized in that the ageing includes a first stage wherein the extrusion is heated with a heating rate above 100°C/hour to a temperature between 100 to 170°C, a second stage in which the extrusion is heated with a heating rate between 5 and 50°C/hour to the final hold temperature, and in that the total ageing cycle is performed in a time between 3 and 24 hours. 2. A process as claimed in claim 1, wherein the molar ratio of Mg/Si is at least 0.75. 3. A process as claimed in anyone of the preceding claims, wherein the molar ratio of Mg/Si is at most 1.25. V" 4. A process as claimed in anyone of the preceding claims, wherein the final ageing temperature is at least 165°C. 5. A process as claimed in anyone of the preceding claims, wherein the final ageing temperature is at most 205°C. 6. A process as claimed in anyone of the preceding claims, wherein in the said second heating stage the heating rate is at least 7°C/hour. 7. A process as claimed in anyone of the preceding claims, wherein the said second heating stage the heating rate is at most 30°C/hour. 8. A process as claimed in anyone of the preceding claims, wherein at the end of the said first heating step the temperature is between 130 and 160°C. 9. A process as claimed in anyone of the preceding claims, wherein the total ageing time is at most 12 hours. 10. A process as claimed in anyone of the preceding claims, wherein during the preheating before ex-trusion the alloy has been heated to a temperature between 510°C to 560°C, after which the alloy is cooled to normal extrusion temperatures. Dated this 6th Day of August, 2001. (RANJNA MEHTA-DUTT) OF REMFRY & SAGAR ATTORNEYS FOR THE APPLICANTS

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