RS49557B - Process for machining aluminum alloy containing magnesium and silicon - Google Patents

Abstract

Process for processing aluminum alloy containing magnesium and silicon, 0.5 - 2.5% by weight of alloying mixture of magnesium and silicon, where the molar ratio of Mg / Si is between 0.70 and 1.25, additional amount of Si is equal to approximately 1/3 of the amount of Fe, Mn and Cr present in alloy in% by weight, the remainder being aluminum, unavoidable impurities and other alloying agents: which, after cooling, is subjected to homogenization, pre-heating, pre-extraction, extraction and aging, before aging at temperatures between 160 and 220 ° C. , which aging after cooling of the extracted product comprises the first step in which the extracted product is heated at a heating rate above 30 ° C / h to a temperature between 100 and 170 ° C and the second step in which the extracted product is heated at a heating rate between 5 and 50 ° C / h to the final holding temperature and the total aging cycle performed between 3 and 24 hours. The application contains 9 more dependent claims.

Description

The invention relates to an aluminum alloy containing 0.5 - 2.5% by weight of an alloying mixture of magnesium and silicon, a molar ratio of Mg/Si between 0.70 and 1.25, the additional amount of Si is equal to approximately 1/3 of the amount of Fe, Mn and Cr present in the alloy, and the rest of the content is aluminum, unavoidable impurities and other alloying agents, which alloy is subjected to homogenization after cooling, preheating before extraction, extraction and aging, where aging takes place at final temperatures between 160 and 220°C.

An alloy of this type is described in WO 95.06759. According to this publication, aging is performed at a temperature between 150 and 200°C, and the heating rate is between 10 and 100°C/h and primarily 10 - 70°C/h. An alternative two-stage heating procedure is suggested, where a temperature maintained in the range of 80 to 140°C is suggested to achieve an overall heating rate within the range indicated above.

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, while this has a negative effect on the drawability of the aluminum alloy. It was previously observed that the phase that increases the hardness of the Al-Mg-Si alloy had a composition close to Mg2Si. It was also known that excess Si gives better mechanical properties.

Later experiments showed that the precipitation sequence is very complex and that apart from the equilibrating phase, the participating phases do not have a stoichiometric ratio of Mg2Si. In the publication S.J. Andersen, et.al, Acta mater, Vol.46 No. 9 p.3283-3298 from 1998, it was proposed that one of the phases for increasing hardness in Al-Mg-Si alloys has a composition close to Mg5Si6.

It is therefore an object of the invention to create an aluminum alloy which has better mechanical properties and better drawability, which alloy has a minimum amount of alloying agents and a general composition which is as close as possible to traditional aluminum alloys. This and other objectives are achieved by aging after cooling the drawn product as a two-speed aging operation that includes the first step in which the drawn product is heated at a heating rate above 30°C/h to a temperature between 100 and 170°C, the second step in which the drawn product is heated at a heating rate between 5 and 50°C/h to a final temperature maintained between 160 and 220°C and the entire aging cycle is performed in a time between 3 and 24 hours. The optimal Mg/Si ratio is the one where all available Mg and Si are transformed into Mg5Si6 phases. This combination of Mg and Si gives the highest mechanical strength with minimal use of alloying elements Mg and Si. It was found that the maximum extraction speed is almost independent of the Mg/Si ratio. Therefore, with an optimal Mg/Si ratio, the sum of Mg and Si is minimized for a given strength requirement, and this alloy will also have the best drawability. By using the composition according to the invention combined with the two-speed aging procedure according to the invention, it is achieved that strength and drawability are maximized with a minimum total aging time.

In addition to the MgsSi6 phase, there is another hardness-enhancing phase that contains more Mg than the MgsSie phase. However, this phase is not as effective and does not contribute as much to the mechanical strength as the MgsSi6 phase. With more Si in the MgsSi6 phase, it will most likely not be a hardenhancing phase, and lower Mg/Si ratios than 5/6 will not be effective.

The positive effect on the mechanical strength of the two-speed aging procedure can be explained by the fact that the extended time at low temperature generally improves the formation of Mg-Si precipitates of higher density. If the entire aging operation is carried out at such a temperature, the total aging time will be beyond practical limits and the performance of the aging furnace will be too low. By slowly increasing the temperature to the final aging temperature, the large number of precipitates initiated at low temperature will continue to grow. The result will be a large number of precipitates and mechanical strength values similar to those obtained by aging at a low temperature, but with a significantly shorter overall aging time.

Two-step aging also gives improvements in mechanical strength, but rapid heating from the first holding temperature to the second holding temperature gives a considerable chance of reversion of the smallest precipitates, with fewer precipitates increasing hardness and hence lower mechanical strength as a result. Another benefit of the two-speed aging procedure compared to normal aging and also two-step aging is that the slow heating rate will ensure a better temperature distribution in the charge. The temperature regime of the drawn products in the filling will be almost independent of the size of the filling, the packing density and the thickness of the walls of the drawn products. The result will be more uniform mechanical properties than with other types of aging procedures.

Compared to the aging procedure described in WO 95.06759 where a slow heating rate starts from room temperature, the two-speed aging procedure will shorten the overall aging time by applying a high heating rate from room temperature to a temperature between 100 and 170°C. The resulting strength will be almost as good when slow heating starts from medium temperature as when slow heating starts from room temperature.

Depending on the intended strength class, various compositions are possible within the general scope of the invention.

Thus, it is possible to have an aluminum alloy with tensile strength in class F19-F22, when the amount of alloying mixture of magnesium and silicon is between 0.60 and 1.10% by weight. For an alloy with tensile strength in class F25 - F27, it is possible to use an aluminum alloy that contains 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 class F29 - F31, it is possible to use an aluminum alloy that contains between 1.10 and 1.80 by weight of an alloying mixture of magnesium and silicon.

Primarily and according to the invention, tensile strength in class F19 (185 - 220 MPa) is achieved with an alloy containing between 0.60 and 0.80% by weight of alloying mixture, tensile strength in class F22 (215 - 250 MPa) with an alloy containing between 0.70 and 0.90% by weight of alloying mixture, tensile strength in class F25 (245 - 270 MPa) with an alloy containing between 0.85 and 1.15 % by weight of alloying mixture, tensile strength in class F27 (265 - 290 MPa) with an alloy containing between 0.95 and 1.25 % by weight of alloying mixture, tensile strength in class F29 (285 - 310 MPa) with an alloy containing between 1.10 and 1.40 % by weight alloying compounds, and tensile strength in class F31 (305 - 330 MPa) with an alloy containing between 1.20 and 1.55% by weight of the alloying mixture.

With the addition of Cu, which as a rule increases the mechanical strength by 10 MPa per 0.10% by weight of Cu, the total amount of Mg and Si can be reduced while still satisfying a higher strength class than Mg and Si additions alone would provide.

For the reasons explained above, it is better for the Mg/Si molar ratio to be between 0.75 and 1.25, and even better between 0.8 and 1.0.

In the primary solution of the invention, the final aging temperature is at least 165°C and at most 205°C. When these recommended temperatures are used it has been found that mechanical strength is maximized while the overall aging time remains within reasonable limits.

In order to reduce the total aging time in a two-speed aging operation, it is better to perform the first aging step at the highest possible heating rate, which as a rule depends on the available equipment. Therefore, in the first heating step, a speed of at least 100°C/h should be used.

In the second heating step, the heating speed must be optimized considering the overall time efficiency and the final quality of the alloy. Therefore, the speed of the second heating should be at least 7°C/h and at most 30°C/h. At heating rates less than 7°C/h the total aging time will be long with low aging furnace performance as a result, and at heating rates greater than 30°C/h the mechanical properties will be less than ideal.

Primarily, the first heating step will be completed at 130 - 160°C and at these temperatures there is sufficient precipitation of the MgsSig phase to achieve high mechanical strength of the alloy. A lower temperature in the first step will lead to an increased overall aging time. The total aging time should be a maximum of 12 hours.

In order to have a drawn product with almost all Mg and Si in solid solution before the aging operation, it is important to control the parameters during drawing and cooling after drawing. With the right parameters this can be achieved with normal preheating. However, using the so-called preheating 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 drawing, after which cooling to the normal drawing temperature is carried out. This will ensure that all the Mg and Si added to the alloy are dissolved. By properly cooling the extracted product, Mg and Si remain dissolved and available to form hardness-enhancing precipitates during the aging operation.

For a low composition alloy, dissolution of Mg and Si can be achieved during the drawing operation without overheating if the drawing parameters are correct. However, the normal preheating conditions of higher composition alloys are not always sufficient to convert all Mg and Si into a solid solution. In such cases, superheating will make the drawing process more robust and will always ensure that all Mg and Si are in solid solution when the profile leaves the press.

Other characteristics and advantages will become clear from the following description of several tests performed on the alloys according to the invention.

Example 1

Eight different alloys with the compositions given in Table 1 were cast into <J>95 mm bars under standard casting conditions for 6060 alloys. The bars were homogenized at a heating rate of approximately 250°C/h, the temperature holding period was 2 hours and 15 minutes at 575°C, and the cooling rate after homogenization was approximately 350°C/h. The bars are finally cut into 200 mm long pieces.

The drawing test was performed on an 800 ton press equipped with a <|>100 mm vessel and an induction furnace for heating the pieces before drawing.

The mold used for the drawability experiment produced a 7 mm diameter cylindrical rod with two ribs 0.5 mm wide and 1 mm high, diametrically placed.

In order to obtain good measures of the mechanical properties of the profile, a special test was performed with a mold that produced a 2<*>25 mm<2> bar. The bars were preheated to approximately 500°C prior to drawing. After extraction, the profiles were cooled in still air for approximately 2 minutes to a temperature below 250°C. After drawing, the profiles were stretched by 0.5%. Storage time at room temperature was controlled before aging. Mechanical properties were obtained using a tensile test.

Complete drawability test results for these alloys are shown in Tables 2 and 3.

For alloys 1-4, which have approximately the same sum of Mg and Si but different Mg/Si ratio, the highest yield rate without cracking is approximately the same at comparable bar temperatures.

For alloys 5-8, which have approximately the same sum of Mg and Si but a different Mg/Si ratio, the highest yield rate without cracking is approximately the same at comparable bar temperatures. However, comparing alloys 1 - 4 which have a lower sum of Mg and Si with alloys 5 - 8, the highest draw rate is higher for alloys 1 - 4.

The mechanical properties of the various alloys aged with different aging cycles are shown in Tables 4 - 11.

As an explanation of these tables, reference is made to Figure 1, where the different aging cycles are shown graphically and identified by letter. In Figure 1, the total aging time is on the x-axis and the temperature is on the y-axis.

Column headings have the following meaning:

Total time = total aging time for the aging cycle

Rm = ultimate tensile stress

Rp02 = fracture stress

AB = elongation at break

Au = proportional elongation

All these data were obtained by standard tensile testing and the numbers shown are the mean values of two parallel samples of drawn profiles.

The following comments are based on these results.

Ultimate tensile stress (U.S.T.) of alloy no. 1 is slightly below 180 MPa after A-cycle aging and 6 hours of total time. With the two-speed aging cycle, the KNI values are higher, but still not above 190 MPa after a 5-hour B-cycle, and 195 MPa after a 7-hour C-cycle. With the D-cycle, the KNI values reach 210 MPa, but not before the total aging time of 13 hours.

Ultimate tensile stress (U.S.T.) of alloy no. 2 is slightly above 180 MPa after A-cycle aging and 6 hours of total time. The KNI value is 195 MPa after B-cycle lasting 5 hours, and 205 MPa after C-cycle lasting 7 hours. With the D-cycle, the KNI value reaches approximately 210 MPa after 9 hours and 215 MPa after 12 hours.

Alloy no. 3', which is closest to the MgsSi6 line on the Mg excess side, shows the highest mechanical properties of alloys 1 - 4. After the A-cycle, the KNI is 190 MPa after a total time of 6 hours. With the B-cycle and after 5 hours, the KNI is close to 205 MPa, and slightly above 210 MPa after 7 hours of the C-cycle. With a D-aging cycle of 9 hours, the KNI is close to 220 MPa.

Alloy no. 4 shows lower mechanical properties than alloys 2 and 3. After A-cycle with 6 hours of total time, KNI is not more than 175 MPa. With a D-aging cycle of 10 hours, the KNI is close to 210 MPa.

These results clearly show that the optimal composition for obtaining the best mechanical properties with the lowest sum of Mg and Si is close to the MgsSiena line on the side of excess Mg.

Another important aspect with the Mg/Si ratio is that a low ratio seems to give a shorter aging time to achieve maximum mechanical properties.

Alloys 5 - 8 have a constant sum of Mg and Si that is higher than in alloys 1 - 4. Compared along the MgsSig line, all alloys 5 6 8 are located on the Mg excess side.

Alloy no. 5 which is farthest from the MgsSie line shows the lowest mechanical properties of the four different alloys 5 - 8. With A-cycle alloy no. 5 has a KNI value of approximately 210 MPa after a total time of 6 hours. Alloy no. 8 has a KNI value of 220 MPa after the same cycle. With a C-cycle and 7 hours of total time, the KNI values for alloys 5 and 8 are 220 and 240 MPa, respectively. With D-cycle and 9 hours KNI values are approximately 225 and 245 MPa.

This again shows that the highest mechanical properties are achieved with alloys closest to the MgsSie line. For alloys 1 - 4, the benefit of the two-rate aging cycle appears to be greatest for the alloys closest to the MgsSi6 line.

Aging times to maximum strength appear to be shorter for alloys 5 - 8 than for alloys 1 - 4. This is expected as aging times decrease with increasing alloy content. Also, for alloys 5 - 8, aging times seem to be slightly shorter for alloy 8 than for alloy 5.

Total elongation values appear to be almost independent of the aging cycle. At the highest strength, the total elongation values, AB, are around 12%, even though the strength values are higher for the two-rate aging cycles.

Example 2

Example 2 shows the ultimate tensile strength of a profile of superheated alloy 6061 bars. Directly heated bars were heated to the temperature shown in the table and drawn at a maximum draw rate before the profile surface was destroyed. The superheated bars are preheated in a gas furnace to a temperature above the solvus temperature for that alloy and then cooled to the normal drawing temperature shown in Table 12. After drawing, the profiles are water cooled and aged in a standard aging cycle to maximum strength. By using an annealing process, the mechanical properties will be generally higher and more consistent than without annealing. Also, with overheating, the mechanical properties practically do not depend on the temperature of the bar before drawing. This makes the drawing process more robust in providing high and consistent mechanical properties, and enables operations at lower alloy compositions with lower safety margins to the required mechanical properties.

Claims (10)

1. Process for processing aluminum alloy containing magnesium and silicon, namely 0.5 - 2.5% by weight of alloying mixture of magnesium and silicon. where the Mg/Si molar ratio is between 0.70 and 1.25, the additional amount of Si is equal to approximately 1/3 of the amount of Fe, Mn and Cr present in the alloy in % by weight, and the rest is aluminum, inevitable impurities and other alloying agents; that the alloy after cooling is subjected to homogenization, preheating before drawing, drawing and aging, wherein the aging takes place at temperatures between 160 and 220°C, characterized in that the aging after cooling of the drawn product comprises a first step in which the drawn product is heated at a heating rate above 30°C/h to a temperature between 100 and 170°C and a second step in which the drawn product is heated at a heating rate between 5 and 50°C/h until the final holding temperature and that the total aging cycle was performed between 3 and 24 hours. 2. The method according to claim 1, characterized in that the Mg/Si molar ratio is at least 0.75. 3. The method according to any of the preceding claims, characterized in that the Mg/Si molar ratio is at most 1.25. 4. The method according to any of the previous requirements, characterized in that the final aging temperature is at least 165°C. 5. The method according to any of the preceding requirements, characterized in that the final aging temperature is at most 205°C. 6. The method according to any of the preceding claims, characterized in that in the second heating step, the heating rate is at least 7°C/h. 7. The method according to any of the previous requirements, characterized in that in the second heating step, the heating speed is at most 30°C/h. 8. The method according to any of the preceding claims, characterized in that at the end of the first heating step the temperature is between 130°C and 160°C. 9. The method according to any of the previous requirements, characterized in that the total aging time is at most 12 hours. 10. The method according to any of the preceding claims, characterized in that during preheating before drawing the alloy is heated to a temperature between 510 and 560°C, after which the alloy is cooled to the normal drawing temperature.