Patent Grant
7025839
This invention relates to the heat treatment of aluminium alloys, that are able to be strengthened by the well known phenomenon of age (or precipitation) hardening.
Heat treatment for strengthening by age hardening is applicable to alloys in which the solid solubility of at least one alloying element decreases with decreasing temperature. Relevant aluminium alloys include some series of wrought alloys, principally those of the 2XXX, 6XXX and 7XXX (or 2000, 6000 and 7000) series of the International Alloy Designation System (IADS). However, there are some relevant age-hardenable aluminium alloys which are outside these series. Also, some castable aluminium alloys are age hardenable. The present invention extends to all such aluminium alloys, including both wrought and castable alloys, and also can be used with alloy products produced by processes such as powder metallurgy and with rapidly solidified products, as well as with particulate reinforced alloy products and materials.
Processes for heat treatment of age-hardenable aluminium alloys normally involve the following three stages:
Ageing conditions differ for different alloy systems. Two common treatments which involve only one stage are to hold for an extended time at room temperature (T4 temper) or, more commonly, at an elevated temperature for a shorter time (for example 8 hours) which corresponds to a maximum in the hardening process (T6 temper). For certain alloys, it is usual to hold for a prescribed period of time (for example 24 hours) at room temperature before applying the T6 temper at an elevated temperature. In other alloys, notably those based on the Al—Cu and Al—Cu—Mg systems (of the 2000 series), deformation (for example by stretching or rolling 5%) after quenching and before ageing at an elevated temperature, causes an increased response to strengthening. This is known as a T8 temper and it results in a finer and more uniform dispersion of precipitates throughout the grains.
For alloys based on the Al—Zn—Mg—Cu system (of the 7000 series) several special ageing treatments have been developed which involve holding for periods of time at two different elevated temperatures. The purpose of each of these treatments is to reduce the susceptibility of alloys of this series to the phenomenon of stress corrosion cracking. One example is the T73 temper which involves ageing first at a temperature close to 100° C. and then at a higher temperature, e.g. 160° C. This treatment causes some reduction in strength when compared to a T6 temper. Another example is the treatment known as retrogression and re-ageing (RRA) which involves three stages, for example 24 hours at 120° C., a much shorter time at a higher temperature (200–280° C.) and a further 24 hours at 120° C. Some such treatments tend to remain confidential to companies that supply the alloys.
It is generally accepted that, once an aluminium alloy (or other suitable material) is hardened by ageing at an elevated temperature, the mechanical properties remain stable when the alloy is exposed for an indefinite time at a significantly lower temperature. However, recent results have shown that this is not always the case. A magnesium alloy, WES4, which is normally aged at 250° C. to achieve its T6 temper, has shown a gradual increase in hardness together with an unacceptable decrease in ductility if subsequently exposed for long periods at a temperature close to 150° C. This effect is attributed to slow, secondary precipitation of a finely dispersed phase throughout the grains of the alloy. More recently certain lithium-containing aluminium alloys, such as 2090 (Al-2.7 Cu-2.2 Li), have shown similar behaviour if exposed for long times at temperatures in the range 60 to 135° C., after being first aged to the T6 temper at 170° C.
The present invention is directed to providing a process for the heat treatment of an age-hardenable aluminium alloy which has alloying elements in solid solution, wherein the process includes the stages of:
This series of treatment stages in accordance with the present invention is termed T6I6, indicating the first ageing treatment before the stage (c) interrupt (I) and the treatment after the interrupt.
Stages (c) and (d) may be successive stages. In that case, there may be little or no applied heating in stage (c). However, it should be noted that stages (c) and (d) may be effectively combined through the use of appropriately controlled heating cycles. That is, stage (c) may utilise a heating rate, to the final ageing temperature Tc, which is sufficiently slow to provide the secondary nucleation or precipitation at relatively lower average temperature than the final ageing temperature Tc.
We have found that, with the heat treatment of the present invention, substantially all aluminium alloys capable of age hardening can undergo additional age hardening and strengthening to higher levels than are possible with a normal T6 temper. Maximum hardness can be increased such as by 10 to 15%, while yield strength (i.e. 0.2% proof stress) and tensile strength can be increased such as by 5 to 10% or, with at least some alloys, even higher, relative to levels obtainable with conventional T6 heat treatments. Moreover, at least in many cases and contrary to usual behaviour after conventional treatments, the increases obtainable with the present invention are able to be achieved without any significant decrease in ductility as measured by elongation occurring on testing alloys to failure.
As indicated, the process of the present invention enables alloys to undergo additional age hardening and strengthening to higher levels relative to the age hardening and strength obtainable for the same alloy subjected to a normal T6 temper. The enhancement can be in conjunction with mechanical deformation of the alloy before stage (a); after stage (b) but before stage (c); and/or during stage (c). The deformation may be by application of thermomechanical deformation; while deformation may be applied in conjunction to rapid cooling. The alloy may be aged in stage (a) directly after fabrication or casting with no solution treatment stage.
The process of the present invention is applicable not only to the standard T6 temper but also applicable to other tempers. These include such instances as the T5 temper, where the alloy is aged directly after fabrication with no solution treatment step and a partial solution of alloying elements is formed. Other tempers, such as the T8 temper, include a cold working stage. In the T8 temper the material is cold worked before artificial ageing, which results in an improvement of the mechanical properties in many aluminium alloys through a finer distribution of precipitates nucleated on dislocations imparted through the cold working step. The equivalent new temper is thus designated T8I6, following the same convention in nomenclature as the T6I6 temper. Another treatment involving a cold working step, again following the process of the present invention, is designated T9I6. In this case the cold working step is introduced after the first ageing period, TA and before the interrupt treatment at temperature TB. After the interrupt treatment is completed, the material is again heated to the temperature TC, again following the convention of the T6I6 treatment.
Similar parallels exist with temper designations termed T7X, as exemplified previously, where a decreasing integer of X refers to a greater degree of overageing. These treatments consist of a two step process where two ageing temperatures are used, the first being relatively low (e.g. 100° C.) and the second at a higher temperature of, for example, 160° C.–170° C. In applying the new treatment to such tempers, the final ageing temperature TC is thus in the range of the usual second higher temperatures of 160° C.–170° C., with all other parts of the treatment being equivalent to the T6I6 treatment. Such a temper is thus termed T8I7X when employing the new nomenclature
It should also be noted that the new treatment can be similarly applied to a wide variety of existing tempers employing significantly differing thermomechanical processing steps, and is in no way restricted to those listed above.
The process of the invention has proved to be effective in each of the classes of aluminium alloys that are known to respond to age hardening. These include the 2000 and 7000 series mentioned above, the 6000 series (Al—Mg—Si), age hardenable casting alloys, as well as particulate reinforced alloys. The alloys also include newer lithium-containing alloys such as 2090 mentioned above and 8090 (Al-24 Li-1.3 Cu-0.9 Mg), as well as silver-containing alloys, such as, 2094, 7009 and experimental Al—Cu—Mg—Ag alloys.
The process of the invention can be applied to alloys which, as received, have been subjected to an appropriate solution treatment stage followed by a quenching stage to retain solute elements in supersaturated solid solution. Alternatively, these can form preliminary stages of the process of the invention which precede stage (a). In the latter case, the preliminary quenching stage can be to any suitable temperature ranging from TA down to ambient temperature or lower. Thus, in a preliminary quenching stage to attain the temperature TA, the need for reheating to enable stage (a) can be avoided.
The purpose of the solution treatment, whether of the alloy as received or as a preliminary stage of the process of the invention, is of course to take alloying elements into solid solution and thereby enable age hardening. However, the alloying elements can be taken into solution by other treatments and such other treatments can be used instead of a solution treatment.
As will be appreciated, the temperatures TA, TB and TC for a given alloy are capable of variation, as the stages to which they relate are time dependent. Thus, TA for example can vary with inverse variation of the time for stage (a). Correspondingly, for any given alloy, the temperatures TA, TB and TC can vary over a suitable range during the course of the respective stage. Indeed, variation in TB during stage (c) is implicit in the reference above to stages (c) and (d) being effectively combined.
The temperature TA used in stage (a) for a given alloy can be the same as, or close to, that used in the ageing stage of a conventional T6 heat treatment for that alloy However, the relatively short time used in stage (a) is significantly less than that used in conventional ageing. The time for stage (a) may be such as to achieve a level of ageing needed to achieve from about 50% to about 95% of maximum strengthening obtainable by full conventional T6 ageing Preferably, the time for stage (a) is such as to achieve from about 85% to about 95% of that maximum strength.
For many aluminium alloys, the temperature TA most preferably is that used when ageing for any typical T6 temper. The relatively short time for stage (a) may be, for example, from several minutes to, for example, 8 hours or more, such as from 1 to 2 hours, depending on the alloy and the temperature TA Under such conditions, an alloy subjected to stage (a) of the present invention would be said to be underaged.
The cooling of stage (b) preferably is by quenching. The quenching medium may be cold water or other suitable media. The quenching can be to ambient temperature or lower, such as to about −10° C. However, as indicated, the cooling of stage (b) is to arrest the ageing which results directly from stage (a); that is, to arrest primary precipitation of solute elements giving rise to that ageing.
The temperatures TB and TC and the respective period of time for each of stages (c) and (d) are inter-related with each other. They also are inter-related with the temperature TA and the period of time for stage (a); that is, with the level of underageing achieved in stage (a). These parameters also vary from alloy to alloy. For many of the alloys, the temperature TB can be in the range of from about −10° C. to about 90° C., such as from about 20° C. to about 90° C. However for at least some alloys, a temperature TB in excess of 90° C., such as to about 120° C., can be appropriate.
The period of time for stage (c) at temperature TB is to achieve secondary nucleation or continuing precipitation of solute elements of the alloy. For a selected level of TB, the time is to be sufficient to achieve additional sufficient strengthening. The additional strengthening, while still leaving the alloy significantly underaged, usually results in a worthwhile level of improvement in hardness and strength. The improvement can, in some instances, be such as to bring the alloy to a level of hardness and/or strength comparable to that obtainable for the same alloy by that alloy being fully aged by a conventional T6 heat treatment. Thus if, for example, the underaged alloy resulting from stage (a) has a hardness and/or strength value which is 80% of the value obtainable for the same alloy fully aged by a conventional T6 heat treatment, heating the alloy at TB for a sufficient period of time may increase that 80% value to 90%, or possibly even more.
The period of time for stage (c) may, for example, range from less than 8 hours at the lower end, up to about 500 hours or more at the upper end. Simple trials can enable determination of an appropriate period of time for a given alloy. However, a useful degree of guidance can be obtained for at least some alloys by determining the level of increase in hardness and/or strength after relatively short intervals, such as 24 and 48 hours, and establishing a curve of best fit for variation in such property with time. The shape of the curve can, with at least some alloys, give useful guidance of a period of time for stage (c) which is likely to be sufficient to achieve a suitable level of secondary strengthening.
The temperature TC used during stage (d) can be substantially the same as TA. For a few alloys, TC can exceed TA, such as by up to about 20° C. or even up to 50° C. (for example, for T6I7X treatment). However for many alloys it is desirable that TC be at TA or lower than TA, such as 20° C. to 50° C., preferably 30 to 50° C., below TA. Some alloys necessitate TC being lower than TA, in order to avoid a regression in hardness and/or strength values developed during stage (c).
The period of time at temperature TC during stage (d) needs to be sufficient for achieving substantially maximum strength. In the course of stage (d), strength values and also hardness are progressively improved until, assuming avoidance of significant regression, maximum values are obtainable. The progressive improvement occurs substantially by growth of precipitates produced during stage (c). The final strength and hardness values obtainable can be 5 to 10% or higher and 10 to 15% or higher, respectively, than the values obtainable by a conventional T6 heat treatment process. A part of this overall improvement usually results from precipitation achieved during stage (c), although a major part of the improvement results from additional precipitation achieved in stage (d).
In order that the invention may more readily be understood, description now is directed to the accompanying drawings, in which:
The present invention enables the establishment of conditions whereby aluminium alloys which are capable of age hardening may undergo this additional hardening at a lower temperature TB if they are first underaged at a higher temperature TA for a short time and then cooled such as by being quenched to room temperature. This general effect is demonstrated in
In stage (a), the alloy is aged at temperature TA. The temperature TA and the duration of stage (a) are sufficient to achieve a required level of underaged strengthening, as described above. From TA, the alloy is quenched in stage (b) to arrest the primary precipitation ageing in stage (a); with the stage (b) quenching being to or below ambient temperature. Following the quenching stage (b), the alloy is heated to temperature TB in stage (c), with the temperature at TB and the duration of stage (c) sufficient to achieve secondary nucleation, or continuing precipitation of solute elements After stage (c), the alloy is further heated in stage (d) to temperature TC, with the temperature TC and the duration of step (d) sufficient to achieve ageing of the alloy to achieve the desired properties. The temperatures and durations may be as described early herein.
In relation to the schematic representation shown in
(a) aged for only 2.5 hours at 150° C.;
(b) quenched into quenchant;
(c) held at 65° C. for 500 hours;
(d) re-aged at 150° C.
The peak hardness is now achieved in the shorter time of 40 hours and has been increased to 144 VHN.
As indicated, the solid line in
Examples of the increases in hardness, in response to age hardening by applying the T6I6 treatment in accordance with the invention are shown in Table 1 for a range of alloys, as well as selected examples of variants of the standard treatments. Typical tensile properties developed in response to T6I6 age hardening according to the invention are shown in Table 2. In each of Tables 1 and 2, the corresponding T6 values for each alloy are presented. In most cases, it will be seen from Table 2 that the ductility as measured by the percent elongation after failure is either little changed or increased, although this is alloy dependent. It also is to be noted that there is no detrimental effect to either fracture toughness or fatigue strength with the T6I6 treatment.
The strain to failure in the comparison of Table 2 for casting alloy 357 appears to be inconsistent with other data presented. However it should be noted that the test batch from which these samples were taken typically display levels between 1 and 8% strain, with a mean of ˜4.5%. Therefore it should be considered that the values presented for the T6 and T6I6 tempers in alloy 357 are effectively equivalent.
Table 3 shows typical hardness values associated with T6 peak ageing, and the maximum hardness developed during stage (d) for the T6I6 condition for the various alloys. Table 3 also shows the time of the first ageing temperature during stage (a) and the typical hardness at the end of stage (a). Additionally, Table 3 shows for each alloy the approximate increase in hardness during the entire TB hold of stage (c), as well as the increase in hardness during the TB hold, after 24 and 48 hours and at different TB temperatures.
Table 4 provides an example of fracture toughness comparison values, comparing the T6 and T6I6 tempers of the various alloys.
Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| PQ4853 | Dec 1999 | AU | national |
This is a continuation of application No. PCT/AU00/01601, filed Dec. 21, 2000.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5076859 | Rioja et al. | Dec 1991 | A |
| 5108520 | Liu et al. | Apr 1992 | A |
| 5690758 | Oh et al. | Nov 1997 | A |
| 5858134 | Bechet et al. | Jan 1999 | A |
| 5879481 | Price | Mar 1999 | A |
| RE36692 | Gupta et al. | May 2000 | E |
| Number | Date | Country |
|---|---|---|
| 297839 | May 1984 | TW |
| 297839 | Feb 1997 | TW |
| 8700206 | Jan 1987 | WO |
| 9218658 | Oct 1992 | WO |
| 9524514 | Sep 1995 | WO |
| 9618752 | Jun 1996 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20030041934 A1 | Mar 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/AU00/01601 | Dec 2000 | US |
| Child | 10172365 | US |
