D.S. Thompson, S.A. Levy and G.E. Spangler, Aluminium, 50, 719. (197 4). 20. ... P.L. Metr, E.H. Spuhler, L.W. Mayer and J.E. Grant, Alcoa Green. Letter 206 ...
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NATIONAAL LUCHT- EN RUIMTEVAARTLABORATORIUM NATIONAL AEROSPACE LABORATORY NLR THE
NETHERLANDS
NLR MP 77031 U
THERMOMECHANICAL TREATMENT OF ALUMINIUM ALLOYS
BY
R.J.H. WANHILL
AND
G.F.J.A.
VAN
GESTEL
-1-
NLR MP 77031 U
THERMO:MECH..A.NICAL TREATMENT OF ALUMINIUM ALLOYS
by R.J.H
Wanhill and G.F.J.A. van Gestel
ABSTRACT The effects of thermomechanical treatment (TMT) on the enginee~ing properties of aluminium alloys are discussed. At present only one type of TMT seems commercially practicable. This is a T3XX type treatment for 2000 series alloy sheet. Of the other kinds of TMT those with the most potential are high temperature ageing final thermomechanical treatments (FTMTS). The main incentive for continued investigation of TMT and the most likely key to its commercial exploitation is the possibility of improving fatigue properties.
( 31 pages)
This paper has been prepared for submission for publication in "Aluminium".
Division: Structures and Materials
Completed
August
Prepared: RJHW/GFJAvG/ ~
Ordernumber
543.501
Typ.
NI3
Approved: HPvL/
~
1977
-2-
INTRODUCTION The structural efficiency of aluminium alloys may be increased in two ways. First, allowable stresses can be raised by innovative design. Examples of this approach are the use of adhesive bonding instead of mechanical fastening, thereby reducing the effective stress concentrations and enabling a higher fatigue strength [1-3] ; and selection of sandwich and laminated concepts, which provide increased fatigue crack propagation resistance by load shedding to uncracked face sheets and laminations
[4-7].
Secondly, the engineering properties can be improved by metallurgical means. An important example of this approach for aluminium alloys is thermomechanical treatment (TMT). In this paper the types of TMT and their effects on a variety of alloys are discussed. The potential of TMT for commercial exploitation is assessed.
TYPES OF THERMOMECHANICAL TREATMENT For aluminium alloys there are two kinds of TMT. Intermediate thermomechanical treatment (ITMT) involves processing in, or near, the recrystallization temperature range to achieve a hot-worked structure largely retained during subsequent thermal treatments, including solution and ageing anneals for precipitation-hardening alloys. Final thermomechanical treatment (FTMT) is characterized by cold-to-warm working, which necessarily limits the temperature of subsequent heat treatment, since recrystallization must be avoided. For precipitation-hardening alloys the working is done after solution annealing and quenching, and before or during the ageing cycle.
ITMT Intermediate thermomechanical treatments were initially established to improve fabrication
[8].
In the last few years ITMTS have aimed
to improve ductility, toughness and stress corrosion resistance of
-37000 series (AlZnMgCu) alloys, especially in the short transverse direction, without decreasing strength as compared to conventionally processed materials [ 9]
.
Inferior transverse properties in conventionally processed materials are attributable to the grain size and shape and the presence of second phases at the grain boundaries. During consolidation intermetallic second phases precipitate at the cast grain boundaries. These intermetallics hinder grain boundary movement such that, especially in 7000 series alloys, subsequent working of the cast ingots results in a pancake structure of unrecrystallized "grains" (composed of subgrains) highly elongated in the working direction. The elongated grain boundaries contain many second phase particles and form a more or less continuous easy fracture path in the plane normal to the short transverse direction. Modern ITMT is based on the observation that the influence of the original cast structure can be minimized by an intermediate recrystallization during the working schedule. The beneficial_effect of this recrystallization is partly due to separation of the grain boundaries from the second phase particles. Also, a fine equiaxed grain structure results, figure
1.
Most of the recent ITMT results
have been reported by Di Russo et al.
[9-11] ,
under the designation
ISML-ITMT. Recrystallization is effected on a partially homogenized ingot for which Cris mainly in solid solution. Then the ingot is fully homogenized, resulting in a fine uniform precipitation of Grrich particles. This precipitation prevents grain boundary migration during subsequent processing. A significantly different treatment, designated FA-ITMT
[12],
involves complete homogenization to
precipitate Cr, followed by slow cooling to precipitate Zn, Mg and Cu as coarse particles. The ingot is then worked at low temperature, recrystallized and homogenized. A schematic of the ITMT variants is given in figure 2.
FTMT
.... Final thermomechanical treatments involving small amounts of
cold work and ageing to T8 tempers have been employed for many years
to increase the strength of 2000 series (AlCuMg) alloys. Recent work has investigated two basic variants of FTMT for 2000 7 5000 (AlMg) and 7000 series alloys. One procedure involves cold work and low temperature a,'_;eing with the intention of retaining ductility and toughness to higher strength levels than obtainable with conventional treatments. The other variant consists of cold-to-warm working and high temperature ageing to provide improved combinations of strength and stress corrosion resistance. Both FTMT variants aim to increase strength by forming a dense uniform dislocation network locked by precipitate phases to resist localized softening. In practice, elevated temperature deformation is more successful in this respect [13] •
MATERIALS SUBJECTED TO TMT Thermomechanical treatments have been applied mainly to aerospace structural alloys, in particular the commercial 2000 and 7000 series. Considerable effort has been made to improve the properties of the widely used 2024 and 7075 alloys. Some results are available for 5000 series alloys and for laboratory -made materials with experimental compositions. Several investigations aimed basically to develop new alloys, such that TMT results were reported for materials with new composition limits, but there were no conventional treatments for comparison. Tables 1-3 list a number of alloys investigated, the types of TMT and properties evaluated. Subdivisions have been made for commercial alloys, new alloy design and laboratory-made alloys. Of the treatments chosen, low temperature FTMT predominates. ITMT is the subject of relatively few studies. This is undoubtedly because such processing would in practice be much more expensive than conventional treatments.
EFFECTS OF TMT ON 2000 SERIES ALLOYS (FTMT ONLY) Table 1 shows that strength and ductility were commonly measured, but only three investigations [13,17,19-21,25]
were
-5-
fairly comprehensive in considering the influence of FrMT on engineering properties of 2000 series alloys. Tables 4 and 5 list qualitatively the effects of post-deformation natural ageing FTl'IIT and post-deformation artificial ageing FTl'IIT on engineering properties. For Al-4Cu, 2014, 2017, 2024, 2219 and 2618 the baselines for comparison are the same alloys conventionally heat treated. For 2048 and UHP 2000 comparisons with 2024 and 2124 baselines were available and are considered the most informative. For sheet 2048 and UHP 2000 optimum combinations of strength and fracture toughness were obtained by cold working and natural (T3XX) or low temperature final ageing (19-21, 25]
•
However, such treatments have the disadvantage of limiting the service temperature and, more importantly, limiting the sheet gauge to less than about 5 mm in order to obtain sufficiently rapid quenching to give good exfoliation resistance [19]. Post-deformation artificial ageing to improve exfoliation resistance resulted in higher strength but a significant decrease in fracture toughness [19,25]
• As a compromise, two approaches have been suggested for
achieving an optimum balance between strength, toughness and resistance to stress corrosion or exfoliation. These are (1) quench, cold work and age to a T86 condition [25], and (2) preage, warm work and age for short times [13, 17, 25]. Figure 3 summarises the effects of ageing and/or FrMT on the fracture toughness of 2024, 2124, 2048 and UHP 2000 alloys. For sheet materials the greater microstructural cleanliness of 2124 [38], 2048 [39]
and UHP 2000 [25]
was responsible for the much higher
fracture toughness (K) of the T3 and T8 conditions as compared to C
2024. In comparison to these higher K values the T3XX FTMTS for 2048 C
increased the strength while maintaining or somewhat increasing the toughness, but all FTMT variations for UHP 2000 greatly reduced the toughness. Thus, the favourable strength-toughness comparisons of UHP 2000 with 2024-T3 in tables 4 and 5 (i.e. equivalent strength with much higher toughness, or equivalent toughness with much higher strength) are essentially due to the increased purity of the UHP 2000. For plate the fracture toughness (~c) of 2048-T851 is superior to that of 2024 and 2124, albeit at slightly lower strength as compared to 2024-T851. For 2024 an elevated temperature FTMT to obtain optimum
-6-
strength, toughness, stress corrosion resistance and stability to thermal exposure resulted in a 16
%increase
in yield strength with
equivalent toughness and stress corrosion resistance as compared to 2024-T851
[13,17] •
A major shortcoming of FTMT for 2000 series alloys is the lack of consistent improvement in constant amplitude fatigue properties. Early results on an Al-4Cu alloy showed that increases in unnotched fatigue streng~h were obtained by cold swaging [23] • However, for 2048
[19]
and UHP 2000
[25]
the smooth and notched fatigue
properties were only equivalent to, or sometimes slightly better than, those for 2024-T3, tables 4 and 5. Limited fatigue crack propagation data indicate that 2048 and UHP 2000 in the T3XX conditions are more resistant than 2024-T3 and 2124-T3 [19-21 1 25] , but the main controlling parameter appears to be final ageing, not TMT: figure 4 shows that crack propagation rates for UHP 2000 were much less for the naturally aged conditions (T36 and T4) than for all the artificially aged FTMTS. Both variants of FTMT are beneficial to strength after thermal exposure [13-17]
, tables 4 and 5. However, the primary creep rates
for 2024 and 2618 were increased [18], possibly because the deformation was cold rather than warm and therefore less able to give a stable dislocation network. On the other hand, stress rupture lives of these alloys were dissimilarly influenced: FTMT increased the stress rupture life of 2024 but decreased that of 2618
[18]
EFFECTS OF TMT ON 5000 SERIES ALLOYS (FTMT ONLY) For Al-Mg base alloys FTMT raises the yield strength while maintaining
[26,28]
or even improving [27] the ductility. In the
most detailed study [28], which considered a number of alloys and processing variations, table 2, the best combination of strength and stress corrosion resistance was obtained with cold rolled and heterogenized Al-5MgZrCrMn. Cold work before ageing also increased the unnotched fatigue strength of commercial (HE30TE) and laboratory (Al-lOMg) alloys [26,27]: especially for the commercial alloy the increase was large.
EFFECTS OF FTMT ON 7000 SERIES ALLOYS Several FTMT investigations were carried out with 7075 alloy, table 3. Working was usually done after preageing, but sometimes after solution treatment [13,17,29,35]. Properties measured were especially strength and ductility, followed by fracture toughness, stress corrosion resistance and fatigue strength. A few data for fatigue crack propagation resistance are available [19,25] • Tables 6 and 7 list qualitatively the effects of low temperature ageing FTMT and high temperature ageing FTMT. Owing to the importance of exfoliation resistance and stress corrosion resistance, obtained by high temperature ageing, the data for low temperature ageing FTMT are more limited. In both tables the baseline for comparison is mostly the same alloy, wherever possible in the peak aged condition. For RX725 and UHP 7000 comparisons with 7475 and 7075 were available and have been preferred. The results for low temperature ageing FTMT indicate that to obtain significant strength increases as compared to conventional heat treatments a preage is necessary, and post-deformation ageing is required after warm working [29] • For high temperature ageing FTMT it appears that warm working provides better combinations of properties than cold working [13,17,19,25,33,34,36], but there is no consensus as to the degree of preageing. For RX725 sheet and several experimental AlZnMgCu alloys in sheet form optimum combinations of strength, toughness and exfoliation resistance were obtained from overageing followed by warm working [19,33,34,36]. On the other hand, an underageing preage gave better strength-toughness combinations for exfoliation-resistant UHP 7000 sheet [25] • A comprehensive study for thick section 7075 and 7049 showed that the optimum FTMT was peak ageing (T6), warm working (plus cold working for 7049) and post-deformation short time ageing, resulting in T6 strength and toughness and T73 stress corrosion resistance [13,17] • Figures 5 and 6 summarise the sffects of ageing and/or FTMT on the fracture toughness of 7075, 7475, RX725, UHP 7000 and several AlZnMgCu alloys. Figure 5 shows that for sheet 7475, RX725 and the experimental AlZnMgCu alloys the various FTMTS offer little improvement over conventionally processed 7475. Large strength-toughness
-8improvements as compared to 7475 were obtainable from UHP 7000 sheet, figure 6 1 but only a few FTMTS were superior to UHP 7000 in the T6 and T76 conditions. In view of these results the main reason for high toughness in RX725, the experimental AlZnMgCu alloys and UHP 7000 is probably a high degree of microstructural cleanliness due to low Fe and Si contents [25,33]
(as in the case of 7475 (44])
rather than the use of TMT. The very high strength obtainable from UHP 7000 also appears to be due to microstructural factors not specifically dependent on TMT [45]. For thick sections figure 5 shows that low temperature ageing FTMT improved the Kic of 7075, but only to levels obtainable from conventionally processed 7475. Also, the stress corrosion resistance after FTMT was not much better than that of 7075-T651 [30,31], table 6. Of more potential is the already mentioned high temperature ageing FTMT involving peak ageing, warm working and short time ageing [13,17] : results for 7075, if repeatable for 7475 without loss of the inherently higher fracture toughness, indicate that it may be possible via FTMT to obtain 7475 with a yield strength Kic
>
> 500
MPa,
40 MPa fi, and T73 stress corrosion resistance.
Figure 7 shows that FTMT does not consistently improve the constant amplitude smooth and notched fatigue strengths of 7000 series alloys, although there is a tendency for fatigue strength to increase with yield strength. This tendency, which is most probably due to intrinsic differences between alloys rather than to FTMT [46], is also reflected in the qualitative rankings of UHP 7000 with respect to 7075-T6 in table 7. For UHP 7000 it was found that very few FTMTS resulted in smooth and notched fatigue properties significantly better than those for UHP 7000-T76, and the overall superiority as compared with 7075-T6 was attributed to greater purity [25]. In addition, FTMT gave little or no improvement in fatigue crack propagation resistance for UHP 7000 (25]: figure 8 shows that the primary factor influencing crack growth rates was the degree of final ageing (T6 compared to T7), such that crack growth rates for UHP 7000-T76 and 7475-T761 were similar to those for FTMT UHP 7000 averaged to T7 conditions.
-9EFFECTS OF ITMT ON 7000 SERIES ALLOYS Table 8 lists qualitatively the effects of ISJY!L-ITMT and FA-ITJV!T on 7000 series alloys. As stated previously, the purpose of ITMT is to increase ductility, toughness and stress corrosion resistance, especially in the short transverse direction, without decreasing strength. The results demonstrate a marked improvement in ductility while maintaining strength [9,10,12], and also a combination of IT.MT with low temperature ageing FT.MT gave a large strength increase without the ductility loss characteristic of FTMT alone (see table 6). ISML-ITMT invariably raises the short transverse fracture toughness, and the smooth specimen stress corrosion resistance of overaged, but not peak age~materials [9,10]. However, there is not much improvement, if any, in threshold stress intensity factors and plateau velocities for pre-cracked specimen stress corrosion, figure 9. FA-IT.MT appears to be at a disadvantage compared to ISJY!LITMT in that fracture toughness was not found to be improved [10].
DISCUSSION Within the last few years much effort by aluminium alloy producers has resulted in the following developments: (1)
Metallurgically cleaner alloys like 2124, 2048 and 7475, possessing higher fracture toughness than older materials. 2124 and 2048 were developed mainly in response to a need for improved elongation and fracture toughness in T851 plate, particularly in the short transverse direction [37,38J, while maintaining the strength, corrosion and fatigue resistance, and elevated temperature stability of 2024-T851. The development of 7475 was an effort to optimise strength and toughness in 7000 series sheet and plate [ 41,44
(2)
J
Alloys like 7010, 7049 and 7050, combining in thick sections improved fracture toughness with high resistance to stress corrosion at strength levels equivalent to T6 in older alloys, table 9.
(3)
Experimental powder metallurgy 7000 series alloys for extrusions and forgings with combinations of strength, fracture toughness
-10and stress corrosion resistance superior to those of commercial ingot metallurgy alloys I44,54,55] • In comparison to these developments, especially (1) and (2), which only involve essentially conventional processing, the commercial exploitation of TMT requires demonstration of significant advantages in those properties limiting structural performance. For sheet applications TMT offers relatively little improvement in combinations of strength and fracture toughness as compared to the improvements obtained from greater microstructural cleanliness. Furthermore, the toughnesses of conventionally processed sheet materials like 2048 and 7475 are sufficiently high that the limiting design criteria for airframe structures in the absence of aerodynamic heating are not strength and toughness but the fatigue properties. Although there is no consistent effect of TMT on either smooth and notched fatigue properties or the resistance to fatigue crack propagation, it does appear that of the various TMTS a 2000 series alloy T3:X:X type treatment is the most promising, particularly since the process requires only cold working after solution treatment, i.e. the additional cost is minimal. The influence of FTMT on 2000 series alloy elevated temperature properties, important for e.g. engine areas, SST and Space Shuttle airframes, needs to be more thoroughly investigated. It is clear that retention of strength after thermal exposure can be much improved by FT.MT, but the reported increase in primary creep rates [18] is unfavourable. For thick sections TMT is more readily applicable to plate than forgings or extrusions. Of the latter products only the simplest geometries would be suitable, and the permissible amount of working is much less than for sheet or plate [29]. For applications with primary requirements of strength, fracture toughness and stress corrosion resistance the improvements in plate material properties offered by the newer, conventionally processed alloys 7010, 7049 and 7050 make it doubtful whether IT.MTS or low temperature ageing FT.MTS are worthwhile, in particular because IT.MTS require considerable extra and costly high temperature processing, and neither ITMTS nor low temperature ageing FTMTS give much improvem~nt in precracked specimen stress corrosion resistance [9,30,31]. High temperature ageing FT.MT involving peak ageing, warm working and
-ll-
short time post-deformation ageing is a promising process for obtaining premium strength and stress corrosion resistance. However, outstanding strength and stress corrosion resistance are also available from high toughness powder metallurgy alloys, irrespective of product form
[55].
For some thick section applications fatigue properties may be determining. Suitable data for comparing materials are limited, but it appears that although unnotched fatigue strengths can vary widely
[56],
the high-cycle notched fatigue strengths of conventionally
processed newer 7000 series alloys are no better than that of 7075 [56-58]. TJ\/IT may offer an improvemer.t in notched fatigue strength, but the evidence so far is dissuasive. There is more reason to anticipate better notched fatigue strengths for powder metallurgy alloys
[44,55].
SUMMARY
At the present time only one type of TJ\/IT seems commercially practicable. This is a T3XX type FTMT for 2000 series alloy sheet in gauges up to about 5 mm. Of the other kinds of TJ\/IT those with the most potential are high temperature ageing FTJ\/I'I'S involving warm working. For 2000 series alloys high temperature ageing FTJ\/IT may enable a significant improvement in elevated temperature properties, and in thick sections a superior combination of strength, toughness and stress corrosion resistance. For 7000 series alloys high temperature ageing FTJ\/IT results in premium strength with excellent stress corrosion resistance. Factors unfavourable to commercial exploitation of 'IMT are the extra costs, particularly if warm working is involved 1 the significant improvements in properties obtainable from conventionally processed newer alloys, the promising results for powder metallurgy alloys which, however, also entail additional processing costs, and the lack of consistent improvement of fatigue properties by TJ\/IT. Nevertheless, it appears that the main incentive for continued investigation
of TJ\/IT and the most
likely key to its commercial exploitation is the possibility of improving fatigue properties.
-12ACKNOWLEDGEMENT This study is a follow-up on a survey for which financial support was provided by the Directorate of Materiel, Air, Royal Netherlands Air Force.
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TABLE 1 2000 SERIES ALLOYS SUBJECTED TO FT.IYIT ALLOY DESIGNATION
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