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A STUDY OF THE MICROSTRUCTURE AND PROPERTIES OF DIE FORGINGS IN
ALUMINIUM-LITHIUM ALLOYS 2091 AND 8090
A. Smith
To cite this version:
A. Smith. A STUDY OF THE MICROSTRUCTURE AND PROPERTIES OF DIE FORGINGS IN
ALUMINIUM-LITHIUM ALLOYS 2091 AND 8090. Journal de Physique Colloques, 1987, 48 (C3),
pp.C3-629-C3-641. �10.1051/jphyscol:1987373�. �jpa-00226604�
JOURNAL DE PHYSIQUE
Colloque C3, supplement au n 0 9 , T o m e 48, septembre 1987
A STUDY OF THE MICROSTRUCTURE AND PROPERTIES OF DIE,FORGINGS IN ALUMINIUM-LITHIUM ALLOYS 2091 AND 8090
A.F. SMITH
Materials Laboratory, Westland Helicopters Ltd, GB-Yeovil, BA20 2YB, Somerset, Great-Britain
ABSTRACT
This paper reports on some of the results from an examination of sample die forgings in aluminium-lithium alloys 2091 and 8090. Whilst direct comparison of properties of the two forgings is complicated by the differing configurations, useful data has, nevertheless, been generated which will provide useful background information for further programmes about to commence.
The present study has revealed density reductions of-8% and -10% respectively for the 2091 and 8090 compositions, compared to current 2xxx series alloys.
With regard to microstructure, th'e 2091 forging exhibited predominantly unrecrystallised grains which contained linear features postulated to originate from a mechanical/deformation twinning type of mechanism. In contrast, the 8090 component showed extensive re.crystallisation.
Microhardness measurements indicated lithium and magnesium depletion at surface locations while optical and electron microscopy suggested that apparent porosity bands on the polished microsections were due to the presence of the equilibrium 6-AlLi phase, particles of which fragment and are removed during metallographic preparation.
Mechanical properties showed significant anisotropy in the 2091 forging and were att~ibuted to texture effects. Highest strengths were obtained in the longitudinal direction, while the lowest appeared at-30° and-70° to the longitudinal direction: long transverse strengths were intermediate. The resulting failure modes are briefly discussed. In contrast, mechanical properties in the 8090 component exhibited a gradual increase in strength from longitudinal to long transverse directions, in keeping with the recrystallised nature of this forging.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987373
INTRODUCTION
Although lithium additions were first made to aluminium as long ago as the 1920's (I), an inability to fully match the properties of non-lithium containing aluminium alloys has prevented their successful commercialization in the past, notwithstanding, in particular, the attempts made with the 2020 alloy of the 19601s (2) which was ultimately withdrawn due primarily to fracture toughness deficiencies. However, recent years have seen a renewed interest in aluminium-lithium based alloys specifically as a means of achieving the weight reductions necessitated by modern aircraft design together with operational considerations and, accordingly, much effort has been devoted to the development of compositions and associated manufacturing technologies in order to eliminate the shortcomings of their predecessors, (3, 4, 5). Employing the 'ingot metallurgy' route, this work has evolved a number of new lightweight precipitation - hardening alloys, most of which are based upon the Al-Li-Cu-Mg-Zr alloy system in which co-precipitation of the 6'-A13~i and st-A12cuMg intermediate phases (6) can produce combinations of strength and fracture toughness which are at least comparable with present aluminium alloys and are clearly superior to the obsolete 2020 composition.
Whilst the 8-10% desired density reduction is a function of chemical composition and is readily achieved in all the new aluminium-lithium based alloys, the attainment of mechanical properties is dependent upon the ability to produce the optimum dispersion of the aforementioned hardening phases.
A1 though the &'-A1 Li phase generally nucleates homogenously throughout the matrix (7,8) nucleation of the sf-~12cullg phase is critically dependent upon the copper and magnesium levels (6). Due to this, the new alloys fall into two broad groups, those in which the above elemental contents are such that S ' - A ~ ~ C U M ~ forms homogenously throughout the matrix and those in which the application of some degree of post-solution treatment cold work is required to introduce a dislocation network on which s'-A1 CuMg can heterogenously nucleate: in the absence of this stimuli, S' precipieation is predominantly at lattice discontinuities such as grain boundaries, where this phase effects minimal strength contributions.
Clearly, choice of alloy is a critical consideration which must be largely determined by the processing routes which can be applied to the component.
Whilst cold work may be applied to mill products by post-solution treatment stretching prior to artificial ageing, cold coining is the analogous process for forgings (9). However, this may not always be possible and as a preliminary to further comparative investigations currently in hand, this paper reports on the microstructure and properties of two aluminium-lithium based alloys, 2091 and 8090, in the form of forgings: neither were subjected to post-solution treatment cold coining. The latter alloy is particularly dependent upon cold working to achieve properties, whilst the former is less SO.
EXPERIMENTAL PROCEDURE
The 2091 component in the form of a precision die forging, fig. 1, was produced by the Cegedur Pechiney subsidiary, Forgeal, from 90 mm diameter bar extruded from 450 mm diameter semi-continuously DC cast billet. Heat treatment of the finished component to T6 temper was achieved by an 8h solution treatment at 526OC, water quenched at 70°C and subsequently artifically aged for 24h at 190°C. . The die forging in 8090 alloy, fig. 2, was produced by HDA Forgings Ltd, UK, from 165 mm diameter billet machined from Alcan-cast rectangular section ingot. Heat treatment to T6 temper was achieved by a 4h solution treatment, PAG quenched and then aged for 16h at 190°C.
Density measurements (conforming to ASTM B311) and subsequent chemical analysis
were taken at three separate locations on each forging. Optical and scanning
electron microscopy studies- were performed on 6 pm diamond finish surfaces
produced by standard metallographic preparation tehniques. Samples for grain
structure examination were etched for 45s in Kellers Reagent and cleaned by
flash immersion in 50% HN03.
Microhardness profiles were obtained from as-polished surfaces employing a 15g load. Where dimensions allowed, tensile tests were carried out using round specimens of 28 mm and 5.64 mm gauge length and diameter respectively; flat specimens of 24 mm gauge length and 3 mm thick x 6 mm wide gauge section were used elsewhere. Figs. 3 and 4 show the relevant sectioning diagrams.
RESULTS AND DISCUSSION
--
Composition and Density
Table 1 details the mean composition values from both sets of triplicate determinations and shows the main alloying elements of lithium, copper and magnesium to lie at the lower end of the specification range for each alloy. The high accuracy of these analyses is indicated in Table 2 where there is an excellent correlation between measured density values and those predicted from the composition-density relationship due to Peel et a1 (6). This data clearly illustrates the dependance of density upon alloy content, and the Li:Cu ratio in particular, the lower value in 2091 resulting in density reductions of -8% over current aluminium alloys whereas a corresponding - 10% decrease is exhibited by 8090.
Fig. 2. Die forging in 8090 alloy
'ig. 1. Precision die forging in 2091 alloy
A Fig. 3. Sectioning diagram for 2091 alloy component
A Fig. 4. Sectioning diagram for 6090
alloy component
Table 1. Mean chemical composition and specification limits
Grain Structure
Typical grain structures are shown in figs. 5a, b and whilst both forgings display a banded appearance typical o f current commercial aluminium-lithium alloys, marked differences are, nevertheless, readily apparent. Although 8090 alloy shows some evidence of the characteristically large and 'pancake' shaped grains attributable primary to the zirconium additions, the extensive recrystallised nature of the material is evidenced by the relatively uniform distribution of fine, equiaxed grains, fig. 5b. As it is known that the original as-cast
forging stock was un-recrystallised, Table 2. Measured and theoretical it follows that the mechanical mean density
working of the material has generated sufficient stored energy to initiate recovery and subsequent recrystallisation during forging and/or solution treatment.
In contrast, relatively large 'pancakef shaped grains predominate in the 2091 alloy, fig. 5a, amongst which are interspersed only occasional regions of equiaxed, recrystallised grains. A distinctive feature of polished and etched microsections from this component, moreover, is the presence of relatively straight and parallel lines across many of the larger grains, while others remain clear. Similar artefacts have been observed in other aluminium-lithium alloys and have frequently been referred to as 'slip linesr arising from post-solution treatment working (10). However, two important points arise here. Firstly, the 'slip' lines in the current material are observed on sections from the as-received component, which being in the T6 temper, would imply that the features originate from the mechanical working of the component during forming and are relatively unaffected by solution treatment. Secondly, the presence of the lines are not in accord with the basic mechanism of slip,
Alloy
2091 8090
Densi t i (g Measured 2.589 2.561
Theoretical
2.591
2.554
as this phenomenon involves lattice movements along a slip plane of an integral number of Burgers Vectors and since lattice plane coincidence.should be identical before and after slip, microsections would not be expected to exhibit evidence of lattice mismatch. As this is at variance with the
observations, an alternative mechanism must clearly be operative and one possibility is that mechanical (deformation) twinning, or a similar process, may be responsible. Although this is an uncommon feature in aluminium, there are, nevertheless, factors which support this in the current alloy. Work on copper based alloys has shown that the propensity to form deformation twins increases with decreasing stacking fault energy which, in turn is inversely proportional to the solute concentration of the alloy (11). An anologous situation may therefore be occuring in the 2091 compositions where the total atomic content of lithium, magnesium and copper additions of -9% may be expected to significantly lower the stacking fault energy compared to 'conventional', more dilute aluminium alloys such as 2014 (total atomic alloying content of - 2.5%) where similar
deformation lines are not
ST
generally observed.
Based upon this hypothesis, the following model is proposed.
The forging of the 2091 component imparts severe deformation to the metal. Due to the strong texture effects characteristic of worked aluminium-lithium alloys, (12) some of the larger grains will be favourably oriented to accommodate this deformation by slip, which leaves no evidence upon polished and etched microsections, as no residual
.
lattice mismatch is involved.
Further, true slip in these
grains leads to dislocation
pile-up at grain boundaries,
thus providing the alergy for
limited recrystallisatlon which
is manifest in colonies of
relatively small, equiaxed
grains at these locations. In
other, unfavourably oriented
grains, slip cannot easily take
alloy forging and (b) 8090 place and instead, deformation
twinning occurs which
effectively eliminates the driving force for recrystallisation during solution treatment. Hence, deformation bands are apparent on microsections due to the inherent lattice discontinuities in this case.
Admittedly, similar features are absent on the 8090 microsections, notwithstanding a similar total solute concentration in atomic terms.
Reference to fig. 1 suggests that the form and dimensions of this particular component would require less severe deformation during forging, the magnitude of the associated resolved shear stress being insufficient to initiate twin formation. A certain level of stored energy may therefore remain through to solution treatment which subsequently promotes the observed widespread recovery and recrystallisation.
Surface Effects
--
A noticeable feature on as-polished microsections through the forgings was a band of apparent sub-surface porosity parallel to the metal surface, but from which it was separated by a pore-free region, fig. 6. Further, a microhardness gradient was detected in the same vicinity, fig. 7, and is indicative of a lithium and magnesium depleted surface layer arising from the strong oxidizing tendencies of these elements when solution treated in air; the alternative use of salt baths minimizes/eliminates these effects.
The inter-relationship between these two features has already been established, the 'porosityt band having clearly been shown to occupy the same region of the surface microhardness profile, irrespective of the depleted depth, which itself is a function of time and temperature
( 1 3 ) . The cause of the 'pores'
has previously been attributed to the "Kirkendall Effect"
arising from the agglomeration of vacancies generated by the diffusion of lithium and magnesium to the surface, the significantly slower diffusing aluminium atoms failing to reverse the flow ( 1 4 , 15).
However, this model fails to adequately explain the pore-free band adjacent to the metal surf ace.
Results from the current work have suggested an alternative explanation. Whilst manual metallographic preparation of samples from the 2091 forging, in particular, gave rise to the surface pores shown in fig. 6, similar optical microscopy of surf aces produced using controlled, automatic polishing techniques revealed additional features having the form of incomplete and distorted circles, but otherwise indistinguishable from the surrounding matrix, fig. 8a. In contrast, their surface and sub-surface appearance in the
Fig. 6. 'Porosity' band at 2091 forging surface.
Fig. 7. Typical microhardness gradient at 2091
forging surface with location of associated
'porosity' band.
scanning electron microscope using a secondary electron image (but also confirmed employing varying accelerating voltages in back-scattered electron mode), fig. 8b, is strongly indicative of distinct particles having a relatively brittle nature, as evidenced by the fragmented edges in fig. 8c. This is in keeping wlth the equilibrium 6-A1Li phase whose occurence is generally deduced from the presence of shallow pits in the optical microstructure but, when remaining intact, exhibits an as-polished appearance which is very similar to that of aluminium (16). The current observations therefore lead to
Fig. 8. As polished surface of 2091 component (a) optical image (b) and (c)secondary electron
images.
the hypothesis that the frequently encountered 'porosity' is only such on the metallographic surface and originates from 6-AlLi phase particles which tend to be removed by the polishing operation. Whilst the localised formation of this compound is not yet fully explained, its presence relative to the aforementioned surf ace microhardness profiles indicates the expected dependence upon specific lithium levels, thereby accounting for the absence of pores in areas immediately adjacent to the metal surface vhere lithium levels would be insufficient for its formation.
Mechanical Properties
The present work has indicated that mechanical behaviour is often dependant upon location and direction within the forgings. Table 3 details the longitudinal mechanical properties within the 2091 forging walls as measured on specimens 1-10 (fig 3).
Excepting the anomolous 0.2%
proof strengths from samples 2
and 6, properties are reasonably
consistent with mean 0.2% PS, TS
and elongations of 464 MPa, 543
MPa and 7% respectively. In
contrast, specific trends are
exhibited by the 2091 forging
base, Fig. 9 illustrates the
difference in longitudinal
properties between the outer and
half width positions, as does
Table 3. 2091 longitudinal mechanical properties fzom the forging 'walls'.
fig. 10 for specimens 17-21 where exceptional respective 0.2% PS, TS and ductility levels of 515 MPa, 580 MPa and 13% are achieved at the latter location.
Aluminium-lithium based alloys characteristically exhibit marked anisotropy due to strong texture effects. The 2091 alloy forging is no exception. Fig.
11 shows longitudinal properties to be noticeably higher than long transverse, although minimum levels occur at
angles of 30' and 70' to the Fig. 9. Longitudinal mechanical properties at longitudinal direction. This, location X in 2091 forging.
however, differs from other aluminium-lithium alloys where
minimum properties occur uniquely at one angle and generally at 55-60' to the longitudinal direction. Whilst specific measurements have not been carried out, it is postulated that (a) the texture introduced during extrusion of the 2091 as-cast ingot has been carried through to the forging and (b) this has been modified by the forging process, together with a superimposed texture arising from movement of metal at approximately 90° to the extrusion direction to form the forging walls.
Fig 12 shows the corresponding fractured 2091 test pieces where the appearance
of the fracture surface clearly varies with test direction and may be
correlated with mechanical properties. Crack propagation in long transverse
specimens (ie. 90') occurs predominantly in an intergranular manner between
both the edges of the elongated, unrecrystallised grains and/or the colonies of
fine recrystallised grains. As the testing angle changes to the longitudinal
direction, failure becomes progressively transgranular through the
unrecrystallised grains and proceeds along lattice discontinuities such as
sub-boundaries and/or deformation bands in a fairly ductile manner, fig. 13 b,
d and this, more tortuous route, is manifest in the higher strengths noted in
fig. 11. However, intermittant changes to an intergranular mode cause partial
u o l ~ ~ ~ " " ' " ' ' ~ " ' ~
0 10 20 30 40 50 60 70 80 TRANSVERSE DISTANCE ACROSS FORGING BASE^^^)
Fig. 10. Variation in mechanical properties as a function of distance across forging base for 2091 forging.
Fig. 11. Anisotropy in 0.2% PS and TS
properties in 2091 forging.
ANGLE TO LONGITUDINAL DIRECTION Ides)separation along the weaker CD-planar boundaries between the elongated 'pancake' shaped grain and result in the 'stepped' appearance of the fracture, figs. 13 a, c, while suitably oriented adjacent grains frequently suffer extended decohesion giving the fractures exhibited by the O0 and 50° samples in fig. 12. Nevertheless, it is noted that the 30° specimen does not conform to the latter pattern and it is postulated that texture effects at this angle lead to easy and widespread slip which readily promotes transgranular failure with much less propensity to extensive delamination. This accounts for the corresponding low strengths and the high ductility which is manifest as distinct surface undulations on the test piece gauge length.
In contrast, the extensive recrystallised nature of the 8090 component is responsible for (a) the generally lower mechanical properties compared to 2091, although these may, to some degree, arise from the greater dependence of the 8090 composition to post-solution treatment cold working and (b) -
o 30. SO* 60' TO" 80 90
the lack of anisotropy,
mechanical properties gradually
ANGLE 1 0 LONGITUDINAL OIRICTION
increasing from the longitudinal
direction to the long
transverse, fig. 14. Fig. 15,
Fig. 12. Fractured 2091 tensile test typical of all 8090 test pieces,
pieces from anisotropy shows the fracture surface of a
studies. sample oriented at 45' to the
longitudinal direction, and
indicates failure to have
Fig. 13. Typical mixed-mode tensile fracture in 2091 alloy ( a ) SEM image of specimen no. 22 fracture surface, (b) SEM image of locationZ, (c) and (d) As above, optical microsections.
occurred predominantly in an intergranular manner, with relatively little delamination effects as seenin corresponding 2091 specimens.
In summary, although the longitudinal and long transverse property levels for
the 2091 forging are attractive and comfortably exceed BSL 77 (2014A-T6)
specification minimum 0.2% PS, TS and elongation ~equirements of 395 MPa, 450
MPa and 6% respectively, this is not always the case at intermediate
orientations and could be the cause of considerable concern to aircraft
designers. Accordingly, it would be most desirable to substantially reduce the
degree of anisotropy, perhaps by the use of fully recrystallised material for
ANGLE TO LONGITUDINAL DIRECTION(de9.)
