Lithium is the only alloying element that increases strength and modulus while reducing density in aluminum alloys.

Harry A. Lipsitt* Wright State Universihj, Dayton, Ohio

Andrew M. Sherman* Ford Research Lab, Dearbom, Michigan

S tiffness is a major design criterion for au­tomotive body structures, which are usu­ally constructed of stamped sheet metal. Because the elastic modulus of metals is

generally proportional to density, this represents one of the limiting factors on the weight savings possible with an aluminum body structure. Thus, it would be helpful if an aluminum automobile body alloy could be designed with a modest increase in elastic modulus.

In addition, it would be advantageous if such a material would possess a modest sh·ength increase in the annealed condition, as well as a reduced den­sity. This article describes a method in which ex­isting handbook and literature data becomes the basis for calculating the compositions of Al-Mg-Li alloys with yield strengths in the range 110 to 145 MPa, densities in the range 2620 to 2590 kg / m3, and elastic moduli in the range 74 to 76 GPa. The structural efficiency (specific stiffness) of these al­loys (Eip) ranges from 28.2 to 29.3 x 106 GPa­m3 I kg. These values are to be compared to the char­acteristics of alloy 5754, the current aluminum alloy for body structures, which typically has a yield strength of 100 MPa, a density of 2680 kg/ m3, a modulus of 70.3 GPa, and a structural efficiency of 26.2 x 106 GPa-m3 / kg. The increase in structural ef­ficiency, combined with the higher strength, could enable the weight of an optimally designed alu­minum body structure to be reduced by about 10%. Moreover, the modified alloys should have fabri­cation characteristics such as formability and spot weldability that are fairly similar to those of con­ventional alloys. *Member of ASM International

T11e Ford Prodigy concept vehicle has an aluminum body structure aluminum exterior panels. Otha materials chosen to reduce weight in a varieh; of components in­clude magnesium, titanium, and carbon fiber composites. Aluminum-lithium alloys have the potential to replace such materials in future production automobiles.

Effects of lithium Several commercial aluminum alloys have been

designed to take advantage of the attributes of lithium, which is the only alloying element that in­creases strength and modulus while decreasing the density of the alloy. However, the existing com­mercial alloys that contain lithium were designed to be quite strong, so they also contain the usual al­loying elements that provide precipitation strength­ening. In addition, the amount of lithium added is usually maximized, consistent with other proper­ties, so that the density and modulus effects are also maximized. As a result, no standard non-heat treat­able lithium-bearing aluminum base alloys are available.

Russian workers h ave developed a series of lithium-bearing alloys, some of which do not con­tain copper. One of these, alloy 1424, is essentially an Al-Mg-Li alloy containing 1.4 to 1.65 wt% Li and 5.6 to 6.0 wt% Mg, along with small amounts of zinc, scandium, and zirconium. The density of this alloy is reported to be 2.52 g I cm3 with an elastic modulus of 77 GPa. This alloy contains a very high magnesium content and is mainly hardened by the metastable Lh &'-phase (Al3Li), which has a small misfit parameter ( - 0.2%) and is fully coherent with the matrix. The&' -phase appears as a fine, uniform, spherical precipitate. The small amounts of scan­dium and zirconium form the Ah(Sc,Zr) phase, which inhibits recrystallization during homoge­nization and solution treatrnents. Maximum

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ADVANCED MATERIALS & PROCESSES/OCTOBER 200 1 37

Table 1 -Effects of lithium in solution in aluminum Modulus of

Lithium, Lithium, elasticity Density p,

age of CAFE standards. To com­pensate for this problem, an alloy with both higher strength and

FJp ~fop, higher stiffness would be ideal. =.::..:.::_ _ _ __:.:..:.:..:.:....__.....:::!.-=..:::..=....._-=:2.._=--_ _....;;~=----...:.._:_----.:..!'---....:..:...- Small additions of lithium to alu-at.% wt.% E, GPa M!,% glcmJ &, %

0 0 66 2.71 1.9 0.496 72 +9 2.67 - 1.5 4.2 1.110 76 +15 2.62 -3.3 5.5 1.475 78 +18 2.59 -4.4

Table 2 - Tensile properties of selected alloys Yield strength, Ultimate tensile

Alloy MPa strength, MPa Elongation, %

Al-2Li 70 125 4 Al-2Li-1Mg 135 220 6 Al-2Li-2Mg 190 300 7 Al-2Li-3Mg 200 330 9 Al-2Li-4Mg 210 350 11 Al-1.38Li (coarse grain) 40 90 17 Al-1.39Li-1Mg (coarse grain) 65 150 14

Table 3 - Tensile properties of selected binary alloys

Alloy/wt% Mg

5005/0.8Mg 5050/1.2Mg 5052/2.5Mg 5154/3.5Mg 5086/4.0Mg 5083/4.5Mg 5056/5.2Mg

Yield strength, MPa

41.4 55.2 89.6

117.2 117.2 144.8 151.7

UTS, MPa Elongation,%

124.1 30 144.8 24 193.0 25 241.3 27 262.0 22 289.6 22 289.6 35

Table 4 - Yield strength vs. magnesium content Magnesium, Magnesium, Magnesium,

(at.%)¥2 Yield strength,

Alloy

5005 5050 5052 5154 5086 5083 5056

TI1is article is based on a paper that is part of the ]ames T. Staley

Honorary Symposium on

Aluminum Alloys,

to be presented during the

ASM International

Materials Solutions

Conference in Novembetin Indianapolis. Visitwurw.

asminternational. org!indy2001 for details.

wt% at.% MPa

0.8 0.887 0.94 41.4 1.2 1.33 1.15 55.2 2.5 2.77 1.66 89.6 3.5 3.87 1.97 117.2 4.0 4.42 2.10 117.2 4.5 4.97 2.23 144.8 5.2 5.74 2.40 151.7

strengthening of this alloy is achieved by a subse­quent precipitation treatment at 150 to 170°C (340°F) for 24 hours. This treatment produces a 8' precipi­tate in the range of 5 to 20 nm. However, the 8'­phase is subject to Ostwald ripening at l20°C (250°F), and this diffusional growth rate is fwther increased under stress.

Alloy design In thinking about a course to follow, it is helpful

to understand that automotive sheet structures are frequently stiffness limited. This means that it may not be possible to form a satisfactory sheet compo­nent of material as thin as needed simply to achieve adequate strength, because the thin sheet compo­nent may not have sufficient rigidity to meet func­tional design goals. The usual approach is to form parts from thicker material than required based on strength considerations, so as to provide the nec­essary mechanical stiffness. However, this leads to an increase in part weight that is not wanted in this

24.3 26.9 29.0 30.1

+10.7 +19.3 +23.9

minum are known to increase both of these properties. Since lithium is the least dense metal in the periodic table, its addition in solid solution re­sul ts in a rapidly diminishing alloy

density. In fact, for each one weight percent add i­tion of lithium, the density of an aluminum alloy is reduced by 3% and the elastic modulus is increased by 6%. Thus lithium additions to aluminum have the potential to yield an alloy with increased strength and stiffness together with a decreased density!

The purpose of this document is to explore the possibility of developing such a sheet material with the design criteria detailed here. The alloy is to have an annealed yield strength of about 120 MPa (17 ksi), be sufficiently ductile and tough at room tem­perature for its designed purpose, and (possibly) be superplastically formable. Ideally, it should be manufactured on common production facilities and cost no more to purchase or to form than current alloys. An additional goal is to have an elastic mod­ulus greater than that of alloy 5754, coupled with a density less than that of 5754, such that the com­bined effect on specific modulus, E/ p, should be ap­proximately a 10% increase.

Let us begin by discussing the properties of Al-Li binary alloys, specifically the effects of lithium in solid solution on the density, elastic modulus, strength, and ductility of the alloys. N oble, Harris, and Dinsdale showed that lithium in solu tion in aluminum produces the effects shown in Table 1.

The grain size of these alloys was not specified, although 0.05%Zr had been added to keep the grain size small. For Table 1, densities were calculated (substituting weight percentage of the elements) using the formula of Peel et al., which for these al­loys simplifies to:

p = 2.71 - 0.01Mg- 0.079Li

Noble et al. also showed that the addition of 2.2 wt% magnesium reduces the elastic modulus of these compositions by about 1 GPa. We might re­calculate the above table for alloys containing about 1 wt% Mg in solid solution, but since we would be reducing the moduli by about 1 GPa and the den­sities by about 0.01 g I crn3, the calculated data for modulus increase, density decrease, and change in specific modulus would be almost unchanged. Thus, for the purposes of this paper, the effect of the smaller amount of magnesium in the alloys shown in Table 2 on the elastic modulus will be neglected.

The tensile properties of selected alloys in the so­lution heat treated condition are shown in Table 2. Data for both binary Al-Li alloys and for ternary Al-Li-Mg alloys are included for reasons that will later become apparent. A brief comparison of these data with those shown in Tables 1 and 3 shows that the effect of grain size is a dramatic one. Since the

38 ADVANCED MATERIALS & PROCESSES/OCTOBER 2001

a large volume fraction of coherent o' precipitates. 220 /•

200 o\O -L 180 ,_ ..... .. "' :!i _, ~,;-.,-r .... ,

~ 160 !v .,:. ~~·/ -s' 140 CD

~ V ! A ~':lv/e <: ~ 120 .. ~ .J~ I$ / .,

100 Ql ]I $ , ;;:: I 80 ~ :uw ; 60 "'~/ 40 liY ' 'h /

1.0 2.0 3.0 4.0 5.0 (Total alloy content) 112

The presence of magnesium improves the duc­tility of Al-Li alloys. Grain refining additions of zir­conium would prevent grain growth during an­nealing, but it has been shown that they have a great effect on both yield and ultimate tensile strength (about 150 MPa). Grain refining additions of man­ganese have also been studied. The addition of man­ganese does not produce as large a strength increase as zirconium, but it also does not prevent recrystal­lization, so a subgrain structure is not present in man­ganese-bearing alloys, and it would be expected that the elevated temperature creep strength would be improved as a result. A manganese addition also re­

Yteld strength vs. the square root of total alloy content ts sults in a higher room temperature ductility in AI-shawn for conventional Al-Mg alloy, Al-2Li-xMg, and several Li-Mg-Mn alloys than when 0.2% zr acts as a grain designed alloys.

refiner. The fracture toughness of solution-treated grain sizes for much of the data in this analysis were and aged alloys containing 2Mg-2Li is expected to not available, it must be assumed that, unless it was be in the range 22 to 24 MPa'-'m. Leaner alloys tested so stated, the grain sizes were all reasonably small. in the annealed condition should be expected to

The data from Table 3 for essentially binary Al- show higher toughness. Mg alloys (5056 may be two-phase) are reworked To summarize the above paragraphs, it appears in Table 4 and plotted in the graph above as yield that to ensure adequate ductility and toughness, it strength versus the square root of the magnesium would be beneficial to have some magnesium in content in atomic percent. A straight line results, as the alloy, as opposed to a simple Al-Li binary. Fur­would be expected for a solid-solution alloy. thermore, the lithium content must be restricted to

Also on the graph are plotted the data from Table some amount less than 5.5 at.% in order to com-2 that are reworked in Table 5 for Al-2Li-XMg al- pletely avoid the presence of a o' precipitate. If loys. A second straight line results, displaced from superplas tic forming ability is preferred, a grain-~~~an~~~ if propriate to the addition of Table 5 - Composition and yield strength o selected Al-Li alloys 2 w t% Li. The second line is Yield essentially parallel to the Magnesium, Magnesium, first, indicating that the ad- ..:.w:..:t..:.:%:__ ___ _ ..=a..::.t. o.:.:Yo:__ _ _ ....:.::.::.:.::..._ __ .:.:..::..:.::_ _ _ ::..:..:_:_:__ _ __:::.:.:.:..:..:..__-=:.:..::..- __.:.__.:._

dition of magnesium to ei- 0 o

Magnesium, Lithium, Lithium, Lithium, strength, (at.%)11'1 wt% at.% (at.%)112 Lt/o MPa

ther alloy base has essen- 1 1.05 tially the same effect on the 2 2.094 yield strength. 3 3.14

0 2 7.35 2.71 2.71 70 1.025 2 7.34 2.71 3.73 135 1.45 2 7.34 2.71 4.16 190 1.77 2 7.33 2.71 4.48 200

From these data, we can 4 4.18 now begin to design alloys .::._------==-----==-----=::..._ __ __:..::._ __ ..:._.::._ _ _:_....:.._ ___ _ that contain various amounts of magnesium and refining element such as chromium or manganese lithium. Since the properties of solid solution alloys should be added, since these do not markedly af­are a linear function of the square root of alloy con- feet the sh·ength of the alloy.

2.04 2 7.32 2.70 4.74 210

tent, we can divide the space between the two lines Based on the above data, it was decided to retain in such a way as to yield the effect of various a magnesium content of 1.2 wt%, as in the case of lithium contents between 0 and 2 w t%. alloy 5050. With this as an initial presumption, alloy

However, before we begin to design alloys, we design curves parallel to that for the Al-Mg binary need to detennine whether the alloy compositions alloys were constructed to yield approximate yield we might select for our purpose are restricted in strengths of 110, 117, and 124 MPa (16, 17, and 18 any way. U1e limit of solid solubility of lithium in ksi). In addition, a design composition based on 2 aluminum is 14 at.% (4.2 wt%) at600°C (1110°F), wt% Mg (necessary to avoid too much lithium) was and the o' solvus is -5.5 at.% Li (1.475 wt%) at also determined. In addition to the alloying addi-1700C, and about 10 at.% at 270°C. The presence tions of magnesiwn and lithium, it is suggested that of magnesium reduces the solubility for lithium 0.25 wt% Cr should also be added to act as a grain in solution in aluminum. For alloys containing refiner. • more than 2 wt% Mg and aged at relatively high temperature, a non-coherent cubic phase is formed . This phase, w hich is Al2LiMg, precipi­tates mainly in the grain boundaries, and has an adverse effect on ductility. The ductility of lean Al-Li alloys remains high (>>30%) at low lithium contents when the alloys are in the annealed con­dition. However, aged binary Al-Li alloys have low ductility and touglmess because of the severe strain localization that arises from the presence of

For more information: Dr. Andrew Sherman, Ford Motor Co., Scientific Research Laboratory, MID 3135, 2101 Vil­lage Rd., Dearborn, MI 48124.

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