2009-JMT-Thermo-mechanical Treatments of Sc- And Mg-modified Al

8/12/2019 2009-JMT-Thermo-mechanical Treatments of Sc- And Mg-modified Al

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ORIGINAL ARTICLE

Thermo-mechanical treatments of Sc- and Mg-modified

AlCu alloy welds

S. R. Koteswara Rao &B. Kamsala Devi &

K. Sreenivasa Rao &K. Prasad Rao

Received: 17 February 2008 /Accepted: 14 January 2009 /Published online: 11 February 2009# Springer-Verlag London Limited 2009

Abstract High-strength heat-treatable aluminum alloy

AA2219 finds application in aerospace industries. Though ithas good weldability, with alternating currenttungsten inert

gas welding, the joint efficiency obtained is only 40%,

particularly in thicker plates. In the present study, an attempt

has been made to improve the weld metal properties by

modifying the chemistry of fusion zone and post-weld

thermo-mechanical treatments. Fillers were made through

casting route by melting conventional 2319 filler with Sc and

Mg. Two levels of Sc (0.3% and 0.6%) and four levels of Mg

(0.3% to 0.6%) were varied. Compressive deformation was

done on the fusion zone of the weld to get three levels of

percentage of reduction (4%, 8%, and 12%). As welded

specimens and welds after compressive deformation, those

were subjected to post-weld aging treatments at 190C for

different periods up to 100 h. Compressive deformation on the

welds made with modified filler of 2319 with Sc and Mg

resulted in significant improvement in the weld metal strength.

Keywords AlCu alloy weld . Scandium .

Compressive deformation . Fusion zone . Aging .

Thermo-mechanical treatment

1 Introduction

High-strength heat-treatable aluminum alloys belonging to

2xxx are attractive materials for aerospace applications. These

applications involve structural components such as rocket

shells, cryogenic tanks, engine casings, and some other small

components. AA 2219 with 6% copper is considered to be the

most weldable among the commercial high-strength aluminum

alloys because of its resistance to cracking. AA 2219 has high

levels of alloying elements and there is excess liquid available

during solidification, which flows into the cracks and heals the

cracks. Though AA 2219 has good weldability, with gas

shielded arc welding, the weld strength is less than half that of

base metal strength [1]. This is true both in autogenous welds

as well as those welded with the matching filler 2319, which

has slightly higher contents of Ti and Zr. The reduction in

strength is due to the dissolution of strengthening precipitates

during melting and high cooling rates involved in welding.

The addition of scandium (Sc) has recently generated

considerable interest to the aluminum welding community.

Small additions of Sc have been found to improve the weld

strength considerably [2]. Mg addition along with Sc has been

proved to improve the beneficial effects of scandium [3].

Therefore, modification of conventional 2319 filler with

Sc and Mg will be advantageous. Furthermore, thermo-

mechanical treatments such as planishing in welds are

expected to exert a beneficial effect with and without post-

planished aging. Compressi ve deformation of welds,

particularly the fusion zone, has found application in the

form of roll planishing of welds to improve the mechanical

propertie s of wel ded joints. It was rep orted by L.I.

Gladstein [4] that crushing or cold working of the fusion

zone by planishing improves the strength of the welded

joint by reducing porosity, changing the residual stress

condition, and weakening the effect of minor inclusions in

Int J Adv Manuf Technol (2009) 45:1624

DOI 10.1007/s00170-009-1936-8

S. R. Koteswara RaoPrincipal, Tagore Engineering College,

Chennai 600048, India

B. Kamsala Devi :K. Prasad Rao

Metallurgical and Materials Engineering Department,

IIT-Madras, Chennai 600036, India

K. Sreenivasa Rao (*)

Metallurgical Engineering Department,

A.U. College of Engineering (A),

Visakapatnam 530003, India

e-mail: [email protected]


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steel weld metals. It was reported by Kurkino [5] that

compressive deformation of the fusion zone, what they called

as roller treatment, offsets a high percentage of longitudinal

and lateral shrinkage by crushing down the seam, relieves the

stress patterns, and smoothens out the weld edges resulting in

better tensile properties. Application of elastic stress during

the aging process of age-hardenable aluminum alloys was

also reported to improve the mechanical properties by alteringthe distribution of strengthening precipitates. This effect was

also observed in AlCu alloys [6].

Keeping the above factors in view, in the present work,

compressive deformation with and without aging treatments

has been carried out on AA 2219 welds.

2 Experimental

The alloys cast to get the modified 2319 composition were

obtained from appropriate mixtures of commercial-purity

aluminum (99.7%), Al2% Sc master alloy, high-purity Cugranules (99.9%), Al5% Ti master alloy, and Al3.5% Zr

master alloy. All of the alloys used for this study were

melted in-house in an electrical resistance furnace. The

casting consists of two parts: cylindrical rods of 6-mm

diameter and 120-mm length and a plate-like (7070 mm)

bottom portion. The castings were machined to remove any

surface contamination, degreased, and cleaned. The rods

were used for welding. The plate portion was used for

doing thermo-mechanical treatment on castings. The cast

plates were subjected to homogenization treatment at 535C

for 1 h in an air-cooled furnace before further treatments.

The compositions of the cast alloys are presented in Table1.

The values correspond to the average of three spectral

analyses and the Sc contents were calculated based on the

amount of AlSc master alloy added to the melt.

A Miller-Synchrowave 350 power source was used to

make gas tungsten arc welds on 2219 plates. All the welds

for this study were made using AC-GTA welding. The

parameters used for welding are given in Table2. Welding

was done by placing the cast filler rods in a rectangular

groove machined in 2219 T87 base plate and running a gas

tungsten arc to make welds of at least 6 mm deep in a plate of

7.5-mm thickness. The fusion zone compositions are given in

Table 3. The specimens were deformed to three different

percentages of deformation (4%, 8%, and 12%) by passing

them in between the steel rollers. In case of weld, rolling was

done in transverse direction such that only the crown gets

flattened. Strength increment after deformation was measured

by hardness testing. The aging temperature range for AA

2219 is 165190C and aging was done at 190C up to 100 h

for both welds and castings. Vickers hardness measurements

were done at regular intervals during aging. Tensile tests wereconducted in a microprocessor-controlled servo hydraulic

universal testing machine (Schenk) with extensometer attach-

ment on subsize samples made according to ASTM E-8 M

standard. Microstructural examinations were carried out using

an optical microscope (Leica, DMLM). Transmission electron

microscopic (TEM) studies were done on welds and casting

to find the distribution and morphology of precipitates before

and after thermo-mechanical treatment.

3 Results and discussion

The microstructures of base metal (2219-T87) and 2319

weld fusion zone microstructure are shown in Fig. 1.

Fusion zone showed the long columnar grains. It has been

shown that it is possible to replace the columnar grains with

equiaxed grains and reduce overall grain size. This is

achieved by creating and maintaining free nuclei in the

molten weld pool by addition of nucleants to the pool. This

is achieved by the addition of Sc into the weld metal.

Scandium is one of the most effective modifiers of a cast

grain structure in aluminum alloys. This is evident from the

fusion zone microstructures of Sc-added welds. These

Table 1 As welded fusion zone tensile strength values

Composition As-welded

Yield (MPa) UTS (MPa) Ductility (%)

2219T87 365 475 19.8

2319 125 229 4.7

0.3 Sc 142 249 9.2

0.3 Sc + 0.4 Mg 158 271 10.4

0.6 Sc + 0.45 Mg 168 275 8.6

Table 2 Tensile test values of weld fusion zones after compressive

deformation

Composition 8% deformed


0.3 Sc 192 280 9.7

0.3 Sc + 0.4 Mg 264 306 7.2

0.6 Sc + 0.45 Mg 266 306 7.4

Table 3 Tensile strength values of weld fusion zones after compres-

sive deformation + aging

Composition 8% deformed + 9 h aging at 190C


0.3 Sc 232 298 10.5

0.3 Sc + 0.4 Mg 272 310 11.9

0.6 Sc + 0.45 Mg 245 299.6 11.1

Int J Adv Manuf Technol (2009) 45:1624 17


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modified composition welds showed equiaxed grains

instead of columnar as in 2319 fusion zone (Fig. 2). When

Sc is added to Al, it forms dispersoids called Al3Sc. It has

L12 structure with scandium atoms at the corners and

aluminum atoms at the face centers. These primary

intermetallic particles act as potent heterogeneous nucle-

ation sites within the melt due to their high coherency andlow interfacial energy with the Al matrix. Scandium

aluminide and aluminum have the same lattice type and the

discrepancy between lattice parameters is about 1.5%.

Therefore, the Al grains can easily nucleate on the surface

of the Al3Sc precipitates. That is the reason why, even with

the 0.3% addition of Sc, the fusion zone microstructure

changed from long columnar grains to equiaxed flower-like

grains. However, addition of 0.6% Sc showed still further

grain refinement. The Al-rich end of AlSc phase diagram

is shown in Fig.3[7]. The presence of 0.6% of Sc comes in

the hyper-eutectic (above 0.55 wt.%) composition. So, for

this composition, the number of primary Al3Sc particlesavailable per unit volume will be higher when compared to

0.3% of Sc. That is the reason why the fusion zone with

0.6% Sc showed more grain refinement.

When compared to the 2319 + Sc welds, 2319 + Sc +

Mg weld fusion zones showed finer grains (Fig. 4). The 0.3

Sc + 0.4 Mg fusion zone showed more refinement

compared to that of 0.3 Sc fusion zone (Fig. 2). However,

when the Sc percentage was hyper-eutectic, with 0.3% Mg

the grain refinement was still significant. This noticeable

grain refinement occurred because of two reasons. Firstly,Mg addition increases the lattice parameter of Al matrix;

this further decreases the lattice parameter discrepancy

between -Al and Al3Sc. Secondly, it has been reported

that Mg clusters act as a nucleation site for Al3Sc

precipitates [8]. Therefore, the number of heterogeneous

nucleation sites per unit volume increases. In addition, Zr

present in the fusion zone forms Al3 (Sc, Zr) phase whose

lattice difference with the Al-matrix is still lower than

Al3Sc. Therefore, Zr increased the effectiveness of modi-

fying action. So, Zr addition reduced the amount of Sc

needed for effective grain refinement. The as-welded fusion

zone hardness values are given in Fig. 5. The base metalhardness was 148 Vickers hardness number (VHN). When

compared to base metal hardness, the fusion zone hardness

value of 2319 weld was 78 VHN, which is only about 50%

Fig. 2 Fusion zone micro structure of 2319 + 0.6 Sc welds showing

equiaxed grains Fig. 3 Al-rich end of AlSc phase diagram

Fig. 1 Fusion zone microstruc-

tures 2219 T87 (a) base metal

and (b) 2319 weld showing

columnar grains

18 Int J Adv Manuf Technol (2009) 45:1624


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of the base metal strength. This drastic reduction is because of

the dissolution of strengthening precipitates during welding.

The TEM pictures for the base metal and 2319 weld are given

in Fig.6. The base metal showed dense distribution of Al2Cu

precipitates, whereas for 2319 fusion zone very few

precipitates were found. This dissolution of precipitates is

because of the large heat input and high cooling rate involved

in arc welding. With addition of 0.3% and 0.6% of Sc, there

was also no increment in fusion zone hardness. Though there

was significant reduction in the grain size in the Sc-added

welds, it was not reflected in hardness values. This could be

because of the nonavailability of a sufficient amount of Al3Sc

strengthening precipitates in the as-welded condition. The

TEM picture for 2319 + 0.6 Sc weld fusion zone is given in

Fig.7. The TEM picture showed some spherical dispersoids.

These rounded precipitates were confirmed earlier as Al3Sc

primary particles [9]. The driving force available for Al3Sc

dispersoids to precipitate in the as-welded condition was very

low. There is no report about the precipitation of Al3Sc

dispersoids at room temperature. Therefore, the rounded

particles in the TEM picture are the primary particles from the

filler. These primary particles acted as heterogeneous nucle-

ation sites and caused good grain refinement, but they were

not significant in improving the fusion zone strength.

Fig. 4 Fusion zone microstructures of weldsa 2319 + 0.3 Sc + 0.4 Mg, b2319 + 0.6 Sc + 0.3 Mg,c 2319 + 0.6 Sc + 0.45 Mg, d 2319 + 0.6 Sc +

0.63 Mg

Fig. 5 As-welded fusion zone hardness values



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For the 2319 + Sc + Mg welds, there was some

improvement in the fusion zone hardness values when

compared to welds with 2319 + Sc welds. For the 2319 + 0.3Sc + 0.4 Mg weld, there was nearly a 20-VHN increment in the

fusion zone hardness. There are two reasons for this increment.

When Mg was added to aluminum, it gave solid solution

strengthening because of atomic misfit. Another reason could

be formation of omega () precipitates by natural aging. The

TEM picture for 2319 + 0.3 Sc + 0.4 Mg weld fusion zone is

given in Fig. 8. In order to determine the relationship between

grain morphology and mechanical properties of the weld

metal, transverse tensile tests (i.e., the test specimens being

perpendicular to the weld direction) were carried out. Tensile

tests were done only for three compositions since there was

no significant variation in the fusion zone hardness values.

The values for 2219 T87 base metal, 2319, and other

modified composition welds are given in Table 1. The yield

strength of 2319 weld was nearly one third when compared

to base metal. In addition, there was a drastic decrease in the

ductility. This poor ductility is because of the columnar

dendritic structure of the fusion zone. For all the welds,

specimens fractured in the fusion zone. With Sc and Sc + Mg

addition, there was a significant increase of about 20 to40 MPa in fusion zone yield strength. The improvement in

percentage of elongation was attributed to the fine-grain

structure of the fusion zone.

The response of the Sc-added weld fusion zone to

compressive deformation is given in Fig. 9. Both 0.3% and

0.6% Sc welds showed a linear increase in fusion zone

hardness with the increase in the percentage of deformation.

For the 0.3% Sc weld, the increase was gradual and peak

hardness was obtained for 12% of deformation. In the 0.6%

Sc weld, there was a 25-VHN increment within 4% of

deformation and then no significant increment with increase

in percentage of deformation. The effect of rolling on

fusion zone hardness values for 2319 + Sc + Mg welds is

given Fig.10. Compared to 2319 + Sc welds, 2319 + Sc +

Fig. 7 TEM of 2319 + 0.6 Sc weld fusion zone

Fig. 6 TEM of a base metal

and b 2319 weld

Fig. 8 TEM of 2319 + 0.3 Sc + 0.4 Mg weld fusion zone



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Mg welds showed an increased response to compressive

deformation, i.e., the increase in the fusion zone hardness

values with increase in percentage of deformation is

significant. All the Sc + Mg welds showed a drastic

increment in fusion zone hardness for even 4% of

deformation. In the case of 0.3 Sc + 0.4 Mg-added weldfor 8% of deformation, there was an increment of about 45

VHN in fusion zone hardness. This increment caused the

fusion zone to gain strength of nearly 85% as that of base

metal 2219 T87. Compared to 0.3 Sc + 0.4 Mg weld, 0.6 Sc +

0.3 Mg weld showed a decreased response to compressive

deformation. By increasing the Mg content from 0.3 to 0.45,

the increase in fusion zone hardness did not also come up to

the level of 0.3 Sc + 0.4 Mg, but for 0.63% addition of Mg

there was a significant increase in fusion zone hardness and

these hardness values were similar to that of 0.3 Sc + 0.4 Mg

welds. Therefore, the point that should be noted here is that

the increase in Sc percentage from 0.3 to 0.6 did not result in

any significant response to compressive deformation.

Actually, with increase in Sc percentage, the response of

fusion zone hardness came down. The reason for this

decreased response could be due to the formation of W

phase with the increase in the percentage of Sc. This

phase forms in the AlCuSc alloys (Fig. 11). The

precipitation and strengthening ability of the W phase is

more uncertain. However, as its formation would decrease

the quantity of and Al3Sc precipitates, it is probable that

its presence is undesirable. Usually, this W phase is

associated with high cooling rates [10]. Since arc welding

process involves a higher rate of cooling, the possibility of

W phase formation is higher.

Therefore, for all the welds, the increase in fusion zone

hardness is attributed by two factors, strain hardening or

work hardening due to compressive deformation and

formation of strengthening precipitates at room temperature

itself by natural aging.

Fig. 9 Effect of compressive deformation on fusion zone hardness

values of 2319 + Sc welds

Fig. 10 Effect of compressive deformation on fusion zone hardness

values of 2319 + Sc + Mg welds

Fig. 11 Isothermal section of Al corner of AlCuSc phase diagram

at 500C

Fig. 12 TEM of deformed 2319 + 0.6 Sc weld fusion zone



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Though strain hardening contributes more to increase in

fusion zone hardness, there could be some increase because

of precipitation at room temperature. To study the effect of

natural aging, transmission electron microscopic studies

were done on some of the deformed samples. The TEM

picture of 2319 + 0.6 Sc weld fusion zone (Fig.12) showed

only the presence of rod-shaped precipitates. EDS (Fig.13)

analysis confirms that these are (Al2Cu) precipitates.

There were no Al3Sc dispersoids. This is because the

driving force available at room temperature is not sufficient

for the Al3Sc dispersoids to form. In addition to Al2Cu, the

TEM picture showed some dark spots. This could be

because of the residual traces of acid, which was used for

the electro-chemical thinning process. Due to improper

cleaning, these traces were left on the specimen. Transmis-

sion electron micrographs were also taken for 2319 + Sc +

Mg welds (Fig. 14). Here, in addition to Al2Cu, there was

another kind of hexagonal-shaped precipitates found. The

compositions of these hexagonal-shaped precipitates were

found to be similar to that of Al2Cu with some amount of

Mg. According to the literature, precipitates are the

modified form of with segregation of Mg in the /

interface. Therefore, in addition to , precipitates also

play a role for the increase in strength. The tensile strength

values for three compositions at 8% deformed condition are

given in Table2. The hardness trend was reflected in tensile

values also. When compared to 0.3% Sc weld, the 0.3%

Sc + 0.4% Mg weld showed an increase of about 70 MPa.

This increment mainly came due to effective strain

hardening in the presence of Mg and the presence of an

additional strengthening precipitate, . Though there was a

good increment in yield strength, the increase in ultimate

tensile strength (UTS) was not significant. Even with an

Fig. 13 TEM EDS of Al2Cu

precipitate

Fig. 14 TEM of deformed 2319 + 0.6 Sc + 0.63 Mg weld fusion

zones

Fig. 15 Effect of direct aging at 190C on fusion zone hardness

values of 2319 + Sc welds



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increase in Sc or Mg percentage, it also did not increase

much. When compared to the as-welded condition, the

increase in yield strength was significant for all composi-

tions and especially for Mg-added welds. The increment

from 158 to 264 MPa for the 0.3 Sc + 0.4 Mg weld is a

noticeable increment. The difference in ultimate tensile

strength was not much for the as-welded and deformed

samples. The decrease in percentage of elongation due to

strain hardening was not much when compared to the as-welded condition. The reason for this good ductility comes

from the fine grain structure.

To study the response of welds for artificial aging, the

welds were subjected to an aging treatment at elevated

temperature (190C) up to 100 h and hardness values were

taken at regular intervals. The response of the 0.3% and

0.6% Sc welds to the aging treatment is given in Fig. 15.

Both welds showed a gradual and very less increase in

fusion zone hardness with increase in aging time, but the

increase in fusion zone hardness with artificial aging was

only about 10 VHN. The response of Sc + Mg welds for

artificial aging treatment is given in Fig. 16. For all thewelds, there was no change in fusion zone hardness values

even for exposures up to 100 h at 190C. An increase in Sc

or Mg content also did not change the response to artificial

aging. Usually, the Al3Sc precipitates are sluggish and the

driving force needed for them to nucleate is also high.

Though there was a nucleation of some and

precipitates, their number was not significant and so were

Fig. 16 Effect of direct aging at 190C on fusion zone hardness of

2319 + Sc + Mg welds

Fig. 17 Effect of deformation + aging (190C) on fusion zone

hardness values of 2319 + 0.3 Sc welds

Fig. 18 TEM of 2319 + 0.6 Sc after deformation + aging

Fig. 19 Effect of deformation + aging (190C) on fusion zone

hardness values of 2319 + 0.3 Sc + 0.4 Mg weld

Fig. 20 TEM of 2319 + 0.6 Sc + 0.4 Mg weld fusion zones after

deformation + aging



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the fusion zone hardness values. So, the effect of direct

aging for the composition-modified welds is not significant

when compared to compressive deformation.

The artificial aging treatment was done for all the welds

to get the maximum advantage of precipitation hardening.

The aging treatment at 190C was done for all the three

percentages of deformations in each composition. Date on

the response of 0.3% and 0.6% Sc welds to the agingtreatment are given in Fig. 17. In case of 0.3% Sc weld,

with the increase in aging time, there was a gradual increase

in the fusion zone hardness and a peak hardness of about

100 VHN was obtained. Though there was a gradual

increment in hardness, the peak hardness value was

maintained even after aging up to 100 h. For the 0.6% Sc

weld, there was a decrease in fusion zone hardness during

the initial stages of aging. With the increase in the aging

time, hardness was of a gradual increment and a maximum

value of about 115 VHN was obtained. After aging for

100 h, a fusion zone hardness value of about 90 VHN was

maintained. The TEM picture of 0.6% Sc weld at the peakaged condition is given in Fig. 18. The TEM clearly

showed the presence of Al3 (Sc, Zr). From the bulls eye

structure, it is clear that during the nucleation of this

precipitate, initially, Al3Zr homogeneously nucleated from

the matrix and on the dislocations created by compressive

deformation. Then, Al3Sc dispersoids formed on the surface

of this Al3Zr precipitates. In addition to this, there could be

a presence of some Al2Cu precipitates. They are not seen in

Fig. 18 since focusing was done in such a way to get the

Al3Sc precipitates. Details on the response of 2319 + Sc +

Mg welds for the artificial aging treatment are given in

Fig.19. For all the compositions, there was a decrement in

the fusion zone hardness values during the initial stages of

aging. With the increase in aging time, there was a gradual

increment and saturation at a reasonably high hardness

value. In all of the cases, peak hardness value of about 120

VHN was obtained. After 100 h of aging, a saturated

hardness value of about 105 VHN was maintained for all

the compositions. During aging, there was a competition

between release of stored energy, recovery, and precipita-

tion. At some point, precipitation exceeded the effect of

recovery and peak fusion zone hardness was obtained. The

distribution of precipitates in the fusion zone after aging is

given in Fig. 20. The precipitation in Sc + Mg welds is a

complicated process. Since different kinds of precipitates

play a role for strength increment, their nucleation and

precipitation kinetics will be different. The possible

precipitates for Sc + Mg composition are Al2Cu, , Al3(Sc, Zr), and possibly some S. Since the Mg content is low,

the possibility of formation of S is less.

The tensile test results for deformed fusion zones

subjected to aging treatment are given in Table 3. The

artificial aging treatment after compressive deformation

resulted in a significant improvement in tensile properties.

This has been attributed to the grain refinement, strength-

ening precipitates, and strain hardening of the fusion zone

caused by the addition of scandium and magnesium to the

conventional filler of AA2319.

4 Conclusions

Modification of conventional 2319 filler with either Sc or

Sc + Mg has proven to be beneficial in refining the fusion

zone microstructures during welding of AA2219 alloy.

With Sc and Mg addition, there was a significant increase in

fusion zone yield strength. Improvement in percentage of

elongation was attributed to fine grain structure of fusion

zone. The effect of direct aging for the composition-modified

welds is not significant when compared to compressive

deformation. The artificial aging treatment after compressive

deformation resulted in a significant improvement in tensile

properties of fusion zone. This has been attributed to the grainrefinement, strengthening precipitates, and strain hardening of

the fusion zone caused by the addition of scandium and

magnesium to the conventional filler of AA2319.

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2009-JMT-Thermo-mechanical Treatments of Sc- And Mg-modified Al