Nanoindentation Measurement of Interfacial Reaction Layers in 6000 Series

Aluminum Alloys and Steel Dissimilar Metal Joints with Alloying Elements*1

Tomo Ogura, Keisuke Ueda*2, Yuichi Saito*3 and Akio Hirose

Division of Materials and Manufacturing Science, Osaka University, Suita 565-0871, Japan

Nanoindentation measurements were successfully applied to the interfacial reaction layers in dissimilar metal joints of 6000 seriesaluminum alloys containing alloying elements to steel in order to characterize their mechanical properties. The nanoindentation hardness of thereaction layer formed at the aluminum side was lower than that formed at the low carbon steel (SPCE) side of the investigated joints. At thealuminum side, the nanoindentation hardness changed by the addition of alloying elements. The hardness of the resulting Al12Fe3Si intermetalliccompound (IMC) (and the same IMC containing Cu) was lower than that of Al3Fe. In comparison with the hardness values obtained from bulkAl-Fe binary series IMCs, it is considered that hardness changes of interfacial reaction layers are derived from the crystal structural changesproduced by the alloying elements. The result of micro-testing of Al-Fe series IMCs indicates that the modification of the interfacial reactionlayer by alloying elements contributes to higher ductility and the improvement of joint strength through crystal structural change.[doi:10.2320/matertrans.L-MZ201112]

(Received October 2, 2010; Accepted December 27, 2010; Published April 20, 2011)

Keywords: dissimilar metal joints of aluminum alloys and steels, interfacial reaction, nanoindentation, diffusion bonding, micro-tensile test

1. Introduction

Reduction of fuel consumption and curbing carbon dioxide(CO2) emissions are key issues being tackled by automotiveindustries to resolve energy problems and global warming.To achieve weight reduction in automobiles at lower cost,hybrid car bodies made from aluminum alloys and steels arefeasible structures, and involve the joining of these dissimilarmetals.1–5) The dissimilar metal joining of 6000 seriesaluminum alloys and steels has been considerably researchedusing several joining techniques, such as spot welding,6) laserwelding7) and friction stir welding (FSW).8) However, it hasbeen a common problem that the formation of a brittlealuminum-rich Al-Fe intermetallic compound (IMC) layer atthe bonded interface causes low strength in aluminum/steeldissimilar metal joints.1) Therefore, microstructual control ofthe interfacial reaction layers is essential to obtain high-reliability dissimilar metal joints. Our previous work reportedthat the interfacial reaction layer was modified by theaddition of silicon and/or copper to 6000 series aluminumalloy through diffusion bonding, and this leads to a higherstrength of an aluminum alloy/low carbon steel (SPCE)dissimilar metal joint for the same thickness of the reactionlayer.9,10) This result suggests that the characteristics of theinterfacial reaction layer would be changed by the addition ofthese alloying elements. Therefore, characterization, such asthe mechanical properties, of IMCs formed at the interfaceduring bonding becomes quite important.

Because nanoindentation techniques enable us to examinethe local deformation and hardness of alloys at the nano-meter-scale, they are a powerful tool for the direct exami-nation of the mechanical properties of micro-scale struc-tures.11,12) Nanoindentation is expected to clarify the

characteristics of the interfacial reaction layers in alumi-num/steel dissimilar joints. In the present work, nano-indentation measurements were therefore applied to theinterfacial reaction layers in dissimilar aluminum/steel jointscontaining alloying elements. From the obtained experimen-tal results, nanoindentation hardness changes are discussedbased on the crystal structures of interfacial reaction layers.Micro-tensile tests were also carried out to evaluate themechanical properties of Al-Fe binary series IMCs.

2. Experimental Procedure

The chemical compositions of the 6000 series aluminumalloys used in the present work are listed in Table 1. For sim-plicity, the Al-0.6%Mg-0.6%Si alloy (in mass%) is desig-nated as the base alloy, whereas the Al-0.6%Mg-1.5%Si, Al-0.6%Mg-0.6%Si-1.0%Cu and Al-0.6%Mg-1.5%Si-1.0%Cualloys are designated as the Si-containing, Cu-containing and(Si+Cu)-containing alloys, respectively. A low carbon steelincluding 0.01%C, 0.15%Mn and 0.01%Si (in mass%)(SPCE) was also prepared. After polishing the surface ofthe samples using emery paper (#2000), the surfaces werecleaned by acetone. Diffusion bonding was carried out in avacuum of <1:0� 10�1 Pa at a temperature at 785 K for aholding time of 1.8 ks with a pressure of 2.5 MPa, whichforms the reaction layers with larger thickness regardless ofthe alloying elements.9,10) The heating rate was 3 K/s. Thebonding apparatus and a specimen used in the present workare shown in Fig. 1. The steel plate was sandwiched between

Table 1 Chemical compositions of the alloys used in this work (in mass%).

Alloy Mg Si Cu Fe Mn Ti Al

Base 0.6 0.6 — 0.18 0.07 0.02 Bal.

Si-containing 0.6 1.5 — 0.18 0.07 0.02 Bal.

Cu-containing 0.6 0.6 1.0 — — — Bal.

(Si+Cu)-containing 0.6 1.5 1.0 — — — Bal.

*1The Paper Contains Partial Overlap with the ICAA12 Proceedings by

USB under the Permission of the Editorial Committee.*2Graduate Student, Osaka University*3Graduate Student, Osaka University. Present address: Nippon Kaiji

Kyokai, Tokyo 102-8567, Japan

Materials Transactions, Vol. 52, No. 5 (2011) pp. 979 to 984Special Issue on Aluminium Alloys 2010#2011 The Japan Institute of Light Metals

two aluminum alloy cylinders and put into a vacuumchamber. The resulting joint sample was embedded inconductive resin, and then cut vertically through the bondedsurface for nanoindentation measurements.

Nanoindentation measurements were performed along theinterface of the joints at 2 mm intervals using an ENT-1100a(Elionix Inc.) instrument. The applied load was 50 mgf andthe load and release times were 10 and 1 s, respectively.Microstructual observation and composition analysis afternanoindentation measurement were performed using electronprobe microanalysis (EPMA; JXA-8700, JEOL Ltd.). Micro-tensile tests were also carried out with a cross-head speed of1 mm/s at R:T:, in order to evaluate the mechanical propertiesof Al-Fe binary series IMCs. The specimen for a micro-tensile test was prepared by wire-cutting, not from thereaction layer but from another bulk Al-Fe IMC specimen, asshown in Fig. 2. Elongation is estimated by measuringdirectly the change of gage length during deformation of aspecimen using a microscope.

3. Results and Discussion

3.1 Nanoindentation hardness changes of the reactionlayers in aluminum alloys/SPCE dissimilar metaljoints

Figure 3(a) shows a typical SEM image at the interface of

the (Si+Cu)-containing alloy/steel joint after nanoindenta-tion measurements. Although no indentations were seen inthe interfacial reaction layer due to the very low indentationdepth (< 50 nm), it can be confirmed that some of theindentations were successfully located within the reactionlayer from the trace of indentations in the matrix (becausethey were located at regular 2 mm intervals). Figures 3(b) and(c) show EPMA mapping results of silicon and copper,respectively. It was found that both silicon and copper wereenriched within the interfacial reaction layer, especially onthe aluminum side, indicating the presence of a copper-containing Al12Fe3Si IMC. The interfacial reaction layers inthis bonding condition are summarized in Table 2, which wasobtained from TEM observations and EDX analyses in ourprevious work.9,10) At the initial stage of bonding, Al3Fe IMCis formed at the aluminum side in the base alloy joint,whereas Al12Fe3Si (containing copper) IMC is formed whenalloying elements are present. Then, at a later stage ofbonding, an Al5Fe2 IMC is additionally formed at the SPCEside of the joint.

The nanoindentation hardness was estimated as a functionof the distance from the interface of all the investigatedjoints, as shown in Fig. 4. The center of the interface isdefined as the boundary between the SPCE and the reactionlayers. The corresponding SEM images are also shown. Thehardness in each interfacial reaction layer was classified intodifferent categories from the results of EPMA mapping. In allthe investigated joints, the hardness of reaction layers ismuch higher than that of the aluminum alloy or the SPCEmatrix, as normally estimated using a conventional hardnesstest. In addition, within the interfacial reaction layers, thehardness of the reaction layer at the aluminum side (blacksymbol) was found to be lower than that at the SPCE side(gray symbol).

Nanoindentation hardness results for the interfacial reac-tion layers are summarized in Fig. 5, together with thecompositions of the corresponding IMCs. At the SPCE side,the hardness value of the interfacial reaction layer is the

(a)

Aluminum alloy 5

15ϕ10

ϕ9

(b)

Infrared Oven

Gas

Pressure control system

Air cylinder

Pressure rod

Vacuum

15

5

Aluminumalloy

ϕ9

1.1

Unit:mmϕ10

SteelThermocouple

Vacuum Pump

and

Evacuation

Work piece

Chamber

Heat

Controller

System

Fig. 1 (a) Schematic illustration of bonding apparatus and (b) dimensions of the specimen used for diffusion bonding.

1.6

0.2

t = 0.3

3.0

0.71.60.7

1.1

Unit: mm

Fig. 2 Dimensions of the specimen used for micro-tensile test.

980 T. Ogura, K. Ueda, Y. Saito and A. Hirose

same, regardless of the alloying elements, showing thatAl5Fe2 IMC is formed in this area, as found in the earlierwork shown in Table 2. On the other hand, the hardness ofthe reaction layer at the aluminum side was changed by thepresence of alloying elements. The hardness in the reactionlayer in the base alloy joint (Al3Fe IMC) is lower than that ofAl5Fe2 as normally measured using micro-Vickers hard-ness.13) It should be noted that the hardness in the reactionlayer of the Al12Fe3Si IMC is lower than that of Al3Fe IMCin the Si-containing alloy. The effect becomes larger byincorporating 2% copper to Al12F3Si IMC in the (Si+Cu)-containing alloy joint.

3.2 Nanoindentation hardness and tensile propertieschanges in Al-Fe binary series IMCs

From the nanoindentation measurement around the inter-

face in aluminum alloys/SPCE joints, it was recognized thatthe hardness of the reaction layers were much higher than thatof aluminum and SPCE matrix (Fig. 4). Moreover, thehardness in the reaction layer of Al12Fe3Si IMC was lowerthan that of Al3Fe IMC for the Si-containing alloy and theeffect was increased by incorporating 2% copper in Al12F3Si(Fig. 5).

To better understand the hardness changes found in thepresent work, nanoindentation measurements were alsoperformed on Al-Fe binary series IMCs. From the phasediagram, it is well known that there are five types of Al-FeIMCs (AlFe3, AlFe, AlFe2, Al5Fe2, and Al3Fe).14) Bulkspecimens of these Al-Fe IMCs were therefore prepared.Nanoindentation hardness results for these Al-Fe series IMCsare shown in Fig. 6. The hardness of Al5Fe2 was the highestin Al-Fe IMCs. With the exception of Al3Fe, the hardnessmonotonously decreased with an increasing atomic ratio ofiron. The hardness values of the Al5Fe2 IMC and Al3Fe IMCare in good agreement with the hardness of reaction layers ofthese materials reported in Fig. 5.

The observed hardness changes are considered to bederived from the crystal structures of the IMCs involved. Thecrystal structures of Al-Fe binary series IMCs are shown inTable 3.15) Aluminum rich-IMCs (Al2Fe, Al5Fe2 and Al3Fe)have a small number of slip planes, causing them to be brittle.In contrast, equivalent or iron-rich IMCs (AlFe and AlFe3)have cubic structures, having a comparatively larger numberof slip planes, which leads them to be ductile. Therefore, it isconsidered that the hardness decrease produced by alloyingelements in the reaction layer at the aluminum side of thejoints (Fig. 5) originates in a change of crystal structure.

To evaluate the effect of these crystal changes on themechanical properties of Al-Fe series IMCs, micro-tensiletests were carried out. Although specimens of AlFe3 andAlFe (40%Al) IMCs were succesfully wire-cut, Al2Fe,Al5Fe2 and Al3Fe IMCs were too brittle to cut. The obtainedstress-elongation curves to fracture are shown in Fig. 7.Some of the noise in the curves was caused not by thematerials, but by electrical signals from the equipment. It isseen that AlFe3 IMC has higher strength and elongation thanAlFe IMC. In particular, the average elongation of AlFe3

IMC is 3.5(%) is more than twice that of AlFe IMC (1.4%),showing good deformability. We recognize that tensileproperties of these Al-Fe IMCs are indirect evidence ofthose of Al12Fe3Si (and containing Cu) IMCs and that more

(a) SPCE

(b) Si

(Si+Cu)-containing

(c) Cu

5µm

Fig. 3 (a) An SEM image of nanoindentation marks around the interface

and (b) and (c) corresponding EPMA mappings in the (Si+Cu)-containing

alloy/SPCE joint bonded at 785 K for 1.8 ks. Nanoindentation marks are

indicated by the arrow.

Table 2 Effects of alloying elements on the reaction layers in aluminum

alloy/SPCE joints.9;10Þ

Reaction layer formed at

initial stage of bonding

Reaction layers formed at

later stage of bonding

Base Al3Fe Al3Fe + Al5Fe2

Si-containing Al12Fe3Si Al12Fe3Si + Al5Fe2

Cu-containing

Al12Fe3Si containing

lower Cu (<0:5%)

Al12Fe3Si containing

lower Cu (<0:5%)

+ Al5Fe2

(Si+Cu)-containing

Al12Fe3Si containing

higher Cu (�2%)

Al12Fe3Si containing

higher Cu (�2%)

+ Al5Fe2

Nanoindentation Measurement of Interfacial Reaction Layers in 6000 Series Aluminum Alloys and Steel Dissimilar Metal Joints 981

study is needed. However, the tensile properties of a coppercontaining Al12Fe3Si IMC is thought to be very similar tothose of the Al-Fe IMCs shown in Fig. 7 based on the resultsfrom nanoindentation hardness measurements and theircrystal structures. Therefore, the high ductility of a coppercontaining Al12Fe3Si IMC would lead to a higher strength ofthe (Sn+Cu)-containing joint.9) The modification of theinterfacial reaction layer by alloying elements is, therefore,considered to contribute the improvement of joint strengththrough changes in the crystal structure.

4. Conclusions

Nanoindentation measurements were successfully appliedto the interfacial reaction layers formed in dissimilar metaljoints of 6000 series aluminum alloys to steel with alloyingelements in order to characterize their mechanical properties.Micro-tensile tests were also carried out to evaluate themechanical properties of Al-Fe binary series IMCs. Theresults obtained in the present work are summarized asfollows.

(b)(a) SPCESPCE

8

10

8

10

5µm 5µmSi-containingBase

-6 -4 -2 00

2

4

6

-6 -4 -2 00

2

4

6

Har

dnes

s, H

/GPa

Base

SPCESn-

containing

SPCE

(d)(c)

Distance from an interface of the joint, l /µm

SPCESPCE

6

8

10

6

8

10

5µm5µm

ess,

H /G

Pa

SPCE

(Si+Cu)-containingCu-containing

-6 -4 -2 00

2

4

-6 -4 -2 00

2

4

Har

dn

Distance from an interface of the joint, l /µm

SPCECu-containing

(Si+Cu)-containing

2

2 2

2

Fig. 4 SEM images and distributions of hardness around the interface in the (a) base alloy, (b) Si-containing alloy, (c) Cu-containing alloy

and (d) (Si+Cu)-containing alloy/SPCE joints bonded at 785 K for 1.8 ks. The hardness of the reaction layer in the aluminum side and the

SPCE side is plotted in black and gray symbols, respectively.

982 T. Ogura, K. Ueda, Y. Saito and A. Hirose

The nanoindentation hardness of the reaction layerformed at aluminum side was lower than that formed atSPCE side in all joints. Moreover, the hardness of Al12Fe3Sicontaining 2% copper IMC was lower than that of Al3Fe atthe aluminum side. The hardness of bulk IMC specimensmonotonously decreased with increasing atomic ratio ofiron from Al5Fe2 to AlFe3. In comparison with the hardnessresult of Al-Fe binary series IMCs, it is considered thathardness changes of interfacial reaction layers are derivedfrom the crystal structural change produced by alloyingelements. The result of micro-testing of Al-Fe series IMCsindicates the modification of the interfacial reaction layer byalloying elements contributes to higher ductility and theimprovement of joint strength through crystal structuralchange.

Acknowledgements

The authors would like to thank Prof. T. Nakano and Dr. T.Ishimoto of Osaka University for their significant help withnanoindentation measurement, and Prof. F. Minami, Prof. M.Ohata and Dr. Y. Takashima of Osaka University for theirsignificant help with micro-tensile test. We deeply acknowl-edge Prof. H. Y. Yasuda of Osaka University for providingmaterial for this research. We are also grateful to Mr. K.Ohmitsu of Osaka University for operating EPMA. This workwas supported by a Grant-in-Aid for Young Scientists (B)No. 21760585, Japan, and Priority Assistance for theFormation of Worldwide Renowned Centers of Research—The Global COE Program (Project: Center of Excellence forAdvanced Structural and Functional Materials Design) fromthe Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Japan.

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8H

ardn

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