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Analysis of intermetallic layer in dissimilar TIGwelding–brazing butt joint of aluminium alloyto stainless steel

J. L. Song, S. B. Lin*, C. L. Yang, C. L. Fan and G. C. Ma

Intermetallic layer of dissimilar tungsten inert gas welding–brazing butt joint of aluminium alloy/

stainless steel has been studied. A visible unequal thickness intermetallic layer has formed in

welded seam/steel interface, and the thickness of the whole layer is ,10 mm. The interface with

Al–12Si filler metal consists of t5-Al8Fe2Si layer in welded seam side and h-(Al,Si)13Fe4 layer in

steel side with the hardness values of 1025 and 835 HV respectively, while the interface with Al–

6Cu filler metal consists of h-Al13(Fe,Cu)4 layer with the hardness of 645 HV. The average tensile

strength of the joint with Al–12Si filler metal is 100–120 MPa, and the fracture occurs at h-

(Al,Si)13Fe4 layer, while the joint with Al–6%Cu filler metal presents high crack resistance with

tensile strength of 155–175 MPa, which reaches more than 50% of aluminium base metal

strength.

Keywords: Aluminium–steel hybrid joint, TIG welding–brazing, Intermetallic layer, Phase identification, Mechanical properties

IntroductionAgainst the background of the required weight reduc-tion in transportation through lightweight construction,the application of hybrid structures between aluminiumalloy and steel has a huge industrial interest.1,2 However,joining of aluminium alloy and steel has great difficultyby fusion welding since mass of brittle intermetallics isformed in the joint. Solid state welding methods, such asfriction welding and friction stir spot welding, have beenused to make these dissimilar metals joint.3,4 However,the shape and size of such solid state joints are extremelyrestricted. For instance, solid state welding processes aregenerally not suitable for butt joints of long lengths inthick materials. In the previous study,5 the authorsreported that dissimilar metals between 5A06 aluminiumalloy and SUS321 stainless steel were butt joinedsuccessfully by tungsten inert gas (TIG) welding–brazingand that the joint has a dual characteristic: in thealuminium alloy side, it is a welding joint, while in thesteel side, it is a brazing joint.

In this process, the base materials and filler metal areheated or melted by TIG arc heat, and the joiningbetween aluminium alloy and steel is based on theinteraction of a liquid aluminium alloy with a solidsteel.6,7 During the interaction time, the solid surfacedissolves into the melt, and intermetallic layers aresubsequently formed at the interface. Usually, theformation of brittle intermetallic layers limits the

mechanical resistance of the transition zone betweendissimilar metals,8 when their thickness exceeded apermissible value. Most of past reports about laser orcold metal transfer arc brazing of aluminium alloy andsteel8–10 indicated that Si additions in the filler metalseffectively control the growth of the Al–Fe intermetalliclayer. Furthermore, the present study shows that Cu,Mn and Ti additions in the filler metals also havebenefits to the Al–Fe intermetallic layer’s property.11,12

However, the detailed observation on the intermetallicsin the TIG welding–brazing butt joint of aluminium alloyand steel has never been reported, and the mechanism foralloying elements in the filler metal to control the growthof the intermetallic layer is currently unknown. In thepresent work, the intermetallics, especially the interme-tallic layer, in the joint, has been followed by observationin SEM and TEM. The mechanical properties of thejoints have been measured under the dynamic ultrami-crohardness tester and SEM in situ tensile tester.Different reaction mechanisms for Si and Cu additionsto control the growth of the intermetallic layer have beenanalysed and compared as well.

ExperimentalMaterials used are 5A06 aluminium alloy and SUS321stainless steel plates in 3?0 mm thickness. The fillermetals used are 4047 Al–12Si and 2319 Al–6Cu weldingwires, with a diameter of 2?5 mm. The chemicalcompositions of the base material and filler metal areshown in Tables 1 and 2 respectively. The maincompositions of modified non-corrosive flux areNocolok flux (KAlF4 and K3AlF6 eutectic), Zn and Snmetal powders, K2SiF6, etc.

State Key Laboratory of Advanced Welding Production Technology,School of Materials Science and Engineering, Harbin Institute ofTechnology, Harbin 150001, China

*Corresponding author, email [email protected]

� 2010 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 11 October 2009; accepted 9 December 2009DOI 10.1179/136217110X12665048207610 Science and Technology of Welding and Joining 2010 VOL 15 NO 3 213

All plates were cut into the size of 100650 mm, andthe surface was cleaned by abrasive paper and acetonebefore experiment. A single V groove was opened in theplates, with a bevel angle of 40u in the steel side and 30uin the aluminium alloy side. The flux suspension (fluxpowder dissolved in acetone) was smeared homoge-neously in a 0?2–0?5 mm thickness on the groove and onboth front and back surfaces of the steel in 10 mmwidth. Tungsten inert gas welding–brazing was carriedout using ac TIG welding source. The welding para-meters were welding current of 135 A, arc length of 3?0–4?0 mm, welding speed of 120 mm min21 and argon gasflowrate of 8–10 L min21. The schematic of aluminium–steel butt TIG welding–brazing process is shown inFig. 1.

After welding, a typical cross-section of the joint wascut and mounted in self-setting epoxy resin in an asclamped condition. Then, the samples were polished to amirror-like surface aspect. Microstructures and compo-sitions of intermetallics in the joint were measured byscanning electron microscopy (SEM) and energy dis-persive spectrometer (EDS). The intermetallics in thejoint were identified by selected area electron diffraction(SAED) in the transmission electron microscope (TEM).The mechanisms for Si and Cu additions to control thegrowth of the intermetallic layer were analysed andcompared as well.

Results and discussion

Microstructures of intermetallic layersSEM images of interface regions A and B, plotted bysquares in Fig. 1, in an aluminium–steel butt joint withdifferent filler metals are shown in Fig. 2. A visibleunequal thickness intermetallic layer has formed in thewelded seam/steel interface, and the whole layer

presents different patterns with different filler metals.With Al–12Si filler metal, as shown in Fig. 2a and b,the interface presents a serrated shape oriented towardsthe welded seam, and the average thickness of theinterface is 6–8 mm. Some reports6–8 have shown that,to obtain effective joining by arc welding–brazing, aconnective intermetallic layer is required, but it maynot grow more than 10 mm because, at that point, itbecomes too brittle for technical purpose. The interfaceat the upper part of interface region A is thicker thanat the lower part of interface region B due to the higherheat input in the upper part. However, with Al–6Cufiller metal, as shown in Fig. 2c–d, the interfacepresents a compact lath shaped structure, and theinterface thickness is 2–4 mm, which is less than thatwith Al–12Si filler metal.

Phase identification of intermetallicsThe intermetallic phases have also been identified bySEM–EDS and TEM–SAED. Figure 3a and b showsthe magnified SEM morphology and the line scanningresults of intermetallic layers with Al–12Si filler metal.The interface consists of two different intermetalliclayers, named layers I and II, from the welded seam tothe steel substrate. Layer I in the welded seam sidepresents a compact faceted structure, while in the steelside, layer II is a needle-like crystal oriented towardslayer I. As can be seen, the consistence of Al and Si issmaller from layers I to II, while Fe, Cr and Ni increasegradually in the layers. Si enriches in the interface andparticipates in the intermetallic layer’s formation.Figure 3c and d are the SAED patterns of layers I andII. By SAED patterns together with the compositions ofthe intermetallic layers measured by EDS, layer I hasbeen identified to be t5-Al8Fe2Si, which has a hexagonalunit cell with a51?2406 nm, b51?2406 nm, c52?6236

Table 2 Chemical compositions of filler metals, wt-%

Elements Si Fe Cu Zn Mn Mg Ti Zr Al

Al–12Si 11?0–13?0 ,0?8 ,0?3 ,0?2 ,0?05 ,0?1 … … Bal.Al–6Cu ,0?2 ,0?3 5?8–6?8 ,0?1 0?2–0?4 0?2–0?4 0?1–0?2 0?1–0?2 Bal.

Table 1 Chemical compositions of base materials, wt-%

Elements C Mn Mg Al Si Cu Zn Ti Ni Cr Fe

5A06 … 0?5–0?8 5?8–6?8 Bal. 0?4 0?1 0?2 0?1 … … 0?4SUS321 0?12 2?0 … … 1?0 … … 0?2 8?0–11?0 17?0–19?0 Bal.

1 Schematic illustration of TIG welding–brazing process

Song et al. Intermetallic layer of dissimilar TIG welding–brazing butt joint of aluminium alloy to stainless steel

Science and Technology of Welding and Joining 2010 VOL 15 NO 3 214

and c/a52?115,13–15 and layer II has been identifiedto be h-(Al,Si)13Fe4, with which nearly 7 wt-%Si isin solid solution and which has a monoclinic unitcell with a51?5489 nm, b50?8083 nm, c51?2476 andb5107?72u.15,16

Figure 4a and b shows the magnified SEM morphol-ogy and the line scanning results of intermetallic layerwith Al–6Cu filler metal. As can be seen, the consistencyof Al and Cu is smaller form welded seam to steelsubstrate, while Fe, Cr and Ni increase gradually in the

a, b with Al–12Si wire; c, d with Al–6Cu wire2 Microstructures of interface regions A and B, as shown in Fig. 1, in aluminium–steel butt joint with different filler

metals

a magnified SEM morphology; b line scanning results of intermetallic layers; c, d TEM diffraction spots in layer I along[010] direction and in layer II along [121] direction

3 Intermetallic layers in aluminium–steel joint with Al–12 wt-%Si wire



layer. At the same time, the slender Cu rich particle,marked by A, precipitates in welded seam near the layerdue to the high cooling rate. Figure 4c and d are theSAED patterns of the particles. By SAED patternstogether with the compositions of the particles measuredby EDS, the slender Cu rich particle is Al2Cutphase formed in the grain boundary of welded seamtwith he (AlzAl2Cu) divorced eutectic reaction,17 andthe intermetallic layer has been identified to beh-Al13(Fe,Cu)4, with which nearly 4 wt-%Cu is insolid solution and which has a monoclinic unit cellwith a51?5489 nm, b50?8083 nm, c51?2476 andb5107?72u.18–20 Moreover, each layer contains somecontents of Cr and Ni elements to replace Fe inintermetallic layers, which is believed to be beneficialto the quality of the layers.11,12

Mechanical properties of jointsVickers microhardness of the interface in joint ismeasured under dynamic ultramicrohardness tester with100 mN loading force and 10 s holding time. Theaverages of five measurements in each material areshown in Fig. 5. The original hardness of welded seamsis 94?4 HV with Al–12Si filler metal and 104?5 HV withAl–6Cu filler metal, and the value of the steel substrate is250 HV. However, the hardness increases quickly in theinterface of the joint. With Al–12Si filler metal, theinterface hardness values are 1025 HV in the t5-Al8Fe2Silayer and 835 HV in the h-(Al,Si)13Fe4 layer. With Al–6Cu filler metal, the interface presents the averagehardness of 645 HV in the h-Al13(Fe,Cu)4 layer, which isless than the hardness value 890 HV of standard Fe4Al13

phase. Some contents of Cu atoms replacing Fe inAl13Fe4 can reduce its hardness greatly, which canreduce its brittleness to enhance the intermetallic layerjoining between welded seam and steel, while Si atoms

dissolve in the h-Al13Fe4 phase to form the super-saturated solid solution, which has less effect on thephase hardness.

SEM in situ tensile test was carried out in order tomeasure the tensile strength of the joints with differentfiller metals, the results of which are shown in Fig. 6.Figure 7a and b shows the fracture position of the jointwith Al–12%Si filler metal. The interface region is theweak zone of the joint, and the fracture occurs ath-(Al,Si)13Fe4 layer, and the average tensile strength ofthe joint is 100–120 MPa. Figure 7c and d shows thefracture position of the joint with Al–6%Cu filler metal.The h-Al13(Fe,Cu)4 layer presents a high crack resis-tance, and the fracture initiates from h-Al13(Fe,Cu)4

layer at the bottom of the joint and derives into theslender Al2Cu particle in the grain boundary of weldedseam at the upper part of the joint; the average tensile

a magnified SEM morphology; b line scanning results of intermetallic layers; c, d TEM diffraction spots in A particlesalong [110] direction and in B particles along [010] direction

4 Intermetallic layers in aluminium–steel joint with Al–6Cu wire

5 Microhardness distribution across interface of joints



strength of the joint reaches 155–175 MPa, whichreaches more than 50% of aluminium base metalstrength.

According to the observations above, (Al,Si)13Fe4 is ahigh brittle phase, and Si addition in the filler metal hasa limited effect on improving the crack resistance ofh-Al13Fe4, while Al13(Fe,Cu)4 presents a high crackresistance, and Cu addition can effectively improve thetensile property of h-Al13Fe4. Comparing (Al,Si)13Fe4

and Al13(Fe,Cu)4, it is easily found that Si and Cuadditions have different reaction mechanisms with Aland Fe atoms, which can be expressed as

8AlzSiz2Fe?Al8Fe2Si (1)

13xAlz13ySiz4Fe? AlxSiy� �

13Fe4 xzy~1ð Þ (2)

13Alz4n Cuz4m Fe?Al13 Fem,Cunð Þ4 mzn~1ð Þ (3)

According to equations (1) and (2), Si additions caneasily react with Fe atoms to form Si–Fe covalent bondsin Al8Fe2Si and (Al,Si)13Fe4 phases. As is known,comparing with Al–Fe bond, the Si–Fe covalent bondalso presents a high brittleness,15 so Al8Fe2Si and(Al,Si)13Fe4 are both of brittle phases. However,according to equation (3), Cu additions in filler metalreact with Al atoms to form Al–Cu bonds inAl13(Fe,Cu)4 phase, and Al–Cu bond presents higherbinding ability than Al–Fe,21,22 so Al13(Fe,Cu)4 presentsa high crack resistance. Moreover, with the addition ofSi in filler metal, the solubility and dissolution rates ofFe in aluminium molten pool increase greatly.23,24 Onthe contrary, Cu additions in filler metal decrease thesolubility and dissolution rates of Fe in aluminiummolten pool.25 Therefore, with Al–Si filler metal, moreFe atoms dissolve into the molten pool to form thickerintermetallic layers than that with Al–Cu filler metal.

Conclusions

1. In aluminium–steel TIG welding–brazing joint, avisible unequal thickness intermetallic layer has formedin welded seam/steel interface, and the whole layerpresents different patterns, and the thickness of thelayers is ,10 mm.

2. The interface with Al–12Si filler metal consists oftwo different intermetallic layers, t5-Al8Fe2Si layer inthe welded seam side and h-(Al,Si)13Fe4 layer in the steelside, which has high brittleness, while the interface withAl–6Cu filler metal consists of h-Al13(Fe,Cu)4 layer,which presents a high crack resistance.

a, b with Al–12Si wire, c, d with Al–6Cu wire7 SEM in situ tensile test images of interface region in aluminium–steel joint with different filler metals before and after

test

6 Tensile strengths of joints with different filler metals



3. The average tensile strength of the joint with Al–12Si filler metal is 100–120 MPa, and the fracture occursat h-(Al,Si)13Fe4 layer, while the joint with Al–6Cu fillermetal presents high crack resistance with the tensilestrength of 155–175 MPa, and the fracture initiates fromh-Al13(Fe,Cu)4 layer at the bottom of the joint andderives into the slender Al2Cu particle in the grainboundary of welded seam at the upper part of the joint.

Acknowledgements

The authors would like to thank State Key Laboratoryof Welding of China, all of the work within which werecarried out. They also appreciate the financial supportfrom the National Natural Science Foundation of China(grant no. 50874033).

References1. C. Y. Lee, D. H. Choi, Y. M. Yeon and S. B. Jung: Sci. Technol.

Weld. Join., 2009, 14, (3), 216–220.

2. M. Staubach, S. Juttner, U. Fussel and M. Dietrich: Weld. Cutt.,

2008, 7, (1), 30–38.

3. M. Kimura, H. Ishii, M. Kusaka, K. Kaizu and A. Fuji: Sci.

Technol. Weld. Join., 2009, 14, (5), 388–395.

4. T. Liyanage, J. Kilbourne, A. P. Gerlich and T. H. North: Sci.


5. J. L. Song, S. B. Lin, C. L. Yang, G. C. Ma and H. Liu: Mater. Sci.

Eng. A, 2009, A509, (1/2), 31–40.

6. S. B. Lin, J. L. Song, C. L. Yang and G. C. Ma: Sci. Technol. Weld.

Join., 2009, 14, (7), 636–639.

7. L. M. Liu and S. X. Wang: Sci. Technol. Weld. Join., 2006, 11, (5),

523–525.

8. G. Sierra, P. Peyre, F. D. Beaume, D. Stuart and G. Fras: Mater.

Charact., 2009, 59, (12), 1705–1715.

9. H. T. Zhang, J. C. Feng, P. He and H. Hackl: Mater. Charact.,

2007, 58, (7), 588–592.

10. G. Sierra, P. Peyre, F. D. Beaume, D. Stuart and G. Fras: Sci.


11. A. O. Mekhrabov and M. V. Akdeniz: Acta Mater., 1999, 47, (7),

2067–2075.

12. S. Seifeddine, S. Johansson and I. L. Svensson: Mater. Sci. Eng. A,

2008, A490, (1/2), 385–390.

13. M. V. Kral, H. R. McIntyre and M. J. Smillie: Scr. Mater., 2004,

51, (3), 215–219.

14. K. A. Nazari and S. G. Shabestari: J. Alloys Compd, 2009, 478, (1/

2), 523–530.

15. Y. Du, J. C. Schuster, Z. K. Liu, R. Hu, P. Nash, W. Sun, W.

Zhang, J. Wang, L. Zhang, C. Tang, Z. Zhu, S. Liu, Y. Ouyang, W.

Zhang and N. Krendelsberger: Intermetallics, 2008, 16, (4), 554–

570.

16. A. E. Kiv, V. I. Ezersky and M. M. Talianker: ‘The stability of

structures with icosahedral local order in Al-based alloys with

transition metals’, Mater. Sci. Eng. A, 2003, A352, (1/2), 100–

104.

17. A. Boag, A. E. Hughes, N. Wilson, A. Torpy, C. MacRae, A. M.

Glenn and T. H. Muster: Corros. Sci., 2009, 51, (8), 1565–

1568.

18. P. R. Goulart, V. B. Lazarine, C. V. Leal, J. E. Spinelli, N. Cheung

and A. Garcia: Intermetallics, 2009, 17, (9), 753–761.

19. S. Miyazki, A. Kawachi, S. Kumai and A. Sato: Mater. Sci. Eng.

A, 2005, A400/A401, (1/2), 294–299.

20. Y. J. Li and L. Arnberg: Acta Mater., 2004, 52, (10), 2945–2952.

21. L. Dubourg, F. Hlawka and A. Cornet: Surf. Coat. Technol., 2002,

151/152, 329–332.

22. G. Rosas and R. Perez: Mater. Sci. Eng. A, 2001, A298, (1/2), 79–

83.

23. Y. Y. Chang, W. J. Cheng and C. J. Wang: Mater. Charact., 2009,

60, (2), 144–159.

24. M. Vybornov, P. Rogl and F. Sommer: J. Alloys Compd, 1997, 247,

(1/2), 154–157.

25. D. Chen, Z. H. Chen, J. H. Chen and P. Y. Huang: J. Alloys

Compd, 2004, 376, (1/2), 89–94.



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