Weldability of CMT Joining of AA6061T6 to Boron Steels...

Introduction

Boron-based hot-forming steels(Ref. 1) and aluminum alloys (Refs. 2–3) have been widely applied for body-in-white construction. Making use ofthese materials not only reduces vehi-cle weight, but it also improves crash-worthiness and decreases gasemissions (Ref. 4). To prevent surfaceoxidation of boron steels during warmforming [i.e., 850°C during the form-ing followed by a cooling rate greater

than 50°C/s (Ref. 5)], various coatingshave been developed. While the coat-ings minimize the surface oxidation,their influence on the weldability ofboron-based steel is a concern. Sinceboth boron-based steel and aluminumalloy are being applied for structuralapplications, it is inevitable toencounter the need to join themtogether. The joining of aluminum to steel isa great challenge because of the largedifferences in thermo-physical proper-

ties between the two materials and es-pecially the formation of brittle Al-Feintermetallic compounds (IMCs) at el-evated temperatures (Ref. 6). The for-mation of Fe-Al phases is necessary forachieving an effectual connection be-tween aluminum and steel, but an ex-cessive formation of Fe-Al IMC resultsin brittleness of the joints (Ref. 7).Thus, in order to suppress the largeformation of Fe-Al IMC, solid-statewelding methods, e.g., diffusion weld-ing (Ref. 8), explosive welding (Ref. 9),friction welding (Refs. 10–12), and ul-trasonic welding (Ref. 13) have beentried to join these dissimilar metalsjoints. Though the IMC could be hin-dered using these processes, high effi-ciency is still lacking, and the shapeand size of such solid-state joints arerestricted. Fusion welding processes are feasi-ble to join dissimilar metals with highefficiency (Ref. 14). From the Al-Fephase diagram (Ref. 15) shown in Fig.1, Fe-Al IMC could be formed betweenAl and Fe during the welding process.However, the brittle Fe-Al IMC couldbe restricted by only melting the alu-minum alloy because of the huge dif-ference in melting points of Al (649°C)and iron (1539°C). Gas tungsten arcwelding (GTAW)-brazing (Ref. 16) andlaser arc welding-brazing (Ref. 17) hadbeen used to connect Al and steel suc-cessfully. A fusion welding methodwith low heat input and highefficiency like cold metal transfer

WELDING RESEARCH

JUNE 2014 / WELDING JOURNAL 193-s

SUPPLEMENT TO THE WELDING JOURNAL, JUNE 2014Sponsored by the American Welding Society and the Welding Research Council

Weldability of CMT Joining of AA6061T6 to Boron Steels with Various Coatings

R. CAO, J. H. SUN, and J. H. CHEN are with Lanzhou University of Technology, Gansu, China. PeiChung WANG is with GM Research and Development Center,Warren, Mich.

ABSTRACT The effect of coatings on the weldability of cold metal transfer (CMT) joining ofAA6061T6 to boron steels was investigated. Samples were fabricated and the weldappearance, microstructure, and composition analyses of the joints were examinedusing scanning electron microscope (SEM), energydispersive spectrometer (EDS), Xray diffraction, and transmission electron microscope (TEM). In addition, thestrengths of various joints were evaluated. It was found that molten zinc from a galvanized coating enhanced the molten aluminum filler metal to the wet boron steelsubstrate, and decreased the heat on the surface of the steel sheet, which made thestrength of the joints decrease. The FeZn coating from galvannealed boron steel wasnot fully molten, and consequently it was of little assistance for the molten aluminumfiller metal to wet the boron steel substrate, leading to poor joint quality.Furthermore, sound CMT weldbraze AA6061T6 to AlSi coated boron steel andAA6061T6 to bare boron steel could not be properly produced due to largeformation of brittle FeAl3 intermetallic during the welding process.

KEYWORDS AA6061T6 • Boron Steels • Coatings • Cold Metal Transfer (CMT)• Weldability • Microstructure • Dissimilar Metals

In this study, the effect of coatings on joining aluminum to steel using a cold metal transfer process was investigated

BY R. CAO, J. H. SUN, J. H. CHEN, AND PeiChung WANG

Cao Supplement new_Layout 1 5/15/14 4:13 PM 93

(CMT) welding-brazing (Ref. 14)might offer a solution for aluminumuse in automobile applications. Thereare many advantages to joiningaluminum to steel by the CMT joiningtechnique (Ref. 18). Importantly, theheat input can be controlled, as well asthe IMC formation and thickness,thereby enabling optimization of thejoint strength. However, it was unfeasible to joinAl and steel with a fusion processwithout a coating because of a largeformation of Fe-Al IMC. The coatinghas a significant role in the welding ofAl and steel. As described earlier,boron steel and aluminum alloys arebeing widely used in the automotiveindustry, and various coatings such asAl-Si and Zn were investigated to pre-vent surface oxidation during thehigh-temperature process (Ref. 19).Thus, it is inevitable to join aluminumto boron steels with various coatings. In this study, the effect of coatingson the weldability of CMT joining

AA6061-T6 toboron steels wasinvestigated. Weldappearance,microstructureanalyses, and ten-sile-shear strengthof the weldedjoints and joiningmechanism at the

braze interface were examined anddiscussed.

Experimental Procedure

Materials

The materials used in the studyincluded 1-mm-thick aluminumAA6061-T6 alloy and 1.5-mm-thickbare, galvanized and galvannealedboron steels. Per our experimentalmeasurements, the compositions ofboron steels are shown in Table 1.Al4043 welding wire with a diameter of1.2 mm was used. Per themanufacturer’s datasheet, Table 2 liststhe compositions of Al4043 weldingwire and AA6061-T6 sheet.

CMT Joining Process

The key feature of the CMT processis the wire motion has been integratedinto the joining process and into the

overall control of the process. The wireretraction motion assists the dropletdetachment during the short circuit,and the metal can transfer into thewelding pool without the aid of theelectromagnetic force. Thus, there isgreat control of heat input and weldspatter (Refs. 20, 21). From the Al-Fe phase diagram (Fig.1), the melting points of aluminumand iron are 649°C and 1539°C,respectively. This huge difference inmelting points makes it difficult tocreate a metallurgical bond betweenthe aluminum alloy and steel viafusion joining processes without form-ing intermetallic compounds. One ap-proach to skirt this issue is to controlthe heat input and keep the steel solidstate during the welding process.Therefore, in this study, a weld-brazeprocess was proposed to joinaluminum 6061-T6 to boron steelswith various coatings. Figure 2 shows the schematic of aweld-braze joint. As shown, themolten aluminum droplet mergedwith molten aluminum base metal toform a welded aluminum-to-aluminum connection, whereas, themolten aluminum wire wet the zinccoating on the steel workpiece to forma brazed aluminum-to-steelconnection. In order to obtain a weld-braze joint, CMT technology wasadopted.

WELDING JOURNAL / JUNE 2014, VOL. 93194-s

WELDING RESEARCH

Fig. 1 — Phase diagram of AlFe (Ref. 17).

Fig. 2 — Schematic of a CMT weldbraze joint of aluminum6061T6 to boron steel.

Table 1 — Chemical Compositions of Boron Steels (wt%)

Materials C S P Si Mn Cr Ni Mo Cu Nb V Ti Al N B Fe

Galvanized and 0.22 0.002 0.012 0.23 1.15 0.17 0.03 0.001 0.01 0.01 0.004 0.03 0.04 0.007 0.002 Bal.galvannealedboron steel

Bare boron 0.14 0.008 0.01 0.23 1.15 0.15 0.05 0.01 0.01 0.01 0.016 0.04 0.06 0.007 0.001 Bal.steel

AlSi boron 0.24 0.003 0.018 0.26 1.18 0.18 0.03 0.01 0.01 0.01 0.003 0.04 0.13 0.008 0.001 Bal.steel

Cao Supplement new_Layout 1 5/15/14 4:13 PM Page 194

Sample Fabrication

For the purpose of avoiding the in-fluence of surface contamination,prior to welding the steel sheet wasdegreased by acetone and tap water.However, to avoid significantporosities produced with the weldingprocess, the Al sheet was degreased byacetone first, and then polished byabrasive cloth, followed by a cleaningwith 5–10% NaOH solution at a tem-perature range of 40° ~ 70°C for 3–7min and rinsed with tap water. Thiswas followed by surface cleaning with30% HNO3 solution at a temperatureof 60°C for 1–3 min and then rinsingwith tap water. The lap-shear joint configuration(Fig. 3A) was fabricated from 200 × 50mm sheets. The arrangement of the testsheets with respect to the welding gunis shown in Fig. 3B. The AA6061-T6workpiece was placed on top of the steelworkpiece in a lap configuration with anoverlap distance of 10 mm. The anglebetween the welding gun and thenormal to the lap joint was 45 deg awayfrom the direction of welding. The weld-ing direction was parallel to the lap jointand was offset from the edge of the Alsheet edge by a deviation distance (D). AFronius arc welding system (CMT 3200)was used to fabricate the samples. Theprocess variables, including the wirefeed speed, welding speed, anddeviation distance of the wire from the edge of the sheet, were developedduring the welding process. A 100%argon shielding gas with a flow rate of15 L/min was used throughout the experiments.

Analytical Analysis

Standard grinding sample prepara-tion procedures were used and mixedsolutions of Nital (i.e., 4 vol-% HNO3

+ 96 vol-% ethanol) were used to etchthe samples. The microstructures andcompositions of the coatings were ob-served and analyzed by scanning elec-tron microscope (i.e., SEM 6700F)equipped with energy-dispersive X-rayspectrometer (EDS). To examine the quality of CMT weld-braze joints, the cross sections of thespecimens were prepared andexamined. The polished aluminum andsteel workpieces were etched by Dix-Keller’s. The microstructures andelement distributions of the weld were

observed and analyzed by SEMequipped with EDS. The phases of thecoating and some cracking interfaceswere confirmed by XRD (i.e., D8ADVANCE) transmission electronmicroscopy (TEM).

Results and Discussion

Coating Analyses

To analyze the phases of thecoatings on three coated hot stamping


WELDING RESEARCH

Fig. 3 — Schematic of lapped aluminumtosteel workpiece. A — Plane; B — side view of the welding gun with respect to the sample.

Fig. 4 — Binary phase diagram. A — AlZn (Ref. 22); B — FeZn (Ref. 23).

Table 2 — Chemical Compositions of AA6061T6 and Filler Metal (wt%)

Materials Si Fe Cu Mn Mg Zn Cr Ti Al

Al6061 0.4–0.8 0.70 0.15–0.4 0.15 0.81.2 0.25 0.04–0.35 0.15 Bal.4043 Al wire 4.5–6.0 0.80 0.30 0.05 0.05 0.10 — 0.20 Bal.

Table 3 — Point Analysis Results of Coating of FeZn Galvannealed Boron Steel Shown in Fig. 5A

Location Al (at.%) Fe (at.%) Zn (at.%) Possible Phase

1 1.4 12.1 86.62 1.1 13.7 85.23 1.1 11.2 87.7 FeZn104 0.8 12.4 86.7

Average 1.1 12.4 86.5

A

A

B

B


steels, Al-Zn, and Fe-Zn (Ref. 22)phase diagrams presented in Fig. 4Aand B, respectively, were referenced.For the Al-Zn phase diagram, thephases at room temperature werecomposed of Al-Zn eutectoid, Zn solidsolution, and Al solid solution.However, for the Fe-Zn phase diagramshown in Fig. 4B, phases at room tem-perature consisted of severalintermetallic phases, such as Γ(Fe3Zn10), Γ1 (Fe5Zn21), δ (FeZn10)and ζ (FeZn13) (Ref. 23). Composition analyses of coatinglayers for galvannealed boron steel,galvanized boron steel, and Al-Si-coated boron steel are shown in Figs.5–7, respectively. As shown in Fig. 5,cross-section examination ofgalvannealed boron steel wasconducted and the results presentedthat a coating with a thickness of~10 mm (Fig. 5A) was formed. X-raydiffraction analyses of the coatingshown in Fig. 5B and C indicated itwas mainly composed of Fe-Zn IMC.

By combining Fe-Zn binary phasediagram shownin Fig. 4B andEDS analysis re-sults of the coat-ing shown inTable 3, it wasfound that the δ-FeZn10 phasewas formed whenFe and Zncontents reached

respectively ~12% and ~87%. Theseresults suggested that the coatinglayer for galvannealed boron steel wasprimarily composed of δ-FeZn10phase. The thickness and compositions ofgalvanized coating on boron steel wereexamined and analyzed, and the resultsare shown in Fig. 6A, B, and C. Asshown in Fig. 6A, the zinc coating layerhad a thickness of ~10 mm. By combin-ing the results shown in Figs. 6B, C, 4A,and Table 4, it was found that when theAl and Zn contents reached ~2–3% and~96–98%, respectively, Zn solidsolutions were formed, which suggestedthat the coating of galvanized boronsteel was mainly composed of Zn solidsolutions. Furthermore, line scan analy-sis results shown in Fig. 6C revealedthat an Al-rich phase was formed at theinterface between the coating and steel.These results indicated that themicrostructure adjacent to the interfaceconsisted of not only Zn but ultrathin

Fe-Al IMC due to the hot-dip galvaniz-ing process, which was consistent withthe results found in Ref. 24.

Figure 7 presented the cross sectionand compositions of Al-Si-coatedboron steel. Careful examination ofthe results shown in Fig. 7A showedthat the coating layer had a thicknessof ~35 mm. By combination of the re-sults shown in Fig. 7B, C, and Table 5,it was found that the coating was pri-marily composed of two layers. The ex-ternal dark reaction layer A, shown inFig. 7A, mainly consisted of FeAl3IMC, and the bright layer B next to thebase metal mainly consisted of AlFeSiIMC. x-ray diffraction analysis resultsof the coating shown in Fig.7B furtherrevealed that the coating of Al-Siboron steel was composed of series Fe-Al and Fe-Al-Si IMC. The weldabilityof the Al-Si boron steel and aluminumsheets are discussed in the nextsection.

Joining AA6061T6 to BoronSteel

To assess the effect of coating on theweldability, weld appearance of weld-braze AA6061-T6 to boron steel was ex-amined first. Figure 8 shows the weldappearance of CMT weld-braze 1.0-mm-thick AA6061-T6 to 1.5-mm-thick bareboron steel. As shown, while the weldmetal barely covered the aluminum sub-strate, it fully spread on the steelsubstrate. This was primarily attributed


WELDING RESEARCH

Fig. 5 — Composition analyses of coating layer of galvannealedboron steel. A — Cross section of the coating layer; B — XRDanalysis of the coating layer; C — line scanning analysis alongline AB shown in Fig. 5A.

A B

C


to the combination of the strong affin-ity of aluminum to steel and melting ofthe aluminum substrate. Becauseenthalpy of mixing of aluminum in iron(i.e., –48 kJ/mole) was less than that ofaluminum in aluminum (i.e., 0kJ/mole), affinity of aluminum inaluminum was weaker than that of Al inFe (Ref. 25). Therefore, the molten alu-minum spread to the steel side ratherthan the aluminum side during welding.

The microstructural variations atthe interface and weld metal of CMTjoined AA6061-T6 to boron steel areshown in Fig. 9. From the cross section

shown in Fig. 9A,the weld metal wasseparated from thebase metal.Moreover, a signifi-cant amount of thecracks wereobserved in the weldmetal. Energy-dispersive spectrom-eter analysis of theregion A shown inFig. 9A was

performed and the results are listed inTable 6. As presented in Table 6 andFig. 9B, massive brittle FeAl3 IMC atthe location 1 was distributed amongα-Al solid solutions (in Region 2) andAl-Si eutectic (in Region 3) in the weldmetal. Furthermore, experimental ob-servations showed that a significantamount of the boron steel was moltenunder an intensive arc heat. Themolten steel reacted with aluminumfiller metal, leading to formation of alarge amount of brittle Fe-Al IMC.These results suggested that the

cracks observed in the weld metallikely resulted from the combinationof the formation of brittle Fe-Al IMCand thermal-induced residual stressesdue to the difference in coefficient ofthermal expansion between steel andaluminum. Figure 9C shows the crack-ing interface shown in Fig. 9A betweenthe weld metal and bare steel sheet. Inaddition, EDS analysis of locations 4and 5 shown in Fig. 9C and XRD analy-sis at the cracking interface betweenthe weld metal and steel sheet wereperformed. The XRD results are shownin Fig. 9D. These results indicated thatmassive brittle Fe-Al IMC existed atthe cracking interface. Thus, soundjoints between AA6061-T6 and bareboron steel could not be formed by theCMT method.

Joining AA6061T6 to AlSiCoated Boron Steel

To sustain the temperature in therange of 950°C during hot stamping,Al-Si-coated boron steel was developedfor automotive applications (Ref. 26).To examine if Al-Si coating wouldaffect the weldability, CMT joiningAA6061-T6 to Al-Si-coated boron steelwas conducted. Figure 10 shows theappearances of the welds. Experimen-tal observations revealed that thewelds split immediately after welding.Figure 10A and B shows the weldappearance of the Al and steel sheet


WELDING RESEARCH

Table 4 — Point Analysis of Coating Layer of Galvanized Coated Boron Steel Shown in Fig. 6A

Location Al (at.%) Zn (at.%) Possible Phase

1 3.2 96.82 2.3 97.73 2.4 97.6 Zn solid solutions4 2.9 97.1

Average 2.7 97.3

Fig. 6 — Composition analyses of coating layer of galvanized boronsteel. A — Cross section of the coating layer; B — XRD analysis ofthe coating layer; C — line scanning analysis along line CD shown in A.

A B

C


sides, respectively. Carefulexaminations of the welded specimensindicated that the poor weld qualitywas likely caused by the formation ofbrittle Fe-Al and Fe-Al-Si IMC, shownin Fig.7B and C, and thermal residualstresses developed during the welding. To validate this observation, thecompositions and phases of the weldmetal and the cracking interface wereanalyzed, and the results are shown inFig. 11 and Table 7. As shown, coarsecolumnar grains with cracks (zone A inFig. 11A) and equiaxed crystal grains

(zone B) wereobserved at theupper and lowerparts of the weldmetal, respectively.The weld metalshown in Fig. 11Amainly consisted oftwo differentfeatures, i.e., darkand bright regions.

The compositions of the dark region(location 1) of the weld metal shownin Table 7 were 96.4 at.-% Al, 2.4 at.-%Fe, and 1.2 at.-% Si, which indicatedthe phases of this weld metal were αa-Al, and some Al-Si eutectic phases (lo-cation 2) distributed among the α-Albase metal. The compositions of thebright region (location 3) of the weldmetal shown in Table 7 are 67.7 at.-%Al, 31 at.-% Fe, and 1.3 at.-% Si, whichsuggested the phases of this weldmetal were primarily FeAl3 IMC. In ad-

dition, there existed a transition zonebetween the dark and bright regions.EDS analysis results showed that thephase of this zone (location 4) wasFeAl3 IMC, too. To further confirm thephases of the cracking interface shownin Fig. 11A, corresponded EDS analy-sis of locations 5 and 6, shown in Fig.11D, on the cracking interface andXRD analysis of the cracking interfacewere performed and the results areshown in Table 7 and Fig. 11E, respec-tively. The results shown in Fig. 11Dand E revealed that a significantamount of brittle Fe-Al IMC existed atthe cracking interface (Fig. 11A and D)between the weld metal and Al-Siboron sheet. From these analyses, weconcluded that Al-Si-coated boronsteel could not be properly joined toAA6061-T6 primarily due to formationof large amounts of brittle FeAl3 IMC.

Joining AA6061T6 toGalvannealed Boron Steel

Figure 12 shows the effect of wirefeed speed on the appearance of CMTweld for 1.0-mm-thick AA6061T6 to1.5-mm-thick galvannealed boronsteel. Tests were conducted with a wirefeed speed of 3 m/min and the resultsshown in Fig. 12A indicated that fillermetal still could not properly wet thesteel surface, and consequently thenarrow weld was formed. With an in-crease of wire feed speed from 3 to 4.5m/min, the weld becamediscontinuous and sporadic. Then, thesurfacing welds were trialed on thesurface of galvannealed boron steelwith wire feed speeds of 4 and 5m/min. As shown in Fig. 12C, themolten aluminum also didn’t wet thesteel substrates well, which is the rea-


WELDING RESEARCH

Fig. 7 — Composition analysis of coating layer of AlSi boron steel. A — Cross section; B — XRD analysis; C — line scanning analysis alongline EF shown in A.

Fig. 8 — Weld appearance of CMT for 1mmthick AA6061T6to 1.5mmthick bare boron steel.

Table 5 — Point Analysis Results of Coating Layer of AlSi Coated Boron Steel Shown in Fig. 7A

Location Al (at.%) Si (at.%) Fe (at.%) Possible Phase

1 72.24 2.60 25.16 FeAl32 44.38 16.16 39.47 AlFeSi3 19.92 5.44 74.65 Fe3Al4 0 0 100 αFe

Table 6 — EDS Analysis Results of Zones 1 – 5 in Fig. 9


1 77.43 1.17 21.40 FeAl32 83.78 16.22 0 AlSi eutectic3 98.3 1.4 0.47 αAl4 28.8 3.2 67.9 FeAl35 29.6 3.4 67.0 FeAl3

A B C


son for the poor appearance of CMTwelded AA6061T6 to galvannealedboron steel. Experimentalobservations indicated that the arcwas unstable and large spatteroccurred during CMT joining. Joiningmechanisms of the joints are discussednext. Before the process optimizationtests were performed, we examinedthe cross-section, microstructure, andline scan analysis of CMT welded lapAA6061-T6 to galvannealed boronsteel joints, and the results are shownin Fig. 13. As shown in Fig. 13A, themolten filler metal was deposited onthe surface of the steel substrate witha poor contact angle of ~115 deg.From Fig. 13A, three different zones(i.e., Zones A, B, and C) wereobserved at the interface between theweld metal and galvannealed boronsteel. EDS analyses of locations 1 to 2

in Zone A shown in Fig. 13B,locations 3 to 5 in Zone B shown inFig. 13D, and locations 6 to 7 in ZoneC shown in Fig. 13F were conducted,and the results are shown in Table 8.As presented in Fig. 13B and Table 8,at the Zone A of bonding interfacebetween the weld metal andgalvannealed boron steel consisted ofFe3Al and FeAl3 IMC with a thicknessof ~5 μm. EDS results showed that re-action layer of Zone B shown in Fig.13D was mainly composed of Zn, Al,and Fe. The aluminum element in themolten weld metal interacted with Fe-Zn coating, and consequently, an in-terface layer consisted of Fe2Al5,Fe4Al13, α-Al and α-Zn was producedduring the welding process. At theZone C in Fig. 13A, severe cracks wereobserved between the weld metal andgalvannealed boron steel. Especiallyfor enlarged Zone C in Fig. 13G, the

interface near the steel is mainlycomposed of FeZn10 compound,which is consistent with the phase inthe coating layer in Fig. 5. Therefore,these results showed that the galvan-nealed coating was not fully moltenso that molten aluminum hardly wet-ted the steel substrates at this zone,and consequently sound CMT weldingof AA6061-T6 and galvannealedboron steel could not be attained.

Joining AA6061T6 toGalvanized Boron Steel

After we had examined the Al-Si-coated and galvannealed boron steel,CMT joining of AA6061-T6 togalvanized boron steel was alsoperformed. Figure 14A presents theappearances of the welds. As shown,welds with low reinforcement wereproduced. Experimental observationsshowed that adequate wetting of themolten filler metal on the substratesand a stable arc were observed. To un-derstand the effect of galvanized coat-ing on the weldability, the crosssection of CMT welded AA6061-T6 togalvanized boron steel was examined,and the results are shown in Fig. 14B.A welded connection was developedbetween the molten filler metal andaluminum substrate on one side, whilea brazed connection was formed


WELDING RESEARCH

Table 7 — EDS Analysis Results of Zones 1–6 in Fig. 11


1 96.4 1.2 2.4 αAl2 90.2 9.3 0.5 αAl + AlSi eutectic3 67.7 1.3 31 FeAl34 66.8 6.4 26.8 FeAl35 65.1 33.6 1.7 FeAl36 53.8 37.5 8.7 Sirich FeAl3

Fig. 9 — A — Cross section of CMT weldfor 1.0mmthick AA6061T6 to 1.5mmthick bare boron steel; B — enlargedview of zone A shown in A; C — crack interface shown in A between the weldmetal and bare steel sheet; D — XRDanalysis at the crack interface shown in Abetween the weld metal and steel sheet.

A B

C D


between the aluminum and steel. Thereaction products along the brazing in-terface can be divided into two zones:zinc-rich Zone A and Fe-Al reactionlayer B. Figure 15 shows thecorresponding SEM micrographs ofthese two zones. The phases of thesezones were determined by EDS, andthe results are shown in Table 9. Microscopy analyses revealed that

the Zn element at the Zone A (i.e., thetoe of a lap weld observed in Fig. 15Aand B) had been driven from the steelsurface toward the regions near theweld toe. EDS analysis of regions 1and 2 in Zone A were performed, andthe results listed in Table 9 indicatedthat they were mainly composed ofbright dendrites of Al-Zn eutectoidand α-Al. Region 1 had a zinc content

of 22.4%, which corresponds to thechemical compositions of theeutectoid fcc-Al + hcp-Zn (Ref. 27). Inaddition, the gray zone (Region 2),which mainly consisted of α-Al existedbetween the grey dendrites of Al-Zneutectoid. Furthermore, some Si gran-ules observed at Region 3 existedamong the Al-Zn eutectoid and αa-Al.The Zn element was known toenhance the wetting of the molten alu-minum alloy and filler metal on thesteel substrate (Ref. 28). Compared toFe, aluminum has a larger affinity toZn (Ref. 29), and consequently, Al-Znphases were favored than the Fe-AlIMC, which is the reason why most Al-Zn eutectoid phases were formed inZone A. At the brazing interface (i.e., ZoneB), the interfacial microstructureshown in Fig. 15C was found to be ho-mogeneous and had a continuousmorphology with a thickness of 2–5μm. The thickness of the reaction layerwas reduced greatly because of the Znevaporation from the arc heat (Ref.14). To identify the elements, distribu-tion at the brazing layer, EDS linescanning of the middle interface B wascarried out and the results are plottedin Fig. 15 D. As observed, the Feelement content decreased whereasthe Al element content increasedalong the interface. In general, it wasfound that the distribution ratio ofFe/Al remained unchanged at the Fe-Al reaction layer, which indicated thatIMC were likely formed in anotherway. The EDS analyses at the Zones 4and 5 of the interface were performed,and the results are shown in Table 9.As shown, the phase (denoted by Zone4) near the steel substrate consisted of38.7 at.-% Al, 60.7 at.-% Fe, and 1.2at.-% Si, while the phase (denoted byZone 5) near the weld metal contained72.2 at.-% Al, 23.9 at.-% Fe, and 3.9at.-% Si, which showed that the possi-ble phases at the brazing interfacewere Fe4Al13 and Fe2Al5 IMC. To fur-


WELDING RESEARCH

Fig. 10 — Weld appearance of CMT weldbraze of 1.0mm thick AA6061T6 to 1.5mm thick AlSi boron steel with a wire feed speed of4.5 m/min. A — Weld surface at Al sheet side; B — detached surface of weld from AlSicoated boron steel sheet side.

Fig. 11 — CMT weld of AA6061T6 to AlSi boron steel. A — Weld metal; B — enlargedview of zone A in A; C — enlarged view of zone B in A; D — cracking interface (shown inFig. 11A) between the weld metal and AlSi boron sheet; E — XRD analysis at the cracking interface shown in Fig. 11D.

A

A

B

B

C

D E


ther confirm Fe-Al IMC phases at thebrazing interface, TEM micrographand selected area diffraction patterns

(SADP) corresponding to the specificregions at the brazing interface wereprepared, and the results are shown inFig. 16. As shown, the intermetallic layer atthe steel/weld metal interfacecontained the grains with differentmorphologies. The finger-like grains,which were approximately perpendicu-lar to the interface, were formed near

the steel matrix, and trapezoidalgrains were produced next to the weldmetal. Electron diffraction patternsperformed on the finger-like grains ofregion B confirmed them to be Fe2Al5phase. The trapezoidal grains of regionA located between the grains and weldmetal were identified as Fe4Al13 phase.In addition, massive dislocations wereobserved in Fe2Al5 phase, which might


WELDING RESEARCH

Fig. 12 — Effect of wire speed on the weld appearance of CMTweldbraze of 1.0mmthick AA6061T6 to 1.5mmthick galvannealed boron steel. A — A wire feed speed of 3 m/min; B — a wirefeed speed of 4.5 m/min; C — surfacing welds with speeds of4m/min and 5m/min.

Fig. 13 — CMT weld of 1.0mmthick AA6061T6 to 1.5mmthick galvannealed boronsteel. A — Cross section; B — microstructure of zone A; C — line scan along yellow HGat interface of Zone A; D — microstructure of zone B; E — line scan along yellow IJ atinterface of Zone B; F — microstructure of Zone C; G — enlarged Zone C.

A

A

B

B

C

C

D

E F

G


have nucleated from the phase bound-aries and prolonged with the growthof the finger-like Fe2Al5 grains. Fromthe aforementioned analyses, thethickness of brittle brazing layer is

smaller than the critical thickness of10 μm (Ref. 30), and this contributedthe formation of a sound AA6061-T6to galvanized boron steel weld. To evaluate the strength of CMTweld-braze AA6061-T6-galvanizedboron steel, quasistatic tests were con-ducted, and the results are shown inFig. 17. The joint strength was definedas the failure load/specimen width,N/mm. For the purpose of comparison,the results of CMT welded AA6061-T6to bare boron steel, Al-Si-coated steeland galvannealed steel with the samedimensions were also included in Fig.17. As shown, the strengths for bothAA6061-T6 to bare boron steel and Al-Si-coated steel joints were relatively lowdue to massive formation of brittle Fe-Al IMCs during the welding process,and the strength for AA6061-T6 to gal-vannealed steel was also relatively weakbecause of the poor bond formed at thebrazing interface. However, for CMTweld-braze AA6061-T6 galvanizedboron steel, the strength was sound(i.e., 208 N/mm), and the failurelocation for this joint was at thealuminum heat-affected zone shown inFig. 18, which indicated that soundCMT weld-braze AA6061-T6 galvanizedboron steel joints were produced.

Weldability of CMT JoiningAA6061T6 to Boron Steel

The aforementioned test resultsand analyses showed that galvanizedboron steel was joined successfully toAA6061-T6 by the CMT process usingAlSi5 feed wire, while the weldabilityof AA6061-T6 to bare boron steel,AA6061-T6 to Al-Si-coated boronsteel, and AA6061-T6 to galvannealedboron steel was poor. From the pointanalysis results of the coating layershown in Tables 3 and 4, Fe-Zn andpure Zn coatings on galvannealed andgalvanized boron steels containedabout 86% and 96% Zn element,respectively. Refer to Fig. 4 (i.e., Al-Znand Fe-Zn binary phase diagrams), themelting points of pure Zn and Fe-Zncoatings were 381°C and 665°C,respectively. Pure Zn coating was read-ily molten under the CMT arc, andmolten filler metal spread on theliquid zinc layer other than the surfaceof the solid steel substrate, which im-proved the wettability and spreadabil-ity of the molten weld metal. In addi-


WELDING RESEARCH

Fig. 14 — CMT weld of 1.0mmthick Al to 1.5mmthick galvanized boron steel under awire feed of 4.5 m/min. A — Weld appearance; B — cross section.

Fig. 15 — Microstructures of CMTweldbraze of 1.0mmthick AA6061T6to 1.5mmthick galvanized boronsteel. A — Zone A; B — enlarged ZoneA; C — braze interface B; D — enlargedzone B; E — line scan at the brazinginterface B.

A

B

A B

E

C D


tion, the Zn coating was vaporized dueto its low boiling point (i.e., 960°C),and the evaporation of Zn likely tookaway a significant amount of heat gen-

erated by the arc, and consequently,the thickness of Fe-Al IMC wasreduced due to lower reaction temper-ature and less reaction time. Further-

more, the results shown in Ref. 27showed that the existence of thin Fe-Al IMC between the coating layer andsteel base metal during the hot-dipgalvanizing process also inhibited thechemical reaction between Fe and Aland obstructed the interdiffusion be-tween Fe and Al when joining Al togalvanized steel. However, under thesame welding condition, Fe-Zn coatingwas not fully molten due to its highmelting point, and consequently hin-dered the wetting of the molten fillermetal on the steel substrate. As a re-sult, welds with poor quality were pro-duced. For CMT welding AA6061-T6 tobare boron steel, massive brittle Fe-AlIMC were formed due to the directmixing of molten filler metal andmolten steel, and the cracks likelyoriginated from the reaction layer andpropagated along the weld metal dueto thermal residual stresses after weld-ing. For CMT welding AA6061-T6 toAl-Si-coated boron steel, a largeamount of Fe-Al IMC were produceddue to the existence of 35-mm-thickFe-Al IMC layer on the coating and lo-


WELDING RESEARCH

Fig. 16 — TEM micrograph and indexed selected area diffraction patterns at thebraze interface. A — Morphology of grains present at IMC resolved in TEM model; B— SAD corresponding to region A; C — SADP corresponding to region A.

Fig. 17 — Tensile strengths of CMT joining ofAA6061T6 to boron steel.

Fig. 18 — Failure location of CMT weldbrazeof AA6061T6 to galvanized boron steel atthe aluminum heataffected zone.

Table 8 — EDS Results of Zones 1–7 in Fig. 13

Location Al (at.%) Si (at.%) Fe (at.%) Zn (at.%) Possible Phase

1 22.5 4 73.5 0 Fe3Al2 69.7 14.4 15.9 0 FeAl33 20.9 1.2 61.8 16.1 Fe2Al5 + Fe4Al13 + αAl + αZn4 34.3 2 57.9 5.8 Richzinc FeAl IMC5 94.9 1.1 2.5 1.5 Richzinc αA16 0 0 13.2 86.8 FeZn107 0 0 13.7 86.3 FeZn10

Table 9 — EDS Results of Zones 1–5 in Fig. 15

Location Al (at.%) Si (at.%) Fe (at.%) Zn (at.%) O (a. %) Possible Phase

1 70.3 2 0 22.4 0 AlZn eutectoid2 92.6 1.8 0 5.6 0 αAl3 1.6 98.3 0 0.1 0 Si granule4 70.5 1.8 27.7 0 0 Fe4Al135 72.2 3.9 23.9 0 0 FeAl3 + Fe2Al5

A

B C


cally molten steel base metal, and theload-bearing capability of the welded-brazed joints decreased drastically,which easily led to the detachment be-tween the weld metal and base Al-Siboron steel under thermal-inducedresidual stresses.

Conclusions

1) The effect of coating on CMTjoining of AA6061-T6 to boron steelhas been investigated. It was foundthat a zinc coating on the steelsubstrate is essential. Pure Zn coatingon galvanized boron steel was moltenunder the arc, and it enhanced thewetting of molten aluminum fillermetal onto the solid steel substrate.Consequently, welds with a smoothappearance and low reinforcementwere produced. 2) Fe-Zn coating from galvannealed

boron steel was barely molten underthe arc due to its high melting point,and consequently it barely enhancedthe molten aluminum filler metal towet the steel substrate. As a result,welds with poor quality wereproduced. 3) During CMT joining AA6061-T6galvanized boron steel, element zincwas evaporated. The heat dissipationfrom the evaporation of the Znreduced the temperature of the weldpool, and consequently decreased thegrowth of brittle Fe-Al intermetallicsbetween the aluminum weld metal andthe steel substrate. 4) Sound AA6061-T6-bare boronsteel and AA6061-T6-Al-Si-coatedboron steel joints could not beproduced by CMT arc weldingtechnique due to a large formation ofthe brittle FeAl3 intermetallic duringthe welding process.

This work was financiallysupported by National Nature ScienceFoundation of China (No. 51265028)and GM Global Research and Develop-ment Center.

1. Karbasian, H., and Tekkaya, A. E. 2010. Areview on hot stamping. Journal of Materials Processing Technology 210 (15): 2103–2118.

2. Miller, W. S., Zhuang, L., and Bottema, J.2000. Recent development in aluminum alloysfor the automotive industry. Materials Scienceand Engineering A 280: 37–49.

3. Gould, J. E. 2012. Joining aluminumSheet in the automotive industry — A 30 Yearhistory. Welding Journal 91(1): 23–34.

4. Naderia, M., Ketabchi, M., Abbasi, M., andBleck, W. 2011. Analysis of microstructure andmechanical properties of different high strengthcarbon steels after hot stamping. Journal of Ma-terials Processing Technology 211: 1117–1125.

5. Lorenz, D., and Roll, K. 2005. Modellingand analysis of integrated hot forming andquenching processes. Proc. Sheet Metal 2005Conf., pp. 787–794.

6. Dong, H. G. 2012. Dissimilar metal joiningof aluminum alloy to galvanized steel with Al-Si,Al-Cu, Al-Si-Cu and Zn-Al filler wires. Journal ofMaterials Processing Tech 212: 458–464.

7. Lee, J. K., Kumai, S., Kawamura, N.,Ishikawa, N., and Furuya, K. 2007. Growth man-ner of intermetallic compounds at the weld in-terface of steel/aluminum alloy lap joint fabri-cated by a defocused laser beam. MaterialsTransaction 48: 1396–1405.

8. Travessa, D., Ferrante, M., and Ouden, G.2002. Diffusion bonding of aluminum oxide tostainless steel using stress relief interlayers. Ma-terials Science and Engineering A 337: 287–296.

9. Acarer, M., and Demir, B. 2008. An inves-tigation of mechanical and metallurgical proper-ties of explosive welded aluminum-dual phasesteel. Materials Letters 62(25): 4158–4160.

10. Taban, E., Gould, J. E., and Lippold, J. C.2010. Dissimilar friction welding of 6061-T6aluminum and AISI steel: Properties andmicrostructural characterization. Materials andDesign 31: 2305–2311.

11. Fukumoto, S., Tsubakino, H., Okita, K.,Aritosh, M.I., and Tomita, T. 1999. Frictionwelding process of 5052 aluminium alloy to 304stainless. Material Science Technology 15(9):1080–1086.

12. Taban, E., Gould, J. E., and Lippold, J. C.2010. Characterization of 6061-T6 aluminumalloy to AISI steel interfaces during joining andthermo-mechanical conditioning. Materials Sci-ence and Engineering A 527: 1704–1708.

13. Tsujino, J., Hidai, K., Hasegawa, A.,Kanai, R., Matsuura, H., Matsushima, K., andUeoka, T. 2002. Ultrasonic butt welding of alu-minum, aluminum alloy and stainless steel platespecimens. Ultrasonic 40: 371–374.

14. Zhang, H. T., Feng, J. C., He, P., Zhang, B.B., Chen, J. M., and Wang, L. 2009. The arc char-acteristics and metal transfer behavior of coldmetal transfer and its use in joining aluminumto zinc-coated steel. Materials Science and Engi-neering A 499: 111–113.

15. Cao, R., Yu, G., Chen, J. H., and Wang, P.C. 2013. Cold metal transfer joining aluminumalloys to galvanized mild steel. Journal of Materi-

als Processing Technology 213: 1753–1763.16. Song, J. L., Lin, S. B., Yang, C. L., and

Fan, C. L. 2009. Effects of Si additions on inter-metallic compound layer of aluminum-steel TIGwelding-brazing joint. Journal of Alloys and Com-pounds 488: 217–222.

17. Peyre, P., Sierra, G., and Deschaux, B. F.2007. Generation of aluminum–steel joints withlaser-induced reactive wetting. Materials Scienceand Engineering A 444: 327–338.

18. Yang, X. R. 2006. Cold metal transferMIG/MAG dip-transfer process for automatedapplications. Arc Welding Machine 36: 5–7.

19. Kima, C., Kang, M. J., and Park, Y. D.2011. Laser welding of Al-Si coated hot stamp-ing steel. Procedia Engineering 10: 2226–2231.

20. Cao, R., Wen, B. F., Chen, J. H., andWang, P. C. 2013. Cold metal transfer joining ofmagnesium AZ31B-to-aluminum A6061-T6. Ma-terials Science and Engineering A 560: 256–266.

21. Pickin, C. G., Williams, S. W., and Lunt,M. 2011. Characterization of the cold metaltransfer (CMT) process and its application forlow dilution cladding. Journal of Materials Pro-cessing Technology 211: 496–502.

22. Alloy Phase Diagram Database™. ASMInternational.

23. Li, Z., Su, X. P., He, Y. H., Tan, Z., andYin, F. C. 2008. Growth of intermetalliccompounds in solid Zn/Fe and Zn/Fe-Si diffu-sion couples. The Chinese Journal of NonferrousMetals 18: 1639–1644.

24. Li, L. Q., Tan, C. W., Chen, Y. B., and Guo,W. 2012. Influence of Zn coating on interfacereactions and mechanical properties duringlaser welding-brazing of Mg to steel. Metallurgi-cal and Materials Transactions 43A: 4740–4754.

25. Eustathopoulos, N., Michael G. N., andDrevet, B. 1999. Wettability at HighTemperatures. Department of Materials Scienceand Metallurgy. University of Cambridge, UK.

26. Ghiotti, A., Bruschi, S., and Borsetto, F.2011. Tribological characteristics of highstrength steel sheets under hot stamping condi-tions. Journal of Materials Processing Technology211: 1694–1700.

27. Murray, J. L. 1986. Massalski T. B. (ed.)Binary Alloy Phase Diagrams. ASM International,Materials Park, Ohio, p. 185.

28. Agudo, L., and Eyidi, D. 2007.Intermetallic FexAly-phases in a steel/Al-alloyfusion weld. J. Mater. Sci 42: 4205–4214. 29. Kato, T., Nunome, K., Kaneko, K., andSaka, H. 2000. Formation of ζ phase at an inter-face between an Fe substrate and a molten 0.2mass % Al-Zn during galvannealing. Acta Mater48: 2257. 30. Abea, Y., Katob, T., and Moria, K. 2009.Self-piercing riveting of high tensile strengthsteel and aluminum alloy sheets using conven-tional rivet and die. Journal of Materials Process-ing Technology 209(8): 3914–3922.


WELDING RESEARCH

Acknowledgments

References


Weldability of CMT Joining of AA6061T6 to Boron Steels...

Documents

Transcript of Weldability of CMT Joining of AA6061T6 to Boron Steels...