Materials and Design 56 (2014) 725–730

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Materials and Design

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Short Communication

Microstructure and mechanical properties of spray formed 7055aluminum alloy by underwater friction stir welding

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.11.071

⇑ Corresponding author. Tel.: +86 0189 14571603; fax: +86 0511 84434793.E-mail address: yongzhao418@just.edu.cn (Y. Zhao).

Yong Zhao a,⇑, Qingzhao Wang a, Huabin Chen b, Keng Yan a

a Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, No. 2 Mengxi Road, Zhenjiang, Jiangsu 212003, Chinab School of Materials Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Road, Shanghai 200240, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 August 2013Accepted 28 November 2013Available online 7 December 2013

Ultra-high strength spray formed 7055 aluminum alloy in which Zn is supersaturated solid solutionrequires strict control of heat input in welding process. In this paper, underwater friction stir weldingis carried out in order to reduce heat input comparing with traditional friction stir welding and furtherimprove the joint performances by varying welding temperature history. Through comparing the thermalcycle curves and distribution of residual stress of the plate welded in different media, the reason why thejoint welded underwater shows a better performance is figured out. The result shows that tensilestrength, hardness and plasticity of underwater welded joint are better than that welded in air. Theunderwater joint has a fine grained microstructure without ‘‘S line’’ defect, a typically distinct boundarybetween the weld nugget zone and the thermal mechanically affected zone and a narrow heat affectedzone. The main strengthening phase in underwater joint is MgZn2 .

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Al–Zn–Mg–Cu series (7XXX) alloys are widely used in the aero-space and automotive industries due to their excellent properties,i.e. high strength, low density and outstanding machinability [1,2].However, a practical limit of about 8 wt% Zn in Al–Zn–Mg–Cualloys is imposed for conventional cast materials due to inherentfoundry problems. Spray formed process enables the content ofZn to be increased considerably and further improves the mechan-ical properties of the alloys [3]. Early in 1990s, industrial developedcountries had developed a new type of ultra-high strength 7XXXaluminum alloy with fine microstructures and high solid solubilityof Zn (above 8%, even up to 14%) through spray formed technology.Increasing Zn content makes the grain of the as-deposited alloysrefine [4]. Its tensile strength is up to 780–830 MPa after properheat treatment.

The property of 7XXX series alloy is dominated by the quantityof g0 phases which consists of Zn and Mg [5]. The spray formed pro-cess can make Zn to be supersaturated solid solution in the Al alloymatrix. However, this kind of alloy is considered ‘‘unweldable’’ bytraditional fusion welding techniques owning to solidificationcracking or severely degraded mechanical properties of the welds.The present researches about spray formed Al alloy just focus on itspreparation technologies, including its heat treatment technolo-gies and the evolution of microstructures during preparationprocess, but the study on its weldability is rarely reported.

Friction Stir Welding (FSW) is a solid-state welding process.Since its invention in 1991 by TWI (Cambridge, United Kingdom)[6], FSW has emerged as a reliable method to join high strengthseries Al alloys [7–9]. Furthermore, the defect features andmechanical properties of friction stir welded dissimilar aluminumalloy sheets are widely investigated [10,11]. Although the basemetal does not melt during FSW process, improper thermal cyclesstill can cause reduction of mechanical properties of the joints. Somore and more studies are focus on controlling the heat input ofFSW. Sakurada et al. [12] were the first one who used submersionin a rotary friction weld for 6061 aluminum alloys. The resultsshowed that it was possible to generate enough friction heat forwelding even though the samples were submerged in the water.Thomas adopted submerged FSW to improve the strength of theFSW joint of 6061 aluminum alloy [13]. Liu et al. [14,15] conducteda study on properties and microstructure of 2219-T6 joints weldedby underwater friction stir welding. The results showed that watercooling environment could improve the strength of joints.

In this paper, ultra-high strength spray formed 7055 aluminumalloy is welded by friction stir welding in air and underwater,respectively. The thermal cycle curves and the distribution ofresidual stress are analyzed. The tensile properties, hardness,microstructures and fracture features of joints are alsoinvestigated.

2. Material and experimental procedures

The base material (BM) used for the experiment was a sprayformed 7055 aluminum alloy. The 4 mm thick sheets were cut into

Fig. 2. The location of thermocouples (AS: advancing side, RS: retreating side).

726 Y. Zhao et al. / Materials and Design 56 (2014) 725–730

250 � 100 mm specimens which were prepared for FSW process-ing. The chemical composition and the mechanical property ofthe base metal are listed in Tables 1 and 2, respectively.

Bead-on-plate friction stir welds were produced with a H13steel tool consisting of a concave 10 mm diameter shoulder and a4 mm diameter pin with the length of 3.75 mm. During the FSW,a constant tile angle of 2.5� was maintained. The welding speedwas 100 mm/min and the rotation speed of the tool was1000 rpm. Friction stir welding experiments were carried out inair and underwater, respectively. The experimental setup of under-water friction stir welding is shown in Fig. 1. The thermal cycletemperature was measured by 8 thermocouples fixed on the work-piece and the locations of the thermocouples are shown in Fig. 2.‘‘AS’’ represents advancing side, and ‘‘RS’’ represents retreatingside. The residual stress of plate was measured after welding. Holedrilling method according to ASTM: E837-13a was applied and thelocation of the strain rosettes on the workpiece was indicated inFig. 3. The Vickers microhardness measurements were conductedby MH-5D hardness tester (load: 100 g, time: 5 s).

The joints were cross-sectioned perpendicular to the weldingdirection for microstructure analyses and tensile tests. The tensiletests were conducted under the guide of GB/T228.1-2010 [16]. Thecross-sections of the metallographic specimens were observed byoptical microscopy (OM) after etching with Keller’s reagent. Theevolution of strengthening precipitation in different experimentconditions was observed by D/max 2550VL/PC X-ray diffraction(XRD). The room temperature tensile properties were tested by aZwick Z020 E-stretching machine, and the results of each jointwere evaluated using three tensile specimens cut from the samejoint. The fracture features of the joints were observed byJSM-6460 scanning electron microscope (SEM).

Table 1Chemical composition of spray formed 7055 aluminum alloy (wt%).

Zn Mg Mn Cr Fe Si Ti Cu Al

7.6–8.4 1.8–2.3 0.05 0.04 0.15 0.1 0.06 2.0–2.6 Bal.

Table 2Mechanical properties of spray formed 7055 aluminum alloy.

Material Thickness(mm)

Heattreatment

Ultimate tensilestrength (Rm/MPa)

Elongation(%)

7055 4 T6 570 12

Fig. 1. Experimental setup of underwater friction stir welding.

Fig. 3. The location of strain rosettes.

3. Results and discussion

3.1. Thermal cycle curves

The thermal cycle curves of welds formed in different media areshown in Fig. 4 and the corresponding meanings and locations ofthe marks are indicated in Fig. 2. Fig. 4(a) profiles the room tem-perature weld initiated at 30 �C. Approximately 80 s later, therotating tool is approaching the test area, and it begins to show asharp increase in temperature. When rotating tool passes by thethermocouples, the temperature at this point of the plate rises tothe maximum (196.5 �C) on advancing side (AS). Once the rotatingtool passes the test area, the curves begin to show a gradualdecrease in temperature. When the welding process is conductedunderwater, the temperature curves become mild and no signifi-cant increase is observed, as shown in Fig. 4(b). The maximumtemperature is 68.5 �C which is 128 �C lower than that of the weldformed in air. It means that water can ensure the plate to be kept ina low temperature environment and extra heat can be taken awayby water in time. After rotating tool passing by, the temperature ofplate can fall to room temperature quickly.

The thermal cycle curves that come from the nearest andfarthest points away from the weld center on the same side arecompared in Fig. 5. Fig. 5(a and b) shows that a significant dropin temperature is observed from the nearest point to the farthestpoint both on AS (from 196.5 �C to 106.4 �C) and RS (from173.6 �C to 90.3 �C) when the joint is welded in air, which showsa very large temperature gradient around weld. A large range

Fig. 4. The thermal cycle curves of weld formed in different media (a) in air and (b) underwater.

Fig. 5. The thermal cycle curves tested by the points nearest and farthest from the weld center on the same side (a) AS in air, (b) RS in air, (c) AS underwater, and (d) RSunderwater.

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variation of temperature would make properties of joint non-uni-form which is detrimental to the performance of weld. While thetemperature of the underwater welding is more steady andchanges little, as shown in Fig. 5(c and d). No matter the weldingprocess is conducted in air or underwater, the temperature on ASis always higher than that of RS. The result is consistent with theconclusion of present study on the thermal cycle of friction stirwelding [17]. Clearly the material on AS is affected by the heatinput more seriously than RS.

3.2. Residual stress distributions

The distribution of residual stress of the joint welded in differ-ent media is shown in Fig. 6. Distribution of residual stress isrelated to thermal cycle and restraint intensity. In this experiment,the deformation of plate is strictly limited by the fixture duringwelding process, so the difference in residual stress mainly gener-

ates by different thermal cycles. According to the results of thermalcycles, the plate welded in air suffers higher temperature and takesa longer time to cool down. If it is in a free state, the plate weldedin air would suffer a serious deformation to respond the heat.However, it is fixed strictly during the whole welding process,which leads to higher residual stress in the plate welded in air.

By contrast, the plate welded underwater just experiences a lowand mild thermal cycle for a short time, and shows lower residualstress on all range and it even shows compression stress in theweld area. Compression stress should come from squeezing actionof rotating tool to the weld. The plate will expand when it is heatedfor a period of time. Compression stress which gained from frictionstir welding process would be released during cooling processwhen the expansion reaches a certain degree, and farther more ittends to be residual tensile stress left in the weld and plate finally.In traditional friction stir welding process, the heat input is usuallyhigh enough to make the plate expand to counteract the

Fig. 6. Distribution of residual stress of the plate welded in different media.

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compression stress. Compared to the traditional FSW joint whichshows tensile stress on all range, compression stress is reservedin weld area of the underwater FSW joint. Residual compressivestress can improve the fatigue strength of the joints.

Fig. 7. Microstructure of join

Fig. 8. Microstructure of joints w

3.3. Microstructures

Figs. 7 and 8 show the cross-section and microstructures ofjoints welded by FSW in air and underwater, respectively. The ‘‘Sline’’ is observed in the weld nugget zone of the joint welded inair, as shown in Fig. 7(b). The ‘‘S line’’ is a kind of weak connectiondefect, which is detrimental to the properties of joints. But ‘‘S line’’is not found in the underwater joint (Fig. 8(b)).

It is known that the black ‘‘S line’’ probably originates from theoxide layer on the plate surface, which is broken up, extruded anddeformed during FSW [18]. The oxidation film is generated soon byfriction heat during FSW in air process even though it has beencleaned thoroughly before welding. When it is welded by under-water FSW, the plate is prevented from atmospheric oxidationand the ‘‘S line’’ defect is eliminated. Furthermore, some researches[19,20] indicate that reducing temperature gradient is also helpfulto eliminate ‘‘S line’’. Underwater FSW has a milder temperaturegradient than that in air and the water can take redundant heataway produced by friction in time which further reduces the pos-sibility of forming ‘‘S line’’.

The AS of the underwater joint is characterized by a typicallydistinct boundary between the nugget zone (NZ) and the thermalmechanically affected zone (TMAZ), as shown in Fig. 8(a). By con-trast the boundary between the NZ and the TMAZ in RS is ratherunclear, as shown in Fig. 8(c). In the joint welded in air, there is

ts welded by FSW in air.

elded by underwater FSW.

Fig. 9. XRD spectra of base metal and FSW joints.

Y. Zhao et al. / Materials and Design 56 (2014) 725–730 729

no clear interface between the NZ and the TMAZ on both side of theweld (Fig. 7(a and c)). In addition, the process of traditional FSWexperiences higher temperature and longer time, so HAZ of thejoint is wider due to a higher heat input. Under the influence ofheat input, HAZ is fuzzy because of variation of grain size andmicrostructure in this area. Water cooling can take the weldingheat away in time, gentle the temperature gradient, restrain heatspreading to base metal and prevent the grain in HAZ fromcoarsening.

3.4. X-ray analysis

XRD spectra of base metal and FSW joints are displayed in Fig. 9.Strengthening phase of base metal mainly consists of MgZn2, Al-CuMg and AlMg4Zn11. AlMg4Zn11 phase is not observed in jointsafter welding. Ref. [21] shows that the g0 phase which consists ofMgZn2 is the main strengthen phase of spray formed aluminumalloy when MgZn2 is supersaturated solid solved in base metalmatrix. However the g0 phase is unstable and MgZn2 is sensitiveto heat input. Zn tends to dissolve out with the decrease of thesolubility of Zn after welding and the quantity of MgZn2 wouldreduce, then g0 phase would turn into g phase. With the change

Table 3Tensile properties of welded joints in different media.

Medium Parameters Ultimate te

F (mm/min) S (rpm)

Underwater 100 1000 495In air 100 1000 430

Fig. 10. Fracture features of different joint

of dissolved state of Zn, strengthening mechanism of joints differsfrom base metal.

Compared with underwater joint, the higher heat input of FSWin air has changed the component of strengthening phases. Thestrengthening phase in the underwater joint is MgZn2, while thatin the joint welded in air is MgZn2 and MgCu2. Although bothtwo phases are Laves Phase working as solid solution strengthen-ing phase, MgCu2 and MgZn2 are different in electron concen-tration and crystal structure which cause different degrees ofgrain distortion and lead to different strengthening effects. MgCu2

generates from the displacement reaction of Cu and MgZn2. Whenthe welding process is conducted in air, Cu tends to be active underhigher heat input and bonds Mg more tightly and easily than Zndoes due to the difference of atomic radius. Ref. [22] proves thatCu would displace Zn from MgZn2 when the aluminum alloy getsadequate heat. MgZn2 and MgCu2 combining strengthening meth-od is less efficient than MgZn2 single strengthening method. MgZn2

tends to transform into MgCu2 under a higher heat input easily. Itwould have a great impact on the property of joint.

3.5. Mechanical properties

The results of tensile test are given in Table 3. The normal jointhas an ultimate tensile strength of 430 MPa, equivalent to 75% ofbase metal strength. By underwater FSW, tensile strength of jointscan be increased to 495 MPa, which is approximately 87% of basemetal strength. The elongation of the underwater FSW jointreaches 7.2% which is higher than that of the normal joint. No mat-ter the joint is welded in air or underwater, the AS is weakened dueto the asymmetrical distribution of heat input of the heat inputduring FSW, with fractures occurring in advance sides.

Fig. 10 shows the fracture features of joints welded in differentmedia. The fracture feature of the sample FSW underwater(Fig. 10(a)) is consisted of dimples with white dots. White dots hin-der the movement of dislocation which improves the strength andductility of grain boundary. However, the fracture feature of jointof traditional FSW shows small dimples and fuzzy torn lines with-out white dots. Its fracture mechanism is close to quasi-cleavagefracture (Fig. 10(b)). Water immersion environment reduces heatinput of FSW and preserves the excellent properties of basematerial furthest.

nsile strength (MPa) Elongation (%) Fracture location

7.2 AS (advance side)4.1 AS (advance side)

s (a) FSW underwater, (b) FSW in air.

Fig. 11. Microhardness distributions of different joints.

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3.6. Microhardness distributions

The values of hardness of the weld formed underwater arehigher than that of weld formed in air, as shown in Fig. 11. Thehardness curve of weld welded in air shows a ‘‘W’’ shape whichcontains a distinct low value area in the thermo-mechanicalaffected zone (THAZ) on both sides. By contrast, the hardness curveof underwater joint is a flat-like area which eliminates the distinctlow value area, and shows a more uniform performance. But nomatter the plate is welded in which media, the lowest value ofhardness always lies on the AS where the fracture takes place.

4. Conclusions

From the above investigations, the main conclusions can besummarized:

(1) Underwater FSW creates a milder and lower thermal cyclethan traditional FSW which is helpful to reserve the excel-lent performance of base metal furthest.

(2) Water environment has reduced the residual stress of thejoint obviously and even reserved compression stress inthe weld.

(3) Underwater FSW eliminates the ‘‘S line’’ defect and theunderwater joint has a clear boundary between the NZ andthe TMAZ on AS. The joint welded in air shows no clearinterface the NZ and the TMAZ on both side of the weld.

(4) Strengthening phase particles of base metal is AlMg4Zn11,MgZn2 and AlCuMg, and AlMg4Zn11 is not found in FSWjoints. The strengthening phase in the underwater joint isMgZn2, while MgCu2 replaces MgZn2 partly in the normaljoint.

(5) Tensile strength of the underwater joint reaches 495 MPa,equivalent to 75% that of the base metal and the elongationis higher than that of the normal joint.

Acknowledgement

The authors are grateful to be supported by the NationalNatural Science Foundation of China (Grant No. 51005153).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.matdes.2013.11.071.

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