Microstructure and Mechanical Study on Laser-Arc-Welded Al­Zn­Mg Alloy

Jiaxing Gu1, Shanglei Yang1,2,+, Qi Xiong1 and Chenfeng Duan1

1School of Materials Engineering, Shanghai University of Engineering Science, No. 333 Long Teng Road, 201620, Shanghai, China2Shanghai Collaborative Innovation Center of Laser Advanced Manufacturing Technology,No. 333 Long Teng Road, 201620, Shanghai, China

Al­Zn­Mg alloy is widely used as lightweight material. Laser-arc hybrid welding is considered to be a suitable joining process foraluminum alloy. In this paper, 3-mm-thick 7075 aluminum alloy was welded by laser-arc hybrid welding method. The microstructure of thewelded joint was analyzed. Microhardness measurement and tensile test were conducted. The fatigue test of the welded joint and base metal wascarried out. And the feature of fatigue crack initiation and propagation were discussed. [doi:10.2320/matertrans.MT-M2019178]

(Received June 27, 2019; Accepted October 21, 2019; Published December 25, 2019)

Keywords: aluminum alloy, welding, microstructure, mechanical

1. Introduction

Al­Zn­Mg series aluminum alloys, the mechanical proper-ties of which are considered to be the most enhanced amongall aluminum alloys.1) 7075 is an typical Al­Zn­Mg seriesalloy widely used in aircraft structures, mold processing, andother mechanical equipment. Traditionally, metal inert gas(MIG) welding is usually used in joining aluminum alloy.However, due to the large heat input, defects such as pores,cracks, and deformations are generated easily; therefore,welded joints (WJs) have low mechanical performance.2)

Laser beam welding (LBW) has been used as an advancedjoining technology due to its many advantages including highenergy density, high welding speed, and narrow heat-affectedzone (HAZ). However, due to aluminum’s high reflectivity tolight, LBW becomes a big challenge.3)

In order to overcome the disadvantages of laser weldingand arc welding, laser-arc hybrid welding (LAHW) wasdeveloped. It has lots of advantages,4,5) such as deeperwelding penetration, more stable and better gap tolerance,which raised increasing interests in the research field. Inrecent years, studies on the hybrid laser-MIG welding are ahot topic in the welding of Al-alloys. Wu et al.6) examinedthe microstructures and mechanical properties in hybrid7075-T6 welds. Hu et al.7) examined the weldability of 7075using a hybrid laser-GMA welding.

Welding is one of the most important way of connectingmaterials. Most of the damage is well known to be caused byfatigue fracture, which is usually the process of formation,propagation, and fracture of fatigue cracks. Therefore,studying the fatigue fracture behavior of materials andwelded joints is of great significance in both theory andpractice.

Shiraiwa et al.8) examines machine learning methods topredict fatigue strength with high accuracy. Liu et al.9)

proposed a fatigue crack initiation life-based model andinvestigated fatigue characteristics of A7N01 alloy weldedjoint. Zhang et al.10) pointed out that hybrid weldingproduced the welds with low percent porosity, grainrefinement, thus improving the fatigue properties. Ghoshet al.11) thought that the fatigue crack of Al­Zn­Mg

weldment was initiated along to the weld center due tothe plenty of pores formation during the welding process.Qiao et al.12) pointed that MgZn2 precipitation in the weldzone can make the dislocation movement slowdown, thuseffectively prevented fatigue crack growth. Wu et al.13)

thought that 7000 series hybrid weld shows a serioussoftening in hardness and poor mechanical properties dueto the evaporation loss of Zn and Cu and precipitate growthtogether with the porosity.

This work uses a fiber laser-MIG welding method to weld3-mm-thick 7075 aluminum alloy. An analysis of themicrostructure of the WJs was conducted. The local-zonemicrohardness and tensile strength of the WJs wereinvestigated intensively to analyze the joint properties. Afatigue test of the WJs was also carried out. The fracturemorphology and crack initiation and propagation of the WJswere observed by metallographic microscope and scanningelectron microscope (SEM), and the initiation and prop-agation features of the cracks were analyzed. The aim is toprovide a theoretical and experimental basis for practicalengineering applications.

2. Experimental Procedure

2.1 Materials and welding processThe base metals (BMs) used in this study were 7075-T6

plates with 3-mm thickness. The filler material was ER5356with a 1.2-mm diameter. The chemical compositions of 7075and ER5356 are listed in Table 1.

The welding experiments were carried out by an IPG YLS-5000 fiber laser machine and a Fronius TPS4000 electric arcwelder controlled by a KUKA industrial robot. A diagram ofthe experimental process is illustrated in Fig. 1(a). Beforewelding, a laser cleaning machine was used to removesurface oil, other metal elements, and oxide film from thesurface to avoid the creation of pores during welding.Throughout the experiments, argon gas with a flow rate of20L/min was applied to protect the molten pool.

To optimize the welding parameters, several hybrid laser-MIG hybrid welding experiments were performed. Accordingto the weld appearance, suitable welding process parametersare presented in Table 2. Figure 2 shows the postweldappearance of the WJs. The weld bead was well formed,+Corresponding author, E-mail: yslyangshanglei@163.com

Materials Transactions, Vol. 61, No. 1 (2020) pp. 119 to 126©2019 The Japan Institute of Metals and Materials

the fusion with the BM was good, the transition was gentle,and no defects were generated, such as incompletepenetration and welding cracks.

2.2 Microstructure observation and mechanical testAfter welding, the cross-section of the weld was polished

and etched by Keller’s reagent (HF:HCl:HNO3 = 1:1.5:2.5).Microstructural examinations were carried out using KeyenceVHX-6000 and Nikon Epiphot 300 optical microscope.

Standard tensile and fatigue specimens (Fig. 1(b)), wereextracted both from the BM and the WJs according to theNational Standard GB2561-88, tested at room temperature bya Zwick/Roell Amsler HB250 electrohydraulic servo-testing

machine to determine the mechanical properties of the WJs.The loading frequency was 15Hz and the stress ratio was 0.1.A Hitachi S-3400N SEM was used to observe the surfacemorphology of the fractures.

Extensive microhardness measurements were conducted asshown in Fig. 2. A 0.981N force was applied for 15 s withthe adjacent indention distance of 0.5mm.

The local-zone samples were sliced parallel to the weldseam every 0.9mm from the weld center (Fig. 1(c)).Considering the size of the BM and weld seam, thesespecimens had a thickness of 0.6mm after polishing (detailedsizes are shown in Fig. 1(d)). The local-zone tests wereperformed at room temperature.

Table 1 Compositions of 7075 aluminum alloy and ER5356 wire (mass fraction, %).

Fig. 1 (a) Schematic diagram of the welding process; (b) Dimensions of the general extracted specimen; (c) Sketch of extraction location;(d) Dimensions of local-zone test specimens.

Table 2 Detailed welding parameters.

J. Gu, S. Yang, Q. Xiong and C. Duan120

3. Results Analysis

3.1 Microstructure analysisFigure 3 shows typical metallographic images of different

regions of the hybrid WJ. Figure 3(b) shows the micro-structure in the fusion zone (FZ), which is a complextransition zone between the weld and the unmelted BM.Therefore, the chemical composition and microstructure inthe FZ are very complicated. The crystal grains close to the

BM are only partially melted and the liquid phase coexistswith the remaining unmelted solid phase, forming thepartially melted zone (PMZ). The grains are all melted inthe weld zone (WZ) near the PMZ and the originalcomposition remains after melting due to the extremely shortresidence time. In the subsequent crystallization process,since the edge of the weld is sufficiently heat dissipated andthe cooling rate is fast, the molten metal is directly nucleatedon the surface of the unmelted crystal grains, thereby forminga columnar crystal structure growing along the heat-dissipation direction.

The grain in the center of the weld is obviously equiaxed(Fig. 3(a)), indicating that free crystallization occurs. In thisarea, the temperature gradient is small and it is less affectedby the edge heat-dissipation conditions. A wide componentsupercooling zone can be formed in the liquid phase. Coarseand coarse dendrites can be formed at the crystallizationfront. As the degree of subcooling continues to increase, newgrains will be produced inside the liquid phase. These grainsare not hindered by the surrounding fluid and can growfreely, resulting in equiaxed crystals in the middle of theweld.

Figure 3(c) and (d) shows the microstructure of theoveraging softened zone (OSZ) and BM, respectively. The7075-T6 aluminum alloy used is a rolled deformed aluminumalloy; therefore, the BM grains are elongated. Fine dispersedblack particles are observed in the strengthening phase. Thegrains in the OSZ become coarse and the strengthening phasealso gathers and grows up, due to the influence of weldingheat, which weakens the original strengthening effect of theBM. Also, according to the grain boundary-strengtheningtheory, the finer the grain size of the metal material, the moregrain boundaries, the strong hindrance of dislocation motion,

Fig. 3 Metallographic image of different regions: (a) Weld center; (b) Fusion zone; (c) OSZ in HAZ; (d) BM.

Fig. 2 Macroscopic appearance of the welded joint and hardness measure-ment position and direction.

Microstructure and Mechanical Study on Laser-Arc-Welded Al­Zn­Mg Alloy 121

and the higher the material strength. Thus, the strength ofOSZ is much lower than that of the BM, which will beanalyzed below.

3.2 Hardness distributionHardness is an important index to measure the perform-

ance of metal materials. It can be understood as the ability ofmaterials to resist elastic deformation, plastic deformation, ordamage, and can be expressed as the antidestructive abilityof materials to resist residual deformation. At the same time,the change of hardness also reflects the change in the WJs’microstructure. Figure 4 describes the distribution of themicrohardness of both top and bottom positions of thetransverse section of the hybrid WJs. The main alloyingelements of 7075 aluminum alloys are Mg and Zn, which areaging-strengthened aluminum alloys, and the main strength-ening phase is MgZn2. The main components of the weldingwire are Al and Mg, and the Zn content in the weld is small;thus, the main strengthening phase MgZn2 is difficult to form.Therefore, the hardness of the welding seam is 92.4HV,which is significantly lower than the HAZ and the BM, whichis 179.8HV.

The temperature field in the HAZ is unevenly distributedduring welding. The closer to the weld, the higher the peaktemperature. Because the welding thermal cycle experiencedby different parts of the HAZ is different, the behavior ofdissolution and aggregation growth of the second-phaseparticles is different, and the microstructure and properties indifferent regions of the HAZ are changed. Near the FZ, thepeak temperature is high, and the second-phase particles arelargely dissolved into the matrix. Then, the extremely fastcooling rate suppresses the precipitation of the second-phaseparticles, resulting in solid solution strengthening. The solidsolution strengthening effect is weaker than the agestrengthening, making the hardness of the zone lower thanthe BM, but higher than the weld. The peak temperatureand cooling rate become lower as distance from the WZincreases. The dissolution of the second-phase-enhancingparticles is insufficient and solid solution strengthening doesnot substantially occur, which reduces the microhardness.Because they are close to the BM, they are less affected by

the welding heat. Therefore, because the second-phaseparticles do not substantially dissolve, the tendency ofaggregation growth is gradually weakened, and the hardnessloss is also smaller until it reaches a stable value.

3.3 Tensile performanceGenerally speaking, there is a certain relationship between

hardness and the strength of the metal. The area with highhardness is also high in strength, but its plasticity andtoughness will decrease accordingly.

Figure 5 shows the tensile test results of 7075 aluminumalloy BM, the overall WJs and different zones, including WZand OSZ. The test results show that the WJs have an ultimatetensile strength of 355MPa, a yield strength of 240MPa, andan elongation after fracture of 2.35%. The tensile strengthand yield strength of the BM are 518MPa and 416MParespectively, which is much higher than that of the WJs. Theelongation after fracture is 11.3%, which is also significantlyhigher than that of the WJs, indicating that the strength andplasticity change significantly after welding. In the test, theWJ specimen was generally broken in the WZ. Thus, the WZis the weakest mechanical region of the WJ. The main reasonis that the mechanical properties of Al­Mg-based filler metalsare much lower than those of 7075 aluminum alloy BMs. TheWZ results showed that the tensile strength, yield strength,and elongation are 317MPa, 218MP, and 3.37%, comparedto 418MPa, 323MPa, and 5.16% of the OSZ, respectively.When it reaches the tensile strength limit of the WZ, the yieldstrength of OSZ and BM have not been reached; therefore,plastic deformation in only the WZ occurred during the WJtest, resulting in lower elongation. Wu et al. showed that thetensile strength of the WJ is 333MPa using a hybrid fiberlaser and pulsed arc heat source system. According to Huet al., a hybrid weld subjected to 1-month natural aging, butwithout solution treatment, shows a maximum strength of343MPa.

3.4 Fatigue propertyAccording to the yield strength of the WJ, the fatigue

performance test of the 7075 aluminum alloy laser-MIGhybrid WJ was carried out under different stress levels. Thecyclic stress corresponding to N = 1 © 107 is taken as thefatigue limit of the WJ.

Fig. 5 Tensile properties of different zones.

Fig. 4 Microhardness profiles of the welded joint.

J. Gu, S. Yang, Q. Xiong and C. Duan122

The power function of the S­N curve is expressed as:

cðNfÞk ¼ ·a ð1Þwhere ·a = (·max ¹ ·min)/2, Nf is the number of cycles tofailure, C is the fatigue strength coefficient, and k is thefatigue strength exponent.

Take the logarithm of eq. (1), then the double logarithmicequation is:

lgð·aÞ ¼ lgðNfÞ þ C ð2ÞThus, in the logarithm coordinates, lg(·a) and lg(Nf ) shows

a linear relationship. The S­N curve fitted with the measuredexperimental data is illustrated in Fig. 6. According toeq. (2), the fitting formulae of the curves for the WJ and BMare calculated, respectively:

lgð·aÞ ¼ �0:0644 � lgðNfÞ þ 2:1443 ð3Þlgð·aÞ ¼ �0:0644 � lgðNfÞ þ 2:3723 ð4Þ

Figure 6 shows that the distribution of fatigue life underdifferent stresses is relatively discrete. The fatigue limit ofthe WJ and BM are 110MPa and 190MPa, respectively,under 1 © 107 cycles.

4. Discussion

Even the small size of pores can have a great impact on thefatigue properties of materials. Therefore, the initiation andpropagation mechanism of microcracks in pores are analyzedin the following paragraph.

4.1 Fatigue crack initiationFigure 7 shows two cracks initiated from two sides of the

pore on the sample surface. According to the SEM imagesof the fractured fatigue specimens, the crack initiation frompores was the predominant cause of fracture for the WJs.

As we can see from Fig. 7, the size of the pore is about100 µm. According to Fig. 1(b), the fatigue specimen parallelsection is 12mm. The width of the hole size is far less thanthe size of the sample; therefore, we can use the infinite platewith a pore defect as the theoretical model to analyze the

crack-initiation mechanism. Based on the above assumptions,the force model of a circular hole in an infinite plate isestablished (Fig. 8).

The plate is subjected to uniform force ·, and there exists asmall circular hole with radius a. The existence of the circularhole must affect the stress distribution. The stress at any pointP(r, ª) near the hole will be much greater than that withoutthe hole, and much greater than that farther away from thehole. This phenomenon is called stress concentration. If ris sufficiently far from the center of the hole, the stressdistribution should be the same as ·. Based on the aboveanalysis, it becomes an external circular force problem ofa thick-walled cylinder with inner diameter a and outerdiameter r.

According to the elastic mechanics knowledge, threeforces are applied to the outer circumference. They are theradial normal stress ·r, ring normal stress ·ª and ring shearstress ¸rª, respectively (Fig. 9). The magnitude of the threeforces are:14)

·r ¼·

21� a2

r2

� �þ ·

2cos 2ª 1� 4

a2

r2þ 3

a4

r4

� �

·ª ¼·

21þ a2

r2

� �� ·

2cos 2ª 1þ 3

a4

r4

� �

¸rª ¼ �·

2sin 2ª 1� 3

a4

r4þ 2

a2

r2

� �

8>>>>>>>><>>>>>>>>:

At the edge of the hole, r = a. When ª = «³/2, ·ª = 3·.At the section perpendicular to the load direction, themaximum stress at the edge of the hole is three times higherthan that without the hole.

Fig. 7 OM image of crack initiation and propagation direction near thepore.

Fig. 8 Diagram of an infinite plate containing a pore.

Fig. 6 S­N curve of the welded joint.

Microstructure and Mechanical Study on Laser-Arc-Welded Al­Zn­Mg Alloy 123

However, according to Fig. 7, there are still somedeviations between the actual crack initiation location atthe stoma and the above model. In addition to the stressconcentration at the edge of the pore, the unevenness of grainstructure and lacking of smoothness at the edges of pore alsoaffects the crack initiation.

4.2 Fatigue crack propagationFigure 10 shows two cracks initiated and propagated from

both sides of a pore perpendicular to the load direction. Aradial plastic zone can be observed at the tip of the cracks.During the crack-propagation process, the high stressconcentration at the crack tip will cause the material tolocally yield and form a plastic deformation zone of a certainsize. The plastic deformation zone is caused by the slip alongthe maximum shear stress plane caused by the shear stress.The main function of plastic deformation at the crack tip isto absorb the plastic deformation, passivate and relax thecrack tip, reduce the stress level of the crack tip, and preventcrack growth.

During each cycle, the fatigue crack propagation is causedby plastic passivation at the crack tip.15) The mechanism isshown in Fig. 11. Under the minimum load, the crack isclosed and the tip is sharp. As we have discussed above,under a tensile load, the crack tip produces a plastic zone dueto stress concentration. Plastic deformation causes the crackto gradually change from a closed state to an open state.When the tensile load increases to the maximum value, thecrack-opening amount is the largest and the correspondingplastic deformation is also the largest. The crack tip slipsalong the direction of maximum shear stress by a doubleslip mechanism, causing plastic passivation. This passivationprocess allows the crack to extend forward a distance. When

the stress is reduced, the plastic deformation will decrease,the crack tip will re-sharp and passivate again duringsubsequent loading process. Thus, fatigue cracks propagateforward.

From Fig. 12(b), the propagation of the crack is notstraightforward during the fatigue process, but propagatesforward in a snake shape. The mechanism of its formation isillustrated in Fig. 12(a). For the crack type I, there exists twoslip sources ¸1 and ¸2 at the crack tip, which can be activatedon the intersecting slip plane. The crack propagation can bedivided into four steps: microcrack is generated when a slipsource (¸1) at the crack tip is first activated. After passivation,it is difficult to expand the microcrack in the originaldirection. A tear is generated in the other direction. Themicrocrack is then passivated again. After multiple cycles, asnake-shaped crack is produced.

The propagation of the crack is mainly through trans-crystalline form. Metallographic observation on the polishedand corroded surface reveals the morphology of the crackacross the crystal (Fig. 13).

Figure 14 shows the SEM images of fatigue fracture ofWJs. Fatigue crack initiation occurs at the surface porositydefect. After initiation, the fatigue crack propagation zone isformed by extending into the sample. The specimen breakswhen the fatigue crack grows to a critical size.

There are three kinds of fracture forms for aging aluminumalloy: namely, slippage zone cracking, intergranular cracking,and dimple cracking. Figure 14(a) shows that although thereis a river-like pattern on the fracture, the “river” extends tothe periphery of the plane and the “river” is relatively shortand discontinuous with small obvious convergence features,

Fig. 9 Diagram of stress distribution near the pore in the plate: (a) Radial normal stress ·r; (b) Ring normal stress ·ª; (c) Ring shearstress ¸rª.

Fig. 10 OM image of plastic zone at fatigue crack tip.Fig. 11 Plastic passivation mechanism of fatigue crack propagation:

(a) Crack closure; (b) Crack starts to open; (c) Significant plasticdeformation; (d) Max plastic deformation; (e) Crack closure.

J. Gu, S. Yang, Q. Xiong and C. Duan124

which is obviously different from the “river pattern” of thecleavage fracture. There are many relatively flat quasicleav-age surfaces on the fracture surface, the small planes areconnected by tearing, and obvious tearing edges can be seen.

Therefore, from the perspective of fracture mechanism, themain mode of the WJ is quasicleavage fracture.

The quasicleavage steps parallel to the crack growthdirection can be clearly seen in Fig. 14(b). The featheryextension features can be seen in Fig. 14(c). Its formation isdue to the extension of fatigue crack propagation from onegrain to another grain, and the dislocation movement is easilyobstructed at the grain boundary. New cracks are generatedby the tensile stress caused by dislocation plug accumulation.In pursuit of minimal energy expenditure, the cracks expandto slightly different surfaces, which causes a feathery feature.These are the obvious characteristics of brittle fatiguefracture, which occurs with quasidissociation.

As we discussed above, since the crack closure cannoteliminate the passivation caused by the maximum stresscompletely, the crack will extend a distance further in thesubsequent process. The traces of such crack propagation

Fig. 12 The propagation of the crack and its formation mechanism: (a) Snake-shaped crack formation step; (b) OM image of fatigue thesnake-shaped crack.

Fig. 13 Metallographic image of a transcrystalline crack.

Fig. 14 SEM image of fatigue fracture. (a) Fatigue initiation zone; (b) Quasi-cleavage steps; (c) Feathery extension features; (d) Fatiguestriation.

Microstructure and Mechanical Study on Laser-Arc-Welded Al­Zn­Mg Alloy 125

can generally be observed on the fatigue fracture, that is, theso-called fatigue striation (Fig. 14(d)).

5. Conclusion

In the present study, the microstructural and mechanicalproperties of laser-MIG welded 7075-T6 aluminum alloyusing ER5356 filler were investigated and the mechanism offatigue initiation and propagation was analyzed. The mainconclusions are summarized as follows:(1) A microstructure analysis revealed that the laser-MIG

hybrid WJ is composed of HAZ, PMZ, and WZ.Equiaxed grains formed in the weld center and columnarcrystal formed near the fusion line. The grains in theHAZ are coarsened and recrystallized in the PMZ.

(2) The hardness distribution of the WJ is not uniform. TheBM has the highest hardness of 179.8HV. The weld hasthe lowest hardness of 92.4HV. The HAZ is dividedinto a solid solution zone and an OSZ, and the OSZ hasa minimum hardness of 140.1HV.

(3) The WZ has an ultimate tensile strength of 317MPa,which is the mechanical weakest area in the whole WJ,compared to 417MPa of the OSZ, which is much lessthan that of the BM (518MPa). The ultimate tensilestrength of the WJ is 350MPa and the fatigue limitat 1 © 107 cycles is 110MPa. The fracture mode isquasicleavage fracture and mainly through a trans-crystalline form.

(4) Crack initiation from pores was the predominant causeof fracture. The maximum stress at the edge of thehole is three times higher than that without the hole.The high stress concentration at the crack tip willproduce a plastic deformation zone. The fatigue crackpropagation is caused by plastic passivation and presenta snake shape.

Acknowledgement

This project is sponsored by the National Natural ScienceFoundation of China (51971129) and the Shanghai NaturalScience Foundation of China (19ZR1421200), also supportedby the Research Innovation Program for Graduate Student ofShanghai University of Engineering Science (E3-0903-19-01144).

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