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Introduction

Age-hardened Al-Zn-Mg-Cu alloys,especially 7075 with T6 temper (7075-T6),have become some of the most widelyused structural materials in the aerospace,high-speed train, and automotive indus-tries because of their attractive mechani-cal properties (Ref. 1). Nonetheless, thehigh heat sensitivity and low eutectic liq-uidus temperature range of 7075-T6 makeit difficult to weld with conventional arcwelding methods. Hybrid laser arc welding(termed hybrid welding) was proposed toachieve a sound joint in both strength and

geometry (Ref. 2). However, the weldingprocess readily degrades the characteris-tics of the precipitation strengthening.This phenomenon has been observed toaccompany a dramatic loss of staticstrength, although the strength can be par-tially recovered by elaborately designingthe postweld aging (Refs. 3, 4). Prior stud-ies have shown that heat-induced soften-ing is primarily caused by the intrinsic dis-solution and growth of precipitates (Refs.

5, 6). To prevent quality loss in 7075-T6joints, solid-state friction stir welding(FSW) is being actively pursued as a po-tential alternative to fusion welding. Un-fortunately, the softening behavior of theweld nugget zone (WNZ) and thermome-chanically affected zone (TMAZ) remainsa major challenge (Refs. 7, 8). Therefore,it is vital to deeply explore the softeningmechanism of fusion hybrid welds.

Physically, the softening can be attrib-uted to three causes. First, the strengthen-ing elements, such as zinc, magnesium,copper, and their hybrids (intermetalliccompounds) in the 7075-T6 matrix tend tobe rearranged during welding, and this re-distribution affects the microstructure andstrength (Ref. 9). Previous papers havemainly focused on the evolution of precip-itation phases in the base metals (Refs.1012), and a number of empirical modelshave been established to correlate thestrength with the precipitates, aging stage,and compositions (Refs. 13, 14). Further-more, the presence of various gas poresdecreases the effective load capacity andhas a detrimental effect on the mechanicalproperties. The relevant literature haspaid more attention to the source, pre-vention, and formation of gas porosity(Ref. 15), and few works have reported theinfluence of this porosity on the staticstrength (Refs. 16, 17). This lack of infor-mation exists because it is difficult to de-tect the exact size, morphology, and distri-bution of pores in the fusion welded joints.Microcracks, inclusions, segregations,tensile residual stress, and filler metal(FM) are also responsible for the compro-mised mechanical properties (Ref. 18).

The softening that arises from theseforegoing factors can seriously underminethe fitness for service of welded aluminumalloy structures (Ref. 19), in addition todeteriorating their load-carrying capacity,especially under variable reversed loads(Ref. 20). The existing literature has rarely

ABSTRACT

The severe strength loss of hybrid welded Al-Zn-Mg-Cu alloy joints such as 7075-T6has been characterized using high-resolution synchrotron radiation X-rays and theo-retical modeling. To elucidate the physical causes of this static strength change, the dis-tribution of strengthening elements such as Zn and Cu and the three-dimensional gasporosity were mapped. The interesting findings included the following: 1) Hybrid weldsonly reach approximately 53% of the ultimate tensile strength of the base phase, andthe lowest strength is situated in the central fusion zone; 2) because of the excessiveevaporation of elemental Zn and the significant inverse segregation of elemental Cu incenral fusion welds, the major strengthening elements gather near the heat-affectedzone; 3) the elastic modulus of hybrid welds is slightly larger than that of their base al-loys, probably as a result of the inhomogeneity in their chemical composition, mi-crostructure, geometry, and residual stress; 4) the pore size ranges from below 0.01 mto approximately 107 m, and it is modeled by the Schwartz-Saltykov method to revealits damaging effects on the static strength; 5) a strength model of 7075-T6 welded buttjoints is established to correlate with the elastic modulus, porosity, and heat inputs, andit coincides well with the computational and experimental results; and 6) in principle,the porosity has little influence on the static strength of hybrid welds, and the modifiedstrengthening structure markedly dominates the overall mechanical properties.

S. C. WU ([email protected]) and, W. H. ZHANG are with State Key Lab of TractionPower, Southwest Jiaotong University, Chengdu,China. X. YU, R. Z. ZUO, and J. Z. JIANG arewith School of Materials Science and Engineer-ing, Hefei University of Technology, Hefei, China.H. L. XIE is with Shanghai Syncrotron RadiationFacility, Shanghai Institute of Applied Physics,Shanghai, China.

KEYWORDS

7075-T6 Aluminum AlloysHybrid Laser Arc WeldingSoftening BehaviorGas PorosityElement LossStrength ModelingSynchrotron Radiation X-Rays

Porosity, Element Loss, and Strength Modelon Softening Behavior of Hybrid Laser Arc

Welded Al-Zn-Mg-Cu Alloy with Synchrotron Radiation Analysis

Gas porosity and alloying elements Zn, Cu, Ti, and Mn were characterized by high-resolution synchrotron radiation analysis, and a new strength model of hybrid

welded 7075-T6 joints was formulated and validated using these results

BY S. C. WU, X. YU, R. Z. ZUO, W. H. ZHANG, H. L. XIE, AND J. Z. JIANG

Wu Supplement March 2013_Layout 1 2/15/13 2:13 PM Page 64

been concerned about the softening of hy-brid welded 7075-T6 joints. Consequently,it is commercially and academically im-portant to quantify the softening behaviorof hybrid welded joints.

This work presents novel elementalcomposition and gas porosity results, andtheir roles in the strength of 7075-T6 buttjoints are identified by measurementstaken using the high-resolution ShanghaiSynchrotron Radiation Facility (SSRF)(Ref. 21). The elemental composition andpores were detected using synchrotron ra-diation micro X-ray fluorescence (SR-XRF) at the 15U beam line (BL15U)and synchrotron radiation X-ray microto-mography (SR-XRM) at the 13W beamline (BL13W), respectively. Relativelyperfect joints were obtained using hybridwelding, and nonheat-treatable ER5356was used as the FM. A quantitative rela-tionship between the strength with theporosity in hybrid welds was establishedbased on the elastic modulus, heat input,and gas porosity. New insights are pro-vided to characterize the heat-inducedsoftening behaviors, which are expected tobe an important complement to the recog-nition of the softening mechanism of hy-brid fusion welded Al-Zn-Mg-Cu joints.

Experimental

Materials

Table 1 shows the chemical composi-tions of the high-strength 7075-T6 alu-minum alloys and the self-developed con-sumable ER5356 (1.2 mm) with yieldstrength of 120 MPa and ultimate tensilestrength (UTS) of 260 MPa. A commercial7075-T6 sheet was sheared into samplesizes of 240 60 2 mm3. Following de-greasing and abrading, the butting sur-faces were carefully cleaned using a

sodium hydroxidesolution at 60C.The samples werethen washed in a di-lute nitric acid solu-tion. To reduce thehydrogen gasporosity as much aspossible, both theBM and FM wereplaced into a dry,clear, well-venti-lated storage space for no more than 24 h.

Hybrid Welding

Hybrid welding was performed by incor-porating a fiber laser (YLR-4000) with a gasmetal arc welding (GMAW, FroniusTPS4000) power source. The parameters ofthe laser were as follows: 0.4-mm focus spotdiameter, 10 kW/mm2 maximum outputpower density, 1.06-m wavelength, and150-mm focal length of the lens. The fillertip was 3 mm away from the laser axis, and99.999% pure argon gas with a flow rate of45 L/min was selected as the shielding gas.To alleviate reflection, the laser was inclinedby approximately 10 deg, and the arc torchwas inclined to the sample plane at a 70-degangle. To prevent any relative movement,the plates were fixed onto a worktable usingparallel slat clamps. Single-pass, butt-jointhybrid keyhole welding was then performedin the rolling direction.

For a good quality of butt joints, a largenumber of hybrid welding experimentswere conducted, and the suitable parame-ters were as follows: Plaser = 2.5~3 kWlaser power, IGMAW = 90~110 A arc cur-rent, about UGMAW = 23 V arc voltage,vtorch=2.5~3 m/min torch travel speed,vfiller= 5.3 m/min for the FM, and = 1mm defocusing distance. laser = 0.60 andGMAW=0.82 were thermal efficiency forthe laser and GMAW heat source, respec-tively, and the heat input was Q =(laserPlaser+GMAWIGMAWUGMAW)/vtorch.

Performance Examination

Briefly, the microstructure was investi-gated using an optical microscope (OM)and a scanning electron microscope (SEM).The morphology and distribution of theheat-induced particles were then examinednear the heat-affected zone (HAZ).

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Fig. 1 Typical SEM macroimage of the complete weld produced byhybrid laser arc welding, in which solid circles on the transverse crosssection of the hybrid joint were used to illustrate the indentation pointsfor the microhardness and white rectangle across the left weld interfacewas drawn to show triangular or polygonal particles. The black solidline near left-curved weld interface was actually the longitudinal crossplane as perpendicular to the x-y plane for the particle analysis by SEM.The z-axis is normal to the transverse cross section.

Fig. 2 Microhardness distribution curves for hybrid welded 7075-T6 alu-minum joints after 10 months natural aging under different heat inputs: lowheat input Q1 = 65520 J/m, medium heat put Q2 = 72080 J/m, and high heatinput Q3 = 86496 J/m.

Fig. 3 Yield strength, ultimate tensile strength, and elongation results fromcross-tensile tests with the same gauge length of 10 mm on received base metal,hybrid welded joints after three months of natural aging, and autogenous laserwelded joints after seven months of natural aging.

Table 1 Measured Nominal Chemical Composition (wt-%) of the 7075-T6 High-StrengthAluminum Alloy and the Self-Developed ER5356 Filler Metal

Elements Zn Mg Cu Ti Mn Cr Fe Si Al

7075-T6 5.54 2.43 1.30 0.05 0.10 0.19 0.25 0.20 Bal.ER5356 0.10 4.80 0.10 0.12 0.15 0.10 0.40 0.25 Bal.


Cross-tensile tests were conductedwith hybrid and autogenous laser weldedjoints that had been naturally aged forthree and seven months, respectively. Thesamples were machined to a 10-mm gaugelength with a 25-mm width based onwelded joint tensile test code (ISO 4136:2001). The facility employed was an MTS809 servohydraulic testing system with aload capacity of 250 kN. The fracturedsurfaces were then examined for the frac-ture mode and distribution of gas poresand microcracks using SEM.

To obtain the hardness, the Vickers mi-crohardness was examined along thetransverse section with a load of 200 g forapproximately 10 s. The natural aging pe-riod was 10 months for present hybridwelded joints. Figure 1 demonstrates thespatial distribution of the pressed points ina complete joint.

Synchrotron Radiation X-Ray

The strengthening elements weremapped in the sectioned welds with a syn-chrotron radiation light source. The highflux and low divergence of the SR-XRDdemonstrate its advantage in quantifyingalloying elements with less damage to theirradiated samples (Refs. 22, 23). The sam-ples were ground and polished to a 1.5-mmthickness, and a maximum energy of 20keV was selected to ensure a 30% trans-mission rate. The spot size was 1.6 1.8

m2 inside the sample with a focus photonflux density of 1.8 1011 phs/(sm2) at 10keV. The fluorescent intensities were meas-ured using a liquid-nitrogen-cooled seven-element energy-dispersive high-puritySi(Li) detector, and the sample wasscanned in the step-by-step mode atBL15U. Using this method, it took a totaltime of 1.5 h to pick up a total of 10,000 pix-els. Unfortunately, the strengthening ele-mental Mg could not be detected becauseof its absorption edge limit.

The formation of pores is a problemwhen joining 7075-T6. Complete avoidanceof pores is unrealistic for fusion welding.High-resolution SR-XRM has become themost advanced tool (Ref. 24) to determinethe exact descriptions of the gas pores, aswell as their size, morphology, position,number, and distribution (Ref. 25). A voxelsize of 0.7 m was chosen as the sample size,representing a compromise between theminimum size of the gas pores and the 2048 2048 CCD detector. Successive two-dimensional (2D) in situ slices were ob-tained at BL13W, and these slices were sub-sequently reconstructed using the Amirasoftware for three-dimensional (3D) im-ages.

It is vital to evaluate the influence of thepores on the mechanical properties. Manymethods have been introduced to deter-mine the 3D size distribution of sphericalpores. Here, the classical Schwartz-Saltykov(SS) method is selected because it is suitable

to the zero-continuity and low fraction ofthe pores (Refs. 26, 27). By characterizingthe gas porosity, the damage to the staticstrength can be evaluated.

Results and Discussion

Softening Behavior

Figure 2 shows the hardness distribu-tion curves along the x-axis illustrated inFig. 1, for three cases of welding heat in-puts Q1 = 65520 J/m, Q2 = 72080 J/m, andQ3 = 86496 J/m. It is clearly observed thatthe welding heat input has a primary effecton the hardness. A higher heat input re-sults in a larger softening region, a rela-tively wider fusion zone (FZ), and a lowerhardness (Ref. 28). Moreover, it was alsofound that the hardness changes slowlyfrom the joint center to the weld interface(WI) and then increases suddenly to thelevel of base metal (BM). The hardnessprofiles fully show that the welded joint ismechanically and compositionally hetero-geneous, which practically makes the FZthe weakest part of the aluminum weldedstructures.

To clearly reveal the serious softeningbehavior of hybrid welded joints, cross-tensile tests with a 25-mm gauge lengthwere conducted for the BM and two typesof welded joints. Tensile results show that al-most all fractures occurred near the PMZrather than the central FZ. It is seen in Fig.

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Fig. 4 XRF maps of elements Zn and Cu inside hybrid welded 7075-T6 joints after five months natural aging for the case of high heat input Q3=86496 J/m, inwhich 20-keV synchrotron radiation micro X-ray fluorescence was used at the 15U beam line. A Element Zn; B element Cu. Scale bars represent normal XRFintensity for the respective element.

A B

A B

Fig. 5 XRF maps of elements Ti and Mn inside hybrid welded 7075-T6 joints after five months natural aging for the case of high heat input Q3=86496J/m, in which 20-keV synchrotron radiation micro X-ray fluorescence was used at the 15U beam line. A Element Ti; B element Mn. Scale bars representnormal XRF intensity for the respective element.


3 that the hybrid joints achieve the lowestyield strength and UTS that is only approx-imately 53% of the strength of the base7075-T6 material, indicating serious soften-ing. In contrast, autogenous laser welds ob-tain a slightly higher UTS because of thelower heat input.

Elemental Mapping

It is known that the 7075-T6 alloyachieves its high strength from a series ofprecipitates such as typical (MgZn2) andT(Al2Mg3Zn) (Refs. 29, 30). The contentand gradient of the alloying elementschange distinctly in hybrid welds and play aprimary role in the microstructure and re-sulting properties. Few studies have exam-ined the softening mechanism from the el-emental perspective. Figure 4 shows thespatial distribution of the major Zn and Custrengthening elements. The elementalmappings are graded using a continuouscolor bar. Each bar is assigned to an ele-mental level, for which red is the highest,yellow is medium, and blue is the lowest.

It can be observed that Zn and Cu have

similar distributions, and their concentra-tion is lowest near the lower reinforce-ment and highest near the HAZ. It is gen-erally thought that owing to the excessiveenergy accumulation during welding, lessZn is found in the FZ because of its lowboiling point. While inverse segregationhappens for the strengthening elementCu, that is to say, the region that solidifiesfirst has the higher Cu (Ref. 31). The ele-ment-induced softening appears to beconsistent with the hardness variationseen in Fig. 2. The relationship and mech-anism between the change of alloying ele-ments such as Cu and Zn with the me-chanical performance of welded jointswere not explored in present researchmainly due to limited information. How-ever, one important point should be notedthat the biggest effect of element variationwould come from the filler dilution. Inother words, the main contribution to thelower content of Zn and Cu is probablythe addition of ER5356 FM into themolten pool.

Ti and Mn are usually considered im-purities in commercial 7075-T6 aluminum

alloys. Nevertheless, they sometimes playuseful roles in either refining grains or form-ing dispersed second phases of MnAl6 and(Cr,Mn)Al12. A small amount of elementalTi in ER5356 helps to form TiAl3 dispersionparticles smaller than 0.5 m and refine theweld microstructure. As a dispersed precip-itate, elemental Mn is effective in prevent-ing the grain growth (Ref. 32). Figure 5shows the distribution maps of Ti and Mn.

The map shows that Ti and Mn exhibit acomparable distribution. The top region ofthe weld has a slightly higher content, and alower content in the lower reinforcement.

The level of Zn is the highest of all otherelements, which is consistent with the alloy-ing contents listed in Table 1. Elemental Niand Co are also identified by the 20-keVsynchrotron micro X-ray spectrums. No fur-ther qualitative efforts are made to analyzethese results.

Microstructure

The strength and hardness of weldedjoints always depend on the elemental re-distribution and microstructural changes

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A

Fig. 6 Optical microscope images of the following: A PMZ (1000); B central FZ microstructures with slightly etched hybrid welded metallographicspecimen after 17 months natural aging. Scanning electron microscope micrographs of the following: C Transverse cross section or oxy plane PMZ mi-crostructures; D PMZ microstructures of the longitudinal cross section as illustrated in Fig. 1 or the plane parallel to oyz with slightly etched hybrid weldedmetallographic specimen after 10 and 11 months natural aging, respectively.

Inside PMZ (OM)B

DC


that occur when the weld experiences anintense thermal history. Figure 6 illus-trates the OM and SEM images of twomutual vertical sections denoted in Fig. 1.These images show the typical solidifica-tion characteristics of columnar dendriticstructures near the HAZ and equiaxeddendritic structures in the central FZ to-gether with the morphology of particles in-side the partially melted zone (PMZ,about 100 m width) near the WI.

Figure 6A shows the coarse grains con-taining a large number of dispersed parti-cles near the PMZ of the transverse crosssection. These particles appear as equilat-eral triangles and polygons with the size var-

ied from 0.1 to 2.5 m when slightly etchedusing Kellers reagent. Figure 6C showstheir morphology more clearly under SEM.These types of precipitates gradually in-crease from either side of the PMZ and fewwere found in both the central FZ and BM.Further studies show that these precipitatescontain a large amount of Zn and a little Fe,which is basically consistent with the ele-mental mappings seen in Fig. 4. Thoroughinformation concerning the chemical com-positions of precipitated phases and theirinfluence on the mechanical properties willbe investigated in another work.

The FZ exhibits finely equiaxed den-dritic structures, as seen in Fig. 6B, which

are attributed to the higher cooling rate.The lower hardness is most likely caused bythe eutectic constituent precipitates or thedisappearance of the Guinier-Preston (GP)zone (Ref. 33). It should be specially notedthat the FM also makes a pronounced con-tribution to the strength of welds due to itsdilution effect of composition.

Figure 6D further shows the morphol-ogy of particles in the longitudinal cross sec-tion running through the PMZ. It is basi-cally concluded that these particles exhibitsome kind of typical spatial shape. It is ofvalue to explore the detailed information ofcomposition, microstructure, and resultingperformance.

Modeling Porosity

The previous sections briefly discussthe softening mechanism from the per-spective of the hardness, strength, and el-emental composition, which is mainly re-sponsible for the lower strength andhardness. The gas pores formed during hy-brid welding present another problem thatcontributes to the loss of mechanical be-havior of hybrid 7075-T6 welds.

Figure 7 shows a stereogram of the gas

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Fig. 7 X-ray microtomography mapping of 3D porosity with detailed size, amount, distribution, morphology, and location inside the hybrid 7075-T6 weld with 40-keV synchrotron radiation XRM at the 13W beam line and SEM fracture morphology micrograph of typical gas porosity under low heat input Q1 = 65520 J/m.

Fig. 8 Size and volume distributions of 3D pore size in hybrid 7075-T6 welds derived by the Schwartz-Saltykov analysis according to the diameters variation [Dmax100.1(i2)~Dmax100.1(i1)]. A The re-lationship between percentage by number; B the correlation of maximum volume V(t) and maxi-mum diameter t.

Fig. 9 The curve that is fit to the relationship betweenelastic modulus Es and heat input Q of hybrid welded7075-T6 aluminum joints measured from the cross-tensile tests.

A

A

B

B

Table 2 The Number of Gas Pores in Each Volume Interval and Resulting Maximum Volume

Interval Porosity diameter ( m) 3D porosity Maximum volume ( m3)

i=1 99.90~74.04 1 1.25107

i=2 74.04~53.49 2 8.50106

i=3 53.49~37.90 7 7.05106

i=4 37.90~24.93 8 2.74106

i=5 24.93~14.64 7 6.49105

i=6 14.65~6.48 289 2.59105

i=7 6.48~0.01 184 1.77105


porosity and fractured surface inside a weld.Many voids and microcracks were observedsurrounding the pores, which might be re-lated to the isolated particles on the den-drite boundary. However, the voids and mi-crocracks are treated as pores forconvenience and simplification. Voids andmicrocracks can act with gas pores underloading, affecting the performance and lifeof the material as a whole (Refs. 34, 35).

According to the SS approach, the num-ber of 3D pores is represented as n volumeintervals of NV(i), where i=1,2n, and the diameters are[Dmax100.1(i2)~Dmax100.1(i1)] for the ithinterval, where Dmax is the diameter of thelargest 3D pore (Ref. 36). Table 2 lists thevolume distribution of the gas pores shownin Fig. 7.

The 3D size distribution of the totalporosity can be readily derived, and thebest-fit curve for the size was created as afunction of the step plot of percentage, asshown in Fig. 8A.

Special attention should be paid to thenegative number of pores in the 3D size dis-tribution, which is primarily caused by in-herent cumulative error (Ref. 37). Based onthe fitted function of v(x), the correlation ofthe maximum volume V(t) and maximumdiameter t can be established as shown inFig. 8B. The total pore volume V(t) can becalculated using the product NvV(t).Thus, the porosity p(t) of the hybrid weldsunder a set of suitable welding parametersis simply expressed as follows:

p(t)=V(t)/V=NvV(t)/V (1)

where V is the volume of the specifiedwelds. It is assumed that V is solely relatedto the welding parameters instead of thesize of the pores. It is suggested that alonger weld pool results in a lower poros-ity inside the welded joints.

Two assumptions are made to correlatethe total porosity p(t) with the effectiveheat input (Q =(1P1+2P2)/v, where 1and 2 are the energy efficiencies of thelaser and arc heat source, respectively; P1and P2 are the total output laser power andarc heat source power; and v is the weld-ing speed): V=a1Q, and NV=a2, where a1and a2 are undetermined constants, and is the total retention time of weld pool. Bysubstituting these equations into Equation1, the following alternative relationshipcan be obtained:

p(t)=cQ2V(t) (2)

where c is an undetermined constant re-lated to the welding process.

It can be clearly seen that Q not onlycan trigger the elemental redistribution inwelds, but it can also drive the productionof gas pores and voids. Therefore, thewelding heat input Q is the principal fac-

tor that determines the mechanical prop-erties of welded joints. The following sec-tion is focused on the detailed quantifica-tion of this relationship between thetensile strength with the heat input Q andthe porosity p(t).

Strength Model

Different models have been estab-lished to nondestructively determine themechanical properties of commercial Al-Zn-Mg-Cu alloys (Refs. 3841). However,little research has been published on therelationship of the strength to variations inthe strengthening alloying elements,porosity, and welding heat inputs.

Elastic Modulus-Tensile Strength Relationship

In the present formulation, the truestress-strain responses of the 7075-T6alloy and its hybrid welded joints are as-sumed to obey similar exponential func-tions; therefore, the offset yield strength0.2 at =0.002 is

0.2 = B(0.002)n (3)

0.2 = 0.002E (4)

where B and n are the strength coefficientand the hardening parameter, respec-tively.

At the uniaxial UTS, the differential ofthe load P is zero, which gives an analyti-cal relationship between the tensilestrength and the spot area A in the formof dA/A=d/ when necking occurs. Ad-ditionally, the volume constraint gives thecorrelation dL/L=dA/A, where L is thespecimen length. Then the strain at thenecking point b can be obtained as follows:

b = n (5)Based on the strain energy theory at agiven strain, is

= Bnn/1+n (6)The relationship of the tensile strength

with the elastic modulus E is given by

/E = nn/(1+n)0.002n1 (7)

Modeling Weld Strength

The mechanical properties of engi-neering materials are generally deter-mined by their chemical composition andmicrostructure, but the properties are alsosensitive to various macro- and microdis-continuities. Gas porosity is considered tobe this type of damage. Therefore, we canestablish a strength model by dividing theoverall property into two main parts: onepart arises from the metallurgical aspect inthe absence of discontinuity, and the otherarises only from microdamage, includinggas porosity, voids, microcracks, and in-clusions, in the absence of metallurgicalmodification of the structures and ele-ments. Therefore, the elastic modulus canbe rewritten as follows:

Ed/Ep=[(1p(t))]2 (8)

where Ed is the corresponding measure ofweld flaws induced by gas pores, and Ep isthe value of the BM. Thus, the practicalelastic modulus Es is expanded as follows:

Es=Ep+(Ed/Ep1)Ep+(Em/Ep1)Ep (9)

where Em is the contribution of the mi-crostructure in terms of chemical compo-sition, and Es can be measured throughtensile tests, as shown in Fig. 9.

The correlation coefficient (R2, with avalue of 0.9230) suggests a good correlationbetween the measured Es with the givenheat inputs. By incorporating Equations79 and establishing a best-fit equation, wecan obtain the following:EE

EE

p t

EE

cQ V t

m

p

s

p

s

p

= + ( ) = + ( )

1 1

1 1

2

2'

210( )

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Table 3 Computed Mechanical Properties of the Hybrid 7075-T6 Welds Based on StrengthModel of Hybrid Welds

Q (J/m) Em (GPa) m (MPa) m / p (%) d (MPa) d / p (%)

Q1 70.18 266.68 50.79 517.98 98.64Q2 72.43 275.23 52.42 519.14 98.86Q3 77.49 294.46 56.08 521.01 99.22

Mean 73.37 278.79 53.10 519.38 98.91

Table 4 Predicted Properties of the Hybrid 7075-T6 Welds

Q (J/m) s(p) (MPa) Error (%) Weakening (%)

Q1 259.56 0.46 49.43Q2 269.27 0.12 51.28Q3 290.37 0.39 55.30


If Equation 7 is applied to the mi-crostructure-induced strength, the gasporosity-induced strength of the welds,and base 7075-T6 materials, the practicaltensile strength can eventually be ex-pressed as follows:

s=p(pm)(pd) (11)

Verification of Strength Model

To verify the proposed strength model,three sets of hybrid laser arc welds wereconducted. The corresponding porosityp(t) from Equation 1, the maximum gaspore diameter t, and the measured ulti-mate strength s(m) were conveniently ac-quired. For the first heat input Q1,p1(t)=0.0117, t1=102 m, ands1(m)=260.77 MPa. For the second heatinput Q2, p2(t)=0.0038, t2=84 m, ands2(m)=269.60 MPa. For the third heatinput Q3, p3(t)=0.0024, t3=107 m, ands3(m)=289.23 MPa. The UTS of the BMis p =525.10 MPa, and this UTS corre-sponds to Ep=66.21 GPa. By substitutingthe data from the above three cases intoEquation 2, the corresponding constantscan be obtained: c1=2589.00, c2=1017.68,and c3=925.55. As a result, the mean con-stant is c=1510.74.

Based on this empirical constant c, wecan expediently predict the porosities forthe three heat inputs: p1(t)=0.0068,p2(t)=0.0056, and p3(t)=0.0039. Finally,we can compute the mechanical strengthsof the welded joints by combining Equa-tions 7 and 8 with 10, in which the harden-ing parameter n is 0.18 for the welded7075-T6 joints. Table 3 lists the calculateddata for Em, m, and d.

The alloying-element-induced mi-crostructure plays the dominant role indefining the static properties of hybridwelds, compared with porosity-induceddiscontinuities. In other words, the essen-tial change of the strengthening phases in-side the welds produces the softening be-havior. However, the mechanicalperformances of the welds are sensitive tointernal discontinuities, such as gas pores,voids, microcracks, inclusions, segrega-tions, and impurities when undergoing re-versed loading and unloading (Refs.4244).

By using the experimental data fromTable 3, the practical ultimate tensilestrength s(p) and softening behavior caneasily be predicted in terms of the com-puted UTS, as given in Table 4.

It is apparent that the predicted mate-rial performance agrees with the experi-mental results, which validates the pro-posed strength model in terms of theelements and porosities, as analyzed fromSR-XRF and SR-XRM, respectively.

There is a nearly linear relationship be-tween the elastic modulus and heat input

for the hybrid welds, compared to that ofthe base 7075-T6 material. It is thoughtthat the inhomogeneity and complexresidual stress leads to greater stiffness.This type of asymmetry might be visiblefrom the weld geometry, mechanical prop-erties, chemical composition, and mi-crostructure. Furthermore, the disconti-nuities resulted from the microcracks aresusceptible to significant stress concentra-tions, which easily generate localized yieldzones, even when the load level is small.

It would be of interest to clarify the fol-lowing issues in the future. First, themechanism of the larger elastic modulusof welded joints is not clear and requiresfurther metallurgical proof and character-ization. Second, the composition andstructure parameters of the newly ob-served phases or particles should be iden-tified, characterized, and related to thecomplex mechanical behaviors inside thePMZ near the HAZ. Finally, the accumu-lated elemental Zn and Cu might corre-late with the evolution of the strengthen-ing phases and microsegregations,formation of hot cracks, and variation ofthe hardness. Relevant results will be pre-sented in subsequent research.

Conclusions

This paper systematically reports a newquantitative perspective on the softeningbehavior of hybrid 7075-T6 butt joints. Sig-nificant physical softening is observed as asteep loss in hardness and strength as a re-sult of the welding process. Elements andgas pores in the welds are measured usingSR-XRF and SR-XRM, respectively. Anew strength model is then formulatedand validated using the experimental re-sults. From the present study, the follow-ing conclusions are obtained:

1. The heat input has great influence onthe strength loss due to welding. A greaterheat input produces a wider softening re-gion, a wider FZ, and, thus, a lower hard-ness. The FZ has the lowest hardness atapproximately 95 HV, which produces anotable decrease in mechanical proper-ties. This trend is consistent with thestrength variation in terms of the UTS.

2. The strengthening elements Zn andCu change dramatically inside the hybrid7075-T6 welds. The HAZ contains the max-imum content, and the minimum is found inthe center of the FZ. Because of its highermelting temperature, Zn evaporates rap-idly, whereas less Cu collects in the FZprobably because of inverse segregation. Itis generally thought that the dilution of FMplays a pronounced effect on the redistrib-ution of strengthening elements.

3. There are a great number of un-known grown precipitates found inside thecolumnar dendritic grain on the PMZ withabout 100 m width. Either side of the

PMZ has few such triangular particleswhose size varies from 0.1 to 2.5 m.These precipitated phases contain a largeamount of Zn and a little Fe.

4. The largest size of the metallurgicalpores varies from 0.1 to 0.2 mm. The for-mulated porosity, p(t)=cQ2V(t), wasfound to be proportional to the negativesquare of the heat input and maximum di-ameter of the gas pores.

5. The microstructure-inducedstrength without discontinuity and poros-ity-induced strength without structuralchanges are treated independently to es-tablish a new strength model. It is foundthat this model is in agreement with theexperimental data from the heat inputsand porosity.

6. The predicted results show that theporosity-induced strength loss has little in-fluence on the overall strength of the 7075-T6 joints, whereas the variation in ele-ment-induced microstructures dominatesthe strength.

7. The elastic modulus of the hybridwelds is greater than that of the base 7075-T6 aluminum material. Additionally, theelastic modulus appears to linearly in-crease with the increase in the heat input.

Acknowledgments

Financial support from the NationalNature Science Foundation of China(Grant No. 51005068), the National BasicResearch Program of China (973 Pro-gram, Grant No. 2011CB 711105-1) andthe Open Research Fund Program of theState Key Lab of Advanced Design andManufacturing for Vehicle Body (GrantNo.: 31115030) is gratefully acknowl-edged.

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