Materials and Design 55 (2014) 335–342

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Materials and Design

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Technical Report

The effect of gas tungsten arc welding and pulsed-gas tungsten arcwelding processes’ parameters on the heat affected zone-softeningbehavior of strain-hardened Al–6.7Mg alloy

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.09.061

⇑ Corresponding author. Address: MME Department, University of Waterloo, 200University Avenue West, Waterloo, ON N2L 3G1, Canada. Tel.: +1 519 8884567x38743; fax: +1 519885 5862.

E-mail address: ahadadza@uwaterloo.ca (A. Hadadzadeh).

Amir Hadadzadeh a,⇑, Majid Mahmoudi Ghaznavi b, Amir Hossein Kokabi b

a Mechanical and Mechatronics Engineering Department, University of Waterloo, Waterloo, ON, Canadab Materials Science and Engineering Department, Sharif University of Technology, Tehran, Iran

a r t i c l e i n f o

Article history:Received 25 June 2013Accepted 25 September 2013Available online 8 October 2013

a b s t r a c t

The heat affected zone (HAZ) softening behavior of strain-hardened Al–6.7Mg alloy welded by gastungsten arc welding (GTAW) process was investigated. Increasing the heat input during welding ledto formation of a wider HAZ. Moreover, the size of the precipitates was increased at higher heat inputs.Consequently, by increasing the heat input, lower strength was obtained for the welding joints. At thesecond stage of the study, pulsed-GTAW (PGTAW) process was employed to improve the strength ofthe joints. It was observed that the overall strength of the welding joints was improved and the fractureduring tensile test was moved from the HAZ to the fusion zone. Moreover, the effect of duration ratio andpulse frequency was studied. For the current study, the duration ratio did not have a significant effect onthe strength and microstructure of the weld, but increasing the frequency led to higher strength of theweld and finer microstructure.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminum alloys are largely used in various parts of theindustry due to their inherently low density, high strength toweight ratio, high electrical and thermal conductivity, corrosionresistance, toughness retain in cryogenic temperatures andeasiness of forming. There are two major categories for thewrought aluminum alloys; heat treatable and non-heat treatablealloys. The strengthening mechanism for the heat treatable alloysis precipitation or age hardening [1] and for the non-heat treatablealloys is solid solution and strain hardening [2]. Al–Mg alloys (5xxxseries) have the highest strength among non-heat treatable alloys[3]. Also these alloys have well corrosion resistance, formabilityand weldability [4,5]. The reason of high strength of the Al–Mgalloys, in addition to the strain-hardening, is the existence ofprecipitates in the microstructure.

A challenge for fusion welding of strain-hardened alloys is theheat affected zone (HAZ) softening of the joint [6]. The effect ofstrain-hardening is completely lost in the fusion zone because ofmelting and solidification and is partially lost in the HAZ owingto recrystallization and grain growth [7–9]. The solution of theissue is reduction of heat input [10]. On the other hand, the full

penetration of the weld should be guaranteed at lower heat inputs.One way to obtain both low heat input and full penetrationsimultaneously is using pulsed current [8,11–16].

The objective of this study was to investigate the HAZ Softeningbehavior of strain-hardened Al–6.7Mg alloy during welding byGTAW process. This alloy is a new alloy from 5xxx series whichhas a high strength due to strain-hardening during manufacturing.The pulsed current was also employed to join the alloy usingpulsed-GTAW process. The effect of pulse parameters were studiedon the strength of the welding joints and improvement of HAZsoftening phenomenon.

2. Experimental procedure

Table 1 shows the chemical composition of the Al–6.7Mg alloyused as the base metal for the current study. The plates wereproduced through a thermo-mechanical process and the alloywas in the H38 condition. The yield strength (YS) and ultimatetensile strength (UTS) of the as-rolled plates were 260 and410 MPa in the rolling direction, respectively. Fig. 1 shows the opti-cal microstructure of the base metal in the rolling direction. As ob-served, the precipitates are extensively elongated in the rollingdirection. The high strength of this alloy is due to presence of pre-cipitates and also large amount of strain-hardening applied to thematerial in the production procedure. One challenge for the arcwelding of this strain-hardened alloy is heat affected zone (HAZ)softening due to recrystallization and grain growth in the HAZ.

Table 1Chemical composition of Al–6.7Mg alloy (in wt%).

Al Mg Mn Fe Si Other

Balancing 6.7 0.58 0.16 0.11 0.26

Table 2Welding parameters used for GTAW.

Specimen I (A) S (mm/s) V (V) Q (kJ/mm)

G1 120 4.39 25 0.68G2 120 5 25 0.6G3 135 4.39 25 0.77G4 135 5 25 0.68G5 150 4.39 26 0.89G6 150 5 26 0.78

Fig. 2. Schematic representation of pulsed current used for PGTAW process.

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The as-rolled plates of Al–6.7Mg were cut with the dimensionsof 350 mm � 150 mm � 2.7 mm. In order to study the effect ofheat input (Q, in kJ/mm) on the HAZ softening of the alloy, at thefirst stage of the study, the bead on plate welds were carried outon the plates using GTAW process with the conditions shown inTable 2. The welding procedures were conducted using anautomatic machine with AA5356 filler metals. All welds werecreated transverse to the rolling direction and full penetrationwas obtained for all welds. The current–voltage relationship ofthe welding machine follows a linear equation;V = 0.0403I + 19.88 (where V is voltage in V and I is current in A).Various heat inputs were obtained by changing welding currentand speed. Heat input was calculated using Eq. (1) [17].

Q ¼ l V : IS� 1000

ð1Þ

where Q is the welding heat input (in kJ/mm), l welding efficiencycoefficient, V welding voltage (in V), I welding current (in A) and Swelding speed (in mm/s). Since all welds were produced using anidentical process, an assumption of l ¼ 1 was made for the sakeof simplicity.

At the second stage of the study, pulsed-GTAW (PGTAW) pro-cess was employed to apply bead on plate welds on the base metalto assess the effect of pulsed current on the improvement of HAZsoftening. The filler metal was identical to the one used in the firststage. Fig. 2 shows a schematic representation of pulsed currentemployed for PGTAW process [12]. There are four basic parameterswhich determine the characterization of pulsed current; peak cur-rent (IP), base current (IB), peak time (tP) and base time (tB) [18,19].The combination of these four parameters leads to other weldingvariables [11,12,18,20]. In the current study, as observed in Table 3,16 conditions for PGTAW were carried out to study the effect ofaverage current (Iavg), duration ratio (T) and pulse frequency(f in Hz) on the HAZ softening behavior and weld strength. Theseparameters were chosen based on full penetration achievement.Average current, duration ratio and pulse frequency are deter-mined using Eqs. (2)–(4), respectively [12,15].

Iavg ¼IBtB þ IPtP

tB þ tPð2Þ

T ¼ tB=tP ð3Þ

Fig. 1. Optical microstructure of the base metal.

f ¼ 1=ðtB þ tPÞ ð4Þ

After welding, the specimens were cut perpendicular to thewelding direction and prepared for microstructural study. Thespecimens were prepared by grinding and polishing followed byetching using the following solution:

� 50 ml Poulton’s solution, 25 ml HNO3, 40 ml of solution of 3 gChromic acid per 10 ml of distilled water.� Poulton’s solution: 50 ml HCl, 15 ml HNO3, 3 ml HF, 5 ml FeCl3

solution.

The microstructure of the welds was then studied using opticalmicroscope. Moreover, tensile test specimens were preparedaccording to ASME QW-462.1 standard [21] and tensile properties(YS and UTS) of welds were obtained by the tensile test. Fig. 3shows the location of microstructure study and tensile test speci-men preparation.

3. Results and discussion

3.1. GTA welded specimens

Fig. 4 illustrates the results of tensile test for various heat in-puts. Joining the Al–6.7Mg alloy by GTAW process, even at lowheat inputs, significantly reduces the strength of the welding jointwith respect to the base metal. Moreover, increasing the heat inputleads to a lower strength welding joint [8]. Investigation of speci-mens after tensile test showed that in all cases fracture occurs inthe HAZ. In other words, the softest part of the welding joint is lo-cated in the HAZ.

In order to characterize the HAZ softening behavior of the joints,the microstructure of the weldment was studied. Fig. 5 shows theoptical microstructure of welding joint for specimens G2 and G5.Six specific regions were recognized in the microstructure; markedA to F. Region A is the dendritic fusion zone and B is the Epitaxial

Fig. 3. Schematic representation of the location of (a) microstructure study (‘+’ signshows the location of the weld metal microstructural study) and (b) tensile testspecimen.

Fig. 4. Effect of GTAW process heat input on the (a) YS and (b) UTS of weld metal.

Fig. 5. Optical microstructure of the welding joint for specimen (a) G2 (Q = 0.6 kJ/mm) and (b) G5 (Q = 0.89 kJ/mm), r is the fusion line and s is the boundarybetween HAZ and BM.

A. Hadadzadeh et al. / Materials and Design 55 (2014) 335–342 337

growth zone. Regions C, D and E represent different areas in theHAZ; C is the coarse grain zone, D is the fine grain zone and E pre-sents the partially recrystallized zone. Zone F is the base metal (notaffected by the heat input) [7].

In the areas near the fusion zone (region C) the thermal cyclecauses recrystallization and grain growth. By getting further fromthe fusion zone, the peak temperature of the thermal cycle andthe time material stays above the effective recrystallization tem-perature decreases. Consequently, in a specific distance to the fu-sion zone recrystallization with no grain growth occurs (regionD) and in the further distances partial recrystallization happens(region E).

The HAZ width (size) is determined at the mid-thickness posi-tion of the welding joint as the distance between lines r and s,referring to Fig. 5. Line r is the fusion line and line s is the bound-ary between HAZ and BM. As expected, increasing the heat input(from 0.6 kJ/mm for specimen G2 to 0.89 kJ/mm for specimenG5) leads to formation of a wider HAZ [9]. Existence of a widerHAZ provides more softened material and consequently thestrength of the welding joint decreases [10].

Despite the formation of a wider HAZ when higher heat input isused, the size of HAZ is not significantly affected by the level of theheat input. Fig. 6 shows the size and morphology of the precipi-tates in the coarse grain zone (zone C) affected by the heat input.By increasing the heat input, the precipitates in the zone C growand smaller precipitates join the bigger ones and the morphologyof them changes from fine granular to coarse elongated ones. Sub-sequently, the areas between the elongated precipitates soften, andthe strength of the material decreases. Therefore, besides the widthof the HAZ, the size and morphology of the grown precipitates is

Fig. 6. Optical micrograph of the precipitates morphology in the coarse grain zone in specimen (a) G2 (Q = 0.6 kJ/mm), (b) G4 (Q = 0.68 kJ/mm) and (c) G5 (Q = 0.89 kJ/mm).

Fig. 7. Improvement of overall (a) YS and (b) UTS of the welding joints by using thepulsed current.

Fig. 8. Optical microstructure of weld created by PGTAW.

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also a determinant parameter in the HAZ softening behavior of thewelding joint.

3.2. Pulsed-GTA welded specimens

3.2.1. Tensile properties improvementEmploying pulsed current in GTAW process changes the behav-

ior of the arc. Referring to Fig. 2, the current changes between IP

and IB which specifies two duty cycles for the arc; ‘‘work period’’

(IP at tP) and ‘‘rest period’’ (IB at tB). During the work period, thehigh level of IP generates high heat input and a ‘‘weld nugget’’ isformed as a reason of base metal melting and filler metal deposi-tion. Continuing the process, once the arc reaches the rest period,the level of heat input decreases and the piled-up heat in the weldmetal is sunk by the base metal. In fact, IB is just held at a sufficientlow level to keep the arc stable [22]. During the rest period, due toheat loss, part of the weld pool solidifies. In the next stage, the fu-sion zone forms and since the electrode has been moved forward, itoverlaps the previous fusion zone. At last, the weld metal is shapedby overlapped individual weld nuggets [22]. Presence of the restperiods during PGTAW process leads to occurrence of lower peaktemperature in the thermal cycle in the HAZ and also less timeabove the effective recrystallization temperature. Hence, higherstrength of weldment is obtained. Fig. 7 illustrates the impact ofpulsed current on the improvement of weld tensile properties.Joining the strain-hardened alloy by using PGTAW process in-creases the overall strength of the joint, specifically the UTS. Theinvestigation of samples welded by PGTAW process after tensiletest showed that fracture happened in the weld metal which isan evidence of HAZ softening improvement by employing pulsedcurrent.

Fig. 8 shows the typical microstructure of a welding joint cre-ated using PGTAW process. A thinner HAZ is formed in compare

Table 3Welding parameters used for pulsed-GTAW.

Specimen IP (A) IB (A) tP (s) tB (s) T f (Hz) Iavg (A) S (mm/s)

P1 130 40 0.5 0.5 1 1 85 5P2 150 95 5P3 170 105 4.5P4 190 115 5P5 130 0.25 0.25 1 2 85 4.9P6 150 95 5.1P7 170 105 4.9P8 190 115 5.1P9 209 0.5 0.8 1.6 0.77 105 4.8P10 235 115 5P11 235 1 2 0.67 105 5P12 265 115 5.1P13 209 0.25 0.4 1.6 1.54 105 4.9P14 235 115 5P15 235 0.5 2 1.33 105 5P16 265 115 5

A. Hadadzadeh et al. / Materials and Design 55 (2014) 335–342 339

to specimen welded by GTAW (see Fig. 5). Moreover, the precipi-tates in the HAZ are finer with a more uniform distribution. Thiscould be the main reason of obtaining higher strength for the jointsmade by PGTAW process.

3.2.2. Effect of average current (Iavg)Samples P1 to P8 were welded with a constant base current

(40 A) and various peak currents (130 to 190 A) to obtain a rangeof average current (see Table 3). The difference between samplesP1 to P4 and P5 to P8 is the pulse frequency. Fig. 9 shows the effectof Iavg on the strength of welding joints for f = 1 Hz and 2 Hz.Increasing Iavg leads to lower strength of the weld, however this

Fig. 9. Effect of average current on the strength of welding joint (a) Y

strength reduction is not as dramatic as the trend observed forGTAW process.

Since for the specimens welded using PGTAW process the frac-ture took place in the weld metal, the microstructure of the fusionzone determines the strength of the joint. Increasing the averagecurrent and consequently the heat input affects the solidificationprocedure of the molten metal in the fusion zone. Recalling thecritical nucleation radius (r⁄) from the theory of solidification(Eq. (5)) [23], the undercooling in the solidifying liquid metal af-fects the number of nuclei. The lower the critical nucleation radius,the higher the chance of growth for the atomic clusters and as a re-sult the number of solidification site increases. So, a finer micro-structure in the solidified metal is obtained.

r� ¼ 2;rTm

DHmDTð5Þ

where r⁄ is the critical nucleation radius, r is the solid/liquid inter-face energy, Tm is the equilibrium melting temperature, DHm is thelatent heat of fusion and DT is the undercooling. Referring to Eq. (5)a determinant parameter for the size of critical nucleus is the und-ercooling occurs in the liquid metal. For an alloy, the constitutionalundercooling presents in the liquid metal (Eq. (6)) [23].

DT ¼ Ti þmc01� k

k

� �1� exp � R

Dx

� �� �� Gx ð6Þ

where Ti is the temperature of the solidification front, m is the liq-uidus slope, c0 is the initial composition of the alloy, k is the distri-bution coefficient, R is the growth rate, D is the diffusion coefficientof the solute in the liquid metal, x is the distance from the startingpoint of the solidification and G is the temperature gradient in theliquid metal.By increasing the heat input during welding, the

S, f = 1 Hz, (b) UTS, f = 1 Hz, (c) YS, f = 2 Hz and (d) UTS, f = 2 Hz.

Fig. 10. Optical microstructure of the weld metal, sample (a) P3 (Iavg = 105 A) and(b) P4 (Iavg = 115 A).

Fig. 11. Effect of duration ratio (T) on the welding joint strength for (a) Iavg ¼Iavg ¼ 115 A;tP ¼ 0:25 s.

340 A. Hadadzadeh et al. / Materials and Design 55 (2014) 335–342

temperature of the liquid metal increases. This temperature eleva-tion in the liquid metal causes higher temperature gradient (G).By increasing the last term in the right hand side of Eq. (6), the con-stitutional undercooling reduces. Recalling Eq. (5), a lower underco-oling causes a larger critical radius which eventually leads toformation of a coarser microstructure. In Fig. 10 the optical micro-structure of the fusion zone for specimens P3 (Iavg = 105 A) and P4(Iavg = 115 A) is shown. The microstructure was studied at the mid-dle of the weld metal at the mid-thickness position (shown by ‘+’sign in Fig. 3). A coarser solidified microstructure is observed forhigher average current (P4) [22] which has lower strength.

3.2.3. Effect of duration ratio (T)In order to study the effect of duration ratio, the average current

was kept constant (105 A and 115 A) for specimens P9 to P16 (seeTable 3) to keep the heat input constant and then the results werestudied for duration ratios of 1, 1.6 and 2. In order to study theduration ratio effect, the samples were divided to four categories:

1. Iavg ¼ 105 A; tP ¼ 0:5 s

105 A;t

2. Iavg ¼ 115 A; tP ¼ 0:5 s

3. Iavg ¼ 105 A; tP ¼ 0:25 s

4. Iavg ¼ 115 A; tP ¼ 0:25 s

P ¼ 0:5 s, (b) Iavg ¼ 115 A;tP ¼ 0:5 s, (c) Iavg ¼ 105 A;tP ¼ 0:25 s) and (d)

Fig. 12. Effect of pulse frequency on the (a) YS (Iavg = 105 A), (b) UTS (Iavg = 105 A), (c) YS (Iavg = 115 A) and (d) UTS (Iavg = 115 A) of the welding joint.

Fig. 13. Optical microstructure of the weld metal for specimen (a) P9 (f = 0.77 Hz)and (b) P13 (f = 1.54 Hz).

A. Hadadzadeh et al. / Materials and Design 55 (2014) 335–342 341

Fig. 11 illustrates the influence of duration ratio on the weldstrength. Increasing duration ratio from 1 to 2 does not affect theweld strength significantly; however, a slight peak in strength isobserved at T = 1.6 for f = 2 Hz. Increase of the duration ratio repre-sents the increase of base time which provides more time for therest period of the arc. Consequently, more heat transfer occursfrom the fusion zone to the base metal and the undercooling rises.Eventually, a finer microstructure should develop [12]. However,this expectation was not achieved for the current study. Appar-ently, the change of duration ratio was not pronounced enoughto affect the cooling behavior of the weld metal.

3.2.4. Effect of pulse frequency (f)Pulse frequency effect was studied for the specimens welded

with the same average current. Fig. 12 shows the results forIavg = 105 A and Iavg = 115 A. Increasing the pulse frequency fairlyimproves the joint strength [19]. Welding the specimens withhigher frequencies causes more vibration in the weld pool and asa consequence the solidified dendrites in the weld pool are broken.Dendrite fragmentation creates several smaller solid particleswhich act as growth sites and finer microstructure is achieved.Fig. 13 shows the optical microstructure of the weld metal forspecimens P9 (f = 0.77 Hz) and P13 (f = 1.54 Hz) at the middle ofthe weld metal at the mid-thickness position. In the microstructureof specimen P9 the equiaxed dendrites are observed while for thespecimen P13 these dendrites have been fragmented and a finermicrostructure has been developed. Presence of this finer micro-structure led to tensile properties improvement.

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4. Conclusions

A strain-hardened aluminum alloy (Al–6.7Mg alloy) waswelded using both GTAW and PGTAW processes. The HAZ soften-ing behavior of the alloy welded by GTAW was analyzed and theeffect of pulsed current parameters on the welding joint strengthand microstructure was studied. The following conclusions weredrawn from this study:

1. Welding the strain-hardened Al–6.7Mg alloy using theGTAW process causes HAZ softening in the welding jointsdue to recrystallization and grain growth in the HAZ. More-over, growth of the precipitates is a reason of strengthreduction. Increasing the heat input affects both the sizeof the HAZ and precipitates.

2. Employing pulsed current for the GTAW process reducesthe heat input and the overall strength of the jointincreases. Meanwhile the overall HAZ width of the weldbecomes narrower and the HAZ softening of the alloy isimproved.

3. By increasing the average current in the PGTAW, thestrength of the weld reduces because of a coarser micro-structure development in the weld metal.

4. Duration ratios used in the current study did not affectthe strength of the welding joint significantly. This seemsto be a reason of small changes in duration ratio whichcould not affect the solidification behavior of the liquidmetal.

5. Higher strength of welding joint was obtained for higherfrequencies. The microstructure development of the weldsuggests that increasing the frequency causes morevibration in the weld pool which causes dendrite frag-mentation. Dendrite fragmentation leads to formationof a finer microstructure and subsequently higherstrength.

Acknowledgements

The authors would like to thank the Research Board of SharifUniversity of Technology for providing of the research facilities.One of the authors (A. Hadadzadeh) would also like to appreciateMr. M. Nasirian and Mr. M R. Razagha in the welding laboratory,Mr. J. Akhgar in the mechanical properties laboratory and Mr. PP. Nasoodi in the metallography laboratory in the Department ofMaterials Science and Engineering, Sharif University of Technologyfor the technical assistance.

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