Materials and Design 30 (2009) 2033–2042

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Materials and Design

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Effect of welding speed on the quality of friction stir welded buttjoints of a magnesium alloy

X. Cao *, M. JahaziAerospace Manufacturing Technology Center, Institute for Aerospace Research, National Research Council Canada, 5145 Decelles Avenue, Montreal, Quebec, Canada H3T 2B2

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

Article history:Received 11 July 2008Accepted 30 August 2008Available online 8 September 2008

Keywords:Non-ferrous metals and alloys (A)Welding (D)Mechanical (E)

0261-3069/$ - see front matter Crown Copyright � 2doi:10.1016/j.matdes.2008.08.040

* Corresponding author. Tel.: +1 514 283 9047; faxE-mail address: Xinjin.cao@cnrc-nrc.gc.ca (X. Cao)

The effect of welding speed ranging from 5 to 30 mm/s on 2-mm butt joint quality of friction stir weldedAZ31B-H24 magnesium alloy was investigated to determine defects, microstructures, hardness and ten-sile properties. High welding speed over a wide range can be used to weld this material at high tool rev-olution rates indicating the great potential of this technique for magnesium alloys. Equiaxed grains wereobserved in the stir and thermo-mechanically-affected zones. The grain size in the stir zone decreaseswith increasing welding speed due to lower heat input. Higher welding speeds produce slightly higherhardness in the stir zone. The yield strength increases with increasing welding speed. The tensile strengthincreases first with increasing welding speed but remains constant from 15 to 30 mm/s. A Hall–Petch lin-ear relationship between yield strength and inverse square root of grain size in the stir zone illustratesthe strong relationship between these parameters.

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1. Introduction

Weight reduction in ground vehicles and aircraft is one of themost crucial measures to improve fuel economy and environment.As the lightest structural alloys, magnesium alloys, have been andwill be widely used mainly due to their low density and highstrength-to-weight ratio. In addition, magnesium alloys have goodcastability, excellent sound damping capabilities, electromagneticinterfere shielding properties, excellent machinability and recycla-bility as well as an abundant supply [1]. However, to further ex-pand the application of magnesium alloys more effective weldingand joining techniques are required. As an emerging joining tech-nique, friction stir welding (FSW) has great potential for magne-sium alloys since it can significantly reduce weld defects such asoxide inclusions, porosity, cracks, and distortions, commonlyencountered in fusion welded joints. To date, however, the majorresearch and development efforts associated with FSW techniquehave focused on aluminum alloys and commercial applications ofFSW for aluminum alloys are now increasing. For magnesium al-loys, however, very limited work has been conducted [2–8].Clearly, comprehensive investigation of FSW for magnesium alloyswould be useful. This investigation is targeted at automotiveapplications where high productivity is required. To maximize pro-ductivity, a tool rotational rate of 2000 rpm was employed to join2-mm thick AZ31B-H24 magnesium alloy. The effect of weldingspeed ranging from 5 to 30 mm/s on the butt joint quality is

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studied in terms of weld defects, microstructures, hardness andtensile properties.

2. Experimental method

The experimental alloy is AZ31B-H24 magnesium alloy sheetswith dimensions of 1200 � 500 � 2.0 mm. The alloy has nominalcomposition of Al 2.5–3.5 wt.%, Zn 0.7–1.3 wt.%, Mn 0.2–1.0 wt.%and the balance Mg. The 300 � 100 � 2.0 mm specimens werecut from the as-received sheets with the end surfaces machinedalong the specimen length. After the faying surfaces and the sur-rounding of the work-pieces were carefully cleaned, the specimenswere attached to the backing plate of a MTS ISTIR FSW equipment.The adjustable welding tool used has a scrolled shoulder with adiameter of 9.50 mm and a right hand threaded pin with a diame-ter of 3.175 mm. The pin was made of H13 steel. The weldingdirection was perpendicular to the roll direction of the work-piece.The butt joints were welded at a tool rotational rate of 2000 rpmclockwise, the maximum available for the FSW machine used inthis work. The tool advancing side (AS) is defined as the side ofthe tool where the local direction of the tool rotation is the sameas that of the welding direction while the tool retreating side(RS) is the opposite. The tilting angle of the pin tool was 0.5� forall the experiments. The welding speed ranged from 5 to 30 mm/s. The pin length was approximately 1.7 mm. The penetrationdepth was controlled through the actual shoulder plunge depthat approximately 0.1 mm. Here, the shoulder plunge depth is de-fined as the distance that the tool shoulder has penetrated the

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2034 X. Cao, M. Jahazi / Materials and Design 30 (2009) 2033–2042

top surface of the work-piece, measured normal to the weld panelsurface.

For each FSWed butt joint, 50 mm at the beginning and 20 mmat the end were removed to exclude possible unstable welding.Three metallurgical specimens were then cut from each joint.These specimens were mounted using cold-setting resin, groundand polished to produce a mirror-like finish. All the specimenswere etched in acetic picral [10 mL acetic acid (99%), 4.2 g picricacid, 10 mL H2O, 70 mL ethanol (95%)] for about 6 s to reveal grainstructures. The macrostructures and microstructures were exam-ined using an Olympus Inverted System Metallurgical Microscope

Fig. 1. Overviews of the butt joints obtained at welding speeds of (a) 5, (b) 10, (c) 15, (dunless indicated otherwise).

GX71 equipped with a digital camera and AnalySIS Five digital im-age software. The pore area was measured using the image analysissystem and the average pore area measured from three transversesections of each joint was reported. The grain size was obtainedaccording to ASTM standard E112 using the linear intercept meth-od. The Vickers microindentation hardness was measured at themid-thickness or from the top to the bottom of the butt jointsusing a Struers Duramin A-300 hardness tester at a load of 100 gforce, a dwell period of 15 s and an interval of 0.3 mm.

For each weld joint, four tensile specimens were machined fromthe central part of the joint according to ASTM E8M-01 standard

) 20, (e) 25 and (f) 30 mm/s (the left is advancing side and the right retreating side

Fig. 2. Cavity defect in the welding nugget obtained at welding speed of 10 mm/s.

X. Cao, M. Jahazi / Materials and Design 30 (2009) 2033–2042 2035

for sheet type material (gauge length 50 mm, width 12.50 mm, andoverall length 200 mm). The specimens were tested at room tem-perature using a 50 kN Instron machine with self-locking gripsand type 2620–604 Instron extensometer (gauge length 50 mm).The crosshead speed was fixed at 2 mm/min. For each tensile test,the data were converted from load and displacement to stress andstrain, respectively, in order to obtain the stress–strain curve andto calculate yield, tensile strength and elongation at rupture.

3. Results and discussion

Fig. 1 shows the transverse sections of all the butt joints at eachwelding speed tested. Visual inspections failed to reveal observa-ble defects. However, all the joints have some microporosity as ob-served at higher magnifications (Figs. 2–4). The indentationsshown in Figs. 3a, b and 5a are due to the microhardness measure-ments. It is interesting to note that most porosity appeared at thesubsurface, usually 20–100 lm from the bottom surface of thespecimens. Occasionally, the porosity was observed in other areas.For example, Fig. 5a shows porosity near the top surface of thebutt joint obtained at a welding speed of 5 mm/s. The area ofthe porosity was measured and its variation with welding speedis shown in Fig. 6. The maximum overall porosity area was ob-served at a welding speed of 20 mm/s. This pore area summedfrom several small micropores as partly shown in Fig. 3 is approx-imately equivalent to one large size pore with a diameter of275 lm. The overall area percentage of the porosity to the weldingnugget is far less than 1% indicating that the porosity level is stillquite low. At lower or higher welding speeds, the porosity area israther low, equivalent to one pore with a diameter of approxi-mately 80 lm. It was reported that the formation of porosity ismainly due to two mechanisms (i) volume deficiency and (ii) inad-equate material flow and mixing [7]. Excessive metal loss maycause subsurface pores which usually occur at the upper half ofthe stir zone on the advancing side. However, the porosity ob-served in this work mainly appeared near the bottom surfaces ofthe specimens. It is more probably formed mainly due to the inad-equate material flow and mixing. For instance, the formation ofvortex may cause pores. It was reported that inadequate stirringand mixing can be caused by too fast joining speed, or inadequatecombination of welding speed and pin tool rotational rate [9–11].When the traversing speed and the tool rotational rate are noteffective enough to stir the plasticized material in front of theprobe and to completely fill the rear of the trailing edge, porescan be formed. The maximum overall porosity area observed at20 mm/s is probably an indicative that inadequate material flowand mixing may appear. Clearly, more experimental and modeling

investigations into the material flow behavior during friction stirwelding may help to understand the formation of the porosity de-fects and thereby decrease the formation of these pores. The poorstirring and mixing can also be caused by low heat input [9], par-ticularly for dissimilar joints. At low heat input (i.e. low tempera-ture) material mixing is difficult, leading to the formation ofdiscontinuity and ultimately initiate pores.

In addition to porosity, some linear crack-type defects were ob-served as shown in Figs. 2, 3e, 4 and 5a. As is well known, kissingbond is the main crack-like defects observed in friction stir welds,initially originating from the entrapped surface oxides present onthe surface of the work-piece [5,7]. The surface oxides in magne-sium alloys usually consist of a collection of separate magnesiacrystals, resembling to coarse sandpaper. This was well observedin the fracture surface of the friction stir welds [6]. Due to themicroscopic roughness or the asperities of the oxide with someabsorbed air, effective metallic bonding can not be well estab-lished at solid state during FSW, leading to the formation of kiss-ing bond defects. If the stirring action is strong enough, moremechanical bonding will be established. If the entrapped oxidefilms are fragmented, the oxides will become particles and willbe dispersed in the stir zone, leading to the formation of solidinclusions. The metallic bond in oxide–oxide, or oxide–metal sur-faces may not be well obtained but mechanical bonding can beestablished at the contact interface under the shear stress duringFSW. Better bonding can be reached at lower heat input (i.e. lowerwelding speed or higher tool rotational rate), resulting in thetransformation of the continuous oxides into dispersed particles.The linear crack-like defects observed in this study, however, par-allel with or perpendicular to the sheet surface, appear at all weld-ing speeds indicating that they may not always be kissing bonddefects. At high welding speed, low heat input may cause lack ofbonding and the formation of kissing bond defects. At low weldingspeed, however, the linear crack-like defects are most probablydue to the lack of bonding or loose structure caused by inadequatematerial flow and mixing. At the bottom surface of the butt joints,some notches were observed as shown in Figs. 5b and 7. The notchdepth is very small, usually less than 50 lm, but it is expectedthat the notches may cause stress concentrations and have signif-icant influence on the fatigue properties of the joints. These rootnotches and subsurface micropores near the bottom surface ofthe work-pieces have seldom been observed in the butt jointswelded using left-hand threaded pin [5,6]. Their formation mightbe related to the upward movement of the material in the stirzone.

Fig. 8 shows some typical microstructures obtained at a weldingspeed of 20 mm/s. The stir zone (SZ) and the thermo-mechanically-

Fig. 3. Weld defects observed at welding speed of 20 mm/s.

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affected zone (TMAZ) mainly consist of equiaxed grains. The heat-affected zone (HAZ) mainly consists of equiaxed grains near theTMAZ side and some elongated grains begin to be observed to-wards the base metal (BM) side. The base metal has both equiaxedand pancaked grains with various sizes (Fig. 8g). The heterogeneityin the grain structure of the base metal may be due to both defor-mation (rolling) and incomplete dynamic recrystallization (partialannealing).

As shown in Fig. 8e–f, the HAZ displays a mixed grain structure,i.e. both equixed and elongated grains were observed. Compared

with the base metal (Fig. 8g), far more equixed grains appear inthe HAZ indicating that partial recrystallization has taken place.The recrystallization temperature for the alloy is approximately205 �C. Thus the temperature in part of the HAZ may be well abovethis value. This is confirmed by some large grains observed in theHAZ due to growth after recrystalization (Fig. 8e–f). It was reportedthat in Al alloys, the grains in the TMAZ are severely deformed, ro-tated, and elongated due to plastic deformation caused by interac-tion with the tool, but usually the grains do not recrystallize [12].In the present work, however, the TMAZ was mainly composed of

Fig. 4. Weld defects at welding speed of 25 mm/s.

Fig. 5. (a) Cavity and (b) notch defects at welding speed of 5 mm/s.

X. Cao, M. Jahazi / Materials and Design 30 (2009) 2033–2042 2037

equiaxed grains indicating that recrystalization had already takenplace.

The equiaxed grains in the stir zone are formed by dynamicrecrystalization [13]. The effect of welding speed on the grainstructure in the stirring zone is shown in Figs. 9 and 10. The grainsize decreases with increasing welding speed. As is well known,heat input and processing temperature decrease with increasingwelding speed. Thus, less time is available at higher welding speedfor grain growth. Moreover, the strain rate in the stir zone ismainly controlled by welding speed, i.e. higher welding speed will

lead to higher strain rate [14]. The decrease in grain size withincreasing welding speed can also be attributed to greater strain-ing of the metal which in turn activates more strain free nucle-ation sites [15]. The greater the nucleation rate, the morecompetitive the grain growth and hence the finer the final grainsize [15]. As shown in Fig. 10, the average grain size in the stirzone is larger than that in the base metal at welding speeds below20 mm/s, i.e. grain growth occurs in the stir zone. Above weldingspeeds of approximately 20 mm/s, the average grain size in thestir zone is slightly smaller than that in the base metal indicating

0.00

0.05

0.10

0 10 20 30 40

Welding Speed (mm/s)

Po

re a

rea

(mm

2 )

Fig. 6. Effect of welding speed on pore area in stir zone.

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that the grains are refined. The grain size distributions along themiddle-thickness of the weld joints obtained at welding speedsof 5 and 30 mm/s are shown in Fig. 11. At the lowest weldingspeed (5 mm/s), the grains become larger from the base metal,through the HAZ and the TMAZ and then to the stir zone. At thehighest welding speed (30 mm/s), the grains become slightlysmaller from the base metal, through the HAZ and TMAZ and thento the stir zone. Clearly, the grain growth or refinement dependson the microstructure of the base metal and the kinetics of recrys-talization and grain growth. For the 2-mm AZ31B-H24 magnesium

Fig. 7. Notches on the bottom surface ob

alloy used in this study, the grain growth or refinement corre-sponds to a critical advancing rate (welding speed/tool revolutionrate) of 0.6 mm per revolution, i.e. the grains in the stir zone willgrow below this threshold but will be refined above this criticalrate.

As indicated in Fig. 11, the hardness along the middle-thicknessof the joints decreases gradually from the base metal through theHAZ and TMAZ and then to the stir zone where the lowest valuewas reached. The base metal has an average hardness of approxi-mately 76. The hardness in the stir zone varies from 80% to 85%of the base metal’s when the welding speed increases from 5 to30 mm/s. It was reported that the decrease of hardness may be cor-related to the grain growth [16]. With the decrease of grain size athigh welding speed, the microindentation hardness in the stir zoneslightly increases as shown in Fig. 10. In this study, however, it wasfound that the hardness in the stir zone is still reduced even if re-fined grains are obtained (Fig. 11b).

Fig. 12 shows the variations of tensile properties with weldingspeed. The yield strength increases with increasing welding speeddue to the decreasing grain size in the stir zone as demonstratedin Hall–Petch relationship (to be discussed later). The tensilestrength also increases with increasing welding speed but re-mains relatively constant above 15 mm/s. The base metal has ten-sile strength of approximately 286 MPa in roll direction [17].Thusthe joint efficiency varies approximately from 69% to 78%. Theelongation increases with welding speed but tends to decreaseabove 10 mm/s. The elongation values are rather low, rangingfrom approximately 1.6–2.4%. The failure of the AZ31B-H24 alloy

tained at welding speed of 15 mm/s.

Fig. 8. (a) Overview of the joint obtained at 20 mm/s. (b–g) High magnification observations of the microstructures in various regions.

X. Cao, M. Jahazi / Materials and Design 30 (2009) 2033–2042 2039

butt joints occurs along the boundary between the stir zone andthe TMAZ or base metal mainly on the advancing side but occa-sionally also on the retreating side. As shown in Fig. 10, the grainsizes in the stir zone decrease slightly from 5.5 to 3.7 lm with

the increase of welding speeds from 5 to 30 mm/s. These grainsin the stir zone have rather similar average size to that of thebase metal (approximately 4.2 lm). Compared with the basematerial, the decreases in tensile strength and elongation are

Fig. 9. Effect of welding speed on grain structure in the stir zone at (a) 5, (b) 10, (c) 15, (d) 20, (e) 25 and (f) 30 mm/s.

0

2

4

6

8

10

0 10 20 30 40

Welding Speed (mm/s)

Gra

in S

ize

(µm

)

50

55

60

65

70

75

80

HV

100

gf

Grain SizeHardness

Hardness for base metal

Grain size for base metal

Fig. 10. Effect of welding speed on grain size and hardness in stir zone.

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probably due to the formation of weld defects. As discussedabove, three types of defects, i.e. porosity, linear crack-like defectsand surface notches were found in this study. All weld specimenssuffered to varying extents from these defects, leading to the de-creases in the tensile properties, in particular the ductility.Although microporosity is one of the main defects, the tensileproperties do not appear to display clear relations with theoverall pore area which is below 1% of the nugget area for eachjoint.

It is well known that yield strength at room temperature de-pends on grain size, according to the Hall–Petch relation, i.e.r0.2 = r0 + Kd�1/2, where r0.2 is the yield stress, r0 is the yield stressrelating to materials of infinite grain size which is similar to that ofsingle crystals, K is a constant representing the grain boundary asan obstacle to slip across the grain boundaries and d is grain size[18]. As shown in Fig. 13, the Hall–Petch linear relationship is

40

60

80

100

-15 -5 5 15

Distance from center (mm)

-15 -5 5 15

Distance from center (mm)

HV

100

gf

HV

100

gf

0

2

4

6

8

10

Gra

in s

ize

(µm

)G

rain

siz

e (µ

m)

Hardness Grain size

WN HAZ +TMAZ

HAZ +TMAZ

BM BM

40

60

80

100

0

2

4

6

8

10

Hardness Grain size

WN

HAZ +TMAZ

HAZ +TMAZ

BM BM

Fig. 11. Grain size and hardness profiles at welding speed: (a) 5 mm/s and (b)30 mm/s.

0

50

100

150

200

250

0 10 20 30 40

Welding speed (mm/s)

Str

eng

th (M

Pa)

0

1

2

3

4

5

Elo

ng

atio

n (%

)

YS UTS El.

Fig. 12. Effect of welding speed on tensile properties.

σ 0.2 = 303d-1/2 + 80

σ 0.2 = 160d-1/2 + 10

σ 0.2 = 286d-1/2 + 27

50

100

150

200

250

300

0.40 0.45 0.50 0.55 0.60

d-1/2 (µm-1/2)

σ0.

2 (M

Pa)

FSP

FSW

Fig. 13. Relation between yield strength (r0.2) and grain size (d).

X. Cao, M. Jahazi / Materials and Design 30 (2009) 2033–2042 2041

verified for the 2-mm FSWed butt joints. In Fig. 13, the results ob-tained by Wang et al. [18] for hot extruded and friction stir pro-cessed (FSP) AZ31B magnesium alloy are also included. Theyconcluded that there exists a weak grain size dependence of yieldstress in the FSPed AZ31B alloy as compared with the hot extrudedspecimens. For the 2-mm FSWed AZ31B-H24 alloy of this study,the Hall–Petch line lies between the hot extruded and the FSPedAZ31B alloy. The slope of the FSWed line has a value of286 MPa lm�1/2, very close to the K value for the hot extrudedspecimen (303 MPa lm�1/2) and much higher than the FSPed spec-imen (160 MPa lm�1/2). Therefore, the FSWed AZ31B-H24 alloyseems to display strong grain size dependence of yield stress com-pared with the FSPed specimens obtained by Wang et al. [18]. This

is probably due to the original material differences in chemicalcompositions, grain structures, etc.

As shown in Fig. 12, the tensile strength remains relatively con-stant with welding speed after 15 mm/s but the yield strength con-tinues to increase. Therefore, the 2-mm magnesium alloy sheetscan be welded at a tool rotational rate of 2000 rpm and weldingspeed ranging from 15 to 30 mm/s or even higher. The highestjoint efficiency (approximately 78%) was obtained at weldingspeed of 30 mm/s in this work. As expected, the tensile strengthmay decrease after a certain welding speed because of theformation of significant kissing bond at higher welding speed (i.e.lower heat input). However, this upper welding speed is not beendetermined as the FSW machine used in this study has a recom-mended limit of 25 mm/s. The maximum speed used in this workis 30 mm/s, already beyond the upper limit of the equipment.Higher welding speeds are thus possible for the 2-mm thin magne-sium alloy sheets indicating the potential of high speed welding formagnesium alloys. This may be especially important for theautomotive industry.

4. Conclusions

The influence of welding speed ranging from 5 to 30 mm/s on2-mm butt joint quality of friction stir welded AZ31B-H24 magne-sium alloy was investigated by examining welding defects, micro-structures, hardness and tensile properties. Some conclusions canbe drawn as follows:

� The grains in the stir zone and the thermo-mechanically affectedzone underwent dynamic recrystallization and the grain shapebecame equiaxed after friction stir welding. In the heat-affectedzone, some partial recrystalization occurred.

� Compared with the base metal, grain growth appears at anadvancing rate (welding speed/tool revolution rate) less than0.6 mm per revolution. Above this threshold, the grains in thestir zone and the thermo-mechanically affected zone are slightlyrefined. The grain size in the stir zone decreases with increasingwelding speed due to lower heat input.

� The hardness decreases gradually from the base metal throughthe heat-affected zone, to the thermo-mechanically-affectedzone and then to the stir zone where the lowest hardness isobtained. Higher welding speed produces slightly higher hard-ness in the stir zone.

� The yield strength increases with increasing welding speed. Thetensile strength increases with increasing welding speed up to15 mm/s but remains constant from 15 to 30 mm/s.

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� The Hall–Petch relationship between yield strength and inversesquare root of grain size in the stir zone holds true indicating thestrong dependence of yield stress on the grain size.

Acknowledgements

This investigation is part of a three-country Canada–China–USAcollaboration under the Magnesium Front End Research and Devel-opment (MFERD) project. Thanks are due to technical officers, M.Guerin for the preparation of FSWed samples and M. Banu forthe tensile testing, K. Ng, co-op student from McGill University,for the metallurgical analyses and T. Shariff, Master student atMcGill University, for the measurement of the grain size.

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