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Metallurgical characterization of pulsed current gas tungsten arc, friction stir and laser beam welded AZ31B magnesium alloy joints
Materials Chemistry and Physics 125 (2011) 686–697
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Metallurgical characterization of pulsed current gas tungsten arc, friction stir and laser beam welded AZ31B magnesium alloy joints G. Padmanaban ∗ , V. Balasubramanian Center for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
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Article history: Received 22 March 2010 Received in revised form 11 August 2010 Accepted 28 September 2010 Keywords: Alloys Welding Electron microscopy Mechanical properties
a b s t r a c t This paper reports the influences of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on mechanical and metallurgical properties of AZ31B magnesium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy and X-Ray diffraction technique were used to evaluate the metallurgical characteristics of welded joints. LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints due to the formation of finer grains in weld region, higher fusion zone hardness, the absence of heat affected zone, presence of uniformly distributed finer precipitates in weld region. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The combination of low density, high specific strength and stiffness, and good castability qualifies magnesium alloys as ideal materials for lightweight structural applications. Most of the commercial magnesium alloys are based on Mg–Al system; and casting is currently the most commonly used production process for magnesium components. While the advancements on improving the properties of the alloys and on components forming technologies have been made in recent years, joining of magnesium alloy components is crucial in many applications. Hence, it is imperative for proper welding procedures to be established. For this reason, there has been an increasing effort in research on welding of magnesium alloys [1–3]. Wrought magnesium alloys are usually welded more easily than certain cast alloys. Mechanical joining process such as riveting and bolting are dominant in connection with wrought magnesium alloys. However, the limited strength of the joints restricts the applications of magnesium parts. Magnesium alloys can be joined using the gas metal arc welding, gas tungsten arc welding, laser beam welding, electron beam welding and solid state welding processes [4]. Gas tungsten arc welding (GTAW) process is a widely used material joining process, for welding magnesium alloys. In GTAW a nonconsumable tungsten electrode, shielded by an inert gas, is used to strike an electric arc with the base metal. The heat
∗ Corresponding author. Tel.: +91 4144 239734/241147; fax: +91 4144 238080/238275; mobile: +91 9443956536. E-mail addresses: [email protected] (G. Padmanaban), [email protected] (V. Balasubramanian). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.09.072
necessary to melt the base metal is provided by the electric arc [5]. Fig. 1 explains the working principle of GTAW process. Fusion zone of gas tungsten arc welded magnesium alloy typically exhibit coarser grains because of the prevailing thermal conditions during weld metal solidification. This often results inferior weld mechanical properties and poor resistance to hot cracking. While it is thus highly desirable to control solidification structure in welds, such control is often very difficult because of the higher temperatures and higher thermal gradients in welds in relation to castings and the epitaxial nature of the growth process. In general, the severity of a number of weld defects can be reduced if the solidification structure is refined. Certain novel welding techniques like pulsed current welding have been employed to improve hot cracking resistance and mechanical properties [6]. Pulsed current gas tungsten arc welding (PCGTAW) is a variation of GTAW process which involves cycling of the welding current from a high level to a low level at a selected regular frequency. Pulsed current is used to increase cooling rate. In pulsed current technique the welding current is pulsed between two levels. The background current used is not enough to melt the base metal but to maintain the arc stable. Peak current instantaneously melts the base plate in a small region for which the surrounding base metal acts as a chill and increases the cooling rate of the spot. With a specific frequency, the series of spots get overlapped and results in a continuous weld bead and these series of pulses agitate the weld pool and increases the cooling rates [7]. Shielding the welding region by inert gas or flux is needed to prevent the fire hazard and risk. Preheating is needed in weldingmagnesium applications because of the degree of joint restraint and metal thickness [8]. Laser beam welding (LBW) has been deemed as a high quality and efficiency process, under its action, all kinds of material will
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Fig. 3. Schematic representation of FSW principle. Fig. 1. Schematic diagram of the GTAW welding process.
Fig. 4. Dimensions of joint configuration.
Fig. 2. Schematic diagram of the laser beam welding process.
be heated to vaporize. The vaporizing pressure presses the molten material ejecting and a small hole (keyhole) is formed in the material being welded and under the focused beam. The keyhole traps the laser beam almost completely. The beam energy is absorbed by the material trough Fresnel absorption of the keyhole wall during multiple reflections of the beam on the wall. Around the keyhole there is a layer of molten material. As the focused beam scans on the material, the keyhole sweeps and the molten material flows around the keyhole from the front to the rare and re-solidified there to form a weld bead [9]. Fig. 2 explains the working principle of LBW process. Laser beam welding is a preferred method for joining magnesium alloys because of low heat input, elevated speed and limited deformation; however, the tendency of developing porosity must be reduced [10]. Recently developed friction stir welding (FSW) is also equally good process for welding magnesium alloys. Problems in fusion welding of magnesium alloys such as solidification cracking, liqua-
tion cracking and porosity are eliminated with friction stir welding due to its solid state nature of the process. This technique is a variant of the friction welding process, but utilizes a rotating tool with a shoulder and a profiled probe that is plunged into the work pieces and traversed along the weld centerline. The motion of the tool generates frictional heat within the work pieces, extruding the softened plasticized material around it and forging the same in place so as to form a solid-state seamless joint [11]. Fig. 3 explains the working principle of FSW process. Significant research is still needed on welding of magnesium alloys to achieve future goals to reduce the vehicle mass and the amount of greenhouse gases [12]. Recently, some studies [13–16] were carried out to evaluate the weldability of magnesium alloys. Microstructure and tensile properties of friction stir welded AZ31B magnesium alloy joint were studied by Afrin et al. [13]. Quan et al. [14] investigated the effect of heat input on microstructure of laser beam welded AZ31B magnesium alloy. The effect of filler wire on tensile properties of gas tungsten arc welded (GTAW) AZ31 magnesium alloy was studied by Liming Liu et al. [15]. Sun et.al. [16] compared GTAW and LBW joints of AZ31 magnesium alloy in terms of weld bead formation. However, a detailed comparison of mechanical and metallurgical properties of GTAW, FSW and LBW joints of AZ31B magnesium
Fig. 5. Macrostructure of welded joints (10×): (a) PCGTAW, (b) FSW and (c) LBW.
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Fig. 6. Optical micrographs of base metal and weld regions: (a) Base metal, (b) PCGTAW-fusion zone, (c) FSW-stir zone and (d) LBW-fusion zone.
alloy has not been reported yet. Hence, the present investigation is aimed to reveal the influences of these three welding processes on mechanical and metallurgical properties of AZ31B magnesium alloy.
Table 1a Chemical composition (wt.%) of base metal AZ31B magnesium alloy. Al
Mn
Zn
Mg
3.0
0.20
1.0
Bal
2. Experimental work The rolled plates of AZ31B magnesium alloy were machined to the required dimensions (300 mm × 150 mm × 6 mm). The chemical composition and mechanical properties of base metal are presented in Tables 1a and 1b. Square butt joint configuration, as shown in Fig. 4, was prepared to fabricate the joints. The plates to be joined were mechanically and chemically cleaned by acetone before welding to eliminate surface contamination. The direction of welding was maintained normal to the rolling direction of base plates. Necessary care was taken to avoid joint distortion and sin-
gle pass welding procedure was applied to fabricate the joints. High purity (99.99%) argon gas was used as shielding gas in GTAW process. A non-consumable, rotating tool made of high carbon steel was used to fabricate FSW joints. The welding conditions and the optimized process parameters used to fabricate the joints are presented in Table 2. The welded joints were sliced and then machined to the required dimension, according to the ASTM E8M-04 standard for sheet type material (i.e., 50 mm gauge length and 12.5 mm gauge width). Two different tensile specimens were prepared to evaluate the
Table 1b Mechanical properties of base metal AZ31B magnesium alloy. Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation in a gauge length of 50 mm (%)
Reduction in cross-sectional area (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Hardness at 0.05 kg load (Hv)
171
215
14.7
14.3
192
0.89
69
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Table 2 Optimised process parameters used to fabricate the joints. Process
PCGTAW
FSW
LBW
Welding machine Rotational speed (rpm) Welding speed (mm/min) Axial force (kN) Laser power (W) Focal position (mm) Peak current (A) Base current (A) Pulse frequency (Hz) Gas flow rate Pulse on time (%) Heat Input (kJ/mm)
Lincoln, USA – 180 – – – 210 80 6 16 lit/min 50 0.85
RV machine tools, India 1600 40 3 – – – – – – – 0.54
Rofin Slab CO2 laser, Germany – 5500 – 2500 −1.5 – – – 20 lit/min – 0.027
transverse tensile properties. The smooth (unnotched) tensile specimens were prepared to evaluate yield strength, tensile strength, elongation and reduction in cross sectional area. Notched specimens were prepared to evaluate notch tensile strength and notch strength ratio of the joints. Tensile test was carried out using a 100 kN, electro-mechanical controlled Universal Testing Machine (Make: FIE-Bluestar, India; Model: UNITEK-94100). The 0.2% offset yield strength was derived from the load-displacement diagram. Vicker’s microhardness testing machine (Make: Shimadzu, Japan
and Model: HMV-2T) was used for measuring the hardness with a 0.05 kg load. The specimens for metallographic examination were sectioned to the required size from the joint comprising weld metal, heat affected zone (HAZ) and base metal regions and polished using different grades of emery papers. Specimens were etched with a standard reagent made of 4.2 g picric acid, 10 ml acetic acid, 10 ml diluted water and 70 ml ethanol to reveal the micro and macrostructure. Microstructural analysis was carried out using
Fig. 7. SEM micrographs of base metal and weld regions: (a) Base metal, (b) PCGTAW-fusion zone, (c) FSW-stir zone and (d) LBW-fusion zone.
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Fig. 8. TEM micrographs of base metal and weld regions: (a) Base metal, (b) PCGTAW-fusion zone, (c) FSW-stir zone and (d) LBW-fusion zone.
an optical microscope (Make: MEIJI, Japan; Model: MIL-7100) incorporated with an image analyzing software (Metal Vision), scanning electron microscope (Make: JOEL, Japan; Model: 5610 LV) and transmission electron microscope (Make: Philips, UK; Model: CM20). Thin foils with a thickness of about 1 mm were cross sectioned to prepared transmission electron microscopy (TEM) samples. After being mechanically ground to approximately 80 m, the foils were further ground to a thickness of 15 m by a dimple machine. Final thinning of the foils was performed by ion milling operated at 5 kV. Energy dispersive spectroscopy (EDS) analysis was carried out to estimate the weight percentage of various elements. To identify the precipitated phase constitution presents in the fusion zone, samples were cut from the weld regions and XRD analysis (Make: RIGAKU, Japan; Model: ULTIMA-III) was carried out. The fractured surfaces of the tensile specimens were analysed using scanning electron microscope at high magnification to study the fracture morphology. 3. Results 3.1. Macrostructure Macrostructure of the cross-section of joints are displayed in Fig. 5. There is no evidence of macro level defects in all the joints.
Due to the variations in heat input of welding processes, an appreciable variation in the width of weld region is evident from the macrostructure of the joints. The macrostructure of PCGTAW joint shows a trapezoidal shaped weld region (Fig. 2(a)). FSW joint shows an elliptical shaped weld nugget region (Fig. 2(b)). The macrostructure of LBW joint shows a narrow keyhole shaped weld region (Fig. 2(c)).
3.2. Microstructure Microstructure of the joints was examined at different locations. The optical micrographs of base metal and weld region of PCGTAW, FSW and LBW joints are shown in Fig. 6. The average grain diameter of the weld region of all the joints were measured by applying Heyn’s line intercept method (ASTM E112-04) [17]. The average grain size of magnesium grains in base metal is about 58 m. From the micrographs, it is evident that fusion zone grains are finer than the base metal, irrespective of welding processes. PCGTAW joint contains coarser grains (38 m) in the weld region compared to LBW and FSW joints (Fig. 6(b)). LBW joint (Fig. 6(d)) contains finer grains (7 m) in the weld region as compared to FSW and PCGTAW joints. The average grain size of magnesium grains in FSW joint is about 13 m.
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Fig. 9. TEM micrographs (dislocation cell structure) of base metal and weld regions: (a) base metal, (b) PCGTAW-fusion zone, (c) FSW-Stir zone and (d) LBW-fusion zone.
Fig. 7(a) shows the SEM image of base metal. It contains coarser grains with Al12 Mg17 intermetallic compounds. SEM micrograph of fusion zone of PCGTAW joint is shown in Fig. 7(b). It is observed that the fusion zone contains coarser grains with more concentration of Al12 Mg17 intermetallic compounds which are not homogeneously distributed in the magnesium matrix. SEM micrograph of stir zone is shown in Fig. 7(c). It reveals that the stir zone contains fine grains with significantly refined Al12 Mg17 intermetallic compounds which are not uniformly distributed in the magnesium matrix. Fig. 7(d) shows the SEM image of fusion zone of LBW joint, which contains finer grains with more dense precipitates. The TEM micrographs displayed in Fig. 8 reveal the size and distribution of precipitates in the base metal, stir zone of FSW, fusion zone of LBW and PCGTAW joints. The base metal shows precipitates free zone (PFZ) since it is in annealed condition. The stir zone of FSW joint shows traces of fine precipitates but mostly dominated by precipitate free zone. The peak temperature reached in FSW is just sufficient to dissolve the precipitates. The weld region of LBW joint shows lot of precipitates all over the matrix. Since LBW is a fast process and related heat input is lower and hence the precipitates are finer compared to PCGTAW joints. The weld region of PCGTAW joint contains coarse and agglomerated precipitates and this is mainly due to higher heat input involved in the process. The dislocation cell
structure observed at very high magnification in the base metal, stir zone of FSW, fusion zone of LBW and PCGTAW are shown in Fig. 9. This clearly indicates that the dislocation density is higher in the fusion zone of LBW joint compared to FSW and PCGTAW joint. 3.3. XRD and EDS analysis The XRD results presented in Fig. 10, confirm the presence of Al12 Mg17 precipitates in weld region along with the traces of Mg2 Zn11 . The EDS results presented in Fig. 11 confirm that the matrix composition is mostly concentrated by aluminium and magnesium elements. However, evaporation of Zn is also observed in LBW and PCGTAW joints due to higher peak temperature experienced by the weld region. 3.4. Hardness The hardness was measured along the mid-thickness line of the cross-section of the joint using Vicker’s microhardness testing machine. Diamond indenter with 0.05 kg load was applied for the Vicker’s microhardness test and the values are presented in Fig. 12. The hardness of base metal (unwelded parent metal) is 69 Hv. The laser beam welded joint showed higher hardness of
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Table 3 Transverse tensile properties of welded joints. Joint type
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation in a gauge length of 50 mm (%)
Reduction in c.s.a (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Joint Efficiency (%)
Base metal PCGTAW FSW LBW
176 151 171 174
215 188 208 212
14.7 7.6 11.8 12.1
12.4 5.9 8.7 9.4
192 156 181 187
0.89 0.85 0.87 0.88
– 87.4 96.7 98.6
76 Hv in the weld region compared to other joints. In PCGTAW joint, reduction in hardness is observed in the HAZ region (61 Hv), due to grain coarsening. In FSW joint the lowest hardness is recorded in the TMAZ region (63 Hv). Presence of elongated and deformed
grains led to softening in this region and this may be the reason for reduction in hardness. Appreciable reduction in hardness is observed at the fusion boundary of LBW joint (68 Hv). The softening caused by annealing effect in these regions may be the reason for decreased hardness. The optical micrographs of Vickers hardness indented samples of the three welded samples are shown in Fig. 13.
3.5. Tensile properties
Fig. 10. XRD results: (a) base metal, (b) PCGTAW, (c) FSW and (d) LBW.
The transverse tensile properties such as yield strength, tensile strength, percentage elongation, notch tensile strength, and notch strength ratio of AZ31B magnesium alloy joints were evaluated. In each condition, three specimens were tested and the average of three results is presented in Table 3. The yield strength and tensile strength of unwelded parent metal are 176 MPa and 215 MPa respectively. But the yield strength and tensile strength of PCGTAW joints are 151 MPa and 188 MPa, respectively. This indicates that there is a 13% reduction in strength values due to PCGTA welding. Similarly, the yield strength and tensile strength of FSW joints are 171 MPa and 208 MPa, respectively which are 3% lower compared to unwelded parent metal. However, the yield strength and tensile strength of LBW joints are 174 MPa and 212 MPa, respectively. Of the three joints, the joints fabricated by LBW process exhibited higher strength values and the difference is approximately 11% higher compared to PCGTAW joints and 2% higher compared to FSW joints. Elongation and reduction in cross-sectional area of unwelded parent metal are 14.7% and 12.4%, respectively. But the elongation and reduction in cross-sectional area of PCGTAW joints are 7.6% and 5.9%, respectively. This suggests that there is a 48% reduction in ductility due to PCGTA welding. Similarly, the elongation and reduction in cross-sectional area of FSW joints are 11.8% and 8.7%, respectively, which are 20% lower compared to the parent metal. However, the elongation and reduction in cross-sectional area of LBW joints are 12.1% and 9.4%, respectively. Of the three joints, the joints fabricated by LBW exhibited higher ductility values and the difference is approximately 37% higher compared to PCGTAW joints and 3% higher compared to FSW joints. Notch tensile strength (NTS) of unwelded parent metal is 192 MPa, but the notch tensile strength of PCGTAW joint is 156 MPa. This reveals that the reduction in NTS is approximately 19% due to PCGTA welding. Similarly, the NTS of FSW is 181 MPa and the NTS of LBW is 187 MPa. Of the three joints, the joints fabricated by LBW process exhibited higher NTS values and the difference is 19% higher compared to PCGTAW and 3% higher compared to FSW. Another notch tensile parameter, NSR, is found to be lesser than unity (>1) for all the joints. This suggests that the AZ31B magnesium alloy is sensitive to notches and they fall into the ‘notch brittle materials’ category. The NSR is 0.89 for unwelded parent metal but it is 0.85 and 0.87 for PCGTAW and FSW joints respectively. Of the three joints, the joints fabricated by LBW process exhibited a relatively higher NSR (0.88). Joint efficiency is the ratio between tensile strength of welded joint and tensile strength of unwelded parent metal. The joint efficiency of PCGTAW joints is approximately 88% and the joint efficiency of FSW joints
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Fig. 11. EDS Results: (a) base metal, (b) PCGTAW, (c) FSW and (d) LBW.
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3.6. Failure modes
Microhardness (Hv) 0.05
90
80
70
PCGTAW FSW LBW
60
50
40 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8
Distance from weld centre (mm)
During tensile test, it is observed that the location of failure in PCGTAW joint is invariably heat affected zone (HAZ). Similarly in FSW joint the location of failure is thermo mechanically affected zone (TMAZ) of advancing side. However, in LBW joint the failure occurred in the transition region (between fusion zone and base metal). Optical microstructure of the above regions are displayed in Fig. 14. The presence of coarse grains and lower hardness are the main reasons for the failure to occur in these regions. SEM fractographs of fractured tensile specimens are shown in Fig. 15. The base metal shows elongated dimples and FSW joint contains finer dimples on the fractured surface. LBW joint and PCGTAW joint show mixed mode failure pattern (dimples with cleavage fracture) and this is consistent with the grain size of respective failure region.
Fig. 12. Hardness Plots.
4. Discussion is 97%. Of the three joints, the joints fabricated by LBW process exhibited the highest joint efficiency (99%) and it is 11% higher compared to the PCGTAW joints and 2% higher compared to FSW joints.
From the tensile test results, it is inferred that the LBW joints are exhibiting superior tensile properties compared to FSW and PCGTAW joints.
Fig. 13. Optical micrographs of Vickers hardness indented samples: (a) PCGTAW-fusion zone, (b) FSW-stir zone and (c) LBW-fusion zone.
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Fig. 14. Optical micrographs of failure regions: (a) PCGTAW–HAZ region, (b) FSWTMAZ region and (c) LBW-transition region.
The grain size of the fusion zone and HAZ are influenced by the heat input of the welding process. Of the three welding processes used in this investigation to fabricate the joints, the PCGTAW process utilized higher heat input compared to FSW and LBW processes. Even though, the heat input was high,
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the reason for the formation of finer microstructure compared to base metal was that the rapid cooling induced by good thermal conductivity and low thermal capacity of magnesium alloy restricted the growth of grains in fusion zone [18]. The microstructure analysis of PCGTAW revealed coarser grains in HAZ (Fig. 14(a)) and the hardness measurement showed very low hardness and fracture also occurred in HAZ during tensile test. This demonstrates that the HAZ is the weakest region in PCGTAW joints [15,19]. From the TEM micrographs, it is understood that the Al12 Mg17 intermetallic compounds are coarse and relatively more dense compared to the base metal (Fig. 8(b)). The dislocation density in PCGTAW joint is lower than the other joints (Fig. 9(b). From the EDS results (Fig. 11(b)), it is found that the PCGTAW joint contains less amount of Zn in the matrix (due to evaporation caused by high peak temperature). All these factors might have deteriorated the tensile properties of PCGTAW joints. In friction stir welding, there is no possibility of formation of a molten weld pool. Due to the frictional heat generated between the tool shoulder and the base metal, the material under the action of the rotating tool attains a plastic state. The axial force applied through the rotating tool causes the plasticized metal to extrude around the tool pin in the vertical direction and get consolidated in the back side when the tool moves forward. Both the stirring and extrusion causes the elongated grains to fragment into smaller grains [20]. Intense plastic deformation and frictional heating during FSW resulted in generation of fine recrystallized grains in the weld region. The microstructure of nugget region consist of dynamically recrystallized magnesium grains with significantly refined Al12 Mg17 intermetallic compounds which are not uniformly distributed in the magnesium matrix. During FSW process, magnesium grains plastically deform with rotation of the tool, but the second phase, which is a brittle phase, is hard to deform. This causes stress concentration on the second phase. When applied stress is higher than the fracture strength of the second phase, fracture takes place and large second phases change into smaller ones gradually. The grain size of TMAZ is coarser than the nugget region (Fig. 14(b)), because of insufficient deformation and thermal exposure [21]. This is also one of the reasons for lowest hardness distribution along the TMAZ region and also reason for failure along the TMAZ region in the advancing side. The microstructure of fusion zone of LBW joint consists of fine equiaxed grains. Due to high welding speed and fast recrystallization the fusion zone is characterized by very fine grains that resulted in increased hardness. A large number of precipitates distributed in the matrix are visible in the FZ which are not observed in the base alloy. Quan et al. [14], reported that during the laser welding process, the melt alloy is quickly solidified and cooled to room temperature, and the maximum solid solubility of Al decreases immediately from 12.7 wt.% to about 2.0 wt.%. The remnant Al will precipitate the ␥-Mg17 Al12 phase. The sharp transition from the base metal to the weld region (Fig. 14(c)) have indicated that there is no HAZ and similar observation was made by other investigators also [22,23]. Appreciable variation in hardness at the fusion boundaries is observed. This effect can indicate that the heat-affected zone that has not been observed under optical microscope, may present at the fusion boundaries. At the outset, the annealing softening in these regions might have decreased the hardness [24]. From the TEM micrographs, it is understood that the Al12 Mg17 intermetallic compounds are more dense, finer and are uniformly distributed throughout the magnesium matrix (Fig. 8(d)). The dislocation density in LBW joint is higher than the other joints (Fig. 9(d)). Due to the above said reasons, the LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints.
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Fig. 15. SEM fractographs of fractured tensile specimens: (a) base metal, (b) PCGTAW, (c) FSW and (d) LBW.
5. Conclusions From this investigation, the following important conclusions are derived. The joint fabricated by LBW process exhibited superior tensile properties compared to FSW and PCGTAW joints. The enhancement in tensile strength is approximately 11% compared to PCGTAW joints and 2% compared to FSW joints. The formation of very fine grains (7 m) in weld region, higher fusion zone hardness (76 Hv), the absence of heat affected zone, the presence of uniformly distributed finer precipitates and higher dislocations in weld region are the main reasons for superior tensile properties of LBW joints compared to PCGTAW and FSW joints. Acknowledgements The authors are grateful to the Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, India for extending the facilities of Material Testing Laboratory to carryout this investigation. The authors wish to place their sincere thanks to Science and Engineering Research Council (SERC), Department of Science and Technology (DST), New Delhi for financial
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Metallurgical characterization of pulsed current gas tungsten arc, friction stir and laser beam welded AZ31B magnesium alloy joints G. Padmanaban ∗ , V. Balasubramanian Center for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
a r t i c l e
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Article history: Received 22 March 2010 Received in revised form 11 August 2010 Accepted 28 September 2010 Keywords: Alloys Welding Electron microscopy Mechanical properties
a b s t r a c t This paper reports the influences of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on mechanical and metallurgical properties of AZ31B magnesium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy and X-Ray diffraction technique were used to evaluate the metallurgical characteristics of welded joints. LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints due to the formation of finer grains in weld region, higher fusion zone hardness, the absence of heat affected zone, presence of uniformly distributed finer precipitates in weld region. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The combination of low density, high specific strength and stiffness, and good castability qualifies magnesium alloys as ideal materials for lightweight structural applications. Most of the commercial magnesium alloys are based on Mg–Al system; and casting is currently the most commonly used production process for magnesium components. While the advancements on improving the properties of the alloys and on components forming technologies have been made in recent years, joining of magnesium alloy components is crucial in many applications. Hence, it is imperative for proper welding procedures to be established. For this reason, there has been an increasing effort in research on welding of magnesium alloys [1–3]. Wrought magnesium alloys are usually welded more easily than certain cast alloys. Mechanical joining process such as riveting and bolting are dominant in connection with wrought magnesium alloys. However, the limited strength of the joints restricts the applications of magnesium parts. Magnesium alloys can be joined using the gas metal arc welding, gas tungsten arc welding, laser beam welding, electron beam welding and solid state welding processes [4]. Gas tungsten arc welding (GTAW) process is a widely used material joining process, for welding magnesium alloys. In GTAW a nonconsumable tungsten electrode, shielded by an inert gas, is used to strike an electric arc with the base metal. The heat
∗ Corresponding author. Tel.: +91 4144 239734/241147; fax: +91 4144 238080/238275; mobile: +91 9443956536. E-mail addresses: [email protected] (G. Padmanaban), [email protected] (V. Balasubramanian). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.09.072
necessary to melt the base metal is provided by the electric arc [5]. Fig. 1 explains the working principle of GTAW process. Fusion zone of gas tungsten arc welded magnesium alloy typically exhibit coarser grains because of the prevailing thermal conditions during weld metal solidification. This often results inferior weld mechanical properties and poor resistance to hot cracking. While it is thus highly desirable to control solidification structure in welds, such control is often very difficult because of the higher temperatures and higher thermal gradients in welds in relation to castings and the epitaxial nature of the growth process. In general, the severity of a number of weld defects can be reduced if the solidification structure is refined. Certain novel welding techniques like pulsed current welding have been employed to improve hot cracking resistance and mechanical properties [6]. Pulsed current gas tungsten arc welding (PCGTAW) is a variation of GTAW process which involves cycling of the welding current from a high level to a low level at a selected regular frequency. Pulsed current is used to increase cooling rate. In pulsed current technique the welding current is pulsed between two levels. The background current used is not enough to melt the base metal but to maintain the arc stable. Peak current instantaneously melts the base plate in a small region for which the surrounding base metal acts as a chill and increases the cooling rate of the spot. With a specific frequency, the series of spots get overlapped and results in a continuous weld bead and these series of pulses agitate the weld pool and increases the cooling rates [7]. Shielding the welding region by inert gas or flux is needed to prevent the fire hazard and risk. Preheating is needed in weldingmagnesium applications because of the degree of joint restraint and metal thickness [8]. Laser beam welding (LBW) has been deemed as a high quality and efficiency process, under its action, all kinds of material will
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Fig. 3. Schematic representation of FSW principle. Fig. 1. Schematic diagram of the GTAW welding process.
Fig. 4. Dimensions of joint configuration.
Fig. 2. Schematic diagram of the laser beam welding process.
be heated to vaporize. The vaporizing pressure presses the molten material ejecting and a small hole (keyhole) is formed in the material being welded and under the focused beam. The keyhole traps the laser beam almost completely. The beam energy is absorbed by the material trough Fresnel absorption of the keyhole wall during multiple reflections of the beam on the wall. Around the keyhole there is a layer of molten material. As the focused beam scans on the material, the keyhole sweeps and the molten material flows around the keyhole from the front to the rare and re-solidified there to form a weld bead [9]. Fig. 2 explains the working principle of LBW process. Laser beam welding is a preferred method for joining magnesium alloys because of low heat input, elevated speed and limited deformation; however, the tendency of developing porosity must be reduced [10]. Recently developed friction stir welding (FSW) is also equally good process for welding magnesium alloys. Problems in fusion welding of magnesium alloys such as solidification cracking, liqua-
tion cracking and porosity are eliminated with friction stir welding due to its solid state nature of the process. This technique is a variant of the friction welding process, but utilizes a rotating tool with a shoulder and a profiled probe that is plunged into the work pieces and traversed along the weld centerline. The motion of the tool generates frictional heat within the work pieces, extruding the softened plasticized material around it and forging the same in place so as to form a solid-state seamless joint [11]. Fig. 3 explains the working principle of FSW process. Significant research is still needed on welding of magnesium alloys to achieve future goals to reduce the vehicle mass and the amount of greenhouse gases [12]. Recently, some studies [13–16] were carried out to evaluate the weldability of magnesium alloys. Microstructure and tensile properties of friction stir welded AZ31B magnesium alloy joint were studied by Afrin et al. [13]. Quan et al. [14] investigated the effect of heat input on microstructure of laser beam welded AZ31B magnesium alloy. The effect of filler wire on tensile properties of gas tungsten arc welded (GTAW) AZ31 magnesium alloy was studied by Liming Liu et al. [15]. Sun et.al. [16] compared GTAW and LBW joints of AZ31 magnesium alloy in terms of weld bead formation. However, a detailed comparison of mechanical and metallurgical properties of GTAW, FSW and LBW joints of AZ31B magnesium
Fig. 5. Macrostructure of welded joints (10×): (a) PCGTAW, (b) FSW and (c) LBW.
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Fig. 6. Optical micrographs of base metal and weld regions: (a) Base metal, (b) PCGTAW-fusion zone, (c) FSW-stir zone and (d) LBW-fusion zone.
alloy has not been reported yet. Hence, the present investigation is aimed to reveal the influences of these three welding processes on mechanical and metallurgical properties of AZ31B magnesium alloy.
Table 1a Chemical composition (wt.%) of base metal AZ31B magnesium alloy. Al
Mn
Zn
Mg
3.0
0.20
1.0
Bal
2. Experimental work The rolled plates of AZ31B magnesium alloy were machined to the required dimensions (300 mm × 150 mm × 6 mm). The chemical composition and mechanical properties of base metal are presented in Tables 1a and 1b. Square butt joint configuration, as shown in Fig. 4, was prepared to fabricate the joints. The plates to be joined were mechanically and chemically cleaned by acetone before welding to eliminate surface contamination. The direction of welding was maintained normal to the rolling direction of base plates. Necessary care was taken to avoid joint distortion and sin-
gle pass welding procedure was applied to fabricate the joints. High purity (99.99%) argon gas was used as shielding gas in GTAW process. A non-consumable, rotating tool made of high carbon steel was used to fabricate FSW joints. The welding conditions and the optimized process parameters used to fabricate the joints are presented in Table 2. The welded joints were sliced and then machined to the required dimension, according to the ASTM E8M-04 standard for sheet type material (i.e., 50 mm gauge length and 12.5 mm gauge width). Two different tensile specimens were prepared to evaluate the
Table 1b Mechanical properties of base metal AZ31B magnesium alloy. Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation in a gauge length of 50 mm (%)
Reduction in cross-sectional area (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Hardness at 0.05 kg load (Hv)
171
215
14.7
14.3
192
0.89
69
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Table 2 Optimised process parameters used to fabricate the joints. Process
PCGTAW
FSW
LBW
Welding machine Rotational speed (rpm) Welding speed (mm/min) Axial force (kN) Laser power (W) Focal position (mm) Peak current (A) Base current (A) Pulse frequency (Hz) Gas flow rate Pulse on time (%) Heat Input (kJ/mm)
Lincoln, USA – 180 – – – 210 80 6 16 lit/min 50 0.85
RV machine tools, India 1600 40 3 – – – – – – – 0.54
Rofin Slab CO2 laser, Germany – 5500 – 2500 −1.5 – – – 20 lit/min – 0.027
transverse tensile properties. The smooth (unnotched) tensile specimens were prepared to evaluate yield strength, tensile strength, elongation and reduction in cross sectional area. Notched specimens were prepared to evaluate notch tensile strength and notch strength ratio of the joints. Tensile test was carried out using a 100 kN, electro-mechanical controlled Universal Testing Machine (Make: FIE-Bluestar, India; Model: UNITEK-94100). The 0.2% offset yield strength was derived from the load-displacement diagram. Vicker’s microhardness testing machine (Make: Shimadzu, Japan
and Model: HMV-2T) was used for measuring the hardness with a 0.05 kg load. The specimens for metallographic examination were sectioned to the required size from the joint comprising weld metal, heat affected zone (HAZ) and base metal regions and polished using different grades of emery papers. Specimens were etched with a standard reagent made of 4.2 g picric acid, 10 ml acetic acid, 10 ml diluted water and 70 ml ethanol to reveal the micro and macrostructure. Microstructural analysis was carried out using
Fig. 7. SEM micrographs of base metal and weld regions: (a) Base metal, (b) PCGTAW-fusion zone, (c) FSW-stir zone and (d) LBW-fusion zone.
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Fig. 8. TEM micrographs of base metal and weld regions: (a) Base metal, (b) PCGTAW-fusion zone, (c) FSW-stir zone and (d) LBW-fusion zone.
an optical microscope (Make: MEIJI, Japan; Model: MIL-7100) incorporated with an image analyzing software (Metal Vision), scanning electron microscope (Make: JOEL, Japan; Model: 5610 LV) and transmission electron microscope (Make: Philips, UK; Model: CM20). Thin foils with a thickness of about 1 mm were cross sectioned to prepared transmission electron microscopy (TEM) samples. After being mechanically ground to approximately 80 m, the foils were further ground to a thickness of 15 m by a dimple machine. Final thinning of the foils was performed by ion milling operated at 5 kV. Energy dispersive spectroscopy (EDS) analysis was carried out to estimate the weight percentage of various elements. To identify the precipitated phase constitution presents in the fusion zone, samples were cut from the weld regions and XRD analysis (Make: RIGAKU, Japan; Model: ULTIMA-III) was carried out. The fractured surfaces of the tensile specimens were analysed using scanning electron microscope at high magnification to study the fracture morphology. 3. Results 3.1. Macrostructure Macrostructure of the cross-section of joints are displayed in Fig. 5. There is no evidence of macro level defects in all the joints.
Due to the variations in heat input of welding processes, an appreciable variation in the width of weld region is evident from the macrostructure of the joints. The macrostructure of PCGTAW joint shows a trapezoidal shaped weld region (Fig. 2(a)). FSW joint shows an elliptical shaped weld nugget region (Fig. 2(b)). The macrostructure of LBW joint shows a narrow keyhole shaped weld region (Fig. 2(c)).
3.2. Microstructure Microstructure of the joints was examined at different locations. The optical micrographs of base metal and weld region of PCGTAW, FSW and LBW joints are shown in Fig. 6. The average grain diameter of the weld region of all the joints were measured by applying Heyn’s line intercept method (ASTM E112-04) [17]. The average grain size of magnesium grains in base metal is about 58 m. From the micrographs, it is evident that fusion zone grains are finer than the base metal, irrespective of welding processes. PCGTAW joint contains coarser grains (38 m) in the weld region compared to LBW and FSW joints (Fig. 6(b)). LBW joint (Fig. 6(d)) contains finer grains (7 m) in the weld region as compared to FSW and PCGTAW joints. The average grain size of magnesium grains in FSW joint is about 13 m.
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Fig. 9. TEM micrographs (dislocation cell structure) of base metal and weld regions: (a) base metal, (b) PCGTAW-fusion zone, (c) FSW-Stir zone and (d) LBW-fusion zone.
Fig. 7(a) shows the SEM image of base metal. It contains coarser grains with Al12 Mg17 intermetallic compounds. SEM micrograph of fusion zone of PCGTAW joint is shown in Fig. 7(b). It is observed that the fusion zone contains coarser grains with more concentration of Al12 Mg17 intermetallic compounds which are not homogeneously distributed in the magnesium matrix. SEM micrograph of stir zone is shown in Fig. 7(c). It reveals that the stir zone contains fine grains with significantly refined Al12 Mg17 intermetallic compounds which are not uniformly distributed in the magnesium matrix. Fig. 7(d) shows the SEM image of fusion zone of LBW joint, which contains finer grains with more dense precipitates. The TEM micrographs displayed in Fig. 8 reveal the size and distribution of precipitates in the base metal, stir zone of FSW, fusion zone of LBW and PCGTAW joints. The base metal shows precipitates free zone (PFZ) since it is in annealed condition. The stir zone of FSW joint shows traces of fine precipitates but mostly dominated by precipitate free zone. The peak temperature reached in FSW is just sufficient to dissolve the precipitates. The weld region of LBW joint shows lot of precipitates all over the matrix. Since LBW is a fast process and related heat input is lower and hence the precipitates are finer compared to PCGTAW joints. The weld region of PCGTAW joint contains coarse and agglomerated precipitates and this is mainly due to higher heat input involved in the process. The dislocation cell
structure observed at very high magnification in the base metal, stir zone of FSW, fusion zone of LBW and PCGTAW are shown in Fig. 9. This clearly indicates that the dislocation density is higher in the fusion zone of LBW joint compared to FSW and PCGTAW joint. 3.3. XRD and EDS analysis The XRD results presented in Fig. 10, confirm the presence of Al12 Mg17 precipitates in weld region along with the traces of Mg2 Zn11 . The EDS results presented in Fig. 11 confirm that the matrix composition is mostly concentrated by aluminium and magnesium elements. However, evaporation of Zn is also observed in LBW and PCGTAW joints due to higher peak temperature experienced by the weld region. 3.4. Hardness The hardness was measured along the mid-thickness line of the cross-section of the joint using Vicker’s microhardness testing machine. Diamond indenter with 0.05 kg load was applied for the Vicker’s microhardness test and the values are presented in Fig. 12. The hardness of base metal (unwelded parent metal) is 69 Hv. The laser beam welded joint showed higher hardness of
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Table 3 Transverse tensile properties of welded joints. Joint type
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation in a gauge length of 50 mm (%)
Reduction in c.s.a (%)
Notch tensile strength (MPa)
Notch strength ratio (NSR)
Joint Efficiency (%)
Base metal PCGTAW FSW LBW
176 151 171 174
215 188 208 212
14.7 7.6 11.8 12.1
12.4 5.9 8.7 9.4
192 156 181 187
0.89 0.85 0.87 0.88
– 87.4 96.7 98.6
76 Hv in the weld region compared to other joints. In PCGTAW joint, reduction in hardness is observed in the HAZ region (61 Hv), due to grain coarsening. In FSW joint the lowest hardness is recorded in the TMAZ region (63 Hv). Presence of elongated and deformed
grains led to softening in this region and this may be the reason for reduction in hardness. Appreciable reduction in hardness is observed at the fusion boundary of LBW joint (68 Hv). The softening caused by annealing effect in these regions may be the reason for decreased hardness. The optical micrographs of Vickers hardness indented samples of the three welded samples are shown in Fig. 13.
3.5. Tensile properties
Fig. 10. XRD results: (a) base metal, (b) PCGTAW, (c) FSW and (d) LBW.
The transverse tensile properties such as yield strength, tensile strength, percentage elongation, notch tensile strength, and notch strength ratio of AZ31B magnesium alloy joints were evaluated. In each condition, three specimens were tested and the average of three results is presented in Table 3. The yield strength and tensile strength of unwelded parent metal are 176 MPa and 215 MPa respectively. But the yield strength and tensile strength of PCGTAW joints are 151 MPa and 188 MPa, respectively. This indicates that there is a 13% reduction in strength values due to PCGTA welding. Similarly, the yield strength and tensile strength of FSW joints are 171 MPa and 208 MPa, respectively which are 3% lower compared to unwelded parent metal. However, the yield strength and tensile strength of LBW joints are 174 MPa and 212 MPa, respectively. Of the three joints, the joints fabricated by LBW process exhibited higher strength values and the difference is approximately 11% higher compared to PCGTAW joints and 2% higher compared to FSW joints. Elongation and reduction in cross-sectional area of unwelded parent metal are 14.7% and 12.4%, respectively. But the elongation and reduction in cross-sectional area of PCGTAW joints are 7.6% and 5.9%, respectively. This suggests that there is a 48% reduction in ductility due to PCGTA welding. Similarly, the elongation and reduction in cross-sectional area of FSW joints are 11.8% and 8.7%, respectively, which are 20% lower compared to the parent metal. However, the elongation and reduction in cross-sectional area of LBW joints are 12.1% and 9.4%, respectively. Of the three joints, the joints fabricated by LBW exhibited higher ductility values and the difference is approximately 37% higher compared to PCGTAW joints and 3% higher compared to FSW joints. Notch tensile strength (NTS) of unwelded parent metal is 192 MPa, but the notch tensile strength of PCGTAW joint is 156 MPa. This reveals that the reduction in NTS is approximately 19% due to PCGTA welding. Similarly, the NTS of FSW is 181 MPa and the NTS of LBW is 187 MPa. Of the three joints, the joints fabricated by LBW process exhibited higher NTS values and the difference is 19% higher compared to PCGTAW and 3% higher compared to FSW. Another notch tensile parameter, NSR, is found to be lesser than unity (>1) for all the joints. This suggests that the AZ31B magnesium alloy is sensitive to notches and they fall into the ‘notch brittle materials’ category. The NSR is 0.89 for unwelded parent metal but it is 0.85 and 0.87 for PCGTAW and FSW joints respectively. Of the three joints, the joints fabricated by LBW process exhibited a relatively higher NSR (0.88). Joint efficiency is the ratio between tensile strength of welded joint and tensile strength of unwelded parent metal. The joint efficiency of PCGTAW joints is approximately 88% and the joint efficiency of FSW joints
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Fig. 11. EDS Results: (a) base metal, (b) PCGTAW, (c) FSW and (d) LBW.
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3.6. Failure modes
Microhardness (Hv) 0.05
90
80
70
PCGTAW FSW LBW
60
50
40 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8
Distance from weld centre (mm)
During tensile test, it is observed that the location of failure in PCGTAW joint is invariably heat affected zone (HAZ). Similarly in FSW joint the location of failure is thermo mechanically affected zone (TMAZ) of advancing side. However, in LBW joint the failure occurred in the transition region (between fusion zone and base metal). Optical microstructure of the above regions are displayed in Fig. 14. The presence of coarse grains and lower hardness are the main reasons for the failure to occur in these regions. SEM fractographs of fractured tensile specimens are shown in Fig. 15. The base metal shows elongated dimples and FSW joint contains finer dimples on the fractured surface. LBW joint and PCGTAW joint show mixed mode failure pattern (dimples with cleavage fracture) and this is consistent with the grain size of respective failure region.
Fig. 12. Hardness Plots.
4. Discussion is 97%. Of the three joints, the joints fabricated by LBW process exhibited the highest joint efficiency (99%) and it is 11% higher compared to the PCGTAW joints and 2% higher compared to FSW joints.
From the tensile test results, it is inferred that the LBW joints are exhibiting superior tensile properties compared to FSW and PCGTAW joints.
Fig. 13. Optical micrographs of Vickers hardness indented samples: (a) PCGTAW-fusion zone, (b) FSW-stir zone and (c) LBW-fusion zone.
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Fig. 14. Optical micrographs of failure regions: (a) PCGTAW–HAZ region, (b) FSWTMAZ region and (c) LBW-transition region.
The grain size of the fusion zone and HAZ are influenced by the heat input of the welding process. Of the three welding processes used in this investigation to fabricate the joints, the PCGTAW process utilized higher heat input compared to FSW and LBW processes. Even though, the heat input was high,
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the reason for the formation of finer microstructure compared to base metal was that the rapid cooling induced by good thermal conductivity and low thermal capacity of magnesium alloy restricted the growth of grains in fusion zone [18]. The microstructure analysis of PCGTAW revealed coarser grains in HAZ (Fig. 14(a)) and the hardness measurement showed very low hardness and fracture also occurred in HAZ during tensile test. This demonstrates that the HAZ is the weakest region in PCGTAW joints [15,19]. From the TEM micrographs, it is understood that the Al12 Mg17 intermetallic compounds are coarse and relatively more dense compared to the base metal (Fig. 8(b)). The dislocation density in PCGTAW joint is lower than the other joints (Fig. 9(b). From the EDS results (Fig. 11(b)), it is found that the PCGTAW joint contains less amount of Zn in the matrix (due to evaporation caused by high peak temperature). All these factors might have deteriorated the tensile properties of PCGTAW joints. In friction stir welding, there is no possibility of formation of a molten weld pool. Due to the frictional heat generated between the tool shoulder and the base metal, the material under the action of the rotating tool attains a plastic state. The axial force applied through the rotating tool causes the plasticized metal to extrude around the tool pin in the vertical direction and get consolidated in the back side when the tool moves forward. Both the stirring and extrusion causes the elongated grains to fragment into smaller grains [20]. Intense plastic deformation and frictional heating during FSW resulted in generation of fine recrystallized grains in the weld region. The microstructure of nugget region consist of dynamically recrystallized magnesium grains with significantly refined Al12 Mg17 intermetallic compounds which are not uniformly distributed in the magnesium matrix. During FSW process, magnesium grains plastically deform with rotation of the tool, but the second phase, which is a brittle phase, is hard to deform. This causes stress concentration on the second phase. When applied stress is higher than the fracture strength of the second phase, fracture takes place and large second phases change into smaller ones gradually. The grain size of TMAZ is coarser than the nugget region (Fig. 14(b)), because of insufficient deformation and thermal exposure [21]. This is also one of the reasons for lowest hardness distribution along the TMAZ region and also reason for failure along the TMAZ region in the advancing side. The microstructure of fusion zone of LBW joint consists of fine equiaxed grains. Due to high welding speed and fast recrystallization the fusion zone is characterized by very fine grains that resulted in increased hardness. A large number of precipitates distributed in the matrix are visible in the FZ which are not observed in the base alloy. Quan et al. [14], reported that during the laser welding process, the melt alloy is quickly solidified and cooled to room temperature, and the maximum solid solubility of Al decreases immediately from 12.7 wt.% to about 2.0 wt.%. The remnant Al will precipitate the ␥-Mg17 Al12 phase. The sharp transition from the base metal to the weld region (Fig. 14(c)) have indicated that there is no HAZ and similar observation was made by other investigators also [22,23]. Appreciable variation in hardness at the fusion boundaries is observed. This effect can indicate that the heat-affected zone that has not been observed under optical microscope, may present at the fusion boundaries. At the outset, the annealing softening in these regions might have decreased the hardness [24]. From the TEM micrographs, it is understood that the Al12 Mg17 intermetallic compounds are more dense, finer and are uniformly distributed throughout the magnesium matrix (Fig. 8(d)). The dislocation density in LBW joint is higher than the other joints (Fig. 9(d)). Due to the above said reasons, the LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints.
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Fig. 15. SEM fractographs of fractured tensile specimens: (a) base metal, (b) PCGTAW, (c) FSW and (d) LBW.
5. Conclusions From this investigation, the following important conclusions are derived. The joint fabricated by LBW process exhibited superior tensile properties compared to FSW and PCGTAW joints. The enhancement in tensile strength is approximately 11% compared to PCGTAW joints and 2% compared to FSW joints. The formation of very fine grains (7 m) in weld region, higher fusion zone hardness (76 Hv), the absence of heat affected zone, the presence of uniformly distributed finer precipitates and higher dislocations in weld region are the main reasons for superior tensile properties of LBW joints compared to PCGTAW and FSW joints. Acknowledgements The authors are grateful to the Department of Manufacturing Engineering, Annamalai University, Annamalai Nagar, India for extending the facilities of Material Testing Laboratory to carryout this investigation. The authors wish to place their sincere thanks to Science and Engineering Research Council (SERC), Department of Science and Technology (DST), New Delhi for financial
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