Indian Journal of Engineering & Materials Sciences Vol. 25, February 2018, pp. 26-32

Microstructural and mechanical properties of friction stir welded pure lead

A Günena*, E Kancab, M Demirb, F Çavdarc, S Mistikoğlub & G Çamb

aMaterials Engineering Department, Faculty of Engineering and Natural Sciences,

Iskenderun Technical University, Hatay, Turkey bMechanical Engineering Department, Faculty of Engineering and Natural Sciences,

Iskenderun Technical University, Hatay, Turkey cDörtyol Vocational High School, Iskenderun Technical University, Hatay, Turkey

Received 2 April 2016; accepted 10 October 2017

The present study focuses on the applicability of friction stir welding to pure Pb plates. The joint performances are evaluated by optical microscopy, microhardness measurements, notch impact tests and three point bending tests. The effects of the tool rotational speed (1500, 2000 and 2500 rev/min) and welding speed (72, 100 and 125 mm/min) on the joint performance are determined. The results indicate that the welded joints exhibit similar or higher mechanical properties than those of the base material. Optimal joint performance was achieved with a rotational speed of 1500 rev/min and welding speed of 100 mm/min.

Keywords: Lead, Toxic effect, Friction stir welding, Joint performance

Lead (Pb) is one of the oldest metals used by man1. Its widespread occurrence, relatively simple extraction and high malleability2 have made it useful to mankind for thousands of years. In addition, it has high corrosion resistance and the ability to react with organic chemicals. It has poor electrical conductivity compared to other metals, yet it has high ductility, and can be reduced (smelted) to its metallic state with relative ease1-3. These properties have resulted in its widespread use in many applications including construction materials, lead-acid batteries, bullets and shot, weights, as part of solders, pewters, and fusible alloys2, as coolants in fast reactors4, and in radiation shielding5. However, Pb is categorized as one of the toxic metals; it is a neurotoxin. Any detectable amount of Pb may cause negative health effects6. If ingested, Pb is poisonous to all animals including humans, damaging the nervous system and causing brain disorders especially in children. Excessive Pb exposure causes blood disorders in mammals. Pb also accumulates both in soft tissues and bones6-8. It is of importance, therefore, to protect industrial workers and the general population from Pb exposure.

Fusion welding of Pb and Pb-alloys is an environmental concern due to toxic effects. Conventional welding processes are often unsafe and

unsatisfactory. There is the risk of inhalation of toxic fumes during the fusion welding process as a result of the evaporation from the metal. In such cases, it might be advantageous to instead utilize solid state welding techniques. Friction stir welding (FSW) is a solid state welding technology, developed by the TWI (The Welding Institute) in 19919-14. The joining technique is relatively simple: a rotating tool equipped with a specially designed pin and shoulder is plunged between edges of work-pieces held under lateral pressure, causing friction. Frictional heat is generated between the wear resistant welding tool (the shoulder and pin) and the work-pieces. The intense heat does not melt the work-pieces but plasticizes the material around the tool. The tool shoulder prevents the plasticized material from running out of the weld area. The rotating tool travels along the joint to be welded (welding line) transferring the softened material around itself, stirring the plates together11-14.

FSW offers a number of attractive advantages when compared to other welding processes, perhaps the most significant being the ability to weld alloys that are difficult or impossible to weld using conventional fusion welding techniques. The FSW process is carried out in the solid-phase, at temperatures below the melting point of the material to be welded, and as a result problems related to re-solidification are not encountered, such as the

_________ *Corresponding author (E-mail: aligunen2013@gmail.com)

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formation of second phases, porosity, embrittlement, and cracking. In addition, the lower temperature of the process enables joining with relatively lower distortion and lower residual stresses compared to the traditional welding processes. FSW is also energy efficient, in that it requires no filler material and, in most cases, does not require the use of a shielding gas. Moreover, FSW lacks the fumes, arc flash, spatter and pollution associated with most fusion welding techniques. For these and many other reasons, FSW has become an attractive joining process for many welding applications11-15.

Initial studies on FSW were mainly focused on Al alloys, but later, following the development of better FSW tools Mg, Cu, some of steels, brasses, Ti and its alloys, Ni-alloys, etc. have been welded by this welding process11,16-53. However, virtually no reports exist in the open literature on the welding of Pb by FSW technique. This situation has triggered this current study.

The main objective of this study is to investigate the effects of FSW parameters, namely rotational speed and welding speed, on the weldability of pure Pb by FSW. The welded joints were characterized by hardness, bend, and notch impact tests. Microstructural changes in the joints were also compared.

Experimental Procedure The material used in this study was cold-rolled

(30%) pure Pb plate with a purity level of 99.97 wt%. The dimensions of plates were 150×60×6.5 mm. A triangular profile welding rod made of AISI 4140 steel was used which had a 16 mm diameter shoulder, a 6.3 mm probe height and triangle with one side of 6 mm. The oxidation film and oil particles on the surface of work-piece were removed before welding. The two 6.5 mm thick sheets were then butt welded. The welding trials were conducted using a CNC milling center. Experimental parameters were varied in the test in the following ranges to determine the best welding conditions: The tool rotational speeds used were 1500, 2000, and 2500 rev/min, and the welding speeds were 72, 100, and 125 mm/min (Table 1).

Metallography specimens were extracted from the welded plates for microstructural examination. The specimens were polished with an alumina suspension (the grain size was 0.3 µm) and then etched with 5% citric acid and 95% ethanol. A detailed microstructural characterization for each welded plate was conducted using optical microscopy to determine the presence of any weld defects and the

microstructural evolution within the stirred zone. All micrographs were taken from the midsections of the joints in the thickness direction. Micro-hardness measurements were conducted on each welded plate to determine the variation in hardness across the stirred zones. Moreover, a bend test (according to ASTM E23) specimens (50×10×5 mm) and ASTM E190 standard notch impact V-Charpy test specimens (55×10×5 mm) were extracted from both base plates and welded plates (minimum three specimens) and tested at room temperature to determine the joint performances. All specimens were bent up to 180º, to determine whether cracking occurs in bending. Results and Discussion

Appearance and microstructural aspects

In order to determine the optimum welding conditions for FSW pure Pb, joints were produced using various weld parameters using tool rotational speeds of 1500, 2000, and 2500 rev/min, and welding speeds of 72, 100, and 125 mm/min. Figure 1 shows surface appearances of the welded joints. The examination of the surface appearance of the joints indicates that the welded joints display excellent surface appearance, without distortion and no macro defects, and that sufficient welding parameters were used. The best surface appearance is observed in the joint obtained using a rotational speed of 1500 rev/min and a welding speed of 100 mm/min (hereafter designated as 1500/100) (Fig. 1a).

Increasing the rotational speed at a constant welding speed leads to an increase in surface roughness (Figs 1a-1c). This can be attributed to the softening of Pb, which cannot maintain its solid shape due to high temperatures and redundant heat input during FSW process. Increasing the welding speed at constant rotational speed leads to a decrease in the width of the weld seam. In addition, the weld zone is

Table 1 — FSW process variables

Joint # Rotational speed

(rpm) Welding speed

(mm/min)

1 1500 72 2 1500 100 3 1500 125 4 2000 72 5 2000 100 6 2000 125 7 2500 72 8 2500 100 9 2500 125

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narrower in the root section than the upper surface due to the high heat input (frictional heat) generated in the upper section by the tool shoulder. Wang et al.51 studied FSW of MB3 alloys and they noted that some FSW parameters provided good surface appearance while others resulted in high surface roughness values and voids behind the tool's direction. The improper appearance on the surface was attributed to the fact that the flow rate of the metal was too low due to the low rotational speed or the high welding speed. Moreover, Sun et al.52 also reported tunnel-like defects in the case of insufficient heat input, and superfluous flash defects in case of redundant heat input in the FSWed Cu-30Zn brass

alloys. This indicates that the metal flow due to the heat input has a great affect on the properties of the surface and internal structure of FSWed joints.

Although the weld nugget mostly consists of dynamically recrystallized zone (DXZ), the presence of TMAZ in this region is also seen in all pure Pb joints produced in contrast to Al-alloys in which the weld nugget consists of two clearly distinct regions DXZ and TMAZ. This can be attributed to the fact that the recrystallization is much more readily induced in pure Pb than in Al-alloys as pointed out by Çam11. Optical micrographs illustrating the microstructures of the base metal and the DXZs of the FSWed pure Pb joints produced at a constant welding speed of

Fig. 1 — Surface appearances of FSWed pure Pb plate joints produced using different weld parameters

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100 mm/min using various rotational speeds are displayed in Fig. 2. It can be seen from this figure that the cold-rolled pure Pb base plate has a typical elongated grain structure (Fig. 2a) with a grain size of several microns, whereas Figs 2b, 2c and 2d show microstructures of DXZs of the FSWed joints, produced with weld parameters of 1500/100, 2000/100, 2500/100, respectively. It is clear from the microstructures that the elongated grain structure was completely converted to a refined equiaxed grain structure in the weld zone. This is due to the dynamic recrystallization taking place during FSW in the DXZ of the cold rolled lead base plates. However, grains evolved in the DXZ are not homogeneous in all the

joints due to the different heat dissipation rates during the welding (Fig. 2). This was also reported by Fujii et al.53 in FSWed Mo alloy joints. They reported that the large plastic deformation leads to the occurrence of dynamic recrystallization in one area before another. As a result, dynamic recrystallization at different times (early or late) and subsequent grain growth eventually result in the formation of coarse and fine grained banded areas. In addition, increasing rotational speed at a constant welding speed leads to a slight coarsening of the recrystallized grains in the DXZ. This can be attributed to the higher heat input resulting from higher rotational speeds.

Figures 3a and 3b show the thermo-mechanically-affected zones (TMAZ) on the retreating sides of the

Fig. 2 — Optical micrographs of (a) base metal and the DXZs of the joints produced with the weld parameters: (b) 1500/100, (c) 2000/100 and (d) 2500/100

Fig. 3 — Optical micrographs of TMAZ regions of the joints obtained using the weld parameters: (a) 2500/72 and (b) 2500/125

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joints produced using the weld parameters of 2500/72 and 2500/125. An inhomogeneous microstructure consisting of both coarse and fine recrystallized grains is also observed in TMAZ as the case in the DXZ51. This is probably due to the fact that recrystallization is easily induced in pure Pb due to its low melting point. Similar results were also reported in friction stir welded Cu and Cu-alloys in which recrystallization is also easily induced48,49. It is also worth pointing out that the grain size in the weld nugget increases with increasing welding speed at constant rotational speeds. This is in good agreement with the fact that the heat input also decreases with increasing traverse speed52.. Mechanical properties

The hardness distribution across the weld area, which is considered to be one of the significant factors affecting the mechanical properties, has been investigated along the cross-sectional centerline of the welded joints. The hardness profiles53 of the FSWed pure Pb joints produced using various rotational speeds and welding speeds are shown in Fig. 4. The highest hardness values were displayed by the joint which was produced using the weld parameters of 2500/72. This was followed by joints obtained with the weld parameters of 2500/100, 2000/72 and 1500/72, respectively. There is no clear correlation between the hardness values measured and the grain

size in the DXZs of the respective joints. Similar results were also reported for FSWed Cu-alloys48,49. Hardness values have in general decline with increasing welding speed. This is in contrast to the case in FSWed Al-alloys where the hardness of the weld region usually increase with the increasing welding speeds due to the lower heat input at higher welding speeds leading to finer grain sizes in the weld region25-26 Furthermore, the hardness values measured in some welded samples (1500/125, 2000/125, 1500/100, 2000/100) were lower than that of base metal. This indicates that the cold rolled pure Pb was annealed by the effect of heat input during the FSW. Similar softening (loss of hardness) in the weld region was also reported for FSWed cold rolled AA5086-H32 Al-alloy plate joints50. According to Fig. 4, the hardness in the welding zone ranges from 32.8 to 42.2 HB for almost all the joints, which is quite similar to that of the base metal, i.e., 36.8 HB. The distribution of hardness was affected by the welding parameters. In some samples the lowest hardness value was observed in welding nugget while in others the highest hardness values measured in this region. The reason for this is that when the traverse speed decreases to 72 mm/min at all rotational speeds used, dynamic recrystallization takes place more completely across the entire weld nugget due to a higher heat input and this apparently leads to a hardening effect.

Lead has a low tensile strength (16-17 MPa), thus it is very difficult to obtain reliable results from tensile test. Therefore, three point bending test and notch impact tests were conducted in order to investigate the influence of the weld parameters on the mechanical properties of the joints. Notch impact test samples prepared according to ASTM E23 standard and three point bending test samples prepared according to ASTM E190 were tested. Some of the tested samples are shown in Fig. 5. Notch impact test results are given in Table 2

Fig. 4 — Hardness distribution along the centerline of the cross-sections of the joints

Fig. 5 — Some tested samples after (a) three point bending test and (b) notch impact test

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In the testing of non-standard bending specimens all specimens were bent up to 180º, with the weld root outside to determine whether cracking occurs or not in this bending condition. No cracking was observed in the bend testing of the joints except one, which was produced with the weld parameters of 2500/72. This sample was the one which displayed the highest hardness values in the weld nugget, apparently leading to cracking in the bend test. Three point bending test results indicate that the joints are reasonably sound and possess relatively high toughness.

When Table 1 is examined, it can be seen that the highest toughness values, i.e., 7, 7, 6, and 6 J/cm2) were obtained for the samples produced with the weld parameters of 1500/125, 2000/125, 1500/100, 2000/100, respectively. Notch impact energy values obtained from these samples were higher than that of the base metal, i.e., 5 J/cm2. This demonstrates that a certain amount of recrystallization occurred in the cold-rolled Pb plate, due to the heat input during FSW process, leading to softening. Thus, recrystallization increased the toughness. The lowest toughness has been exhibited by the samples produced using the weld parameters of 2500/72, 2500/100, 1500/72, and 2000/72, which displayed the highest hardness values in the weld nugget. The low toughness value determined for the joint produced using the weld parameters 2500/72 is also in good agreement with the bend testing of this joint in which cracking occurred. The notch impact test results also indicate that toughness generally decreases with increasing hardness in the weld region. Conclusions

The following conclusions can be drawn from the present study: (i) Defect free butt joints were easily obtained in

cold worked commercially pure Pb plates, 6.5 mm thick, by means of FSW using an AISI 4140

alloy tool. High quality butt joints free from defects or imperfections have been produced using a large window of weld parameters. This material has the lowest melting point (327°C) of those that have been friction stir welded metals so far.

(ii) The joints showed a microstructure consisting of recrystallized grains in the weld nugget. The microstructure of the weld nuggets of all the joints contained both coarse grains and fine grains, in varying proportions, both in the DXZ and TMAZ.

(iii) The joint produced at a rotational speed of 1500 rev/min using a welding speed of 100 mm/min exhibited the best joint performances.

(iv) Grain refinement was achieved in the weld nuggets of all the joints produced at high welding speeds. As expected, it was demonstrated that the lower the heat input during welding the finer the grain size in the stir zone.

(v) No significant cracking was observed in three point bending test with the exception of one specimen indicating that the joints are reasonably sound.

(vi) Notch impact test results suggest that friction stir welded pure Pb plates exhibited satisfactory notch toughness values.

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