Free Access
Issue
Metall. Res. Technol.
Volume 116, Number 1, 2019
Article Number 118
Number of page(s) 8
DOI https://doi.org/10.1051/metal/2018106
Published online 25 January 2019

© EDP Sciences, 2019

1 Introduction

Ultra low carbon steels are widely used in the automotive industry due to reasonable combination of high strength and formability [13]. Specific or complex deep drawing operations with a high plastic strain ratio, good elongation, high hardening index and low yield to tensile strength ratio can be performed by this type of steel. These properties provide an advantage to the application of sheet stamping in automobile industries. Joining of low carbon steel for multi-component fabrication in an automobile industry is very important. Joining of steel using arc welding provides distortion and the surface finish of the joint is not acceptable and electron beam welding needs high energy input for joining process. Thus, friction stir welding is most appropriate for joining. Presently, continuous research and development are being executed to improve the weld properties. Initially, friction stir welding was performed on aluminium and its alloys. Later, friction stir welding was applied on magnesium and its alloys due to both materials low melting point. Recently, friction stir welding has proven to be perfectly appropriate for welding high-melting temperature alloys, such as Fe- [46], Ni- [7], and Ti-based alloys [8].

Literature reports that the friction stir welding of interstitial free steel plates was carried out at 800 rpm rotational speed and 50 mm/min traverse speed with tungsten-based alloy tool and they report the ultimate tensile strength (UTS) and impact toughness at room temperature of the joint were ∼274 MPa (86% of base metal) and 28 Joule, respectively [9]. Panda et al. [10] reports the laser welded joint of interstitial free steel and achieved maximum tensile strength of ∼253.4 MPa (80% of base metal). Matsushita et al. [11] reports that the friction stir welding of advanced high strength steel with 1.6 mm thickness was carried out with tungsten carbide based material tool at 200 to 600 rpm rotational speeds and 10–60 mm/s traverse speeds with 3° tilt angle. They also report that less than 100% joint efficiencies exhibited due to softening in TMAZ and HAZ. Miles et al. [12] reports that the friction stir spot welding has been shown to be a viable method of joining ultra-high strength steel, both in terms of joint strength and process cycle time. Kim et al. [13] reported that friction stir welding joint of advanced high strength steel was performed at the tool rotation speed of 800 rpm and traverse speed of 180 mm/min and also reported that temperature at the SZ was around 900 °C. Sarkar et al. [14] reported that DP 590 steel sheets were joined by friction stir spot welding, a range of rotational speeds from 400–2400 rpm and traverse speeds from 0.03 to 3.8 mm/s were exercised to attain defect free welds in 1.6 mm thick sheets. Literature reports that friction welded joints of accumulative roll-bonding ultra low carbon steel (20 ppm carbon) was carried out at 400 rpm rotational speed and 400 mm/min traverse speed with 3° tool tilt angle. They also report that stir zone had maximum hardness value with respect to that of other zone due to strain relaxed at stir zone as compared to as received material [15]. Atapour et al. [16] reports that the friction stir welding of stainless steel was carried out at constant rotational speed of 800 rpm with various travel speeds of 50, 100, and 150 mm/min and they also report that the corrosion properties of friction stir welded joint was highly influenced by the welding parameters in mixture of 0.1 M H2SO4 and 0.1 M NaCl solution.

In the present study friction stir welding of ultra low carbon steel was carried out at different tool rotational speeds of 300–900 rpm and 30 mm/min traverse speed. The present study also focused the microstructure of different regions, electrochemical study at the stir zone and the mechanical properties of the joints.

2 Experimental

In the present study, ultra low carbon (ULC) cold rolled steel was welded using a friction stir welding (FSW) machine. 140 mm × 70 mm × 3 mm plates were prepared with help of shaping machine from the received plates. The chemical composition of ULC steel was Fe–0.004C–0.05Mn–0.006Si–0.007S–0.008P–0.002N–0.05Ti (wt.%). Welding was carried out along rolling direction. Prior to the welding, the edge of the flat steel plate was machined using milling machine and finished by a surface grinder to achieve a flat surface and then mounted in a jig to make a butt joint. The gap between the two plates was maintained nil (in naked eye estimate) with an appropriate clamping. The shoulder and pin of FSW tool were made of tungsten carbide (WC) material and holder of the tool was made of high speed steel. A schematic diagram of FSW tool dimension is shown in Figure 1.

The shoulder of the tool having 25 mm diameter and conical pin of 5 mm diameter and 2.7 mm height was used for FSW (Fig. 1). Normal load of ∼5 kN and tool tilt angle (2°) were kept constant during processing. The FSW joints were carried out at tool rotational speed of 300 to 900 rpm in steps of 150 rpm and traverse speed was 30 mm/min. The welding direction of ULC steel was parallel to the rolling direction of the plates and the tool pin was placed at the middle of the two plates.

The samples processed at different rotational speeds were first cut from their respective welding joints along the transverse direction and mounted with mounting powder, mixed with a binder. Then the samples were belt grounded, followed by paper polishing (from grit size 120 to 1800) after which they were coarse and fine polished in the rotating polishing wheel with coarse and fine polishing abrasive (SiC powder). Then, diamond polishing was done and a mirror like finish was obtained. Each specimen was cleaned with water and subsequently with alcohol and then they were etched with 5% Nital and dried properly for microscopic examination. The microstructures of properly etched surfaces (different region of the welding) of the samples were examined in optical microscope. Tensile test, fracture surface study, microhardness test and electro-chemical tests were also done. The microstructure of the samples for grain size of different regions was analysed using Carl Zeiss micro-imaging GmbH (Axiovision, Axiovert 40 MAT) software. First, the grain boundaries were differently coloured and form the image with annotations, the average grain size was automatically measured. The average grain size of each specific zone for the different welded specimen was compared. For the transmission electron microscopy (TEM) observations, parallel to the welding direction (WD), thin foils were cut from the centre of the weld and twin-jet electro-polishing was carried out using an electrolyte of mixture of 90% acetic acid and 10% perchloric acid solution. The different regions of thinned disc samples were subsequently observed in a TEM (Philips Technai G-2 with EDAX facility) at 200 kV operating voltage.

Tensile properties of the welded joints were evaluated in a tensile testing machine (Instron 4204) at a crosshead speed of 1.66 × 10−3 mm/s at room temperature. The tensile test specimens were prepared from the friction stir welded (FSWed) plates as per ASTM ID: E8M-11 using abrasive water jet cutter (Fig. 2). All the samples were cut along transverse direction to the welded joint so that the joint was in the centre of the tensile specimens. Four samples were tested at each process parameter to check the reproducibility of results. The microhardness measurement was carried out for the welded samples using a diamond micro-indenter with a 20 gf load for 15-second duration. Fracture surfaces of the samples were observed in secondary electron mode in SEM (JEOL JSM-7610F).

The base metal and welded sample were used as working electrodes for electrochemical study, they were individually joined at one end with an insulated copper wire using a conducting Teflon epoxy. The exposed surface of specimens was ground and polished with different grades of SiC grit papers up to 2400 grit finish, polished over the diamond abrasive wheel and washed with double distilled water and acetone.

An Origalys potentiostat/galvanostat/ZRA was used to conduct the electrochemical measurements. The experiments were carried out at room temperature in naturally aerated solutions without stirring. A saturated calomel electrode was used as a reference electrode and a graphite rod was used as a counter electrode. Open Circuit Potential (OCP) time curves were recorded for base metal and joints in 0.1 mol/L HCl solution from immersion up to 5 h. After 5 h of immersion, optical microscopy was performed on the specimens in order to ascertain the extent of corrosion and the surface morphologies of the corroded samples. In a parallel set of experiments, after allowing the OCP values to stabilize for about 5 h, Electrochemical Impedance Spectroscopy (EIS) measurements were performed on the system. A frequency sweep was applied from 100 KHz to 10 MHz with initial voltage of 10 mV. Following the EIS experiment, a potentiodynamic polarization sweep was applied to the system under investigation from −1000 mV to +1000 mV in the cathodic to anodic direction. A scan rate of 1 mV/s was used for the polarization.

thumbnail Fig. 1

Schematic diagram of FSW tool dimension.

thumbnail Fig. 2

Schematic diagram of the tensile specimen.

3 Results and discussion

The macro images of FSWed joints at various tool rotational speeds are shown in Figure 3. Macrophotographs show that the joints preparations were successful under the experimental conditions. Typical volumetric defects like worm holes, tunnels, pin holes and voids encountered in FSW are not present in these macrographs. The frictional heat generation were enough to plasticize the ULC steel and caused good materials flow leading to proper coalescence. However, at 300 rpm joints, there was existence of some defect at bottom part of the weld, i.e. cavity and crack. The reasonable explanation was the improper welding parameters resulted in the unsmooth metal flow, and those metals behind the pin could not fill the cavity left before the pin. Therefore, these defects occurred due to low heat input. It is important that the tunnel defect was not observed for all the FSWed joints. These defects were attributable to the combination of weld parameters: insufficient rotational speed with excessive traverse speed at constant speed. In these cases, the welded parts were properly mixed together and hence tunnel was not observed for all the joints.

Optical images of FSW joints are shown in Figures 4 and 5. Figure 4a shows the retreating side (RS) and Figure 4b shows advancing side (AS) of FSWed sample at 300 rpm rotational speed. In both RS and AS, three different welding regions (i.e. stir zone (SZ), thermomechanically affected zone (TMAZ) and heat affected zone (HAZ)) were observed. Figure 5 shows the grain sizes of different regions of FSW joint. The elongated ferrite grains (Fig. 5c and d) were observed in the thermomechanical affected zone due to the plastic deformation along the shear direction during rotation of tool. At HAZ (Fig. 5e and f), the ferrite grains were coarser as compared to the base metal and the grain size increased with the increase in tool rotational speed. However, the very fine and more or less equiaxed grain (Fig. 5a and b) were observed in the stir zone due to dynamic recovery and dynamic recrystallization. Figure 6 shows the average grain size increased with increasing the tool rotational speeds. It has been demonstrated that the grain size of the stir zone was affected mainly by thermomechanical factors including degree of deformation and the peak temperature during welding. Higher rotational speeds cause to higher deformation and peak temperature. The peak temperature at the stir zone was described by Arbegast [17] and is given by the following equation: T is the temperature at the stir zone; Tm is the melting temperature of the steel, α and K are constant (vary between 0.04–0.06 and 0.65–0.75, respectively), ω is the rotational speed (rpm), V is traverse speed (mm/min). The peak temperatures are given in Table 1. The peak temperature is 790 °C at 300 rpm rotational speed and is 900 °C at 900 rpm. It was clear that the temperature produced at stir zone was above the recrystallization temperature. Thermocouple was inserted near to the stir zone for measuring the temperature value experimentally. At tool rotational speeds of 300 and 900 rpm the measured temperature value were 668 °C and 846 °C respectively.

The TEM images of the as received steel and welded joint at 600 rpm are shown in Figure 7. Figure 7a showed the ferrite lath along with dislocation in the as received steels. The welded joint at the SZ was equiaxed ferrite grains along with dislocation were observed (Fig. 7b). However, at the TMAZ elongated ferrite grain along with high dislocation are exhibited in Figure 7c. The TEM images of the HAZ (Fig. 7d) showed larger ferrite grain due to grain growth. At the TMAZ, fine scale precipitates were observed along the ferrite lath boundaries as indicated in Figure 7e. These precipitates indicated the MC (Fig. 7f) type to be TiC due to the presence of 0.05 wt.% Ti, i.e., in hyper-stoichiometric level with respect to the N content (0.002 wt.%).

The microhardness profiles along the cross section of the different region of FSW joints are shown in Figure 8. The hardness of the stir zone was higher as compared to the as received steel plates. Both sides of the TMAZ exhibited lower hardness value as compared to that of the stir zone. The maximum hardness values were 203 ± 3 and 201 ± 5 HV at tool rotational speeds of 300 and 450 rpm, respectively. Maximum hardness values were exhibited at the SZ for lower rotational speed due to the smaller grain size. According to the Hall-Petch linear relationship, finer grain size causes higher hardness, as finer grains consist of more grain boundary, which are the major hindrances to the dislocation slips [18]. Hardness value decreased with the increase in rotational speed due to the increase in grain size at SZ. At TMAZ and HAZ of the FSWed joint, the hardness values gradually decreased with the increase in tool rotational speed.

Figure 9 shows the variation in the tensile strength and yield strength (YS) of the FSWed joints with the change in tool rotational speed at constant tool traverse speed. At 300 rpm rotational speed, both the tensile strength and YS are lower. With the rise in tool rotational speed to 450 rpm, the joint strength (both UTS and YS) increased and attained the maximum value. At this rotational speed, the grain size of the stir zone was very fine due to recrystallization of the grains and finer grain promoted better mechanical properties, when the joints were processed at 300 rpm, the stir zone grain size was finer than that of the joint processed at 450 rpm; however the joint strength is lower than that of 450 rpm. At 300 rpm rotational speed, the void and crack (Fig. 3a) was observed at the weld region due to insufficient rotational speed at constant axial force. In this case, the welded parts could not be properly stirred and mixed together [19]. At the rotational speed of 600 rpm and above, the joint strength (both UTS and YS) gradually decreased and attained its lower value at 900 rpm processing rotational speed; the UTS and YS were 251 MPa and 195 MPa, respectively. Increase in rotational speed attributed to the grain growth of the recrystallized grain due to higher heat generation. This was also responsible for the increase in grain size at the TMAZ and HAZ.

Figure 10 reveals the variation in percentage elongation of the FSW joints with the change in tool rotational speed. It was observed that percent of elongation increased with the increase in tool rotational speed. The minimum values of percent elongation were obtained at 300 rpm due to the presence of voids and crack defects in weld region and also maximum dislocation density because of less heat generation. The percent elongation increased with the increase in tool rotational speed and attained its maximum values at 900 rpm processing rotational speed due to coarsening of the grain of all the regions. Higher rotational speeds produced sufficient heat for metallurgical transformation such as grain coarsening and lowering of the dislocation density at the SZ of the FSWed joints [20]. At 450 rpm, the strength was higher due to the grain refinement at the stir zone as compared to other rotational speed. The elongation was higher at 900 rpm as compared to the other rotational speed due to higher heat input which influences the grain growth.

The tensile fractures of the welded joints are shown in Figure 11. The tensile fracture indicated that at the lower rotational speeds (at 300 and 450 rpm) fracture occurred at the stir zone of the welded joint. However, the fracture occurred through the HAZ at higher rotational speed (600 rpm and above).

The tensile fracture surface of welded joint at different rotational speed specimens are shown in Figure 12. It was clear that dimples were observed in a wider area for all the fracture surface of welded joint. The dimple sizes were smaller for lower the rotational speed and it gradually increased with the increase in the tool rotational speed. At peak temperature, i.e. at higher rotational speed, welded joint resulted dimples and a fewer larger voids. The fracture surface indicated that all the fractures were ductile in nature.

Figure 13 shows the electrochemical polarization curves for base metal and welded joints at different rotational speeds. Electrochemical parameters obtained from the polarization plots are given in Table 2. The base metal has higher corrosion rate as compared to that of the welded region of all samples and the welded region corrosion rate decreased with increasing tool rotational speeds. The corrosion rate was higher for base metals due to excessive strain energy absorbed during the cold working. Corrosion rate was lower (i.e. 4.6 × 10−5 mm/year) at 900 rpm rotational speed due to the formation of larger grain size at the welded region. The nyquist plots for weld samples and base metal are shown in Figure 14 and it was analyzed using the equivalent circuit are given in Figure 15. Rsol represents the electrolyte resistance and other resistance, R2 represents the film resistance, R1 corresponds to the resistance inside the film pores, Q includes the pseudo capacitance of the film, expressed using the constant phase element (CPE) and C stands for the double layer capacity [21].

The variations of film resistance (R2) for welded joints are given in Table 3. The film resistance of base metal was lower than that of the welded joint and the resistance of the welded joint decreased with the increase in tool rotational speed. Figure 16 shows the optical micrographs of welded region after immersion in solution. It could be clearly observed that the extent of corrosion was higher in welded samples processed at lower rotational speed than that of higher rpm welded sample in solution. More numbers of smaller and larger pits were observed in the sample processed at 450 rpm. However, at 900 rpm, sample had less number of pits as compared to welded sample processed at 450 rpm due to lower corrosion rate.

thumbnail Fig. 3

Macrophotograph of the welded joint processed at (a) 300 rpm, (b) 600 rpm and (c) 750 rpm.

thumbnail Fig. 4

Optical photograph of the welded joint at 450 rpm tool rotational speed at (a) retreating side and (b) advancing side.

thumbnail Fig. 5

Optical photograph of the welded joints at (a) 450 rpm, SZ; (b) 750 rpm, SZ; (c) 900 rpm, TMAZ; (d) 600 rpm, TMAZ; (e) 300 rpm, HAZ; (f) 450 rpm, HAZ.

thumbnail Fig. 6

Grain size of different regions of the welded joints at different rotational speeds.

Table 1

The theoretical calculation of maximum temperature at the SZ of FSW.

thumbnail Fig. 7

TEM images of (a) as received steel, and welded joint of 600 rpm at (b) SZ, (c) TMAZ at AS, (d) HAZ, (e) enlarged view of (c), (f) EDS of the precipitated (arrowed in figure (e)).

thumbnail Fig. 8

Microhardness profiles along the cross section of FSW joints at different rotational speed.

thumbnail Fig. 9

Variation of YS and UTS with tool rotational speed of the FSW joints as compared to the base metal.

thumbnail Fig. 10

Variation of elongation with tool rotational speed of the FSW joints as compared to base metal.

thumbnail Fig. 11

Tensile specimen of the welded joints after fracture.

thumbnail Fig. 12

Fracture surfaces of the FSW joints processed at (a) 300 rpm, (b) 450 rpm, (c) 900 rpm tool rotating speed.

thumbnail Fig. 13

Polarization curve for different tool rotational speed of the FSW joint in 0.1M HCl solution.

Table 2

Values of cyclic voltammetry experiments belonging to base metals and welded samples.

thumbnail Fig 14

Nyquist plots of the friction stir welded joints at different rotational speed.

thumbnail Fig. 15

The equivalent circuit for quantitative evaluation of EIS spectra.

Table 3

Resistance values for EIS analysis of base metal and joint samples.

thumbnail Fig. 16

Optical micrographs of the welded region after immersion in solution (a) 450 rpm, (b) 900 rpm.

4 Conclusions

In the present study ultra low carbon steels were joined by friction stir welding in the range of 300–900rpm in steps of 150rpm tool rotational speeds, and 30mm/min traverse speed. Important conclusions are summarized below:

  • from the macrostructure, the defect free joints was observed, processed at 450 rpm and above rotational speeds. However, cavity and crack was observed at bottom part of the weld at 300 rpm rotational speed. Both the retreating and advancing sides, three different welding regions (i.e. SZ, TMAZ and HAZ) were observed. The grain size increased with the increase in tool rotational speed for all the region of the joint;

  • maximum UTS of ∼336 MPa and YS of ∼250MPa were obtained at 450 rpm tool rotational speed. At 600rpm and above rotational speed, the joint strength (both UTS and YS) gradually decreased. The percent elongation gradually increased with the increase in tool rotational speed and attained its maximum value at 900rpm processing rotational speed. Microhardness distribution indicated that the maximum hardness was obtained at the stir zone and higher hardness value was observed at lower tool rotational speed;

  • the corrosion rate at the stir zone decreased with the increase in tool rotational speed. The higher corrosion resistance was obtained at tool rotational speed of 900 rpm due to coarser grain in the stir region.

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Cite this article as: Ishita Koley, Sukumar Kundu, Satish V. Kailas, Friction stir welding of ultra low carbon steel: microstructure, mechanical properties and electrochemical study, Metall. Res. Technol. 116, 118 (2019)

All Tables

Table 1

The theoretical calculation of maximum temperature at the SZ of FSW.

Table 2

Values of cyclic voltammetry experiments belonging to base metals and welded samples.

Table 3

Resistance values for EIS analysis of base metal and joint samples.

All Figures

thumbnail Fig. 1

Schematic diagram of FSW tool dimension.

In the text
thumbnail Fig. 2

Schematic diagram of the tensile specimen.

In the text
thumbnail Fig. 3

Macrophotograph of the welded joint processed at (a) 300 rpm, (b) 600 rpm and (c) 750 rpm.

In the text
thumbnail Fig. 4

Optical photograph of the welded joint at 450 rpm tool rotational speed at (a) retreating side and (b) advancing side.

In the text
thumbnail Fig. 5

Optical photograph of the welded joints at (a) 450 rpm, SZ; (b) 750 rpm, SZ; (c) 900 rpm, TMAZ; (d) 600 rpm, TMAZ; (e) 300 rpm, HAZ; (f) 450 rpm, HAZ.

In the text
thumbnail Fig. 6

Grain size of different regions of the welded joints at different rotational speeds.

In the text
thumbnail Fig. 7

TEM images of (a) as received steel, and welded joint of 600 rpm at (b) SZ, (c) TMAZ at AS, (d) HAZ, (e) enlarged view of (c), (f) EDS of the precipitated (arrowed in figure (e)).

In the text
thumbnail Fig. 8

Microhardness profiles along the cross section of FSW joints at different rotational speed.

In the text
thumbnail Fig. 9

Variation of YS and UTS with tool rotational speed of the FSW joints as compared to the base metal.

In the text
thumbnail Fig. 10

Variation of elongation with tool rotational speed of the FSW joints as compared to base metal.

In the text
thumbnail Fig. 11

Tensile specimen of the welded joints after fracture.

In the text
thumbnail Fig. 12

Fracture surfaces of the FSW joints processed at (a) 300 rpm, (b) 450 rpm, (c) 900 rpm tool rotating speed.

In the text
thumbnail Fig. 13

Polarization curve for different tool rotational speed of the FSW joint in 0.1M HCl solution.

In the text
thumbnail Fig 14

Nyquist plots of the friction stir welded joints at different rotational speed.

In the text
thumbnail Fig. 15

The equivalent circuit for quantitative evaluation of EIS spectra.

In the text
thumbnail Fig. 16

Optical micrographs of the welded region after immersion in solution (a) 450 rpm, (b) 900 rpm.

In the text

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