Microstructure and mechanical properties evolution of

Materials and Design 91 (2016) 378–387

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Microstructure and mechanical properties evolution of friction stir spot welded high-Mn twinning-induced plasticity steel M.M.Z. Ahmed a,b,⁎, Essam Ahmed a,b, A.S. Hamada b,c, S.A. Khodir d, M.M. El-Sayed Seleman a,b, B.P. Wynne e a

Suez and Sinai Metallurgical and Materials Research Center of Scientific Excellence (SSMMR-CSE), Suez University, Suez, Egypt Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, P.O. Box 43721, Suez, Egypt Centre for Advanced Steels Research, University of Oulu, P.O. Box 4200, FI-90014 Oulu, Finland d Central Metallurgical Research and Development Institute, P.O. Box 11421, Helwan, Egypt e Department of Materials Science and Engineering, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK b c

a r t i c l e

i n f o

Article history: Received 10 September 2015 Received in revised form 22 November 2015 Accepted 1 December 2015 Available online 2 December 2015 Keyword: TWIP Steel Friction stir spot welding Microstructure EBSD Mechanical properties

a b s t r a c t Friction stir spot welding of high-Mn twinning-induced plasticity steel was studied. Welds were made at different tool rotation speeds and constant plunge rate and dwell time. The microstructure evolution was examined by optical microscopy, scanning electron microscopy and electron backscattered diffraction technique. In addition, the microhardness distribution and tensile-shear load bearing capacity were measured. The friction stir spot welding process successfully produced high integrity completely defect-free joints at all the proposed welding parameters. However, the complex plastic deformation and high thermal cycle experienced had a significant effect on the weld region, which consisted of three distinct zones. The flow transition zone, stir zone and torsion zone were all characterized by a recrystallized grain structure. The heat affected zone was characterized by a coarse grain structure as a result of grain growth caused by the high thermal cycles experienced. The hardness was significantly affected by friction stir spot welding, resulting in a softened region in the joint area. The softening increased as the rotation rate increased. The maximum peak tensile shear load of 13 kN was obtained at 750 rpm, and a considerable amount of extension was obtained in all the joints with a maximum of 4 mm at 500 rpm. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction The innovation of new materials, such as dual phase (DP), transformation-induced plasticity (TRIP), and twinning-induced plasticity (TWIP) steels, play an essential role in the transportation industries of the 21st century [1]. TWIP steels exhibit an excellent tensile strength–ductility combination due to the formation of deformation twinning under mechanical loading [2–5]. The deformation twin boundaries act as strong barriers for the dislocation movement, resulting in grain refinement strengthening via reduction in dislocation mean free path (the dynamic Hall–Petch effect). This, in turn, leads to relatively high rate of work hardening delaying the onset of necking, leading to a significant ductility increase. Consequently, an excellent tensile strength–ductility combination is attained [6–10]. Alloys attracting much attention for the automotive industry are High-Mn TWIP steels, because of their high energy absorption and the crash safety property, which is more than twice that of conventional high ⁎ Corresponding author at: Suez and Sinai Metallurgical and Materials Research Center of Scientific Excellence (SSMMR-CSE), Suez University, Suez, Egypt. E-mail address: [email protected] (M.M.Z. Ahmed).

http://dx.doi.org/10.1016/j.matdes.2015.12.001 0264-1275/© 2015 Elsevier Ltd. All rights reserved.

strength steels [11,12]. These properties guarantee the safety in highstrain rate deformation of order 103 s−1, such as automotive crash safety systems and military vehicle armor [13,14–16]. In addition their desirable properties ensure weight saving, which represents a straightforward strategy to improve fuel economy and environmental protection [17]. Research on welding is one of the most important topics related to TWIP steels, particularly in the automotive industry as automobile bodies and other structural components are assembled almost entirely by welding [18,19]. The formation of chemical inhomogeneities as a result of Mn evaporation, dilution, and micro-segregation are the main determinative issues associated with the fusion welding of TWIP steels [20, 21]. Evaporation is due to the increasing manganese vapor pressure with increasing temperature. For instance, in the liquid state at 1500 and 1600 °C, the vapor pressure of manganese can reach 2000 and 4770 Pa, respectively [22]. Dilution effects come during the dissimilar welding of TWIP steels to other steels where buoyancy forces (density differences) and Maragoni convection (surface tension gradients and thermocapillary forces mean chemistry stability cannot be maintained [23]. The high manganese content of typical TWIP steels is also responsible for the segregation phenomena, which leads to enrichment of the

M.M.Z. Ahmed et al. / Materials and Design 91 (2016) 378–387

379

Fig. 1. Thermo-mechanical cycle of the studied high Mn TWIP steel.

liquid phase with manganese and thus to a corresponding depletion of the solidifying austenitic phase leading to the presence of secondary phases, such as eutectic (Fe, Mn)3C cementite [24,25]. Resistance spot welding is considered as one of the dominant methods of auto body assembly [26]. However, conventional resistance spot welding has disadvantages, such as the consumption of tools during joining, large heat distortion, and poor weld strength in joints; porosity defects cannot be avoided by laser spot welding; riveting will increase the weight of components and the drilling needed will increase the cost [27]. Hence, new spot welding processes, such as friction stir spot welding (FSSW), are essential. FSSW is a novel solid-state process that has recently received considerable attention from various industries, including automotive sectors, due to the many advantages over resistance spot welding and riveting [28–30]. A typical FSSW set up consists of a rotating tool with a probe

Fig. 4. a) Stress–strain curves of the BM-TWIP steel, b) IPF coloring OIM map of the base material with the high angle grain boundaries (HAGBs) N 15° is superimposed in black lines. White areas are non-indexed due to the particles. Triangle coloring key is indicated on the map.

Fig. 2. Scheme and macrographs of the tool geometry used in the friction spot stir welding of the TWIP steel showing: a) the dimensions in mm, b) tool before FSSW, and c) after FSSW.

Fig. 3. FSSWed specimen with two backing slices to ensure the axial loading during the tensile–shear test.

380


Fig. 5. Macrographs of the top surface of the FSSWed TWIP steel joints at a) 500 rpm, b) 750 rpm, and c) 1000 rpm.

that is plunged into two overlapped sheets of metal to be joined. The rotational speed and downward force of the tool generates localized heat as the tool interacts with the sheets. The frictional heat generated by the rotation tool softens the materials around the tool and forms a solid state joint between upper and lower sheets [31]. The FSSW parameters, such as the tool rotation rate, tool shoulder plunge depth, and dwell time, determine the heat generation, joint formation, and mechanical properties [32–35]. Therefore, to apply this technique, the process parameters should be optimized to obtain improved mechanical properties compared to resistance spot welding [36,37]. FSSW has been applied in joining of galvanized steel [38], low carbon steel [31,39], mild steel [40] advanced high-strength steels [41], DP600 steel [42], and TRIP steels [43,44]. Mazzaferro et al. [44] reported that the strength of the welds depended on a complex interaction between a number of geometrical features and process parameters. The highest joint strength was observed for the lowest tool rotational speed and the highest dwell time combination of welding parameters. Therefore, it is essential to understand the effect of the FSSW process parameters, such as rotational speed, dwell time, and plunge depth, on the weld quality characteristics [45,46]. The present paper focuses on the microstructure evolution and the mechanical properties of friction stir spot welded TWIP steel. 2. Experimental procedure The material used in this study was High Mn TWIP steel (nominal composition 0.6C–22Mn–0.3%V (in wt.%)), produced by ArcelorMittal, Metz, France, received in the form of 50% cold rolled 1.5 mm thick sheet. The as-received material was then subjected to heat treatment under argon atmosphere as outlined in Fig. 1. Tensile properties of this base material (BM) were then measured with a tensile specimen of gauge length of 120 mm and cross-sectional area of 20 width mm

1.5 mm thickness at strain rate of 5 × 10−4 s−1, using a Zwick Z 100 universal tensile testing machine equipped with a high resolution extensometer. FSSW was used to manufacture lap shear joints from 30 mm wide by 100 mm long plates with a 30 mm overlap. The tool was tungsten carbide (WC), Fig. 2, with 20 mm shoulder diameter, 5 mm pin diameter, and 2.8 mm pin height. Welding was performed at rotation rates of 500, 750, and 1000 rpm with a constant plunge rate of 0.05 mm/s and dwell time of 10 s. Note, the WC tool exhibited no wear and high dimension stability after each FSSW weld. The as-welded joints were characterized using optical microscopy (OM), scanning electron microscopy (SEM), tensile shear testing (three samples for each FSSW condition), and Vickers microhardness testing. For the OM and SEM investigations, the transverse cross section of the joints were prepared according to the standard grinding and polishing procedures and then etched using a nital solution. Microstructure of the base and welded material was also characterized using electron back scattered diffraction (EBSD) in a Quanta FEG 250 SEM equipped with Hikari EDAX-EBSD camera controlled by orientation image microscopy data collection software (OIM DC 7.2). For EBSD, the samples were mechanically polished and further electropolished for 30 s at a temperature of 15 °C. The electrolyte solution contained 5 vol.% of perchloric acid, 15 vol.% of acetic acid, and 80 vol.% ethanol. OIM data collection was carried out for an area of 400 μm × 185 μm in the FSSWed zone using 0.5 μm step size and an area of 100 μm × 50 μm in the BM using 0.1 μm step size, all data were collected at 20 kV acceleration voltage and a working distance of 15 mm. Then OIM data is processed using TSL OIM analysis 7.2 software. The lap-shear tensile tests were conducted at room temperature on a computerized universal testing machine (Instron model 4208 of 300 kN) at a constant crosshead speed of 5 × 10−2 s−1. Two packing slices were adhesively joined to the weld specimen to ensure the axial loading of the test specimen, as shown in Fig. 3. The Vickers microhardness measurements were performed on a grid at 0.5 mm spacing in x and y directions. Four rows in the y direction were performed, which allowed for microhardness map plotting.

3. Results and discussion 3.1. BM-TWIP steel characterization The tensile properties of the BM show the well-known TWIP steel combination of high strength and elongation with a 0.2% offset yield strength of 516 MPa, ultimate tensile strength of 1170 MPa and elongation to failure of 72%. The microstructure of the BM was fully austenitic, Fig. 4b, with an average grain size of 3 μm, as measured using the linearintercept method (ASTM E112). It should be noted, however, that there was a reasonably large range of grain sizes with grains of 10 μm diameter clearly identifiable.

Fig. 6. Optical macrographs of transverse cross sect a) 500 rpm, b) 750 rpm, and c) 1000 rpm. Dashed lines show the HAZ width. ND is the normal direction, TD is the transverse direction and WD is the welding direction of FSW axes.


381

Fig. 7. Optical microstructures of the FSSWed TWIP steel at rotation speed 750 rpm for different regions: (a) and (b) at the top adjacent to the surface of the tool probe, (c) and (d) underneath the tool probe.

3.2. FSSWed joints macro and microstructures The macro features of the FSSWed joint are given in Figs. 5, 6 and 7 for the top view, transverse cross sections and a magnified macrograph for the 750 rpm joint, respectively. Fig. 5 shows the top view of the joints welded at the different rotation speeds. The top view clearly shows defect-free joints with a clear heat effect on the surface appearance around the spot, which increases in size with increasing rotation rate. Fig. 6 shows optical macrographs of the transverse cross section of the joints. It can be observed that the interface between the overlapped sheets is moved upward due to the complex plasticized material flow produced by the rotation of the tool. In a study of material flow during FSSW [47], it has been reported that a three distinct regions are developed in the weld region after the rotating shoulder comes in contact with the upper sheet. They are the flow transition zone under the tool shoulder, the stir zone around the probe, and the torsion zone under the tool probe. These three zones can be clearly observed in the macrographs presented in Fig. 6. Furthermore, it can be observed that the heat affected zone (HAZ) width increases with increasing rotation speed, as indicated by the superimposed dashed lines. A good approximation to the energy input into the weld is presented in Eq. (1) [48]. For each weld condition the calculated energies are presented in Table 1. As expected, the energy input increased with increasing the rotation rate, which can be considered the main contributor in the observed HAZ width increase [48]

Power ¼ τ

2πr 60

ð1Þ

where, τ and r are the FSSW tool torque (Nm) and rotation speed (rpm), respectively. To clearly identify the three distinct regions in the weld area, a magnified macrograph of the joint produced at 750 rpm is presented in Fig. 7, with associated zoomed in areas (a, b, c and d) at the positions indicated in the macrograph. Fig. 7a and b show the stir zone that formed around the tool probe and Fig 7c and d shows the torsion zone that formed underneath the tool probe. It can be observed that both zones are very thin, around 200 μm thick and have dynamically recrystallized grain structure. Also, it can be noted that the stir zone is almost continuous around the tool probe of the same thickness, while the torsion zone has irregular thickness. Fig. 8 shows the optical microstructure at the stir zone (Fig. 8a) and the torsion zone with their nearest HAZ parts (Fig. 8b, c) for the joint produced at 1000 rpm rotation speed. This shows the typical observation for all welds that both the stir zone and the torsion zone has a new modified grain structure, whereas the HAZ has only appeared to coarsen from the BM. Also, it can be observed that the stir zone is almost homogeneous in terms of the grain structure across the whole joint, however, the grain structure in the torsion zone is inhomogeneous and consists of both fine and coarse grain structures, as can be seen by Table 1 Torque, power, and energy values for FSSW conditions at 10 s dwell time. Rotation rate, rpm

Torque, Nm

Power, J/s

Energy, J

500 750 1000

85 63 59

4448 4945 6175

44,480 49,450 61,750

382


Fig. 8. Optical micrographs at different zones of FSSWed TWIP steel at 1000 rpm. a) adjacent to the tool probe and b, c) under the tool.

Fig. 9. Optical microstructures on the cross-sections of FSSW joints welded at different rotation speeds: a) 500 rpm under the shoulder, b) 500 rpm in HAZ near the stir zone, c) 750 rpm under the shoulder, d) 750 rpm in HAZ near the stir zone, e) 1000 rpm under the shoulder, and f) 1000 rpm in HAZ near the stir zone.


383

Fig. 10. Bond ligament dimensions of FSSWed TWIP steel at a) 500 rpm, b) 750 rpm, and c) 1000 rpm.

Fig. 11. SEM micrographs of FSSWed TWIP steel a) at the stir zone adjacent to the FSSW tool, b) at the torsion zone below the FSSW tool, and c) IPF coloring OIM map relative to ND in the stir zone of the TWIP steel FSSWed at 750 rpm with the HAGBs N 15° is superimposed in black lines. Triangle coloring key is shown in Fig. 4b.

Fig. 12. SEM micrographs of FSSWed TWIP steel after electropolishing a) at the stir zone and b) HAZ.

384


Fig. 13. Vickers microhardness distribution maps across the transverse cross section of the FSSW joints a) 500 rpm, b) 750 rpm, and c) 1000 rpm. Lowest hardness indicated with arrows.

the relatively fine grains in Fig. 8b and the relatively coarse grains in Fig. 8c. This can be attributed to non-uniformity in the amount of torsion deformation experienced under the tool probe due to some geometrical effects. Fig. 9 shows the optical microstructure of the three joints at the flow transition zone under the shoulder and at the HAZ near the stir zone. The grain structure in the flow transition zones (Fig. 9a, c and e) is relatively coarse due to the high temperature and high amount of

Fig. 14. Tensile shear testing results of FSSWed TWIP steel. a) Load extension curves and b) variation of peak load and extension with rotation rate.

strain induced by the shoulder. It can be observed that twins starts to appear at 1000 rpm rotation rate (Fig. 9e). The grain structure in the HAZ near the stir zone (Fig. 9b, d and f) is also relatively coarse grain structure but some twins start to appear at 750 rpm rotation speed (Fig. 9d) and their density increased at 1000 rpm (Fig. 9f). SEM and EBSD investigations of the microstructure features of the joints are presented in Figs. 10–12. The bond ligament, using SEM micrographs (Fig. 10), was approximately 550 μm in both FSSW joints at 500 rpm and 1000 rpm, while it was only about 180 μm in the welded joint at 750 rpm. Fig. 11a and b shows the thin layer fine grain structure in the stir zone and the torsion zone below the FSSW tool, as well as grain coarsening within the HAZ. The current observation in the HAZ is in agreement with observation of Razmpoosh et al. [2] in their study of resistance spot welding of TWIP steel. An EBSD map at a region similar to that shown in Fig. 11a is shown in Fig. 11c. The map shows a relatively fine grain structure in the very thin layer adjacent to the FSSW tool (top left corner of the map) and then significant grain coarsening in the HAZ. This coarsening is mainly due to the high thermal cycle experienced during FSSW, which caused dissolution and/or coarsening of the vanadium carbide (VC) second phase particles, reducing their grain pinning effect. In comparison to the BM grain size that is about 3 μm, the grain size observed in the HAZ is significantly greater. However, the grain size in both the stir zone and the torsion zone is also relatively coarse. These relatively coarse grains observed in the stir zone and the torsion zone have formed due to the severe plastic deformation at high temperature experienced in both zones, which is expected to cause dynamic recrystallization. Hajian et al. [49] in their study of microstructure evolution of friction stir processed AISI 316L austenitic stainless steel suggested that the microstructure in the stir zone was mainly dominated by discontinuous dynamic recrystallization and also found some evidences of continuous dynamic recrystallization and partial static recrystallization in the stir zone. However, because significant coarsening is observed in the highest temperature regions of the HAZ in this work, the key mechanism for grain coarsening is suggested to be due to dissolution or coarsening of the second phase precipitates. To investigate the coarsening and dissolution of the VC particles, the joints transverse cross section was examined using SEM after electropolishing process, which only attacks the base material and leaves the particles behind. Fig. 12 a and b shows the distribution of the second phase particles in the stir zone and in the HAZ respectively. It can be observed that


385

Fig. 15. Optical macrographs of the top (left) and bottom (right) sheets of tensile shear over all fracture surface appearance of FSSWed TWIP steel a) 500 rpm, b) 750 rpm, and c) 1000 rpm.

the stir zone is almost free from particles, which indicates dissolution has occurred whilst the HAZ has high density of coarse particles, indicating particle coarsening has occurred. It has been reported [50] that precipitation hardening of high-Mn TWIP steel using vanadium carbide occurs mainly by precipitates smaller than 30 nm formed in recrystallized grains whilst larger precipitates formed in non-recrystallized grains do not contribute much in hardening. Also, it has been reported [51] that the dissolution temperature of the VC precipitates is around 950 °C. Accordingly it can be said that both the stir zone and HAZ are almost completely free from the nano size precipitation hardening VC particles due to high thermal cycle experienced upon FSSW, which can result in a temperature higher than 1000 °C in the vicinity of the tool. Both the dissolution and the coarsening allow the grain coarsening to occur in the stir zone and HAZ according to the temperature experienced. 3.3. Mechanical properties The mechanical properties of FSSWed TWIP steel were evaluated using Vickers microhardness testing and tensile shear testing. The Vickers microhardness distributions in 2D maps are shown in Fig. 13. The hardness measurements were carried out in the transverse cross section of the joint measured at steps of 0.5 mm in x and y directions. It can be seen that the BM has the highest hardness of 300 HV due to the fine grain structure and the presence of VC precipitates. The hardness then decreases towards the stir zone, with the lowest hardness in the HAZ, similar to the behavior of friction stir welded precipitation

hardened aluminum alloys [52–54]. Also, it can be observed that with increasing tool rotation from 500 to 1000 rpm, the hardness of the HAZ decreases from 220 HV to 200 HV, respectively (blue region in the map). Thus it can be said that the softening in the HAZ and the stir zone is mainly due to the loss of the main strengthening mechanisms operating in the BM, namely; second phase precipitates and grainrefining mechanisms. It is well known that with increasing the tool rotation rate, the heat generation in the weld zone increases and this is agreed with the energy calculation using Eq. (1) above. Hence, increasing the rotation rate from 500 rpm to 1000 rpm results in a significant softening and increases the HAZ width, as shown in Fig.13. Due to the dissolution and/or coarsening of the nano size VC precipitates [50] as well as the significant grain coarsening in the HAZ, which resulted in grain size around 20 μm in the HAZ and slightly smaller in stir zone, this hardness behavior is not surprising. It has been reported that the mechanical properties of the TWIP steel is decreased by increasing the grain size [19] and also by the loss of precipitation hardening precipitates [50,51]. Fig. 14a shows the load-extension curves for the three joints produced at different rotation rates, and Fig. 14b shows the variation of peak load and extension with the rotation rate. As mentioned above, the joints were completely free of any type of defects and this is clearly observed in the tensile shear results. The three joints showed a considerable amount of extension before failure, varying from 3 to 4 mm resulting in peak load variations from about 13 kN to about 7 kN, respectively. It should be noted here that the max peak load occurred at 750 rpm rotation rate and the max extension obtained at 500 rpm

386


Fig. 16. SEM micrographs of tensile shear fracture surface appearance of FSSWed TWIP steel a) 500 rpm, b) 750 rpm, and c) 1000 rpm.

rotation rate. This result implies that the peak load is not significantly dependent on the bond ligament width, as the joints produced at 500 and 1000 rpm have about 550 μm bond ligament width and that produced at 750 rpm has only 180 μm bond ligament width. In comparison with the of tensile shear results obtained by Razmpoosh et al. [2] in their study of resistance spot welding of TWIP steel, FSSW has resulted in higher integrity joints, free of defects at all conditions, and with higher peak loads. Examples of both the bottom and top sheets after tensile shear failure showing the overall fracture locations of the three joints are illustrated in Fig. 15. As clearly seen, the failure looks like a ductile failure separation of the two sheets, which indicates a strong joint has been formed between the two sheets. It is also clear that failure occurs in the HAZ, which is consistent with the significant grain growth observations and hardness reductions in the HAZ. To further investigate the fracture surfaces SEM was used to examine the failure surfaces, Fig. 16. All fracture surfaces have a ductile dimpled fracture confirming the absence of embrittlement in these weld joints at the investigated ranges of rotation speeds. It has been reported that [19] the yield strength of high Mn TWIP steel decreases with increasing grain size, for example from 600 MPa at 3 μm grain size to about 350 at 20 μm grain size. The size of the grains observed in the HAZ is around 20 μm and higher, which means the yield strength is decreased in the HAZ allowing for ductile fracture to occur. The presence of a relatively coarse and soft austenite grain structures in the HAZ eliminate any change in the failure mechanism with change in the rotation speeds.

4. Conclusions Macrostructures, microstructures and fracture surfaces of friction stir spot welded high Mn TWIP steel have been investigated using OM, SEM and EBSD. Microhardness and tensile-shear behavior were also investigated. The main conclusions are as follows: • TWIP steel sheets that were FSSWed at different rotational speeds of 500, 750, and 1000 rpm exhibited defect-free joints with a clear heat effect on the surface appearance around the spot region that extended into a large area with the increase of the rotational speed from 500 to 1000 rpm. • Three distinct microstructure regions are developed in the weld region. They are the flow transition zone under the tool shoulder, the stir zone around the probe, and the torsion zone under the tool probe. Furthermore, the HAZ width of TWIP steel weldments increases with the increase of rotational speed form 500 to 1000 rpm as a result of increasing the energy input from 44.48 to 61.75 kJ. • WC tool exhibits no wear and high dimension stability after FSSW experiments of TWIP steel sheets. • In general, microhardness values of FSSWed TWIP steel weld zone are lower than the microhardness values of the base TWIP steel and with increasing tool rotation speed from 500 to 1000 rpm, the hardness of the weld zone decreases and HAZ width increases. • For all the weldments, a considerable amount of extension ranging from 3 to 4 mm before failure was observed. A maximum peak tensile


shear load of 13 kN was obtained for a weld made at a rotation speed of 750 rpm whereas the maximum extension of 4 mm was reached for a weld made at a rotation speed of 500 rpm. • The fracture surfaces of all joints have a ductile dimpled fracture confirming the absence of embrittlement in welded joints.

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