Microstructural and mechanical characterization of a Ti-containing TWIP steel welded joint through GTAW process

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/srin.202000129. This article is protected by copyright. All rights reserved Microstructural and mechanical characterization of the Ti-containing TWIP steel welded joint produced by GTAW process H. Hernández-Belmontesa, I. Mejíaa*, J.A. Benitob, J.M. Cabrera a,b a Instituto de Investigación en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Hidalgo, Edificio “U-3”, Ciudad Universitaria, 58030-Morelia, Michoacán, México. b Department of Materials Science and Engineering EEBE, Universidad Politecnica de Catalunya, c/Eduard Maristany 10-14, 08019-Barcelona, Spain. * Corresponding author. Tel.: +52 (443) 322 3500 Ext: 4009; Fax: +52 (443) 322 3500 Ext: 4010. E-mail address: [email protected], i.mejia.g[email protected] Accepted Article steel welded joint produced by Accepted Article steel welded joint produced by H. Hernández Accepted Article H. Hernández Instituto de Investigación en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Accepted Article Instituto de Investigación en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Hidalgo, Edificio “U Accepted Article Hidalgo, Edificio “U Accepted Article Department of Materials Science and Engineering EEB Accepted Article Department of Materials Science and Engineering EEB c/Eduard Maristany 10 Accepted Article c/Eduard Maristany 10 This article is protected by copyright. All rights reserved Abstract The aim of this work is to study the microstructural and mechanical changes generated in a welded joint of Ti-containinig TWIP steel by Gas Tungsten Arc Welding (GTAW) process. For this purpose the welding parameters that allowed the union of 5.6 mm thick butt plates with full penetration and without filler material were investigated. Microstructural changes were examined by light optical metallography. Segregation and second-phase precipitated particles were investigated by scanning electron microscopy and electron-dispersive spectroscopy. Phase transformations were evaluated using X-ray diffraction. Finally, the mechanical behavior was assessed by Vickers microhardness and micro-tensile tests. In general, the welded joint showed dendritic structure formation in the fusion zone (FZ) and equiaxed grain growth in the heat affected zone (HAZ). The FZ showed high degree of segregation, where Mn, Si and C segregated in the interdendritic regions, while Al preferentially segregated in dendritic areas. On the other hand, no effect on the austenite stability was noticed. The welded joint shows an increase in microhardness, which is associated with the formation of precipitated particles. Finally, the micro-tensile results showed a decrease in UTS in the FZ, while the welding interface and HAZ showed an increase in UTS compared to the base material. Keywords: Twinning induced plasticity (TWIP) steel; Ti microalloying element; Welding; Heat input; Microstructure. 1. Introduction One of the main objetives of the automotive industry is to improve vehicle safety providing larger space, enhanced performance, better reliability and comfort. As a result, the weight in the vehicles has increased, which in turns increases CO2 emissions to the environment. Accordingly weight reduction issue is a priority in the automobile manufacturing [1]. Consequently, the automotive industry is interested in the application of high-strength steels and/or light-weight alloys [2]. Technological advances regarding these new steels present advantages compared to conventional ferritic-pearlitic steels. Nowadays, fully austenitic steels or austenite-containing multi-phase steels, known as Advanced High-Strength Steels (AHSS) are developed and introduced in the body-inwhite car structure. These steels present an optimal combination of strength and formability [3] among which Twinning-Induced Plasticity (TWIP) steels are one of the most representative and promising ones. TWIP steels present high-Mn content and small additions of alloying elements such as Al, Si and C, allowing an austenitic microstructure at room temperature. These steels are characterized by low stacking fault energy (SFE) that allows twinning as a deformation controlling mechanism (the so-called TWIP effect). This mechanism promotes an excellent work-hardening Accepted Article t optical metallography Accepted Article t optical metallography investigated by scanning electron microscopy and e Accepted Article investigated by scanning electron microscopy and e transformations were ev Accepted Article transformations were ev assessed by Vickers microhardness and Accepted Article assessed by Vickers microhardness and dendritic structure formation in the fusion zone (FZ) and equiaxed grain growth in the heat affected Accepted Article dendritic structure formation in the fusion zone (FZ) and equiaxed grain growth in the heat affected zone (HAZ). The FZ showed high degree of segregation Accepted Article zone (HAZ). The FZ showed high degree of segregation interdendritic regions, while Al preferentially segregated in dendritic areas. On the other hand, Accepted Article interdendritic regions, while Al preferentially segregated in dendritic areas. On the other hand, effect on the austenite stability was noticed Accepted Article effect on the austenite stability was noticed which is associa Accepted Article which is associa ted with the formation of precipitated partic Accepted Article ted with the formation of precipitated partic showed a decrease in Accepted Article showed a decrease in UTS Accepted Article UTS UTS compared to the base material. Accepted Article UTS compared to the base material. T Accepted Article T winning induced plasticity Accepted Article winning induced plasticity icrostructure. Accepted Article icrostructure. Accepted Article Introduction Accepted Article Introduction One of the main objetives of Accepted Article One of the main objetives of enhanced Accepted Article enhanced performance, Accepted Article performance, has increased, which Accepted Article has increased, which in turns Accepted Article in turns issue is a priority Accepted Article issue is a priority industry is interested in the application of high Accepted Article industry is interested in the application of high Accepted Article Technological advan Accepted Article Technological advan ces regarding Accepted Article ces regarding pearlitic steels. Nowadays Accepted Article pearlitic steels. Nowadays known as Advanced High Accepted Article known as Advanced High This article is protected by copyright. All rights reserved rate as a result of the generation of deformation twins, which gradually reduce the effective sliding distance of the dislocations, resulting in a sort of dynamic Hall-Petch effect [1-4], combining tensile strength up to 1100 MPa and elongation up to 90% simultaneously [5]. Currently TWIP steels are designed with the following chemical contents: 15-25 wt% Mn, 0-3 wt% Si, 0-3 wt% Al, 0-1 wt% C and eventually secondary additions of Cr, Cu, N, Nb, Ti and/or V [3]. However, the industrial applications of TWIP steels has been limited due to, among other facts, the lack of knowledge on their welding behavior. Indeed preliminary results reports indicate that the high Mn and C content generate undesirable effects in the different welding zones [6-10]. Since good weldability is required to guarantee the use of the TWIP steels in industrial applications, these undesirable effects should be considered in order to take benefit of the high potential of TWIP steels in the automobile structures [8]. The most important problems reported with welding of TWIP steels are related to segregation, dendritic grain formation in the fusion zone (FZ), hot cracking, reduction of mechanical properties and occasionally phase transformations [7, 9, 10]. The latter can be associated to the high degree of segregation during melting, which in turns enriches the liquid phase with Mn and C, generating instability of the austenitic phase leading to unexpected phase transformations, such as eutectic (Fe,Mn)3C cementite, (γ-M3C) eutectic phase, δ/ α-ferrite or εmartensite [4, 11]. Different authors have also studied the embrittlement in Zn-coated TWIP steels, e.g. Ashiri et al. [8] reported that TWIP steels are susceptible to liquid metal embrittlement (LME) when they are subjected to the resistance spot welding process. Saha et al. [12] reported that the cracks are located in the heat affected zone (HAZ) adjacent to the fusion line and fully filled with solidified liquid metal from the weld nugget. Therefore, the presence of a low melting liquid film along grain boundaries generates lack of plastic compatibility between adjacent grains enhacing the crack appearance. On the other hand, the reduction of mechanical properties in the FZ and HAZ is also generated by microstructural changes as a consequence of the heat input in the welding processes. These changes are due to formation of dendritic grains in FZ and equiaxed grains growth in the HAZ [6]. In consequence one way to minimize these problems is the use of low heat input welding processes. Because TWIP steels in the annealed condition display relatively low yield stresses, microalloying elements, carbide and nitride formers, can be added. These elements influence the microstructure and mechanical properties by solid solution and the formation of precipitated phases. ReyesCalderon et al. [13] studied the effect of microalloying elements such as Nb, V and Ti on the hot flow behavior of TWIP steel. They determined that their addition generates a slight increase in the flow stresses at high temperature being the Ti microalloyed TWIP steel the one exhibiting the highest stress values. They also found that additions of V and Ti in TWIP steels promoted grain refinement. It is very important to keep in mind that microalloying particles can pin the grain Accepted Article applications of TWIP steels has been limited due to, among other facts, the lack of knowledge on Accepted Article applications of TWIP steels has been limited due to, among other facts, the lack of knowledge on their welding behavior. Indeed preliminary resul Accepted Article their welding behavior. Indeed preliminary resul generate undesirable effects in the different welding zones Accepted Article generate undesirable effects in the different welding zones required to guarantee the use of the TWIP steels in industrial applications, these undesirable effects Accepted Article required to guarantee the use of the TWIP steels in industrial applications, these undesirable effects should be considered in order to take benefit of the high potential of TWIP steels in the automobile Accepted Article should be considered in order to take benefit of the high potential of TWIP steels in the automobile ] Accepted Article ] . Accepted Article . The most important problems reported with welding of TWIP steels are related to Accepted Article The most important problems reported with welding of TWIP steels are related to segregation, dendri Accepted Article segregation, dendri tic grain formation in the fusion zone (FZ), hot cracking, reduction of Accepted Article tic grain formation in the fusion zone (FZ), hot cracking, reduction of mechanical properties and occasionally phase transformations Accepted Article mechanical properties and occasionally phase transformations associated to the high degree of segregation during melting, which in turns enriches the liquid phase Accepted Article associated to the high degree of segregation during melting, which in turns enriches the liquid phase with Mn and C, generating instability of the austenitic phase leading to unexpected phase Accepted Article with Mn and C, generating instability of the austenitic phase leading to unexpected phase transformations, such as eutectic (Fe, Accepted Article transformations, such as eutectic (Fe, 4 Accepted Article 4 , Accepted Article , 11 Accepted Article 11 ] Accepted Article ] . Accepted Article . Different authors have also studied the embrittlement in Zn Accepted Article Different authors have also studied the embrittlement in Zn e.g. Ashiri et al. Accepted Article e.g. Ashiri et al. [ Accepted Article [ 8 Accepted Article 8 ] Accepted Article ] reported that TWIP steels are susceptible to liquid metal embrittlement (LME) Accepted Article reported that TWIP steels are susceptible to liquid metal embrittlement (LME) when they are subjected to the resistance spot welding process. Saha et al. Accepted Article when they are subjected to the resistance spot welding process. Saha et al. cracks are located in the heat affected zone (HAZ) adjacent to the fusion line and fully filled with Accepted Article cracks are located in the heat affected zone (HAZ) adjacent to the fusion line and fully filled with solidified liquid metal from the weld nugget. Therefore, the presence of a low melting liquid film Accepted Article solidified liquid metal from the weld nugget. Therefore, the presence of a low melting liquid film along grain boundaries generates lack Accepted Article along grain boundaries generates lack crack appearance. On the other hand, the reduction of mechanical properties in the FZ and HAZ is Accepted Article crack appearance. On the other hand, the reduction of mechanical properties in the FZ and HAZ is also generated by microstructural changes as a consequence of the heat input in the welding Accepted Article also generated by microstructural changes as a consequence of the heat input in the welding sses. These changes are due to formation of dendritic grains in FZ and equiaxed grains growth Accepted Article sses. These changes are due to formation of dendritic grains in FZ and equiaxed grains growth Accepted Article [ Accepted Article [ 6 Accepted Article 6 ] Accepted Article ] . Accepted Article . In consequence Accepted Article In consequence welding processes. Accepted Article welding processes. Because TWIP steels in the annealed Accepted Article Because TWIP steels in the annealed elements, carbide and nitride formers, can be added. These elements influence the Accepted Article elements, carbide and nitride formers, can be added. These elements influence the This article is protected by copyright. All rights reserved boundaries and avoid grain growth, but these particles can be dissolved at high temperatures, allowing then grain growth. This large grain size promotes in turns a delay on the onset of dynamic recrystallization (DRX), a softening mechanism taking place at high temperatures which is also a grain refiner [14]. Research works on microalloyed TWIP steels welding are very scarce. It is for this reason that the present work aims primarily to study the microstructure and mechanical properties of a Ti microalloyed TWIP (TWIP-Ti) steel welded by the gas tungsten arc (GTAW) process. 2. Experimental details 2.1. Base material The present Fe-22Mn-1.8Al-1.2Si-0.57C-0.014N-0.022Ti (wt%) Ti-containing TWIP steel was melted in the Foundry Lab of the Metallurgical Research Institute-UMSNH (Mexico) using high purity raw materials, in a 25 kg capacity induction furnace. Rectangular specimens were extracted from the ingots and homogenized at 1200 ºC and then hot-rolled to 60 % thickness reduction. They were slow cooled down to room temperature. Additionally, specimens were reheated at 1200 ºC and hot-rolled to 50 % thickness reduction, followed by solution heat treatment at 1200 ºC and then water quenched. The heat and thermomechanical treatments were performed in order to generate a microstructural conditioning of the steel, modifying the dendritic structure of the solidification, and obtaining a refined equiaxial austenitic grain microstructure. Mechanical properties of the studied TWIP-Ti steel in the as-solutioned condition were yield strength (YS) of 380 MPa, ultimate tensile strength (UTS) of 739 MPa, elongation at fracture of 78% and Vickers microhardness of 395 ± 19.4 HV0.5. 2.2. Welding Parameters The dimensions of the pair of plates for weldability test were 85 mm in length, 45 mm in width and 5.6 mm in thickness. The welding joint was carried out using the Gas Tungsten Arc Welding (GTAW) semi-automatic process without filler material and butt joint. In this case, direct current with reversed polarity, arc-length of 1.5 mm, tungsten electrode EWTh-1 of 1.587 mm diameter and Ar as shielding gas were used. The operating variables and heat input used in the weld joint are shown in Table 1. Equation 1 [15] was use to calculate the heat input: 𝑄 = �� � 𝑓…………………………………………….(1) Accepted Article properties of a Ti microalloyed Accepted Article properties of a Ti microalloyed 2. Experimental d Accepted Article 2. Experimental d etails Accepted Article etails aterial Accepted Article aterial Fe Accepted Article Fe - Accepted Article - 22Mn Accepted Article 22Mn - Accepted Article - 1.8Al Accepted Article 1.8Al melted in the Foundry Lab of the Metallurgical Research Institute Accepted Article melted in the Foundry Lab of the Metallurgical Research Institute purity raw materials, in a 25 kg capacity induction furnace. Rectangular specimens were Accepted Article purity raw materials, in a 25 kg capacity induction furnace. Rectangular specimens were from the ingots and homogenized at 1200 ºC and Accepted Article from the ingots and homogenized at 1200 ºC and were slow cooled down to room temperature. Additionally, specimens were reheated at 1200 ºC and Accepted Article were slow cooled down to room temperature. Additionally, specimens were reheated at 1200 ºC and rolled to 50 % thickness reduction, followed by s Accepted Article rolled to 50 % thickness reduction, followed by s water quenched. Accepted Article water quenched. The heat Accepted Article The heat microstructural conditioning Accepted Article microstructural conditioning obtaining a refined equiaxial austenitic grain microstructure. Accepted Article obtaining a refined equiaxial austenitic grain microstructure. Ti steel in the as Accepted Article Ti steel in the as - Accepted Article - solutioned condition were yield Accepted Article solutioned condition were yield (UTS) Accepted Article (UTS) of 739 MPa, elongation at fracture of Accepted Article of 739 MPa, elongation at fracture of 2.2. Welding Parameters Accepted Article 2.2. Welding Parameters The dimensions of the Accepted Article The dimensions of the pair of plates Accepted Article pair of plates 5.6 mm in thickness. The welding joint Accepted Article 5.6 mm in thickness. The welding joint semi Accepted Article semi - Accepted Article - automatic Accepted Article automatic with reversed polarity, arc Accepted Article with reversed polarity, arc Ar as shieldi Accepted Article Ar as shieldi ng gas were used. The operating variables and heat input used in the weld joint are Accepted Article ng gas were used. The operating variables and heat input used in the weld joint are This article is protected by copyright. All rights reserved where Q is the heat input (kJ/mm), V is the voltage (V), I is the electric current (A), v is the displacement speed (mm/s) and f is the electric arc efficiency. Table 1. Operating variables used in the GTAW process of the TWIP-Ti steel. Electric current (A) Voltage (V) Displacement speed (mm/s) Heat input (kJ/mm) 91 10.92 1.16 0.574 Additionally, the temperature distribution through the HAZ was measured at 7, 27, 37 and 42 mm from the welding interface by means of K-type thermocouples. 2.3. Characterization in as-solutioned and post-welding condition The microstructural analysis of the TWIP-Ti steel in as-solutioned and post-welding condition was carried out using light optical microscopy (LOM). For this purpose, samples were prepared through chemical etching, where the interest areas were revealed by using nital 4% for 5 s and a solution of Na2S2O5 (2 g) in 100 ml of distilled water for 15 s. The average grain size in the HAZ was measured using the Sigma Scan Pro 8 software. Segregation and precipitated particles in the welded joint was determined using point and elemental mapping chemical analysis by Electron Dispersive Spectroscopy (EDS) using a Scanning electron microscope (SEM) JEOL JSM-7600F. Phase transformations were evaluated by X-ray diffraction (XRD) using Cu-K radiation (λ= 1.5402 Å) in a D5000 Siemens diffractometer with the following operating conditions: 2θ range from 20 to 120°, step of 0.02° and time of 1 s per step. Vickers microhardness tests in as-solutioned and post-welding condition were performed using a Wilson microhardness tester with load of 500 g, by indentations along the cross section of the weld bead, in the middle of the sample thickness. The distance between indentations was of 250 µm in the FZ, 200 µm in the HAZ and 300 µm in the base material. Micro-tensile tests were also carried out in the as-solutioned and post-welding condition (particularly in the FZ, close to the welding interface (WI) and in the HAZ areas) with specimens of the geometry and dimensions shown in Figure 1. The specimens were prepared by wire electrical discharge machining (EDM). The micro-tensile tests were conducted in a Microtest DEBEN testing machine at a constant speed of 2 mm/min. Accepted Article Accepted Article Accepted Article Accepted Article Accepted Article Additionally, the temperature distribution through the HAZ was measured at 7, 27, 37 and 42 mm Accepted Article Additionally, the temperature distribution through the HAZ was measured at 7, 27, 37 and 42 mm from the welding interface by means of K Accepted Article from the welding interface by means of K 2.3. Characterization in as Accepted Article 2.3. Characterization in as The microstructural analysis of the TWIP Accepted Article The microstructural analysis of the TWIP using light optical microscopy (LOM). For this purpose, samples were prepared through Accepted Article using light optical microscopy (LOM). For this purpose, samples were prepared through etching Accepted Article etching , where the inte Accepted Article , where the inte (2 g) in Accepted Article (2 g) in 100 ml of Accepted Article 100 ml of measured using the Sigma Scan Pro 8 software. Segregation and preci Accepted Article measured using the Sigma Scan Pro 8 software. Segregation and preci joint was determined using point and elemental mapping chemical analysis by Electron Dispersive Accepted Article joint was determined using point and elemental mapping chemical analysis by Electron Dispersive Spectroscopy (EDS) using a Scanning electron microscope (SEM) JEOL JSM Accepted Article Spectroscopy (EDS) using a Scanning electron microscope (SEM) JEOL JSM Accepted Article transformations were evaluated by X Accepted Article transformations were evaluated by X a D5000 Siemens diffractometer with the following operating conditions: 2θ range from 20 to 120°, Accepted Article a D5000 Siemens diffractometer with the following operating conditions: 2θ range from 20 to 120°, step of 0.02° an Accepted Article step of 0.02° an d time of 1 s Accepted Article d time of 1 s Vickers microhardness tests in as Accepted Article Vickers microhardness tests in as microhardness tester Accepted Article microhardness tester bead, in the middle of the sample thickness. The distance between Accepted Article bead, in the middle of the sample thickness. The distance between the FZ, 200 µm in the Accepted Article the FZ, 200 µm in the Accepted Article HAZ Accepted Article HAZ tensile tests were also carried out in the as Accepted Article tensile tests were also carried out in the as particularly in the Accepted Article particularly in the FZ, Accepted Article FZ, Accepted Article close to Accepted Article close to the geometry and dimensions shown in Accepted Article the geometry and dimensions shown in discharge machining (EDM). The Accepted Article discharge machining (EDM). The This article is protected by copyright. All rights reserved Figure 1. a) Geometry and dimensions of the specimens used for the micro-tensile test. b) Specimens extraction location. 3. Results and discussion 3.1. Base material microstructure The microstructure of the TWIP-Ti steel in the as-solutioned condition consists of fully austenite equiaxial grains with annealing twins (see Figure 2a). The average grain size was 64 ± 35 μm. A heterogeneous distribution can be observed as grain sizes range from 15 to 286 µm, which could be associated with the presence of abnormal grains. The statistical analysis showed that grain size has a log-normal distribution, as shown in Figure 2b, with a marked right hand tail as consequence of the latter abnormal grains. These heterogeneous grain size distribution can be associated with the interference between the recrystallization (annealing) kinetics and the precipitation dissolution of the microalloying elements [16]. It is well-known that Ti allows the formation of precipitates at high temperatures, which can inhibit grain growth [17]. Mejia et al. [18] showed the effect of Ti as a grain refiner in TWIP steels, reporting that a TWIP steel microalloyed with Ti exhibited a grain size of 63 ± 32 μm while the TWIP steel without microalling element showed a grain size of 104 ± 23 μm after austenization at high temperatur es. Most probably the present annealing temperature was not enough to completely dissolve the TiN particles, and therefore some abnormal grain growth took place. This explains the apparently large standard deviation in the grain size distribution. Accepted Article Figure 1 Accepted Article Figure 1 . Accepted Article . a) Accepted Article a) Geometry Accepted Article Geometry 3. Results and discussion Accepted Article 3. Results and discussion 1. Base material microstructure Accepted Article 1. Base material microstructure The microstructure of Accepted Article The microstructure of the Accepted Article the equiaxial grains with annealing twins (see Accepted Article equiaxial grains with annealing twins (see heterogeneous distribution can be observed as grain sizes range from 15 to 286 µm Accepted Article heterogeneous distribution can be observed as grain sizes range from 15 to 286 µm associated with the pre Accepted Article associated with the pre sence of Accepted Article sence of normal distribution, as Accepted Article normal distribution, as abnormal grains. These heterogeneous grain size distribution can be Accepted Article abnormal grains. These heterogeneous grain size distribution can be interference between the recrystallization Accepted Article interference between the recrystallization the microalloying elements Accepted Article the microalloying elements Accepted Article high temperatures, which can inhibit grain growth Accepted Article high temperatures, which can inhibit grain growth a grain refiner in TWIP steels, reporting that a TWIP steel microalloyed with Ti exhibited a grain Accepted Article a grain refiner in TWIP steels, reporting that a TWIP steel microalloyed with Ti exhibited a grain size of 63 ± 32 μm while the TWIP steel without microalling element showed a grain size of 104 ± Accepted Article size of 63 ± 32 μm while the TWIP steel without microalling element showed a grain size of 104 ± 23 μm after austenization at high temperatur Accepted Article 23 μm after austenization at high temperatur not enough to completely dissolve the TiN particles, and therefore some abnormal grain growth Accepted Article not enough to completely dissolve the TiN particles, and therefore some abnormal grain growth took place. This explains the apparently large standard deviation in the grain size distribution. Accepted Article took place. This explains the apparently large standard deviation in the grain size distribution. Accepted Article This article is protected by copyright. All rights reserved Figure 2. Microstructure of the studied TWIP-Ti steel in as-solutioned condition. a) LOM micrograph, and b) Grain size histogram. 3.2. Post-welding microstructure The welding of the TWIP-Ti steel revealed a uniform joint, where the welding bead presented a complete penetration and good aspect, free of porosity and cracking. The different zones generated during the welding process are showed in Figure 3, where three clear zones, namely the fusion zone (FZ), the welding interface (WI) and the heat affected zone (HAZ) are clearly depicted. The FZ shows a fully austenitic dendritic microstructure formed by columnar and equiaxial grains. The equiaxed dendritic grains are mostly noticed in the center of the weld bead, while columnar dendritic grains are perpendicularly oriented to the WI, i.e., the transition region between melted and non-melted areas (see Figures 3a and 3b). It is particularly interesting to notice the relatively small width of this FZ, ~ 3000 m, which is due to the low heat input used during the welding process. Accepted Article Figure 2 Accepted Article Figure 2 . Accepted Article . Microstructure Accepted Article Microstructure welding microstructure Accepted Article welding microstructure The welding of the TWIP Accepted Article The welding of the TWIP complete penetration and good aspect, free of porosity and cracking. The different zones generated Accepted Article complete penetration and good aspect, free of porosity and cracking. The different zones generated during the welding Accepted Article during the welding process Accepted Article process zone (FZ), the welding interface (WI) and the heat affected zone (HAZ) are clearly depicted. The Accepted Article zone (FZ), the welding interface (WI) and the heat affected zone (HAZ) are clearly depicted. The FZ shows a fully austenitic Accepted Article FZ shows a fully austenitic equiaxed dendritic grain Accepted Article equiaxed dendritic grain dendritic grains are perpendicularly oriented Accepted Article dendritic grains are perpendicularly oriented melted Accepted Article melted areas Accepted Article areas (see Accepted Article (see Accepted Article width of this FZ, ~ Accepted Article width of this FZ, ~ Accepted Article This article is protected by copyright. All rights reserved Figure 3. Microstructures of the TWIP-Ti steel welded joint. a) Welding cord section, b) Dendritic grain in the FZ, c) Welding interface (WI), and d) HAZ extension of equiaxial grain growth. On the other hand, the microstructural changes in the HAZ consist of coarsen austenitic grains, as compared to those of the TWIP-Ti steel in the as-solutioned condition (see Figure 3c and 3d). The HAZ presented grain sizes up to 155 µm, very close to the transition region between melted and non-melted region (see Figure 3d), due to the grain growth caused by the heat generated during the welding process. Due to the austenitic nature of this steel, the HAZ length can not be simply derived by observing areas where microstructural phase change may have taken place. Alternatively it can be derived by optical observation of the grain size and hardness profiles. In this sense, Table 2 shows the average grain size at different distances from the weld bead center. As expected, the zone adjacent to the welding interface showed the largest grain size of 76 ± 32 μm (1.8 2 mm from centerline of the weld bead). From that point the grain size was rapidly lowering down to achieve a Accepted Article Microstructures of the TWIP Accepted Article Microstructures of the TWIP grain in the FZ Accepted Article grain in the FZ , c) Welding Accepted Article , c) Welding On the other hand, the microstruct Accepted Article On the other hand, the microstruct compared to those of the TWIP Accepted Article compared to those of the TWIP presented grain sizes Accepted Article presented grain sizes melted region (see Accepted Article melted region (see Accepted Article This article is protected by copyright. All rights reserved similar value to the one of the base material (taking account of the large standard deviation). In other words, and taking account of the grain size profile, the HAZ extension seems to be very narrow (~ 450 m), as shown in Figure 3d. On the other hand, the smallest grain size, corresponding to 50 ± 27 μm, was observed at 5 mm from the weld bead center (see Table 2). Table 2. Average grain size at different distances from the weld bead center. Distance from weld bead center (µm) -25000 -14000 -5000 -3370 -2520 -1820 0 5000 14000 25000 Average grain size (µm) 64 72 60 67 67 76 50 61 56 Standard deviation 25 37 34 38 36 32 27 24 28 It is worth noting that the grain size on both sides of the weld bead showed slight differences, which can be associated to the at least two factors: i) Heterogeneity of the TWIP-Ti steel grain size in the as-solutioned condition, and, ii) A non-uniform heat input distribution. It is well-known that the solidification of metallic alloys involves the formation of crystals with a preferred growth direction due to heat flux from the system to the surroundings, which promotes instability of the solidification front as a result of constitutional supercooling in the molten metal and the dendritic structure formation generated [19, 20]. Furthermore, the constitutional supercooling in the center of the weld bead is larger than that close to the weld interface. Thus, equiaxed dendrites nucleated and grown from the weld centerline, blocking columnar dendritic growth [21]. On the other hand, different research works refer to the excessive equiaxial grain growth in the HAZ of TWIP steels, using different fusion welding processes such as laser and resistance spot welding [21, 22]. It is worth highlighting that although the HAZ of TWIP-Ti steel showed grain growth, this growth was not excessive, which can be associated to the low heat input used in the semi-automatic GTAW process, and the grain growth inhibition effect of Ti as a microalloying element. [17, 18]. 3.3. Segregation and precipitated particles Figure 4 displays the chemical composition profiles and elemental mapping images of the alloying elements in the FZ of the TWIP-Ti steel welded joint. Mn, Al, Si and C were considered in this analysis. It is a general observation that Al segregated towards the interior of the dendrites, while Mn, Si and C segregated towards the interdendritic zone (see Figure 4a). On the other hand, the variations of the alloying elements were quantified by means of EDS point analysis and compared Accepted Article Table 2. Accepted Article Table 2. Average grain size Accepted Article Average grain size Distance from Accepted Article Distance from center (µm) Accepted Article center (µm) - Accepted Article - 25000 Accepted Article 25000 Accepted Article Accepted Article Accepted Article Accepted Article Average grain Accepted Article Average grain 64 Accepted Article 64 Accepted Article Accepted Article Accepted Article Accepted Article 25 Accepted Article 25 Accepted Article Accepted Article Accepted Article Accepted Article Accepted Article Accepted Article Accepted Article Accepted Article It is worth noting that the grain size on both sides of the weld bead showed slight differences, which Accepted Article It is worth noting that the grain size on both sides of the weld bead showed slight differences, which can be associated to the Accepted Article can be associated to the at least two Accepted Article at least two ed Accepted Article ed condition Accepted Article condition , Accepted Article , and, Accepted Article and, solidification of metallic alloys involves the formation of crystals with a preferred growth direction Accepted Article solidification of metallic alloys involves the formation of crystals with a preferred growth direction due to heat flux from the system to the surroundings, which promotes instability of the Accepted Article due to heat flux from the system to the surroundings, which promotes instability of the olidification front as a result of constitutional supercooling in the molten metal and the dendritic Accepted Article olidification front as a result of constitutional supercooling in the molten metal and the dendritic structure formation generated Accepted Article structure formation generated the weld bead is large Accepted Article the weld bead is large r Accepted Article r than that close to Accepted Article than that close to grown from the weld centerline, blocking columnar dendritic growth [ Accepted Article grown from the weld centerline, blocking columnar dendritic growth [ different research works refer to the Accepted Article different research works refer to the Accepted Article using different fusion welding Accepted Article using different fusion welding highlight Accepted Article highlight ing Accepted Article ing that although the HAZ of TWIP Accepted Article that although the HAZ of TWIP not excessive, w Accepted Article not excessive, w hich Accepted Article hich can be Accepted Article can be and the Accepted Article and the grain growth inhibition effect of Ti as a microalloying element. Accepted Article grain growth inhibition effect of Ti as a microalloying element. This article is protected by copyright. All rights reserved Figure 10. Precipitated particles in TWIP-Ti steel. a) As-solutioned condition, b) Fusion zone (FZ), and c) Heat affected zone (HAZ-5 mm). 3.6. Micro-tensile tests. The micro-tensile tests from samples extracted in different welding zones of the TWIP-Ti steel in the as-solutioned (base material) and post-welding condition are shown in Figure 11. The stress and strain values of the TWIP-Ti steel in as-solutioned condition were used as reference for the mechanical behaviour analysis in the different welding zones. In this condition the base material exhibited a YS of 385 MPa and UTS of 739 MPa with an elongation to rupture of 78 %, while the sample extracted from the FZ showed a YS of 372 MPa and UTS of 669 MPa with an elongation to rupture of 79.9 %. In general, this zone exhibited the lowest values of UTS, which corroborates that the high heat input in the TWIP-Ti steel welded joint significantly affects the tensile strength. Contrary to the FZ, the zone closer to the WI exhibited the highest values of YS of 426 MPa and UTS of 802 MPa with elongation to rupture of 76 %. The UTS increased 8.2 % while the elongation decreased 2.5 % at the weld interface with respect to the base material, as shown in Figure 11. On the other hand, the HAZ samples extracted at 5 and 30 mm from the weld bead center also showed an increase in the UTS with values between 760-768 MPa, which is 3.9 % Accepted Article . Precipitated particles in TWIP Accepted Article . Precipitated particles in TWIP tensile tests. Accepted Article tensile tests. tensile tests from samples extracted in different Accepted Article tensile tests from samples extracted in different solution Accepted Article solution ed Accepted Article ed (base material) Accepted Article (base material) strain values of the TWIP Accepted Article strain values of the TWIP mechanical behaviour analysis in Accepted Article mechanical behaviour analysis in Accepted Article YS Accepted Article YS of 385 MPa Accepted Article of 385 MPa sample extracted from the Accepted Article sample extracted from the rupture of 79.9 %. In general, this zone exhibited the lowest values of UTS, which corroborate Accepted Article rupture of 79.9 %. In general, this zone exhibited the lowest values of UTS, which corroborate the high heat input in the TWIP Accepted Article the high heat input in the TWIP Accepted Article This article is protected by copyright. All rights reserved higher than in the base material. In general, the HAZ samples showed an increase in the elongation to rupture with values between 79.5-82 % at 10 mm from the weld bead center, which is 5 % greater than the base material. However, at 5 mm from the weld bead center, the elongation decreased 6.4 %, being this zone the most critical one, with an elongation to rupture of 73 %, the lowest one in all areas tested. On the other hand, the greatest elongation to rupture was exhibited at 30 mm from the weld bead center with a value of 82 % (see Figure 11). It is important to note that the YS in the HAZ is higher compared to the BM and FZ, but lower than near the WI (see Figure 11b). The differences in UTS and elongation to rupture can be associated to different factors such as chemical composition variation, solidification condition and thermal process experienced by the welding [27]. This effect can be explained in terms of either solid solution or second-phase particles strengthening (i.e. the interaction of dislocations with different obstacles) [32]. Figure 11. Micro-tensile tests in TWIP-Ti steel in as-solutioned (base material) and post-welding condition. a) Engineering stress-strain curves, and b) YS, UTS and elongation to rupture. In this way, the dendritic microstructure in the FZ and the presence of particles such as AlN, MnS and Ti (C,N) located in the dendritic grain boundaries, explains the lowest strength of this zone. Contrary, equiaxial grains in the HAZ with the presence of fine particles of Ti (C,N) and TiC distributed inside the grains explains the excellent mechanical behavior in this region. In any case, the large ductility values, and the absence of brittelness in the welded areas is an indication of the excellent mechanical behavior that welded joints of TWIP steels can present, provided correct welding parameters are employed. This fact is associated to the intrinsic good work hardening ability of this type of steels. Figure 12 shows the SEM micrographs of the fractured surfaces. The type fracture shown by the micro-tensile specimens in as-solutioned and post-welding conditions was ductile failure as Accepted Article in the HAZ is Accepted Article in the HAZ is high Accepted Article high The differences in UTS and elongation to rupture can be associated to different factors such Accepted Article The differences in UTS and elongation to rupture can be associated to different factors such as chemical composition variation, solidification condition and thermal process experienced by the Accepted Article as chemical composition variation, solidification condition and thermal process experienced by the . Accepted Article . This effect can be explained in terms of either so Accepted Article This effect can be explained in terms of either so strengthening (i.e. the interaction of dislocations with different obstacles) Accepted Article strengthening (i.e. the interaction of dislocations with different obstacles) Accepted Article Micro Accepted Article Micro - Accepted Article - tensile Accepted Article tensile condition. a) Engineering s Accepted Article condition. a) Engineering s the Accepted Article the dendritic Accepted Article dendritic N) located in the dendritic g Accepted Article N) located in the dendritic g equiaxial grain Accepted Article equiaxial grain Accepted Article distributed inside the grains explains the Accepted Article distributed inside the grains explains the the large ductility values, and the absence o Accepted Article the large ductility values, and the absence o Accepted Article This article is protected by copyright. All rights reserved illustrated by the evident nucleation, growth and coalescence of dimples and voids. This behavior is clearly associated to the high ductility shown in the micro-tensile tests. On the other hand, specimens with greater deformation exhibited a larger number of dimples (see Figures 12a and 12b) than samples depicting less deformation capacity (see Figures 12c and 12d). Figure 12. SEM micrographs of fracture surface of TWIP-Ti steel: a) As-solutioned condition, b) Fusion zone, c) FZ/HAZ interface, and d) HAZ at 30 mm from the weld bead center. Figure 13 shows SEM images of the microstructural evolution during micro-tensile tests in the base material condition and different welding zones (FZ and HAZ). Deformation twins are readily shown as thin bands of a few tenths of micrometers wide. However, deformation twins were not evident in all austenitic grains which can be explained in terms of the crystallopgraphic misorientation between the grains and the applied external force. On the other hand, it can be seen that some zones have different systems and density of twinning, e.g., in the as-solutioned condition has a parallel bands twinning system (see Figures 13a and 13b) while in the FZ shows a laddershaped twinning pattern, i.e., secondary twins appears within the primary one (see Figures 13c and 13d). In the case of the HAZ, a combination of twinning patterns is shown, i.e., parallel and ladderAccepted Article SEM micrographs of fracture Accepted Article SEM micrographs of fracture Fusion zone, c) Accepted Article Fusion zone, c) FZ/HAZ Accepted Article FZ/HAZ shows SEM images of the microstructural evolution during micro Accepted Article shows SEM images of the microstructural evolution during micro material condition and different welding zones (FZ and HAZ). Deformation twins are readily Accepted Article material condition and different welding zones (FZ and HAZ). Deformation twins are readily shown as thin bands of a few tenths of micrometers wide. Accepted Article shown as thin bands of a few tenths of micrometers wide. evident in all austenitic grains which can be explained in terms of the crystallopgraphic Accepted Article evident in all austenitic grains which can be explained in terms of the crystallopgraphic Accepted Article This article is protected by copyright. All rights reserved shaped bands (see Figures 13e and 13f). Furthermore, the presence of precipitated particles and cavities was also observed in the austenitic matrix. These cavities may be the result of the detachment of precipitated particles due to plastic deformation. Typically, fusion welding processes cause loss of the mechanical properties. However, in the welding of the studied TWIP-Ti steel was observed that mechanical properties do not suffer a significant loss, which can be associated to the twinning effect that was generated independently of the different microstructures of the welded joint and the TiC and Ti (C,N) particles formation. Wang et al. [33] suggest that according to the generated structures by the deformation twins, there are three types of grains in plastically deformed TWIP steels: i) Type I grains that contain few or no deformation twins, ii) Type II grains that exhibit a well-developed twin structure along one active twinning system, and iii) Type III grains that have well-developed twin structure along more than one active twinning systems. This depends of the type of the microstructure of the TWIP steel, i.e., equiaxial, columnar or dendritic, as well as the orientation and size of grains, which is evident in the different welding zones of the studied TWIP-Ti steel, particularly in the FZ and HAZ where the mechanical behavior is different. Accepted Article twinning effect that was generated independently of the different microstructures of the welded Accepted Article twinning effect that was generated independently of the different microstructures of the welded joint and the TiC and Ti (C,N) particles formation. Accepted Article joint and the TiC and Ti (C,N) particles formation. ed structures by the deformation twins, there are three types of grains in p Accepted Article ed structures by the deformation twins, there are three types of grains in p TWIP steels: Accepted Article TWIP steels: i) T Accepted Article i) T ype I grains that contain few or no deformation twins, Accepted Article ype I grains that contain few or no deformation twins, exhibit a well Accepted Article exhibit a well - Accepted Article - developed twin structure along one Accepted Article developed twin structure along one that have well Accepted Article that have well - Accepted Article - developed twin structure along more than one active twinning systems Accepted Article developed twin structure along more than one active twinning systems Accepted Article of the type of the microstructure Accepted Article of the type of the microstructure orientation and size of grains Accepted Article orientation and size of grains Accepted Article steel, particularly in the FZ and HAZ where the mechanical behavior is different. Accepted Article steel, particularly in the FZ and HAZ where the mechanical behavior is different. This article is protected by copyright. All rights reserved Figure 13. Deformation twinning in micro-tensile specimens in the as-solutioned and post-welding conditions. a), b) As-solutioned; c), d) Fusion zone; e), f) Heat affected zone. Conclusions Based on results obtained from the metallographic, structural and mechanical characterization of the Ti-containing TWIP steel welded joint, the following conclusions can be drawn: Accepted Article . Deformation twinning in micro Accepted Article . Deformation twinning in micro conditions. a) Accepted Article conditions. a) , b) As Accepted Article , b) As Accepted Article This article is protected by copyright. All rights reserved 1. The high content of alloying elements in the TWIP-Ti steel causes the formation of the dendritic structure and Mn and C segregation in the fusion zone (FZ). 2. The heat input in welded joint by the GTAW process caused very limited grain growth in the heat affected zone (HAZ). 3. No phase transformations of austenite were observed with the welding parameters used to join the present TWIP-Ti steel, which is indicative of good weldability. 4. Second-phase particles (TiC, Ti(C,N) and AlN) were formed in the FZ and HAZ of the welded joint promoting precipitation hardening effect. 5. The microstructural changes in the FZ of the welded joint generated a slight decrease in the ultimate tensile strength (UTS) compared to the base material, while the elongation to rupture slightly increased. 6. In general, the heat affected zone (HAZ) exhibits excellent ultimate tensile strength (UTS) and elongation to rupture, which is associated to the deformation mechanism by twinning and the fine precipitated particles formation. 7. The elongation to rupture values in the different weld zones are little affected and remain high, which guarantees a high mechanical performance of the welded joint. 8. No brittle behavior was observed in the welded area, a feature associated to the intrinsic work hardening ability of these steels. This behavior is also an indication of the optimal behavior of TWIP steels to welding processes when approppiated welding parameters are selected. Acknowledgments: Authors would like to thank the National Council of Science and Technology (CONACyT-Mexico) for the support during the project CB-2012-01-0177572. The present research project was also funded by the Coordinación de la Investigación Científica-UMSNH (Mexico) (CIC-1.8). H.H.B’s studies were sponsored by the National Council of Science and Technology (Consejo Nacional de Ciencia y Tecnología-México), N.B. 421680. JMC especially thanks CONACyT for partially funding his sabbatical leave at IIM-UMSNH. Technical assistance by Dr. Casas at UPC’s facilities is also acknowleged. References [1] B.C. De Cooman, K. Chin and J. Kim, High Mn TWIP steels for automotive applications. In: New Trends and Developments in Automotive System Engineering (Eds: M. Chiaberge), InTech, Shanghai, 2011, pp. 101-128. [2] M. Tisza, Key Eng. Mater. 2014, 381, 137. Accepted Article No phase transformations Accepted Article No phase transformations the present TWIP Accepted Article the present TWIP - Accepted Article - Ti steel Accepted Article Ti steel phase particles (TiC, Ti(C,N) and AlN) were formed in the FZ and HAZ of the welded Accepted Article phase particles (TiC, Ti(C,N) and AlN) were formed in the FZ and HAZ of the welded joint promoting precipitation hardening effect. Accepted Article joint promoting precipitation hardening effect. 5. The microstructural changes Accepted Article 5. The microstructural changes ultimate tensile strength (UTS) compared to the base material, while the elongation to Accepted Article ultimate tensile strength (UTS) compared to the base material, while the elongation to slightly increased. Accepted Article slightly increased. 6. In general, the heat affected zone (HAZ) exhibits excellent ultimate tens Accepted Article 6. In general, the heat affected zone (HAZ) exhibits excellent ultimate tens elongation to rupture, which is associated to the deformation mechanism by twinning and the Accepted Article elongation to rupture, which is associated to the deformation mechanism by twinning and the precipitated particles formation. Accepted Article precipitated particles formation. The elongation to rupture values in the different weld zones are little affected and remain Accepted Article The elongation to rupture values in the different weld zones are little affected and remain hich guarantees a high mechanical performance of the welded joint Accepted Article hich guarantees a high mechanical performance of the welded joint 8. No brittle behavior was observed in the welded area, a feature a Accepted Article 8. No brittle behavior was observed in the welded area, a feature a hardening ability of these steels. This behavior is also an indica Accepted Article hardening ability of these steels. This behavior is also an indica Accepted Article TWIP steels to welding processes when approppiated welding parameters are selected Accepted Article TWIP steels to welding processes when approppiated welding parameters are selected Acknowledgments: Accepted Article Acknowledgments: Authors would like to thank the National Council of Science and Technology (CONACyT Accepted Article Authors would like to thank the National Council of Science and Technology (CONACyT for the support during the project CB Accepted Article for the support during the project CB funded by the Coordinación de la Investigación Científica Accepted Article funded by the Coordinación de la Investigación Científica studies were sponsored by the National Council of Science and Accepted Article studies were sponsored by the National Council of Science and Ciencia y Tecnología Accepted Article Ciencia y Tecnología - Accepted Article - Accepted Article México), N.B. 421680. JMC especially thanks CONACyT for partially Accepted Article México), N.B. 421680. JMC especially thanks CONACyT for partially funding his sabbatical leave at IIM Accepted Article funding his sabbatical leave at IIM is also acknowleged. Accepted Article is also acknowleged. This article is protected by copyright. All rights reserved [3] B.C. De Cooman, Y. Estrin and S.K. Kim, Acta Mater. 2018, 142, 283. [4] O. Grässel, L. Krüger, G. Frommeyer and L.W. Meyer, Int. J. Plast. 2000, 16, 1391. [5] M.Y. Demeri, in Advanced High-Strength Steels: Science, Technology, and Applications, First ed. (Eds; ASM International), 2013, pp. 1-312. [6] I. Mejía, H. Hernández-Belmontes and C. Maldonado, MRS Adv. 2017, 2, 3899. [7] J. Yoo, B. Kim, Y. Park and C. Lee, J. Mater. Sci. 2015, 50, 279. [8] R. Ashiri, M. Shamanian, H.R. Salimijazi, M.A. Haque, J. Bae, C. Ji, K. Chin and Y. Park, Scr. Mater. 2016, 114, 41. [9] H. Hernández-Belmontes, I. Mejía and C. Maldonado, MRS Proc. 2016, 1812, 35. [10] L. 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Accepted Article , 114, 41. [9] H. Hernández Accepted Article [9] H. Hernández - Accepted Article - Belmontes, I. Mejía and C. Maldonado, Accepted Article Belmontes, I. Mejía and C. Maldonado, Accepted Article L. Mujica, S. Weber, C. Thomy and F. Vollertsen, Accepted Article L. Mujica, S. Weber, C. Thomy and F. Vollertsen, Accepted Article Roncery, S. Weber and W. Theisen, Accepted Article Roncery, S. Weber and W. Theisen, D.C. Saha, I. Chang and Y. Park, Accepted Article D.C. Saha, I. Chang and Y. Park, F. Reyes Accepted Article F. Reyes - Accepted Article - Calderón, I. Mejía, A. Boulaajaj and J.M. Cabrera, Accepted Article Calderón, I. Mejía, A. Boulaajaj and J.M. Cabrera, A.I. Fernández, P. Accepted Article A.I. Fernández, P. Uranga, B. López and J.M. Rodriguez Accepted Article Uranga, B. López and J.M. Rodriguez S. Kou, in Accepted Article S. Kou, in Welding Metallurgy Accepted Article Welding Metallurgy M. Sabet, A. Zarei Accepted Article M. Sabet, A. Zarei - Accepted Article - Hanzaki and S. Khoddam, Accepted Article Hanzaki and S. Khoddam, B. Mintz and D.N. Crowther, Accepted Article B. Mintz and D.N. Crowther, I. Mejía, F. Reyes Accepted Article I. Mejía, F. Reyes - Accepted Article - Calderón and J.M. Cabrera, Accepted Article Calderón and J.M. Cabrera, W. Kurz and D.J. Fisher, Accepted Article W. Kurz and D.J. Fisher, Accepted Article Publications Ltd), Accepted Article Publications Ltd), 1998 Accepted Article 1998 K. Easterling, Accepted Article K. Easterling, Introduction to the Physical Metallurgy of Welding Accepted Article Introduction to the Physical Metallurgy of Welding Accepted Article Butterworth Accepted Article Butterworth - Accepted Article - Heinemann), Accepted Article Heinemann), [21] W. Guo, Z. Wan, Q. Jia, L. Ma, H. Zhang, C. Tan and P. Peng, Accepted Article [21] W. Guo, Z. Wan, Q. Jia, L. Ma, H. Zhang, C. Tan and P. Peng, Accepted Article Accepted Article Accepted Article Accepted Article Saha Accepted Article Saha Saha Accepted Article Saha , Accepted Article , S. Accepted Article S. Han Accepted Article Han , Accepted Article , K.G. Accepted Article K.G. Accepted Article D. Kim, K. Han, B. Lee, I. Han, J.H. Park and C. Lee, Accepted Article D. Kim, K. Han, B. Lee, I. Han, J.H. Park and C. Lee, This article is protected by copyright. All rights reserved [28] C. Lee, Y. Jaehong, S. Kim, Y. Park and J. Choi, Characteristics of the hot cracking and segregation behaviour in the high manganese steels welds. In: Proceedings of the 1st International Conference on High Manganese Steels, Seoul. F-7, 2011. [29] G. Frommeyer, E.J. Drewes and B. Engl, Rev. Met. Paris. 2000, 97, 1245. [30] A. Dumay, J.-P. Chateau, S. Allain, S. Migot and O. Bouaziz, Mater. Sci. Eng. A. 2008, 483484, 184. [31] R.E. Smallman and A.H.W. Ngan, Physycal Metallurgy and Advanced Materials Engineering, Seventh ed. (Eds: Butterworth-Heinemann), 2007, pp. 1-672. [32] J.C. Lippold, Welding Metallurgy and Weldability, First ed. (Eds: John Wiley & Sons), Inc., Hoboken, New Jersey, 2015, pp. 1-442. [33] D. Wang, K. Wang, Z. Li, X. Wang, X. Wang and F. Han, Mater. Sci. Eng. A. 2015, 636, 396. Table captions: Table 1. Operating variables used in the GTAW process of the TWIP-Ti steel. Table 2. Average grain size at different distances from the weld bead center. Table 3. Peak temperature and cooling rate in the HAZ of the TWIP-Ti steel welded joint. Figure captions: Figure 1. a) Geometry and dimensions of the specimens used for the micro-tensile test. b) Specimens extraction location. Figure 2. Microstructure of the studied TWIP-Ti steel in as-solutioned condition. a) LOM micrograph, and b) Grain size histogram. Figure 3. Microstructures of the TWIP-Ti steel welded joint. a) Welding cord section, b) Dendritic grain in the FZ, c) Welding interface (WI), and d) HAZ extension of equiaxial grain growth. Figure 4. Distribution of the alloying elements in the fusion zone (FZ) of the TWIP-Ti steel welded joint. a) Elemental mapping of chemical elements distribution, b) Concentration profiles for Mn, Al, Si and C, c) Mn segregation pattern, and d) Si segregation pattern. Figure 5. Precipitated particles in the welded joint of TWIP-Ti steel. a) AlN, b) Ti(C,N), c) MnS, and d) TiC particles. Figure 6. X-ray diffraction patterns of TWIP-Ti steel in as-solutioned and post-welding condition. Figure 7. Average value and profile microhardness of the TWIP-Ti steel in as-solutioned and postwelding condition. Figure 8. Thermal cycle in the welded joint of TWIP-Ti steel through GTAW process. Figure 9. JMatPro® v9.1 predictions of TWIP-Ti steel. a) Equilibrium phase diagram, and b) Second-phases prediction. Accepted Article Accepted Article 484, 184. Accepted Article 484, 184. Accepted Article R.E. Smallman and A.H.W. Ngan, Accepted Article R.E. Smallman and A.H.W. Ngan, Accepted Article Seventh ed. (Eds: Butterworth Accepted Article Seventh ed. (Eds: Butterworth J.C. Lippold, Accepted Article J.C. Lippold, Welding Metallurgy and Weldability Accepted Article Welding Metallurgy and Weldability Hoboken, New Jersey, Accepted Article Hoboken, New Jersey, D. Wang, K. Wang, Z. Li, X. Wang, X. Wang and F. Han, Accepted Article D. Wang, K. Wang, Z. Li, X. Wang, X. Wang and F. Han, Table captions: Accepted Article Table captions: Operating variables used in the GTAW process of the TWIP Accepted Article Operating variables used in the GTAW process of the TWIP Average grain size at different distances from the weld bead center Accepted Article Average grain size at different distances from the weld bead center Peak temperature and cooling rate in the HAZ of the TWIP Accepted Article Peak temperature and cooling rate in the HAZ of the TWIP Figure captions: Accepted Article Figure captions: . a) Geometry Accepted Article . a) Geometry Accepted Article Specimens extraction location. Accepted Article Specimens extraction location. Microstructure Accepted Article Microstructure , and b) Grain size histogram. Accepted Article , and b) Grain size histogram. Microstructures of the TWIP Accepted Article Microstructures of the TWIP grain in the FZ, c) Welding interface (WI), and d) HAZ extension of equiaxial grain growth. Accepted Article grain in the FZ, c) Welding interface (WI), and d) HAZ extension of equiaxial grain growth. . Distribution of the alloying elements in the fusion zone (FZ) of Accepted Article . Distribution of the alloying elements in the fusion zone (FZ) of joint. a) Elemental mapping of chemical elements distribution, b) Concentration profiles for Mn, Al, Accepted Article joint. a) Elemental mapping of chemical elements distribution, b) Concentration profiles for Mn, Al, Si and C, c) Mn segregation pattern, and d) Si segregation pattern. Accepted Article Si and C, c) Mn segregation pattern, and d) Si segregation pattern. Accepted Article . Precipitated particles in the welded joint of TWIP Accepted Article . Precipitated particles in the welded joint of TWIP and d) TiC particles. Accepted Article and d) TiC particles. This article is protected by copyright. All rights reserved Figure 10. Precipitated particles in TWIP-Ti steel. a) As-solutioned condition, b) Fusion zone (FZ), and c) Heat affected zone (HAZ-5 mm). Figure 11. Micro-tensile tests in TWIP-Ti steel in as-solutioned (base material) and post-welding condition. a) Engineering stress-strain curves, and b) YS, UTS and elongation to rupture. Figure 12. SEM micrographs of fracture surface of TWIP-Ti steel: a) As-solutioned condition, b) Fusion zone, c) FZ/HAZ interface, and d) HAZ at 30 mm from the weld bead center. Figure 13. Deformation twinning in micro-tensile specimens in the as-solutioned and post-welding conditions. a), b) As-solutioned; c), d) Fusion zone; e), f) Heat affected zone. This research work studies microstructural and mechanical changes of Ti-containing TWIP steel welded joint by GTAW semi-automatic process in 5.6 mm thick butt plates without filler material and full-penetration. No phase transformations occur, which is indicative of good weldability. TiC and Ti(C,N) precipitate in FZ and HAZ. Welded joint exhibits excellent UTS and elongation, which is associated to twinning deformation. Accepted Article Fusion zone, c) Accepted Article Fusion zone, c) interface, Accepted Article interface, . Deformation twinning in micro Accepted Article . Deformation twinning in micro conditions. a), b) As Accepted Article conditions. a), b) As - Accepted Article - solutioned; c), d) Fusion zone; e), f) Heat affected zone Accepted Article solutioned; c), d) Fusion zone; e), f) Heat affected zone This research work studies Accepted Article This research work studies welded joint by GTAW Accepted Article welded joint by GTAW semi Accepted Article semi Accepted Article penetration. Accepted Article penetration. No phase transformations occur, Accepted Article No phase transformations occur, precipitate in FZ and HAZ. Accepted Article precipitate in FZ and HAZ. is associated to Accepted Article is associated to twinning deformation Accepted Article twinning deformation Accepted Article