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Stationary shoulder friction stir welding of Ti-6Al-4V of 7mm thickness was conducted with varying welding speeds and rotation speeds. Variant selection analysis was carried out based on the inherited α phase texture and the reconstructed β phase texture. The weld surfaces became significantly smoother with increasing welding speed and decreasing r...
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... were carried out on an Oxford-HKL EBSD system attached within a Sirion FEGSEM microscope at 20 kV accelerat- ing voltage and spot size of 5. EBSD data were represented by the orientation imaging maps (OIM) consisting of inverse pole figure coloring maps, band contrast maps and pole figures. Pole figures are the representation of texture. Fig. 1 shows photographs of the weld surfaces for all the five welds. All the welds had relatively flat surfaces, but, as shown in Fig. 1, there are rings associated with the rotation of the tool which are more defined at the slowest traverse speed. Surface profile mea- surements are shown in Fig. 3. From visual inspection it was quite clear ...
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... spot size of 5. EBSD data were represented by the orientation imaging maps (OIM) consisting of inverse pole figure coloring maps, band contrast maps and pole figures. Pole figures are the representation of texture. Fig. 1 shows photographs of the weld surfaces for all the five welds. All the welds had relatively flat surfaces, but, as shown in Fig. 1, there are rings associated with the rotation of the tool which are more defined at the slowest traverse speed. Surface profile mea- surements are shown in Fig. 3. From visual inspection it was quite clear that all the three welds with constant rotation speed had a flat surface with obvious rings which must have been associated with ...
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... high temperature bcc phase. GB5 and 2 belong to the same prior grain 1 and 1 is inside 3. {0001} pole figures from 1, 2 and GB1 in Fig. 7(c) displays that 1, 2 and GB5 have different orientations from each other, this is an example for the laths between the adjacent prior grains that do not share a common {0001} pole. It was demonstrated in Fig. 1(c) that GB5 exhibits an orientation as a compromise of the orientations of the colonies 1 and 2 [20]. There was no grain boundary misorientation between GB6 and the colony 3 as indi- cated in Fig. 7(d). It can be seen from Fig. 7(e) that 1 (purple) has different orientation from 3 (dark cyan). The {110} pole figures from 1 and 2 in Fig. ...
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... the AS of the SZ of all the three welds E, B and C. In this work, the difference of the pole intensity between {0001} pole figure and the corresponding {110} pole figure of the six poles (1-6) is given as the ratio (≥1) of pole intensity of each {0001} pole figure versus that of the corresponding {110} pole figure or vice versa ({0001} ↔ {110} ). Fig. 10(a) shows the ratio of {0001} ↔ {110} pole intensity for all the six poles of Weld E. Similarly, Fig. 10(b-c) shows the difference of the pole intensity for Weld B and Weld C in terms of ratio. A key of the six poles (1, 2, 3, 4, 5, and 6) is shown at the bottom of the ...
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... {0001} pole figure and the corresponding {110} pole figure of the six poles (1-6) is given as the ratio (≥1) of pole intensity of each {0001} pole figure versus that of the corresponding {110} pole figure or vice versa ({0001} ↔ {110} ). Fig. 10(a) shows the ratio of {0001} ↔ {110} pole intensity for all the six poles of Weld E. Similarly, Fig. 10(b-c) shows the difference of the pole intensity for Weld B and Weld C in terms of ratio. A key of the six poles (1, 2, 3, 4, 5, and 6) is shown at the bottom of the ...
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... selection intensity was calculated as the ratio of {0001}/{110} pole intensity in Fig. 4. And Fig. 11 shows the variant selection intensity as a function of the prior grain size on the central horizontal line for all the five welds. It can be seen that as the prior grain size increases, the average thickness of the laths decreased significantly, the variant selection intensity becomes stronger, this shows agree- ment with Obasi et al. [23] who found that the relative free growth of the dominant {1, , 2} {90 • , 30 • , 0 • } texture com- ponent into an "empty" grain contributes to the stronger variant selection with increasing prior grain size. ...
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... 82 m and 62 m have lower variant selection intensity. However, the decrease of thickness of the laths was related to a weak variant selection for a titanium alloy [15]. The prior grain size increases with increasing heat input from Weld E, Weld D to Weld C, Weld B and Weld A as the decrease of the traverse speed and the increase of rotation speed (Fig. 12). It was reported that the tensile strength and yield strength decreased significantly with the increase of the prior grain size and the thick- ness of laths [27,28], therefore, smaller prior grain size and finer laths will improve mechanical properties of titanium alloys ...
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... due to extra heat input or greater cooling rate at plate surfaces. Moreover, it should be noted that the blank regions have the highest hardness of 480 HV, whereas the hardness of the BM is about 360 HV and the other weld zones have average hardness of 380 HV, this could be attributed to the infiltration of the pin material W-Re. It is shown in Fig. 13(b) that the blank regions observed in the SZ bottom of Weld A at the AS are the regions that cannot be reconstructed because the and the phase do not obey the Burgers orientation relation, the unreconstructed blank regions could also be probably due to the refined prior grains ...
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... is seen from Figs. 13 and 14 that the hardness is the lowest in the regions where a large quantity of the adjacent grains have {110} crystal planes paralleling or nearly paralleling to each other. This result shows agreement with Bhattacharyya et al. [26] who reported that the grain boundaries with colonies having common basal planes {0001} resulted in softer ...
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Citations
... Stationary shoulder friction stir welding (SSFSW) has been widely used in joining many structural engineering materials, such as aluminum alloys and titanium alloys [2,3]. The process parameters of the SSFSW determined the temperature distribution, material flow, phase transformation, and recrystallization processes during the welding process, which affects the mechanical properties of the weld to some extent. ...
During the friction stir welding process, the motion of the stirring tool is divided into two parts: the rotational motion and the lateral feed, both of which influence the temperature distribution, material flow, and slip rate. The effect of the rotational motion in numerical simulations is usually the focus of research, but the changes in material flow, slip, and heat generation due to welding speed are usually neglected. In this study, the numerical model of stationary shoulder friction stir welding (SSFSW) was developed to optimize the effect of welding speed on heat generation and material flow based on the instantaneous velocity center (IVC), and the relationship between n/v and IVC were discussed. The instantaneous velocity center distance was inversely proportional to n/v, and its magnitude affected the difference in temperature between the AS and RS.
... In addition, the lower tensile strength will result from the larger grain size and thicker lamellae embedded within the prior grain; grain size and lamellae thickness increase as a tool the rotational speed increases, thus lowering the strength of the weld. It is consistent with previous research showing that the tensile strength of Ti-6Al-4V increases with decreasing α-plate thickness and grain size [59][60][61]. This phenomenon is also true for individual welds at the top, middle and bottom regions because grain size reduces from top to bottom along the thickness direction. ...
The light metal titanium alloys find extensive usage is in industries where weight is a significant factor. Research in friction stir welding (FSW) aimed at producing strong joints without adding extra weight since machines like aircraft and automobiles must have as little weight as possible to improve their fly-to-buy ratio. This study reports 10 mm thick titanium alloy plates that are FSWed by varying rotational speed and tool traverse speed using the W-La2O3 tool. Due to the heat produced during the welding process, each material region experiences a different thermal cycle, which significantly affects microstructural changes. The peak temperature during FSW exceeded the β-transition temperature, causing phase transformations in the stir zone (SZ). A lamellar structure was observed in the SZ, and a transition line region (TLR), Bimodal, or duplex microstructure obtained consists of (α + β) phase. The SZ grain size decreases along the thickness direction, tensile strength increases, and reaches 89–102% of the base material. A lower hardness value is found in the SZ than in the base material (BM). The tensile fracture surface is observed to have a honeycomb-like structure or dimples, representing ductile fracture.
... However, a large temperature gradient along the thickness of the weld was generated in conventional FSW of titanium alloy [6][7][8]. Thus, the stationary shoulder friction stir welding (SSFSW) was invented and developed by TWI to overcome the problem of large temperature gradient in conventional FSW [9][10][11][12]. The tool system of SSFSW consists of internal rotating tools and an external stationary shoulder, which remains stationary but only sliding over the surface of workpiece during the welding process, the pin rotates and the shoulder is ''still'' relative to the pin. ...
The effect of stationary shoulder friction stir welding (SSFSW) on temperature gradient and material flow was investigated to optimize the SSFSW process. A three-dimensional numerical model of heat generation and material flow was established by using computational fluid dynamics, and thermo-physical phenomena of SSFSWed Ti-6Al-4V were quantitatively analyzed in terms of heat generation, heat transfer, material flow and viscosity. The temperature gradient was more uniform in a narrow stir zone produced by the SSFSW process. The distribution of velocity was studied, and instantaneous velocity center of tool was divided into two different velocity regions to study the material flow. The simulation results were verified by experimental thermal cycles of calculated position of model, which was in accordance with the experimental results.
... However, reducing the vertical force to 20 KN has resulted in more surface roughness again in Figure 5(c). Jiang et al. 45 investigated the SSFSW of Ti-6Al-4V alloy and reported that the weld surfaces became significantly smoother with increasing welding speed and decreasing rotation speed. Li et al. 46 investigated the joint feature of AA2024 welded with external stationary shoulder assistance and reported that the assisted external stationary shoulder is beneficial to the joint formation, by which arc corrugation, flash and void can be eliminated or diminished, compared with the conventional FSLW joints. ...
The stationary shoulder friction stir welding (SSFSW) technique promotes new opportunities for welding material. In the current work, a special setup of SSFSW was designed and manufactured in only three parts according to the homemade friction stir welding (FSW) machine dimensions and specifications. This setup with minimum parts is easily adjustable beside it facilitates fast control of the tool pin length. This gives the design more advantages than the previous designs published for the same purpose. Two different parameters were used with the SSFSW setup to butt weld samples of AA7075-T6 of 5 mm thick. Various welding travel speeds of 25, 50 mm/min to 75 mm/min and different values of Z-force of 20, 25 KN, and 50 KN were used to evaluate the effect of the SSFSW parameters on the joints quality and properties. The results confirmed that sound joints were produced without internal or surface defects. In addition, the joints are characterized with a smooth surface finish compared to the conventional FSW joints. A narrow heat affected zone around the nugget due to the elimination of heat generation by the shoulder. The tensile strength of welded samples increases with increasing the vertical force with the maximum tensile strength of 418.7 MPa at a rotational speed of 600 rpm, a welding speed of 50 mm/min, and 50 KN Z-force. This implies that the vertically applied force upon SSFSW is an important parameter in controlling the joint properties. Analysis of the hardness distribution across the weld cross-section shows a slight reduction in the heat-affected zone.
... Both Zhang et al. [16] and Edwards et al. [17,18] found that, the hardness of the SZ increased at a temperature higher than the β transus temperature with an elongation less than half of the BM. To improve the weldability and joint properties of FSW of titanium alloys, stationary shoulder FSW (SSFSW) was invented by TWI in 2004 to solve the problems of gradient temperature distribution, especially the surface overheating, along the thickness direction of the titanium joints due to its low thermal conductivity [19], and a relatively homogenous microstructure along the direction of plate thickness was reported by Russell et al. and Wynne et al. [20,21] with excellent forming and good joint performance. Davies [22] and Jiang [23,24] also observed the uniformed distributed microhardness along the plate thickness with varying welding parameters. ...
The effect of modifying microstructure and texture by electric current on mechanical properties of electrically assisted stationary shoulder friction stir welded Ti6Al4V alloy were investigated. The width of the 'bowl-shaped' stir zone gradually increased with the increasing current value and texture of the joints changed abruptly with an increasing current from 0 to 300 A. The macrozones consisting of similar orientated grains in the heat-affected zone have the highest deformation degree and the lowest hardness, while macrozones were not seen in the stir zone of both the friction-stir welded joints and the electrically assisted joints. Why the tensile strength and elongation reached the highest at a current of 300 A was explained in terms of microstructure and texture. ARTICLE HISTORY
... The electron micrographs were captured and EDX point scanning was performed by a scanning electron microscope (SEM). EBSD tests were carried out by a SEM-EBSD system [17]. Transmission electron microscopy of thin foils were prepared to examine the microstructure at high magnifications and to analyze substructures and dislocations. ...
The effect of electric current on microstructure, texture, deformation behavior and tensile properties were investigated for the electrically assisted friction stir welding process of Ti-6Al-4 V alloy. The base material has strong prismatic texture {101¯0} < 112¯0 > linked to strong prismatic slip system {101¯0} < 112¯0>. Texture of the stir zone is randomly distributed with the weak basal {0001} < 112¯0 > texture owing to {101¯2} < 101¯1 > twinning. The introduction of current into the welding process has resulted in the alleviation of the abrupt change of microstructure between the surface, center and bottom of the stir zone, the decrease in texture strength and more uniformly distributed strain. The improvement of tensile properties without loss of ductility for the electrically assisted joints was mainly attributed to microstructure and strain homogeneity, beta phase dissolution and random orientations.
... The array of studies reported the welding of titanium alloys using different techniques, including friction stir welding [6][7][8][9][10][11], ultrasonic spot welding [12], tungsten inert gas welding [13], electron beam welding [14], fusion welding [15,16] and explosion welding [17,18]. Though, conventional fusion welding and electron beam welding of titanium alloys generally cause reduced mechanical properties at the joint and require prolonged welding time due to the vacuum cycle. ...
Titanium alloys are supreme structural materials primarily due to their high specific strength. However, their wide use is largely restrained by the high cost of raw titanium compared to other metals commonly used in structural alloys. Layered structures of titanium alloys allow substantial increase of the material utilisation ratio and therefore draw significant attention. The rational ways of layered parts fabrication are bonding or joining of individually optimised layers into a final complex structure. The use of friction welding to join the parts is one of the most attractive ways of achieving a desirable result, since it is a solid state and near-net-shape process that modifies the structure of connected parts only locally. The study goal was to validate feasibility of the layered structures of Ti-6Al-4V (Ti-64) alloy and metal matrix composite (MMC) on its base with 10% of TiC fabricated by rotary friction welding (RFW) and linear friction welding (LFW). Both initial structures, Ti-64 and MMC, were made using low-cost blended elemental powder metallurgy. RFW and LFW were successfully used to bond the sections of the alloy and its composite. TiC particles stabilise the structure and are not fragmented by friction welding under used processing parameters.
... The dominant mechanism of variant selection is that the nucleation of grain boundary α (GBα) will be limited by the orientation of neighbouring parent β grains. Since the nucleation and growth of α plates with the same orientation as the GBα are preferable compared to other variants, only several specific α variants can be formed within one parent β grain [13,17,18]. To elaborate, when adjacent β grains with a common <110> β pole, the c-axis of the newly transformed GBα will along the common <110> β pole. ...
... To elaborate, when adjacent β grains with a common <110> β pole, the c-axis of the newly transformed GBα will along the common <110> β pole. Under this mechanism, Jiang et al. [18] and Obasi et al. [14,19] reported that as prior β grain size increases, the variant selection intensity becomes stronger. They speculated that it was because specific GBα grains governed by adjacent parent β grains with a common <110> β pole can grow freely into an "empty" grain. ...
... Assuming no other influences, the nucleation of GBα will be homogeneous. However, it has been reported that if neighboring β grains with a common <110> β pole, <0001> pole of GBα will be parallel to the common <110> β pole [13,17,18], leading to the specific GBα nucleates at the boundary. Subsequently, the fresh α grains with the same orientation will form by leeching on to the pre-existing GBα and the α colonies will be developed. ...
Local crystallographic orientation characteristics of grains with different sizes in a ZrTiAlV alloy after interrupted cooling from β phase region are investigated by electron backscatter diffraction (EBSD) technique. A statistical analysis of EBSD data shows that bigger parent β grains present weaker variant selection than smaller grains. The relevant influences are studied by comparing the nucleation behavior of grain boundary α (GBα) and succedent growth stage in varisized grains. In-depth analysis indicates that the deviation between pole of GBα and common pole of adjacent parent β grains is responsible for the smaller degree of variant selection inside the bigger parent β grains. The formation of more than one GBα type at the β/β grain boundary with common pole in bigger grains results in obstructing each other for these variants during the following growth stage, while only one type of GBα nucleate at the β/β grain boundary with common pole in smaller grains and it can freely grow into the interior of the grains, leading to stronger variant selection in smaller β grains.
... e requirement of the high-performance airplane accelerates the application of titanium alloys due to their high specific strength together with the extraordinary corrosion resistance [1,2]. It is well known that the precipitate of the α phase from β grains directly affects the properties of the titanium alloys, which results in a considerable amount of interest in the precipitation mechanism of the α phase [3][4][5][6]. ...
... During the β to α phase transformation process, the α precipitates are well known to be formed according to the Burgers orientation relationship (BOR), which implies the parallelism of dense planes and direction of both phases: (011) β //(0001) α and [1][2][3][4][5][6][7][8][9][10][11] β // [11][12][13][14][15][16][17][18][19][20] α [7,8]. Because of the symmetry of the α and β phases, 12 crystallographic α orientations can form in a single parent β grain, called 12 α variants. ...
... When all α variants in a β grain occur with equal statistical probability, the transformation is said to evolve without variant selection [3]. However, it is unable to observe all 12 variants in a β grain in real alloys, which means the variant selection is always taking place during transformation in titanium alloys [6,9,10]. e variant selection will then result in the strong α transformation texture. Due to the high anisotropy, the mechanical properties of titanium alloys depend strongly on the texture of the α phase caused by the variant selection [11,12]. ...
The effect of the changing of the local composition of the β matrix on the precipitation of the α phase has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates based on the Ti-5.04Al/Ti-1.52Mo (at.%) diffusion couple. The results showed that the composition gradient was formed from one side of the diffusion couple to another side after diffusion annealing. Followed by a secondary heat treatment process, it was found for the first time that all 12 variants formed in a single β grain in the diffusion zone in the Ti-5.04Al/Ti-1.52Mo diffusion couple, which indicated that the changing of the local composition of the β matrix significantly weakened the α variants selection behavior.
Aluminum alloy is one of the important materials in the field of automotive lightweight research; in order to meet the current demand for welding process of new energy power battery shell, this paper adopts static shoulder friction stir welding (SSFSW) on 3-mm-thick AA3003-H14 to conduct welding test; to explore the influence of welding process parameters on the forming quality, microstructure evolution and mechanical properties of the joint are investigated. The test results indicate that, at a constant welding speed of 500r/min, rotational speeds ranging from 2000 to 4000r/min yield defect-free welded joints with satisfactory forming quality. The cross section of SSFSW weld primarily consists of the weld nugget zone, the surrounding thermo-mechanically affected zone, and the heat-affected zone. The width of the weld seam and the overall grain size decrease as the welding speed increases. The distribution curve of joint microhardness exhibits a “W” shape, with the lowest hardness occurring in the backward side of the heat-affected zone and the highest hardness appearing in the forward side of the weld core area. Optimal tensile properties are observed when the welding speed is 1000 mm/min and the stirring head rotates at 4000r/min, resulting in a tensile strength of 157.79 MPa, which is equivalent to 92.8% of the base material's strength. A joint with favorable tensile properties can be achieved when the value of ω/v falls between 3 and 6. The fracture location corresponds to the region of lowest hardness in the joint, predominantly located at the junction of the thermo-mechanically affected zone and the heat-affected zone on the backward side of the joint. This fracture is characterized by evident toughness.
