# Mechanical and Microstructural Investigation on AZ91B Mg Alloys with Tool Tilt Variation by Friction Stir Welding

Authors: S. Yaknesh; Mohammed Tharwan; Rajasekaran Saminathan; N. Rajamurugu; K. B. Prakash; Ankit; Manoj Kumar Pasupathi; Atul Sarojwal; Dawit Tafesse Gebreyohannes
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

By modifying the angle of tilt of 2 distinct tool pin shapes, both tapered with and without threaded structures, TPT tool pin with threads and TPNT tool pin with no threads, the mechanical behaviour and microstructural attributes were examined for the joints of AZ91B alloy formed by friction stir welding. Spindle torque and unique forces operating laterally and axially (referred to asY-force and X-force, respectively) were created at the junction of the tool and plate, which were monitored and assessed in order to know the influence of degrees of tilt. When the tool’s angle of tilt advances, torque and forces of the spindle also show an increasing trend that is important to friction stir welding. The spindle torque and Y-force produced by the TPT tool are higher than those of the TPNT tool, whereas the X-force associated with the TPT tool is less. Discrepancies in forces grow along the concurrent increase in the angle of tilt for the TPNT as well as TPT tools, exhibiting opposite behaviour. The onion ring-type structure was visualized in the threaded tool pin structure TPT due to the high flow of plasticized metal compared to TPNT tool joints, for which the onion ring shape is not visible. With the TPT tool, the joint made at a 0° angle of tilt had a peak tensile strength of 232 MPa.


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## Body

## 1. Introduction

Alloys of magnesium (Mg) fall into the group that belongs to low dense, metallic characteristics, engineering materials and exhibit a variety of extensive and appealing properties such as exceptional electromagnetic shielding features, distinguished thermal conductivity, high degree of recyclability, greater specific strength, easily cast-able, and larger specific stiffness [1–3]. The presence of these qualities has enhanced the use of Mg alloys in a range of manufacturing sectors, especially aerospace, marine, automotive, railroads, and electronics. On the other hand, Mg alloys have a limited range of uses because they are hard to shape at low room temperatures. This is because they have a densely packed hexagonal structure [4, 5].Welding is the most important method for manufacturing in a variety of production industries and its applications. As well, identifying suitable welding processes for Mg alloys will be extremely beneficial in compensating for these alloys’ lower formidability and extending their applicability as engineering materials in a variety of sectors [6, 7]. As a result, it is critical to establish a suitable procedure for welding Mg alloys, particularly those in the AZ series, as they are generally used in a variety of production industries [8, 9].Because friction stir welding (FSW), a high eminence, very effective, and environmentally friendly solid-state welding technique, is the best method to weld Mg alloys, FSW quickly avoids widespread solidification-related difficulties (such as constituent loss, grain roughening, and surface alteration) [10–12]. Moreover, because FSW is capable of combining Mg alloys without melting them, it can reasonably avoid various traditional fusion welding defects (such as air voids, joint tumours, fractures, and ejection) [13, 14]. When using the FSW technique to weld magnesium alloys, the induced heat would be substantially lower than the heat generated by the typical procedure of welding. Conditions after a joint are likely to end up with a big drop in deformation and residual stress [15].The quality of weldments is determined by an intricate combination of process-based parameters (tool angle of tilt, tool speed, rate of plunge, tool traverse speed, vertical downwards force, tool pin design, etc.), material properties that are vital for the weldment (shear and yield stress, hardness, strength, etc.), and cooling technique [5, 16]. In FSW, materials to be welded (typically flat plates) must be rotated by the welding tool to achieve a precise strain of the plasticized material, and therefore, the generated torque at the junction between the work specimen and tool plays a vital role [10, 17]. As it lowers through the specimen for joining, the FSW tool often encounters Y-directional forces. When the tool pin moves in the direction of welding, it also feels a longitudinal force called the X-force. This force is mostly caused by the resistance of the plate’s plasticized metal over the tool pin region [7, 13].A considerable amount ofY-force is an unavoidable need for efficient welding because, at the trailing edge of the tool pin, heat production and plasticized metal combining are improvised. Aside from these factors, the shape of the tool is also very important. It affects how much heat is generated, how much the metal deforms when it is softened, and how the metal mixes at the weldment line where the plates meet, which in turn affects the quality of the joint [4, 18].Yang et al. [18] examined the AA6061-T6 plates by FSW using seven tool pins with varied geometry. The shoulder shape of the tool and tool pin was found to have a vital part in heat generation, at the plunging phase the tool torque fluctuation as well the nugget zone area estimation. A tool that has fluted shoulders and corrugated pins produces grains that are highly refined in the weldment region, but pins with flat faces produce faulty junctions with nonhomogeneous shaped grains in their NZ. Kumar et al. [19]’s FSW was used to produce junctions of various aluminium alloys (Aluminium 6061 and Aluminium 5083) using tools with 3 independent pin geometries. It was found that joints made of different kinds of aluminium alloy with a tapered tool pin and threaded profiles were the strongest.In major accordance with regard to tool pin shape impact on FSW [20–22], from the literature, it is clear that FSW is produced by changing the speed of the tool, and the tool tilt angle is either fixed or not considered. There are extremely less works of literature. References [23–25] have made reviews on the FSW regarding the effect of tool tilt angle. For instance, Zhai et al. [23] the outcome of angle of tilt on production of heat, metal flow behaviour, and field temperature during friction stir with regard to tool pin shape impact on FSW, it is clear from the literature that FSW is produced by changing the speed of the tool, and the tool tilt angle is either fixed or not considered [20–22]. There is extremely little literature that has reviewed the FSW regarding the effect of tool tilt angle [23–25]. For instance, Zhai et al. [23] the outcome of the angle of tilt on the production of heat, metal flow behaviour, and field temperature during friction stir welding was explored both experimentally and numerically. When compared to a 0° angle of tilt, a slanted angle of 2.5° provides a forceful forging effect at the trailing side, hence boosting the maximum temperature with the concomitant escalation in the plasticized metal flow [26]. The FSW of AZ31B and AZ91C magnesium alloys was tested, and the joints had a strength of 186 MPa, which is 78.81 percent higher than that of AZ91C and 70.72 percent higher than that of AZ31B, another parent metal. Intermetallic phased elements, like Mg17Al12, can be found in different places on fractured surfaces. These elements have made the nugget zone more fragile and decreased the joint’s tensile strength.The maximum tensile strength of the aluminium base metal, which was manufactured at a 20° tilt angle, was improved by 75% [27]. At the upper tilt angle, the increased heat production and axial force cause the separation of steel fragments on the aluminium matrix and the formation of the intermetallic complex Al-Fe. An investigation was conducted into the effects of FSW tool rotation speed (rpm), travelling speed (V), tilt angle (tilt), and plunge depth on material flow, joint tensile strength, flexural strength, impact energy, and hardness. The joint with the highest strength is achieved at 2200 rpm with a travelling speed of 105 mm/min at 2.5° tilt and a plunge depth of 1.2 mm [28]. The greatest values for tensile strength, flexural strength, and impact energy were 82% (about 55 MPa), 76% (about 61 MPa), and 75% (about 19 kJ/m2, respectively). According to fractographic research, the surface has diverging lines that grow outside of the fracture start zone [29]. The effects of tilt angle, plunge depth, and tool offset were examined in the friction stir welding of poly (methyl methacrylate) T-joints. The greatest flexural and tensile strengths of friction stir welded joints were approximately 93% and 90% of as-received polyethylene at 0 mm offset, 2° tilt angle, and 0.2 mm plunge depth added (methyl methacrylate) [30].It was tested on a different FSW polycarbonate (PC) sheet and an aluminium alloy AA5058 lap. The highest values of tensile and bending strengths were achieved at 68.2% (46 MPa) and 69.4% (60 MPa), respectively. Fractography showed that the AA5058 alloy/PC contact broke during the tensile test below and in the middle of the stir zone. In another investigation, the effects of submerged temperatures (three levels of 273 K, 298 K, and 333 K) on the metallurgical and mechanical characteristics of the joints were analysed [31]. A lower hardness was attained in submerged welding. However, it can be boosted by processing it with warm water. The butt-joint of AA5083 Al-Mg alloy and A441 AISI steel sheetshad been accomplished in ambient air without overcooling, and the examination was performed by immersing them in the cooling water medium at three different temperatures of 0°C, 25°C, and 50°C [32]. Based on the results of experiments, all of the samples fail at the joint interface along the IMC layer when the cooling medium is at room temperature and has the best tensile strength (310 MPa) and elongation (13%).In a nut-shell, it can be ascertained that FSW with tool variation of dimension and angle of tilt work is limited, thereby examining the tool tilt effect, and the shape of the tool should be investigated critically. This work focuses on evaluating the angle of tilt and two unique tool pin shapes with and without threads.


## 2. Materials and Methods

The entire research project was carried out using friction stir welding equipment, which is a versatile semiautomated machine. AZ91B Mg alloy plates with rectangular dimensions of 110 × 50 × 6 mm were stacked in the specially designed fixture to achieve a but type of junction throughout the operation. Table1 lists the constituents of this AZ91B rectangular plate, as well as the appropriate weight percent and mechanical parameters.Table 1
AZ91B properties and compositions.
Name of the alloyAlZnNiCuMnSiFeMgTensile valueYield value% of extensionAZ91B3.10.850.0050.050.310.080.005Balance263 (MPa)178 (MPa)11.7The tools employed in this study were made of high-speed steel M42 grade that had been 63HRC hardened. In this research, tools with 2 different tool pin shapes are used, which are cylindrically tapered pins with threads (TPNT) and without threads. Figure1 shows a view of the two unique tools being used.Figure 1
Picture of tool pins. (a) Tapered tool pin cylindrical shape with threads (TPT). (b) Tapered tool pin cylindrical shape without threads (TPNT).As shown in Figure1, all of the tools used in this work have the identical tool dimensions, which include a taller and bigger external shoulder (2 cm in diameter and 5 cm in length), trailed by a smaller length of tool and shoulder diameter (1.5 cm in all dimensions). With the help of a single-factor experimental design [27], the main goal of the research is to look at how the angle of tilt affects the production of different torques and forces by two different-shaped tools. Because of this, process variables like tool speed (1000 rpm) and tool traverse (1.5 mm/sec), which were found in the preexperimental work, were kept the same throughout the study.The spindle torque,Y-force, and X-force are measured using a strain gauge-type load cell installed into the bottom of a specially designed clamp. Figure 2 depicts a pictorial representation of the work, along with the depiction of the Y-force and X-force directions, rectangular plates, the AZ91B control board, the friction stir welding machine, and the load cell. The load cell used is a semiautomated model, which has a fixture that is unique for maintaining plates together.Figure 2
Research work setup.A dwell period of 3 seconds, a plunging rate of 25 mm/sec, and a plunge depth of 0.2 mm were all considered for analysis. The majority of the experimental studies [33–36] show that defect-free friction stir welded connections were obtained when an angle of tilt of less than 3°-4° was used. At the same time, several researchers [37, 38] have shown that an angle of tilt of 0° is desirable for high-quality joints. Based on those previous works, the angles of tilt of 0°, 1°, and 2° were used in this study.


## 3. Results and Discussion

A set of tools, TPT and TPNT, were used to join AZ91B-Mg alloys together. In this section, the tool tilt effect, spindle torque vertical force acting downwards (Y-force), and force met by the tool pin axially (X-force) are analysed with macrostructural characteristics of welded specimens being investigated, followed by microstructural analysis through SEM images. Tensile characteristics were evaluated and compared with the parent alloy material of the specimens.Figure3 illustrates the torque deviation induced in the spindle torque vertical force acting downwards(Y-force) and the force met by the tool pin axially (X-force) with regard to the used tool tilt angle. Once the transverse initiates after the dwell completion, the force and torque numerical mean values are considered for the analysis. The beginning of the transition phase (moment at unstable forces and torque) had also been taken into interpretation with the average value, since the difference between them is not very big, and the overall drift of friction stir weldments is to all mean values.Figure 3
(a) Torque induced in the spindle, (b) longitudinal force (X-force) in tool pin, and (c) (Y-force) acting vertically downwards in reference to the angle of tool tilt.
(a)(b)(c)The weldment made with various tool pin shapes with its grain size in the nugget region is briefed in Table2. The nominal diameter of the grain size is calculated by the mean linear intercept approach. From this table, it is clear that as the tool pin angle rises, there is a proportionate grain size increase at the nugget zone. Joints made with TPT have smaller and finer grain sizes when compared to joints made with tapered pins without threads. The weldment made from a tapered tool pin with a threaded shape at 0° produced more refined and smaller grains at the nugget zone, whereas the AZ91B weldment fabricated at 2° of tilt using the TPNT tool produced a bigger grain structure at the nugget zone.Table 2
Grain size in the nugget zone.
Shape of tool0° angle1° angle2° angleTPNTSize of the grain13.1μm15.7μm13.9μmTPTSize of the grain10.2μm12.1μm13.2μmEDX analysis of the weldments TPT tool at 0, 1, and 2 degrees of tilt reveals a large peak of oxygen (O), as well as miniature apexes of carbon (C), zinc (Zn), and a massive apex of magnesium (Mg) and aluminium (Al). These samples contained silicon (Si) particles as small as a few micrometres in size. A frequent scenario for AZ91B is the occurrence of higher percentages of aluminium (Al) and lower percentages of zinc (Zn). Al and Mg are important alloying ingredients for improving weldment characteristics [12, 19]. In this study, detailed research on the creation of specific strength-increasing components was not done because the main goal of the EDX as well as SEM tests was to confirm the occurrence of materials within the nugget region caused by different angles of tilt.Tensile fractography was used in this research to examine the mechanism of fracture in which the constructed joints collapsed. Table3 shows that the joints produced with the TPT tool at 0° and 2° degrees of tilt and the joints manufactured with the TPNT tool at 0° degrees of tilt had dent-like variations in shapes, i.e., gaps that are visible at broad tear rims, suggesting that the specimen has undergone a fracture in ductile mode. Similarly, the cracked top layer or surface of weldments manufactured at 1° and 2° with the TPNT tool reveals the occurrence of dimple kinds of abysses or voids and flat surfaces, which are featured in the fracture type and the trans-granular type. Simultaneously, joint surfaces produced from the fracture specimen with 1° of tilt with the TPT tool show the existence of many layered surfaces and platforms. The brittle type of the fracture causes the broken surfaces of the joints mentioned above to have these unique features.Table 3
SEM images of fractured zone.
Tool type and tool tilt angleFractured surface pictureFractures area SEM picsTPNT tool and 0° tool tilt angleTPNT tool and 1° tool tilt angleTPNT tool and 2° tool tilt angleTPT tool 0° tool tilt angleTPT tool 1° tool tilt angleTPT tool 2° tool tilt angle


### 3.1. Force and Torque Generation

Once the transverse initiates after the dwell completion, the force and torque numerical mean values are considered for the analysis. The onset of the transition phase, when forces and torque were unstable, was also considered with a mean value since their connected disparity is insignificant, and the overall drift of friction stir weldments to all mean values. From Figure2, it can be seen that when using a 0° angle of tilt, both the TPT and TPNT tools are induced to a smooth Y-force, which is in the vertical direction with a spindle torque too. Similarly, with 1° and 2° of tilt, the two tool pin designs experienced an increase in spindle torque and Y-force during the early conversion phase. The primary cause of such behaviour can be linked to the requirement for a massive amount of torque, and the force at the primal phase is for more metal plasticization, but the upsurge in tilt of tool at the shoulder region at weldment heat is not generated at the required quantity [20, 39].Moreover, as the degree of tilt increases, so does the amount of metal to be moved. However, the thermal salving of the material achieved has been insufficient for that large amount of metal compared to other welding methods. It circuitously adds toX-force abnormalities. The changes related to torque as well as forces appear to hurt the tool’s ability to last. Since the TPT tool achieved more torque and Y-force than the TPNT tool, the TPT tool had a lower mean value of X-force for all of the utilized degrees of tilt. This, in turn, causes additional heat to be generated, resulting in greater metal plasticization. Moreover, the plasticized metal flow is improvised by the support of threads in the profile of the TPT tool, reducing the force, i.e., X-force stressing the tool pin. In the usage of TPNT at 2° angle of tool tilt, a small decrease in X-force is noticed, but the value of X-force gets an increase in its value when working with a TPT tool from 0° to 1° angle of tilt. It clarifies the process where there is a considerable improvement in thermal plasticization, and the related metal flow has been reached at a specific degree of tilt, which is the threshold value above which the torque and Y-force get a high value, leading to a high value of frictional heat generation in the weldment [15, 23]. For the same reason, inconsistencies have been reduced at a higher tilt of the tool in TPT tools, despite the tool pin having a greater amount of softened material.Due to the presence of the threads within the tool pin design, i.e., TPT tool, it precisely clobbers the plasticized metal flow, which ensures the proper movement of the material flow. Contrarily, the typical TPNT tool without the threads supports the greater quantity of plasticized metal flow. At higher tilt angles, the threaded tool properly maintains the plasticized metal flow by generating more heat through friction, which proportionally increases the thermal plasticizing effect of the weldment specimen, thereby improving the flow of metal flow is actuated by the tool pin with threads. But as the tool tilt angle gets bigger, the frictional heat generated by the TPNT tool isn’t in the right range or isn’t enough. This causes the weldment to have uneven spots or holes. This is the reason behind the variation inx-force at the higher tilt of the TPNT tool, and fluctuations are noted in the x-force for the prolonged feasibility of tool shape and metal flow [40]. The larger discrepancies, nonconformities, and irregularities of the material flow around the pin as well as the higher variations in the stress are put on the pin when the tool moves.


### 3.2. Microstructure and Macrostructure

Table4 illustrates the cross-sectional macrostructure in joints manufactured using tools with tapered pins with threads (TPT) tool and without threads (TPNT) tool. Except for the zone of agitation of the joint manufactured at 0° angle of tilt, the rest of the 2 weldments made using the TPNT tool don’t show onion ring characteristics. At the very instant, this onion kind of circle had been seen very commonly within the tool stir region produced by the TPNT tool [41]. The onion ring shape comes from the way through the softened metal flow. From Table 4, it may be deduced that the TPNT tool with a 0° angle of tilt has onion ring-type characteristics in its nugget zone. At the same time, when the degree of tilt increases, the presence of these onion ring-type characteristics within the nugget regions of the joints manufactured through the TPNT becomes less noticeable. In the FSW process, the metal is driven out by the tool shoulder by its motion from the side of initiation to the descending side. The metal is then squeezed using the pin of the tool and driven across the surfaces of the tool pin from the side of descendance to initiation. A portion of the ejected metal migrates beneath the pin and to the side of progress. The utilized tool’s shoulder on the progression sideways likewise forces the softened material downhill via the progression region of the tool [27].Table 4
Macrostructural cross-sectional views.
Tool tilt angleTaper pin tool no threads (TPNT)Taper pin tool with threads (TPT)0°1°2°The interactions between the two metal flows in the progression region (a downward flow of weldment metal caused by the shoulder region and a metal flow from the rebuttal area to the tool pin surface area) have led to this onion-ring-shaped pattern at the nugget region [11, 25]. When using the TPNT tool at a 0° angle of tilt, the generated X-force is more consistent than at other degrees of tilt. Similarly, at a 0° angle of tilt, the Y-force parameter value is less. As a consequence, at a 0° angle of tilt, the tool shoulder and pin created the plasticized metal flow and prevented severe contact of these flows of metal, resulting in onion ring-type features. At 0° and 2° of tilt, the pin’s flow of plasticized metal is less stable because the X-force changes are getting bigger.TheY-force has surpassed the X-force in dominance and is continuously expanding. Because the tool pin and shoulder interacted, the metal flowed in a way that caused a deviation in the pin-generated plasticized metal flow [24]. This meant that onion ring-like features didn’t form in the nugget zone. A distinct zone of the nugget may be observed by increasing the angle of tilt from 0° to 1°. Because the resulting forces were insufficient to breakdown the oxide layer on the neighbouring sides of the surface of the AZ91B base plate, a kissing bond formed. Similarly, the downward movement of the plasticized metal was insufficient and subsided root gap formation. But when the angle of tilt is changed from 1° to 2°, it can be noted that in the nugget region, the root gaps are removed. The onion circles could be seen in the nugget zone of all manufactured joints with the TPT tool as well. It is clear in all tool tilt angles that the plasticized metal flow was proper and steady due to the presence of the threaded structure. The TPT tool’s onion ring characteristics show an amorphous layer.The onion ring’s inclined characteristics are not immaculate and have been separated into many distinctive layers [7, 13]. In the bottom section of such onion circles, it could be observable that all of the joints are constructed at three unique degrees of tilt, i.e., 0°, 1°, and 2°. However, the top portion of the onion circles has been poorly perceptible in joints at 0° as well as 1° of tool angle. The phenomenon demonstrates that there is 0° to 1° of tilt. The plasticized metal flow was protruding in the upper region of the nugget zone, which disordered the flow of the metal in between the layers of the metal which is guided by the tool shoulder. In a 2° angle of tilt, such an onion circle structure had been visualized in the upper region of the nugget zone. That proves that when the angle of tool tilt increases the shoulder-driven and pin-driven metal flow, a balance is attained. The variability in the X-force is modest in these joints manufactured at 2° of tilt, suggesting a significantly more continuous plasticized metal flow generated by the welding tool pin. In comparison with the other degrees of tilt, the 2° angle of tilt X-force mean value is higher, which indicates the tool pin is more effective in pushing the plasticized metal [33, 42].The other deduction is that when the angle of tilt increases from 0° to 1°, the breadth of the nugget zone expands, regardless of the shape of the tool pin used. However, increasing the angle of tilt from 1° to 2° results in a lessening in the breadth of the nugget area for both pin designs. Because of the increasedY-force as well as spindle torque at greater degrees of tilt, a greater input of heat was obtained, resulting in the salving of the material [28]. It might have happened as a result of an increase in sliding contact between the base plate and the instrument in use. Moreover, with an increase in the angle of tilt, the plasticized flow of metal increases in the downward direction. This caused lessening of the metal deformation at the nugget zone in direction parallel to the cross of the weldment joint cross-section proportionally reducing the width of the nugget zone formed by 2° angle of tilt. The formation of an equal-sized and uniform grain structure in the nugget zone is due to dynamic recrystallization [38].With an upsurge in the used tool tilt, the size of the grains may be observed to increase concurrently from Table2. When the joints manufactured with the TPT tool were compared to the tools fabricated with the TPNT tool, fine-sized grains were discovered. This demonstrates that the tool pin’s threads had a vital role to play in improving the plasticized metal flow [43]. The Y-force and spindle torque values continue to increase with the tool tilt angle.The generation of frictional heat increases, reaching a peak temperature and a longer cooling period due to the increase inY-force as well as torque parameters [20, 23]. As a result, the duration of exposure to the heat in the nugget region has increased. Under these favourable circumstances, alloys of magnesium are normally more responsive to grain development, which has led to the rise in grain size with regard to tool tilt [13, 40].


### 3.3. SEM and EDAX

The SEM images taken at the area of the boundary between the original alloy and nugget zone, the thermo-mechanically transmuted area, and the zone of nugget in joints are detailed in Table5. The presence of higher percentages of oxygen, aluminium, and other elements shows the presence of intermetallic components of Mg17Al12. The presence of substantial quantities of oxygen (O), aluminium (Al), and carbon (C) in these intermetallic elements was due to a phenomenon known as reaction-based diffusion that happens within the elements of oxygen and carbon existing within the parent metal (AZ91B) and utilized tool. Due to frictional heat generation, the base metal surface of the AZ91B alloy at the welding interface region is prone to high plastic deformation. Mg clings to the surface of the tool pin as a consequence of the bonding between the HSS tool surface and the plasticized magnesium alloy.Table 5
SEM images.
Tool type and angle of tool tiltBoundary region—nugget zone and parent materialThermo-mechanically renovated regionNugget zoneTPNT tool and 0° tool tilt angleTPNT tool and 1° tool tilt angleTPNT tool and 2° tool tilt angleTPT tool 0° tool tilt angleTPT tool 1° tool tilt angleTPT tool 2° tool tilt angleAt higher temperatures, a reaction-based diffusion phenomenon occurs on the surface of a high-speed steel tool, generating Mg17Al12 brittle intermetallic components on the tool pin’s surface [15, 21]. These brittle intermetallic elements breakdown off the pin area and become coagulated within the nugget region during subsequent joining of the AZ91B plates at greater temperature and stress. There are brittle intermetallic mixtures in the nugget region, such as Mg17Al12, which is fabricated by the TPT tool at 0°, 1°, and 2° angles of tilt due to the great number of variations in X-force employed on the tool pin and the larger confrontation against the plasticized metal flow around the tool pin surface. Table 6 shows an EDX spectroscopy analysis of the FSW weldments, the effect of frictional heat produced by the tool, and different degrees of deformation caused by changes in the tool tilt and the tool shape profile, which can be used to understand microstructural characteristics.Table 6
EDX images.
Tool type and angle of tool tiltEDX image of the nugget zoneTPNT tool and 0° tool tilt angleTPNT tool and 1° tool tilt angleTPNT tool and 2° tool tilt angleTPT tool 0° tool tilt angleEDX image of the nugget zoneTPT tool 1° tool tilt angleTPT tool 2° tool tilt angleIn the same instant, welding joints fabricated by the TPNT tool pin at 0°, 1°, and 2° angles of tool tilt at the nugget zone of specimen traces of carbon and oxygen were not observed. An EDAX examination of the nugget zone of the welded joints reveals the presence of lower quantities of silicon, manganese, and other elements. They are the base metal alloying elements (AZ91B), and the presence of these components verifies the occurrence of intermetallic in the combination of materials Mg-Si-Mn at the weldment nugget zone [16, 35].


### 3.4. Tensile Fractography

The specimens of the welded joints were submitted to tensile testing to evaluate the influence of the tool pin shape, degree of tool tilt angle, and corresponding demonstrated values in tensile strength. From Figure4, it is to be noted that joints normally, regardless of the degree of tool tilt used, weldments from the TPT tool display higher effectiveness in tensile characteristics than the ones produced with the TPNT tool. It is also worth noting that the TPT tool demonstrates a consistent and incessant decrease in tensile-related parameters while simultaneously increasing the angle of tilt. Such a characteristic is not visible in joints formed with the TPNT tool. Weldments from the TPT tool at an angle of tilt of 0° had the maximum value of tensile strength-related characteristics. At 2° of tilt, the joint made with the TPT tool has the least amount of tensile strength.Figure 4
Comparison of ultimate tensile strength with tool pin angles.Inferring from Figure4, the increase in the angle of tilt showed a decreasing trend in tensile strength for the TPT tool, with a maximum value of 232 MPa, and about 88.2% compared to the parent alloy material was exhibited by the weldment made with a 0° angle of tilt and the TPT tool. The grain size at the nugget region increased with respect to the angle of tilt generated by the TPT tool, as presented in Table 3. Moreover, for joints made with the TPNT tool, the differences in grain size in the nugget zone do not follow a consistent pattern in relation to the degree of tilt used. The grain size of the joint produced at 1° angle of tilt utilizing a TPNT tool is greater than the grain size created at 0° and 2° tool tilt angles. The least strength (124 MPa) from tensile tests is shown by the weldment specimen fabricated at 1° angle of tool tilt with TPNT tool pin of all the manufactured welding specimens because of the occurrence of large grains that are large at the nugget zone [22, 44]. The larger structure of the grains at the nugget zone in the weldment made with the TPNT tool at a 1° angle of tilt is quickly explained by the variation in the amount of frictional heat generated, the formation of secondary phase intermetallic constituents, and the flow of softened material, all of which are affected by the shape of the tool pin and its effect on the amount of plastic deformation [31].Specimens retrieved from joints manufactured with the TPT tool at 1° and 2° degrees of tilt had fractures at their nuggets. The joint sample manufactured with the TPT tool at 0° angle of tilt, on the other hand, has fractured at its thermo-mechanical reformed area on the side of progression. Because the weldment is brittle, the stress intensity at the nugget zone is not linear. This causes the weldment to have low values during tensile testing, which causes the nugget zone to break at 1° and 2° of tilt of the TPT tool.Specimens taken from joints constructed with the TPNT tool at 0° angle of tilt underwent fracture by ductile mode in their thermo-mechanical transformed zone. It is evident from Figure4 that this joint has displayed a significant value in mechanical strength in comparison to two other joints constructed at 1° and 2° degrees of tilt (using the TPNT tool). Deep inspection of the cracked weldment made by tilting it at an angle of 1° (using the TPNT tool) shows that the tensile fracture followed a unique curved shape. Typically, this fracture or failure would have started at the zone of the nugget base as the zone had the minimum stirring effect by the tool and spread in the form of a kissing bond, making the failure rate high in testing for tensile strength [18, 36].For the weldments manufactured with the TPT tool, a declining trend in tensile strength can be noted, along with an increase in the angle of tilt (as shown in Figure4). Any macrostructural abnormalities were not found for the joints manufactured utilizing the TPT tool at 0° angle of tilt, and on the outside of the nugget zone, this type of weldment encountered failure (in the thermo-mechanically transformed zone on the advancing section). Tensile strength ratings for joints prepared at 1° of tilt (using the TPNT tool) and 2° of tilt (using the TPT tool) were the lowest of any produced joints. The existence of intermetallic elements that are stiff and tough in character was the primary cause of these low tensile strength values [10, 25]. Table 6 also shows that most of the welded samples have cracks right away in the nugget zone, except for the sample of the weld made with a 0° angle of tool tilt and a TPT tool. This sample has a crack on the side where the thermomechanical transformation happened.


## 3.1. Force and Torque Generation

Once the transverse initiates after the dwell completion, the force and torque numerical mean values are considered for the analysis. The onset of the transition phase, when forces and torque were unstable, was also considered with a mean value since their connected disparity is insignificant, and the overall drift of friction stir weldments to all mean values. From Figure2, it can be seen that when using a 0° angle of tilt, both the TPT and TPNT tools are induced to a smooth Y-force, which is in the vertical direction with a spindle torque too. Similarly, with 1° and 2° of tilt, the two tool pin designs experienced an increase in spindle torque and Y-force during the early conversion phase. The primary cause of such behaviour can be linked to the requirement for a massive amount of torque, and the force at the primal phase is for more metal plasticization, but the upsurge in tilt of tool at the shoulder region at weldment heat is not generated at the required quantity [20, 39].Moreover, as the degree of tilt increases, so does the amount of metal to be moved. However, the thermal salving of the material achieved has been insufficient for that large amount of metal compared to other welding methods. It circuitously adds toX-force abnormalities. The changes related to torque as well as forces appear to hurt the tool’s ability to last. Since the TPT tool achieved more torque and Y-force than the TPNT tool, the TPT tool had a lower mean value of X-force for all of the utilized degrees of tilt. This, in turn, causes additional heat to be generated, resulting in greater metal plasticization. Moreover, the plasticized metal flow is improvised by the support of threads in the profile of the TPT tool, reducing the force, i.e., X-force stressing the tool pin. In the usage of TPNT at 2° angle of tool tilt, a small decrease in X-force is noticed, but the value of X-force gets an increase in its value when working with a TPT tool from 0° to 1° angle of tilt. It clarifies the process where there is a considerable improvement in thermal plasticization, and the related metal flow has been reached at a specific degree of tilt, which is the threshold value above which the torque and Y-force get a high value, leading to a high value of frictional heat generation in the weldment [15, 23]. For the same reason, inconsistencies have been reduced at a higher tilt of the tool in TPT tools, despite the tool pin having a greater amount of softened material.Due to the presence of the threads within the tool pin design, i.e., TPT tool, it precisely clobbers the plasticized metal flow, which ensures the proper movement of the material flow. Contrarily, the typical TPNT tool without the threads supports the greater quantity of plasticized metal flow. At higher tilt angles, the threaded tool properly maintains the plasticized metal flow by generating more heat through friction, which proportionally increases the thermal plasticizing effect of the weldment specimen, thereby improving the flow of metal flow is actuated by the tool pin with threads. But as the tool tilt angle gets bigger, the frictional heat generated by the TPNT tool isn’t in the right range or isn’t enough. This causes the weldment to have uneven spots or holes. This is the reason behind the variation inx-force at the higher tilt of the TPNT tool, and fluctuations are noted in the x-force for the prolonged feasibility of tool shape and metal flow [40]. The larger discrepancies, nonconformities, and irregularities of the material flow around the pin as well as the higher variations in the stress are put on the pin when the tool moves.


## 3.2. Microstructure and Macrostructure

Table4 illustrates the cross-sectional macrostructure in joints manufactured using tools with tapered pins with threads (TPT) tool and without threads (TPNT) tool. Except for the zone of agitation of the joint manufactured at 0° angle of tilt, the rest of the 2 weldments made using the TPNT tool don’t show onion ring characteristics. At the very instant, this onion kind of circle had been seen very commonly within the tool stir region produced by the TPNT tool [41]. The onion ring shape comes from the way through the softened metal flow. From Table 4, it may be deduced that the TPNT tool with a 0° angle of tilt has onion ring-type characteristics in its nugget zone. At the same time, when the degree of tilt increases, the presence of these onion ring-type characteristics within the nugget regions of the joints manufactured through the TPNT becomes less noticeable. In the FSW process, the metal is driven out by the tool shoulder by its motion from the side of initiation to the descending side. The metal is then squeezed using the pin of the tool and driven across the surfaces of the tool pin from the side of descendance to initiation. A portion of the ejected metal migrates beneath the pin and to the side of progress. The utilized tool’s shoulder on the progression sideways likewise forces the softened material downhill via the progression region of the tool [27].Table 4
Macrostructural cross-sectional views.
Tool tilt angleTaper pin tool no threads (TPNT)Taper pin tool with threads (TPT)0°1°2°The interactions between the two metal flows in the progression region (a downward flow of weldment metal caused by the shoulder region and a metal flow from the rebuttal area to the tool pin surface area) have led to this onion-ring-shaped pattern at the nugget region [11, 25]. When using the TPNT tool at a 0° angle of tilt, the generated X-force is more consistent than at other degrees of tilt. Similarly, at a 0° angle of tilt, the Y-force parameter value is less. As a consequence, at a 0° angle of tilt, the tool shoulder and pin created the plasticized metal flow and prevented severe contact of these flows of metal, resulting in onion ring-type features. At 0° and 2° of tilt, the pin’s flow of plasticized metal is less stable because the X-force changes are getting bigger.TheY-force has surpassed the X-force in dominance and is continuously expanding. Because the tool pin and shoulder interacted, the metal flowed in a way that caused a deviation in the pin-generated plasticized metal flow [24]. This meant that onion ring-like features didn’t form in the nugget zone. A distinct zone of the nugget may be observed by increasing the angle of tilt from 0° to 1°. Because the resulting forces were insufficient to breakdown the oxide layer on the neighbouring sides of the surface of the AZ91B base plate, a kissing bond formed. Similarly, the downward movement of the plasticized metal was insufficient and subsided root gap formation. But when the angle of tilt is changed from 1° to 2°, it can be noted that in the nugget region, the root gaps are removed. The onion circles could be seen in the nugget zone of all manufactured joints with the TPT tool as well. It is clear in all tool tilt angles that the plasticized metal flow was proper and steady due to the presence of the threaded structure. The TPT tool’s onion ring characteristics show an amorphous layer.The onion ring’s inclined characteristics are not immaculate and have been separated into many distinctive layers [7, 13]. In the bottom section of such onion circles, it could be observable that all of the joints are constructed at three unique degrees of tilt, i.e., 0°, 1°, and 2°. However, the top portion of the onion circles has been poorly perceptible in joints at 0° as well as 1° of tool angle. The phenomenon demonstrates that there is 0° to 1° of tilt. The plasticized metal flow was protruding in the upper region of the nugget zone, which disordered the flow of the metal in between the layers of the metal which is guided by the tool shoulder. In a 2° angle of tilt, such an onion circle structure had been visualized in the upper region of the nugget zone. That proves that when the angle of tool tilt increases the shoulder-driven and pin-driven metal flow, a balance is attained. The variability in the X-force is modest in these joints manufactured at 2° of tilt, suggesting a significantly more continuous plasticized metal flow generated by the welding tool pin. In comparison with the other degrees of tilt, the 2° angle of tilt X-force mean value is higher, which indicates the tool pin is more effective in pushing the plasticized metal [33, 42].The other deduction is that when the angle of tilt increases from 0° to 1°, the breadth of the nugget zone expands, regardless of the shape of the tool pin used. However, increasing the angle of tilt from 1° to 2° results in a lessening in the breadth of the nugget area for both pin designs. Because of the increasedY-force as well as spindle torque at greater degrees of tilt, a greater input of heat was obtained, resulting in the salving of the material [28]. It might have happened as a result of an increase in sliding contact between the base plate and the instrument in use. Moreover, with an increase in the angle of tilt, the plasticized flow of metal increases in the downward direction. This caused lessening of the metal deformation at the nugget zone in direction parallel to the cross of the weldment joint cross-section proportionally reducing the width of the nugget zone formed by 2° angle of tilt. The formation of an equal-sized and uniform grain structure in the nugget zone is due to dynamic recrystallization [38].With an upsurge in the used tool tilt, the size of the grains may be observed to increase concurrently from Table2. When the joints manufactured with the TPT tool were compared to the tools fabricated with the TPNT tool, fine-sized grains were discovered. This demonstrates that the tool pin’s threads had a vital role to play in improving the plasticized metal flow [43]. The Y-force and spindle torque values continue to increase with the tool tilt angle.The generation of frictional heat increases, reaching a peak temperature and a longer cooling period due to the increase inY-force as well as torque parameters [20, 23]. As a result, the duration of exposure to the heat in the nugget region has increased. Under these favourable circumstances, alloys of magnesium are normally more responsive to grain development, which has led to the rise in grain size with regard to tool tilt [13, 40].


## 3.3. SEM and EDAX

The SEM images taken at the area of the boundary between the original alloy and nugget zone, the thermo-mechanically transmuted area, and the zone of nugget in joints are detailed in Table5. The presence of higher percentages of oxygen, aluminium, and other elements shows the presence of intermetallic components of Mg17Al12. The presence of substantial quantities of oxygen (O), aluminium (Al), and carbon (C) in these intermetallic elements was due to a phenomenon known as reaction-based diffusion that happens within the elements of oxygen and carbon existing within the parent metal (AZ91B) and utilized tool. Due to frictional heat generation, the base metal surface of the AZ91B alloy at the welding interface region is prone to high plastic deformation. Mg clings to the surface of the tool pin as a consequence of the bonding between the HSS tool surface and the plasticized magnesium alloy.Table 5
SEM images.
Tool type and angle of tool tiltBoundary region—nugget zone and parent materialThermo-mechanically renovated regionNugget zoneTPNT tool and 0° tool tilt angleTPNT tool and 1° tool tilt angleTPNT tool and 2° tool tilt angleTPT tool 0° tool tilt angleTPT tool 1° tool tilt angleTPT tool 2° tool tilt angleAt higher temperatures, a reaction-based diffusion phenomenon occurs on the surface of a high-speed steel tool, generating Mg17Al12 brittle intermetallic components on the tool pin’s surface [15, 21]. These brittle intermetallic elements breakdown off the pin area and become coagulated within the nugget region during subsequent joining of the AZ91B plates at greater temperature and stress. There are brittle intermetallic mixtures in the nugget region, such as Mg17Al12, which is fabricated by the TPT tool at 0°, 1°, and 2° angles of tilt due to the great number of variations in X-force employed on the tool pin and the larger confrontation against the plasticized metal flow around the tool pin surface. Table 6 shows an EDX spectroscopy analysis of the FSW weldments, the effect of frictional heat produced by the tool, and different degrees of deformation caused by changes in the tool tilt and the tool shape profile, which can be used to understand microstructural characteristics.Table 6
EDX images.
Tool type and angle of tool tiltEDX image of the nugget zoneTPNT tool and 0° tool tilt angleTPNT tool and 1° tool tilt angleTPNT tool and 2° tool tilt angleTPT tool 0° tool tilt angleEDX image of the nugget zoneTPT tool 1° tool tilt angleTPT tool 2° tool tilt angleIn the same instant, welding joints fabricated by the TPNT tool pin at 0°, 1°, and 2° angles of tool tilt at the nugget zone of specimen traces of carbon and oxygen were not observed. An EDAX examination of the nugget zone of the welded joints reveals the presence of lower quantities of silicon, manganese, and other elements. They are the base metal alloying elements (AZ91B), and the presence of these components verifies the occurrence of intermetallic in the combination of materials Mg-Si-Mn at the weldment nugget zone [16, 35].


## 3.4. Tensile Fractography

The specimens of the welded joints were submitted to tensile testing to evaluate the influence of the tool pin shape, degree of tool tilt angle, and corresponding demonstrated values in tensile strength. From Figure4, it is to be noted that joints normally, regardless of the degree of tool tilt used, weldments from the TPT tool display higher effectiveness in tensile characteristics than the ones produced with the TPNT tool. It is also worth noting that the TPT tool demonstrates a consistent and incessant decrease in tensile-related parameters while simultaneously increasing the angle of tilt. Such a characteristic is not visible in joints formed with the TPNT tool. Weldments from the TPT tool at an angle of tilt of 0° had the maximum value of tensile strength-related characteristics. At 2° of tilt, the joint made with the TPT tool has the least amount of tensile strength.Figure 4
Comparison of ultimate tensile strength with tool pin angles.Inferring from Figure4, the increase in the angle of tilt showed a decreasing trend in tensile strength for the TPT tool, with a maximum value of 232 MPa, and about 88.2% compared to the parent alloy material was exhibited by the weldment made with a 0° angle of tilt and the TPT tool. The grain size at the nugget region increased with respect to the angle of tilt generated by the TPT tool, as presented in Table 3. Moreover, for joints made with the TPNT tool, the differences in grain size in the nugget zone do not follow a consistent pattern in relation to the degree of tilt used. The grain size of the joint produced at 1° angle of tilt utilizing a TPNT tool is greater than the grain size created at 0° and 2° tool tilt angles. The least strength (124 MPa) from tensile tests is shown by the weldment specimen fabricated at 1° angle of tool tilt with TPNT tool pin of all the manufactured welding specimens because of the occurrence of large grains that are large at the nugget zone [22, 44]. The larger structure of the grains at the nugget zone in the weldment made with the TPNT tool at a 1° angle of tilt is quickly explained by the variation in the amount of frictional heat generated, the formation of secondary phase intermetallic constituents, and the flow of softened material, all of which are affected by the shape of the tool pin and its effect on the amount of plastic deformation [31].Specimens retrieved from joints manufactured with the TPT tool at 1° and 2° degrees of tilt had fractures at their nuggets. The joint sample manufactured with the TPT tool at 0° angle of tilt, on the other hand, has fractured at its thermo-mechanical reformed area on the side of progression. Because the weldment is brittle, the stress intensity at the nugget zone is not linear. This causes the weldment to have low values during tensile testing, which causes the nugget zone to break at 1° and 2° of tilt of the TPT tool.Specimens taken from joints constructed with the TPNT tool at 0° angle of tilt underwent fracture by ductile mode in their thermo-mechanical transformed zone. It is evident from Figure4 that this joint has displayed a significant value in mechanical strength in comparison to two other joints constructed at 1° and 2° degrees of tilt (using the TPNT tool). Deep inspection of the cracked weldment made by tilting it at an angle of 1° (using the TPNT tool) shows that the tensile fracture followed a unique curved shape. Typically, this fracture or failure would have started at the zone of the nugget base as the zone had the minimum stirring effect by the tool and spread in the form of a kissing bond, making the failure rate high in testing for tensile strength [18, 36].For the weldments manufactured with the TPT tool, a declining trend in tensile strength can be noted, along with an increase in the angle of tilt (as shown in Figure4). Any macrostructural abnormalities were not found for the joints manufactured utilizing the TPT tool at 0° angle of tilt, and on the outside of the nugget zone, this type of weldment encountered failure (in the thermo-mechanically transformed zone on the advancing section). Tensile strength ratings for joints prepared at 1° of tilt (using the TPNT tool) and 2° of tilt (using the TPT tool) were the lowest of any produced joints. The existence of intermetallic elements that are stiff and tough in character was the primary cause of these low tensile strength values [10, 25]. Table 6 also shows that most of the welded samples have cracks right away in the nugget zone, except for the sample of the weld made with a 0° angle of tool tilt and a TPT tool. This sample has a crack on the side where the thermomechanical transformation happened.


## 4. Conclusions

This research reveals the variation of the angle of tool tilt using two varied tool pin structures on varied friction stir weldments of AZ91B alloy, such as mechanical strength, spindle torque, forces encountered, and microstructural characteristics. The following are the inferences presented in this study endeavour.(i)
At the interface of weldment between the AZ91B plates, spindle torque and forces of friction were produced, which increased with the increase in the angle of tilt. Regardless of the tool pin design used, a smooth distribution of spindle torque and FSW-associated forces was observed at a 0° angle of tilt. Because of the increased depth of dive at the trail’s edge,Y-force increases with the angle of tilt. Because of the use of the TPT tool, the tool-workpiece area of contact has expanded, which induces high Y-force and spindle torque values irrespective of the degree of tilt.(ii)
The TPT tool had anX-force value of less compared to the force value produced by the TPNT tool because the high torque and Y-forces produced by the TPT tool gave rise to the evolution of a high frictional heat, which promptly improved the plasticized flow of the metal, which decreased the forces resisting that are induced in the tool pin. These things have also indirectly helped to make the oscillations of the joining force less.(iii)
Onion ring-like characteristics were noticed in weldments created with the TPT tool. Onion ring-type characteristics can’t be easily viewed in joints created using the TPNT tool. The increased plasticized metal flow caused by the tool pin with threaded structure TPT has supported the formation of the onion ring shapes in the joint of the weldment. The thickness of the nugget zone of manufactured joints has decreased as the angle of tilt has gone up.(iv)
The weldments made with the TPT tool showed an increasing trend in the grain size as the angle of tilt increased at the nugget region.The tensile characteristics of joints manufactured with the TPT tool show a diminishing trend with an increase in the angle of tilt. The maximum tensile strength of 232 MPa, i.e., 88.2% of the parent material tensile strength value, was achieved by a 0° angle of tilt by the TPT tool.



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*Source: 8311413-2022-12-05.xml*