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Abstract
This study aims to extrude a portion of an Al sheet through a steel hole shaped like a rivet to assemble an Al and a preholed steel sheet by a mechanical interlock. This will be accomplished by joining the two materials together. AA6061 and AISI1006 sheets were assembled using a lap arrangement, with the steel preholed sheet placed below the aluminum sheet. A rivet head die was placed beneath the hole of the steel sheet. To attach the parts, a spinning tool used friction stir spot extrusion to force the Al through the die and into the steel hole. The influence of the hole diameter, tool depth of plunge, and revolving speed on the dimensions of the rivet head (thickness and diameter) and joint strength of shear were analyzed by the design of the experiment’s technique. The joint microstructure was investigated. An extrusion of the Al metal in the form of a rivet was conducted successfully. The diameter of the hole in the steel sheet revealed a critical influence on the joints’ force of shear and the dimensions of the rivet head. Raising the tool depth of the plunge increases the joint’s strength of shear. Due to the shearing of the extruded aluminum at the steel hole surface, the joined sheets failed. This occurred without the produced rivet head being detached. A mechanical interlock was the joining mechanism without intermetallic compounds forming between the two materials. AISI 1006 preholed steel and AA6061 sheets were assembled for the first time by extruding the Al with a flat rivet shape. The joint’s shear strength efficiency was 94.6%.
1. Introduction
Reducing fuel consumption, toxic emissions, and weight attracts excellent attention in industrial applications. Aluminum alloys with the same design largely replaced steel in many industrial, transportation, and shipbuilding uses to decrease fuel consumption and overall structure weight [1]. Aluminum alloys are characterized by high thermal conductivity, low density, high strength, and good corrosion resistance [2]. Different welding or joining techniques were used to allow the use of aluminum alloys in various industrial applications [3]. At the moment, there are a variety of joining procedures, each of which has its own set of restrictions amount for combining multimaterial structures [4, 5]. When it comes to combining materials that are somewhat different from one another, they are often too rigid and inflexible to respond rapidly to changing needs.
On top of that, these joining techniques are plagued by issues such as differing thermal expansion coefficients, chemical incompatibility, and uneven stiffness. In addition, traditional joining procedures have difficulties, as metals are frequently not very weldable and are not distinct [6]. Because of this, attempts are being made to create new and creative bonding methods and further enhance current joining operations that are distinguished by their adaptability and can thus respond to differences in batches or changes in operation circumstances [7].
A mechanical connecting procedure without heat input, self-piercing riveting (SPR) [8] is beneficial for combining two or more metals with an auxiliary bonding device [9]. With SPR, a nondetachable joint may be easily made in only one step. A blank holder, punch, and die are necessary for establishing riveted joints. There are essentially four steps to the whole procedure. The rivet and its surrounding blank holder first secure the joining components to the die. The rivet drills its prehole in the punch-sided sheet due to the punch’s increased feed rate. Interlocks are formed when rivets undergo plastic deformation and extend radially into die-sided sheets. This leads to the formation of a link that is both form-fit and force-fit. The blank holder and punch are returned to their original positions [10].
Concerning the mechanical jointing of metallic sheets, the SPR technique is a tried-and-true cold-forming method widely used in joining metal sheets, especially steel and aluminum [11]. The SPR process is relatively immune to characteristics influencing other joining techniques, such as spot-welding procedures. To provide only one example, it is not contingent on the melting points of the alloys. Its use has grown across various industries, notably in the aerospace and automotive industries [12]. Up to this point, it has been shown that apart from aluminum and steel, various comparable and different material stacks may be mechanically coupled utilizing correctly designed SPR methods.
Abe et al. [13] used the SPR method to join AA5052 to mild steel. They observed that the penetration through the lower sheet and its necking determine the defects of joints. Abe et al. [14] used the SPR technique to combine AA5052 with several kinds of alloyed steel. The bonding strength of tensile was less than 590 MPa.
Clinching (CL) [15] has been described as combining several profile pieces, tubes, or overlapping sheet metal using a punch and die to create at least one joining partner. The process begins with the joining partners partly interspersed, then shear forming partially displaces the sheet material from its original plane, and finally, stamping creates a junction via radial material flow (DIN, 2003b-09) [16]. Once the blank holder has fastened the parts to be combined, the setup procedure begins. The punch is guided through the pieces until it hits the die’s base. The next stage involves reducing the thickness of the joining components due to the growing joining force, which in turn initiates a radial flow of material that fills the die shape. A narrow gap forms between the two connecting partners due to this radial flow of material. The blank holder is used as a stripper during the return stroke. Based on the forming die’s specifications, the connecting element may be created in closed or split dies. The stress state generated around the joints differs in these two cases [17].
Another process called clinch riveting (CR) technology [18] is similar to the CL and SPR. A solid rivet is plastically deformed to produce the lock, which comprises connected sheet metals [19]. The lock is created using CL technology, such as a stiff punch and the appropriate die. The SPR process involves punching holes in the top sheet metal to insert the rivet. It secures the sheet metals by fastening the rivet to the underside of the material. Alternatively, the CR method secures the solid rivet in the top sheet metal by forcing it into place. The metal sheet at the top is fastened to the substance at the bottom. The joint is formed in this manner.
In recent years, friction stir spot welding (FSSW) and friction stir welding (FSW) methods have been commonly used in welding aluminum alloys together with steel. The FSW is a solid-state method that involves forming at the interface between welded dissimilar metals intermetallic compounds (IMCs) [20]. Pourali et al. [21] investigated the influence of FSW variables on IMC generation. St37 steel and AA1100 lap joints were performed with different tool welding speeds and rotating. IMCs of FeAl and Fe3Al formed at joint interface, according to the welding settings, with a thickness ranging from 4 to 93 μm. Low rotating and high welding speeds developed one type of IMCs (FeAl) of 4–25 μm thickness. Helal et al. [22] welded AA6061 to ultra-low-carbon steel sheets with a lap joint configuration using the FSW procedure. The welding method achieves constant revolving speed and a wide range of welding speeds. IMCs of the FeAl were observed at joint interfaces.
Increasing the speed of welding strengthened the joints by decreasing the IMCs’ layer thickness. Jamshidi Aval and Loureiro [23] improved the AA7075-AISI304 FSW joint quality using a DR-FSW procedure. A unique tool design was used in the DR-FSW and compared with the conventional FSW (C-FSW) process. The DR-FSW process resulted in refining grains of the nugget zone, reducing the microstructure change of the AA7075 and the number of IMCs in comparison to the C-FSW. Wang et al. [24] derived a theoretical model of FeAl IMC formation during the FSW of AA6022 to DP600 steel. A scribe tool was used to weld the two materials, which lowered the thickness of the IMC layer to a nanoscale. Anaman et al. [25] fabricated a butt joint of AA5052-H32 to dual-phase steel using the FSW technique, with a tool offset toward the steel side. The stir zone of the joint consisted of three regions: top, middle, and bottom layers, which included an aluminum matrix with steel fragments IMCs of the FeAl and steel, respectively. Also, the IMCs were observed between the two materials at the interface region.
Moreover, The IMCs formation mechanism occurs in the FSSW technique of aluminum and steel. Fereiduni et al. [26] observed a critical layer thickness of 2.3 mm of IMC at the interface joint of AA5083 to steel alloy sheets. The joints were carried out using FSSW. Thin and thick IMC exhibited higher and lower joint strength, respectively. Dong et al. [27] joined an aluminum alloy of type Novelist AC 170 PX to a galvanized steel sheet using a refilled FSSW technique. The joint interface exhibited an IMC of Zn and O on the aluminum side, with a width of 0.68 μm. Chen et al. [28] high-strength steel was used to fill the keyhole in the FSSW of the AA6061 welding. Thin layers were observed at the hook structure. Hsieh et al. [29] explored the development of IMCs during low-carbon steel FSS fusion welding to AA6061. Two types of IMCs were observed: Fe4Al13 and Fe2Al5. Approximately, the IMCs’ thickness ranged between 1.5 and 25 μm. Shen et al. [30] observed that the joint strength of coated steel to AA5754, which was welded by the FSSW technique, was highly affected by IMCs.
Recently, to avoid the brittle and hard IMCs, the aluminum alloys are joined with steel without stirring between the two materials. Cai et al. [31] investigated the differences in joint characteristics between two techniques: the resistance spot welding technique (RSW) and the SPR. In light of the findings, it was determined that the SPR had a greater degree of joint distortion than the RSW.
Huang et al. [32] used the self-riveting friction process to bond AA6082 with QSTE340TM steel. The joining mechanism occurred by flowing the aluminum metal by a revolving tool through a preholed steel. A value of 3.21 kN was attained for the average joint fracture load. In the presence of a diffusion layer, a metallurgical bonding took place between the two varieties of materials. Lazarevic et al. [33] analyzed the friction stir method in joining AA6014 to AISI5182. The joining mechanism occurred by heating the aluminum and forming it through the steel holes, with the two materials locking together mechanically. There were two types of fractures that caused the joints to fail: peeling of the aluminum sheet and braze fracture. Evans et al. [34] to join low-carbon steel with AA6061, a process known as friction stir extrusion (FSE) was used. By using the mechanical interlocking mechanism, this approach was able to eliminate the development of IMCs for the two materials. Huang et al. [35] investigated the joint properties of type AA6082 to QSTE340 TM steel. A self-riveting friction stir lap welding method was adopted to fill preholes of steel sheets with deformed aluminum. The strength of the joint exceeded the C-FSW by 23%. Hussein et al. [36] extruded AA5052 through AISI1006 steel predrilled and threaded using a frictional forming procedure. The joint strength was most affected by the depth to which the tool was plunged.
This work utilized a novel friction stir spot extrusion joining method to join aluminum alloy and steel without any IMC formation. A portion of the aluminum upper sheet was extruded into a predrilled steel sheet with a rivet shape to prevent the dislocation of aluminum during the shear loading. To conduct the tests and analyze how the joining factors affected the joint characteristics, the experimental method, design of the experiment (DOE), was adopted as the design. The joint’s microstructure was examined.
2. Experiment Details
2.1. Specimens
Low-carbon steel type AISI-1006 and aluminum alloy AA 6061-T6 sheets were used with 1.5 and 2.4 mm thicknesses, respectively. Their chemical composition, according to the manufacturer’s MSDS datasheet, is presented in Table 1. The mechanical characteristics per specimen are summarized in Table 2. A precise thickness balance was achieved by selecting a steel sheet with a thickness of 1.5 mm; with a thinner steel sheet, the rivet head’s neck might be sliced with less shear force, leading to a misguidance from the true strength of the joint. From an industrial and economic perspective, it is not a good indication that thicker Al sheets are required to fill the hole and die gap when thicker steel sheets are employed.
| Element | Ti% | Cu% | Zn% | Cr% | Fe% | Si% | Mn% | Mg% | Al% |
|---|---|---|---|---|---|---|---|---|---|
| AA 6061-T6 (wt.) | 0.11 | 0.09 | 0.19 | 0.19 | 0.49 | 0.74 | 0.39 | 0.59 | Bal. |
| Element | Si% | Cu% | Ni% | Cr% | P% | S% | C% | Mn% | Fe% |
| AISI-1006 (wt.) | 0.06 | 0.08 | 0.07 | 0.05 | 0.039 | 0.049 | 0.077 | 0.31 | Bal. |
| Shear strength (MPa) | Tensile strength (MPa) | Yield strength (MPa) | |
|---|---|---|---|
| AA6061-T6 [40] | 203 | 254 | 249 |
| AISI1006 [41] | 230 | 330 | 280 |
In any case, spot friction stir works well with thinner materials [37, 38]. It has been settled on this thickness for the Al because it was the bare minimum needed to seal the hole and rivet head while retaining a respectable amount of joint strength. However, many typical engineering and industrial applications fall within the range of these two thicknesses (2.4 mm and 1.5 mm). Hussein et al. [36] used 2.4-mm Al and 1.4-mm steel sheets to be joined using this technique without a rivet head. They drilled a hole of Ø 4.8 mm in the steel sheet and then had one inner M6 prethreaded. They partially formed the upper aluminum sheet by extruding it via the hole in the steel sheet and penetrating it through the thread slots. The steel and extruded aluminum were able to lock together mechanically. Another study by Abdullah et al. [39] used 2.4-mm Al and 1.4-mm steel sheets to be joined using the same technique without a rivet head. This time, a stepped holed was drilled (Ø4 mm through hole and Ø8 mm hole at a depth of 0.5 mm) at the same axis.
Following the American Welding Society (AWS C1.1 M/C1.1:2012) standard specification, each specimen was constructed with 25 × 100 mm2 dimensions. This was done to ensure that the area of the lap joint was 25 × 25 mm2. The steel specimen was holed with a specific diameter (d) at the center of the lapped joint area at 12.5 mm from the sheet’s edge, as shown in Figure 1. The extruding pinless tool was made of H13 alloy steel, having a diameter of 10 mm and 80 mm in length. A K-type thermocouple was attached to one side of the Al upper sheet to measure the process temperature. The thermocouple was fixed at 12.5 mm from the sheets’ end and connected to a temperature data logger.
Figure 1
2.2. Configuration
The assembly included five steps, as illustrated in Figures 2(a), 2(b), 2(c), 2(d), 2(e). Joint configuration was achieved by arranging a steel specimen on top of a circular die set on a lower steel fixture, placing an aluminum specimen on top of the steel, and attaching a collar to the aluminum sample. Suitable bolts tighten the upper fixture with the steel fixture; see Figures 2(f) and 2(g). The collar and die were manufactured from carbon steel. The circular collar was bored with a hole diameter of 11 mm, allowing the tool to rotate freely. For the circular die, a hole with a flat surface was bored having 1 mm depth and 6 mm diameter. Full-hardened H13 tool steel (57 HRC) was used to manufacture the revolving tool, having a diameter of 10 mm (less than the collar hole diameter).
Figure 2 (a)
Figure 2 (b)
Figure 2 (c)
Figure 2 (d)
Figure 2 (e)
Figure 2 (f)
2.3. Process Components’ Functions
During the revolving friction of the lower tool layer to the Al sheet, enough heat was generated to soften the metal. The tool can also be plunged through the Al sheet to penetrate and extrude through the steel hole. During the operation, the circular collar was used to avoid the development of a flash directed outward. The function of the die was to form the extruded aluminum with a riveted head shape below the steel sheet’s lower surface. The fixtures were used during the joining operation to prevent sheets from sliding.
2.4. Principle of Joining
Figure 3 presents the two materials joining stages. The joint was achieved in two stages: preheat and plunging of the tool. In the first stage, the revolving tool touches the upper layer of Al during a preheating time of 10 s. The temperature steadily rises with time due to the rotational friction between the Al sheet and the tool. The Al metal under the revolving tool surface becomes softer due to the heat input. The second stage represents plunging the revolving tool via the softened Al under the tool pressure effect. Following its passage through the die hole, the aluminum that has been softened flows or extrudes through the steel sheet hole. Depending on the characteristics of the assembling procedure, the extruded aluminum will enter the hole in the steel surface. The process parameters determine the diameter and thickness of the rivet head, which is formed by extruding Al via a die hole.
Figure 3 (a)
Figure 3 (b)
Figure 3 (c)
2.5. Process Parameters
It investigated how the quality of the joining was affected by three different process factors: the size of the hole, depth of tool plunge, and rotating speed. For each parameter, three levels were taken into account. The experiment conditions for each level of the parameter were planned as per the DOE approach. Using a MINITAB program, a Taguchi procedure was adopted to prepare the variable level by L9 runs per attempt. Accordingly, nine runs were designed to achieve the process of joining, as illustrated in Table 3. A preheating duration of 10 s was consistently used throughout all trials. Three rotating speeds (1120, 1400, and 1800 rpm), three plunging depths (0.5, 1, and 1.5 mm), and three-hole diameters (2, 3, and 4 mm) were used in this study. At each plunging depth, three runs with different rotating speeds were conducted to reveal the effect of the tool’s rotating speed on joint quality.
| Run no. | Hole diameter, d (mm) | Plunging depth (mm) | Rotating speed (RPM) |
|---|---|---|---|
| 1 | 2 | 0.5 | 1120 |
| 2 | 2 | 1.0 | 1400 |
| 3 | 2 | 1.5 | 1800 |
| 4 | 3 | 1.0 | 1120 |
| 5 | 3 | 1.5 | 1400 |
| 6 | 3 | 0.5 | 1800 |
| 7 | 4 | 1.5 | 1120 |
| 8 | 4 | 0.5 | 1400 |
| 9 | 4 | 1.0 | 1800 |
Figure 4 depicts an example of the combined specimens in upper and lower views. The rivet head shape under the steel surface was generated when the Al was extruded through the steel sheet hole. In addition, no flash of Al has developed on the Al sheet top surface.
Figure 4
3. Results and Discussion
3.1. Rivet Head Dimensions
Figure 5 illustrates the variation of each run-formed rivet head dimensions (diameter and thickness). For all the samples, the aluminum metal extrudes via the hole in the steel sheet, passing to the hole of the die and shaping a rivet head with variable dimensions. The thickness of the rivet heads varied between 0.4 and 1 mm, while the diameter was between 2.4 and 5.4 mm. The dimension deviation of the rivet head exhibited the same behavior: The size and thickness increased gradually in samples 1 to 5. In contrast, the other samples (6–9) exhibited alternative values with the same behavior. Consequently, the rivet head dimensions change depending on the join process parameters.
Figure 5 (a)
Figure 5 (b)
The rivet head diameter (see Figure 6) and thickness (see Figure 7) were analyzed with DOE. The main effect plot, Figures 6(a) and 7(a), indicates that the dimensions of the rivet head increased with rising the tool plunging depth and the hole diameter in the steel sheet. Rising the tool depth of the plunge raised the pressure applied on the softened metal, which increased the extruded Al amount via the steel and die hole [36, 42]. Compared to the smaller diameter, the big hole diameter in the steel sheet made it possible for a greater quantity of the softened Al to flow through. Pareto chart Figures 6(b) and 7(b) indicate that the most significant influence on rivet head size was the size of the steel hole, followed by the depth of plunging and tool rotation speed. All rotational speeds produced enough heat to soften the Al alloy. The amount of the softened Al required sufficient pressure and outlet to flow in the die hole. The amount of the flowed aluminum determined the rivet head dimensions.
Figure 6 (a)
Figure 6 (b)
Figure 7 (a)
Figure 7 (b)
Table 4 shows the estimated relationships between the experimental parameters used in the DOE analysis for the thickness and diameter of the rivet heads. The predicted data were plotted in Figure 8 under the same experimental conditions and compared with the measured dimensions of the rivet head. The predicted relations of the rivet dimensions agreed well with the measured dimensions.
| Term | Coefficients of hole diameter | Coefficients of thickness |
|---|---|---|
| Constant | −0.599894 | −0.173974 |
| Diameter of hole (mm) | 1.07 | 0.1994587 |
| Rotating speed (RPM) | 0.000360112 | 7.09945e − 05 |
| Depth of plunging (mm) | 0.969974 | 0.189 |
Figure 8 (a)
Figure 8 (b)
3.2. Shear Test
Shear forces of the joints at fracture were recorded and plotted in Figure 9(a). The data ranged from 550 to 2420 N. Minimum shear forces were observed at samples 1, 2, and 3, which joined at minimum steel hole diameter (d = 2 mm). There was an increase in joint shear stress as the diameter of the steel holes became larger. A higher steel hole diameter results in more extruded Al and a higher shear force at the joints [43]. All joints had shear strengths lower than those of the weakest component (AA6061), as shown in Figure 9(b). This is because the AA6061 alloy is exposed to high levels of deformation at the stirring zones (SZs) under the tool at elevated temperatures.
Figure 9 (a)
Figure 9 (b)
Work hardening is also referred to as strain hardening. This causes the alloy to become more resistant to deformation. When an aluminum alloy is formed through plastic deformation, a phenomenon known as strain hardening may take place [44]. Aluminum’s grain structures move against one another in regions called slip planes due to plastic deformation [45]. When dislocations are in close proximity to one another, the strain field around them acts to repel one another and slow their motion [46–48].
In Sample 2, which had a lower steel hole diameter (d = 2 mm), the maximum shear strength was 196 MPa. The larger hole diameter increased the penetrating of extruded Al via the steel hole’s inner surface compared to the small hole diameter. The higher amount of penetrating Al prevents joint dislocation and increases the shear joint strength [42]. Sample 7 revealed the highest shear force of 2420 N; this is attributed to the biggest hole diameter (d = 4 mm) in the steel sheet, allowing more metal to be involved in resisting the shear loading. Its shear strength recorded was 192 MPa, with a joint efficiency of 94.6% compared to the 203 MPa shear strength of the base Al sheet metal.
Figure 10 illustrates the analysis of the joint’s DOE force of shear. According to the plot of the main effect, the shear force increased as the hole diameter expanded. The hole size was shown to have the most significant impact on the shear force, as seen by the chart of Pareto. The force of shear was least affected by the tool’s rotational velocity and depth of penetration [49].
Figure 10 (a)
Figure 10 (b)
DOE analyzed the joints’ shear strength; see Figure 11. As the plunge depth increased, the shear strength was reduced. This may be consoled to the fact that the increase in the depth of the plunge increases the heat input and applied pressure on the extruded aluminum, which reduces the shear strength [50].
Figure 11 (a)
Figure 11 (b)
According to the DOE analysis, the load of shear and strength relations of the joints were predicted by the experimental variables; see Table 5. Figure 12 depicts the expected relationships between the variables. The shear force formula that was predicted showed a relatively high degree of concordance with the shear forces that were observed for the samples. The expected shear strength formula was in agreement with all the samples, with inaccuracies that were virtually disregarded.
| Term | Force coefficient of shear | Strength coefficient of shear |
|---|---|---|
| Constant | −849.9987 | 225.9989 |
| Diameter of hole (mm) | 764.998 | −7.159934 |
| Rotating speed (RPM) | −0.050012 | −0.01399974 |
| Plunging depth (mm) | −117.0014 | −14.99324 |
Figure 12 (a)
Figure 12 (b)
3.3. Joined and Fractured Surface Features
Joined and fractured sample surfaces are illustrated in Table 6. There is evidence from the top Al surfaces that the collar was successful in preventing the creation of the Al flash, which reduced the metal losses during the joining process compared with the C-FSSW [42]. A collar trace is observed in all samples at the Al upper surface, which is a sign of a slight softening around the tool due to the heat generated from the friction caused by the revolving tool on the top Al layer. No signs of die trace were revealed at the rivet head of samples (1, 2, 3) because of its low hole diameter of 2 mm, causing brutal metal flow through the hole of the steel sheet. The rivet head images of the assembled sheets indicate that the metal extruded successfully via the die hole passing through the hole in the steel sheet.
 |
When the bottom of the spinning tool rubs against the top of the Al sheet, it creates the frictional heat needed to transform the metal into a solid state. To extrude the Al through the hole, the combined heat and supplied pressure were used. In the case of this particular sort of join, temperature was seen as a critical element. During the joining process, a combination of frictional heat and thermomechanical causes the Al to soften and be passed through the steel hole. Consequently, the steel sheet is mechanically interlocked with the Al sheet by a rivet head [51]. When the temperature is raised to a degree where it approaches the melting degree of the alloy, the material becomes too soft, a layer of IMC is formed, and the mechanical characteristics of the resulting joining are degraded. A minimal amount of heat, much below the melting point of aluminum, was used to construct a high-quality junction by allowing the plasticized Al to flow via the hole [3]. The shearing of a portion of the extruded Al layer at the area of the Al specimens’ lower surface was the mode of failure for all samples. Shearing the extruded Al at the surface of the Al lower sheet was the sole mode of failure observed in the tested samples. Despite pullout modes being a common failure mode in prior research, none of the tests for all runs experienced it [36, 39]. There was no rivet head failure during any test runs, suggesting that the rivet head retained sufficient strength to withstand the shear loading. However, because every run failed at the same spot (the neck), the rivet head proved greater strength than the neck. The neck is the distance the metal is located inside the steel hole. The extent of the connected region created by the mechanical interlock between the two metals was a significant factor in determining the shear strength of the samples.
The amount of the extruded Al differs from one sample to another based on the joining conditions. The die trace on the rivet head indicated that the softened Al touched the die surface, as shown in Sample 5. Extruded Al shearing in the joint cross-sectional center without detaching the rivet head was the likely cause of failure, as demonstrated by the failed surfaces of the tested assembled joints [52]. The steel sheets’ inner surfaces indicated that the softened Al approximately filled the steel hole. All samples failed at the parting line located at the mating surfaces of the sheets.
All rivet heads remained in their extruded position after the shear loading test, indicating that no pullout occurred on the rivet head, which is a sign of a good locking mechanism between the rivet head and the steel sheet. All fractured rivet heads in all samples revealed a ductile fracture mode caused by the shearing action during the shear loading test, as it makes sense when comparing them with its measured shear strength that revealed high values that were near the shear strength of the base Al metal of 203 MPa. In addition, all rivet heads for all samples, as shown in Table 6 (second column), showed larger diameters before fracture than the diameter of the fractured extruded Al portion shown in the third and fourth columns in Table 6. This confirms that the extruded metal in all run cases successfully flows through the steel hole and then moves in a lateral and radial orientation inside the die opening located directly below the hole in the steel sheet after passing the hole edge. The steel hole and the die opening were at the same centerline. Another essential issue that one can observe from Table 6 is the fracture behavior of the extruded portion, as shown in both the third and fourth columns. For example, in Run 1, the fractured extruded portion on the inner steel surface side (fourth column) revealed a tiny necking behavior as a minor pull out of the metal was observed, resulting in salience with a small clearance between the extruded portion and the hole of the steel sheet indicating that the extruded portion has been subjected to tensile stresses before fracture by the shearing action of the shear loading test resulting a smaller diameter because of the necking phenomena causing the appearance of the clearance. When comparing this issue to the inner aluminum surface side (third column), the shape of the fractured extruded portion was flattened with the sheet surface. These two issues of the salience appearance on the steel side and the flattening on the Al side may be attributed to the deviation in mechanical characteristics between the two zones, as the flattened metal portion on the aluminum side is subjected to an intensive stirring action caused by the rotating tool during the plunging stage resulting lower mechanical properties [6, 7, 38]. On the other hand, the rivet head was subjected to pure plastic deformation through the steel hole and inside the die opening, resulting in better mechanical properties due to the strain hardening effect [53]. All run tests exhibited the same behavior as shown in the third and fourth columns of Table 6.
3.4. Metal Flow and Microstructure of Joints
The joint cross-sectional microstructure of Run 7, representing the best welding condition, is shown in Figure 13(a). The higher input heat and applied pressure at the assembly stage resulted in a plastic deformation in the Al surface, with a convex shape at the trace of the tool [26]. The formed Al metal was highly affected by the input heat under the tool surface, which had approximately a symmetrical shape, as shown in the dark zone of the microstructure feature. The extruded metal’s flow is observed near the steel hole surfaces toward the rivet dead die.
Figure 13
The process of the top sheet’s deformation is responsible for the development of heat caused by friction and the plastic distortion that follows from that occurs beneath the tool plunge. The mechanism that governs the stirring that occurs under the effect of a spinning tool directly touching the stir spot is the heat flux due to friction transferred from the top sheet and the additional plastic deformation that occurs beneath the downward plunging tool. However, it has been discovered that the SZ, which is formed like a U shape, is only present on the top sheet; see Figures 13(b) and 13(c). It is comparable to the deformation zone with a bowl shape that may be seen at the stir location while probeless FSSW is performed [37, 54]. The lower sheet hole area is pushed down partially due to the downward forces of the metal flow through the hole, causing a minor bending of the hole edges because there is no support from the die under it; see Figures 13(d) and 13(e). Following Figures (13(f) and 13(h)), the SZ is situated over the border, separating the two metals inside the top. One may see a pattern of mixing that is almost symmetrical inside the SZ and shifts a little to the left side due to a minimum misalignment between the axis of the hole of the lower sheet and the tool axis. Significant plastic deformation and the resulting metallurgical transformations have occurred due to the stirring. Almost uniform plasticization and extrusion have occurred inside the confines of the plasticized zone (PDZ), which is accessible via the bottom sheet’s predrilled hole; see Figures 13(g) and 13(i). The plunging downward tool influences the extrusion of the PDZ, which is the area that is located under the center of the SZ.
Plasticization and softening of the PDZ are processes that occur due to the heat transferred from the SZ. While the tool is being lowered into the stir location, the plasticized top metal is being extruded via the hole that has been previously bored. In contrast to the material in the PDZ, which is believed to experience radially inward motion into the predrilled hole, the metal in the SZ has a tendency to follow a circular path with the revolving tool. This is because the PDZ is located in the peripheral zone. The thermomechanical affected zone (TMAZ) lies in the stir spot’s side walls, which has developed due to the rubbing action of the stir tool’s lateral surface on the top sheet; see Figure 13(f).
There has been significant mechanical deformation in this area. TMAZ is directly impacted by the plastic deformation and heat flux brought on by the stir tool. Only the SZ, PDZ, and TMAZ are considered integral components of the top sheet. The lower sheet serves as a basis for the development of the zones that were stated before. The heat flux significantly impacts these zones, causing significant structural changes.
Due to the intense plastically distorted grains and the heat flux from high friction, the grains around and in the SZ of the joined metals go through recrystallization [53]. Static recrystallization occurred in the area that is being exposed to heat via frictional forces as the heat-affected zone (HAZ) and PDZ, while dynamic recrystallization occurred in the area that is directly touching the stir tool’s face (SZ) [6]. In dynamic recrystallization, excessive plastic distortion and heat caused by friction work together. When static recrystallization is taking place, heat flux is the only determinant. According to the literature, dynamic recrystallization is common in alloys of aluminum due to their high energy of fault stacking and rapid rate of recovery [55]. After being subjected to frictional heat flux, the TMAZ has been subjected to grain recovery. On top of the backing die is where the bottom of the lower sheet is located. Because of this, the rate at which heat is lost from the bottom film is much quicker than that of the top sheet. The flux of heat and plastic distortion both have a role in determining the difference in grain size that may be detected in different zones. There is a rising order to the grain size measurements taken in these zones. In the HAZ area of the top sheet, one can detect the coarse, irregular grains with a diameter of 76.3 μm.
The HAZ area undergoes a heat cycle without plastic deformation [55]. The HAZ grains experienced grain growth and static recrystallization due to the heat flux generated by friction and received from the upper sheet/tool contact, as shown by their size relative to the parent metal grains. In the SZ, small, uniform grains with diameters ranging from 2.4 to 5.9 μm are found; see Figure 13(h). These small grain size of this zone provides the dark color appearance shown in this zone at this image magnification percentage. The SZ has undergone dynamic recrystallization due to plastic deformation caused by frictional heat flux and stirring. Another source also reported a similar finding: Mishra and Ma [55], Reimann et al. [56], and Chiou et al. [57]. The growth of grain annihilation is governed by the degree of plastic distortion, as seen by the forming of coarse grains in the HAZ and fine grains’ development in SZ.
The final geometry of the grain’s flow was like a flange or circular disk under the lower layer of the steel specimen. However, extruded Al was able to fill the steel hole without any visible flaws, such as cracks, porosity, or voids, as seen in Figure 13(f). This was accomplished by the aluminum’s ability to penetrate through the steel hole. Figures 13(g) and 13(i) show the extruded Al microstructure as it flows via the hole at either edge; this process mechanically interlocks the two surfaces, resulting in the joining of the two materials. The black dotted arrows on the joint’s left side (see Figure 13(g)) show the flow of grains throughout the joining process; this side is similar to the right. Close to the bored hole, which is thought of as fixed ends, the grain flow was close to the steel surfaces. Under normal conditions, the distorted grains would flow via the free area of the steel hole and the open space under the bottom sheet created by the die, and they would concentrate close to the fixed surfaces. This was because of the load that was applied.
As a result, the grains of aluminum that had been thermally plasticized could penetrate through the surrounding hole surface of the steel, resulting in the formation of a small circular disk or flange (similar to a rivet head) below the steel sheet. Because of this mechanism, the two sheets mechanically interlocked at the interface line [36, 39, 58]. The load the rotating tool applies in a vertical direction to the steel surface is reacted by the fixed hole edge of the steel surface located close to the joining zone. This reaction is directed toward the aluminum grains. HAZ was produced as a consequence of the interaction between the load that was applied and the heat that was introduced into the aluminum grains that were located close to the joining area [27]. According to the fact shown in Figure 13(i), both the left and right sides exhibited similar microstructure behavior, which is depicted in Figure 13(g). Based on the microstructure pictures, it was determined that no IMC was present at the line of interface between the two metals [20]. In addition, at the interface line, the extruded Al flowed through the steel hole without any faults throughout the process.
The barreling effect is presented in the extruded portion, as shown in Figures 13(g) and 13(i). The barreling effect occurs for two reasons: (1) forces caused by friction at the interface between the die and the workpiece. During compression, the metal can be considered as a fluid, and just like a fluid, the layers of material close to the die surface (lower extruded part) and the steel sheet surface (upper extruded part) experience frictional forces much more than the layers in the center of the formed circular disk or flange. So, the contact surfaces resist the deformed layers close to them. (2) Due to the nonuniformly distribution of temperature of the hot metal, there is a specific quantity of heat transfer that occurs between the metal workpiece and the dies. This happens when the contact surfaces with the extruded portion are colder than the workpiece. When this occurs, the layers of metal that are closer to the die surfaces get colder than the layers of metal that are located in the middle. Consequently, the thermal deformation is nonuniform in the metal, as the colder material is less prone to deformation and will expand less than the hotter material in the middle.
3.5. Joint X-Ray Diffraction (XRD)
The XRD tested the interface line of Sample 7 to examine the formation of IMCs between the two joined materials; see Figure 14. The chart indicates that the joining process took place between the two metals without the development of an IMC. Two different peaks of aluminum and iron are observed, indicating that the interface line consists of the base joined materials. Accordingly, the mechanism of joining happened due to the mechanical interlock between the two metals [26].
Figure 14
3.6. Temperature Measurement
To measure the temperature during the jointing procedure, a temperature data logger was utilized in conjunction with a type “K” thermocouple. The thermocouple was located at the side edge of the upper Al sheet at a distance of 12.5 mm from the sheet’s end and a height of 1.2 mm above the interfacing line between the Al and steel sheet. The process temperature was measured at a second response time and saved in a 4 GB SD card memory inside the data logger. After each joining process was executed, the temperature data were collected per sample. To ensure accurate temperature readings, three samples were measured per run. Two of these three samples were measured by the temperature data logger, and a temperature multimeter measured the remaining sample.
When compared to the temperature in the room, the temperature rose at a quick rate during the first period. The position at which the spinning tool made contact with the aluminum specimen is the beginning point, and this step symbolizes that location. The temperature steadily rose with each passing second [59] until the peak temperature (150°C–240°C) was reached after the joining process. A consequence of this was that the plasticized aluminum metal could pass through the steel hole. A good junction was formed between the connected materials when plasticized aluminum metal was forced into the steel’s hole slightly below the tool surface [3]. The temperature dropped progressively as the frictional surface was removed, and the revolving tool was lifted upward from the aluminum specimen after about 20 s. The temperature obtained during the joining procedure was about half of the melting point of AA6061. Conventional methods, including friction stir, use temperatures above 80% of AA6061’s melting point [60].
Figure 15(a) depicts the maximum temperature calculated at each run. Figure 15(b) illustrates the temperature history for the best jointing condition of Run 7. The jointing procedures consist of two steps. In the first stage, the tool is fed at a 16 mm/min feed rate through the lap joint of two sheets to report the rising temperature. The temperature is raised at 13.1°C/s rate of heating. After welding, the lap joint is subjected to air cooling at a rate of 3.3°C/s, to achieve a continuous temperature decline. This step is known as the drawing-out stage.
Figure 15 (a)
