Keywords

23.1 Introduction

Joining is a crucial technology for creative and sustainable manufacturing among many manufacturing technologies. Combining several materials to form a multimaterial component is in high demand to achieve more optimum lightweighting components with high performance and also the trend of trying to integrate even more features into each component is fulfilled. Many industries such as aeronautics and automotive are in need of joining dissimilar materials to lower the weight of the end products in order to improve energy efficiency, mobility, and agility [1]. Joining of dissimilar metals using traditional jointing techniques can be an issue, owing to significant variations in material’s mechanical, chemical, thermal, and physical characteristics give rise to difficulties such as metallurgical precipitation, mechanical joint properties deterioration, heat-affected zone (HAZ), intermetallic compounds (IMCs) formation, distortion and defects [2, 3]. But in recent years, various derivative technologies of friction stir welding (FSW) for dissimilar materials joining have been developed in a way to mitigate these challenges along with achieving higher joining performance. Mostly in prior reports, individual aspects of FSW are reported. In this paper, the latest potential variants of FSW such as FSE, FSS, FSD, and FSI that are adopted to join dissimilar materials are studied and reviewed the fundamental principles, research progress, and efficiency of each process. The paper ends with a conclusion and future scope of the novel derivative technologies of FSW.

23.2 Variants of FSW

23.2.1 Friction Stir Extrusion (FSE)

A detailed overview of the challenges for dissimilar joining of aluminum and steel are compared to friction stir spot welding (FSSW) and standard FSW is provided by Haghshenas et al. [4]. To address the challenges, a new method is introduced for joining dissimilar materials especially steel and aluminum through friction stir extrusion (FSE) process [5, 6]. The Welding Institute (TWI) has introduced FSE back in 1993 [7] and developed very little until the patent lapsed in 2002. The FSE technique is part of the FSP technology, built after the FSW. FSE is a process of solid-state recycling (SSR) for synthesizing a material that produces extruded products in one step by transforming waste into advanced bulk materials employing thermomechanical and mechanical processing. SSR reduces energy consumption that is required for remelting in the conventional phase.

The procedure includes plunging a die, which rotates in a hole chamber that contains a billet of the material extruded. The forces of friction in between the billet and the die decay in heat lead to softening the metal and the extrusion channel creates a plastic flow on the die axis of the rotation. Figure 23.1 displays a process sketch of FSE. A transition layer that heats the chip but is not homogenized like a continuum is found that is far from the tool interface. The material extruded possesses the room temperature at the end of the cycle with an air cooling. The key geometric variable of FSE procedure is the ratio of extrusion (or) extrusion ratio, i.e., the ratio of the chamber–diameter and the extruder, in a comparable way with traditional extrusion procedure. The option of a higher extrusion ratio will prevent the critical bonding of the extruded wire at the center, which leads to the formation of defects [8]. The forces of extrusion along with the rotational speed of the tool are the key factors that affect the process especially the surface quality [9]. The application of a consistent extrusion force makes it possible to adjust the plunge speed to the local material flow stress. It is evident that extrusion only happens when the raw material meets the appropriate temperature and strain levels. The capacity for this approach can be used by different materials, such as Al, Mg, or steel [10,11,12].

Fig. 23.1
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Schematic diagram of FSE process [47]

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Friction stir back extrusion (FSBE) is also one of the many frictions stir processing (FSP) variants which are produced via FSW, which is in the form of spiral friction stir processing (SFSP) with an aim to fabricate strong ductile tubes. Many studies also demonstrated FSBE by the fixed chamber and rotating the die and producing rod, tube, and cable. The viability and ability of the FSBE process produce tubular samples without evidence of internal defects or voids. The FSBE is also widely used to transform metal chips into wires, replacing the traditional melt cycle, as an SSR process [13]. Using this definition, a rotating tool with a defined axial feed and feed rate is plunged into a cylindrical sample. The tool's later movement pushes the material outside as in the back extrusion, whereas friction at the interface of the sample/tool produces enough heat to suppress and deform. Furthermore, the stirring effect under significantly higher pressure forces the material to be severely deformed by plastic and refines the grain structure. The concept of FSBE is shown in Fig. 23.2.

Fig. 23.2
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Schematic diagram of FSBE process [14]

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From previous studies, it is evident that FSE [9, 15, 16] and friction stir back extrusion (FSBE) [17,18,19,20,21] process can be used for recycling the metal chips to fabricate good surface quality wires, also with the presence of small internal voids and non-homogeneous microstructure. In both cases, samples have a fine recrystallized grain microstructure, with grain size growing after excessive speed increase of rotation along with the increased strength when compared with the base material. For materials like magnesium alloys or even aluminum alloys, whose ductility can be smaller than steel, this is very advantageous. It is anticipated to enhance the efficiency of the tubes before and after hydroforming with improved ductility. Few researchers are therefore interested in FSBE for the production of ultra-fine meso- and microscale tubes. In a similar line of SSR investigations, researchers are enabling direct recycling into the semi-finished product of metal scraps using different processes such as FSE [22] and FSBE [23].

23.2.2 Friction Stir Scribe Welding (FSS)

Processes of solid-state joining such as FSW facilitate the welding of several materials or else viewed as unweldable. Through friction-based approaches, joints across different metals are demonstrated in the past. Due to extreme variations of the melting temperatures for each component, the metallurgical immiscence of certain blends makes the joining of dissimilar materials very complicated [24]. Friction stir scribe welding (FSS) process is proposed by the Pacific Northwest National Laboratory to solve the problems caused by chemical incompatibilities and the differences in melting temperature among different material combinations. FSS enables the fusion of different materials in a lap configuration where both chemically and mechanically bonding materials allows the fitting and the improvement of the welding effect. FSS is an alteration in the process of FSW that a pin tip is inserted with a scribe [25]. The scribe is usually made of tungsten carbide (WC). The scribe works by impacting the base material sheet of a lap weld of dissimilar material as an extra tool configuration as illustrated in Fig. 23.3. The geometry of the FSS tool is carefully designed as shown in Fig. 23.4. FSS combines high melting temperature material focused matching and controlled and localized extrusion of the material of low melting temperature at the interface of both the materials. The simultaneous process generates a mechanically interlocked joint which is created due to the combined action at a relatively low melting point temperature of the material with a lower melting temperature. The process solid-state nature is thus maintained and the complications caused by traditional melt-solidifying weld processes are eliminated. This facilitates FSS to allow very different metals to get welded and also allows different materials like polymers to weld into metals or composites. Few studies of FSS joints for various material combinations have been reported, such as polymer to Al [26, 27], carbon fiber-reinforced polyamide to Al [28], steel to Al [29,30,31,32,33,34], and a computational approach is also reported in [35]. A cross-sectional observation showed the change in the IMC layer thickness during the process of welding with a scribe trace. Steel/Al joint fractography is evident that regionally formed IMC at the interface for fracturing welds via a welded interface.

Fig. 23.3
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Schematic penetration of FSS tool into a lap joint of dissimilar material (above) and tools of FSS (below) [36]

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Fig. 23.4
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Single (above) and double (below) scribe cutter FSS tool [46]

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Fig. 23.5
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FSD technique illustration and tooling demonstrating metallurgical bonding and mechanical interlocking in a dovetail groove [42]

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23.2.3 Friction Stir Dovetailing (FSD)

While there is a wide amount of research to join steel to Al metallurgically, only limited studies have found thickness measurements for steel or Al over 6 mm [37,38,39]. This is mainly because the thin sheets joining techniques are usually not well suited to thick plates. The newly evolved FSD technique [40] therefore fills a major gap in the literature published. Glue and dovetails are employed in woodworking to safely join the wood pieces, where a similar approach is adopted by FSS but in metals. Steel dovetail grooves are deformed by the Al to create a mechanical interlock with a specially designed tool. The mechanical interlocks formed by FSD between the steel and Al further enhanced by in situ metallurgical bonds while joining [42]. Simultaneously, the tool also rubs the bottom of the dovetail to form an IMC or thin metallurgical bond that “glues” the metals into the dovetail. Metallurgical bonding and mechanical interlocking combined through advanced technology in a single phase of the FSD process that creates high ductility and strength joints compared to joints produced by other friction stir techniques. Before the joint breaks, the FSD joined material can stretch over half a centimeter demonstrating ductility five times greater than aluminum and steel, combined with other friction-based techniques.

FSD is demonstrated in the lap configuration on the AA6061-T651 attached to MIL-DTL-12560 J Rolled Homogeneous Armor (RHA) [41]. In the course of FSD, plastic deformed Al enters into dovetail grooves and on the underlying RHA surface, it is premachined to form a mechanical interlock, while the WC tip along with the interface of the RHA-Al produces localized heat and leads to a metallurgical bond. Figure 23.5 depicts an FSD technology of a single dovetail groove in a lap configuration, cut into the RHA. Thermocouples of type-k are incorporated in the tooltip where the WC is contained in the H13 tool. At specific locations of the tool, these thermocouples are soldered to constrain the intermetallic growth through temperature control. Pin threads are designed in a way to push the material into the dovetail while scrolling attribute to collect material on the shoulder to prevent the defects on the surface and inner wormholes from forming. FSD has been performed using an H13 steel tool that hardened to HRC from 45 to 48. A shoulder diameter of 38.1 mm, pin diameter of 15.85 mm (close to the shank), length of 11 mm with 3 flats (apart of 120°), 2.12 mm/revolution threaded, 9° frustum shaped, and a 3.18 mm/revolution of convex scrolled are included in the tool. It can be inferred from the mechanical and microstructural data that FSD is an assuring novel method for joining a thick steel-Al portion. With the aid of the FSD technique, the formation of the intermetallic compounds (IMCs) layer is controlled on the basis of localized deformation of solid state. IMCs are not unusual in processing metal at high temperatures as traditional welding but are not very often useful or controllable [42].

Later, a similar line of work AA7099 to Ni–Cr-Mo is joined using an improved FSD double-pass approach along with the previous single-pass approach. The double-pass approach is to form the metallurgical bonding of AA6061 to RHA with a silicone-enriching IMC in the dovetail and the second pass ought to be done by the FSD to establish a lap joint for the AA6061 to AA7099 [43]. Authors concluded that FSD is capable of implementing metallurgical bonding and mechanical locking and simultaneously, to extrude different alloys of aluminum (AA7099, AA6061) in dovetail grooves in RHA plates to form a lap joint. The AA6061 asymmetrical material flow on the retreating and advancing sides led to asymmetric joint results. To estimate the mechanical performance of the thick steel-Al joints that are processed using FSD, modeling and simulation approach is developed in [44]. At the Al corner of the dovetail, it predicted the failure of the FSD single-pass joints without IMCs. The dovetail neck failure location is predicted for the joints of single-pass FSD connections with IMCs. The failure position is expected at the loading side of HAZ/TMAZ (thermomechanically affected zone) of the Al for triple and double-pass joints of FSD with IMCs. The average predictable Al thickness, provided by the FSD joint configurations simulated for this task without driving fault, is 44.45 mm and 17.78 mm for FSD triple and double-pass joints, respectively.

23.2.4 Friction Stir Interlocking (FSI)

Friction stir interlocking (FSI) is a modern, evolved solid-phase technique [45]. FSI can be adopted for joining lightweight metals to ceramics, thermoset plastics, non-metals, and composites. There are two FSI approaches currently used to bind metal plates and sheets to non-metals. The first method is to incorporate metal inserts into non-metallic elements and then FSW of the metal plate or sheet to the metal insert directly. The second includes inserting metal pins like some kind of rivet into the non-metal and after this FSW of the metal plate or sheet to the metal insert directly to result in forming mechanical fastener. This new technology makes it possible to quickly and uniformly create numerous interlocks in one pass and tend to offer lower costs and improved efficiency in processes compared to traditional metal to non-metal fasteners, such as spot welding, riveting, and pillaring. FSI also limits the pitting and galvanic corrosion which can in composite materials between carbon fibers and metal fasteners and this can be attributed to the generation of heat during FSP resulting corrosion barriers.

23.3 Conclusions

In this review, potential variants of FSW such as FSE, FSS, FSD, and FSI are investigated. Also, their fundamental principles, research progress, and efficiency of each process are studied and reviewed. Major conclusions drawn from the review include that FSE and FSBE processes can be used for recycling the metal chips to fabricate good surface quality wires, sometimes with presence of small internal voids and non-homogeneous microstructure. FSS joint fractography is evident that regionally formed IMC at the interface of steel/Al for fracturing welds via welded interface. Metallurgical bonding and mechanical interlocking combined through advanced technology in a single phase of the FSD process that creates high ductility and strength joints compared to joints produced by other friction stir techniques. Before the joint breaks, the FSD joined material can stretch over half a centimeter demonstrating ductility five times greater than aluminum and steel, combined with other friction-based techniques that made it unique and preferable. Although FSI is a recent technology that makes it possible to quickly and uniformly create numerous interlocks in one pass and tend to offer lower costs and improved efficiency in processes compared to traditional methods, from the literature, it is evident that FSD is a promising process for efficient and low cost joining and even the FSI process can be considered as a noteworthy process. In this regard, further investigations are highly needed for FSD and FSI technologies to explore the joining possibilities of various materials.