Haris Ali Khan The Harold and Inge Marcus Department of Analyses of Friction Stir Riveting Industrial and Manufacturing Engineering, Penn State University, State College, PA 16801 Processes: A Review e-mail: [email protected] This study presents detailed analyses of variant joining processes under the category of Jingjing Li1 friction stir riveting (FSR) that are applied to assemble similar or dissimilar materials by Mem. ASME integrating the advantages of both friction stir process and mechanical fastening. It cov- The Harold and Inge Marcus Department of ers the operating principle of FSR methods along with the insights into various process Industrial and Manufacturing Engineering, parameters responsible for successful joint formation. The paper further evaluates the

Penn State University, researches in friction stir-based riveting processes, which unearth the enhanced metal- Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021 State College, PA 16801 lurgical and mechanical properties, for instance microstructure refinement, local e-mail: [email protected] mechanical properties and improved strength, corrosion, and fatigue resistance. Advan- tages and limitations of the FSR processes are then presented. The study is concluded by Chenhui Shao summarizing the key analyses and proposing the potential areas for future research. Department of Mechanical [DOI: 10.1115/1.4036909] Science and Engineering, University of Illinois at Urbana-Champaign, Keywords: friction stir, joining, dissimilar materials, process physics, mechanical Urbana, IL 61801 properties e-mail: [email protected]

1 Introduction partly pierces the lower sheets before undergoing the effect of the lower upsetting , which radially dislocates the hollow rivet end Today’s manufacturing industry is inclined toward evolving outward [13]. Simultaneously, the work material is displaced to new technologies to meet rapidly changing needs [1]. Among fill any created voids. The entire process generates a mechanical those, weight savings and increasing fuel efficiency emerged as interference and interlocking. However, two-sided access is man- particular demands [2] in industries like aviation and automotive. datory for this process. Details of the process are illustrated in Lightweight materials such as Mg and Al alloys, carbon , Fig. 1(c) [14]. In recent years, mechanical riveting methods such and polymer composites are widely used in combination for as SPR and BR are widely used in automotive industry for joining achieving the desired results [3,4]. These manufacturing advance- of dissimilar materials [15,16]. ments call for new efficient joining technologies. Dissimilar mate- The institute is the pioneer in inventing friction stir rial joining poses more challenges than similar materials because welding (FSW) in 1991 [17]. Friction stir spot welding (FSSW) is of difference in their mechanical, chemical, and thermal proper- a derivative of this process [18]. In FSSW, the material pieces are ties. Nonetheless, dissimilar materials are difficult to be joined in fastened by a welding through both sides. The welding tool is multimaterial structures using conventional fusion-based welding brought into contact with the parts at a desirable rotational speed methods. Mechanical , e.g., bolting and riveting, can be (RS). Consequently, frictional heat is generated. The tool is fur- used independently as well as in combination with adhesives ther driven into the work material under pressure until it partially [5,6]. Current mechanical fastening substitutes for joining materi- penetrates through both the work materials. The frictional heat als are solid riveting, blind riveting (BR), and self-piercing rivet- generated during the process results in softening and subsequently ing (SPR), to name a few [7]. joining of work materials [19–23]. The process is explained in Solid riveting is a conventional fastening technique [8] where a Fig. 1(d) [24]. In automotive industry, FSW of aluminum and solid rivet is placed inside a predrilled hole. The rivet shank is utilized for manufacturing of vehicle components for deforms under an axial compressive load to fill the hole/cavity instance, the trunk hinge on the Mazda MX-5 [25] and the hybrid and forms the rivet clinch (Fig. 1(a) [9]). BR [10] is one type of steel/aluminum sub frame presented by Honda on the 2013 solid riveting; however, in this process, more intricate rivets are Accord [26]. used. A blind rivet has two components, and shank. The From the previously mentioned discussion, it can be inferred mandrel is a long rod with an increased diameter at one end; and that there are several mechanical riveting or friction stir mecha- the shank comprises a hollow tube with a flat cap on the end. The nisms for dissimilar materials. In addition, technologies including internal diameter is capable of housing the mandrel. The three adhesive bonding, laser welding, or other solid-state methods steps of BR are predrilling, placing, and pulling [11]. The first (e.g., ultrasonic welding) are being applied in joining dissimilar step, predrilling, is to drill a hole on the work materials with a materials. However, there is a continuous rise in the expansion of diameter larger than the rivet’s body diameter. The second step, substitute joining technologies to minimize the limitations of con- placing, as the name suggests, involves a blind rivet seating into the temporary processes, such as joint performance, production time, predrilled hole. The third step, pulling, is to drag the mandrel of the and cost. This results in evolution of new techniques, which can blind rivet until the mandrel breaks off. Figure 1(b) [12] gives a meet the stringent manufacturing requirements, particularly in schematic representation of the process. In a self-piercing riveting, robustness and ease of implementation. Therefore, this paper aims a hollow rivet is driven into the specimen materials that are incor- at analyzing and comprehending an emerging joining technique, porated in a shaped die. Initially, the rivet is driven into the upper FSR processes, which integrates the advantages of both friction surface of the work materials because of the pressure applied stirring and mechanical riveting. FSR are novel joining processes, (through the press ). At this stage, the rivet penetrates which eradicate the necessity of predrilling for rivet insertion, and through the upper sheets of materials. At the same time, it also thus surmount the difficulties in hole alignment. The processes are fast, only taking a few seconds to form a joint, and ready with implementation of robot systems. There are different variants in 1Corresponding author. Manuscript received February 16, 2017; final manuscript received May 19, 2017; use for FSR processes, which are friction riveting, friction self- published online July 18, 2017. Assoc. Editor: Wayne Cai. piercing riveting (F-SPR), friction bit joining (FBJ), two-sided

Journal of Manufacturing Science and Engineering SEPTEMBER 2017, Vol. 139 / 090801-1 Copyright VC 2017 by ASME Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021

Fig. 1 Schematic of different joining processes: (a) solid riveting [9], (b) blind riveting [12], (c) self-piercing riveting [14], and (d) friction stir welding [24] friction stir riveting by , and friction stir blind riveting (FSBR). This paper covers a comprehensive review of the differ- ent FSR processes starting from the description of different proc- esses and fundamentals of the process design, followed by the analytical modeling studies, the underlying process parameters, different bond formations, and characterization of affected zones; presents research on mechanical behavior of FSR joints, behavior under corrosive environment; and in the end, concludes with syn- opsis and future outlook.

2 Process Physics and Experimental Setup This section will cover important aspects related to process physics involved in the accomplishment of FSR processes. Each process is discussed individually, and processes sharing common- ality are grouped together.

2.1 Friction Riveting. Many researchers joined various dis- similar materials by exploiting the advantages of friction stirring and mechanical fastening. Researchers at Helmholtz-Zentrum, Geesthacht, Germany [27] invented friction riveting process (called FricRiveting) to join hybrid thermoplastic-metal structures Fig. 2 Schematic illustration of FricRiveted process [28]: (a) through spot connections. One or more thermoplastic components fixturing of joining materials, (b) axial movement of the rotating are joined with metal by inserting a round profiled (or plain) rivet into polymeric partner(s), (c) increase of axial force and metallic rivet in them. In this process, the surface of the plastic of the rivet, and (d) anchor formation of deformed rivet component is pressed by a rotating metallic rivet (Fig. 2(a))at tip and consolidation of joint

090801-2 / Vol. 139, SEPTEMBER 2017 Transactions of the ASME high rotational speeds. The applied axial force and high rotational moving rotating rivet interacts with the material; (2) hot riveting speed generate frictional heat, thereby creating a plasticized/ stage in which the rotating rivet penetrates into the upper work- molten film around the rivet tip (Fig. 2(b)). The frictional heat sheet, and meanwhile softens and inserts into the bottom material results in softening of materials, thus allowing the rivet to penetrate for interlock formation; (3) friction stage where the downward further. The tip of the rivet softened at the culmination of the heat- movement of the rivet is stopped; however, it keeps spinning for a ing stage, owing to a local temperature rise. This marks the begin- preset time to produce additional heat required for solid-state join- ning of forging phase, where the axial force is increased and the ing; and (4) off-stage where the rotation movement is halted to rotation is decelerated. The rivet tip thrusts the remaining soften generate a static contact among the rivet and encompassing mate- material to the flash; however, the rivet faces resistance from the rials. Figure 3 gives the schematic of the process. colder material and is deformed during the process to adopt a wider The hybrid friction-stir riveting method, developed at the Uni- diameter (Fig. 2(c)). After consolidation under pressure, anchoring versity of Toledo, Toledo, OH [1,31], is very similar to F-SPR. In and adhesive forces hold the joint together (Fig. 2(d))[28]. this process, a joint is formed by spinning and pressing a self- piercing rivet into sheet metals. Once the cycle is completed, the rivet is left in the sheets to form a joint. Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021 2.2 Friction Self-Piercing Riveting (F-SPR). Researchers have used F-SPR process to join Al–Mg alloys [29,30]. The pro- cess is similar to self-piercing rivet other than the step where rivet 2.3 Friction Bit Joining (FBJ). Friction bit joining was rotates at high speeds and penetrates to soften the materials. Fur- developed by Mega Stir in which a consumable bit was used as ther, the shank of rivet flares and locks into the bottom workpiece the friction stir tool [32]. FBJ process mainly involves two steps, similar to conventional SPR process. Consequently, a joint forms, cutting and friction [33–35]. Initially, the consumable bit cuts which consists of the mechanical joining mechanism of SPR along through the top workpiece (Fig. 4(b)), which is followed by a join- with the solid-state joining technique of FSSW. The entire tech- ing procedure where the friction heat results in heating the bit and nique is distributed into four stages, namely (1) rivet feed stage work materials (Fig. 4(c)). As a result, the bit acts as filler material where worksheets are fastened on the fixture, and the downward to combine the sheets. In the first step (cutting), the consumable

Fig. 3 Schematic diagram of friction self-piercing riveting process: (a) rivet feed stage, (b) hot riveting stage, (c) friction stage, and (d) off stage [29]

Fig. 4 Schematic illustrations of FBJ process [35]: (a) lap joint before joining, (b) cutting step, (c) joining step, and (d) finished joint

Journal of Manufacturing Science and Engineering SEPTEMBER 2017, Vol. 139 / 090801-3 brought into contact at top and bottom sheets. Because of the stir- ring action, the softened materials flow into a predrilled hole at the middle sheet in the second step (Fig. 5(b)). In the third step, the are retracted from the work pieces leaving behind long rivetlike structures at both top and bottom sheets (Fig. 5(c)). Unlike other FSR procedures, this process uses two pinless FSSW tools, which feature a convex taper with scroll-like features.

2.5 Friction Stir Blind Riveting (FSBR). FSBR was invented by General Motors, Detroit, MI, in 2006 [39–41], which is a blend of friction stir riveting and BR processes. In FSBR, a rotating blind rivet is moved downward and brings in contact

with work materials. The generated frictional heat from the inter- Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021 action between the rivet and the work materials softens the work Fig. 5 Two-sided friction stir riveting by extrusion process materials, thereby permitting the rivet to be driven into under [36]: (a) plunged, (b) dwell, and (c) retraction reduced axial force. The blind rivet is finally pulled once it is fully penetrated into the work materials [41,42]. FSBR eradi- cates the necessity of predrilling as in the traditional BR process while retaining the advantage of one-sided accessibility. The process is explained through a schematic diagram in Fig. 6 [43], and the process has been used for joining similar as well as dissimilar materials. Researchers successfully joined a metal (Al or Mg) with a glass fiber-reinforced plastic (GFRP) sheet in lap joint configuration through a process termed as spin-blind-riveting (SBR) [44]. The process integrates flow drilling and conventional blind riveting. In this process (as shown in Fig. 7), a rotating rivet is simultaneously pressed into the sheets by a penetration force and generates fric- tional heat. The metal because of the rivet movement forms a sleeve analogous to flow drilling [45]. The formed sleeve is then pressed against the GFRP sheet and thermoplastic matrix begins to melt due to heat transfer by conduction. Because of the molten Fig. 6 Steps of the FSBR process: (a) contacting, (b) friction polyamide matrix, the fibers can be expatriated by the rivet rather stir riveting (FSR), (c) blind riveting (BR), and (d) completion [43] being damaged. Once the rivet is entirely penetrated in work mate- rials, the rivet mandrel is pulled back, and formation of mandrel head in rivet body occurs. SBR process is similar to FSBR process bit is rotated at relatively slow speeds. However, after cutting, the by considering the rivet movement and the final joint formation. bit rotation speed is increased for more heat generation and subse- From the previously mentioned discussion, it is evident that quent strong bond formation. The purpose of spindle speed settings there are different processes available under the umbrella of term is to achieve two-pronged effects. Lower speeds help the bit to main- “friction stir riveting.” Every process is distinct from the other in tain its cutting edge necessary for proper cutting, whereas higher process setup and implementation. However, the underlying prin- speeds result in softening the bit material (due to frictional heat gen- ciple for each process is similar, which is the utilization of fric- eration) and bonding with work material. The joining bit is separated tional heat generated by tool–workpiece interaction to allow an from the weld at the end of the joining process (Fig. 4(d)). easy penetration of mechanical rivet or form a special material flow. However, the last stage of every process is different, which 2.4 Two-Sided Friction Stir Riveting by Extrusion Process. defines the role of the rivet in the joint. A brief summary of all the In 2015, a new method was developed for joining dissimilar mate- FSR processes is presented in Table 1. rials and termed as two-sided friction stir riveting by extrusion. The method is an extension of one-sided friction stir extrusion process [36]. Two-sided FSR by extrusion combines elements of friction-stir extrusion (FSE) [37] and rotating anvil friction stir 3 Process Mechanics Requirements spot welding (RAFSSW) [38]. Two-sided FSR by extrusion uses From the previous discussion, it is clear that except friction-stir FSE idea of extruding material by friction stirring, but does so at a extrusion process, in all other processes, the tool, e.g., the rivet or single spot with the two-sided RAFSSW process. In this process, the bit, is clamped inside a fixture in a , which is also first in plunging process (Fig. 5(a)), the two FSSW tools are used to drive it. This condition necessitates the elimination of any

Fig. 7 Schematic of the SBR process [44]

090801-4 / Vol. 139, SEPTEMBER 2017 Transactions of the ASME Table 1 Summary of FSR processes

Process Acronym Inventors Process description aMaterials

Friction FricRiveting GKSS-Forschungszentrum, Geesthacht,  A metallic rivet is friction stirred into ther-  Ti6Al4V/PPV riveting Germany, 2007 moplastic workpiece materials and forms  Ti grade 2/PEI-GF [27,28,47–49,53,59,60,65,66,69,74] the joint through anchoring (increasing  Al 2024-T351/PEI diameter) of metallic rivet  Al 2024-T351/PC  One-sided access is required  Ti grade 3/PEEK-CF

Friction self- F-SPR University of Toledo, Toledo, OH, 2012,  A commercial self-piercing rivet is friction  Al6061-T6/MgAZ31B piercing rivet- Shanghai Jiao Tong University, China, 2013 stirred into the work materials and forms  Al7075-T6 /Mg AZ31B ing (also [1,29–31] the joint through the mechanical  Al5052-H34/Steel SPCC called hybrid interlocking Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021 friction stir  Two-sided access and die are required riveting) Friction bit FBJ Brigham Young University, Megastir Tech-  A consumable bit made of tool steel per-  Al5754-O/UHSS DP980 joining nologies LLC, 2014 [32–35,61,73,76,83] forms preliminary cutting on the top mate-  Al7075-T6/UHSS DP980 rial and becomes filler material to join the bottom sheet under frictional heat  The bit has a compatible chemical compo- sition with bottom material  The bit is stronger than the top material Two-sided — Department of Mechanical Engineering,  The structure contains three work pieces,  Al6061-T6/Low carbon steel friction stir Vanderbilt University, Nashville, TN, 2016 where the middle sheet has a predrilled  MgAZ31B/Al 5005-H34 riveting by [36] hole. The top and bottom work materials extrusion are softened and extracted through the pre- process drilled hole in the middle sheet by two rotating FSSW tools and resulting in joint formation with a rivet like structure Friction stir FSBR General Motors Corporation, Detroit, MI, 2006  Joint is formed by friction stirring and sub-  CFRP/Al 5754-O blind riveting [39–43,50,55–58,62,64,67,68,70–72,75,82] sequently blind riveting  Al 5754-O/Mg AZ31B-H32  One-sided access  Mg AZ31B-H32/CFRP  AZ31B/HSS DP600  Al 5005-H34/Al 380  Al6022/Al6111  MgAM60/AA6022  MgAM60/AA6082 Spin-blind- SBR GESIPA Blindniettechnik GmbH,  The process is similar to FSBR process  GFRP/Al AW-5754 riveting Moerfelden-Walldorf, Germany, 2015 [44]  GFRP/Mg AZ31 aHave been tested till date. comparative motion or slippage among the tool and the spindle FSR processes. The models were implemented to optimize the fixture [46]. Failure of abiding this requirement results in two sit- process parameters by correlating the forces with the performance uations. (1) Only relative rotational motion (torque) occurs. In this indicators. Altmeyer et al. [47], when investigating FricRiveting case, the compromised energy transfer from the fixture to the tool process, deduced the mechanical energy relationship as a conse- ultimately reduces the tool rotation speed to zero. This scenario is quence of forces from rotational motion only. The authors proved also devoid of any friction stirring, and the entire process only that translational forces had the negligible impact on mechanical results in penetration, which eventually causes tool failure. (2) energy input. Blaga et al. [48] derived an analytical model to Translational relative motion along the feed direction is involved. define the anchoring efficiency of the rivet in FricRiveting pro- This case results in pressing the fixture against the tool head cess. The authors called the model as “the volumetric ratio,” and before it fully penetrates the work materials. In addition to these used it to link the anchoring efficiency with the tensile strength of two generic failure modes during the FSR processes, for FSBR the joints. Amancio-Filho et al. [49] studied the thermal degrada- and SBR processes, there is an additional failure mode. The man- tion of a polymer under increasing rotational speeds using Fourier drel body of a blind rivet has an indentation for easy tensile break- transform infrared spectroscopy, gel permeation chromatography, off. This notch can degrade the torsional strength resulting in and X-ray computer microtomography in the FricRiveted zone. shear rupture of the mandrel before it can fully penetrate into the Based on the results, the researchers correlated the molecular samples. This failure mode occurs when the necessary penetration weight measurements with thermal degradation phenomenon. Min torque exceeds the design torque limit of the mandrel. et al. [50] utilized the force and torque considerations from solid Torque and transverse force are two important components that mechanics theory [51] to develop analytical models for FSBR pro- act in synergy for an efficient FSR process. Deviation from their cess. The developed models were used to compute the material optimum conditions will result in either damaging the equipment penetration force, material removal rate, and torque during the or a weak joint. In addition, torque and transverse force vary FSBR process. The models have the potential to extend further to throughout the FSR processes according to different contact con- other FSR or friction stir drilling processes, where material ditions between rivet head and the workpiece. removal is involved. The underlying assumptions in the Min’s model, derived from Ref. [52], suggested that there could be three possible contact conditions between the rivet and the work materi- 4 Analytical Modeling of the Processes als, which are (1) pure sliding, (2) absolute sticking, and (3) mixed Different analytical models were proposed by researchers to sliding and sticking in any friction stir process. The researchers estimate the generation of frictional heat and resultant forces in utilized those assumptions to assess the contact condition by

Journal of Manufacturing Science and Engineering SEPTEMBER 2017, Vol. 139 / 090801-5 equating the ratios of torque to penetration force in FSBR [41]. tensile loading. The process parameters were rotational speed The authors proposed torque to penetration force ratio for evaluat- (RS), friction pressure (FP), forging pressure (FoP), and friction ing the communication condition in the FSBR process. They con- time (FT). The evaluated parameters were the mushrooming effi- cluded that initial rivet penetration was pure sliding which later ciency, which was defined as a measure of anchoring efficiency of changed to the mixed contact beyond a critical penetration depth. the rivet in the base plate, the rivet penetration depth (T), the In the pure sliding condition, a linear relationship between mate- mechanical energy input, and the pull-out force (Fpull-out). The rial removal rate and the penetration force or torque was observed. authors suggested that lower RS, FT, and FP values were suitable This relationship was found independent of the rotation speed but for less mechanical energy inputs to avoid likely thermal degrada- was greatly influenced by the feed rate. tion of polymer matrix. Amancio-Filho et al. [59] described the Amancio-Filho [53] suggested an analytical model to calculate rivet geometry in anchoring zone (AZ) as the most important the heat input during FricRiveting of unreinforced aluminum/ parameter and linked it with the mechanical performance of joints. thermoplastics. The model allowed the calculation of the total They argued that anchoring region experienced the bulk of the load heat generated; however, the model was restricted only to thermo- faced by the joint. They further formulated that anchoring zone plastic materials as the constitutive model was based on the shear geometry was substantially affected by the frictional heat in addition Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021 rates and variations in viscosity of the molten polymer. to the deformation force borne by the rivet during the forging phase. Available analytical models cover the estimation of rivet By evaluating the thermal degradation of Al 2024/polyetherimide anchoring efficiency, frictional force, and torque required during (PEI) FricRiveted joints, it was established in Ref. [60] that the rota- the FSR processes. However, the models do not account for the tion speed appeared as a prime contributing factor in governing the varying frictional heat generation due to the interaction of rivet rate of heating, heating time, and temperature change. An increasing and work material during the process. The heat development trendintemperatureandheatingrateswasestablishedwithhighrota- regime during FSR processes is an extremely intricate phenom- tional speeds, and the heating time increased only slightly. enon, which involves solid friction and viscous dissipation (inter- Miles et al. [33] found that in friction-bit joints, the joint nal shearing of macromolecules) between the work and rivet strengths were profoundly influenced by material and features of materials. A comprehensive thermomechanical model catering for the joining bit in addition to the spindle speeds and feed rates. thermal softening and phase changes in work material during the Squire et al. [61] in their research work of joining high strength FSR processes is not available. The present models can be aug- steel (HSS) and Al alloys through FBJ found that increasing spin- mented by incorporating the thermal effects to obtain a superior dle speed and feed rates had a positive influence on joint strength. estimation of the associated forces. However, it was found that after a certain rpm (3250), there was a sharp decline in the tensile strength. The researchers attributed this phenomenon to the excessive heat generation. 5 Evaluation of Essential Process Parameters For friction stir riveting by extrusion, Evans et al. [36] deter- The successful joint formation through the friction stir proc- mined a noteworthy variance in the joint strength once the two esses depends on performing the operations on their optimized aluminum sheets were only extruded through the hole but not parameters [54]. Available literatures give an insight on the com- joined as compared to when they were extruded and joined. The bination of different parameters to assess the feasibility of the pro- joints made through joined and extruded aluminum sheets dis- cess. Studies conducted by Min et al. [55] explored the process played superior strengths compared to the joints where the alumi- parameters for FSBR joints with two different shank diameter riv- num sheets were only extruded. ets and various Al sheets. The authors used three different Extensive investigations have been devoted to evaluate the pro- settings of spindle speed and feed rate (for each parameter) to cess windows and parameters influencing the performance of FSR optimize the process parameters. The study suggested that maxi- processes. Spindle speeds and feed rates along with the tool mum penetration force and peak torque increased with feed rate. design emerged as important considerations for all FSR methods. However, increasing the spindle speed had a reverse influence on However, these parameters (spindle speeds and feed rates) need to maximum penetration force and peak torque. Gao et al. [56] con- be optimized within a process window for an FSR process. firmed the importance of rivet design, parameters like off-axis Improper selection of these parameters may result in excessive angle, and rivet cap diameter, which significantly contributed or insufficient heat generation along with the undesired plunging/ toward the joint strength. The findings showed the inconsequential cutting force, which affects the bond formation. The gap between effect of rivet diameter on joint strength. Lathabai et al. [57] stud- work materials is a common defect, which is caused by the mate- ied the effect of rivet design on force and torque parameters for rial flow under improper process parameters. Also, the configura- FSBR joints. They suggested that penetration force required in tion of materials in a joint and rivet geometry serves as a major case of blind rivets with hollow mandrel heads was much lower contributing factor for the processes like FSBR, SBR, and FricRi- than the one with solid mandrel heads. Wang [58] probed the pro- veting. In general, the softer work material is preferred to be cess parameter effects on the FSBR process. The materials used placed above the other one to facilitate the material flow, and in for this purpose were Al, Mg, and carbon fiber-reinforced polymer some cases, form the mechanical interlocking. (CFRP) composite materials. FSBR joints were made using differ- ent stacking sequences of the materials. Through the experimental results of tensile tests and torque and force measurements, the 6 Characterization of Stirred Region authors found the configuration of the material sheets as the most Understanding the microstructural changes in stirred area is important factor for FSBR joint. Feed rate also appeared as a pri- important as it develops the foundation for successful studies into mary contributing factor in quality issues like the gap between the thermomechanical modeling and mechanical performance of the work materials and rupture at the bottom sheet. However, spindle FSR processed joints. Min et al. [62] examined the microstructural speed was not as important as configuration and feed rate. development of an FSBR aluminum alloy sheet (AA6111) In Ref. [44], Podlesak et al. discovered that high-quality joints were through electron-backscattered diffraction (EBSD) technique. The formed by SBR process when the spindle speed was higher than a crit- researchers found distinct zones near the rivet, which were one ical value, i.e., 3500 rpm, with a stable process time. The researchers stir zone (SZ), three thermomechanical-affected zones (TMAZs), recommended these settings for all material combinations. They dis- and one heat-affected zone (HAZ). HAZ and TMAZ terminolo- covered that a lower spindle speed resulted in less frictional heat and gies represented the same meaning as in an FSW process [63]. consequently worse plastic deformation especially in the case of Mg. The amount of shear deformation varied in all TMAZs, whereas Blaga et al. [48] used a full-factorial design with a center point with the increasing distance form rivet hole characterized the to assess the influence of the process parameters in FricRiveted HAZ formation. Through EBSD, it was found that only the SZ joints on the joint formability and mechanical behavior under underwent recrystallization. Microhardness of HAZ was found to

090801-6 / Vol. 139, SEPTEMBER 2017 Transactions of the ASME be on the lower side than the parent base material. Temperature interface in MTMAZ. PTMAZ was between the MTMAZ and and the tangential shear decreased with the increasing dis- PHAZ, characterized by a weld line with consolidated polymer. It tance from the hole edge. These phenomena resulted in lowering was inferred from the analysis that interfaces were mainly held by of the amount of deformation in the three TMAZs. Figure 8 [62] adhesive forces. Rodrigues et al. [66] revealed microstructure depicts the grain structure of various zones. changes in a friction-riveted joint of Al 2024-T351/polycarbonate Croom et al. [64] investigated the distributions of fiber, metal (PC), such as different microstructural zones in the anchoring inclusion, and pore volume fractions in the stirred region of a zone and grain realignment in the MTMAZ. However, no micro- CFRP in an FSBR joint by using micro X-ray computed tomogra- structural changes were visible in MHAZ, compared with the base phy in combination with volumetric digital image correlation material. The authors observed that there was no noticeable (V-DIC). The authors found significant microstructural differen- change in polycarbonate as it was amorphous and transparent. ces between stirred region and the bulk material. The study results The changes in metals are related to the grain structure modifi- showed an apparent increase in volume fractions of fiber, metal cation caused by phase transformation. The changes occur due to inclusion, and pore in the stir zone. An interesting observation softening and shear deformations. Based on the grain refinement was the existence of sporadically spread large size metal inclu- and adjustments, the researchers categorized the affected regions Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021 sions. Large size metal compositions were aluminum, which in various zones—mainly stir zone, thermomechanical, and heat believed to be coming from aluminum workpiece placed as the affected zones. In a frictionally stirred thermoplastic polymer, top sheet. The authors pointed out that the presence of aluminum melting is the main observation, which may have the adhesive inclusions was owing to the high operating temperatures occurred effect on the metal joined next to it. during the process. Amancio-Filho [65] characterized the microstructural zones of FricRiveted joints between amorphous polymers and aluminum. 7 Bond Formability in FSR Joints Five distinct microstructural zones (Fig. 9) namely, the anchoring To understand the mechanical behavior of the FSR joints, a zone (AZ) of the deformed rivet tip, the polymer heat affected detailed insight of different bonds formability during the joint for- zone (PHAZ), the polymer thermomechanically affected zone mation is essential. Gao et al. [56] tried to interpret the interface (PTMAZ), the metal heat affected zone (MHAZ), and the metal morphology of the FSBR joint. The authors cited that frictional thermomechanically affected zone (MTMAZ) were identified. heat that was generated throughout the process might have caused The characterization was conducted based on the temperature and the formation of the rivet to work material bonding and interwork high deformation rates. The researchers found metal–polymer piece welding. However, their results exhibited minor indication of a direct association between frictional heat generation and joint strength. Wang et al. [67] stated that during the FSBR process, the bottom material tried to flow up. The authors articulated that in the case of bottom material being stronger than the upper work material, it penetrated into the upper sheet and resulted in the for- mation of a mechanical interlocking at the interface of two layers. On the other hand, a gap formed between two sheets if the bottom material was softer, as the lower material could not be pierced into an upper layer. Authors performed experiments with different configurations of CFRP, Al, and Mg sheets. They found that CFRP/Al and Mg/Al configurations had different mechanical interlocking. CFRP layer on the top created a mechanical inter- locking around the rivet body since the residue was corn shaped, which caused the bottom material to flow up with an angle. How- ever, aluminum or magnesium on the top could not generate mechanical interlocking around the rivet body because their resi- due shapes were concaves, and the material flows were different. Additionally, the mechanical interlocking of Mg/Al configuration was about 1 mm away from the rivet body. Intermetallic com- pound (IMC) layer was not observed in FSBR joint. Figure 10(a) illustrates the mechanical interlocking bond formation for CFRP/ Fig. 8 EBSD microstructure of the frictionally penetrated Al configuration [67]. AA6111 specimen showing different microstructural zones Miles et al. [35] used a thin intermediary film of interstitial free along with their dimension [62] steel between the cast iron and the aluminum to yield adequate frictional heating for bond formation in FBJ process. The researchers observed between the joining bit and cast iron because of the steel plate. In another research work [34], Miles et al. while joining HSS and Al alloy through FBJ found that the bonding area of the materials was larger than the characteristic weld nugget diameter of a resistance spot weld steel used in automotive applications. They argued that the increased bond diameter helped in improving the joint strength in compari- son to spot welding. Good metallurgical bonding between the steel and Al alloy was seen (Fig. 10(b)) and validated by micro- chemistry analysis. However, no intermetallic phases were found between Fe and Al. In F-SPR process [29] (Fig. 10(c)), IMC layer was observed at different stirred regions along with the thick part of mixed materials. The regions became thicker with increasing dwell time. Fig. 9 Schematic representation of typical microstructural In summary, in addition to mechanical connection, the bonding zones found in FricRiveting joints: PHAZ, PTMAZ, MHAZ, and phenomena include mechanical interlocking and chemical bond- MTMAZ [65] ing, e.g., IMC and non-IMC layers. The bond formation is highly

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Fig. 10 Bond formation in different FSP: (a) mechanical interlocking in CFRP/Al joint due to FSBR [67], (b) interfacial bonding in Fe/Al joint due to FBJ [34], and (c) bond formation in F- SPR [29] dependent on the material flow through the process, which in turn of the displacement occurred due to thinner AA6111 workpiece tear- depends on the process dynamics and the tool design. ing. The authors concluded that in AA6111/AA6022 joints, mechani- cal properties were governed by the properties of the AA6111 8 Mechanical Behavior of FSR Joints workpiece, which was the fragile constituent material in the joint. Amancio-Filho et al. [69] classified five failure modes for a Fri- For the optimal utilization of any structure, it is imperative to cRiveted thermoplastic material joint subjected to static tensile comprehend its mechanical behavior when subjected to various loading. They are given as follows: (1) Through the rivet (type I), loading conditions. This section covers various aspects of mechanical in which ductile fracture occurred in the metallic rivet outside the characterization for FSR joints. joint. (2) Rivet pulling out with a back plug (type II) took place because of crack nucleation at the rivet deformed tip in the 8.1 Classification of Failure Modes. Extensive literatures anchoring zone. The term “back plug” referred to the leaving part were reported in identifying the failure modes of FSR joints under dif- once the rivet was pulled out. (3) Full rivet pulling out (type III) ferent loading conditions. In Ref. [44], the researchers found lateral, was hallmarked with the complete removal of the rivet leaving shear-out, and bearing failure as the failure modes for spin riveting. behind a hole of similar diameter as of deformed end. (4) Rivet Wang et al. [67] classified the failure modes of different config- pulling out (type IV) was characterized by large deformations at urations of FSBR lap-shear joints under static tensile loads. The the rivet tip with small insertion depths, which caused the anchor- authors used an in situ acoustic emission analysis technique to ing zone close to the material surface. (5) Rivet pulling out with evaluate the commencement and growth of damage. For as- secondary cracking (type V) involved an intricate failure mecha- fabricated CFRP/Al FSBR joints, cleavage and tension failures nism [63]. In this failure mode, nucleation was observed at differ- occurred in the CFRP workpiece. Three different failure modes ent positions nearby the anchoring zone. The initial phases of were observed for as-fabricated FSBR Mg/Al joints, i.e., tension, crack promulgation resembled type IV failure, but the final frac- shearing, and bearing followed by cleavage patterns. The domi- ture occurred as typified as type III failure. Figure 11 describes nant mode of failure was tension mode. Likewise, for the as- different failure modes [69]. fabricated FSBR CFRP/Mg joints, failure occurred either at the Two-sided friction stir riveting by extrusion processed joints CFRP (tension failure) or at the Mg side (mixed failure of tension depicted shearing of the aluminum extrusion at both edges of the and shearing). Moreover, a rivet pullout was also observed. Min steel along with a breaking of the aluminum/steel bond [36]. In et al. [68] classified the failure modes of FSBR AA6022/AA6111 most of the cases, aluminum shearing occurred at both the top and joints. They found that the thicker AA6022 workpiece underwent the bottom of the steel hole leaving a deformed, cylindrical sec- minimal bending deformation during tensile testing and the majority tion of aluminum in the hole.

090801-8 / Vol. 139, SEPTEMBER 2017 Transactions of the ASME Fig. 11 Failure modes in FricRiveted joints [69] Downloaded from http://asmedigitalcollection.asme.org/manufacturingscience/article-pdf/139/9/090801/6405942/manu_139_09_090801.pdf by guest on 25 September 2021 Different FSR processes show various failure modes because of reduction in microhardness with respect to the parent metal was the different material interactions across the interfaces. Also, the fas- observed. However, the decrease in the values was smaller for the tening mechanism in different FSR processes (e.g., tail in MHAZ than TMAZ. Moreover, regions in the vicinity of the rivet FSBR and anchoring zone in FricRiveting) changes the distribution experienced an increase in microhardness compared to the base of forces in the joints, thereby resulting in different failure modes. material. The authors associated these changes in hardness in the PHAZ with physical aging and hardening. The phenomenon 8.2 Determination of Mechanical Properties. Mechanical occurred as a result of loss of structural water, coming as a by- properties are important characteristics in estimating a joint per- product when different monomers joined together to form polymer. formance under different service conditions. The preceding para- Min et al. [68] carried out microhardness tests for the top sheet graphs provide an insight of mechanical performance of FSR joints. (AA6111) and presented the results in the form of relative hard- ness values, where the interrupted and complete joints were meas- 8.2.1 Tensile Strength Studies. All published literatures on ured at four different layers through the thickness. In an FSR joints conclude that the resultant joint through FSR is of intermittent joint, layer-I (layer closest to the AA6111 top surface) higher structural strength than many other joining methods, such had the maximum hardness. The authors related this phenomenon as adhesive bonding. Min et al. [70] investigated the tensile with the massive deformation of upper surface of the AA6111 strength of Mg/Al FSBR joints (manufactured at different spindle workpiece that was near the shank. Min et al. [70] also investi- speeds and feed rates) and compared them with the ones having gated the hardness profile of AM60 alloys. The authors explained predrilled holes. They used three materials cast Mg AM60, rolled that the increase of hardness phenomenon in FSBR stir zone was 1.5 mm thick Al AA6022, and extruded 3.15 mm thick Al AA6082 due to frictional penetration and tail forming (expansion of rivet’s for the study. FSBR joints in all the cases were found to be of supe- tail once the mandrel is pulled out) process. Croom et al. [64] rior tensile strength compared to the joints with predrilled holes. explored the local mechanical properties in the stirred region of Based on the analysis, tensile strength was linked to parameters CFRP. V-DIC was used for 22 and 44 MPa compression loads in such as tail forming process, frictional penetration, material match- the near rivet hole specimen to calculate the axial strain. The ing, and sheet position. Min et al. [71] also found superior tensile results revealed that enhanced metal inclusion and fiber contents strength for CFRP/CFRP and CFRP/Al FSBR joints. Zhang et al. improved the material stiffness in the stir zone. Also, localized [72] studied HSS/Mg alloy FSBR joints. They found that tensile bands under high axial compression were detected on the rivet strength varied with changing feed rate and material configuration. hole surface highlighting a nonuniform deformation in different It was noted that with increasing feed rates, load-carrying capability stirred zone areas, which was related to kinking introduced by and percentage elongation increase for FSBR samples. localized buckling (Fig. 12)[64]. In Ref. [66], the tensile strength of the AA2024-T351/polycarbonate 8.2.3 Performance Evaluation Under Cyclic Loading. Fatigue (PC) FricRiveted joints exhibited excellent values of ultimate ten- life performance consideration is necessary as the joints undergo sile forces in comparison to riveted joints, i.e., 68.4% greater dynamic loads throughout their service life. Moreover, it helps in strength. The similar range of superior tensile strength values was devising the fail-safe methods. In that regard, Gao et al. [56] per- also reported by Amancio-Filho [65] for AA2024-T351/polyetherimide formed the fatigue life comparison of Al 5052-H32 FSBR joints (PEI) FricRiveted joints. Blaga et al. [48] used tensile testing to with the similar material joints made using resistance spot weld- investigate the mechanical anchoring behavior of FricRiveting ing and blind riveting processes. Experimental results concluded joints. The researchers measured the contact volumes between the that the fatigue life of FSBR was one to two orders of magnitude polymer and the rivet to define the mechanical anchoring. FBJ higher than spot welding and blind riveting process. Fatigue test- and SPR joints [73] made of Al 7075-T6/dual phase (DP) 980 ing of Al7075/DP980 steel FBJ [75] joints revealed that the failure steel alloy were compared in terms of shear strength. Results occurred in the aluminum material while the FBJ bit and the weld revealed that FBJ average lap shear strength was 6.4 KN in com- zone remained intact and undamaged. It was concluded from the parison of 5.0 KN shear strength of SPR joints. experiments that FBJ bonds had improved fatigue properties com- Reported literatures reveal that FSR processes usually depict pared to the base aluminum material. higher strength compared to conventional fastening techniques. Available studies regarding mechanical properties show Although all the researchers have cited metallurgical/mechanical improved tensile and fatigue strengths compared to conventional bond formation as the primary contributor to this improved per- riveting mechanisms, but most of the studies are focusing on link- formance, different reasons were proposed for the bond formation ing the mechanical properties to process window optimization. in each individual FSR process. Limited literature is available in establishing the relationship among microstructure evolution, mechanical properties, and the 8.2.2 Investigation of Local Mechanical Properties. In the lit- process mechanics, which inhibits the development of comprehen- erature, to evaluate the softening and thermal effects of the pro- sive understanding of the FSR processes. cess in stirred zone, microhardness tests were conducted for different FSR processes. In Ref. [74], researchers discovered that the microhardness of a friction riveted polymer metal joint was 9 Corrosion Resistance of FSR Joints associated with microstructural changes due to thermomechanical A thorough understanding of mechanical joints under corrosive processing. For metallic rivet portions within the worksheets, a environment is vital, particularly when dissimilar materials

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Fig. 12 Calculated axial strain ezz at (a) 130 and (b) 260 N compression loads of CFRP composite after FSBR. Axial strain concentration on the rivet hole surface is marked with (*) [64]. involved, which increase the likelihood of galvanic corrosion at  The methods are capable of joining both similar and dissimi- the joint interface [76–80]. FSR joints are also susceptible to this lar materials with a wide variety of material selections. phenomenon. Li et al. [81] examined the behavior of FSBR Mg/  They are highly suitable for batch production and automated Al joints under severe marine environment. The authors found feeding system. through Fourier transform infrared spectroscopy and X-ray pow-  FSR processes require a shorter span of process times includ- der diffraction (XRD) analyses that FSBR Mg/Al joints experi- ing post processing and sample preparations as compared to enced both crevice and outside of the crevice corrosion. Mg other joining methods, e.g., adhesive bonding combined with passivated inside the crevice, and substantial corrosion was found welding. on Al. Within the crevice, hydrogen-evolution cathodic reaction  FSR joints have relatively high strengths and large displace- yielding OHÀ resulted in high pH environment. The additional ments before fracture. OHÀ then prevented ClÀ in the crevice, thereby further increasing  FSR processes allow the development of newer products and the passivation of Mg. Outside crevice, minimal corrosion was sophisticated design that were previously not possible with observed on Al, contrary to a significant amount of corrosion on conventional joining processes. Mg. The authors suggested that preventions should be taken to Like any other process, FSR processes also have some limita- protect both Al and Mg metals when a crevice is formed. In addi- tions. The primary limitations are described below: tion, the authors found that at the interface, the rivet was free of corrosion indicating little or no seepage of moisture from the envi-  The nature of processes only permits production of spotlike ronment due to the tight contact between rivet and work materials. joints. Researchers studied the corrosion behavior of spot joined Al  FSR processes are limited to opting for optimized worksheet 7075-T6/DP980 steel alloy by FBJ [82]. The study was conducted configurations for high quality joints. considering two scenarios: (1) with adhesives in the joint (weld-  Friction-riveted joints like other bonded and riveted joints bonding specimens); and (2) without adhesives (FBJ only speci- cannot be re-opened. mens). Using tensile testing, it was instituted that adhesive-FBJ bonded specimens showed higher strengths than the FBJ samples. The researchers found a large crevice between the steel and the 11 Synopsis and Future Outlook aluminum sheets only in FBJ specimens, providing an entry path for corrosive material. The path was closed because of the exis- The paper presents a detailed review of FSR processes, which tence of the adhesive for the weld-bonding joints, therefore result- includes process physics, mechanical behavior, process parameters, ing in better corrosive resistance. microstructural studies, corrosion behavior, process modeling, pro- In addition to galvanic corrosion, which is believed to be the cess advantages, and limitations. major corrosion mechanism in the dissimilar material joint, crevice The techniques under “FSR” umbrella are promising joining corrosion should be considered in FSR joint since the crevices are usu- techniques that synthesize the advantages of friction stirring and ally found between work materials. The size of crevice between the mechanical fastening. All FSR processes rely on frictional heat rivet and work materials, however, is much smaller than the conven- generation resulted from the interaction of the rivet and joining tional riveting where predrilling hole is required; and thus, minimize material. The frictional heat leads to softening the material, the corrosion of rivet itself. If the rivet does not corrode significantly, thereby allowing sufficient stirring, which subsequently causes the joint maintains its majority of the strength. Nevertheless, corrosion mechanical interlocking where material flow acts as an additional behavior of FSR joints needs further intensive investigations. bonding mechanism to riveting itself. In some cases, metallurgical bonding between the rivet and work materials can also occur. Thereby, improved mechanical properties of FSR joints are usu- 10 Advantages and Limitations of the FSR Processes ally seen when compared to conventional riveting method. The process parameters, including spindle speed, feed rate, and work The study brings forth various advantages of FSR processes, material stacking sequence, emerge as crucial factors for defect- some of which are summarized below: free joints. Rivet geometry and material are also important consid-  Most of the FSR processes eliminate the need for a predrilled erations for the process’s success. In the current FSR processes, hole, thereby reducing the difficulties in laminating the mul- most rivets are commercially available. Special rivet geometry tiple holes during joining. design is still in the early investigation stage. Three major

090801-10 / Vol. 139, SEPTEMBER 2017 Transactions of the ASME microstructural zones are usually observed in FSR processes. Stir [13] Booth, G. S., Olivier, C. A., Westgate, S. A., Liebrecht, F., and Braunling, S., zone is characterized by refined grain structures; thermomechani- 2000, “Self-Piercing Riveted Joints and Resistance Spot Welded Joints in Steel and Aluminum,” SAE Paper No. 2000-01-2681. cally affected zone experiences medium deformations and tempera- [14] Wang, J. W., Liu, Z. X., and Shang, Y. Y., 2011, “Self-Piercing Riveting of ture rises and is characterized by deformed grains; and heat-affected Wrought Magnesium AZ31 Sheets,” ASME J. Manuf. Sci. Eng., 133(3), p. 031009. zone involves only temperature rises without apparent grain structure [15] Yasube, Y., Kishimoto, K., Kato, T., and Mori, K., 2010, “Mechanical Clinch- change but sometimes precipitate coarsening happens. ing of Hot-Dip Zinc-Aluminum Alloy Coated Steel Sheets,” J. Jpn. Soc. Tech- nol. 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