Zengqing CAO, Yngjie ZUO,b,*

a School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China

b School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, China

KEYWORDS

Abstract With the increasing applications of novel materials and structures in new-generation aircraft, conventional joining techniques in aircraft component assembly are greatly challenged. To meet those challenges,the electromagnetic riveting(EMR)technique was developed as an advanced joining tool, which exhibits obvious advantages in the assembly of new-generation aircraft. In this paper,the riveting principle of EMR was analyzed,and its development history and status were presented in detail. Then, equipment features of three typical EMR systems were given. Moreover,three important applications of EMR were covered,i.e.,composite structure riveting,titanium rivet and large-size aluminum rivet riveting, and interference fit bolt installation. Specially, a novel strengthening method for mechanical linking holes based on EMR was also presented, which can significantly improve the fatigue behaviors of mechanical joints. Finally, open questions in the EMR field were discussed,and some recommendations for future work were also made.This paper can be useful for optimizing the joint designs of aircraft components and improving the level of aircraft maintenance.

1. Introduction

In order to ensure future competitiveness in the aeronautic field, new-generation aircraft are extensively required to have a reduction in structure weight and production, and an increase in the service life and maneuverability. To meet those challenges, light-weight and high-strength materials such as composite materials and titanium alloys are finding increasing use in aircraft structural components.1–3However, uses of those novel materials also bring about many technical challenges for aircraft assembly.Typically,joining composite components is much more complex and critical compared with joining conventional metals, which is due to the brittleness,anisotropy,and low transverse strength of composite.1,4,5Titanium alloy is hard-forming at room temperature,resulting that joining titanium alloy components also involves many technical challenges.6,7Moreover, as new-generation aircraft increase in size to reduce operating cost and increase effective load capability, so do the requirements for component joining due to increases of component size and load transfer. Hence,advanced joining methods and tools are necessary to be developed to improve the current levels of design and assembly of new-generation aircraft.

Mechanical fastening techniques are dominant in joining of aircraft component design currently to take their potential advantages of suitability for highly loaded laminates,inspectability, detachability, and repairability.8–11Among them, riveting techniques are widely used in aircraft assembly.12,13However, conventional riveting techniques, such as press riveting and pneumatic riveting,are prone to cause riveting damage in composite structures, and have difficulty in forming hard-forming materials or large-size aluminum rivets,which relatively limits the improvement of the joining level of new-generation aircraft components.

To simplify,accelerate,and improve the quality of rivet formation in contrast to conventional riveting methods, the electromagnetic riveting (EMR) technique was developed to take its advantages of rapid deformation,large impact force,deformation stability, and easy control.14–16During the EMR progress,uniform interference can be obtained,thus facilitating its applications in composite structure riveting; moreover, adequate riveting forces of EMR can also overcome the difficulty in the deformation of titanium alloy rivets or large-size aluminum rivets at room temperature.17,18With recent developments in the EMR technique, EMR has been used to realize the potential values of composite, titanium alloy, or largesize components in some aerospace assembly processes as a novel riveting technique.19,20

This paper focuses on the EMR technique and its applications. The riveting principle of EMR is analyzed firstly, and then the development history and status of EMR are presented.Finally,four important applications of EMR are characterized,including composite structure riveting,titanium rivet and large-size aluminum rivet riveting, interference fit bolt installation, and strengthening of mechanical linking holes.This paper can be useful for optimizing joint designs of aircraft components and improving the level of aircraft maintenance.

2. Principle and characteristics of EMR

Fig.1 Schematic of the EMR progress.

Electromagnetic dynamic load has been used in many fields such as electromagnetic launch and electromagnetic gun. A schematic of the EMR progress is shown in Fig.1.The riveting force of EMR is based on Lorentz force,which is converted by electromagnetic energy.15,17During the EMR progress, a DC power source is used to charge the system capacitanceCfirstly.A magnetic field around the master coil is established by the discharge from the system capacitance, and then a current in the slave coil is induced by this magnetic field, setting up a magnetic field around the slave coil. Finally, the interaction of the magnetic fields around the slave coil and the master coil generates a dynamic Lorentz’s load. After the generation, the dynamic load acts on the rivet in the form of stress wave through the driving head, which leads to a finished rivet configuration with a high-speed loading rate.

In the EMR progress, the discharge energyEstored in the system capacitance is

whereUis the discharge voltage.The riveting energyErcan be obtained by

where κ is the energy conversion coefficient of EMR, which manly depends on the discharge voltage and the geometric structures and materials of the master coil and the slave coil.The riveting force of EMRFacting on the slave coil is obtained by

whereJandBare the current density and the magnetic flux density, respectively. The riveting force of EMR is a function of time, which was described as14

whereRandLare the resistance and inductance of the RLC oscillating-loop installation, respectively.

The riveting principle gives EMR advantages of an accurate and repeatable output force, easy control, and a high loading rate, which result in exceptional consistency in rivet upset dimensions and operating efficiency.21,22Indeed, EMR involves rapid deformation leading it to be superior to conventional riveting techniques in riveting quality. Typically,during the EMR progress, the hole expansion of a riveted joint is more homogeneous than those of conventional riveting techniques, which results in a better exhibition in the fatigue life of riveted joints.23–25The riveted joint of EMR also exhibit an excellent joining quality compared with those of other riveting methods in mechanical property tests and microstructure observations.Besides, EMR can effectively reduce installation time and noise levels, thus reducing worker fatigue.21,22Property comparisons between EMR and typical riveting methods are listed in Table 1.

3. Development history of EMR

In 1960s, the Boeing Company firstly conducted research in EMR to meet the technique challenges in pneumatic riveting,and then developed the first handheld high-voltage EMR system in 1971.25Since the cost of a high-voltage EMR system was high, and a highly-charged voltage (higher than 5000 V)could create security risks for operators, the application of EMR in aircraft component assembly was limited.

With the development of both capacitance and control technology, a handheld low-voltage EMR system with a charged voltage lower than 500 V was developed in 1988.15Moreover, the Electroimpact Inc. was also built by Peter Z.focusing on the development of an EMR system and the study of EMR applications. The use of a low-voltage EMR system significantly reduced the cost of equipment, improved the service life and safety factor of equipment, and also met many technique challenges involved in high-voltage EMR. After the development, the low-voltage EMR system was quickly and widely applied in some aerospace assembly processes.

Considering the limitations of handheld EMR systems such as poor riveting consistency and high labor intensity, automatic riveting machines with a low-voltage EMR head weredeveloped in mid 1990s.As expected,automatic EMR systems could significantly increase the riveting quality and riveting efficiency; however, their production and maintenance costs could be very high. Currently, America, Russia, and China are the three main countries involved in researching the EMR technique. The EMR development stages of these three countries are list in Table 2.

Table 1 Property comparisons between EMR and typical riveting methods.19,26

4. EMR systems

In the past 60 years, a series of EMR systems has been developed in response to the demand of aircraft component assembly for better process control and reliability, as well as better joining quality of riveted joints,which have a wide application in many generation aircraft such as A320,ARJ21,B747,A380,B787, etc. Typically, EMR systems can be divided into three types:handheld EMR systems,semi-automatic EMR systems,and automatic EMR systems.

Handheld EMR systems are designed for use with a variety of solid alloy rivets. Those systems can be used in formations of both headed and slug rivets due to electronic synchronization of actuators,and allow an operator to form rivets in a safe and efficient manner.22,27Fig.2 details two typical handheld EMR systems. Since handheld EMR systems are designed for hand riveting, they are required to be effective, light, and portable. To achieve those goals, those systems are normally developed with an increased upper limit of the voltage range such as 1000 V to maximize the output force.Besides,the output force produced is also dependent on the size of the master coil; therefore, handheld EMR guns with different sizes of master coils are designed to meet the requirements of the formation of rivets with a variety of sizes and materials,as shown in Fig.3. To minimize the recoil and mass, a spring-damper system is normally incorporated to a handheld EMR gun.Moreover, computers are used to control handheld EMR systems to obtain a high repeatability of rivet formation by reducing the dependence upon the operator’s skill.However,even if computer-controlled EMR systems are used,the riveting quality of handheld riveted joints is still significantly affected by the operator’s skill. If not used properly, handheld EMR systems can result in a poor consistency in rivet upset dimensions.

To further improve the joining quality of riveted joints in EMR, semi-automatic EMR systems (see Fig.4)32and automatic EMR systems(see Fig.5)33have been developed.Thosesystems are normally designed to automatically drill and install rivets and interference fit bolts in varieties of aircraft components, such as tank panels, wing panels, fuselages, and spars.Compared with handheld EMR systems, those systems offer advantages of higher repeatability of riveting quality, lower joining rejection rate and labor intensity,and higher efficiency.Despite those potentials, their overall complexities and high requirements in technology result in high production and maintenance costs, which limit their applications in aircraft assembly.

Table 2 Development history of EMR.19,25–31

Fig.2 Schematics of typical handheld EMR systems.

In EMR,the forming angle of the rivet die α has an important influence on the formation of the rivet head, which can control the radial component of the flow of material in the rivet head.34Without a carful design of the forming angle of the rivet die, crack formation can occur in the rivet head.14The geometrical structure of a rivet die in EMR and rivet dies with typical forming angles are shown in Fig.6,35wherehanddare,respectively,the depth and diameter of the groove of the rivet die.A smaller forming angle can lead to a larger interference size, while leading to a smaller-diameter rivet head. The optimal design of a rivet die depends on the requirement of riveted structures,and is essential to produce high-quality riveted joints.

5. Typical applications of EMR

With advantages over conventional riveting techniques, the EMR technique has been applied in the aerospace industry as an advanced joining tool. Typically, EMR exhibits obvious advantages in composite structure riveting, titanium rivet and large-size aluminum rivet riveting, and interference fit bolt installation. Moreover, EMR provides a novel strengthening method for mechanical linking holes to increase the fatigue life of mechanically fastened joints in aircraft structures.

5.1. Composite structure riveting

Composite materials, like carbon fiber-reinforced polymer(CFRP),have been increasingly used in aircraft structures.36,37However, composite joining remains unavoidable due to the structural complexity and size of aircraft components and load transfer requirement, which represents a potential weak point and is required for careful design.20,38Due to the non-uniform interference, conventional riveting techniques are prone to cause serious bearing damage in composite structures during the formation of rivets (see Fig.7(a)), which severely limits their application in joining composite structures. To reduce the damage,bolted joints instead of riveted joints are currently dominantly used in composite structures,39,40unfortunately,leading to overweight structures and high costs. With the advantage of uniform interference, EMR allows composite structures to be riveted by reducing the riveting damage,41–43which can obviously reduce the weight and cost of the joint.As shown in Fig.7(b),the formations of rivet bars are uniform and no obvious damage is caused in CFRP during the EMR progress, which can lead to a high quality of riveted joints.

Fig.3 Handheld EMR guns.

Despite the potential advantages of EMR,riveting parameters of composite riveted joints in EMR, such as the shank length of rivets,the clearance between the rivet and the hole, and the forming angle of the rivet die,still need to be carefully designed to optimize the joint design.

To further reduce the riveting damage,the EMR technique using a washer is developed. As shown in Fig.8,in whichFis the riveting force,a washer is used to restrain the forming of a rivet bar on the tail head side in EMR, which can increase the forming uniformity of the overall rivet bar and significantly reduce the damage. Normally, the washer is fabricated from titanium alloy to prevent corrosion caused by the electric potential between the washer and CFRP.By using the washer,EMR is allowed to be better applied in joining of composite structures. However, the use of a washer also leads to an increase in the weight of riveted joints due to the washer’s own weight, which can cause an overweight structure. Moreover, the small inside diameter of the washer can result in a small or even zero interference, which limits the potentials of EMR in the quality improvement of riveted joints, while the large inside diameter of the washer can result in a large interference size likely causing a damage in CFRP. Hence, to optimize the joint design, the thickness, inside diameters, and outside diameters of the washer need to be carefully designed in EMR.

5.2. Titanium rivet and large-size aluminum rivet riveting

Similar to composite materials, titanium alloys are also more and more widely used in new generations of aircraft due to their characteristics of low density, excellent corrosion resistance, and superior strength-to-weight ratio.2,44,45However,due to the dense-hexagonal crystal structure, formability of titanium alloy rivets, such as TA1 and TC4 titanium alloy rivets with a 4-mm diameter, is poor at room temperature,which has limited their application in aircraft manufacturing.17In order to form those titanium rivets, rivets need to be heated to improve their formability in conventional riveting methods, which can be complex and expensive, and even cause burn of the riveted joint. EMR has been proven to be an effective mean of installing titanium rivets at room temperature,46,47as shown in Fig.9. The formation mechanism of titanium rivets in EMR is based on a high strain rate.48With a high loading rate in EMR, the high-strain rate formation process of titanium rivets can be considered as an adiabatic state, and the soft effect of the high strain rate contributes to the formation of the rivets. Moreover, EMR can also provide an adequate force for the formation of titanium rivets. In EMR, the plastic deformation of titanium rivets are mainly concentrated at the shear band (see Fig.10),which is reported as the adiabatic shear band (ASB).18,49Despite the potentials of EMR, if riveting parameters, such as the shank length of the rivet and the riveting voltage,are not designed properly in EMR, the ASB can evolve in the crack damage in the tail head, leading to a poor joining quality of riveted joints.

Fig.4 Typical semi-automatic EMR systems.

Fig.5 Typical automatic EMR systems.33

Fig.6 Schematics of typical rivet dies in EMR.35

To reduce operating cost and increase effective load capability, some new-generation aircraft are required to be increased in size, especially for some commercial aircraft.Increasing the aircraft size leads to an increased use of large-size aluminum rivets (the diameter larger than 6 mm).Large-size aluminum rivets can cause severe difficulties in the formation using conventional riveting methods due to their requirement for a high forming force. Based on the loading mechanism,EMR can provide an adequate force in the formation of large-size aluminum rivets,besides,the advantages of a high loading rate and one-step forming in EMR can also increase the formability of large-size rivets by reducing the cold hardening of a rivet during the riveting progress. Typical 8-mm-diameter aluminum rivets formed by EMR are shown in Fig.11. It has been proven that EMR is an effective mean for the formation of aluminum rivets up to a 10 mm diameter.

5.3. Interference fit bolt installation

Interference fit is proven to significantly increase the ultimate failure load and the fatigue life of jointed structures by reducing stress concentration factors and oscillatory stresses.50–52The installation of interference fit bolts can cause an installation resistance and damage in joints due to the interference,which has an important influence on the quality of bolted joints.53–55To achieve the optimal fatigue life,a large interference size is designed,increasing the requirement in the installation progress of an interference fit bolt by resulting in a large installation resistance. The use of thick laminates can also bring the challenge of interference fit bolt installation by significantly increasing the installation resistance. Moreover, composite structures are prone to be damaged in the installation progress of interference fit bolts, which limits the realization of the maximum potential benefits of composites.53,56Hence,conventional installation methods such as pneumatic riveting or press riveting are greatly challenged.

Compared with conventional methods, EMR can provide an adequate force and high installation speed for interference fit bolt installation of both thick laminates and highinterference fit joints.57,58The use of EMR can reduce the installation resistance (see Fig.12) and the damage in jointed structures, especially in composite structures, during the interference fit bolt installation progress,which is due to the advantages of high loading rate and one-step installation in EMR.Typical applications of EMR in installing interference fit bolts in joints with a large interference fit size,composite joints,and thick laminates are shown in Fig.13. However, the related research is rare currently,and the installation mechanism using EMR is still not well understood.

To achieve a better installation quality of interference fit bolts, special installation and support heads of a bolt are required in EMR. As shown in Fig.14, an installation head is used to fix a bolt before the installation of the bolt and then drive the bolt finishing the installation;meanwhile,the support head is used to support jointed structures to reduce their deformations caused by the installation resistance.

Fig.7 Composite structure riveting.26

Fig.8 EMR technique using a washer.

Fig.9 Titanium structures riveted by EMR.

5.4. Strengthening of mechanical linking holes

To improve the fatigue behaviors of mechanically-fastened joints in aircraft structures,a variety of strengthening methods have been developed, such as interference fitting,62–64cold extrusion strengthening,65,66,38shot peening,67and laser shock strengthening.68,69With the development of EMR, it is found that EMR can also be useful in strengthening of a mechanical linking hole and significantly increase the fatigue life of the hole. As shown in Fig.15, plates with a hole strengthened by EMR were found to exhibit a longer fatigue life than those without strengthening.70

Fig.11 8-mm-diameter aluminum rivets formed by EMR.

Fig.12 Peak installation resistances during the interference fit bolt installation progress installed by EMR and press riveting,respectively.

The mechanism of EMR strengthening is similar to that of the cold extrusion strengthening method.71–73In EMR strengthening, a plastic deformation zone around the mechanical linking hole is formed and provides favorable residual stress to reduce the local amplitude of fatigue load around the hole. The strengthening progress of EMR is shown in Fig.16. In the first step, the EMR gun generates a dynamic Lorentz’s load. After the generation, the load acts on the plate in the form of stress wave through the strengthening die, and a local plastic deformation zone is formed.Here, the peak of the dynamic Lorentz’s load can be up to 2×105N, providing an adequate force for the strengthening of a variety of metal materials. In step 2, the plate is drilled at the location of the plastic deformation zone. Since the diameter of the plastic deformation zone is larger than that of the twist dill, after drilling, a part of the plastic deformation zone is still retained around the hole. This plastic deformation zone around the hole contributes to an increase in the fatigue life of the joint by providing favorable residual stress to reduce the local amplitude of fatigue load around the hole.Finally, the plate can be used for bolted joining with a high resistance to fatigue.

A schematic of EMR strengthening is depicted in Fig.17.Based on the special loading mechanism, EMR has the advantages of easy control, large impact force, high stability,and low cost over conventional strengthening methods. However, as a novel strengthening method, EMR strengthening still involves a number of technical challenges. Typically,the strengthening mechanism is not fully understood; moreover, the influences of strengthening parameters, such as the shape of the strengthening head, the loading rate, and the strengthening time, on the fatigue life of joints are still unknown. Thus, to improve fundamental understanding of EMR strengthening, more studies are necessary to be conducted.

Fig.13 Typical applications of EMR in interference fit bolt installation.59,60

Fig.14 Installation and support heads of an interference fit bolt in EMR.61

Fig.15 Fatigue life improvement using the EMR strengthening method.70

Fig.16 Strengthening progress of the EMR strengthening method.

Fig.17 Schematic of the EMR strengthening method.

6. Conclusions

To meet the challenges in the assembly of new-generation aircraft, the EMR technique was developed, which exhibited significant advantages in joining novel materials and structures compared with conventional joining methods. This paper focused on the EMR technique. The riveting principle was analyzed, and the development history and status of EMR were presented in detail. Equipment features of three typical EMR systems, i.e., handheld EMR systems, semi-automatic EMR systems, and automatic EMR systems, were given.Moreover, three typical applications of the EMR were covered, including composite structure riveting, titanium rivet and large-size aluminum rivet riveting,and interference fit bolt installation. Finally, a novel strengthening method for mechanical linking holes based on the EMR technique was discussed, which can significantly improve the fatigue behaviors of mechanically fastened joints in aircraft structures.Although EMR provides an advanced joining tool,some technique problems still exist.For instance,the recoil force of EMR systems is high during the riveting progress of large-size rivets, and the servicing life of the master coil is normally short. Moreover,designs of riveting parameters are much stricter in EMR applications. To further improve the EMR technique, ongoing work can be focused on the following:

(1) Improvement of EMR systems: highly automatic EMR systems with high reliability need to be developed to meet the increasing requirements in aeronautics and astronautics manufacturing fields.

(2) Riveting and interference fit bolt installation of composite structures: many more experimental and numerical studies on EMR applications in riveting and interference fit bolt/pin installation of composite structures are necessary to improve composite joining designs.

(3) Dynamic installation method for interference fit bolts:the installation mechanism and installation parameter optimization based on the EMR technique need to be studied.

(4) EMR strengthening method for mechanical linking holes: the strengthening mechanism, strengthening parameter optimization,and development of strengthening equipment all need to be studied.

Acknowledgement

This work was funded by the Major National Research Project of Numerical Control Machine and Basic Manufacturing Equipment of China (No. 2016ZX04002004-008).