Approach to Interference Riveting Process Control of Aircraft Automatic Drilling and Riveting

Interference fit riveting is an effective way to improve the fatigue life of aircraft.The accurate control of riveting interference of aircraft automatic drilling and riveting equipment is achieved by process parameters including upsetting force and upset head height.It is valuable for aircraft manufacturing engineering.An approach to interference riveting process control based on the analysis of interference riveting stress field is proposed.According to assembly structure,the upsetting force is calculated by the material property and interference fit level,and the upset head height is deduced by the upsetting force.The experimental result shows that the interference fit level can be controlled accurately by the upsetting force and upset head height,and then,the quality of aircraft automatic riveting can be improved.The proposed approach is verified by the good match between the predicted result and the experimental result.

A Comparative Study of Friction Self-Piercing Riveting and Self-Piercing Riveting of Aluminum Alloy AA5182-O

In this paper,self-piercing riveting(SPR)and friction self-piercing riveting(F-SPR)processes were employed to join aluminum alloy AA5182-O sheets.Parallel studies were carried out to compare the two processes in terms of joint macrogeometry,tooling force,microhardness,quasi-static mechanical performance,and fatigue behavior.The results indicate that the F-SPR process formed both rivet–sheet interlocking and sheet–sheet solid-state bonding,whereas the SPR process only contained rivet–sheet interlocking.For the same rivet flaring,the F-SPR process required 63%less tooling force than the SPR process because of the softening effect of frictional heat and the lower rivet hardness of F-SPR.The decrease in the switch depth of the F-SPR resulted in more hardening of the aluminum alloy surrounding the rivet.The higher hardness of aluminum and formation of solid-state bonding enhanced the F-SPR joint stiffness under lap-shear loading,which contributed to the higher quasi-static lap-shear strength and longer fatigue life compared to those of the SPR joints.

An Overview of Self-piercing Riveting Process with Focus on Joint Failures, Corrosion Issues and Optimisation Techniques

Self-piercing riveting(SPR)is a cold forming technique used to fasten together two or more sheets of materials with a rivet without the need to predrill a hole.The application of SPR in the automotive sector has become increasingly popular mainly due to the growing use of lightweight materials in transportation applications.However,SPR joining of these advanced light materials remains a challenge as these materials often lack a good combination of high strength and ductility to resist the large plastic deformation induced by the SPR process.In this paper,SPR joints of advanced materials and their corresponding failure mechanisms are discussed,aiming to provide the foundation for future improvement of SPR joint quality.This paper is divided into three major sections:1)joint failures focusing on joint defects originated from the SPR process and joint failure modes under different mechanical loading conditions,2)joint corrosion issues,and 3)joint optimisation via process parameters and advanced techniques.

Design of Drilling and Riveting Multi-functional End Effector for CFRP and Aluminum Components in Robotic Aircraft Assembly

To fulfill the demands for higher quality,efficiency and flexibility in aviation industry,a multi-functional end effector is designed to automate the drilling and riveting processes in assembling carbon fiber reinforced polymer(CFRP)and aluminum components for a robotic aircraft assembly system.To meet the specific functional requirements for blind rivet installation on CFRP and aluminum materials,additional modules are incorporated on the end effector aside of the basic processing modules for drilling.And all of these processing modules allow for a onestep-drilling-countersinking process,hole inspection,automatic rivet feed,rivet geometry check,sealant application,rivet insertion and installation.Besides,to guarantee the better quality of the hole drilled and joints riveted,several online detection and adjustment measures are applied to this end effector,including the reference detection and perpendicular calibration,which could effectively ensure the positioning precision and perpendicular accuracy as demanded.Finally,the test result shows that this end effector is capable of producing each hole to a positioning precision within ±0.5 mm,aperpendicular accuracy within 0.3°,a diameter tolerance of H8,and a countersink depth tolerance of±0.01 mm.Moreover,it could drill and rivet up to three joints per minute,with acceptable shearing and tensile strength.

Study of galvanic corrosion and mechanical joint properties of AZ31B and carbon-fiber–reinforced polymer joined by friction self-piercing riveting

A new testing methodology was developed to quantitively study galvanic corrosion of AZ31B and thermoset carbon-fiber–reinforced polymer spot-joined by a friction self-piercing riveting process.Pre-defined areas of AZ31B in the joint were exposed in 0.1 M NaCl solution over time.Massive galvanic corrosion of AZ31B was observed as exposure time increased.The measured volume loss was converted into corrosion current that was at least 48 times greater than the corrosion current of AZ31B without galvanic coupling.Ninety percent of the mechanical joint integrity was retained for corroded F-SPR joints to 200 h and then decreased because of the massive volume loss of AZ31B。

Friction self-piercing riveting(F-SPR)of aluminum alloy to magnesium alloy using a flat die

Friction self-piercing riveting(F-SPR)process based on a pip die has been invented to solve the cracking problems in riveting high-strength and low-ductility light metals,such as magnesium alloys,cast aluminum,and 7 series aluminum alloys.In this paper,in order to solve quality issues caused by the misalignment between rivet and pip-die in F-SPR,a flat-die based F-SPR process was proposed and employed to join 1.27 mm-thick AA6061-T6 to 3 mm-thick AZ31B.The results indicate that a 1.0 mm die distance is effective to avoid rivet upset and insufficient flaring.As the feed rate increases,the heat input in the whole process decreases,resulting in a larger riveting force,which in turn increases both the bottom thickness and interlock amount.Besides,solid-state bonding,including Al-Mg intermetallic compounds(IMCs),Al-Mg mechanical mixture,and Al-Fe atom interdiffusion was observed at the joint interfaces.The upper Al layer was softened,but the lower Mg layer was hardened,and both sheets exhibited a narrowed affected region with the increase of feed rate,while the rivet hardness shows no obvious change.Three fracture modes appeared accompanying the variations in lap-shear strength and energy absorption as the feed rate increased from 2 mm/s to 8 mm/s.Finally,the F-SPR process using a flat die was compared to those using a pip die and a flat bottom die to show the advantage of flat die on coping with the misalignment problem.

A Laser Line Scanner Based Hole Position Correction Mechanism for Automatic Drilling and Riveting in Aircraft Assembly

The low-stiffness of aircraft skins may results in the differences between aircraft actual parts and their theoretical models,which will consequently affect the accuracy of automatic drilling and riveting in aircraft assembly.In this paper,a novel approach of hole position correction using laser line scanner(LLS)is proposed to assign a single row of holes on the parts’surfaces.First,we adopt a space circle fitting method and the random sample consensus(RANSAC)to obtain the precise coordinates of center of the datum holes’coordinates.Second,LLS is calibrated by the laser tracker,and the relations between the LLS coordinate system and the tool coordinate system(TCS)can be calculated.Third,the kinematics model of the automatic riveting machine is established based on a two-point referencing strategy proposed in this paper.Thus,the positions of the holes to be drilled can be adjusted.Finally,the experimental results show that in TCS the measurement error of LLS is less than 0.1 mm,and the correction error of the hole position is less than 0.5 mm,which demonstrates the reliability of our method.

Friction-based riveting technique for AZ31 magnesium alloy

A new friction-based riveting technique, Rotating Hammer Riveting(RHR), is demonstrated to fully form AZ31 Mg rivet heads in a mere 0.23 s. Heat and pressure generated through severe plastic deformation during the process was sufficient to form the Mg rivet head without the need for a pre-heating operation. Due to preliminary twinning and followed by dynamic recrystallization, AZ31 Mg grains in the rivet head were refined during RHR, which enhance the formability of Mg rivets by triggering grain boundary sliding and reducing plastic anisotropy of Mg. In addition, RHR joints showed a metallurgical bond between the rivet head and top AZ31 Mg sheet, which eliminates a significant pathway for corrosion.

Embedment Effect on Eliminating Damage of CFRP Pull-riveting Process by Simulation Study

The rivet joints have been widely applied in aerospace and vehicle fields.During the joining process of the carbon fiber reinforced plastic(CFRP)laminates,the pre-tightening force of pulling-rivet was the key factor to ensure the connection performance.To predict the impact of clamping loads on stress and failure of laminates,the value of stress and damage evolution of the wall of a hole under the pre-tightening force were simulated by the finite element method.The results of the simulation showed that excessive clamping force led to the damage and failure of CFRP in the hole edge.Connection performance together with progressive failure process and failure modes of CFRP laminates with various pre-tightening forces were investigated.A kind of metal embedded parts embedded in the laminates was designed to reduce the damage by the simulation study.Simulation results showed that embedment reduced the failure and damage efficiently.The embedment reduced about 64%of the maximum stress.

Flexible servo riveting system control strategy based on the RBF network and self-pierce riveting process

As more and more composite materials are used in lightweight vehicle white bodies,self-pierce riveting(SPR)technology has attracted great attention.However,the existing riveting tools still have the disadvantages of low efficiency and flexibility.To improve these disadvantages and the riveting qualification rate,this paper improves the control scheme of the existing riveting tools,and proposes a novel controller design approach of the flexible servo riveting system based on the RBF network and SPR process.Firstly,this paper briefly introduces the working principle and SPR procedure of the servo riveting tool.Then a moving component force analysis is performed,which lays the foundation for the motion control.Secondly,the riveting quality inspection rules of traditional riveting tools are used for reference to plan the force-displacement curve autonomously.To control this process,the riveting force is fed back into the closed-loop control of the riveting tool and the riveting speed is computed based on the admittance control algorithm.Then,this paper adopts the permanent magnet synchronous motor(PMSM)as the power of riveting tool,and proposes an integral sliding mode control approach based on the improved reaching law and the radial basis function(RBF)network friction compensation for the PMSM speed control.Finally,the proposed control approach is simulated by Matlab,and is applied to the servo riveting system designed by our laboratory.The simulation and riveting results show the feasibility of the designed controller.

Hole surface strengthening mechanism and riveting fatigue life of CFRP/aluminum stacks in robotic rotary ultrasonic drilling

Carbon fiber reinforced plastic(CFRP)and aluminum stacks are widely used in aviation industry due to light weight and high performance.Millions of rivet holes need to be drilled on body materials,and more than 80%of fatigue cracks occur at the connection holes,so the damage and residual stress of hole surface have crucial effect on the riveting fatigue life of CFRP/aluminum stacks and the flight performance.Recently,robotic rotary ultrasonic drilling(RRUD)technology is a promising method to machine the stacks.However,the hole surface strengthening mechanism in RRUD and the service performance of the riveting joint are not verified.Thus,in this paper,the hole surface strengthening mechanism of RRUD for CFRP/aluminum stacks is investigated,a theoretical residual stress model is established,and the fatigue life experiment of riveted joints is conducted.Firstly,analysis on residual stress in RRUD is carried out with consideration of strengthening force and cutting temperature.Residual stress model is established based on the calculation of elastic stress,plastic stress and stress release.Validation experiment results show that ultrasonic vibration changes residual stress from tensile stress to compressive stress.At the same time,comparative damage analysis of CFRP hole exit and hole surface in robotic conventional drilling(RCD)and RRUD is presented.Finally,fatigue strength experiments of riveted joints are conducted for performance verification.Experimental results indicate that fatigue life of single-hole riveted joints is increased by 68%with ultrasonic vibration,and four-hole riveted joint arranged according to aerospace design standards is increased by more than 86%.

Framework on robotic percussive riveting for aircraft assembly automation

Presented in this paper is a framework for the implementation of a robotic percussive riveting system, a new robot application for aircraft assembly. It is shown here that a successful robot application to the automation of a process requires in-depth research of the process and the interaction with the robot. For this purpose, a process plan- ning-driven approach is proposed to guide a robot applica- tion research. A typical process planning will involve a list of key considerations including： process sequence, process parameters, process tooling, and process control. Through this list, a number of key research issues are identified for robotic percussive riveting, such as rivet pattern planning, rivet time determination, rivet tooling design and rivet insertion control. The detailed research on these issues has effectively created know-how for the successful implemen- tation of our robotic percussive riveting system.

Mtuli-objective Optimization Method for Automatic Drilling and Riveting Sequence Planning

There are numerous riveting points on the large-sized aircraft panel, irregular row of riveting points on delta wing. It is essential to plan the riveting sequence reasonably to improve the efficiency and accuracy of automatic drilling and riveting. Therefore, this article presents a new multi-objective optimization method based on ant colony optimization （ACO）. Multi-objective optimization model of automatic drilling and riveting sequence planning is built by expressing the efficiency and accuracy of riveting as functions of the points＇ coordinates. In order to search the sequences efficiently and improve the quality of the sequences, a new local pheromone updating rule is applied when the ants search sequences. Pareto dominance is incorporated into the proposed ACO to find out the non-dominated sequences. This method is tested on a hyperbolicity panel model of ARJ21 and the result shows its feasibility and superiority compared with particle swarm optimization （PSO） and genetic algorithm （GA）.

Electromagnetic riveting technique and its applications

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.

Riveting Process Modeling and Simulating for Deformation Analysis of Aircraft＇s Thin-walled Sheet-metal Parts

The riveting joint is one of the important joint methods to permanently fasten two thin-walled sheet-metal parts. It is most ba- sic to efficiently analyze and estimate the deformation of the riveting joint for the performance, fatigue durability and damage of the riveting structure in the aircraft. This paper researches the riveting process mathematics modeling and simulating to more accurately analyze deformation of thin-walled sheet-metal parts. First, the mathematics and mechanics models for the elastic deformation, plastic deformation and springback of the rivet are built by mechanics theory. Second, on the basis of ABAQUS system, a finite element system, an instance made up of the rivet and two thin-walled sheet-metal parts of aluminum alloy is used to analyze and simulate the stress and deformation. What＇s more, a comparison is made between the results obtained by the mathematics and mechanics models and those by finite element method （FEM）. The models are proved true by the calculating and simulation results of the instance.

Influence of die geometry on self-piercing riveting of aluminum alloy AA6061-T6 to mild steel SPFC340 sheets

The self-piercing riveting (SPR) process was used to join 2.0-mm-thick aluminum alloy 6061-T6 and 1.2-mm-thick mild steel SPFC340 sheets. SPR joints produced with a conventional flat-bottom die and conicalsection dies were investigated both experimentally and numerically. Lap shear tests were conducted under quasistatic conditions to evaluate the load-carrying capability of these SPR joints. The effect of variation in die geometry (such as variation in the die groove shape, cone height, and die radius) on the main mechanical response of the joints, namely the peak load and energy absorption, was discussed. The results showed that SPR joints produced with the conical-section dies exhibited a failure mode similar to those produced with a conventional die. All the joints failed by tearing of the top steel sheet. Cracks that occurred in the bottom aluminum alloy 6061-T6 sheet around the rivet leg were a result of tangential tensile stress. The cone height of a conical-section die is the most important parameter affecting the surface quality of Al/steel SPR joints. Conical-section dies with a moderate convex can ensure a good surface quality during the SPR process. In addition, SPR joints with single conical-section die allow higher tensile strength and energy absorption compared to those with double conical-section die.

Numerical Study on Die Design Parameters of Self-Pierce Riveting Process Based on Orthogonal Test

The concave die design of self-pierce riveting(SPR) is of critical importance for product quality. The optimization of concave die parameters based on orthogonal test is proposed to explore the relationship between self-pierce riveted joint quality and die parameters. There are nine independent die parameter factors in orthogonal test and each factor has 4 levels. In order to evaluate the interlock and neck thickness, we carry out numerical simulations by the software DEFORM-2D. Then, the primary and secondary factors that affect the joint quality have been found out by means of range analysis. Finally, an optimization scheme is brought forward to design concave die in SPR process, which indicates that the joint has higher quality than that of former orthogonal tests.This work can be extended by a detailed mechanical and fatigue analysis for the joint quality of SPR process.

Nonlinear Study on the Mechanical Performance of Built-Up Cold-Formed Steel Concrete-Filled Columns under Compression

Given their numerous functional and architectural benefits,such as improved bearing capacity and increased resistance to elastic instability modes,cold-formed steel(CFS)built-up sections have become increasingly developed and used in recent years,particularly in the construction industry.This paper presents an analytical and numerical study of assembled CFS two single channel-shaped columns with different slenderness and configurations(backto-back,face-to-face,and box).These columns were joined by double-row rivets for the back-to-back and box configurations,whereas they were welded together for the face-to-face design.The built-up columns were filled with ordinary concrete of good strength.Finite element models were applied,using ABAQUS software,to assess mechanical performance and study the influence of assembly techniques on the behavior of cold-formed columns under axial compression.Analytical approaches based on Eurocode 3 and Eurocode 4 recommendations for un-filled and concrete-filled columns respectively were followed for the numerical analysis,and concrete confinement effects were also considered per American Concrete Institute(ACI)standards for face-to-face and box configurations.The obtained results indicated a good correlation between the numerical results and the proposed analytical methodology which did not exceed 8%.The failure modes showed that the columns failed due to instabilities such as local and global buckling.

Modeling and simulation of percussive impact for robotic riveting system

Riveting is one of the major joining methods used in assembly, and the robotic riveting has been grad- ually introduced into aircraft industry. In this paper, a method is presented for modeling and simulation of per- cussive robotic riveting. In percussive riveting, vibration always exists. When an impact force is employed, a forced vibration will be induced. If it resonates with a robot nat- ural frequency, the vibration will cause damage to the robot. The main content of this paper is divided into three parts. Firstly, a robot dynamic model is established to compute the driving torque for each joint. Secondly, vibration responses under impact are analyzed for the percussive riveting process. Thirdly, the effect of riveting on robot vibration is studied over the robot workspace. The purpose of this paper is to discuss the suitable regions for riveting where the robot vibration is very minimal. It is shown that based on the presented method an appropriate trajectory can be planned for robotic riveting.

A Novel Friction Stir Spot Riveting of Al/Cu Dissimilar Materials

In order to eliminate the disadvantages of the keyhole in conventional friction stir spot welding joint and attain the highstrength lap joint of Al/Cu dissimilar metals,a novel welding technique,named as friction stir spot riveting(FSSR),was proposed.A pinless tool and an extra filling stud were employed.The Al/Cu spot joints without keyhole defect were achieved by the FSSR.A Cu anchor-like structure was formed,which greatly increased the mechanical interlocking between the upper Al sheet and lower Cu sheet.The thin intermetallic compounds containing CuAl2 and CuAl at the Al/Cu interface strengthened the joining interface between the Al sheet and the Cu stud.Increasing rotating velocity increased frictional heat and plastic deformation and then eliminated the interfacial joining defects.The FSSR joint with the maximum tensile shear load of 3.50 kN was achieved at a rotating velocity of 1800 rpm and a dwell time of 20 s,whose fracture path passed through the softened region of upper Al sheet.In summary,the novel FSSR technique has the advantages of strong mechanical interlocking and metallurgical bonding between dissimilar materials,thereby attaining the high-strength spot joint.