Mechanical Joining Processes Research Articles

A State-of-the-Art Review on Advanced Joining Processes for Metal-Composite and Metal-Polymer Hybrid Structures.

Multi-materials of metal-polymer and metal-composite hybrid structures (MMHSs) are highly demanded in several fields including land, air and sea transportation, infrastructure construction, and healthcare. The adoption of MMHSs in transportation industries represents a pivotal opportunity to reduce the product’s weight without compromising structural performance. This enables a dramatic reduction in fuel consumption for vehicles driven by internal combustion engines as well as an increase in fuel efficiency for electric vehicles. The main challenge for manufacturing MMHSs lies in the lack of robust joining solutions. Conventional joining processes, e.g., mechanical fastening and adhesive bonding involve several issues. Several emerging technologies have been developed for MMHSs’ manufacturing. Different from recently published review articles where the focus is only on specific categories of joining processes, this review is aimed at providing a broader and systematic view of the emerging opportunities for hybrid thin-walled structure manufacturing. The present review paper discusses the main limitations of conventional joining processes and describes the joining mechanisms, the main differences, advantages, and limitations of new joining processes. Three reference clusters were identified: fast mechanical joining processes, thermomechanical interlocking processes, and thermomechanical joining processes. This new classification is aimed at providing a compass to better orient within the broad horizon of new joining processes for MMHSs with an outlook for future trends.

Development of General data-based Process Models for Self-Pierce Riveting

The determination of ideal process parameters for mechanical joining processes such as self-pierce riveting currently requires a comprehensive understanding of the process, the availability of the materials to be joined and the corresponding system technology. General process models can simplify the use of these joining technologies, accelerate development cycles and thereby reduce the effort for implementation into production. In this paper, the development of general data-based process models for the mechanical joining method self-pierce riveting with semi-tubular rivet is described. Extensive experimental and numerical investigations with more than 2300 joint combinations for steel and aluminum sheets with tensile strengths between 240 - 1020 MPa were generated for the building of the models. Based on these results, different meta-models are fused into general data-based process models for the self-pierce riveting process in order to show the general relationships between material properties, process parameters and joining results. The paper discusses the acquisition of the experimental and numerical data, the statistical methods for evaluation and the application of the data-based process models.

Hybrid Joining by Induction Heating of Basalt Fiber Reinforced Thermoplastic Laminates

Induction heating was used to join basalt fiber reinforced polymer laminates (BFRPL) using the process called inductive contact joining (ICJ). Two other mechanical joining processes, nut and bolt (NB) and two-piece hollow riveting (2PR), were compared to ICJ. The obtained joints were evaluated using tensile shear tests and by analyzing fractured surfaces. Furthermore, simulation of the ICJ process was used to estimate the effective parameters. Joints produced by ICJ had superior joint strength compared to joints manufactured by 2PR. In addition, during ICJ, the BFRPL fibers were not damaged and the strength of the base material was maintained. The tensile shear forces of the ICJ process exceeded 3.5 kN and 2.5 kN for a joined, sandblasted aluminum sheet with BFRPL and for joining BFRPL to itself, respectively. Further optimization potential of the ICJ process was discovered during the investigation, so that potentially higher joint strengths and shorter processing times can be expected, making the process interesting for future industrial applications.

Study on Effect of Shapes of Serration of Joining Plane on Joining Characteristics for the Aluminum-Steel Multi-Materials Press Joining Process.

Recently, mechanical joining processes have received much attention for joining multi-materials. In particular, these processes have a great demand in the automobile industry for weight reduction. The press-fitting process is a representative mechanical joining process. In this process, the shape of the interfacial serration on the joining plane is very important because it has a significant effect on the joining strength. In this study, the characteristics of the aluminum–steel press joining process were investigated according to the shape of the interfacial serration of the joining plane. The deformation of the material and the forming load were investigated by conducting finite element analysis. In addition, the unfilled height of the material, joining force, and torque were measured experimentally.

Combined process of hot stamping and mechanical joining for producing ultra-high strength steel patchwork components

Abstract A combined process of hot stamping and mechanical joining for producing ultra-high strength steel patchwork components was developed. The main blank and patch were uniformly heated side by side without overlapping in a furnace, and then, the overlapped main blank and patch were hot-stamped. An appropriate layer of intermetallic compounds was generated on the surface of the patchwork part by uniform heating. The main blank locally came in the space of the die step due to buckling, and interlocks for joining the main blank and patch were created. To increase the amount of interlock, both edges of the main blank are fixed with the stoppers in the latter stage of forming, and then, the main blank is compressed in the planar direction with the punch and die. For a length difference for main blank of 16 mm and a space length of die step of 5.1 mm, the gap between the main blank and patch was almost zero, the main blank and patch were successfully laser-welded, and the laser-welded patchwork part had sufficient collapse load and punch stroke. Tapered and zinc-coated patchwork parts were successfully produced. Another mechanical joining process using a punched patch was developed.

Influence of process parameters on forming quality of Cu-Fe joints by laser shock hole-clinching

Laser shock hole-clinching is a novel mechanical joining process in which the shock wave pressure induced by a high-energy pulsed laser is employed as a punch. This process is quite suitable for the joining of two dissimilar materials with significant differences in plasticity and strength. However, the effect of key process parameters on the forming quality of joints by laser shock hole-clinching is less concerned. In this study, the influence of laser power density, spacer height, number of laser pulse, and initial grain size of metal foils on the forming quality of Cu-Fe joints was experimentally investigated. The characteristics of interlock value and thinning rate distribution under various process parameters were systematically examined. It is revealed that laser shock hole-clinching process can be divided into the confined bulging and interlock formation stages. Based upon the examination of substandard joints, the defects in laser shock hole-clinching process are classified which contains no interlock, nonuniform interlock, bottom and neck fracture. It is found that the thinnest position of joints locates at the region in contact with the upper corner of the joining partner II in which extensive plastic deformation occurs related to the force acting during clinching process. The interlock value increases significantly as the enlargement of initial grain size, whereas the deviation also becomes larger due to the inhomogeneous plastic deformation for the metal foil with coarse grains. Moreover, the ratio of spacer height to the thickness of the joining partner II affects the thinning rate distribution and interlock value of joints. The spacer height equaling to the thickness of the joining partner II is recommended within the given parameters.

Effect of Blank Holder Force and Edge Radius on Joining Strength in Flat-Clinching Process

A lightweight structure has been often mentioned in the automotive industry due to the increasing cost of energy as well as environmental legislation. Multi-material design is applied to reduce the weight of a car body. To join dissimilar sheet materials, it is quite difficult to achieve by welding processes. A flat-clinching process is one of the mechanical joining processes, which have join-ability of dissimilar sheet materials by plastic deformation. The purpose of this paper was to study the clinch-ability to join AA1100-to-AA1100 aluminum, AISI1010-to-AISI1010 steel, and AISI1010 steel-to-AA1100 aluminum sheets. Furthermore, the effect of both blank holder force (BHF) and edge radius (BHR) in the flat-clinching process on a joining strength was investigated. A shear strength test was conducted to examine the joining strength. The results show that the flat-clinching process can be successfully applied to join the similar sheet materials with the formation of interlocking. However, to achieve the joint of the dissimilar sheet materials, the AISI1010 steel sheet must be on the punch side. Also, both the BHF and the BHR significantly influent on the joining strength.

Modeling the Failure Behavior of Self-Piercing Riveting Joints of 6xxx Aluminum Alloy

Self-piercing riveting (SPR) is a mechanical joining process which is applied for joining similar and dissimilar lightweight materials in modern car body manufacturing. For qualifying SPR joints, cross sections must be investigated with respect to predefined quality features. Thus, numerous tests must be carried out in order to determine the maximum load capacity of SPR joints for different load angles. The growing number of materials used for the body-in-white requires a reliable and time efficient routine for predicting the joining behavior and the load capacity of SPR joints. In this study, the load capacity of three SPR joints was investigated numerically and experimentally using so-called KS2 samples. The results of axisymmetric two-dimensional finite element simulations (Hönsch et al in J Phys Conf Ser 1063:1-6, 2018) are the basis for three-dimensional simulations of the destructive testing procedure. The experimental setup of destructive testing was modeled using the FE software Simufact Forming 15. The numerically determined load capacity was validated with experimental data. Comparing the failure modes and the force–displacement curves revealed good agreement of simulations and experiments. Therefore, the presented simulation is a powerful tool for predicting the behavior of SPR joints under different load cases.

Parametric Investigation of Effect of Abnormal Process Conditions on Self-Piercing Riveting

Self-piercing riveting (SPR) is one of the mechanical joining processes, and its application to Al/Fe dissimilar materials combination, which is hard to weld, is expanding in the automotive industry. The main process parameters in SPR are types of rivet and die, setting force, and rivet setting speed. Previously, the relationship between the main process parameters and output parameters such as cross-sectional characteristics and joint strength has been studied to optimize the SPR process. In practical applications, there are unexpected and abnormal process conditions such as poor fit-up, angular misalignment, edge offset distance, and inaccurate setting and pre-clamping forces, and their effects on the joining quality have not been discussed. In this study, parametric investigation was performed using an experimental design on SPR joints for 1 mm-thick high strength steel (590 DP) and 2 mm-thick Al alloy (Al5052-H32). The main effect of each level of the abnormal process parameters on the output parameters was statistically investigated, and the analysis of variance was performed for each abnormal process parameter. In the range of abnormal process conditions applied, the set force was the most significant factor affecting the output parameters, and the effect of pre-clamping force on the output parameters was the least significant.

Innovative joining technology for the production of hybrid components from FRP and metals

Abstract The joining of fiber-reinforced plastics (FRP) currently poses particular process engineering challenges. Adhesive bonding or mechanical joining is normally used for such joints. However, bonding processes require complex measures for surface pre-treatment, fixing concepts and usually requires long curing times. On the other hand, mechanical joining processes such as screws or semi-hollow punch rivets lead to significant damage to the reinforcing fibers. Although different approaches for the production of plastic/metal hybrids are being pursued, there is still a need for a joining process that adequately meets the technical and economic challenges of joining metal and FRP. An innovative joining concept bypasses the problems of conventional joining techniques by using small-scale form-fitting elements in combination with established resistance welding processes. The investigations presented here include temperature measurements of the welds by means of contact thermocouples and analysis of the matrix material after welding by IR spectroscopy. The mechanical properties of the joint were verified in static shear tensile tests on coupon samples and in 3-point bending tests on geometries close to the component. The investigations carried out so far show high static bond strengths, which can be increased by coordinated welding parameters and component geometries. The joining process offers many possibilities with regard to the design freedom of the joining zone, since the pin structures can be flexibly adapted with regard to their arrangement, number and geometry, so that different requirements can be met. The Multi-Spot-Joint generates a quasi-ductile post-break characteristic. Thermal damage to the plastic matrix can be avoided by the targeted use of particularly short resistance welding processes. No damage to the plastic matrix could be detected during the IR spectroscopy.

Extension of the Conventional Press Hardening Process by Local Material Influence to Improve Joining Ability

Press hardened structural components are a key factor in lightweight car design and thus in reducing vehicle mass while increasing crash safety. The use of quenched 22MnB5 (Usibor 1500) has been established in hot sheet forming for the production of safety-relevant car body components. In order to expand the field of application for press-hardened components, a process-reliable joining technique is essential. Ultra-high-strength components can be joined with other parts in car bodies using the resistance spot welding process. Here, challenges like uneven welding lens formations with an incorrect connection in multi-sheet joints arise. Mechanical joining processes, for example self-pierce riveting, can only be used to a limited extent due to the high hardness of the hardened parts. For this purpose, an annealing treatment is often carried out in order to reduce the strength of the material after press hardening. Another possibility to create softened areas is the introduction of local deformation in the austenitic material. The phase areas in the continuous cooling transformation diagram are shifted to shorter cooling times, which enables the development of deformation-induced ferrite. The local thinning and softening improves joinability by means of mechanical and thermal joining processes but increases the forming force. Therefore, in this study the process of hot forming and local deformation is first pre-estimated using numerical simulation. The required force for the deformation and an optimal positioning, as well as the possible number of deformation punches, are investigated. Furthermore, the first experimental results of the feasibility of locally thinned and softened sheets are presented. In addition, joining tests by resistance spot welding and self-pierce riveting are carried out on the generated specimen to illustrate the practical effectiveness of the local thinning and the use of deformation-induced ferrite for critical joints.

Experimental Evaluation for the Interference-fit Electromagnetic Rveting Joint with Headless Rivet

Electromagnetic riveting (EMR) technology as a relatively new mechanical joining process has many advantages compared to traditional riveting methods. However, it needs to be investigated whether the traditional pressing riveting can be replaced by EMR process in aerospace manufacturing engineering. Therefore, in this paper, the differences of riveting quality between automatic riveting and handheld EMR were compared by riveting and mechanical experiments including the uniformity of interference fit, shear and pull-off strength and fatigue life of the joints respectively. It was observed that the interference fit of the riveting joint using EMR is more uniform along the axial direction of rivet than that of automatic riveting. In addition, the overhang of the rivet was difficult to control by handheld EMR, so the quality of rivets for EMR was poorer than that of automatic riveting. Thus, the shear and pull-off strength and fatigue life of EMR joints were affected by the quality of rivets, and a longer rivet is needed for the EMR.

Joining of Taper Nut and Bolt to Ultra-high Strength Steel Sheet by Punching

A mechanical joining process of taper nuts and bolts to ultra-high strength steel sheets by punching was developed, because resistance projection welding generally used for joining of nuts and bolts has low joinability for the ultra-high strength sheets. In this process, the sheet is punched with the taper nut and bolt, and the nut and bolt are joined by ironing and expanding the punched hole. The upper surfaces of the nut and sheet are aligned by controlling the stroke in order to attach other parts. No pre-punching is required for the sheet, whereas higher strength than the sheets is necessary for the nuts and bolts. For the die-quenched steel sheet having a tensile strength about 1500 MPa, the joint strength was about two times higher than that specified in the Japanese Industrial Standard of weld nuts. For the sheets above 780 MPa in tensile strength and above 1.6 mm in thickness, sufficient joint strength was obtained.

Hybrid joining: Numerical process development of flat-clinch-bonding

Abstract Clinching is one of the most commonly used mechanical joining processes in the automotive industry. In it, two or more overlapping metal sheets are locally formed so that a form- and force-closed joint with a die-side protrusion is established. Because of this protrusion, conventional clinching technology cannot be used for visible areas (e.g., automotive body shells) or functional surfaces (e.g., sliding surfaces). Flat-clinching is a special type of clinching for producing one-sided planar joints. Using this technology, it is possible to join the same or different kinds of materials (e.g., metal and metal, metal and plastic, or metal and wood). The flat-clinch-bonding process combines flat-clinching and adhesive bonding. After applying the adhesive, metal sheets are mechanically fixed by flat-clinching, thus guaranteeing the assembly handling strength. This allows continuous processing and shortens process time. The final operational strength of the assembly is achieved after complete adhesive curing. This paper describes the development of flat-clinch-bonding using numerical simulations. Special attention is paid to material characterization and implementation of material data in the simulation model. After calibrating the simulation model using reference experiments, the process of flat-clinch-bonding is investigated. The focus is on minimizing the size of adhesive pockets, as they prevent interlocking. The knowledge gained from the simulations is subsequently experimentally proven. Also, the joint strength of the novel process variant is experimentally determined and compared to the strength of an elementary adhesive bond.

Mechanical joining of sheets to tubes by squeeze-grooving

The purpose of this paper is to present a mechanical joining process to produce sheet-tube connections in a single stroke, at room temperature. The proposed solution is an enhancement of a previous joining by squeeze-grooving process that makes use of two independent mandrels and a customized spacer to control the inner joint radius and to improve its overall aesthetics. The presentation is illustrated with selected examples retrieved from experimentation and finite element modeling. Results show that the new proposed sheet-tube connections are characterized by a better redistribution of material within the joints and a very significant improvement of the pull-out strength. The proposed process also allows connecting a sheet to a tube close to the tube ends, which is impossible to achieve by means of the original joining by squeeze-grooving process.

Experimental and numerical study of the joinability of sheets by sheet-bulk forming

The authors present an innovative mechanical joining process that allows to connect perpendicular sheets to one another. This study is focused on joining similar and dissimilar sheets, based on sheet-bulk forming technology, and it is supported by experimental data and numerical simulation. Destructive tensile tests of different joined materials were performed to determine the maximum force that the joints are capable to withstand without failure. The joining technique should be chosen according to the materials joint combination. The two-stages technique could be a valid option to overcome the clearance between the plastically deformed polycarbonate tenon and aluminium mortise.

Joining of Bar and Nut to Die-Quenched Steel Sheet by Punching

A mechanical joining process of a bar and nut to a hard hot-stamped steel sheet was developed, because the resistance projection welding generally used for joining of bolts and nuts to hot-stamped sheets has low joinability. In this process, a slightly smaller hole than a bar and nut was made in the sheet with the bar and nut having a small round or chamfer at the bottom corner, and then the bar and nut were inserted into the hole. The punched hole was ironed with the bar and nut during the insertion, and thus the joint strength became high. No pre-punching is required for the sheet, whereas higher strength than that for the sheet is necessary for the bar and nut. For the bar and nut having a radius of 1.0 mm at the bottom corner, the ironed surface was the largest, and thus the maximum joint strength was obtained.

Processing and tooling considerations in joining by forming technologies; part A—mechanical joining

Joining by plastic deformation, also called joining by forming techniques, has been developed as a powerful tool for joining a broad range of similar and dissimilar materials in an environmentally friendly manner. Generally, the joining by forming techniques fall into two distinctive categories, namely mechanical joining and solid state welding. In this paper, different methods of joining by forming are studied from processing and manufacturing points of view. The article is arranged in three parts. Mechanical joining processes will be discussed in this part A. Various processes in this category have been addressed individually. Principles of the processing, applicable materials, process variants, joint mechanical behavior including static and dynamic performance, and various optimization techniques including experimental, numerical, and analytical approaches are the fields of concentration in this study.

Modelling and Characterisation of a Servo Self-Piercing Riveting (SPR) System

SPR is a cold mechanical joining process in which multiple sheets of material are riveted together without the need for a predrilled hole. It works by pushing a typically semi-tubular rivet into a target stack of material, during which the plastic deformation of the material and rivet are such that a mechanical lock is formed within the material stack. The process is used extensively in the automotive industry in car body construction, and is a competing technology to more established joining techniques such as resistance spot welding. As part of the ongoing development of the technique, there is a strong need to understand and simulate the dynamics of the process. In this work, a lumped parameter model of the SPR system with a non-parametric model of the joint is presented. Simulated results are compared with experimental data for a given joint configuration. Furthermore, the model is used to highlight the significance of the compliances within the system. It is shown that during rivet insertion, the stiffness of the C-frame structure is an influential factor in determining the dynamic response of the system. The results provide the basis for a more comprehensive sensitivity analysis into the factors which affect the quality of the resulting joint.

Effects of Process Parameters on Crack Inhibition and Mechanical Interlocking in Friction Self-Piercing Riveting of Aluminum Alloy and Magnesium Alloy

Friction self-piercing riveting (F-SPR) process has shown advantages over fusion welding, solid state welding, and traditional mechanical joining processes in joining dissimilar materials. Because of the thermo-mechanical nature of F-SPR process, formation of the joint is determined by both riveting force and softening degree of materials to be joined. However, it is still not clear that how exactly the riveting force and generated frictional heat jointly influence mechanical interlocking formation and crack inhibition during F-SPR process. To address these issues, F-SPR process was applied to join 2.2 mm-thick aluminum alloy AA6061-T6 to 2.0 mm-thick magnesium alloy AZ31B. The correlation of riveting force, torque responses, and energy input with joint quality was investigated systematically under a wide range of process parameter combinations. It was found that a relatively greater final peak force and higher energy input were favorable to produce sound joints. Based on that, a two-stage F-SPR method was proposed to better control the energy input and riveting force for improved joint quality. The joints produced by the two-stage method exhibited significantly improved lap-shear strength, i.e., 70% higher than traditional self-piercing riveting (SPR) joints and 30% higher than previous one-stage F-SPR joints. This research provides a valuable reference for further understanding the F-SPR joint formation mechanism and conducting process optimization.