9year XXV, no. 3/2016

KeywordsHybrid riveting, innovative process, metallic materials,

experiments

1. IntroductionResearch on rivet joining techniques currently concern

the international scientific community in the field, which is reinforced by scientific papers published in the main flow of information, namely patents protected in advanced countries [1] - [6].

The focus is on research of joining processes, on their mathematical modeling, joints characterization, process monitoring, as well as the identification of new applications, particularly in priority areas.

Worldwide, there is interest in the industrial environment to implement research into new methods of joining by riveting especially in the automotive industry, aviation, railway, construction.

Assembly operations by riveting are used especially where, for various reasons, welded joints are not recommended: the construction of components for aircraft, automotive components, construction of bridges, truss, bodywork, in rail transport - by carriage fabrication, etc. [7].

Cases where riveting joining techniques are indicated:- Joints subjected to vibration loads [8];- Joints subjected to high dynamic efforts [9].- Weld assembly of metals difficult to weld [8];- Assembly of metal constructions profiles [8];- Assemblies of parts from different materials [10].For these reasons, riveting is still one of the main methods

of assembly in the construction of aircraft, as well as some special structures.

In this paper concerns of ISIM Timisoara are presented for research, development, promotion and implementation of a hybrid process of riveted joints, which results in the combination of mechanical clamping - friction welding.

Research and preliminary obtained results were based on joining methods proposed worldwide.

A method for joining by riveting metal materials, that is currently relevant, is Friction Stir Riveting patented by specialists of General Motors Corporation [11], developed by the University of Toledo [12], intended for joining similar and dissimilar light metal materials.

An alternative process of riveting was patented by researchers at GKSS Research Centre GmbH Geesthacht, Germany [13], as a new joining technique in solid state, applicable to bonding polymeric or thermo-plastic materials using bolts (rivets) of various metal alloys. These processes can be cost-effective and ecological alternatives to spot welding processes [14], [15].

2. Hybrid joining process of mechanical grip and friction welding

To obtain quality bonding, applying the new method of riveting with hybrid effect (mechanical grip and friction welding) involves complying to specific process conditions, technological parameters, but also specific solutions regarding rivet geometry, details on how to fix it and ensure its technological movements, effective process running, materials to be joined, technical characteristics of the equipment used.

Mainly, the operations that the operator must ensure and the equipment for applying the method of riveting with hybrid effect, except those related to positioning and fixing the materials to be joined, are shown in the Figure 1.

Figure 1. Operating mode of the riveting joining process with hibrid effect.

Figure 2 shows the principle of a working sequence with respect to the materials to be assembled, the riveting process with hybrid effect (mechanical clamping and friction welding). Before the start, the rivet is lowered and maintained at the desired / prescribed position (Fig. 2a). This sequence is necessary to verify whether the joining will take in the set place. The next step is turning the rivet in contact with the base material BM1, without advance motion, to preheat the material (Fig. 2b). Preheat the BM1 contributes to plastifying and easier mixing the materials in the following sequences of the process and also to reduce the loads that act on the rivet and machine. After preheating, the movement of advance on the vertical of the rivet is applied (with the movement of rotation - Fig. 2c),

Modern methods of joining by riveting R. Cojocaru; L.N. Boţilă; V. Verbiţchi; C. Ciucă; I.A. Perianu

National Research & Development Institute for Welding and Materials Testing - ISIM Timisoara, Romania

E-mail: rcojocaru@isim.ro

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until reaching the prescribed penetration depth, correlated with a pressing force of the rivet shoulder on the BM1 (Fig. 2d). The optimum downforce is determined experimentally according to the characteristics of the couple of materials BM1 and BM2, respectively the geometric and dimensional characteristics of the rivet. After fulfilling these conditions (values of penetration depth, respectively downforce), the movements of vertical advance, respectively rotation of the rivet are both commanded off.

Figure 2. Ongoing of work sequences of the joining process by riveting with hybrid effect

Expertise of the research carried out for the development of the FSW (Friction stir welding) process stated the concept that at the joints made by friction, therefore also in the case of riveting with hybrid effect (mechanical joint - friction welding), the following elements could be significant, as characteristics of the techniques used for the application:

• Control of the downforce of the active element (FSW tool, rivet, pieces by classical friction welding) throughout the bonding process. In the case of riveting with hybrid effect, the control of the downforce is very useful, especially when the rivet head comes into contact with the plate located above.

• As it is a process which also produces a friction joint, as a result of the plasticization and mixing of materials, it is important to know and be able to hold a real-time control over the temperature at which the important sequences of the process are conducted.

• Contrive and execution, especially of the bed frame and moving elements of the equipment used in the application of the process must be completed in a robust structure and secure operation (vibrations introduced into the system during the process to be minimum values)

• Base materials (parts to be joined) must be properly positioned and fixed firmly, so that during the process no changes occur in the initial relative position of these

3. Program of experimentsBased on the presented considerations on the new hybrid

process of riveting, a preliminary experimental program was

developed, aimed mainly to the practical substantiation of the innovative ideas set.

The main purpose of the preliminary experimental program was knowing and explaining the mechanism of joint resulting from the application of the new riveting process, identifying and defining factors of influence on the process, determining the technological parameters of the process.

3.1. Technique of the experiments

Technical solutions have been developed, starting from the FSW 4-10 welding machine, the existing equipment of ISIM, and the resulted system for joining by riveting is shown in Figure 3.

The machine was completed with a system for monitoring the forces and temperatures developed during the riveting process with hybrid effect.

The control system for monitoring the forces is designed to measure the thrust downforces (the vertical component), developed in the riveting operation.

Figure 3. System for friction riveting with hybrid effect.

The components of the system for hybrid riveting with friction effect (Figure 3) are the following:

1 - welding machine type FSW 4-10;2 - jig for positioning and fixing the parts to be joined;3 - rivet with port-rivet device;4 - monitoring system of forces (pos. 4a - with force transducer

fixed in a device on the FSW machine) and temperatures (pos. 4b - with thermocouples), using a data-logger, a voltage source module and a PC for data acquisition, storage and processing (pos. 4c).

5 - temperature monitoring system using infrared thermography (pos. 5) – with thermographic camera placed on a positioning and fixing device, using a PC for data acquisition, storage and processing.

These features of the machine have generated the premises that the FSW welding machine with the specific adaptations can become a functional model usable in the laboratory experiments for the development of the process of joining by riveting with hybrid effect (mechanical clamping and friction welding).

The main features that the hybrid joining system provides are: fixing and technological movements of the rivet by the set values, positioning and fixing the parts to be joined by riveting, real-time measurement and control of the downforce of the rivet shoulder on BM1; temperature measurement in the pre-set areas,

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other than the surface of BM1 using thermocouples; measuring temperature at the surface of BM1 in the rivet shoulder area, respectively on the cylindrical surface of the rivet.

3.2. Constructive solutions for rivets

Figure 4 shows the rivets constructive solutions used by specialists from the University of Toledo, USA, for implementing the method of Friction Stir Riveting [16].

Figure 4. Constructive solutions for Friction Stir Riveting [16].

Figure 5 presents a cross section from a friction stir riveting joint.

To make robust riveted joints rivets configurations were developed which produce a large area during operation (W in Figure 5) of the material undergoing plastic deformation and FS mixing friction and an extended area of overlap / mechanical interconnection between the rivet and the base material (L).

Figure 5. Transversal cross section of friction stir riveting joint [16].

The logic behind this concept is the fact that the rivet penetrates the base material and the concave shape of the rivet head helps to target the material displaced back toward the trunk of the rivet, helping to fill holes or pores type defects that may arise during the riveting process. Following studies and experiments, it was observed that this type of rivet design is not implementable due to difficulties arising from their processing (especially for applications in thin sheets). The conclusion from these studies was to abandon this type of rivet.

Given the results obtained worldwide at ISIM Timisoara there were used different designs for rivets, which were tested in some preliminary experiments.

Following these tests, promising preliminary results that may underlie the development of complex research were obtained using rivets design presented in Figure 6.

Figure 6. Rivet design - ISIM.

3.3. Experiments

Preliminary experiments were performed for hybrid joining process - mechanical clamping - friction welding proposed for development within the project.

During experiments there were used M6 threaded rivets, 8-10 mm in length, shoulder diameter of 16 mm, made from X155CrVMo12 steel (thermal treated / non treated) and sheets for rivet joining (base materials - BM) made from EN AW 1200, EN AW 6082 aluminum alloys, Cu 99 copper, with thicknesses s1 = 5 mm and s2 = 6 mm.

X155CrVMo12 (AISI D2, DIN 1.2379) material used for making the rivets, is a tool steel with high carbon, chrome, molybdenum and vanadium content with high wear resistance which varies depending on the carbon and vanadium content. The chemical composition is presented in Table 1 [17].

AISI D2 material has high hardness, good thermal and dimensional stability, used to create FSW tools respectively for rivets related to the process addressed in this project.

Base materials have not been submitted to pre-process preparation processing (e.g. BM1 holes drilled for rivet through passing). Figure 7 shows the macroscopic appearance

Table 1. Chemical composition and properties for AISI D2 steel [17].

Chemical composition [%]C Cr Co Mn Mo P Si S V

1.40 - 1.60 11.00 - 13.00 ≤ 1.00 ≤ 0.60 0.70 - 1.20 ≤ 0.03 ≤ 0.60 ≤ 0.03 ≤ 1.10Hardness after quenching and tempering

150°C 200°C 250°C 300°C 350°C 400°C61 - 62 HRC 60 - 61 HRC 59 - 60 HRC 56 - 57 HRC 55 - 56 HRC 55 - 56 HRC

Figure 7. Joint macroscopic aspect.

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of an experiment in which plates made of aluminum alloy EN AW 1200 were used, thickness s1 = 5 mm and s2 = 6 mm, the speed of the rivet n = 1450 rev/min, advance speed of the rivet v = 20 mm/min, anticlockwise rotation.

Macroscopic analysis reveals the following aspects:• it was conducted a “mechanical grip” joint type by placing

material BM1 and BM2 between the turns of thread (rivet)• it was achieved a friction joint of BM1- BM2, in the area of

interference between the two materials through the action of the rivet through the combination of two movements which characterize the process (rotation + vertical displacement- rivet penetration BM1 and BM2).

• it was found that at higher advance speeds, the movement of penetration of the rivet in BM1, respectively BM2, the axial resistant force to advance of the rivet in the material increases proportionally. Also contributing to the growth of this resistant force is the anticlockwise rotation of the rivet (correlated with the metric thread). These high resistant forces contribute to the shear the rivet at approx. 1.5 mm from the shoulder.

The experiment showed that in addition to the actual riveting carried out by means of the rivet (mechanical joining) a friction joint was also performed. So we can speak of a combined riveting + friction welding joint, obtained by applying a single process.

The resulting samples from the experiment with the parameters presented were subjected to breaking and shearing tests. Shearing occurred at a force Fmax = 5700N. In order to investigate the effect of friction riveting compared with conventional mechanical fastening a screw assembly of the two materials was also carried out (through-hole in the first sheet and threaded hole in the second sheet). At shear fracture testing, maximum tear force was Fmax = 5000N.

By comparison of the two joints it was observed that:• breaking strength was higher by approx. 14% in the case

of friction riveting hybrid effect of the sheets, compared with the case where joining was achieved by mechanical fastening (screw).

• at hybrid friction joining there is considerable less work carried out by elimination of the following operations:

- drilling through hole Ф 6,5 mm in the sheet above - drilling through hole Ф 4,8 mm in the sheet below - M6 threading o the hole in the sheet below

It is estimated that the execution time for the joint made by the process of friction stir riveting is reduced by approx. 300-400%, with approx. the same material consumption and with minimum facilities.

In another experiment, overlapping sheets of aluminum alloy EN AW 6082 6 mm thick were used. The rivets were made of X155CrVMo12 steel with M6 threaded length of the active part of 10 mm. In this case the used rivet was not heat treated. The used rotational speed was n = 1450 rev / min, the speed of vertical displacement of the rivet is raised progressively up to a max. 20 mm / min. The rotation of the rivet was counter clockwise. At macroscopic analysis (Figure 8) it was observed that the rivet was broken in the joint where the mechanical fastening area of the rivet and BM1, respectively BM2, the contact line between BM1 and BM2 and a friction welded area are highlighted.

Also it was observed a smaller removal of base materials in the contact area than the copper plates, the space created having the size of approx. 0,25-0,3 mm. Macroscopic analysis brings out all the defining elements relating to the characteristics

of the joint for the new riveting process proposed within the project. It is observed in this experiment also that rivet shearing

occurred at approx. 0,5-1,0 mm under the rivet head (in the base material).

4. ConclusionsRiveting joining techniques have a great potential for

development and increase of the degree of applicability in top industrial applications.

Hybrid effect riveting process - mechanical gripping - friction welding is an innovative variant of joining light metal materials.

Preliminary experiments were conducted or hybrid effect friction riveting which demonstrated that the idea regarding the new method proposed is viable and applicable for aluminum alloys base materials (EN AW 1200, EN AW 6082) with thickness 5 mm and 6 mm and rivets made of steel X155CrVMo12 (AISI D2, DIN 1.2379).

Preliminary tests of applying the new method of hybrid effect friction riveting allowed acquiring of knowledge and obtaining preliminary technical data on production parameters, configuration of rivets, influence factors, etc. This data is the starting point for the development of complex research programs regarding the development of the new hybrid riveting process.

It is necessary that further research will consolidate the application techniques of variants for rivet joining proposed, controlling and finalizing process conditions as main influence factors (e.g. removing the causes that favour rivet shearing at finalization of the actual riveting process) in close correlation with characteristics of materials subjected to the process, developing optimized technologies / applications.

AcknowledgementsThe paper was developed on the basis of preliminary results

achieved in the project PN 16 08 101 entitled “Development of new, innovative and environmentally friendly processes of joining advanced materials using unconventional techniques “, financed under the ISIM Timisoara ”Nucleu” Program (contract 7N / 2016 -2017) by the National Authority for Scientific Research and Innovation - ANCSI

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Figure 8. Macroscopic aspect of joint X155CrVMo12 rivet – EN AW 6082 base materials, thickness 6 mm.

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[2]. R. Stevenson, Pei-Chung Wang – Patent No. US 6,892,924, B2, 2005 – Processing rivet and method for friction stir riveting, General Motors Corporation, Detroit[3]. Pei-Chung Wang, R. Stevenson – Patent No.US 7,862,271, B2, 2011 – Friction stir rivet method of joining, General Motors Corporation, Detroit[4]. J.R. Trinick - Patent No.US 8,898,880, din 2014 – Self-piercing riveting, Flintshire, GB[5]. R.S. Wright, et. al - Patent No.US 9,162,332, din 2015 – Method and apparatus for automated multi-drilling and multi-rivet machine, Boeing Company, Chicago[6]. S.F. Golovashchenko- Patent No.US 9,168,581, din 2015 – Electro-hydraulic rivet and method of riveting and welding parts, Ford Global Techologies[7]. http://www.sim.tuiasi.ro/wp-content/uploads/Ianus-Organe-de-masini-partea-I-curs.pdf[8]. www. nituirea.beep.com/files/nituirea.ppt[9]. http://tehnologiidimitrieleonida.wikispaces.com/file/view/T14+Nituirea.pdf[10]. http://www.desen.utcluj.ro/curs/asamblari sudate+nituite.pdf[11]. R. Stevenson, Pei-Chung Wang – Patent No. US 6,988,651, B2, 2006 – Friction Stir Rivet Drive System and Stir Riveting Methods, General Motors Corporation, Detroit

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