Version 2002 European Aluminium Association (auto@eaa.be) 1

Joining Mechanical joining Table of contents 5 Mechanical joining ................................................................................................................... 2

5.1 Introduction ....................................................................................................................... 2 5.2 Comparison of properties of some typical mechanical joints ........................................... 4 5.3 Example: Hardtop Porsche 911 Cabrio............................................................................ 5 5.4 Example: Self-piercing riveting of dissimilar materials ..................................................... 6 5.5 Example: Joining by screws ............................................................................................. 7 5.6 Hemming .......................................................................................................................... 8

5.6.1 Process principle....................................................................................................... 8

5.6.2 Flat and "Rope" hem ................................................................................................. 9 5.7 Bolting ............................................................................................................................ 10

5.7.1 General remarks on bolting with fasteners and inserts .......................................... 10

5.7.2 Press nuts and bolts Features and process ........................................................ 11 5.8 Clinching ......................................................................................................................... 12

5.8.1 Principle methods and application potentials ......................................................... 12

5.8.2 Single-step clinching Process steps .................................................................... 13 5.8.3 Quality-criteria for a connection .............................................................................. 14

5.8.4 Design criteria ......................................................................................................... 15 5.9 Self piercing riveting ....................................................................................................... 16

5.9.1 Self piercing riveting Principle .............................................................................. 16 5.9.2 Design criteria ......................................................................................................... 17

5.9.3 Quality-criteria ......................................................................................................... 19

5.9.4 Self-piercing bolts and nuts .................................................................................... 20

5.9.5 Riveting process Self-piercing nuts ..................................................................... 21 5.9.6 Types of self-piercing bolts and nuts ...................................................................... 22

5.10 Blind riveting ................................................................................................................. 23

5.10.1 Blind rivet systems Rivets, Nuts and bolts ......................................................... 23 5.10.2 Blind rivet and riveting process ............................................................................. 24

5.10.3 Blind rivet types..................................................................................................... 26

5.10.4 Blind riveting bolts and nuts .................................................................................. 27

5.10.5 Riveting process Blind riveting bolts and nuts ................................................... 29 5.10.6 Types of blind riveting bolts and nuts ................................................................... 31

5.11 Self threading screws ................................................................................................... 32

Version 2002 European Aluminium Association (auto@eaa.be) 2

5 Mechanical joining

5.1 Introduction Literature:

Hahn, O.; U. Klemens, Fgen durch Umformen - Nieten und Durchsetzfgen - Innovative Verbindungsverfahren fr die Praxis. Dokumentation 707, Dsseldorf: Verlag und Vertriebsgesellschaft, 1996, ISBN 3-930621-56-8

Klemens, U. and Hahn, O.: Nietsysteme, 1994, ISBN 3-922293-32-8 Ostermann, F. and 8 Co-authors, Aluminium Materials Technology for automobile

construction, English translation edited by Roy Woodward, London Mechanical Engineering Publications Limited, 1993, ISBN 0 85298 880 X

Ostermann, F., Anwendungstechnologie Aluminium, Berlin, Heidelberg, London, New York, Tokyo: Springer-Verlag, 1998, ISBN 3-540-62706-5

Section of hardtop Porsche 911: blind-riveting nuts and self-piercing rivets

Source: LWF-Univ. Paderborn

Because of the sensitivity of work-hardened and age-hardened aluminium alloys for the heat input from fusion and resistance spot welding and due to the effects of oxide films on RSW electrode life the use of "non-thermal" joining techniques have gained particular importance in automotive applications. Mechanical joining techniques like bolting, self-piercing riveting, blind riveting, clinching and the combination of these techniques with adhesive bonding have, therefore, been developed to substitute the traditional resistance spot welding. The different mechanical joining methods have found application in the joining of sheet components and also for use in joining of extrusions and castings. Some examples are depicted in the figure below.

Version 2002 European Aluminium Association (auto@eaa.be) 3

Source: ATZ

These methods are described in the following subchapters.

Version 2002 European Aluminium Association (auto@eaa.be) 4

5.2 Comparison of properties of some typical mechanical joints

The table below gives a brief survey of static and fatigue properties of typical mechanical joints in carbody sheet materials. Values illustrate some effects of type of sheet alloy and typical thickness on tensile properties and are not meant for design calculations. The fatigue values are R=0.1

Typical mechanical properties of some mechanical joining methods

Source: Niedermeier, Alcan

Contrary to screw bolt joints, mechanical joints are generated by local plastic deformation of either the fastener or the work-piece or both. For this reason joint properties depend strongly on the chosen tooling and fastener parameters. Please contact the tool or fastener supplier for details.

Version 2002 European Aluminium Association (auto@eaa.be) 5

5.3 Example: Hardtop Porsche 911 Cabrio Description: Materials:

exterior panel: AA6016-T4, 1.2 mm interior panel: EN AW-5182-O, 1.5 and 2.25 mm

Joining methods: Mechanical joints:

self-piercing rivets: 200, press nuts: 27, screw bolts: 12,

Adhesive tape: 2000 mm (in combination with self-piercing rivets), Stud welding: 10.

Porsche 911 Hardtop

Courtesy: Porsche AG, LWF Paderborn

Version 2002 European Aluminium Association (auto@eaa.be) 6

5.4 Example: Self-piercing riveting of dissimilar materials

Aluminium side impact beam with steel sheet stampings joined by riveting (Opel Astra)

Source: Alcan

Same as above figure: details of joints

Version 2002 European Aluminium Association (auto@eaa.be) 7

5.5 Example: Joining by screws Combination of aluminium extrusion with magnesium die castings and die cast / forged aluminium connecting pieces joined by screw bolts. The figure shows the instrument panel support of the DaimlerChrysler A-class model.

Version 2002 European Aluminium Association (auto@eaa.be) 8

5.6 Hemming

Flat hem of alloy 6016-T4

5.6.1 Process principle Literature:

Baartman, R., Atzema, E.H., Bottema, J.: Optimisation of the hemming process for AA6016-T4 Aluminium Body Sheet. Proc. ISATA98, 98NM056, (1998)

Hemming, bordering and folding methods are fundamental joining techniques for the manufacturing of automotive structures. Hemming belongs to the cold forming processes. The quality of the hem w.r.t. minimum bend radius depends on:

the aluminium alloy, the heat treatment of the aluminium, the forming history of the sheet / part, the thickness of the aluminium sheet and the forming process parameters.

Step 1: bending 90, Step 2: hemming 45, Step 3: folding 180

Source: Anwendungstechnologie Aluminium

Version 2002 European Aluminium Association (auto@eaa.be) 9

5.6.2 Flat and "Rope" hem The figures below show two possible designs of hemming:

The type 1 shows the standard geometry of flat hemmed sheets. The type 2 geometry ("Rope Hem") should only be chosen if the ductility of the

aluminium sheet is critical.

In order to maintain the bead (rope hem) radius during the final folding operation, the punch

can be designed with an inclined surface with an angle to the horizontal which can be varied, depending on the sheet thickness and the minimum allowable inside bending radius, see figure below.

Forming a ''Rope Hem''

Source: K. Siegert, TALAT Lecture 3706

Version 2002 European Aluminium Association (auto@eaa.be) 10

5.7 Bolting

5.7.1 General remarks on bolting with fasteners and inserts See also:

AAM Joining 1 Fusion welding > Stud welding AAM Joining 5 Mechanical joining > Self piercing riveting AAM Joining 5 Mechanical joining > Blind riveting > Blind riveting bolts and nuts

Bolted or threaded connections for the attachment of equipment to aluminium components and structures may be achieved by simply bolting through the aluminium part in a comparable way as for conventional blind riveting. It is often necessary to provide internal support if bolting through a closed section such as for the engine or suspension attachments to the body front rails. This support can be with tubes or extrusions fixed inside the section to prevent the section from collapse under high installation loads. Bolted connections can also be achieved with threaded studs and nuts fixed to the aluminium part. Aluminium threads are not recommended for situations where frequent removal for service is required, but can be applied for lightly loaded connections for internal trim, electrical harnesses, equipment attachment etc. Care must be taken if bolting material combinations are used, which are critical with respect to galvanic corrosion. Except for stainless steel, all steel inserts assembled into aluminium parts must be coated to prevent galvanic corrosion. Insert manufacturers can supply a range of suitable coatings. Sealants, gaskets or protective coatings may be required in severe corrosive environments, whereas simple surface treatment of the steel and/or the aluminium may be adequate in a dry internal environment. Aluminium welded studs and nuts are available (s. LINK) that may be welded directly to the aluminium parts by an electric arc welding process. Alternatively, steel threaded studs and nuts may be installed for applications where higher strength or frequent dismantling may be necessary. The selection of insert type depends upon the strength (torque) required and whether access is only possible from one side (blind) or both sides of the aluminium part. Steel inserts for studs and nuts are available that can be installed in pre-pierced holes in the aluminium part. The insert installation can sometimes be incorporated in the press line after the forming, trimming and piercing operations for a stamped part. Of course, they can be also installed separately at any stage in the assembly sequence including in-process and in-service repair. Fixed studs and nuts are also available that do not require pre-pierced holes and which can be installed in a single operation in the press line or as a separate operation. Fixed nut inserts are available that are sealed to prevent any leakage through the joint. Some fixed stud and nut inserts leave a raised element on the opposite side that must be allowed for in the design of subsequent assembly of the part. Due to the large variety of possible solutions, no performance values can be quoted here, please refer to product suppliers for more detail information.

Version 2002 European Aluminium Association (auto@eaa.be) 11

5.7.2 Press nuts and bolts Features and process

double sided access not necessary pre-punching or drilling necessary high load bearing capability of screw threads in thin panels with special blind rivet nuts additional properties, e.g. gas- and water tightness

Version 2002 European Aluminium Association (auto@eaa.be) 12

5.8 Clinching

5.8.1 Principle methods and application potentials Clinching involves the joining of two materials by local forming and does not require an extra fastener. A punch forms the two materials to be joined into a die. A button is formed on the underside and provides an interlock between the sheets. Clinching works with aluminium combinations, with multi-material combinations, like aluminium and steel, and also with pre-coated or galvanized materials. Clinching can also be combined with a sealant or an intermediate layer that acts as a sound dampener. The clinching process doesn't build any thermal stresses into the work-piece, so a clinched joint performs well where there's the potential of thermal fatigue or heat. The two main principles in clinching are single stroke and double stroke. Single-stroke clinching requires special tool sets for each set of parameters, especially sheet thickness. While double-stroke clinching can adapt to a range of thicknesses, it requires a larger capital investment and is difficult to integrate into the stamping press line.

Version 2002 European Aluminium Association (auto@eaa.be) 13

5.8.2 Single-step clinching Process steps Single-step clinching is the most commonly used clinching method in automotive joining operations. The process sequence is illustrated (below) in 4 steps:

Single-step clinching without cutting

Step 1:The punch and blank holder move downward, the work pieces are clamped and fixed by spring force of the blank holder. Step 2: By action of the punch the material flows into the bottom die cavity forming a cup. The process parameters and dimensions of the punch and die are finely tuned to the sheet thicknesses of the work pieces. This insures that no material is laterally drawn into the joint from surrounding area. Step 3: Finally, the thickness of the cup's bottom is reduced by upsetting and the material forced into the die groove and in lateral direction, forming the necessary undercut. Step 4: After reaching a preset maximum force (force control) or a preset displacement (stroke controlled), the punch is retracted and the clamping force relieved. The joint connection requires no finishing.

Version 2002 European Aluminium Association (auto@eaa.be) 14

5.8.3 Quality-criteria for a connection For round button non-cutting clinching techniques the strength of the connection is determined by the magnitude of the undercut and the neck thickness. These values are influenced by the tool dimensions, such as the punch diameter and the depth and diameter of the die cavity, as well as by the setting of the displacement limits for the upper-die. The residual bottom thickness correlates well with the joint strength and can be used as a non-destructive quality control measurement. A larger undercut can be achieved by reducing the residual bottom thickness. However, to avoid overloading the tools and work piece due to excessive joining forces, a compromise between maximum joint strength and tool life is required.

Quality criteria for a clinched connection

Version 2002 European Aluminium Association (auto@eaa.be) 15

5.8.4 Design criteria For best results when dissimilar materials are being joined the rivet is generally applied from the direction of the thick sheet into the thin sheet, or from the high strength material into the low strength material. Various suppliers offer tooling designs which can be optimised for the special automotive applications. The strength of a clinched joint depends basically on four main factors:

the type of aluminium alloy of the work pieces, the sheet thickness, the clinch button size (the diameter should be as large as possible), the surface condition of the material a completely dry, grease free surface will give

a stronger joint then if the surface is oily or wet (a minimum lubrication, however, avoids adhesion and significantly improves the tool life).

Rules of joining direction for different materials and materials' thickness:

"thick sheet into thin sheet" or

"high strength into low strength" The flange width "D" - distance from the edge where the clinch spot is to be placed - must be sufficient to ensure that there is material to contain the deformed clinch spot and sheet.

The button may otherwise burst out of the edge of the flange or cause distortion in the joint. Proper overlap of the layers to be joined and a correct flange width will also help ensure proper alignment between the work-piece, punch and die. A pre-clamping step may be helpful if joining a flange width close to the minimum width is to be undertaken. The clinch spots should be spaced to avoid contact with previously driven clinch spots or the strained area immediately around them. Placing several clinch spots too near to each other may cause distortion or some bending of the joint. A pre-clamping step can help to minimise this. A sufficient number of clinching spots must, however, be used to guarantee the overall design strength of a section. A precise relationship between part fit-up, alignment and joint quality is not easy to quantify. However, good control of these two variables will help ensure that the layers of material to be joined are drawn together properly as the clinch spots are driven and set. In addition, force will not be diverted into pressing parts.

Version 2002 European Aluminium Association (auto@eaa.be) 16

5.9 Self piercing riveting

5.9.1 Self piercing riveting Principle Self-piercing riveting requires no pre-drilling of holes into the mating work pieces. A punch and die are used to complete the joining operation in a single step. With solid rivets, the punch drives the rivet which pierces the sheet plies completely. Using semi-tubular rivets, the punch drives the rivet which pierces the top sheet and is set into the work-piece by partially piercing the bottom layer. A shaped die on the underside reacts to the setting force and causes the rivet tail to flare within the bottom sheet. This produces a mechanical interlock which includes the added rivet joining element and creates a button in the bottom sheet. The length of the rivet tail, hole diameter and hole depth to shank diameter, and the design of the tooling mainly determine the final shape of the rivet and of the button on the underside of the joint. There is a wide choice of rivet forms. The rivet is generally semi-tubular but may also be solid. The self piercing rivets are generally made of steel coated with AlZn powder to prevent galvanic corrosion. In addition, also aluminium self piercing rivets can be used in special cases.

Version 2002 European Aluminium Association (auto@eaa.be) 17

5.9.2 Design criteria Countersunk heads can provide a flush finish in the top sheet and even ensure colour matching to organic-coated or pre-painted material. For best results when dissimilar materials are being joined the rivet is generally applied from the direction of the thin sheet into the thick sheet, or from the low strength material into the high strength material. Various suppliers have developed rivet and tooling designs which can be tailored to the special automotive application. Also materials such as organic-coated and pre-painted aluminium sheets which are usually non-weldable can be joined by self-piercing riveting. Riveting of pre-finished material can eliminate the need for post-joining painting of parts. The ability to join dissimilar materials such as aluminium, steel, plastics and composites opens up new exciting opportunities for this process. Rules for self-piercing riveting of different materials and thickness:

"thin sheet into thick sheet" or

"low strength into high strength". Estimating the length L of a semi tubular rivet:

3 mm rivet diameter: L3 = thickness of sheet plies + 2.5mm

5 mm rivet diameter: L5 = thickness of sheet plies + 3.5mm The flange width D - i.e. distance from the edge to where the rivet is to be placed - must be sufficient to ensure that there is enough material to contain the deformed rivet and sheet. The button may otherwise break out of the edge of the flange or cause distortion. Proper overlap of the layers to be joined and a correct flange width will also help ensure proper alignment between the work-piece, punch and die. A pre-clamping step may be helpful if joining a flange width close to the minimum width. Rivets should be spaced to avoid contact with neighbouring rivets or the strained area immediately around them.

Since the rivets are made of harder material than the work pieces, riveting over an existing joint may result in serious damage to the tooling. Placing several rivets too near to each other may cause distortion or some bending of the joint. A pre-clamping step can help to minimise this. Poor fit-up and alignment may reduce joint performance and accelerate tool wear.

Version 2002 European Aluminium Association (auto@eaa.be) 18

Recommended Joint Design

Flange width should be sufficient to contain deformed rivet and sheet Ensure adequate spacing between rivets Ensure good fit up of stampings.

A precise relationship between part fit-up, alignment and joint quality is not easy to quantify. However, good control of these two variables will help ensure that the layers of material to be joined are drawn together properly as the rivets are driven and set.

Version 2002 European Aluminium Association (auto@eaa.be) 19

5.9.3 Quality-criteria In self-piercing riveting with semi-tubular rivets as well as with solid rivets, the strength of the joint is determined by the amount of undercutting as shown in the figures below. Since the tool and rivet dimensions are carefully tuned to each other and to the joint thickness, the amount of interlock is determined by the "compression measure" (s. figures below), which can serve as a non-destructive quality criterion when compensated for the position of the rivet head within the joint.

The figures below show quality relevant measures for joints with semi-tubular and solid rivets.

Semi-tubular rivet

Solid rivet

Version 2002 European Aluminium Association (auto@eaa.be) 20

5.9.4 Self-piercing bolts and nuts Characteristic features:

double sided access necessary without pre-punching or drilling necessary high load bearing capability of screw threads in thin panels great resistance to static and dynamic loads no damage of coatings

The figures show terms of a self piercing nut and bolt and the geometrical parameters.

Terms of self-piercing nuts and self-piercing bolts

Geometrical parameters

Version 2002 European Aluminium Association (auto@eaa.be) 21

5.9.5 Riveting process Self-piercing nuts The riveting process for self-piercing nuts is illustrated in the figure below. The process for self-piercing bolts works in analogy.

Riveting process: self-piercing nuts

Step 1: Blank and self-piercing nut are both positioned. Step 2: In a single-step mounting process, the joining element punches a hole through the sheet blank and in combination with the die generates a positive connection with the material of the sheet blank. In the process, material flows into a circumferential groove. Step 3: The punched slug is pressed out of the sheet blank by an ejector. Step 4: The joint has been created. As a rule, the geometry of the nut does not undergo any alteration. The joint is flush on one side.

Version 2002 European Aluminium Association (auto@eaa.be) 22

5.9.6 Types of self-piercing bolts and nuts The standard form of the self-piercing nut has a rectangular geometry, which ensures a torsion protected joint via a positive connection. Depending on the design type of the nut elements, the sheet blank is to be prepared with or without a bead. Rotational symmetrical self-piercing nuts require grooves and/or a beam for a positive connection for a torsionally strong seat in the sheet blank. Round shoulder nuts are used to attach dynamically highly-loaded components. Positive and non-positive connections, which can be loaded from both directions, are achieved via beading. Self-piercing bolts are joined in one self-punching step in a manner such that a plane bolting surface results. Acting forces from operating loads can be equally well accepted in both traction and compression directions.

Types of self-piercing bolts and nuts

Version 2002 European Aluminium Association (auto@eaa.be) 23

5.10 Blind riveting

5.10.1 Blind rivet systems Rivets, Nuts and bolts See also:

AAM Joining 5 Mechanical joining > Self piercing riveting Literature:

Grandt, J.: Blindniettechnik, 1994, ISBN 3-478-93115-0 Grandt, J.: Blindnietgewindesysteme: Typen, Verarbeitung, Einsatzbereiche.

Landsberg/Lech: Verl. Moderne Industrie, 1998. (Die Bibliothek der Technik; Bd. 159) ISBN 3-478-93174-6

Only rivet systems amenable to fully mechanised fabrication are preferably applied in the automotive industry. Rivet technologies can be subdivided into two groups: 1. rivet systems requiring pre-punched holes. 2. self piercing systems which require no pre-punched holes. The first category includes standard (upsetting) rivet systems. However, blind riveting fasteners, which can be applied from one side only, are or greater importance in automotive applications. For information of self-piercing riveting systems, see Links. Blind riveting is particularly useful joining technique for repair work on aluminium automotive structures, often combined with adhesive bonding. Characteristic features of blind riveting systems:

double sided access not necessary, pre-punching or drilling necessary, high load bearing capability of screw threads in thin panels, with special blind rivet nuts additional properties, e.g. gas and water tightness.

Version 2002 European Aluminium Association (auto@eaa.be) 24

5.10.2 Blind rivet and riveting process Literature:

Grandt, J.: Blindniettechnik, 1994, ISBN 3-478-93115-0 The blind rivet consists of the rivet sleeve equipped with a tool pin. While the shaft of the tool pin is discarded after setting, the pin head remains permanently attached, see figures below.

Typical blind rivet details and forms

Version 2002 European Aluminium Association (auto@eaa.be) 25

Blind riveting process

The blind rivets are classified in categories A, B, C and D (s. Lit.). For structural purposes the C and D categories have to be chosen.

Version 2002 European Aluminium Association (auto@eaa.be) 26

5.10.3 Blind rivet types Blind rivets are available in different designs. The selection depends on the respective requirements, e.g. component material and strength, examples are:

Standard blind rivet, Multirange blind rivet, Impervious blind rivet, Split blind rivet, Press-tab blind rivet, Pull-through blind rivet and Drive-in blind rivet.

Blind rivet types

Version 2002 European Aluminium Association (auto@eaa.be) 27

5.10.4 Blind riveting bolts and nuts Blind-riveting nuts and bolts are thread-bearing insert fasteners. They are inserted into a pre-punched hole from one side and efficiently and rapidly set with a processing tool. The figures show terms and geometrical parameters of blind rivet nuts and bolts.

Terms of blind-riveting nuts and bolts

Version 2002 European Aluminium Association (auto@eaa.be) 28

Geometrical parameters

Version 2002 European Aluminium Association (auto@eaa.be) 29

5.10.5 Riveting process Blind riveting bolts and nuts Setting of blind riveting bolts and nuts proceeds in analogy to the setting of blind rivets, but requires a rotary action to release the inserted fastener from the chuck or mandrel.

Riveting process: blind rivet bolts

Source: LWF, University of Paderborn

Version 2002 European Aluminium Association (auto@eaa.be) 30

Riveting process: blind rivet nuts

Version 2002 European Aluminium Association (auto@eaa.be) 31

5.10.6 Types of blind riveting bolts and nuts Blind-riveting nuts and bolts are set from one side without the need for additional finishing work. The figure shows different types of bolts and nuts.

Types of blind rivet bolts and nuts

Version 2002 European Aluminium Association (auto@eaa.be) 32

5.11 Self threading screws Due to the lightweight philosophy of aluminium automotive structures, the single aluminium components, i.e. stamped sheets, die castings and extrusions are designed with thin gauge dimensions. This typically means for

aluminium sheets: 1 1.5 mm aluminium die casts: 1.5 2.5 mm aluminium extrusions: 1.8 2.5 mm

Regarding screw joints this thin gauge design causes a limited load bearing length of the screw. Different methods are used to increase the load bearing strength:

creating a collared hole by plastic forming of the sheet: - flowing drill screw, flow hole forming, collar forming

use of additional fastening elements like - pressed nuts, spring nuts, etc.

5 Mechanical joining5.1 Introduction5.2 Comparison of properties of some typical mechanical joints5.3 Example: Hardtop Porsche 911 Cabrio5.4 Example: Self-piercing riveting of dissimilar materials5.5 Example: Joining by screws5.6 Hemming5.6.1 Process principle5.6.2 Flat and "Rope" hem

5.7 Bolting5.7.1 General remarks on bolting with fasteners and inserts5.7.2 Press nuts and bolts Features and process

5.8 Clinching5.8.1 Principle methods and application potentials5.8.2 Single-step clinching Process steps5.8.3 Quality-criteria for a connection5.8.4 Design criteria

5.9 Self piercing riveting5.9.1 Self piercing riveting Principle5.9.2 Design criteria5.9.3 Quality-criteria5.9.4 Self-piercing bolts and nuts5.9.5 Riveting process Self-piercing nuts5.9.6 Types of self-piercing bolts and nuts

5.10 Blind riveting5.10.1 Blind rivet systems Rivets, Nuts and bolts5.10.2 Blind rivet and riveting process5.10.3 Blind rivet types5.10.4 Blind riveting bolts and nuts5.10.5 Riveting process Blind riveting bolts and nuts5.10.6 Types of blind riveting bolts and nuts

5.11 Self threading screws