Joining for Manufacturing of Lightweight Vehicle Structures

This paper was presented at 'Materials for Energy-Efficient Vehicles/Glass Technologies in the Automotive Industries', 29th International Symposium on Automotive Technology & Automation, Florence Italy, 3-6 June 1996.

Part 1: Joining technology implications for lightweight vehicles

A significant amount of work has been expended by both automotive manufacturers and material suppliers in the development of materials and manufacturing procedures for lightweight vehicle structures. This has covered an extensiveevaluation of the use of aluminium alloys, lightweight steel structures and the incorporation of plastics and composite materials into the vehicle body assembly. Joining technology is an integral part of the manufacturing process andeffort has been spent to develop and demonstrate the suitability of various processes for application into lightweight structure fabrication.

However, there still remains a number of issues which need to be examined in order to improve the confidence of automotive manufacturers in the exploitation of joining technologies required for the fabrication of lightweightstructures in high volume manufacturing environments. These issues can be broadly divided into the following topics:

• Design
• Materials
• Processes/equipment
• Performance/quality
• Environmental impact

Areas where future work will need to be carried out in order to achieve successful implementation of lightweight assemblies into the industrial production of vehicles are summarised in the following tables:

Part 2: Current welding techniques for aluminium alloys in vehicle body manufacture

Resistance spot welding

Based on the considerable experience gained in resistance spot welding of steel sheets in vehicle body manufacture, there is an impetus to transferring the technology directly to aluminium alloy sheets. However, there are a numberof factors to consider in making the switch which depend, to a large extent, on the physical characteristics of the aluminium alloy.

These can be summarised as follows:

• Primary services; mains (x3), air (higher), water (x2)
• Equipment costs; transformer (x1.5), gun/pedestal machine (higher), dressing equipment (more use)
• Consumables; electrodes (x5-10)

In addition to the changes in equipment required, the surface condition of the aluminium alloy is important in governing weld consistency and electrode life. In the automotive industry mechanical or chemical removal of the oxideimmediately prior to welding is not considered to be an acceptable procedure in the case of high volume production. This means that spot welding should be carried out on as-received material which could be in mill-finished form or withspecial surface pretreatments. The surface resistance of these materials is likely to exhibit some variability. This variability can cause welding inconsistency and limited electrode lives, which for high quality welds should be in therange 10-100µW and maintained within 10-25µW of that value. ^[1].

For these reasons, there is a considerable momentum towards looking at alternative techniques in aluminium alloy based vehicles.

Arc Welding

Gas shielded arc fusion welding (e.g. MIG and TIG welding) processes have become firmly established as reliable methods for joining of aluminium alloys.

MIG (GMAW) welding, in particular, has been demonstrated as a consistent process for welding of aluminium alloys, which, coupled with robotic manipulation is suitable for welding of complex joint geometries as found in spaceframestructures. However, the procedures developed for high quality welds involve removal of oil or lubricants and mechanical or chemical removal of the oxide prior to welding. Again, this is not considered to be desirable for the massproduction automotive industries. It has been shown that MIG welding, in particular, can be performed on un-cleaned aluminium alloys through adjustment of normal welding procedures but higher levels of porosity are likely to beencountered. This, coupled with the lack of penetration control in thin sheet welding and the level of distortion produced by the arc welding processes which affects dimension tolerancing, makes arc welding less attractive in thesearch for a joining process suitable for mass production of low weight vehicle body assemblies.

Adhesive Bonding

Adhesive technology is also making inroads in applications in vehicle body assembly for both steel and aluminium alloys. Adhesives can provide joint sealing capabilities, reduced vibration, improved stress distribution and fatiguestrength, and the ability to join dissimilar materials ^[2]. However, adhesive bonding alone is characterised by low peel strengths. A combination of adhesive and a point joining method (e.g. spot welding or mechanical fastening) can improve the situation. Otherfactors which are of concern for mass production vehicle manufacture are the need for surface preparation of aluminium alloys, doubts about the durability of adhesively bonded joints, and the consistency and environmental issuesconcerned with adhesive dispensing.

Part 3: Developments in alternative joining techniques for aluminium alloys in vehicle body manufacture

The following joining techniques can be considered as candidates for addressing the need to develop suitable methods for assembly for aluminium alloy vehicle bodies in mass production:

• Mechanical fastening
• Laser welding
• Friction stir welding

Each of the techniques is described, covering the principles, the types of properties achieved and the types of application in vehicle body assembly which may be suitable for these processes.

Mechanical Fasteners

The exploitation of mechanical fastener technology in automotive body assembly has been accelerated through the development of self-piercing riveting and press joining methods, which can be automated in a similar way to spot weldingand do not require a pre-drilled hole in the sheets. Self-piercing riveting involves driving a tubular rivet into the sheets to be joined, between a punch and die in a press tool. The rivet is expanded in the lower sheet, normallywithout piercing it, and forms a mechanical interlock, see Fig.1. Press joining (or clinching) is a similar operation but without the rivets, see Fig.2. A punch deforms the sheets into a die which is specially designed to permitinterlocking of the sheets in a button formed on one side of the sheet. For vehicle body applications, the sheets are generally not pierced through in the making of the joint.

Fig. 1. Schematic representation of self piercing riveting operation. 

Fig. 2. Schematic representation of the press joining operation.

Work at TWI has included comparative studies of commercially available self-piercing riveting, clinching and spot welding. Single spot samples have been produced and subjected to peel and tensile loading. Cross sections ofrepresentative joints are presented in Fig.3.

Fig. 3. Representative sections of lap joints in 1.6mm thick 5182 aluminium alloy.

a) Press joint. 

b) Self piercing rivet. 

c) Resistance spot weld.

Average peel and shear failure loads for the self-piercing rivets, press joints and spot welds are presented below for the 1.6m thick 5182 aluminium alloy:

The results showed that the self-piercing rivets achieved greater peel and shear failure loads than the spot welds on average, although there is an influence of rivet design. These higher strengths could be attributed to thestrength of the steel rivets used. For press joints, peel and shear strengths of less than half that obtained with self-piercing rivets and spot welds were achieved.

Both the self-piercing rivets and press joints have been demonstrated on dissimilar material combinations such as aluminium/steel joints and on lightweight aluminium/plastic sandwich material; both of which are impossible to joinusing resistance spot welding.

The use of adhesives in the joint line is also increasingly attractive in combination with mechanical fastener systems, which avoids some of the problems of use of adhesives alone and provides the capability for sealing of joints.These hybrid joints are already being used in the production of aluminium alloy based vehicles in Europe.

For the mechanical fastening techniques, there is still concern about the in-service performance for automotive structures especially with regard to corrosion, fatigue and impact. For corrosion of self-piercing riveted joints,special coatings for the steel rivets have been developed which are reported to avoid problems with galvanic corrosion. Work is on-going to evaluate the fatigue and impact performance of mechanical fastener and hybrid joints includingdesign of specimens to relate test performance to in-service performance.

Laser Welding

There has been considerable interest in the application of laser welding to aluminium alloys, where advantages may be gained in terms of low general heat input, low distortion, high welding speeds and the potential for automation ^[4]. These advantages are already being exploited in the assembly of vehicle body components in coated steel (e.g. tailored blanks). For aluminium alloys in vehicle bodies, there are two areas in which lasers maybe applied, welding of sheet materials as an alternative to spot welding, adhesive bonding or mechanical fasteners or welding of 'spaceframe' structures consisting of extrusions and castings as an alternative to arc fusion techniques.For both areas, two types of laser need to be considered; CO ₂ and Nd:YAG.

Laser welding of aluminium alloy sheet

The two main joint configurations found in vehicle body manufacture are butt welds, which are used in tailored blank production and lap joints, used for welding such items as radiator supports, door window frames, and hem flanges indoors.

For butt welds which may be considered for tailored blank application, weld formability is a major issue. Work has been carried out using CO ₂ and Nd:YAG lasers to produce butt welds in 5754 aluminium alloy, (1.6 mm thick) and assessing tensile and formability properties (using a biaxial bulge test).

A photograph of a butt welded sample after biaxial bulge testing is shown in Fig.4. The results indicate that with appropriate welding conditions, an acceptable weld formability may be achieved for some tailored blank applications.At present, a 5 kW CO ₂ laser is capable of attaining welding speeds of about 5m/min whereas a 2 kW Nd:YAG laser can reach speeds of around 1-2m/min.

Full and partial penetration welds can be produced in lap joints using CO ₂ and Nd:YAG lasers at similar conditions and speeds to butt welds. An example of a full penetration lap joint in 1.6 mm thick 5754 aluminium alloy is shown in Fig.5. The mechanical properties of such joints, areto an extent, governed by the weld width at the interface. A weld width of equal to or greater than the sheet thickness is desirable. It is also possible to join dissimilar alloy types (e.g. a outer skin panel of 6000 series materialto inner panel of 5,000 series material).

Fig. 4. CO ₂ laser welded butt joint in 1.6 mm thick 5754 aluminium alloy subjected to biaxial bulge test for formability. 

Fig. 5. CO 2 laser welded lap joint in 1.6 mm thick 5754 aluminium alloy.

Laser welding of 'spaceframe' structures Developments in spaceframe concepts for vehicle assembly have led to widespread interest in the application of welding techniques to extrusions and castings. Laser welding offers the possibility of producing joints in these types ofcomponents with lower distortion and lower heat input than MIG welding and work has been carried out to investigate welding procedures on representative extrusions and extrusion/casting joints. Both CO 2 and Nd:YAG lasers can be considered for these applications, although the Nd:YAG laser and fibre-optic beam delivery would have some advantages in terms of each of manipulation around what may be complex jointgeometries. The results of trials carried out on 2.0 mm thick 6060 extrusions and of an extrusion to cast joint are presented in table form below:

The susceptibility to cracking of the welds was of concern in the laser welding of extrusions, and wire feed procedures have been developed using an Al-Si (4047) based wire for the welding of extruded profiles, as shown inFig.6. A cross-section of a typical laser weld between an extrusion and casting is shown in Fig.7. The main concern when welding Al-Si based castings is the occurrence of porosity in the weld which appeared to be related to the quality ofthe cast material. In conclusion, laser welding can offer a versatile option for vehicle body assembly in aluminium alloys which is less dependent on surface preparation than conventional welding techniques and can produce low distortion structures.However, further work is necessary on real components to establish tolerances to production variables and to assess service performance of laser welded assemblies.

Fig. 6. CO ₂ laser welded extruded profile using filler wire.  

Fig. 7. CO ₂ laser weld in aluminium alloy extrusion to casting.

Fig. 8. Schematic representation of the friction stir welding process.

Friction Stir Welding

Friction stir welding of aluminium alloys is a recent innovation, patented by TWI. The process works by insertion of a rotating tool into the aluminium alloy sheets to be welded at the close butted interface and moving the workpiecewhilst continuing tool rotation, see Fig 8. This action causes plastic flow of the material around the tool which produces an autogenous solid-phase weld with low distortion. The technique offers a new, low cost alternative to fusionwelding techniques due to the low power requirements, no gas shielding and no special joint edge preparation ^[5]. In addition, the process is repeatable, can be easily monitored and does not produce any major safety hazards, such as fume or radiation. As the technique can be used in conjunction with conventional millingmachines, its immediate area of application is for the production of long straight-line welds found in the welding of aluminium extruded profiles and tailored blanks.

Fig. 9. Transverse section of friction stir weld in 3.2 mm thick 6082 aluminium alloy.

Friction Stir Welding has been applied in material thicknesses from 1.6 mm to 12.7 mm. Materials at the lower end of the thickness range are more relevant for vehicle body assembly and speeds of 0.6m/min for 1.6 mm thick and0.3m/min for 3.2mm thick 6082 aluminium alloy have been obtained. A cross section of a typical weld is shown in Fig.9. The process has also been shown to be tolerant to gaps of up to 10% of material thickness. However, at present, thetechnique requires rigid clamping against a backing bar to prevent weld metal breakout, if full penetration welds are required. In addition, a hole is left at the end of each run which may need filling if run-off plates cannot be used.Work in the near future will concentrate on developing fade out procedures to eliminate problems with the stop/start regions.

Work is continuing to examine different aluminium alloys and thicknesses, and to develop tool designs to achieve higher welding speeds. Further extensions to three dimensional processing are being considered for future applications,such as aluminium alloy spaceframes. In conclusion, friction stir welding offers a low cost alternative to conventional arc welding whilst avoiding the problems associated with fusion welding techniques. This will make a significantimpact in fabrication techniques for aluminium alloys for a wide range of industrial sectors in the future.

Conclusions

Work carried out at TWI has demonstrated that several alternative techniques exist or are being developed to meet the requirement for consistent and reliable joining of mass production aluminium alloy vehicle bodies. Three of thesetechniques (mechanical fasteners, lasers and friction stir welding) are likely to make an impact in industrial processing over the next 5 years.

Mechanical fastening techniques could replace the traditional resistance spot welding process for a range of components. Self-piercing rivets could be used in load bearing applications and press joints in areas of less loading. Itis also likely that combinations of adhesives and mechanical fasteners will be used. Laser welding has potential for application in the following areas:

• Production of butt welded tailored blanks, which require high speed, low distortion welding, with good formability properties.
• In lap joints where low overall heat input is required.
• In the manufacture of complex joints in spaceframe structures made up of extrusions and castings.

Friction stir welding could be applied in the manufacture of straight-line welds in sheet and extrusions as a low cost alternative to arc welding (e.g. in the fabrication of truck floors or walls). The development of robotisedfriction stir welding heads could extend the range of applications into three dimensional components.

Acknowledgment

This work was funded through the Core Research Programme of TWI, (supported by the U.K. Department of Trade and Industry and Member Companies of TWI), the EU-194 programme on Industrial Applications of High Power Lasers and theMembers of Group Sponsored Project 5651 'Development of the Friction Stir Technique for Welding Aluminium Alloys'.

The authors would like to thank the following colleagues for their contributions; Mr C J Dawes, Mr I A Jones, Mr M Whittaker, Mr G Muggridge, Mr D G Staines and Mr N Almond.

References

Published with permission of Automotive Automation Limited, Croydon, UK (e-mail - 100270.1263@compuserve.com