Space Frame Joining Techniques - Laser Welding

YAG laser systems are increasingly broadening the scope of their industrial application in the automotive industry. Laser welding has a number of inherent advantages over arc-welding which makes the process worthy of consideration for spaceframe applications: -

• Very little thermal distortion
• High processing speeds
• High quality welds
• Conducive to automation – in particular being able to transmit a beam along a fibre optic cable.

It cannot be ignored however that, as to be expected, there are limitations associated with the process as well. These are as follows: -

• Relative expense of capital equipment is very high.
• Process suffers from limited penetration depths which may restrict the use of lasers in some areas.
• The process can experience crack sensitivity problems – sometimes compensated for by the introduction of filler wires high in silicon.
• Highly focussed beam makes the process intolerant to gaps in joints.
• Risks to operators in terms of potential eye damage means stringent safety precautions need to be adopted.

Despite these limitations, the prominence of laser welding in the automobile industry is growing. This is generally because of the superior quality of the weld finish, potential material savings and the flexibility the process offers particularly with respect to new designs and the use of the optical fibre beam delivery system.

Ref. Barnes T, Pashby I, (1998)

Audi have taken the laser welding process a stage further and developed a laser/MIG hybrid welding process. They believe this achieves diverse synergy effects by combing both processes in order to extend the limits of current thermal jointing processes, that is in terms of efficiency, seam quality and process reliability. The hybrid process is used to weld various functional panels onto the lateral roof frame with seam length runs up to 4.5m.

Ref. Youson M (2002)

There are several other welding techniques that could be considered and discussed for automobile manufacture and in particular with respect to aluminium materials. Spot welding and laser welding are the dominant techniques in the industry but there is evidence that hybrid techniques such as the laser/MIG are in development. This suggests that the suitability of laser welding technology, without further innovation in the process to support the manufacture of aluminium bodies, cannot be a foregone conclusion.

Space Frame Joining Techniques - Spot Welding

Space frames present a considerable challenge for fabrication in volume production and ultimately the success of this design approach depends heavily on how well associated fabrication processes lend themselves to volume production.

Some of the considerations that need to be taken into account when looking at the fabrication processes are: -

• Operating costs
• Cycle times
• Reliability
• Quality

Spot Welding

The conventional technology associated with steel monocoque designs in terms of joining techniques is spot welding. This technique has the benefits of being a very well known, extensively proven technology with which the industry is highly familiar and has considerable experience.

There are however significant problems with regard to spot welding aluminium in terms of accessing both sides of the joint and breaking down the surface oxide layers present on the aluminium.

For steel, a single phase 50-60Hz AC welder is sufficient to produce a high quality reliable joint.

For aluminium however there is a need for higher current devices with stiffer electrodes capable of delivering the higher forces necessary to break down the oxide film.  The consequence of this is that the welders are heavier and bulkier which in turn leads to accessibility problems when trying to position the equipment to the weld points on a spaceframe.

A further implication of the use of higher currents and electrode forces is the need for more frequent tip dressing as a result of rapid electrode wear due to over-heating.

Table showing the effect of material on single phase 50Hz AC resistance spot-welding parameters for 0.9mm body sheet Ref. Barnes T & Pashby I, (2000)

Parameters	Bare Aluminium	Bare Steel	Zinc Coated (hot-dipped)
Weld Time (50Hz cycles)	3	7 - 10	9 - 12
Current range (kA)	18.0 - 23.0	7.0 - 10.0	9.0 - 10.0
Force (kN)	4.1 – 5.0	1.9 – 2.6	2.2 – 2.9

Typically in automobile manufacture, spot welders are used in conjunction with highly versatile robot arms and also as part of large multi-welders capable of supplying simultaneously up to 240 welds. This can reduce fabrication time significantly but means that such equipment tends to be designed for use on specific parts and therefore cannot be readily adapted for use in fabricating spaceframes.

This information tends to suggest that mechanical fastening techniques and adhesive bonding would be a more practical route with respect to joining aluminium.

Aluminium Versus Steel in Automobile Manufacture

Below is a table that compares some of the properties of steel against those of a typical aluminium alloy.

	Steel	Aluminium Alloy (Al,Mg,Si)
Modulus of elasticity	190,000 – 220,000 N/mm2	60,000 – 80,000 N/mm2
Strength	290 –470 N/mm2	280 – 350 N/mm2
Density	7.85 kg/dm3	2.7 kg/dm3
Coefficient of thermal expansion linear 200C	12.6 mm/mm-0C	23.2 mm/mm-0C

Ref. Carle D. and Blount G. (1999) - modified to include CTE

Aluminium is available in sheet form, as complex castings and in extruded sections that can also be quite complex in form without the need to seam weld (a typical requirement for steel). It is the ductility of aluminium over steel that permits this more flexible extruded option, ductility being the characteristic of a material that allows it to be formed or rather deformed to the required shape.

Aluminium alloys are equal in strength to steel but have a lower rigidity due in part to a lower modulus of elasticity. The consequence of this is that to obtain higher rigidity in aluminium structures various techniques or approaches are required through the manufacturing and assembly processes that will provide the higher rigidity levels required.

Increasing the wall thickness of the aluminium will compensate to an extent for the low modulus of elasticity whilst still providing the benefit of reduced mass overall, this is because the density of aluminium is approximately 1/3 that of steel (however see below ref. bending & torsion members). The resultant mass reduction for equivalent rigidity is approximately 50% where the aluminium is 1.44 times as thick as the steel.

The use of castings for single piece complex components provides the potential for increasing torsional stiffness by up to 20% and the use of extruded box sections eliminates the rigidity losses of conventional spot welded seams.

Ref. Edmans L (1993)

It is important to consider the significant effects of the differences in moduli and usable thickness for aluminium. Similar tensile test loads of 180MPa applied to both mild steel and aluminium alloy, where the steel has a thickness of 1mm and the aluminium 1.6mm, show the aluminium to be close to its yield strength but when density is taken into account, the aluminium displays a greater 'mass specific strength'.

What this means is that the performance of the aluminium for both yielding and buckling is typically 100% better than for mild steel equivalents.

The mechanical properties of aluminium alloys are less strain rate sensitive than those of mild steel and generally they do not suffer from low impact toughness under high speed impact conditions. For comparable impact energy absorption to that of mild steel an aluminium form of similar outside dimensions will be about half the steel's weight but because aluminium has a 1/3rd the stiffness and density of steel the specific elastic moduli of both materials is about the same.

The consequences of this is that simply increasing the aluminium thickness does not provide any weight saving compared to steel in bending and torsion members unless there is an increase in the section perimeter, conversely for panels subject to out of plane bending, large savings are possible.

So ideally to optimise the stiffness and rigidity of aluminium components, it is necessary to increase the perimeter for improvements to 'in plane' stiffness and increase wall thickness for improvements in 'out of plane' stiffness.

The second modern era aluminium intensive vehicle was the Porsche 928S which used Al-Mg-Si sheet alloy (AA6016). Its steel unibody structure was replaced by aluminium stampings and was joined by spot welding. The only significant design change was to modify the rocker rails creating deeper sections to compensate for the lower stiffness modulus of aluminium. This factor tends to support the previous discussion.

Ref. Wheeler M. J. (1997)

A further factor that is important with respect to aluminium is the need to heat treat it to achieve the optimum strength characteristics of the material and this should ideally be done after forming the material in it's lower strength condition (typically T4) in order to ease that particular process.

Steel on the other hand does not require further heat treatment and can be relatively easily formed in its final condition. This is particularly true with respect to stamping sheet metal where due to the characteristics of aluminium the sheet has a tendency to tear during the stamping process, especially in its final heat treated condition (typically T6), and has different springback characteristics to steel which would need to be accommodated in any tool design

One final point is with respect to the weldability of aluminium, a feature of aluminium is that it is resistant to corrosion and the reason for that is that the surface of the material forms into a tight knit aluminium oxide film which prevents oxygen reacting with the base material beneath it.

Where on the one hand this provides the benefit of corrosion resistance, it does at the same time make the material more difficult to weld due to the oxide film interfering with any welding processes, mainly because the oxide has a higher melting point than the parent material.

Ref. Narraway (1988)

Risk Factors

It is important to examine and compare both the attraction and the physical properties of alternative materials in order to understand the potential risks involved in changing from one material to another. There are 2 primary reasons for this: -

• The risk of technical failure.
• The risk of market failure

Both types of risk can result in a cost to the company with the most immediate expense being related to development costs, especially if the product fails or results in an inferior product for technical reasons. There are numerous reasons why a product can fail technically, these range from material properties simply failing to meet the requirements of the product design specification to adopting inappropriate manufacturing techniques or not meeting the design constraints with respect to cost of manufacture.

Equally important however, is the risk of market failure, this assumes that the specification has been satisfied in technical terms but that the product fails to find a market. This concerns the more qualitative aspects of the product design or material choice rather than the quantative and relates to the appeal of the product to the consumer. Get this wrong and the product will still fail even if the technical aspects of the design specification are met.

Ref. Smith, Preston G & Reinertsen, Donald G.(1991)

There are clear differences in the properties of aluminium versus steel which can, for the automobile industry, on first impressions and from a technical perspective, appear to be resolvable through a simple increase in wall thickness. However on further investigation it becomes apparent that there are additional aspects which need to be considered in order to optimise the use of aluminium for weight reduction e.g. increasing the section perimeters, utilising complex shapes, how to overcome potential spot welding problems and so on. The immediate suggestion here is that a straightforward replacement of steel components with aluminium is unlikely to be a satisfactory solution.