Innovation Process Steps

1. Innovation Process - Identifying an Original Idea

Generally for an idea to become an innovation there needs to be an element of originality which would result ideally in a product or process which is cheaper, more reliable or which is less complex.

Note: It is important to understand that an innovation is not just and initial idea or spark of ingenuity, it is actually a product that is conceived and taken to full commercial release i.e. it becomes a product that is sold commercially. So the innovation process describes the stages required to achieve that objective.

Action Required

At this stage a good deal of effort needs to go into establishing that the idea is original. This is achieved by conducting a series of searches for evidence that no one else has had the same or a similar idea particularly if they have progressed the idea before you. The main types of searches that would need to be conducted are as follows: -

1. Product search – look at what else is on the market.
2. Patent search – looking at patents that have been registered which may encompass the idea both in the UK and internationally.

Both of the above are geared towards establishing if prior art exists and that the idea is not original.

Potential Problems

The main problem is that this is a huge task, prior art is all encompassing with respect to mediums, geographical location and time but can determine whether a patent is accepted or rejected.
Discovery of prior art cannot be ignored, either a completely new idea must be developed or at least a change of course back to originality must be undertaken.
It can also be costly with no guarantee of complete success.

2. Innovation Process - Is There a Demand for the Idea

Originality on its own is no guarantee of successful innovation, there must be an existing or potential demand for the end product or process

Action Required

It must be established if anyone actually needs the idea, what the potential market price of the product will be versus manufacturing costs and whether the idea has significant advantages over existing methods or products.

Investigations need to be carried out looking at competitive products and prices. Market research involving potential customers will need to be done and a search of relevant published information should be conducted.

Potential Problems

Be wary of disclosure when discussing the idea with potential customers or sponsors.
Make sure information sources are reliable.

3. Innovation Process - Will the Idea Work

Modelling the idea where possible will help determine if it actually works.

Action Required

If possible make a prototype; ideally using detailed drawings to support manufacture.
Establish manufacturing costs and be prepared to have the information available for presentation to potential sponsors.

Potential Problems

Producing prototypes & drawings can be expensive.

4. Innovation Process - Is It Worth Selling

The potential return must be greater than the risk involved getting it to market. It’s a good idea at this stage to seek financial assistance.

Action Required

Seek help from a professional or professional body: -

• Innovation centres
• Business Links
• Local Council
• Various sources of sponsorship, private or commercial should be considered

Update market and originality searches.

Potential Problems

Be prepared to accept reasonable investment /advise costs. Being greedy may discourage potential support groups.
Ensure offered assistance is worth associated costs.

5. Innovation Process - Intellectual Property

Without establishing the unique aspect of the idea and when it occurred there will be nothing to sell.

Action Required

Ensure there is enough information in place, drawn, photographed or written, so that someone familiar with the technology or skill will be able to manufacture it.

Ensure all relevant forms of legal protection are taken out: -

• Copyright
• Design registration
• Design Right
• Trade Marks
• Confidentiality
• Patents

Potential Problems

If the information is vague or ambiguous, legal protection is much more difficult.
Establishing & defending legal protection can be very expensive.
Be sure the idea has a market before committing to expensive legal protection.

6. Innovation Process - Financial Expectations

Financial expectations need to be realistic.

Action Required

Ensure any assessment periods negotiated with interested companies are well-defined and documented in terms of time scales and payments.
Be prepared to work out an outline deal covering royalties and pre-production payments.
Be ready to complete negotiations but ensure a legal representative deals with final details.

Potential Problems

Don’t try and renegotiate once the outline is agreed this may result in a withdrawal.
The cost of manufacture should be no more than 25% - 30% of established market price in order for all parties to realise a reasonable profit margin.

7. Innovation Process - Commercial Strategy

Depends on a product which is good enough to sell, this is the last chance to assess whether it is.

Action Required

Be prepared for rejection and if necessary to go into manufacture yourself if you can’t identify a suitable company to approach or those that you do are not interested.

Be sure the product is priced correctly in line with market expectations established earlier.
If taking on the role of manufacturer ensure professional advice is sought and considered.

Potential Problems

Large companies with well-established development strategies are unlikely to be responsive.
Be aware of potential disclosure situations at this late stage.
Going into production yourself is a whole new ball game, be sure all aspects of this approach are understood.

Innovation Processes and Practices

Innovation Process Books

Acceptance Sampling Versus Statistical Process Control

Quality Assurance (SPC) versus Quality Control (Acceptance Sampling) is the subject of the debate that examines which approach should be taken when manufacturing products and striving to attain quality levels that are deemed to be acceptable to both manufacturer and customer.

There are no clear cut answers, there are arguments for and against using one or the other, using both or using neither.

Quality comes at a cost and generally it is accepted that quality assurance methods which include SPC are more complex and therefore more expensive to set up, but once in place tend to have lower running costs and are geared towards ‘zero defects’ rather than an acceptable defect level.

Ref. Edwards & Endean (1990)

Does this mean then that SPC is the answer to which quality process to use. Unfortunately it is not as straightforward as that, one factor that cannot be ignored is that SPC is only effective when a process is deemed to be in control i.e. that the process has been set up correctly and is inherently capable of producing components to meet the design specification.

In order to ensure the process is in this condition it is necessary to take early samples of components produced, measure critical features and use the results to make a decision about the process. It has been argued that this is very similar to Acceptance Sampling and that the data could in fact be used for this purpose as well as to establish if the process is under control. In other words it presents a case for combining the 2 methods.

Edward G Schilling discusses an ABC plan which also supports the view that there is synergy between SPC and acceptance sampling and that they can be combined to present a scenario where there is minimal risk to the consumer.

The ABC plan is subject to several constraints :

• Acceptable quality levels are not utilized
• Acceptance number of zero
• Simplicity

The plan progresses through 3 stages :

• Stage A – control being established
• Stage B – capability being established
• Stage C – capability being maintained

There are various rules associated with the implementation of this plan which are designed to promote continual improvement and learning, but it is accepted that the plan is a prototype and is subject to further development.

Ref. Schilling (1994)

W. Edwards Deming condemns the use of acceptance sampling and proposes all or nothing inspection. Although what he is really objecting to is the misuse of acceptance sampling and the suggestion that a proportion of defective components is acceptable.

In contradiction to this statement it is suggested that Acceptance Sampling is valid for processes that are in a state of chaos and until they reach the point where a process becomes stabilized.

Ref. Sower et al (1993)

There are arguments for both processes either independent of one another or combined. In cases of mass production it is clear that dependent on the situation, who the customers, are what agreements have been reached etc. then each and every option could be assessed to be the most appropriate.

A more modern approach is a move to Total Quality Management which was discussed at the end of the article on Statistical Process Control. What the plan represents is a totally different approach to manufacturing which requires a stepped culture change and a belief in the individual working in a team environment. An approach most modern day management structures would tend to adhere to.

The concept of ‘Poka-Yoke’ otherwise known as ‘mistake-proofing', an approach presented by Shigeo Shingo and refers to the need to prevent defects occurring in the first place by examining the process to determine what can cause problems and introduce mechanisms that will prevent this from happening.

This can be likened to the SPC approach of avoiding defects before they happen but differs in that once proper processes are established there should no longer be a need for ongoing monitoring.

The move to more flexible and adaptable quality systems has become more supportable given that manufacturing has had to become ever more flexible and fluid as the demands of society have moved from acceptance of limited choices of mass produced products to a situation where the individual is looking for variability in the choices they are offered to suit their own personal requirements.

Also the advent of CNC machining and other new manufacturing techniques has provided the means for supporting this level of flexibility. The result of this is smaller production runs and frequent re-tooling to produce the smaller production batches required. Neither SPC or Acceptance Sampling lend themselves to this scenario readily, so alternative methods have had to be identified and developed.

Statistical Process Control

The basic idea behind SPC is that the products of any manufacturing process vary, one from another, in 2 distinct ways: -

• Variation that is inherent in the process.
• Variation induced by some external factor.

As long as the process chosen is capable of maintaining the tolerances required by the product specification, the first type of variation should not result in defective components.

Externally induced variations are less predictable e.g. a chipped tool may result in sudden deterioration that could affect both surface finish and dimensional accuracy.

SPC puts great emphasis on studying processes to characterise inherent variability so that when variations occur for other reasons they can be detected quickly and adjustments made to the process before defective components are produced.

Variables & Attributes

There are 2 different characteristics associated with any product which must be treated differently with respect to product quality. These are variables and attributes.

Variables tend to be thought of as the properties of a material e.g. surface texture, dimensions etc. and these tend to have a range of values which have upper and lower limits.

Attributes can be thought of more as observable defects e.g. surface defects, porosity etc. and these tend to be present or absent, acceptable or unacceptable.

The major difference between variables and attributes is that variables will always be specified as some ideal, whereas it is possible for a customer to specify zero as the only acceptable level for a particular attribute. Ref. Edwards & Endean (1990)

Controlling Product Variables

The first step in meeting this objective is to establish process capability, firstly to determine if the chosen process can produce components to the required standard and secondly to determine the precise nature of the inherent variability.

The tools of SPC are the normal distribution curve used in conjunction with 3 important parameters: -

• The mean or average of the values measured
• The range – difference between highest and lowest readings measured
• The standard deviation, which is derived by formula.

The normal distribution has useful properties which are exploited in process control, i.e.

• The distribution is symmetrical
• The mean coincides with the most frequently occurring reading
• The number of readings falling within any part of the curve is related to the standard deviation.

Knowing the mean and standard deviation of a variable measured on a sample of products provides the function of predicting the number of products that are likely to be made with a value of more than say 2 (standard deviations) above or below the mean.

By taking the initial samples over a very short time frame the effects of any externally induced variability can be considered insignificant. Also an assumption is made that each sample follows a normal distribution curve despite the small sample size.

Taking these factors into account it can be estimated from the areas under the normal distribution, the likelihood of making products outside the specified tolerances, or in other words is the process capable.

Having established the process is capable, the next objective is to look at how future performance of the process can be judged.

This is typically done using control charts, the most common of which are based on the mean and range values. The first indicates how the process is behaving relative to initial settings and the second helps detect when additional factors are affecting random variability.

Limits are put on the control charts to provide an indication when either the range or mean has moved sufficiently far way from the target to increase the probability of making out of tolerance components. Typically the convention is to set control lines so that the probability of a data point falling outside by chance alone is 1 in 1000. Ref. Edwards & Endean (1990)

Controlling Product Attributes

This can only be done if the customer is prepared to accept a finite number of defective products given a known parameter.

Attribute sampling is similar to acceptance sampling but with a difference that the number of defects is used to decide if the process is still in control rather than whether the lot should be accepted or rejected.

When controlling by attributes, it is a shift in the number of defects in a product or the defective products in a sample that is the trigger for action. There is no upper limit to the number of defects possible so Poisson distribution is used to establish the probability of finding ‘x’ number of defects in a sample.

Similar to acceptance sampling the relationship established is used to calculate the probability of finding a particular number of defects in a product and from that the probability of finding more or less than a given number. It is this information which is used to decide the positions of control lines on a control chart.

Other Issues to Consider

A conclusion that can be drawn is that every production scenario must be examined in its own right before a decision on which is the most appropriate quality process to select can be made. There will be scenarios where neither SPC, Acceptance Sampling or a combination of the two is appropriate e.g. bespoke small batch production. If neither acceptance sampling or SPC are appropriate then it needs to be considered what other options are available to try and maintain acceptable quality levels?

An aspect which has not been touched on so far is the need to ensure the specification is correct, clearly if a component is tightly toleranced it will be more difficult to meet the specification. The question must then be asked does the component need to be toleranced so tightly or will it be capable of functioning as required with more open tolerances.

Another aspect which should be considered is the more modern approach to quality of ‘Total Quality Management’. This effectively refers to the workforce at every level of a manufacturing organisation taking individual responsibility over the quality of goods produced.

Discussions and publications by John S. Oakland, Crosby, Deming and Juran could be consulted for implementing total quality management systems in an organisation. In particular Deming's 14 point plan.

Acceptance Sampling for Quality Control

When applying acceptance sampling as your approach to quality, a small batch of components are measured or observed and a decision to scrap or accept is made. This approach is dependent on statistical sampling techniques that use the data collected on a small number of samples to be extrapolated to predict the likelihood of large numbers of products meeting the design specification.

The decision to accept or reject is based on the idea that a certain number of defective items can be tolerated.

When is Acceptance Sampling Effective.

To consider when acceptance sampling is going to be effective it is necessary to look at 2 possible extremes.

If the cost of 100% inspection > the cost of all defective components, it is cheaper not to inspect.

If the cost of 100% inspection < cost of the customer finding 1 defect, it is cheaper to inspect 100%.

When the cost of inspection lies between these 2 extremes, acceptance sampling is said to become effective. This does however conveniently omit to take into account the possible damage to reputation, legal liability and potential loss of business as a result of the customer receiving defective goods. In other words the qualitative aspects of doing business are ignored in favour of the quantative aspects.

Ref. Edwards & Endean (1990)

How Does Acceptance Sampling Work

Acceptance sampling is formed around probability theories from which it can be predicted what percentage of a lot of components will be acceptable given the results from the observations made on a sample batch from that lot.

Batches whose samples show fewer than a specified number of defectives are going to be accepted, so the probability of finding fewer than that number of defectives in the sample needs to be known. This is determined by the sum of probabilities up to that for the number of defectives specified.

For example: -

If we want to find the probability of finding 1 defective component in a lot of 20 given 3 samples then the equation is

P = 1/20 = 0.95, so the probability of finding fewer than 1 defective in a sample of 3 is

0.95x0.95x0.95 =0.8574

This means that rejecting samples which have at least one defective could result in nearly 15% of all acceptable batches being rejected.

However what we want to know is what would be the risk of allowing batches with more than the permissible number of bad components to get through at either threshold of acceptance/rejection.

To find out requires calculating all relevant values of p, the fraction of defective components in the batch.

Rather than calculate this information every time, there are appropriate national and international standards available that provide the information in the form of extensive tables that detail the size of samples and the levels of defectives to accept or reject in order to provide the desired probabilities of detecting bad batches of products.

There is a finite probability of passing unacceptable batches and rejecting acceptable ones. Inspection procedures are arranged to minimise this risk to either manufacturer or customer.

Ref. Edwards & Endean (1990).

Aluminium Cars, the Market and Production Volumes

The target market for an all aluminium car would be very much dictated by company policy and business strategy of the automobile manufacturer. For example when Audi produced the A8, which was a relatively expensive, lower volume, luxury automobile, the company strategy was to demonstrate that the material aluminium is fundamentally suitable for vehicle body manufacture and to illustrate the ease with which body repairs on aluminium vehicles could be carried out. Daily volumes for the A8 ran up to about 80 vehicles a day and because the A8 was at the higher priced end of the market it was felt that the increased costs associated with the initial excursion into using aluminium structures could be more easily absorbed.

For the A2 on the other hand, which was a smaller family type hatchback, the emphasis was on production technology and on validating the process of high volume production.

Daily production for the A2 ran at around 300 vehicles a day and the production processes have a degree of automation similar to that utilised in the production of steel bodied cars which is around 85% and is markedly different to the 25% automation level initially employed on the production of the A8, although steps were taken to increase this level of automation to a similar level to the A2.

In order to achieve the production volumes required at acceptable costs and quality levels, Audi entered into agreements with strategic business partners, identified through their work on the A8. This was with a view to improving the material properties of the aluminium alloys they used i.e. optimising the materials for specific functions to help overcome shortfalls identified in their previous exposure to production of aluminium vehicles such as removing the need to perform straightening operations during the production process.

One of these relationships was with Alcan, one of the main producers of aluminium. This has resulted in Alcan becoming one of the biggest suppliers of aluminium parts for the A2, not only supplying finished components such as extrusions and castings but also developing and supplying the specialist materials previously discussed.

Audi also sought to improve the production processes they had identified as most appropriate for aluminium car production again by entering into strategic business partnerships with the suppliers of specialist welding, bonding and mechanical fixing equipment.

Besides the business partnerships they entered into, Audi sought to develop core skills in the production of aluminium cars, in their case this centred around the ability to produce very high quality, close tolerance outer skin components on a volume basis.

Having invested somewhere in the region of 300 million Euros in plant and capital equipment Audi have committed to the further and increasing use of aluminium in their car designs not just for the models mentioned but across the whole range of vehicles produced by Audi. This effectively means that the use of aluminium will increase in all market sectors that Audi supply to.

Ref. Dr. Wolfgang Ruch (2001)

Audi have demonstrated that Aluminium cars are a viable alternative to steel cars in both medium and high volume production. For the high volume option where margins would be tighter they needed to achieve high levels of innovation both in the development of the car and of the supporting manufacturing processes in order to make this a viable enterprise. Without access to the company figures on sales and profit margins the assumption that they have managed to meet a market need and have done that cost effectively has to be made and is supported by the evidence of further investment in capital equipment required to raise the automation level for the A8 model.

A clear part of Audi’s approach was also to answer the make or buy questions as part of their business strategy making good use of external expertise whilst developing their own core skills in areas they felt they could excel, getting this balance right is also crucial to success in terms of where and how capital is invested in order to keep an edge over competitors and to be able to compete in the market place.

Tha last Audi A2 left the productions line at Neckarsulm in August 2005 but set the standard for aluminium cars manufactured on a high volume production basis.

Space Frame Joining Techniques - Mechanical Fasteners

There is a large range of mechanical fasteners available of which self piercing rivets and clinch joints have been identified as having a high potential for use in the automotive industry as a viable alternative to spot welding and to supplement the use of adhesive bonding for joining structural frameworks.

They are both essentially cold forming operations which can join two or more pieces of material mechanically. See below: -

This figure illustrates the self piercing rivet process. Ref. Barnes T, Pashby I, (1998)



The rivet is designed to both pierce and form a permanent fastening within the materials being joined. Having pierced the upper sheet of the material, the rivet expands in the lower sheet, usually without piercing it, to form a mechanical interlock. This is a process widely employed by Audi in the production of ASF (Audi Space Frame) structures.

The clinch joint is very similar in that it involves the deformation of the material being joined to form a mechanical interlock. The main difference is that clinching does not use a rivet.

This figure illustrates the clinching process. Ref. Barnes T, Pashby I, (1998)




In both processes the advantage of not piercing all the way through means that the integrity of the joint is maintained with respect to moisture ingress.

Also both processes compare favourably to spot welding with respect to production criteria, in that they share similar limitations and advantages but have the added advantage of being considered low energy, safe processes.

The main drawback of riveting compared to spot welding is that it introduces additional consumable items and therefore weight into the process, but on the positive side dissimilar metals can be joined. Clinching also has this characteristic without adding a further consumable but is considered to provide a slightly less effective joint in terms of overall strength.

Ref. Barnes T, Pashby I, (1998)

Jaguar have used over 3000 self piercing rivets in the body of the Jaguar XJ aluminium riveted and adhesive bonded monocoque bodied luxury car with no spotwelds. This was achieved by developing a relationship with Henrob, a specialist producer of riveting solutions.

The process development between Jaguar and Henrob claims to have produced some world firsts in terms of riveting techniques, these included the joining of multi-layer material combinations using a comprehensive range of riveting equipment and the introduction of lightweight riveting tools for favourable robotic handling.

Together Henrob and Jaguar went through a series of prototype builds and worked on experimental joining solutions which included static, fatigue and corrosion testing of joints whilst also identifying cost effective rivet coating solutions that would address problems associated with meeting new European legislation on the “end of life” for vehicles.

The final result was a process that produces highly consistent joint properties as well as giving enhanced fatigue properties that can be monitored non-destructively by an integral process monitoring system thus helping to streamline production and reduce inspection intervals.

Ref. Anon (2002)

The use of mechanical fastening techniques combined with adhesive bonding techniques does seem to have provided a solution to the specific problems associated with aluminium component assembly and in some respects suggests that this method of assembly has the potential to provide a superior product in comparison to the traditional steel spot welded monocoque designs both in terms of process and of product performance.

In Jaguars design, the retention of an aluminium monocoque design is significant in that it differs from the approach taken by the other main producers of aluminium bodies i.e. Audi and Honda, it would be interesting to explore the reasons behind this a little further.

Space Frame Joining Techniques - Adhesive Bonding

Adhesive bonding tends to be considered as a low cost option for joining spaceframes, but process automation requirements and the need for viscosity compensation to provide consistent application tend to contradict this view.

There are however a number of advantages associated with the use of adhesives for the joining of structural components that leads to the use of this method being retained particularly with respect to joining aluminium spaceframes.

• No distortion as would typically result from arc welding
• Improved joint stiffness due to the continuous bond, as opposed to local joint contact, and more uniform stress distribution.
• Good energy absorbing characteristics combined with noise and vibration dampening properties.
• Dual purpose, provides mechanical strength and seals against moisture and debris ingress.
• Smooth joints reduce stress concentration at the joint edges providing good fatigue resistance.
• High strength in shear.
• Dissimilar metals can be joined without leading to galvanic corrosion.

The limitations associated with adhesive bonding are numerous but the main factors that effectively rule it out as a stand alone process relate to the inability to use non destructive methods to check the strength of the bond and the unknown effects of exposure to different environments over time.


Clearly it would be very risky to rely entirely on the adhesive bond without knowing its real strength or understanding how it will endure in all potential environments it might be exposed to. There are also further limitations as listed below: -

• Epoxy or solvent based adhesives typical of the type used can be hazardous to health and require suitable fume extraction systems, protective clothing and storage facilities to protect against the risk of fire.
• Investment in equipment is risky because there is a possibility that these substances may be banned in the future.
• Heat curing is a necessary stage in the process.
• Limited shelf life of adhesives requires that adequate batch management procedures are in place.
• Adhesive dispensers require regular routine maintenance to keep them clean.
• For aluminium, surfaces need to be carefully prepared to ensure a good bond is achieved.

Despite all the limitations adhesive bonding is still being adopted, however automobile manufacturers have typically elected to provide some sort of mechanical re-enforcement that can combine with the use of adhesives and provide solutions to the inherent problems associated with this method of joining.

Audi for example, among other methods, use a joining technique called roller type hemming where rollers secured to a robot arm bend an outer panel over an inner panel which when combined with a hem-bonding adhesive provides a powerful connection. These adhesive joints are then hardened using a process called ‘inductive gelling’ which uses an electric field to target the specific hem-bonded zones.

Ref. Youson M (2002)

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.