We are part of the biggest global technology and social upheaval right now, and collision repair is very much part of that change. In this article we shall take a look at an area where technology impact has changed the collision repair business for ever – body materials.
The body beautiful
Once upon a time vehicle body structures were made from mild steel alloy with typical yield strengths of 180 MPa, that could be pressed via multiple stage forming into all sorts of shapes without splitting, thanks to the press tool designers. The steel gained strength thanks to mainly the form of the panel rather than any sort of work hardening – where the form could not deliver the required strength the sheet steel thickness was increased.
However, from around 1997 onwards this situation changed with the arrival of higher strength steel alloys. The first application of these ‘boutique’ steels were in B pillar reinforcements, but rapidly spread to include the A pillar as well as the sill reinforcements. Why? To increase structural performance not only for stiffness (handling, stability, and noise, vibration and harshness), but also the relatively new crash test procedures from NCAP (US) as well as Euro NCAP (erm… Europe!).
Thus greater performance could be delivered without the weight increase compared with engineering the same level of performance with mild steel alloy.
So last century
Taking a snap shot of vehicle body engineering across new cars in 2000, we can see that some older models which were still in production had almost nothing but mild steel alloy, whereas the ‘cutting edge’ new model programmes had around 70% mild steel alloy.
Contrast this with 2016, where most vehicles have less than 30% mild steel alloy – a change which has spread across the entire passenger car, light commercial vehicle and bakkie sectors in less than a decade.
Why? The relative cost of the body structure as a proportion of the whole vehicle has fallen, allowing the selective use of higher strength steel alloys – and indeed non-ferrous materials – to be used to offset vehicle weight. There are some basic economics which show why steel alloy has been and will continue to play a big part in vehicle structures.
Whilst aluminium alloy is more tolerant of high proportion of recycled material than steel production, each kg of material would have to be recycled around eight times to get parity with the amount of energy used to make steel alloy the same number of times. Carbon fibre reinforced plastic for mass market applications is still in development, and thanks to new thermo plastic resins replacing thermoset resins it can be recycled too. Magnesium alloys have very limited mass market applications (typically instrument panel cross car support members, centre console support members, hood slam panel assemblies) and this situation will remain until the raw material cost reduces significantly….
A few high end vehicles have used aluminium alloy intensive body structures but this is not due to weight saving alone – the same effect could be achieved at higher tooling cost with steel alloys, but where production volumes are modest the use of aluminium is a good compromise to achieve the desired goal. The added bonus is for the consumer who perceives aluminium alloy to be ‘high tech’, ‘rust free’ and different.
We can see the mass market vehicle structure uses a skeletal form, in which load bearing members can be made from a variety of steel and aluminium alloys – although steel has and will continue to dominate. This collection of panels assembled together into a series of box members provides the primary load path for the platform stability which is required not only for rise/handling but also for good noise, vibration and harshness (NVH) performance. The same members are also optimised to direct impact energy from the point of collision throughout the remaining vehicle structure.
In the case of skin panels there has and will continue to be a push towards weight saving, and this is manifest in panel thickness reduction along with substitution for aluminium alloy. In both cases there is a demand to maintain panel stiffness for NVH as well as resistance to the infamous car park denting creators.
Super materials, in aluminium and steel alloys….
In the case of aluminium alloy it is now common to find a sandwich material where the core is made from a lower strength alloy and the two skins are made from high grade aluminium alloy. The beauty is the resultant sandwich that can be pressed as easily as the core material, but delivers almost the same strength as the higher strength aluminium alloy.
The higher strength 6000 and 7000 series aluminium alloys are typically used for aerosapce load bearing skins, and are prone to rapid work as well as age hardening. This limits how much form can be put into the panels, making the materials nominally unsuitable for automotive applications. One should note the higher strength aluminium alloy ‘skins’ are each less than a fraction of a millimetre thick, and have a higher magnesium content than the ‘core’ alloy.
Note! When refinishing damaged panels the outer skin can be removed during the process of taking off the high spots. Due to the differing magnesium content of the aluminium alloy, this may induce paint system finish issues in later life. JLR for example, state that reforming such panels is not permitted, and the factory supplied coat should not be removed from new panels either.
For steel alloys the development of materials that could be pressed as easily as mild steel and yet deliver significantly higher strength has been the focus of companies around the world for more than two decades.
Let’s take a look at two families of steel alloy commonly used on vehicles – dual phase (DP) and press hardened steel alloys.
Dual Phase steel alloy
The steel alloy family is a mass market dream in that it can be formed via single or multi stage pressing just like mild steel, with minimal work hardening. The raw material is produced by taking the rolled sheet steel alloy to orange heat so that the grain structure is in the austinitic phase, and then quenched at a specific rate to induce ferrite in the grain structure. When the desired proportion of ferrite is achieved, the steel alloy is then rapidly quenched to turn the remaining austenite to martinsite. As the sheet is then used to press parts, the softer ferrite takes the strain and induces the first level of tension – or work hardening – due to the differential grain structure. The second stage of work hardening comes during the painting process where vehicle manufacturing facilities typically use 170 degrees C for around 20 minutes to bake the finish. This adds to the first stage of work hardening by initiating rapid age hardening.
Typically dual phase steel alloys have a yield strength from 220 to 590 MPa, although even stronger versions are available. Typical applications include ultra thin skin panels (0.6 mm thick, around 220 MPa yield strength) through to structural box member panels (around 590 MPa yield strength).
Note! The Dual Phase process is not reversible in the vehicle repair environment. Repairing such materials is possible with limited cold working, but use of heat will destroy the grain structure. This means it might be possible to repair a door skin, but not a structural member in this way. Major structural members should be sectioned and new parts welded in, using the guidelines from the vehicle manufacturer – the allowable heat degradation along the section joints has been engineered by the vehicle manufacturer to permit such operations.
Press hardened steel alloy
The process was invented by Swedish Steel to produce immensely strong steel alloy which could be used selectively for high load paths where conventional solutions – making the panel thicker – were not possible due to weight and bulk considerations. The process uses a specifically formulated rolled sheet steel alloy which is then heated to 850 degrees C (orange heat), and then pressed inside a male/female press tool which is selectively cooled at a rate of 50 degrees C per second whilst the panel is inside it. This converts the austinite phase grin structure to mainly tough martensite grain structure.
The process typically produces formed panels in one hit, so offsetting the increased time to produce panels by eliminating the need for multiple stage pressings. In addition such panels initially had a lot of surface scale, and so would not be used for ‘A’ class exterior surfaces. However, since the process was first used commercially around 2001, it has been evolved to include tailor blanks (steel alloy for press hardening joined to another type of steel alloy before pressing) and tailor blank rolled panels (the same as tailor blank with the addition of varying the thickness of the finished panel).
Note! The press hardened steel alloy process is not reversible in the vehicle repair environment. Repairing such materials is possible with limited cold working, but use of heat will destroy the grain structure. This means it might be possible to cold pull a sill or B pillar if the deflection is one or two millimetres, but structural members should be sectioned and new parts welded in, using the guidelines from the vehicle manufacturer – the allowable heat degradation along the section joints has been engineered by the vehicle manufacturer to permit such operations.
Lessons for all
The upshot of looking into just three materials typically found on vehicles today is doing anything that ‘feels right’ or ‘looks right’ without the information from the vehicle manufacturer is problematic. Gone are the days of the beautifully tolerant material – mild steel – since the more complex multi material body structure is not only with us now, but extends across all vehicle segments. Preparation for repairs is now very important to ensure a successful outcome.
This article was based on a presentation by the author given to the Cross Industry Conference held on 22 April, 2016. Automotive Refinisher enabled the author to present this subject in South Africa.
Next Edition – part 2: How motor insurance has developed in Europe, as a preview of what could happen in South Africa, and new developments underway in automotive manufacturing which will migrate to collision repair.
Auto Industry Consulting is an independent provider of technical information to the global collision repair industry. Products include EziMethods, our online collision repair methods system and Auto Industry Insider, our collision repair industry technical information website. For more information please visit the websites: www.ezimethods.com and www.autoindustryinsider.com or contact email@example.com