“Managing mixed materials and modern vehicles”
January 1, 2017
We’ve seen more change in vehicle structures technologies over the past 10 years than we had ever seen before. The growth of Advanced High Strength Steel (AHSS), the number of aluminum-intensive vehicles and the expanded use of carbon fiber and other composites are significant factors in how we repair today’s vehicles. Historically, we’ve been focusing on either a “steel vehicle” or an “aluminum-intensive vehicle” – that is going to change, significantly, over the next several years. The landscape of collision repair will continue to change, rapidly, and we must keep pace to ensure complete, safe, quality repairs. Soon, we’ll be working on mixed-material vehicles that will require access to information, new tools, equipment, techniques, proper training, and a new skill set.
The Cadillac CT6 used a significant amount of aluminum and steel in the vehicle structure.
There are at least a couple of factors that have gone into the extensive changes we’re seeing in today’s vehicles; most significantly weight reduction and safety. The vehicle makers are in a daily struggle against two opposing forces. How do they add all of the safety and ‘creature comfort’ items that customers demand, while meeting stringent (and getting more stringent) Corporate Average Fuel Economy (CAFÉ) requirements? The array of options on today’s vehicles is vast: heated and cooled seats, rear-seat infotainment systems, center console coolers, and even vacuums are just a few of the options available on an increasing number of vehicles. Add to that the significant number of Advanced Driver Assist Systems (ADAS) such as adaptive cruise control, lane keep assist, blind spot monitoring, active park assist, and many others, and the vehicle makers have a weight problem. All of these options, the computers electronics to run many of them, and the wiring that powers them, all add a significant amount of weight to the vehicle.
At the same time, the vehicle makers also have to design stronger vehicles to protect vehicle occupants. The new Insurance Institute for Highway Safety (IIHS) 25 percent small overlap crash test has posed a significant challenge to the vehicle makers. When the test was first released, many vehicles yielded dismal results. The vehicle makers quickly scrambled and re-engineered their model line up to be more successful in the new crash test. Design changes, including additional reinforcements, again added some weight to the vehicle.
Notice the extensive use of aluminum castings, stamping and extrusions, as well as ultra-high-strength steel (UHSS), in the side aperature of the Cadillac CT6.
All of the weight that is added to the vehicle, adversely affects the fuel economy, imposed by government regulations, the vehicle makers must meet. Vehicle makers have a few options for reducing that weight:
- Build smaller vehicles. While we’ve seen a significant increase in the number of small cars on the market, trucks, SUVs, and crossovers continue to be some of the most popular vehicles on the roads of America. The vehicle makers must produce a significant number of these smaller vehicles to offset the sales of larger vehicles, to meet their CAFÉ requirements.
- Decrease engine size. Smaller engines are more fuel-efficient, but many customers also want horsepower and towing capabilities, especially in the truck, SUV, and crossover hungry U.S. market. This is one reason we’re seeing a lot more turbo-charged engines on many of todays’ vehicles. Prior to its release, Ford estimated that 56 percent of 2015 F-150 sales would include one of their tubocharged, Ecoboost engines, while their 5.0 liter engine would only account for roughly 28 percent of sales.
- Reduce the weight of the vehicle structure. This is a key area and the reason why we see a lot more high- and ultra-high strength steel, aluminum, magnesium, and composites on today’s vehicles.
Reducing the weight of the structure is an area that today’s vehicle design engineers focus on, daily. Historically, those engineers have looked to leverage a variety of either high- and ultra-high-strength steels or aluminum for the vehicle structure. The cars and trucks that we repair on a daily basis are, more-often-than-not, either a “steel vehicle” or an “aluminum-intensive vehicle.” That is beginning to change and will continue to change at a much more rapid pace over the next several years.
Tomorrow’s (and some of today’s) design engineers are going to choose the material they believe is best suited for a particular application and won’t likely focus on one type of material. Tomorrow’s vehicle will be a mixed-material vehicle and we’re going to need to adapt our approach to repairing those vehicle.
Today’s mixed-material vehicle
There are a few vehicles that are already employing, or had previously employed, this approach to vehicle design and construction. Several years ago, BMW introduced their GRAV structure on the 5- and 6-series vehicles and Audi leveraged a similar approach on the TT. Today, we’re seeing BMW leverage steel, aluminum, and carbon fiber on the new, carbon-core, 7-series. Additionally, Cadillac used both steel and aluminum throughout the structure of their flagship, the CT6. For futures mixed-material vehicle applications, you may want to keep an eye on these vehicle makers.
One of the first steps in the development of any repair plan should be to identify the material(s) used by the vehicle maker. This will provide the foundation for a solid repair plan. Fortunately, the number of vehicles that provide detailed information on material types continues to increase. Unfortunately, the information isn’t always located in the same area of the body repair manual and may be a bit challenging to find. However, with time and experience, you’ll quickly become familiar with how different vehicle makers lay out their repair information, making it easier to find in subsequent visits to their repair websites.
Once you’ve identified the type(s) of materials that are going to be involved in the repair, the next step is to determine what is repairable and what will require replacement. Again, the best first step is to refer to the vehicle’s body repair manual. An increasing number of vehicle makers offer a wealth of repairability guidelines for their vehicles. The number of vehicle makers that allow for straightening of high- and ultra-high strength steel and aluminum, is decreasing, so part replacement will become quite common. The amount of straightening allowed for mixed-material vehicles will likely be even less. More-often-than-not, if the structure of a mixed-material vehicle is damaged, part replacement is going to be required.
If no repair guidelines exist, you’ll have to rely on your expertise and previous training to make an effective repair decision. If the vehicle has damage to mild steel or aluminum stampings, some straightening may be an option. If there is damage to aluminum extrusions, aluminum castings, high- or ultra-high-strength steel, or carbon fiber, replacement will likely be the only option.
The I-CAR kink vs. bend rule was designed for steel, not aluminum, so it likely wouldn’t apply on a mixed-material vehicle; but, if the part in question is steel, the rule may be applicable. However, the kink vs. bend rule has also changed over the years, based on the types of steels found on today’s vehicles. The number of damaged parts that would qualify as bent has dramatically decreased, while the number of damaged parts that would now be considered a kink has increased. Why has the number increased? The kink vs. bend rule is no longer as simple as it sounds. Part of the original definition of a kink is: a part is considered kinked if, after straightening, there is a permanent area of deformation which will not return to its original state and shape without the use of excessive heat. Kinked parts may also have visible cracks or tears in the metal.
A part may not look like it is kinked, but it may not be able to be straightened without cracking. The stronger the steel, the less repairable it becomes. For example, a slight bend in a UHSS part may crack when straightened, if it is able to be straightened at all. As a best practice, do not straighten parts that are above 600 MPa unless specific documentation exists from the vehicle maker. This is due to possible cracking and tearing of the part and possible damage to adjacent panels. If straightening is allowed by the OEM, a dye penetrant should be used to ensure no micro-cracking has occurred in the steel. Some vehicle makers do not recommend straightening UHSS parts that are above 600 MPa due to cracking and tearing of the part and damage to adjacent panels.
If the part is bent, but the thickness or strength of the metal will not allow for straightening without leaving an area of permanent deformation, then replacement, either partial or complete, would be appropriate.
Tools, equipment and part replacement
While many of the tools and equipment you work with today will be useful for mixed-material vehicles, you will need to invest in some new equipment, if you’re not already working on aluminum-intensive vehicles today, or doing MIG brazing.
While vehicle makers may be able to join dissimilar metals together using spot welding, friction-stir welding, or other ‘fusion’ methods, that type of equipment will not be available for repairs; at not least for the foreseeable future. When attaching dissimilar materials to one another, most repair procedures are going to include mechanical fasteners, in conjunction with adhesives. Blind rivets, and in some cases, self-piercing rivets (SPR) and adhesives are going to be common place in replacement attachment methods. Not only is rivet bonding an effective attachment method, it also provides a barrier between dissimilar materials, especially aluminum and steel. While the likelihood of galvanic corrosion is minimal, rivet bonding provides additional protection. It also helps with noise, vibration, and harshness (NVH) and general corrosion protection.
Factory installed SPRs will often be replaced with blind rivets during repair.
Repairs may not always require joining dissimilar materials and there will many cases where a dissimilar material is sandwiched between two similar materials; for example, an aluminum or carbon fiber reinforcement between two pieces of steel. In those cases, squeeze-type resistance spot welding (STRSW) will likely be the preferred attachment methods. However, not all areas will be accessible for spot welding arms. In those cases, conventional GMA plug welding may be allowed, or MIG brazing may be required. Honda is one vehicle maker that requires MIG brazing when attaching 1,500 MPa steel where spot welding arms won’t reach.
Again, to ensure you’re using the proper tools, equipment, and attachment methods, it is imperative that you refer to the vehicle maker procedures. Not doing so will create an inferior and, quite possibly, a failed repair.
All of the tools and equipment needed for repairing tomorrow’s vehicles won’t be as effective without proper training. Fortunately, I-CAR has a couple of new courses available to help. I-CAR now offers an in-shop, hands-on, Rivet Bonding (RVT01) course and an in-shop, hands-on, MIG Brazing Hands-On Skill Development (BRZ02) course. Both of these courses use your equipment to train you and your team on how to properly use these advanced joining techniques.
I-CAR is also in the process of developing a new suite of courses on new vehicles and new technology to share important information on which vehicle makers are leveraging which materials and repair methods. Many of these new courses will be offered online and will be accessible 24/7.
Working on vehicles with mixed materials can be complicated, but with the proper information, training, tools, equipment, and techniques you can ensure complete, safe, quality repairs for our customers. Always research the vehicle maker information before beginning repairs and ensure you’re following the vehicle maker’s procedures for repair and replacement.