OEM lightweighting efforts to improve fuel economy have introduced advanced high-strength steels, aluminum, and complex joining methods that fundamentally change how vehicles must be repaired, requiring collision shops to master sophisticated new procedures and material-specific techniques.
- Vehicle weight reduction of 10% improves fuel economy by 6-8%, driving automakers to use stronger, lighter materials
- Modern vehicles use multiple grades of high-strength and ultra-high-strength steels (up to 1,500 megapascals) replacing traditional mild steel
- Hot-stamped steel is heated, formed while hot, and rapidly cooled to create extremely strong components for B-pillars and side sills
- Advanced materials require tighter control over forming, heating, and joining processes with less tolerance for error than conventional steels
- Modern vehicle structures combine multiple material types including aluminum bonded to steel, each requiring different repair approaches and expertise
The drive to reduce vehicle weight has played an important role in the increased use of stronger steels, aluminum and more complex joining methods, according to Michael Worswick, professor emeritus and adjunct professor Department of Mechanical and Mechatronics Engineering of the University of Waterloo, who specialized in automotive materials, forming and crashworthiness.
The founding director of the Waterloo Forming and Crash Lab, said the shift has made modern vehicle structures considerably more complex than those produced a generation ago.
“The complexity and range of materials that OEMs are using in cars has increased considerably,” Worswick told Collision Repair.
Much of that change can be traced to the long-running effort to reduce vehicle mass without sacrificing crash protection, durability or performance. Fuel-economy and emissions requirements helped put pressure on automakers to build lighter vehicles, while reducing mass also improves efficiency because less energy is required to move the vehicle.
“Car companies have been pushed by increased crash safety standards, regulatory and emmissions standards as well as performance requirements to reduce weight,” Worswick said.
The U.S. Department of Energy estimates that reducing a vehicle’s weight by 10% can improve fuel economy by about 6% to 8%. Automakers have generally pursued that goal in two ways: by substituting lower-density materials such as aluminum, or by using stronger steels that can provide the required structural performance with less material.
“That generally means either higher strength or lower density,” Worswick said. “That’s where aluminum alloys came in.”
That search for strength at lower weight has transformed automotive steel. Vehicles that once relied heavily on relatively conventional mild steels now use a range of high-strength, advanced high-strength and ultra-high-strength grades in different parts of the structure.
“The materials were nowhere near the strength levels we see today,” Worswick said of vehicles produced several decades ago.
Those older steels were also less complicated to work with. They generally involved less alloying and offered wider tolerances during forming and joining, which made welding and structural repair more forgiving.
“It was simply easier to repair,” Worswick said. “The welding procedures, for example, were considerably simpler.”
As manufacturers pushed further into lightweighting, however, they began using increasingly sophisticated alloys that required tighter control over how they were formed, heated and joined.
“As the drive to reduce vehicle weight has progressed, increasingly sophisticated alloys requiring more careful processing have been introduced,” Worswick said.
The challenge is not limited to the properties of one material. Modern vehicle bodies can combine multiple grades of steel with aluminum and other materials, each selected for a different structural purpose.
“You have different alloy combinations and even different material combinations, including aluminum bonded to steel,” Worswick said.
That variety has made joining more demanding. Even a familiar process such as resistance spot welding can require tighter control when advanced steels are involved. Manufacturers may have to account for problems such as joint brittleness or liquid metal embrittlement, a form of cracking that can occur under certain welding conditions.
“Joining complexity has gone up tremendously,” Worswick said.
The reason those details matter is that a vehicle structure is designed not simply to be strong, but to deform in a controlled way during a collision. Materials in a crumple zone have to absorb energy while maintaining the behaviour engineers intended.
“Bendability becomes very important in a crash,” Worswick said. “When materials have to fold over on themselves in a crumple zone, a lot of care goes into that.”
The same principle applies to the joints holding the structure together. During an impact, welds and other connections are subjected to rapid deformation and have to maintain sufficient performance as the surrounding structure changes shape.
“You have to consider what a weld will experience during an impact, when deformation is happening at a very high rate,” Worswick said. “Its tolerance to those conditions is really important.”
That is where the manufacturing changes begin to matter to collision repair. Worswick, who cautions that he is not a collision repair specialist and had not reviewed recent OEM repair procedures, said the increased sophistication required to build the vehicle would logically affect how it can be repaired.
“If it affects the manufacturing specifications, it obviously has to carry through to repair,” he said.
Hot-stamped steel is one example of how far automotive material processing has moved from conventional sheet metal. In the most common process, steel is heated, formed while hot and then rapidly cooled in a water-cooled die. The process allows manufacturers to produce extremely strong structural components.
“You take the steel to a red-hot condition, form it while it is hot and then quench it in the die,” Worswick said.
Hot-stamped steel is now commonly used in areas such as B-pillars and side sills, where high strength is particularly important for protecting the passenger compartment.
“You see it commonly in components such as B-pillars and side sills,” Worswick said.
Some hot-stamped automotive steels have nominal strengths of about 1,500 megapascals, far above the strength levels associated with traditional mild steel. Their coatings and heat treatment can also affect how they respond to welding and other repair processes.
“It’s ultra-high-strength steel,” Worswick said. “A nominal grade can be around 1,500 megapascals. That’s really strong.”
Aluminum has followed a parallel path. Its lower density offers manufacturers another way to reduce mass, but its use also brings different forming, joining and repair requirements.
