SAE - Aluminum Auto-Body Joining

Organization: SAE
Publication Date: 11 November 2015
Page Count: 96
scope:

Introduction

Heavy-duty pickup trucks have sustained United States and global sales numbers over the past decade, and even have experienced increased sales in 2015. However, their future market viability will be affected by the implementation of new United States Federal CAFE 2 environmental standards. Gas and diesel engine, as well as powertrain technology, have matured, and the remaining innovative solutions for increasing fuel efficiency are limited. Achieving lighter weight is one of the last options available to meet CAFE 2 standards while retaining the popular size, horsepower, and utility offered by heavy-duty pickup truck vehicles. Lighter weight translates to higher fuel efficiency.

It should be noted that the decade-long sustained sales occurred through a period when gas prices peaked at historic levels in the United States. While sales did decline when average gas prices spiked at $4.49 a gallon1 in July 2008, sales rapidly recovered in subsequent years. Additionally, new heavy-duty truck sales were offset by increased owner retention of their older heavy-duty trucks. The number of heavy-duty pickup trucks in the United States stayed constant. Some of the loyalty for these types of vehicles can be attributed to engine and powertrain technology innovations that have improved fuel efficiency while retaining the owners' desired horsepower and torque. However, consistent new vehicle sales and retention of these highly utilitarian vehicles through gas price volatility reflect a degree of owner insensitivity to fuel cost increases. While owners may be insensitive to fuel cost, federal fuel efficiency requirements are driving producers of heavy-duty pickup trucks to find ways to meet future fuel efficiency standards. Meeting the seemingly contradictory requirements of higher federal fuel efficiency standards while supplying the customer's desire for size, utility, horsepower, and torque has led producers to leave the comfort of steel and look to other lightweight materials.

The move to lighter-weight materials is not limited to heavyduty pickup trucks, nor is it new to road vehicles. Hybrid and electric vehicle producers recognize the relationship between energy storage, energy consumption, and weight. In the early versions of these vehicles, size was reduced to minimize weight. In gas-driven vehicles, size was also reduced, resulting in small cars that have not translated as expected to popularity and sales. A minimum size meets human comfort and utility requirements, and that, in turn, drives demand.

It is a fact of physics: larger vehicles are safer than smaller vehicles. Consumers shopping for a fuel-efficient vehicle are already gravitating toward smaller cars. By doing so, they put themselves at greater risk of injury or death in an accident. Smaller, lighter cars are generally not as safe as larger, heavier cars.

Large vehicles have longer hoods and bigger crush zones, which gives them an advantage in frontal crashes. In studies conducted by the Insurance Institute for Highway Safety (IIHS), a heavier vehicle will typically push a lighter one backward during the impact. As a result, less force will be placed on the occupants of the heavier vehicle and more on those in the lighter vehicle, according to IIHS. Historically, the rates of driver deaths per million registered vehicles have been higher for the smaller and lighter vehicles.

Although aluminum reduces weight in larger vehicles, and the overall weight of the vehicle is reduced enough to increase fuel efficiency to meet the future federal guidelines, the vehicles would retain a weight advantage in a collision. The addition of aluminum or hybrid aluminum/ iron materials would also lower the center of mass for these types of vehicles because steel would remain a major component of the chassis. A lower center of mass relative to height would lower the center of gravity, make a more stable vehicle, and reduce instances of rollover.

Two material options available to reduce weight are carbon fiber composites and aluminum. Carbon fiber composite materials have migrated into many vehicles over the past decade. Recent examples are the BMW i3 mostly composite automobile and Lamborghini Veneno Roadster and Sesto Elemento concept car; some of their vehicles contain as much as 80% carbon fiber composite material. Carbon fiber is quite expensive when compared to steel or aluminum, and it resists high-rate manufacturing processes. Additionally, carbon fiber is not recyclable. It works well to provide the light weight and stiffness desired for performance, high-end vehicles but does not translate well when affordable vehicle cost and high-rate production are desired.

Aluminum is one least-cost alternative to reduce weight across all vehicle types. While aluminum is a least-cost alternative material to reduce weight, it is not without transition-to-production challenges for high-rate production across all road vehicle types. Aluminum has been used in cars for many years in limited applications on body parts such as hoods, trunk lids, and tailgates. Some aluminum side panels exist on a few vehicle types. The application of aluminum to hoods, trunk lids, and tailgates represents attached parts that are bolted onto the main structure of the vehicle. The application of aluminum when combined with steel and integrated into the main structural components of a vehicle presents a significant design/engineering and manufacturing challenge to an industry deeply rooted in steel. To an industry characterized by factories filled with thousands of robots spitting sparks as they weld together steel pieces and parts, aluminum is an alien material fraught with unfamiliar manufacturing processes.

Aerospace has long recognized the advantages of aluminum to reduce weight while providing a more formable material than steel. The "goodness" of aluminum's characteristics is offset by its need for more complex manufacturing processes and its sensitivity to corrosion and damage during assembly. Aluminum can be welded, and in certain applications traditional welding methods provide a satisfactory joining solution. For most parts and pieces made of aluminum and joined to other aluminum or steel parts and pieces, bonding and fastening are the preferred and most prevalent methods.

Before the transformation of mostly steel vehicles into mostly aluminum vehicles occurs, new design criteria must be considered and incorporated. Design complexity increases when replacing steel with aluminum, or combining aluminum and steel into a unified assembly. Aluminum is a softer material. Structural components that were formerly steel require considerable redesign to provide the strength, stiffness, and durability necessary to meet vehicle safety and performance requirements. Simply replacing an aluminum part for a steel part in the bill of material will not work.

Complicating the engineering solution for transitioning a vehicle from steel to aluminum or from steel to a steel/ aluminum combined assembly is the joining method. Joining steel-to-steel using automated welding technology is a well understood manufacturing process with known outcomes within the automotive industry. Transitioning to bonding, fastening, and other means to hold all the vehicle pieces together presents a design and engineering challenge. The proper size, positioning, placement, and penetration depth of fasteners contribute to the strength, durability, and performance of a vehicle. Trying to simply transfer spot weld positions from a previous process to fastener placement is unacceptable. Also complicating the design and engineering challenge of bonding and fastening aluminum pieces and parts together are the new manufacturing challenges for access and automation of the new processes required for aluminum integration into the vehicle. Automating the fastener or bonding process is much more complex than automating spot welding.

If the bonding process is employed, bond-line placement and assessment criteria have to be included into the vehicle engineering and design. Bonded aluminum strength properties along the bond-line are highly dependent on many factors including material preparation and cleanliness, uniformity of the bond-line, controlled application, voids, and temperature/humidity at time of application. This complicates a historically less-thanpristine steel manufacturing environment.

Fastening aluminum structure to other aluminum pieces or to steel adds complexity to the design engineering and manufacturing challenge. Corrosion is the enemy of aluminum. Aerospace has long known that given the smallest access to entry, corrosion in aluminum will find an opening and propagate. Corrosion in aluminum parts migrates, often in a clandestine way. Many times it remains unseen and continues to degrade the strength of the material until part or assembly failure occurs. A fastenedtogether mostly aluminum vehicle has a thousand places for entry and propagation of corrosion, which can spread rapidly. Each fastener in aerospace, and by extension road vehicles, must be meticulously protected to prevent entry of corrosion.

Simulation and modeling to evaluate and predict galvanic corrosion can provide needed information during the design phase to reduce or eliminate the effects of corrosion. Other corrosion inhibitors include various coating applications that provide an effective barrier to moisture penetration to the metal. Manufacturing enhancements such as clean rooms are necessary additions for the preparation and application of coatings and paint to aluminum.

Welding options are applicable to eliminate fasteners that are being considered for joining automotive body parts. Friction stir welding was considered long ago to be a viable option to replace fasteners on aluminum aircraft. For the aircraft industry, the rapid replacement of aluminum with high-strength carbon fiber composites killed the migration of this type of technology onto the airframes of many of today's airplanes. It remains a viable option to join certain piece parts of automobile bodies as more aluminum makes its way into the road vehicles of tomorrow.

Another limitation and design/engineering consideration is aluminum's softer material properties. Again, aerospace designers have recognized that the soft and formable properties of aluminum need to be stiffened when they are incorporated into areas that require both stiffness and light weight. One way to accomplish stiffness while retaining the desired light weight of an aluminum part is to incorporate a core sandwiched between two pieces of aluminum. A lighter-weight part is produced, but with the added benefit of stiffness. The inclusion of a core for targeted parts provides the attributes of stiffness combined with light weight where both are needed.

New metals are emerging into the mainstream that effectively coalesce iron and aluminum into a hybrid lighterweight material that provides stiffness beyond a purely aluminum material. Some of these materials are entering production and offer one viable option to a purely aluminum vehicle. They provide stiffness with only a marginal weight penalty.

This book will address some innovative solutions to mitigate the challenges of migrating aluminum onto the bodies of heavy pickup trucks and other passenger and utility vehicles. It is organized chronologically from design/ engineering, simulation, low-cost aluminum raw material production, hybrid materials, corrosion, and options for joining the various aluminum or aluminum/steel pieces into an assembled vehicle. It also includes a chapter on the inclusion of core between aluminum sheets to increase stiffness.

Adding aluminum to reduce the weight of road vehicles is not without its challenges and cost. The addition of aluminum is estimated to add up to 500 U.S. dollars to the cost of an automobile previously made of steel. That cost is offset by improved fuel economy and the added safety and comfort the driver receives from retaining the utility of the larger vehicle. Consumers want an automobile that provides comfort, safety, power, and the utility that the auto-miniaturization path to fuel economy cannot provide.

Lowering the cost of integrating aluminum material into a previously mostly steel vehicle using mature and robust manufacturing processes will occur. As the application spreads and the learning curve matures, a robust and understood manufacturing process will evolve to incorporate the technologies, materials, and processes described in this book. They do not encompass all that must be addressed as the auto industry transitions to a mix of metals that will make up the vehicle of the future. But understanding these options starts the journey toward a better understanding of options and solutions to add aluminum as a material option. The material called aluminum, which once dominated the aerospace market and is now in decline, may see a rebirth as the future material of choice on road vehicles.

References

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