Lightweight Materials for the Transportation Industry
Several newsworthy items crossed The Doctor’s desk late last year: the announcement that Alcoa officially opened the world’s largest aluminum-lithium plant in Lafayette, Ind., (IHDaily NewsBrief, Oct. 15); the release of The Aluminum Quick Reference App (ASM International, Oct. 16); and word that Alcoa has signed a $1 billion contract with Boeing to supply multiple aluminum components over the next several years (Oct. 20).
These announcements piqued The Doctor’s interest in learning more about the battle being waged between aluminum and alternative materials in the transportation industry. To understand who will win and why, one must look at the drivers this industry faces to stay technologically relevant as well as the lightweight material options, properties and costs. Let’s learn more.
Fuel consumption, greenhouse gas (GHG) emissions and global dimming/warming are the principle drivers for change in the automotive industry. For every 100 kg of weight reduction in an automobile, a fuel savings of 0.3-0.5 L per 100 kilometers is possible with a reduction in CO2 emissions of 0.8-1.1 kg per 100 kilometers.
The interest in moving to aluminum and other lightweight alloys in automobiles is fueled in large part by government regulations/mandates, cost (performance versus cost in dollars saved per kilogram), safety (i.e., How light should we go?) and performance (acceleration, braking, handling, noise, vibration and harshness). Other options are conventional low-carbon steel, with a cost advantage while targeting weight reduction; and HSLA steels and AHSS (advanced high-strength steels, or so-called “lightweight steel”), where weight reduction is achieved by thinner/lighter material made possible by higher strength.
Aluminum advantages are in the total amount of aluminum used per vehicle, life-cycle/recycling advantages, space frame versus monocoque (i.e., skin or “egg shell”), loading and forming/joining (e.g., the absence of spot welds, use of rivets, etc.).
An example is the Corvette Z06 (Fig. 1), which is ranked as Car & Drivers’ number-one performance-ranked coupe and convertible. It can achieve 650 horsepower in a choice of two transmissions: a 7-speed manual or an 8-speed paddle-shift automatic. The driver-centric Z06 includes technologies like a Driver Mode Selector, which allows for customization of vehicle performance dynamics with the turn of a knob. On or off the track it is available with an industry-exclusive Performance Data Recorder, which records high-definition video with telemetry overlays on playback.
Another example is the number-one selling truck in the U.S., the Ford F150, which has undergone a dramatic redesign taking advantage of the best of steel and lightweight alloys. Aluminum alloy 6463 is used in the body and bed to reduce the total vehicle weight by 320 kg (705 pounds).
BMW has introduced an Al-Mg die-cast engine block, which is 15% lighter than a comparable all-Al alloy engine. Both BMW and GM have an all Mg block (Fig. 2) under development that is 25% lighter than an all-Al engine.
Aluminum has a long history in aviation (Table 1). Fuel consumption, GHG emissions sustainability and global dimming/warming are also the principle change drivers in the aerospace industry. Aluminum’s dominance is being challenged today by GFRPs (graphite-fiber reinforced plastics), carbon/carbon composites and aluminum-lithium alloys.
For example, the Orion spacecraft (Fig. 3) has olive-green aluminum-lithium metal panels designed to be covered by an advanced version of the thermal protection tiles that were earlier used on the space shuttle. Design challenges such as pressure testing cracks during weld strength testing have yet to be fully addressed.
Alloying with lithium reduces structural mass by three effects:
• Displacement: A lithium atom is lighter than an aluminum atom. Each lithium atom then displaces one aluminum atom from the crystal lattice while maintaining the lattice structure. For every 1% (by weight) of lithium added, the density of the resulting alloy is reduced by 3% and increases in stiffness by 5%. This effect works up to the solubility limit of lithium in aluminum, which is 4.2%.
• Strain hardening: Introducing another type of atom into the crystal strains the lattice, which helps block dislocations. The resulting material is stronger, which allows less of it to be used.
• Precipitation hardening: When properly aged, lithium forms a metastable Al3Li phase (d’) with a coherent crystal structure. Precipitates strengthen the metal by impeding dislocation motion during deformation.
Finally, all-aluminum combat ships have been introduced and are designed to be fast, maneuverable and modular – or, as they like to say in the Navy, lethal, flexible, survivable and affordable. Aluminum in the 5xxx series (e.g., 5086, 5083, 5456, 5454) and 6xxx series alloys (e.g., 6005, 6061, 6082) are in use. The properties and attributes of aluminum that make it an attractive choice are its high strength-to-weight ratio; density (one third of steel); corrosion resistance (although pitting and crevice-corrosion problems have arisen); weldability; ease of forming, bending and machining; availability and diversity of semi-finished products; high thermal and electrical conductivity; nonmagnetic properties; and recyclability.
What stands out in this discussion is that the materials are changing and with them our role as heat treaters. Moving forward, changes in the alloys, product form, function and specifications will influence (and perhaps dictate) the way in which we will perform heat treatment.
1. Herring, Daniel H., Aluminium Battles Alternative Lightweight Materials in the Transportation Race, Presentation at theSECO/WARWICK Aluminum Seminar, October 2014
2. Professor Joseph Benedyk, Illinois Institute of Technology/Thermal Processing Technology Center and Editor Light Metal Age magazine, technical contributions and private correspondence
3. DOE briefing on Automotive Magnesium R&D for Lightweighting, 2014.
4. Chicago Auto Show, 2014
5. Aluminium Exposition, Dusseldorf GmbH, 2014
6. Aeromat 2015
7. Commander Fred Latrash, U.S. Navy (retired), Dr. Brett Conner, Alcoa Defense and Derek Novak, ABS Americas, Webinar on “Advantages of Aluminum in Marine Applications,” 2014