Summary: Discusses 4 strategies to lightweighting (material selection, structural optimization, architected materials and multifunctionality) and their convergence enabled by Additive Manufacturing
Weight and the Marketplace
The Ford Model T that first rolled off production lines in 1907, weighed about a third of today’s Tesla Model 3. Our cars today are significantly heavier than they were a century ago. In fact, on average, automotive weight for a given class (car, SUV, truck) has changed little over the past two decades. Which makes sense when you consider that car weight by itself is not a primary selection factor for most automotive consumers, as opposed to performance, safety and features. The commercial aviation industry, on the other hand, has embraced lightweight materials to a far greater degree, presumably since as humans we will always seek the lowest price on comparative choices of a fleeting nature (such as an airline ticket) while de-prioritizing recurring future costs on long-term purchases such as a car. If we had to pay for 10 years of fuel costs up-front when we purchased a car, I am willing to bet cars would be much more fuel efficient than they are today. This is equivalent to payload costs in the burgeoning commercial space industry (currently, on average about $10,000 per pound), and is arguably a key reason why Additive Manufacturing (AM) has been adopted early on in the aerospace industry. But when it comes to the automotive industry, I have always been a skeptic about its implementation of AM for light-weighting cars for consumers.
It was thus a pleasant surprise for me to read the recent news about GM (General Motors) creating an Additively Manufactured seat post bracket that claimed a 40% reduction in weight. Costs were not discussed in the article and it will likely be several years before these brackets are implemented in cars that roll off dealerships across the world. Nonetheless, this is a promising development because it represents a step in a direction several in the AM community are currently working hard to penetrate – that of mass production.
Light-weighting strategies have been explored by engineers for over a century now. With AM technologies entering the production floor, this feels like a good time to take stock of four different strategies to light-weighting, and take a peek into the possible future of this field in the context of AM.
1. Material Selection
Light-weighting is most readily thought of as a material selection problem. Every materials science engineer and most mechanical engineers have seen charts similar to the one shown in Figure 1 that enables selection of a material to achieve certain performance targets (strength, modulus etc.) against the density of that material. The lowest density material that meets all the design requirements is, all other things being equal, the best candidate for the task. All other things are rarely equal of course, and other issues such as manufacturability (ductility for example) and cost also come into play and can dominate selection considerations.
2. Structural Optimization
Weight is however the result of the combination of material and structure. Once a material is selected, a further opportunity is thus to leverage design to drive down the total weight of the structure in question. By “structural” optimization, I refer to design optimization performed at the level of an assembly or individual parts through removal of material (as shown with Topology Optimization in Figure 2), or through a consolidation of structures into fewer components. Both strategies result in significant weight savings. Simulation driven design tools pre-suppose a material selection, so the design process tends to follow material selection, but can be iterative to allow for selection among a range of material choices.
3. Architected Materials
Architected, or cellular materials, enable tuning of properties locally to a higher degree of precision than achievable with bulk optimization. In bone implants, for example, this can be of great benefit since it allows for the mimicking of bone’s stiffness by making local variations that would be hard to enable otherwise. However, as a strategy for light-weighting alone, it is arguable whether these materials by themselves are a superior option to topology optimization of the bulk, in particular given practical concerns such as integration with assemblies, manufacturing variances and defect inspection. Even setting aside these practicalities, the choice of bulk optimization versus cellular materials is likely strongly down to the actual form of the structure in question and its application. Cellular materials have clear advantages when applied to sandwich panels, for example, but may not be preferable to topology optimization for an engine mount bracket. What cellular materials do enable however, is the aforementioned tunability, and as we will discuss next, multi-functionality.
In the context of light-weighting, multi-functionality represents the opportunity to use materials and structures in ways that ultimately offset part counts and assembly devices such as fasteners, and thereby accomplish weight reduction. A notional depiction of multi-functionality is in the wing schematic adapted from a 2016 review paper by Schaedler and Carter, shown in Figure 4. The core function of the wing is to generate lift. However, of interest to us from a light-weighting standpoint are the structures that constitute the wing. These structures must be resilient in all anticipated environmental conditions, but they can also be modified in ways to optimize the location of the center of gravity, and/or for thermal management or energy storage. Such level of multi-functionality, if it is to be sought within a monolithic component, greatly benefits from an ability to control properties locally, as can be achieved with architected materials. It also requires a blending of these local structures with continuous topologies such as the skin of the wing and internal tubing.
AM and Convergence
The key point of this post is not just to point out four different approaches to light-weighting, but make the point that AM is enabling opportunities for optimizing across the interactions of these otherwise disparate, series-operational entities. Thus, it is possible that a selection of a material of sub-optimal properties (driven by cost, for example) is offset by the development of a brand new design paradigm (such as architected materials) that is enabled via AM, or through the addition of a second material in specific locations that help the overall structure meet requirements. Most generally, access to AM allows us to pose the following question: How best would we co-optimize materials, structure and function in an age of multi-material, high design fidelity Additive Manufacturing?
The above separation of light-weighting approaches into 4 distinct strategies is thus superficial in the age of functional-part-capable AM. It is also superficial in the natural world, where these four strategies are employed together for light-weighting – the bones of birds being an obvious example. In the context of light-weighting, AM does have the limitation at the present moment of having a shorter list of materials to select from, especially when using them in conjunction – and overall cost remains a challenge in the domain of mass production. But it is hard to see these challenges remaining in play a decade from now. Whether that will translate into lighter vehicles on our roads will probably belong more to the realm of social economics than engineering – but at least the solutions will be at a metaphorical arm’s length.