Size limit is perhaps one of the most commonly asked questions about the commercial fabrication of Liquidmetal alloys. “How big can we make it?” If you are familiar with typical commercially molded parts from Liquidmetal alloy you may have already observed a common theme; parts a few inches in any dimension and typically thin walled sections. The questions about size typically arise when engineers and designers become aware of the mechanical properties of Liquidmetal alloys: twice the strength of steel, high hardness, corrosion resistance, lustrous as-molded finishes, and high elasticity, among others. It’s not surprising that alloys with similar mechanical properties would be desired for high-performance structural applications such as aircraft bodies, I-beams, bridges, and car bodies. This post aims to help consumers understand the uses and clarify a few limitations of Liquidmetal alloys.
Michael Ashby, a professor at Cambridge, England, has developed a strategy for the selection of particular materials for a specific application, a process called Materials Selection for Mechanical Design. In this method, all material properties are plotted against one another on axes displaying different mechanical properties (strength vs. toughness or cost vs. density, for example).
These plots help designers select a material (out of thousands of options) for a specific set of mechanical, thermal, electrical, cost, or other requirements. Liquidmetal alloys are often put on these plots, so-called Ashby maps, because they occupy regions of “material property space” not usually occupied by other materials. For instance, Liquidmetal alloys have the strength of steel but the elasticity of polymers, so on a plot of elasticity vs. strength, the alloys stand alone.
These types of plots help describe the optimal applications for Liquidmetal alloys, while simultaneously showing other applications that are not well-suited. Therefore, to understand the size at which Liquidmetal alloys can be manufactured, one has to consider the critical design features for these alloys; cost, glass-forming ability, and manufacturability.
If nothing else, the cost of metallic glass-forming alloys prevents large parts from being manufactured effectively (specifically heavy parts). Liquidmetal alloys are predominately made from zirconium and titanium, which are inherently higher-cost elements compared to aluminum and iron. This is a restriction on all high-performance Ti-alloys in addition to metallic glasses. For example, Ti is not used in I-beams for buildings because it costs approximately 10 times more than steel! This does not mean that Ti-alloys aren’t useful; it just means there are some applications where steel is a better selection, while the converse is also true.
Glass-forming ability (GFA) is also another factor to consider when developing large applications for Liquidmetal alloys. GFA describes the maximum thickness that a particular metallic glass-forming alloy can be fabricated without it crystallizing. Liquidmetal alloys must be formed at cooling rates sufficiently high that the formation of the more stable crystalline phase can be avoided. As a part cools from the liquid state during molding, the center of the part experiences the slowest cooling rate, because it is furthest from the cold mold material. As the molded part gets thicker, the cooling rate at the center gets slower and slower until it eventually is insufficient to form a glass. Over thirty years of design have resulted in the current Liquidmetal alloys, some of which can be molded up to an inch thick! However, this explains why large structural components, such as bridges, buildings, and cars cannot be practically fabricated from today’s metallic glasses. The most important thing to note is that while metallic glasses have limited thickness, there is no theoretical limit to how wide they can be. Large panels or sheets of Liquidmetal alloys are possible, even though they are not currently being commercially manufactured.
Commercial applications for Liquidmetal alloys are perpetually being developed and current parts are limited by the size of the mold cavity in Liquidmetal Technologies’ proprietary molding machine. In other words, parts can only be as large as the molds (or as large as the maximum material that can be melted at one time). As commercial applications for Liquidmetal alloys increase, a more expansive complement of companies will be involved, bringing their experience and capital equipment to bear to increase the size and volume of parts that can be made. When this occurs, it is quite likely that Liquidmetal alloys will be as widespread as crystalline Ti-alloys, like Ti-6Al-4V.
So to summarize, and answer the original question, “How big can we make it?” the answer is a part weight up to 80 grams (100 grams maximum total shot size of which 80 grams is usable). Parts with wall thicknesses between 0.6 and 4.0mm thick are optimal for current fabrication. If one were to look around at the number of small metal parts that are in this size range, it’s seemingly endless. Thus, so too are the possible applications of Liquidmetal alloys.