Metals (steel and aluminium) belong to the series of very first materials used for packaging. Although they have superior qualities for packaging, they are limited in terms of forming and shaping an interesting geometry. On the other hand, metal packaging has a strong and stable structure.
Plastics have revolutionized industrial design due to their versatile processability, however their relatively low strength has hampered their use in structural innovations, like designers do in glass. However glass is breakable. The “ideal” material would offer a desirable combination of superior structural properties and the ability to be precision shaped into complex geometries. Here we enter the bulk metallic glasses (BMGs) era.
Conventional metallic materials have a crystalline structure consisting of single crystal grains of varying size arranged in a micro-structure, in other words they have a highly ordered arrangement of atoms. Bulk metallic glasses have a disordered atomic-scale structure in contrast to most metals. Certain oxide mixtures (e.g. silicate glasses), have such sluggish crystal nucleation and growth kinetics, that the liquid can be readily under-cooled far below the melting point of crystals (e.g. a quartz crystal). At deep under-cooling, these oxide melts undergo a “glass transition” and freeze as vitreous solids.
These bulk metallic glasses (BMG’s) have unusual properties. They are typically much stronger than crystalline metal counterparts (by factors of 2 or 3), are quite tough (much more so than ceramics), and have very high strain limits. As a new class of engineering materials, BMG’s offer an opportunity to revolutionize the field of structural materials with combinations of strength, ductility, toughness, and processability.
Recently a Yale University research group has blow moulded a recently developed alloy of bulk metal glasses (BMG). By blow moulding the BMGs, the team was able to perform shaping, joining and finishing in one step that took less than a minute.
BMGs which have superior mechanical properties, can, apparently, be blow moulded like plastics. The key to the enhanced processability of BMG formers is their amenability to thermoplastic forming. This allows complex BMG structures, some of which cannot be produced using any other metal process, to be shaped precisely.
“This could enable a whole new paradigm for shaping metals,” says Jan Schroers, an associate professor of mechanical engineering at Yale. “The superior properties of BMGs relative to plastics and typical metals, combined with the ease, economy and precision of blow moulding, have the potential to impact society just as much as the development of synthetic plastics and their associated processing methods have in the last century”.
The team blow moulded the alloys at low temperatures and low pressures, where the bulk metallic glass softens dramatically and flows as easily as plastic but without crystallizing like regular metal. It’s the relatively low temperatures and low pressures that allowed the team to shape the BMGs with unprecedented ease, versatility and precision, Schroers said. Temperatures are around 430ºC, and pressures are also low, around 1 atmosphere.
In order to carefully control and maintain the ideal temperature for blow moulding, the team shaped the BMGs in a vacuum or in fluid. “Vacuum levels can be very low, just to reduce thermal conductivity of the environment,” says Schroers. “Lots of shapes do not require vacuum at all. The liquid process is for very complex shapes.”
“The trick is to avoid friction typically present in other forming techniques,” Schroers said. “Blow moulding completely eliminates friction, allowing us to create any number of complicated shapes, down to the nano-scale.”
Blow moulding of BMGs also results in very smooth surfaces while providing the ability to pattern surfaces. Expansion under plane strain conditions takes place during the free expansion stage. During this expansion stage the action of surface tension alone smoothes perturbations to approximately 10 µm. Once the BMG touches the mould, no more lateral strain takes place due to stick conditions between the BMG and the mould. However, even when the BMG is in contact with the mould, the normal stress component still results in normal strains as long as the length scales involved are small compared to the thickness of the deforming BMG. The presence of a normal component results in an outstanding surface finish, and it can also be utilized to pattern the surface, allowing the surfaces to be integrated into the blow mould process.
With blow moulding, high strength bulk metallic glasses can be formed in a manner similar to plastics when the specifics of these alloys are considered. This allows for shaping complex geometries in an economical and precise manner, including designs, which can not be produced with any other metal processing method.
The materials cost is about the same as high-end steel, Schroers said, but can be processed as cheaply as plastic. The alloys are made up of different metals, including zirconium, nickel, titanium and copper.