March 01, 2011

Stronger Than Steel but Moldable as Plastic

A team led by Jan Schroers, a materials scientist at Yale University, has shown that some recently developed bulk metallic glasses (BMGs)-metal alloys that have randomly arranged atoms as opposed to the orderly, crystalline structure found in ordinary metals-can be blow molded like plastics into complex shapes that can't be achieved using regular metal, yet without sacrificing the strength or durability that metal affords. Their findings are described online in the current issue of the journal Materials Today.

Materials Today journal - Thermoplastic blow molding of metals

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While plastics have revolutionized industrial design due to their versatile processability, their relatively low strength has hampered their use in structural components. On the other hand, while metals are the basis for strong structural components, the geometries into which they can be processed are rather limited. The “ideal” material would offer a desirable combination of superior structural properties and the ability to be precision (net) shaped into complex geometries. Here we show that bulk metallic glasses (BMGs), which have superior mechanical properties, can be blow molded 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 net shaped precisely.

Metals are the most widely used structural material, spanning length scales from not, vert, similar100 nm to not, vert, similar100 m in applications where a combination of strength and ductility is required. Compared to plastics, however, metals exhibit limited processability.

Properties vs. processability compared via the temperature-dependent strength for conventional steel, SPF alloys, plastics, and BMGs. Conventional metals are represented by a 1045 steel, SPF alloys by aluminum based 2004 SPF, BMGs by Zr44Ti11Cu10Ni10Be25 and Pt57.5Cu14.7Ni5.3P22.5, and PET (polyethylene terephthalate) was chosen as an example system for plastics. The temperature dependent strength (flow stress) is calculated for the fluids from σ = η·3var epsilon˙ for a strain rate of 10−1 sec−1. The ideal processing region is defined by the lowest processing pressure where effects such as turbulent flow, wetting, and gravitational influences can be neglected on the time scale of the experiment. This region can be accessed by plastics and also by some of the recently developed highly processable BMGs, but not by conventional metals or even by SPF alloys. Compared to plastics, such BMGs exhibit a room temperature strength which is two orders of magnitude higher. Thus, BMGs can be considered high strength metals that can be processed like plastics.

The origin of the superb processability of (thermo)plastics is the gradual softening from a solid-like material (glass) below the glass transition temperature, Tg, to a liquid-like material (supercooled liquid) when heated above Tg. From a processing point of view, an ideal material would flow under a forming pressure, which is low, yet sufficiently large that turbulent flow is avoided and gravity and wetting effects can be neglected on the time scale of the process (Fig. 1). Such a desired processing window of forming pressure lies between 10−5 and 1 MPa and can be readily accessed with (thermo)plastics. Plastic processing is typically carried out at viscosities of 10^3–10^6 Pa·s and strain rates of 10^−2 to 10^1 sec^−1. In the simplest case of a Newtonian flow, viscosity, η, translates into the flow stress (or forming pressure), σ, according to σ = η·3var epsilon˙, where var epsilon˙ is the strain rate. Most conventional metals cannot be processed within the ranges described above. They are either processed in their crystalline state, where even at elevated temperatures they possess high strength, or in a highly fluid liquid state above their melting temperature. This results in turbulent flow, wetting effects, and possible segregation, as well as undesirable microstructures on subsequent solidification. Thus, compared to plastic processing, most metal processing methods yield inferior results in terms of versatility of shapes, precision, and economics. These limitations have spurred research focused on processing metals in a softer

"These alloys look like ordinary metal but can be blow molded just as cheaply and as easily as plastic," Schroers said. So far the team has created a number of complex shapes-including seamless metallic bottles, watch cases, miniature resonators and biomedical implants-that can be molded in less than a minute and are twice as strong as typical steel.

The materials cost 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.

The team blow molded 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 low temperatures and low pressures that allowed the team to shape the BMGs with unprecedented ease, versatility and precision, Schroers said. In order to carefully control and maintain the ideal temperature for blow molding, the team shaped the BMGs in a vacuum or in fluid.

"The trick is to avoid friction typically present in other forming techniques," Schroers said. "Blow molding completely eliminates friction, allowing us to create any number of complicated shapes, down to the nanoscale."

Schroers and his team are already using their new processing technique to fabricate miniature resonators for microelectromechanical systems (MEMS)-tiny mechanical devices powered by electricity-as well as gyroscopes and other resonator applications.

In addition, by blow molding the BMGs, the team was able to combine three separate steps in traditional metal processing (shaping, joining and finishing) into one, allowing them to carry out previously cumbersome, time- and energy-intensive processing in less than a minute.

"This could enable a whole new paradigm for shaping metals," Schroers said. "The superior properties of BMGs relative to plastics and typical metals, combined with the ease, economy and precision of blow molding, 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."

Integration of shaping, joining, and finishing into one processing step

Blow molding 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 mold, no more lateral strain takes place due to stick conditions between the BMG and the mold. However, even when the BMG is in contact with the mold, 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 (see, e.g., Fig. 2a), and it can also be utilized to pattern the surface, as demonstrated in Fig. 4c, allowing functionalization of the surfaces to be integrated into the blow mold process.

Fig. 4. By reducing the heat losses during deformation, shapes that cannot be produced with any other metal processing method can be precisely net shaped within less than 1 minute through TPF based blow molding (a and b). Expansion of Zr44Ti11Cu10Ni10Be25 pre-shapes as shown in Fig. 3a result in hollow, low symmetry shapes that are seamless. (b) These include thin-walled shapes incorporating undercuts. Surface patterning and functionalization can be integrated into the blow molding processing step. (c) Joints can be created by blow molding around fastener sites, resulting in a mechanical bond. (d) Surface patterning or finishing can also be integrated into one processing step with the blow molding.

Besides surface patterning, joining can also be integrated as a processing step during blow molding. In conventional metal processing, an additional processing step is required to join parts, which creates stresses and affects mechanical properties. Applying conventional joining techniques to BMGs is particularly problematic due to the metastable nature of their amorphous structure[25] and [26]. Alternatively joints can be created by blow molding around fastener sites, resulting in a mechanical bond (Fig. 4c).

With blow molding, high strength bulk metallic glasses can be formed in a manner similar to plastics when the specifics of these alloys are considered. This allows one to net shape complex geometries in an economical and precise manner, including shapes, which can not be produced with any other metal processing method. Furthermore, blow molding of BMGs enables combination of three traditional processing steps (shaping, joining, finishing) into one processing step. The superior properties of BMGs relative to plastics and typical structural metals, combined with the ease, economy, and precision of blow molding, have the potential to impact society in a manner similar to the development of synthetic plastics and their associated processing methods.

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