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Thereby, a more homogenous stress distribution during deformation is established and, in addition, the overall strain required is reduced. In the processing of (thermo)plastics, this strategy has led to a vast array of complex shapes, one prominent example of which is the soda bottle. In order to blow mold the rectangular-shaped bottle shown in Fig. 3a from a disc, a strain of ~4500 % is required. Using the pre-shape shown in Fig. 3a, however, the maximum required strain is reduced to less than 1000 %. Fig. 3a depicts the FEM expansion under a pressure difference of 0.1 MPa of the pre-shaped bottle with Zr44Ti11Cu10Ni10Be25 material properties. The final shape is achieved after only 54 sec. When the same pre-bottle was experimentally expanded using even higher pressures (up to 0.3 MPa) and longer times (up to 300 sec), complete filling of the mold could not be achieved (Fig. 3c), despite a suggested theoretical maximum strain exceeding 2500 %.Blow molding comparison of BMGs, plastics, and SPF alloysTo understand the limited formability observed experimentally during blow molding, the BMGs are compared with plastics and SPF metals in Table 1. The processing temperatures and pressures for some BMGs are comparable to those of plastics. Even though the maximum strain that can be achieved with BMGs is limited, its value can exceed 10 000 % and thus it does not impose a practical limitation. In addition, resistance to thinning for BMGs is comparable to plastics, and significantly larger than for SPF metals19. This comparison suggests that BMGs are as suited for blow molding as plastics are. However, in the experimental realization of blow molding, temperature variations are always present. Such temperature variations within the experimental setup affect a BMG’s viscosity and therefore its deformation resistance. The temperature dependence of the viscosity is quantified through the steepness index, ms = d l—og— dTg— (/η—/TG —)–⎥⎥⎥T=Tg (G: Shear Modulus, Tg: glass transition temperature)20. Within this categorization scheme, plastics are considered fragile, exhibiting a viscosity which changes rapidly with temperature, while BMG formers are considered moderately strong liquids21. The reduced temperature sensitivity of BMGs’ viscosity suggests that BMGs should hold a processing advantage over plastics for blow molding. The time scale over which temperature gradients in the processing environment affect the blow molded material is controlled by the material’s thermal conductivity. The thermal conductivity of BMG is about two orders of magnitude higher than for plastics used in blow molding (see, for example10). Therefore, a temperature gradient within the experimental setup translates significantly faster into a temperature change within the deforming BMG than in the deforming plastic. Since the temperature gradients are always negative due to convective heat Fig. 3 (a) FEM of the expansion of a Zr44Ti11Cu10Ni10Be25 pre-shape. Assuming the same processing conditions used in experiments (p = 105 Pa, T = 460 °C) and Newtonian behavior (σ = η⋅3ε ⋅ ) as the constitutive equation with a viscosity of η = 3.1×106 Pa•s, complete expansion into the final shape is achieved after 54 seconds. (b) Even when the experimental pressure is increased to 3×105 Pa and the processing time is increased up to 300 seconds, filling is still incomplete. (c) Effect of processing environment on the expansion process.
When processing in vacuum (outer surface), heat losses are significantly reduced, resulting in large strains and the FEM-predicted cross-sectional profile. When processing in air, convection causes the deforming BMG to cool, which increases its flow stress, thereby slowing down the expansion kinetics and limiting the achievable strain. Unlike the cross-sectional profile predicted by FEM in vacuum (Fig. 2c), here the thinnest sections are close to the edges where cooling is the lowest due to the proximity to the heater. (e-f) Schematics of the temperature distribution during blow molding of SPF metal, polymer, and BMG. The SPF alloy (e) is in thermal equilibrium with the environment due to the long processing time of approximately 30 minutes and the high thermal conductivity. The opposite is true for polymers (f) where the low thermal conductivity results in adiabatic conditions on the time scale of the process (<10 s). BMGs on the other hand are in-between these conditions. They develop a temperature gradient with a magnitude determined by the difference between the temperature of the heater and the environment (g). transfer to the environment, the temperature of the BMG specimen decreases during blow molding (Fig. 3b). Consequently, in order to blow mold BMG formers into shapes with a complexity comparable to plastics, temperature gradients within the setup must be reduced to a significant larger extent than is required for plastic processing. This is not only required to achieve large overall strains but also replicate small features, where the stress decreases according to σ = ΔpS/4t (Δp: forming pressure, S: span width, t: BMG’s thickness), leading to a reduced strain rate in those regions. One approach to reducing the temperature gradients involves thermally decoupling the setup from the environment by processing in a vacuum (Fig. 3c), which results in larger possible strains than when processed in air (Fig. 3d). Alternatively, a liquid medium can be used, which reduces temperature variations due to its high thermal mass and convective flow. Even though the thermal conductivity for SPF alloys is even higher than the thermal conductivity for BMGs, the fact that forming is carried out on a timescale of ~30 minutes allows the alloy to equilibrate with the processing environment (Fig. 3e). For the BMG (Fig. 3g) the timescale (~1 minute) is too short to equilibrate, yet too long, considering its thermal conductivity, to achieve adiabatic conditions like for plastics (Fig. 3f).Fig. 4 illustrates the application of strategies to reduce temperature gradients; complex geometries can be net shaped (within 1 min) through BMG blow molding when temperature uniformity is established (Figs. 4a & b). The low forming pressure required, together with the BMG’s ability to replicate extremely small features22, and its drastically reduced solidification shrinkage compared to conventional alloys (<0.5 % vs. 5 %10), result in the highest dimensional precision of blow molded BMG parts. Integration of shaping, joining, and finishing into one processing stepBlow 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 approximately23 10 μm. Once the BMG touches the mold, no more lateral strain takes place due to stick conditions between the BMG and the mold.