Precise Measurements of Thermophysical Properties of Liquid Ti–6Al–4V (Ti64) Alloy On Board the International Space Station

Abstract

Titanium alloys generally exhibit high strength and toughness within a large composition and temperature range. Combined with their high corrosion resistance and low density, titanium alloys are important for many applications, ranging from lightweight constructions over aerospace applications to biomedical implants. [1][2][3][4][5] One of the most commonly used titanium alloys is the alpha-beta titanium alloy Ti64 (Ti-6Al-4V, in wt% or Ti-10.2Al-3.6V, in at%). Developed in 1954, it is an established alloy and a prototype for a large group of newer alpha-beta Ti-Al alloys with good strength to weight ratio. [2,5,6] Different fabrication routes to produce parts and components from Ti64 exist (casting, additive manufacturing, metal injection molding, etc.), mainly including the presence of the liquid phase. [6] Ti64 is also very commonly used in 3D printing and additive manufacturing technologies that continue to evolve rapidly.The final microstructure, defect density, residual stresses, surface roughness, porosity, and consequently the mechanical properties of Ti64 parts are depending on the used process conditions. [7][8][9][10][11][12] Manufacturing processes can be optimized by simulations of heat and mass flow during the manufacturing process. [13][14][15][16][17] Also, the microstructure evolution, including porosity and other defects, can be predicted by numerical methods. [18] These simulations need precise thermophysical property data of the simulated alloy as input parameters. [5,[13][14][15][16][17][18] Although Ti64 is a widely applied alloy, not too much literature data are available for its thermophysical properties in the liquid phase. Also, there are some disagreements in the published literature concerning the thermophysical property data, such as thermal conductivity or specific heat capacity. One reason is that the measurement of thermophysical properties of liquid Ti alloys using classical thermoanalytical equipment (containerbased methods) is challenging or even impossible. The inevitable contact of the material with the container walls leads to contaminations and parasitic container-wall reactions that will prevent precise measurements of thermophysical properties. [19] We present measurements of Ti64 using the electromagnetic levitator (ISS-EML) inside the European science module "Columbus" on board the International Space Station (ISS). This containerless method is the methodology of choice for precise measurement of thermophysical properties of hightemperature reactive liquid metallic melts. The microgravity environment is used to diminish the strong forces necessary

Investigating Thermophysical Properties Under Microgravity: A Review

“…[117] However, only with the longer μg times on board the ISS, benchmark measurements of the full set of thermophysical properties were possible. [59] For this sample, the complete set of thermophysical properties was obtained, including the electrical resistivity, surface tension, viscosity, mass density, specific heat, and also total hemispherical emissivity. [59] The molar specific heat capacity and the total hemispherical emissivity of Ti64 is shown in Figure 8.…”

“…[59] For this sample, the complete set of thermophysical properties was obtained, including the electrical resistivity, surface tension, viscosity, mass density, specific heat, and also total hemispherical emissivity. [59] The molar specific heat capacity and the total hemispherical emissivity of Ti64 is shown in Figure 8. The literature values for the mass density in the liquid phase given in a handbook [118] differ by about 30% from the values obtained by us [59] and by Li.…”

“…[59] The molar specific heat capacity and the total hemispherical emissivity of Ti64 is shown in Figure 8. The literature values for the mass density in the liquid phase given in a handbook [118] differ by about 30% from the values obtained by us [59] and by Li. [119] The results for the thermal conductivity in the liquid phase, obtained by Boivineau et al [120] using pulse heating experiments, are in agreement with our values, but differ from the literature data obtained by thermal diffusivity measurements.…”

“…The comparison of cooling curves in vacuum and in argon was used to verify a new description of heat loss in gas, which adds the ability to determine the total hemispherical emissivity not only under vacuum conditions but also under Ar gas conditions. [59] With regard to Zr-based alloys-e.g., Zr-based BMGs and industrial Zr-based alloys-the thermophysical properties of pure zirconium are also of general interest for comparison and modeling approaches. Figure 10a shows a typical melt cycle used to perform surface tension and viscosity measurements that demonstrates the accurate temperature control of the ISS-EML.…”

“…The industrially widely applied alloy Ti64 was also investigated in the ISS-EML due to its importance for biomedical implants, for automotive and for the aerospace industries. [59] Even though the alloy is known for long time, the high chemical reactivity in the liquid phase makes the precise determination of all its thermophysical properties reliant on container-less methods, such as pulse heating, [60] electrostatic, [61] or electromagnetic levitation, [56] where a number of thermophysical quantities can only be precisely measured under long-duration microgravity. [59] Modern steels are used in a large variety of applications.…”