High Temperature Dilatometry on Tungsten

Refractory metals such as tungsten belong to a small class of metals that have a high melting point, density, tensile strength, electrical conductivity, and a low thermal expansion coefficient. Tungsten (W) is used in nuclear control rods and rocket nozzles for its high melting point, and is used in other fields for its chemical and mechanical properties. Tungsten is fabricated through the process of powder metallurgy- where metal powders are compacted and then heated, in a process known as sintering, to achieve a higher density. A master sintering curve (MSC) is a versatile model that is used to optimize the sintering process. MSCs are used to determine ideal sintering conditions based on the density desired; they combine the effects of temperature and time into a single parameter that relates temperature profiles to a predicted final density and provide an activation energy of sintering. These curves are typically generated using dilatometry as the density of the sample is measured at every point in the temperature program. However, this approach has not been widely utilized due to the high temperature tungsten needs for sintering. Here we show the results of high-temperature dilatometry on a tungsten pellet to produce a master sintering curve from one experiment. A green tungsten pellet was produced by pressing 1.5g of 1-5𝜇m powder at 1 metric ton with no binders or lubricant in a 6mm die. The green density was 63% of the theoretical density of W. We performed dilatometry using a Netzsch 402c push-rod dilatometer equipped with a graphite furnace capable of testing at 2000℃. 3% hydrogen with balanced argon was used to purge the sample holder. Shrinkage measurements are shown below in Figure 1. We found that heating the tungsten pellet at a ramp rate of 2℃/min, reaching 1950℃, then cooling the pellet at a ramp rate of 20℃/min resulted in tungsten having a higher final density of 77.74%. Based on the shrinkage data and using the activation energy for W sintering from literature, a MSC will be calculated for this experiment. The MSC is modified by substituting the densification ratio and densification parameter to identify the regions where shrinkage occurs. Furthermore, a MSC will be produced to analyze the shrinkage resulting from different ramp rates of tungsten.