Improvement of thermo-physical data of pure Cu, Fe12Cu and W8Ni2Cu during and after sintering using kinetic modelling
Wolfgang Hohenauer, Daniel Lager and Ijaz Ul Mohsin
The design of de-binding and sintering steps in a powder metallurgical part production, the prediction of the final shape of sintered part, and the estimation of stress during the sintering process are based on the knowledge of the transient and spatial thermo-physical properties of the precursor. Powders from pure Copper, a pre-mixed heavy alloy 90 W8Ni2Cu and a pre-mixed 88Fe12Cu – all of them representing processing materials under technological treatment – are studied in detail regarding to their thermo-physical properties ρ(T), cp(T), a(T) and λ(T) under de-binding and sintering related conditions. Technologically these three material systems represent a solid phase sintering material (pure Cu) and liquid phase sintering materials (pre-mixed heavy alloy 90 W8Ni2Cu and pre-mixed 88Fe12Cu respectively) as well. To regard these materials as processing materials, thermo-kinetic modelling is used to improve the examination of their thermo-physical data from standard measuring techniques as pushrod dilatometry, dynamic scanning calorimetry (DSC) and laser flash technique. Thermo-kinetic models further can be used to formulate implicit material laws for later use in calculation tools to simulate these technological processes.
Measurements of thermal mass, density and specific heat intrinsically capture the change of chemical composition and structure. The use of flash methods to determine the thermal diffusivity a(T) requires the exact knowledge of the actual geometry of the measured sample at any time of measurement. Thus the shrinkage of the sample has to be taken into account to obtain sufficient diffusivity data. As mentioned before the shrinkage history of the part is driven from the heat treatment during the production steps. Hence during the diffusivity measurements a temperature history regarding closely to the thermal conditions in the production routine has to be used. The transient shrinkage of the sample during flash measurement is calculated from a thermo-kinetic model which is established from a set of dilatometer measurements performed with geometrically stepped heating rates. Doing this, corrected flash data represent the thermal diffusivity factoring the actual T(t)-correlated density and the geometry of the part during heat treatment.
Concerning their thermo-physical data ρ(T), cp(T), a(T) and λ(T) the material systems pure copper, pre-mixed heavy alloy 90 W8Ni2Cu and pre-mixed 88Fe12Cu are investigated in detail. Details of the measuring techniques are explained. The relevant thermo-kinetic models and their kinetic parameters are referred. It is shown that maximum correction of thermal transport data at maximum sintering temperature range about some 50% lower referred to the un-corrected data.
Keywords: Kinetic modelling, de-binding, sintering, solid state, liquid state, rate controlled process, density, specific heat, thermal diffusivity, thermal conductivity, uncertainty.