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Thermal Conductivity of Metal Powders During De-Binding and Sintering Under Technologically Relevant Heating Rates

W. HOHENAUER, D. LAGER, I. UL MOHSIN

Abstract


The prediction of the net shape of a sintered part and the estimation of stress during the sintering process is based on the knowledge of the transient spatial densification. The transient spatial densification itself can be described with thermokinetic models of the de-binding and sintering process as well. Both processes are strongly determined from the local transient temperature history within the part. The required thermo-physical data: density ρ(Τ), specific heat cp(Τ), and thermal conductivity λ(Τ) have to be determined for a “material in progress” regarding that a part during de-binding and sintering consists of a continuously transforming material. Measurements of thermal mass, density and specific heat intrinsically capture the significant densification of the sintered material and the thermal response thereof. The use of flash methods to determine the thermal diffusivity a(Τ) requires the exact knowledge of the actual geometry of the measured sample. The shrinkage of the sample has to be taken into account to obtain sufficient diffusivity data. Hence for diffusivity measurements a temperature history regarding to the production routine is used. Transient shrinkage is calculated from a thermo-kinetic model which is established from a set of dilatometer measurements performed with geometrically stepped heating rates. Thus corrected flash data represent the thermal diffusivity correlated with the actual density and geometry of the part as well during sintering. Based on this optimized thermo-physical data, a realistic transient temperature field in the sample can be calculated. This provides a significant improvement in the through-process-modelling of any powder-metallurgical based technology. Metal injection moulding (MIM) was used to manufacture parts from three different material systems. These material systems represent solid state sintering (pure Cu) and liquid phase sintering as well (pre-mixed heavy alloy 90W-8Ni-2Cu and premixed 88Fe-12Cu respectively). The thermo-kinetic model of pure copper, it’s kinetic parameters, and the thermo-physical data ρ(Τ), cp(Τ), a(Τ) and λ(Τ) are given. Maximum correction of diffusivity data at maximum sintering temperature is near 60% relative to the corrected value

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