Thermal diffusivity of materials during sintering process
W. Hohenauer, D. Lager and I. Ul Mohsin
Both, the prediction of the net shape of 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 a thermo-kinetic model of the sintering process which is strongly determined from the local transient temperature history within the sintered part. The required thermophysical data: density ρ, specific heat cp, and thermal conductivity λ have to be determined for a “material in progress” regarding that a sintering material is a continuously transforming material. The measurement of thermal 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 requires the exact knowledge of the actual thickness 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 kinetic model which is established from a set of dilatometer measurements performed with geometrically stepped heating rates. Thus obtained corrected data representing thermal diffusivity correlated with the actual density of the part during sintering. Based on this optimized thermophysical data, a realistic transient temperature field can be calculated. This provides a significant improvement in the through process- modelling of any powder-metallurgical based technology. Maximum correction of diffusivity data at maximum sintering temperature was about 50% referred to the corrected value.
Keywords: Thermal diffusivity, sintering.