Influence of the green density on the dewaxing behaviour of uniaxially pressed powder compacts

The effect of the green density on the dewaxing behaviour of two different uniaxially pressed iron based powder metallurgy alloys has been characterised by means of non-destructive mechanical and microstructural techniques, i.e. impulse excitation technique and synchrotron tomography. The elastic and damping properties of the green and dewaxed specimens pressed to two different densities have been evaluated. It was found that the as-pressed green specimens with lower density possessed higher stiffness and lower damping than the high density ones. This behaviour has been explained on a microstructural basis, considering the damage generated in the green specimens during the ejection step. The elastic and damping properties of the specimens after dewaxing strongly depended on their initial green density. Increased elastic properties (and lower damping) were observed for the low density specimens with respect to their green counterparts, while the opposite behaviour was observed for the specimens with higher green density (i.e. decreased stiffness and slightly higher damping after dewaxing). 'In situ' monitoring of the evolution of the elastic and damping properties of the materials has been carried out by means of the high temperature impulse excitation technique to better understand the behaviour observed. Additionally, thermogravimetric analysis has been carried out to correlate the mass loss events with the relevant features of the HT-IET curves. It was demonstrated that the damage generated in the green specimens during ejection is amplified during dewaxing for the high density compacts, while some degree of pre-sintering is responsible for the increased properties of the lower density specimens