Preparation and Microstructural Characterisation of Metal Injection Moulded Powder Metallurgical Components (MIM) in the Green and Sintered Conditions

Abstract

Summary The aim of this work was to develop a reproducible method of preparation for green and brown bodies which, after injection moulding and sintering, would permit an entire MIM component to be microscopically examined for internal inhomogeneities such as porosity and segregation. The particular challenge was to prepare the fragile MIM microstructure prior to the sintering process free of artefacts. Three different methods of preparation (variations in the polishing substrates and suspensions used) were compared. To evaluate the quality of the resulting preparation, the microscopically determined volume fraction of metal particles was chosen. This was determined after set preparation times using quantitative microstructural analysis (QMA/QGA) and evaluated during the various stages and times of the preparation process. The feedstock composition which had a defined volume fraction of 68 % was used as the reference value. The preparation of the brown sintered body proved to be the most successful. The volume fraction determined microscopically measured 67 %. The duroplastic infiltration mounting resin penetrates into the open pore structure effectively stabilising and fixing the powder particles. In contrast the preparation of the green body proved to be much more difficult as the breaking out of the metal powder particles from the comparatively soft thermo-plastic binder was not able to be prevented completely. The volume fraction measured microscopically was 62 % which is about 6 % less than the actual value. The chemical removal of the binder from the surface of the green body significantly improved the overall result of the preparation as it was then possible to effectively fix the powder particles by subsequent infiltration with the duroplastic mounting resin, as in the case of the brown body. In addition to the qualitative appraisal of the microstructural specimens, quantitative microstructural analysis proved to be a valuable instrument for systematically and objectively evaluating the results of the preparation and for developing the preparation processes. Using the methods of preparation developed it is now possible to carry out qualitative examinations on almost artefact free microstructural specimens prior to the lengthy and costly thermal processing.