Stainless steels have played an important role in the development of MIM technology,and for this reason,many of the theoretical studies regarding the technological parameters were development with Stainless steels.The reason for this was the larger range of application for this family of materials,which was more than any other material in the period that MIM was being developed.
All of the steps of the process were investigated,including the influence of binder components and the rheological parameters.Of particular interst were those works that studied the effect of surface additives on the interaction with the other binder components and the rheology of the feedstock.Thanks to those kinds of studies,the use of surface agents like stearic acid has been widely adopted in numerous to understand feedstock formulations.
An important set of studies was performed to understand the influence of the morphology of powders and the sizes and granulometric distribution .The morphology of the particles is an important factor that influences the rheology,so the availability in the market of water-atomised and gas-atomised powders of different sizes and distributions allows study of this important factor.
Using similar sizes,the results obtained for both kinds of powders were very similar ,although they were slightly better for gas-atomised powders.One possible advantage of using the water-atomised powders is that,when mixed with gas-atomised powders,the shape relation during sintering and dimensional control can be improved.Also,water-atomised powders are cheaper than their equivalent grades gas-atomised.
Even more important to the rheology than the morphology is the size of the particles.By using different gas-atomised powders with different sizes,it was found that up to an averange of 45um,there is no important change in the properties ,but with larger particles sizes,the packing features can be improved.Large particles can produce some negative effects,like higher sintering temperature,galling in the screw check ring of molding,worse surface finish and worse shape retention ,but with the benefit of lower cost.Most of the studies were performed with the most-used powders,but there are some works related to phase-hardened steels and special processes such as MIM.Powder loading is another important factor affecting rheological behaviour and technological parameters.