Orlando Scalzo

Impact of binders on viscosity of low-pressure powder injection molded Inconel 718 superalloy

Rheological behavior of powder-binder mixture has a direct impact on the successful mold filling for parts obtained from powder injection molding. In this study, the impact of binders on rheological properties of feedstocks was investigated. The experiments were conducted on several feedstocks obtained by mixing of Inconel 718 powder with wax-based binder systems. Their rheological and thermal properties were investigated using rotational rheometry and differential scanning calorimetry techniques, respectively. It was demonstrated that a large amount of ethylene vinyl acetate (EVA) should be added to paraffin wax (PW) to produce a thickening effect of the mixture. At low shear rate, the mixing of high-viscosity paraffin waxes with a low-viscosity PW produces a similar thickening effect denoted with the EVA. In addition, the feedstocks containing only PW demonstrate a pseudoplastic behavior. It was also shown that an addition of only 1 vol% of stearic acid (SA) in PW generates an important decrease in viscosity, and further increases of this constituent induce no effect on rheological behavior. From a rheological perspective, the best candidate feedstocks are the mixtures containing PW and SA while feedstocks based on PW, beeswax or containing a small amount of EVA could be also considered as good.

Segregation measurement of Inconel 718 feedstocks used in low-pressure metal injection molding

Low-pressure metal injection molding (LP-MIM) is an advanced manufacturing technology where a wax-based feedstock is injected into a complex shape before densification heat treatments. Feedstock is generally designed to minimize segregation, maximize flowability, maximize the strength of the molded component, maximize the solid loading potential and ease of debinding. In this study, the emphasis is placed on the evaluation of the effect of segregation on different wax-based Inconel 718 superalloy feedstocks used in LP-MIM. In powder metallurgy, particle or phase segregation generates a fluctuation of the particle distribution in powder-binder mixtures from point to point. Such demixing generally occurs before or during the injection process, and can lead to the formation of defects such as cracks, distortions or heterogeneous shrinkage of the sintered parts. Different wax-based feedstocks were poured in cylindrical hot molds (95°C), maintained in molten state for 1 minutes or for 60 minutes, and rapidly cooled to room temperature. The specimens were then extracted from the top and bottom regions of each cylindrical part. A thermogravimetric analysis technique was used to measure the volume fraction of powder at these two locations in order to quantify the degree of segregation in green parts. The best candidate feedstocks minimizing segregation are the mixtures containing only paraffin wax, or those containing paraffin wax and ethylene vinyl acetate combined. An increase in the time spent in the molten state and the use of beeswax or stearic acid promote the powder-binder separation of feedstocks.

Molding properties of Inconel 718 feedstocks used in low-pressure powder injection molding

The impact of binders and temperature on the rheological properties of feedstocks used in low-pressure powder injection molding was investigated. Experiments were conducted on different feedstock formulations obtained by mixing Inconel 718 powder with wax-based binder systems. The shear rate sensitivity index and the activation energy were used to study the degree of dependence of shear rate and temperature on the viscosity of the feedstocks. The injection performance of feedstocks was then evaluated using an analytical moldability model. The results indicated that the viscosity profiles of feedstocks depend significantly on the binder constituents, and the secondary binder constituents play an important role in the rheological behavior (pseudoplastic or near-Newtonian) exhibited by the feedstock formulations. Viscosity values as low as 0.06 to 2.9 Pa·s were measured at high shear rates and high temperatures. The results indicate that a feedstock containing a surfactant agent exhibits the best moldability characteristics.