Investigation on the physical properties of magnesium feedstocks for metal injection moulding

Abstract

Magnesium and its alloy are found to be extremely biocompatible and have similar properties to natural bone. This makes them an attractive material for the manufacture of biodegradable parts such as biomedical implant. As biomedical implants are rather small and complex in shape, the metal injection moulding (MIM) technique seems to be well suited for the near net shape mass production of such parts. This research investigated the physical properties of the magnesium feedstocks for metal injection moulding process. The detail study on the feedstocks behavior was conducted including critical loading determination, mixing kinetics, rheology study, green molded properties, solvent debinding process, and sintering. The binder used in this study were paraffin wax (PW), palm stearin (PS), stearic acid (SA), zinc stearate (ZS), high density polyethylene (HDPE), waste rubber (WR), and waste plastic (WP). The critical powder volume concentration (CPVC) of Mg powder was conducted using oil absorption test at room temperature. In this study, the rheological properties and behaviors of magnesium metal injection moulding feedstock was investigated using capillary rheometry. After injection moulding, the density, strength, and morphology of the green molded part was investigated using Archimedes density concept, 3 point bending test, and scanning electron micrograph, respectively. The effect of the leaching time and temperature on the solvent debinding process of Mg metal injection moulding (MIM) green part has been investigated. In this study, both soluble binder, paraffin wax and stearic acid molecules were removed from the Mg green part by immersing compact parts in heptane solution. Then, the solvent debinding rate has been investigated under the conditions of different leaching time, temperature, and surface area to volume ratio. The weight loss percentages of paraffin wax and stearic acid were calculated and the pores structure was analyzed by scanning electron micrograph. The effective diffusivity and activation energy of the soluble binder have also been studied. Sintering process has been carried out in 2 cycles which are debinding cycle and sintering cycle. For debinding cycle, the temperature was 450 °C with heating rate 1K/min and soaked for 1 hour. While the sintering cycle was set at 640 °C with heating rate of 5K/min and sintered for 8 hours. The result obtained for CPVC was 69 vol.% and the optimum powder loading was at 65 vol.%. The rheological results exhibited the pseudoplatic behavior and suggested feedstocks containing PWPEWPSA in the binder system as the best feedstocks. The optimum temperature was 60 ºC with immersion time of 360 minutes. Effective diffusivity was higher at dissolution stage as compared to diffusion stage. The dissolution activation energy (Q) was about 3-5 times higher than diffusion activation energy. The density of obtained sintered part was 1.134 g/cm3.