In-situ formation and additive manufacturing of nano particulate reinforced metal matrix composites by aser metal deposition
Final Report Abstract
Nano particle reinforced nPRMMC can be produced via LMD on the basis of micro-scaled powders avoiding the addition of nano particles which are difficult to handle and harmful to human health. This was shown for Ni super alloys with additions of TiC. The nano particles derive from added micro particles either by partly melting and/or complete melting and resolidification in a size range of 25 —200 nm. Mixing of matrix powder and additive particles is best done by ball mixing where the small carbide particles stick on the surface of the large matrix particles. A maximum of 10 wt.-% TiC could be incorporated. Higher amounts reduced the flow ability of the powder which restricts homogeneous powder feeding. With appropriate parameters bulk volumes with only few defects can be built up. The main defects are pores with a maximum diameter of 50 pm. Adding 10 wt. % TiC also leads to some cracking near the surface of the volumes. The microstructure consists of columnar dendrites of the matrix material. The large particles are located at the grain boundaries while nano-scaled particles are also located in the dendrites. These particles exhibit a good bonding to the matrix. The amount of small particles increases with increasing energy input due to increased melting of the original particles. The particles have a slight grain refinement effect due to restricted grain growth or acting as nuclei. The tensile strength is significantly increased also at elevated temperatures while the ductility is decreased. But only samples with 10 wt.% TiC content showed extreme brittle behavior. TEM investigations revealed that the nano particles can act as obstacles for dislocation movement. The oxidation resistance at high temperatures up to 800 °C is significantly increased with the addition of the particles compared to the pure Ni alloy.
