Dispersion strengthened metal matrix composites in additive manufacturing

Abstract

In this work, metal matrix composites and their manufacturing process by Direct Metal Laser Sintering (DMLS) have been investigated. DMLS is an additive manufacturing method in which raw material in form of powder is melted by a scanning laser beam layer by layer to form a solid part directly from 3D-CAD model. The matrix material in the composites studied was EOS StainlessSteel 316L powder with chemical properties according to AISI 316L high chromium stainless steel standard. Two different types of titanium carbide (TiC) powder were used as reinforcements, both of them of the micrometric size scale. Both of the reinforcements were added to the matrix metal powder in three different concentrations and mixed by mechanical mixing. The mixing homogeneity of the composite powders was investigated by visual inspection and field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis. Solid parts of all the mixtures were manufactured using EOS M100 DMLS system with standard EOS StainlessSteel 316L parameters. Mechanical properties of the composite materials were tested with hardness-, impact-, tensile- and wear testing and their corrosion resistance properties were inspected by salt water immersion and electrochemical testing. Structural characterization was performed of the samples using optical microscopy, density measurements and FE-SEM + EDS analysis. Wear test surfaces and fracture surfaces of selected samples from impact and tensile testing were also characterized by FE-SEM + EDS. The tests concluded that the tensile strength, wear resistance and hardness of the 316L steel were significantly improved by the TiC particle addition. The particle addition, on the other hand, decreased ductility and impact toughness of the base metal. The reinforcement concentration had nearly linear effect on all of these properties. In the immersion tests, the composite samples showed no different corrosion behavior than the pure 316L steel. The electrochemical tests showed similar potential levels initiating corrosion in the composites and the base metal, while the particles affected the composites’ ability to recover from corrosion damage. In structural characterization minimal reaction between the matrix and the particles were observed. The results of the work are very promising, as increases in mechanical properties were greater than those documented in most previous studies with similar composites. The properties are partly due to the DMLS manufactured parts’ unusually fine microstructure, which is not achieved with conventional manufacturing methods.