Rapid Solidification of Alloys and Selective Laser Melting of Metal Matrix Composites for Abrasive Applications

Abstract

Advanced tools with superabrasive grains like diamond and cubic boron nitride are widely applied for machining high strength materials. The emerging Selective Laser Melting process has the potential to embed the superabrasives into such tools, so-called Metal Matrix Composites. Commercially available matrix alloys were developed for the conventional processes like casting, brazing which have cooling rates of several orders of magnitude lower than Selective Laser Melting. The present work focuses on developing new alloys for Metal Matrix Composites by Selective Laser Melting. Computationally assisted alloy screening is a cost-effective mean to narrow the alloy compositions of interest. To pre-screen alloys, rapid solidification experiments were carried out on the Cu-Sn-(Ti) and Ni-Cr-Si systems, which mimic the primary metallurgical process taking place in Selective Laser Melting. The results allow the understanding of the effects of cooling rates and compositions on the microstructural formation. In addition, in-situ synchrotron X-ray diffraction helps track phase transformations during the rapid solidification and the underlying mechanisms. Based on the results, a test alloy was selected for Selective Laser Melting. Sets of fundamental processing parameters were investigated to search for the processing window. Two superabrasives, i.e. diamond and cBN, were tested to study the effect of physical properties on the consolidation. It turned out that rapid solidification experiments enabled to estimate microstructure in as-built parts by observation of the same phases in the rapid solidification tests. The superabrasives are soundly bonded to the matrix by the formation of Ti carbides, borides and nitrides. Diamond content varies from 2 to 20 vol.%. Composites with 2 vol.% diamond can be free of cavities while density of defects significantly increases with diamond fractions. Using of Ti coated diamond showed limited improvement of density. However, 10 vol.% of cBN particles were successfully embedded into the final parts with far less defects. Besides, Finite Element Modelling was employed to support the understanding of the consolidation of the melt-diamond suspension. Due to the high thermal conductivity and volumetric heat capacity of diamond, abnormal heat transfer accounts for the occurrence of a large amount of defects and the poor mechanical performance.