Laser melt injection of hard ceramic particles into Al and Ti alloys ― processing, microstructure and mechanical behaviour

摘要

The conditions for a successful Laser Melt Injection (LMI) of SiC and WC particles into the melt pool of Al8Si and Ti6Al4V alloys were studied experimentally and theoretically by FEM calculations. The laser employed is a high power Nd:YAG. The formation of a relatively thick aluminium oxide layer on the Al melt surface was confirmed during in-situ melting in an Environmental Scanning Electron Microscope (ESEM) at temperatures above 900℃. This oxide layer acts as a barrier for particle penetration but it can be dissolved in the melt at temperatures above 1250℃ and consequently it opens a "window" for particle injection. The finite element analysis of the laser melting process of Al-Si alloy leads to the conclusions that the laser scan velocity has only a small influence on the substrate temperature distribution in the vicinity of the laser beam and that the size of extended part of laser melt pool (which is the best place for injection) is extremely small. Therefore, in contrast to Ti-alloys an extension of a melt pool size behind the laser beam, which serves as an effective instrument for successful LMI of ceramic particles, is not a successful processing route in the case of Al alloys. The relationship between microstructure, tensile and wear properties has been investigated for SiC/Al-Si and WC/Ti-Al-V metal matrix composite layers. Although the presence of hard and brittle phases formed during the laser processing decreases the tensile strength in comparison with substrate materials, a massive improvement of wear resistance of both layers is confirmed. As in-situ (ESEM) tensile tests show the crack-initiation process in the WC/Ti-Al-V layer strongly depends on the type of WC powder used in the laser melt injection process.