J.A. Vreeling

Ti–6Al–4V strengthened by laser melt injection of WCp particles

The laser melt injection (LMI) process has been explored to create a metal–matrix composite consisting of 80 µm sized WC particles embedded in a Ti–6Al–4V alloy. In particular the influences of the processing parameters, e.g. power density, scanning speed and powder flow rate, on the dimensions and microstructure of the laser track have been examined. The microstructure was investigated by advanced transmission electron microscopy including energy filtering techniques and scanning electron microscopy with an integrated electron back-scatter diffraction/orientation imaging microscopy. Typical dimensions of a single laser track are a width of 1.8 mm and a depth of 0.7 mm. The volume fraction of the WC particles is about 0.25–0.30. An important finding is that the particle distribution is homogeneous and that the particles are injected over the whole depth and whole width of the melt pool. Only occasionally a crystal orientation relation between WC, W2C and TiC is observed. A substantial increase in wear resistance was observed, i.e. 0.5 × 10–6mm3/Nm for the WCp laser embedded and 269 × 10–6mm3/Nm for the untreated Ti–6Al–4V alloy at the same contact stress (20 MPa).

LASER MELT INJECTION IN ALUMINUM ALLOYS: ON THE ROLE OF THE OXIDE SKIN

In this paper the method of laser melt injection of SiC particles into an aluminum substrate is investigated both experimentally and theoretically. An extremely small operational parameter window was found for successful injection processing. It is shown that the final injection depth of the particles is controlled mainly by the temperature of the melt pool rather than by the particle velocity. A theoretical model that takes into account the wetting behavior and the particle penetration processes is developed on the basis of the observed particle velocity, thickness and area fraction of oxide skin that partially covers the surface of the heated aluminum melt pool. The model reveals the role of the oxide skin: it is relatively strong at low temperature and acts as a severe barrier for the injection process. It was found that preheating the aluminum substrate results in a higher temperature of the melt pool and partial dissolution of the oxide skin, through which the injected particles are able to penetrate.

In-situ microscopy investigation of failure mechanisms in Al/SiCp metal matrix composite produced by laser embedding

Laser surface treatments are suitable techniques for improving the mechanical, tribological and chemical properties of metal surfaces. In the laser injection process the laser beam interacts primarily with the substrate and to a lesser extent with the particles, which are simultaneously injected into the melt pool produced by the absorbed laser power density. Metal Matrix composite (MMC) layers with interesting properties and very good connection to the metal substrate may be prepared by a selection of suitable combination of metallic substrate and ceramic particles. In this study Al/SiC MMC layers prepared by laser injection of SiC particles into Al substrate were mechanically tested to reveal the weakest structural components from a mechanical point of view. The objective is to obtain accurate information about the initiation and propagation of cracks in such heterogeneous structures by microscopic inspections of the sample surface during the loading. A combination of techniques was used to characterize the nucleation and progress of fracture: i.e., in-situ microscopy observations during testing and by conventional fractographic methods after failure.

EBSP study of reaction zone in SiC/Al metal matrix composite prepared by laser melt injection

The reaction zone in SiC/Al metal matrix composite layer prepared by Laser Melt Injection process is studied by Electron Back-Scatter Diffraction. Special attention is dedicated to the sample preparation process and also to the automatic indexing procedure when patterns of back-scattered electrons are evaluated during the surface scanning by electron beam. The orientation relationship between ceramic particles and phase formed in the reaction zone was observed by both transmission electron microscopy and by EBSD.

Failure of WCp/Ti-6Al-4V layer prepared by laser melt injection

Metal Matrix Composite layers consisting of WC particles (similar to 80 pin diameter) incorporated into a Ti-alloy matrix by the so-called Laser Melt Injection process were mechanically tested. Standard tensile tests as well as in-situ scanning electron microscope observations of tensile stressed surfaces were performed. Crack initiation and crack propagation processes were observed at initial and final stages of failure. Cracks initiate always in the ceramic particles. Intergranular brittle fracture of the central part of the WC particles or brittle decohesion along the WC/W2C interface form the initial failure. The crack propagates further inside the Ti matrix through brittle fracture of individual TiC dendrites and induces new cracks in the ceramic particles in the front of the main crack tip, Ductile fracture of the metal matrix, that creates the resulting fracture surface, is the final stage of the failure process. Internal tensile stresses formed during the laser processing are superimposed on the external tensile stress, which causes that local failure of the MMC layer starts at a relatively low external stress. The toughness of the Ti matrix makes the final failure process ductile.

Reinforced SiC/Al composite layer produced by laser particle injection

SiC particles with a mean size of 80 mu m were injected into Al substrate:by the laser particle injection process with the aim to improve the surface properties of aluminium. Experimental difficulties induced by the big difference in absorptivity of laser energy between Al and SiC lead to an extremely small operational window of processing parameters. A combination of parameters of Nd:YAG laser beam, SiC powder stream and Al substrate pre-heating temperature which leads to the formation of single laser tracks was found experimentally. The injection depth is discussed from the point of view of a simple injection model. Optical, scanning and (high-resolution) transmission electron microscopy were used to study the microstructure of the produced particle reinforced metal matrix composite. Partial thermal decomposition of SiC and reaction with liquid Al lead to the formation of new phases presented in the laser tracks, detected as Al4C3 and elemental Si.