V. Ocelík

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).

SiCp/Ti6Al4V functionally graded materials produced by laser melt injection

Abstract With a well-controlled laser melt injection (LMI) process, for the first time the feasibility is demonstrated to produce SiC particles (SiCp) reinforced Ti6Al4V functionally graded materials (FGMs). SiCp are injected just behind the laser beam into the extended part of the laser melt pool that is formed at relatively high beam scanning velocities. The process allows for the minimization of the decomposition reaction between SiCp and Ti6Al4V melt, and also leads to FGMs of SiCp/Ti6Al4V instead of a homogeneous composite layer on Ti6Al4V substrates. An injection model is designed based on the temperature/viscosity field of the laser pool for a deeper understanding of the mechanism of formation of the FGMs with LMI. The model is based on finite element calculations of the temperature field in the melt pool, physical considerations of the LMI process and it is supported by experimental observations. Three types of reaction layers are observed around SiCp, namely a thin monocrystalline TiC layer, a cellular polycrystalline TiC layer and a thick mixed layer of TiC with Ti5Si3. Among them, only the monocrystalline TiC layer exhibits particular orientation relationships (ORs) to the SiCp lattice, i.e. (111)TiC∥(0001)SiC and 〈110〉 TiC ∥ 〈1 1 00〉 SiC or ( 111 ) TiC ∥( 10 1 2 ) SiC and 〈1 1 0〉 TiC ∥ 〈1 2 10〉 SiC . These two kinds of TiC reaction layers act as a barrier against the interfacial reaction and its swift formation during rapid cooling hinders the dissolution of SiCp in the Ti-melt.

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.

Microstructure and Properties of TiB/Ti-6Al-4V Coatings Produced With Laser Treatments

TiB/Ti-6Al-4V metal-matrix composite (MMC) layers were produced on Ti-6Al-4V substrates by laser cladding. A TiB2/Ti powder mixture was used as a precursor to obtain a dispersion of TiB needles in the Ti alloy matrix, with the aid of an exothermic reaction between TiB2 and Ti. A eutectic microstructure was obtained that consisted of an extremely homogeneous dispersion of TiB eutectic needles in the Ti alloy matrix, having a volume fraction as high as 0.33. Also, an equilibrium-like microstructure was found, consisting of a dispersion of both primary and eutectic TiB needles inside the Ti alloy matrix. An analysis of the geometry of the layers was performed and proved successful in determining the percentage of B. Further, it correctly predicted the variation of atomic B content as a function of laser power. The relative wear resistance coefficient, defined as the wear coefficient of the uncoated matrix divided by that of coating, shows an improvement by a factor as high as 1500 for the eutectic microstructure.

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.

Microstructural characterization of Co-based coating deposited by low power pulse laser cladding

A detailed microstructural study of Stellite 6 coating deposited on a low carbon ferritic steel substrate using preplaced powder method and low power Nd:YAG pulse laser is performed. The grain structure and solidification texture of the coating are investigated by orientation imaging microscopy (OIM) and scanning electron microscopy. In addition, the effect of consecutive pulses on the microstructure of the coating is examined. The orientation relationship (OR) at coating/substrate interface and the solid state phase transformation in heat-affected zone are studied as well as the Vickers microhardness profile measurement in order to support the microstructural observations. An important conclusion is reached that the shape of solidification front during pulsed laser cladding is similar to the shape of solidification front during continuous cladding with a doubled laser beam scanning speed. Further, OIM reveals the Greninger–Troiano OR between the face centered cubic coating and bcc substrate grains. It is concluded that at the moment of solidification epitaxial growth of the grains in the coating occur on the austenitic grains of the substrate and that an austenite–ferrite transformation occurs in the heat-affected zone upon subsequent cooling.

Electron Microscopy Characterization of Ni-Cr-B-Si-C Laser Deposited Coatings

During laser deposition of Ni-Cr-B-Si-C alloys with high amounts of Cr and B, various microstructures and phases can be generated from the same chemical composition that results in heterogeneous properties in the clad layer. In this study, the microstructure and phase constitution of a high-alloy Ni-Cr-B-Si-C coating deposited by laser cladding were analyzed by a combination of several microscopy characterization techniques including scanning electron microscopy in secondary and backscatter imaging modes, energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The combination of EDS and EBSD allowed unequivocal identification of micron-sized precipitates as polycrystalline orthorhombic CrB, single crystal tetragonal Cr5B3, and single crystal hexagonal Cr7C3. In addition, TEM characterization showed various equilibrium and metastable Ni-B, Ni-Si, and Ni-Si-B eutectic products in the alloy matrix. The findings of this study can be used to explain the phase formation reactions and to tune the microstructure of Ni-Cr-B-Si-C coatings to obtain the desired properties.

Fundamental and applied aspects of laser surface engineering

Abstract The contribution to this jubilee issue of IJMR concentrates on the analysis of laser surface treatments. In particular laser cladding processes using coaxial and side set-ups are evaluated to determine the optimal processing window that prioritizes the clad quality and the efficiency of the coating method. The microstructural features in these so-called processing maps are illustrated with Inverse Pole Figure (IPF) mapping, texture pole figures and Electron Backscatter Pattern (EBSP) techniques. The distribution of stresses inside Co-based laser coating is measured with the so-called three-dimensional X-ray Diffraction microscopy (3DXRD) and it is found very dispersive, i. e. stresses may change from one grain to another considerably. The mean value of observed hydrostatic stress in all grains at specific laser track depth is gradually changed from tensile state in the upper part of the coating to the compressive state in its lower part close to the substrate. On the other hand the average value of shear stress component does not show any substantial change with depth.

Scratch test induced shear banding in high power laser remelted metallic glass layers

Laser remelted surface layers of a Cu-based metallic glass forming alloy have been produced with fully amorphous depths up to 350 mu m for single track widths of around 1.3 mm and have been checked by transmission of synchrotron radiation. They have been subjected to indentation hardness and scratch testing, and the development of shear bands in both situations has been addressed. During the cross-sectional hardness indentation tests, Vickers values of over 735 HV2 have been found through the depth of the treated layer, and the scratch testing has revealed extremely low friction coefficient values (<0.02 at 10 N in single-pass and 0.02 at 18 N multi-pass regimes against a diamond stylus). The shear band formation has been related to both scratch test speed (strain rate) and load (contact stress) by methods such as atomic force microscopy measurements and subsequent Surface roughness characterization by a height-height correlation function.

High energy density processing of a free form Nickel-alumina nanocomposite.

Nanosized alumina (Al2O3) powders had been successfully fabricated by a simple polymer solution route employing polyvinyl alcohol (PVA) as an organic carrier. The fabricated alumina powders had an average particle size of 6.1 nm with a high specific surface area of 99.5 m2/g. As well, the alumina powders were fully crystallized to alpha phase at a relatively low temperature of 1000 degrees C. The PVA polymer contributed to a soft and porous microstructure of the calcined alumina powders, and ball-milling process with the porous powders was effective in making nanosized alumina powders. In addition, the content and degree of polymerization of the PVA affected the development of crystallization and powder properties. In this study, the simple polymer technique and milling process for the fabrication of nanosized alumina powders are introduced, and the effects of PVA on the property of the synthesized alumina powders are observed. For the study, the characterizations of the synthesized powders are conducted by using XRD, TEM, particle size analyzer, and nitrogen gas adsorption.