Interfacial adhesion of laser clad functionally graded materials

Specially designed samples of laser clad AlSi40 functionally graded materials (FGM) are made for evaluating the interfacial adhesion. To obtain the interfacial bond strength notches are made right at the interface of the FGMs. In-situ microstructural observations during straining in a field-emission gun environmental scanning electron microscopy reveal different failure modes of the FGMs and substrate. Mapping of strain fields using digital imaging correlation shows a gradual transition of deformation over the interface region and softening effects in the heat-affected zones of the FGM tracks. The strengthening of the FGM is dominated by the size of the Al halos around the particles, in accordance with a dislocation pile-up model.

Toughening mechanism for Ni-Cr-B-Si-C laser deposited coatings

Laser deposited coatings were made from Colmonoy 69 Ni-Cr-B-Si-C alloy and Nb-modified Colmonoy 69 using laser cladding with powder injection. Addition of Nb was done to decrease the structural scale of Cr boride precipitates by providing Nb-rich nucleation agents. The purpose of the study was to evaluate the viability of microstructural refinement as a toughening mechanism for Ni-Cr-B-Si-C alloys. The results show that although a significant refinement of the Cr-rich precipitates while preserving the original level of hardness could be induced in these alloys by a suitable addition of Nb, cracking susceptibility of the deposits was not decreased. This is attributed to the continuous network of hard eutectics providing an easy route for crack growth. The outcome of this work points out that an effective toughening mechanism for Ni-Cr-B-Si-C alloys should include not only refinement of the hard precipitates, but also modification of the eutectic structure. (C) 2013 Elsevier B.V. All rights reserved.</p

Microstresses and microstructure in thick cobalt-based laser deposited coatings

Microstresses in a thick laser clad Co-based coating on steel substrate were investigated with 3D X-ray microscopy using an intense synchrotron microfocused beam. The microstructure was examined with tight microscopy and field emission scanning electron microscopy equipped with X-ray energy dispersive spectroscopy and Electron Back Scattering Diffraction (orientation imaging microscopy). Microhardness and scratch resistance variations inside the coating are related to the local microstructure influenced by additional heating and by melt convection during the laser track overlapping. The residual microstrains were accessed with a high spatial resolution defined by the size of the synchrotron microbeam. Type 11 residual strains and stresses on the level of individual grains and dendrites were analyzed in terms of tensor invariants, hydrostatic and von Mises shear stress, along the depth of a slightly diluted clad track. The upper part of the coating shows a constant spread of hydrostatic stresses between -500 and 500 MPa; towards the bottom of the track the spread of these stresses increases almost linearly with depth. A correlation between the microstructural features and the spread of the hydrostatic microstresses was found. It is concluded that microstresses in individual neighboring grains are inhomogeneously dispersed. (c) 2007 Elsevier B.V. All rights reserved

Microscopy study of ripples created on steel surface by use of ultra short laser pulses

This paper concentrates on ripples on the surface of steel that arise from lasermaterial interaction. In particular we have observed two different sets of ripples on steel samples that were machined by 210 fs laser pulses with 800 nm wavelength at normal incidence. Small ripples were found with spacing of about 250 nm lying longitudinal to the vector of laser beam polarization. Big ripples exhibited at a much larger distance of about 500 nm and they are perpendicular to the polarization vector. The laser treated surfaces were investigated with\ud Scanning Electron, Confocal and Atomic Force Microscopy. The laser-material interaction could be divided into three subsequent steps: absorption of laser light via electron gas excitations, transfer of heat into the lattice followed by a thermal expansion of material. From our microscopic observations it is concluded that the small ripples are formed by solidification of liquid material present as a thin layer near the interface of solid bulk material

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

Thick Co-based coating on cast iron by side laser cladding:Analysis of processing conditions and coating properties

The objective of this work was to create Co-based coatings (compositionally close to Stellite 6) on compacted graphite and gray cast iron,substrates with a high power laser (2 kW continuous Nd:YAG) cladding process. The relationships between the relevant laser cladding parameters (i.e. laser beam scanning speed, laser power and powder feeding rate) and the main geometrical characteristics of a single laser track (height, width, dilution, etc.) were examined. A gradual variation of a single processing parameter was used for an appropriate experimental analysis and statistical correlations study between main processing parameters and geometrical characteristics of an individual laser track. These relations lead to the design of a laser cladding processing map that can be used as a guideline for the selection and further tuning of proper processing parameters for laser cladding of extensive layer. The coatings with thickness from 1.0 to 3.3 mm were created on flat substrates without cracks and other major defects. The nucrostructural features of these coatings were studied using optical microscopy, scanning electron microscopy (Philips XL30 FEG), EDS (EDAX) and XRD. Mechanical properties were determined using microhardness measurement, scratch test (CSM Revetest) analysis at room temperature and using the tribotesting (CSM HT Tribometer) at room and elevated (up to 525 degrees C) temperatures. (c) 2006 Elsevier B.V. All rights reserved

Creep-induced structural changes in Ni-Si-B amorphous alloy

The influence of the stress annealing on the reversible structural relaxation of a Ni-Si-B amorphous ribbon was studied. Creep-induced structural changes in the amorphous structure were derived from anisothermal DSC and dilatometric experiments. It is demonstrated that considerable enthalpy and specimen length variations associated with the reversible structural relaxation are observed after previous creep at higher temperatures.Исследовано влияние отжига под напряжением на обратимую структурную релаксацию полоски аморфного сплава Ni-Si-B. Изменения аморфной структуры, вызванные ползучестью, исследовались экспериментально с использованием дифференциальной сканирующей калориметрии в неизотермических условиях и дилатометрии. Показано, что значительные изменения энтальпии и длины, связанные с обратимой структурной релаксацией, наблюдаются после предварительной ползучести при повышенной температуре

The fcc-bcc crystallographic orientation relationship in AlxCoCrFeNi high-entropy alloys

This paper concentrates on the crystallographic-orientation relationship between the various phases in the Al-Co-Cr-Fe-Ni high-entropy alloys. Two types of orientation relationships of bcc phases (some with ordered B2 structures) and fcc matrix were observed in Al0.5CoCrFeNi and Al0.7CoCrFeNi alloys at room temperature: (1 -1 0)(bcc)//(200)(fcc), [CM](bcc)//[001](fcc), (b) (1 -1 1)B2//(2 - 2 0)(fcc), [011]B2//[11 root 2](fcc). (C) 2016 Elsevier B.V. All rights reserved.</p