Papers by Author: Fjodor Sergejev

Abstract: Friction stir welding (FSW) is employed primarily for metals characterized by poor weldability at fusion welding: aluminium, magnesium, titanium and copper alloys as well as stainless steels. The focus of the study was on the feasibility of application of WC-based hardmetal 85WC-Co and TiC-based cermet 80TiC-NiMo as potential tool materials for FSW of copper. The single-pass welding trials of Cu sheets were performed using a vertical milling machine. For better understanding of interactions between the tool and workpiece at welding temperature EDS line scans across the interfaces tool-workpiece after welding as well as after diffusion tests were performed. It was concluded that both tested ceramic-metal composites did not failure during multiple plunges and during the total transverse welding distance of 10 m. Also, significant tool wear was not observed after such a welding distance. The possibility of producing visually defect-free welds using tools from WC- and TiC- based ceramic-metal composites was proved and also mutual diffusion of elements across the interface tool-workpiece was discussed.

Abstract: Friction stir welding is a novel and promising joining process and most common welding tool failure is transformation of geometry caused by wear. In our point of view, this is adhesion wear. The lathe testing of adhesion wear was conducted to compare wear resistance of following materials: Co-25wt%Cr, TiC-25wt%Ni/Mo and WC-6wt%Co. According to characteristics of investigated materials, they are capable to become alternatives for conventional frictional stir welding (FSW) tool materials. Adhesive wear tests were performed by turning aluminium alloy AW6082-T6 at low speed – travel length, turning speed and feed rate were selected to simulate FSW conditions. The adhesive wear was determined as the change of the geometry of the cutting edges of the tool measured using SEM images. Most promising tool material in terms of adhesion wear resistance is WC-Co hardmetal. The two main stages of wear were distinguished: at first, the appearance of intensive adhesion wear followed by steady state wear. Surface fatigue wear complements development of the adhesive wear.

Abstract: In this work the influence of sintering method on the surface fatigue of carbide composite was studied. The research focuses on WC-15wt%Co hardmetals prepared using different powders and different sintering techniques and as a result different microstructure: conventional WC+Co powder and novel reactive powder type W+C+Co are sintered using vacuum furnace, compression sintering (sinterhipping) and spark plasma sintering (SPS method). As tungsten carbide is a common material for cold forming punches and surface degradation causes punch failure [1], the tool life can be significantly extended by material surface fatigue life improvement. It is expected that SPS production route of WC-15wt%Co hardmetal will conclude in better microstructure, more even average grain size distribution and smaller residual porosity, and respectively better mechanical and surface fatigue properties compared to conventional production routes. There are some expectations related to the reactive sintering production routes, as this technique promotes the fine microstructure and better mechanical properties.

Abstract: Physical Vapour Deposited (PVD) coatings are used in wide range of industrial applications where requirements differ. For example, in cutting applications adhesive-abrasive wear along with high contact stresses prevail and PVD coating with thickness of ~2 μm are used. In forming applications adhesive wear usually dominates and relatively thick PVD coatings (~5 μm) are preferred. For both the applications coatings are subjected to cyclic stress and therefore it is a point of interest to learn the behaviour of PVD coatings with different thickness under cyclic loading. Cracking resistance and fatigue properties of gradient TiCN on hard metal substrate was evaluated by means of the cyclic Vickers indentation method. Hard metal was chosen as a substrate material to avoid pile-up effect and support the hard coating during indentation. The results of the single indentation Vickers test show that secondary radial and circumferential cracks appear in tested coatings already after the first indent. With increasing cycles the cracks grow up to a critical crack length after which the crack length doesn’t increase further. The tested coating thickness has no significant effect on cracking behaviour.

Abstract: The investigated brush-plated gold and silver coatings are used for repairing the commutators of generators and sliding contacts. Tensile residual stresses generated in the plated coatings were determined by the curvature method and by instrumented indentation testing of a thin-walled open ring substrate, as described in our earlier papers. These stresses relax over time and their dependence on relaxation time was approximated by a linear-fractional function. The Young ́s modulus and nanohardness of the coatings were determined. The surface structure and cross section of the coated substrates were studied.

Abstract: The investigated brush-plated silver and gold coatings are used for refining the surface properties of electric apparatuses. Tensile residual stresses generated in the plated coatings were determined with a thin-walled ring substrate using the curvature and instrumented indentation techniques. These stresses relax over time; the dependence of relaxation time was approximated by a linear-fractional function. The modulus of elasticity and the nanohardness of the coatings were determined by nanoindentation. The surface morphology and structure in cross-section of the coated substrates are presented.

Abstract: In the present article, the laser hardening of the carbon steel C45, previously coated by the physical vapour deposition (PVD) process, is studied. The (Al,Ti)N-G and nACo® (nc-Al_xTi_1-xN/α-Si₃N₄) coatings were applied. Nd:YAG laser with the laser beam power density of 1945 W/cm² and scan speed of 300 mm/min was used for hardening process. Laser hardening lead to the formation of hardened layer under both coatings, consisting of austenite and ferrite. The approximate depth of the hardened layer and maximal microhardness was approximately 0.2 mm and 955 HV0.05 and 0.1 mm and 520 HV0.05 in the case of the (Al,Ti)N-G and the nACo® coating, respectively. After laser hardening the sliding wear of the (Al,Ti)N-G coating decreased by 1.25 times and of the nACo® coating by 1.05 times.

Abstract: Nickel-hardened gold and silver coatings were brush-plated from a commercial SIFCO Dalic Solution (Gold Hard Alloy), Code SPS 5370, and Silver Hard Heavy Build, Code SPS 3080, on unclosed thin-walled copper ring substrates. To determine residual stresses, the conventional curvature method (common among the electrodeposition methods) was applied, where the substrate was coated with certain thickness and then the slit increment (bending deflection) of the substrate was measured as an experimental parameter. Residual stresses on gold coatings were also determined by X-ray diffraction (XRD) based on the sin2 method. The values of residual stresses determined by the curvature method and by the XRD technique were comparable. Relaxation of residual stresses was observed. An equation for approximation of the change of residual stresses was applied assuming that the dependence of residual stresses on relaxation time is linear-fractional. The surface morphology and microstructure of the coatings was studied by means of scanning electron microscopy (SEM). The magnitudes of the modulus of elasticity and of the nanohardness of the coatings were obtained by instrumented indentation.

Abstract: Aluminium 6061 has proven to be a suitable alloy as a basis for producing metal matrix composites (MMC). These MMCs have a low specific weight combined with a relatively good specific stiffness and high specific strength. The hardness and compressive strength of Al composites can be increased by reinforcing bulk material with nano particles. However the ductility of such alloys is relatively low, therefore one of the applications for such light alloys could be wear applications. In many wear conditions such as erosive or abrasive wear at normal impact angles the surface wear resistance plays a significant role. The surface fatigue properties have not been widely studied for such nanoparticle reinforced aluminium composites. The nano-reinforced composite materials were produced by means of high-energy milling (HEM) of nano-sized reinforcement particles together with a metallic matrix powder, followed by hot pressing. By utilizing up to 6 wt% multiwalled carbon nanotubes (MWCNT) as reinforcement the hardness of Al6061 MMC has been increased from 45 HV10 up to 317 HV10, compressive yield strength from 58 MPa up to 660 MPa and indentation modulus from 60 GPa up to 90 GPa compared to hot pressed Al6061. Surface fatigue tests were conducted at impact (dynamic) loading conditions using a hardened steel sphere as indenter. The Wöhler-like curves are plotted to estimate the surface fatigue. The surface fatigue indents were photographed by the aid of light optical microscopy (LOM) and analysed by image analysis software and optical profilometry (OP).

Abstract: Current paper handles the comparison of impact wear and sliding wear properties of the hard PVD single layer TiN and Ti(C,N), multilayer (Ti,Al)N and nanocomposite FiVIc® coatings on the plasma nitrided low-alloy 42CrMo4 steel. All the studied coatings demonstrated a relatively high impact wear resistance at the low (104) and medium (105) number of impacts, however, all the studied coatings vanished at the high number of impacts (106). Most extensive wear among the coatings during the sliding wear test was observed for the (Ti,Al)N coating, the FiVIc® showed the least extensive wear; the most extensive wear of the counterbody (hardened steel ball) was registered for the (Ti,Al)N coating, the lowest – for the FiVIc® and Ti(C,N) coatings. The principle wear mechanism of coatings was tribooxidation and mild abrasion, of the counterbody – plastic deformation