Vladimir O. Drozdov

The Effect of TiN Nanoparticles on the Structure and Mechanical Properties of the Porous–Solid Steel Joint Formed by Laser-Beam Welding Влияние наночастиц TiN на структуру и механические свойства лазерного соединения пористой стали с монолитной

—The formation of a crystal structure and properties of a welded joint between a porous stainless steel plate and a solid plate of greater thickness with the use of nanomodifying additives has been experimentally investigated. To obtain a high-quality butt joint of the plates, the end layer of the solid plate was found to have been penetrated. The laser beam axis is shifted from the boundary of the plate joint by some distance, which is necessary to compensate the (material of) metal during the melting of the porous plate. The application of nanomodifying additives reduces the grain size of the weld structure, which, in turn, has a positive effect on the mechanical properties of the resulting joint. The results of tensile strength tests have shown that the failure of nanomodified samples, unlike unmodified ones, occurred only in the main porous metal. The average value of the tensile strength is 89.5 MPa.

One of the most promising methods of spraying protecting coating is the laser coating with strengthening phases, alloying or modifying elements. The purpose of this study is the creation of composite wear and corrosion resistant coatings by additive technology of laser coating for oil and mining industries. The possibility of obtaining complex coating with a strengthening phase is shown. Two variants of coating were considered. Variant 1: support plate made of 12Х18Н10Т stainless steel was cladded with sublayer made of Ni + Cr powder, afterwards the upper cladding layer made of Х23Н5М3ГС steel of Cr-Ni-Мn system was formed with various content of strengthening WC phase in the form of VK-12 powder. Variant 2: support plate made of carbon steel was cladded with four layers of self-fluxing cladding powder PS-12NVK-01 of Ni-Cr-Fe / WC system with addition of nanomodification of powder TiN + Y2O3 + Cr + Fe. The specialists established the dependence of microstructure, structural-phase components and micro hardness of cladded layer from the quantity of WC strengthening phase. The various content of WC strengthening phase (from 10 to 30 %) does not influence the micro hardness of the upper layer significantly – it’s average value is ≈ 700 HV The laser influence on powder mixture leads to partial deterioration of WC strengthening phase, which leads to chemical reaction of tungsten cobalt powder and stainless steеl with formation of complex intermetallic compounds and carbides. Small addition of nanosized modifier TiN + Y2O3 (0,1 % of weight) in a composite mixture with coating power PS-12NVK-01 allows increasing the hardness of protective coating by 1.4, and wear-resistance – by 2 times in comparison to the coatings with similar characteristics, obtained without nanomodifying additions.

In the paper, the influence of heat treatment on the mechanical characteristics of welds obtained by laser welding is investigated. Changes in the microstructure and strength properties depending on various modes of heat treatment of the welds are investigated. As a result, full heat treatment provides a 1.5 times increase in the tensile strength of the weld in comparison with the strength of the alloy as delivered from the factory.

The paper presents the results of numerical and experimental studies of the process of obtaining a permanent joint of two plates of heterogeneous metals that cannot be welded in the usual way: alloy Grade 4 and steel AISI 321 using a laser beam and an intermediate composite insert. The composite insert was obtained by explosion welding of four thin plates of titanium (Grade 4), niobium, copper, and steel (AISI 321). The insert was placed between the welded plates of titanium and steel, and the steel plate was welded with the steel part of the insert, and the titanium plate was welded with the titanium part of the insert. The plates were welded using a CO₂ laser. The connection of metals with the help of explosion is carried out without their melting, so the formation of the brittle intermetallics does not occur in most cases. This ensures the greatest strength of the joints as compared to the joints obtained by other welding methods. To analyze the distribution of thermal fields in the composite insert and welded plates, a numerical study was conducted of the laser welding of steel and titanium plates with the corresponding parts of the insert. The purpose of the study was to determine the rational parameters of welding (laser beam power, speed of its movement, size and position of the focal spot), at which there was no complete melting of the steel and titanium parts of the insert during through penetration of the welded plates. The experimental part of the work is devoted to analysis of formation of the internal boundaries and microstructure of the composite insert and the strength of the permanent joint. It is shown that as a result of the explosion welding, weld seams of different wavelike configuration are formed. The most pronounced wavelike boundary is observed in the steel-copper connection, since these materials have a face-centered cubic lattice and are easily subjected to plastic deformation. At the contact boundaries of the plates, transition diffusion zones with different widths (from 5 to 40 μm) and element concentrations are formed. The hardness in the boundary diffusion zones is higher than in the connected metals, which is due to the diffusion interaction of the materials adjacent to each other. It has been established that the tensile strength of the composite insert is comparable to the maximum strength of Grade 4 alloy (456–511 MPa), and the failure in most cases occurred over the least durable component of the composite material, which is the copper plate, whose strength was significantly increased by cold hardening during explosion welding and diffusion of elements of the contacting plates.

Production of welded bimetallic structures of titanium and steel using a laser beam is a very urgent and important task in the shipbuilding, airspace and power engineering. Laser welding using an intermediate insert is one of the ways to solve this problem. In this paper, we present the results of experimental studies of formation of the structure and properties of composite insert, obtained by explosion welding, after its application at laser welding steel with titanium. A study of a four-layer composite insert obtained by explosion welding showed that it has no brittle intermetallic phases and defects in the form of cracks and pores. The boundaries between the plates to be welded in the composite insert have a characteristic wavy structure with narrow zones of mutual diffusion penetration of elements of the adjacent metals. It is established that the strength of the composite insert is comparable with the maximum strength of Grade 4 alloy, and the destruction of the product during the tensile tests in most cases occurred along the weakest component of the composite insert, i.e. the copper layer, whose strength was significantly increased due to the hardening that took place in the explosion welding.

The paper concerns the possibility of obtaining a lasting permanent joint of dissimilar metals: technically pure titanium and stainless steel using laser welding and an intermediate composite insert. The insert was a four-layer composition of plates of steel, copper, niobium, and titanium welded by explosion. The material layers used in the insert prevented the molten steel and titanium from mixing, which excluded the formation of brittle intermetallic compounds, such as FeTi and Fe2Ti. The optimization of explosion welding parameters provided a high quality of the four-layer composition and the absence of defects in the area of the joint of insert plates. The results of strength tests showed that values of the ultimate strength and yield of the permanent joint with the composite insert welded by explosion are comparable to the strength characteristics of titanium.

The paper presents the experimental investigation of the creation of complex composition corrosion-resistant coatings with low abrasive wear which are used in oil industry, by the method of laser cladding additive technology. Availability of the additive laser cladding of the complex composition corrosion-resistant coatings with a strengthening phase is shown. Optimal conditions for the cladding layers creation regarding the laser radiation parameters and radiation direction are found. The changes in micro-hardness, micro-structure and structure-phase components are established against the strengthening phase content in the upper clad layer. Different content of the strengthening phase in the upper layer from 10% to 30% does not change the micro-hardness of the upper layer and is on the average 700 HV. The laser irradiation of the powder mixture containing the strengthening phase results in the partial rupture of the WC strengthening phase. Hence the chemical interaction of the tungsten-cobalt powder mixture and stainless steel occurs and results in complex carbide formation.