Selected Properties of High Entropy Alloys Based on the AlFeMnNbNiTi System

The aim of this work was to study the impact of various fabrication methods used to prepare high entropy alloys based on the AlFeMnNbNiTi system. Chemical composition was customized to ensure a solid solution structure with precipitation of the Laves phase. The three manufactured alloys were prepared by melting, but with the use of various input materials and different furnaces in protective atmospheres. After the melting process, heat treatment was carried out. Structures of obtained materials were analyzed by means of a Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) mapping. Mechanical properties were represented by Vickers hardness. In this paper, impact of the use of low purity input materials is shown, as well as differences in structure resulting from the utilization of different melting furnaces.

Hardening of Additive Manufactured 316L Stainless Steel by Using Bimodal Powder Containing Nanoscale Fraction

The particle size distribution significantly affects the material properties of the additively manufactured parts. In this work, the influence of bimodal powder containing nano- and micro-scale particles on microstructure and materials properties is studied. Moreover, to study the effect of the protective atmosphere, the test samples were additively manufactured from 316L stainless steel powder in argon and nitrogen. The samples fabricated from the bimodal powder demonstrate a finer subgrain structure, regardless of protective atmospheres and an increase in the Vickers microhardness, which is in accordance with the Hall-Petch relation. The porosity analysis revealed the deterioration in the quality of as-built parts due to the poor powder flowability. The surface roughness of fabricated samples was the same regardless of the powder feedstock materials used and protective atmospheres. The results suggest that the improvement of mechanical properties is achieved by adding a nano-dispersed fraction, which dramatically increases the total surface area, thereby contributing to the nitrogen absorption by the material.

Skew Rolling of Bimetallic Rods

The present article reports selected results of a preliminary study of the process of skew rolling of bimetallic rods. The experiments were conducted using a numerically controlled three-roller skew rolling mill. During the tests, bimetallic rods were rolled from billets whose cores and outer sleeves (bushings) were made of different types of steel. The results demonstrate that the proposed method can be successfully used in the production of bimetallic rods. However, proper fastening of the two materials depends on the geometrical parameters of the billets, and the quality of bimetallic rods depends on the heating method used. When the rods are heated without protective atmospheres, the surface layer of the core gets decarburized and the surfaces of the materials being joined together are oxidized, which hinders the welding process and adversely affects the physical and chemical properties of such products. The results of numerical modeling indicate that the material near the surface tends to flow, which may have a negative impact on the welding process. In addition, the distribution of stress in the tool–workpiece contact zone may make welding of the materials difficult. The results reported in this paper are preliminary and constitute a prelude to a more detailed analysis of bimetallic rod rolling.

History, Developments and Trends in the Heat Treatment of Steel

Ferrous alloys (steels and cast irons) and their heat treatment have attracted a great amount of basic and applied research due to their decisive importance in modern industrial branches such as the automotive, transport and other industries. Heat treatment is always required for these materials, in order to achieve the desired levels of strength, hardness, toughness and ductility. Over the past decades, many advanced heat- and surface-treatment techniques have been developed such as heat treatment in protective atmospheres or in vacuum, sub-zero treatment, laser/electron beam surface hardening and alloying, low-pressure carburizing and nitriding, physical vapour deposition and many others. This diversity of treatment techniques used in industrial applications has spurred a great extent of research efforts focused on the optimized and/or tailored design of processes in order to promote the best possible utilization of material properties. This special journal issue contains a collection of original research articles on not only advanced heat-treatment techniques—carburizing and sub-zero treatments—but also on the microstructure–property relationships in different ferrous alloys.

Examination of the surface layer composition formed on ML19 magnesium alloy during melting at protective gas atmospheres

Currently the most common method of the magnesium alloys flux free melting is the melting under the gas protective atmosphere. This atmosphere consists of inert carrier gas with low addition of active gas. The ML19 casting magnesium alloy contains Y and Nd that enough active. The interaction of such alloys with gas protective atmospheres is poorly studied and has serious practical importance. Sulfur hexafluoride (SF6) has a great influence on the global warming and because of that its application is limited. As a result, the number of countries cross over to HFC-R134a as the active gas. This paper presents the investigation of the effect of gas protective mixtures consisting of carrier gas (argon of nitrogen) and active gas (SF6 or HFC-134a) on the composition of protective layer formed on the surface of ML19 magnesium alloy melt. It was developed a special laboratory setup providing the contact of the protective gas mixture with the alloy during heating, melting and solidification of the samples and preventing the influence of the surrounding atmosphere. The loss of the alloying elements was negligible but in the case of using nitrogen as a carrier gas the Y and Nd content in alloy was lower than if the argon is used. If SF6 is used as an active gas, the Zr content in alloys was lower. Composition and thickness of oxide film that formed in both SF6 and HFC-R134a protective atmospheres are mostly the same. The surface film is consist of magnesium fluoride (MgF2) with admixtures of oxides, fluorides and nitrides of zirconium, yttrium and magnesium. The key difference of protective layer phase composition if HFC-R134a used as an active gas is presence of the large amount carbon in the form of compounds and in a free state. Additionally, it was established that using of HFC-134a in protective atmosphere requires more careful dosage given the fact of its percentage in the gas mixture of more than 1 vol.% leads to severe corrosion of the crucible inner surface during the melting.

Brazing in SiH4-Doped Inert Gases: A New Approach to an Environment Friendly Production Process

Engineering under protective atmospheres or in vacuum allows the production of materials and components, where the absence of oxygen is an essential requirement for a successful processing. Ideally, joining or coating of (and with) metallic materials needs oxide free material surfaces, in order to achieve durable joints or coatings. Using the established technology of brazing in controlled atmosphere, fundamental physical mechanisms for deoxidation of metal surfaces are presented and the role of oxygen and water residue in the process atmosphere is analyzed. Furthermore, the doping of gases with monosilane for generating virtually oxygen-free process atmospheres is introduced and its advantages for an oxygen-free production are discussed.

Production of Ultrafine Grained Hardmetals by Electrical Resistance Sintering

In this work, powders of cemented ultrafine WC-6 wt.% Co were consolidated. The feasibility of the medium frequency electrical resistance sintering (MF-ERS) technique were studied to prevent WC grain growth during consolidation. Porosity and hardness were measured at different zones of the MF-ERS compacts. The compacts showed a slight inhomogeneity in their properties across their section, but it was controlled by choosing suitable values of the processing parameters. The optimal values for the material studied were current intensities between 7 and 8 kA and sintering times between 600 and 800 ms. The main achievement using this consolidation method was that sintered compacts essentially maintained the initial WC grain size. This was attained to processing times of less than 2 s, and without the need for using protective atmospheres.

The effect of protective atmosphere on decarburisation of the heat-treated surface of steel

In heat-treating furnaces, many different types of protective atmospheres are used. This article researches the effect of protective atmospheres on the quality of the surface layer of bolts during the process of heating to reach the temperature of hardening. For this research, we produced specimens that were annealed in the furnace with two different types of protective atmosphere, i.e. in atmospheres of endothermic gas and nitrogen. After hardening and tempering, we measured the hardness of the specimens and investigated the microstructure. We measured the hardness profile from the surface to the inside of the product. We found that the hardness of the surface of the tested product was lower while using protective atmosphere of nitrogen due to the occurrence of ferrite. The depth of the decarburised layer in this atmosphere reached up to 70 mm, where predominantly there was a microstructure of ferrite on the surface, and then, with depth, an increasingly mixed microstructure of ferrite and martensite was found. The depth of the decarburised layer for sample treated in endothermic gas was minimal (i.e. 10 mm) on the surface.