No 2 (2021)

Foamed metals are promising materials with a unique combination of mechanical and operational properties: low specific gravity, low thermal conductivity, ability to absorb acoustic and electromagnetic vibrations, and the ability to deform under a constant load. Currently, the most used methods for producing foamed aluminum and foamed magnesium are methods based on mixing gas or porophore into molten aluminum and forming a porous structure during the solidification of the aluminum melt. An alternative to this technology is the formation of a porous structure through the use of soluble granules that pre-fill the mold and after impregnating the granules with molten metal and solidifying the castings, they are leached. The work aims to determine the influence of casting modes and the size of granules on the depth of impregnation of granular filling with metal melt during the formation of porous aluminum castings. The authors proposed the technique for calculating the depth of impregnation of granular filling when producing castings of porous non-ferrous metals based on the calculation of melt cooling when moving along the thin-walled channel. The calculations made it possible to determine the depth of impregnation and establish the allowable wall thickness of the casting of porous aluminum, depending on the size of the granules used, the speed of the melt in a form, the mold temperature, and the temperature of molten aluminum. The study identified that to increase the depth of impregnation and obtain porous aluminum castings with thinner walls, it is advisable to increase the diameter of the salt granules and not the temperature and hydrodynamic modes of casting. The authors carried out calculations and identified the influence of the casting regimes and the diameter of the granules on the depth of mold impregnation to obtain porous castings from promising magnesium alloys.

The paper considers the issues of ensuring the uniformity of strain of axisymmetric long-dimensional samples during thermal force processing (TFP), which is the simultaneous application of force and temperature effects for comprehensive improvement of geometric characteristics and physical and mechanical parameters of the workpiece material. This technology is used at various stages of technological processes of parts manufacturing, but its main task is to ensure the axis straightness and the specified distribution of residual technological stresses at the procuring stage. The disadvantage of TFP is that the axial deformation proceeds nonuniformly along the workpiece axis. The core process parameter is the deformation, the control of which is a key factor ensuring the TFP efficiency. The authors studied the plastic strain distribution over the sections of long-length workpieces with different deformation degrees. The study involved the assessment of strain uniformity over the workpiece sections, taking into account the stage of the stress-strain relation at the end of the loading cycle. Based on the concepts of plastic deformation as an auto-wave process, the authors selected the range of technological modes corresponding to the most uniform strain distribution along the workpiece axis with complete processing of the entire workpiece volume. This range corresponds to the stage of parabolic hardening of the plastic flow curve with the formation of the maximum number of stationary zones of localized plasticity. Rheological modeling allows identifying the control points that specify the boundaries of the plastic flow curve stages at various loading parameters, including temperature. To improve the reliability of determining the actual deformation under production conditions, the authors proposed modernizing the TFP process monitoring method by fixing the deformation on a limited workpiece section using the optical technique. The statistical analysis of the strain distribution over the sections for the samples confirms the correctness of this approach. The application of the proposed control method will ensure the most uniform distribution of plastic deformation due to the reliable enter of the workpiece deformation to the range of strain values corresponding to the stage of parabolic hardening of the plastic flow curve.

The ultrasonic fatigue testing (USFT) is an effective method for rapid determination of the fatigue properties of structural materials under high cycle (≥10⁶ cycles) loading. However, the occurrence and accumulation of fatigue damage with this test method remain uncertain due to the limitations of the existing measurement methods. Currently used monitoring methods allow detecting the fatigue cracks, but only in the late stages of failure. Despite the superior sensitivity to localized processes in materials, the use of the acoustic emission (AE) method in ultrasonic testing is extremely difficult due to the presence of resonant noise. This work aimed to suppress resonant noise and extract the signal for early detection of fatigue damage. The authors tested the samples of the AlSi9Cu3 aluminum alloy under the asymmetric cyclic loading (R=0.1) at a resonant frequency of 19.5 kHz with a non-threshold AE registration. The fracture surfaces were analyzed by electron and optical microscopy. The authors processed AE by two different methods: (1) the digital filtering method consisted of detecting resonant noise and removing it from the spectrum; (2) the φ-function method consisted of differentiating the spectrogram by time. The processed spectrograms were integrated by the frequency with further extraction of the AE events using the threshold method. The digital filtering method revealed a correlation between AE signals and fatigue damage, whereas the undamaged control sample showed no signals. The φ-function technique demonstrated ambiguous results, showing high AE activity on the control sample.

Amorphous alloys based on metal components demonstrate a unique ability to realize plastic deformation under the influence of external mechanical stresses. Influenced by substantial degrees of plastic deformation in alloys, one can observe shear bands (SB) in the form of rough lines on the polished surface of the sample. The concept of shear band formation in amorphous metallic glasses varies greatly from plastic deformation processes in crystalline metals and alloys. Unlike crystalline metals, amorphous metallic glasses can exist in a spectrum of structural states with accompanying mechanical, thermodynamic, and physical properties of materials. The formation and evolution of shear bands control the fluidity and plasticity of almost all metallic glasses at room temperature, and in many cases, the formation of dominant shear bands rapidly leads to failure. The literature does not contain any rigorous quantitative description of SB main parameters, which could adequately describe in the analytical form the process of plastic deformation of amorphous alloys, similar to the dislocation and disclination theories of plastic deformation of crystals. An open question remains how the transition from macroscopic deformation to severe plastic deformations of amorphous alloys affects the key SB characteristics. In this work, using the method of optical profilometry, the author studied in detail the quantitative characteristics of the steps formed by shear bands on the surface of deformed samples of the massive amorphous alloy Zr₆₀Ti₂Nb₂Cu_18.5Ni_7.5Al₁₀ after high-pressure torsion (HPT) and after rolling. The study identified that the design of shear bands depends on the deformation method and showed that the magnitude of deformation had the controlling effect on the shear bands thickness (the height of the steps). The transition from deformation by rolling (e=0.4) to plastic deformation during HPT (e=2.6) leads to the threefold increase in the power of shear bands and the average distance between them.