Dmitry G. Eskin

Mechanical properties in the semi-solid state and hot tearing of aluminium alloys

This review represents a comprehensive coverage of results reported in the literature over last 50 years on the methods of studying hot tearing and mechanical properties of semi-solid aluminium alloys; the mechanical properties of these alloys in the semi-solid state; and hot tearing criteria. While compiling this review, the authors attempted to include in it all available sources including quite a few works never published in English before. The review consists of three parts. The first part introduces the reader to the phenomenon of hot tearing. The second part describes different techniques for testing metallic alloys in the semi-solid state and summarizes reported results on strength and ductility of semi-solid model and commercial aluminium alloys. The third part describes the methods for assessing hot tearing susceptibility of aluminium alloys, gives the results on hot cracking of various aluminium alloys and discusses different hot tearing criteria.

The Application of External Fields to the Manufacturing of Novel Dense Composite Master Alloys and Aluminum-Based Nanocomposites

The possibility of producing dense and concentrated master alloys containing nanosized Al2O3 by shock-wave compacting is demonstrated. Different conditions of shock-wave process are discussed. The data of master alloys characterization are presented. The nanostructured master alloys have high density and are convenient for metallurgical handling. It is found that the use of such a master alloy with nanoceramic particles facilitates the particle introduction into the aluminum melt. The ultrasonic treatment performed during and after the introduction of the master alloy into the melt further leads to uniform distribution of strengthening nanoparticles and improvement of alloy strength and ductility. Experimental results are shown and discussed.

In situ observation and analysis of ultrasonic capillary effect in molten aluminium

An in situ synchrotron radiographic study of a molten Al-10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode. This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.

PIV quantification of the flow induced by an ultrasonic horn and numerical modeling of the flow and related processing times

The flow in a confined container induced by an ultrasonic horn is measured by Particle Image Velocimetry (PIV). This flow is caused by acoustic streaming and highly influenced by the presence of cavitation. The jet-like experimentally observed flow is compared with the available theoretical solution for a turbulent free round jet. The similarity between both flows enables a simplified numerical model to be made, whilst the phenomenon is very difficult to simulate otherwise. The numerical model requires only two parameters, i.e. the flow momentum and turbulent kinetic energy at the position of the horn tip. The simulated flow is used as a basis for the calculation of the time required for the entire liquid volume to pass through the active cavitation region.

Characterizing the cavitation development and acoustic spectrum in various liquids

A bespoke cavitometer that measures acoustic spectrum and is capable of operating in a range of temperatures (up to 750°C) was used to study the cavitation behaviour in three transparent liquids and in molten aluminium. To relate these acoustic measurements to cavitation development, the dynamics of the cavitation bubble structures was observed in three Newtonian, optically transparent liquids with significantly different physical properties: water, ethanol, and glycerine. Each liquid was treated at 20kHz with a piezoelectric ultrasonic transducer coupled to a titanium sonotrode with a tip diameter of 40mm. Two different transducer power levels were deployed: 50% and 100%, with the maximum power corresponding to a peak-to-peak amplitude of 17μm. The cavitation structures and the flow patterns were filmed with a digital camera. To investigate the effect of distance from the ultrasound source on the cavitation intensity, acoustic emissions were measured with the cavitometer at two points: below the sonotrode and near the edge of the experimental vessel. The behaviour of the three tested liquids was very different, implying that their physical parameters played a decisive role in the establishment of the cavitation regime. Non dimensional analysis revealed that water shares the closest cavitation behaviour with liquid aluminium and can therefore be used as its physical analogue in cavitation studies; this similarity was also confirmed when comparing the measured acoustic spectra of water and liquid aluminium.

Thermal strain in the mushy zone for aluminum alloys

The parameters in a recently developed constitutive equation for macroscopic thermal strain in the mushy zone have been determined for the commercial alloys A356, AA2024, AA6061, and AA7075 in addition to an Al-4 wt pct Cu alloy. The constitutive equation for macroscopic thermal strain in the mushy zone reflects that there is no thermal strain in the solid part of the mushy zone at low solid fractions and that the thermal strain in the mushy zone approaches thermal strain in the fully solid material as the solid fraction increases toward 1. The development of thermal strain in the mushy zone is determined by combining experimentally measured contraction of a cast sample with thermomechanical stimulations. Experiments were performed at cooling rates in the range from 2 to 5.5 °C/s. The solid fractions when the tested alloys start to contract,gsth, are in the range from 0.63 to 0.94. Grain refinement increasesgsth for all the tested alloys. For most of the tested alloys the thermal strain in the mushy zone increases rapidly to the same level as thermal strain in fully solid material once the solid fraction becomes higher thangsth.

Synchrotron quantification of ultrasound cavitation and bubble dynamics in Al–10Cu melts

Knowledge of the kinetics of gas bubble formation and evolution under cavitation conditions in molten alloys is important for the control casting defects such as porosity and dissolved hydrogen. Using in situ synchrotron X-ray radiography, we studied the dynamic behaviour of ultrasonic cavitation gas bubbles in a molten Al-10 wt%Cu alloy. The size distribution, average radius and growth rate of cavitation gas bubbles were quantified under an acoustic intensity of 800 W/cm(2) and a maximum acoustic pressure of 4.5 MPa (45 atm). Bubbles exhibited a log-normal size distribution with an average radius of 15.3 ± 0.5 μm. Under applied sonication conditions the growth rate of bubble radius, R(t), followed a power law with a form of R(t)=αt(β), and α=0.0021 &β=0.89. The observed tendencies were discussed in relation to bubble growth mechanisms of Al alloy melts.

Relationship between Solidification Microstructure and Hot Cracking Susceptibility for Continuous Casting of Low Carbon and High Strength Low Alloyed Steels : a Phase-Field Study

Hot cracking is one of the major defects in continuous casting of steels, frequently limiting the productivity. To understand the factors leading to this defect, microstructure formation is simulated for a low-carbon and two high-strength low-alloyed steels. 2D simulation of the initial stage of solidification is performed in a moving slice of the slab using proprietary multiphase-field software and taking into account all elements which are expected to have a relevant effect on the mechanical properties and structure formation during solidification. To account for the correct thermodynamic and kinetic properties of the multicomponent alloy grades, the simulation software is online coupled to commercial thermodynamic and mobility databases. A moving-frame boundary condition allows traveling through the entire solidification history starting from the slab surface, and tracking the morphology changes during growth of the shell. From the simulation results, significant microstructure differences between the steel grades are quantitatively evaluated and correlated with their hot cracking behavior according to the Rappaz–Drezet–Gremaud (RDG) hot cracking criterion. The possible role of the microalloying elements in hot cracking, in particular of traces of Ti, is analyzed. With the assumption that TiN precipitates trigger coalescence of the primary dendrites, quantitative evaluation of the critical strain rates leads to a full agreement with the observed hot cracking behavior.

Ultrasonic degassing of aluminium alloys: basic studies and practical implementation

Abstract Ultrasonic processing is known to be an efficient means of aluminium melt degassing and structure modification with additional benefits of being economical and environment friendly. The present paper reports on the kinetics of ultrasonic degassing and regassing of foundry aluminium alloys and on pilot scale degassing trials. Efficiency of ultrasonic degassing is compared with conventional Ar rotary degassing. Direct measurements of hydrogen concentration in the melt by Foseco Alspek-H probe are used along with reduced pressure test. The effects of ultrasonic processing on porosity are studied using three-dimensional X-ray tomography.

Ultrasonic processing of molten and solidifying aluminium alloys: overview and outlook

Ultrasonic melt processing attracts since the 1930 a lot of interest both from academic researchers and industry. In the last 10 years the interest to ultrasonic melt processing grew with regard to understanding the underlying mechanisms of previously established effects, developing numerical models of ultrasonic cavitation and the development of nanocomposite technology. This review paper summarises the mechanisms involved in the ultrasonic melt processing, including cavitation, flows, nucleation, activation, fragmentation and their consequences for degassing, structure refinement and particle dispersion. Some typical mistakes made by researchers in performing experiments and in interpretation of the results are discussed. New advanced methods of studying ultrasonic treatment and phenomena are considered. The paper also gives an outlook to future developments and challenges. This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.