Upscaling environment-friendly cavitation melt treatment (UltraMelt2)

Lead Research Organisation: Oxford Brookes University
Department Name: Faculty of Tech, Design and Environment

Abstract

Our use of metals is so important that it defines periods of human civilisation - from the Bronze Age (c. 3600 BC) to the Iron Age (c. 1100BC). With our present-day mastery of metals and alloys, the mounting emphasis is now on resources and the environment. The metals industry is looking at new ways to produce lighter, stronger materials in a sustainable, economical and pollution-free manner. Ultrasonic cavitation treatment offers a route to meet these goals. Ultrasonic treatment of the second commonest structural metal, aluminium, causes degassing through the evacuation of dissolved gases that lead to porosity, grain refinement to assist formability, dispersion and distribution of solid or immiscible phases to improve mechanical properties during recycling etc. In spite of the benefits, transfer of this promising technology to industry has been plagued by difficulties, especially in treating large volumes of liquid metal typical in processes such as 'Direct Chill' continuous casting for ingot production. Fundamental research is needed to answer the following practical questions: what is the optimum melt flow rate that maximises treatment efficiency whilst minimizing input power, cost, and plant complexity? What is the optimum operating frequency and acoustic power that accelerates the treatment effects? What is the optimum location of an ultrasonic power source in the melt transfer system in relation to the melt pool geometry? Answering these questions will pave the way for widespread industrial use of ultrasonic melt processing with the benefit of improving the properties of lightweight structural alloys, simultaneously alleviating the present use of polluting (Cl, F) for degassing or expensive (Zr, Ti, B, Ar) grain refinement additives.

Capitalising on the unique expertise gained by the proposers during the highly successful UltraMelt project (22 publications), this research aims to answer the challenge of efficiently treating large liquid volumes by developing a comprehensive numerical model that couples all the physics involved: fluid flow, heat transfer, solidification, acoustics and bubble dynamics. Greenwich will lead the development of an improved cavitation model, based on the wave equation and conservation laws, and applied to the two-phase problem of bubble breakup and transport in the melt, and its interaction with solid inclusions (e.g. the solidification front of an aluminium alloy or of any intermetallic impurities present). To improve the efficiency of the ultrasonic cavitation treatment in flowing metal, a launder conduit will be used. The sensitivity of the process with respect to different adjustable parameters (source power, frequency, time in the cavitation zone, baffle location ...) will be examined with parallel computations in a 3D model of melt flow in the launder.

This computer model will be validated by experiments in both transparent liquids and aluminium. Water and transparent organic alloy experiments will use a PIV technique by Oxford Brookes University to measure the size, number and positions of bubbles and compared these with the numerical predictions. Mechanisms of intermetallic fragmentation and particle cluster breakup will be observed in real time using a high speed camera at Brunel University and X-ray radiography at the Diamond Light Source facility. Mechanical properties of intermetallic impurities at temperatures relevant to melt processing will be measured using unique nano-indentation technique in collaboration with Anton Paar Ltd. Cavitation pressure measurements in launder conduits will be conducted at Brunel University and the empirical observations will be compared with model predictions. The fully-developed model will be used to optimise the ultrasonic melt treatment in melt flow during direct-chill casting and verified using pilot-scale facilities at AMCC (Brunel, with support of Constellium) and industrial-scale facilities at Kaiser Aluminum.
 
Description A numerical model for describing acoustic flows and cavitation have been improved. Experimental setups have been developed to study the interaction of melt flow with ultrasonic sources with the aim to optimise the residence time and efficiency of ultrasonic processing; as well as to provide data for computer simulations. Improved understanding of the effects of impact load from collapsing of acoustically generated cavitation bubbles on crack propagation and surface fragmentation; For the first time the prime fragmentation mechanism of intermetallics due to interaction with cavitating bubbles was revealed using ultra-high speed camera observations. The role of shockwaves upon implosion of cavitation bubbles in fragmentation of intermetallics has been demonstrated by in-situ studies and analytical modelling. The unique experimental nanoindentation measurements allowed us to apply fracture mechanics to the analysis of fragmentation of intermetallics under cavitation.
Exploitation Route The experimental results so far will be used to improve the computer models being developed within the project.
Sectors Aerospace, Defence and Marine,Chemicals,Energy,Environment,Manufacturing, including Industrial Biotechology

 
Description Calibration and data acquisition 
Organisation NPL Ltd
Country United Kingdom 
Sector Private 
PI Contribution We provide datasets for the analysis.
Collaborator Contribution NPL provides assistance in calibration of cavitometers as well as solving issues with data acquisition. This enhances their expertise in measuring acoustic parameters in liquid metals.
Impact Calibration and recommendations
Start Year 2018
 
Description High temperature nanohardness 
Organisation Anton Paar Ltd
Country United Kingdom 
Sector Private 
PI Contribution We provide large samples of intermetallics extracted from Al alloys (up to 1-2 mm)
Collaborator Contribution Anton Paar measures nanohardness and extract mechanical properties at high temperatures, 600 to 700 C.
Impact A technique to grow large intermetallic particles and extract them have been developed. The samples have been sent to Anton Paar.
Start Year 2018
 
Description Ultra-high speed camera observations 
Organisation University of Glasgow
Country United Kingdom 
Sector Academic/University 
PI Contribution We have identify for the first time the prime responsible mechanism for the fragmentation of intermetalic crystals in a sonicated environment
Collaborator Contribution Allowed us to use their state of the art facility providing knowledge and expertise
Impact 2 conference papers are already published in the relevant proceedings; a number of journal papers is under development; more experiments are scheduled for this year
Start Year 2019
 
Description Special Issue 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact A Special Issue (SI) on Ultrasonic Cavitation Treatment (UST) of Metallic Alloys was edited by Iakovos Tzanakis and Dmitry Eskin with original contributions from leading research groups around the globe. SI scrutinise the use of UST in technology of high-quality metallic alloys with improved mechanical properties, assess the driving mechanisms of cavitation-induced effects, such as grain refinement, degassing, wetting, particle distribution etc. and examines the possibility of achieving high efficiency in processing of large melt volumes through technological approaches for the commercial implementation.
Year(s) Of Engagement Activity 2020
URL https://www.mdpi.com/journal/materials/special_issues/ultrasonic_metallic_alloys
 
Description Symposium at TMS Annual Meeting 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A symposium "Ultrasonic processing of liquid and solidifying alloys" is organised at the TMS Annual Meeting in San Antonio (TX, USA) in March 2019. This is a one-day symposium with 14 presentations and expected 50-professionals audience.
Year(s) Of Engagement Activity 2019