Innovative solution for degassing aluminum melts in industry

Significance 

Aluminum alloys are attractive for a number of applications due to their high specific strength, and therefore developing efficient processing technologies is a major goal for materials engineers. Casting is one of the many stages of the manufacturing of aluminum alloy products and it involves the handling and pouring of molten metal into shape molds. However, the presence of porosity in most castings presents a challenge considering its detrimental effects on the post-processing steps and the final mechanical performance of the aluminum products. Porosity is formed during the solidification stage and is often difficult to remove once formed. Thus, it is important to eliminate or at least reduce the causes of porosity before casting to increase weld quality and produce high-quality casting products. Two main factors determining the quality of aluminum melts are dissolved hydrogen and oxide bi-films with poor wettability, both due to the reaction of the liquid aluminum surface with the ambient moisture. Therefore, removing hydrogen and bi-films before casting, i.e. degassing, is a crucial step in ensuring high melt quality and reducing the formation of porosity during solidification.

Among the available molten aluminum degassing methods, the industry uses rotary degassing due to its simplicity and improved performance compared to other available methods. For this method, the degassing efficiency is dependent on the effective size reduction and distribution of the gas bubbles inside the molten metal. However, strategies for improving the degassing efficiency in conventional rotary degassing are still not well understood. For example, while increasing rotor speed could reduce the size of bubbles and accelerate the removal of hydrogen, higher speeds can cause vortex and surface turbulence leading to reabsorption of new oxides and hydrogen. The increase of gas flow rate that could solve such problems is also limited in terms of efficiency. Additionally, redesigning the rotary impeller with the assistance of computer modeling, which has been deemed as a promising approach for process optimizing, has proved to be inadequate for achieving full degassing efficiency.

To this note, Dr. Jaime Lazaro-Nebreda, Dr. Jayesh Patel and Professor Zhongyun Fan from Brunel Centre for Advanced Solidification Technology (BCAST), at Brunel University London (UK), have developed an innovative melt conditioning technology that can help industry to significantly increase the degassing efficiency of aluminum melts in comparison with current rotary degassing units.

The technology is based on advanced high shear melt conditioning (HSMC) and consist of a rotor-stator configuration with a controlled gas injection, which allows the rotor speed to be increased without vortex generation and at the same time a more effective dispersion of bubbles and oxide bi-films in the molten metal. The processing parameters, in terms of rotor speed and gas flow rate have been optimized by water modeling under controlled representative conditions for later testing on a A356 aluminum alloy melt. The castings obtained with the HSMC degassed melt have been examined for microstructural defects and also for mechanical properties performance. The process efficiency has been compared to that obtained with conventional degassing methods. The outcome of this work is published in the Journal of Materials Processing Technology.

Compared to the conventional techniques, the present technology is much faster in removing oxide bi-films and hydrogen that causes porosity without generating vortex or surface turbulences because it can operate at much higher speeds. In addition, very low gas flow rates are required to achieve effective bubble dispersion, which is of great benefit in improving process efficiency and reducing processing costs.

The resulting melts exhibit high quality for long after processing. Unlike rotary degassing, aluminum melts processed by HSMC degassing do not require covering fluxes. Furthermore, the resulting castings have less defects, low porosity levels and significantly improved mechanical properties.

In summary, the possibility of improving degassing efficiency and mechanical properties of aluminum alloys castings via innovative HSMC degassing technology has been successfully validated in this study. Overall, HSMC degassing technology outperformed the conventional degassing methods like rotary degassing in terms of efficiency, cost implications, weld quality, and mechanical properties of the resulting castings were significantly improved. This makes HSMC degassing and an excellent alternative for improving melt quality and casting productivity in different industries. The authors, in a statement to Advances in Engineering, said that the new technology can be easily implemented in the industry by replacing the existing rotary impeller design with minimal impact on the casting process operations.

Innovative solution for degassing aluminum melts in industry - Advances in Engineering

BCAST-Advances in Engineering

BCAST is an academic research centre at Brunel University London focusing on both fundamental and applied research on solidification of metallic materials.

BCAST was first established in October 2002, and is now one of the world’s strongest solidification research groups. It comprises 100+ members, from academics, research fellows, postgraduate research students, technicians and administration staff.

Research interests span from fundamental research, to technological developments and industrial applications. The activities are well supported by EPSRC, TSB, EU and a network of industrial companies, including Constellium, JLR and Aeromet to list but a few. In particular BCAST strives to realise a sustainable metals market and provide industries with environmentally friendly metal processing solutions.

Since 2015, and based at BCAST, the Future LiME Hub, a national centre of excellence in liquid metal engineering, continues on the success of the EPSRC Centre LiME (set in 2010). The long-term vision of the Hub is full metal circulation, in which the global demand for metallic materials is met by a full circulation of secondary metals (with only limited addition of primary metals each year) through reduced usage, reuse, remanufacture, closed-loop recycling and effective recovery and refining of secondary metals (as illustrated below). This represents a paradigm shift for metallurgical science, manufacturing technology and the industrial landscape. This vision is shared by both our academic spokes and our industrial collaborators.

Between 2016 and 2018, BCAST also opened the AMCC (Advanced Metal Casting Centre) and the AMPC (Advanced Metal Processing Centre), two industrial research buildings with extensive range of factory-level metal casting/processing facilities and the critical supporting research facilities for developing advanced metallic materials, as well as to underpin component performance testing and for process modelling and simulation.

https://www.brunel.ac.uk/research/Centres/BCAST

https://www.lime.ac.uk/

About the author

Dr. Jaime Lazaro Nebreda is a Research Fellow at BCAST.

He holds a degree in Physics and a PhD in Materials Science from University of Valladolid (Spain). He joined BCAST in 2014 to work on the technological development and fundamental understanding of the BCAST multi-purpose physical melt treatment technologies for aluminium and magnesium alloys.

His research focuses on aluminium alloy scrap recovery, purification and recyclability for a full metal circulation, advanced characterization, manufacturing process optimization, and the technology developments which allow for an enhanced melt cleanliness, including the removal of dissolved gasses (degassing), accumulated elements like iron, manganese, and non-metallic impurities. He has also participated in various, EU and the UK funded, industrial collaboration projects.

About the author

Dr. Jayesh Patel is the Head of Technology Applications at BCAST.

He holds a degree in Materials Engineering and a PhD in Materials Science from Brunel University (UK). Since 2006 he has been working at BCAST, with focus on the development and industrialization of the BCAST melt conditioning technologies for the non-ferrous metals industry.

His main research interests are on the preparation of BCAST technologies for commercial application in industry, including direct chill casting, twin roll casting and high pressure die casting. He is responsible of the design and manufacturing improvements of the technologies, the international public relations with industry and also the training and commissioning of the equipment in commercial production environments. He has also participated in various industrial collaboration projects including 5 Innovate UK funded projects.

About the author

Professor Zhongyun Fan is the Founder and Director of BCAST and the principal investigator/director of the EPSRC Future LiME Hub.

He obtained his first degree in Metallurgy from University of Science and Technology, Beijing (China) and his PhD in Materials Science and Engineering from University of Surrey (UK). He has published over 400 scientific papers with an H-Index of 49 and a total currently of 9010 citations. He has initiated, managed and delivered research projects funded from a wide range of grant sources as principal investigator, including 12 EPSRC grants, 3 HEFCE grants, 6 DTI/TSB/IUK grants, 2 EU grants and a number of industrial contracts, with grants totalling over £45M.

He is also the co-chairmen of the Casting and Solidification Society (IOM3), Board Member of the Light Metals Division (IOM3), a Fellow of the Institute of Materials, Minerals and Mining (IOM3) and the Institute of Cast Metal Engineers (ICME) as well as a member of the American Society of Materials (ASM) and the EPSRC Metals College. He was the recipient of the Elegant Work Prize (1995), the Cook/Ablett Award (2003) and Dowding Medal and Prize (2012) of the Institute of Materials, Minerals and Mining (IOM3). His research is focused on understanding and modelling the early stages of solidification, covering Prenucleation, Heterogeneous Nucleation, Grain initiation and Grain refinement, and also on developing innovative techniques for melt treatment and metal casting.

Reference

Lazaro-Nebreda, J., Patel, J., & Fan, Z. (2021). Improved degassing efficiency and mechanical properties of A356 aluminium alloy castings by high shear melt conditioning (HSMC) technologyJournal of Materials Processing Technology, 294, 117146.

Go To Journal of Materials Processing Technology

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