Treating molten metal with ultrasound is cleaner and more efficient than using argon rotary degassing to produce high-quality castings, according to scientists at Brunel University London.
Molten aluminum alloys at 700°C (1292°F) naturally contain a high percentage of dissolved hydrogen—left untreated, the resulting solid metal is highly porous, the university explained. Argon rotary degassing—the most widely-used method of hydrogen removal—is energy-intensive and requires costly components.
In pilot-scale trials funded by EU Framework Seven Programme, the research team, led by Professor Dmitry Eskin of the Brunel Centre for Advanced Solidification Technology, found that ultrasound was just as efficient as the standard method but produced far less waste material (dross) and was much greener.
“We know from industry that the price of argon gas continues to rise because making it requires producers to liquefy air, which takes a lot of energy,” said Eskin in a statement announcing the findings. “There are also issues with the graphite impellers used. If they break in use, the entire batch of alloy is contaminated and useless.
“Our pilot-scale research with quantities of up to 150 kg (330 lb) confirmed earlier laboratory tests that a moving ultrasound probe could achieve the similar end results in terms of the resulting casting quality to using argon but with the advantages of not relying on expensive and fragile graphite rods and expensive gas which cannot be captured and recycled.
“The five-fold reduction of the amount of dross created is another benefit. Recovering useable metal from dross is also an expensive and energy-intensive process that involves electrolysis.”
The process also can lead to improvements in material attributes, according to Brunel spokesman Mark Howard.
“U/S (ultrasound) treatment significantly improved both ductility and helped grain refinement which is of interest for alloys where grain refiners are not available,” Howard shared with SAE Magazines. “It’s aluminum alloys where the issue of degassing is most important—otherwise you have to do a lot of heat treatment/rolling, which is of course costly. Ductility improvement is important because it raises the prospect of being able to produce vehicle panels from initial melt feedstock.”
Eskin has been working on this project for about three years now, but solidification science has been big at Brunel for 20 years or more, according to Howard.
Scaling up the trials to the half-ton level is the next goal of the research program. Eskin’s team is working ultimately to introduce cost-effective ultrasound degassing earlier in the production cycle.
“Dmitry is hoping to scale up to 500-kg melts this year and to introduce U/S degassing between initial melt and ladle, which will have a longer timescale. Because it can be retrofitted, it’s really up to industry on speed to foundry floor,” said Howard.
The ultimate aim is to replace components that are currently machined from treated billets to ones than can be cast, Howard shared.
“Economic drivers like producing lighter engines and lighter [vehicle] bodies are pushing process improvements in producing higher-quality alloys much further back in the production cycle to where alloys are first smelted,” Eskin explained. “Ultrasound treatment holds out the promise of being able to degas effectively and continuously, and we have already made some steps toward achieving this on the pilot level.”