A summary of the Blasch article on static and rotary degassers and a ceramic with pores around 5 microns.
Hydrogen in the melt becomes porosity in the casting.
The size of the first bubble decides how much of it you get out.
Aluminum is degassed to remove dissolved hydrogen, which otherwise comes out of solution on solidification and leaves porosity in the cast part. The Blasch article argues that the most efficient degassing produces a large number of small bubbles, and that the pore size of the ceramic sets the initial bubble size: a ceramic with pores around 5 microns, about a tenth of conventional porous plugs, makes finer bubbles and keeps metal out of the part.
| Item | Detail |
|---|---|
| Title | Improved Ceramic Material for Aluminium Degassing |
| Publication | Simple Solutions, section Systems Integration, an online publication for metalcasting and die casting in North and South America, in English and Spanish |
| Year | 2017 |
| Author | Philip Geers, Molten Metal Market Manager, Blasch Precision Ceramics |
Hydrogen dissolves readily in molten aluminum, and its solubility depends on temperature and pressure. Moisture in the air reacts with the melt surface, and alloying elements such as magnesium make it easier for hydrogen to enter. On solidification the hydrogen leaves solution and forms pores and voids, which means lower part quality and more scrap.
Static systems sit in the floor of the vessel: porous plugs, also called spargers, and some lance types. They reach a large share of the melt from below, have no moving parts and suit larger vessels where a longer service life justifies the installation cost.
Dynamic systems use a rotor on a shaft driven from above. Gas runs down the shaft, which turns at 200 to 400 rpm, and leaves through or under a head shaped to shear the bubbles. They suit smaller vessels such as foundry crucibles that cannot be fitted with a bottom plug.
A conventional porous plug is a solid ceramic cone inside a metal shell, with pores of 50 to 100 microns. The shell adds cost and must be Inconel when chlorine is in the gas. The body gives no control of gas direction, the large pores make large initial bubbles, and gas has to flow constantly to keep metal out of the pores.
Rotary shafts and heads have long been graphite, which machines easily but oxidizes at the metal and air interface, and multi hole heads plug when idle. Silica parts do not oxidize, but they react with aluminum and form dross.
The Blasch process produces ceramic bodies with a continuous pore network and pores around 5 microns, with adjustable permeability and non wetting additives. In the article, a silicon carbide and alumina composite with these pores is proposed for both approaches:
Most degassing improvements work on moving bubbles around, while this one starts earlier and makes each bubble smaller before it leaves the ceramic.
If you fight porosity in die castings, compare your current degasser by four questions from the article: how small the bubbles are, whether the gas reaches the whole melt, whether the part is attacked by aluminum, air or treatment gas, and how easily it is installed and serviced.
The article also suggests that a ceramic head able to make small bubbles on its own may reduce the need to shear bubbles or allow lower rotor speeds. That is presented as a possibility, not as a measured result.
Blasch makes rotary degassers, porous plugs and lances in this material, part of its degassing and die casting range. The material family is described on the page for alumina bonded silicon carbide.
At the same gas flow, smaller bubbles mean many more bubbles and more surface in contact with the melt, which the article names as the key criterion for efficiency.
Static plugs suit larger vessels and treat the melt from below without moving parts. Rotary degassers suit smaller vessels such as crucibles that cannot take a bottom plug.
Without it, gas escapes through the sides and bottom of a solid porous body. The shell also carries the gas piping. Cast threads and internal voids can make it unnecessary.
The article is about aluminum. The Blasch product range lists non wetting degassing parts for aluminum, copper, zinc and Galvalume melts.
Send the vessel type, the alloy, the gas mix and how your current degasser fails. A Blasch engineer suggests the part and the design.
