Degassing in aluminium sand casting

Published - 16th Nov 2017

Previously in this blog, we have looked at the issue of gas porosity in metal casting. Porosity takes the form of small pin holes in the casting, which can have a detrimental effect on the mechanical and physical properties of the final product.

Gas porosity may occur if hydrogen is introduced into the molten metal when it is poured into the mould. Hydrogen can be drawn in through moisture or water vapour in the atmosphere. Degassing is used to address this problem and it is carried out as a standard part of the Haworth Castings’ casting process.

In this blog, we will be looking at the science behind degassing and the different techniques available.

Click on the image above, or here to watch a video which shows the full pouring process in aluminium sand casting, including the degassing stage.

Aluminium – a highly versatile metal

As we have discovered in the past, aluminium is one of the most abundant and widely used metals in industry due to its excellent weight-to-strength ratio. However, it is also a very reactive metal that actively dissolves hydrogen on contact. Hydrogen can be introduced during the casting process in a variety of ways – through atmospheric humidity, moisture on the pouring ladle or mould, and even in the furnace.

Hydrogen gas is soluble in aluminium in its molten state. However, as the metal cools and solidifies, the gas can create porosity (pin holes) in the metal casting. The size of these pores varies, depending on the composition of the alloys and the solidification conditions.

Degassing techniques

This problem can be resolved by purging the molten metal with a dry, soluble gas before the pouring stage. The gas reacts with the hydrogen and then dissipates from the molten metal.

Argon and nitrogen are commonly used to degas metals. At Haworth Castings, we use nitrogen.

The process can be carried out using various techniques, including degassing fluxes (tablets) and static lances. One of the most effective methods, which we use at Haworth Castings, involves a rotary degassing unit.

This unit looks very similar to a standard paint mixer, with a rotating impeller that introduces nitrogen gas bubbles into the molten metal. The degassing time is dependent on various factors, including the choice of aluminium alloy and the temperature of the molten metal.

Whatever the variables, our highly experienced engineers ensure the right degassing technique is used for each individual casting project.

With a wealth of expertise, our foundry engineers provide the full range of casting services. Find out more by contacting us today on +44 (0)1794 512685 or email us: sales@haworthcastings.co.uk