It’s not uncommon for defects to arise during the aluminium casting process. Some are so small as to be tolerable, and some can be easily repaired. Others, though, are so severe that we have no recourse but to throw the casting out and start again.
Of course, this is far from ideal. Fortunately, we have numerous ways to treat defects.
Here are the five main types of irregularities found in aluminium castings:
- Shrinkage – Shrinkage defects occur during the solidification process. Open shrinkage defects are found on the surface of castings, whereas closed shrinkage defects are formed within the casting, when isolated pools of liquid are allowed to form within the solidified metal.
- Gas Porosity – Liquid aluminium can hold large volumes of dissolved gas, whereas solid aluminium cannot. As a result, the gas can form bubbles within the metal as it cools, reducing the overall strength of the casting. Gas porosity is most commonly caused by dissolved hydrogen in the molten metal.
- Pouring Metal Defects – A few things can go wrong when pouring molten metal. The liquid metal may not fill the entire mould cavity, resulting in an unfilled portion. Alternatively, two fronts of metal may not fuse properly within the mould cavity, resulting in a weak spot.
- Metallurgical Defects – Sometimes, the chemical composition of the metal does not allow for optimum cooling conditions. This can result in hot spots – hard areas on the surface of the casting that cooled quicker than the surrounding metal.
- Mould Defects – Metal is weaker when it’s hot. If the mould has been poorly designed, it can cause residual stresses in the material as it cools, resulting in hot cracking.
How Do We Treat Casting Defects
Our approach to treating irregularities in our castings will vary depending on the nature of the defect. But as usual, prevention is better than a cure.
At Harrison Castings, we have a dedicated, onsite laboratory that is committed to ensuring that all of our castings have the finest metallurgical properties. In this way, we reduce the chances of defects even occurring in the first place.
We to continuously monitor our mechanical processes, our sand compositions, and our alloys. As a result, we have been able to abandon any inefficient processes that have historically resulted in lower quality castings, whilst perfecting any processes that lead to higher yields and superior metallurgical properties.
Our extensive metal control process incorporates spectrographic analysis, density indexing, and rotary degassing.
Rotary degassing involves making use of inert gasses such as nitrogen in order to eliminate the presence of hydrogen in the molten aluminium, which singularly serves to reduce the risks of both shrinkage and gas porosity defects.
Gas porosity defects are further reduced through a process called density indexing. We subject a sample of molten aluminium to a reduced pressure test (RPT), where it is allowed to solidify in a vacuum. The reduced pressure enhances the size of the porosity formed.
Comparing this sample to a sample that was allowed to solidify under atmospheric pressure allows us to calculate the density index of our molten aluminium. We draw from this information to improve the efficiency and the effectiveness of the subsequent degassing process.
In short, thanks to our ongoing commitment to quality control, we have greatly reduced the chances of defects even occurring in the first place. And should they ever occur, we will simply take it as a lesson, and use it to improve our processes even more.
In this way, you can rest assured to receive nothing but the best from Harrison Castings.