Close control of the cast structure is a major requirement in the production of high quality aluminium alloy castings. The most effective way to provide a fine and uniform as-cast grain structure is to add nucleating agents to the melt to control crystal formation during solidification.
Grain Refiners with a number of different relationships i.e. the titanium to boron, titanium to carbon or titanium to aluminium.
Grain refinement in any wrought alloy can be discussed as to its overall ‘requirement’, feed rate and final application necessity.
Sure, the grain refinement manufacturing companies will insist on its use and quote some supporting statistics, faster casting speeds, greater fluidity, better mechanical properties, improved surface quality etc etc…. But in reality, for an extrusion product that goes through multiple heat cycles and is squeezed through a die and pulled and stretched does an 'accurate' grain size post cast really matter and therefore do we really need to add so much refining agent.
Some thoughts of mine for discussion. I have spent a significant amount of time looking at the need for grain refinement and curtailing its use to the bare minimum while achieving good castability while achieving the customers specification.
Balancing the feed rate accurately can save millions of dollars per year for the large casthouses and organisations.
Many casthouses and process engineer frequently over ‘inoculated’ the cast, with a 'just in case' mentality.
When what were really saying is “heck, I don’t buy the stuff so what difference does it make, better to be safe than sorry right”? No, not at all.
Over feeding grain refinement serves no purpose at all and in fact can be detrimental to the product increasing the TiB2 “inclusion” levels and possibly introducing detrimental oxides Al2O3 into the billet, depending upon the supplier of the raw material.
Compounding the fact that were just wasting money and adding to the overall product conversion cost!
In practical terms the best choice of grain refining agent and addition rates depend upon several factors, being.
1) The supplier
2) Casting process, casting speed and metal temperature
3) Composition of the alloy
4) Furnace Ti levels prior to inline feeding
5) Flow rate of the metal
6) Diameter of the billet (greater the billet diameter the higher the consumption), all molds being equal
7) Final product application
Lets first look at the mechanisms to grain refine and take an example of AlTiB 5/1. Firstly, what does that actually mean "AlTiB5/1".? Well, it relates to the ratio of Aluminium, Titanium and Boron.
In this example the amount of Titanium is 5% and 1% is Boron additive with the remainder being hopefully pure aluminium (or as near as possible). You will see similar compositions of 3:1 and 5:0.2 and so on.
Aluminium in its liquid form (T°C above 660°C) requires a substrate for heat extraction for the mechanism of solidification to exist, turning our liquid into the solid form we require for a semi finished products. In this case an extrusion billet. This transition can be the primary cooling effect of the mold wall of the container, an inclusion or ‘the self-imposed’ inclusion that is boron from the refining agent.
The solidification principles are predominantly the same, well to a point.
The boride particles in the grain refiner act as the nucleation substrate, the borides are present as insoluble particles, typically 1-2μm in size.
These boron particles exhibit a thin layer of Al3Ti, it is upon this excess titanium that the nucleation and growth of the aluminium crystals begin.
“Excess” titanium is required to make the refiners work this can be added in the refining coil and also as a bulk refiner in the furnace. (typically 100 to 150ppm) works well. The balance between furnce Ti and RGF (Rod Grain Feeder) is the key to success!
I have seen over the years that different types of grain refiners and even different suppliers have varying levels of efficiency. A typical average grain size per refiner addition can be seen below in the chart from W.A. Schneider et al. - Light Metals 2003.
If you’re not feeding the grain refinement in the range above then something is wrong and this is costing you a lot of money.
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