Grain Refining : Ray Bignell
Introduction
In as cast aluminium, grains can be between 5 and 10mm in size and would give poor product performance. Examples of poor product performance are poor structures, porosity and analysis uniformity. Grain refining as the term suggests is achieved by adding very small amounts of TiBAl master alloy generally in the form of rod which reduces the size of the grains to a fraction of a mm. The benefits that a grain refining result in:
1. Increased Casting speed.
Casting speed is generally in two steps, the speed at the start of the cast and at run or steady state. In technical terms casting speed is the heat input side of the heat balance equation and the water flow is the heat extraction, these levels set by the mould technology in question. With this in mind therefore the start speed is set at a safe level, to avoid bleed out and one where the start of the cast is crack free. The casting speed at the start is therefore set a high as possible to get a crack free butt and it has been proven that a grain refined ingot butt will allow higher start speeds than a non grain refined butt. The same is true for the casting speed in steady state where a grain refined structure will allow a higher cast speed than a non refined system.
2. More Uniform Structure
Grain refined ingots will have a uniform structure throughout the ingot so from top to bottom and from core to edge will have grains that are very similar is size and shape.
3. Uniform deformation during downstream manufacturing
Ingots with a uniform structure therefore will give uniform mechanical properties of the wrought products as ingot deformation and recrystalisation is improved during the hot rolling operation.
4. Uniform Chemistry
A fine grain refined structure will give a uniform distribution of elements across the ingot and improved response during homogenisation.
5. Improved surface finish on sheet products
A fine grained sheet, plate or extrusion will give good anodising response with no signs of steaks that can be associated with large columnar grains.
6. Improved porosity in plate products
Hydrogen is ejected during solidification so bubbles are left at the grain boundaries so again a finer structure has more uniform porosity. Porosity is also known to influence tearing during hot rolling,
7. Heat Treatment response
Linked to mechanical properties a fine grained sheet or any other wrought product will give an enhanced response when subjected to a thermal treatment. Such treatments could be an annealing cycle for soft alloys or solution treatment and precipitation treatment for heat treatable alloys.
A non grain refined ingot or an interruption to a grain refiner supply will give what is known as a columnar structure. It is imperative that a fine equiaxed grain structure is obtained during the casting operation so that the ingot or billet produced can meet the wrought material specification that is required by end users.
Grain refinement is required right at the start of the process and be interrupted throughout the casting process. Any interruption will give a columnar or most commonly referred to feathered or twinned structure.
Solidification liberates heat so we would expect to see a plateau in the melt temperature time curve. In practice cooling below the equilibrium freezing point to required to nucleate the first dendrites. As dendrites grow heat is liberated and the temperature rises. The temperature drop required is termed as undercooling and is a measure on how difficult it is the nucleate the first dendrites. Typically grain refined alloys have very low undercooling compared to non grain refined alloys because the action of the grain refiner to bring on nucleation. Following the temperature rise it will fall again as heat is extracted.
When casting the first solid nuclei are close to the mould wall. These start to grow and form an outer equiaxed zone. Dendrites growing parallel or opposite the heat flow advance more rapidly. Other dendrite orientations are overgrown due to mutual competition leading to the formation of a columnar zone. Beyond a certain stage in their development branches break off and become detached and grow independently and become equiaxed.
Nucleation
To undergo homogeneous nucleation large undercooling needs to be observed but never is this seen in practice as the alloys produced will be solid id we wait for the undercooling to take place! In the absence of undercooling a method to obtain homogeneous nucleation is where grain refiners are used which add sites for nucleation to take place. In this case solid particles of TiB2 or TiC form the grain refiner and other solid constituents will too serve as nucleation sites for grain growth.
At the start of nucleation the structure is similar to the structure of aluminium. This process is known as lattice matching and whilst this is important for nucleation other contributing factors such as chemical effects, phases and segregations are known to assist. It is known that the more highly alloyed products for instance 2xxx and 7xxx are easier to grain refine than high purity versions.
The mechanism of grain refinement is not a complex process and many theories have been put forward since the evolution of the technique back in the 1930’s. In all probably four or five grain refining methods have evolved but worth noting is that most of the theories have to rely upon particles to nucleate grains and whilst in detail each are slightly different they all appear to be similar to the operator on the shop floor.
TiB2 Nucleants with Ti residual. This is where TiB2 particles act as substrates for TiAl3 which then nucleate alpha aluminium grains. TiB2 remain active and show no sign of fade and in time transform to (TiAl)B2. This can be regarded as the most popular of the methods to grain refine ingots.
TiAl3 Nucleants. This is where undissolved TiAl3 acts as nucleant. These TiAl3 particles only survive about 1 to 2 minutes before complete dissolution. This method is generally used for surface critical applications.
TiC Nucleants with residual Ti. This technique uses the same thinking as TiB2 nucleates but do not display agglomeration and are resistant to poisoning by Zirconium.
Residual Ti with no intentional nucleants. Raising solute titanium to 0.05% to 0.08% will lower the partition coefficient low enough to bring on constitutional under cooling. Nucleation occurs from oxides, walls and junk in the alloy.
Residual Ti only with no intentional nucleants. This is where we rely on adding sufficient to be in the peritectic region of the grain boundary and utilise the peritectic hulk theory where particles transform directly into the aluminium grain via the peritectic reaction.
As most cast houses use TiB2 rod with solute Ti the mechanism of grain refinement can be explained and really embodies the current thinking offered by the experts in the field.
Common grain refiners contain TiB2 and Al3Ti so probably the most accepted explanation is that a layer of TiAl3 has to be present on the TiB particle for to be capable of providing a nucleation site in alpha (FCC) aluminium. It is thought that the coating of the TiB particle occurs during the manufacturing of the grain refiner but the mechanism has yet to be explained.
Grain refiners contain both TiAl3 and TiB2. It is known that TiAl3 particles measure 80 microns (a grain of salt is 80 microns) and they need about one minute to dissolve. TiB particles typically measure between 1 and 5 microns and require a certain number if discrete particles to generate grains that survive. This is where that grain refiner comes into play and is added in the form of AlTiB rod to the molten metal flow from the furnace. The rod addition rate is calculation is base on the number of discrete particles required not the Ti added form the rod itself. To explain nucleation particles are provided from the rod whereas the total titanium content arises from other inputs to the furnace.
For all casting operations and the majority of alloys it is important to add titanium master alloy to the melt to maintain a surface layer on the TiB2 particles. The role of the Titanium, commonly known as solute titanium is required to obtain the surface layer on the TiB particles. It has been proved that 0.005% or 50ppm is sufficient for commercial alloys. To add titanium to a melt waffle plates containing Aluminium 6% or 10% Titanium are generally added to the furnace but other sources such as Aluminium 75% Ti compacts are now available. The titanium in the form of TiAl3 dissolves rapidly in the melt and can be measured when chemical analysis is carried out.
The role of solute titanium is twofold. Firstly along with other alloying elements assists in the provision of nucleation sites and dendritic growth. Secondly these alloying elements exert a growth restriction effect called constitutional undercooling so give more opportunity for nucleation events. With this in mind therefore if the undercooling is large then nuclei are present in this area so further nucleation and growth of equiaxed grains can occur ahead of the columnar front. This process continues until a fine equiaxed structure is obtained.
Growth restriction values m(k-1) can be estimated from phase equilibrium diagrams and from m (the slope of the liquidus) and k (solid composition divided by liquid composition).
To confirm therefore that the role of titanium is twofold for nucleation and growth resistance. If grain refining is a problem, altering the liquidus slope to achieve undercooling is difficult in practice so increasing solute titanium in the furnace will increase growth restriction.
The table summarises the effect of element concentration on the growth restriction and that titanium has a highly disproportional effect and 0.1% is equivalent to 4% silicon.
Taking all of this into consideration the grain refining strategy for an individual alloy will depend on the alloy chemistry. In most cases TiB2 rod is added to the trough but the amount of titanium added to the furnace will depend on the growth restriction factor of the alloy. In general terms:
Before the introduction of the in “launder rod addition” grain refinement was achieved by adding a 5%Ti 1% B waffle to the furnace. Typically between1kg – 1.5kg/tonne was the typical addition and whilst this gave fine equiaxed structures were obtained several operational problems arose. Such problems were that of “fade” whereby the grain refining efficiency decreased over time, agglomeration of TiB2 particles and poisoning of the grain refiner come to mind.
The reason for the fade is that the TiB2 from the waffle agglomerated which forming a sludge which fell to the hearth of the furnace. Stirring the furnace partly regenerated the grain refiner but not to such a marked extent when compared with the efficiency following the initial waffle addition. Furnaces were generally static design so the stirring was normally performed after three hours but this in itself was not ideal as this would lead to the formation of spinels (hard oxides). Furnaces today generally are tilting design and therefore cleaner and with the TiB waffle addition now in the dim and distant past as grain refining technology improves fade is no longer an issue.
The poisoning of grain refiners is well documented and in the main is a problem in alloys that contain zirconium. Whilst this does not affect the majority of the recipients of this presentation it is a real problem for the “hard” alloy producers. Zirconium is added to 7xxx series alloys to promote recrystalisation after hot rolling. Added as a 5% -10% Zr –Al master alloy the temperature of the furnace needs to be above 730°C to avoid zirconium aluminide, ZrAl3. This element is also very sludgy and blocks inline filters and mould distribution devices and in situations like this the drop has to be halted and remedial steps taken. Poisoning of the grain refiner was more of an issue with furnace waffle additions as the TiB was in the furnace and in contact with the elemental Zr in the metal. In such cases ZrB was formed rather than TiB2, this compound will not nucleate FCC aluminium. As the rod type grain refiners were introduced together with a furnace addition of titanium Zr poisoning is no longer reported as an issue.
Chromium is too regarded as a possible candidate for poisoning but with the rod addition and solute Ti route and like zirconium is no longer reported.
In silicon containing alloys, contents above 3% have the tendency to form TiS2 on the surface of the TiB2 so effecting the nucleation force of the boride particle so it is common to use a lot higher solute titanium levels in these group of alloys.
βeta
Aluminium in its purest form is very soft (poor growth conditions) and the earliest work suggested that alloying with other elements strengthen the alloy and in some cases the alloy can show strengthening by applying a thermal treatment. It has been previously discussed that these alloying elements assist in nucleation and to further this subject a measure of the constitutional effect for cellular to dendritic growth is known as β. This sums the total solute effect on the melt phase diagram:
Many aluminium alloys suffer from hot tearing (cracking) during casting due to high thermal stresses developed during solidification. The type, size and morphology of intermetallics formed during the final stages of solidification can affect hot tearing tendency.
Grain refiner additions, impurity levels and melt cleanliness have all recently been shown to individually affect secondary intermetallic phase selection in Al alloys. In turn, the type, size and morphology of such intermetallics can significantly affect the ability to carry out downstream processing and the mechanical properties of final components. In strong alloy the volume % secondary phase is known to have an effect on the fracture mechanics of the plate so too is the Fe bearing phase. The understanding of alloy is an entirely separate discussion but demonstrates the need to control the whole of the process for each from melting/alloying, grain refinement and casting and homogenisation.
Common available commercial Al-Ti-B grain refiners in rod form are:
Al-3-Ti-1B
Al-5Ti-1B
Al-3Ti-0.2B
Al-5Ti-0.2B
Al-10Ti-0.4 B
For furnace titanium additions these are commercially available as:
Al-6Ti Waffle
Al-10Ti Waffle
Al-70Ti Compact
Used in the form of rod enables that additions are accurately controlled and by precise machine control, they do not fade and do not contaminate the furnace. Injection of rod is generally just after the furnace exit (but not at the spout) and against the flow from the furnace. Rod additions are always before inline devices so that adequate contact time can be allowed to allow complete dissolution. Just before the start it is quite common to lay an ingot or several feet of ALTiB rod so that any in line processes are grain refined and that the butt of the ingot is too grain refined. This not only helps to avoid start cracks but also means that the grain refinement process starts immediately.
As we are a customer of the supplier of grain refiners the supplier must provide release data on the products supplied. His tests must show that the grain refiner is effective and free form coarse particles and oxides. It is also important that the correct chemistry is supplied and the rod diameter is correct. These products are very expensive and over grain refining products serves no purpose at all and could form AlTiB2 agglomerates which are detrimental to product quality.
SEM of boride cluster showing alkali (K,P) peaks form source material with Ti and Al peaks. Worth noting is the O peak which indicates agglomerate probably formed around an oxide film.
Grain refiner manufacturers have to routinely test their products to check for the efficiency, boride size and cleanliness.
Such tests include:
On line thermal analysis to check for undercooling. (Alu-Delta)
Aluminium Association TP1
Alcoa Directional cold finger test.
Certificates supplied with grain refiners should always contain this information. Trial for new suppliers materials or changes to their current rod should first be approved by the customer(s) effected from this this trial so any change in their downstream process can be linked to the trial, similarly if the trial aids and benefits. Also consult the changes through your respective technical - Quality - or change management group before full implementation is made.
The above are results for the TP1 test ranging from 1 which is a columnar structure through to rating number 10 which is a fine equiaxed. Products with a number 10 rating are required for all applications.
As mentioned previously grain refiners can create problems the most acclaimed if the premature blocking of filtration equipment as TiB2 clusters can combine with oxide films to form spinels which are large in size. It is also known that grain refiners can give fir tree structure in 1xxx and vertical folds in 5xxx alloys. This just supports both quality control and correct addition rates and certified machinery are required when rod refiners are being used.
One alternative to AlTiB rod refiners are titanium carbon systems, these refiners were developed initially from “super6” rod refiners which contained very small amounts of carbon. This grain refiner contains both TiAl3 which is soluble and TiC which is insoluble and do not agglomerate like the TiB2 equivalent. They are known not to produce vertical folds in 5xxx and 1xxx can still exhibit fir tree. As TiC does not agglomerate this feature can be used to increase the life of filtration devices and in strong alloy can be used to have better control of grain size. Such are the issues with TiB grain refiners over time the use of TiC will increase.
Conclusions
To obtain a fine equiaxed structure a source of potent nuclei together with adequate growth restriction factor in the alloy. It is well proved that titanium added to the furnace is the best growth restrictor and should always be used no mater how small the addition is. Grain refiner rod used without solute titanium will not be as efficient as with. A grain refining strategy should be for each alloy group shall be devised and should be documented in SOP’s that should be adhered to.
Remember grain refiners are expensive. Adding too much is detrimental to product quality and is a complete waste.
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George