Aluminum Grain Refiner and Sr Modifier: What Each Does and How to Add It

An aluminum grain refiner and a strontium modifier solve two different problems in the same casting. The refiner controls the size and shape of the primary aluminum grains, while the modifier changes the eutectic silicon. Both are added as master alloys. Both lose effect if the melt sits too long, and they can interfere with each other. This guide covers what each one does, how to add it, why it fades and where hardeners fit in.

What an aluminum grain refiner does to the microstructure

Without treatment, a hypoeutectic Al-Si alloy often solidifies with coarse, columnar or large equiaxed grains. In one published test, untreated A356 cast in a steel mold showed a grain size of 3,120 μm. A refiner adds a large number of nucleation sites. Many small equiaxed grains then grow at once, instead of a few large ones.

The most common refiner is the Al-5Ti-1B master alloy. According to a review chapter on grain refining and the columnar-to-equiaxed transition, Al-5%Ti-1%B decomposes in the melt into Al3Ti and TiB2 particles. In Al-7%Si, the Al3Ti dissolves quickly. A thin titanium-rich layer then forms on the TiB2 surface and acts as the nucleation substrate.

Finer grains bring practical benefits on the shop floor:

• Better feeding during solidification, so shrinkage porosity is spread out and smaller.
• Lower tendency to hot tearing in long or restrained sections.
• More uniform response to heat treatment and machining.
• A more even surface after anodizing or polishing.

Silicon complicates the picture. The same chapter notes a catch in Al-7%Si. There, titanium reacts with silicon to form an (Al,Si)2Ti phase, which is a poor refining agent. For this reason, foundries casting high-silicon alloys often trial a boron-rich aluminum grain refiner instead of a standard Ti-B grade. The right choice depends on the alloy, so test before you switch.

Al-Sr modifiers and the eutectic silicon

A modifier works on a different phase. Unmodified eutectic silicon grows as coarse, acicular plates. These plates act as crack starters and lower elongation. Strontium, usually added as an Al-10Sr master alloy, changes those plates into a fine fibrous structure.

The effect on fatigue is documented. A study on porosity and fatigue in A356 from the Advanced Casting Research Center concludes that Sr-modified alloys show longer fatigue lives than unmodified alloys. The same work also shows that casting defects such as pores and oxide films cut fatigue life sharply. In other words, modification only pays off when the melt is also clean and degassed. Our article on graphite rotors for aluminum degassing covers that side of melt treatment.

An aluminum grain refiner and a modifier are usually used together. As a result, the real question is how they interact. That interaction explains most of the fade problems we discuss below.

Adding an aluminum grain refiner: rod, waffle or combined alloy

The physical form of the master alloy should follow the process, not the other way around. The three common options are:

Combined master alloys make chemistry control simpler. However, their ratio matters. In a 2022 study in Archives of Metallurgy and Materials, Al-3%B-3%Sr refined the grains and fully modified the eutectic silicon in A356. Al-4%B-1%Sr gave even smaller grains, but the eutectic silicon stayed coarser and acicular. So the same family of products can favor one effect over the other.

Whatever the form, a few habits keep results repeatable. Weigh each addition rather than counting pieces. Add the aluminum grain refiner at a consistent point in the cycle, record the time, and stir. Above all, confirm the result with a spectrometer check of titanium, boron and strontium. Add a grain-size test piece when the alloy or supplier changes.

Fade, settling and poisoning

Every treatment loses strength with time. The same 2022 paper explains that agglomeration and sedimentation of Al3Ti, TiB2 and SrB6 particles reduce refining efficiency, especially when the melt is held for 60 to 120 minutes. It also cites earlier work on Al-5Ti-1B. There, the density difference of the Al3Ti and TiB2 particles was a main factor in the loss of efficiency.

Boron and strontium can also react with each other. When they form SrB6, the strontium tied up in that compound is no longer free to modify the silicon. In the study, the alloy with more SrB6 showed larger grains and coarser eutectic silicon. After 120 minutes of holding, SrB6 and AlSiSr compounds had collected at the bottom of the casting. Notably, stirring the melt before pouring improved both grain size and silicon morphology.

In practice, fade and poisoning point to a short list of controls:

1. Add the aluminum grain refiner as late as the process allows, and avoid long holds after treatment.
2. Stir or gently agitate before pouring when metal has been standing.
3. Avoid stacking high boron with high strontium unless your trials show it works.
4. Keep in mind that strontium is lost over time, so recheck it after long holds or remelts.
5. Track returns: remelted gates and risers carry their own Ti, B and Sr, which shifts the balance.

Hardeners and alloying tablets for chemistry correction

An aluminum grain refiner or a modifier changes structure. Hardeners change chemistry. A hardener is a master alloy that carries an element such as copper, manganese, iron, chromium or titanium in a form that dissolves at normal melt temperatures. Without it, high-melting elements would need far more time and heat to go into solution.

Alloying tablets are the compact alternative. They are pressed from metal powder with a small amount of flux. Therefore, they sink and dissolve without a separate melting step. Tablets suit small corrections in holding furnaces and crucibles. Master alloy ingots suit larger, routine additions.

Both need the same discipline as refiners: a verified certificate of analysis, dry storage, weighed additions and a spectrometer check afterwards. If you are still choosing the base alloy, our overview of aluminum alloy series and their applications is a useful starting point.

AL-LINX supplies master alloys, including grain refiners, modifiers and hardeners, as well as alloying tablets. To verify additions on the floor, a spark spectrometer (OES) closes the loop.

FAQ

Can I use the same aluminum grain refiner for every alloy?

Not always. Standard Al-5Ti-1B works well in many wrought and low-silicon alloys. In high-silicon casting alloys, titanium can react with silicon and lose effect, so boron-based or combined grades deserve a trial.

Why did my modification disappear after a long hold?

Strontium is lost over time, and part of it can be tied up as SrB6 when boron is present. Particles also settle. Recheck strontium and stir before pouring.

Rod or waffle?

Rod suits continuous feeding into a launder. Waffle suits batch additions to a furnace, ladle or crucible.

Need refiners, modifiers or hardeners to your specification? Send the alloy, the target chemistry and your addition method, and we will quote several items in one request. Request a quote for master alloys and alloying tablets.