The meanings of metamorphic treatment, grain refinement, and primary phase refinement or morphology change are different, and sometimes the term metamorphic treatment is used to summarize these contents.

(1) Deterioration treatment. For simple binary Al Si alloys, such as Z102 containing 11%~13% Si, it is a typical eutectic alloy. Its organization consists of coarse needle shaped (should be layered) silicon and a eutectic of alpha (Al) solid solution, as well as a small amount of block shaped primary silicon (with alloy composition biased towards the upper limit). The mechanical properties of the alloy are not high, with a tensile strength not exceeding 140 MPa and an elongation of less than 3%. If a modifier (2-3%) containing Na or NaF is added to the melt before pouring for modification treatment, it seems that the eutectic point is shifted to the right and the eutectic temperature is lowered after modification treatment, causing the originally slightly hypereutectic alloy to become a hypoeutectic alloy. The primary silicon disappears, the eutectic is refined, and a primary α (Al) solid solution appears. The mechanical properties of the alloy have been improved, with a tensile strength of 180 MPa and an elongation of over 8%. It can even be pressure processed into plates and strips.

Using sodium salt as a modifier is prone to moisture absorption, difficult to store, and difficult to control the amount of sodium added. Castings are prone to porosity, and the duration of the modification effect is short. After the modification treatment is completed, it must be poured within 30 minutes. Later, it was discovered that not only sodium, but also strontium had the same effect and had a long shelf life, known as long-acting spoilage agents. Therefore, using sodium salt (NaF) or Al Sr intermediate alloy, it is obvious that Al Sr intermediate alloy is superior, easy to store, easy to control the amount added, and has a long shelf life.

The effect of adding phosphorus is opposite to that of sodium. Originally a eutectic alloy, it obtains a hypereutectic structure, which increases the amount of bulk primary silicon and improves the wear resistance of the alloy. The eutectic cast aluminum alloy used for producing pistons should be treated with phosphorus modification.

Using antimony for modification treatment does not show significant changes in the alloy casting structure, similar to the unmodified structure. However, after solid solution aging, the structure undergoes significant changes, especially the strength and toughness of the alloy are significantly improved.

(2) Refine grain size. Adding a small amount of titanium to aluminum alloy to refine the alpha (Al) grains can improve both the mechanical properties and processability of the alloy. The process of refining the structure of aluminum alloy by adding titanium is also known as modification treatment.

Grain refining agents must be nucleation promoters. When metals and alloys solidify into crystals, they undergo nucleation and growth processes. The formation of crystal nuclei can be divided into homogeneous nucleation and heterogeneous nucleation. Homogeneous nucleation must ensure that the entire melt has the same degree of undercooling. In actual production, it is impossible to achieve this. There are always places (such as the mold wall) that cool quickly, and some places (such as the center of the casting) that cool slowly, resulting in different degrees of undercooling and varying nucleation times. During production, it is expected to add a substance that can be used as a crystal nucleus to achieve a uniform and fine structure in the casting. Adding a small amount of titanium or a small amount of titanium and boron to aluminum alloy can achieve this goal. In addition, cooling rate is also an important factor, which not only affects nucleation but also controls the growth rate of crystals. Only by considering both nucleation (including external crystal nuclei) and cooling rate can a uniform and fine microstructure be obtained.

(3) Change the initial appearance. There are often primary needle like (flake like) compound phases in aluminum alloys, which have a significant impact on the alloy's properties, especially its mechanical properties. If Al3Fe is a needle like phase, the mechanical properties will decrease. If Fe: Si=4:1, AFeSi phase is formed, which changes the shape and significantly improves the mechanical properties, especially elongation. Adding manganese can also alter the morphology of iron containing phases.

In AI Mg alloys, when the magnesium content is high, the β (Mg5Al8) phase becomes coarse and exists at grain boundaries. Without solid solution treatment, the mechanical and corrosion resistance properties are poor. However, solid solution treatment requires a long time. If a small amount of zirconium is added, the β phase can be refined, which is beneficial for heat treatment.

In aluminum alloys, there are occasionally impurities of low melting point metals (such as Pb, Sn, Bi, etc.) present, which weaken the grain boundaries and cause melting at slightly higher temperatures; For example, aluminum contains sodium, causing "sodium embrittlement". Adding a small amount of rare earth elements to turn these metal impurities into compounds can eliminate the problems present in grain boundaries.

Therefore, it can be said that in the process of melting and casting, the elements or intermediate alloys that can improve the alloy structure, refine the alloy grains, change the size, morphology or distribution of primary phases, and remove harmful phases at grain boundaries are all called modifiers, and this process can be called modification treatment.

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