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What are the solid solution, aging, and homogenization heat treatment processes for aluminum alloys

Whether it's airplanes in the sky, high-speed trains on the ground, or mobile phones in the hand, lightweight and sturdy aluminum alloy has become the cornerstone of modern manufacturing. To achieve outstanding performance of aluminum alloy, precise heat treatment processes are indispensable. Among them, homogenization treatment, solid solution treatment, and aging treatment are the three most core technologies.

1. Homogenization Treatment

1.1. Professional definition

Homogenization treatment, also known as homogenization annealing, refers to heating aluminum alloy ingots or castings to a higher temperature close to the eutectic temperature (but lower than the melting temperature of low melting point eutectic, usually lower than the solid solution line temperature) after solidification, and holding them for a long time (up to several hours to tens of hours), and then cooling them in the furnace. The purpose is to eliminate dendrite segregation (intragranular segregation) inside the ingot through atomic diffusion, making the chemical composition and microstructure distribution more uniform.

1.2. In layman's terms, this is like noodles. In freshly kneaded dough, the distribution of flour (aluminum matrix) and water (alloying elements) is uneven, with many small lumps (component segregation). You put it aside to let it rise (keep it warm for a long time), allowing the moisture to slowly and fully penetrate into each flour particle, ultimately obtaining a smooth dough with uniform composition and consistent inside and outside, so that good bread can be made later (rolled or extruded into good profiles).

1.3. Main purpose

Eliminate dendrite segregation and chemical composition inhomogeneity in ingots.

Improve the plasticity of the ingot, prepare for subsequent hot rolling, hot extrusion and other hot deformation processing, and prevent cracking. Dissolve some non-equilibrium solidified low melting eutectic phases to improve the thermal workability of the material and the performance of the final product.

2. Solution Treatment

2.1. Professional definition

Solid solution treatment refers to heating aluminum alloy to a high-temperature single-phase region (where the soluble strengthening phase or transition phase in the alloy can fully dissolve into the matrix α - Al solid solution), holding for a sufficient time to maximize the dissolution of alloy elements (such as Cu, Mg, Si, Zn, etc.) into the aluminum matrix, forming a supersaturated solid solution, and then cooling at a fast enough rate (usually water quenching) to suppress solute atom precipitation, thereby obtaining a metastable single-phase microstructure at room temperature.

2.2.Simply put, it's like making a cup of instant coffee. You pour hot water (heating) into the coffee grounds (alloy elements), stir (keeping warm), and let the coffee grounds completely dissolve in the water. Then quickly put it in the refrigerator to freeze (quench), and the coffee froze before it could settle out. In the end, you get a cup of ice coffee with extremely high concentration and all flavors "locked" inside (supersaturated solid solution).

2.3. The main purpose is to prepare for subsequent aging treatment and is a prerequisite for obtaining high-strength precipitation strengthening effect.

Improve the plasticity and toughness of the alloy, making it easier for subsequent plastic processing.

3. Aging Treatment

3.1. Professional definition

Aging treatment refers to the process of holding aluminum alloys that have undergone solid solution quenching at a lower temperature (usually 120 ° C-190 ° C) for a certain period of time, so that solute atoms in the supersaturated solid solution precipitate in the form of dispersed small strengthening phases (such as GP zone, θ 'phase, and η'), thereby significantly improving the strength and hardness of the alloy. According to the processing temperature, it can be divided into natural aging (stored at room temperature) and artificial aging (heated and insulated).

3.2. In layman's terms, it's like letting that frozen cup of coffee slowly thaw. During the thawing process (low-temperature heating), the supersaturated coffee powder (solute atoms) inside begins to slowly and uniformly solidify into extremely fine particles (strengthening phase) and precipitate out. These countless small particles are everywhere, greatly enhancing the taste and richness (strength and hardness) of this' coffee '.

3.3. Main purpose: Strengthening. By precipitating strengthening phases, the strength, hardness, and yield limit of the alloy are significantly improved. Adjust the comprehensive mechanical properties of the alloy to meet the needs of different application scenarios.

Essential difference: Understanding the relationship and goals among three parties with just one image

A more vivid understanding:

You can see these three steps as a progressive process of "mixing first, dissolving then, and strengthening later".

Homogenization is a fundamental pre-treatment aimed at ingots with inherent deficiencies, laying a solid foundation for all subsequent processing.

Solid solution+aging is an inseparable strengthening combination punch, usually carried out continuously, targeting semi-finished or finished parts that have undergone "postnatal strengthening". Solid solution is' empowerment ', while aging is' release'.

To sum up, 

homogenization concerns the macroscopic uniformity of components.

The focus of solid solution and aging is to achieve ultimate strength through microscopic precipitation.