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The extrusion process and characteristics of the extrusion structure of aluminum alloy profiles

1. Main characteristics of aluminum alloy extrusion process

Squeezing is a process of obtaining profiles, tubes, or bars with specific cross-sectional shapes by applying tremendous pressure to a heated aluminum ingot (billet) to undergo plastic deformation within the confines of a mold. Its main characteristics are as follows:

1.1 Extremely high flexible production capacity ("near final shape" molding)

·By simply changing the mold, products with extremely complex shapes and huge cross-sectional differences can be produced on the same equipment. This is incomparable to other plastic processing methods such as rolling and forging.

1.2. Strong triaxial compressive stress state

·The billet is surrounded by the extrusion tube and mold, and is subjected to strong hydrostatic pressure. This greatly improves the plasticity of metals, enabling some low plasticity and difficult to deform aluminum alloys (such as 2-series and 7-series high-strength alloys) to be successfully processed and formed.

1.3. High production efficiency and high material utilization rate

·Continuous production of large-sized products, followed by simple cutting. Compared with mechanical processing, flash edge waste is less and the yield is higher.

1.4. Improve material organization and performance

·Key feature: Through large plastic deformation, coarse second phases and dendrites in the as cast microstructure can be broken, significantly refining the grain size and improving the density and mechanical properties of the product.

1.5. The product has high dimensional accuracy and good surface quality

·Modern extrusion technology can produce high-precision products, and due to the smooth surface of the mold and the protection of the oxide film, the surface of the extruded material is usually smooth.

1.6. Main limitations

·Uneven organization and performance: There is a significant organizational gradient along the length and cross-section of the product (caused by uneven deformation).

·Geometric waste (compression residue) is inevitable: after each extrusion, a portion of the billet is left in the extrusion cylinder, causing waste.

·High mold cost and limited lifespan: Complex mold design and manufacturing are difficult, and they are prone to wear and tear under high temperature and pressure.

2. The influence of extrusion process on the microstructure of aluminum alloy

Squeezing is a dynamic thermo mechanical coupling process that fundamentally changes the microstructure of the original ingot, with a profound impact.

2.1. Impact on grain structure

·Broken as cast microstructure: Completely breaking the coarse columnar crystals and equiaxed crystals formed by casting, forming a fibrous microstructure elongated along the extrusion direction.

·Dynamic response and recrystallization:

·Dynamic response: When extruded at lower temperatures or higher speeds (such as in 6-series alloys), dislocations undergo rearrangement to form a subgrain structure, and the structure still maintains an elongated shape, but the interior is composed of small subgrains. This state combines strength and toughness well.

·Dynamic recrystallization: At higher temperatures or greater degrees of deformation, new equiaxed grains are formed through nucleation and growth. Complete recrystallization will soften the material and reduce its strength.

·Common incomplete recrystallized structures after extrusion: fine recrystallized equiaxed crystals at the edges, and elongated unrecrystallized fibrous structures at the center. This is caused by uneven deformation.

2.2. Effects on second phase particles (precipitation phase, inclusion phase)

·Crushing and refinement: Coarse casting precipitates (such as Mg ₂ Si, Al ₂ Cu, etc.) and brittle impurities (such as compounds formed by Fe and Si) are crushed and spheroidized under strong shear deformation, and distributed in a chain like manner along the extrusion direction.

·Dissolution and Re precipitation: Squeezing at high temperatures causes some soluble second phases to dissolve into the matrix. The rapid cooling after extrusion (online quenching) creates supersaturated solid solution, creating conditions for subsequent aging strengthening (such as T5 and T6 states). This "deformation heat treatment" effect is an important reason for obtaining high strength in extruded aluminum alloys.

2.3. Form a strong texture

·Due to unidirectional large deformation, the grain orientation tends to be consistent, forming a strong texture. This leads to significant anisotropy in material properties: strength and toughness along the extrusion direction are usually better than in the transverse direction.

2.4. Affects the surface layer structure (producing "coarse crystal rings")

·Phenomenon: In many extruded profiles that can be strengthened by heat treatment (such as 6 series and 7 series), a layer of coarse recrystallized grain rings often appears in the edge area.

·Cause: The friction between the surface metal and the mold is large, the deformation is severe, and the temperature rise is higher. At the same time, it is strongly sheared by the sizing band of the mold hole, providing sufficient energy and nucleation sites for recrystallization. Coarse grained rings can reduce the fatigue performance and surface treatment (such as anodizing) quality of materials.

2.5. Welding performance

·During the extrusion of hollow profiles using a splitter die, the segmented metal flow can be completely re welded in the welding chamber under high temperature and pressure, forming a dense "weld line". A qualified extrusion process can make the performance of the welding zone close to that of the substrate.

3. How key process parameters affect organization

3.1. Extrusion temperature:

·Excessive temperature: Coarse grains are prone to overburning, resulting in the formation of coarse grain rings and a decrease in strength.

·Low temperature: high deformation resistance, increased extrusion force, easy to cause uneven structure, surface cracks, and insufficient dynamic recrystallization.

3.2. Squeezing speed:

·Speed too fast: The deformation heat cannot dissipate in time, resulting in an increase in actual temperature, which may cause coarse grains and surface overheating patterns.

·Slow speed: Low production efficiency and possible uneven deformation due to temperature drop.

3.3. Squeezing ratio (degree of deformation):

·The larger the compression ratio, the more complete the deformation, the better the refinement effect of the structure, and the more uniform the performance. But the squeezing force also increases accordingly.

3.4. Mold design:

·Directly affecting the uniformity of metal flow. Poorly designed molds can lead to significant differences in flow velocity between the center and edges, resulting in defects such as distortion and waves, and exacerbating tissue unevenness.

3.5. Cooling method (online quenching):

·The quenching speed determines the supersaturation of the solid solution, which directly affects the subsequent aging strengthening effect. Uneven cooling can result in residual stress and deformation.

summary

The core feature of aluminum alloy extrusion process is "large deformation+hot processing". Its core impact on the organization is:

·Positive aspects: Refining grain size, crushing second phase, achieving solid solution, laying the foundation for obtaining excellent comprehensive performance.

·In terms of challenges, it is inevitable to introduce issues such as organizational non-uniformity, anisotropy, and surface coarse grain rings.

Therefore, the goal of modern extrusion technology is to precisely control process parameters (temperature, speed, cooling) and mold design, while leveraging its advantages in microstructure refinement, to maximize the suppression of non-uniformity and defects, thereby producing high-performance and high uniformity aluminum alloy extrusion products. This is also why extruded materials are widely used in aerospace, transportation, and building structural fields that require extremely high strength and lightweight requirements.