Part two: Coarse grain zone, Aluminum alloy extrusion defect analysis and quality control method

Part two: Coarse grain zone, Aluminum alloy extrusion defect analysis and quality control method

Coarse grain zone, Aluminum alloy extrusion defect analysis and quality control method

Coarse grain zone causes and solutions

Coarse grain zone

The coarse grain zone is an annular coarse grain region formed on the periphery of the extruded product and is a structural defect of the extruded product.

The grain size in the coarse grain zone may exceed 10 to 100 times the original grain size and up to 800 to 1500 btm.

It causes the mechanical properties of the product lowered, and the fatigue resistance is lower than that of the central region, the crack is often generated in the coarse grain zone.

When the aluminum profile having such a defect is stretch-straightened, the surface becomes rough and the surface is not beautiful after anodizing.

Some aluminum alloy extruded products form a coarse recrystallized grain structure along the periphery of the product on a low-fold test piece after solution treatment, which is called a coarse grain zone.

Due to the different shape and processing method of the product, ring, arc and other forms of coarse grain zone can be formed.

The depth of the coarse grain zone is gradually reduced from the trailing end toward the front end to completely disappear.

The formation mechanism is a sub-grain region formed on the surface layer of the product after hot extrusion, and forms a coarse recrystallized grain region after heat solution treatment. 

The causes for coarse grain zone：

1. Extrusion uneven deformation;

2. Heat treatment temperature is too high, the holding time is too long;

3. The chemical composition of the ingot is unreasonable;

4. The general heat treatable strengthening alloy has a coarse grain zone after heat treatment, especially the type of 6A02, 2A50 alloy profile and bar is the most serious, can not be eliminated, only can be controlled within a certain range;

5 The extrusion deformation is small or insufficient deformation, or in the critical deformation range, easy to produce coarse grain zone. 

Solutions for coarse grain zone

1. The inner wall of the extrusion container is smooth and clean, reduce the friction during extrusion;

2. The deformation is as full and uniform as possible, and the process parameters such as temperature and speed are reasonably controlled;

3. To avoid the solution treatment temperature is too high or the holding time is too long;

4. Extrusion with multi-cavity mold;

5. Extrusion by backward extrusion and hydrostatic extrusion;

6. Produced by solution treatment-drawing-aging method;

7. Adjust the total alloy composition and increase the recrystallization inhibition element;

8. Make the extrusion at higher temperature;

9. Homogenized for some alloy ingots.

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