Coarse crystal ring
Some aluminum alloy extruded products form a coarse recrystallized grain structure area along the periphery of the product on the low magnification test piece after solution treatment, which is called a coarse grain ring. Due to the different shapes and processing methods of the products, ring-shaped, arc-shaped and other forms of coarse-grained rings can be formed. The depth of the coarse-grained ring gradually decreases from the tail to the front and disappears completely. The primary formation mechanism is the sub-grain region formed on the surface of the product after hot extrusion, and the coarse recrystallized grain region is formed after heating and solution treatment.
The main cause of coarse crystal ring
1. Uneven extrusion deformation
2. The heat treatment temperature is too high and the holding time is too long, so that the grains grow up;
3. The chemical composition of the gold is unreasonable;
4. After heat treatment, general heat-treatable strengthening alloys have coarse-grained rings, especially the shapes and bars of 6a02, 2a50 and other alloys are the most serious, which cannot be eliminated and can only be controlled within a certain range;
5. The extrusion deformation is small or the deformation is insufficient, or in the critical deformation range, and it is easy to produce a coarse crystal ring.
Prevention method
1. The inner wall of the extrusion cylinder is smooth and clean, forming a complete aluminum sleeve to reduce the friction during extrusion;
2. The deformation should be as full and uniform as possible, and the process parameters such as temperature and speed should be reasonably controlled;
3. Avoid the solution treatment temperature is too high or the holding time is too long;
4. Extrusion with porous die;
5. Extrusion by reverse extrusion method and static extrusion method;
6. Produced by solution treatment-drawing-aging method;
7. Adjust the total gold composition and increase the recrystallization inhibitory element;
8. Use higher temperature extrusion;
9. Some alloy ingots are not homogenized, and the coarse grain ring is shallow during extrusion.
Layered
This is a skin delamination defect formed when the metal flow is relatively uniform, and the surface of the ingot flows into the product along the interface between the mold and the front end elastic zone. On the transverse low-magnification test piece, it appears that there is a defect of different layers at the edge of the cross-section.
The main cause of stratification
1. There is dust on the surface of the ingot or the ingot has large segregation aggregates instead of car skin, metal tumors, etc., which are easy to form layers;
2. There are burrs on the surface of the blank or dirt such as oil stains, sawdust, etc., which are not cleaned before extrusion;
3. The position of the die hole is unreasonable, close to the edge of the extrusion cylinder;
4. The extrusion tool is seriously worn or there is dirt in the bushing of the extrusion cylinder, which cannot be cleaned and replaced in time;
5. The diameter difference of the extrusion pad is too large;
6. The temperature of the extrusion cylinder is much higher than that of the ingot.
Prevention method
1. Reasonable design of molds, timely inspection and replacement of unqualified tools;
2. Unqualified ingots are not installed in the furnace;
3. After cutting the residual material, it should be cleaned up without sticking to the lubricating oil;
4. Keep the lining of the extrusion cylinder intact, or clean the lining in time with a gasket.
