Main Surface Defects of Aluminum Alloy Extruded Materials and Their Elimination Methods

Aluminum alloy profiles come in many varieties and specifications, with many production processes, complex technologies and high requirements. Various defects will inevitably occur during the entire production process of casting, extrusion, heat treatment finishing, surface treatment, storage, transportation and packaging.

Causes and elimination methods of surface defects:

1. Layering

Cause:

The main reason is that the surface of the ingot is stained with oil and dust, or the working part of the front end of the extrusion barrel is greatly worn, causing the accumulation of dirty metal around the elastic zone of the front end. It is formed when the sliding surface of the elastic zone is rolled into the periphery of the product during extrusion. It usually appears at the tail end of the product. In severe cases, it may also appear in the middle end or even the front end of the product. There are also unreasonable die hole arrangements, too close to the inner wall of the extrusion barrel, excessive wear or deformation of the extrusion barrel and extrusion pad, etc., which may also cause layering.

Elimination method:

1) Improve the cleanliness of the ingot surface.

2) Reduce the surface roughness of the extrusion cylinder and the mold, and promptly replace the extrusion cylinder and extrusion pad that are severely worn and out of tolerance.

3) Improve the mold design and make the die hole position as far away from the edge of the extrusion cylinder as possible.

4) Reduce the difference between the diameter of the extrusion pad and the inner diameter of the extrusion cylinder, and reduce the residual dirty metal in the lining of the extrusion cylinder.

5) Keep the lining of the extrusion cylinder intact, or use a gasket to clean the lining in time.

6) After cutting the remaining material, it should be cleaned and no lubricating oil should be allowed.

2. Bubbles or peeling

Cause:

The cause is that the internal structure of the ingot has defects such as looseness, pores, and internal cracks, or the extrusion speed is too fast during the filling stage, and the exhaust is not good, causing air to be drawn into the metal product.

Production reasons for bubbles or peeling include:

1) The extrusion cylinder and extrusion pad are worn and out of tolerance.

2) The extrusion cylinder and extrusion pad are too dirty and stained with oil, moisture, graphite, etc.;

3) There are too many deep shovel grooves on the surface of the ingot; or there are pores, blisters, loose tissue, and oil stains on the surface of the ingot. The hydrogen content of the ingot is higher;

4) The barrel was not cleaned when replacing the alloy;

5) The temperature of the extrusion cylinder and the extrusion ingot are too high;

6) The ingot size exceeds the allowable negative deviation;

7) The ingot is too long, filled too fast, and the temperature of the ingot is uneven;

8) The die hole design is unreasonable. Or cutting the remaining materials improperly;

Elimination method:

1) Improve the level of refining, degassing and casting to prevent defects such as pores, looseness, cracks and other defects in the ingot;

2) Reasonably design the matching dimensions of the extrusion cylinder and the extrusion pad; check the size of the tool frequently to ensure that it meets the requirements.

3) The extrusion pad cannot be out of tolerance;

4) When replacing the alloy, the cylinder should be cleaned thoroughly;

5) Slow down the speed of the extrusion and filling stage;

6) Keep the surfaces of tools and ingots clean, smooth and dry to reduce lubrication of the extrusion pad and mold;

7) Strict operation, correct cutting of residual materials and complete exhaust;

8) The ingot gradient heating method is used to make the head temperature of the ingot high and the tail temperature low. When filling, the head deforms first, and the gas in the cylinder is gradually discharged through the gap between the pad and the extrusion cylinder wall;

9)Check equipment and instruments frequently to prevent excessive temperature and excessive speed;

10) Reasonably design and manufacture the tooling and mold, and design the guide holes and diverter holes with an internal slope of 1° to 3°.

3. Extrusion cracks

Cause:

The occurrence of cracks is related to the stress and flow of the metal during the extrusion process. Taking surface periodic cracks as an example, the constraints of the mold shape and the effect of contact friction hinder the flow of the blank surface. The flow velocity in the center of the product is greater than the flow velocity of the outer metal, so that the outer metal is subject to additional tensile stress, and the center is subject to additional compressive stress. The generation of additional stress changes the basic stress state in the deformation zone, causing the axial working stress of the surface layer (the superposition of basic stress and additional stress) may become tensile stress. When this tensile stress reaches the actual fracture strength limit of the metal, inwardly expanding cracks will appear on the surface, its shape is related to the speed of the metal through the deformation zone.

Elimination method:

1) Ensure that the alloy composition meets the specified requirements, improve the quality of the ingot, minimize the content of impurities in the ingot that will cause a decrease in plasticity, and minimize the sodium content in high magnesium alloys.

2) Strictly implement various heating and extrusion specifications, and reasonably control the extrusion temperature and speed according to the material and characteristics of the product.

3) Improve the mold design, appropriately increase the length of the mold sizing belt and appropriately increase the fillet radius of the cross-section corners. In particular, the design of the mold bridge, soldering station chamber, and corner radius must be reasonable.

4) Improve the homogenization effect of the ingot and improve the plasticity and uniformity of the alloy.

5) When conditions permit, use methods such as lubrication extrusion, cone die extrusion or reverse extrusion to reduce uneven deformation.

6) Regularly inspect instruments and equipment to ensure normal operation.

4. Orange peel

Cause:

The main reason is that the internal structure of the product has coarse grains. Generally, the coarser the grains, the more obvious they are. Especially when the elongation is large, this kind of orange peel defect is more likely to occur.

Prevention methods:

To prevent the occurrence of orange peel defects, the main thing is to select the appropriate extrusion temperature and extrusion speed and control the elongation. Improve the internal structure of the ingot and prevent coarse grains.

5. Dark spots

Cause:

The main reason is that the cooling rate at the contact point between the thick-walled part of the profile and the heat-resistant felt (or graphite strip) is much smaller, and the solid solution concentration is significantly smaller than elsewhere. Therefore, the internal structure is different and the appearance shows a dark color.

Elimination method:

The main method is to strengthen the cooling of the discharging table and not stop at one place when reaching the sliding table and cooling bed, so that the products can be in contact with the heat-resistant felt at different positions to improve uneven cooling conditions.

6. Tissue stripes

Cause:

Due to the uneven structure and composition of extruded parts, band-like lines in the extrusion direction appear on the products. Generally appear in areas where wall thickness changes. This can be determined by corrosion or anodizing. When changing corrosion temperatures, banding may sometimes disappear or change in width and shape. The cause is due to uneven macroscopic or microstructure of the ingot, insufficient homogenization of the ingot or incorrect heating system for extruded product processing.

Elimination method:

1) The ingot should be refined to avoid using coarse grained ingots.

2) Improve the mold, select the appropriate shape of the guide cavity, and trim the guide cavity or mold sizing belt.

7. Longitudinal welding line

Cause:

It is mainly caused by the structural difference between the welded part of the metal flow and other parts of the metal in the extrusion die. Or it may be caused by insufficient aluminum supply in the mold welding cavity during extrusion.

Elimination method:

1) Improve the design of the bridge structure and welding cavity of the split combined mold. Such as adjusting the split ratio-the ratio of the split hole area to the extruded product area, and the welding cavity depth.

2) To ensure a certain extrusion ratio, pay attention to the balance between extrusion temperature and extrusion speed.

3) Do not use casting chains with oil stains on the surface to avoid mixing lubricants and foreign matter into the welding joint.

4) Do not apply oil on the extrusion cylinder and extrusion pad and keep them clean.

5) Appropriately increase the length of the remaining material.

8. Horizontal welding lines or stop marks

Cause:

The main reason is that during continuous extrusion, the metal in the mold is poorly welded to the front end metal of the newly added billet.

Elimination method:

1) Sharpen the blade of the scissors used to cut the remaining material and straighten it.

2) Clean the end surface of the billet to prevent lubricating oil and foreign matter from mixing in.

3) Increase the extrusion temperature appropriately and extrude slowly and evenly.

4) Reasonably design and select tool molds, mold materials, size coordination, strength and hardness.

9. Scratches, scratches

Cause:

The main reason is that when the products are transported horizontally from the slide-out table to the finished product sawing table, hard objects protrude from the cooling bed and scratch the products. Some of them occur during loading and transportation.

Elimination method:

1) The mold sizing belt should be smooth and clean, and the mold empty tool should also be smooth.

2) Check carefully when installing molds to avoid using molds with small cracks. Pay attention to the fillet radius when designing the mold.

3) Check and polish the mold work belt promptly. The mold hardness should be uniform.

4) Frequently check the cooling bed and finished product storage table. They should be smooth to prevent hard protrusions from scratching the products. The guide path can be properly lubricated.

5) When loading, spacers that are softer than the finished product should be placed, and transportation and lifting should be done smoothly and carefully.

10. Metal pressing

Cause:

The main reason is that the alumina slag generated at the empty knife position of the mold adheres to the extruded product and flows into the discharge table or slide out table and is pressed into the surface of the extruded material by the rollers. During anodization, no oxide film or indentations or pits are formed where the metal is pressed.

Elimination method:

1) Smooth the sizing belt and shorten the length of the sizing belt.

2) Adjust the empty knife of the sizing belt.

3) Change the layout of the die holes and try to avoid placing the flat surface of the product underneath and in contact with the rollers to prevent alumina slag from being pressed in.

4) Clean the surface and ends of the ingot and avoid metal shavings in the lubricating oil.

11. Other surface defects

Cause:

1) During the melting and casting process, the chemical composition is uneven, with metallic inclusions, pores, and non-metallic inclusions, the internal structure of the oxide film or metal is uneven.

2) During the extrusion process, the temperature and deformation are uneven, the extrusion speed is too fast, the cooling is uneven, and the structure is uneven in the contact with graphite and oil.

3) The mold design is unreasonable and the transition between the sharp corners of the mold is not smooth. The empty knife is too small and scratches the metal, the mold is poorly processed, has burrs and is not smooth, and the nitriding treatment is not good. The surface hardness is uneven and the work belt is not smooth.

4) During the surface treatment process, the bath liquid concentration, temperature, and current density are unreasonable, and the acid corrosion or alkali corrosion treatment process is improper.

Elimination method:

1) Control the chemical composition, optimize the casting process, strengthen purification, refinement and homogenization.

2) The ingot homogenization process requires rapid cooling.

3) Reasonably control the extrusion temperature and speed to ensure uniform deformation and use a reasonable ingot length.

4) Improve the design and manufacturing methods of the mold, increase the hardness of the mold working belt, and reduce the surface roughness.

5) Optimize the nitriding process.

6) Strictly control the surface treatment process to prevent secondary damage or pollution to the surface during acid corrosion or alkali corrosion.

Edited by May Jiang from MAT Aluminum

23 Jan, 2024