There are numerous varieties and specifications of aluminum alloy profiles, with many production processes, complex technology and high requirements. Throughout the entire production process, including melting and casting, extrusion, heat treatment and finishing, surface treatment, storage, transportation and packaging, various defects are inevitable.
Causes and elimination methods of surface defects:
I. Layering
Cause of occurrence
The main reason is that the surface of the ingot is contaminated with oil and dust, or the working part at the front end of the extrusion cylinder is worn out significantly, resulting in the accumulation of dirty metal around the elastic zone at the front end. It is formed when the sliding surface along the elastic zone is drawn into the periphery of the product during extrusion. It usually occurs at the tail end of the product, but in severe cases, it may also appear at the middle end or even the front end of the product. Unreasonable arrangement of die holes, being too close to the inner wall of the extrusion cylinder, as well as excessive wear or deformation of the extrusion cylinder and extrusion pad may also cause delamination.
Elimination method
1. Enhance the cleanliness of the ingot surface.
2. Reduce the surface roughness of the compression cylinder and die, and replace the extrusion cylinder and extrusion gasket that are severely worn and out of tolerance in a timely manner.
3. Improve the mold design and position the die holes 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 to minimize the residual dirty metal in the inner lining of the extrusion cylinder.
5. Keep the inner lining of the extrusion cylinder intact or clean it in time with a gasket.
After shearing the residual materials, they should be cleaned up thoroughly and must not be contaminated with lubricating oil.
Two. Bubbles or peeling
Cause of occurrence
The cause of this is that the internal structure of the ingot has defects such as porosity, pores, and internal cracks, or the extrusion speed during the filling stage is too fast, resulting in poor exhaust and drawing air into the metal product.
The production-related causes of bubbles or peeling include:
1. The wear of the extrusion cylinder and extrusion pad exceeds the tolerance.
2. The extrusion cylinder and extrusion pad are too dirty, contaminated with oil, moisture, graphite, etc.
3. There are too many and too deep grooves on the surface of the ingot. Or the surface of the ingot has pores, sand holes, loose structure and oil stains. The hydrogen content of the ingot is relatively high, etc.
4. The inside of the cylinder was not cleaned thoroughly when replacing the alloy.
5. The temperature of the extrusion cylinder and the extrusion ingot is too high;
6. The ingot size exceeds the allowable negative deviation;
7. The ingot is too long, filled too quickly, and the temperature of the ingot is uneven.
8. The design of the die holes is unreasonable. Or improper cutting of leftover materials;
Elimination method
1. Enhance the levels of refining, degassing and casting to prevent defects such as porosity, porosity and cracks in ingots;
2. Reasonably design the fit dimensions of the extrusion cylinder and the extrusion gasket; Regularly check the dimensions of the tools to ensure they meet the requirements. If a dog's belly appears on the extrusion cylinder, repair or replace the inner lining of the extrusion cylinder that is worn out beyond the tolerance in a timely manner.
3. The extrusion pad must not exceed the tolerance.
4. When replacing the table metal, the cylinder should be thoroughly cleaned.
5. Slow down the speed during the extrusion filling stage;
6. Keep the surfaces of tools and ingots clean, smooth and dry to reduce lubrication of extrusion pads and dies.
7. Operate strictly, correctly shear the residual material and completely vent.
8. By adopting the gradient heating method for ingots, the temperature at the head of the ingot is high while that at the tail is low. During filling, the head deforms first, and the gas inside the cylinder is gradually discharged through the gap between the gasket and the extrusion cylinder wall.
9. Regularly inspect the equipment and instruments to prevent excessive temperature and excessive speed.
10. Design and manufacture the tools and dies reasonably. The flow guide holes and flow distribution holes should be designed with an inner slope of 1° to 3°.
Iii. Extrusion Cracks
Cause of occurrence
The formation of cracks is related to the force and flow conditions of the metal during the extrusion process. Taking surface periodic cracks as an example, the constraint of the die shape and the effect of contact friction hinder the flow on the surface of the billet. The flow rate at the center of the product is greater than that of the outer metal, thus causing the outer metal to be subjected to additional tensile stress and the center to be subjected to additional compressive stress The generation of additional stress alters the fundamental stress state within the deformation zone, making it possible for the axial working stress on the surface layer (the superposition of fundamental stress and additional stress) to become tensile stress. When this tensile stress reaches the actual fracture strength limit of the metal, inward-expanding cracks will appear on the surface, and their shape is related to the speed at which the metal passes through the deformation zone.
Elimination method
1. Ensure that the alloy composition meets the specified requirements, improve the quality of ingots, minimize the content of impurities in ingots that can cause a decrease in plasticity, and in high-magnesium alloys, reduce the sodium content as much as possible.
2. Strictly implement all heating and extrusion specifications. According to the metal and characteristics of the products, reasonably control the extrusion temperature and speed.
3. Improve the mold design, appropriately increase the length of the mold sizing band and the radius of the fillet at the corners of the cross-section, especially the design of the mold bridge, welding chamber and corner radius should be reasonable.
4. Enhance the homogenization effect of ingots and improve the plasticity and uniformity of alloys.
5. When conditions permit, methods such as lubricating extrusion, conical die extrusion or reverse extrusion should be adopted to reduce uneven deformation.
6. Regularly patrol and inspect the instruments and equipment to ensure their normal operation.
Four. Orange peel
Cause of occurrence
The main cause of this is the coarse grains in the internal structure of the product. Generally, the coarser the grains, the more obvious the defect. Especially when the elongation is relatively large, this kind of orange peel defect is more likely to occur.
Prevention methods
To prevent the occurrence of defects in orange peels, it is mainly necessary to select an appropriate extrusion temperature and speed, and control the elongation rate. Improve the internal structure of the ingot to prevent coarse grains.
V. Black spots
Cause of occurrence
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 lower, and the solid solution concentration is significantly lower than in other areas. Therefore, the internal structure is different, which is manifested as a darker color on the appearance.
Elimination method
The main method is to enhance the cooling of the discharge table. When reaching the sliding table and the cooling bed, they should not stop at the same place. Let the products come into contact with the heat-resistant felt at different positions to improve the uneven cooling conditions.
Vi. Organizational Stripes
Cause of occurrence
Due to the uneven structure and composition of the extruded part, the product shows band-like patterns in the extrusion direction. It usually occurs in areas where the wall thickness changes. It can be identified through corrosion or anodizing treatment. When the corrosion temperature is changed, the band-like streaks may sometimes disappear or change in width and shape. The causes are due to the uneven macroscopic or microscopic structure of the ingot, insufficient homogenization treatment of the ingot, or incorrect heating system during the processing of extruded products.
Elimination method
1. The ingots should undergo grain refinement treatment to avoid using coarse-grained ingots.
2. Make mold improvements, select an appropriate shape for the flow guide cavity, and adjust the flow guide cavity or mold sizing band.
Vii. Longitudinal welding line
Cause of occurrence
It is mainly caused by the structural differences between the welded parts and other parts of the metal flow in the extrusion die. Or it could be caused by insufficient supply of aluminum in the die welding cavity during extrusion.
Elimination method
1. Improve the design of the bridge structure and the welding cavity of the split combination mold. Such as adjusting the split ratio (the ratio of the area of the split hole to the area of the extruded product) and the depth of the welding cavity.
2. It is necessary to ensure a certain extrusion ratio and pay attention to the balance between extrusion temperature and extrusion speed.
3. Do not use cast chains with oil stains on the surface to prevent lubricants and foreign substances from mixing into the welding joints.
4. Do not apply oil to the extrusion cylinder and extrusion gasket and keep them clean.
5. Appropriately increase the length of the residual material.
Viii. Transverse weld lines or stop marks
Cause of occurrence
The main reason is that during continuous extrusion, the metal inside the die does not weld well with the metal at the front end of the newly added billet.
Elimination method
Sharpen the scissors used for cutting the leftover materials and straighten them.
2. Clean the end face of the billet to prevent the mixture of lubricating oil and foreign objects.
3. Appropriately increase the extrusion temperature and extrude at a slow and uniform speed.
4. Reasonably design and select tools and dies, die materials, dimensional fit, strength and hardness.
Ix. Scratches and marks
Cause of occurrence
The main reason is that when the products are transported laterally from the sliding table to the finished product sawing table, hard objects protrude from the cooling bed and scratch the products. Some cases also occur during loading and handling.
Elimination method
The sizing band of the mold should be machined to be smooth and clean, and the empty tool of the mold should also be machined to be smooth.
2. When installing the mold, a careful inspection should be carried out to prevent the use of molds with fine cracks. When designing molds, attention should be paid to the radius of fillets.
3. Inspect and polish the working band of the mold in a timely manner, and the hardness of the mold should be uniform.
4. Regularly inspect the cooling bed and the finished product storage platform. They should be smooth to prevent any hard protrusions from scratching the products. The guide path can be lubricated appropriately.
5. When loading materials, softer spacers than the finished products should be placed. Transportation and hoisting should be carried out smoothly and carefully.
X. Metal Indentation
Cause of occurrence
The main reason is that the alumina slag generated at the empty knife position of the die adheres to the extruded product, flows into the discharge table or the slide table, and is pressed into the surface of the extruded material by the roller. During anodizing, the area where the metal is pressed in does not form an oxide film or indentations or pits.
Elimination method
Smooth the sizing band and shorten the length of the sizing band.
2. Adjust the empty knife of the sizing band.
3. Alter the layout of the die holes to avoid placing the flat surface of the product at the bottom in contact with the rollers as much as possible, so as to prevent the alumina slag from being pressed in.
4. Clean the surface and ends of the ingot thoroughly, and avoid metal shavings in the lubricating oil.
Xi. Other surface defects
Cause of occurrence
The causes generated during the casting process. The chemical composition is uneven, with metal inclusions, pores, non-metal inclusions, oxide films, and uneven internal structure of the metal.
2. Causes generated during the extrusion process. The temperature and deformation are uneven, the extrusion speed is too fast, the cooling is uneven, and the structure at the contact points with graphite and oil stains is uneven.
3. Reasons related to tools and molds. The mold design is unreasonable, and the transition of the sharp corners of the mold is not smooth. The empty tool is too small and scratches the metal. The die processing is poor, there are burrs and the surface is not smooth. The nitriding treatment is not good. The surface hardness is uneven and the working band is not smooth.
4. Reasons related to surface treatment. The concentration of the bath solution, temperature and current density are unreasonable, and the acid corrosion or alkali corrosion treatment process is inappropriate.
Elimination method
1. Control the chemical composition, optimize the casting process, enhance purification, refine treatment and homogenize treatment.
2. Rapid cooling is required for the homogenization treatment of ingots.
3. Reasonably control the extrusion temperature and speed to ensure uniform deformation and adopt a reasonable ingot length.
4. Improve the design and manufacturing methods of molds, enhance the hardness of the working band of molds, and reduce surface roughness.
5. Optimize the nitriding treatment process.
6. Strictly control the surface treatment process to prevent secondary damage or pollution to the surface during acid or alkali corrosion.
