1. shrinkage
At the tail end of some extruded products, upon low-power inspection, there is a trumpet-like phenomenon of disjointed layers in the middle of the cross section, which is called shrinkage.
Generally, the shrinkage tail of forward extrusion products is longer than that of reverse extrusion, and the shrinkage tail of soft alloy is longer than that of hard alloy. The shrinkage tail of forward extrusion products is mostly manifested as an annular non-combined layer, while the shrinkage tail of reverse extrusion products is mostly manifested as a central funnel shape.
When the metal is extruded to the rear end, the ingot skin and foreign inclusions accumulated in the dead corner of the extrusion cylinder or on the gasket flow into the product to form a secondary shrinkage tail; when the residual material is too short and the shrinkage in the center of the product is insufficient, a type of shrinkage tail is formed. From the tail end to the front, the shrinkage tail gradually becomes lighter and disappears completely.
The main cause of shrinkage
1) The residual material is too short or the length of the product tail does not meet the requirements. 2) The extrusion pad is not clean and has oil stains. 3) In the later stage of extrusion, the extrusion speed is too fast or suddenly increases. 4) Use a deformed extrusion pad (a pad with a bulge in the middle). 5) The temperature of the extrusion barrel is too high. 6) The extrusion barrel and the extrusion shaft are not centered. 7) The surface of the ingot is not clean and has oil stains. Segregation tumors and folds have not been removed. 8) The inner sleeve of the extrusion barrel is not smooth or deformed, and the inner lining is not cleaned in time with a cleaning pad.
Prevention methods
1) Leave residual material and cut tail according to regulations 2) Keep the tools and dies clean 3) Improve the surface quality of the ingot 4) Reasonably control the extrusion temperature and speed to ensure smooth extrusion 5) Except in special circumstances, it is strictly forbidden to apply oil on the surface of tools and molds 6) Cool the gasket properly.
2. Coarse grain ring
On low-magnification test pieces of some aluminum alloy extruded products after solution treatment, a coarse recrystallized grain structure area is formed along the periphery of the product, which is called a coarse grain ring. Due to different product shapes and processing methods, coarse grain rings in ring, arc and other forms can be formed. The depth of the coarse grain ring gradually decreases from the tail end to the front end until it disappears completely. The formation mechanism is that the sub-grain area formed on the surface of the product after hot extrusion forms a coarse recrystallized grain area after heating and solution treatment.
The main causes of coarse grain ring
1) Uneven extrusion deformation 2) Too high heat treatment temperature and too long holding time cause grain growth 3) Unreasonable alloy chemical composition 4) Generally, heat-treatable strengthening alloys will produce coarse grain rings after heat treatment, especially 6a02, 2a50 and other alloys. The problem is most serious in the types and bars, which cannot be eliminated and can only be controlled within a certain range 5) The extrusion deformation is small or insufficient, or it is in the critical deformation range, which is prone to produce coarse grain rings.
Prevention methods
1) The inner wall of the extrusion cylinder is smooth to form a complete aluminum sleeve to reduce friction during extrusion. 2) The deformation is as full and uniform as possible, and the temperature, speed and other process parameters are reasonably controlled. 3) Avoid too high a solution treatment temperature or too long a holding time. 4) Extrusion with a porous die. 5) Extrusion by reverse extrusion and static extrusion. 6) Production by solution treatment-drawing-aging method. 7) Adjust the full gold composition and increase recrystallization inhibition elements. 8) Use higher temperature extrusion. 9) Some alloy ingots are not treated uniformly, and the coarse grain ring is shallow during extrusion.
3. Stratification
This is a skin delamination defect formed when the metal flows evenly and the surface of the ingot flows into the product along the interface between the mold and the front elastic zone. On the horizontal low-magnification test piece, it appears as a non-combined layer defect at the edge of the cross section.
The main causes of stratification
1) There is dirt on the surface of the ingot or there are large segregation aggregates on the surface of the ingot without car skin, metal tumors, etc., which are prone to layering. 2) There are burrs on the surface of the blank or oil, sawdust and other dirt stuck on it, and it is not cleaned before extrusion. Clean 3) The position of the die hole is unreasonable, close to the edge of the extrusion barrel 4) The extrusion tool is severely worn or there is dirt in the extrusion barrel bushing, which is not cleaned and not replaced in time 5) The diameter difference of the extrusion pad is too large 6 ) The extrusion barrel temperature is much higher than the ingot temperature.
Prevention methods
1) Reasonably design the mold, check and replace unqualified tools in a timely manner 2) Do not install unqualified ingots into the furnace 3) After cutting the remaining material, clean it up and do not allow lubricating oil to stick to it 4) Keep the lining of the extrusion barrel intact, Or use a gasket to clean the lining in time.
4. Poor welding
The phenomenon of weld stratification or incomplete fusion at the weld of hollow products extruded by a split die is called poor welding.
The main causes of poor welding
1) Small extrusion coefficient, low extrusion temperature, and fast extrusion speed 2) Unclean extrusion raw materials or tools 3) Oiling of molds 4) Improper mold design, insufficient or unbalanced hydrostatic pressure, unreasonable diversion hole design 5) Oil stains on the surface of the ingot.
Prevention methods
1) Appropriately increase the extrusion coefficient, extrusion temperature, and extrusion speed 2) Reasonably design and manufacture the mold 3) Do not oil the extrusion cylinder and extrusion gasket and keep them clean 4) Use ingots with clean surfaces.
5. Extrusion cracks
This is a small arc-shaped crack on the edge of the horizontal test piece of the extruded product, and periodic cracking at a certain angle along its longitudinal direction. In mild cases, it is hidden under the skin, and in severe cases, the outer surface forms a serrated crack, which will seriously damage the continuity of the metal. Extrusion cracks are formed when the metal surface is torn by excessive periodic tensile stress from the die wall during the extrusion process.
The main causes of extrusion cracks
1) Extrusion speed is too fast 2) Extrusion temperature is too high 3) Extrusion speed fluctuates too much 4) The temperature of extruded raw material is too high 5) When extruding with porous dies, the dies are arranged too close to the center, resulting in insufficient metal supply in the center, resulting in a large difference in flow rate between the center and the edge 6) Ingot homogenization annealing is not good.
Prevention methods
1) Strictly implement various heating and extrusion specifications 2) Regularly inspect instruments and equipment to ensure normal operation 3) Modify mold design and carefully process, especially the design of mold bridge, welding chamber and edge radius should be reasonable 4) Minimize the sodium content in high magnesium aluminum alloy 5) Perform homogenization annealing on the ingot to improve its plasticity and uniformity.
6. Bubbles
The defect in which the local surface metal is continuously or discontinuously separated from the base metal and appears as a round single or strip-shaped cavity protrusion is called a bubble.
The main causes of bubbles
1) During extrusion, the extrusion cylinder and extrusion pad contain moisture, oil and other dirt. 2) Due to the wear of the extrusion cylinder, the air between the worn part and the ingot enters the metal surface during extrusion. 3) There is contamination in the lubricant. Moisture 4) The ingot structure itself is loose and has pore defects. 5) The heat treatment temperature is too high, the holding time is too long, and the atmosphere humidity in the furnace is high. 6) The gas content in the product is too high. 7) The extrusion barrel temperature and the ingot temperature are too high.
Prevention methods
1) Keep the surfaces of tools and ingots clean, smooth and dry 2) Properly design the matching dimensions of the extrusion cylinder and extrusion gasket. Check the tool dimensions frequently. Repair the extrusion cylinder in time when it becomes bloated, and the extrusion pad cannot be out of tolerance. 3) Ensure that the lubricant is clean and dry. 4) Strictly abide by the extrusion process operating procedures, exhaust air in time, cut correctly, do not apply oil, thoroughly remove residual materials, and keep the blank and tool mold clean and free from contamination.
7. Peeling
in which local separation occurs between the surface metal and the base metal of aluminum alloy extruded products.
The main reason for peeling
1) When changing the alloy for extrusion, the inner wall of the extrusion barrel is adhered to the bushing formed by the original metal and is not cleaned properly. 2) The extrusion barrel and the extrusion pad are not properly matched, and there is local residual metal lining on the inner wall of the extrusion barrel. 3) Lubricated extrusion barrel is used for extrusion. 4) Metal is adhered to the die hole or the die working belt is too long.
Prevention methods
1) When extruding a new alloy, the extrusion barrel must be thoroughly cleaned. 2) Reasonably design the matching dimensions of the extrusion barrel and the extrusion gasket, frequently check the tool dimensions, and the extrusion gasket must not exceed the tolerance. 3) Clean the residual metal on the mold in time.
8. Scratches
The mechanical scratches in the form of single stripes caused by the contact between sharp objects and the surface of the product and the relative sliding are called scratches.
The main causes of scratches
1) The tool is not assembled correctly, the guide path and workbench are not smooth, there are sharp corners or foreign objects, etc. 2) There are metal chips on the mold working belt or the mold working belt is damaged 3) There are sand or broken metal chips in the lubricating oil 4) Improper operation during transportation and handling, and the lifting equipment is not suitable.
Prevention methods
1) Check and polish the mold working belt in time 2) Check the product outflow channel, which should be smooth and lubricate the guide appropriately 3) Prevent mechanical friction and scratches during transportation.
9. Bumps and bruises
The scratches formed on the surface of products when they collide with each other or with other objects are called bumps.
The main causes of bumps and bruises
1) The structure of the workbench, material rack, etc. is unreasonable. 2) The material baskets, material racks, etc. do not provide proper protection for metal. 3) Failure to pay attention to handling with care during operation.
Prevention methods
1) Operate carefully and handle with care. 2) Grind off sharp corners and cover the baskets and racks with pads and soft materials.
10. Abrasions
The scars distributed in bundles on the surface of an extruded product caused by relative sliding or dislocation between the surface of an extruded product and the edge or surface of another object are called abrasions.
The main causes of abrasions
1) Severe mold wear 2) Due to the high ingot temperature, aluminum sticks to the die hole or the die hole working belt is damaged 3) Graphite, oil and other dirt fall into the extrusion barrel 4) The products move against each other, causing surface scratches and uneven extrusion flow, resulting in the product not flowing in a straight line, causing scratches on the material, the guide path, and the workbench.
Prevention methods
1) Check and replace unqualified molds in time 2) Control the heating temperature of the raw material 3) Ensure that the extrusion cylinder and the raw material surface are clean and dry 4) Control the extrusion speed and ensure uniform speed.
11. Mold Mark
This is the mark of longitudinal unevenness on the surface of the extruded product. All extruded products have mold marks to varying degrees.
The main cause of mold marks
Main reason: The mold working belt cannot achieve absolute smoothness
Prevention methods
1) Ensure that the surface of the mold working belt is bright, smooth and without sharp edges. 2) Reasonable nitriding treatment to ensure high surface hardness. 3) Correct mold repair. 4) Reasonable design of the working belt. The working belt should not be too long.
12. Twisting, bending, waves
The phenomenon of the cross section of the extruded product being deflected in the longitudinal direction is called twisting. The phenomenon of the product being curved or knife-shaped and not straight in the longitudinal direction is called bending. The phenomenon of the product being continuously undulating in the longitudinal direction is called waving.
The main causes of twisting, bending and waves
1) The die hole design is not well arranged, or the working belt size distribution is unreasonable 2) The die hole processing accuracy is poor 3) The appropriate guide is not installed 4) Improper die repair 5) Improper extrusion temperature and speed 6) The product is not pre-straightened before solution treatment 7) Uneven cooling during online heat treatment.
Prevention methods
1) Improve the level of mold design and manufacturing 2) Install appropriate guides for traction extrusion 3) Use local lubrication, mold repair and diversion or change the design of diversion holes to adjust the metal flow rate 4) Reasonably adjust the extrusion temperature and speed to make the deformation more uniform 5) Appropriately reduce the solution treatment temperature or increase the water temperature for solution treatment 6) Ensure uniform cooling during online quenching.
13. Hard Bend
A sudden bend in an extruded product somewhere along its length is called a hard bend.
The main cause of hard bending
1) Uneven extrusion speed, sudden change from low speed to high speed, or sudden change from high speed to low speed, or sudden stop, etc. 2) Hard movement of products during extrusion 3) Uneven extruder work surface
Prevention methods
1) Do not stop the machine or change the extrusion speed suddenly. 2) Do not move the profile suddenly by hand. 3) Ensure that the discharge table is flat and the discharge roller is smooth and free of foreign matter, so that the finished product can flow smoothly.
14. Pockmarkes
This is a surface defect of the extruded product, which refers to the small, uneven, continuous flakes, point-like scratches, pitting, metal beans, etc. on the surface of the product.
The main causes of pockmarks
1) The mold is not hard enough or is uneven in hardness and softness. 2. The extrusion temperature is too high. 3) The extrusion speed is too fast. 4) The mold working belt is too long, rough or sticky with metal. 5) The extruded material is too long.
Prevention methods
1) Improve the hardness and hardness uniformity of the die working zone 2) Heat the extrusion barrel and ingot according to the regulations and use an appropriate extrusion speed 3) Rationally design the die, reduce the surface roughness of the working zone, and strengthen surface inspection, repair and polishing 4) Use a reasonable ingot length.
15. Metal Pressing
During the extrusion production process, metal chips are pressed into the surface of the product, which is called metal intrusion.
The main causes of metal pressing
1) There is something wrong with the end of the rough material; 2) There is metal on the inner surface of the rough material or the lubricating oil contains metal debris and other dirt ; 3) The extrusion cylinder is not cleaned and there are other metal debris: 4) Other metal foreign objects are inserted into the ingot; 5) There is slag in the rough material.
Prevention methods
1) Remove burrs on the raw material 2) Ensure that the raw material surface and lubricating oil are clean and dry 3) Clean out metal debris in the mold and extrusion barrel 4) Select high-quality raw materials.
16. Non-metallic press-in
The pressing of foreign matter such as black stone into the inner and outer surfaces of extruded products is called non-metallic pressing. After the foreign matter is scraped off, the inner surface of the product will show depressions of varying sizes, which will destroy the continuity of the product surface.
The main causes of non-metallic press-in
1) The graphite particles are coarse or agglomerated, contain water or the oil is not mixed evenly. 2) The flash point of the cylinder oil is low. 3) The ratio of cylinder oil to graphite is improper, and there is too much graphite.
Prevention methods
1) Use qualified graphite and keep it dry 2) Filter and use qualified lubricating oil 3) Control the ratio of lubricating oil and graphite.
17. Surface Corrosion
The defects of extruded products without surface treatment, which are caused by chemical or electrochemical reaction between the surface and the external medium, are called surface corrosion. The surface of the corroded product loses its metallic luster, and in severe cases, gray-white corrosion products are produced on the surface.
The main causes of surface corrosion
1) The product is exposed to corrosive media such as water, acid, alkali, salt, etc. during production, storage and transportation, or is parked in a humid atmosphere for a long time. 2) Improper alloy composition ratio
Prevention methods
1) Keep the product surface and the production and storage environment clean and dry 2) Control the content of elements in the alloy
18. Orange peel
The surface of the extruded product has uneven wrinkles like orange peel, also known as surface wrinkles. It is caused by the coarse grains during extrusion. The coarser the grains, the more obvious the wrinkles.
The main cause of orange peel
1) The ingot structure is uneven and the homogenization treatment is insufficient. 2) The extrusion conditions are unreasonable, resulting in large grains of the finished product. 3) The amount of stretching and straightening is too large.
Prevention methods
1) Reasonably control the homogenization process 2) Make the deformation as uniform as possible (control the extrusion temperature, speed, etc.) 3) Control the amount of tension and correction not to be too large.
19. Unevenness
After extrusion, the area where the thickness of the product changes on the plane appears concave or convex, which is generally not visible with the naked eye. After surface treatment, fine dark shadows or bone shadows appear.
The main causes of unevenness
1) The mold work belt is improperly designed and the mold repair is not in place. 2) The size of the shunt hole or front chamber is inappropriate. The pull or expansion force of the profile in the intersection area causes slight changes in the plane. 3) The cooling process is uneven, and the thick-walled part or the intersection part The cooling rate is slow, resulting in varying degrees of shrinkage and deformation of the plane during the cooling process. 4) Due to the huge difference in thickness, the difference between the structure of the thick-walled part or transition zone and that of other parts increases.
Prevention methods
1) Improve the level of mold design, manufacturing and mold repair 2) Ensure uniform cooling rate.
20. Vibration marks
Vibration marks are horizontal periodic stripe defects on the surface of extruded products. It is characterized by horizontal continuous periodic stripes on the surface of the product. The stripe curve matches the shape of the mold working belt. In severe cases, it has an obvious concave and convex feel.
The main causes of vibration marks
shaft shakes forward due to equipment problems, causing the metal to shake when it flows out of the hole. 2) The metal shakes when it flows out of the mold hole due to mold problems. 3) The mold support pad is not suitable, the mold rigidity is poor, and shaking occurs when the extrusion pressure fluctuates.
Prevention methods
1) Use qualified molds 2) Use appropriate support pads when installing the mold 3) Adjust the equipment.
21. Inclusions Main causes of inclusions
The main causes of inclusions
Because the included blank contains metal or non-metal inclusions, they are not discovered in the previous process and remain on the surface or inside of the product after extrusion.
Prevention methods
Strengthen the inspection of billets (including ultrasonic inspection) to prevent billets containing metal or non-metallic inclusions from entering the extrusion process.
22. Water marks
Light white or light black irregular water line marks on the surface of products are called water marks.
The main causes of water marks
1) Poor drying after cleaning, resulting in residual moisture on the surface of the product 2) Residual moisture on the surface of the product caused by rain and other reasons, which was not cleaned in time 3) The fuel of the aging furnace contains water, and the moisture condenses on the surface of the product during the cooling of the product after aging 4) The fuel of the aging furnace is not clean, and the surface of the product is corroded by burned sulfur dioxide or contaminated by dust. 5) The quenching medium is contaminated.
Prevention methods
1) Keep the product surface dry and clean 2) Control the moisture content and cleanliness of the aging furnace fuel 3) Strengthen the management of quenching media.
23. Gap
The ruler is superimposed transversely on a certain plane of the extruded product, and there is a certain gap between the ruler and the surface, which is called a gap.
The main cause of the gap
Uneven metal flow during extrusion or improper finishing and straightening operations.
Prevention methods
Design and manufacture molds rationally, strengthen mold repair, and strictly control extrusion temperature and extrusion speed according to regulations.
24. Uneven wall thickness
The phenomenon that the wall thickness of the same size extruded product is uneven in the same cross section or longitudinal direction is called uneven wall thickness.
The main causes of uneven wall thickness
1) The mold design is unreasonable, or the tooling assembly is improper. 2) The extrusion barrel and the extrusion needle are not on the same center line, resulting in eccentricity. 3) The inner lining of the extrusion barrel is worn too much, and the mold cannot be firmly fixed, resulting in eccentricity. 4) The wall thickness of the ingot blank itself is uneven, and it cannot be eliminated after the first and second extrusions. The wall thickness of the rough material is uneven after extrusion, and it is not removed after rolling and stretching. 5) The lubricating oil is unevenly applied, resulting in uneven metal flow.
Prevention methods
1) Optimize tool and die design and manufacturing, and reasonably assemble and adjust 2) Adjust the center of the extruder and extrusion tool and die 3)
Select qualified billet 4) Reasonably control process parameters such as extrusion temperature and extrusion speed.
25. Expansion (parallel)
The defect of the two sides of the extruded profile products such as groove-shaped and I-shaped products sloping outward is called flaring, and the defect of sloping inward is called paralleling.
The main causes of expansion (parallel)
1) Uneven metal flow rate of the two “legs” (or one “leg”) of the trough or trough-like profile or I-shaped profile 2) Uneven flow rate of the working belt on both sides of the trough bottom plate 3) Improper stretching and straightening machine 4) Uneven cooling of the online solution treatment after the product leaves the die hole.
Prevention methods
1) Strictly control the extrusion speed and extrusion temperature 2) Ensure the uniformity of cooling 3) Correctly design and manufacture the mold 4) Strictly control the extrusion temperature and speed, and correctly install the mold.
26. Straightening marks
The spiral stripes produced when the extruded product is straightened by the upper roller are called straightening marks. All products straightened by the upper roller cannot avoid the straightening marks.
The main causes of straightening marks
1) There are edges on the straightening roller surface 2) The curvature of the product is too large 3) The pressure is too high 4) The angle of the straightening roller is too large 5) The product has a large ovality.
Prevention methods
Take appropriate measures to adjust according to the causes.
27. Stop marks, momentary marks, bite marks
of the product perpendicular to the extrusion direction produced during the extrusion process are called bite marks or instantaneous marks (commonly known as “false parking marks”).
During extrusion, the attachments that are stably attached to the surface of the working belt will instantly fall off and adhere to the surface of the extruded product to form patterns. The horizontal lines on the working belt that appear when extrusion stops are called parking marks; the horizontal lines that appear during the extrusion process are called instantaneous marks or bite marks, which will make a sound during extrusion.
The main cause of stop marks, moment marks, and bite marks
1) The heating temperature of the ingot is uneven or the extrusion speed and pressure change suddenly. 2) The main part of the mold is poorly designed or manufactured or assembled unevenly or with gaps. 3) There is an external force perpendicular to the extrusion direction. 4) The extruder runs unsteadily and there is creeping.
Prevention methods
1) High temperature, slow speed, uniform extrusion, and keep the extrusion pressure stable 2) Prevent external forces perpendicular to the extrusion direction from acting on the product 3) Reasonably design the tooling and mold, and correctly select the material, size, strength and hardness of the mold.
28. Inner surface abrasion
The abrasion on the inner surface of the extruded product during the extrusion process is called inner surface abrasion.
The main causes of inner surface scratches
1) There is metal stuck on the extrusion needle 2) The temperature of the extrusion needle is low 3) The surface quality of the extrusion needle is poor and there are bumps and scratches 4) The extrusion temperature and speed are not well controlled 5) The ratio of extrusion lubricant is improper.
Prevention methods
1) Increase the temperature of the extrusion barrel and extrusion needle, and control the extrusion temperature and extrusion speed. 2) Strengthen the filtration of lubricating oil, check or replace the waste oil regularly, and apply oil evenly and in an appropriate amount. 3) Keep the surface of the raw material clean. 4) Replace unqualified molds and extrusion needles in time, and keep the surface of the extrusion mold clean and smooth.
29. Unqualified mechanical properties
If the mechanical properties of extruded products, such as hb and hv, do not meet the requirements of technical standards or are very uneven, it is called unqualified mechanical properties.
The main causes of unqualified mechanical properties
1) The main elements of the alloy’s chemical composition exceed the standard or the ratio is unreasonable 2) The extrusion process or heat treatment process is unreasonable 3) The quality of the ingot or bad material is poor 4) The online quenching does not reach the quenching temperature or the cooling speed is not enough: 5) Improper artificial aging process.
Prevention methods
1) Strictly control the chemical composition according to standards or formulate effective internal standards 2) Use high-quality ingots or blanks 3) Optimize the extrusion process 4) Strictly implement the quenching process system 5) Strictly implement the artificial aging system and control the furnace temperature 6) Strictly Temperature measurement and temperature control.
30. Other factors
In short, after comprehensive management, the above 30 defects of aluminum alloy extruded products have been effectively eliminated, achieving high quality, high yield, long life, and beautiful product surface, bringing vitality and prosperity to the enterprise, and achieving significant technical and economic benefits.
Post time: Dec-12-2024
