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What Is A Void In Aluminum Extrusion

What Is A Void In Aluminum Extrusion

What Is A Void In Aluminum Extrusion


At the tail end of some extruded products, after low magnification inspection, there is a horn-like phenomenon in the middle part of the cross section, which is called shrinking tail. Generally, the tail of the forward extrusion product is longer than that of the reverse extrusion, and the soft alloy is longer than the hard alloy. The shrinkage of the forward extruded product is mostly in the form of an annular disjoint layer, and the shrinkage of the reverse extruded product is mostly in the shape of a central funnel. The metal is extruded to the back end, and the ingot skin and foreign inclusions accumulated on the dead corner of the extrusion cylinder or the gasket flow into the product to form a secondary shrinkage; when the residual material is too short and the center of the product is insufficiently fed, the form a type of abbreviation. From the end of the tail to the front, the tail gradually becomes lighter and disappears completely. The main cause of shrinkage

  • 1. The residual material is too short or the length of the cut end of the product does not meet the regulations;
  • 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 squeeze pad (a raised pad in the middle);
  • 5. The temperature of the extrusion cylinder is too high;
  • 6. The extrusion cylinder and the extrusion shaft are not aligned;
  • 7. The surface of the ingot is not clean, there are oil stains, segregation tumors and folding and other defects are not removed;
  • 8. The inner sleeve of the extrusion cylinder is not clean or deformed, and the inner lining is not cleaned with a cleaning pad in time.

Prevention method

  • 1. Leave residue and cut tails as required;
  • 2. Keep the molds clean;
  • 3. Improve the surface quality of the ingot;
  • 4. Reasonably control the extrusion temperature and speed to ensure smooth extrusion;
  • 5. Except for special circumstances, it is strictly forbidden to apply oil on the surface of the tool and mold;
  • 6. The gasket is properly cooled.

Coarse-Grained 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.


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.

Poor Welding

The phenomenon of weld delamination or incomplete welding of the hollow product extruded by the split die at the weld is called poor welding. The main cause of poor welding

  • 1. The extrusion coefficient is small, the extrusion temperature is low, and the extrusion speed is fast;
  • 2. Extrusion wool or tools are not clean;
  • 3. Oil the mold;
  • 4. Improper mold design, insufficient or unbalanced hydrostatic pressure, unreasonable design of shunt holes;
  • 5. There is oil stain on the surface of the ingot.

Prevention method

  • 1. Properly increase the extrusion coefficient, extrusion temperature and extrusion speed;
  • 2. Reasonable design and manufacture of molds;
  • 3. The extrusion cylinder and extrusion gasket are not oiled and kept clean;
  • 4. Use ingots with clean surfaces.

Extrusion Cracks

This is a small arc-shaped crack on the edge of the transverse test piece of the extruded product, and periodic cracking at a certain angle along its longitudinal direction, which is hidden under the epidermis in light cases, and serrated cracks in the outer layer in severe cases, which will seriously damage the continuity of the metal. Extrusion cracks are formed when the metal surface is torn apart by the excessive periodic tensile stress of the die wall during the extrusion process. The main cause of extrusion cracks

  • 1. The extrusion speed is too fast;
  • 2. Extrusion temperature is too high;
  • 3. The extrusion speed fluctuates too much;
  • 4. The temperature of the extruded wool is too high;
  • 5. When the porous die is extruded, the die arrangement is too close to the center, so that the central metal supply is insufficient, so that the difference between the center and the edge flow rate is too large;
  • 6. The ingot homogenization annealing is not good.

Prevention method

  • 1. Strictly implement various heating and extrusion specifications;
  • 2. Frequent inspection of instruments and equipment to ensure normal operation;
  • 3. Modify the mold design and carefully process it, especially the design of the mold bridge, welding room and edge radius, etc. should be reasonable;
  • 4. Minimize sodium content in high magnesium aluminum alloys;
  • 5. The ingot is homogenized and annealed to improve its plasticity and uniformity.


The local skin metal is continuously or discontinuously separated from the base metal, and it is manifested as a circular single or strip-shaped cavity raised defect, called a bubble. The main cause of bubbles

  • 1. When extruding, the extrusion cylinder and extrusion pad have dirt such as moisture and oil;
  • 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 moisture in the lubricant;
  • 4. The ingot structure itself has loose and porosity 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 hydrogen content in the product is too high;
  • 7. Extrusion cylinder temperature and ingot temperature are too high.

Prevention method

  • 1. The surfaces of tools and ingots should be kept clean, smooth and dry;
  • 2. Reasonably design the matching size of the extrusion cylinder and the extrusion gasket, check the size of the tool frequently, repair the extrusion cylinder in time when there is a big belly, and the extrusion gasket should not be out of tolerance;
  • 3. Ensure that the lubricant is clean and dry;
  • 4. Strictly abide by the extrusion process operation process, exhaust in time, cut correctly, do not apply oil, completely remove the residual material, keep the blank and the mold clean and not polluted.


This is the phenomenon of local separation between the skin metal and the base metal of the aluminum alloy extrusion product. The main cause of peeling

  • 1. When the alloy is replaced and extruded, the inner wall of the extrusion cylinder is adhered to the bush formed by the original metal, which is not cleaned properly;
  • 2. The extrusion cylinder and the extrusion pad are not properly matched, and the inner wall of the extrusion cylinder is lined with local residual metal;
  • 3. It is extruded by lubricating extrusion cylinder;
  • 4. There is metal on the die hole or the working belt of the die is too long.

Prevention method

  • 1. Thoroughly clean the extrusion cylinder when extruding the alloy;
  • 2. Reasonably design the matching size of the extrusion cylinder and the extrusion gasket, check the tool size frequently, and the extrusion gasket cannot be out of tolerance;
  • 3. Clean up the residual metal on the mold in time.


The single-striped mechanical scars caused by the contact between the sharp objects and the surface of the product during relative sliding are called scratches. The main cause of scratches

  • 1. Improper assembly of tools, unsmooth guides and worktables, 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 inappropriate spreader.

Prevention method

  • 1. Check and polish the mold working belt in time;
  • 2. Check the outflow channel of the product, it should be smooth, and the guide path can be properly lubricated;
  • 3. Prevent mechanical rubbing and scratching during handling.

Mold Marks

This is a trace of longitudinal unevenness on the surface of the extruded product, and all extruded products have die marks to varying degrees. The main cause of mold marks The main reason: the mold working belt cannot achieve absolute smoothness. Prevention method

  • 1. Ensure that the surface of the mold working belt is clean, smooth and free of sharp edges;
  • 2. Reasonable nitriding treatment to ensure high surface hardness;
  • 3. Correctly repair the mold;
  • 4. The working belt should be designed reasonably, and the working belt should not be too long.

Twist, Bend, Wave

The phenomenon in which the cross section of the extruded product is angularly deflected in the longitudinal direction is called twisting. The phenomenon that the product is curved in the longitudinal direction or the knife shape is not straight is called bending. The continuous undulating phenomenon that occurs in the longitudinal direction of the product is called a wave. The main causes of twisting, bending, and waves

  • 1. The design and arrangement of the die holes are not good, or the size distribution of the working belt is unreasonable;
  • 2. Poor machining accuracy of die holes;
  • 3. The proper guide is not installed;
  • 4. Improper mold 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 method

  • 1. High level of mold design and manufacturing;
  • 2. Install a suitable guide, traction and extrusion;
  • 3. Use local lubrication, mold repair and diversion or change the design of the shunt hole 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.

Hard Bend

The sudden bending of an extruded product somewhere in the length direction 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, and sudden stop;
  • 2. Rigidly move the product during the extrusion process;
  • 3. The working surface of the extruder is uneven.

Prevention method

  • 1. Do not stop at random 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 table is smooth, without foreign matter, and the combined product is unobstructed.

Hemp Noodles

This is the surface defect of the extruded product, which means that the surface of the product is small and uneven, continuous flakes, spot scratches, pits, metal beans, etc. The main cause of pockmark

  • 1. The hardness of the mold is not enough or the hardness is uneven;
  • 2. Extrusion temperature is too high;
  • 3. The extrusion speed is too fast;
  • 4. The working belt of the mold is too long, rough or stuck with metal;
  • 5. The extruded wool is too long.

Prevention method

  • 1. Improve the hardness and hardness uniformity of the mold working belt;
  • 2. Heat the extrusion cylinder and the ingot according to the regulations, and use the appropriate extrusion speed;
  • 3. Reasonably design the mold, reduce the surface roughness of the working belt, and strengthen the surface inspection, repair and polishing;
  • 4. Use a reasonable ingot length.

Metal Pressing

During the extrusion process, metal chips are pressed into the surface of the product, which is called metal pressing. The main causes of metal intrusion:

  • 1. The wool ends are faulty;
  • 2. The inner surface of the wool is stuck with metal or the lubricating oil contains metal debris and other dirt;
  • 3. The extrusion cylinder is not cleaned, and there are other metal debris;
  • 4. The ingot is sunk into other metal foreign objects;
  • 5. There is slag inclusion in the wool.

Prevention method

  • 1. Remove the burrs on the wool;
  • 2. Ensure that the surface of the wool and the lubricating oil are clean and dry;
  • 3. Clean up the metal debris in the mold and extrusion cylinder;
  • 4. Select high-quality wool.

Non-Metal Pressing

Foreign matter such as stone black is pressed into the inner and outer surfaces of the extruded product, which is called non-metallic indentation. After the foreign matter is scraped off, the inner surface of the product will show depressions of different sizes, which will destroy the continuity of the surface of the product. The main cause of non-metallic intrusion

  • 1. The graphite particle size is coarse or agglomerated, containing moisture or oil, and the stirring is uneven;
  • 2. The flash point of cylinder oil is low;
  • 3. The ratio of cylinder oil and graphite is improper, and there is too much graphite.

Prevention method

  • 1. Use qualified graphite and keep it dry;
  • 2. Filter and use qualified lubricating oil;
  • 3. Control the ratio of lubricating oil and graphite.

Surface Corrosion

Extruded products that have not undergone surface treatment, after the chemical or electrochemical reaction between the surface and the external medium, cause the defect caused by the local damage of the surface, which is 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 cause of surface corrosion

  • 1. The products are exposed to corrosive media such as water, acid, alkali, salt, etc. during production, storage and transportation, or parked in a humid atmosphere for a long time;
  • 2. Improper alloy composition ratio;

Prevention method

  • 1. Keep the product surface and production and storage environment clean and dry;
  • 2. Control the content of elements in the alloy.

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, and the grains of the finished products are coarse;
  • 3. The amount of stretching and straightening is too large.

Prevention method

  • 1. Reasonably control the homogenization process;
  • 2. The deformation should be as uniform as possible (control the extrusion temperature, speed, etc.)
  • 3. Control the amount of tension correction not to be too large.


After extrusion, the area where the thickness of the product changes on the plane appears concave or convex. Generally, it cannot be observed with the naked eye. After surface treatment, fine shadows or bone shadows appear. The main cause of unevenness

  • 1. The mold working belt is not properly designed, and the mold repair is not in place;
  • 2. The size of the shunt hole or the pre-chamber is not suitable, and the force of pulling or expanding the profile in the cross area causes a slight change in the plane;
  • 3. The cooling process is uneven, and the cooling speed of the thick-walled part or the intersecting part is slow, resulting in different degrees of shrinkage and deformation of the plane during the cooling process;
  • 4. Due to the huge difference in thickness, the difference between the thick-walled part or the transition area organization and other parts of the organization increases.

Prevention method

  • 1. Improve the level of mold design, manufacture and mold repair;
  • 2. Ensure uniform cooling speed.

Vibration pattern

This is a periodic streak defect transverse to the surface of the extruded product. It is characterized by horizontal continuous periodic stripes on the surface of the product, and the stripe curve is consistent with the shape of the mold working belt. The main cause of vibration

  • 1. The extrusion shaft moves forward and shakes due to equipment reasons, which causes the metal to shake when it flows out of the hole;
  • 2. The metal shakes when it flows out of the die hole due to the mold;
  • 3. The mold support pad is not suitable, the mold rigidity is not good, and the vibration occurs when the extrusion force fluctuates.

Prevention method

  • 1. Use qualified molds;
  • 2. Appropriate support pads should be used when the mold is installed;
  • 3. Adjust the equipment.


The main cause of inclusions Since the inclusion billet has metal or non-metal inclusions, it was not found in the previous process, and remained on the surface or inside of the product after extrusion. Prevention method Strengthen the inspection of the billet (including ultrasonic inspection) to prevent the billet containing metal or non-metallic inclusions from entering the extrusion process.

Water Marks

The light white or light black irregular waterline marks on the surface of the product are called water marks. The main cause of water marks

  • 1. The drying is not good after cleaning, and there is residual moisture on the surface of the product;
  • 2. The residual moisture on the surface of the product caused by rain and other reasons has not been cleaned up in time;
  • 3. The fuel of the aging furnace contains water, and the water 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 the sulfur dioxide after combustion or polluted by dust;
  • 5. The quenching medium is polluted.

Prevention method

  • 1. Keep the surface of the product dry and clean;
  • 2. Control the moisture content and cleanliness of the aging charge;
  • 3. Strengthen the management of quenching medium.

The Gap

The ruler is horizontally superimposed 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 method Design and manufacture molds reasonably, strengthen mold repair, and control extrusion temperature and extrusion speed in strict accordance with regulations.

Uneven wall thickness

Extruded products of the same size have thin or thick walls in the same section or longitudinal direction, and the phenomenon is called uneven wall thickness. The main cause of uneven wall thickness

  • 1. The mold design is unreasonable, or the tool and mold assembly is improper;
  • 2. The extrusion cylinder and the extrusion needle are not on the same centerline, forming eccentricity;
  • 3. The lining of the extrusion cylinder is worn too much, and the mold cannot be firmly fixed, resulting in eccentricity;
  • 4. The uneven wall thickness of the ingot blank itself cannot be eliminated after the first and second extrusions. The uneven wall thickness of the wool after extrusion is not removed after rolling and stretching;
  • 5. The lubricating oil is unevenly applied, which makes the metal flow unevenly.

Prevention method

  • 1. Optimize the design and manufacture of tooling and molds, and rationally assemble and adjust;
  • 2. Adjust the center of the extruder and the extrusion die;
  • 3. Select qualified blanks;
  • 4. Reasonable control of extrusion temperature, extrusion speed and other process parameters.

Expand (And) Mouth

The defect that the two sides of the extruded profile products such as groove and I-shaped are inclined outward is called flaring, and the defect that is inclined inward is called parallel opening. The main causes of expansion (consolidation)

  • 1. The metal flow rate of the two “legs” (or one “leg”) of the trough or similar trough profiles or I-shaped profiles is uneven;
  • 2. The flow rate of the working belt on both sides of the groove bottom plate is uneven;
  • 3. Improper stretch straightening machine;
  • 4. After the product is out of the mold hole, the online solution treatment is unevenly cooled.

Prevention method

  • 1. Strictly control the extrusion speed and extrusion temperature;
  • 2. Ensure the uniformity of cooling;
  • 3. Correctly design and manufacture molds;
  • 4. Strictly control the extrusion temperature and speed, and install the tool and die correctly.

Straightening marks

The helical streaks produced when the upper roll of the extruded product is straightened are called straightening marks, and straightening marks cannot be avoided for any product straightened by the upper roll. The main cause of straightening marks

  • 1. There are edges on the roller surface of the straightening roller;
  • 2. The bending of the product is too large;
  • 3. Too much pressure;
  • 4. The angle of the straightening roller is too large
  • 5. The product has a large ovality.

Prevention method Take appropriate measures to adjust according to the cause.

Stop marks, instant impressions, bite marks

Stop extrusion during extrusion to produce stripes on the surface of the product and perpendicular to the extrusion direction, called stop marks; line or strip stripes on the surface of the product and perpendicular to the extrusion direction during extrusion , known as bite marks or instant impressions (commonly known as “fake parking marks”) During extrusion, the attachments that are stably adhered to the surface of the working belt are instantly detached and adhered to the surface of the extruded product to form patterns. The horizontal stripes of the working belt that appear when the extrusion is stopped are called parking marks; the stripes that appear during the extrusion process are called instant impressions or bite marks, and they will make a sound during extrusion. The main causes of stop marks, instant marks, and bite marks

  • 1. Uneven heating temperature of ingot or sudden change in extrusion speed and pressure;
  • 2. The main parts of the mold are poorly designed and manufactured, or the assembly is uneven and there are gaps;
  • 3. There is an external force perpendicular to the extrusion direction;
  • 4. The extruder does not run smoothly, and there is a phenomenon of crawling.

Prevention method

  • 1. High temperature, slow speed and uniform extrusion, the extrusion force remains stable;
  • 2. Prevent the external force in the vertical extrusion direction from acting on the product;
  • 3. Reasonable design of tools and molds, correct selection of mold materials, size matching, strength and hardness.

Scratches on the inner surface

The scratches on the inner surface of the extruded product during the extrusion process are called inner surface scratches. The main cause of internal surface abrasion

  • 1. The extrusion needle is stuck with metal;
  • 2. The temperature of the extrusion needle is low;
  • 3. The surface quality of the extrusion needle is poor and there are bumps;
  • 4. The extrusion temperature and speed are not well controlled;
  • 5. Improper ratio of extrusion lubricant;

Prevention method

  • 1. Increase the temperature of the extrusion cylinder and extrusion needle, and control the extrusion temperature and extrusion speed;
  • 2. Strengthen lubricating oil filtration, check or replace waste oil frequently, and apply oil evenly and appropriately;
  • 3. Keep the wool surface clean;
  • 4. Replace unqualified molds and extrusion needles in time, and keep the surface of extrusion molds clean and smooth.

Unqualified Mechanical Properties

The mechanical properties such as hb and hv of extruded products do not meet the requirements of technical standards or are very uneven, which is called mechanical properties incompatibility. The main causes of unqualified mechanical properties

  • 1. The main elements of the chemical composition of the alloy 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 billet 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 and Control Measures

  • 1. Strictly control the chemical composition according to the standard or formulate an effective internal standard;
  • 2. Use high-quality ingots or billets;
  • 3. Optimize extrusion process;
  • 4. Strictly implement the quenching process system;
  • 5. Strictly implement the artificial aging system and control the furnace temperature;
  • 6. Strict temperature measurement and temperature control.

Other Factors

In short, after comprehensive treatment, 30 kinds of defects of the above-mentioned aluminum alloy extrusion products can be effectively eliminated, high quality and high yield, long life, and beautiful product surface, creating a brand, bringing vitality and prosperity to the enterprise, and having significant technical and economic benefits.

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