Analysis and quality control of main defects in th

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Analysis of main defects in aluminum alloy extrusion process and quality control methods

at the tail end of some extruded products, after low magnification inspection, there is a phenomenon that the interlayer in the middle of the section looks like a horn, which is called tail shrinkage. It is often seen that there are two kinds of situations, i.e. the first kind of tail shrinking or the second kind of tail shrinking. One kind of shrinkage tail is located in the central part of the product, showing wrinkled cracks or funnel-shaped holes. The second kind of shrinkage tail is located in the area of 1/2 of the radius of the product, showing annular or crescent cracks. Sometimes 0 There are continuous or discontinuous cracks or crack traces in the interlayer at mm, which is called the third kind of tailing by some people

generally, the tailing of forward extrusion products is longer than that of reverse extrusion, and the length of soft alloy is longer than that of hard alloy. The shrinkage tail of forward extrusion products is mostly annular stratified, and the shrinkage tail of reverse extrusion products is mostly central funnel-shaped

the metal is extruded to the rear end, and the ingot skin and foreign inclusions accumulated on the dead corner of the extrusion barrel or gasket flow into the product to form a secondary shrinkage tail; When the residue is left too short and the central feeding of the product is insufficient, a kind of tail shrinkage is formed. From the tail forward, the shrinking tail gradually becomes lighter and even disappears completely

the main causes of tail shrinkage

1, the residue is too short or the length of the product tail cutting does not meet the requirements

2. The extrusion pad is not clean and greasy

3. In the later stage of extrusion, the extrusion speed is too fast or suddenly increases

4. Use the deformed extrusion pad (the pad raised in the middle)

5. The temperature of extrusion cylinder is too high

6. The extrusion cylinder and extrusion shaft are misaligned

7. The ingot surface 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 smooth or deformed, and the lining is not cleaned with a cleaning pad in time

prevention methods

1. Leave residues and cut tails as required

2. Keep the tools and dies clean

3. Improve the surface quality of ingots

4. Reasonably control the extrusion temperature and speed to extrude smoothly

5. Except under special circumstances, it is strictly forbidden to apply oil on the surface of tools and molds

6. The gasket shall be properly cooled

II. Coarse crystal ring

some aluminum alloy extruded products form coarse recrystallized grain structure areas along the periphery of the products on the low magnification test pieces after solution treatment, which is called coarse crystal ring. Due to the different shapes and processing methods of products, ring, arc and other forms of coarse-grained rings can be formed. The depth of the coarse-grained ring gradually decreases with the tail end to the front end, and then disappears completely. The formation mechanism is that the sub grain area formed on the surface of the product after hot extrusion and the coarse recrystallized grain area formed after heating and solution treatment

the main causes of coarse-grained ring

1, uneven extrusion deformation '

2, the heat treatment temperature is too high, the holding time is too long, so that the grain grows

3. Unreasonable chemical composition of faecal gold

4. Coarse crystal rings are produced in general heat treatable strengthening alloys after heat treatment, 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. Small or insufficient extrusion deformation, or in the critical deformation range, is easy to produce coarse-grained rings

prevention method

1. The inner wall of the extrusion cylinder is smooth and smooth, forming a complete aluminum sleeve, reducing the friction during extrusion

2. The deformation shall be as full and uniform as possible, and the process parameters such as temperature and speed shall be reasonably controlled

3. Avoid too high solid solution treatment temperature or too long holding time

4. Extrude with porous die

5. Extrusion by reverse extrusion and static extrusion

6. Production by solution treatment drawing aging method

7. Adjust the composition of the whole gold and increase the recrystallization inhibiting elements

8. Adopt higher temperature extrusion

9. Some alloy ingots are not homogenized, and the coarse-grained ring is shallow during extrusion

III. layering

this is a kind of skin delamination defect formed when the metal flow is relatively uniform in Ningbo, an important gathering place of China's new material industry, and the ingot surface flows into the products along the interface between the mold and the front elastic zone. On the transverse low magnification test piece, there is a defect of delamination at the edge of the section

the main causes of layering

1, there is dust on the surface of the ingot, or there is a large segregation aggregate on the ingot without turning, and metal tumors are easy to produce layering

2. There are burrs or greasy dirt, sawdust and other dirt on the surface of the blank, which are not cleaned up before extrusion

3. The position of die hole is unreasonable, close to the edge of extrusion cylinder

4. The extrusion tool is seriously worn or there is dirt in the extrusion cylinder bushing, which is not cleaned up and not replaced in time

5. The diameter difference of extrusion pad is too large

6. The temperature of extrusion barrel is much higher than that of ingot

prevention methods

1. Reasonably design the mold, and timely check and replace unqualified tools

2. Unqualified ingots are not fired

3. After cutting the residue, it should be cleaned and no lubricating oil should be stuck

4. Keep the lining of the extrusion cylinder intact, or clean the lining with gaskets in time

IV. poor welding

the phenomenon of weld delamination or incomplete welding at the weld of hollow products extruded with shunting die is called poor welding

the main causes of poor welding

1. The extrusion coefficient is small, the extrusion temperature is low, and the extrusion speed is fast

2. Extruded wool or tools are not clean

3. Oil the mold

4. Improper mold design, insufficient or unbalanced hydrostatic pressure, and unreasonable design of shunt hole

5. There is oil stain on the surface of ingot

prevention methods

1. Appropriately increase the extrusion coefficient, extrusion temperature and extrusion speed

2. Reasonably design and manufacture molds

3. The extrusion cylinder and gasket shall not be oiled and kept clean

4. Adopt ingots with clean surface

v. extrusion crack

this is a small arc-shaped crack at the edge of the transverse test piece of the extruded product. It has a certain angle of periodic cracking along its longitudinal direction. It is hidden under the skin when it is light, and serrated cracking is formed in the outer surface layer when it is serious, which will seriously damage the metal continuity. The extrusion crack is formed by the tearing of the metal surface under the excessive periodic tensile stress of the die wall during the extrusion process

the main causes of extrusion cracks

1, the extrusion speed is too fast

2, the extrusion temperature is too high

3, the extrusion speed fluctuates too much

4. The temperature of extruded raw material is too high

5. When the multi hole die is extruded, the die arrangement is too close to the center, so that the central metal supply is insufficient, so that the flow velocity difference between the center and the edge is too large

6. Ingot homogenization annealing is not good

prevention methods

1. Strictly implement various heating and extrusion specifications

2. Patrol the instruments and equipment frequently to ensure normal operation

3. Modify the mold design and carefully process, especially the design of mold bridge, welding chamber and corner radius should be reasonable

4. Try to reduce sodium content in high magnesium aluminum alloy

5. The ingot is homogenized and annealed to improve its plasticity and uniformity

VI. bubbles

local skin metal is separated from the base metal continuously or discontinuously, which shows the defect of circular single or strip cavity bulge, which is called bubbles

the main causes of bubbles

1. During extrusion, the extrusion cylinder and extrusion pad contain dirt such as water 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 water in the lubricant

4. The ingot structure itself has loose and 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 hydrogen content in the product is too high

7. The extrusion barrel temperature and ingot temperature are too high

prevention methods

1. Keep the surface of tools and ingots clean, smooth and dry

2. Reasonably design the matching size of the extrusion cylinder and the extrusion gasket, and often check the tool size. When the extrusion cylinder has a big belly, it should be repaired in time, and the extrusion gasket should not exceed the 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 residues, and keep the blanks and tooling clean and free from pollution

VII. Peeling

this is the phenomenon of local separation between the skin metal and the base metal of aluminum alloy extruded products

the main causes of peeling

1. During alloy replacement extrusion, the inner wall of the extrusion cylinder is adhered with the bushing formed by the original metal, which is not cleaned up

2. The extrusion cylinder is not properly matched with the extrusion pad, and there is local residual metal lining on the inner wall of the extrusion cylinder

3. Use lubricating extrusion cylinder for extrusion

4. There is metal stuck on the mold hole or the mold working belt is too long

prevention method

1. Thoroughly clean the extrusion cylinder when replacing the alloy extrusion

2. Reasonably design the matching size of the extrusion cylinder and the extrusion gasket, and often check the tool size, and the extrusion gasket cannot exceed the tolerance

3. Clean the residual metal on the mold in time

VIII. Scratch

mechanical scars distributed in a single strip when sharp objects are in contact with the surface of the product and slide relatively are called scratches

the main causes of scratches

1. The tool assembly is incorrect, the guide path and workbench are not smooth, there are sharp corners or foreign objects, etc

2. Metal chips are stuck on the mold working belt or the mold working belt is damaged

3. There are sand particles or broken metal chips in the lubricating oil

4. Improper operation and lifting appliance during transportation

prevention methods

1. Timely check and polish the mold working belt

2. Check the outflow channel of the product, which should be smooth and the guide path can be properly lubricated

3. Prevent mechanical scratches and scratches during handling

IX. knock injury

the scars formed on the surface of products due to the collision between products and other objects are called knock injury

the main causes of knock and bruise

1, the structure of workbench, material rack and so on is unreasonable

2. Improper protection of metal by material basket and material rack

3. Do not handle with care during operation

prevention methods

1. Operate carefully and handle gently

2. Polish off the sharp corners, and cover the basket and rack with skids and soft materials

X. scratch

the bunched scars on the surface of the extruded product caused by relative sliding or dislocation after the surface contacts with the edges or surfaces of other objects are called scratch

the main causes of abrasion

1, serious die wear

2. Because the ingot temperature is too high, the die hole is stuck with aluminum or the working belt of the die hole is damaged

3. Dirt such as graphite and oil fall into the extrusion cylinder

4. The products move with each other, causing the surface scratch and uneven extrusion flow belt, causing the products not to flow in a straight line, resulting in the scratch of materials and guides and worktables

prevention methods

1. Timely check and replace unqualified molds

2. Control the heating temperature of wool

3. Ensure that the surface of extrusion cylinder and wool is clean and dry

4. Control the extrusion speed to ensure uniform speed

Xi. Die marks

this is the longitudinal uneven mark on the surface of extruded products. All extruded products have die marks of different degrees

main causes of die marks

main causes: the working belt of the die cannot be absolutely smooth

prevention methods

1. Ensure that the surface of the mold working belt is smooth, smooth and free of sharp edges

2. Reasonable nitriding treatment to ensure high surface hardness

3. Correct mold repair

4. Reasonably design the working belt

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