characteristics, causes and remedies of main defects in aluminum alloy die castings

name of defects

characteristics and remedies of defects

causes

preventive measures and remedies

1. unqualified chemical composition

the contents of main alloy elements or impurities do not meet the technical requirements, which were found during chemical analysis or spectral analysis of the samples.

1. Incorrect calculation of ingredients, too little consideration of burning loss of elements, incorrect calculation of ingredients, etc. 2. Raw materials and recycled materials are put into use if their compositions are inaccurate or have not been analyzed;

3. It is not allowed to weigh the ingredients;

4. There are problems in feeding, such as adding less or more and omitting materials;

5. The storage of materials is chaotic, resulting in mixing;

6. The melting operation was not carried out according to the process, and the temperature was too high or the melting time was too long, so it was difficult to burn seriously;

7. The chemical analysis is inaccurate.

1. For metals with serious oxidation burning loss, the batching should be calculated according to the upper limit of technical standards or the upper limit of empirical burning loss value; After batching and core-checking;

2. Check whether the weighing, chemical analysis and spectral analysis are correct;

3. Calibrate the weighing instrument regularly and disable it inaccurately;

4. The raw materials required for batching are separately marked and stored, and used in sequence;

5. Strengthen the storage of raw materials with clear marks and orderly storage;

6. It is forbidden to overheat the alloy liquid or the melting time is too long;

7. After analysis before use, if the analysis is unqualified, the composition should be adjusted immediately, and the burden should be supplemented or diluted;

8. Smelting sediment and waste materials above grade 2 are re-refined and added for use, and the proportion should not be too high;

9, pay attention to the waste or use process, there are sand, lime, paint mixed.

2, blowhole

The large or small holes on the surface or inside of the casting have regular shapes; There are two types: decentralized and centralized; It can be found after the casting is X-rayed or machined.

1. The charge contains water vapor, which increases the concentration of water vapor in the furnace;

2. The furnace is large, and it has not been dried or dried completely after medium repair;

3. The alloy liquid is overheated, resulting in serious oxygen gettering;

4. The oxygen in the melting furnace and ladle tool is not dried;

5. The release agent is sprayed excessively or contains a large amount of gas;

6. The exhaust ability of the mold is poor;

7. The water content in coal, gas and oil exceeds the standard.

1. It is forbidden to load the furnace burden with moisture, and dry it at the hearth before loading;

2. Stoves, crucibles and tools are not allowed to be used before drying;

3. Pay attention to the overheating of molten aluminum, and adjust the furnace to the heat preservation state during downtime;

4. Refiners, slag removers, etc. are not allowed to be used before drying, and it is forbidden to stir the alloy liquid violently when using;

5. Strictly control the content of calcium;

6. Choose a release agent with a small amount of volatile gas, and pay attention to the low proportion and spraying amount;

7. gases such as undried chlorine gas and solids such as undried chlorine salt shall not be used.

3. Vortex holes

Small holes in the casting or large holes at the confluence of alloy liquid flow. It can be seen in machining or X-ray fluoroscopy.

1. The direction in which the alloy liquid is introduced into the cavity is incorrect, which washes away the cavity wall or core, generating eddy current and enveloping the air;

2. The injection speed is too fast, and gas is involved from the pouring port;

3. The inner gate is too thin, and the alloy liquid moves too fast, resulting in spraying and splashing, which blocks the exhaust tank prematurely;

4. The position of the exhaust slot of the mold is wrong, or the outlet section is too small, which makes the exhaust ability of the mold poor and the air cushion back pressure of the cavity large;

5. The position of the cavity in the mold is too deep, and the position of the exhaust slot is improper or too few;

6. The gap between the punch and the pressing chamber is too small, and when the punch returns too quickly, a vacuum is formed, and the uncondensed alloy liquid is pumped back to form pores; Or the punch returns too quickly;

7. The capacity of the press chamber is large and the amount of alloy liquid poured is too small.

1. Change the direction or position of the injection of the alloy liquid into the cavity, so that the alloy liquid first enters the deep and high part of the cavity or the wide part of the bottom layer, and the cavity air in this part is pressed into the exhaust groove, and the exhaust groove cannot be blocked before the alloy liquid fills the cavity;

2. Adjust the injection speed and quick pressure position, and shorten the second-speed distance as much as possible on the premise of enrichment;

3. Increase the entrance thickness of the inner gate under the condition of ensuring that no splashing and spraying can occur and the cavity can be filled;

4. Strengthen the exhaust capacity of the cavity: (1) The position of the exhaust groove should be considered not to be blocked by the alloy liquid that enters first; (2) Add an overflow trough, and pay attention to that the joint between the overflow trough and the workpiece should not be too thick, otherwise it will be blocked prematurely and the surrounding air holes will be generated; (3) Using the mosaic structure, the parting surface is designed as a zigzag parting surface, which solves the problem of difficult exhaust of the deep cavity; (4) Increase the cross-sectional area of the rear end of the exhaust chute, with the thickness of the front end generally being .5-.2mm, and the rear end being thickened to .4mm.

5. According to the heating and exhaust conditions of various parts of the casting, properly spray paint, and blow dry the accumulated water after spraying, so as to avoid wet mold closing;

6. Enlarge the gap between the punch and the pressing chamber to about .1mm, and appropriately extend the holding time;

7. Adjust the position of the lower punch of the vertical die casting machine, or increase the amount of alloy injected in the too bad chamber.

4. Shrinkage and porosity

Dark gray holes with irregular shapes appear on the casting; The concentrated large holes are called shrinkage cavities, and the scattered small holes with loose honeycomb structure are called shrinkage porosity. Found in appearance inspection or X-ray before or after machining.

1. During the condensation process of the alloy, the interior of the casting is not fed by the alloy liquid, resulting in blowholes;

2. The pouring temperature of alloy liquid is too high;

3. The specific pressure of injection is too small;

4. The design structure of the casting is unreasonable, and there are thick and large transition parts, lugs, bosses, etc. whose thickness and cross section change too dramatically.

1. Improve the casting structure, and try to avoid thick and large transition parts or lugs and bosses with too drastic changes in thickness and cross-section. If this is not possible, we can adopt hollow structure or insert design, and increase the cooling of their positions.

2. The pouring temperature of the alloy liquid can be appropriately reduced on the premise of ensuring that the casting does not produce cold insulation and under-casting;

3. Properly increase the pressurization pressure and increase the compaction effect;

4. Adding .15 ~ .2% grain refiner such as metallic titanium into the alloy liquid to reduce the tendency of shrinkage cavity formation of the alloy;

5. Use alloy varieties with small volume shrinkage and linear shrinkage, or adjust the alloy liquid to reduce its shrinkage or modify the alloy.

6. increase the cross-sectional area of internal pouring to ensure the solidification of the casting under pressure and prevent the premature solidification of internal pouring from affecting the pressure transmission.

5. External shrinkage

(sag)

The sag on the surface of the casting, the thick plane, the inner corner and the vicinity of the shrinkage cavity can be seen directly, and some surfaces are attached with a thin layer of aluminum, which is the same as the ordinary sag after removing this layer.

1. The shrinkage of the alloy is too great;

2. The design structure of the casting is unreasonable, and there are hypertrophy parts with different cross-sectional areas;

3. The cross-sectional area of the inner gate is too small or the flow direction of molten aluminum is too chaotic;

4. The specific pressure of injection is small;

5. The poor exhaust ability of the mold makes the back pressure of the cavity cushion large, and the air is compressed between the mold wall and the casting.

1. Use alloy with small shrinkage or modify it to refine its grains and reduce its shrinkage;

2. Improve the design structure of the casting, and try to avoid the thick and large parts where the two walls are connected with different thickness sections. If it is inevitable, it can be changed into a hollow structure or an insert structure;

3. appropriately increase the cross-sectional area of the inner gate;

4. Properly increase the injection specific pressure;

5. Improve the exhaust capacity of the mold:

(1) Increase the exhaust slot;

(2) Add overflow tank, etc.

6. Install a cooling device at the shrinkage cavity, and increase the spraying amount of release agent at its position.

6. Cracks

There are linear or wavy cracks on the surface of the casting, and the cracks are mostly dark gray. Under the action of external force, the cracks are widened, which can be found before and after sandblasting or before and after machining.

1. The alloy itself has a large shrinkage, a wide quasi-solid temperature range or a small amount of crystals or poor strength and toughness in the quasi-solid temperature range;

2. There are deviations in the chemical composition of the alloy: (1) The zinc content or copper content in Al-Si series and Al-Cu series alloys is too high; (2) When the magnesium content in Al-Mg alloy is too high or between 3.5 and 5.5; (3) The content of iron and sodium in the alloy is too high; (4) The content of silicon in Al-Cu series and Al-Mg series is too low; (5) The content of harmful impurity elements is too high, which reduces the plasticity of the alloy;

3. The structural design of the workpiece is unreasonable, including severe transition parts with different thicknesses, large bosses and lugs, and linear stiffeners in circular or frame structures.

4. The alloy is mixed with low melting point alloy;

5. The design structure of the mold is unreasonable, and the position of the inner gate is improper, which washes the cavity wall or core, causing local overheating or hindering the contraction of the alloy liquid;

6. It is too late to open the mold after pouring;

7. The mold temperature is too low.

1. Choose or switch to alloy varieties with small shrinkage, narrow quasi-solid temperature range, large amount of * * * crystals formed during crystallization, or high high temperature strength;

2. Adjust the alloy composition to the specified range

(1) Reduce the zinc and copper contents in Al-Si series and Al-Cu series alloys;

(2) adding aluminum ingots to dilute the magnesium content in the alloy; (3) strictly control the sodium content, and the sodium content in the Al-Si alloy should be controlled at about .1 ~ .14%.

(4) Add Al-Si alloy to the alloy to increase the silicon content;

(5) strictly control the content of harmful impurities in the alloy within the range specified by technical standards;

3. Improve the design structure of the casting, and try to avoid severe transition parts, large bosses, lugs, and linear stiffeners in circular or frame structures. If it is inevitable, it can be changed into a hollow structure or an insert structure;

4. Improve the design structure of the mold, correctly design the position and direction of the inner gate, and avoid the cracks and deformation caused by local overheating or hindering the shrinkage of the casting;

5. Strictly control the content of low melting point metals;

6. Pay attention to opening the mold within the appropriate time.

7. properly raise the working temperature of the mold and core, and slow down the cooling rate of the alloy liquid.

8. properly reduce the pouring temperature;

9. Adjust the core and ejector pin to ensure that the castings are pushed out in parallel and evenly;

1. Increase the casting fillet and demoulding inclination at the excessive position.

7. Deformation or warping

The shape and size of the casting have changed, which is beyond the tolerance range of the drawing. During the appearance inspection, measurement or marking of the castings before and after machining, it is found that

1. The design structure of the castings is unreasonable, resulting in uneven shrinkage of all parts of the castings;

2. The casting is subjected to resistance during shrinkage cooling;

3. The time from pouring to mold opening is too short and the cooling is too fast;

4. During the process of ejection during die casting, the casting is biased;

5. The alloy itself has large shrinkage, wide quasi-solid temperature range and poor high-temperature strength.

1. If possible and necessary, improve the design structure of the casting, such as changing the section thickness, avoiding the transfer parts with different thickness and unreasonable bosses, lugs, ribs, etc., and try to design the hypertrophy parts into hollow structures or mosaic structures;

2. Improve the design structure of the mold to eliminate the unreasonable structure that hinders the shrinkage of the casting;

3. Extend the mold retention time to prevent the casting from being deformed due to chilling;

4. Always check the moving parts of the mold to prevent the product from being deformed due to mold reasons (such as jamming and deformation);

5. According to the complexity of the structural shape of the casting, if it is difficult to eliminate the deformation, we can consider using an alloy with small shrinkage and high high temperature strength or adjusting the alloy composition (for example, if the silicon content in the Al-Si alloy is above 15%, the shrinkage of the casting will become very low;

6. It is forbidden to pile up complicated die castings during heat treatment furnace charging or packing. Try to avoid deformation caused by unbalanced internal stress caused by mechanical processing;

7. Reasonably increase the number of thimbles, arrange thimbles' positions, and ensure the balance of ejection;

8. Change the pouring and drainage system, such as adding cooling water at the position of thick, large and deep cavities, to achieve heat balance distribution;

9. When the deformation is not large, it can be corrected by mechanical or manual methods.

8. Slag holes

There are irregular open holes or hidden holes on the surface and inside of the casting, the surface is not smooth, and all or part of the holes are filled with slag, which can be found when the casting is visually inspected and X-rayed before and after machining.

1. The furnace charge itself has been oxidized or adhered with sundries;

2. The flux composition is impure;

3. The coating is too thick;

4. The slag removal in refining is not in place, and there are too many oxidized slag inclusions;

5. The metal hydraulic casting temperature is too low, and the fluidity is poor, so that silicon exists in a free state and becomes slag inclusion;

6. When the content of silicon in Al-Si alloy exceeds 11.5, and the content of copper and iron is also too high, silicon will precipitate in a free state and form slag inclusion;

7. Unreasonable design or poor temperature control of the furnace leads to serious oxidation of molten metal on the surface;

8. Scoop the scum together when scooping;

9. There is too much graphite in the paint or punch particles or the graphite is damaged and falls off.

1. It is strictly forbidden to use oxidized and unblowed burden with oil and water;

2. Choose or prepare the flux strictly according to the process;

3. Choose a good coating with a reasonable proportion;

4. Choose good slag remover and refining agent and use them reasonably;

5. Properly increase the pouring temperature of alloy liquid to prevent silicon from existing in a free state;

6. With high magnesium aluminum alloy, .1% beryllium can be added to reduce oxidation.

7. When the content of copper and iron is high, the content of silicon should be properly controlled not to exceed 1%, and the temperature of alloy liquid should be appropriately increased;

8. The molten metal stays in the crucible for too long (which is introduced in the ingot data), so the alloy liquid should be refined again;

9. Pay attention to prevent the damaged graphite crucible from falling into the molten metal;

1. Choose better punch particles;

11. Before using the coating, the coating should be fully stirred evenly to make the graphite in a suspended state without lump;

12. When scooping alloy liquid, the slag on the liquid surface should be removed first.

9. The surface of the cold spacer

is that the surface of the casting is not fused, and the matrix is divided into narrow gaps with smooth surface. There are two kinds of penetration and non-penetration, and this gap has a tendency to continue to develop under the action of external force, which can be found by visual inspection.

1. The pouring temperature of alloy liquid is too low;

2. The chemical composition of the alloy is unqualified, which reduces the fluidity of the alloy;

3. The injection speed is too slow;

4. There are too many internal gates leading into the cavity;

5. The flow path of the alloy liquid in the cavity is too long, the cavity is narrow and the cooling is too fast;

6. The exhaust capacity of the mold is too poor, and the back pressure of the air cushion in the cavity is too large, which prevents the liquid flow from merging.

1. increase the pouring temperature and mold temperature of the alloy liquid and improve the fluidity of the alloy liquid (such as modification and refinement treatment);

2. Control ingredients of ingredients, and test their fluidity after blending;

3. Properly increase the injection speed and specific pressure;

4. appropriately increase the cross-sectional area of inner gates and reduce the number of inner gates to reduce the collision of alloy liquids;

5. Improve the exhaust capacity of the mold, reasonably arrange the position and number of exhaust slots, and reduce the back pressure of the air cushion in the cavity;

6. Fully refine the alloy liquid to reduce the oxidation degree of the alloy liquid, thus improving its fluidity; Prevent the alloy liquid from overheating.

7. Improve the gating system to prevent the runner from running too long.