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Aluminum Castings Common Defects And Their Causes Analysis And Prevention

Wuxi Hoohi Engineering Co.,Ltd | Updated: Sep 06, 2017

A stomata

Features: 1. hole wall surface is generally smooth, with a metallic luster. 2. Individual or flocked or present in subcutaneous castings. 3. The flue gas is yellowish.

Analysis of the reasons: 1. Liquid metal pouring gas involved in the alloy liquid after solidification in the form of pores in the castings. 2. Subcutaneous porosity generated by the metal after the reaction of the mold. 3. The slag in the alloy solution or the gas adhering to the scale is mixed with the alloy liquid to form pores.

Preventing methods: 1. Prevent air from getting involved during pouring. 2. alloy liquid into the cavity before the filter in addition to alloy liquid in the slag, oxide and bubbles. 3. Replace the casting material or add the coating layer to prevent the alloy liquid from reacting with the mold. 4. Allow the welding parts to clean up the defect after the repair welding.

Second, pinhole

Features: 1. Uniform distribution of holes in the entire section of the casting (diameter less than 1 mm). 2. Frozen fast part of the small number of holes, solidification of the slow number of parts of the hole is also large. 3. In the eutectic alloy in a circular hole, in the solidification interval of the alloy was elongated hole. 4. In the X-ray film was a small black spots in the fracture was discontinuous milky white pits.

Analysis of the reasons: alloy in the liquid state of the molten gas (mainly hydrogen), in the alloy during the solidification process from the alloy and the formation of the form of uniform distribution of holes.

To prevent the method: 1. Alloy liquid state thoroughly refined degassing. 2. In the solidification process to increase the rate of solidification, to prevent the dissolved gas from the alloy precipitation. 3. Casting under pressure in the solidification, to prevent the alloy dissolved gas precipitation. 4. Burden, auxiliary materials and tools should be dry.

Three, loose

Features: 1. Sponge-like non-tight organization, serious shrinkage. 2. The surface of the hole is rough pits, grain large. 3. Fracture gray or light yellow, heat treatment after the gray, light yellow or black. 4. More in the hot section produced. 5. In the X-ray film was cloudy fluorescent examination was dense small bright spots.

Analysis of the reasons: 1. Alloy liquid degassing is not clean to form a gas loose. 2. The final solidification site is insufficient. 3. Mold local overheating, too much water, poor exhaust.

To prevent the method: 1. To maintain a reasonable solidification sequence and fill. 2. charge clean. 3. Place cold iron in loose parts. 4. In the part of the welding can allow the defective parts can be cleaned up after welding.

Four, mixed

By the paint, modeling materials, refractories and other mixed into the alloy liquid and the formation of the casting surface or the internal composition of the composition with different particles.

Analysis of reasons: 1. foreign matter mixed with liquid alloy and poured into the mold. 2. Poor refining results. 3. The outer surface of the mold cavity surface or the molding material is peeled off.

To prevent the method: 1. carefully refined and pay attention to slag. 2. Smelting tool The coating layer should be firmly attached. 3. The pouring system and cavity should be cleaned. 4. The charge should be kept clean. 5. Surface inclusions can be polished to remove, if necessary, can be covered welding.

Five, slag

Features: 1. Oxidation slag to floc exists in the casting inside, the fracture was yellow or gray. 2. Flux slag is dark brown dot-like, slag removed after the smooth surface of the hole. After exposure to the air for some time, sometimes the corrosion characteristics.

Analysis of reasons: 1. Refining treatment after the slag is not clean. 2. Refined metamorphosis after the standing time is not enough. 3. The pouring system is unreasonable and the secondary oxide is involved in the alloy liquid. 4. After refining the alloy liquid agitation or contamination.

To prevent the method: 1. Strict implementation of refined metamorphic casting process requirements. 2. The molten metal should be injected into the mold smoothly during pouring. 3. Charge should be kept clean, back to the furnace treatment and use should be strictly abide by the technical procedures.

Six, crack

Features: 1. Cracks are linear or irregular curves. 2. The hot crack section is characterized by oxidation, no metallic luster, and more in the hot section of the sharp corners of the inside, the thickness of the cross-section, often and loose symbiotic. 3. The surface of the broken metal is clean.

Analysis of the reasons: 1. Casting parts of the cooling is not uniform. 2. Casting solidification and cooling process by the outside resistance and not free to shrink, the internal stress exceeds the strength of the alloy and produce cracks.

To prevent the method: 1. As far as possible to maintain the order of solidification or solidification at the same time, reduce internal stress. 2. Refine the alloy structure. 3. Select the appropriate pouring temperature. 4. Increase the casting of the concession.

Seven segregation

Features: 1. In the melting process crucible bottom and the upper part of the chemical composition is not uniform. 2. The first solidified part of the casting and the chemical composition of the post-solidification site are not uniform.

Analysis of the reasons: 1. Precipitation phase of the alloy and the solute concentration of the liquid phase is different. In most cases, the liquid phase solute is enriched and is too late to diffuse the chemical composition of the solidified part. 2. During the process of melting some of the compounds formed by the density and density of the mother liquor is different from a certain temperature range, along the bottom of the crucible chemical composition caused by uneven.

To prevent the method: 1. Smelting process to strengthen the mixing and the appropriate standing. 2. Appropriate increase in solidification cooling rate.

Eight, segregation of the tumor

Features: 1. In the casting surface to form tumor-like protrusions. 2. Metallographic examination is generally low melting eutectic enrichment area.

Analysis of the reasons: when the central part of the casting has not yet solidified, the casting surface shrinkage center is not solidified liquid phase through the surface layer of leakage and production.

To prevent the method: 1. Properly reduce the pouring temperature. 2. Appropriate increase in the cooling capacity of the mold. 3. Extended opening time.

Nine, the composition of ultra-poor

Features: Chemical composition exceeds the upper limit or below the lower limit, the impurity element exceeds the permissible upper limit.

Analysis of the reasons: 1. Intermediate alloy or prefabricated alloy composition is not uniform or component analysis error is too large. 2. Charge calculation or ingredient weighing error. 3. Smelt operation improper, easy to oxidize the element burned too much. 4. Smooth mixing uneven, easy to segregation elements uneven distribution.

To prevent the method: 1. Furnace before the analysis of substandard ingredients may be appropriate to adjust. 2. When the final inspection fails, it may be consulted with the design and use department.

Ten, cold separated

Features: cast metal is separated by oxide, not completely separated into one, that is, into a cold. Seriously become less cast. The cold fringes often appear on the top wall of the casting, on thin horizontal and vertical surfaces, at thick corners or on the ribs. Cold and more like lines.

Analysis of the reasons: 1. Inappropriate pouring system, so that the alloy liquid process is too long, or confluence concentrated in the thin parts. 2. Mold exhaust bad, so that the alloy flow blocked, surface oxidation, not a good fusion. 3. The pouring temperature is too low. 4. Casting improper structure, such as the outer fillet is too small, poor alloy flow.

To prevent the method: 1. Improve the casting system design, to avoid the alloy liquid process is too long to prevent the concentration of concentrated in the sink site. 2. Take the inclined pouring so that the flow is smooth. 3. To improve the mold exhaust capacity, strict control of sand moisture. 4. Appropriate increase in pouring temperature and casting speed. 5. To improve the casting structure design, to avoid the fillet is too small.


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