Die Casting Defects - Causes and Solutions
Contents:
Blowholes, open holes, pinholes 2. Cut and washes, swell, drops, metal penetration, rat tail 4. Cold shut, misrun, slag inclusion 5. Hot tears, hot spot.
- Casting defect.
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The defect caused due to misalignment of upper and lower part of the casting and misplacement of the core at parting line. It is the enlargement of the mold cavity because of the molten metal pressure, which results in localised or overall enlargement of the casting.
Therefore, if the casting is less than perfect, but still useful and in tolerance, the imperfections should be deemed "discontinuities". Leave a Reply Cancel reply Your email address will not be published. Other ingredients can be added to the mixture to cause the dross to float to the top where it can be skimmed off before the metal is poured into the mold. It is the holes created on the external surface or inside the casting. There is various reason or sources which is responsible for the defects in the cast metal. Misruns and cold shuts are closely related and both involve the material freezing before it completely fills the mold cavity. Other possible defects are gas porosity, shrinkage porosity, hot tears, and flow marks.
Types of Patterns in Casting Process. Types of Moulding Sand.
for die caster to have knowledge on the type of defect and be able to identify the exact root cause, and their remedies. 2. CASTING DEFECT CAN BE. Keywords: Casting defects and their root causes, remedies for casting defects. . pouring temperature and increasing the speed of die.
Difference Between Forging and Casting. When gases entrapped on the surface of the casting due to solidifying metal, a rounded or oval cavity is formed called as blowholes. These defects are always present in the cope part of the mold. Drop defect occurs when there is cracking on the upper surface of the sand and sand pieces fall into the molten metal.
Fluxing means addition of a substance in molten metal to remove impurities. After fluxing the impurities from the molten metal can be easily removed. These casting defects appear as an uneven and rough surface of the casting. When the size of sand grains is larges, the molten fuses into the sand and solidifies giving us metal penetration defect.
Because of this the molten metal penetrates in the molding sand and we get rough or uneven casting surface.
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They are very small holes of about 2 mm in size which appears on the surface of the casting. This defect happens because of the dissolution of the hydrogen gases in the molten metal. When the molten metal is poured in the mold cavity and as it starts to solidify, the solubility of the hydrogen gas decreases and it starts escaping out the molten metal leaves behind small number of holes called as pinholes.
The formation of cavity in the casting due to volumetric contraction is called as shrinkage cavity. Types of Chips in Metal Cutting. Mechanical Properties of Materials. It is a type of surface defects and a line on the surface can be seen. When the molten metal enters into the mold from two gates and when these two streams of molten metal meet at a junction with low temperatures than they do not fuse with each other and solidifies creating a cold shut appear as line on the casting. They require a nucleation point, so impurities and dissolved gas can induce closed shrinkage defects.
The defects are broken up into macroporosity and microporosity or microshrinkage , where macroporosity can be seen by the naked eye and microporosity cannot. Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools. Nitrogen , oxygen and hydrogen are the most encountered gases in cases of gas porosity.
What is Casting Defects – Types, Causes and Remedies?
In larger casting they can be up to a millimeter 0. To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as argon [10] or carbon dioxide , [11] or under a flux that prevents contact with the air.
To minimize gas solubility the superheat temperatures can be kept low. Turbulence from pouring the liquid metal into the mold can introduce gases, so the molds are often streamlined to minimize such turbulence. Other methods include vacuum degassing , gas flushing , or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form a dross that floats to the top. For instance, oxygen can be removed from copper by adding phosphorus ; aluminum or silicon can be added to steel to remove oxygen.
Hydrogen is normally produced by the reaction of the metal with humidity or residual moisture in the mold. Drying the mold can eliminate this source of hydrogen formation. Gas porosity can sometimes be difficult to distinguish from microshrinkage because microshrinkage cavities can contain gases as well. In general, microporosities will form if the casting is not properly risered or if a material with a wide solidification range is cast. If neither of these are the case then most likely the porosity is due to gas formation.
Tiny gas bubbles are called porosities, but larger gas bubbles are called blowholes [14] or blisters. Such defects can be caused by air entrained in the melt, steam or smoke from the casting sand , or other gasses from the melt or mold. Vacuum holes caused by metal shrinkage see above may also be loosely referred to as 'blowholes'.
Proper foundry practices, including melt preparation and mold design, can reduce the occurrence of these defects. Because they are often surrounded by a skin of sound metal, blowholes may be difficult to detect, requiring harmonic, ultrasonic , magnetic , or X-ray i. Pouring metal defects include misruns , cold shuts , and inclusions. A misrun occurs when the liquid metal does not completely fill the mold cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mold cavity, leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or cross-sections that are too narrow.
The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mold cavities. Misruns and cold shuts are closely related and both involve the material freezing before it completely fills the mold cavity.
These types of defects are serious because the area surrounding the defect is significantly weaker than intended. Fluidity affects the minimum section thickness that can be cast, the maximum length of thin sections, fineness of feasibly cast details, and the accuracy of filling mold extremities. There are various ways of measuring the fluidity of a material, although it usually involves using a standard mould shape and measuring the distance the material flows.
Fluidity is affected by the composition of the material, freezing temperature or range, surface tension of oxide films, and, most importantly, the pouring temperature. The higher the pouring temperature, the greater the fluidity; however, excessive temperatures can be detrimental, leading to a reaction between the material and the mold; in casting processes that use a porous mould material the material may even penetrate the mould material. The point at which the material cannot flow is called the coherency point.
The point is difficult to predict in mold design because it is dependent on the solid fraction, the structure of the solidified particles, and the local shear strain rate of the fluid. Usually this value ranges from 0. An inclusion is a metal contamination of dross , if solid, or slag , if liquid.
These usually are impurities in the pour metal generally oxides , less frequently nitrides , carbides , or sulfides , material that is eroded from furnace or ladle linings, or contaminates from the mold. In the specific case of aluminium alloys, it is important to control the concentration of inclusions by measuring them in the liquid aluminium and taking actions to keep them to the required level.
There are a number of ways to reduce the concentration of inclusions.
Casting defect - Wikipedia
In order to reduce oxide formation the metal can be melted with a flux , in a vacuum , or in an inert atmosphere. Other ingredients can be added to the mixture to cause the dross to float to the top where it can be skimmed off before the metal is poured into the mold. If this is not practical, then a special ladle that pours the metal from the bottom can be used. Another option is to install ceramic filters into the gating system. Otherwise swirl gates can be formed which swirl the liquid metal as it is poured in, forcing the lighter inclusions to the center and keeping them out of the casting.
There are two defects in this category: Hot tears, also known as hot cracking , [21] are failures in the casting that occur as the casting cools. This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools.
Proper mold design prevents this type of defect.
aluminum die casting defects
Hot spots are areas on the surface of casting that become very hard because they cooled more quickly than the surrounding material. This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal. In die casting the most common defects are misruns and cold shuts. These defects can be caused by cold dies, low metal temperature, dirty metal, lack of venting, or too much lubricant.
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Other possible defects are gas porosity, shrinkage porosity, hot tears, and flow marks. Flow marks are marks left on the surface of the casting due to poor gating, sharp corners, or excessive lubricant. A longitudinal facial crack is a specialized type of defect that only occurs in continuous casting processes.