Casting defects have been a significant problem for foundry enterprises. The absence of a resolution to this issue will have ramifications for the quality of castings. Casting enterprises are responsible for the production of machine tool castings, which are susceptible to various casting defects. These defects include wear, scratches, sand holes, pinholes, cracks, deformation, reduced hardness, and damage. The utilization of welding in the repair of defects can be categorized as follows: Initially, argon arc welding is employed in the fabrication of precision castings, encompassing alloy steel and stainless steel fine castings, as well as aluminum alloy die castings. Its utilization surpasses that of conventional welding methods, which rely on argon arc welding machines. A subset of manufacturers specializing in mold fabrication and repair employ welding machinery to address defects in molds.
POROSITY
The term “porosity” is defined as the property of a material that allows it to be penetrated by pores or cavities.
The presence of gas within the liquid metal shell prior to the timely escape of liquid metal from the casting, resulting from hole class defects, is a critical factor in understanding the behavior of liquid metal under pressure. The inner wall of a porosity is characterized by a smooth, bright, or slightly oxidized surface. The generation of porosity in the casting will reduce its effective bearing area, and the surrounding porosity will cause stress concentration and reduce the casting’s impact and fatigue resistance. Porosity has been demonstrated to reduce the density of castings, which consequently necessitates the discarding of a proportion of castings that are required to withstand the hydrostatic test. Furthermore, the porosity of the castings, along with their corrosion and heat resistance, has been demonstrated to have a detrimental effect.
In order to prevent the production of porosity, it is necessary to reduce the gas content of the metal liquid, increase the permeability of the sand, and increase the air vent in the highest part of the cavity.
In the production process, heat treatment, shot blasting, cleaning, or machining of ductile iron parts often results in the presence of components with a diameter ranging from 0.5 to 3 millimeters. These components are characterized by a spherical, elliptical, or pinhole-like smooth inner wall structure. The distribution of these pores, measuring 2 to 3 millimeters, is referred to as subcutaneous porosity and is typically found beneath the casting skin.
The formation of subcutaneous porosity is attributable to the magnesium-containing iron surface tension and the facile formation of an oxide film. These phenomena impede the discharge of precipitation and invasion gases, which have a certain effect. Consequently, these gases are retained in the subcutaneous porosity. Furthermore, the solidification characteristics of ductile iron paste in the gas channel are blocked earlier, which also promotes the formation of subcutaneous porosity defects.
STICKY SAND
A layer of sand adheres to the surface of the casting, making it difficult to remove. This phenomenon is referred to as “sticky sand.” The presence of sticky sand has been shown to have a detrimental effect on the appearance of castings. Furthermore, it has been demonstrated that this phenomenon increases the workload of the casting cleaning and cutting process. In addition, the life of the machine is also affected.
In order to prevent the adhesion of sand, it is necessary to add coal powder to both the sand and the coating applied to the casting surface. This will result in the creation of an anti-sticking sand paint.
The phenomenon of vanishing mold casting sand sticking can be categorized into two distinct classifications: mechanical sand sticking and chemical sand sticking. The phenomenon of mechanical sand sticking can be attributed to the infiltration of metal liquid into the fine interstices between sand grains. This process occurs when the metal liquid’s infiltration pressure surpasses the gas counterpressure within the voids and the additional pressure induced by the metal liquid’s surface tension. The infiltration of sand grains is a phenomenon that occurs when a layer of dense coating is applied to the outer surface of a foam mold. This layer can play a role in preventing the infiltration of metal liquid, thereby effectively preventing the casting of mechanical sand sticking. Additionally, there have been reports of mechanical sand sticking. Chemical sand sticking is primarily attributable to the material’s low refractoriness and low melting point. During the pouring process into a high-temperature liquid metal, the material can be readily melted by the liquid metal, resulting in the formation of knots and other defects. Consequently, the implementation of a refractory coating between the liquid metal and the material is highly advantageous to prevent chemical sand sticking.
SAND ENTRAPMENT
The defects of grooves and scars formed on the surface of castings are very easy to be produced in wet casting of thick and large flat castings.
Most of the parts of the casting sand in contact with the upper surface of the sand mold, the upper surface of the cavity by the role of the liquid metal radiation heat, easy to arch and warping, when the warped sand layer by the liquid metal flow constantly flushing may be fractured and broken, to stay in place or be brought into other parts. The larger the upper surface of the casting, the larger the volume expansion of the sand, the greater the tendency to form sand traps.
SAND HOLE
Inside or on the surface of the casting is filled with the sand hole type defects.
INSUFFICIENT LILLING AND INCOMPLETE POURING
Insufficient liquid metal filling capacity, or poor filling conditions, cause the molten metal to stop flowing before the cavity is filled, resulting in casting defects such as incomplete pouring or cold separation. In the case of incomplete pouring, the casting will not achieve a complete shape; while in the case of cold separation, although the casting can have a complete shape, due to the presence of uncompletely fused seams, the mechanical properties of the casting are severely damaged.
Prevention of incomplete pouring and cold separation: Increase the pouring temperature and pouring speed.
CAVITY FORMATION
Irregular and rough surface holes are formed inside the casting, and the small and dense holes within them are called cavity formation;
Prevention methods:
1. Improve the casting structure to minimize thermal joints as much as possible
2. Design the gating and riser system reasonably to achieve sequential solidification
3. Select an appropriate casting temperature
4. Improve the smelting process to reduce gas content and oxides
5. Ensure the casting volume and perform reasonable secondary pouring
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