What Is a Casting Defect?

There are many types of casting defects. Analyzing the casting defects and their causes, inspecting the quality of the casting, and repairing the defects of the casting are very important to optimize the casting production process.

It mainly includes appearance quality, intrinsic quality and use quality. Appearance quality refers to the surface roughness, surface defects, dimensional deviation, shape deviation, and weight deviation of the casting; intrinsic quality mainly refers to the chemical composition, physical properties, mechanical properties, metallographic structure, and holes, cracks, inclusions, Segregation and other conditions; the quality of use refers to the working durability of castings under different conditions, including wear resistance, corrosion resistance, cold and heat resistance, fatigue, shock absorption and other properties, as well as processability such as machinability and weldability.

The quality of castings has a great impact on the performance of mechanical products. For example, the wear resistance and dimensional stability of machine tool castings directly affect the accuracy and service life of machine tools; the sizes of various types of pump impellers, casings, and internal cavity of hydraulic parts, the accuracy of profile and surface roughness directly affect the pump And the development of hydraulic system's working efficiency, energy consumption and cavitation; the strength and resistance to chilling and heat resistance of the internal combustion engine cylinder block, cylinder head, cylinder liner, piston ring, exhaust pipe, etc., directly affect the engine's working life.

There are many factors that affect the quality of castings. The first is the design process of castings. When designing, in addition to determining the casting geometry and size according to the working conditions and metal material properties, the rationality of the design must be considered from the perspective of the casting alloy and casting process characteristics, that is, the obvious size effect and solidification and shrinkage , Stress and other problems to avoid or reduce the occurrence of defects such as segregation, deformation and cracking of castings. The second is to have a reasonable casting process. That is, according to the casting structure, weight and size, casting alloy characteristics and production conditions, the appropriate parting surface and shape, core making method are selected, and the casting ribs, cold irons, feeders, and pouring systems are reasonably set. To ensure the quality of castings. The third is the quality of the raw materials for casting. The quality of metal charge, refractory, fuel, flux, modifier, and foundry sand, molding sand binder, paint and other materials are not up to standard, which will cause defects such as pinholes, pinholes, slag inclusion, and sand sticking in the casting, affecting the appearance quality of the casting. And internal quality, casting will be scrapped in severe cases. The fourth is the process operation. It is necessary to formulate reasonable process operation procedures, improve the technical level of workers, and make the process procedures correctly implemented.

In casting production, the quality of castings should be controlled and inspected. First of all, it is necessary to formulate process codes and technical conditions from raw materials, auxiliary materials to the control and inspection of each specific product. Each process is strictly controlled and inspected in accordance with the process code and technical conditions. Finally, quality inspection is performed on the finished castings. Equipped with reasonable testing methods and suitable testing personnel. Generally speaking, the appearance and quality of castings can be judged by comparing the sample surface roughness; fine cracks on the surface can be inspected by coloring method and magnetic particle method. The internal quality of the casting can be checked and judged by audio, ultrasonic, eddy current, X-ray and gamma rays. ^[1]

Sand casting casting defects include: cold insulation, insufficient pouring, porosity, sand sticking, sand trapping, trachoma, and swelling.

1) Insufficient cold insulation and pouring

Liquid metal has insufficient filling capacity, or the filling conditions are poor. Before the cavity is filled, the metal liquid will stop flowing, which will cause insufficient casting or cold insulation defects. When the casting is insufficient, the casting can not obtain the complete shape; although the casting can obtain a complete shape during cold insulation, the mechanical properties of the casting are seriously damaged due to the incompletely fused joints. Prevent insufficient pouring and cold separation: increase pouring temperature and pouring speed.

2) Stomata

Gases did not escape in time before the liquid metal was crusted, and holes were generated in the castings. The inner wall of the stomata is smooth, bright or slightly oxidized. After the pores are generated in the casting, the effective bearing area will be reduced, and stress concentration will be caused around the pores, which will reduce the impact resistance and fatigue resistance of the casting. Porosity can also reduce the compactness of the casting, causing some castings that require hydrostatic testing to be scrapped. In addition, pores also have an adverse effect on the corrosion resistance and heat resistance of the casting. Prevent the generation of pores: reduce the gas content in the metal liquid, increase the permeability of the sand mold, and add an air riser at the highest point of the cavity.

3) Sticky sand

A layer of hard-to-remove sand on the surface of the casting is called sticky sand. Sand sticking not only affects the appearance of the casting, but also increases the workload of casting cleaning and cutting, and even affects the life of the machine. For example, sticky sand on the surface of the cast teeth is easy to be damaged. If there is sticky sand in the parts of the pump or engine, it will affect the flow of fluids such as fuel oil, gas, lubricating oil and cooling water, and it will stain and wear the entire machine. Preventing sand sticking: adding coal powder to the molding sand, and applying anti-sticking sand coating on the surface of the mold.

4) Sand inclusion

The grooves and scars formed on the surface of the casting are extremely easy to produce when casting thick and flat plate castings with wet molds.

Most of the sand inclusions in the casting are in contact with the upper surface of the sand mold. The upper surface of the cavity is susceptible to arching and warping due to the radiant heat of the metal liquid. When the raised sand layer is continuously washed by the metal liquid flow It may be broken, left in place or taken into other parts. The larger the upper surface of the casting, the larger the volume expansion of the molding sand, and the greater the tendency to form sand inclusions.

5) The trachoma is filled with hole defects such as molding sand inside or on the surface of the casting.

6) Defects caused by the expansion of the casting mold wall due to the pressure of the molten metal during the pouring of the expanded sand. In order to prevent sand swelling, the strength of the sand mold, the rigidity of the sand box, the box pressing force or the tightening force when closing the box should be increased, and the pouring temperature should be appropriately reduced to make the surface of the metal liquid crust early to reduce the metal liquid's impact on the mold pressure. ^[1]

The inspection of castings mainly includes dimensional inspection, visual inspection of appearance and surface, chemical composition analysis, and mechanical performance tests. For castings that require more important or easy to cause problems in the casting process, non-destructive inspection is required. It can be used for ductile iron castings Nondestructive testing technologies for quality testing include liquid penetration testing, magnetic particle testing, eddy current testing, radiographic testing, ultrasonic testing, and vibration testing.

First, the detection of casting surface and near surface defects

1) Liquid penetration detection

Liquid penetration testing is used to check various opening defects on the casting surface, such as surface cracks, pinholes and other defects that are difficult to find with the naked eye. The commonly used penetrant test is coloring test, which is to wet or spray a colored (usually red) liquid (penetrant) with high penetrating ability on the casting surface. The penetrant penetrates into the opening defects and quickly wipes the surface penetrant. Layer, and then spray the easy-to-dry display agent (also called developer) on the casting surface. After the penetrant remaining in the opening defect is sucked out, the display agent will be dyed, which can reflect the shape of the defect, Size and distribution. It should be pointed out that the accuracy of the penetration detection decreases with the increase of the surface roughness of the tested material, that is, the better the light is on the surface, the better the detection accuracy is. The accuracy of the surface polished by the grinder is the highest, and even intergranular cracks can be detected. In addition to coloration detection, fluorescence penetration detection is also a commonly used liquid penetration detection method. It needs to be equipped with ultraviolet light for irradiation observation, and the detection sensitivity is higher than coloration detection.

2) Eddy current detection

Eddy current testing is suitable for inspecting defects below the surface that are generally not deeper than 6 to 7 mm. There are two types of eddy current detection: placement coil method and feed-through coil method. When the test piece is placed near the coil where the alternating current passes, the alternating magnetic field entering the test piece can induce a eddy current (eddy current) flowing in the test piece in a direction perpendicular to the excitation magnetic field. Generate a magnetic field opposite to the direction of the excitation magnetic field, so that the original magnetic field in the coil is partially reduced, thereby causing a change in the coil impedance. If there is a defect on the casting surface, the electrical characteristics of the eddy current will be distorted, so that the existence of the defect is detected. The main disadvantage of eddy current detection is that it cannot directly display the size and shape of the detected defect. Generally, only the position and depth of the surface where the defect is located can be determined. In addition, it is less sensitive to the detection of small opening defects on the surface of the workpiece than penetration detection.

3) Magnetic particle detection

Magnetic particle inspection is suitable for detecting surface defects and defects several millimeters deep below the surface. It requires DC (or AC) magnetizing equipment and magnetic powder (or magnetic suspension) to perform the inspection operation. Magnetizing equipment is used to generate magnetic fields on the inner and outer surfaces of the casting, and magnetic powder or magnetic suspension is used to display defects. When a magnetic field is generated within a certain range of the casting, a defect in the magnetized area will generate a leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic powder is attracted, so that the defects can be displayed. The defects displayed in this way are basically defects that cut across the magnetic field lines, and they cannot be displayed for the long-type defects parallel to the magnetic field lines. Therefore, the magnetization direction needs to be constantly changed during operation to ensure that defects in unknown directions can be checked. .

Detection of internal defects in castings

For internal defects, common non-destructive testing methods are radiographic and ultrasonic testing. Among them, the ray detection effect is the best. It can obtain intuitive images reflecting the type, shape, size and distribution of internal defects, but for large castings with large thickness, ultrasonic detection is very effective and can accurately detect the location of internal defects. , Equivalent size and distribution.

1) Ray inspection (micro focus XRAY)

X-ray inspection generally uses X-rays or -rays as the radiation source. Therefore, equipment and other ancillary facilities are required to generate radiation. When the workpiece is exposed to the radiation field, the radiation intensity of the radiation will be affected by the internal defects of the casting. The intensity of the radiation emitted through the casting changes locally with the size and nature of the defect, forming a radiographic image of the defect, which is imaged and recorded by radiographic film, or detected in real time through a fluorescent screen, or detected by a radiation counter. Among them, the method of radiographic film recording is the most commonly used method, which is commonly referred to as radiographic inspection. The defect image reflected by radiography is intuitive, and the defect shape, size, number, plane position and distribution range are all It can be displayed, but the depth of the defect is generally not reflected, and special measures and calculations need to be taken to determine it. The International Casting Industry Network has applied the method of ray computer tomography. Due to the relatively expensive equipment and high use cost, it cannot be popularized, but this new technology represents the future development direction of high-definition ray detection technology. In addition, the micro-focus X-ray system using an approximate point source can actually eliminate the blurry edges generated by larger focus devices and make the image outline clear. The digital image system can improve the signal-to-noise ratio of the image and further improve the sharpness of the image.

2) Ultrasonic testing

Ultrasonic inspection can also be used to inspect internal defects. It uses sound beams with high-frequency acoustic energy to propagate inside the casting, and when it encounters internal surfaces or defects, it reflects and finds defects. The amount of reflected acoustic energy is a function of the directivity and nature of the inner surface or defect and the acoustic impedance of such reflectors, so the acoustic energy reflected by various defects or inner surfaces can be used to detect the presence of defects, wall thickness or surface The depth of the defect. Ultrasonic testing is a widely used non-destructive testing method. Its main advantages are: high detection sensitivity, which can detect small cracks, and large penetration ability, which can detect thick section castings. Its main limitations are: difficulty in interpreting the reflection waveforms of broken defects with complex contour dimensions and poor directivity; undesired internal structures, such as grain size, microstructure, porosity, inclusion content, or fine dispersion Precipitates and the like also hinder the interpretation of the waveform; in addition, reference to a standard test block is required for detection. ^[2]

The results of casting quality inspection are usually divided into three categories:

The most fundamental way to solve the shrinkage defect of castings is "thermal balance". The method is:

(1) The rapid solidification is performed at the thick places and thermal joints formed by the machine tool casting structure, which artificially causes the temperature field of the machine tool casting to be basically balanced. Internal and external cold iron is used, and zircon sand, chromite sand or special coating with large heat storage is used locally.

(2) Reasonable process design. The pouring gate is located at the opposite wall of the casting of the machine tool, and it is scattered for several hours. The first molten metal entering the thick wall is solidified first, and the thin wall is solidified later, so that the solidification is basically achieved everywhere. For machine wall castings with uniform wall thickness, multiple internal runners and air vents are used. There are many sprues, which are dispersed and evenly distributed, so that the overall heat is balanced. The air vents are thin and numerous, which means that the exhaust is smooth and plays a role of heat dissipation.

(3) Change the position of the runner

(4) The use of large-capacity modeling materials is extremely important for the production of anti-wear products with lost foam! Chromite sand replaces other sands with small heat storage, such as quartz sand, and will achieve good results. It is better to finish micro-seismic!

(5) Low temperature fast burning, open pouring system. Fill the mold quickly, smoothly and evenly. This depends on the piece.

(6) Ductile iron's machine tool has high strength, surface hardness 90, and sand box rigidity, which is beneficial to eliminate shrinkage.

(7) When a riser is needed, first move the hot riser and leave the heat festival. If the riser is placed on the heat section, the size of the riser will definitely be increased, forming "hot heating". If it is not good, not only the shrinkage is difficult to remove, but also concentrated shrinkage, which also reduces the process yield.

(8) Both the tilted placement of the casting mold and alloying benefit. Eliminating the shrinkage defect of machine tool castings is a complex process of recognition and implementation. The basic principle of "thermal balance" should be used to scientifically analyze the carcass castings, formulate a reasonable process plan, relocate the appropriate modeling materials, tooling, and correct operation and standardization. Then the shrinkage defect of any machine tool casting can be solved.

Due to various factors, defects such as pores, pinholes, slag inclusions, cracks, and pits often occur. Common repair equipment is argon arc welding machine, resistance welding machine, cold welding machine and so on. For casting defects with low quality and appearance requirements, argon arc welding machines and other welding machines with high heat generation and fast speed can be used to repair them. However, in the field of precision casting defect repair, due to the large influence of argon welding heat, the deformation of the casting, reduced hardness, trachoma, local annealing, cracking, pinholes, wear, scratches, undercuts, or insufficient bonding force will be caused during repair. Secondary defects such as stress damage. The cold welding machine just overcomes the above shortcomings. Its advantages are mainly reflected in the small heat affected area, no preheating of the casting, cold welding repair at room temperature, so no deformation, undercuts and residual stress, no local annealing, and no change in the metal of the casting. Organization status. Therefore, the cold welding machine is suitable for repairing the surface defects of precision castings. Cold welding has a welding repair range of 1.5-1.2mm. The welding repair point is repeatedly melted and piled up. In the process of repairing large-area defects, the repair efficiency is the only factor restricting its wide application. For large defects, the composite application of traditional welding repair process and casting defect repair machine is recommended. But sometimes we do nt have a lot of defects, so we do nt need to invest a lot of cost. We can fix it with some repairing agents. For example, for iron materials, we can use JS902 to repair it. It can be used later if it is not used up. This can save costs for our manufacturers, let our foundry manufacturers invest more funds to improve the quality of the products themselves, and let users create more wealth. ^[3-4]

The casting defect repairing machine can perform the functions of depositing, sealing, repairing and repairing defects such as wear, scratches, pinholes, cracks, defect deformation, reduced hardness, trachoma, and damage to metal workpieces. Simple operation, low heat input, no heat input during the instant of surfacing, so the mold does not deform, anneal, undercut and residual stress, and does not change the metal structure of the mold or product. The repair precision is high, the coating thickness is from a few micrometers to a few millimeters, only need to be sanded and polished. It can also perform various machining processes such as turning, milling, planing, grinding, and post-processing such as electroplating. High power argon gas protection, can work for a long time. Applicable substrates: including low carbon steel, medium carbon steel, mold steel, stainless steel, tool steel, cast iron, cast steel, cast aluminum, aluminum alloy, copper alloy, nickel alloy, carbon tungsten alloy, etc., and all conductive conductors that can conduct electricity .

The principle of the casting defect repairing machine is to use a charging capacitor to discharge in an ultra-short time of ^10-6 to ^10-5 seconds in a cycle of ^10-3 to ^10-1 seconds. The contact area between the electrode material and the mold will be heated to 8000 ~ 25000 ° C, and the molten metal in the plasma state will transition to the surface layer of the workpiece in a metallurgical manner. Due to the alloying effect with the base material, it diffuses into the workpiece and infiltrates, resulting in a high-strength bond. The reason for the cold welding (low heat input) of the casting defect repair machine is that the discharge time is extremely short compared to the next discharge interval. The machine has sufficient relative stop time, and the heat will diffuse to the outside through the mold body. There is no heat build-up at the processing site. Although the temperature of the mold stays almost at room temperature, the temperature of the electrode tip can reach about 25000 ° C due to instantaneous melting. Although the casting defect repairing machine has a low heat input, it still has a strong binding force. This is because the welding rod momentarily generates metal droplets and transitions to the contact part with the base metal. Imaging results in a strong, diffuse diffusion layer. It exhibits high binding and will not fall off.