Blog 31: Different Types of Casting Defects:?

Introduction:

Casting defects are flaws or irregularities that interfere with a component’s ability to achieve quality standards. A number of factors, including as faulty casting equipment, ineffective casting procedures, and material failure, can result in casting imperfections.

Some of these casting defects might be disregarded or dismissed, but others might result in poor casting, odd shapes, and poor functionality. They must be avoided or removed for the final product to perform better.

Casting defects come in a variety of forms, but they can be broadly divided into four groups as follows:

  • Metallurgical defects
  • Defects Due to heat
  • Mold-material casting flaws
  • Casting shape defects.

Also read: Different Types of Casing Processes

The following list includes several casting defects:

Metallurgical Defects

These casting defects occur majorly due to problems related to the metal material. There are five different types of metallurgical defects, including:

1. Porosity Defects

These kinds of internal die casting faults are often difficult to find. They frequently appear on metal surfaces with minute divots, crevices, or air pockets. Porosities often form when air becomes trapped in the metal during the casting process. This may result in a mould filling too slowly or leaving holes at the top of the die. Therefore, the resulting component might have had less structural resistance. There are two major porosity issues:

  • Gas porosity
  • Porosity due to shrinkage
Gas Porosity

As the cast metal hardens inside the mould, the amount of gas it can contain when it’s still liquid decreases. As a result, the metal has a propensity to leak gas. Bubbles get stuck inside the metal because the gas can’t pass through the mould readily. These bubbles appear to be:

  • Pinholes. The tops of the die-cast pieces have these minute perforations. They are generally seen in groups close to the casting’s surface upon visual inspection.
  • Blowholes. They are bigger holes that develop inside the cast workpiece. Contrary to pinholes, they are not visible to the naked eye. Therefore, you’ll need to perform an x-ray, ultrasonic, harmonic, or magnetic study to find them.
  • Hole openings. These are the various types of blowholes that can form on the surface of a workpiece. When the metal is placed into the mould, air becomes trapped, which leads to their development. Open holes resemble superficial scars in appearance.

These imperfections have a circular or oval shape on the surface of castings. They often have smooth, brilliant white or bright yellow surfaces. As a result, after the casting process, casting faults related to gas porosity can be found through visual inspection. On the other hand, you need an X-ray machine to search for this fault on non-machined surfaces.

Causes

  • The Gases Used to Fill Metal Alloys Filling dies with molten metal alloy quickly and under high pressure is a step in the metal casting process. Gas turbulences may therefore result from a molten alloy’s inability to flow steadily and orderly. Excessive casting parameters, gating system blind areas, and subpar washer design all frequently cause turbulence. These turbulences may cause you to notice certain porosity issues.
  • The molten alloy emitted hydrogen. This is one of the main causes of casting porosity. The contamination of the molten metal or steel alloy by moisture from the air or the smelting apparatus may increase the production of hydrogen. As the cast cools and hardens, hydrogen is released and creates gas porosities as a result of a reduction in its solubility.
  • Gases generated by mould release agents. In many cases, when a mould release agent comes into touch with a molten metal alloy, it heats up. They consequently degrade and emit gas, which causes porosity flaws. Gas volatilization could also happen if mould releasing agents are used in excess.

Remedies

  • To avoid hydrogen generation, utilise metal alloy ingots that are clean and dry.
  • Use appropriate casting parameters, such as injection pressure and speed.
  • Keep the smelting temperature under control to avoid overheating.
  • Use runners and sprues that are long enough (at least 50mm). By doing this, you can guarantee a steady and sufficient flow of gases.
  • The mould release agent needs to be of the finest calibre and used in moderation.
Shrinkage Porosity

Gas porosity has rounded, smooth surfaces, whereas shrinkage porosity has jagged, angular edges. Usually, metals shrink as they cool and solidify. However, if the hollow shrinks unevenly, the component’s structure is altered, or holes are left within, it becomes a defect. Typical shrinkage porosity faults include the following:

  • Open shrinkage defects. On the surface of the cast product, they show as holes or dips (caved surfaces) (pipes). Air is drawn into the mould as the metal alloy shrinks unevenly, which causes an open shrinkage.
  • Closed shrinkage faults. Where the molten metal has been heated unevenly, these appear as holes inside the casting. They are available in both small and large sizes. Macro shrinkages are visible to the eye as jagged lines or marks. Microscopes, on the other hand, can see the minute holes, which appear angular.
  • Warping. This shrinkage, which takes place during or after metal solidification, modifies the component’s shape and dimensions. They curl the metal’s flat or significant portions.

Causes

  • The metal is concentrated heavily in some parts of the mould.
  • Injection pressure is too low.
  • Badly designed gates and runners.
  • Asymmetrical metal solidification.
  • Variations in temperature across the molten metal.
  • A soaring downpour temperature.

Remedies

  • Employ straightforward casting geometries with enhanced runner and gating designs.
  • Using simulation software to improve the runner will ensure optimal cavity filling.
  • Boost the pressure of the metal injection.
  • To ensure proper heat dissipation, insert cooling coils, ribs, or internal chills.
  • Scrub the metal surface to get rid of any dirt.

2. Sinks

A press-down impact known as a “sink” does not precisely duplicate the design of the mould on the casting surface. On the casting surface, sinks appear when there is a sub-surface cavity. Along the areas with thick walls on the metal surface, these depressions can be seen. Sinks reflect light, making it easy for visual inspections to identify these casting surface faults.

Causes
  • Varying shrinkage during solidification is frequently caused by the uneven wall thickness of castings.
  • The casting solidifies gradually when the die is partially overheated.
  • Low injection pressure;
  • inadequate die cavity venting;
  • inadequate pressure-holding duration;
  • poor feeding effects as a result of gas compressing between the cavity surface and the liquid metal surface.
Remedies
  • To achieve equal wall thickness, optimise the casting’s structural design.
  • Increase the venting efficiency of the die cavity.
  • Boost metal injection pressure
  • Prevent overheating the die cavity and extend the metal’s time under pressure.

3. Slag Inclusions

Slag inclusions firmly adhere to the casting, decreasing the mechanical characteristics of the finished product. On the casting surface, it typically occurs when non-metallic substances form irregular crusts. An unevenly formed pocket or ribbon-like entrapment with sharp ends is the casting fault. Additionally, inclusions may appear as sand, nails, dirt, or oxides. Despite being only a few millimetres thick, they are visible to the unaided eye.

Causes
  • Pouring and solidification of slag-containing liquid metal.
  • An inefficient gating system.
  • The solidification process results in the formation of insoluble intermetallic compounds, which segregates them in the leftover liquid.
  • Adding an alloy that hasn’t melted down completely.
Remedies
  • Regular and thorough die cleaning.
  • Make the gating system’s design better.
  • Employ pure molten metal.
  • Use a little amount of alloy material and track how the metal alloys change.

4. Dross

Dross is the name for a metal loss that occurs when nonferrous metals are cast. During the melting process, metal oxidises, creating layers of contaminants and scum that float to the surface. The surface mixing of the base metal’s oxide with other metals results in the impurities.

Causes
  • Reaction to thermite.
  • The formation of oxides as a result of the reaction between the air in the mould and the liquid metal’s high temperature.
Remedies
  • Prevent any stirring that could cause oxidation or a thermite reaction.
  • Lower the temperature of the metal.
  • Shorten the melting period.

5. Soldering

Soldering is a common casting mistake that occurs throughout the casting process. It takes place when molten metal sticks to the surface of the die chamber and stays there even after the casting is taken out. As a result, there may be extra or missing material in some casting portions. Visual inspection can be used to distinguish soldering.

Causes
  • Damages, insufficient hardness, or low cavity roughness.
  • An slanted injection caused by an improper ejection mechanism.
  • Overheating the dies or the molten metal alloy.
  • Using low-quality mould release agents.
  • An alloy with insufficient amounts of the iron element
Remedies
  • Enhance the die cavity’s roughness and fix any damage.
  • Modify the gate design and draught angles.
  • By improving the ejection mechanism, ensure balanced ejection.
  • Keep an eye on and manage the temperatures of the molten alloy and die.
  • Make use of top-notch mold-release products.
  • Use an iron element that is adequate with a 0.8% to 1.1% margin.

Defects Due to Heat

There are five casting defects caused by heat, including hot tears, cold shut, and thermal fatigue.

1. Hot Tears

When the material being cast is partially solid and partially liquid, a hot tear is a discontinuity that develops during the solidification phase of the casting operation. Because a material will typically desire to contract as it solidifies, a hot tear might happen. A hot rip will develop if some force or restraint prevents this contraction; this can make the casting more susceptible to corrosion and failure.

Causes
  • Thermal contractions on the surface of the metal.
  • Insufficient pouring temperatures for metal.
  • Incorrect gate placement and solidification procedure.
Remedies
  • Use quality fillet at intersections.
  • Accurately place the gates close to the die.
  • Use common solidification techniques.
  • Ensure that the material is uniformly thick.

2. Cold Shut

Cold shut faults are cracks with rounded edges. They develop as metal enters the mould through various openings. But the metal is too cold to fully meld and create a lovely item. The outcome is a cold shut, which weakens the casting and occurs when round-edged cracks run through the middle of the workpiece.

Causes
  • Inadequate gating system design.
  • Low temperatures for the metal in liquid form.
  • Less fluid metal in the molten state.
  • The casting’s thinnest parts.
Remedies
  • Make the gating mechanism more effective to avoid constrained cross pathways and guarantee proper metal flow.
  • Increase the mold’s gas permeability.
  • Raise the temperature at which metal is poured to avoid early solidification.

3. Thermal Fatigue

This fault is frequent in aluminium die casting because of the enormous thermal stress that is exerted on the dies. Thermal stresses are produced by the heating and cooling of the surface during lubricant injection, ejection, and spraying. Because die-casting dies are susceptible to severe thermal and mechanical loads, repetitive cycles of extremely low and high temperatures will induce structural weakness and cracks in a component.

Causes
  • Constantly raising the temperature of the die.
  • Corners and edges with inadequate radiuses.
  • The gradient of temperature is too close to the gate.
Remedies
  • Provide edges and corners with allowable radii.
  • Slightly lessen the temperature difference at the gate.
  • Give the die some time to cool.

Mold Material Casting Defects

There are 7 types of casting defects caused by unsuitable mold material. Read on.

1.      Cuts and Washes

When molten metal washes away some of the moulding sand, surplus metal collects in these regions. When this occurs, the sand loses its ability to withstand erosion. Low bulges can be seen along the workpiece’s surface as a result of cuts and washes. Typically, they lean toward the end where there was greater pouring pressure on the liquid metal.

Causes
  • The high-velocity pouring of molten metal allows extra metal to pass past the gate, resulting in surplus metal in some sections of the mould.
Remedies
  • Ensure that the gating mechanism is properly designed.
  • Boost the core and mold’s tensile strength.
  • The core sand should be given more binders.

2. Fusion

Another flaw in sand casting is called fusion, which happens when sand particles mix with the flowing liquid metal. As a result, the casting develops a thin coating of hard, glassy crust that is tightly fastened to it. The low refractoriness of sand is the primary contributor to this problem. Sand’s low refractoriness prevents it from withstanding the liquid metal’s high temperature. A fusion defect results as a result.

Causes
  • Sand or clay with low refractoriness;
  • Sand mixed with metal particles.
  • The metal pouring temperature is too high.
Remedies
  • Lower the temperature at which molten metal is poured;
  • Increase the refractoriness of moulding materials.

3. Runout

When molten metal seeps from the mould, runouts occur. As a result, the mould cavity won’t be entirely filled. You frequently receive missing or imperfectly cast components as a result. The appearance is often smooth, but occasionally you may see irregular and harsh runouts.

Causes
  • A faulty mold or die casting machine.
Remedies
  • Create precise casting moulds.
  • Examine and repair damaged moulds before casting.
  • When designing moulds, choose premium raw materials that can endure high temperatures.

4. Swells

The mould wall frequently moves back due to metallicostatic forces, expanding the workpiece’s size. In other words, the casting will swell past the specified volume, manifesting as little, smooth bulges. Its feeding and machining requirements will consequently rise, and metal waste may result.

Causes
  • Low strength mold.
  • Improper ramming of the sand mold.
Remedies
  • Design molds to withstand molten metal pressure to prevent the mold wall from moving backward.
  • Use stronger, well-rammed molds.

5. Drops

These flaws manifest when the casting is still molten. They happen when lumps or particles of loose moulding sand fall into the mould cavity from the cope’s surface. As a result, the surface of the casting develops an unnatural and unevenly formed projection. Drops may also leave the metal’s surface looking soiled and unusable.

Causes
  • Low sand strength.
  • Insufficient fluxing of the molten metal.
  • Soft ramming.
  • Absence of reinforcement of sand projection in the cope.
Remedies
  • Use sand with higher strength.
  • Removing the molten metal impurities with proper fluxing.
  • Provide harder ramming.
  • Reinforce the sand projections with gaggers or nails.

6. Metal Penetration

This happens when the molten metal enters cracks or spaces in the moulding sand. The main cause of this is that the moulding sand’s grain size is too large, which prevents the liquid metal from flowing properly. As a result, the casting surface will be uneven and rough, which will reduce its aesthetic appeal. Metal penetration’s effects are clearly visible to the unaided eye.

Causes
  • Using sand with high permeability and low strength.
  • Soft ramming of sand.
  • Using coarse or large sand grains causes more extreme metal penetration.
  • Lack of mold wash.
Remedies
  • Use small-sized, high-strength grain.
  • Ensure hard ramming and low permeability of sand.

7. Rat Tails

On castings, rat tails take the form of erratic lines or fissures. When the molten metal is too hot, a compression failure on the surface of the mould cavity causes these deformities. Heat-induced expansion of the molten sand causes the mould wall to migrate rearward and cave in. The casting surface will exhibit a thin line mark during the procedure. On the surface of the component, you could detect crisscrossing little lines if this type of failure occurs repeatedly throughout the casting process.

Causes
  • Poor expansion capabilities of the sand.
  • Improper mold design.
  • Excessive metal pouring temperature.
Remedies
  • Add combustible additives to molding sand.
  • Reduce the metal pouring temperature.
  • Design mold to be less hard to ensure adequate expansion.

Casting Shape Defects

Not right casting shape can cause two types of defects: mismatches and flash.

1. Mismatches

Mismatches are casting flaws that result from the mold’s lower and top portions being out of alignment. It could happen if the mold’s cope and drag are not properly aligned before the metal is poured. Mismatches come in two flavours: mould shift and core shift. Mold shift, which happens when the mould is out of alignment, frequently manifests as a horizontal displacement. Core shift, on the other hand, manifests as a vertical displacement and results from the core’s alignment being incorrect.

Causes
  • Loose box pins.
  • Wrong pattern dowel pins.
  • Placing the upper part and lower part of the mold wrongly causes misalignment.
Remedies
  • Ensure proper match plate pattern alignment and mounting.
  • Use the right molding box and closing pins.

2. Flash

Any extra or undesirable material that appears as thin, asymmetrical events on the separation line of a die-casting object is referred to as a flash. Usually, a thin film of metal forms on the faces of the separation and, after being remelted, turns into dross. Flashes can be recognised visually since they frequently have greater sizes.

Causes
  • Insufficient clamping force.
  • Inadequate cleaning of the parting surface.
  • Too high injection speed and filling temperature.
  • Insufficient strength of the die material.
  • Deformed or worn-out hinges of the casting machine.
Remedies
  • Adjust the casting parameters such as injection speed, filling temperature, and clamping force.
  • Ensure regular maintenance of casting machines.
  • Clean up the die cavity and parting surface.

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