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Analysis of Casting Defects

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Introduction

Metal casting is one of the direct methods of manufacturing the desired geometry of component. It is one of the primary processes for several years and one of important process even today in the 21st century.

Today, casting applications include automotive components, spacecraft components and many industrial & domestic components.

Casting is a very versatile process and capable of being used in mass production. The size of components is varied from very large to very small, with intricate designs. Out of the several steps involved in the casting process, moulding and melting processes are the most important stages. Improper control at these stages results in defective castings, which reduces the productivity of a foundry industry.

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1. Casting defects are often very difficult to characterize due to the intrinsic complexity of the casting process.

2. All defects will fall into one or more of the established categories of casting defects.

3. The proper identification of a particular defect is the prerequisite to correcting and controlling the quality of castings.

4. The nature of a casting defect can only be determined by correctly categorizing the shape, appearance, location and dimensions of the defect.

5. Once appropriately classified, the possible causes can be examined and the corrective action can be taken.

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Casting has often been referred to as "a process that really knows how to keep a secret." Even in a controlled process, defects in the output can occur which defy rational explanation.

The complexity of the process is the result of integrating the varied disciplines of physics, thermodynamics and chemistry.

When these factors are combined with the problems associated with the high temperature alloys, the root cause of a casting defect can truly become a mystery.

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Casting defects in a controlled process will generally fall into one of two categories.

1. Defects can be chronic, i.e., long-standing, which require a remedy through a process change usually via designed experiments

2. sporadic, which are sudden adverse changes in the normal capability of the process. Sporadic defects are probably the most troublesome for casters. In a controlled process defects do not just happen, they are caused. "If a defect occurs, measures must be adopted to eliminate its cause and prevent its repetition."

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Defect Characterization

Two distinct journeys must be taken to correct sporadic defects.

the diagnostic journey from symptom to cause the remedial journey from cause to remedy

There is a temptation to attempt to diagnose a defect by the possible causes; but, an incorrect diagnosis of the root cause can lead to an incomplete or incorrect remedy of the problem. It is important to correctly identify the defect symptoms prior to assigning the cause to the problem.

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In general, a casting defect is defined as an observable and unplanned variation of a specification.

The identity of a particular casting defect is based upon the specific shape, appearance, location and dimension or profile of the irregularity.

The proper identification of a specific defect is the prerequisite to correcting and controlling the quality of castings.

A problem solving approach that is characterized by the unique "morphology of the defects is more logical than one based upon the causes since it requires no prior assumptions to be made

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Categories of casting defects

1. Metallic Projections 2. Cavities 3. Discontinuities 4. Defective Surface 5. Incomplete Casting 6. Incorrect Dimensions or Shape7. Inclusions or Structural Anomalies

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DEFECTS IN CASTING

1. Gas defects2. Shrinkage cavities3. Molding Material Defects4. Pouring Metal Defects5. Metallurgical defects

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Gas Defects

Blow Holes Open Blows Air Inclusion Pin Hole Porosity

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Blow Holes and Open Blows Cavities, mostly with smooth walls; bright or oxidised. In the case of large blowholes with corrugated walls too.

Always spherical form, even in large expansions (then always flat cavities). Frequently accompanied by slags and oxides

Typical characteristics: Rounded shapes, location always in the upper part of the castings and beneath cores or undercuts. Individual blowholes or large accumulations, then mostly with different sizes. Preferred defect locations: In parts of the casting in the top of the mould, often above larger cores. In thick parts, but also in thin walls.

Apart from presence of moisture, they occur due to lower to venting and lower permeability of the mould

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Blow Holes and Open Blows

Remedial measures: - Adequately dry cores- Larger proportion of new sand - Better gas permeability- Smaller proportion of binder- Check casting system - Reduce moisture content of the sand - Add a little oxide to the melt

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Pin Hole Porosity

Agglomeration of small, rounded cavities up to around 5 mm in size. Surface of the cavities mostly smooth and polished, occasionally with graphite. On contact with the atmosphere mostly clearly visibly pinholes in the casting skin; then the surface is often oxidised Typical characteristics: All cavities have the same size and the long form is always oriented perpendicular to the surface, arranged at outer edges in the line bisecting the angles. Preferred defect locations: Outer edges and corners, as well as workpiece surfaces, defects always in the immediate vicinity of the surface. Often at mould parting lines or transitions to cores, then mostly arranged like strings of pearls/beads.

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Pin Hole Porosity

Remedial measures:- Optimise mould material Water content Nitrogen content Residual moisture of the blacking- Nitrogen content in iron; set-point value < 80 ppm

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Internal Shrinkage cavities

Defect is almost always identifiable during machining. Zones of visibly "loose" microstructure, occasionally sponge-like structure or an agglomeration of numerous small pores. Remedial measures:- Check the gate and feeder system - Reduce the casting temperature- Carry out a solidification simulation

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Dispersed shrinkage

Accumulation of small, crack-like cavities. Only visible after machining. Cross-sections up to around 8 mm long and 1-2 mm wide, up. All cavities are the same size and always oriented perpendicular to the surface, arranged at the outer edges of the lines bisecting the angles to 2 cm deep. Remedial measures: - Minimise core sand intake - Reduce water content - Extend mixing time - Increase casting temperature

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Molding Material Defects

CUTS AND WASHESThickening of the casting at one place.

Typical characteristics: Sand washout, combined with sand and /or slag inclusions because of low strength or molten metal flowing at high speedPreferred defect locations: Mostly close to the gate or on edges, which tend to heat Remedial measures: - Choose binder with good properties at high temperatures - Greater compaction of the mould sand - Avoid high flow speeds and long pouring times - Improve gate design to reduce turbulence and increasing the size of the gates

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Metal Penetration

More or less thick sand adhesions on the casting, which are held fast on the workpiece by iron penetrations. The iron between the grains cannot always be identified by the naked eye. Widespread, frequently cushion-shaped form.Can often not be removed by blasting. Penetrating adhesions in blackened moulds and cores can often be cracked off the workpiece using a chisel

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Metal Penetration

Typical characteristics: Metal between sand grains. Can often only be identified with a microscope.

Preferred defect locations: Thick parts, zones of inadequately compacted mould material and zones, in which cores lie in the hottest parts. Preferably in the lower parts of the mould (higher casting pressure).

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Mis Runs and Cold shuts

Metal unable to fill the mold cavity completelyCold shut is caused when two metal streams while meeting in the mold cavity do not fuse together properly, thus causing a discontinuity or weak spot.Remedial measures:- Increase casting temperature - Increase casting speed and make gates larger

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Dross

Irregularly shaped interruption in the material

Typical characteristics:

Dark scars, foamy dark surfaces, very finely distributed.

Dross worsens the mechanical properties, Dross mainly

consists of magnesium oxy-silicates and magnesium

sulphides, and is a product of the reaction of magnesium

with oxygen, sulphur and silicon.

Preferred defect locations:

In the upper casting surface or beneath cores. Mostly in

cast iron with nodular graphite, often only visible after the

casting skin has been removed.

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Dross

Remedial measures: Dross cannot be prevented, only reduced.Melts:- High temperature - Low residual Mg contents- Clean melting of pure raw materials- Highly concentrated Mg master alloys or pure Mg- Gas purging- Refining slags (melt cleaner)

In the work pieces:- Draw off and restrain treatment slags (large pouring basin, optimum casting system)- Filters are very effective - Dry, closed moulds

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Mismatch in mold

Unintentional protruding casting parts.

Typical characteristics: Parts of the casting are displaced relative to each other.

Preferred defect location: On the parting line.

Remedial measures:

- Check box guides, mould assembly and closure equipment- Check models

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Mould damage

Irregular, widespread thickening of the outside or inside of the casting. After the pattern has been stripped, areas of the subsequent cast contour are broken off in the mould material area of the mould cavity. Typical characteristics:Enlargement of the wall thicknesses or change in contour, the broken off areas of the mould material are then found in the casting as mould material inclusions, if the defect is caused when the mould is assembled and closed by pressing core prints or in the mould joint.

Preferred defect locations:Joint face or core prints

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Mould damage

Remedial measures:

-Re-open the mould after assembling and

closing and check for pressure points and

broken off edges.

-Check pattern and core box, core print

allowance

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Rough casting surface / Roughness

Roughness must be assessed relative to the grain size of the casting selected. Under certain circumstances a workpiece cast in coarse sand with fully uniform surface must be assessed as being smooth, although it is rougher than a “rough area” on a workpiece cast in fine grained sand. Preferred defect locations: Close to gate, parts deep in the mould, zones with inadequate compactionRemedial measures: - Use finer sand - Reduce water content - Increase compaction pressure - Reduce casting temperature

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Rough casting surface / Roughness

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