DBT 206 FOUNDRY TECHNOLOGY Lecture 7 Defects in

DBT 206 FOUNDRY TECHNOLOGY [Lecture 7 -Defects in Casting]

Casting defects • Failures of castings can occur from: ▫ service conditions ▫ improper design ▫ materials selection ▫ manufacturing deficiencies ▫ or a combination of these • Defects may occur due to one or more of the following reasons: ▫ Fault in design of casting pattern ▫ Fault in design on mould & core ▫ Fault in design of gating system & riser ▫ Improper choice of molding sand ▫ Improper metal composition ▫ Inadequate melting temperature ▫ Rate of pouring ▫ Pouring temperature

Casting Quality • There are numerous opportunities for things to go wrong in a casting operation, resulting in quality defects in the product • The defects can be classified into TWO broad categories: (1) General defects - common to all casting processes (2) Defects related to sand casting process – the use of sand

Basically. . classification of casting defects. . 7. Incorrect dimensions or shape 1. Metallic projections 2. Inclusions 6. Incomplete casting Classification of casting defects 3. Cavities 5. Discontinuities 4. Defective surface

(1) General Defects (common in all casting process) (a) Misrun - A casting that has solidified before completely filling mold cavity causes i) Fluidity of the molten metal is not sufficient ensure sufficient fluidity - degree of superheat, viscosity, heat transfer to surrounding ii) Pouring temperature too low iii) Pouring too slow iv) Cross section of the mold cavity is too thin (b) Cold shut - Two portions of metal flow together but there is a lack of fusion due to premature freezing - Causes similar to misrun

(1) General Defects (c) Cold shot - Metal splatters during pouring causing the formation of solid globules & become entrapped in casting prevention: - pouring procedures – avoid splattering - gating system design that avoid splattering (d) Shrinkage cavity Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze -Often occurs near the top of the casting – look like a “pipe” - can be solved by proper riser design

(1) General Defects (e) Microporosity - a network of small voids distributed throughout the casting Causes; i) localized solidification shrinkage of the final molten metals in the dendritic structure ii) gases Prevention; - Ensure adequate molten metal - Use chills (external or internal) - Steep temperature gradient (f) Hot tearing (hot cracking) -Occurs when the casting is restrained from contraction by an unyielding mold during the final stages of solidification or early stages of cooling after solidification - separation of a metal at a point of high tensile stress caused by the metal’s inability to shrink naturally Prevention; - compounding the mold to be collapsible

(2) Sand Casting Defects (swells) (b) Pin holes - Formation of (c) Sand wash/swells – many small gas cavities irregularity in the surface of (a) sand blow (<2 mm) at or slightly below the castings Contour of Balloon‑shaped gas surface of casting the erosion is formed in the cavity caused by release - Causes: surface of the final cast part of mold gases during release of gases during Causes: pouring - erosion of the sand mold - Occurs at or below during pouring Prevention; the casting surface near the top of the - Use good melting & fluxing - enlargement of the mould cavity due to molten metal casting practices. pressure - Causes: - Reducing the moisture content of molding sand & - Insufficient ramming low permeability, poor increasing its permeability - Insufficient weight venting, high - Cope lifts moisture content of - Promote faster rate of the sand mold - Rapid pouring solidification *sand blow – hembus pasir *pin holes – lubang jarum

(2) Sand Casting Defects (d) scab – rough areas on the surface of the casting due to encrustrations of sand & metal Causes; (e) Penetration – surface -portions of the mold surface defects occurs when the flaking off during solidification fluidity of the liquid & becoming embedded in the metal is high casting surface penetrates into the sand -expansion of surface layers of mold or sand core the sand mould – weak - Upon freezing, the bonding casting surface consists -hard & uneven ramming of a mixture of sand grains & metal -Sand grain not uniform prevention; Prevention; - Harder packing of the sand mold - use smaller grain sand - Reduce moisture in sand *penetration *scab - kuping penusukan (f) Mold shift – caused by sidewise relative displacement of cope and drag – results a step in the cast product at the parting line causes; buoyancy of the molten metal *buoyancy – keapungan/daya apungan (Archimedes principle)

(2) Sand Casting Defects /Fin/flash (h) Mold crack/fins/flash – mold strength is insufficient crack (g) Core shift – similar to develops into which liquid mold shift, but its is the metal can seep to form a core that is displaced “fin” on the final casting excessive amounts of metal -The displacement is - or occur at the parting line usually vertical or core sections Mould - caused by buoyancy of & cores incorrectly the molten metal assembled - High metal pressure due to too long sprue - Insufficient weight - Improper clamping (i) Blow holes – Smooth & round holes (<3 mm) perceptible on the casting surface. Causes; - generation of gas or entrapped air in the mould cavity - Insufficient permeability of molding or core sand - Hard ramming - High moisture content - Pouring temperature too low Prevention; - The moisture content in sand must be well adjusted - Sand of the proper grain size should be used - Ramming should not be too hard - Increase pouring temperature *blow holes– lubang hembus

(3) Inclusions: Mold or core inclusions, slag, dross, and flux inclusions. • Mold or core inclusions • Come from sand or mold dressings & are usually found close to the surface • Slag, dross, and flux inclusions • Smooth depressions or cavities on the upper surface of casting. • Causes – incorrect gating system and poor fluxing of metal. Inclusions

Typical Classification of Casting Defects

Typical Classification of Casting Defects 7. Incorrect dimensions or shape 1. Metallic projections 2. Inclusions 6. Incomplete casting Classification of casting defects 3. Cavities 5. Discontinuities 4. Defective surface

Thank You …Q & A session…