Casting defects and design for casting

Every casting contains something. The useful question has never been whether a casting has indications, but which ones, where, and how big — and the great majority of them are decided by the shape of the part long before the metal is melted.

A sectioned and polished aluminium casting under a bench inspection lamp, the cut face showing a dark angular shrinkage void at the thermal centre of a heavy boss, with a jeweller loupe and a polishing wheel on the bench beside it. — Angular, dendritic, and sitting in the heaviest section. That shape says feeding, not melt practice.

The five that account for most of it

A chart of five common casting defects. For each, a small sectioned sketch shows its appearance — shrinkage porosity as angular dendritic voids at the thermal centre of a heavy section, gas porosity as rounded evenly distributed holes, a cold shut as an unfused seam where two metal fronts met, an inclusion as a dark irregular non-metallic particle, and a hot tear as a ragged crack at a sharp internal corner. Beside each sketch the usual cause and the usual cure are listed, separated into design cures and foundry cures. — Shape tells you the cause. Angular voids in the thermal centre are a feeding problem; round ones spread through the section are a melt problem. They are not fixed the same way.

Defect	Appearance	Real cause
Shrinkage porosity	Angular, dendritic, at the thermal centre	The last metal to freeze could not be fed
Gas porosity	Rounded, dispersed	Gas dissolved in the melt came out on freezing
Cold shut	An unfused seam, often on a thin wall	Metal lost too much heat before the cavity filled
Inclusion	Dark, irregular, non-metallic	Slag, oxide film or eroded mould material
Hot tear	Ragged crack at a sharp corner	Contraction restrained while the metal was still weak

Hot spots, and the rule that removes most of them

A diagram of solidification and hot spots. The upper row shows three junctions — an abrupt thin wall meeting a heavy boss, an L junction with a sharp internal corner, and a cross junction of four ribs — with the inscribed circle drawn at each to show the local thermal mass, and the isolated hot spot shaded in each case. The lower row shows the same three junctions redesigned — the boss cored out and ribbed, the corner radiused, and the cross junction staggered into two offset T junctions — with the inscribed circle reduced and the hot spot eliminated. Arrows show the freezing front moving progressively towards the feeder in the improved versions. — A hot spot is a place where a larger circle fits. Cored bosses, blended radii and staggered junctions all do the same thing — shrink that circle until nothing needs feeding.

Solidification runs from thin to thick. So a heavy section surrounded by thin walls is left as an isolated pocket of liquid with nowhere for feed metal to come from, and it forms shrinkage porosity at its centre.

The rule that prevents most casting defects is a single sentence: keep the wall thickness uniform and blend every change gradually.

Where a heavy feature is unavoidable, the aim is to remove the thermal mass, not to feed it:

Core out the boss and carry the load with ribs
Stagger a cross junction into two offset T junctions
Radius every internal corner — a sharp corner is both a hot spot and a hot tear site
Taper sections so the freezing front runs towards the feeder

Getting the metal in without spoiling it

A liquid metal surface oxidises instantly. Turbulence folds that oxide film into the body of the casting, where it behaves as a crack that was cast in.

So a gating system is not judged on whether it fills the cavity. It is judged on whether the stream stays full and quiet: tapered sprue, bottom filling, filters, no free fall. A cavity filled turbulently is full of defects that did not have to exist.

Thin sections and long flow paths are where cold shuts appear — two fronts meeting after losing too much heat to fuse. The seam looks cosmetic. It carries no load whatsoever.

Inspecting, and agreeing what counts

The defects that matter in a casting are internal, and a perfect surface says nothing about the thermal centre. That is why radiography is specified on critical castings, with acceptance by comparison against reference radiographs at a stated severity level.

What to take away

Angular voids at the thermal centre are a feeding problem; round dispersed ones are a melt problem.
Hot tears come from restrained contraction at sharp corners while the metal is weak.
Cold shuts are unfused seams on thin sections and carry no load.
Inclusions are oxide film and slag folded in — usually by turbulent filling.
The single best rule is uniform wall thickness with blended transitions.
Handle a heavy boss by coring it out and ribbing it, not by feeding it.
Stagger cross junctions; radius every internal corner.
Gating is judged on a full, quiet stream, not on whether the mould filled.
The defects that matter are internal — specify radiography where it counts.
Agree acceptance by zone and severity before production.