Investment Casting Defect

Investment Casting Defects Analysis and Remedies

　　In last article,we have introduced one of the biggest defects of investment casting—Crack. Beside it,there is also others defects happened during the production of investment precision castings.Here,we will introduce them ,analysis cause and provide the basic remedies solution for them.Foundry production is a complex multi-step process.It is avoidless that some investment casting defects are occurred due to some wrong operations. Once happened, How could we prevent it from happening?

Here we list 9 big investment casting defects Analysis and Remedies methods

1. Gas hole

Gas hole is the hole that exists in the casting surface or internal area. When the casting cools and solidifies, bubbles form because the solid form of the metal cannot hold as much gas as the liquid form. These bubbles appear on a casting as rounded, circular cavities or holes. It can be round, oval, or irregular shape, sometimes is an air mass made up of many gas holes. Internal gas holes is usually pear-shaped. Shape of choking hole is irregular, and the surface is rough. Gas nest is casting dents on the surface, the surface is much more smooth. Surface gas holes can be found by appearance inspection, but, subsurface gas holes can only be tested after machining.

Three types of casting defects related to gas hole.

Pinholes	Pinholes, also sometimes referred to as porosity, are very tiny holes (about 2 mm) usually found in the cope (upper) part of the mold, in poorly vented pockets.casting defects They usually appear in large numbers together, either at the surface or just below the surface of the casting. They are always visible to the naked eye and don’t require equipment to identify.
Subsurface blowhole	Blowholes, or simply blows, are larger cavities than pinholes. A subsurface blowhole appears on the inside of a cast and usually isn’t visible until after machining.Subsurface blowholes can be difficult to detect before machining, requiring harmonic, ultrasonic, magnetic or x-ray analysis.
Open holes	These blowholes appear on the surface of the cast and are easier to detect than subsurface blowholes.

Investment Casting Defect Causes and Analysis:

Poor venting of mold and cores

Preheating temperature of casting mould is too low, and cooling of liquid metal is too fast when it goes throught the pouring system.

Bad coating material, poor exhaust, even volatile or break down the gas itself.

Holes and pits are exsited in the surface of mold cavity, when pouring liquid metal, the gas in the holes and pits are rapidly expanding, and compress gas liquid metal, thus to form choking hole.

Surface of mold cavity is rust, and did not clean up.

Raw material (sand core) is improperly stored, without preheating before use.

Poor deoxidizer, improper dosage or improper operation, etc.

Insufficient drying of mold and cores

Investment Casting Defect Remedies Method

Mould should be fully preheat, the granularity of coatings (graphite) shoulds not be too thin, permeability needs to be good.

Using tilt pouring way when casting.

Raw materials should be stored in ventilated, dry place, and should be preheated when used.

Choose deoxidizer (magnesium) with better deoxidization effect.

Pouring temperature should not be exorbitant.

Incorporate good fluxing and melting practices: melt metal in a vacuum, in an environment of low-solubility gases or under a flux that prevents contact with the air

Increase gas permeability of sand: coarser sands have a higher permeability

Increase permeability of mold and cores. Allow air and gas to escape from the mold cavity

Dry out molds and cores before use and store dry

Increase rate of solidification by reducing metal temperature during casting

2.Shrinkage

　　Shrinkage occurs because metals are less dense as a liquid than a solid. Shrinkage cavity is the surface rough hole in the casting surface or internal, slight shrinkage cavity is many scattered small shrinkage cavity, namely shrinkage. It is a depression in a casting which occurs during the solidification process. The grains of shrinkage cavity are big and thick. Often occur in the casting spruce, riser root, thick area, the connecting area between thin & wall thickness, ect. Shrinkage porosity appears with angular edges, compared to the round surfaces of gas porosity. Cavities might also be paired with dendrite fractures or cracks. Large shrinkage cavities can undermine the integrity of the casting and may cause it to eventually break under stress.

Two types of Shrinkage in Investment casting defects.

Open shrinkage and Closed shrinkage

Open shrinkage	These are open to the atmosphere. Air compensated as the shrinkage cavity forms. Caved surfaces are shallow, open shrinkage defects that form across the surface of the casting.
Closed shrinkage	Also known as shrinkage porosity, closed shrinkage defects form within the casting. Macro shrinkage can be viewed with the naked eye, but micro shrinkage cannot. Closed shrinkage defects usually appear at the top of hot spots, or isolated pools of hot liquid.

Shrinkage Causes