Defects in Investment Casting – Part 1
Due to the complexity of the casting process there are numerous opportunities for things to go wrong resulting in casting defects, or undesired irregularities in a metal casting process. Some defects can be tolerated, others can be repaired but some must be eliminated. In order to ascertain the nature of the defect; examine the possible causes and take the appropriate corrective action, its location; appearance; shape and dimension must be determined.
All defects will fall into one or more of the following categories.
Classified as a negative defect, inclusions can be the result of foreign particles, usually non-metallic, such as slag, contained within a casting. The inclusion may also be formed by missing areas of metal in the casting resulting in small, round, irregular or angular craters. Additionally inclusions maybe depressions or cavities of various sizes; typically superficial with traces of ceramic and refractory material.
There are many possible causes of inclusions; some can occur early in the casting process such as cracks in the wax mould allowing bits of ceramic to get into the mould cavity; in some circumstances over hanging ceramic material on the shell can break off during the casting process.
Also referred to as a shrinkage crack, hot tear defects present as a jagged crack with an irregular path which can occur when the molten metal is restricted from contracting by the ceramic shell mould during cooling or solidification. Sometimes the casting design may need to be modified where possible to reduce major sectional changes and prevent hot tears.
Classified as a negative defect type, misrun can occur when the metal is unable to fill the mould cavity completely, leaving a very smooth, rounded edge. Likely causes are the metal is too cold; shell too cold or fill rate too slow.
Coldshuts can occur when two metal streams do not fuse together properly in the mould cavity, leading to discontinuity or a weak spot inside the casting and takes on the appearance of a crack. Potential causes are low pouring temperatures, slow pouring speeds or the shell is too cold.
Gas or porosity appears as a round, smooth walled cavity; it may exhibit a slightly oxidised surface with varying diameter and is the formation of bubbles within the casting during cooling. This occurs because most liquid materials can hold a large amount of dissolved gas, when the metal solidifies the gas expels. Gas porosity may present itself on the surface of the casting as porosity or the pore maybe trapped inside the metal.
A run out occurs when molten metal leaks out of the mould during pouring, resulting in parts not forming correctly leaving voids, in most instances it is smooth in appearance but can also be rough and irregular. Potential causes are thin ceramic shells, dewax cracking of shells or handling in the foundry.
Taking on a jagged or linear appearance, shrinkage defects can occur when casting sections change (e.g. thick or too thin); at the feed gate to the casting; or when standard feed metal is not available to compensate for shrinkage as the thick metal solidifies during cooling.
Categorised as a positive defect, splash can be caused by molten metal ‘splashed’ onto the casting. Fortunately, this type of casting can usually be removed without damage to the part(s). In some cases this defect can be caused by thin or weak areas of the shell failing during dewax or casting, allowing metal to leak into the void of the shell.
Referred to as both a negative and positive defect type, wax damage can often be mistaken for a misrun. However, wax damaged parts form sharp edges as opposed to the more rounded edges seen in a misrun. As the name suggests, one possible cause happens when the wax pattern has been mishandled (negative) but it is also possible to have positive defects due to wax drips from wax assembly.
Not always detectable by the naked eye but can be detected by gauging, a bulge can be identified by its gradual thickening of the casting wall. Potential causes are wax patterns too close resulting in ceramic shell bridging or metallostatic pressure being too high.
Understanding casting defects and the causes is essential to managing casting quality, in part 2 we will explore potential solutions to the defects explained above.