Casting Defect Basics: Shrinkage
Thinking back over my years in the foundry industry, the most common questions asked by buyers, designers, and machinists are related to explanations of casting defects, their causes, and remedies. You will encounter casting defects during your tenure as a buyer, so it is important that you become familiar with the defects that are commonly seen by casting customers.
There are two main categories of defects to be aware of: those related to foundry processes and those related to casting design (we’ll consider metallurgical-related defects as out-of-scope for this discussion). The difficult part in analyzing defects is that sometimes both process and design can be found to be contributors to the root cause of the defects, so the investigation can become quite involved. One important thing to keep in mind is that repetitive defects are generally easier to solve than those that exhibit a more sporadic nature or are one-off types.
End-users of castings tend to categorize the most common casting defects as porosity. While this is directionally correct, the term porosity is generic in nature and doesn’t give the foundry enough detail to start a meaningful investigation into the root cause of the defect. The best practice is to have your inspection team provide the foundry with as much descriptive information as possible. Photos of the defect should show its location on the casting including a clear and well-lit close-up photo of the defect itself.
Also, be sure to include any traceability information for the casting, such as the production date code and mold cavity number. If possible, return a casting that shows the defect to the foundry for their review, or hold one of the castings at your facility so a foundry representative can come in and review the issues with your team.
In my experience, the most common defects that you are likely to encounter are shrink, gas, and inclusions. These all exhibit themselves as a type of porosity but have completely different root causes and corrective actions. Please be careful naming the defect until you are very familiar with the various types. The reason for being cautious is that defects have different root causes, so if you inform the foundry that you are encountering sand defects during machining, but the defect you find is actually shrinkage, the foundry will be working on the wrong solution.
Let’s first consider the shrink defect. The most common type of shrinkage is generally uncovered during machining. This internal defect is caused by a lack of liquid feed metal during the solidification process. It will occur in sections of the casting where isolated thick sections are present, such as bosses or pads. It is also prevalent in areas of the casting where thick sections change to thinner sections. When the shrinkage is uncovered during machining, the area of shrink will be dark and have a rough surface appearance; viewing under a microscope or with a hand lens will show the presence of dendrites, which are very often fern-like in appearance. Shrink can vary in size from very small (just a few specks) to gross shrinkage that leaves a cavity you could stick your finger in. Shrinkage can also occur on the external surface of castings, showing up as depressed areas that have drawn down during solidification.
What remedies can the foundry take to eliminate shrinkage? Since this is related to a lack of feed metal during solidification, the casting engineering team will review the gating and risering systems and potentially make changes to ensure sufficient feed metal is available to provide a sound casting. In cases where the area of shrinkage can’t be easily reached with a riser, the foundry may use chills. A chill is a metal slug that is placed in the mold in contact with the casting surface and essentially becomes a heat sink. When the metal is poured into the mold, the liquid metal touching the chill solidifies faster and can eliminate or reduce the tendency for shrink in that localized area.
In my next column, I’ll cover more common casting defects that fall under the umbrella of porosity, including gas and inclusions. CS