Porosity from casting is an ever present problem to be checked for. Different processes are used for different metals/alloys and end use requirements. Turbine blades for jet engines used to be cast by he investment ( lost wax) method and 100% inspected. They had a high failure rate. Rimming steel, as made in bulk for rolling to thin sections ( car bodies, white goods) needed the initial outer layer solidifying cleanly and directionally so chilling was one method to speed up that. The body of the ingot ( the last to solidify) was encouraged to effervesce and allow gases to escape. Subsequent rolling welded up the remaining internal porosity. Any porosity at the surface would simply split open during rolling. We made engineering steels and used surface crack detection and ultrasonic methods for sub surface defect detection. This allowed us to prevent any suspect batches getting through that would compromise future use/safety. Cast metals can also carry over impurities from the melting/refining. Microscopic examination is used to identify this type of thing. A scale of cleanliness is used to evaluate it. Compare what you see down the microscope to standard photographs. The bottom line is stipulating the manufacturing process and quality standard to apply. The more safety critical the tighter the requirements. Finishing techniques for a cast item must be in place to ensure it doesn’t carry over defects. One thing to notice is when a supplier describes a product as “ machined from billet stock”. This means the item has been rolled ( usually) to change the internal structure away from cast to a wrought one. A stronger, less defect prone product and with some enhanced directional properties. Lining a brake cylinder with a suitable material is a sensible option that reduces the need for enhanced casting requirements. Accept a level of “ defect” and counter it. Reduces initial casting cost/ intricacy with a more robust finish and hopefully cheaper.
