Final answer:
Ultrasonic inspection is the correct nondestructive testing method for locating surface cracks in aluminum alloy castings or forgings, leveraging its ability to detect density changes and suitable for measuring coating thicknesses.
Step-by-step explanation:
The appropriate method for locating surface cracks in an aluminum alloy casting or forging is ultrasonic inspection, which is a nondestructive testing (NDT) technique. Since ultrasound reflects well from any large change in density, it can detect cracks and voids that are too small to be seen with x-rays.
Ultrasonic testing is common in both industry and the military for ensuring the integrity of critical components. This method is also beneficial for applications such as measuring the thickness of coatings, as it can accurately determine the dimensions of various layers without damaging the item being inspected. Therefore, using ultrasound for nondestructive evaluation is an effective means to identify surface cracks in aluminum alloy castings or forgings.