By Andrew Ramsey
CT Specialist/X-Ray Centre of Excellence
Nikon Metrology Inc.
Web site: nikonmetrology.com
X-ray micro-computed tomography (µm CT) has the accuracy, resolution, speed, and flexibility manufacturers require to provide measurement details unattainable by other metrology means (think porosity and density mapping, for example). Applications are diverse and growing, dealing with metals, exotic alloys, composites, and single-crystal materials in a variety of industries. Software tools enable the analysis of part volume against the CAD model, either via direct volume-to-CAD comparisons, or through geometric dimensioning and tolerance measurements.
There are no radioactive sources in X-ray micro CT; rather, electrons are produced from a hot filament similar to a light bulb and accelerated at high voltage, creating a beam of electrons reaching speeds up to 80% of the speed of light. The electron beam is focused by a magnetic lens onto a metal target, producing a spot typically between 1–5 µm in diameter. The sudden deceleration of the charged electrons when they hit the metal target produces 99.3% heat and 0.7% X-rays.
These X-ray emanate from the region where the electron beam hits the target. The size of the X-ray spot size is determined by the acceleratingvoltage and the focusing power of the electron lens. The more power in the X-rays, the more penetrating the X-ray beam.
X-rays travel in a straight line, through the object being inspected, and onto a detector. The object absorbs some of the X-rays (denser objects absorbing more), leaving only a portion to reach the detector. At low X-ray energies (<60 kV), differences of absorption along the X-ray path to the detector are detected and shown as a shadow image. At higher X-ray energies (60–225 kV), absorption and scatter occurs. This scatter generally reduces contrast in the image. With X-ray energies above 225 kV, scatter becomes an increasing problem for certain detectors. Above 225 kV, scatter can be rejected from the detected signal by a linear detector, although throughput decreases (fewer images per hour). At greater than 300–400 kV, scatter is the dominant contrast mechanism.
Amorphous silicon flat-panel detectors have a fluorescent screen which converts the X-ray energy into light to form an image on an array of light-sensitive diodes. Electronics allow this image to be read by a computer. These panels can have pixel sizes over a wide range of sensitivities up to 16 bits (64k grey levels).
The sensitivity of the detector relates in part to the size of the X-ray source. A lot of high-power X-rays sources are minifocus, in the range of 1 mm across. This limits the resolution of images to that of the detector. A very fine detector is needed to get high resolution, and no magnification is possible. Microfocus means the size of the X-ray source is only a few microns across. With a microfocus source, a standard detector can be used and geometric magnification can be used to gain a higher resolution image. Generally, microfocus sources are only available up to 225 kV, although some manufacturers produce 300, 320, and 450 kV microfocus systems.
Combining the penetrating power of X-rays and the power of the computer produces computed tomography. The fundamental setup includes an X-ray source, the object being measured, and a detector. Thousands of digital images can be produced from a single sample, and each two-dimensional pixel in each image contributes to a three-dimensional voxel as computer algorithms reconstruct 3-D volumes. The result is a 3-D volumetric map of the object, where each voxel is a 3-D cube with a discrete location (X,Y,Z) and a density (r).
Not only is the external surface information known, such as with a 3-D point cloud from laser scanning, but internal surfaces and additional information about what is in between the surfaces from the fourth dimension (density) are provided. Furthermore, "slices" produced by the process and accompanying software can yield much information without destroying the part.
Image intensity, then, becomes the basis for measuring the sample. In CT, what’s being measured is the linear attenuation of the X-rays, or how much one unit of length of material reduces X-ray intensity. Better understanding the rules of X-ray micro CT can not only open the door to production cost savings and productivity improvement, knowing when to break them can provide even further process flexibility. ME
This article was first published in the July 2011 edition of Manufacturing Engineering magazine. Click here for PDF.