A microfocus computed tomography (CT) system from Nikon Metrology is being used by BorgWarner Poland to improve research and development of turbochargers for passenger cars, light trucks, and commercial vehicles. The high-power (450 kV) X-ray equipment is able to penetrate the dense materials used in turbocharger production, allowing the internal material quality of castings to be checked nondestructively, and the integrity of welded assemblies to be inspected.
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In addition, dimensional data for specific components is acquired more quickly than is possible with a coordinate measuring machine (CMM), both from external and internal dimensions.
Euro 6 emissions regulations are due to take effect in Europe in 2014, which will further cut the amount of harmful gases and particulates allowed in a vehicle’s exhaust. In response, engine manufacturers and their suppliers are deploying ever more advanced technology in the design and development of air management systems. The goal is not only to reduce pollution, but to also improve fuel economy and enhance vehicle performance.
BorgWarner's three production plants in the Podkarpackie Science and Technology Park in the Rzeszów area of Southern Poland include a production facility constructed in 2009 with the capacity to produce more than one million turbochargers annually, to be used in petrol and diesel engine cars built throughout Western and Eastern Europe. A new, recently opened technical center on the same campus serves BorgWarner Turbo Systems' (BWTS) turbocharger production by providing application engineering and design, simulation, testing, validation, and material analysis. This development significantly broadened U.S.-owned BorgWarner's engineering, research, and development capabilities within Europe.
Combined NDT defect analysis and dimensional inspection
Nikon Metrology's XT H 450 microfocus CT system was installed at the new technical center in February, 2014. "We buy our turbocharger parts, ranging in size from aluminum compressor discs to stainless steel or cast iron housings, from a number of different sources," says Łukasz Krawczyk, team leader/material laboratory manager. "Before we put an assembled turbocharger onto an engine emulator for endurance and thermo-mechanical testing, we need to check the quality of the individual components and sub-assemblies. Previously we did this by sectioning sample castings and machined prototypes and checking them on a CMM, but that meant we were wasting valuable prototype or pre-series components. Additionally, the parts we were testing were representative examples from the same batch, rather than the ones we actually inspected, which were of course destroyed."
Krawczyk continues, "Now we know that the components under test are only the ones we inspected dimensionally and, in the case of castings, for the presence of porosities or inclusions as well."
Overall, much more information is available now than through previous testing, enabling more rigorous analysis. What's most, cost savings are achieved because parts can be reused for further tests. Software enables correlation of any inspected volume against a CAD model or a master sample, either via direct comparison or through geometric dimensioning and tolerancing (GD&T) measurements.
In castings, for example, it is possible to ascertain the location and size of a void or crack, find the likely cause of the fault, and determine whether it is due to the type or quality of the material or the component design.
Also, a bearing assembly can be X-rayed to check that all components are present, avoiding the cost of dismantling. The electron beam weld that joins the impeller to the shaft can be inspected to check for porosity and mechanical integrity, a job that is impossible to do visually.
According to Krawczyk, CT has become much more widely accepted as an inspection technology. It is so flexible that the BWTS team prefers to use it whenever possible over CMMs or other on-site metrology equipment.
Background to CT
Today's manufacturers face increasingly shorter lead times for introducing new products at lower cost; consequently, the number of prototype iterations is fewer. Destructive testing is no longer desirable because a multitude of tests need to be carried out on a single prototype. Tactile or scanning CMM inspections provide dimensional insight of outer dimensions but can only investigate complex internal structures if the sample is cut or disassembled. CT offers a solution that is easy to use, fast, and provides detailed insight for dimensional, material structure, and assembly inspection, resulting in faster problem solving and more effective decision making.
CT is fundamentally a simple process. An object is placed on a rotating stage between an X-ray source and a detector, which acquires simple 2D radiographic images of the object as it rotates. After the object has revolved 360 degrees, the 2D X-ray images are reconstructed into a 3D volumetric map of the object. Each element is a 3D pixel (voxel) which has a discrete location and a density. Not only is external surface information acquired, as with a 3D point cloud from laser scanning, but data on internal surfaces is also revealed. By mapping the density, information is provided on what is between the surfaces.
The X-ray tube is at the core of a CT system. Several different open or closed tube designs exist, but essentially an X-ray source consists of a cylinder in which there is a filament (similar to a light bulb) at one end, together with a high-voltage cathode and anode, a magnetic lens, and a metal target, normally tungsten.
A current is applied to the filament, which causes it to heat up and emit electrons. The electrons are repelled by the cathode and attracted to the anode by the high-voltage field, which accelerates the electrons to up to 80 percent of the speed of light toward the end of the tube. Before they leave it, the electron beam is focused onto the target material using an electromagnetic lens. The electrons slam into the target, and 99 percent of the energy is expended in heating it.
Less than one percent of that energy produces X-rays that are generated in a cone beam from the target. The higher the voltage applied, the more energy is in the beam; consequently, the more power is transferred to the target, the larger the spot size and the more X-ray power is produced.
A limitation of CT in industrial applications is that high material density, especially of metals, tends to attenuate the X-rays. Many system suppliers only offer microfocus sources up to 225kV, while their more powerful sources are mini-focus, producing more X-ray flux but with a spot size an order of magnitude larger, reducing the accuracy of the data collected. A microfocus source is needed to acquire accurate and detailed CT data for most high-accuracy industrial CT applications.
The Nikon Metrology XT H 450 delivers 450W of continuous power, without any restriction on measurement time, while maintaining a small spot size of 50 to 113 microns and delivering a scatter-free CT volume with 25-micron repeatability and accuracy. Samples weighing up to 100 kg can be inspected within a 400 x 600 x 600 mm working envelope, providing a combination of 3D NDT defect analysis and dimensional inspection in a single, highly productive facility.
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