A 2D image of a cast iron turbocharger

A 2D image of a cast iron turbocharger.

High-Voltage CT Magnifies the Effect of NDT

Computed tomography is transforming non-destructive testing for critical parts, like automotive turbochargers for BorgWarner 450-kV X-ray equipment Combined NDT for inspection, analysis Targeting, Selecting CT Tools

A microfocus computed tomography (CT) system developed by Nikon Metrology is being used by a BorgWarner auto parts plant in southwestern Poland to improve research and development of cast iron turbochargers it produces for passenger cars, light trucks, and commercial vehicles. The 450-kV X-ray equipment penetrates dense materials needed in cast products of this type, allowing the internal structure and quality to be inspected non-destructively.

In addition, dimensional data for specific components is collected more quickly than is possible with a coordinate measuring machine (CMM), both from external and internal dimensions. With Euro 6 emissions regulations taking effect now, aiming to reduce further the level of gases and particulates allowed in vehicle exhaust, manufacturers of engines and their suppliers are deploying more advanced technology in the design and development of air management systems. Their goal is not only to reduce pollution, but also to improve fuel economy and enhance vehicle performance.

BorgWarner has three plants at an industrial campus at Bielsko-Biala, Poland, including the turbocharger plant that was completed in 2009 to produce over 1 million units annually. These are supplied to automakers building gas and diesel engine vehicles across Europe. More recently, a technical center opened on the same campus to support BorgWarner’s turbocharger production with application engineering and design, simulation, testing and validation, as well as material analysis. This development broadened BorgWarner’s engineering and R&D capabilities considerably in Europe.

It is at the technical center that the Nikon Metrology XT H 450 microfocus CT system was installed in February. Lukasz Krawczyk, the team leader and manager of the material laboratory, said, “We buy in our turbocharger parts, ranging in size from aluminum compressor discs to stainless steel or cast iron housings, from a number of different sources.

“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.

“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,” he said.

Overall, much more information is available to the inspectors than previously, so their analysis can be more rigorous, and the overall cost of quality control is improved because inspected parts can be reused for further tests. Software makes it possible to correlate the inspected volume against a CAD model, or a master sample, either via direct comparison or through geometric dimensioning and tolerancing (GD&T) measurements.

Ascertaining, Locating Voids

“In castings, for example, it is possible to ascertain the location and size of a void or crack emanating from it and determine the likely cause of the fault and whether it is due to the type or quality of the material or the component design,” according to Krawczyk.

“Also a bearing assembly can be X-rayed to check that all components are present, avoiding the cost of dismantling,” he continued. “The electron-beam weld that joins an impeller to the shaft can be inspected to check for porosity and mechanical integrity, a job that is impossible to do by visual inspection.”

Krawczyk said that CT has become much more widely accepted of late as an inspection technology and is so flexible that his center uses it wherever possible, preferring it to CMMs and other metrology equipment that is also available.

Computed tomography is a radiographic technology, with roots in medical imaging.  Tomographic images (i.e., slices) of specific areas of an object are collected from volume of two-dimensional X-ray images taken from different perspectives or angles, and these cross-sectional images are combined to depict a three-dimensional version of the internal arrangement of the object under inspection.

The number of new systems developed by various metrology and/or X-ray technology suppliers confirms Krawczyk’s contention that CT has grown in use for manufacturing quality control. For example, Yxlon recently introduced a new series of industrial CT scanners with a coordinated software platform for non-destructive testing and measurement. Its emphasis is on the intuitive design and functionality of the technology, available for use by operators, technicians, or inspectors with varying degrees of expertise in material or physical engineering.

Demanding More QC, Faster

Manufacturers’ quality control requirements grow more demanding, in part, because lead-times for introducing new products at lower cost is growing shorter, and so the number of prototype iterations is fewer. Destructive testing is undesirable, so numerous tests must be conducted with a single prototype. Scanning or touch CMM inspections provide external dimensional detail only; to gather details of complex internal structures the sample must be cut open or disassembled. CT is a fast and simple solution to these issues, and gathers details on dimensions and material structures, so inspection is faster and quality evaluations are better informed.

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 turned through 360 degrees, the 2D X-ray images are reconstructed into a 3D volumetric map of the object. Each element is a 3D pixel (voxel) that 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 revealed and 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. Different open- or closed-tube designs are available, 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.) Nikon designs open tube sources that allow the filament to be replaced regularly, lowering the cost of ownership versus closed-tube sources.

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 up to 80% 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% of the energy is expended in heating it.

Less than 1% 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, more power is transferred to the target, and the larger the spot size the more X-ray power will be produced.

High material density, in particular metals, will attenuate X-rays more, which is a limitation for industrial CT. The Nikon Metrology XT H 450 addresses this by supplying 450 W of continuous power, with no restriction on measurement time, maintaining a small spot size (50-113 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 400×600×600 mm working envelope, providing a combination of 3D NDT defect analysis and dimensional inspection in one package.

In Poland, Krawczyk and his colleagues evaluated five potential suppliers of high-power CT systems before they selected the Nikon Metrology 450-kV system. He recalled that it offered an ideal specification for BorgWarner’s applications, producing a higher level of image detail for more comprehensive analysis and measurements.

It also was the best investment, considering that the system included a flat panel detector and a curved linear diode array (CLDA), not one or the other. He noted that it is easy to alternate between detectors to suit the level of resolution required and the material being inspected.

A flat panel is best for obtaining an image of a complete component, and it is preferable for quick scanning to detect defects.

On the other hand, CLDA takes a one-dimensional section image to build a more detailed picture of a part. This technique is better for preventing X-ray beam scatter when dealing with denser materials, such as cast iron turbine housings. The latter mode is also used for metrology, due to the high level of detail generated.

There is another cost-saving aspect to using the XT H 450. The price of filaments is low and the machine operator, not a service technician, can exchange them.

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