A micro-focus 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 non-destructively and the integrity of welded assemblies to be inspected. In addition, dimensional data for specific components is acquired more quickly than is possible with a co-ordinate measuring machine (CMM), both from external and internal dimensions.
With Euro 6 emissions regulations 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, manufacturers of engines 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 also to improve fuel economy and enhance vehicle performance.
BorgWarner’s three production plants on the Podkarpacki Science and Technology Park in the Rzeszów area of southern Poland includes a production facility constructed in 2009 with the capacity to produce more than one million turbochargers annually. They are used in petrol and diesel engine cars built throughout Western and Eastern Europe. A new Technical Centre recently opened on the same campus to serve BorgWarner Turbo Systems’ turbocharger production by providing application engineering and design, simulation, testing and validation as well as material analysis. This development significantly broadened US-owned BorgWarner’s engineering, research and development capabilities within Europe.
NDT and dimensional inspection
It is in the new Technical Centre in Poland that the Nikon Metrology XT H 450 micro-focus CT system was installed in February 2014. Lukasz Krawczyk, Team Leader / Material Laboratory Manager, said, “We buy in our turbocharger parts, ranging in size from aluminium 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 only 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 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 than previously, enabling more rigorous analysis, and money is saved as 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 GD&T (geometric dimensioning and tolerancing) measurements.
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.
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.
Mr Krawczyk said that CT has become much more widely accepted of late as an inspection technology and is so flexible that they use it wherever possible in preference to CMMs and other metrology equipment on site.
Selection of the CT system
Five potential suppliers of high power CT systems were reviewed by Mr Krawczyk and his team. The Nikon Metrology 450 kV micro-focus system was selected, as it had an ideal specification for BorgWarner’s applications, producing a higher level of image detail for more comprehensive analysis and measurements.
It also provided value for money, as both a flat panel detector and a curved linear diode array (CLDA) were included in a single system. It is easy to swap 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 is the preferred mode for scanning quickly to detect defects. CLDA, on the other hand, takes a one-dimensional section image to build a more detailed picture of a part.
This technique is ideal for preventing X-ray beam scatter when dealing with denser materials such as those used for 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 they can be exchanged by the machine operator, without having to call in a service technician, lowering expenditure on maintenance. Mr Krawczyk also points to excellent service from the equipment producer, with rapid response to technical issues and requests for advice.