Turbine blades are designed for optimum aerodynamics and mass center location, and constitute of advanced metal alloy castings and composites to increase strength, resist extreme temperature, and avoid corrosion. Dense metal alloy materials set specific challenges for the inspection of turbine blades. Inaccuracies in blade geometry and positioning may cause energy conversion efficiency loss and untimely blade failure. To guarantee optimum blade position and aerodynamic operation, tight tolerances apply to both the geometry and alignment of turbine blades.

Internal air cooling allows turbine blades to operate under extremely high temperatures up to around 1000°C. Blade wall thickness inspection is key to ensure optimum strength/cooling trade-off across the entire aero foil surface of blades. In addition, structural imperfections, including cracks and inclusions, may put blade lifetime at risk.

 Challenges when inspecting turbine blades

• Non-destructive inspection: Laser scanning, X-ray and micro-CT is used to image and qualify the internal structure of turbine blades. Turbine blades are rather expensive samples.
• High inspection accuracy: both the freeform aerofoil surface of blades as well as specific features require accurate verification. In addition, internal walls thickness is subject to tight geometric deviations.
• Dense material: inspection of dense material requires powerful x-ray source that allows x-rays to travel through the blade.
• X-ray scattering: the inspection of dense materials potentially leads to x-ray scattering, which may cause inferior image quality.
• Fast inspection: turbine blades are inspected at different stages in the prototyping and production process: after concluding molding and specific machining and finishing steps.
• Fast CT reconstruction: in a production environment, it is important to quickly obtain inspection results.
• Larger specimens: turbine blades are somewhat larger in size and require an inspection cabinet that offers sufficiently space.

Traditionally, turbine blades or film radiography are verified through touch sensors based CMM inspection. However, blade manufacturers increasingly use laser scanning to verify the shape of the aerodynamic blade surface and the dimensions of milled planes and flanges, drilled holes and alignment notches. to gain full insight in internal structure, wall thickness and cooling holes the latest technology is to use X-ray and CT inspection.

Compared to traditional tactile inspection, LC and XC laser scanners capture much more point data in a shorter time frame, and embed measurement and analysis in an automatic digital inspection process. At the same time, non-contact measurement requires little or no blade pre-alignment, and eliminates probe compensation when scanning freeform blade surfaces. By capturing thousands of inspection points per second, Focus software is able to apply far more accurate line fitting to determine the positions of notch edges and verify the angles between notches. Laser scanning and Focus point cloud processing software are the cornerstones of a much faster and operator-independent digital inspection process.

Innovative X-ray systems up to 450kV set a new reference for turbine blade measurement and NDT of small to medium castings. At the core of this powerful equipment is a 450kV micro-focus source, providing superior resolution and accuracy available with a flat panel or Curved Linear Diode Array (CLDA) detector The
CLDA detector optimizes the collection of X-ray images by eliminating the scatter phenomena that typically influence 2D radiographs of blades and other metal parts. It is a flexible system that deals with small to large metal parts such as blades, castings etc.

Typical applications:

Application 1 : Detailed analysis of the internal structure of turbine blades

Besides inspecting the aerofoil surface of the blades, it is important to verify the internal structure of the blades. Industrial micro-CT traces inclusions, cracks and other material imperfections before committing to multiple machining process steps. Accurate blade profile images highlight the precise thickness of internal walls at any location along these walls.

Application 2 : Automated pass/fail inspection of blades

The entire inspection procedure can be defined and automated up-front. As a result, a series of turbine blades can be inspected automatically, granting a pass or fail label to each inspected blade.