Paul J. Zombo

Dielectrometers and magnetometers suitable for in-situ inspection of ceramic and metallic coated components

Quasistatic spatial mode (QSM) sensing is a new form of nondestructive evaluation developed to address the increasing need for quantitative materials characterization. Two types of QSM sensors and measurement methods are described: QSM magnetometry and QSM dielectrometry. These measurement methods were invented at the MIT Laboratory for Electromagnetic and Electronic Systems and are being developed at JENTEK Sensors, Inc. for specific applications, such as coating porosity characterization. The sensors discussed here are the meandering winding magnetometer and the inter-digital electrode dielectrometer. These sensors are thin and flexible, permitting inspection of complex and difficult-to-access surfaces. Using continuum electromagnetic models of the sensor field interactions with multiple layered media, repeatable and quantitative measurement of physical and geometric properties are obtained. The specific application addressed here is the characterization of coating and component condition for turbine blades. This includes measurement of thickness and porosity for both ceramic and metallic coatings. Future research will focus on age degradation monitoring as well.

Development of X-Ray Diffraction Methods to Examine Single Crystal Turbine Blades

Technological advances in recent years have led to the development of single crystal, nickel based alloys for turbine blade applications. Among the problems encountered with single crystal turbine blades is the determination of the overall crystalline perfection of the final blades. The existing method relies upon chemical etching and visual inspection. While the problems associated with visual inspection are self evident, there are intricacies associated with the etch process. Following visual inspection of the etched blades, those which meet with approval are then “unetched.” That is, a mechanical peening process is used to remove the shiny finish produced from the etch. The final step in the crystal perfection/orientation process for single crystal turbine blade inspection is to determine the crystallographic orientation of a “good” blade by performing Laue x-ray diffraction, in the back-reflection configuration, at one point on the blade. It is interesting to note that none of the inspection processes above can determine the crystalline perfection of the interior of the blades; as will be discussed below, this is a legitimate concern.

Development of Ultrasonic Inspection for a Bonded Superalloy Blade

Directionally solidified multigrain and single crystal airfoils have been used in aircraft gas turbines for over ten years and are currently found in aircraft engine-derivative gas turbines used for land-based power generation. However, the adoption of this technology for large land-based gas turbines is just underway and is not a simple scale-up. One approach to produce the large blades required for this application involves casting the blade in two separate halves and then bonding these halves together using the transient liquid phase bonding (TLPB) process [1]. This process results in a number of internal bond surfaces at the ribs. The condition of these bond surfaces must be determined prior to the blade entering service.

ULTRASONIC EVALUATION OF TRANSIENT LIQUID PHASE BONDING IN SINGLE CRYSTAL SUPERALLOY CASTINGS

Transient liquid phase bonding (TLPB) is an effective means for joining high performance metal components. It differs from welding and conventional brazing in that it produces very little chemical segregation or microstructural demarcation at the bond-line. The method of transient liquid phase bonding was originally developed in the 1970’s [1] and has been used in the joining of titanium and nickel based superalloy components. In this method, a bonding alloy containing a melting point suppressing element is sandwiched between the parent metals to be joined. The temperature is raised to a point where the bonding alloy melts but the parent metals remain solid. The melting point suppressing element then diffuses away from the bondline, thus raising the melting point and solidifying the bond. Since the temperature never exceeds the melting point of the parent metal, single crystals may be joined without destroying their crystalline structure.

Distance Amplitude Correction Factors for Immersion Ultrasonic Measurements through Curved Surfaces

Near net-shaped forgings offer significant advantages for component manufacture, including less material waste and reduced costs for machining to final shape. However, curved entry surfaces on near net shape forgings create complications for ultrasonic inspection methods. In immersion ultrasonic testing, entry surface curvature causes ultrasonic beam focusing or defocusing, which affects the detection sensitivity to interior material flaws, such as voids and inclusions, as compared to inspection through planar surfaces.