Beijing Institute of Aeronautical Materials, P.O.Box 81‐6, 100095
Some doubtful defects in DD6 single crystal turbine blades were studied through experiments in this article. The research group collected some samples of blades and cut them, comparing with the corresponding radiographic films, then concluded that those indications were not internal defects in metal castings, but inner‐surface defects. Since the collection of turbine blades was hard because of they were costly, the conclusion of this article had limitation. If more casting samples could be collected in future, the research would be carried out deeper and further.
X‐ray testing, turbine blade, single crystal
The key point in engine is turbine and compressor. Whatever commercial high by‐pass ratio
turbofan engine, or military low by‐pass ratio turbofan engine, the core machine is needed, and with the best blades. Blades include turbine blades and compressor blades. Generally, turbine blades are required to run thousands hours in the execrable condition of 1500℃ and huge centrifugal force of nearly 15000/min. Turbine blades should have high working temperature, large loading and complex stress, which need the materials with good heat resistance, impact resistance, fatigue resistance, corrosion resistance, and damage tolerance, etc. In 1970’s, some foreign countries developed directional solidification single crystal super‐alloy, which solved completely the life problem for turbine blades in bad working condition of high temperature and high pressure. Boeing 747, 767 equipped with JT9D engine of U.S uses single crystal alloy of PWA1422, with life of more than 9600 hours. F‐100 engine of F‐15 uses the blades of first generation of directional solidification alloy, but the characterization for the second generation single alloy PWA1484 and the third generation Re‐neN6, is much better than the first generation. We can see that Airbus planes and Boeing planes fly in sky day and night, with the engines working reliably. The life of some CFM‐56 engines could be 14 thousands hours.
DD6 is the second generation Ni‐based single crystal super‐alloy in China, which has some advantages such as high high‐temperature strength, good comprehensive properties, stable structure, and so on. Comparing with the first generation of Ni‐based single crystal super‐alloy DD3, temperature bearing capability was improved about 40℃. Compared with the second generation of single crystal alloy used widely abroad, the properties of tensile, stress rupture, antioxidation and heat corrosion resistance, reach their level with some of them even better, because low content of rhenium has predominance of low cost. This kind of alloy is suitable for producing the parts of high temperature, such as gas turbine working blades with complicated endocoele which run under 1100℃ temperature.
Figure 1 shows that British researchers repaired the defects on single crystal blades with ESD
(electro spark deposition). ESD is a micro‐weld surfacing process used for localized coating nd build up repair. TWI adapted an ESD system for mechanized operation, to improve coating quality whilst increasing deposition rate. 
Status of x‐ray testing for single crystal blades
There is not many related information found about NDT status of single crystal blades. Some of them are as bellowing. In 1991, the researchers from Defense Metallurgical Research Laboratory of India studied single crystal by using a divergent beam x‐ray diffraction technique, which helps in observing defects in single crystal but also in determining the crystal orientation. What’s more, this method was proposed for observing images of sections of single crystal components.
In 1995, Howmet led a project which was supported by a team including ABB, Pratt & Whitney, and Purdue University etc. The project is scheduled over period of 34 months with completion in April 1998. Four technology thrust areas were included: low‐sulfur alloys, casting processes development and understanding, post‐cast process development and improvement, casting defect tolerance level. Postcast process development and improvement includes investigation of varying heat treatment and hot isostatic pressing treatment on large test specimens, conducting metallographic examinations, performing mechanical property tests to determine effects of varying treatments, and developing inspection techniques for detection of porosity and internal defects. Fig 2 shows the three kinds of blades in this project: single crystal (l), directionally solidified (centre), and equiaxed (r).
Experiment Study for black lines, white lines and drop shadow on radiographic films of DD6
single crystal blades
The producing process of DD6 single crystal blade is very complicated, which has fine structure. When taking x‐ray testing, inspectors could not determine some indications on films, so there were some curbstone conclusions, such as black lines (Fig 3a), white lines (Fig 3b), and drop shadow (Fig 3c). All the 209 blades of 12 batches tested in the whole year of 2007 were added up, to get the conclusion that, 6.7% blades radiographs with black lines, 5.3% blades radiographs with white lines, and 15.8% blades radiographs with drop shadow. However the producing branch was not satisfied with such result, they hoped us giving certain judgment of defects or un‐defects. Therefore, the testing branch implemented a series of experiments to the question.
From materials properties and casting technique, we analyzed the possible defects in DD6 single blades. There is only one crystal grain for a single crystal super alloy cast, which grows continuously from the basis after the grain is determined. The perfect structure of single crystal blade shall have the root, body and tip which are consisted of polyphase monocrystalline without any defects. The crystal direction shall be <100>, and the offset from the principal axis stress direction of blade shall less than 10°. Micro hole and cavity may be produced during casting process, some research shows that the drift angle of crystal direction is important for expansion (non‐shape) of casting micro‐cavity. Besides, if minim air enters during moulding process, the defects of holes or cavities are easily generated inside metal.
Indications of black lines and drop shadow on radiographs are the locations darker than surrounded area, but with different shapes. Black line shows generally like continuous, long strip shape, and dark thick lines with smooth edge, and they may be long or short. Most of them exist among the burble poles of trailing edge. But drop shadow usually shows as separated, low ratio of length to width, and dark area, with edge of smooth or not, similar to the dregs or air cavity. They may exist on trailing edge or leading edge.
In order to know what these possible defects really are, there were totally 11 blades collected to be cut for the experiment, which are the scrapped blades with relevant indications. Comparing the section of cut blades with the corresponding radiographs. Some of the results show like Fig 4.
We know that ceramic core was used to form the inner structure of blade, and the working
condition of blades during moulding process and solidification process is very complicated. The
core bears the bending force under the metal pressure, and the surface of core receives the
impact from metal liquid flow when moulding. However, the structure of the inner surface of
blade leads to make machinery processing impossibly, so the inner surface finish is only decided by finish of the core.
Therefore, black lines are considered to be caused probably from surface mistake of alumina base ceramic core when casting, and discontinuities on core surface might lead to the discontinuities on surface of metal after leaching core.
Other side, shadow is risen probably from un‐smoothies of wax mould surface. The bulge on wax mould is presented as concave of inner surface on blades.
Currently, since we were failed to collect the blade with white lines indicated on radiograph, what we could do was to discuss and analyze through communication with producing department. After exchange of viewpoints from both sides, white lines were considered as arisen from outflow of metal liquid caused by core dehiscence during casting, thus light‐color line was formed on inner surface. Similar to redundant metal inside the hollow castings, typical radiograph of redundant metal is shown as Fig.5.
From the above, black lines, drop shadow and white lines are not the inner metallurgical
defects, but the surface defects. Generally, outside surface defects on blade can be recognized by vision test, but the inside surface defects can hardly observed because of the very complicated inside lumen structure of DD6 single crystal blade. However, the locations with surface defects (including outside and inside surface) have also inner defects usually. So in practical testing, inspectors do know the possibilities for superposition of such surface defects with inner defects, they will stop the blades discreetly.
This article makes an elementary experiment research and discussion for X‐ray testing of
DD6 single crystal blades. As the blades are precious, the collection is relative hard, thus the
conclusion of the article is limited to some degree. In future, study and analysis will be performed furthermore provided that more relevant blades are available.
 Flying high – with the aerospace industry. TWI Bulletin, Sep.‐Oct. 2003
 A divergent beam diffraction method for study of single crystal, Satyapal Singh and A M Sriramamurty,
 Manufacturing Technology for Scale‐Up of Single Crystal Turbine Airfoils.htm