X-ray detection limits in Hastelloy X components made with Selective Laser Melting

The most promising non destructive technique (NDT) for complex geometry parts made by additive manufacturing appears to be x-ray radiography and computer tomography (CT). Both techniques are very well established NDT methods. But little is known about the defect detection limits in dense component manufactured by Selective Laser Melting. Here we present preliminary results to help understand the X-ray defect detection limits in Hastelloy X parts manufactured by SLM [1].

The detection of defects in SLMed parts is complicated by the fact that internal pores are effectively filled with un-melted metal powder. These are expected to be more difficult to detect than gas pores in castings foo instance. In the case of narrow elongated defects such as cracks, the orientation of the defects with respect to the X-ray beam is also expected to influence the detectability of the defect. This problem can be overcome by taking X-ray images from different angles and is therefore not an issue with computed tomography, where the sample is rotated with respect to the X-ray beam, multiple images are taken from different angles and the in formation about the absorption of the sample can be reconstructed in 3D. [2]

It is important to understand the X-ray defect detection limits in Hastelloy X parts manufactured by SLM, including the effects of wall thickness, defect type, size, orientation and defects that are filled with loose metal powder. 

[1]

In order to determine the smallest resolvable defect sizes for different types, sizes and orientations of defects in different thicknesses of Hastelloy X parts manufactured by SLM, a series of rod-shaped, sphere-shaped and disc/coin-shaped defects were embedded into each step of a staircase sample manufactured on an EOS M280 production SLM machine with five different steps of 0.8mm, 1.5mm, 3mm, 5mm and 10mm thickness. Spherical defects are chosen to represent porosity and coin-shaped defects are chosen to represent cracks. The custom-designed rod-and coin-shaped defects are embedded at vertical, horizontal and 45deg orientations with respect to the thickness direction of the steps so that their largest dimensions (i.e. rod length and coin diameter) are aligned at 0deg, 90deg and 45deg angles in relation to the incident X-ray beam direction, respectively. 

Polychromatic X-ray radiography is performed to establish the detection limit of defects in slabs of Hastelloy X, depending on the total thickness of the slab. Both conventional film radiography and computed radiography with phosphor plates are used.

Conventional film radiography in general displays higher resolution, while the ability to record digital images (with possibly higher dynamic range) makes computed radiography ideally suited for quantitative post-processing. The thicker slabs display a larger absorption and therefore appear brighter in the picture.

[1]

In this case, the basic trends and defect detection limits shown for both types of radiography are quite similar. All the results show that the minimum detected defect size increases with increasing step thickness. Furthermore, the minimum detected defect size also increases when the defects are oriented in a way that decreases the defect thickness in the incident X-ray beam direction (i.e. horizontal and 45deg orientations). Overall, the smallest detectable defect sizes over a wide range of accelerating voltages and exposure settings were as follows:

• Spheres with a diameter of 0.2mm. 
  
• Rods with a length of 0.2mm and a diameter of 0.05mm (vertically oriented). 
  
• Discs with a diameter of 0.3mm and a thickness of 0.015mm (vertically oriented).
  

Indications of the degree of surface roughness and the geometry of the holes are also obtained. The higher resolution attained by tomography is evident. Nevertheless, absorption 2D radiography represents a more convenient, fast and cost-effective way of sample characterisation. For both conventional and computed radiography, it is expected that the contrast should be increased by reducing the bandwidth of the X-rays

When polychromatic X-rays are used to image a sample, the measurable contrast depends on both defect size/shape and slab thickness. Experimental data suggest that the minimum detectable contrast is about 1–2% when using X-rays with a very broad spectrum. Better contrast and resolution is attainable by using 3D computed tomography.

References
[1] https://asm.confex.com/asm/aero15/webprogram/Paper39611.html
[2]  Non destructive Evaluation of Additive Manufacturing - State-of-the-Discipline Report - NASA/TM–2014-218560 - Jess M. Waller, Bradford H. Parker, Kenneth L. Hodges, Eric R. Burke, James L. Walker – Nov 2014