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3-Dimensional Imaging of Macroscopic Defects in Aluminum Alloys

Image Reconstruction in 3D using Milling and In Situ Optical Microscopy

The investigation of macroscale defects in aluminum (Al) alloys with a rapid 3-dimensional (3D) imaging approach is described in this report. Aluminum (Al) alloys play an important role in the production of aircraft and vehicles, as well as products in other industries. Defects present in the Al alloy used for the production of aircraft, vehicles, or other products can have a significant effect on their quality, performance, and lifetime. Characterization of Al alloys in 3D can lead to a better understanding of defect formation and how to minimize or eliminate it. To produce 3D images of defects in non-transparent Al alloy, the samples are milled in small steps (10 µm). Optical microscopy 2D images of the surface are recorded after a certain thickness of alloy (50 µm) is removed. From these 2D images, a 3D image of the Al alloy defect is reconstructed. The entire workflow for Al alloy sample preparation and 2D image recording is completed in a very short time (1.5 hours) using a single instrument, the EM TXP surface preparation system from Leica Microsystems. Typically, the completion of such a workflow with more common sample preparation methods would be 3 to 6 times longer. Finally, 3D image reconstruction is done using the LAS X 3D Visualization software.

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Table of Content

Introduction

Background

Aluminum (Al) alloys, which are lightweight and strong, are commonly used in the aerospace and automotive industries [1]. They offer significant advantages in terms of cost-effective manufacturing of aircraft and vehicles, while enabling greater fuel economy during operation. However, the occurrence of potential defects in Al alloys during part/component or even alloy production must be taken into account. Such defects can have a significant effect on part/component quality and performance. In the last decade, better methodologies for characterizing alloy defects and microstructure in 3-dimensions (3D) have been developed [2]. Characterization in 3D can provide important insights into the mechanisms behind macroscopic and microscopic defect formation. Data about alloy defects retrieved from 3D images can help to prevent or eliminate them.

3D imaging approach for defects

To make 3D image reconstructions of defects in opaque Al alloys using optical microscopy, a method to remove the material step-by-step and record images of the sample surface in regular intervals is needed.

Efficient workflow for milling and imaging

Polishing the Al alloys is impractical, because the metal tends to smear, rendering polishing very time consuming. Instead of polishing, it is easier to mill the Al alloy, as it produces a good quality surface in much less time.

When preparing the Al alloy sample for milling, it also becomes quickly impractical to remove the sample after each milling step to record an image with an external microscope, such as a higher resolution compound microscope. Mounting, demounting, and re-mounting the sample, not to mention any re-adjustment needed to find the correct area, each time an image is acquired makes the approach very inefficient.

Both milling and in situ imaging of the Al alloy samples can be done with the EM TXP target surfacing system (refer to figure 1). It is used for sample preparation prior to examination by optical microscopy (OM) or scanning (SEM) or transmission (TEM) electron microscopy. Milling, sawing, grinding, and polishing can all be done with this instrument, so users achieve faster sample preparation workflows. The moveable pivot arm of the EM TXP system allows an optimal angle for observation and imaging of the top face and front face (cross section) of the sample. For in situ imaging, meaning the sample is never removed during preparation, the EM TXP system is equipped with a standard M80 stereo microscope. To record images, a digital camera, such as the IC90 E CMOS camera, can be installed.

Materials and Methods

Al alloy sample

An Al alloy sample with a size of 10 x 10 x 10 mm was used. A macroscopic 3D welding defect was visible in the alloy sample before any preparation was started.

Milling and imaging of Al alloy

The EM TXP surfacing system was used for the Al alloy sample preparation. The sample preparation was carried out only with diamond milling and no polishing. Milling of the Al alloy was done in step sizes of 10 µm. After each time 50 µm of Al alloy (5 steps of 10 µm) was milled away, a 2D OM image of the surface with the pivot arm at 45° was recorded (refer to figure 2). All images were taken with the M80 stereo microscope and IC80 HD digital camera. The Al alloy sample was cleaned before imaging with alcohol and blown dry with compressed air. The total thickness of the Al alloy material removed via milling was 2 mm, corresponding to a total of 40 images in 2D. Image reconstruction in 3D from the stack of 40 alloy surface images was performed with the LAS X 3D Visualization software.

Results

Workflow efficiency

The total time to complete the entire workflow was just 1.5 hours. The workflow included:

  • mounting of the Al alloy sample for preparation by milling and
  • recording 40 images of the sample each time 50 µm of material was milled away.

When using more common methods for alloy sample preparation, normally the workflow completion is much longer (4-10 hours).

3D images of defect

Milling of the Al alloy sample produced high quality surfaces where the defect was clearly visible. The 40 stereo microscope images of the sample in 2D allowed good quality 3D image reconstruction with the LAS X 3D Visualization software. The 3D images of the defect have sufficient resolution for precise volume and surface area measurement (refer to figure 3). The defect volume was determined to be 429,000 µm3 (0.429 x 10-3 mm3 equivalent to 0.429 nanoliters) and its surface area to be 545,000 µm2 (0.545 mm2). Higher 3D image resolution can be obtained if 2D images are recorded more frequently after fewer milling steps, e.g., if after every 2 milling steps an image is recorded, then only 20 µm of alloy (2 steps of 10 µm) would be removed.

Summary and Conclusions

This report described a method to image macroscale defects in non-transparent aluminum (Al) alloys in 3-dimensions (3D) with milling and in situ optical microscopy.

Because Al alloys are important for the manufacture of aircraft and vehicles, as well as other products, thorough alloy characterization can help to minimize or eliminate defects which can affect quality and performance.

3D-images of a defect in an opaque Al alloy were produced using the following approach:

  • An Al alloy sample with a macroscopic defect was milled in small steps (10 µm thickness);
  • 2D optical microscopy images of the milled alloy surface were recorded after each time a thickness of 50 µm was removed;
  • A total of 2 mm of alloy was milled away, yielding a total of 40 images in 2D; and
  • A 3D image of the alloy and defect was reconstructed from the stack of 2D images.

Completion of the entire workflow, from initial Al alloy sample preparation to recording of all 40 alloy surface images took 1.5 hours, about 3-6 times faster than more common sample preparation methods. The determined defect volume was 0.429 x 10-3 mm3 (0.429 nanoliters) and surface area was 0.545 mm2.

The entire workflow was performed with the following instrumentation and software from Leica Microsystems:

It should be noted that other Leica digital cameras are available for use with the M80 microscope if a higher digital resolution is required for imaging.

The 3D imaging method using the EM TXP system with integrated microscope allows a much faster execution of the entire workflow. The user has the possibility to use polishing, cutting, sawing, drilling, or milling for sample preparation and choose the most efficient method. In situ imaging of the sample after each milling step eliminates any sample transfer between the preparation tool and an external microscope. As a result, there is no need for any re-adjustment during the work steps, because the sample remains always aligned with both the mill cutter and microscope.

The 3D image of the alloy defect reconstructed from the stack of 2D images had sufficient resolution to enable accurate volume and surface area measurement of the defect.