The Additive Manufacturing (AM) industry has been exploding over the last several years with new materials and processes to produce more complex and reliable parts for critical applications.
Due to this rapidly growing field with new developments and needs, it becomes extremely important to be able to understand the materials characteristics along with the process variations which can impact the final product.
From incoming inspection of raw materials to final inspection of the product, there are several points along the way for Light Microscopy to be a valuable tool in understanding how the process is working.
Additionally, Light Microscopy is also extremely useful in the development of new processes or in failure analysis investigation.
We will look at several applications in Light Microscopy for use in Powder Bed Fusion which is one process widely used in metals. This method provides some challenges but also some distinct advantages.
Fundamentally, the AM process known as Powder Bed Fusion, discussed here, consists of several insertion points along the way from powder to the final product where microscopy can provide valuable insight.
Below are several of these insertion points:
#1 Powder Material Analysis
For the metal powder bed fusion process, incoming powders of various metals and exotic alloys need to be inspected for size, shape, density and defects. Size distribution(s) needs to be checked to ensure proper operation of equipment.
The shape is considered to be best when spherical which leads to maximum powder bed density and flowability. Bed density is how well the particles fit tightly together. If all powder particles are of the same size there will be gaps between them.
Oftentimes two or three-size particles (bimodal or trimodal) are used to fill gaps when melting, to create flows to fill the smallest gaps for fully dense material.
- Light microscopy can provide powder size distribution and shape (sphericity / roundness) through use of image analysis. This is usually for powder sizes =/> 5micron (ISO/ASTM 52907) in brightfield transmitted light. Transmitted light is blocked by the particles and produces a shadow like image of high contrast for analysis. See below.
- Brightfield reflected light allows observation of powder particle’s surfaces for inspection of powder defects such as surface voids, contaminants, etc.
Metallic powder imaged in Brightfield transmitted light microscopy
Metallic powder imaged in Brightfield reflected light microscopy
#2 Process Analysis
Additive Manufacturing for metals is amazingly performing real-time metallurgy. Layer by layer is melted and solidified, something not seen in any other type of traditional machining.
Each part really is a one-of-a-kind outcome where problems can arise. To produce highly critical parts, one has to spend large amounts of time and money to insure proper and consistent processes for their product.
Even more complex metallurgy is available where the metal characteristics can be designed to function differently in different areas of the same part.
To ensure high levels of dependable parts production, a number of inspection methods are required, some of which may include destructive testing of a part per each build.
- Metallographic microscope techniques can be used to view and size grain structures and layering effects of cross sectioned polished parts using reflected Brightfield or Crossed Polarized light.
- Voids can be viewed and analyzed. Shapes or the voids/pores can tell the story of what process problems are causing them – examples are unmelted powder, oxides and moisture as examples.
- Contaminants can get into the process and cause a variety of problems. Understanding and tracing these unwanted particles to the source is of high importance, and light microscopy is used to visualize and aid identification.
- Delamination (layer separation), fatigue cracks and micro cracks can be seen in the reflected light microscopy of polished cross sections and can point to process issues and their solutions.
Brightfield reflected light image of longitudinal cross section laser tracks
SLM (Selective Laser Melting) spherical voids in aluminum alloy caused by moisture.
Contaminant (magenta color) in reflected light polarization
#3 External Defect Analysis
Part during production, undergo a number of steps such as heat treatment, part removal, surface modification and cleaning which can cause defects.
- External critical areas can be inspected visually utilizing a stereomicroscope looking for damaged areas such as voids, pits, edge deformation for example.
#4 Surface Analysis
The surface of parts directly after production often have to some degree of surface roughness.
This may require further processes such as sand blasting, shot peening, electropolishing or a vibratory technique to produce a surface finish required for proper functionality.
- Using stereomicroscopy with various types of illumination we can visualize the effects of the surface finish method and also gauge how it compares to a known good surface. In addition, it can be used also to check for and damage that this surface finishing technique may be causing as an unwanted by product.
There are many other Additive Manufacturing methods and materials too numerous to mention. But they all have a common need for analysis and inspection by utilizing Light Microscopy.
Motic family of upright compound microscopes (starting with the PA53) and stereo microscopes (the SM7) offer ideal solutions to the examinations touched on above.
|The PA53MET holds all brightfield, darkfield and DIC options along with easy-to-use software for measurement and recordkeeping. The microscope also offers motorized xyz for more advanced imaging applications.|
|The SM7 stereo microscope is a great choice for external/surface analysis. Offering APO objectives with top of the line color correction and resolution, the stereo microscope is a great choice for samples with larger depth differences.|
If you are interested in an online demo or imaging your samples, feel free to contact us here.
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