Sample Preparation for Additive Manufacturing: Essential Steps for Successful 3D Printing
Sample Preparation for Additive Manufacturing: Essential Steps for Successful 3D Printing
When it comes to sample preparation for additive manufacturing, you need to ensure that you avoid false positives and accurately identify defects. Unlike traditionally manufactured metal parts, 3D printing of metal components involves unique microstructure and inherent porosity characteristics. Thus, it is essential to use appropriate metallography techniques for 3D printing of metal components.
This article is designed to guide you through the essential steps for metallography of 3D printing metals. You will learn appropriate techniques for sectioning 3D printing metal components, grinding, and polishing. You will also learn appropriate techniques for porosity analysis of 3D printing metals. You will learn appropriate techniques for metallography of 3D printing metals and avoid false positives and defects.
Understanding AM Metallography Requirements
Why Sample Preparation Differs for 3D Printed Parts
Metallography of 3D printing metals is different from metallography of traditionally manufactured metal components. 3D printing metal components involve unique characteristics, and metallography of 3D printing metals is subject to anisotropic effects. These effects mean that 3D printing metal components exhibit different properties based on direction. Melt pool boundaries of 3D printing metals involve overlapping thermal zones, and rapid thermal cycles produce residual stresses throughout 3D printing metal components. 3D printing metal components often involve sophisticated internal structures, especially for medical devices.
At the material support interfaces, there are other complications such as the existence of non-fused zones, contaminations, and microstructural variations because of the extremely different thermal treatment that the material has undergone during the process of 3D printing. Improper cutting with band saws, which causes heat generation, distorts the material, causes premature relaxation of stresses, or induces unwanted phase transformations, is another problem that has to be addressed with care. For example, unwanted heat generation may change the martensitic phase in 3D titanium alloys, which fundamentally alters the measurement that you are trying to achieve.
| Quality Metric | Metal 3D Printing (AM) | Traditional Casting |
| Surface Roughness (µm) | 5 – 10 | 20 – 50 |
| Porosity Level (%) | < 0.5 | 1 – 2 |
| Tensile Strength (MPa) | 1000 – 1200 | 800 – 1000 |
| Fatigue Life (Cycles) | 150,000+ | 120,000 |
| Defect Rate (%) | 0.1 – 1 | 5 – 12 |
| Material Waste Generation | Low (5-10%) | High (30-50%) |
| Energy per kg of Material (kWh) | 20 – 30 | 40 – 60 |
| Certification Compliance | ISO/AS9100 Easy | Variable |
| Sustainability Score (Out of 10) | 9 | 6 |
Table 1: Comparative Quality and Sustainability Metrics of Metal 3D Printing versus Traditional Casting.
Porosity Preservation vs. Artifact Creation
The most important problem that has to be addressed in the process of sample preparation in the context of 3D printing is the ability to differentiate between real porosities and the creation of unwanted artifacts that may occur during the process of preparation. Research has shown that the porosities that are created may reduce to merely one-third of the actual porosities that existed in the material, which is caused by the process of over-polishing, which smears the softer metal matrix material over the microscopic porosities, hiding them from microscopic examination.
The process of manual preparation causes devastating problems, which may be attributed to the unwanted creation of artifacts that occur because of the non-uniform pressure that is exerted on the material, leading to over-polishing, which causes the sharpened pore edges to round off, causing harder inclusions to be polished out from the softer metal matrix material, or, conversely, the creation of pull-outs because of the high forces that are exerted, which may be misinterpreted as the actual lack of fusion porosities that may be observed under high magnification.
Directional Build Effects on Microstructure
The microstructure and the defects that occur in the material may vary depending on the direction in which the material has been built, and thus, to be able to successfully examine the defects that occur in the process of selective laser melting, the material has to be sectioned both parallel and perpendicular to the build direction.
Defect Types in Metal AM Parts
Lack of Fusion (LoF) pores are created at boundaries where there is insufficient laser energy density and/or scanning speeds were too rapid. These irregularly shaped, highly angular pores contain unmelted powder material and have a dramatic effect on fatigue life.
Keyhole porosity happens when there is too much heat input. Too much laser energy or too low a scanning speed can cause extreme melt pool boiling, leading to the formation of deep capillary cavities. When the melt does not have time to drain, the keyholes shut, and gas can be trapped deep in the melt track as tubular pores.
Gas pores are spherical and have a smooth surface. Gas pores are created by trapped gasses in the atmosphere or in the powder particles. Due to the high cooling rates of 10^5 K/s, the gas gets trapped in the melt as it solidifies.
| Defect Classification | Microscopic Morphology | Primary Physical Cause | Typical Location in Microstructure |
| Lack of Fusion (LoF) | Irregular, sharply angular | Insufficient energy input, shadowing from spatter | Layer boundaries, grain boundaries |
| Keyhole Pores | Elongated, deep, tubular | Excessive energy input, keyhole collapse | Deep within the central melt track |
| Gas Pores | Highly spherical | Trapped inert gas in powder or build atmosphere | Randomly distributed throughout matrix |
| Balling | Spherical metallic droplets | Poor wetting, high surface tension dominating flow | Surface tracks, interlayer boundaries |
Table 2: Comprehensive Taxonomy of Common LPBF Additive Manufacturing Defects
Pre-Preparation Planning and Sectioning
Defining Inspection Objectives Before Cutting
Where you cut affects what you can learn about the object under inspection. Before you make your first cut with a saw, define your inspection objective with precision. Do you want to determine porosity percentages? Do you want to detect LOF voids? Do you want to examine melt pool boundaries? Do you want to examine grain structures? Do you want to do phase analysis? Do you want to determine crack initiation sites? Different inspection objectives require different sectioning strategies and different preparation steps.
Selecting Section Orientation for Build Direction Analysis
The structure and defects in a metal vary with the direction in which the object was built and the scanning strategy that was used to make the object. In order to accurately analyze the structure and defects in the object, you should section specimens parallel and perpendicular to the build direction. Critical zones near the surface, layer boundaries, and cracks require strategic sectioning in these areas.
Automated Sample Preparation | Cut Impact, Save Time & Costs
Precision Cutting Methods for Complex AM Geometries
Standard band sawing and wire EDM machines are not precise enough to cut and section metallographic cross-sections of objects created using additive manufacturing techniques. These machines generate too much heat and can destructure the object under inspection and pull out secondary phases in the object. In addition, 3D printing can now create topology-optimized organic structures that cannot be machined using conventional techniques and equipment.
For robust and larger specimens such as aerospace engine blocks, the SERVOCUT series 302-AA, 302-MA, 402-AA, 502, and 602-AX, as well as METACUT 302, are equipped with engineered silicon carbide or alumina abrasives. The cutting feed rates are constantly controlled by automated feed systems that adjust cutting feed rates to reduce thermal load on the material. For intricate parts with micro-thin cross-sections, cutting of wafers is often required.
Controlling Heat and Deformation During Sectioning
Cutting wheels with material-specific compositions are implemented to reduce burning and deformation of materials during cutting. For materials such as titanium and its alloys, which are heavily incorporated in aerospace and medical printing applications, cutting is often a challenge because of its extremely low thermal conductivity. Heat is concentrated at the cutting edge and does not dissipate easily into the material. The TRENO-Ti cutting wheel is developed to handle such thermal concentration during cutting by chemically tailoring it for clean and cool cutting operations. Aluminum cutting also requires ultra-low feed rates of 100 µm/sec and high cutting blade speeds of 2800 RPM to ensure zero thermal damage during cutting operations.
| Specific Cutting Requirement | Metkon Equipment Series | Recommended Consumable | Application within Additive Manufacturing |
| Large/Dense AM Builds | SERVOCUT (302-AA to 602-AX) | TRENO Series | Sectioning bulk engine parts, heavy tool-steel builds. |
| Titanium Components | SERVOCUT / METACUT 302 | TRENO-Ti | Aerospace brackets, implants; utterly prevents thermal phase damage. |
| Delicate Lattices / Thin Walls | MICRACUT (152, 200-S, 202) | DIMOS Diamond Discs | Precision sectioning of fragile internal geometries, multi-material prints. |
| High-Volume Production QC | SERVOCUT 602-AX / AX-R | TRENO-DUR | Continuous, automated batch sectioning for large-scale AM facilities. |
Table 3: Comprehensive Metkon Cutting Solutions Matrix for Additive Manufacturing.
Mounting, Grinding and Polishing AM Samples
Vacuum Impregnation for Porous Structures
The mounting process is also crucial after cutting and is required for uniformity during grinding operations. Additionally, it is useful in preventing edge rounding of fragile sections and is vital for fragile features that are easily damaged during grinding operations. For porous structures such as AM materials containing porosity, impregnation is required during grinding operations to ensure that pore edges are not damaged during grinding operations. Cold mounting with epoxy resin is often required for such operations. However, hot compression mounting using equipment such as ECOPRESS 102 is often required for robust and dense materials such as Inconel 718 and 316L stainless steel, where thermal damage is not a concern.
Progressive Grinding Stages for Different AM Alloys
The grinding is carried out through progressive stages of abrasives from coarse to fine. For AM alloys, grinding up to 2500 grit SiC paper is carried out without introducing new artifacts. The grinding is carried out for precise durations of 30 seconds per stage. This is vital for ductile materials as they are liable to smear.
For the initial material removal phase, Metkon relies on high-quality Silicon Carbide (SiC) papers (classified by strict FEPA P standards ranging from coarse P60/250µ to ultra-fine P2000/10µ). However, for high-throughput, modern AM preparation, MAGNETO Diamond Grinding Discs are overwhelmingly preferred and utilized. MAGNETO discs offer incredibly fast material removal rates (achieving a total grinding time of roughly 2 minutes) and require only plain water as a lubricant, eliminating the need for messy slurries. Most importantly for additive manufacturing parts, which often contain varying hard and soft phases, MAGNETO discs provide exceptionally high edge sharpness and excellent absolute planarity. This ensures that both hard particulate phases and the softer metal matrix are ground completely flat simultaneously, entirely preventing the destructive artifact of hard-phase pull-out.
Diamond Polishing for Melt Pool Visibility
The diamond polishing suspensions are highly effective in revealing the boundaries of the melt pool clearly. The boundaries of the melt pool are like microscopic fingerprints of the laser track on the material.
Final Polish Techniques for Porosity Analysis
The final polishing should be carried out without over-polishing, as it affects the accuracy of results considerably. Studies have shown that the appearance of porosity reduces to one-third of its true values as smeared material flows over microscopic pores.For the final critical polishing phase, Metkon’s premium DIAPLUS series of advanced diamond suspensions (available in highly concentrated water-based monocrystalline or polycrystalline formulations) provides extraordinarily rapid material removal and superior, mirror-like surface finishes. Polycrystalline diamonds, in particular, feature multiple cutting edges that constantly fracture to reveal new sharp points, vastly improving cutting efficiency on hard AM alloys while completely preventing the metal smearing that so often obscures critical micro-porosity. As is well known in the industry, utilizing fewer, highly targeted polishing steps with premium DIAPLUS suspensions produces vastly superior results compared to laboratories that extend polishing times endlessly hoping for a better finish.
Choosing the Right Grinding &; Polishing Machine for Your Lab
Electrolytic Polishing for Ductile Materials
The mechanical polishing of ductile materials like aluminum is inevitable even with high-grade polishing materials. The ELOPREP 102 equipment dissolves the material atom by atom without causing smears and artifacts on the surface.
| Metallographic Preparation Stage | Metkon Equipment Utilized | Core Function / Applied Technology | Specific Benefit for Additive Manufacturing |
| Modular Grinding | FORCIPOL (102/202) + FORCIMAT | Controlled, automated platen rotation and pneumatic force application. | Entirely eliminates manual uneven pressure and edge rounding. |
| Automated Polishing | VELOX 102 / VELOX 102-JR | Fully automated, high-volume, programmable closed-loop processing. | Unmatched reproducibility across shifts; strictly prevents over-polishing artifacts. |
| Abrasive Platform | MAGNETO Diamond Discs | Extremely fast, perfectly flat planar grinding utilizing a magnetic base. | Exceptional high edge sharpness; strictly prevents hard-phase particulate pull-out. |
| Electropolishing | ELOPREP 102 | Non-mechanical, atom-by-atom anodic chemical dissolution. | Completely eradicates mechanical smearing on soft AM alloys (Aluminum, Copper). |
Table 4: Comprehensive Metkon Grinding, Polishing, and Electrolytic Preparation Matrix.
Etching, Imaging and Defect Quantification
Chemical Etching to Reveal Melt Pool Boundaries
The polished surface is able to reveal only macroscopic pores and unmelted powder particles on its surface. The chemical etching technique is applied on the polished surface of the material by applying acid or alkali solutions directly on the polished surface of the material. The grain boundaries and different metallurgical phases and crystal orientation possess different chemical potential values. This results in different and overlapping melt pool boundaries on the surface of the material. These are microscopic fingerprints of the laser track on the material surface. The analysis of the overlapping patterns of these boundaries will reveal whether the laser power and speed were synchronized during printing or not. The epitaxial growth of grains is also revealed on the surface of the material as a result of layer boundaries and heat-affected zones around localized defects.
Metallographic Etching: Key Techniques, Safety Tips &; Choosing the Right Etchant
Optical and SEM Microscopy for Defect Analysis
Optical microscopy can be used to observe the patterns of the melt pool, LOF voids, and cracks. SEM microscopy can be used to obtain higher magnification for the analysis of pore morphology, unmelted particles, and microcracks.
Automated Image Analysis for Porosity Measurement
Human interpretation of images can cause unacceptable variability in the results of the quality control process. In modern defect analysis of SLM parts, automated digital image processing replaces human interpretation. The software in the Imagin Series incorporates microscopes with high-resolution cameras for image processing. The software provides objective analysis for the purpose of evaluating images. Imagin Mesura 200 is used for precise measurement in dimensional analysis. The software can be used to measure the geometric severity of LOF. The software automatically separates darker-colored voids from the metallic matrix. The software can be used to accurately measure porosity area percentage without human estimation.
Conclusion
Sample preparation for additive manufacturing processes can have a direct impact on the accuracy of your results in the quality assessment process. In fact, every step in the process of precision cutting, porosity analysis, and automated analysis must be conducted in accordance with special protocols. Your investment in cutting equipment, polishing processes, and directional cutting can help you gain accurate knowledge about the microstructural characteristics of 3D printed metal. Mastering the process can help you distinguish between actual defects in the metal and false indicators.
FAQs
Q1.
Why can’t I use standard cutting and polishing techniques on 3D-printed metal parts? Standard band sawing techniques are inappropriate because they can generate too much heat, leading to deformation and changes in the phase structure of the metal, as well as untimely relaxation of the residual stresses in the metal parts. Inappropriate polishing can smear metal on the pores, leading to a reduction in the apparent porosity to as low as one-third of its real value.
Q2.
What is the difference between lack of fusion pores and gas pores in additive manufacturing? Lack of fusion pores are irregularly shaped with sharp angular boundaries, occurring at layer boundaries due to insufficient laser energy or too high scanning speeds. They contain un-melted powder particles and are severe stress concentrators in the metal parts. Gas pores, on the other hand, are spherical with smooth boundaries and occur due to the presence of atmospheric gases or gasses in the powder particles that are trapped in the solidifying matrix due to high cooling rates.
Q3.
Why is vacuum impregnation necessary when mounting porous samples produced by the AM process? Vacuum impregnation helps the epoxy to penetrate the open porosity and pores in the sample, as well as the cracks and fissures that exist in the sample. This helps to stabilize the pore structures so that they do not break off or crumble while grinding and polishing the sample.
Q4.
How does the direction in which a 3D printing process builds affect the microstructure of the produced parts? The defects and the microstructure produced in the 3-D printing process vary significantly with the direction in which the process builds the object because of the directional nature of the laser beam in the process. This makes the structure anisotropic, meaning that the structure changes with direction. Therefore, to assess the real condition of the structure, it must be cut in both directions.
Q5.
What role does chemical etching play in analyzing 3-D printing produced metal structures? Chemical etching helps to reveal the invisible structures that exist in the 3-D printing produced metal structures that cannot be revealed using other techniques such as polishing and etching. Using chemical etching, you can analyze whether the laser power and speed were sufficiently matched in the process to produce the desired structure and whether epitaxial grain growth and heat effects occurred in the structure.
