AM Ti-6Al-4V (SLM/EBM)

AM Ti-6Al-4V is a hard (36-42 HRC, ~350 HV) titanium alloy produced by laser or electron beam powder bed fusion. The microstructure is fundamentally different from wrought Ti-6Al-4V due to the rapid layer-by-layer solidification process. Columnar prior-beta grains grow epitaxially across multiple build layers, oriented along the thermal gradient (typically parallel to the build direction). Within these columnar grains, the fine-scale alpha morphology depends on cooling rate and thermal history. SLM (laser) produces fine acicular alpha-prime martensite in the as-built condition; EBM (electron beam) produces alpha+beta Widmanstatten structure due to the elevated build chamber temperature. Melt pool boundaries, visible as arc-shaped etching contrast lines, are a key feature unique to AM material. Build orientation relative to the cross-section plane must always be recorded, as the microstructure is highly anisotropic. Before etching, examine the as-polished surface to characterize porosity type and distribution: lack-of-fusion (irregular shape, often between layers), keyhole (spherical, from excess energy), and gas porosity (small spherical, from powder feedstock). Titanium is reactive and prone to contamination; avoid contact with iron-containing tools and media.

Use a precision abrasive cut-off saw with an alumina (Al2O3) blade designed for titanium and reactive metals. SiC blades can also be used. Apply generous coolant flow to prevent overheating, which can cause alpha-case formation on the cut surface. Cutting speed: 200-300 RPM with moderate feed rate. AM Ti-6Al-4V is slightly harder than wrought due to the martensitic (SLM) or fine Widmanstatten (EBM) structure. For build characterization, take cross-sections in multiple orientations: XY plane (perpendicular to build direction) reveals melt pool geometry and scan strategy; XZ or ZX planes (parallel to build direction) reveal columnar prior-beta grains, layer bands, and epitaxial growth. Leave 2-3 mm allowance for grinding to remove the heat-affected zone from cutting. Label each section with its orientation relative to the build direction.

Cold mounting with castable epoxy is preferred to avoid any thermal effects on the metastable martensitic structure (SLM material). The high melting point (1660C) means hot compression mounting at 150-180C will not melt the sample, but SLM alpha-prime martensite can begin to decompose above ~400C, so standard compression mounting is acceptable from a thermal standpoint. However, cold mounting with low-shrinkage epoxy provides better edge retention for examining the as-built surface roughness and near-surface porosity. For porosity analysis, vacuum impregnation with fluorescent epoxy is strongly recommended; this fills all connected porosity and allows quantitative porosity measurement. Edge-retaining mounting compounds are essential when examining the AM surface condition or surface-connected defects.

AM Ti-6Al-4V is hard (36-42 HRC) and requires appropriate grinding media. Use SiC papers or diamond grinding discs with water lubrication. Disc speed: 250-300 RPM. Apply moderate pressure (25-35 N per 30 mm sample). Titanium is prone to smearing, so use fresh, sharp abrasives and avoid excessive pressure which embeds abrasive particles. For SLM material, the fine martensitic structure resists scratching but is prone to subsurface deformation. For EBM material, the coarser alpha+beta is slightly easier to grind.

Grinding sequence:

• 120 grit SiC: Remove sectioning damage and heat-affected zone (30-60 seconds). Moderate pressure. Remove enough material to eliminate any alpha-case from cutting.
• 240 grit SiC: Remove previous scratches (30-45 seconds). Ensure all 120-grit scratches are removed.
• 320 grit SiC: Further refinement (20-40 seconds). Rotate specimen 90 degrees.
• 400 grit SiC: Continue refinement (20-40 seconds). Fresh paper essential.
• 600 grit SiC: Final grinding step (20-40 seconds). Ensure uniform scratch pattern.

Rotate specimen 90 degrees between each step. Use complementary rotation. Water lubrication must be continuous; titanium smears readily on dry or worn papers. Ultrasonic cleaning between steps is recommended to remove embedded abrasive particles. For as-built surfaces with significant roughness, start at 80 or 120 grit and allow extra grinding time to establish a flat plane.

Titanium requires careful polishing to avoid smearing the soft alpha phase. Use napless or low-nap cloths with diamond suspensions.

Diamond polishing sequence:

• 9 micrometer diamond: 3-5 minutes on a napless synthetic pad with moderate pressure (20-30 N per 30 mm sample). Use oil-based diamond suspension for titanium (water-based can cause staining).
• 3 micrometer diamond: 3-5 minutes on a napless synthetic pad with moderate pressure (15-25 N). Continue using oil-based lubricant.
• 1 micrometer diamond: 2-3 minutes on a short-nap pad with light pressure (12-20 N).

Final polishing:

• 0.05 micrometer colloidal silica: 2-4 minutes on a soft chemical-mechanical polishing pad with light pressure. Colloidal silica provides both mechanical and chemical action on titanium, producing excellent surface quality. Alternatively, a mixture of colloidal silica with 10% H2O2 enhances the chemical-mechanical action. Vibratory polishing with colloidal silica for 4-8 hours gives the best results for revealing fine AM microstructural features (melt pool boundaries, alpha lath orientation).

For porosity analysis, stop before final polishing (after 1 micrometer diamond) and image the as-polished surface. Over-polishing can round pore edges and reduce measured porosity. For microstructural analysis, the full polishing sequence through colloidal silica is required.

AM Ti-6Al-4V responds to standard titanium etchants, but the unique AM microstructural features (melt pool boundaries, columnar prior-beta grains, layer bands) require careful etching technique. Always examine the as-polished surface first for porosity characterization.

Kroll's Reagent (Chemical Etching) - Primary choice:

• Composition: 1-3 ml HF, 2-6 ml HNO3, 100 ml H2O
• Application: Immerse or swab for 5-15 seconds. Start with short times (5 seconds) and increase gradually. AM microstructures can etch differently than wrought.
• Reveals: Alpha-prime martensite needles (SLM), alpha+beta Widmanstatten laths (EBM), prior-beta grain boundaries, melt pool boundaries (as arc-shaped contrast lines), and layer bands. The columnar prior-beta grains appear as elongated grains parallel to the build direction in XZ/ZX cross-sections.
• Rinse: Water, then ethanol. Dry with warm air.

Polarized Light Microscopy - Complementary technique:

• Application: After light etching with Kroll's (3-5 seconds), examine under polarized light with a sensitive tint plate.
• Reveals: Prior-beta grain orientation and morphology with color contrast. Columnar grains sharing the same crystallographic orientation appear as the same color. Excellent for visualizing the extent of epitaxial growth across build layers. Also reveals grain texture (preferred orientation) inherent in AM builds.

10% HF in Water (Chemical Etching) - For stronger contrast:

• Composition: 10 ml HF, 90 ml H2O
• Application: Immerse for 3-10 seconds. More aggressive than Kroll's.
• Reveals: Stronger contrast of melt pool boundaries and prior-beta grain boundaries. Useful when Kroll's does not provide sufficient contrast.

AM-specific etching strategy: For melt pool boundary revelation, a light etch (3-5 seconds Kroll's) is often sufficient; over-etching obscures the subtle melt pool contrast. For prior-beta grain boundary mapping, a slightly longer etch (10-15 seconds) or successive short etches may be needed. Different etch times may be required for SLM vs EBM material due to the different phase compositions.

Safety: HF is extremely dangerous. Use full PPE including face shield, HF-resistant gloves, and lab coat. Work in a fume hood. Have calcium gluconate gel available for emergency treatment of HF burns.