AM Inconel 718 (SLM)

AM Inconel 718 is a hard (38-44 HRC as-built, ~380 HV), work-hardening nickel superalloy produced by selective laser melting. The microstructure features columnar dendrites oriented along the build direction, with interdendritic Laves phase and NbC carbides that are unique to the AM solidification process. Melt pool boundaries appear as arc-shaped contrast lines where dendrite growth direction changes. The material work-hardens during grinding, requiring sharp abrasives and moderate pressures. The as-built condition shows strong microsegregation (Nb, Mo enrichment in interdendritic regions); after homogenization heat treatment, the Laves phase dissolves and subsequent aging produces gamma-prime and gamma-double-prime strengthening precipitates. Preparation approach differs between as-built and heat-treated conditions: as-built is slightly softer but has harder Laves phase particles causing relief; heat-treated is uniformly harder but more homogeneous. Always characterize porosity on the as-polished surface before etching. Build orientation relative to the cross-section must be documented.

Use an abrasive cut-off saw with an alumina (Al2O3) blade designed for nickel alloys. SiC blades will dull quickly on this work-hardening alloy. Apply generous coolant flow. Cutting speed: 200-300 RPM with moderate, steady feed rate. Do not pause or dwell during cutting, as IN718 work-hardens rapidly and a hardened layer will accelerate blade wear. For AM build characterization, section in multiple orientations: XY plane (perpendicular to build) reveals melt pool geometry, scan strategy, and laser track overlap; XZ or ZX planes (parallel to build) reveal columnar dendritic structure, epitaxial growth, and layer banding. Leave 2-3 mm allowance for grinding away the deformed and heat-affected cutting zone. Precision diamond wafering saws also work well at 100-200 RPM with light load.

Both cold and hot compression mounting are acceptable. The high melting point (1336C) and thermal stability of IN718 mean that compression mounting at 150-180C will not alter the microstructure, even in the as-built condition. Hot compression mounting with phenolic or epoxy-phenolic resin is convenient and provides good edge retention. Cold mounting with low-shrinkage epoxy is preferred when examining as-built surface roughness or near-surface defects. For porosity analysis, vacuum impregnation with fluorescent epoxy is recommended to fill and highlight all porosity (connected and surface-breaking). Edge-retaining mounts are essential for examining the AM surface condition and near-surface microstructure.

AM IN718 is hard (38-44 HRC) and work-hardens significantly during grinding. Use fresh, sharp SiC papers or rigid diamond grinding discs. Disc speed: 250-300 RPM. Apply moderate, consistent pressure (25-35 N per 30 mm sample). Do not use excessive pressure; the work-hardening effect increases surface hardness and creates deeper subsurface deformation that is difficult to remove in subsequent steps. Use contra-rotation (platen and holder in opposite directions) which is more aggressive and reduces work hardening compared to complementary rotation for this alloy.

Grinding sequence:

• 120 grit SiC: Remove sectioning damage (30-60 seconds). Moderate, consistent pressure. Do not dwell.
• 240 grit SiC: Remove previous scratches (30-45 seconds). Fresh paper essential for this work-hardening alloy.
• 320 grit SiC: Refinement (20-40 seconds). Rotate specimen 90 degrees.
• 400 grit SiC: Continue refinement (20-40 seconds).
• 600 grit SiC: Final grinding step (20-40 seconds). Ensure uniform scratch pattern with no deep scratches from previous steps.

Rotate specimen 90 degrees between steps. Thorough ultrasonic cleaning between steps prevents cross-contamination from embedded particles. For as-built surfaces with significant roughness, start at 80 or 120 grit with extra time.

The work-hardening behavior requires careful polishing technique. Use diamond suspensions on appropriate pads.

Diamond polishing sequence:

• 9 micrometer diamond: 3-5 minutes on a napless composite pad with moderate pressure (20-30 N per 30 mm sample). The hard Laves phase particles (as-built) or carbides can cause scratching; use consistent pressure.
• 3 micrometer diamond: 3-5 minutes on a napless synthetic pad with moderate pressure (15-25 N). Monitor for relief around interdendritic Laves phase in as-built material.
• 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. The chemical-mechanical action of colloidal silica is effective on nickel alloys and helps remove the work-hardened surface layer. Vibratory polishing with colloidal silica for 4-8 hours produces excellent results for revealing fine dendritic substructure and melt pool boundaries.

For porosity analysis, stop after 1 micrometer diamond and image the as-polished surface. Over-polishing rounds pore edges and affects quantitative measurements. For Laves phase distribution analysis, the as-polished surface after colloidal silica already shows good phase contrast.

AM IN718 responds to standard nickel alloy etchants, but the AM-specific features (melt pool boundaries, dendritic substructure, Laves phase, microsegregation) may require different etchants or conditions than wrought material. Always examine the as-polished surface first for porosity and phase contrast.

Waterless Kalling's Reagent (Chemical Etching) - Primary choice for general AM microstructure:

• Composition: 5 g CuCl2, 100 ml HCl, 100 ml ethanol
• Application: Immerse for 5-30 seconds. Start with 5 seconds and build up gradually. As-built AM material may etch faster than wrought due to microsegregation.
• Reveals: Melt pool boundaries (as arc-shaped contrast lines), columnar dendritic structure, interdendritic Laves phase (appears as bright particles along dendrite boundaries), prior solidification grain boundaries. In heat-treated material, reveals grain boundaries and any remaining carbides.
• Rinse: Ethanol, then dry with warm air.

Marble's Reagent (Chemical Etching) - For enhanced dendritic contrast:

• Composition: 10 g CuSO4, 50 ml HCl, 50 ml H2O
• Application: Immerse for 5-20 seconds. Produces copper deposition that highlights dendrite cores vs interdendritic regions.
• Reveals: Dendritic solidification structure with excellent contrast. Nb/Mo-enriched interdendritic regions appear different from dendrite cores. Good for visualizing microsegregation patterns and melt pool overlap zones.

Electrolytic 10% Oxalic Acid (Electrolytic Etching) - For Laves phase and sensitization:

• Composition: 10 g oxalic acid, 100 ml H2O
• Application: Electrolytic at 3-6 V DC for 5-30 seconds. Stainless steel cathode.
• Reveals: Laves phase distribution (preferentially attacked), NbC carbides, grain boundaries in heat-treated material. Excellent for quantifying Laves phase content in as-built vs heat-treated conditions. Also reveals any sensitization or Nb-depleted zones around Laves phase particles.

AM-specific etching strategy: For melt pool boundary mapping, use a very light chemical etch (Kalling's, 3-5 seconds). For dendritic substructure, Marble's reagent gives the best contrast. For Laves phase quantification and comparison between as-built and heat-treated conditions, use electrolytic oxalic acid at consistent parameters. Sequential etching (light chemical followed by electrolytic) can reveal multiple feature types on the same specimen.

Safety: Use fume hood for all etching. Standard PPE including gloves and eye protection. CuCl2 solutions are toxic; dispose properly.