Appearance
An additively manufactured metal part that comes off the build plate is not finished — it still needs holes for coolant channels, hydraulic passages, fastener locations, and assembly features. Deep hole drilling is often the most efficient way to create these features, especially for long, small-diameter passages that cannot be built vertically in the AM process. But AM materials are not wrought materials: they have different microstructure, different hardness, different thermal conductivity — and they require different drilling strategies.
AM Materials for Post-Process Drilling
Common AM Materials
| AM Material | Process | Typical Hardness (As-Built) | Hardness (Heat Treated/HIP) | AM Characteristics Affecting Drilling | Typical Applications |
|---|---|---|---|---|---|
| Ti-6Al-4V | DMLS / LPBF | 310–350 HV | 340–380 HV (annealed) | Anisotropic — columnar grains — acicular alpha microstructure | Aerospace — medical — structural |
| Inconel 718 | DMLS / LPBF | 320–380 HV | 400–480 HV (aged) | Fine dendritic structure — Laves phases — hard carbides | Aerospace — turbine — high-temperature |
| 316L stainless | DMLS / LPBF | 200–240 HV | 180–220 HV (annealed) | Fine cellular structure — higher yield strength than wrought | Chemical — food — medical — general |
| AlSi10Mg | DMLS / LPBF | 110–130 HV | 80–100 HV (T6) | Fine Si precipitates — higher strength than cast | Automotive — aerospace — thermal |
| Maraging steel (18Ni300) | DMLS / LPBF | 320–380 HV (as-built) | 540–620 HV (aged) | Fine martensitic structure — very hard after aging | Tooling — molds — high-strength |
| Co-Cr (ASTM F75) | DMLS / LPBF | 350–450 HV | 400–500 HV (HIP) | Hard — abrasive — carbide phases | Medical implants — dental — wear-resistant |
| HX (Hastelloy X) | DMLS / LPBF | 200–260 HV | 220–280 HV (solution treated) | Fine cellular structure — moderate work hardening | Aerospace — chemical processing |
| Copper (C18150, GRCop-84) | DMLS / LPBF | 80–120 HV | 100–150 HV (aged) | High thermal conductivity — soft — burr-prone | Heat exchangers — combustion chambers |
| Al-6061 (AM) | DMLS / LPBF | 100–120 HV | 90–110 HV (T6) | Lower density than wrought — porosity concerns | Lightweight structural — thermal |
AM Material Characteristics vs Wrought
| Characteristic | AM Material | Wrought Material | Drilling Implication |
|---|---|---|---|
| Microstructure | Fine cellular or columnar grains — anisotropic | Equiaxed grains — isotropic — texture from rolling | AM material may drill differently in X, Y, Z orientation — anisotropic tool wear |
| Hardness variation | Can vary ±10–20% within a part due to build geometry variation | Uniform ±2–5% | Inconsistent cutting forces — potential diameter variation |
| Porosity | 0.1–1% typical (can be higher in non-optimized builds) | < 0.01% | Interrupted cutting at pores — edge chipping risk — coolant ingress into pores |
| Residual stress | High — especially in as-built condition — varies with build geometry | Lower — relieved during forming | Stress relief may be needed before drilling to prevent distortion |
| Surface condition (as-built) | Rough (Ra 5–20 µm) — partially melted particles | Smooth (Ra 0.4–1.6 µm) | Rough surface affects drill entry — may cause drill skidding |
| Thermal conductivity | Lower than wrought (fine grain boundaries scatter phonons) | Higher — standard value | Higher cutting temperature — reduced tool life |
| Ductility | Lower than wrought (fine grain structure reduces elongation) | Higher — standard | More brittle chips — easier chip breaking — but more edge chipping risk |
Pre-Drilling Preparation
| Preparation Step | Purpose | Recommendation | Notes |
|---|---|---|---|
| Stress relief anneal | Reduce residual stress — prevent distortion during drilling | Per material: Ti-6Al-4V: 650–750°C for 2 hours. 316L: 450–550°C for 1 hour. Inconel 718: 980°C solution treat + age. Maraging steel: 820°C solution treat + age | Stress relief before drilling is critical for thin-wall AM parts |
| Hot isostatic pressing (HIP) | Close internal porosity — improve material uniformity | Per material spec — typically 1000–2000 bar at 80–95% of melting temperature | Eliminates porosity — improves drill consistency significantly |
| Surface machining of reference faces | Create flat entry surface — eliminate as-built roughness | Machine a flat spot face at drill entry (0.5–1 mm depth) | Prevents drill skidding — ensures perpendicular entry |
| Drill bushing installation | Guide drill at entry — prevent wander | Use hardened steel bushing — press fit into fixture | Recommended for AM parts with rough as-built surfaces |
| Identify build orientation | Determine drilling direction relative to build layers | Note whether drilling parallel or perpendicular to build plane | Drilling through layer boundaries vs along layer direction affects tool life and surface finish |
| Non-destructive testing (if needed) | Verify internal quality before drilling | X-ray CT or ultrasonic inspection | Identify porosity or inclusion locations that will affect drilling |
Tool Selection
| AM Material | Carbide Grade | Coating | Geometry Features | Coolant Pressure |
|---|---|---|---|---|
| Ti-6Al-4V | Micro-grain (0.5–0.8 µm) | AlTiN or AlCrN | Sharp edge (5–10 µm radius) — high rake (12–15°) — polished flutes | 80–120 bar |
| Inconel 718 | Ultrafine-grain (0.2–0.5 µm) | AlCrN or TiAlN | Sharp edge (10–15 µm) — moderate rake (8–12°) — variable helix for chatter reduction | 80–150 bar |
| 316L stainless | Micro-grain | AlTiN or TiCN | Sharp edge (5–10 µm) — high rake (12–15°) — polished flutes | 60–100 bar |
| AlSi10Mg | Micro-grain | DLC or uncoated | Very sharp edge — high rake (15–20°) — polished flutes — chip breaker geometry | 40–80 bar |
| Maraging steel 300 | Ultrafine-grain | AlCrN or TiAlN | Standard edge (15–25 µm) — moderate rake (8–10°) | 80–120 bar |
| Co-Cr | Ultrafine-grain (or PCD for finishing) | AlCrN or CVD diamond | Sharp edge — moderate rake (8–10°) — wear-resistant grade | 80–120 bar |
| HX (Hastelloy X) | Ultrafine-grain | AlCrN | Sharp edge (10–15 µm) — moderate rake (8–12°) | 80–120 bar |
| Copper (C18150) | Micro-grain | DLC or uncoated | Very sharp edge — high rake (15–20°) — polished flutes | 40–80 bar |
Drilling Parameters
Parameters by Material and Condition
| AM Material | Condition | Cutting Speed (m/min) | Feed Rate (mm/rev) | Expected Tool Life (m) | Notes |
|---|---|---|---|---|---|
| Ti-6Al-4V | As-built | 20–30 | 0.02–0.05 | 5–12 | Lower speed due to variable hardness |
| Ti-6Al-4V | HIP + annealed | 30–45 | 0.03–0.06 | 10–25 | Speed can be higher in uniform material |
| Inconel 718 | As-built | 12–20 | 0.02–0.04 | 3–8 | Very tough — watch for work hardening |
| Inconel 718 | Solution + aged | 15–25 | 0.02–0.04 | 5–12 | Higher hardness but more uniform |
| 316L stainless | As-built | 25–40 | 0.03–0.06 | 8–20 | Higher strength than wrought 316L |
| 316L stainless | Annealed | 35–50 | 0.04–0.07 | 15–30 | Closer to wrought behavior |
| AlSi10Mg | As-built | 60–100 | 0.05–0.10 | 20–50 | Easy drilling — but stringy chips |
| AlSi10Mg | T6 | 50–80 | 0.04–0.08 | 15–35 | Higher hardness — slightly lower tool life |
| Maraging steel | Solution treated | 25–35 | 0.03–0.06 | 10–20 | Drill before aging if possible |
| Maraging steel | Aged (540–620 HV) | 15–25 | 0.02–0.04 | 3–8 | Very hard — significantly reduced tool life |
| Co-Cr | As-built | 15–25 | 0.02–0.04 | 3–8 | Abrasive — use PCD for finishing |
| Co-Cr | HIP | 15–25 | 0.02–0.04 | 3–8 | Similar to as-built — porosity reduced |
| Copper | As-built | 40–80 | 0.04–0.08 | 15–30 | Burr-prone — use sharp tools |
Parameter Adjustments for AM vs Wrought
| Condition | Adjustment | Reason |
|---|---|---|
| AM material — as-built, not HIP | Reduce cutting speed 20–30% vs wrought equivalent | Internal porosity causes interrupted cutting — variable hardness increases tool load |
| AM material — HIP treated | Reduce cutting speed 10–15% vs wrought equivalent | HIP closes pores but finer grain structure makes material slightly harder than wrought |
| AM material — heat treated | Use wrought parameters for that hardness level | If hardness matches wrought, parameters are similar — but monitor tool life |
| Drilling parallel to build layers | Reduce feed 10–20% | Drilling through layer boundaries creates variable cutting force — lower feed stabilizes process |
| Drilling perpendicular to build layers | Standard parameters — may need higher pressure | Layer boundaries are perpendicular to hole axis — less effect on cutting continuity |
| As-built surface at entry | Reduce entry feed 50% — use spot face | Rough surface causes drill skidding — spot face creates flat entry |
Quality Considerations
| Quality Issue | AM-Specific Cause | Mitigation | Inspection Method |
|---|---|---|---|
| Hole diameter variation | Local hardness variation from build geometry — porosity near hole surface | HIP treatment before drilling — reduce feed rate — use reamer after drill | Air gauge — CMM |
| Surface finish — rough | Porosity exposed at hole surface — smearing of partially melted particles | Increase coolant concentration — reduce feed — use finishing pass or reamer | Profilometer — optical inspection |
| Burr — larger than expected | High ductility in un-heat-treated AM material — low thermal conductivity | Heat treat before drilling — reduce feed at exit — use back-up support | Visual — burr gauge |
| Drill skidding at entry | As-built surface roughness — angled surface from build orientation | Spot face entry surface — use drill bushing — reduce entry feed | Visual entry condition |
| Tool life — shorter than wrought | Abrasive microstructure — carbides — oxides — variable hardness | Reduce cutting speed — verify coating suitability — consider HIP before drilling | Tool life tracking |
| Hole straightness deviation | Residual stress relief during drilling — anisotropic material stiffness | Stress relief before drilling — increase guide bushing support | Laser measurement — CMM |
| Edge chipping at hole entry/exit | Porosity near surface — brittle microstructure | Reduce feed at entry and exit — use chamfer | Visual — microscope |
FAQ
How does additive manufacturing affect deep hole drilling compared to wrought materials?
Additive manufacturing affects deep hole drilling through several material differences: microstructure — AM materials have fine cellular or columnar grain structures (from rapid solidification) rather than the equiaxed grains in wrought material — this makes AM materials typically 10–20% harder than their wrought equivalents at the same nominal composition. Porosity — AM parts contain 0.1–1% porosity even in well-optimized builds — this causes intermittent micro-interruptions in the cut as the drill passes through pores — creating tool edge micro-chipping and variable cutting forces. Residual stress — AM parts have high residual stress from thermal cycling during the build — drilling into a stressed part can cause the material to distort as stress is relieved — especially in thin-wall sections. Anisotropy — AM materials have different properties in X, Y, and Z build directions — drilling parallel to build layers produces different chip formation and tool wear than drilling perpendicular to layers. The practical effect is that drilling AM materials typically requires 10–30% lower cutting speed than the wrought equivalent, plus consideration of pre-drilling heat treatment (stress relief or HIP).
What preparation does an AM part need before deep hole drilling?
Preparation steps for drilling AM parts: stress relief anneal (the most important preparation — AM parts have high residual stress from the build process — drilling into an as-built part without stress relief can cause distortion as the material relaxes — especially in thin-walled parts. Stress relief temperature depends on the material — typically 450–750°C for 1–2 hours). Hot isostatic pressing (HIP) — if the part will have drilled holes near the surface or if hole surface finish is critical — HIP closes internal porosity, making the material more uniform and eliminating the porosity-related issues described above. Surface spot facing — machine a flat entry surface at the drill entry point — AM surfaces are rough (Ra 5–20 µm) and can cause drill skidding — a 0.5–1 mm deep spot face creates a perpendicular entry surface. Drill bushing — use a hardened guide bushing in the fixture — provides additional guidance at entry for rough AM surfaces. Non-destructive testing — if the part is expensive or mission-critical — X-ray CT or ultrasonic inspection before drilling identifies porosity or inclusion locations that could cause problems during drilling.
What cutting parameters work for drilling AM Ti-6Al-4V vs wrought Ti-6Al-4V?
For AM Ti-6Al-4V: as-built (not HIP treated) — cutting speed 20–30 m/min, feed rate 0.02–0.05 mm/rev (reduce speed 20–30% vs wrought wrought Ti-6Al-4V parameters of 35–50 m/min). The lower speed compensates for the variable hardness, internal porosity, and residual stress in as-built AM material. HIP-treated AM Ti-6Al-4V — cutting speed 30–45 m/min, feed rate 0.03–0.06 mm/rev (HIP closes porosity and produces more uniform material — speed can be closer to wrought but still 10–15% lower). Wrought Ti-6Al-4V — cutting speed 35–55 m/min, feed rate 0.04–0.08 mm/rev (standard parameters for annealed wrought material). Key differences: AM material requires 10–30% lower cutting speed, 20–30% lower feed rate (especially if porosity is present — lower feed reduces edge chipping at pore boundaries), higher coolant pressure (80–120 bar for AM — 60–80 bar for wrought — higher pressure helps evacuate discontinuous chips from porosity interruptions), and coated carbide tools with AlTiN or AlCrN coating for both AM and wrought — but AM may benefit from a sharper edge (5–10 µm radius vs 10–20 µm for wrought) to reduce cutting forces in the variable-hardness material.
What quality issues are specific to drilling AM parts?
Quality issues specific to AM parts: surface finish degradation from exposed porosity — as the drill passes through a pore, the pore wall is exposed on the hole surface — creating a surface defect that a profilometer detects as a spike in Ra. This is cosmetic for many applications but can be a functional issue for sealing surfaces or fatigue-critical components. Diameter variation from localized hardness differences — AM parts can have hardness variation of ±10–20% due to build geometry differences (thick sections cool differently than thin sections — different hardness means different drill deflection). Burr formation — some AM materials in the as-built condition have different ductility than wrought — burr size and shape can be different and unexpected. Drill skidding at entry — the rough as-built surface does not provide a stable entry for the drill — the drill skids across the surface before starting to cut — causing entry Location error. Distortion during drilling — residual stress in the AM part can cause the part to distort as material is removed — the hole location shifts, or the part warps.
Should AM parts be heat treated before deep hole drilling?
Yes — stress relief anneal is recommended before drilling AM parts. The high residual stress from the AM build process can cause part distortion when material is removed during drilling — especially in thin-wall sections or parts with asymmetric geometry. Stress relief temperatures vary by material: Ti-6Al-4V — 650–750°C for 2 hours in argon or vacuum (prevents oxidation). 316L stainless — 450–550°C for 1 hour in air. Inconel 718 — 980°C solution treat + age (if final mechanical properties are required). Maraging steel — 820°C solution treat + age (drill in solution-treated condition if possible — aging to full hardness makes drilling very difficult). Hot isostatic pressing (HIP) is recommended for parts where hole surface quality is critical or where the part will see fatigue loading — HIP closes internal porosity, producing a more uniform material that drills more consistently. The HIP cycle typically combines high temperature (80–95% of melting temperature) with high pressure (1000–2000 bar) — it is expensive but eliminates porosity-related drilling problems. For parts that will be heat treated to final hardness anyway — drill in the soft (solution-treated or annealed) condition and heat treat after drilling.
Additive manufactured metal parts bring unique challenges to deep hole drilling — anisotropic microstructure, internal porosity, residual stress, and variable hardness. Successful drilling requires: pre-drilling preparation (stress relief anneal minimum — HIP for critical parts — spot facing for entry), reduced cutting parameters (10–30% lower cutting speed than wrought equivalents — especially for as-built material), coated carbide tools with sharp edges, and higher coolant pressure to manage interrupted cuts from porosity. The key to success is treating AM material as a different material — not assuming it behaves like its wrought counterpart. This article reflects industry practice as of 2026.