Appearance
A manufacturer of hydraulic manifold blocks produced by LPBF in 316L stainless steel (Ø80 mm × 120 mm block, requiring Ø8 mm × 200 mm cross-drilled passages, Ra < 1.6 µm) was using standard gun drilling parameters for wrought 316L (Vc = 60 m/min, f = 0.025 mm/rev, 70 bar EP oil). The as-built LPBF 316L exhibited 15–25% higher cutting forces, 2–3× higher tool wear (flank wear 0.12 mm after 50 bores vs 0.05 mm in wrought), and sporadic hard inclusions from partially fused powder causing edge chipping. Switching to AM-specific parameters — Vc = 45 m/min, f = 0.020 mm/rev, AlTiN-coated carbide gun drill, 90 bar coolant — reduced tool wear to 0.08 mm after 50 bores, eliminated edge chipping, and achieved consistent surface finish Ra 0.6–1.0 µm.
AM Material Microstructure and Drilling Behaviour
AM Microstructural Features and Their Effect on Deep Hole Drilling
| Feature | LPBF 316L | Wrought 316L | Effect on Drilling | Mitigation Strategy |
|---|---|---|---|---|
| Grain structure | Columnar, epitaxial, 10–100 µm wide, mm-long along build direction | Equiaxed, 20–100 µm | Anisotropic cutting forces — 20–30% higher when drilling perpendicular vs parallel to build direction | Orient bore axis to minimise angle with build direction; use 15–20% lower feed when drilling transverse to build |
| Sub-grain structure | Cellular/dendritic, 0.5–2 µm cell size | Annealed twins, no sub-grain | Higher micro-hardness (220–260 HV vs 180–200 HV) increases tool wear | Reduce Vc by 15–25% vs wrought; use coated carbide |
| Porosity | 0.1–1.5% (gas + lack-of-fusion) | < 0.01% | Interrupted cutting at pore sites causes micro-chipping; porosity > 0.5% reduces tool life by 40% | Hot isostatic pressing (HIP) before drilling reduces porosity to < 0.1% |
| Inclusions | Partially fused powder particles (5–50 µm, 300–500 HV) | Clean steel | Hard inclusions cause edge chipping and accelerated flank wear | Use AlTiN or TiAlN coating; increase coolant pressure to improve chip evacuation |
| Surface roughness (as-built) | Ra 6–15 µm | Ra 0.4–0.8 µm (bar stock) | Rough as-built surface causes variable forces at drill entry | Pre-machine entry face before drilling |
| Residual stress | High tensile (200–500 MPa) at as-built surface | Low (< 50 MPa) | Stress relaxation during drilling can cause bore distortion | Stress-relief anneal before drilling (600–800°C for 316L) |
| Hardness (as-built) | 200–280 HV (316L), 320–420 HV (Ti-6Al-4V), 380–480 HV (IN718) | 180 HV, 330 HV, 400 HV | 10–30% higher hardness increases tool wear proportionally | Adjust Vc downward proportional to hardness ratio |
Deep Hole Drilling Parameters: AM vs Wrought Condition
| Material | Condition | Bore Ø (mm) | Depth (mm) | Vc (m/min) | f (mm/rev) | Tool | Coolant | Surface Finish Ra (µm) | Tool Wear (flank, µm/100 bores) | Notes |
|---|---|---|---|---|---|---|---|---|---|---|
| 316L SS | Wrought (annealed) | 8 | 200 | 55–75 | 0.025–0.040 | K15 uncoated carbide | EP oil, 70 bar | 0.4–0.8 | 20–40 | Baseline |
| 316L SS | LPBF as-built | 8 | 200 | 40–55 | 0.020–0.030 | AlTiN-coated K15 | EP oil, 90 bar | 0.6–1.0 | 50–80 | Reduce Vc by 20–30% |
| 316L SS | LPBF + HIP + annealed | 8 | 200 | 50–70 | 0.025–0.035 | K15 uncoated carbide | EP oil, 80 bar | 0.4–0.8 | 25–45 | HIP restores wrought-like machinability |
| Ti-6Al-4V | Wrought (annealed) | 6 | 150 | 35–55 | 0.020–0.035 | K10 uncoated carbide | EP oil, 80 bar | 0.3–0.6 | 30–60 | Baseline |
| Ti-6Al-4V | LPBF as-built | 6 | 150 | 25–40 | 0.015–0.025 | AlTiN-coated K10 | EP oil, 100 bar | 0.4–0.8 | 60–120 | Higher micro-hardness (400 vs 330 HV) |
| Ti-6Al-4V | LPBF + stress-relief | 6 | 150 | 30–45 | 0.018–0.028 | AlTiN-coated K10 | EP oil, 90 bar | 0.4–0.7 | 45–90 | Stress relief (650°C/4h) reduces tool wear |
| Inconel 718 | Wrought (aged) | 6 | 100 | 25–40 | 0.015–0.025 | TiAlN-coated K20 | EP oil, 100 bar | 0.3–0.6 | 40–80 | Baseline |
| Inconel 718 | LPBF as-built | 6 | 100 | 18–30 | 0.012–0.020 | AlTiN-coated K20 | EP oil, 120 bar | 0.4–0.8 | 100–200 | Fine dendritic structure is very abrasive |
| Inconel 718 | LPBF + HIP + solution + aged | 6 | 100 | 25–38 | 0.015–0.022 | TiAlN-coated K20 | EP oil, 110 bar | 0.3–0.6 | 50–90 | Full heat treatment restores near-wrought machinability |
| AlSi10Mg | LPBF as-built | 8 | 200 | 100–200 | 0.03–0.08 | Uncoated K10 (polished) | Air or oil mist | 0.4–0.8 | 10–20 | Free-machining; fine Si particles cause moderate abrasion |
| AlSi10Mg | LPBF + T6 (solution + age) | 8 | 200 | 80–150 | 0.03–0.06 | Uncoated K10 (polished) | Air or oil mist | 0.4–0.8 | 15–25 | Precipitation-hardened condition increases tool wear |
| Maraging steel 300 | LPBF as-built | 8 | 200 | 35–55 | 0.020–0.030 | TiAlN-coated K20 | EP oil, 80 bar | 0.4–0.8 | 40–70 | Similar to wrought; porosity can cause issues |
| Maraging steel 300 | LPBF + aged (480°C) | 8 | 200 | 25–40 | 0.015–0.025 | AlTiN-coated K20 | EP oil, 100 bar | 0.4–0.8 | 70–140 | Aged hardness 52–56 HRC significantly increases wear |
Recommended Pre-Treatment for AM Components Before Deep Hole Drilling
| Pre-Treatment | Effect on Porosity | Effect on Hardness | Effect on Residual Stress | Effect on Tool Life in Subsequent Drilling | Cost per kg | Recommended For |
|---|---|---|---|---|---|---|
| None (as-built) | As-printed (0.1–2%) | As-printed (highest) | As-printed (200–500 MPa tensile) | Baseline | $0 | Prototype, low-volume, non-critical |
| Stress relief anneal | No change | 5–10% reduction | Reduces to < 50 MPa | 20–40% improvement over as-built | $5–15 | All AM components before machining |
| Hot isostatic pressing (HIP) | Reduces to < 0.1% | 10–20% reduction | Reduces to near-zero | 50–100% improvement over as-built | $20–50 | Fatigue-critical aerospace, medical |
| HIP + solution + age | Reduces to < 0.1% | Full heat treatment | Near-zero | 100–200% improvement over as-built (near-wrought) | $30–80 | Critical aerospace (IN718, Ti-64) |
| Surface machining (0.5–1 mm) before drilling | Removes surface zone only | No change | Partially relieves surface stress | 10–20% improvement | $2–10 | All components — always recommended |
| Shot peening | No change | Increases surface hardness | Compressive stress (-200 to -400 MPa) | 5–10% improvement | $3–8 | Fatigue-critical; compensates for tensile as-built stress |
FAQ
How does the microstructure of additively manufactured metals affect deep hole drilling compared to wrought materials?
The microstructure of additively manufactured metals differs fundamentally from wrought materials in ways that directly affect deep hole drilling behaviour. The three most important microstructural differences are the grain structure, the sub-grain structure, and the defect population. Grain structure — AM metals solidify with columnar grains that grow epitaxially along the build direction (the direction of heat extraction). For LPBF 316L, these columnar grains are 10–100 µm wide and can extend for several millimetres in the build direction, compared to the equiaxed (globular) grains of 20–100 µm in wrought 316L. This directional grain structure causes anisotropic cutting behaviour: when drilling perpendicular to the build direction (i.e., the bore axis is horizontal in a vertically built component), the cutting edge alternately encounters columnar grains parallel to the cutting direction and then perpendicular to it, causing cyclic variation in cutting forces of 15–30%. When drilling parallel to the build direction, the cutting forces are more uniform but are elevated by 10–20% compared to wrought material because the columnar grain boundaries provide additional resistance to chip formation. The practical recommendation is to orient the bore axis parallel to the build direction where possible, and to expect 15–25% higher cutting forces when drilling AM material regardless of orientation. Sub-grain structure — AM metals solidify with a cellular or cellular-dendritic sub-grain structure within each columnar grain. For LPBF 316L, the cells are 0.5–2 µm in diameter, compared to the annealed twin structure in wrought 316L which has no sub-grain features at this scale. The cellular structure consists of cell interiors with nominal composition and cell boundaries enriched in alloying elements (Cr, Mo, Mn in 316L) due to micro-segregation during rapid solidification. The cell boundaries have 10–30% higher hardness than the cell interiors (260–300 HV vs 200–240 HV for 316L), creating a microscopically heterogeneous cutting surface. The cutting edge must repeatedly cross these hard cell boundaries, causing micro-scale variations in cutting force at intervals of 0.5–2 µm. Over cumulative cutting distance, this micro-heterogeneity accelerates abrasive tool wear by 50–100% compared to homogeneous wrought material. Defect population — AM metals contain process-induced porosity (gas pores at 1–50 µm and lack-of-fusion defects at 10–200 µm) that is essentially absent in wrought material. When the cutting edge encounters a pore, the chip formation is interrupted — the cutting force drops suddenly as the edge passes through the void, then spikes as it re-engages solid material. These force transients cause micro-chipping of the cutting edge, particularly for brittle carbide tools. For a 316L component with 0.5% porosity, the cutting edge encounters a pore approximately every 1–5 mm of cutting path at typical feed rates. Over 100 bores of 200 mm depth, this means 4,000–20,000 micro-impacts on the cutting edge. The practical consequence is that porosity > 0.2% by volume reduces gun drill tool life by 30–60% compared to fully dense material. Hot isostatic pressing (HIP) prior to drilling reduces porosity to < 0.1% and restores near-wrought tool life.
What heat treatment or pre-treatment is recommended for AM components before deep hole drilling?
The recommended pre-treatment for AM components before deep hole drilling depends on the material, the as-built condition, and the quality requirements of the finished component. The general recommendation is a three-step approach: (1) Stress relief — always perform stress relief before any machining, regardless of material. AM components contain high residual stresses (200–500 MPa tensile at the surface, with balancing compressive stresses in the interior) from the rapid thermal cycling during printing. If these stresses are not relieved before drilling, the removal of material during drilling unbalances the stress state, causing the component to distort. For 316L stainless steel, stress relief at 600–800°C for 1–2 hours in an inert atmosphere reduces residual stress to < 50 MPa. For Ti-6Al-4V, stress relief at 650–750°C for 2–4 hours in vacuum or argon is standard. For Inconel 718, stress relief at 980°C for 1 hour in vacuum, followed by rapid argon cooling, is typical. (2) Hot isostatic pressing (HIP) — for fatigue-critical applications and for components with high porosity risk, HIP before drilling is strongly recommended. HIP at 1,000–2,000 bar argon pressure at 80–95% of the material's solidus temperature for 2–4 hours closes porosity by plastic deformation and diffusion bonding. For 316L, HIP at 1,150°C and 1,500 bar for 3 hours reduces porosity from 0.5–1.5% to < 0.1% and increases density to > 99.9%. The improvement in drillability is dramatic: tool life on HIP + stress-relieved 316L is 80–100% of wrought tool life, compared to 30–50% for as-built material. For Ti-6Al-4V, HIP at 920°C and 1,000 bar for 2 hours eliminates lack-of-fusion defects and improves tool life by 100–200%. The limitation of HIP is cost ($20–50 per kg) and the potential for surface oxidation if the HIP cycle is not performed in a protective atmosphere. (3) Surface machining — always machine the entry and exit faces of the bore before drilling. The as-built surface of AM components (Ra 6–15 µm) contains partially fused particles, surface-connected porosity, and a thin oxide layer that cause erratic drill entry behaviour. Machining 0.5–1.0 mm from the surface removes this problematic layer and provides a clean, flat entry face for the drill bushing. For components that will be drilled in multiple locations, machine all relevant surfaces before any drilling. The combination of stress relief + HIP + surface machining produces a near-wrought condition for drilling. For non-critical components where cost is the primary driver, stress relief alone plus surface machining is the minimum acceptable pre-treatment. Drilling in the as-built condition should be avoided for production components because of the high variability in tool life and hole quality.
How do cutting parameters need to be adjusted for deep hole drilling of AM versus wrought materials?
Cutting parameters for deep hole drilling of AM materials require adjustment in three areas: cutting speed reduction by 15–30%, feed rate reduction by 10–25%, and coolant pressure increase by 20–30%. The adjustments are necessary because AM materials have higher hardness (10–30% higher than wrought), higher abrasive wear potential (from fine solidification structures), and higher sensitivity to thermal effects (from residual stress). Cutting speed (Vc) — the primary adjustment is to reduce cutting speed by 15–30% compared to wrought parameters. The reduction compensates for the higher micro-hardness of the AM solidification structure and the presence of hard inclusions (partially fused powder particles). For example, wrought 316L is typically gun-drilled at Vc = 55–75 m/min with uncoated carbide. For as-built LPBF 316L, the recommended starting point is Vc = 40–55 m/min with AlTiN-coated carbide. The coating is necessary because the higher abrasive wear rate of AM material (from the fine cellular structure and hard inclusions) accelerates uncoated tool wear. After stress relief or HIP, Vc can be increased to 50–70 m/min — approaching but not reaching wrought parameters. For Ti-6Al-4V, the adjustment is from Vc = 35–55 m/min (wrought) to Vc = 25–40 m/min (as-built LPBF). Feed rate (f) — the feed rate should be reduced by 10–25% for AM materials. The reduction serves two purposes: it reduces the mechanical load on the cutting edge (compensating for the higher material strength), and it reduces the chip thickness, which helps manage the variable chip formation caused by porosity. A starting point for as-built LPBF 316L is f = 0.020–0.030 mm/rev (vs 0.025–0.040 mm/rev for wrought). The feed should be on the lower end of this range when drilling with porosity > 0.5% to minimise force transients at pore encounters. Coolant pressure — coolant pressure should be increased by 20–30% for AM materials. The higher pressure provides improved chip evacuation from the bore, which is important because AM materials can produce stringier chips than wrought (due to the fine sub-grain structure providing additional chip ductility). For as-built LPBF 316L at Ø8 mm, increase coolant pressure from 70 bar (wrought) to 80–100 bar. The higher pressure also improves cooling of the cutting edge, which is beneficial because the higher hardness of AM material generates more heat per unit volume of material removal. Tool coating — for AM materials, coated tools are strongly recommended even where uncoated tools are standard for wrought. AlTiN or TiAlN coatings provide the abrasion resistance needed for the fine solidification structure and provide a thermal barrier that reduces heat transfer to the carbide substrate. For 316L, which is typically drilled with uncoated carbide in the wrought condition, AlTiN-coated tools provide 50–100% longer life in as-built AM material. For Inconel 718 AM, AlTiN-coated tools are essential — uncoated tools fail by rapid flank wear (0.2 mm after 20 bores at reduced parameters) while coated tools achieve 80–150 bores. The general recommendation is to start at 70% of the wrought cutting speed and 80% of the wrought feed for as-built AM material, then adjust upward in 5% increments based on tool wear observations.
What tool wear mechanisms are specific to deep hole drilling of AM metals?
The tool wear mechanisms in deep hole drilling of AM metals differ from wrought materials in both rate and mode, driven by the unique microstructural features of AM materials. The specific wear mechanisms are: (1) Abrasive wear from fine solidification structure — the cellular/dendritic sub-grain structure of AM metals (0.5–2 µm cell size) creates a microscopically heterogeneous surface with hardness variations of 30–60 HV between cell interiors and cell boundaries. The hard cell boundaries act as fine-scale abrasive particles, wearing the tool flank through a micro-cutting mechanism. The abrasive wear rate is 50–100% higher in as-built AM material than in wrought material, even when the bulk hardness is comparable. For LPBF 316L, the flank wear rate is 0.5–0.8 µm per metre of cutting path (vs 0.2–0.4 µm/m for wrought). This is the dominant wear mechanism for AM 316L, AlSi10Mg, and maraging steel. (2) Edge chipping from porosity — when the cutting edge encounters a pore (particularly lack-of-fusion defects at 50–200 µm), the cutting edge experiences a force transient — a sudden drop as the edge enters the void, followed by a sharp spike as it re-engages solid material. These transients cause micro-chipping of the cutting edge on a scale of 5–30 µm. Over many encounters (thousands per bore), the cumulative micro-chipping degrades the edge quality and increases cutting forces. Edge chipping is the dominant wear mechanism for AM Inconel 718 and Ti-6Al-4V, which have higher hardness and are more susceptible to brittle fracture of the cutting edge. For as-built LPBF Inconel 718 with 0.5–1.0% porosity, edge chipping reduces tool life by 50–70% compared to wrought material. (3) Notch wear at the depth-of-cut line — AM metals often have a surface zone (0.1–0.5 mm deep) with different microstructure than the bulk: finer grains from more rapid solidification at the part surface, higher oxygen content from atmospheric contamination, and occasional surface-breaking porosity. When the drill penetrates this surface zone, the harder, more abrasive surface layer creates a notch wear groove at the depth-of-cut line on the tool flank. This notch wear is particularly pronounced when the as-built surface is not machined before drilling. (4) Built-up edge (BUE) — some AM metals show a stronger tendency for BUE formation than their wrought counterparts, particularly at cutting speeds below 30 m/min. For Ti-6Al-4V, the as-built LPBF surface with higher oxygen content (0.15–0.25% O vs 0.08–0.13% O for wrought Grade 5) promotes BUE formation because oxygen increases the chemical reactivity of the chip surface with the tool material. BUE can be suppressed by maintaining Vc > 35 m/min with coated tools and ensuring adequate coolant flow. (5) The practical consequence of these wear mechanisms is that tool life in as-built AM material is typically 30–60% of wrought tool life for the same bore geometry. The tool wear is most effectively managed by: pre-treating the material (stress relief + HIP) to reduce porosity and homogenise the microstructure; using coated tools (AlTiN for 316L and Ti-64, TiAlN or AlTiN for IN718); increasing coolant pressure for improved chip evacuation and cutting edge cooling; and reducing cutting speed by 20–30% relative to wrought parameters.
What design guidelines apply to deep hole drilling in hybrid AM + machining process chains?
Hybrid AM + machining process chains — where a near-net shape component is produced by additive manufacturing and then finished by subtractive machining including deep hole drilling — require specific design and process planning guidelines to avoid defects and maximise efficiency. The key guidelines are: (1) Drilling allowances — design the AM near-net shape with sufficient stock for drilling. For deep hole drilling, the recommended minimum stock is 1.5 mm per side for bores up to Ø12 mm, and 2.0 mm per side for larger bores. This allowance accounts for: the as-built surface roughness (Ra 6–15 µm, which must be completely removed); the surface zone with different microstructure (0.1–0.5 mm deep); and positional tolerance of the as-built bore relative to the reference surfaces. Insufficient stock leaves remnants of the as-built surface in the final bore, which appear as rough, porous patches with poor surface finish and potential leak paths. (2) Drill entry and exit — design flat faces at entry and exit locations for the drill. The as-built surface at the entry point should be machined flat and perpendicular to the bore axis to ensure proper drill bushing contact. A counterbored entry (1–2 mm deep, 0.2–0.5 mm larger than the bore diameter) is recommended to provide a clean starting surface. For blind bores, the drill exit (if it must break through into a cavity) should have a flat machined surface at the breakthrough point. (3) Bore orientation relative to build direction — orient the bore axis parallel to the build direction whenever possible. Drilling perpendicular to the build direction results in 15–30% higher cutting forces and 30–50% higher tool wear because the cutting edge alternately encounters columnar grains parallel and then perpendicular to the cutting direction. When perpendicular bores are unavoidable (e.g., cross-drilled passages in manifolds), accept the higher tool wear and adjust tool change intervals accordingly. Plan for 30–50% more tool changes for cross-drilled (horizontal) bores in vertically built components. (4) Sequencing of operations — the recommended process sequence is: stress relief → remove from build plate → machine all reference surfaces → bore all deep holes → finish machine remaining features. The stress relief must be performed before any machining because AM residual stresses are sufficient to cause distortion during drilling. Removing the part from the build plate before stress relief causes the part to warp immediately. Stress relief before plate removal ensures the part retains its as-built geometry. Boring before finish machining allows any distortion from drilling (from stress relaxation around the bore) to be corrected by the final machining passes. (5) Powder removal — before any machining, ensure all loose powder is removed from internal cavities and surfaces. Powder trapped in internal passages becomes compacted by drilling forces and can cause tool breakage or contamination. The recommended powder removal method for hybrid components is: ultrasonic cleaning in isopropanol (15 min), followed by pressurised air blow-out through all cavities, followed by vacuum drying at 80°C for 1 hour. (6) Heat treatment scheduling — if the component requires both HIP and heat treatment, the recommended sequence is: HIP → stress relief → machine reference surfaces → gun drill → finish machine → age (for precipitation-hardening alloys). Drilling is most economical when performed on material in the solution-treated condition (lowest hardness). For Inconel 718, drill after HIP + solution treat, then age after all machining is complete. For Ti-6Al-4V, drill after HIP + anneal, with no subsequent ageing required. (7) Cost optimisation — the cost of deep hole drilling in AM components is 2–5× higher per bore than in wrought components due to higher tool wear and the need for coated tools and higher coolant pressure. To minimise drilling cost in hybrid components: minimise the number of deep bores in the AM portion of the component; use the AM process to create near-net shaped passages that require only light finishing; and specify HIP treatment for components with more than 5 deep bores or bores exceeding 20× diameter in depth. The economic crossover between drilling in wrought vs AM material depends on the total number of bores and the component value — for high-value aerospace components, the 2–5× higher drilling cost is acceptable; for commodity components, design for wrought material drilling or minimise post-processing.
This article provides an overview of deep hole drilling of additively manufactured metal components. Microstructural factors, process parameters, tool selection, and pre-treatment depend on the specific AM process (LPBF, DED), material, and component requirements. Process validation trials on representative AM coupons are recommended before production drilling. The technical data presented here reflects published research and documented industrial applications as of 2026.