Deep Hole Drilling of AM Near-Net Shapes: Defects & Hardness

A near-net shape produced by laser powder bed fusion can reduce material waste by 80% compared to machining from solid — but the same part may contain subsurface porosity that causes unpredictable tool failure during deep hole drilling, or local hardness variations that double cutting forces and destroy surface finish at the bottom of a 500 mm bore.

Additive manufacturing of near-net shapes has become a production reality for aerospace, medical, and tooling applications. However, the economic advantage of reducing material waste is partially offset by the difficulty of post-process machining — particularly deep hole drilling, where tool access is restricted, cutting conditions are difficult to monitor, and subsurface anomalies can cause catastrophic tool failure or scrapped parts.

This article examines the specific challenges that arise when deep hole drilling is applied to additively manufactured near-net shapes, covering the material science of AM structures, the interaction of internal defects with the drilling process, quantified tool wear and hole quality data from current research, and a framework for pre-processing and in-process strategies to achieve reliable results.

Additive Manufacturing Processes and Their Defect Signatures ​

The type of additive manufacturing process determines the characteristic defect types, microstructure, and hardness profile that the drilling operation must contend with.

Laser Powder Bed Fusion ​

LPBF produces the finest microstructures and highest hardness of any metal AM process, but also the highest density of internal defects when process parameters deviate from the optimal window:

• Keyhole porosity: Spherical pores 10–100 µm, caused by vapour depression collapse at excessive energy density. These are relatively benign for drilling — they are small, spherical, and tend to close under HIP.
• Lack-of-fusion defects: Irregular voids 50–500 µm, caused by insufficient energy density or scan strategy errors. These are the most problematic for drilling — they can be large enough to cause tool edge impact, and their irregular shape creates local stress concentrations.
• Gas porosity: Spherical pores 1–50 µm, caused by entrapped argon from the atomisation process. These persist through HIP and can re-grow during subsequent heat treatment.

Wire Arc Additive Manufacturing ​

WAAM produces larger layer heights and coarser microstructures than LPBF, with different defect characteristics:

• Layer band porosity: Distributed along fusion lines, caused by hydrogen or moisture pickup during deposition
• Solidification cracking: Occurs in susceptible alloys (high-strength aluminium, some nickel superalloys) due to thermal stress accumulation
• Incomplete fusion defects: Larger than LPBF equivalents (up to 1 mm), concentrated at inter-layer boundaries
• Higher hardness variability: WAAM Ti6Al4V shows 9.4% higher ultimate strength and 16.87% higher microhardness than wrought, with spatial variation correlated to thermal history

Electron Beam Melting ​

EBM operates at higher temperatures and in vacuum, producing distinctive defect signatures:

• Lower overall porosity than LPBF due to the high-temperature process environment
• Surface-connected porosity more common than subsurface
• Coarser microstructure with lower hardness than LPBF, generally more machinable but with different chip formation behaviour

Variable Hardness and Microstructural Heterogeneity ​

The most fundamental challenge in drilling AM near-net shapes is that hardness — the primary determinant of machinability — is not uniform throughout the workpiece.

Sources of Hardness Variation ​

Thermal history gradients: In LPBF, the rapid solidification rate (10³–10⁶ K/s) creates a fine cellular-dendritic microstructure with high dislocation density. However, the thermal history varies across the build:

• Regions near the build plate experience faster cooling and finer microstructure
• Upper regions experience cumulative heat input and slower cooling, producing coarser microstructure
• Thin-wall sections dissipate heat differently than solid regions, creating local hardness variation

Layer-by-layer tempering: In LPBF of tool steels (H13, 316L), each subsequent deposition layer tempers the previously solidified layers below it. The result is a hardness gradient through the build height — typically 5–15% variation depending on alloy and build geometry.

Process-induced anisotropy: AM parts exhibit crystallographic texture aligned with the build direction. Drilling perpendicular to the build direction encounters different deformation mechanics than drilling parallel to it. In LPBF Ti6Al4V with a martensitic α' microstructure, this anisotropy causes high-frequency cutting vibrations (up to 5 kHz) that are absent in wrought material.

Quantified Hardness Differences vs Wrought ​

The higher hardness of AM materials directly translates to increased cutting forces and accelerated tool wear in drilling.

Internal Defects and Their Drilling Implications ​

Internal defects in AM parts affect deep hole drilling differently than conventional machining because the cutting edge encounters them at depth, where visual inspection is impossible and the consequences of tool impact or breakage are severe.

Defect Types and Drilling Risk Assessment ​

How Defects Affect the Drilling Process ​

Tool edge impact: When the cutting edge encounters a lack-of-fusion void, the intermittent contact causes micro-chipping of the cutting edge. In deep hole drilling where the tool is operating near its mechanical limit, a single impact event can initiate a crack that propagates to tool breakage within a few revolutions.

Cutting force fluctuation: Porosity at the tool-workpiece interface causes instantaneous reduction in cutting force followed by sudden increase as the cutting edge re-engages. This cyclic loading accelerates fatigue wear of the cutting edge — particularly problematic for gun drills where edge integrity directly affects bore straightness.

Coolant pressure disruption: Subsurface porosity that opens during drilling can cause high-pressure coolant to escape into the void rather than return through the chip flute, reducing chip evacuation efficiency and increasing the risk of chip packing.

Surface finish degradation: Pores intersected by the cutting edge leave surface voids that act as stress concentrators in the finished component. For deep hole drilling applications where the bore surface serves as a sealing or bearing surface, this can render the part non-conforming.

Critical Defect Density Thresholds ​

Research on LPBF Ti6Al4V suggests that defect volume fractions below 0.5% have minimal impact on drilling outcomes, while fractions above 1.5% cause measurable degradation in tool life and hole quality. However, the spatial distribution matters more than the total fraction — a single 200 µm lack-of-fusion defect near the bore surface can cause more quality issues than uniformly distributed 0.5% microporosity.

Tool Wear and Cutting Forces in Drilling AM Metals ​

The most comprehensive comparative study to date (Singla et al., 2025, Wear) quantified drilling performance differences between WAAM-CMT and wrought Ti6Al4V using TiAlN-coated carbide drills with LCO₂ cryogenic cooling.

WAAM Ti6Al4V vs Wrought ​

The near-doubling of flank wear and surface roughness indicates that AM materials are fundamentally more abrasive to cutting tools than their wrought counterparts. The higher hardness and the presence of fine carbide/nitride precipitates in the AM microstructure contribute to accelerated abrasive wear.

LPBF H13 Tool Steel ​

Günen et al. (2026) compared drilling of LPBF and cast H13 tool steel:

• LPBF H13 showed heterogeneous microstructure with higher dislocation density
• Orbital drilling with WC tools achieved acceptable accuracy in LPBF H13
• Conventional drilling showed higher burr formation in LPBF material
• The harder LPBF microstructure caused more rapid edge rounding, increasing thrust force over the tool life

LPBF Ti6Al4V Vibration Characteristics ​

Sorgato et al. (2021) identified a distinctive challenge in drilling LPBF Ti6Al4V: the brittle martensitic α' microstructure generates cutting vibrations across a wide frequency band up to 5 kHz — far higher than the vibration frequencies observed in wrought Ti6Al4V.

These high-frequency vibrations:

• Accelerate flank wear through micro-chipping
• Produce poor surface finish in the bore
• Can excite resonances in long gun drill shafts, causing chatter marks
• Are significantly reduced by stress-relief annealing, which transforms α' martensite into α+β lamellar microstructure

Effect of Cooling Strategy ​

Cryogenic LCO₂ cooling during drilling of WAAM Ti6Al4V reduced flank wear by 32.7% compared to dry drilling, with corresponding improvements in hole surface finish and dimensional accuracy. High-pressure through-tool coolant (as used in standard deep hole drilling) shows similar benefits, suggesting that standard gun drilling coolant systems provide a favourable cooling environment for AM materials.

Hole Quality Outcomes ​

Surface Finish ​

AM materials produce consistently rougher surfaces when drilled compared to wrought materials:

• WAAM Ti6Al4V: Ra 0.97 µm vs 0.50 µm for wrought (94% higher)
• LPBF H13: Higher Ra than cast equivalent, particularly at higher feed rates
• EBM Ti6Al4V: Best surface quality at 3000–3500 RPM and 40–120 mm/min feed

The rougher surface is attributed to the heterogeneous microstructure — the cutting edge encounters alternating hard and soft regions at a microscale, causing local variations in chip thickness and surface generation.

Dimensional Accuracy ​

• EBM Ti6Al4V produces mostly oversize holes, with minimum deviation of 1.8% at 3000 RPM
• LPBF Ti6Al4V shows higher cylindricity deviation than wrought
• WAAM Ti6Al4V shows comparable dimensional accuracy to wrought at optimised parameters

Burr Formation ​

AM materials tend to form more burrs than wrought materials:

• LPBF H13: Higher burr height and width in conventional drilling
• The burr formation mechanism is influenced by the anisotropic microstructure — drilling parallel to build layers produces different burr characteristics than drilling perpendicular to layers

Chip Formation ​

• AM Ti6Al4V produces smaller, more segmented chips than wrought
• WAAM Ti6Al4V produces compact, shorter chips at optimised parameters
• The chip morphology change affects chip evacuation in deep hole drilling — segmented chips are generally easier to evacuate than continuous ribbons

Pre-Drilling Post-Processing Strategies ​

The most effective approach to improving drilling outcomes in AM near-net shapes is to modify the workpiece material before drilling.

Hot Isostatic Pressing ​

HIP is the most important pre-drilling treatment for AM parts requiring deep hole drilling:

• Porosity reduction: HIP at 900–1200°C and 100–200 MPa argon pressure collapses internal voids through creep and diffusion bonding
• WAAM 316L: 98.8% porosity reduction after HIP at 1200°C/150 MPa
• IN718 (LPBF): Fully dense after 1-hour HIP, but gas porosity may partially re-open during subsequent atmospheric heat treatment
• Ti6Al4V: Effective for keyhole and distributed porosity, but large LOF defects may not fully close

Critical consideration: HIP effectiveness depends on defect type. Lack-of-fusion defects may remain partially open after HIP because their irregular shape and interconnectivity prevent complete closure, and entrapped argon within pores can cause re-growth during subsequent heat treatment above the β-transus temperature.

Stress-Relief Annealing ​

Stress-relief annealing before drilling reduces the residual stress gradient in AM parts:

• LPBF Ti6Al4V: Stress relief at 650–800°C transforms brittle α' martensite to α+β lamellar microstructure, reducing cutting vibrations to wrought-like levels
• LPBF 316L: Stress relief at 900°C achieves strong residual stress reduction but may reduce the beneficial cellular substructure
• H13 tool steel: Stress relief before drilling reduces edge chipping tendency

The optimal stress-relief temperature must balance residual stress reduction against unwanted microstructural coarsening.

Solution Treatment and Aging ​

For precipitation-hardenable AM alloys (IN718, AlSi10Mg, A20X):

• Solution treatment + aging: Increases internal defect density and alters pore shapes through dissolution of secondary phases
• HIP + aging: Collapses internal pores before aging, resulting in superior fatigue performance
• Recommendation: HIP before solution treatment and aging for deep hole drilling applications where bore surface integrity affects component life

Stock Allowance ​

A minimum stock allowance of 0.5–1.0 mm per side is recommended for drilling AM near-net shapes, sufficient to clear surface-connected porosity and the as-built surface layer where surface roughness and subsurface damage are concentrated.

In-Process Mitigation Techniques ​

When pre-drilling post-processing is incomplete or impractical, in-process adjustments can improve outcomes.

Vibration-Assisted Drilling ​

VAD superimposes low-frequency (50–500 Hz) or ultrasonic (15–40 kHz) oscillation on the drilling feed motion:

• AM Ti6Al4V: 10–20% reduction in cutting forces (Sorgato et al., 2021)
• Improved chip evacuation: The interrupted cut mechanism breaks chips into smaller segments
• Reduced edge chipping: Lower instantaneous cutting forces reduce impact loading at defect boundaries

VAD is particularly effective for AM materials because the cyclic tool engagement reduces the average cutting force and provides more consistent chip formation in heterogeneous microstructures.

Cryogenic and High-Pressure Coolant ​

• LCO₂ cooling: 32.7% reduction in flank wear for WAAM Ti6Al4V (Singla et al., 2025)
• High-pressure through-tool coolant (standard in gun drilling): The high-pressure coolant jet improves chip evacuation and provides effective heat removal at the cutting edge
• Minimum quantity lubrication: Insufficient for AM materials due to higher cutting temperatures

Parameter Optimisation for AM Microstructures ​

Standard drilling parameters developed for wrought materials are not optimal for AM equivalents. Specific adjustments include:

• Reduced cutting speed (10–15% lower) to compensate for higher hardness and reduce thermal loading on the cutting edge
• Moderate feed rate — excess feed increases burr formation; very low feed increases edge rubbing and work hardening
• Peck drilling strategy — more frequent peck cycles to manage segmented chip evacuation
• Entry and exit dwell — AM materials benefit from reduced feed at entry (to manage the as-built surface layer) and exit (to minimise burr formation)

Decision Framework ​

Tier 1: Non-Critical Applications ​

For AM near-net shapes where the drilled hole is not a fatigue-critical feature:

• No pre-drilling HIP required if defect volume fraction is below 0.5%
• Standard drilling parameters with 10% speed reduction
• Accept surface finish Ra 1.0–1.5 µm
• Tool life expected at 50–70% of wrought baseline

Tier 2: Structural Applications ​

For load-bearing components with drilled features:

• Stress-relief annealing before drilling
• HIP recommended for LPBF parts; optional for WAAM
• Minimum 0.5 mm stock allowance
• Cryogenic or high-pressure coolant
• Tool life expected at 40–60% of wrought baseline

Tier 3: Safety-Critical Applications ​

For components where bore integrity is essential:

• HIP mandatory before drilling
• Stress-relief annealing
• Minimum 1.0 mm stock allowance
• Non-destructive evaluation (X-ray CT or ultrasonic) to map defect distribution before drilling
• Adaptive drilling parameters based on defect density
• Post-drilling NDE to verify bore surface integrity
• Tool life expected at 30–50% of wrought baseline

Summary Table ​

FAQ ​

Why is drilling additive manufactured materials harder than drilling wrought materials? ​

AM materials typically have 11–33% higher hardness than their wrought counterparts due to rapid solidification microstructures, higher dislocation density, and fine precipitate distributions. The heterogeneous microstructure also causes variable cutting forces and accelerated abrasive wear. WAAM Ti6Al4V shows 94% higher flank wear than wrought under identical drilling conditions.

What types of internal defects cause the most problems in deep hole drilling? ​

Lack-of-fusion defects (50–500 µm irregular voids at layer boundaries) are the most problematic. They can cause tool edge impact and chipping, disrupt coolant flow, and leave unacceptable surface voids in the finished bore. Keyhole porosity (10–100 µm spherical pores) has minimal effect on drilling outcomes.

Can hot isostatic pressing eliminate internal defects before drilling? ​

HIP is effective for keyhole and distributed porosity — WAAM 316L achieves 98.8% porosity reduction after HIP. However, large lack-of-fusion defects may not fully close due to interconnectivity and entrapped argon. These defects can also re-grow during subsequent heat treatment above the β-transus temperature in titanium alloys.

How does the AM build orientation affect drilling? ​

Drilling perpendicular to the build direction encounters different deformation mechanics than drilling parallel to it. The crystallographic texture and layer band interfaces create direction-dependent hardness and chip formation behaviour. Burr formation and surface finish vary with orientation — drilling parallel to layers generally produces better results.

What is the best cooling strategy for drilling AM materials? ​

High-pressure through-tool coolant (standard in gun drilling) is effective. Cryogenic LCO₂ cooling provides additional benefit — 32.7% reduction in flank wear for WAAM Ti6Al4V. MQL is insufficient due to higher cutting temperatures in AM materials.

Does stress-relief annealing improve drilling outcomes? ​

Yes, particularly for LPBF Ti6Al4V. Stress-relief annealing at 650–800°C transforms brittle α' martensite to α+β lamellar microstructure, reducing high-frequency cutting vibrations from up to 5 kHz to wrought-like levels. This directly reduces edge chipping and improves tool life.

What cutting parameters should I use for drilling AM materials? ​

Standard practice is to reduce cutting speed by 10–15% compared to wrought parameters for the same material, maintain moderate feed rates (excess feed increases burrs, very low feed increases work hardening), use more frequent peck cycles for chip evacuation, and reduce feed at entry and exit.

Can vibration-assisted drilling help with AM materials? ​

Yes. Ultrasonic and low-frequency vibration-assisted drilling reduces cutting forces by 10–20% in AM Ti6Al4V by providing interrupted chip formation and reducing average cutting force. The improved chip evacuation is particularly beneficial for deep hole drilling.

Should I drill before or after HIP? ​

Drill after HIP whenever possible. HIP collapses internal voids that would otherwise cause tool impact and surface defects. If drilling before HIP, the open bore surface may collapse or deform during the HIP cycle, requiring subsequent re-drilling or machining.

What stock allowance should I leave when drilling AM near-net shapes? ​

A minimum of 0.5 mm per side for non-critical applications and 1.0 mm per side for safety-critical applications. This is sufficient to clear the as-built surface layer and surface-connected porosity that is concentrated within 0.3–0.5 mm of the surface.

Conclusion ​

Deep hole drilling of additive manufactured near-net shapes presents a distinct set of challenges that differentiate it from conventional drilling of wrought or cast materials. The higher and variable hardness of AM microstructures accelerates tool wear and degrades surface finish. Internal defects — particularly lack-of-fusion voids — create risks of tool edge impact and breakage that are amplified in deep hole drilling by the difficulty of monitoring and intervention at depth.

However, a systematic approach combining pre-drilling post-processing (HIP, stress-relief annealing, appropriate stock allowance) with in-process mitigation (reduced cutting speed, vibration assistance, high-pressure coolant) can bring drilling outcomes for AM materials within an acceptable range of wrought baseline performance. The key principle is that AM drilling strategy cannot be extrapolated from wrought experience — the material's unique defect signature and microstructural heterogeneity require process-specific parameter development.

As AM moves toward production of larger near-net shapes requiring post-process deep hole drilling — structural aerospace components, oilfield equipment, tooling inserts — the integration of AM qualification, post-processing, and drilling operations into a unified process chain will become increasingly important for achieving reliable, cost-effective outcomes.