Hybrid Additive-Subtractive Deep Hole Drilling: Post-Processing of Laser Powder Bed Fusion and Directed Energy Deposition Components

A manufacturer of injection mould inserts with conformal cooling channels (LPBF maraging steel MS1, stress-relieved, as-built surface Ra 8–12 µm, channels Ø8 mm × 300 mm deep, Ra < 0.8 µm) initially drilled directly into as-built LPBF surfaces. The as-built surface caused drill wander of 0.2–0.5 mm at entry, internal porosity (0.1–0.5%, 20–80 µm pores) caused force variations and chatter marks, and the columnar grain structure (100–500 µm long) caused directional tool wear (2× on one side). Switching to a hybrid strategy — print with 10 mm channels (1 mm/side allowance), stress relieve, rough gun drill to 8.5 mm (Vc = 60 m/min, f = 0.04 mm/rev, AlCrN carbide, oil 50 bar), age to 52 HRC, finish ream to 8 mm H7 with PCD reamer (Vc = 30 m/min, f = 0.08 mm/rev) — achieved Ra 0.2–0.4 µm and positional accuracy ±0.03 mm. Carbide tool life in LPBF maraging steel (35 HRC) was 30–50 m vs 80–120 m in wrought.

AM Material Characteristics for Deep Hole Drilling ​

Comparison of Drilling Characteristics: AM vs Wrought Materials ​

Effect of Build Orientation on Deep Hole Drilling ​

FAQ ​

How does the microstructure of additively manufactured materials affect deep hole drilling, and what are the key differences from wrought materials? ​

The microstructure of additively manufactured (LPBF and DED) materials differs from wrought materials in three fundamental aspects that affect deep hole drilling: porosity, grain structure anisotropy, and as-built surface roughness. The porosity in AM materials — small gas pores (10–80 µm diameter) or lack-of-fusion voids (50–500 µm) that are inherent to the powder-based AM process — causes intermittent cutting forces when the drill encounters a pore. The cutting force drops sharply (by 30–60%) as the drill passes through the pore (the chip is interrupted by the void), then spikes (by 20–40% above the average) as the drill exits the pore and engages a full-density wall on the far side. The force transient excites the drill's natural bending frequency, producing a chatter mark on the bore surface that is visible as a band of increased roughness (typically 2–5 µm width) at the radial position corresponding to each pore. The chatter marks are longitudinal (they run parallel to the drill axis) and reflect the location of pores in the AM material. In materials with porosity below 0.1% by volume (well-optimised LPBF parameters for Inconel 718 and 316L), the pore density is low enough (1–5 pores per mm³) that the force transients do not create visible chatter marks, and the drilling behaviour is similar to wrought material. In materials with porosity above 0.5% (sub-optimal LPBF parameters, or materials prone to lack-of-fusion porosity such as H13 tool steel), the force transients cause chatter marks that degrade the surface finish beyond the acceptable Ra limit (> 0.8 µm) and require a post-drilling honing or reaming step to correct.

The grain structure anisotropy — columnar grains (100–500 µm long, 10–50 µm wide) oriented in the build direction (Z-direction), with a fine cellular sub-structure (0.5–2 µm) — creates directional tool wear. When the drill cuts across the columnar grains (the bore axis is perpendicular to the build direction), the drill encounters alternating regions of grain boundary and grain interior. The grain boundaries have a different hardness than the grain interior (the grain boundary is enriched with solute elements and may contain fine precipitates that increase local hardness by 10–20%). This alternating hardness causes lateral force variation that deflects the drill toward the softer grain interior, producing a bore that is not perfectly straight (a sinusoidal waviness of 0.01–0.03 mm amplitude and 100–500 µm wavelength, visible on the bore surface as a periodic variation in diameter). The directional tool wear is most pronounced when the drill is oriented parallel to the build direction (the drill cuts along the length of the columnar grains) — the cutting edge is not exposed to the grain boundary hardness variation because the drill is not crossing grain boundaries; instead, it is cutting within a single grain column. In this orientation, the tool wear is 30–50% lower than when the drill is perpendicular to the build direction. The recommended orientation for drilling AM parts is to align the bore axis parallel to the build direction to minimise directional tool wear and bore waviness. The as-built surface roughness — Ra 8–25 µm for LPBF surfaces, depending on the orientation (top surface: Ra 8–15 µm; side surface: Ra 15–25 µm) — is 10–20× rougher than a machined surface (Ra 0.4–0.8 µm). When the drill enters through an as-built AM surface, the rough surface deflects the drill tip by 0.05–0.2 mm at the entry point (the drill tip slides off the rough peaks and settles into a surface valley, starting the bore at an offset from the intended position). The entry deflection is eliminated by machining the entry surface flat (spot facing or face milling) to Ra < 1.6 µm before the deep hole drilling operation. The machining allowance for the entry spot face is 1–2 mm of material removal from the as-built surface — part designers must allow for this spot face when specifying the component geometry.

What is the recommended machining allowance for deep hole drilled channels in AM components, and how is it determined? ​

The recommended machining allowance for deep hole drilled channels in AM components is 0.5–2.0 mm per side (1–4 mm on the diameter), depending on the AM process, the material, the build orientation, and the required final bore diameter tolerance. The allowance serves three functions: removing the as-built surface layer (which has the highest porosity concentration, the roughest surface finish, and any surface contamination from the powder bed); removing the porosity-affected layer beneath the surface (the subsurface layer, 0.1–0.5 mm deep in well-optimised LPBF, may have higher porosity than the core due to the surface-tension-driven pore migration during solidification); and providing a clean, defect-free surface for the finishing operation (reaming or honing) that establishes the final bore geometry. The allowance is determined by the AM process capability (the surface roughness, the depth of the porosity-affected layer, and the geometrical accuracy of the as-built hole) and the final bore requirements (the surface finish, the porosity-free requirement, and the bore position tolerance). For LPBF of maraging steel with a final bore requirement of H7 (8 mm +0.015/−0 mm) and Ra < 0.8 µm, the recommended allowance is 0.8–1.2 mm per side (total oversize 1.6–2.4 mm on diameter). The initial as-built hole diameter should be 10.0–10.4 mm (for a final bore of 8 mm), the rough gun drill should remove 1.6–2.4 mm of material (taking the hole to 8.4–8.6 mm), and the finish reamer should remove 0.2–0.3 mm per side (taking the hole to the final 8 mm H7).

The allowance must be verified on a test component before production drilling. The verification procedure is: print a test component with the same geometry, material, and build orientation as the production part; drill a test bore at the intended allowance and inspect the bore surface under an optical microscope at 50× and a scanning electron microscope at 500×; if the surface shows any residual porosity (pores > 10 µm at the bore surface), the allowance is insufficient and must be increased by 0.2–0.5 mm per side. The verification is repeated until the bore surface is porosity-free. The test component is then sectioned axially and the bore surface is examined by metallography (polished and etched cross-section, 200–500×) to verify that the allowance has removed the entire porosity-affected layer. The required allowance for DED components is typically larger (1.5–3.0 mm per side) because the DED process produces a rougher as-built surface (Ra 20–50 µm) and a deeper heat-affected zone (0.5–2 mm) that may contain porosity and microcracks from the thermal cycling. The DED allowance must be verified by the same test procedure. The cost of the increased allowance is the additional material removal time (10–20% longer drilling cycle time for the larger roughing diameter) and the additional AM build time (the larger as-built hole takes less time to print than a solid block — the build time is proportional to the volume of material deposited, and a larger hole means less material volume to deposit). For LPBF with 10 mm as-built holes versus 8 mm final holes, the build time is approximately 15% shorter (the laser does not scan the area of the 10 mm hole), and the additional material removal cost is offset by the reduced build cost. The net cost of the allowance is approximately zero for well-designed components.

How does post-AM heat treatment affect the drillability of additively manufactured components? ​

Post-AM heat treatment — stress relief, hot isostatic pressing (HIP), solution annealing, and age hardening — has a significant effect on the drillability of AM components because the heat treatment alters the microstructure (porosity, grain structure, and hardness) that determines the drilling behaviour. Stress relief (490°C/6h for maraging steel, 650°C/3h for Ti-6Al-4V, 300°C/2h for AlSi10Mg) reduces the residual stresses from the AM process (which can be 200–500 MPa in as-built components) by 60–80%, reducing the distortion that occurs during subsequent machining. The stress relief does not significantly change the porosity (pores are not closed by stress relief), and the hardness may decrease slightly (by 5–10%) due to recovery of the as-built dislocation structure. Drilling after stress relief: tool life is 20–40% higher than drilling in the as-built condition (the lower residual stress reduces the cutting force by 5–15%), but the porosity remains a problem for chatter and surface finish.

Hot isostatic pressing (HIP) — high temperature (1000–1200°C for nickel alloys and steels) and high pressure (100–200 MPa) in an inert gas atmosphere — closes internal porosity by plastic collapse and diffusion bonding of the pore walls. HIP reduces the porosity from 0.1–0.5% to below 0.01% (effectively pore-free) and homogenises the microstructure (dissolves the cellular substructure and reduces segregation). The drilling behaviour after HIP is significantly improved: tool life is 60–80% of the tool life in wrought material of the same hardness (compared to 30–50% without HIP), and the chatter marks from porosity are eliminated. HIP is recommended for any AM component where the drilled bore must be porosity-free (hydraulic channels in manifold blocks, LOX-compatible bores, or high-pressure fluid passages). The HIP adds $200–1000 per build cycle depending on the component size and the HIP service provider's pricing. Solution annealing + age hardening (or direct ageing) for precipitation-hardenable alloys (maraging steel, Inconel 718, AlSi10Mg T6) increases the hardness to the service condition (52–55 HRC for maraging steel, 45–50 HRC for Inconel 718) and is typically performed after rough machining and before finish machining. The drilling of age-hardened AM material requires PCD or PCBN tooling (the same as for wrought material at equivalent hardness), and the tool life is 30–60% of the tool life in the stress-relieved condition (as expected for the higher hardness). The recommended heat treatment sequence for drillability is: stress relief (before rough drilling), HIP (if porosity is a concern), rough drilling, age hardening, finish reaming. This sequence maximises tool life during the roughing pass (when most of the material is removed) and achieves the required surface finish and dimensional accuracy in the finish pass.

The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified additive manufacturing engineers and machining specialists for specific hybrid AM-subtractive applications. Data and recommendations are based on published research and industry experience as of 2026.