Abrasive Flow Machining
Also known as: AFM, Abrasive Flow Finishing, Extrude Hone
Abrasive Flow Machining (AFM) forces a semi-solid viscoelastic abrasive medium through or across workpiece surfaces under hydraulic pressure (5–60 bar). The medium — abrasive particles (SiC, Al₂O₃, or B₄C, grit 60–220 mesh) suspended in a silicone or polyborosiloxane polymer carrier — acts as a self-conforming lapping tool. Material removal is highest at flow restrictions (channels, passages, corners), naturally concentrating abrasion where it is most needed. One bidirectional stroke (media extruded through and back) constitutes one cycle. Typical processes: 5–50 cycles at 5–30 bar for internal channel finishing; 50–200 cycles for precision surface finishing.
Why AM parts need this
AFM is the only practical method for finishing complex internal channels, lattice interiors, and undercut surfaces in AM metal parts. These geometries — heat exchanger passages, conformal cooling channels, turbine fuel nozzles, hydraulic manifolds — are inaccessible to all mechanical finishing tools. As-built LPBF internal surface roughness is Ra 10–35 µm (partially fused powder particles adherent to walls). AFM reduces this to Ra 0.5–3 µm in 10–30 cycles, improving flow coefficient (Cv) by 15–40% and reducing pressure drop across complex manifolds. Extrude Hone (Kennametal) invented AFM in 1968 and remains the dominant equipment OEM; the process parameters are not publicly disclosed at media-recipe level, requiring independent estimation for pre-process planning.
Achievable surface finish
Typical input Ra
15 µm
Achievable Ra
0.2–1 µm
Results depend strongly on channel geometry, alloy, and media selection. Published data: LPBF IN718 channels Ra 12 µm → Ra 0.8 µm in 20 cycles (Abate et al., 2022). Ti-6Al-4V: Ra 18 µm → Ra 1.2 µm (UT Austin repository).
Key parameters
Low pressure (5–15 bar): delicate lattices, thin walls. Medium (15–40 bar): standard channel finishing. High (40–60 bar): hard alloys, heavy stock removal.
Coarse (60–80 mesh): heavy stock removal, Ra >5 µm target. Medium (100–120): balanced. Fine (180–220): final finish, Ra <1 µm.
Ra reduction is logarithmic with cycle count — most improvement in first 10–20 cycles. Beyond 50 cycles, diminishing returns unless changing to finer media.
Higher viscosity = slower flow = more abrasive contact per pass = finer finish. Lower viscosity = higher flow velocity = faster material removal.
Minimum practical channel: ~0.5 mm (media must flow). Restriction ratio (Achannel/Acylinder) controls abrasive action intensity.
Compatible AM processes
Compatible materials
Limitations
- Only works on through-passages — blind holes or dead-end channels cannot be finished by AFM (no flow path)
- Geometry must allow media to exit — closed internal cavities are not processable
- Material removal is preferential at restrictions — uniform stock removal across varying cross-sections requires careful fixture design
- Tight channel radii (<1 mm) may not achieve target Ra due to media bridging at bends
- No commercially available planning software — all process parameters are proprietary to Extrude Hone / Kennametal
- Minimum channel diameter ~0.5 mm for practical media flow
- Not suitable for polymer AM parts
Related tools
Providers
Ra improvement data from Abate-2022 (peer-reviewed, SCIE) and UT Austin AFM-AM repository. Parameter ranges from Extrude Hone published product literature (manufacturer tier) and Kennedy-2020 AFM review.
All post-processing