am-post-processing-metal

name	am-post-processing-metal
description	Metal AM post-processing — stress relief, HIP (hot isostatic pressing), machining AM parts, surface finish improvement (media blasting/electropolishing/vibratory), wire EDM, support removal, quality inspection.
metadata	{"priority":7,"promptSignals":{"phrases":["AM post-processing","additive manufacturing post-processing","HIP additive manufacturing","3D printing post-processing","stress relief AM","metal AM support removal","LPBF post-processing"],"minScore":3}}

Metal AM Post-Processing — Complete Skill

Overview of Post-Processing Steps

Typical sequence for LPBF (Laser Powder Bed Fusion):

1. Stress relief (heat treatment on build plate)
2. Part removal from build plate (wire EDM or bandsaw)
3. Support structure removal
4. Heat treatment (HIP, solution anneal, age-harden)
5. Machining (critical surfaces)
6. Surface finishing
7. Inspection and testing

Stress Relief Heat Treatment

Purpose: reduce residual stresses from rapid thermal cycling during building; prevents distortion after removal from plate

Ti-6Al-4V (LPBF): Stress relief: 800–900°C × 2h in vacuum or argon; FC or slow cool Result: partial β→α transformation; stress < 100 MPa retained (vs. 500–700 MPa as-built) HIP: 920°C × 100 MPa × 2h → complete healing of porosity

IN718 (LPBF): Stress relief: 870°C × 1h in inert atmosphere Then: solution anneal 980°C × 1h + age (718°C × 8h + 621°C × 8h) Aged properties match wrought specification (API 5CRA)

316L stainless (LPBF): Stress relief: 900°C × 1h in vacuum; rapid cool No precipitation hardening available; properties determined by microstructure

AlSi10Mg (LPBF): T6-like heat treatment: solution 525°C × 6h → quench → age 160°C × 6h Or T5: direct age (no solution); retains fine microstructure from AM; higher yield

Hot Isostatic Pressing (HIP)

Purpose: simultaneously close internal porosity AND apply heat treatment Conditions: typically 100–200 MPa argon; 1000–1400°C depending on material Cycle: ramp up (100–200°C/hr); hold 2–4h; slow cool (furnace, 2–5°C/min)

Effectiveness: Gas porosity (< 50 μm): effectively eliminated (> 99% reduction) Lack-of-fusion (LOF, irregular >100 μm): partially closed; small LOF fully closed; large LOF may remain Critical: HIP does NOT fix surface-connected pores (no pressure gradient to drive closure)

HIP + Solution + Age (combined thermal process): For Ni superalloys: HIP at solution temperature → single step; reduces cycle time Example: IN713: HIP at 1185°C/170 MPa/4h + FC to 1000°C/AC + age 870°C/20h

Material qualification with HIP: ASTM B883 (PM parts via HIP); NADCAP accreditation for aerospace HIP; AMS 2801 for Ti HIP

Part Removal from Build Plate

Wire EDM (Electrical Discharge Machining): Most common; precise kerf (0.3–0.5 mm); no mechanical force → no distortion EDM after stress relief (parts stress-relieved on plate first) Band saw / circular saw: faster; cheaper; more distortion risk; use for non-critical parts

Build plate material: SS 316, H13 tool steel, Ti-6Al-4V Mismatch in CTE between part and plate → stress during cooling → EDM minimizes this effect

Support Structure Removal

Design for removal (DfAM): minimize supports; use self-supporting angles (< 45°) Support types: block supports (solid), tree supports, lattice supports, skin supports

Manual removal: Needlenose pliers, chisels, hammers; acceptable for external accessible supports EDM: for internal supports in channels; no mechanical contact Electrochemical machining (ECM): dissolves conductive supports selectively

Support removal from internal channels: Design channel diameter large enough for tool access; or use removable support materials Inconel supports in Ti parts: problematic (galvanic); use same material or design out

CNC Machining of AM Parts

Challenges:

• Surface finish Ra = 5–25 μm (LPBF); needs Ra < 1.6 μm for most engineering surfaces
• Hardened/workhardened layer (affected zone): 0.1–0.5 mm thick
• Residual stress → distortion during machining
• Internal porosity near surface → interrupted cuts

Machining parameters: Ti-6Al-4V (AM): V_c = 40–80 m/min; f = 0.08–0.15 mm/rev; a_p = 0.3–1.0 mm Similar to wrought Ti but hardness may be higher → reduce speed 10–20% Coolant: flood coolant essential for Ti (prevents thermal damage)

Fixturing AM parts: Non-standard shapes → custom fixtures or 5-axis adaptive fixturing Soft jaws machined to match part surface

Thread machining: thread milling or tapping; avoid thread-cutting in AM near surface defects

Surface Finishing Methods

Abrasive Blasting (Glass Bead, Alumina)

Removes loose powder; improves Ra from 15 to 5–8 μm Media: glass beads (gentler), alumina (more aggressive) Imparts compressive residual stress (shot peening effect if high energy) Cost: low; fast; batch capable

Vibratory Finishing / Tumbling

Parts in bowl with abrasive media + water/compound; oscillating or rotating Ra improvement: 5–8 μm → 1–3 μm Extended cycle (24–48 hr): Ra < 1 μm for some geometries Limitation: rounded edges; cannot reach internal channels < 15 mm

Abrasive Flow Machining (AFM/Extrude Hone)

Abrasive putty forced through internal channels under pressure (5–20 MPa) Polishes internal passages; turbine blades, injection nozzles, implants Ra improvement: 5 μm → 0.4 μm in channels

Electropolishing

Electrochemical removal; anodic dissolution in acid electrolyte Ra improvement: 2 μm → 0.2–0.5 μm Excellent for SS and Co-Cr; removes sharp peaks selectively Limitation: geometry-dependent (non-uniform removal at corners/recesses); exposed porosity

Chemical Etching

Similar to electropolishing but without current; isotropic Removes thin surface layer uniformly; can reach internal features Risk: undercutting at pores → exposes subsurface porosity

Laser Polishing

Post-build remelting with defocused laser; local reflow smooths surface Ra improvement: 10 μm → 3–5 μm Can be done on specific surfaces without affecting others; in-situ on AM machine (some vendors)

Heat Treatment + Blasting (Ti-6Al-4V aerospace)

ASTM F3001/AMS4999: stress relief + HIP + solution anneal + age → then blasting Final Ra: < 0.8 μm for mating surfaces; ≤ 1.6 μm for general surfaces

Inspection and Quality

Dimensional: CT scanning (XCT) for internal geometry; CMM for external Internal porosity: XCT (most complete); destructive cross-section; X-ray film Surface finish: profilometry (contact or optical); Sa, Sz parameters (ISO 25178) Mechanical testing: tensile, fatigue, hardness from witness samples (same build/batch)

ASTM standards for metal AM: ASTM F2924: Ti-6Al-4V powder for additive manufacturing ASTM F3001: Ti-6Al-4V ELI for medical; same for aerospace under AMS4999 ASTM F3056: IN718 for AM ASTM F3055: IN625 for AM

Qualification: build parameter qualification → witness specimens → destructive testing → process release Aerospace: per AS9100D; medical: per ISO 13485; nuclear: per NRC guidance

Output

Provide: stress relief cycle (T [°C], time [h], atmosphere), HIP cycle (T [°C], P [MPa], time [h]), machining sequence and parameters for critical surfaces (V_c [m/min], f [mm/rev], coolant), surface finishing method with Ra target [μm], inspection requirements (XCT/CMM/profilometry), applicable ASTM specification, and estimated post-processing time [hr] and cost breakdown.