LPBF Ti-6Al-4V Bracket: Lack-of-Fusion at the Downskin and an Overdue HIP Decision
An LPBF Ti-6Al-4V bracket weighs 320 g and is built two per plate at 40 µm layer thickness, 250 W laser power, 800 mm/s scan speed, 60 µm hatch, 90° stripe rotation, on the eighth powder reuse cycle. Build time is 28 hours. Archimedes density is 99.2%. CT of three parts shows 0.3–0.8 mm elongated indications lying parallel to the build plate in the 35° downskin region of the mounting boss; the rest of the part is clean. As-built downskin roughness is 14 µm Ra. The Ø12 H7 bore carries a 0.6 mm machining allowance. Tensile testing gives 1,080 MPa UTS but 8% elongation against a 10% minimum. The approved post-processing route is stress relief followed by HIP at 920 °C / 100 MPa / 2 h. A 30-piece qualification lot is due in six weeks and the customer has asked whether the parts can ship after HIP.
1. Classify the indications and explain why they concentrate at the 35° downskin. What parameter and build-strategy changes would you make first, and why not simply increase laser power globally?
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Classification. Elongated indications parallel to the build plane, clustered at a low-angle downskin and absent elsewhere, are characteristic of lack-of-fusion (LoF), not gas porosity. Gas pores are spherical, randomly distributed, and typically sub-100 µm; these are irregular, aligned with the layer boundary, and spatially tied to a specific geometry feature.
Why the 35° downskin. The downskin is the region where each layer overlaps only partially with the previous layer. As the angle from horizontal decreases, the fraction of the melt pool that sits over loose powder rather than solid metal increases. Powder conducts heat roughly an order of magnitude worse than solid Ti-6Al-4V, so heat accumulates, the melt pool deepens and destabilizes, and the layer-to-layer remelt depth becomes inconsistent. Combined with a 40 µm layer and 800 mm/s at 250 W, the volumetric energy density is marginal, so any further local loss of conduction produces incomplete fusion. The 35° value matters: below roughly 45° from horizontal the overhang area per layer grows quickly, and at 30–35° the standard downskin parameters are usually insufficient without contour remelting or support.
First changes. (1) Apply dedicated downskin parameters - lower scan speed, higher power, or a second remelt pass on downskin contours - rather than changing the bulk parameter set. (2) Reduce layer thickness locally or globally to 30 µm to increase remelt overlap. (3) Reorient the boss so the critical face sits at ≥45° from horizontal, or add supports that conduct heat away from the downskin.
Why not global power increase. The bulk of the part is already at 99.2% density and meets UTS. Raising power everywhere overheats thin sections, increases spatter-driven gas porosity, worsens residual stress and distortion, and changes the bulk microstructure - invalidating the tensile data you already have. Change the region that is failing.
2. Will HIP fix the elongation shortfall and the CT indications? State the sequence you would run and the risk that HIP does not solve the problem.
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The elongation shortfall. HIP at 920 °C / 100 MPa is below the beta transus for Ti-6Al-4V (roughly 995 °C) and is a reasonable route to close internal porosity and recover ductility by removing the stress concentrators that initiate early fracture. It commonly moves elongation from the 6–8% as-built range toward 10%+. But it is not guaranteed: if the LoF indications are large and numerous in the boss, or if the microstructure is dominated by acicular alpha prime from fast cooling, HIP alone may not reach 10%. Plan for the test to fail rather than assuming it will pass.
The CT indications - the critical risk. HIP closes porosity only if the void is internal and can be collapsed by hydrostatic pressure. LoF at a downskin is frequently surface-connected or lies within a few hundred micrometres of the surface. A surface-connected defect is not healed by HIP; it is merely not improved, and it may be opened further by machining. The boss also gets 0.6 mm of material removed from the bore, which will expose or intersect near-surface defects that CT saw as internal on the as-built part.
Sequence.
- Machine the 0.6 mm allowance and the H7 bore first, then re-inspect by CT or X-ray. Inspecting before machining assesses a geometry that will not exist in service.
- HIP only after machining, so any newly exposed porosity is visible rather than hidden.
- Post-HIP, re-run CT on the same parts plus tensile coupons built in the same orientation and on the same plate.
- If surface-connected LoF is confirmed, HIP is the wrong control - the fix is the process parameter and orientation change, and the affected lot is scrap or concession.
Do not present the six-week date as achievable until step 1 has been run.
3. Define the evidence package you would require before releasing the 30-piece qualification lot, including how powder reuse is handled.
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Build and process evidence.
- Documented parameter set for bulk, downskin, contour, and remelt, with the machine and calibrations recorded; a parameter change after qualification restarts the qualification.
- Build layout and orientation record for every plate, including witness coupons and tensile bars built in the same orientation as the critical boss.
- Powder traceability: reuse cycle count, particle size distribution, chemistry including oxygen and nitrogen, and a defined maximum reuse cycle validated by test data. The eighth reuse cycle is exactly the sort of variable that silently shifts density - chemistry drift in O and N raises strength and drops ductility, which is consistent with 1,080 MPa and 8% elongation. Cap reuse at a level supported by tensile data, not by convenience.
Inspection evidence.
- CT or X-ray on 100% of the 30 parts after machining, with acceptance criteria written as defect type, size, and location, not just 'density acceptable.'
- Dimensional inspection of the H7 bore and critical features with CMM, including the as-built-to-machined datum scheme.
- Surface condition on downskin, including support removal marks and any near-surface indications.
Material evidence.
- Tensile per build, at minimum, with elongation meeting 10% including the confidence allowance; chemistry including O/N/H; microstructural evaluation for alpha prime and prior-beta morphology.
Post-processing evidence.
- HIP cycle charters with temperature, pressure, and hold time, plus furnace qualification.
- A destructive section of one part per build through the boss, since CT has resolution limits and the section is the only way to confirm defect morphology.
Release rule. Release the lot only if all three builds meet every criterion; a single build failure means re-qualification, not a waiver for the good plates.