The short answer
**CT inspection** after SLM parts is not a default “scan the whole build,” and it is not a substitute for [HIP](https://3dwk.com/resources/when-to-specify-hip-after-slm-parts). X-ray computed tomography is a way to look for internal porosity, lack-of-fusion, trapped powder, or closed channels that a probe cannot reach. A usable callout names the **region of interest**, a **voxel size** (or the smallest indication you must resolve), the **acceptance**, and the **sequence**—as-built, after HIP, or after a skim. Silence on ROI and resolution is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the scan time alone.
This guide is for buyers packaging Ti-6Al-4V or similar SLM parts where the RFQ says only “CT as needed.” Illustrations use an **Example part** only—not a customer fitting and not a certificate. Sibling notes live in [HIP after SLM](https://3dwk.com/resources/when-to-specify-hip-after-slm-parts) and [SLM support-removal and skim notes](https://3dwk.com/resources/slm-support-removal-and-skim-notes); here we focus on **when a CT pack is worth calling out**, not on inventing a porosity spec.
What CT after SLM means for quoting
A **CT callout** names the method, the ROI (a channel, a wall, a datum boss—not “the whole part” unless you mean it), a voxel size or minimum detectable indication, the acceptance (pore diameter, volume fraction, or no lack-of-fusion in the ROI), how many pieces, and whether the scan is for integrity, for trapped powder, or also for dimensional use. Writing “CT to applicable standards” is not enough. Process pages for titanium SLM ([SLM Ti-6Al-4V aerospace](https://3dwk.com/en/slm/ti-6al-4v/aerospace.html), [SLM Ti-6Al-4V](https://3dwk.com/en/slm/ti-6al-4v.html), [SLM AlSi10Mg aerospace](https://3dwk.com/en/slm/alsi10mg/aerospace.html)) assume the inspection pack is visible when lots mix as-built cosmetic ribs with internal galleries.
Do not bury “NDT as needed” inside a notes cloud. Put **CT: ROI CHANNEL C AND WALL AT DATUM A, VOXEL ≤ 0.05, NO PORES ≥ 0.3 IN ROI, AFTER SUPPORTS + HIP, BEFORE SKIM OF A** on the PDF and mirror it in RFQ text. Background: [first-article inspection reports](https://3dwk.com/resources/first-article-inspection-reports), [escape-hole notes for hollow SLM parts](https://3dwk.com/resources/escape-hole-notes-slm-hollow-parts), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote).
When the scan is worth calling out
Specify CT when an internal channel or closed wall cannot be probed, when a first-article lot must show porosity or lack-of-fusion against a written limit, or when you need a check after [HIP](https://3dwk.com/resources/when-to-specify-hip-after-slm-parts) to see what remains. Skip a blanket full-part scan on a prototype whose internal volume is not critical, and do not use CT as a silent stand-in for a CMM on external datums unless the pack is actually a metrology-CT setup.
Trapped powder in a hollow or lattice is a different question from porosity in a pressure wall—[escape holes](https://3dwk.com/resources/escape-hole-notes-slm-hollow-parts) and CT can complement each other, but they are not the same line. [Residual-stress and warpage notes](https://3dwk.com/resources/slm-residual-stress-warpage-notes-for-buyers) belong beside the sequence when a skim after HIP will move the surface you just scanned.
Ti-6Al-4V powder and process planning bands sit in the common SLM titanium range under **rates.v1.1** material tables; material cost is rarely the CT-pack story—ROI, voxel size, and sample size are.
Sequence, ROI, and what CT will not do
Name whether the scan is as-built, after support removal, after HIP, or after the [skim](https://3dwk.com/resources/slm-support-removal-and-skim-notes) of a datum. A scan meant to catch lack-of-fusion in a channel should happen before that channel is hidden by a weld or a press-in liner. CT will not, by itself, close porosity—that is a HIP or process question—and it will not replace a written acceptance.
Tolerance and inspection traps
Calling “CT as needed,” asking for “no porosity” with no ROI or size limit, or demanding a 10 μm voxel on a large fitting without saying why, all force **needs_manual**. Default cosmetic ribs to as-printed visual checks; reserve CT for named internal regions and a stated indication size. Related: [ISO 2768 general tolerances on CNC RFQs](https://3dwk.com/resources/iso-2768-general-tolerances-cnc-rfq) when a skimmed datum is later CNC finished, [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
Illustration 1 shows an **Example part** Ti-6Al-4V SLM fitting with labeled CT on channel C and the wall at datum A versus cosmetic ribs left to visual checks.
Example part walkthrough
**Example part:** Titanium Ti-6Al-4V SLM fitting with an internal channel C and a wall at datum A, CT after support removal and HIP and before the skim of A, voxel ≤ 0.05 mm, no pores ≥ 0.3 mm in the ROI, cosmetic ribs as-printed, no invented certificate. With STEP + **CT ROI C + WALL AT A, VOXEL ≤ 0.05, NO PORES ≥ 0.3, AFTER HIP BEFORE SKIM**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent “CT as needed, no porosity” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“scan as needed”) versus a labeled ROI-voxel-sequence pack and the clearer estimate path.
What widens bands fastest
Unlabeled full-part scans, “no porosity” with no size or ROI, a voxel far finer than the acceptance, and CT demanded before the HIP / skim sequence is named. Quantity breaks help unit minutes but do not erase inspection-pack ambiguity—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost) and [Ti-6Al-4V CNC versus SLM](https://3dwk.com/resources/ti6al4v-cnc-vs-slm-cost-lead-time).
What to do next
Mark the RFQ with an **ROI, voxel or indication size, acceptance, and sequence relative to HIP and skim**, then upload on [quote-new.php](https://3dwk.com/quote-new.php). Packaging checklist: CT is inspection, HIP is a process, and external datums still need their own measurement plan. Service pages: [3D printing](https://3dwk.com/3d-printing-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** min/mid/max as estimates, not invoices. If the UI shows **needs_manual**, answer the CT-pack questions rather than forcing a fake price.
Common questions
Is CT the same as HIP?
No. HIP is a densifying process; CT is an inspection method. You may want CT after HIP to see what remains, but “HIP as needed” does not imply a scan, and a scan does not close pores.
Do I need to CT every piece in the lot?
Only if you say so. Many packs CT a first article or a sample, then stay with process control. Write the sample size. A 100% fine-voxel scan on a large lot will widen the band.
Can CT replace a CMM on datum A?
Not by default. Metrology CT is a different setup from a porosity scan. If datum A is a skimmed CNC face, keep a conventional dimensional plan unless you explicitly buy dimensional CT.
What voxel size should I write?
Write a size that can resolve the indication you will accept—for the Example part, voxel ≤ 0.05 mm against a 0.3 mm pore limit in the ROI. A much finer voxel “just in case” is not a free upgrade.