What Causes Porosity in Laser Cladding?
Porosity in powder-fed laser cladding may come from gas in the powder or melt pool, or an unstable vapour depression. Irregular voids at track, layer or substrate interfaces may instead indicate lack of fusion—not gas pores, but incomplete bonding. Classify the defect by its morphology and location before changing process settings.
A dark indication in a polished cladding cross-section is not enough to diagnose the cause. A small rounded pore inside a deposit and an irregular, locally planar void at an interface may both be described informally as “porosity”, but they point to different process conditions. The first question is whether the indication is a gas-filled pore or evidence of incomplete fusion. Treating the two as the same defect can lead to corrective changes that do not address the root cause.
This guide focuses on powder-fed laser cladding and coaxial laser-based directed energy deposition (L-DED). The mechanisms discussed are relevant to these powder-fed processes, but their relative importance depends on the alloy, powder production route, delivery system, nozzle geometry, laser parameters and component geometry. The findings should not be transferred automatically to laser powder bed fusion or wire-fed deposition.
What Types of Porosity Occur in Laser Cladding?
During a porosity investigation, distinguish gas pores, keyhole-related pores and void-like lack-of-fusion defects. Gas and keyhole-related pores involve gas trapped in the solidifying material. Lack of fusion is different: it is incomplete melting or bonding between deposited tracks, layers or the substrate, and may appear as an irregular or planar void in a cross-section. These patterns are useful diagnostic clues, not proof of the defect mechanism.
Gas pores
Gas pores are commonly rounded or elliptical and may occur within the deposited layer without following a track boundary. They form when bubbles remain in the melt pool until the metal solidifies. Possible sources include gas trapped inside feedstock particles, gas associated with powder delivery, gas released by material-specific reactions, or voids opened when an earlier deposit is remelted. Melt-pool flow and the advancing solidification front determine whether bubbles move, merge, escape or become trapped. Zhang and colleagues directly observed several of these behaviours in a 2024 in-situ X-ray study of DED.
Lack-of-fusion defects
Lack-of-fusion defects are usually irregular, elongated or locally planar. They are often found between neighbouring tracks or layers, or at the interface between the cladding and substrate. They indicate that the materials did not melt and join sufficiently across the boundary; they are not gas pores, even when a polished cross-section shows a void-like indication. Low effective heat input, excessive travel speed, an unsuitable powder feed rate, poor track overlap or an incorrect focal position can contribute. Surface oxides or contamination may also interfere with wetting and bonding, depending on the material system.
A repair-relevant study of 316L clads reported small, rounded trapped-gas pores within the deposited material and larger, irregular lack-of-fusion defects at the clad-to-base-material interface. This example shows why both morphology and location matter. The findings apply to the specific 316L materials and deposition systems studied and should not be treated as a universal rule (Revilla & De Graeve, 2022).
Keyhole-related pores
A keyhole-related pore can form when high local energy density creates a deep vapour depression in the melt pool. If the depression becomes unstable and collapses before solidification, a void may be trapped. Consider this mechanism only when the process conditions support vapour-depression formation; a round pore by itself does not prove a keyhole origin. Powder-fed DED also differs from laser powder bed fusion because powder delivery, laser attenuation and nozzle geometry affect the melt-pool conditions.
A 2024 review of coaxial powder-fed L-DED discusses gas pores and lack-of-fusion defects as separate defect categories. It identifies potential gas sources such as powder-borne gas and metal vapour, and describes lack-of-fusion indications at track overlaps, layer interfaces and the coating-to-substrate bond zone (Mo et al., 2024).
What Causes Gas Pores in Laser Cladding?
Gas trapped inside the metal powder
Some powder feedstock carries internal gas-filled pores from its production route. In gas-atomised powder, a particle may contain a small internal pore that is not visible from its external surface. When the particle melts, the gas can enter the melt pool. If the bubble does not escape before the surrounding metal solidifies, it can remain in the deposited layer. A visually sound powder surface therefore does not rule out internal porosity.
Zhang et al. (2024) used in-situ X-ray imaging and modelling to follow gas bubbles during DED. Their observations showed bubbles transferring from gas-atomised powder into the melt pool; some escaped, some merged, and others were captured by the solidification front. When a defect appears after a powder-batch change, include powder history and feedstock characterisation in the investigation rather than assuming that a laser-setting change will solve it.
Gas carried into the melt pool with the powder stream
In coaxial powder-fed systems, carrier gas transports particles through the delivery line and nozzle. The gas is part of the intended powder-delivery process, so its presence alone should not be treated as the cause of porosity. Flow stability, particle velocity, powder-stream convergence, nozzle alignment and stand-off can affect how particles enter and interact with the melt pool. Check for abnormal flow, leaks, restrictions or a misaligned stream alongside the gas supply.
This does not mean that shielding gas is automatically the cause of every pore. Carrier gas and shielding gas have different functions, and a machine may have separate flow paths. For a fuller explanation, see the guide titled What Gas Is Used in Laser Cladding?
Gas released from the material or a previous deposit
The substrate and previously deposited tracks may already contain small pores or other discontinuities. When a later pass remelts that material, some existing voids can open into the new melt pool and be transferred or redistributed. Material-specific reactions may also release gas. Moisture is one possible contributor in certain powder and alloy systems; reviews of laser metal deposition discuss powder drying where it is relevant (Cheng et al., 2022). Do not assume moisture is the cause for every alloy or every pore.
Bubbles trapped before the melt pool solidifies
A bubble does not automatically rise out of a molten track. Its movement depends on size, buoyancy, melt-pool convection, viscosity and the time available before solidification. The 2024 X-ray study observed bubbles moving with the melt flow, merging, escaping or being captured by the advancing solidification front. This is why “slow the cooling” is not a universal cure: the flow field, bubble size and melt-pool geometry also matter.
Can Laser Power Cause Porosity in Laser Cladding?
Yes, but the relationship is not simply “more power means fewer pores” or “less power means fewer pores”. The result depends on how much energy reaches the melt pool relative to the powder feed rate, travel speed, spot size and material response.
If effective energy input is too low, powder may not melt fully and the melt pool may not remelt the previous track or substrate sufficiently. These conditions favour lack-of-fusion defects. At the other extreme, excessive local energy density can produce metal vapour and an unstable vapour depression in some process windows, creating a different route to pore formation. A change in laser power may therefore reduce one defect mechanism while increasing another.
A 2023 study of blue-laser DED of AlSi10Mg used high-speed imaging and X-ray computed tomography to distinguish lack-of-fusion defects, spherical gas pores and larger irregular pores. The authors mapped defect types against processing conditions for their specific material and setup. This supports the need to diagnose the defect before adjusting parameters; it is not a parameter recipe for other lasers, alloys or cladding heads (Wang et al., 2023).
When investigating porosity, review laser power together with scanning speed, powder feed rate, spot size, stand-off distance, track overlap and substrate condition. Use the recorded parameter set as a baseline and change variables systematically. Do not increase laser power simply because a cross-section contains pores.
How Can You Identify the Source of Porosity?
Start with the observed indication, then work backwards through the process. The table below is a screening guide: it helps prioritise checks, but morphology alone cannot prove the origin of a pore or void.
| Observed clue | Possible interpretation | What to check next |
| Small, rounded pores within the deposited layer | Gas trapped in the melt pool or feedstock-related porosity | Powder batch and particle condition, powder handling, sections from multiple locations, and process stability |
| Irregular or elongated void-like indications at track or layer boundaries | Possible lack of fusion (incomplete bonding, not necessarily a gas pore) | Laser focus, travel speed, powder feed rate, track overlap, remelting depth and surface condition |
| Void-like indications concentrated at the coating-to-substrate interface | Possible lack of fusion, incomplete bonding or interface contamination | Substrate preparation, first-layer parameters, oxide condition and interface metallography |
| Pores appear after a powder-batch change | Possible feedstock-related change | Compare powder certificates, particle-size distribution, handling/storage history and representative powder samples |
| Pores vary with process settings or position | Possible melt-pool or powder-delivery instability | Review machine logs, nozzle condition, powder-stream alignment and repeatable test tracks |
For a reliable diagnosis, prepare cross-sections from more than one location and, where practical, in more than one orientation. Record indication size, shape, area fraction and position relative to track boundaries and the substrate. A single section can miss defects or make a three-dimensional void appear misleadingly small. In production troubleshooting, first confirm the indication and classify it; compare affected and unaffected samples; check the powder batch, storage history, feeder and nozzle; review the process log; then run controlled trials, changing one variable at a time where practical. Reinspect with the same method and record the result.
Metallography or X-ray CT: which inspection method should you use?
Metallography is a practical first step for examining local pore morphology, track overlap and interface bonding. It is destructive and represents only the section prepared, so section location and preparation quality matter. X-ray computed tomography can reveal the three-dimensional distribution of internal pores without sectioning the entire specimen, but achievable resolution depends on component size, material attenuation, equipment and the smallest defect that must be detected. CT is not automatically necessary for every production part; select the inspection method against the component specification and risk.
How Can You Prevent Porosity in Laser Cladding?
Prevention is most effective when it targets the mechanism identified during diagnosis. Use the checks below as a process-development sequence, not as a universal parameter prescription.
Check powder condition and batch consistency
Review the powder certificate, alloy designation, particle-size range and production route. Compare batches when a defect appears suddenly. Follow the powder supplier’s storage and drying instructions for the specific alloy; do not apply an arbitrary baking temperature or time. If internal powder porosity is suspected, request feedstock characterisation or compare representative powder samples using a suitable inspection method.
Prepare the substrate and previous layer
Remove oil, grease, rust, loose oxide, machining residue and other contaminants using a method compatible with the substrate and the qualified procedure. Make sure the surface is dry before deposition. Inspect previously deposited tracks before building another layer; discontinuities in an earlier layer may be remelted or carried into subsequent material.
Stabilise powder delivery and the cladding head
Check the feeder, hoses, powder line, nozzle and powder-stream alignment. Confirm that the selected powder is compatible with the feeder and delivery line, and look for blockage, leakage, bridging or abnormal powder-flow fluctuation. Where possible, verify repeatability over a representative run rather than relying on a single short test.
The OurCladding powder feeder for PTAW and laser cladding page describes the available feeder configuration and published feed specifications. Use these details to screen compatibility with the powder and required delivery rate, then verify actual feed stability with the selected material and machine setup. A feeder’s nominal specification is not a substitute for process qualification.
The laser cladding head overview describes the connection between the laser path and powder-delivery component. During troubleshooting, inspect the drawing and instructions for the actual head and nozzle; coaxial and side-feed designs do not share identical gas paths, powder trajectories or adjustment requirements.
Optimise the process as a group of linked variables
Use controlled trials to adjust laser power, scanning speed, powder feed rate, spot size, stand-off and track overlap. Change one variable at a time where practical and record the result. Confirm that the powder is fully melted and that the melt pool remelts the previous track or substrate as required, while avoiding conditions that create an unstable vapour depression. Establish the process window for the actual alloy, powder, cladding head and component geometry.
For gas-related checks, distinguish carrier-gas delivery from shielding coverage and consult the instructions for the specific cladding head. The OurCladding guide to laser cladding gas selection explains the different functions of powder-transport gas and local shielding; use the actual machine gas-path diagram when checking flow.
Validate the coating before releasing the process
Confirm improvements with the inspection method required by the application. Depending on the component, this may include visual examination, metallographic sections, radiography or CT, hardness testing and other mechanical or corrosion tests. Record the powder lot, substrate condition, machine configuration, parameter set, inspection method and acceptance result so that a successful process can be reproduced.
Can Porosity in a Finished Laser-Clad Layer Be Repaired?
Sometimes, but the decision depends on pore or void location, size, distribution, coating thickness, component function and the applicable acceptance criteria. A small isolated indication in a non-critical area may be treated differently from a connected pore network or an incomplete-bonding defect at a highly stressed coating interface. For pressure-containing, fatigue-critical or safety-related components, follow the governing specification and approved repair procedure rather than judging acceptability by appearance alone.
A repair may require removing the affected coating to sound material, preparing the area again and redepositing material with a qualified procedure. Simply remelting the surface does not guarantee that internal pores or interface defects will disappear; it can also change dilution, microstructure and residual stress. Reinspect the repaired area using the method required by the component specification.
Need an Automated Laser Cladding System for Your Application?
Porosity control depends on more than the laser source. Powder feeding, cladding-head and nozzle design, motion control, workholding and process parameters must be matched to the material and component. OurCladding supplies automated laser cladding equipment and powder-feeding solutions for industrial surface repair and wear protection.
Explore the OurCladding automated laser cladding machine and review whether the laser source, powder feeder, cladding head, motion system and workholding configuration suit your substrate, powder grade, component dimensions and coating requirements. For an application review, contact the OurCladding technical team with the workpiece drawing, base material, powder specification and target coating thickness.
Frequently Asked Questions
What is the main cause of porosity in laser cladding?
There is no single cause. Gas may enter with the powder or remain trapped in the melt pool, while insufficient melting can create lack-of-fusion defects. The likely cause depends on the indication’s morphology, location, distribution and the material/process combination.
Can moisture in metal powder cause porosity?
Moisture can contribute to gas-related defects in some powder and alloy systems, but it is not the explanation for every pore. Follow the powder supplier’s storage and drying instructions, and distinguish moisture-related gas from internal pores already present in the powder.
Does increasing laser power reduce porosity?
It may reduce lack of fusion when effective energy input is insufficient, but excessive local energy density can create an unstable vapour depression in some conditions. Power must be optimised together with scanning speed, powder feed rate, spot size, stand-off and the cladding-head setup.
Can shielding gas cause pores in laser cladding?
Gas-flow conditions can influence powder transport and the atmosphere around the melt pool, but carrier gas and shielding gas have different functions. Do not blame shielding gas automatically. Check the actual gas paths, flow stability, nozzle geometry, powder condition and defect location before changing gas settings.
What is the difference between gas pores and lack-of-fusion defects?
Gas pores are gas-filled voids, commonly rounded and found within the deposited material. Lack-of-fusion defects are caused by incomplete melting or bonding and are more often irregular or elongated at track, layer or substrate interfaces. They may look like voids in a cross-section, but they are not the same defect mechanism. Shape and location are clues, not a definitive diagnosis.
How do you detect internal pores in a laser-clad coating?
Metallographic cross-sections reveal local pore shape, track overlap and interface condition. X-ray radiography or computed tomography can investigate internal defects, depending on component geometry, material attenuation and the required detection resolution.
Technical References
The studies below support the defect mechanisms and diagnostic points discussed in this article. Experimental findings are specific to the materials, equipment and conditions reported by each research team; they should not be treated as universal machine settings.
Zhang, K., Chen, Y., Marussi, S., et al. (2024). “Pore evolution mechanisms during directed energy deposition additive manufacturing.” Nature Communications, 15, 1715. https://doi.org/10.1038/s41467-024-45913-9
Mo, B., Li, T., Deng, L., Shi, F., Liu, W., & Zhang, H. (2024). “Mechanisms and influencing factors of defect formations during laser-based directed energy deposition with coaxial powder feeding: a review.” Virtual and Physical Prototyping, 19(1), e2404155. https://doi.org/10.1080/17452759.2024.2404155
Cheng, J., Xing, Y., Dong, E., et al. (2022). “An Overview of Laser Metal Deposition for Cladding: Defect Formation Mechanisms, Defect Suppression Methods and Performance Improvements of Laser-Cladded Layers.” Materials, 15(16), 5522. https://doi.org/10.3390/ma15165522
Wang, A., Wei, Q., Tang, Z., et al. (2023). “Effects of processing parameters on pore defects in blue laser directed energy deposition of aluminum by in and ex situ observation.” Journal of Materials Processing Technology, 319, 118068. https://doi.org/10.1016/j.jmatprotec.2023.118068
Revilla, R. I., & De Graeve, I. (2022). “Microstructural Features, Defects, and Corrosion Behaviour of 316L Stainless Steel Clads Deposited on Wrought Material by Powder- and Laser-Based Direct Energy Deposition with Relevance to Repair Applications.” Materials, 15(20), 7181. https://doi.org/10.3390/ma15207181
Post time: Nov-13-2026