Titanium alloys are widely used in aerospace, marine engineering, automotive, medical, and other high-performance applications because of their low density, high specific strength, and good corrosion resistance. However, relatively poor wear resistance can limit their service life when surfaces are exposed to abrasion, particles, or repeated contact.

Laser cladding provides a way to modify titanium alloy surfaces while maintaining metallurgical bonding with the substrate. A particularly interesting approach is to use nitrogen as a reactive shielding gas during titanium alloy laser cladding. Under suitable processing conditions, nitrogen can enter the molten titanium alloy and promote the in-situ formation of titanium nitrides.

Experimental research using TC4 (Ti6Al4V) powder on TC27 titanium alloy demonstrates this effect. Compared with argon-shielded cladding, nitrogen-shielded coatings developed nitrogen-rich structures and titanium nitride phases, achieved microhardness above 600 HV0.3, and showed substantially improved wet abrasive wear resistance.

Why Do Titanium Alloys Need Surface Wear Protection?

Advantages of Titanium Alloys

Titanium alloys combine relatively low density with high specific strength and good corrosion resistance. These characteristics make them attractive where weight reduction and mechanical performance are both important.

Their applications extend across aerospace, automotive, medical, defense, and marine engineering industries.

The Wear Resistance Limitation of Titanium Alloys

Despite these advantages, titanium alloys can suffer from poor wear resistance. This becomes particularly important when components operate in abrasive environments.

Wet abrasive wear is especially relevant to equipment used in industries such as oil and gas, chemical processing, and marine engineering, where hard particles and liquids may act together on the component surface.

Improving the surface hardness without replacing the entire titanium component therefore has considerable engineering value.

What Is Titanium Alloy Laser Cladding?

Titanium alloy laser cladding uses a concentrated laser beam to melt deposited alloy powder together with a shallow region of the substrate. The molten material rapidly solidifies and forms a coating metallurgically bonded to the base metal.

Compared with many conventional surface modification processes, laser cladding offers precise energy control, a relatively small heat-affected zone, limited workpiece deformation, and considerable flexibility in selecting the deposited material.

For titanium components, the process can therefore be used to modify the surface while retaining the properties of the underlying substrate.

Why Is Titanium Alloy Laser Cladding Challenging?

High Chemical Activity of Titanium

Titanium is chemically active at elevated temperatures. During laser cladding, the molten pool and heated powder are exposed to temperatures at which interaction with the surrounding atmosphere becomes important.

Atmosphere control is therefore a critical part of titanium laser processing.

Shielding Gas Selection

Argon is commonly used to provide an inert atmosphere during titanium alloy laser cladding. However, shielding gas does not necessarily have to act only as an inert barrier.

The experimental work discussed here demonstrates that nitrogen can deliberately participate in the cladding process and alter the resulting coating composition and microstructure.

Rapid Solidification and Phase Transformation

Laser cladding involves rapid heating followed by rapid cooling. These thermal conditions can produce phase transformations that differ from those obtained through conventional manufacturing processes.

In argon-shielded TC4 cladding, for example, the experimental coating contained β phase together with directional needle-like α′ martensitic structures.

Conventional Argon-Shielded Titanium Laser Cladding

One common strategy for improving the wear resistance of titanium alloys is to use argon shielding while introducing hard reinforcement materials into the cladding powder.

Previous approaches have investigated WC, SiC, B4C, cBN and other reinforcement systems. During processing, these materials may remain as hard particles or participate in reactions that generate carbides, borides, nitrides, silicides, or intermetallic phases.

The resulting metal matrix composite coatings can significantly improve wear resistance. However, they generally depend on additional reinforcement materials being incorporated into the feedstock.

A Different Approach: Using Nitrogen to Generate Hard Phases In Situ

Nitrogen-shielded laser cladding offers another route.

Instead of relying only on externally added hard particles, nitrogen surrounding the laser processing zone can interact with molten titanium. Under suitable conditions, nitrogen enters the molten alloy and participates in the formation of titanium nitride phases during solidification.

The process can be summarized as:

Titanium alloy powder + nitrogen atmosphere + laser energy → nitrogen-containing molten pool → in-situ titanium nitride formation → harder cladding layer

This makes the shielding atmosphere an active part of material formation.

Materials Used in the Titanium Laser Cladding Experiment

TC27 Titanium Alloy Substrate

The experimental substrate was TC27 titanium alloy:

Ti-5Al-4Mo-6V-2Nb-1Fe

The specimens had a diameter of 135 mm and a thickness of 10 mm.

TC4 (Ti6Al4V) Powder

The first cladding material was commercially available spherical TC4 powder.

The powder had:

• Particle size: 45–100 μm
• Apparent density: 2.49 g/cm³

TC4 is essentially a Ti6Al4V alloy system and was used as the principal cladding material.

TC4 + 4% Mo Powder

A second feedstock was prepared by mixing:

96 wt.% TC4 + 4 wt.% pure Mo

The Mo powder had a particle size below 55 μm and a stated purity of 99.8%.

This second material allowed the influence of a small Mo addition to be evaluated under both argon and nitrogen shielding conditions.

Laser Cladding Equipment Used in the Experiment

6 kW Fiber Laser

The system used a 6 kW multimode continuous-wave fiber laser with a wavelength of 1070 nm.

The actual experimental laser power was lower than the rated laser capacity.

Robotic Laser Cladding System

A six-axis industrial robot moved the laser cladding head. Robotic motion provides the flexibility required to control the position and orientation of the processing head relative to the workpiece.

Powder Feeding System

Powder was supplied using a rotary quantitative pneumatic powder feeder. Argon served as the powder carrier gas during all four experimental conditions.

Laser Cladding Head

The system used a cladding head based on internal focusing, reflective laser output, and annular powder delivery. The head configuration was selected with internal-bore laser cladding requirements in mind.

An important feature of the setup was control of the relationship between the laser beam, powder convergence position, workpiece surface, and shielding gas region.

How Nitrogen Is Introduced into the Laser Cladding Process

Laser Beam and Powder Convergence

The distance between the nozzle and the cladding surface was adjusted so that the powder convergence point remained at an appropriate position within the effective laser processing region.

This positioning affects both powder melting and interaction between the molten material and the surrounding gas.

Shielding Gas Column

For nitrogen-shielded experiments, the powder convergence region was positioned within the nitrogen gas column.

This created an environment in which the heated and molten titanium alloy powder could interact directly with nitrogen.

Interaction Between Nitrogen and Molten Titanium

Under the experimental conditions, nitrogen entered the molten titanium alloy. During subsequent cooling and solidification, nitrogen-rich regions developed into characteristic dendritic structures.

The study also proposed that nitrogen molecules within the laser interaction region may dissociate, facilitating nitrogen incorporation into the heated and molten titanium alloy.

Laser Cladding Process Parameters

Four experimental groups were used while maintaining the same primary laser processing parameters.

The four groups were:

T-A: TC4 powder + argon shielding
T-N: TC4 powder + nitrogen shielding
TM-A: 96% TC4 + 4% Mo + argon shielding
TM-N: 96% TC4 + 4% Mo + nitrogen shielding

These values represent the conditions of this specific experiment and should not be interpreted as universal parameters for titanium laser cladding.

Argon vs Nitrogen Shielding in Titanium Laser Cladding

The comparison revealed that changing the shielding gas affected more than atmospheric protection.

With argon, the TC4 cladding layer exhibited microstructural characteristics associated with rapid solidification of the titanium alloy.

With nitrogen, nitrogen was incorporated into the molten material and nitrogen-containing phases formed during solidification.

The result was a substantial difference in microstructure, hardness, and abrasive wear performance.

Microstructure of Argon-Shielded TC4 Laser Cladding

β Phase and α′ Martensite

The argon-shielded T-A coating contained β phase and clearly directional needle-like α′ structures.

The α′ phase is associated with martensitic transformation during rapid cooling. The observed microstructure therefore reflects the high cooling rates characteristic of laser cladding.

Microstructure of Nitrogen-Shielded TC4 Laser Cladding

Dendritic Microstructure

When nitrogen replaced argon as the shielding gas, the coating microstructure changed significantly.

The T-N sample exhibited obvious dendritic crystallization rather than the pronounced needle-like α′ structure observed in the argon-shielded TC4 coating.

Nitrogen Enrichment

SEM-EDS analysis confirmed substantial nitrogen within the dendritic regions.

One analyzed region of the T-N coating contained a nitrogen atomic fraction of 22.87%, demonstrating that nitrogen had entered the material during laser processing.

In-Situ Formation of Titanium Nitride During Laser Cladding

Formation of Ti₄N₃−x

X-ray diffraction analysis identified Ti₄N₃−x in the nitrogen-shielded coating.

This provides direct evidence that nitrogen was not functioning only as an external shielding medium. It participated in the material-forming process and contributed to the generation of titanium nitride within the cladding layer.

Nitrogen-Rich Dendritic Structure

A similar effect occurred when 4% Mo was added to the TC4 powder.

The nitrogen-shielded TM-N sample also developed dendritic structures. One EDS measurement of this region showed:

Nitrogen: 32.91 at.%
Titanium: 64.06 at.%

XRD analysis again identified Ti₄N₃−x.

The results therefore show in-situ nitride formation in both the TC4 and TC4 + Mo material systems under the tested nitrogen-shielded conditions.

Effect of Mo Addition on Titanium Laser Cladding

Mo is a β-phase stabilizing element in titanium alloys.

After adding 4% Mo, the argon-shielded TM-A coating did not show the same obvious needle-like α′ transformation observed in the T-A coating.

The wear tests also indicated that the small Mo addition could reduce material loss under the tested conditions. However, the most significant improvement in hardness and abrasive wear resistance remained associated with nitrogen shielding and nitride formation.

How Nitrogen Shielding Changes Laser Cladding Hardness

The microhardness results clearly demonstrate the effect.

Both nitrogen-shielded coatings exceeded 600 HV0.3, substantially higher than their corresponding argon-shielded coatings.

The nitrogen-shielded coatings also reached approximately 50 HRC in macroscopic hardness testing.

Why Does Nitrogen Increase Titanium Cladding Hardness?

Titanium Nitride Hard Phase Formation

The first important mechanism is the in-situ formation of titanium nitride.

Ti₄N₃−x was detected in the nitrogen-shielded coatings, and the nitrogen-rich dendritic regions were associated with the higher measured hardness.

Interstitial Strengthening by Nitrogen

Nitrogen incorporated into titanium can also act as an interstitial solute.

The study notes an atomic-radius ratio of nitrogen to titanium of approximately:

0.70 / 1.32 = 0.55

Because this value is below 0.59, dissolved nitrogen can occupy interstitial positions in the titanium alloy and contribute to increased hardness.

The measured hardness is therefore associated with both nitrogen-containing phase formation and nitrogen dissolved within the titanium alloy.

Wet Sand Abrasive Wear Testing

Why Wet Abrasive Wear Matters

Wear resistance depends strongly on the actual wear mechanism and service environment.

For components exposed to mixtures of liquids and abrasive particles, conventional dry sliding tests do not necessarily represent operating conditions. Wet sand abrasive wear is particularly relevant to some oil and gas, chemical, and marine applications.

ASTM G105 Wear Test

The coatings were therefore evaluated using a wet sand-rubber wheel abrasive wear test based on ASTM G105.

The test conditions included:

Rubber wheel diameter: 178 mm
Wheel speed: 240 r/min
Applied load: 230 N
Wear distance: 3355 m
Abrasive slurry: 1.5 kg quartz sand + 0.95 kg deionized water

Material loss was determined by weighing the specimens before and after testing.

How Nitrogen Shielding Improves Wear Resistance

Nitrogen-shielded coatings experienced substantially lower wear loss than their corresponding argon-shielded coatings.

For TC4 powder:

T-N / T-A relative wear resistance = 2.6026

For TC4 + 4% Mo powder:

TM-N / TM-A relative wear resistance = 2.3883

Under these specific wet abrasive wear conditions, the nitrogen-shielded TC4 coating therefore provided approximately 2.60 times the relative wear resistance of the argon-shielded coating, while the nitrogen-shielded TC4 + Mo coating achieved approximately 2.39 times the corresponding argon-shielded result.

Relationship Between Microstructure, Hardness and Wear Resistance

The experimental results establish a clear process–structure–property relationship:

Nitrogen shielding → nitrogen incorporation → nitrogen-rich dendritic structure → in-situ Ti₄N₃−x formation → increased hardness → reduced abrasive wear

This relationship is important because it shows that laser cladding performance is controlled not only by laser power or powder composition. The processing atmosphere can directly influence the chemistry and phase composition of the final coating.

Nitrogen vs Argon for Titanium Alloy Laser Cladding

This does not mean nitrogen should universally replace argon in titanium laser cladding. The result applies to the specific material system and processing conditions investigated.

Where chemical stability or avoidance of nitrogen pickup is required, a reactive nitrogen atmosphere may be inappropriate. Shielding gas selection must therefore follow the required coating composition and final performance.

Advantages of In-Situ Nitride Formation

The experimental approach demonstrates an alternative to adding large quantities of external hard ceramic particles.

By using a reactive nitrogen atmosphere, hard titanium nitride phases can form during the laser cladding process itself.

Potential technical advantages include direct formation of reinforcement within the deposited alloy, significant surface hardening, improved abrasive wear resistance, and the ability to modify coating properties through control of the processing atmosphere.

The key concept is in-situ material formation rather than simply depositing a premixed hard-particle coating.

Key Process Factors for Nitrogen-Shielded Titanium Laser Cladding

Laser Power

Laser power determines the thermal energy available for melting the powder and substrate. It must be coordinated with scanning speed, spot size, and powder feed rate.

Scanning Speed

Scanning speed influences interaction time, melt pool behavior, and thermal history. It cannot be optimized independently of laser power.

Powder Feed Rate

The powder feed rate must provide sufficient material while allowing effective melting and stable deposition.

Laser Spot Size

Spot size affects energy distribution and the dimensions of the interaction zone. The experiment used a 3 mm laser spot.

Nitrogen Flow Rate

Nitrogen must provide an effective atmosphere around the interaction region. The experimental shielding flow was 30 L/min, but this value should not be treated as a universal setting for other systems.

Powder-Laser Convergence Position

For reactive nitrogen cladding, the relationship between the powder convergence point, laser beam, and nitrogen gas column is particularly important.

The experimental arrangement positioned the powder interaction region within the nitrogen flow so that heated and molten titanium powder could interact with nitrogen.

Laser Head Stand-Off Distance

Stand-off distance influences laser focus, powder convergence, and gas coverage simultaneously.

For this reason, maintaining a stable nozzle-to-workpiece distance is important not only for deposition geometry but also for controlling the gas–powder–laser interaction.

Why Shielding Gas Can Become a Process Variable

In conventional thinking, shielding gas primarily prevents undesirable reactions between the melt pool and the surrounding atmosphere.

Reactive laser cladding introduces another possibility.

The gas can become part of the material-processing strategy:

Shielding atmosphere → melt pool chemistry → phase formation → microstructure → hardness → wear performance

The nitrogen-shielded titanium experiment is a clear example. Nitrogen was intentionally allowed to interact with molten titanium, resulting in nitrogen-containing hard phases and significantly different coating properties.

Shielding gas should therefore be considered a potential process variable when reactive material formation is the objective.

Potential Applications of Nitrogen-Shielded Titanium Laser Cladding

The approach is particularly relevant to research on titanium components requiring improved resistance to abrasive wear.

Potential engineering directions include titanium components used in oil and gas equipment, chemical processing systems, and marine engineering where abrasive particles may be present in wet environments.

However, the reported experiment evaluated material specimens rather than validating every one of these specific industrial components. Application-specific qualification would still be required.

Limitations of the Experimental Results

The results were obtained using a defined combination of TC27 substrate, TC4 or TC4 + 4% Mo powder, specific laser parameters, powder delivery conditions, and argon or nitrogen shielding.

The reported 2.60× and 2.39× relative wear resistance improvements therefore apply to the tested wet sand abrasive wear conditions.

Wear resistance is not a fixed intrinsic property independent of service conditions. Different loads, abrasive materials, temperatures, counterfaces, or corrosion environments can produce different wear mechanisms.

Similarly, the experimental parameters should not be copied directly to a different titanium alloy, geometry, laser system, or coating thickness without process development.

Practical Lessons for Titanium Alloy Laser Cladding

This study highlights an important principle for developing titanium laser cladding processes: coating performance should be considered as the result of the entire material and process system.

A useful engineering sequence is:

Substrate material → cladding powder → shielding atmosphere → laser parameters → powder/gas interaction → melt pool chemistry → solidification → microstructure → hardness → wear performance

Optimizing only laser power or scanning speed may therefore be insufficient. Powder chemistry, shielding atmosphere, laser head configuration, stand-off distance, and powder convergence must work together.

Future Development of Reactive-Gas Laser Cladding

Reactive-gas laser cladding creates opportunities to use the processing atmosphere as an additional tool for controlling coating composition.

For titanium alloys, further development can focus on controlling nitrogen uptake, regulating nitride content and distribution, optimizing reactive-gas flow, and studying how different titanium alloy compositions respond to nitrogen-assisted processing.

Another important direction is to connect process parameters more precisely with phase formation and performance under specific wear conditions.

This could help move reactive-atmosphere laser cladding from experimental material development toward more application-specific surface engineering.

Conclusion

Nitrogen-shielded laser cladding provides an interesting route for improving the surface hardness and abrasive wear resistance of titanium alloys.

Under the investigated processing conditions, nitrogen entered the molten titanium alloy and produced nitrogen-rich dendritic structures containing Ti₄N₃−x. The nitrogen-shielded TC4 and TC4 + 4% Mo coatings achieved microhardness values above 600 HV0.3, significantly exceeding corresponding argon-shielded coatings.

The improvement was also reflected in wet sand abrasive wear testing. Relative wear resistance reached approximately 2.60 times the argon-shielded TC4 coating and 2.39 times the argon-shielded TC4 + 4% Mo coating.

More importantly, the results demonstrate a broader principle in laser cladding: shielding atmosphere can be more than a protective medium. Under controlled conditions, it can participate directly in melt pool metallurgy and become a tool for engineering coating microstructure and performance.