Standardized Laser Metal Deposition Protocol For Tribological Evaluation Of Ceramic Modified TI6Al4V Matrix Composites

Summary

This protocol standardizes laser metal deposition for fabricating ceramic modified TI₆Al₄V matrix composites reinforced with B₄C and BN. It integrates controlled deposition, microstructural characterization, hardness testing, and dry sliding wear evaluation to systematically correlate laser energy input with reinforcement retention and tribological performance.

Abstract

This protocol outlines a laser metal deposition method for creating modified titanium-6Aluminum-4Vanadium (TI₆Al₄V) matrix composites reinforced with boron carbide and boron nitride to enhance hardness and wear resistance. The approach involves promoting in situ phase formation while retaining partially unmelted ceramic particles for effective reinforcement. The process integrates controlled laser-material interaction to enable chemical reactions and preserve ceramic phases within the titanium matrix. Composite fabrication is achieved using a directed energy deposition (DED) laser metal deposition (LMD) system with a coaxial powder feeding mechanism for uniform powder delivery and enhanced melt pool stability. This method allows for tailored microstructural evolution through reaction-driven reinforcement and particulate strengthening. In contrast to traditional surface engineering techniques, this method offers metallurgical bonding, compositional control during deposition, and microstructural manipulation through energy input adjustment. Composite layers were produced at 1400 W and 2000 W with constant scanning speed and powder feed rate. Microhardness (HV_0.5) and ASTM G99 dry sliding wear tests were conducted at loads of 15 N and 25 N. Samples produced at 1400 W exhibited higher average hardness and lower wear volume than those produced at 2000 W. The protocol establishes a standard systematic approach for correlating laser energy density with reinforcement retention, microstructure development, and tribological performance.

Introduction

Additive manufacturing is a powerful method for creating titanium-based alloys and composites with customized microstructures and mechanical properties^1,2. Laser metal deposition (LMD) and selective laser metal deposition (SLMD) are directed-energy deposition processes that enable the precise delivery of metallic or composite powders into a laser-generated melt pool for layer-by-layer fabrication and bonding to the substrate. Titanium–6Aluminum–4Vanadium (TI₆Al₄V) remains one of the most widely used titanium alloys in aerospace, biomedical, and industrial applications due to its high specific strength, corrosion resistance, and biocompatibility^3–5. However, despite its favorable bulk mechanical properties, TI₆Al₄V exhibits moderate hardness and relatively poor wear resistance under sliding contact conditions^6,7.

Previous investigations using laser metal deposition and related directed energy deposition processes have demonstrated that laser processing parameters strongly influence microstructure, phase formation, and mechanical behavior^8,9. Surface engineering strategies, including laser surface treatment¹⁰, nitriding, coatings, and conventional laser cladding^{11,12,13,14,15,16}, have also been employed to improve surface hardness and wear resistance. However, diffusion-based treatments typically yield thin, modified layers with limited load-bearing capacity, whereas mechanically bonded coatings may be susceptible to cracking or delamination under high contact stresses. In addition, many laser processing studies report improved hardness or wear performance without providing detailed procedural guidance that links laser parameters to reinforcement retention, phase evolution, and tribological response. These limitations restrict its use in components subjected to severe friction, repetitive contact loading, or abrasive environments¹⁷.

LMD offers advantages over powder bed systems, such as flexibility in feedstock composition, localized reinforcement addition, and real-time energy input control, making it ideal for creating functionally modified surfaces and metal matrix composites^{18,19,20,21,22,23}. Figure 1 illustrates a laser metal deposition (LMD) Process Overview showing coaxial powder delivery, melt pool formation, clad zone, fusion zone, and heat-affected zone with 50% overlapping tracks. The protocol presented here addresses this gap by providing a reproducible method for fabricating modified titanium matrix composites using Laser Metal Deposition and systematically correlating laser energy input with microstructural development and wear behavior. The workflow integrates powder preparation, laser deposition, microstructural characterization, hardness testing, and tribological evaluation. By controlling laser power during deposition, the method enables adjustment of melt pool dynamics, dilution, reinforcement decomposition, and in situ. phase formation, which collectively influence hardness and wear performance. Figure 2 illustrates dominant wear mechanisms typically observed in monolithic TI₆Al₄V, providing context for evaluating reinforced systems.

Modified reinforcement strategies have shown promise for enhancing the mechanical and tribological properties of titanium alloys by combining multiple ceramic phases^23,24,25. In this protocol, boron carbide (B₄C) and boron nitride (BN) are incorporated as reinforcements during laser metal deposition. Boron carbide provides high intrinsic hardness and promotes in situ titanium carbide (TiC) formation during laser processing, while boron nitride contributes to titanium nitride (TiN) formation and may impart solid lubrication effects during sliding^26,27,28. Compared with nitriding and conventional coatings, the present laser-based approach produces a metallurgically bonded reinforced layer rather than a diffusion-limited or mechanically adhered surface film. Unlike traditional cladding processes, reinforcement incorporation and phase formation occur in situ. and can be controlled directly by adjusting laser power, allowing systematic evaluation of microstructural evolution and tribological response^28,29. Laser power selection is particularly critical, as it governs melt pool temperature, reinforcement dissolution, porosity formation, and residual stress development^30,31. The simultaneous introduction of these reinforcements enables combined strengthening and friction modification within the deposited composite layer^32,33,34.

The overall goal of this protocol is to enable researchers and industry to fabricate modified TI₆Al₄V matrix composites using laser metal deposition and to quantitatively evaluate how controlled variations in laser power influence microstructure, hardness, and wear behavior. This method is appropriate for researchers seeking to improve the surface performance of titanium alloys for aerospace components, biomedical implants, and high-load sliding systems that require enhanced hardness and wear resistance.

Protocol

1. Material preparation (Table of Materials)

1. Prepare TI₆Al₄V Substrates
   1. Cut TI₆Al₄V plates to dimensions of 102 mm × 102 mm × 7 mm using a precision abrasive cutting machine.
   2. Grind the substrate surfaces sequentially using silicon carbide (SiC) papers of 240, 400, 600, and 800 grit under running water to remove oxides and machining marks.
   3. Sandblast the surfaces using 120 µm alumina particles at 0.4 MPa for 60 s to increase surface roughness.
   4. Clean the substrates in acetone for 10 min using ultrasonic agitation.
   5. Dry the substrates with compressed air.
   6. Store the cleaned substrates in a desiccator until deposition.
2. Prepare hybrid composite powder (Three powder feeding strategy)
   1. Weigh gas atomized TI₆Al₄V powder (15–53 µm), boron carbide (B₄C) powder (5–20 µm), and hexagonal boron nitride (BN) powder (1–10 µm) according to the desired weight fraction.
   2. Attach a third-hopper feeder system.
   3. Load the blended composite powder into a three-hopper feeder system.
   4. Set the total powder feed rate to 10 g/min for deposition.

2. Fabricate a composite using laser metal deposition

CAUTION: Laser metal deposition uses high-power laser radiation and molten metal. Operators must wear laser safety goggles and protective clothing. Operate the system in a controlled environment with proper ventilation.

1. Configure the laser metal deposition system
   1. Use a laser metal deposition (LMD) system equipped with a high-power fiber laser and a coaxial powder delivery nozzle.
   2. Mount the TI₆Al₄V substrate securely on the build platform.
   3. Establish argon shielding at 15 L/min to prevent oxidation during processing.
   4. Set the laser spot diameter to 2.5 mm.
   5. Set the scanning speed to 1.0 m/min.
2. Deposit composite layer
   1. Set the laser power to 1400 W.
   2. Deliver the composite powder at a rate of 10 g/min through three hopper coaxial feeders.
   3. Apply a cladding layer with a 50% overlap between tracks.
   4. Maintain constant standoff distance and consistent powder flow throughout deposition.
   5. Allow the deposited samples to cool naturally to room temperature under continuous argon shielding.

3. Prepare metallographic samples

CAUTION: Hydrofluoric acid (HF) is highly corrosive and toxic, and all waste containing HF must be neutralized and disposed of according to institutional hazardous chemical safety protocols.

1. Perform all etching procedures inside a certified chemical fume hood.
   1. Wear acid-resistant gloves, a face shield, and a lab coat during handling.
   2. Immerse the polished sample in the reagent for 10–15 s.
   3. Rinse immediately with distilled water.
   4. Dry using warm air.
2. Section and mount samples
   1. Section the fabricated samples perpendicular to the cladding direction using a precision cutter.
   2. Mount the sections in conductive mounting resin.
3. Grind and polish samples
   1. Grind the mounted samples sequentially using 240, 400, 600, 800, and 1200 grit SiC papers.
   2. Polish the samples using 6 µm, 3 µm, and 1 µm diamond suspensions.
   3. Rinse the polished samples with ethanol.
   4. Dry the samples using warm air.
4. Etch samples
   1. Prepare Kroll’s reagent by mixing 2 mL of hydrofluoric acid, 6 mL of nitric acid, and 92 mL of distilled water.

4. Perform microstructural and phase characterization

1. Conduct optical microscopy.
   1. Examine the etched samples using an optical microscope.
   2. Identify the cladding zone, fusion zone, and heat-affected zone.
2. Perform scanning electron microscopy (SEM)
   1. Operate the scanning electron microscope at an accelerating voltage of 15–20 kV.
   2. Analyze reinforcement distribution, interfacial bonding, and microstructural features.
3. Conduct X-ray diffraction (XRD)
   1. Perform XRD using Cu-Kα radiation (λ = 1.5406 Å).
   2. Scan over a 2θ range of 20°–90° with a step size of 0.02°.
   3. Identify phases using standard diffraction databases.

5. Measure microhardness

1. Calibrate the Vickers microhardness tester prior to testing.
2. Perform Vickers hardness testing using a 500 g load applied for 15 s. Measure at five equidistant points across the clad cross-section.
3. Perform at least five indentations along the cladding center line.
4. Maintain a minimum spacing of three times the indentation diagonal between adjacent indents.
5. Calculate and report the average hardness value.

6. Perform dry sliding wear testing

1. Use a reciprocating ball-on-disc tribometer.
2. Select a 6 mm diameter tungsten carbide counter body.
3. Apply a normal load of 15 N.
4. Set the sliding speed to 0.1 m/s.
5. Maintain a wear track radius of 5 mm.
6. Conduct the test for a total sliding distance of 1000 m under dry conditions at room temperature.
7. Record the coefficient of friction continuously during testing.
8. Measure wear track dimensions using SEM and surface profilometry.
9. Calculate wear volume and wear rate using geometric relations.

Results

Microstructural characterization
Laser metal deposition produced continuous cladding layers metallurgically bonded to the TI₆Al₄V substrates at both 1400 W and 2000 W. Figure 3 shows a representative SEM micrograph of the hybrid TI₆Al₄V+ B₄C + BN composite fabricated at 2000 W. Distinct cladding, fusion, and heat-affected zones (HAZ) were observed. The HAZ exhibited acicular martensitic structures. Reinforcement particles were ...

Discussion

This protocol demonstrates the influence of laser power on microstructure, phase formation, hardness, and wear performance in modified TI₆Al₄V composites reinforced with boron carbide and boron nitride. The specimen fabricated at 1400 W exhibited an average hardness of 572.0 ± SD HV_0.5 (n = 5), compared to 538.2 ± SD HV_0.5 (n = 5) at 2000 W, corresponding to approximately a 6% increase in hardness at the lower laser power. Both reinforced coatings significantly exceed...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ti-6Al-4V Alloy (Substrate)	Ti–6 wt.% Al–4 wt.% V	Plate (102 × 102 × 7 mm)	Base substrate for laser metal deposition
Ti-6Al-4V Powder	Gas-atomized Ti–6Al–4V	15–53 µm powder	Matrix material for composite cladding
Boron Carbide (B4C)	B4C ceramic	5–20 µm powder	Primary hard reinforcement; enhances hardness and wear resistance
Hexagonal Boron Nitride (BN)	h-BN ceramic	1–10 µm powder	Secondary reinforcement; grain refinement and wear reduction
Composite Powder Blend	3.8 wt.% Ti-6Al-4V + 0.1 wt.% B4C + 0.1 wt.% BN	Homogeneously blended powder	Feedstock for hybrid TI6Al4V matrix composite
Shielding / Carrier Gas	Argon (99.99% purity)	Gas	Prevents oxidation during laser deposition
Counter-body Material	Tungsten Carbide (WC)	6 mm diameter ball	Counterface for dry sliding wear tests
Abrasive Media	Silicon Carbide (SiC)	240–1200 grit papers	Grinding and surface preparation
Sandblasting Media	Alumina (Al2O3)	~120 µm particles	Surface roughening prior to deposition
Process Control Agent	Ethanol	Liquid	Prevents powder agglomeration during ball milling
Etchant	Kroll’s Reagent (HF + HNO3 + H2O)	Chemical solution	Reveals microstructure for metallographic analysis