1. Bibliographic Information

1.1. Title

Improving surface quality of LDED thin-wall Ti-6Al-4V alloy with ultralow influence on superficial layer via femtosecond laser polishing

1.2. Authors

• Li Zhang (Zhejiang University; Harbin Institute of Technology)
• Wentai Ouyang (Zhejiang University)
• Shuowen Zhang (Zhejiang University)
• Xiaoxiao Chen (Zhejiang University)
• Chunhai Guo (Zhejiang University)
• Rujia Wang (Zhejiang University)
• Xiaoming Duan (Harbin Institute of Technology)
• Xiaodong Yang (Harbin Institute of Technology)
• Wenwu Zhang (Ningbo Institute of Materials Technology and Engineering, CAS)
• Liyuan Sheng (PKU-HKUST Shenzhen-Hongkong Institution)

1.3. Journal/Conference

Journal of Materials Processing Technology

• Comment: This is a highly reputable and influential journal in the field of manufacturing and materials engineering, specifically focusing on the processing techniques of materials.

1.4. Publication Year

2025 (Published at UTC: 2025-10-24)

1.5. Abstract

This paper addresses the poor surface quality of Ti-6Al-4V alloy components with thin-wall structures manufactured by Laser Direct Energy Deposition (LDED). Conventional polishing methods (mechanical or nanosecond laser) often cause structural deformation or thermal damage due to the material's sensitivity and the component's fragility. The authors propose using Femtosecond Laser Polishing (FLP) as a solution.

• Method: FLP was applied to LDED thin-wall plates and compared with Nanosecond Laser Polishing (NLP).
• Results: FLP reduced surface roughness ($R_a$) from 37.24 μm to 4.97 μm. The oxide layer and Heat Affected Zone (HAZ) were extremely thin (<400 nm and 5 μm, respectively), with no structural deformation. In contrast, NLP caused severe oxidation, cracking, and deformation.
• Conclusion: FLP is established as a viable, high-precision polishing method for critical thin-wall aerospace components.

1.6. Original Source Link

/files/papers/693d96abfab55b0207482a87/paper.pdf (Officially Published)

2. Executive Summary

2.1. Background & Motivation

• The Problem: The "low-altitude economy" (e.g., drones, flying taxis) requires lightweight vehicles. Ti-6Al-4V titanium alloy is ideal due to its high strength-to-weight ratio and corrosion resistance. Laser Direct Energy Deposition (LDED) is an additive manufacturing technique used to build these parts, especially thin-wall structures. However, LDED produces parts with very rough surfaces (melting tracks) that are unsuitable for immediate use.
• Why it's important: Rough surfaces degrade performance (fatigue life, aerodynamics).
• Current Limitations:
  • Mechanical Polishing (Grinding/Milling): Difficult because thin walls vibrate and deform under force. Titanium is also "difficult-to-machine" (sticky, hard).
  • Conventional Laser Polishing (Nanosecond/Continuous Wave): These rely on melting the surface to smooth it. For thin walls, this high heat input causes thermal distortion, deep oxidation, and microcracks upon rapid cooling.
• Innovation: The paper introduces Femtosecond Laser Polishing (FLP). Unlike nanosecond lasers that melt material, femtosecond lasers use ultrashort pulses to "cold ablate" or ionize material, theoretically minimizing heat transfer and preventing damage to the delicate thin-wall structure.

2.2. Main Contributions & Findings

1. Feasibility of FLP: Demonstrated that FLP can effectively polish LDED Ti-6Al-4V thin walls in an air environment, reducing roughness by 86.7% without inert gas protection.

2. Superior Quality vs. NLP:

   • FLP: Created a minimal oxide layer (<400 nm) and negligible Heat Affected Zone (HAZ ~5 μm). No macroscopic deformation.
   • NLP: Resulted in thick oxide layers (>200 μm), deep HAZ (>800 μm), dense surface cracks, and significant structural bending.

3. Mechanism Revelation: The paper clarifies that FLP works by selectively removing peaks via ionization (ablation) with minimal thermal accumulation, whereas NLP relies on a large molten pool that suffers from severe oxidation and shrinkage stress.

4. Performance Verification: Wear tests showed that FLP surfaces have improved wear resistance compared to the as-built state, attributed to the formation of Laser Induced Periodic Surface Structures (LIPSS).

   The following figure (Figure 1 from the original paper) schematically illustrates the core difference: Nanosecond laser polishing (left) induces deep thermal stress and cracks, while Femtosecond laser polishing (right) achieves a "cold" finish with minimal influence.

   该图像是一个示意图，展示了脉冲激光抛光 (FLP) 的过程。FLP 相较于纳秒激光抛光 (NLP)，能有效降低表面粗糙度，减少烧蚀深度，并控制氧化层及热影响区的厚度。该方法展现出在空气环境下抛光 LDED Ti-6Al-4V 薄壁部件的优越性。

3. Prerequisite Knowledge & Related Work

3.1. Foundational Concepts

• LDED (Laser Direct Energy Deposition): An additive manufacturing (3D printing) process where a laser melts metal powder as it is deposited by a nozzle, building a part layer-by-layer. It typically leaves a rough, wavy surface ("melting tracks").
• Ti-6Al-4V: The most common titanium alloy (Titanium with 6% Aluminum and 4% Vanadium). It is strong but chemically reactive (oxidizes easily at high temps) and has low thermal conductivity (heat stays in the cutting zone, causing damage).
• Thin-Wall Structure: Components where the wall thickness is small relative to other dimensions. These are mechanically unstable (prone to vibration/bending) and thermally sensitive (cannot dissipate heat easily).
• Laser Pulse Width (Femtosecond vs. Nanosecond):
  • Nanosecond ($10^{-9}$ s): The pulse is long enough for heat to transfer from electrons to the atomic lattice, causing melting and boiling. It is a thermal process.
  • Femtosecond ($10^{-15}$ s): The pulse is shorter than the time required for heat transfer. Material is ionized into plasma directly (sublimation) before significant heat spreads. It is considered "cold ablation."
• HAZ (Heat Affected Zone): The area of the base material that was not melted but had its microstructure and properties altered by the heat of the process. A large HAZ is usually undesirable as it introduces weakness or brittleness.
• LIPSS (Laser Induced Periodic Surface Structures): Nanoscale ripple patterns that form on a surface treated with ultrashort pulsed lasers (like femtosecond lasers) due to the interference of light waves.

3.2. Previous Works & Technological Evolution

• Mechanical Machining: Traditional milling/grinding causes tool wear and surface defects on Titanium due to its poor machinability.
• Continuous/Nanosecond Laser Polishing: Previous studies (e.g., Xu et al., Obeidi et al.) used these thermal lasers. While they reduce roughness (via remelting and self-leveling), they create deep remelting layers (>70-200 μm). For thin walls, this heat causes residual stress and cracks (as noted in Fig 1c of the paper).
• Femtosecond Laser Applications: Prior work applied FLP to coatings or ceramics (Qian et al., Chen et al.) to achieve high precision. This paper extends FLP to the specific challenge of LDED Ti-6Al-4V thin walls, hypothesizing that its "cold" nature will solve the deformation and cracking issues.

3.3. Differentiation Analysis

• Vs. Mechanical: FLP is non-contact, eliminating tool wear and mechanical force-induced deformation.
• Vs. Nanosecond Laser (NLP): NLP relies on the flow of molten metal (surface tension smoothing). FLP relies on the removal (ablation) of peaks. This paper proves that for thin-wall Ti-6Al-4V, the removal mechanism is superior because it avoids the disastrous oxidation and thermal stress associated with the molten pool of NLP.

4. Methodology

4.1. Principles

The core principle is selective material removal via ultrafast laser ablation. By controlling the laser's focus and energy, the process aims to ablate (vaporize/ionize) the high peaks of the rough LDED surface while leaving the valleys relatively untouched, thereby flattening the surface profile without heating the bulk material.

The following schematic (Figure 18 from the original paper) summarizes the physical mechanism comparison between the two laser types developed in this study:

该图像是示意图，展示了激光扫描过程中的温度和微观结构状态、激光扫描后的应力状态及最终状态。分别比较了纳秒激光和飞秒激光对Ti-6Al-4V合金表面的影响，以及氧化层和热影响区（HAZ）的形成与演变。

4.2. Core Methodology In-depth

The methodology consists of sample preparation, parameter optimization, the polishing process itself, and extensive characterization.

Step 1: LDED Sample Preparation

• Equipment: 4000 W Continuous Wave (CW) fiber laser.
• Material: Ti-6Al-4V powder (spherical, 50-150 μm).
• Process: Thin-wall plates ($130 \times 10 \times 3$ mm) were built. A synchronized induction heating ($600^\circ$C) was used during building to control stress, then cooled to room temperature.

Step 2: Femtosecond Laser Polishing (FLP) Strategy

• Laser Source: HR-Femto-IR-20035 system (1030 nm wavelength, 317 fs pulse width).
• Optimization Strategy: The authors conducted orthogonal experiments to find the best parameters.
  • Defocusing: They found that Positive Defocusing (+5 mm) was superior.
    • Reason: In positive defocusing, the laser beam converges before the surface and diverges as it hits. The "peaks" of the rough surface are physically closer to the focal point (higher energy density) than the "valleys." This naturally creates a higher ablation rate on peaks, flattening the surface.
  • Multi-pass Scanning: A single pass is insufficient. The optimal process involved 20 scanning layers to gradually shave down the roughness.

Key FLP Parameters (Table 3 from paper):

• Laser Energy Density: $3.12 \text{ J/cm}^2$ (Sufficient to ablate but not burn).

• Defocusing Distance: $+5 \text{ mm}$.

• Scanning Speed: $200 \text{ mm/s}$.

• Hatch Distance: $0.01 \text{ mm}$ (Overlap between laser lines).

  The following figure (Figure 5 from the original paper) shows the step-by-step evolution of the surface profile during the 20-layer polishing process:

  该图像是图表，展示了不同层数处理后LDED Ti-6Al-4V合金的表面形貌。图（a）显示原始样品与经过2至20层处理样品的表面轮廓；图（b）为高度变化曲线；图（c）为各层数样品的三维表面图。结果表明，表面粗糙度随处理层数的增加而有所改善。

Step 3: Nanosecond Laser Polishing (NLP) Comparison

To prove the superiority of femtosecond pulses, two NLP control groups were set up:

1. NLP-12: Nanosecond laser at 12 W. This matches the single pulse energy of the FLP setup.
2. NLP-100: Nanosecond laser at 100 W. This uses the maximum power to approximate high-energy polishing, though it cannot match the peak power density of femtosecond pulses.

• Note: Both NLP experiments were conducted in air, identical to the FLP environment, to strictly test the laser-material interaction without gas shielding variables.

Step 4: Wear Rate Calculation

To evaluate the mechanical performance of the polished surface, wear tests were conducted. The authors used the following standard formula to calculate the Wear Rate ($W$):

$$W = \frac { V } { F _ { N } \bullet S }$$

Symbol Explanation:

• $W$: The specific wear rate ($\text{mm}^3 / (\text{N} \cdot \text{m})$).
• $V$: The volume loss ($\text{mm}^3$). This is calculated by measuring the cross-sectional area of the wear track (using a 3D profilometer) and multiplying it by the track length.
• $F_N$: The applied normal load (Newtons). In this experiment, $15 \text{ N}$.
• $S$: The total sliding distance (meters). In this experiment, $180 \text{ m}$.

5. Experimental Setup

5.1. Material & Samples

• Target: LDED Ti-6Al-4V thin-wall plate.
• Initial State: Surface roughness $R_a \approx 37.24 \text{ μm}$. Covered in a native oxide layer from the LDED process.
• Geometry: Thin wall ($3 \text{ mm}$ thick), making it highly susceptible to thermal warping.

5.2. Evaluation Equipment & Metrics

1. Surface Topography:
   • Equipment: Laser Scanning Confocal Microscope (LSCM).
   • Metric: Surface Roughness ($R_a$), which measures the average deviation of the surface height from the mean line.
2. Structural Deformation:
   • Method: Macroscopic imaging and profile contour scanning to detect bending.
3. Microstructure:
   • SEM (Scanning Electron Microscope): To see grain structure and cracks.
   • EBSD (Electron Back-Scatter Diffraction): To analyze crystal orientation, grain boundaries, and phase distribution ($\alpha$ vs $\beta$ Ti).
   • TEM (Transmission Electron Microscope): For nanoscale characterization of the oxide layer and dislocations.
   • XRD (X-Ray Diffraction): To identify phase composition (oxides vs metal) on the surface.
4. Mechanical Properties:
   • Microhardness: Vickers hardness tester (Hv) along the depth of the cross-section.
   • Wear Test: Reciprocating sliding test (Silicon Nitride ball, 15N load, 10Hz, 30 min).

5.3. Baselines

• As-built: The raw LDED surface (Control).
• NLP-12 & NLP-100: Nanosecond laser polishing (Comparison to show why femtosecond is necessary).
• Hot-forged Ti-6Al-4V: Standard industrial material (Reference for wear performance).

6. Results & Analysis

6.1. Surface Topography & Roughness

The FLP method achieved a dramatic improvement in surface quality compared to the baseline and NLP methods.

• As-built: $R_a = 37.24 \text{ μm}$. Very rough melting tracks.

• FLP: $R_a = 4.97 \text{ μm}$. 86.7% reduction. The wavy melting tracks were removed, replaced by fine LIPSS (periodic nanostructures).

• NLP-12: $R_a \approx 35 \text{ μm}$ (No significant improvement).

• NLP-100: $R_a = 52.91 \text{ μm}$ (Worse than initial). The surface was destroyed by excessive melting and oxidation, forming large irregular bulges.

  The following figure (Figure 4 from original paper) visually compares the surface topographies. Note the smoothness of FLP (b) versus the chaotic, cracked surfaces of NLP (c, d).

  该图像是插图，展示了用于提升LDED Ti-6Al-4V合金表面质量的飞秒激光抛光和纳秒激光抛光系统的对比。图中左侧（a和b）为飞秒激光抛光系统与工艺原理，中间（c）为抛光前后表面的对比图，右侧（d）展示了纳秒激光抛光设备。图表（e、f、g）则展示了不同能量密度、扫描速度及聚焦距离对表面粗糙度影响的实验数据和分析，图中关键线条标注了不同测试线（A、B、C）与“as-built”状态。

The following figure (Figure 2 from original paper) provides a clear bar chart summary of the roughness reduction.

该图像是插图，展示了 femtosecond 激光抛光 (FLP) 对 LDED Ti-6Al-4V 薄壁合金表面粗糙度的改善效果。通过选择性去除熔化轨迹的峰区域，FLP 将表面粗糙度降至 4.97 μm，相比之下，常规纳秒激光抛光 (NLP) 导致表面出现密集裂纹和较差的表面质量。

6.2. Structural Deformation

• FLP: No macroscopic deformation observed. The thin-wall plate remained straight.

• NLP: Significant bending deformation occurred. The thermal stress from the large molten pool and rapid cooling caused the thin wall to warp.

  The following figure (Figure 6 from original paper) illustrates the deformation. Notice the significant curvature in the blue and green lines (NLP) compared to the flat red line (FLP).

  该图像是图表，展示了不同激光处理方法下的弯曲变形分析。上部分为不同处理状态的宏观形态，包括原始状态（As-built）、超快激光抛光（FLP）、和纳秒激光抛光的两种参数（NLP-12 和 NLP-100）。下部分为Z位移与距离的关系曲线，比较了这几种状态在相同条件下的变形差异。

6.3. Microstructure & Oxidation Analysis

This is the most critical technical finding. The authors used SEM, EBSD, and TEM to analyze the depth of laser influence.

6.3.1. Oxide Layer & HAZ Depth

• FLP Results:
  • Oxide Layer: Extremely thin, limited to the outer 400 nm (nanometers).
  • HAZ (Heat Affected Zone): Only 5 μm deep.
  • Implication: The bulk material properties are preserved. The laser influence is strictly "skin-deep."
• NLP Results:

  • Oxide Layer: Very thick (>200 μm for NLP-100). Composed of brittle Ti-oxides and Al-oxides.

  • HAZ: Deep penetration (>200 μm for NLP-12, >800 μm for NLP-100).

  • Implication: Severe material degradation. The thick oxide layer is brittle and cracks easily (as seen in SEM images).

    The following figure (Figure 13 from original paper) shows the TEM analysis of the FLP sample. Note the scale bar in (c) is 200 nm, showing the incredibly thin oxide layer.

    该图像是多幅显微图，展示了 LDED Ti-6Al-4V 合金的不同结构特征，包括再熔层、热影响区以及基体等。图中显示了不同区域的相变情况，特别是氢氧化物的分布与组织结构。还可见厚度为 65 μm 的再熔层，和相应的细节图（如图 d 和 e 所示）。

6.3.2. Phase & Crystal Structure

• FLP: Retained the original Basket-weave microstructure ($\alpha'$ phase). The rapid interaction prevented significant phase transformation or grain growth.

• NLP: Induced Columnar grain growth vertical to the surface. The heat allowed grains to grow and re-orient. High density of dislocations (GNDs) accumulated at grain boundaries due to thermal stress, leading to cracking.

  The following table (Table 4 from the original paper) presents the surface elemental composition, confirming the oxidation levels. Note the high Oxygen (O) in As-built and NLP samples vs. lower O in FLP.

Table 4: Element content on the surface of different samples (at%)

6.4. Mechanical Performance (Wear & Hardness)

• Microhardness:
  • FLP: Hardness profile remained consistent with the base material (~460 Hv), confirming no thermal softening or hardening deep in the material.
  • NLP: Showed extreme variations. The surface was very hard (due to brittle oxides) but prone to failure.
• Wear Resistance:

  • FLP: Showed the lowest friction coefficient (started at 0.12) and stable wear behavior.

  • Reason: The LIPSS (nanostructures) acted as a micro-texture to reduce friction, and the removal of the brittle oxide layer prevented abrasive wear debris.

  • NLP: High friction and wear rates due to the surface cracking and spalling of the thick oxide layer.

    The following figure (Figure 17 from original paper) displays the wear test results, including friction coefficients (a) and wear track profiles (b).

    该图像是包含多个图表和显微图的组合，展示了不同抛光方法（手动抛光、FLP、NLP-12、NLP-100、热锻）对LDED Ti-6Al-4V合金表面质量的影响。图中包括摩擦系数随时间变化、截面剖面、磨损率以及抛光后的表面形貌和成分分布。实验结果显示FLP在改善表面粗糙度方面具有明显优势，且热影响区和氧化层较小。

7. Conclusion & Reflections

7.1. Conclusion Summary

This paper successfully establishes Femtosecond Laser Polishing (FLP) as a superior method for post-processing LDED Ti-6Al-4V thin-wall components.

1. Effective Roughness Reduction: Achieved an 86.7% reduction in roughness ($R_a$ 37.24 $\to$ 4.97 μm).
2. Ultralow Thermal Influence: By utilizing the "cold ablation" mechanism of femtosecond pulses, the process limited the oxide layer to <400 nm and the HAZ to 5 μm.
3. Structural Integrity: Completely avoided the warping and cracking that plagues conventional Nanosecond Laser Polishing (NLP).
4. Mechanism: FLP works by ionizing and removing material peaks, whereas NLP relies on melting, which introduces fatal thermal defects in thin-wall reactive alloys.

7.2. Limitations & Future Work

• Efficiency: The paper notes that FLP is a layer-by-layer removal process involving 20 scans. While precise, the removal rate is relatively low compared to bulk melting methods. This might be a bottleneck for mass production.
• Future Directions: The authors suggest further research into the fatigue performance of these polished components. While wear was tested, fatigue is critical for aerospace parts, and the residual stresses (even if low) need to be quantified in that context.

7.3. Personal Insights & Critique

• Innovation: The application of positive defocusing to preferentially ablate peaks is a clever, physically grounded strategy that maximizes the efficiency of the FLP process.
• Rigor: The comparison with NLP is excellent. By matching single pulse energy (NLP-12) and maximizing power (NLP-100), the authors rigorously isolated the effect of pulse width (time) as the decisive factor.
• Applicability: This research is highly relevant for the aerospace industry. As drones and EVTOLs (Electric Vertical Take-off and Landing aircraft) become more common, the need for lightweight, high-quality titanium parts will surge. This method solves a specific "last-mile" manufacturing problem for these parts.
• Critique: The paper claims "ultralow influence," which is true compared to NLP, but a 5 μm HAZ still exists. For extremely fatigue-sensitive parts, even this minor modification might need post-processing (e.g., etching). Additionally, the cost of femtosecond laser equipment is significantly higher than nanosecond lasers, which is a practical barrier not discussed in depth.