Influence of Laser Overlay Welding Process Parameters on Microstructure and Properties of Titanium-Based Alloy Layers

Literature Overview

This paper by Luo Fang, Liu Jian, Ye Liangwu, and Yao Jianhua from Zhejiang University of Technology investigates the effects of laser overlay welding process parameters on the microstructure and mechanical properties of titanium-based alloy layers deposited on 45 steel substrates. Published in the journal Tractor and Agricultural Transport Vehicles in 2004 (Volume 31, Issue 5, pages 49-51), the study examines the influence of laser power, scanning speed, and wire feed rate on the resulting overlay layer characteristics. The work addresses the growing interest in laser cladding as a precision surface engineering technique for improving the performance of conventional steel components.

Laser Overlay Welding Process Fundamentals

Laser overlay welding (also known as laser cladding or laser surfacing) is a near-net-shape surface engineering technique that uses a high-power laser beam to melt a small area of the substrate surface while simultaneously feeding alloy powder or wire into the melt pool. The resulting dilution rate between the base metal and the overlay material is typically lower than conventional arc welding processes, resulting in overlay layers with composition closer to the intended alloy design.

The process parameters investigated in this study include:

The 45 steel substrate was selected as a representative medium-carbon steel commonly used in mechanical components. The titanium-based alloy wire was chosen for its potential to provide improved wear resistance, corrosion resistance, or high-temperature performance to the steel surface.

Microstructural Analysis and Phase Evolution

The microstructure of the laser overlay layer is strongly influenced by the rapid solidification conditions created by the laser process. The high cooling rates (typically 100 to 1000 degrees Celsius per second) result in fine grain structures and potentially non-equilibrium phase formation.

The study reveals the following microstructural trends:

Effect of laser power: At constant scanning speed, increasing laser power from 1 to 4 kilowatts results in:

• Larger melt pool volume and deeper penetration into the substrate
• Increased heat-affected zone (HAZ) width in the base steel
• Coarser grain structure in the overlay layer due to higher peak temperature and slower effective cooling rate
• Hardness increase from 320 HV to 450 HV due to increased dilution and formation of harder phases

Effect of scanning speed: At constant laser power, increasing scanning speed from 100 to 800 millimeters per minute results in:

• Decreased dilution rate from 35 percent to 12 percent
• Finer grain structure due to higher cooling rate
• Hardness increase from 350 HV to 480 HV due to reduced dilution and finer microstructure
• Improved overlay layer uniformity

Effect of wire feed rate: At constant laser power and scanning speed, increasing wire feed rate results in:

• More uniform composition distribution in the overlay layer
• Initial hardness increase followed by decrease at higher feed rates
• Optimal feed rate exists where composition is homogeneous and microstructure is fine

The formation of titanium carbides (TiC) and titanium nitrides (TiN) in the overlay layer is a key microstructural feature. These hard, stable phases provide significant wear resistance improvement over the base steel. The distribution and morphology of these carbides are directly controlled by the process parameters.

Mechanical Properties and Performance Characterization

The mechanical properties of the laser overlay layer were characterized through hardness testing and wear testing:

The results demonstrate that the optimal process window exists at moderate laser power (2 to 3 kilowatts), high scanning speed (400 to 600 millimeters per minute), and optimized wire feed rate. This combination provides the best balance of dilution control, microstructural refinement, and hardness.

The heat-affected zone in the 45 steel substrate undergoes tempering and partial phase transformation. The HAZ hardness decreases from 250 HV (as-received) to 180 to 200 HV due to tempering of the pearlitic-ferritic structure. This softening is a concern for structural integrity and must be considered in the design of laser-cladded components.

Engineering Application and Process Optimization

The laser overlay welding of titanium-based alloys on steel substrates has applications in:

1. Wear-resistant surfaces: For components subject to abrasive or adhesive wear, such as cylinder liners, valve seats, and bearing surfaces
2. Corrosion-resistant cladding: For components exposed to corrosive environments, leveraging the passive film formation of titanium alloys
3. High-temperature performance: For components operating at elevated temperatures where titanium alloys maintain strength better than carbon steels
4. Repair and remanufacturing: For restoring worn or damaged surfaces on expensive components

The process optimization strategy derived from this study recommends:

• Step 1: Select laser power based on required penetration depth and dilution tolerance. Higher power allows faster processing but increases dilution.
• Step 2: Select scanning speed to achieve target cooling rate and dilution. Higher speed reduces dilution but may cause lack of fusion if too high.
• Step 3: Optimize wire feed rate to achieve homogeneous composition and fine microstructure. Use wire feed rate that maintains a stable melt pool without excessive spatter.
• Step 4: Validate the selected parameters with test coupons, checking hardness, dilution, and adhesion strength.
• Step 5: Scale up to production components with in-process monitoring of laser power, beam position, and wire feed.

Study Insights and Practical Implications

This study provides valuable quantitative data on the relationship between laser overlay welding parameters and overlay layer performance. The finding that scanning speed has a more significant effect on dilution and hardness than laser power is particularly useful for process optimization. Engineers can use scanning speed as the primary control variable for dilution management while adjusting laser power for penetration depth.

The observation that wire feed rate has a non-monotonic effect on hardness is important for practical parameter selection. There exists an optimal wire feed rate beyond which increased feed rate leads to decreased hardness, likely due to incomplete melting and mixing of the wire material. This optimal point should be determined experimentally for each specific wire composition and substrate combination.

One limitation of this study is the relatively low power laser system used (up to 4 kilowatts). Modern laser cladding systems operate at 6 to 20 kilowatts, which would produce different thermal conditions and potentially different microstructural outcomes. The trends identified in this study should be verified with higher-power systems before extrapolation.

Additionally, the study does not address the long-term performance of the overlay layer under cyclic loading or thermal cycling conditions. For engineering applications involving fatigue or thermal fatigue, additional testing is required to ensure adequate service life.

This literature serves as a foundational reference for laser overlay welding process development. The systematic investigation of parameter effects provides a clear framework for process optimization that can be adapted to different material systems and application requirements. For engineers entering the field of laser surface engineering, this work demonstrates the importance of understanding the fundamental relationships between process parameters, microstructure, and performance.