Effect of Laser Overlay Welding Process Parameters on Microstructure and Properties of Ti-Based Alloy Layers

Literature Overview

The paper by Luo Fang, Liu Jian, Ye Liangwu, and Yao Jianhua from the Department of Mechanical and Electrical Engineering at Zhejiang University of Technology Zhijiang College, published in Tractors and Agricultural Transport Vehicles (Vol. 31, No. 5, 2004, pp. 49-51), investigates the influence of laser overlay welding process parameters on the microstructure and properties of titanium-based alloy layers deposited on 45 steel substrates. The study systematically varied the laser power, scanning speed, and wire feed speed using a dedicated welding wire, and characterized the resulting deposit microstructure, hardness, and dilution rate. The key findings reveal that laser power and scanning speed have significant effects on the heat-affected zone size, grain morphology, and hardness, while wire feed speed influences the uniformity of the deposit and produces a non-monotonic hardness response.

Process Parameter Effects and Microstructural Analysis

The laser overlay welding process is characterized by a high energy density and rapid cooling rate, which results in fine-grained microstructures and minimal dilution compared to conventional arc welding processes. The study's systematic variation of process parameters provides valuable insights into the process-structure-property relationships.

The following table summarizes the observed effects of each parameter:

Parameter	Effect on HAZ	Effect on Microstructure	Effect on Hardness	Effect on Dilution
Increasing laser power	HAZ enlarges	Grain coarsens	Increases	Increases
Increasing scanning speed	HAZ narrows	Fine grains maintained	Increases	Decreases
Increasing wire feed speed	Minimal change	More uniform distribution	First increases then decreases	Decreases

The effect of laser power is straightforward: higher power delivers more energy to the workpiece, enlarging the molten pool and the HAZ, and promoting grain growth due to the higher peak temperature and longer solidification time. The resulting coarser microstructure generally leads to higher hardness due to the increased volume fraction of hard phases, but it may compromise toughness and fatigue resistance.

The effect of scanning speed is inverse to that of laser power in terms of energy input per unit length. Higher scanning speeds reduce the heat input, resulting in a narrower HAZ and finer microstructure. The dilution rate decreases because the wire feed rate remains constant while the weld pool volume is reduced, meaning a smaller proportion of base metal is mixed into the deposit. The increased hardness at higher scanning speeds is attributed to the finer grain structure and reduced dilution, which preserves the high alloy content of the Ti-based wire in the deposit.

The non-monotonic effect of wire feed speed on hardness is more complex. At low feed speeds, the deposit may be too thin and the dilution rate is high, leading to lower hardness. As the feed speed increases, the deposit becomes thicker and more uniform, and the dilution decreases, resulting in higher hardness. However, beyond a certain feed speed, the wire may not be fully melted, leading to incomplete fusion, porosity, and reduced hardness. This optimal feed speed represents a balance between deposit thickness, melt quality, and dilution control.

Engineering Practice and Process Optimization

For practical laser overlay welding of Ti-based alloys on carbon steel substrates, the process parameters must be selected to achieve the desired combination of dilution rate, microstructure, and hardness. The dilution rate is a critical parameter because it directly affects the chemical composition and properties of the deposit. In Ti-based overlay applications, the dilution rate should typically be kept below 20 percent to ensure adequate titanium content and the associated high-temperature strength and oxidation resistance.

The following process window guidelines can be derived from the study's findings:

Parameter	Recommended Range	Rationale
Laser power	Moderate to high	Sufficient energy for complete melting
Scanning speed	High	Low dilution, fine microstructure
Wire feed speed	Moderate	Complete melting, adequate deposit thickness
Shielding gas	Argon or helium	Prevent oxidation of Ti in weld pool
Interpass temperature	Below 150 °C	Avoid excessive grain growth

The use of a dedicated welding wire is essential for achieving the desired Ti-based alloy composition in the deposit. The wire composition must be carefully matched to the target deposit properties, considering the expected dilution from the 45 steel base. The shielding gas atmosphere must be maintained to prevent oxidation of the titanium, which would form brittle TiO2 inclusions that degrade the deposit properties.

Key Reflections and Implications

The study provides a systematic understanding of the process parameter effects in laser overlay welding of Ti-based alloys, which is valuable for engineers developing laser cladding and overlay welding processes for high-temperature and wear-resistant applications. The non-monotonic behavior of hardness with wire feed speed is a particularly important finding that underscores the need for careful process optimization rather than simply maximizing individual parameters. The findings also highlight the importance of dilution control in laser overlay welding, as the dilution rate directly governs the deposit composition and properties. For steel pipe and fitting applications, laser overlay welding of Ti-based alloys could be applied to critical components such as valve seats, turbine blades, and heat exchanger tubes operating in high-temperature and corrosive environments, provided the process parameters are optimized according to the principles established in this study.