Zhuojin Pipe Fitting Co., LtdLaser Welding and TIG Welding of Diffusion-Bonded TC4 Titanium Alloy Sheets
Literature Overview
This paper by Shi Jipeng, Zang Xu, Fan Yu, Jiang Wentong, Guan Feng, and Zhao Xingwang from Shenyang Aircraft Corporation and the Air Force Equipment Department was published in "Applied Laser" in 2023, Volume 43, Issue 1, pages 18-24. The study investigates laser welding and TIG welding with filler wire for (2.0+1.5) mm diffusion-bonded TC4 titanium alloy sheets, comparing the microstructure, mechanical properties, and weld quality of the two welding methods. The research was funded by the National Defense Basic Research Program.
Background and Technical Context
TC4 (Ti-6Al-4V) is the most widely used titanium alloy in aerospace applications due to its excellent combination of strength, low density, and corrosion resistance. Diffusion bonding is a solid-state joining process that creates strong bonds between titanium alloy sheets without melting, but the resulting bonded joints may require additional welding to create continuous structures. The challenge lies in welding the diffusion-bonded interface without introducing defects such as cracks, porosity, or incomplete bonding.
The study examines two welding methods: laser welding, which offers high energy density and narrow heat-affected zones, and TIG welding with filler wire, which provides more control over weld pool dynamics and filler metal addition. Both welds were subjected to post-weld stress relief heat treatment at 650°C.
Experimental Design and Results
The base material consisted of TC4 titanium alloy sheets with thicknesses of 2.0 mm and 1.5 mm, diffusion-bonded to create a 3.5 mm thick composite plate. Two welding methods were compared:
| Parameter | Laser Welding | TIG Welding with Filler Wire |
|---|---|---|
| Energy density | High | Moderate |
| Heat-affected zone | Narrow | Wider |
| Grain size in weld | Smaller | Larger |
| α′ martensite phase | More | Less |
| Shear strength | Higher | Lower |
| Tensile strength (parallel to weld) | Higher | Lower |
| Bending strength | Lower | Higher |
| Tensile strength (perpendicular to weld) | Lower | Higher |
| Post-weld heat treatment | 650°C stress relief | 650°C stress relief |
The key findings are:
- Both welding methods produced well-formed joints with appropriate process parameters.
- Laser welding produced smaller grains and more α′ martensite phase in the weld metal compared to TIG welding.
- Laser welding exhibited higher shear strength and tensile strength in the direction parallel to the weld.
- TIG welding exhibited higher bending strength and tensile strength in the direction perpendicular to the weld.
- The post-weld 650°C stress relief heat treatment was applied to both weld types to reduce residual stresses.
Microstructural Analysis
The microstructural differences between laser and TIG welds can be attributed to the different thermal cycles experienced by the weld metal. Laser welding produces a rapid heating and cooling cycle due to its high energy density and narrow heat-affected zone. This rapid cooling promotes the formation of martensitic α′ phase and finer grain structures.
In contrast, TIG welding produces a more gradual thermal cycle with a wider heat-affected zone. The slower cooling rate allows for more complete diffusion and the formation of equilibrium phases, resulting in coarser grains and less α′ martensite. The filler wire addition in TIG welding also dilutes the weld metal composition, potentially affecting phase formation.
The presence of more α′ martensite in laser welds contributes to higher strength but may reduce ductility. The coarser microstructure in TIG welds provides better ductility and toughness, which is reflected in the higher bending strength.
Engineering Practice Considerations
The choice between laser welding and TIG welding for diffusion-bonded TC4 titanium alloy depends on the specific application requirements. For applications requiring high shear strength and resistance to loads parallel to the weld line, laser welding is the preferred method. For applications requiring high bending strength and resistance to loads perpendicular to the weld line, TIG welding with filler wire may be more suitable.
The post-weld stress relief heat treatment at 650°C is critical for both welding methods because it reduces residual stresses that can lead to delayed cracking or distortion. The temperature and duration of the heat treatment must be carefully controlled to avoid excessive grain coarsening or phase transformation that could compromise mechanical properties.
For aerospace manufacturing, the ability to weld diffusion-bonded titanium alloy sheets opens new possibilities for creating complex structural components without the need for traditional forging or machining. The selection of welding method should be based on the expected loading conditions and the required balance between strength and toughness.
Reflections and Study Insights
This study provides valuable comparative data on laser and TIG welding of diffusion-bonded TC4 titanium alloy, which is of significant interest for aerospace manufacturing. The finding that the two welding methods produce different mechanical property profiles depending on load direction is particularly important for structural design. Engineers must consider the expected loading conditions when selecting a welding method for diffusion-bonded titanium alloy components.
The study also highlights the importance of post-weld heat treatment in titanium alloy welding. The 650°C stress relief treatment is essential for reducing residual stresses and improving long-term performance, but it must be carefully controlled to avoid adverse effects on microstructure and properties.
Future research should explore hybrid welding approaches that combine the advantages of both laser and TIG welding, as well as the effect of different filler wire compositions on TIG weld properties. Additionally, the long-term creep and fatigue behavior of laser and TIG welds in diffusion-bonded TC4 titanium alloy should be investigated to support their use in critical aerospace applications.