Zhuojin Pipe Fitting Co., LtdComparative Effects of Two TIG Welding Processes on Mechanical Properties of LD10 Aluminum Alloy Joints
Literature Overview
This paper by Kang Ju and colleagues, jointly authored by researchers from Tsinghua University's Key Laboratory of Advanced Forming and Manufacturing and Capital Aerospace Machinery Company, was published in the Transactions of the Welding Institute of China in 2014 (Volume 35, Issue 3, pages 109-112). The study investigates the influence of two different TIG welding strategies on the mechanical properties of 5.5 mm thick LD10 (2A14-T6) aluminum alloy butt joints.
Material Background
LD10 (equivalent to 2A14-T6) is an Al-Cu-Mg alloy widely used in aerospace structural components due to its excellent specific strength and fatigue resistance. The T6 temper (solution treated and artificially aged) provides the material with its peak mechanical properties. However, welding introduces a severe thermal cycle that disrupts the age-hardened microstructure, leading to significant property loss in the heat-affected zone (HAZ). This is the fundamental challenge addressed by the study.
Welding Process Configurations
Two TIG welding strategies were compared:
Two-Side Three-Pass Welding:
- Welding is performed from both sides of the joint.
- Three total weld passes are deposited (one root pass and two fill passes, or two passes from one side and one from the other).
- Each point in the HAZ experiences three distinct thermal cycles.
Single-Side Two-Pass Welding:
- Welding is performed from only one side.
- Two total weld passes are deposited.
- Each point in the HAZ experiences fewer thermal cycles.
Thermal Cycle Analysis
Thermocouples were embedded at multiple locations to measure the thermal cycles experienced by different regions of the joint. The key finding is that in the two-side three-pass configuration, the same point in the HAZ undergoes three successive thermal cycles, each reaching temperatures well above the T6 aging temperature (approximately 175-190 degrees Celsius for 2A14). This repeated thermal exposure causes progressive over-aging of the Al2Cu and Al2CuMg precipitates, resulting in a dramatic hardness reduction.
| Property | Two-Side Three-Pass | Single-Side Two-Pass |
|---|---|---|
| Number of thermal cycles in HAZ | 3 | 2 |
| HAZ width | Wider | Narrower |
| Over-aged zone hardness | Significantly reduced | Moderately reduced |
| Tensile strength | Lower | 358.3 MPa |
| Fracture location | Over-aged HAZ | Weld metal region |
| Weld porosity | Lower | Higher |
| Grain boundary second phase | Less pronounced | More pronounced |
| Joint ductility | Better | Reduced |
Mechanical Property Analysis
The tensile strength results reveal a fundamental trade-off between HAZ and weld metal quality. The single-side two-pass weld achieves a higher tensile strength of 358.3 MPa because the HAZ experiences less over-aging and retains more of its age-hardened strength. However, this comes at the cost of increased porosity and more pronounced grain boundary second-phase distribution in the weld metal, which reduces the joint's ductility.
In contrast, the two-side three-pass weld has a weaker HAZ due to severe over-aging, and fracture initiates in this region during tensile testing. Despite the lower tensile strength, the weld metal itself is cleaner with less porosity, resulting in better overall ductility.
Microstructural Mechanisms
The mechanical behavior differences can be attributed to the following microstructural features:
HAZ Over-Aging: The repeated thermal cycles in the three-pass configuration cause complete dissolution and re-precipitation of strengthening phases, resulting in coarse, ineffective precipitates. The hardness in the severely over-aged zone can drop to 30-40 HV compared to 100-110 HV in the peak-aged base metal.
Weld Metal Porosity: The single-side two-pass configuration may introduce more porosity due to the higher heat input per pass and potentially less effective gas shielding from the back side. Porosity acts as stress concentrators and reduces the effective load-bearing cross-section.
Grain Boundary Second Phase: In the weld metal, the solidification microstructure includes dendritic arms with inter-dendritic segregation of Cu and Mg, which form grain boundary second-phase films during cooling. These films reduce grain boundary cohesion and contribute to reduced ductility.
Engineering Practice Implications
For aerospace applications where LD10/2A14-T6 is used, the choice between these two welding strategies must be guided by the specific design requirements:
- If maximum tensile strength is the priority, the single-side two-pass approach is preferred, but porosity control must be rigorously maintained through meticulous gas shielding and filler wire selection.
- If ductility and fracture resistance are more critical, the two-side three-pass approach provides a cleaner weld metal, but the HAZ weakness must be addressed through post-weld aging treatment.
- A post-weld solution treatment followed by artificial aging (PWHT) can partially restore HAZ properties, but this is often impractical for large assemblies.
Reflections
This study highlights the inherent tension between HAZ property retention and weld metal quality in aluminum alloy welding. The multi-pass strategy that protects the HAZ through lower per-pass heat input inevitably introduces more thermal cycles, worsening over-aging. Conversely, the fewer-pass strategy preserves HAZ properties but compromises weld metal cleanliness. This trade-off has no simple resolution and requires case-by-case evaluation based on the specific service requirements of the joint.
The study also underscores the importance of thermocouple placement in thermal cycle measurement. The number and location of thermocouples directly affect the accuracy of the thermal cycle history reconstruction, which in turn influences the interpretation of microstructural evolution and property degradation.