Microstructure and Properties of TIG Arc-Assisted Laser Welded TA2 Titanium Thin-Walled Straight Seam Pipe

Literature Overview

This study investigates the microstructure and mechanical properties of TA2 titanium thin-walled straight seam pipe welded using a TIG arc-assisted laser welding process. TA2 (equivalent to commercially pure Grade 1 titanium) is widely used in aerospace, chemical processing, and medical applications due to its excellent corrosion resistance, low density, and good formability. The study addresses the specific challenges of welding thin-walled titanium pipes, where the heat input must be precisely controlled to avoid excessive distortion, burn-through, and microstructural degradation. The TIG arc-assisted laser welding approach combines the deep penetration capability of laser welding with the wider fusion zone and better process stability of TIG welding, offering a promising solution for this challenging application.

Core Technical Points

The TIG arc-assisted laser welding process uses a laser beam as the primary heat source for deep penetration and a TIG arc as a secondary heat source to widen the fusion zone and improve process stability. The laser power is typically in the range of 1–5 kW, while the TIG arc current is 50–150 A. The combined process produces a weld with a moderate penetration depth and a fusion zone width that is wider than laser-only welding but narrower than TIG-only welding. This combination is particularly advantageous for thin-walled pipes where laser-only welding can produce excessive keyhole effects and porosity, while TIG-only welding produces excessive heat input and distortion.

The microstructure of the weld in TA2 titanium is governed by the cooling rate and the thermal cycling history. TA2 is an alpha-phase titanium alloy with a melting point of approximately 1668 °C. During welding, the weld metal solidifies from a beta-phase liquid, and the cooling rate determines whether the primary alpha phase forms as Widmanstätten plates, acicular alpha, or equiaxed alpha. The study demonstrates that the TIG arc-assisted laser welding process produces a cooling rate in the range of 50–200 K/s, which results in a fine acicular alpha microstructure in the weld metal and a mixed alpha-beta microstructure in the heat-affected zone (HAZ).

Microstructural Analysis of the Weld and Heat-Affected Zone

The study reveals that the weld metal microstructure consists primarily of acicular alpha phases that form during rapid cooling from the beta-phase solidification range. The acicular alpha plates are typically 1–5 μm wide and 50–200 μm long, growing perpendicular to the heat flux direction. This fine acicular microstructure provides a good combination of strength and toughness, which is critical for the fatigue performance of thin-walled pipe joints.

The HAZ microstructure exhibits a gradient of microstructural features as a function of distance from the fusion boundary. In the region immediately adjacent to the fusion boundary — the coarse grain HAZ (CGHAZ) — the alpha grains are significantly coarsened due to the high peak temperature, and the microstructure consists of coarse Widmanstätten alpha plates. Further from the fusion boundary — the fine grain HAZ (FGHAZ) — the peak temperature is lower, and the microstructure consists of finer acicular alpha phases. The transition between the CGHAZ and FGHAZ is sharp and marks the boundary where the peak temperature exceeds the beta transus (approximately 900 °C for TA2).

The Effect of TIG Arc Assistance on Microstructure

A key finding of the study is that the TIG arc assistance significantly modifies the microstructure compared to laser-only welding. The additional heat input from the TIG arc increases the peak temperature in the HAZ and reduces the cooling rate in the weld metal, which promotes the formation of equiaxed alpha phases in addition to the acicular alpha. This mixed microstructure provides improved ductility and fatigue resistance compared to the fully acicular microstructure produced by laser-only welding.

The study also shows that the TIG arc assistance reduces the porosity content in the weld metal. Laser-only welding of titanium is prone to porosity due to the keyhole effect, where vaporization of the molten metal creates a deep keyhole that can collapse and trap gas bubbles. The TIG arc assistance reduces the keyhole depth by providing additional heat input that widens the melt pool, thereby reducing the aspect ratio of the keyhole and minimizing the risk of keyhole collapse and porosity formation.

Mechanical Property Evaluation

The mechanical properties of the TIG arc-assisted laser welded TA2 pipe joints were evaluated through tensile testing, hardness mapping, and microstructural examination. The results show that the weld metal exhibits a yield strength of approximately 280–320 MPa and an ultimate tensile strength of 350–400 MPa, which is slightly higher than the base metal (yield strength 240–280 MPa, UTS 300–350 MPa). This slight strength increase in the weld metal is attributed to the fine acicular alpha microstructure that provides higher dislocation density and more effective solid solution strengthening.