Microstructure and Mechanical Properties of TIG Welded Joints of BTi-62421s Alloy Plate

Literature Overview

This paper, published in Mining and Metallurgical Engineering (Vol. 33, Issue 5, 2013, pp. 119-123), authored by Peng Xiaomin from Hunan Institute of Engineering and Sun Wei and Xia Changqing from Central South University, investigates the microstructure and mechanical properties of TIG welded joints of BTi-62421s titanium alloy plate. The research examines the effects of pre-weld heat treatment, welding process parameters, and post-weld heat treatment on the welded joint microstructure and properties. This work was supported by the National Natural Science Foundation of China (Grant No. 51101054) and Hunan Institute of Engineering Research Startup Project.

Core Technical Content

BTi-62421s is a near-α titanium alloy designed for high-temperature applications, with a nominal composition of Ti-6Al-2Sn-4Zr-2Mo-0.1Si. The alloy exhibits excellent creep resistance and high-temperature strength, making it suitable for aerospace and power generation applications. However, the alloy's complex phase system and sensitivity to thermal history present significant challenges for welding.

Optimized Welding Process Parameters

The study identifies the following as the optimal TIG welding parameters for BTi-62421s alloy plate:

Parameter	Optimized Value
Welding current	110 A
Welding speed	5 mm/s
Main nozzle argon flow	20 L/min
Tail shield argon flow	12 L/min
Welding configuration	Single-side welding
Electrode	Pure tungsten, 3.2 mm
Joint preparation	Square or V-groove

Microstructural Evolution

The microstructure of the welded joint exhibits distinct characteristics across different zones:

Zone	Microstructure	Characteristics
Weld metal	Acicular martensite (α' martensite)	Fine needle-like α' phase, high strength but low ductility
HAZ (coarse grain)	Primary α + β transformation + acicular martensite	Mixed structure with varying phase fractions
HAZ (fine grain)	Primary α + transformed β	Retains some parent structure
Base metal	Equiaxed α + lamellar α+β	Parent microstructure, unaffected by welding

The formation of acicular martensite in the weld metal is attributed to the rapid cooling rate following welding, which suppresses the diffusion-controlled α+β transformation and promotes the diffusionless martensitic transformation of the β phase. This is a common phenomenon in near-α titanium alloys and represents a key challenge for achieving acceptable ductility in welded joints.

Effects of Pre-Weld Heat Treatment

The study compares two pre-weld heat treatment conditions:

Pre-Weld Treatment	Description	Effect on Weld Metal
Double annealing	Solution treatment + aging	Limited effect on weld microstructure
Stress relief annealing	Low-temperature annealing	Limited effect on weld microstructure

The finding that pre-weld heat treatment has limited influence on weld metal microstructure is expected, as the welding thermal cycle effectively resets the microstructure in the weld zone regardless of the parent material condition. However, double annealing of the base metal contributes to improved overall joint ductility by providing a more ductile HAZ.

Effects of Post-Weld Heat Treatment

Post-weld heat treatment (PWHT) is identified as the most effective method for improving joint ductility. The optimal PWHT condition is 700°C for 2 hours followed by air cooling.

Condition	Tensile Strength (σb)	Elongation (δ)	Strength Ratio	Ductility Ratio
As-welded	1033 MPa	5.5%	96.5% of base metal	38.5% of base metal
After PWHT (700°C/2h)	~980 MPa (estimated)	7.1%	~92% of base metal	49.5% of base metal

The PWHT promotes the α' → α + β phase transformation, which converts the brittle martensitic structure into a more ductile equiaxed α + lamellar β structure. This transformation significantly improves ductility at the expense of a modest reduction in strength.

Mechanical Property Analysis

The as-welded joint exhibits excellent strength retention (96.5% of base metal tensile strength) but significantly reduced ductility (38.5% of base metal elongation). This strength-ductility trade-off is characteristic of martensitic weld metals in titanium alloys and represents a fundamental challenge for welded joint design.

The post-weld heat treatment at 700°C/2h improves ductility to 49.5% of base metal while maintaining acceptable strength levels. This represents a significant improvement in joint toughness and is likely essential for applications where fatigue resistance or impact toughness are design drivers.

Engineering Practice Integration

For engineering applications of BTi-62421s alloy, the following considerations are important:

Process selection: TIG welding with the identified parameters provides a viable approach for fabrication, but the narrow process window requires careful control.
PWHT necessity: Post-weld heat treatment is strongly recommended for all structural applications to achieve acceptable ductility and toughness.
Design considerations: The reduced ductility of the as-welded joint must be accounted for in design calculations, particularly for fatigue-critical applications.
Inspection requirements: The acicular martensitic structure is susceptible to cracking under certain conditions, so thorough non-destructive inspection is essential.

Study Insights and Implications

This research provides comprehensive guidance for the TIG welding of BTi-62421s alloy, a high-performance titanium alloy with demanding service requirements. The key insight is that while the as-welded joint achieves excellent strength retention, the ductility deficit is significant and must be addressed through post-weld heat treatment. The identified PWHT condition of 700°C/2h represents a practical solution that balances strength and ductility. For engineering practice, this study should be considered a reference for process qualification of BTi-62421s welded joints, with the understanding that PWHT is not optional but mandatory for structural applications. The research also highlights the importance of understanding the microstructural evolution during welding, as the formation of brittle martensitic phases is a common challenge in near-α titanium alloys that can be mitigated through appropriate heat treatment.