Microstructure and Mechanical Properties of TC4 Titanium Alloy MIG Weld Joints After Post-Weld Heat Treatment

Literature Overview

This paper by He Yifan, Chen Donggao, Zhang Long, Wang Dafeng, Shao Zhiwen, and Ma Liangchao from the Ningbo Branch of China Ordnance Science Institute, published in Iron, Steel, Vanadium and Titanium in 2021, investigates the effects of post-weld heat treatment (PWHT) on the microstructure, residual stress, and mechanical properties of TC4 (Ti-6Al-4V) titanium alloy MIG weld joints. The study employed blind hole method, tensile testing, impact testing, metallographic examination, and scanning electron microscopy to characterize the weld joints in both as-welded and heat-treated conditions.

Core Technical Findings

Residual Stress Reduction

The blind hole method was used to measure residual stresses before and after PWHT. The results showed significant stress relief:

Condition	Longitudinal Residual Stress	Transverse Residual Stress
As-welded	~350–400 MPa (tensile)	~300–380 MPa (tensile)
After PWHT (optimal)	Average 70.1 MPa	Average 74.2 MPa

The residual stress reduction of approximately 80% is substantial and has important implications for fatigue life and stress corrosion cracking resistance.

Microstructural Evolution

Region	As-Welded Microstructure	After PWHT (650°C + 2 h)
Base metal	α + β lamellar	Slightly coarsened α + β
Weld metal	α' acicular martensite	Coarsened α' with some β phase recovery
HAZ	α + α' mixed	Partially transformed; coarsened α'

The as-welded weld metal consists of acicular α' martensite, which forms due to the rapid cooling rates achieved during MIG welding of titanium alloy. This martensitic structure provides high strength but limited ductility and toughness.

Mechanical Property Response

Property	As-Welded	650°C + 2 h PWHT	650°C + 3 h PWHT
Tensile strength	Higher than base metal	Slightly reduced	Further reduced
Elongation	Lower than base metal	Improved	Decreased (grain coarsening)
Impact energy	95% of base metal	Reduced	Further reduced
Fracture mode	Brittle (at base metal)	Ductile	Ductile but coarsened

Heat Treatment Parameter Optimization

The study reveals a critical balance in PWHT parameter selection:

Parameter	Effect	Optimal Range
Temperature	Higher temperature promotes α' decomposition and β phase recovery	600–650°C for TC4
Time	Longer time promotes grain coarsening and over-softening	2 h optimal; 3 h excessive
Cooling rate	Slower cooling promotes equilibrium microstructure	Furnace cool preferred

The optimal PWHT condition of 650°C for 2 hours provides the best balance between stress relief, microstructural refinement, and mechanical property retention. Extending the holding time to 3 hours leads to excessive grain coarsening, which reduces elongation and impact energy despite further stress relief.

Engineering Practice Implications

For titanium alloy pipe welding in aerospace, chemical processing, and defense applications, the PWHT considerations are critical:

Application	PWHT Requirement	Key Concern
Aerospace structural pipes	Mandatory per ASME BPV Section III	Stress corrosion cracking resistance
Chemical processing pipes	Recommended for CRA applications	HIC/SSC resistance
Defense ordnance components	Mandatory per specification	Fatigue life and reliability
Medical implants	Often required	Biocompatibility and fatigue resistance

The finding that the as-welded joint fracture occurs at the base metal rather than at the weld or HAZ indicates that the weld metal is the strongest region. However, this also means that the joint's ductility is limited by the brittle α' martensite in the weld metal. PWHT addresses this by partially transforming the martensite into a more ductile α + β structure.

Study Insights

This paper provides a comprehensive characterization of PWHT effects on TC4 MIG weld joints, with particular value in the quantitative residual stress measurements using the blind hole method. The key engineering insight is that PWHT is not a uniform improvement—all properties do not improve simultaneously. The reduction in residual stress is accompanied by a reduction in impact energy, and the improvement in elongation is limited by grain coarsening at extended holding times. This trade-off must be carefully managed in engineering practice, where the specific application requirements dictate the optimal PWHT parameters. For pipe welding applications subject to cyclic loading, the stress relief benefit likely outweighs the slight impact energy reduction, but for applications subject to impact or drop loading, the as-welded condition may be preferable if the residual stress can be managed through other means such as peening or vibration stress relief.