HAZ Liquidation Cracking Analysis in Overlay Wear-Resistant Layer on High-Pressure Turbine Blade Tips

Literature Overview

This paper by Song Wenqing et al., published in Electric Welding Machine (2014, Vol. 44, No. 6, pp. 89-93), documents a critical quality failure and its resolution in the production of high-pressure turbine blade tips for an active aero-engine program. The authors from AECC Shenyang黎明 Aero-Engine Group, Institute of Metal Research (Chinese Academy of Sciences), and Shenyang University of Technology describe a systematic investigation of cracking in the heat-affected zone (HAZ) of overlay wear-resistant layers on turbine blade crowns, ultimately achieving a first-pass weld quality rate exceeding 95%.

Problem Description

During engine ground testing, penetrant testing revealed excessive crack indications in the HAZ of overlay wear-resistant layers on multiple high-pressure turbine blade tips. The problem was severe enough to cause batch production failures, with significant quantities of blades scrapped due to the same cracking defect. This represented a critical technical bottleneck constraining the production schedule and engine certification process.

Failure Analysis Methodology

The investigation followed a systematic failure analysis protocol:

Non-destructive testing: Penetrant testing (PT) identified crack locations and extent
Metallographic examination: Cross-sectional preparation revealed crack morphology and orientation
Scanning electron microscopy (SEM) fractography: Fracture surface analysis determined crack initiation sites and propagation mechanism
Microstructural characterization: Identification of the susceptible microstructural features in the HAZ

Crack Mechanism Identification

The analysis confirmed that the defect was liquidation cracking (also termed liquefaction cracking or solidification cracking) in the welding HAZ. This is a particularly insidious defect because:

It occurs in the partially melted zone of the base material, not in the weld metal itself
It is driven by the presence of low-melting-point eutectics that remain liquid while the surrounding matrix has solidified
It is exacerbated by thermal gradients and restraint stresses during cooling
It is often not visible on the surface, requiring cross-sectional examination for detection

The mechanism can be described as follows:

During overlay welding, the heat cycle partially melts the grain boundaries of the base material in the HAZ.
Low-melting-point phases (such as carbides, intermetallics, or impurity-rich eutectics) at the grain boundaries remain liquid.
As the HAZ cools, the solid matrix contracts while the liquid films at grain boundaries create tensile stresses.
When the tensile stress exceeds the cohesive strength of the partially solidified grain boundary liquid, liquidation cracks form.
The cracks propagate along the grain boundaries, often extending significantly beyond the immediate weld zone.

Root Cause Analysis

The investigation identified multiple contributing factors:

Contributing Factor	Description	Effect on Cracking
Casting microstructure	Coarse grain boundaries with segregated impurities	Provides preferential crack paths
Welding heat input	Excessive thermal energy from welding parameters	Increases the width of the partially melted zone
Thermal gradient	Steep temperature gradient during cooling	Increases tensile stress in HAZ
Restraint	Blade geometry provides high restraint to contraction	Increases cracking susceptibility
Base material composition	Susceptible to low-melting-point eutectic formation	Provides liquid films at grain boundaries

Corrective Measures Implemented

The engineering team developed a comprehensive corrective action plan:

Casting process optimization: Modified the casting parameters to produce finer grain structure and reduce grain boundary segregation. This included optimizing the pouring temperature, cooling rate, and possibly adding grain refiners.
Welding process adjustment: Reduced the heat input by optimizing the welding current, voltage, and travel speed. The goal was to minimize the width of the partially melted zone while maintaining adequate fusion with the overlay.
Water-cooled fixture: Introduced a water-cooled backing fixture to control the cooling rate and reduce the thermal gradient in the HAZ. This also helped to reduce the overall distortion of the blade tip.
Inspection method revision: Re-evaluated the weld inspection criteria and acceptance standards, establishing more appropriate criteria for crack detection and acceptance.

Results and Validation

The corrective measures produced dramatic results:

First-pass weld quality rate: Improved to above 95% (from a previously unacceptable level)
Production volume validated: Over 2,000 blade tips produced and tested successfully
Field performance: Continuous tracking of blade tips in engine field service revealed no quality issues with the wear-resistant overlay
No recurring failures: The problem was completely resolved without recurrence

Engineering Lessons and Reflections

This case study offers several valuable lessons for welding engineers working on critical components:

HAZ cracking is often more severe than weld metal cracking: In overlay welding applications, the base material HAZ may be more susceptible to cracking than the weld metal itself, particularly when the base material has a casting microstructure with grain boundary segregation.
Multi-disciplinary approach is essential: The successful resolution required input from casting metallurgy, welding engineering, materials characterization, and quality assurance. No single discipline could have solved the problem alone.
Process interaction effects: The cracking was not caused by a single factor but by the interaction of casting microstructure, welding parameters, thermal management, and geometric restraint. Addressing only one factor would have been insufficient.
Inspection criteria must match the defect mechanism: The original inspection criteria may have been designed for a different type of defect (such as porosity or lack of fusion) and were not optimized for detecting liquidation cracking. Revising the inspection methodology was as important as improving the welding process.
Field validation is the ultimate proof: The 2,000+ blade tips produced and tracked in service without quality issues provides the strongest evidence that the corrective measures were effective. Laboratory testing alone would not have been sufficient for certification.

Implications for Similar Applications

The lessons from this case study apply broadly to overlay welding applications on critical components, including:

Turbine blade shrouds and tips (aero-engines, industrial gas turbines)
Steam turbine blade tips (power generation)
Jet engine nozzle guide vanes
Compressor blade leading edges
Any application where overlay welding is performed on cast superalloy or cast iron substrates

The key principle is that HAZ liquidation cracking must be actively prevented through a combination of base material control, welding parameter optimization, thermal management, and appropriate inspection. A single corrective action is rarely sufficient; the problem requires a systems-level approach that addresses all contributing factors simultaneously.