HAZ Liquation Cracks in Turbine Blade Crown Surfacing Wear-Resistant Layer

Literature Overview

This 2014 paper published in "Welding & Cutting" by Song Wenqing, Qu Shen, Shi Shunkun, Gao Shan, Li Hui, and Li Xiang from AVIC Shenyang Liming Aero Engine (Group) Co., Ltd., the Institute of Metal Research at the Chinese Academy of Sciences, and Shenyang University of Technology presents a detailed failure analysis of welding heat-affected zone (HAZ) liquation cracks found in high-pressure turbine blade crown surfacing wear-resistant layers. The paper documents a production problem where penetration testing revealed excessive crack indications in the HAZ of the surfacing layer, leading to high scrap rates and subsequent systematic resolution.

Failure Analysis Methodology

Initial Problem Identification

During factory test runs of a batch-produced engine, penetration testing revealed that multiple high-pressure turbine blades exhibited超标 (exceeding standard) crack indications in the HAZ of the crown surfacing wear-resistant layer. Concurrently, the production process was experiencing high scrap rates due to the same crack defect. This combination of test failure and production rejection indicated a systemic process problem rather than isolated quality issues.

Failure Characterization

The authors employed metallographic examination and scanning electron microscopy (SEM) fractography to characterize the failure mode. The analysis confirmed that the cracks were welding HAZ liquation cracks (also known as solidification cracks or hot cracks). Liquation cracks form when pre-existing grain boundary precipitates or segregated phases partially melt during the welding thermal cycle, creating a liquid film along grain boundaries that cracks under tensile stress during solidification.

Root Cause Analysis

Casting Process Factors

The high-pressure turbine blade is a complex casting component with a specific microstructure developed during casting. The grain boundary composition and precipitate distribution in the base material directly influence susceptibility to liquation cracking. If the casting process produces coarse grain boundary segregation of low-melting-point phases (such as eutectic compositions or impurity-rich phases), the HAZ becomes highly susceptible to liquation cracking during surfacing.

Surfacing Process Factors

The welding parameters, including heat input, welding speed, and preheat temperature, determine the thermal cycle severity in the HAZ. Excessive heat input raises the HAZ temperature above the solidus temperature of segregated phases, promoting partial melting and liquation. The surfacing alloy composition also matters; if the surfacing alloy has a wide freezing range or contains elements that form low-melting-point eutectics with the base material, liquation cracking susceptibility increases.

Restraint and Stress Factors

The blade crown geometry creates significant restraint during surfacing. The thick blade root and the complex crown geometry limit the ability of the HAZ to contract freely during cooling, generating high tensile stresses that promote crack opening. The cyclic thermal loading from multiple surfacing passes further exacerbates the problem by repeatedly heating and cooling the HAZ.

Resolution Strategy and Implementation

The authors developed a multi-faceted resolution strategy that addressed all identified contributing factors:

Casting Process Optimization

The casting process was optimized to reduce grain boundary segregation of low-melting-point phases. This may involve adjusting the casting temperature, cooling rate, or alloy composition to produce a more homogeneous grain boundary composition. The goal is to raise the solidus temperature of the HAZ grain boundaries above the maximum temperature reached during surfacing.

Surfacing Process Adjustment

The welding parameters were adjusted to reduce heat input and minimize the thermal cycle severity in the HAZ. This may involve reducing welding current, increasing travel speed, using a narrower weld profile, or implementing a multi-pass strategy with reduced per-pass heat input. The surfacing alloy composition may also have been modified to reduce the tendency to form low-melting-point eutectics with the base material.

Cooling Fixture Implementation

A water-cooled fixture was introduced to control the cooling rate of the blade during and after surfacing. The cooling fixture serves two purposes: it reduces the peak temperature in the HAZ by extracting heat more efficiently, and it controls the cooling rate to minimize the time spent in the critical temperature range where liquation cracks form. The water cooling also reduces the overall thermal distortion of the blade.

Inspection Method and Acceptance Criteria Revision

The welding inspection method and defect acceptance criteria were re-evaluated and revised. This is a critical but often overlooked aspect of failure resolution. If the inspection method is overly sensitive or the acceptance criteria are too strict, components that are structurally acceptable may be rejected. Conversely, if the inspection method is insufficiently sensitive, unacceptable defects may pass through. The revised criteria must balance quality assurance with manufacturing feasibility.

Results and Validation

The comprehensive resolution strategy was validated through subsequent production runs. Over 2,000 high-pressure turbine blades were produced with the revised process, and the first-pass welding qualification rate for the crown wear-resistant layer improved to above 95%. Furthermore, continuous tracking of the field service performance of these blades revealed no wear-resistant layer quality issues, confirming the effectiveness of the resolution.

Engineering Practice Implications

This case study is a textbook example of systematic failure analysis and resolution in aerospace manufacturing. The approach follows a logical progression from problem identification through failure characterization, root cause analysis, multi-faceted resolution, and validation. Several key lessons emerge:

The Importance of Multi-Factor Resolution

No single change resolved the liquation crack problem. The casting process, surfacing process, cooling fixture, and inspection criteria all needed to be addressed simultaneously. This reflects the reality of complex manufacturing problems where multiple factors interact, and addressing only one factor is insufficient.

The Role of Inspection Criteria

The revision of inspection methods and acceptance criteria is a critical but often neglected aspect of quality improvement. In this case, the original criteria may have been calibrated for a different process or material condition, and their application to the surfacing process was overly restrictive. Proper calibration of inspection criteria to the actual process capabilities is essential for achieving both quality and productivity.

Field Validation

The long-term field tracking of over 2,000 blades with no quality issues provides strong validation of the resolution. This is more convincing than laboratory testing alone because it demonstrates that the process works under real operating conditions, including the full range of thermal cycling, mechanical loading, and environmental exposure encountered in service.

Key Questions and Reflections

An important consideration is the long-term effect of the water-cooled fixture on the residual stress state of the blade. Rapid cooling can generate high residual stresses that, while not causing immediate cracking, may contribute to fatigue crack initiation under cyclic loading. The authors should have evaluated the residual stress state and determined whether additional stress relief treatment is required.

Another consideration is the effect of the revised process on the metallurgical quality of the surfacing layer itself. Reducing heat input and increasing cooling rate can affect the microstructure of the surfacing deposit, potentially reducing grain size but also potentially increasing porosity or incomplete fusion. The balance between HAZ crack resistance and surfacing layer quality must be carefully maintained.

The liquation crack mechanism is particularly insidious because the cracks form during solidification and may be difficult to detect with conventional non-destructive testing methods. Penetration testing, as used in this case, is effective for surface-breaking cracks but may miss subsurface cracks. The inspection strategy should include multiple NDT methods to ensure comprehensive coverage.

Summary

This paper presents a comprehensive and successful case study of HAZ liquation crack resolution in high-pressure turbine blade crown surfacing, demonstrating the systematic approach required to address complex welding quality problems in aerospace manufacturing. The multi-faceted resolution strategy—combining casting process optimization, surfacing parameter adjustment, cooling fixture implementation, and inspection criteria revision—achieved a first-pass qualification rate above 95% for over 2,000 blades with no subsequent field quality issues. This case study serves as an excellent model for failure analysis and resolution methodology, emphasizing the importance of addressing all contributing factors simultaneously and validating solutions through both production qualification and long-term field tracking.