Formation & Detection Of Central White Block Defects in TC11 Titanium Bars

TC11 titanium alloy is a high-temperature Ti-Al-Mo-Zr-Si series titanium alloy capable of long-term service at 500 °C. It features low density, high specific strength, excellent heat resistance and creep resistance, and is widely applied in core load-bearing components for aerospace equipment. With the upgrading of aerospace equipment, stricter requirements have been imposed on the microstructural and mechanical property stability of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium bars. Internal metallurgical defects will reduce finished product yield and service safety.

1. Macro Morphology

Elongated bright white blocks lie on bar central axes. Etching exposes their pale gray-white fuzzy edges, separating them from cracks and inclusions. Less than 1 mm wide, severe defects run through bars and require sectional etching for detection.

2. Microstructure and Chemical Composition

Against uniform α+β matrix, white blocks are chaotic β-rich, α-poor regions. Element segregation raises Ti and reduces Al, Mo inside the flaws, without pores. This inherent segregation defect greatly weakens material properties.

1. Primary Axial Segregation Generated during Ingot Melting

During vacuum arc remelting of TC11 alloy, Mo and Zr exhibit poor diffusion capacity. Uneven chemical composition arises from rapid melting speed, fluctuating process parameters, inconsistent batching and mixing, and low electrode density. During solidification, titanium crystallizes first, while high-melting-point alloying elements accumulate at the ingot core, forming an axial alloy-depleted segregation band.

2. Segregation Defects Amplified by Uneven Forging and Rolling

Sufficient deformation only occurs on the surface layer during forging and rolling, while the core undergoes inadequate deformation, failing to homogenize inherent segregation. High temperature at the core results in coarsened β grains and suppressed α-phase precipitation. Heavy reduction and rapid forging elongate punctate segregation into long strip bright white microstructures at the bar center.

3. Bright White Block Defects Stabilized by Heat Treatment

When heat treatment temperature approaches or exceeds the β-transus temperature, the core alloy-depleted zones fully transform into β phase. Due to insufficient alloy content, α phase cannot precipitate during cooling, leaving only coarse single β microstructures, which form permanent color-differentiated bright white blocks against the normal dual-phase α+β matrix.

1. Low-Magnification Metallographic Etching

Samples are polished and etched for direct visual observation, which clearly characterizes the morphology and size of bright white blocks and distinguishes them from other defects, serving as the basis for defect grading. However, this destructive sampling method has low efficiency and cannot realize full inspection; it is only applicable to sampling evaluation and process verification.

2. Conventional Pulse Ultrasonic Testing

A non-destructive full-inspection technique suitable for mass production. Its sensitivity is relatively low, leading to frequent missed detection of tiny bright white blocks. In addition, it struggles to distinguish segregation from porosity and other defects, resulting in high false positive rates, and is only used for preliminary screening.

3. Ultrasonic Phased Array Testing

The acoustic beam can accurately scan the bar core with high detection sensitivity, capable of identifying micro-defects as small as 0.8 mm. It supports defect positioning, quantitative measurement, and 2D/3D imaging with low missed detection rates, and is widely adopted for full inspection of high-end aerospace titanium materials. Its drawbacks include high equipment cost and complicated inspection procedures, yet it is gradually replacing conventional ultrasonic testing.

4. Microscopic Characterization

This technique comprises high-magnification metallography, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and microhardness testing. It enables analysis of phase microstructure, elemental segregation, and property deterioration to clarify defect formation mechanisms. As a destructive testing method, it is only used for defect tracing and process optimization, and cannot be deployed for batch inspection.

• Melting Process: Implement uniform batching and mixing, strictly control electrode density, stabilize melting parameters with segmented speed regulation to extend diffusion time of high-melting-point elements, and optimize ingot cooling to reduce axial segregation at the source.
• Hot Working Process: Adopt stepped forging at 980 °C → 960 °C → 950 °C, rationally allocate single-pass deformation to fully penetrate the core, break segregated microstructures and refine β grains.
• Heat Treatment Process: Precisely control temperature to avoid exceeding the β-transus temperature, and conduct segmented heat treatment to promote uniform α-phase precipitation and mitigate uneven microstructure and composition.
• Inspection Control System: Establish a three-level inspection system: full inspection via ultrasonic phased array testing, paired with low-magnification metallographic sampling inspection, and microscopic analysis of defective samples to form a closed-loop management covering defect detection and root cause tracing.

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