Black Streak Defects in Gr5 Titanium Bar Rolling

Gr5 titanium alloy is the most widely used α+β titanium alloy with excellent performance, serving as a key structural material in different fields. Black streak defects tend to occur during the bar rolling process, impairing surface quality, causing inhomogeneous microstructure and properties, and reducing product qualification rate and service safety.

 

 

(1) Ingot Composition Segregation

Uneven diffusion of alloying elements leads to local enrichment of β-phase stabilizing elements and insufficient α-phase stabilizing elements. It forms segregation bands that develop into black streaks after rolling due to differences in microstructure and hardness.

 

Impurities such as Fe, Si and C introduced by raw materials or smelting segregate at grain boundaries. It forms discontinuous or continuous black streaks after rolling, and microcracks may occur in severe cases.

 

Insufficient solidification feeding results in internal defects. It retains microstructural differences after rolling and appear as black streaks after oxidation and corrosion.

 

(2) Mismatched Rolling Process

Excessively high temperature causes grain coarsening and weakened grain boundaries; excessively low temperature results in poor plasticity and high deformation resistance, making the surface prone to microcracks and folds that turn into black streaks after oxidation.

 

Unreasonable roll profile and large speed difference lead to asynchronous deformation, generating shear stress on the surface and inducing microstructural distortion and microcracks that present as black streaks after oxidation and corrosion.

 

Insufficient lubrication and contaminated emulsion tend to cause aluminum pick-up and scratches; incompletely removed oxide scales are pressed into the surface, forming black streaks.

 

Uneven cooling rate leads to asynchronous microstructure transformation, forming microstructural difference bands that appear as black streaks after pickling or heat treatment.

 

(3) Surface Contamination and Medium Reaction

Deteriorated or poorly filtered emulsion retains titanium chips and oil stains, which carbonize at high temperatures to form hard-to-clean black streaks.

High-temperature oxidation causes uneven oxide scales; local over-etching or insufficient pickling leads to color difference; incomplete neutralization triggers local corrosion and blackening.

 

(4) Equipment and Tooling Defects

Worn, titanium-stuck or cracked rolls imprint periodic black streaks on the bar surface.

Misaligned guides and uneven roller tables scratch or fold the bar surface, and the damaged areas blacken after oxidation and corrosion.

High equipment vibration leads to unstable rolling, aggravating surface compositional and microstructural inhomogeneity and producing black streaks.

 

 

(1) Visual and Machine Vision Surface Inspection

Inspection under sufficient light: Black streaks are continuous or discontinuous dark streaks along the rolling direction, with clear boundaries and no obvious unevenness. Low cost and intuitive, but low efficiency and easy to miss fine defects.

 

Linear array cameras and multi-angle light sources are adopted for 360° imaging of bars. Black streaks are identified through grayscale comparison, 3D point cloud and other algorithms, enabling size measurement, positioning and classification. The efficiency is 5–10 times that of manual work, suitable for batch online inspection.

 

(2) Non-Destructive Testing

• Ultrasonic Testing

Ultrasonic waves reflect or attenuate at defect interfaces, and defect location and depth are determined based on waveforms.

Detects internal segregation, porosity and microcracks, and can locate near-surface defects with a depth of less than 1 mm.

• Eddy Current Testing

Compositional and microstructural inhomogeneity changes eddy current impedance, and defects are identified through abnormal signals.

Rapidly screens surface and near-surface black streaks with a depth of less than 0.5 mm, and distinguishes segregation from mechanical damage.

• Penetrant Testing

Penetrant seeps into open defects and becomes visible after developing.

Judges whether black streaks are accompanied by open defects such as microcracks and folds, mostly used for final finished product inspection.

 

 

(1) Source Prevention and Control

Select high-purity sponge titanium and high-quality aluminum-vanadium master alloy ; accurately proportion and multi-stage mix materials to control composition error within ±0.1%. Adopt three-time vacuum consumable arc melting and optimize the process, combined with homogenization annealing to reduce segregation; conduct flaw detection on ingots, and unqualified blanks are prohibited from use.

 

(2) Process Optimization

Adopt a sectional heating mode to control heating rate and temperature difference, preventing overheating and burning. Ensure finishing rolling temperature and reasonably allocate deformation during rolling; maintain equipment in a timely manner, and optimize lubrication and descaling. Cool uniformly after rolling, and anneal finished products to relieve stress, homogenize microstructure and reduce black streaks.

 

(3) Terminal Treatment

Adopt a complete cleaning process of alkaline washing, pickling, neutralization and pure water rinsing to remove oil stains and oxide scales and prevent corrosion residues.

 

online screening with machine vision + eddy current, and manual visual inspection + sampling ultrasonic testing before finished product warehousing to ensure full qualification.

 

Bars with slight segregation black streaks can be used after turning and polishing; bars with brittle segregation, impurities and microcracks are directly scrapped without downgrading.

 

 

For more details about Gr5 titanium bars, reach out anytime :Sam.Rui@bjrh-titanium.com.