Analysis of Various Crack Types in Titanium Alloy Forging

Titanium alloys are widely used in aerospace, shipbuilding, and biomedical fields due to their high specific strength, corrosion resistance, and high-temperature resistance. However, cracking defects are prone to occur during the forging process, seriously affecting product quality and production efficiency. This article systematically reviews common cracking types in titanium alloy forging, combining typical cases with key process control points to provide technical reference for the industry.

End Face Cracking: The "Fatal Wound" of Initial Forging

End Face Cracking is one of the most common defects in titanium alloy forging, often occurring during the ingot upsetting or drawing stages. Its characteristic feature is a crack that propagates radially along the end face of the billet, and in severe cases, can prevent further forging. The main causes include:

Residual Metallurgical Defects: Incomplete removal of shrinkage cavities at the head of the ingot or the cold shut at the tail can become crack sources under forging pressure. For example, a TC4-LC titanium alloy ingot developed through-hole cracking on the side during the first drawing fire due to incomplete removal of subsurface pores.

Uncontrolled temperature gradients: During upsetting, the contact between the end face and the hammer anvil causes rapid heat dissipation. During drawing, the cooling rate at the bulged portion of the end face exceeds 30°C/s, causing localized brittleness.

Uneven deformation: Excessive reduction in a single pass or excessive deformation speed hinders metal flow in the core of the end face, resulting in sunken cracks. In a TA15 titanium alloy bar measuring approximately 85mm in diameter, internal cracks up to 12mm deep were detected in the core due to excessive drawing speed.

Preventative measures: Use ultrasonic testing to thoroughly remove ingot defects. Cover the billet end face with insulation wool during upsetting, control the reduction per pass to ≤15mm, and optimize the hammer anvil preheat temperature to ≥300°C.

 

Folding cracking: A hidden "surface killer"

Folding cracking typically results from disturbed metal flow during the forging process and manifests as layered defects on or within the billet. The formation mechanisms can be categorized into three types:

Initial defects: Ingots with a height-to-diameter ratio ≥ 2.5 or residual grooves from intermediate sampling, which cause metal folding along the defects during upsetting. A TB6 titanium alloy billet developed folding cracks up to 8mm deep after forging due to unpolished sampling grooves.

Process errors: The billet tilts during sawing, resulting in a sudden change in cross-section. Failure to polish sharp corners during 180° flipping and continued processing can cause folding.

Auxiliary process defects: Machining tool marks, oxide scale intrusion, and other defects can expand into folds during subsequent forging.

A typical case: During die forging of an aircraft engine disc, oxide scale was not cleaned from the parting surface, resulting in excessive fold depth and a 30% scrap rate. Solution: Strictly implement the "three inspections" system (self-inspection, mutual inspection, and specialized inspection), performing dye penetrant testing on the billet surface to control fold depth to ≤0.5mm.

 

Tearing and Internal Cracks: A Deeper "Organic Crisis"

Tears often occur during the tensile deformation stage, manifesting as transverse cracks. Their root causes are:

Uncontrolled deformation parameters: Excessive reduction or excessive reduction rate in a single pass leads to uneven metal flow. In one TB6 titanium alloy slab, due to a single-side reduction of 60mm, the tear depth exceeded half the plate thickness.

Tooling wear: Wear on the anvil edge causes stress concentration. In another TC4-DT titanium alloy step shaft, deformation of the anvil caused tearing at the step transition.

Internal cracks are hidden within the billet and are commonly found in small-gauge materials (Ø ≤ 90mm) or difficult-to-deform alloys (such as Ti3Al and Ti2AlNb). Their formation is related to the following factors:

Metallurgical segregation: Segregation of refractory elements such as tungsten and molybdenum leads to localized plasticity reduction. During flaw detection of a TA15 titanium alloy, internal cracks were discovered in the core, and analysis showed that they were caused by Nb segregation.

Failure of temperature management: Low chamfering temperatures or reverse forging resulting in temperature gradients exceeding 50°C. A certain Ti60 alloy developed longitudinal internal cracks exceeding 200mm in length at the chamfer due to overly rapid water cooling.

Process optimization: A multi-directional forging process (upsetting-stretching-upsetting cycles) was adopted, with intermediate annealing performed when the cumulative deformation exceeded 70%. An infrared thermal imaging system was installed to ensure that the billet temperature difference remained below 30°C.

 

Brittle cracking: The "Achilles' Heel" of high-temperature alloys

Difficult-to-deform high-temperature titanium alloys (such as TC19 and IMI 834) are extremely sensitive to temperature and are prone to brittle cracking during forging:

Excessively low final forging temperature: Below the recrystallization temperature, the metal's plasticity drops sharply. A certain high-temperature titanium alloy test material, with a final forging temperature of only 980°C, nearly broke due to cracking.

Heating process defects: Excessively rapid heating rates resulted in a temperature gradient greater than 100°C between the ends and the center. A Ti3Al ingot suffered localized brittle fracture during heating due to uneven insulation wrapping.

Improper cooling methods: Post-forging water cooling caused stress concentration. During rounding of the TC19 alloy, longitudinal cracks developed due to differential cooling rates at the chamfered edges.

Prevention and control strategies: Implement a staged heating process (e.g., three holding stages at 600°C, 800°C, and 1000°C), maintaining the final forging temperature within 50°C of the β transformation point. For difficult-to-deform alloys, utilize asbestos cladding. For a TA12A alloy, the forging yield rate increased from 63.29% to 71.45% through asbestos cladding.

 

Surface Cracks and the Alpha Brittle Layer: Hidden "Performance Killers"

Surface cracks are often caused by excessively low final forging temperatures or prolonged die contact time. A titanium alloy shell was found to have through-cracks during rough machining. The root cause was the formation of an oxygen-rich alpha layer (up to 0.2mm thick) during isothermal annealing after die forging, which increased the surface hardness by 30% and significantly increased brittleness.

Solution:

Lubricant Application: Use glass lubricant during press die forging to reduce friction between the billet and the die; shorten the contact time between the billet and the lower die to ≤2s during hammer forging.

Atmosphere Control: Maintain a slightly oxidizing atmosphere (O₂ content ≤0.5%) in the furnace during forging or heat treatment. Vacuum anneal parts with excessive hydrogen content.

 

Preventing and controlling cracking in titanium alloy forging requires a comprehensive approach across the entire metallurgical, process, and equipment chain. The risk of cracking can be significantly reduced by optimizing the heating temperature profile (e.g., controlling the initial forging temperature 150-250°C above the β-transformation point), implementing multi-directional forging processes, and strengthening online ultrasonic testing (frequency ≥ 2 times per fire). In the future, with the application of digital twin technology in forging process simulation, the prediction and control of titanium alloy cracking will move towards higher precision, providing more reliable material support for high-end equipment manufacturing.