Hydrogen-Induced Cracking in Titanium Plate Welding

In titanium plate welding cracks, hydrogen is recognized as an "invisible killer." The formation of cracks due to hydrogen is not a single or accidental event but rather a chain reaction formed through multiple interconnected stages: "dissolution → diffusion → precipitation → cracking." The core of the issue can be dissected from two main dimensions: titanium's hydrogen storage characteristics and hydrogen-induced cracking mechanisms.

I. Root Cause: Titanium's "Special Sensitivity" to Hydrogen
Titanium's crystal structure and chemical properties determine its characteristics of "strong adsorption + drastic solubility changes" towards hydrogen, forming the "inherent foundation" for hydrogen-induced cracking:

• Strong Hydrogen Storage Capacity: Titanium is a typical "hydrogen storage metal." Its atomic structure contains numerous interstitial sites that can accommodate hydrogen atoms. Under high welding temperatures (molten pool temperature: 1600–1800°C), hydrogen dissolves extensively into liquid titanium through arc ionization and surface adsorption, with solubility reaching several tens of ppm—far higher than at room temperature.

• "Cliff-like Drop" in Solubility: During cooling, hydrogen solubility in titanium plummets. At room temperature, solubility is only 5–10 ppm, significantly lower than at high temperatures. If the cooling rate is too fast (e.g., air cooling, windy welding conditions), hydrogen exceeding the solubility limit cannot escape in time and is forcibly precipitated as hydrides (primarily TiH₂).

• "Embrittlement Nature" of Hydrides: TiH₂ is a typical brittle phase with high hardness and almost zero plasticity. Its precipitation disrupts the originally continuous crystal structure of titanium—akin to embedding "rigid fragments" into the "ductile titanium matrix." This drastically reduces the plasticity of the titanium plate (from 20%–30% to below 5%) and its crack resistance, laying the groundwork for cracking.

II. Core Mechanisms: Three Key Pathways of Hydrogen-Induced Cracking in Titanium Plate Welding
Hydrogen does not directly "tear" the titanium plate but induces cracks step by step through different mechanisms, synergized with welding stress and cooling conditions. These mechanisms can be categorized into three types:

1. Hydride Embrittlement Cracking: The Most Direct "Hard Damage" Cracking
   During welding, titanium in the molten pool and heat-affected zone (HAZ, temperature: 500–1000°C) absorbs a large amount of hydrogen. Upon cooling, hydrides preferentially precipitate in stress concentration areas, directly causing local embrittlement and cracking:

• Concentrated Precipitation Locations: Hydrides tend to aggregate at grain boundaries and dislocation-dense areas in titanium—grain boundaries are "weak zones" with loosely arranged atoms where hydrogen atoms easily accumulate. Welding stress generates numerous dislocations in titanium, which "trap" hydrogen atoms, forming local high-hydrogen zones that eventually precipitate TiH₂.

• Cracking Process: TiH₂ at grain boundaries forms a "brittle film," severing the bonding force between grains. TiH₂ around dislocations acts as "microcrack nuclei." Under the influence of residual welding stresses (e.g., shrinkage stress, structural stress), microcracks rapidly propagate, eventually forming macroscopic cracks. This type of cracking often occurs shortly after welding, concentrated in the HAZ or near the fusion line, with cracks typically extending along grain boundaries.

2. Hydrogen-Induced Delayed Cracking: The Hazardous "Lurking" Failure
   Some titanium plates show no visible defects after welding but suddenly crack after hours, days, or even during initial use. The core lies in the "delayed diffusion + stress-driven" action of hydrogen:

• Continuous Hydrogen Diffusion: After welding, residual stress exists inside the titanium plate. Driven by stress, hydrogen slowly diffuses and accumulates in stress concentration areas (e.g., tiny pores, inclusion edges, lack of fusion zones)—this process takes time, so cracks do not appear immediately.

• Critical Cracking Conditions: When the local hydrogen concentration exceeds a critical value (typically >15 ppm), two key changes occur: first, a large amount of TiH₂ precipitates, embrittling the local area; second, hydrogen atoms combine into hydrogen molecules, forming a "high-pressure hydrogen cavity" at the stress concentration site (volume expansion generates internal pressure).

• Crack Initiation and Propagation: When the pressure from the hydrogen cavity plus residual stress exceeds the fracture strength of titanium, microcracks initiate at the stress concentration site. These microcracks gradually propagate along the distribution direction of hydrides, ultimately leading to sudden cracking. This type of crack is highly concealed and more hazardous, especially for titanium components in critical fields like aerospace and medical devices, potentially causing safety incidents.

3. Hydrogen Porosity-Induced Cracking: The "Accomplice" Effect of Hydrogen
   Hydrogen is also a primary source of porosity in titanium plate welding, and porosity further "amplifies" hydrogen's cracking effect:

• Formation of Hydrogen Porosity: Hydrogen dissolved in the high-temperature molten pool, if its escape rate during cooling is slower than the solidification rate of the molten pool, becomes trapped in the titanium matrix, forming round or oval hydrogen pores.

• "Stress Concentration Effect" of Porosity: Stress at the edges of pores can be 3–5 times higher than in the surrounding matrix, making them natural "crack-sensitive zones." Hydrogen preferentially diffuses and accumulates at pore edges, on one hand precipitating TiH₂ to embrittle the edges, and on the other increasing hydrogen pressure inside the pores. Under this dual action, microcracks easily initiate at pore edges and propagate along the direction of hydrides, eventually connecting with other microcracks to form macroscopic cracking.

Summary: The Core Logic of Hydrogen-Induced Cracking
Hydrogen-induced cracking in titanium plate welding is essentially a closed loop: "Titanium's hydrogen storage characteristics → hydrogen dissolution and precipitation → hydride embrittlement/hydrogen diffusion and accumulation → stress-driven crack propagation." Titanium’s strong hydrogen adsorption and drastic changes in solubility make it susceptible to hydrogen intrusion and brittle phase precipitation during welding. Welding stress and cooling rates act as "catalysts," accelerating crack initiation and propagation. To prevent and control such cracking, the core approach involves three aspects: "cutting off hydrogen sources (e.g., cleaning workpieces, using high-purity shielding gas), controlling hydrogen precipitation (e.g., optimizing cooling rates), and actively removing hydrogen (e.g., post-weld dehydrogenation treatment)," thereby reducing hydrogen intrusion and retention at the root cause.