In-depth Analysis of Titanium Alloys: The Mysteries of Heat Treatment Processes and Structural Transformations(I)

In the fields of aerospace, medical equipment, high-end equipment manufacturing, etc., titanium alloy has become an indispensable key material due to its excellent strength, corrosion resistance and lightweight characteristics. The excellent performance of titanium alloys is inseparable from the precise regulation of the heat treatment process and the complex organizational changes that occur in the process. Now, we will explore the core knowledge of titanium alloy heat treatment and organizational transformation in depth, and uncover the technical code behind this “space metal”.

The Four Core Heat Treatment Processes Of Titanium Alloy

During the processing of titanium alloy, the internal stress and tissue state will directly affect its final performance.In order to optimize material properties and meet the needs of different application scenarios, common heat treatment processes mainly include stress relief annealing, complete annealing, solution treatment and aging treatment. Each process has its own unique purpose and operating points.

1. Stress Relief Annealing: Eliminate Internal Stress And Stabilize The Material State

After the hot forging, casting, cold deformation processing, cutting, welding and other processes of titanium alloy, internal stress is prone to occur inside the material. If it is not eliminated in time, it may cause deformation and cracking of the workpiece, affecting the service life. The core purpose of stress relief annealing is to remove these internal stresses through the regulation of specific temperature and time.

For heat-treatable titanium alloys, recrystalline temperature is usually used for annealing, and the ”recovery" mechanism (that is, through vacancy and dislocation movement, the second type of internal stress generated by deformation is eliminated) is used to achieve stress relief. The choice of annealing temperature and time is essential. It needs to be accurately adjusted according to the type of titanium alloy, the processing method, and the size of the internal stress. It is necessary not only to ensure the full release of the internal stress, but also to avoid fluctuations in material properties caused by excessive heating.

2. Complete Annealing: Optimize The Organization And Improve Plasticity

Complete annealing, also known as recrystalline annealing, its core goal is to obtain a recrystalline structure of titanium alloy, greatly improve the plasticity of the material, and lay the foundation for subsequent processing or use.At present, most α-titanium alloys and α+β duplex titanium alloys are put into use in a fully annealed state, and the complete annealing processes of different types of titanium alloys are significantly different.

* α-titanium alloy: The annealing temperature must be strictly controlled at 120~200℃ below the phase transition point. Too high temperature will lead to roughening of the grains and reduce the strength of the material; too low temperature will make the recry-stalline incomplete and the plasticity will not meet expectations.Since the cooling speed has little impact on the organization and performance of α-titanium alloy, air cooling is mostly used in actual production, which is easy to operate and low cost.

* Near-α titanium alloy and α+β duplex titanium alloy: The annealing process will not only recrystalline, but also accompanied by changes in the content and morphology of the α and β phases. Therefore, the determination of the annealing temperature and cooling method is more complicated. It is necessary to combine the alloy composition and the target performance. Comprehensive design, in order to balance plasticity and strength.

* Metastable β-titanium alloy: Complete annealing is usually combined with solution treatment. The annealing temperature is generally 80~100℃ above the α+β/β phase transition point. The alloy structure is fully transformed through high temperature treatment to prepare for subsequent performance regulation.

3. Solution Treatment: Construct A Metastable Phase To strengthen “Power Storage”

The key to solution treatment is to allow titanium alloys to obtain metastable phases that can be strengthened by aging, such as α' martensitic, α"martensitic or metastable β phase. These metastable phases will decompose during the subsequent aging process to produce fine equilibrium phases, which significantly improve the hardness and strength of the material through the "precipitation strengthening effect".

There are strict standards for the selection of the solution temperature, which is usually lower than the α+β/β phase transition point of 40~100℃. This temperature interval can not only ensure the acquisition of nascent α and β phases, but also avoid excessive roughening of β grains and ensure the overall performance of the material. The cooling methods after the solution are mainly water quenching and oil quenching. Among them, water quenching is more widely used due to its fast cooling speed and good strengthening effect.

4. Aging Treatment: Release The Strengthening Potential And Lock In Excellent Performance

Aging treatment is the “next key step” of solution treatment. Its core is to allow the metastable phase formed by rapid cooling after solution treatment to gradually transform into an equilibrium phase at a specific temperature.In this process, it will be accompanied by reactions such as metastable phase decomposition and supersaturated alpha phase decomposition. It is these changes that give titanium alloy the ability to strengthen heat treatment, and ultimately make the hardness, strength and other properties of the material meet the design requirements.