What is the purpose of using a high-temperature box resistance furnace for TiBw/TA15 composite billets?

The primary purpose of using a high-temperature box resistance furnace in this context is to thermally condition the jacketed TiBw/TA15 composite billets to a specific state suitable for extrusion. By heating the material to approximately 1423 K—a temperature above the beta transformation point—the process lowers the force required to shape the metal and prepares the internal microstructure for significant physical change.

The furnace does not simply heat the material; it creates the thermodynamic prerequisites for severe plastic deformation. By ensuring a uniform internal temperature and reducing deformation resistance, this step is critical for enabling dynamic recrystallization during the extrusion process.

Critical Thermal Objectives

Lowering Deformation Resistance

The immediate mechanical goal of the pre-treatment is to reduce the material's yield strength. TiBw/TA15 composites are inherently strong and resistant to shaping at room temperature.

Heating the billet significantly decreases this resistance. This allows the extrusion machinery to deform the billet without exceeding force limits or fracturing the material.

Ensuring Thermal Uniformity

The "box resistance" furnace design is utilized to create a stable, pervasive heat environment.

It is vital that the temperature is consistent not just on the surface, but throughout the core of the billet. Uniform heating prevents internal stress gradients that could lead to cracking or uneven flow during the extrusion process.

Microstructural Implications

Crossing the Beta Transformation Point

The specific target temperature of 1423 K is chosen to push the titanium matrix past its beta transformation point.

At this temperature, the crystal structure of the alloy changes. This phase transformation improves the ductility of the material, making it more amenable to the severe shape changes involved in extrusion.

Enabling Dynamic Recrystallization

The high temperature provides the necessary energy for dynamic recrystallization to occur.

As the material is deformed, the crystal grains are stretched and damaged. The thermal energy allows the material to generate new, strain-free grains simultaneously, effectively "healing" the microstructure during the process and preventing failure.

Understanding the Trade-offs

The Necessity of Jacketing

The primary reference notes that the billets are jacketed before entering the furnace.

While the furnace provides necessary heat, high temperatures (1423 K) can cause severe oxidation or contamination of the titanium surface. The jacket acts as a sacrificial barrier, protecting the composite from the furnace atmosphere.

Balancing Temperature and Grain Growth

There is a delicate balance to be struck regarding the holding time and temperature.

While high heat improves workability, excessive time at temperatures above the beta transus can lead to coarse grain growth. If the grains grow too large before extrusion, the final mechanical properties of the composite may be compromised.

Making the Right Choice for Your Goal

When configuring your pre-treatment parameters, consider your specific manufacturing priorities:

• If your primary focus is process stability: Prioritize a sufficient holding time to guarantee the core reaches 1423 K, ensuring uniform flow and preventing machinery overload.
• If your primary focus is microstructural refinement: Tightly control the temperature just above the beta transformation point to enable recrystallization without inducing excessive grain growth.

Successful extrusion relies on achieving a thermal state where the material is soft enough to flow but structured enough to recrystallize.

Summary Table:

Parameter	Objective	Importance for TiBw/TA15
Target Temperature	1423 K (Above Beta Transus)	Enhances ductility and lowers yield strength for shaping.
Heating Method	Box Resistance Furnace	Ensures stable, uniform heat across surface and core.
Thermal State	Uniform Internal Temp	Prevents cracking and internal stress gradients during flow.
Microstructure	Dynamic Recrystallization	Repairs grain damage during deformation to prevent failure.
Surface Protection	Jacketing	Protects the composite from oxidation at high temperatures.

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