What are the forging methods for titanium forgings
Titanium and its alloys hold a key position in aerospace, energy, and medical fields due to their high specific strength, corrosion resistance, and biocompatibility. However, titanium's high chemical activity, low thermal conductivity, and high resistance to deformation require its forging process to transcend the limitations of traditional metalworking.
Free Forging: A Flexible Solution for Basic Billet Production
Free forging, which uses hammering or a press to freely deform a billet between upper and lower anvils, is the most fundamental process in titanium forging production. Its core features are simple tooling and equipment, high versatility, and low cost, making it suitable for single-piece or small-batch production. Free forging can eliminate casting defects and improve mechanical properties, but its reliance on manual labor results in low forging precision, large machining allowances, and difficulty in forming complex structures. Therefore, it is primarily used in the billet production process for large forgings, such as forging billets into stepped bars or simple shapes such as round or rectangular shapes, laying the foundation for subsequent finishing.
Die Forging: The "Mainstream Path" of Precision Forming
Die forging restricts metal flow by enclosing the die, significantly improving the dimensional accuracy and surface quality of forgings. It is a core process for mass production of titanium forgings. Based on the die structure, die forging can be divided into the following three categories:
Open Die Forging (Flash Die Forging): The die is equipped with flash grooves. Metal initially fills the die cavity, and the excess flows into the flash grooves, forming transverse flash. As the flash thins and the temperature drops, the resistance to metal flow increases, forcing more material into the die cavity. This process is suitable for mass production of complex forgings, but requires subsequent flash removal, resulting in low material utilization.
Closed Die Forging (Flashless Die Forging): The die is sealed on all sides, and metal is ejected only through longitudinal flash burrs. Material utilization can reach over 90%. Closed die forging requires strict die strength and temperature control, but it can achieve high precision (tolerance ±0.2mm) and low surface roughness (Ra ≤ 1.6μm), making it suitable for producing forgings with high precision requirements.
Extrusion Die Forging: Combining the characteristics of extrusion and die forging, hollow or solid forgings are produced through forward or reverse extrusion. Extrusion die forging can refine grains and increase material density, but it requires large equipment investment and a complex process.
Specialty Die Forging: A Technological Tool for Breaking Through Complex Structures
For deep cavities, thin walls, or special-shaped structures that are difficult to achieve with traditional die forging, specialty die forging technology uses multi-directional loading or isothermal control to break through the deformation limits of titanium alloys:
Multi-directional Die Forging: On a multi-directional die forging machine, combined vertical and horizontal loading forces the metal to flow outward from the center of the die cavity, achieving a single-step forming of complex structures. This process can form deep cavities with rib aspect ratios ≥10:1, avoiding weld defects caused by step-by-step forging.
Isothermal Die Forging: The die is heated to the same temperature as the billet (typically 30-50°C below the β-transformation temperature) and forging is completed under constant temperature conditions. Isothermal die forging reduces deformation resistance and is suitable for producing high-precision, thin-walled forgings (wall thickness ≤ 2mm). However, it requires a high-precision temperature control system (temperature fluctuation ≤ ±3°C) and heat-resistant die materials.
Segmental die forging: For extremely large forgings (such as rocket nozzles with a diameter ≥ 3m), segmented die forging or backing plate die forging is used to reduce equipment tonnage requirements. Segmental die forging can produce extremely large forgings on medium-sized hydraulic presses, but requires optimized segment interface design to avoid stress concentration.
Innovative Processes: Frontiers in Performance Optimization
As the performance requirements for titanium alloys increase, innovative processes are constantly emerging:
Beta Forging: Forging above the beta transformation temperature can improve creep resistance and fracture toughness of forgings, but strict temperature control is required to avoid beta brittleness.
Superplastic Forging: Superplastic treatment creates fine, equiaxed grains in the material, combined with isothermal forging to achieve large deformations (elongation can reach 300%-500%), making it suitable for producing forgings with extremely complex shapes.
Multi-directional Forging Cycle: Through multiple forging cycles, deformation distribution is optimized, microstructure uniformity is enhanced, and deformation per cycle is controlled between 50% and 80%, resulting in grain refinement and the elimination of casting defects.
The selection of forging processes for titanium forgings requires a comprehensive consideration of part structure, performance requirements, production costs, and equipment availability. From the flexible billet production of open die forging to the precision forming of specialized die forgings, to the performance optimization of innovative processes, each technology carries a key breakthrough in the transformation of titanium alloys from "difficult-to-machine materials" to "high-performance structural components."
