Titanium and its alloys possess numerous exceptional properties, including low density, high specific strength, excellent corrosion resistance, and good biocompatibility. They are widely utilized across various sectors such as aerospace, petrochemicals, biomedical engineering, and national defense. Through long-term technology introduction, independent research and development, and application promotion, China's titanium industry has entered a phase of rapid development, with steadily increasing output, solidifying its position as a major player in the global titanium industry. In recent years, the demand for titanium and its alloys has continued to grow, their service environments have become more diverse, forming processes have grown increasingly complex, and requirements for material performance specifications have been continually rising.
Plastic processing is a manufacturing technique that utilizes external force to induce plastic deformation in materials, thereby achieving desired shapes, microstructures, and properties. Common plastic processing methods include forging, rolling, extrusion, drawing, and spinning. However, titanium and its alloys exhibit increased strength and hardness alongside decreased plasticity and toughness during plastic deformation. Processing can also easily lead to issues like surface cracking, oxidation, and excessive roughness, which can adversely affect the mechanical properties, corrosion resistance, and subsequent assembly precision of components. In recent years, emerging superplastic forming (SPF) technology has seen widespread application, significantly improving the formability challenges of titanium alloys.
Currently, plastic processing technologies encompass both traditional and novel methods. Selecting the appropriate plastic processing technique is crucial for enhancing the quality of titanium and titanium alloy products. This article reviews the research progress and application status of major plastic processing techniques for titanium and its alloys (forging, rolling, extrusion, etc.) and offers perspectives on future development trends.
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Traditional Plastic Processing Techniques
Forging
Forging is a common method for processing metal components. It involves applying pressure to cause plastic deformation, thereby obtaining parts with the desired shape and microstructure. Due to the crystalline structure characteristics of titanium alloys, their products are highly sensitive to forging parameters (e.g., temperature, deformation amount), requiring strict process control. During forging, titanium grains reorganize and become denser, internal impurities and voids are eliminated, and internal stresses are partially relieved.
Consequently, material uniformity, purity, mechanical properties, and surface quality are improved. However, its drawbacks are also evident: relatively low production efficiency, significant challenges in process control, and difficulty in processing parts with complex geometries.
Rolling
Rolling is favored for its low cost and operational convenience. Metal materials undergo severe deformation during rolling, and adjusting process parameters can alter their microstructure and mechanical properties. Based on processing temperature, rolling is classified into hot rolling and cold rolling. Hot rolling, performed at elevated temperatures, can eliminate defects in the billet and allows for large deformations. In regions of high deformation, dynamic recovery and recrystallization occur. Cold rolling typically does not induce recovery or recrystallization but can enhance material strength and surface finish. It is generally used as a final step in producing sheets and strips.
Additionally, annealing can be incorporated during rolling to control deformation, or bending distortions can be directly corrected. In recent years, rolling technology in China has developed rapidly, becoming a crucial forming method for titanium and titanium alloy products like plates, bars, and tubes. Compared to forging, rolling offers higher efficiency, greater product precision, and significantly lower production costs, making it suitable for low-cost manufacturing of titanium alloys. Plates are categorized by thickness into thick plates (>4.76 mm) and thin plates (≤4.76 mm). Thick plates are hot-rolled to final dimensions, while thin plates may undergo hot rolling followed by cold rolling, or more efficient methods like pack rolling, hot pack rolling, or coil-to-coil production. Bars are primarily produced using large-deformation processing techniques.
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Extrusion
Extrusion forming, by applying triaxial compressive stress, enables large plastic deformation and improves the comprehensive performance of products. It offers advantages such as wide applicability, high production efficiency, and relatively simple process flow, making it a common method for producing titanium alloy tubes and bars. Based on the relationship between metal flow direction and punch movement, extrusion can be classified into forward (direct) extrusion, backward (indirect) extrusion, combined extrusion, and radial extrusion. Due to the severe springback of titanium and its alloys, their extrusion deformation process is more complex than that of other alloys, making extrusion temperature and process parameters particularly important.
Compared to forging and rolling, extrusion more easily achieves deformation of low-ductility metals and bonding of dissimilar metals. It also avoids the need for expensive full sets of die designs, offering high processing efficiency and production flexibility. However, due to the unique physical and chemical properties of titanium alloys, issues like temperature rise, increased deformation resistance, and die sticking can occur during extrusion. Selecting appropriate lubrication methods and lubricants is key to effectively reducing extrusion force, extending die life, and improving product quality, and has become a critical technology in titanium alloy extrusion production. Furthermore, factors such as extrusion die design and process parameters influence the quality of titanium alloy profiles.
Key parameters include extrusion ratio (λ), billet heating temperature, and extrusion speed. The extrusion ratio is related to alloy type, extrusion method, product requirements, and press capacity. Glass lubricants can effectively protect the billet during heating and provide lubrication during extrusion. Additionally, extrusion speed affects not only product properties and surface quality but also extrusion force. Excessive speed can lead to uneven metal flow; suitable extrusion speeds are generally below 200 mm/s.
Novel Plastic Processing Techniques
Titanium and its alloys are characterized by high deformation resistance and complex microstructural evolution during hot working. Conventional plastic processing techniques like forging, rolling, and extrusion often struggle to form complex shapes. Superplastic Forming (SPF) technology effectively addresses this issue. It is a highly efficient component manufacturing technique suitable for materials, like certain titanium alloys, that exhibit superplasticity under specific intrinsic and extrinsic conditions. Using SPF can not only reduce manufacturing costs but also significantly improve forming efficiency. It has become an important method for titanium alloy processing and is widely used in the aerospace sector.
Main SPF methods include superplastic stretch forming, superplastic die forging, superplastic extrusion, and superplastic blow forming (gas pressure forming). SPF offers advantages such as large deformation, absence of necking, low flow stress, and good formability. Severe Plastic Deformation (SPD) techniques can improve material strength and toughness, achieving enhanced comprehensive properties.
Major SPD methods include High-Pressure Torsion (HPT), Friction Stir Processing/Welding (FSP/FSW), Equal Channel Angular Pressing (ECAP), Accumulative Roll Bonding (ARB), and Multi-Directional Forging (MDF). Moreover, to meet stringent material requirements in different service environments, researchers have combined SPF with traditional techniques like forging, rolling, extrusion, and drawing, developing various composite deformation technologies that have been extensively studied. In recent years, the scope of SPF research has continually expanded, but the depth of investigation remains insufficient. Much work is still at the theoretical and experimental stage. Further exploration of the intrinsic mechanisms and processing rules of superplastic forming for titanium alloys is needed, along with improvements in processing methods, equipment, component quality, production efficiency, and expansion of application ranges.
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Diffusion Bonding (DB), also known as diffusion welding, is a solid-state welding technique where materials are brought into contact under certain temperature and pressure, achieving a tight bond through prolonged atomic diffusion. It enables large-area joining with minimal residual stress. When a material's superplastic forming temperature is close to its diffusion bonding temperature, SPF and DB can be completed in a single heating/pressure cycle to produce locally or integrally stiffened structures or more complex monolithic components. This has evolved into the Superplastic Forming/Diffusion Bonding (SPF/DB) process. SPF/DB technology has been widely researched and applied in aviation, offering benefits such as: ① Forming multiple parts into a monolithic structure in one heating cycle, reducing costs; ② Large, crack-free deformation with minimal residual stress and high forming precision; ③ Excellent overall performance of the structure, with enhanced fatigue and corrosion resistance.
Current demands in aerospace, automotive, and high-tech industries for component processing emphasize lightweighting, high strength-toughness, precision, high efficiency, and sustainability. Many precision plastic forming processes require dedicated dies and can be energy-intensive. Incremental forming technology has attracted attention for overcoming the drawbacks of traditional precision forming, such as high die specificity and energy consumption. However, there are relatively few research reports on titanium alloy incremental forming. Existing processes suffer from drawbacks like susceptibility to forming defects, poor forming stability, and complex, expensive equipment.
There are numerous plastic processing methods for titanium and its alloys, each with its own advantages and disadvantages. The appropriate process should be selected based on specific requirements, generally following these principles: reliable and simple operation; meeting product performance needs; low process cost. As the application fields for titanium and its alloys continue to expand, efficient, high-quality, and low-cost new technologies and processes (such as SPF, SPF/DB, composite incremental forming, etc.) are being continuously developed and researched. With ongoing in-depth research into new titanium alloy processing technologies and techniques, product quality and competitiveness will continue to improve.
