Analysis On Forming Processes And Mechanical Performance Of Titanium Alloy Elbows

Titanium alloy elbow forming quality is critical to pipeline system safety, reliability and service life. Its high yield strength, large yield ratio, narrow plastic deformation range and high-temperature sensitivity to O, N, H contamination limit traditional forming processes.

 

 

 

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(I) Welding Forming Process

• Segmented (Shrimp Waist) Welding: Includes multi-section welding, featuring low equipment requirements and capability for large-diameter elbows. However, it has numerous weld seams, is easy to stress concentration, shows poor corrosion resistance, is liable to leakage, and has a short service life.

 

• Press Plate Welding: Reduces the number of weld seams and slightly improves corrosion resistance but still carries welding risks. It is only used in small quantities for emergency or non-critical pipelines.

 

(II) Stamping Forming Process

• Cold Stamping: Suitable for small-diameter, thin-walled, and small-batch production, offering low cost and simple processes. Its defects include large springback, wall thickness reduction, and surface wrinkling.

 

• Hot Stamping: Adaptable for thick-walled and medium-diameter elbows, ensuring good forming quality. The risk lies in easy oxidation due to improper heating protection, which affects performance.

 

(III) Hot Push Extrusion Forming Process

• Process Core: Based on the principle of constant volume during metal plastic deformation, the mandrel controls deformation, and the compressed metal in the inner arc flows to the diameter expansion area to ensure uniform wall thickness. The elbows have a smooth appearance, high dimensional accuracy, and can be produced continuously in batches.

 

• Forming Key Points: The pipe blank is in a stress state of axial/radial compression and circumferential tension, with deformation concentrated in the belly. Controlling the heating temperature, pushing speed, and mandrel parameters can avoid defects like uneven wall thickness, wrinkling, and cracking.

 

• Application Achievements: The process developed by Northwest Institute for Nonferrous Metal Research can produce seamless elbows with a maximum diameter of φ219, whose performance meets ASTM B363-95, solving the drawbacks of welding and stamping forming.

 

(IV) Casting Forming Process

• Defects: Prone to porosity, shrinkage cavities, and inclusions; wall thickness ≥5mm is incompatible with conventional pipelines; rough surface increases transmission resistance.

 

• Shortcomings: Corrosion resistance is inferior to that of wrought titanium alloy, and the cost is relatively high.

 

(V) Other Special Forming Processes

• Cold Extrusion: Suitable for high-precision and small-diameter elbows, featuring good appearance, high dimensional accuracy, and uniform wall thickness. It has high requirements for molds and pipe blanks, resulting in high costs.

 

• Mandrel Hot Forming: Capable of producing elbows of various diameters, realizing synchronous expansion and bending during heating and pushing, requiring subsequent heat treatment and machining.

 

• UO Method: Titanium plates are pressed into shape, focusing on large-diameter and thick-walled elbows, suitable for working conditions where pipe blanks cannot be used.

 

 

(I) Influence on Mechanical Properties

• Welding Forming: Weld seams are weak; coarse grains in the heat-affected zone lead to reduced performance; residual stress is likely to induce stress corrosion cracking.

 

• Stamping Forming: Work hardening results in increased hardness and decreased toughness; springback accumulation in cold stamping causes residual stress, affecting structural stability.

 

• Hot Push Extrusion Forming: Uniform hot deformation refines the structure, and heat treatment eliminates stress, achieving the optimal mechanical uniformity. The strength is the same as that of the base metal, and the elongation rate is ≥15%, meeting the requirements of key pipelines.

 

(II) Influence on Corrosion Resistance

• Welding: Discontinuous oxide film and defects in weld seams lead to poor corrosion resistance and easy corrosion leakage.

 

• Stamping: Surface defects damage the oxide film, aggravating local corrosion.

 

• Hot Push Extrusion: Inert gas protection prevents oxidation, resulting in an intact oxide film and smooth surface. The corrosion resistance is the same as that of the base metal, adapting to strong corrosion working conditions; uniform wall thickness can prevent stress corrosion and extend service life.

 

(III) Influence on Dimensional Stability

• Segmented (Shrimp Waist) Welding: Cumulative errors from multi-section welding cause uneven angles and curvature, making installation difficult.

 

• Stamping Forming: Elastic springback leads to dimensional deviations, requiring shaping to increase costs.

 

• Hot Push Extrusion Forming: Precise shape control by the mandrel ensures that dimensional tolerances meet HG/T3651-1999 and ANSI B16.9, with excellent installation adaptability.

 

 

• Process Selection

For key corrosion-resistant pipelines, hot push extrusion seamless elbows are preferred.

Cold extrusion is used for small-batch, high-precision, and small-diameter elbows.

Press plate welding can be used in small quantities for non-critical/emergency scenarios .

Segmented (shrimp waist) welding elbows are prohibited.

 

• Performance Optimization

Optimize hot working parameters through numerical simulation to improve precision.

Upgrade oxidation protection technology.

Integrate forming and heat treatment to enhance quality and efficiency.

 

• Quality Control

Comply with HG/T3651-1999 and implement full-process control to ensure safe operation.

 

Ruihang Group as a professional manufacturer, supply the high-quality titanium and titanium alloy products. For more details,please contact us via the Email: Sam.Rui@bjrh-titanium.com