As a key process in the manufacturing of complex structural parts, titanium alloy plate superplastic forming technology has important applications in aerospace, automotive industry and other fields. This paper provides a professional explanation and technical analysis of the three main methods of this technology: vacuum forming, pneumatic forming (blow molding) and molding (coupling molding).
Vacuum forming method: low-pressure precision forming
Vacuum forming is essentially to use the atmospheric pressure difference to realize plate forming, which belongs to the category of low-pressure forming, which can be divided into punch die method and concave die method.
Punch method: The plate heated to superplastic temperature is adsorbed on the punch die with the internal shape characteristics of the part, which is suitable for deep cavity parts that require high dimensional accuracy on the inside. In the manufacture of spacecraft precision structural parts, this method can effectively control the profile accuracy and wall thickness distribution to meet the dimensional stability requirements under extreme working conditions.
Concave die method: The plate is adsorbed on the concave die with the shape of the part, and is mainly used for shallow cavity parts with high dimensional accuracy. In the field of automotive exterior parts, this method can ensure good surface quality and shape consistency, which is conducive to the realization of lightweight and integrated molding.
Technical features and limitations: Vacuum forming pressure of only approx. 0.1 MPa relies on the superplastic behavior of the material for parts with thin plates (typically < 2 mm thick) and smooth curvature variations. For parts with large thicknesses or complex structures, the forming capacity is limited, and process optimization and material modification are required to expand their application range.
Air pressure forming method (blow molding method): flexible forming under controllable air pressure
Pneumatic pressure forming applies controlled pressure through inert gas (such as argon) to gradually fit the plate in the superplastic state into the mold, which is divided into two categories: free blow molding and mold blow molding.
Free blow molding: No mold is required, the plate is freely expanded by air pressure, and is often used for spherical, hood-shaped and other axisymmetric parts. Its advantages are low mold cost and short cycle time, but shape control relies on process parameter adjustment, suitable for trial production or small batch production.
Mold Blow Molding:
Punch forming: Air pressure acts on the outside of the plate, causing it to wrap around the punch. The internal shape of the part has high accuracy and large depth-to-width ratio, but the difficulty of demolding and material utilization are low, and the bottom is easy to thicken.
Concave mold forming: Air pressure acts on the inside of the plate to make it fit into the concave model cavity. The shape of the part has high accuracy, easy demolding, and high material utilization, but the aspect ratio is limited and the bottom thickness is relatively small.
Process advantages: Pneumatic forming can adjust the pressure in the range of 0.3–2.0 MPa to accommodate more complex geometries and larger deformations. The friction is small and the stress state is uniform during the deformation process, which is conducive to improving the consistency of material forming limit and mechanical properties of parts.
Molding method (coupling molding): high-precision contact forming
Molding is pressurized with the upper and lower dies closed, and is formed at a very low strain rate (typically 10⁻⁴–10⁻³ s⁻¹) at superplastic temperatures. While high-precision, high-surface quality parts are theoretically available, the following challenges are presented:
The mold needs to have good thermal stability and creep resistance at high temperatures, and is commonly made of nickel-based alloys or ceramic materials.
The mold fit accuracy requirements are extremely high, especially for complex profiles, the processing difficulty and cost increase significantly;
During the forming process, the friction and temperature distribution between the plate and the mold must be strictly controlled to avoid local thinning or cracking.
Therefore, this process is currently mostly used for experimental research or specific high-precision parts, and industrial application still needs further breakthroughs in mold technology and lubrication conditions.
Process selection and prospects
In actual production, the process selection should be made according to the structural characteristics, precision requirements, batch and cost of parts:
Vacuum forming: suitable for shallow cavity or deep cavity precision parts of thin plates, focusing on cost control and surface quality;
Pneumatic forming: suitable for complex three-dimensional shapes, medium and heavy plates and structural parts that require uniform deformation;
Molding: Currently limited to testing and small-batch high-precision parts, there may be room for improvement with the development of mold technology in the future.
Superplastic forming technology is developing in the direction of composite processes (such as hot forming-superplastic composite forming), intelligent process control (based on numerical simulation and real-time monitoring), and the development of new titanium alloys (high-strain rate superplastic materials), which will further expand the application prospects of titanium alloys in the field of lightweight and integrated structures.