The titanium sheet is well-known and appreciated for its high strength-to-weight ratio and excellent corrosion resistance. It is used for diverse applications, especially in the aerospace and automotive industries, being formed through a wide range of processes. This article discusses the titanium sheet forming and its characteristics.
High strength titanium sheet formed at room temperature
One of the most commonly-used titanium alloys is Ti-6A1-4V, an alloy of high strength and whose forming behavior has been studied at room temperature. In order to determine its mechanical properties, as well as the minimum bending radius, scientists have resorted to both tensile tests and swing folding trials. Moreover, they have analyzed the springback behavior through V-die bending tests.
The tests revealed that this particular titanium alloy has a reduced formability, with a high tendency to springback. The material hardening is low as well, which means that stamping or deep drawing are not possible at room temperature. Scientists have also determined that Ti-6A1-4V can be roll formed at room temperature, resulting in simple longitudinal sections.
The formability of the titanium alloy can be improved through roll forming. More importantly, the tendency to springback decreases, especially in comparison through the one observed in the V-die bending tests.
Roll forming reduces the risk of structural defects, representing a potential solution for titanium sheet forming at room temperature. The resulting sections could be useful for applications in different industries, including the aerospace and automotive ones.
Prior inspection is necessary before the actual forming
The titanium sheet has to be inspected before the forming process, regardless of which option the manufacturer might choose. The inspection can identify structural defects but also check how uniform and thick the sheet is (certain standards have to be met). The manufacturer might also analyze how flat it is, paying attention to additional elements, such as strength and hardness. Last, but not least, the bending behavior will be analyzed.
A careful analysis of the titanium sheet will reveal whether the critical regions have been affected by scratches or other marks, such as from grinding (the titanium sheet is quite sensitive, so it must be handled with extra care). If there are scratches presented, these should be removed through grinding.
The titanium sheet has to be properly cleaned before forming. The removal of oxide facilitates the forming process but this is not the only aspect to take into consideration. It is important to eliminate any traces of grease, oil or dirt; in fact, any chemical or residue present on the surface can have a negative influence on the forming process, especially the one performed at high temperatures (hot forming).
In order to ensure the best results for the forming process, the titanium sheet is handled with clean gloves made from cotton. These are worn before and after the forming process, ensuring that no residue will reach the sheet.
Special attention must be paid to sheet grinding, which is performed to finalize the thickness of the sheet but can leave marks. Also, the shearing of the titanium sheet has to be performed with extra care, as there is a risk of cracks occurring.
Cold forming is a process employed not only for commercially-pure titanium but also for the majority of titanium alloys (to a limited extent). Among the titanium alloys which can be formed through this process, there are: Ti-15V-3Sn-3Cr-3A1, Ti-3A1-8V-6Cr-4Zr-4Mo and Ti-8A1-1Mo-1V.
These titanium sheets can be cold formed using the standard process but one has to make sure that the bend is of a larger radius than in hot forming. Also, the stretch flange must be shallower. In the situation that cold forming is used for other alloys, this will lead to excessive springback. Moreover, stress relieving is required, as well as additional power.
The stretch forming of titanium and its alloys can occur at low temperatures (without being heated). However, it must be taken into consideration that the formability of titanium sheets is best at lower forming speeds. In order to reduce stress corrosion and keep the risk of cracks down at a minimum, manufacturers might resort to both stress relieving and hot sizing.
Upon heating the titanium sheet, there are a number of significant changes that take place. The formability increases and the tendency to springback is reduced, allowing for enhanced modifications.
Hot dies are however used for more severe forming. What happens is that the flat titanium sheet is placed in the die and heated to the desired temperature. Slow pressure is applied and the titanium sheet is removed after a period of time. The titanium alloy Ti-6A1-4V is hot formed at temperatures of approximately 730ºC.
It is worth mentioning that the best results, in terms of uniformity and ductility, are obtained when the temperatures go above 540ºC. At higher temperatures, titanium alloys demonstrate a property known as superplasticity; however, they also present a higher risk for more severe contaminations.
If temperatures over 870ºC are used, the forming must be performed in a vacuum or in a protective atmosphere (for example, argon might be used). In general, the superplastic forming can take place at temperatures between 870º and 925ºC, but the temperature has to be monitored at all times. Hot sizing is used to correct shape and size inaccuracies.
As opposed to the previous method, hot forming has a unique advantage to offer. It improves the uniformity of the titanium sheet and the yield strength, as long as both the forming and sizing temperature is over 540ºC.
Superplastic forming is a process used for the fabrication of sheet metal components, which are further employed by the aerospace industry. These structures have a light weight and a high level of efficiency to offer.
For this process, there is a computer system used to keep the gas pressure under control. The most commonly-used titanium alloy for superplastic forming is Ti-6A1-4V. As for the limitations of the process, it is worth mentioning that the required tools have to be heat resistant. Longer preheating times are required, as well as a protective atmosphere (argon).
There are different forming processes that can be employed, including blow forming, thermoforming, vacuum forming and deep drawing. The superplastic forming process can be enhanced with diffusion bonding. Contour roll forming is also preferred for the titanium sheet, with special attention being paid to the bend radius (allowable limit). Roll forming remains one of the most cost-advantageous methods for forming the titanium alloy sheets, which will be further used in the aircraft industry.