Tube forming technologies based on internal forming pressures, such as hydroforming or hot tube gas forming, are state of the art to manufacture complex closed profile geometries. However, materials with excellent specific strengths and chemical properties, such as titanium alloys, are often challenging to shape due to their limited formability. In this study, the titanium alloy Ti-3Al-2.5V was processed by superplastic hot tube gas forming to manufacture a helically shaped flex tube. The forming process was investigated in terms of process simulation, forming tool technology and process window for the manufacturing of good parts. Within a simulation study, a strain rate optimized forming pressure–time curve was defined. With the newly developed tool design, forming temperatures up to 900 °C and internal forming pressures up to 7 MPa were tested. A process window to manufacture good parts without necking or wrinkling has been successfully identified. The experiment data showed good agreement with the numerical simulations. The detailed study of the process contributes to an in-depth understanding of the superplastic forming of Ti-3Al-2.5V during hot tube gas forming. Furthermore, the study shows the high potential of superplastic hot tube gas forming of titanium alloys for the manufacturing of helical flex tubes and bellows.

https://www.mdpi.com/2075-4701/10/9/1150/pdf

Process Development for a Superplastic Hot Tube Gas Forming Process of Titanium (Ti-3Al-2.5V) Hollow Profiles

Deformation behaviors of four-layered U-shaped metallic bellows in hydroforming

Metal bellows are widely used in piping systems, aerospace, automobile and other industries due to their favourable properties including absorption of expansion, light weight and flexibility. In this paper, the fittability was presented to evaluate the convolution shape precision of the metal bellows. By establishing a finite element model of bellows hydroforming process during the bulging and forming stages, the influence of internal pressure, axial feeding and feeding loading path on the wall thickness variation and fittability of one-convolution bellows was investigated. On that basis, the hydroforming process of multi-convolution bellows was studied, and an experiment was carried out. The results showed that with the increase in internal pressure, the wall thickness of the bellows thinned overall, and the fittability of the root zone of the bellows gradually deteriorated. Some region near the crown zone did not fit the dies well when the internal pressure is small. To improve the fittability of the bellows, the actual axial feeding should be a bit less than the theoretical axial feeding. It is of importance in developing the hydroforming technique and improving the hydroforming quality of bellows.

Influence of process parameters on thinning ratio and fittability of bellows hydroforming

Tube die-less forming technology benefits from its characteristics of reducing die processing and material consumption. It is a flexible forming technology in line with today's world outlook on development and will be popularized in aerospace, nuclear energy, petrochemical and other fields. This chapter first introduces the principle of tube die-less forming. It then expounds the die-less forming process of complex section tube fittings with groove characteristics, bellows and large-diameter thick wall guide tube details to guide the processing of these three kinds of parts and other metal tube fittings. 

Flexible Tube Die-Less Forming

The mapping between the two symmetrical unusual complex geometries is used to correlate the parameters by conformal transformation technique. To locate the actual localized stress points (normal and shear) in the thin-walled corrugated tubes (U-shape bellows convolution) geometry, physical correspondence and relationship is compared to orthotropic plate. The transformation of the associated components of membrane stress moment's components between plate and bellows is discussed. The distributed stress-points on the top, center, and bottom parts of the convolution are technically defined and associated nature of the stresses has been confirmed by correlation. The positioning of the stress- points on the convolution geometry surfaces at the top, middle and bottom parts correspondingly in comparison with plate signifies the pressure-based meridional stresses, circumferential membrane stress, and deflection-based meridional stresses respectively. The stress defined theoretical conformation maps are well verified with FEA simulation and confirms the location with type of the stresses by comparing it with the experimental results. The result obtained by both the methodologies found in close match and well verifies the identification and nature of the associated stresses. This study is very useful to compute the stresses for the complex symmetrical shaped metal bodies made up of the thin walled thickness. Complicated shaped bodies could be compared with the analogous simple geometry plates to define the stress-strain locations. The study is more useful in the case of the existence of the plane stress conditions of the bodies. 

Normal and shear stress distributed points on thin walled corrugated tubes by conformal transformation technique

Reinforced s-shaped bellows is a kind of typical reinforced metal bellows that can withstand high pressure. Hydroforming process and its simulation technology of reinforced s-shaped bellows were investigated in this paper. The hydroforming technology of reinforced s-shaped bellows is a severe plastic deformation process. Most area of the bellows is in the state of severe plastic yielding with a maximum equivalent plastic strain of 30.6%. At the same time, the wall thickness shows a linear thinning trend from wave trough to peak. The control of hydraulic pressure is a key factor of hydroforming process. The best hydroforming hydraulic pressure is 12 MPa, which is in good agreement with the designed shape. At last, the key factors affecting plastic strain and wall thickness thinning of the hydroforming process were revealed. Plastic strain and wall thickness thinning of the large-pitch bellows in the hydroforming process are obviously higher than those of the small-pitch ones. The waveform parameters directly affect the hydroforming process. However, the influences of initial wall thickness and number of layers of the bellows were not obvious. The maximum thinning rate of the bellows basically maintains around 20%.

Jianfeng Han

Papers

Study on the hydroforming technology of reinforced s-shaped bellows