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Gum Metals are β-Ti alloys that present only non-toxic elements in their composition, low elastic modulus, good formability, and excellent biocompatibility. The composition of these alloys needs to fulfill some electronic criteria to present their typical properties. In addition, during their processing, Gum Metal must be significantly deformed, which can be obtained by cold rolling or swaging. Over the last years, several new processing routes and alloy compositions have been proposed to meet these requirements, expanding the range of properties for Gum Metal, and increasing their extent of application. Although being relatively new, Gum Metal has been extensively studied. However, there is a lack of a review that encompasses the findings obtained over the years, as well as its future perspectives. This review aims to critically discuss and analyze Gum Metal development, with a special focus on new compositions, processing routes, and applications. Moreover, future perspectives are explored as well. Graphical abstract

The production of batteries often involves the joining of multilayered foils to a conductive tab. In the present study, a solid state spot welding method, the refill friction stir spot welding (refill FSSW), was employed for welding multilayered commercially pure aluminum (CP–Al) foils to 2024-T3 Al alloy sheets (tab). Defect-free welds with high lap shear strength (LSS) are obtained. In the stirred zone (SZ), 2024 Al alloy is only found within the shoulder refill region, which is greatly refined due to dynamic recrystallization. Unlike overlap joints in thicker sheets, a thermo-mechanically affected zone also forms above the SZ in the probe refill region. “Stop-action” experiments have been conducted to describe joint formation. The presence of the 2024 Al alloy tabs resulted in significant changes to material follow behavior and the formation of microstructural zones not previously observed in conventional refill FSSW overlap welds in thicker sheets. Most welds failed in SZ pull-out mode during LSS test. The cracks initiate at the interface between the upper sheet and the first layer of the multilayered foils and then propagate upward and circumferentially outside the SZ. Welds of multilayered Al foils produced by refill FSSW show low contact resistance. The decomposition of electrochemically active materials in batteries can be avoided by maintaining a minimum distance from the spot-weld. The produced welds with flat appearance, high mechanical properties, and potential to meet industry requirements imply that refill FSSW is a promising welding technique for battery production.

In this work, a Ti–29Nb–13Ta–4.6Zr–xO Gum Metal with two significantly different oxygen levels (388 and 3570 ppm) was investigated during deformation. The alloys were compressed during in situ high-energy X-ray diffraction using three different strain rates, 10−4, 10−3, and 10−1 s−1, in order to evaluate their influence on phase stability and mechanical properties. The influence of oxygen on the deformation process was also studied. Deformation takes place by twinning, stress-induced, and reverse martensitic transformation and was observed, for some samples, a spinodal decomposition of the β-phase during elastic deformation. The mechanical properties were similar for the different rates employed when considering the same oxygen level. The alloy with a higher amount of oxygen, however, showed a substantial increase in mechanical strength, with a yield strength of around 680 MPa, which is more than three times higher than for the specimen with 388 ppm of oxygen.

The exact calculations of the stress and strain distributions based on the controlling equations for a forming process with large deformation are often difficult. To circumvent such difficulties, some analytical methods such upper-bound analysis and slip-line field theory have been established by making a number of simplifying assumptions regarding the material properties and deformation modes. In this work an analytical model based on the upper-bound theory was successfully developed to predict material flow pattern and maximum process loads for an Equal Channel Angular Pressing (ECAP) die with circular cross-section and an intersecting channel angle of 120°. Based on the model, the power dissipated on all frictional and velocity discontinuity surfaces were determined and optimized in order predict the maximum process force as function of the channel geometry and the material plastic behavior. To validate the developed model, the ECAP die were produced and used to determine experimental load-displacement curves of AA6061-T6 specimens with different lengths. A good correlation between theoretical and experimental results was observed. In addition, the constant friction factor demonstrated to have a strong effect on the relative extrusion pressure.

As part of an ongoing process to fully assess the potential for friction stir welding (FSW) to be used in the aerospace industry, an attempt was made to produce double sided FSW joints at traverse speeds equal or higher than 5 mm/s of AA2050-T84 12.7 mm thick plates for high-volume production applications. With an emphasis on weld quality, the local and global mechanical properties were evaluated and correlated with microstructure of the welding area. Sound welds with no volumetric defects were obtained for tool traverse speeds up to 12 mm/s, resulting in yield and ultimate tensile strengths corresponding to 65% and 77% of base material, respectively. The metallurgical and mechanical characterization demonstrate that density of Cu-rich precipitates has a first order effect on micro-hardness variation. In the stir zone the dissolution temperature of this precipitate is achieved and its volume fraction is greatly reduced. The remaining precipitates seems to be partially dissolved and undergone a significant thickening at the welding zones in which the process temperature has not reached the dissolution temperature. Weld fractures after tensile tests were observed to start in the region of hardness minima.

In this paper, the Gum Metal Ti–29Nb–13Ta-4.6Zr-xO with two different oxygen contents (2200 and 4600 ppm) was prepared by high-pressure torsion (HPT) and their microstructural evolution, thermal stability, physical and mechanical properties were investigated for specimens prepared under different processing conditions. An increase in the β-phase stability at the expense of αʺ, as well as in the hardness was observed with the combined effect of oxygen addition and HPT processing. The increase in hardness was attributed to the microstructural refinement, to the creation of a high density of defects, and to the stress-induced ω-phase formation, which were all obtained due to the severe plastic deformation. Besides that, oxygen solid solution hardening provides a great increase in hardness. Vickers microhardness of approximately 400 HV was obtained for samples with higher oxygen content and deformed by HPT. The elastic modulus did not change significantly for the different oxygen levels, with values of 53–66 GPa. The typical Gum Metal properties were attained in the present paper, and they are directly related to its deformation mechanisms such as slip, stress-induced αʺ-martensite as well as reverse martensitic transformation (αʺ → β). The unique Gum Metal properties combined with the obtained increase in hardness make these two alloys very well suited for biomedical applications such as implants.

In this work, theoretical composition design and thermo-mechanical treatments were combined in order to improve the mechanical compatibility of a metastable β-type Ti–29Nb–13Ta–4Mo (wt%) alloy. Also, the influence of Mo and O alloying elements were investigated. By applying cold rolling and low temperature aging (673K during 20 and 100 min) different behaviours in microstructure and mechanical properties were identified. Alloys with Ultimate Tensile strength-UTS = 854 MPa and Elastic Modulus = 65 GPa were successfully fabricated. The microstructure responsible for that behaviour consisted mainly of β-matrix and ω-precipitation obtained via cold rolling plus short-time aging at low temperature, i.e. 673K for 20 min. These precipitates increase the strength of the material by hindering the motion of dislocations while the β-matrix with relatively high content of β-stabilizers (such, Mo) gives rise to the observed low elastic modulus. By extending aging time (100 min), a higher UTS is reached, but a slight increase in Elastic Modulus is also observed. The addition of oxygen increased mechanical and physical properties by favouring ω-phase formation. Whereas, Mo content increased β-stability, and in solution treated condition, only β-phase could be observed.

In this work, the Ti-29Nb-13Ta-4.6Zr-0.07O (TNTZO) alloy was processed by high-pressure torsion (HPT), and its microstructure, phase stability, elastic behavior, mechanical properties and deformation mechanisms were determined for samples prepared under different conditions. An increase in β-phase stability relative to martensite and a significant hardening was observed after severe plastic deformation. The increase in β-phase stability is attributed to the sharp increase in the system’s Gibbs free energy that occurs when the α"-laths reach a critical nanometer-size during the HPT processing. This system’s Gibbs free energy increase triggers the occurrence of the α"→ β reverse martensitic transformation. The greater hardness can be related to three factors: microstructural refinement; high dislocation density and stress-induced formation of the ω-phase. Plastic deformation is based on conventional slip, detwinning, stress-induced martensite formation, as well as the reverse martensitic transformation. The reverse martensitic transformation observed during HPT corresponds to a recently discovered mechanism that has not been well addressed in the literature until now. In this paper, this new mechanism is clearly detected and detailed discussed based on a systematic Rietveld refinement, in-depth microscopy, and calorimetry.

The present study assessed the influence of refill friction stir spot welding (RFSSW) process on the microstructure and electrochemical corrosion behavior of the AA5754-H22 alloy surface of an AA5754-H22/Ti-6Al-4 V overlapped joint. The results demonstrate that the RFSSW process promotes substantial microstrutural changes along the welded joint, which, in turn, reflect directly on the mechanical properties and corrosion behavior of distinct welding regions. The potentiodynamic polarization tests and electrochemical impedance spectroscopy measurements indicate that the stir zone region has better corrosion resistance than the other RFSS welded regions [heat affected zone and base metal] due to higher homogeneity and microstructure refinement. The achieved results are evidence that the RFSSW process is an interesting alternative for the welding of overlapping dissimilar joints.