Leonardo Contri Campanelli

Parameters optimization for friction spot welding of AZ31 magnesium alloy by Taguchi method

Friction spot welding (FSpW) is a solid state welding process suitable for producing spot-like joints, especially in lightweight materials, which are particularly interesting due to the weight saving potential. The plunging of an especially designed non-consumable and rotating tool creates a connection between overlapped sheets through frictional heat and plastic deformation. Minimum material loss is observed, and therefore a fully consolidated joint with flat surface (no keyhole) is obtained. In the current study, the effect of FSpW parameters, such as rotational speed, plunge depth and dwell time, on lap shear strength of AZ31 magnesium alloy joints was investigated. The optimization of input process parameters was carried out through Taguchi approach of DOE. Analysis of variance was applied to determine the individual importance of each parameter. Main effect plots were used to indicate the best levels for maximizing lap shear strength. The results show that tool plunge depth has the higher effect on the weld strength, followed by rotational speed and dwell time.

Effect of an amorphous titania nanotubes coating on the fatigue and corrosion behaviors of the biomedical Ti-6Al-4V and Ti-6Al-7Nb alloys

An array of self-organized TiO2 nanotubes with an amorphous structure was produced on the biomedical Ti-6Al-4V and Ti-6Al-7Nb alloys, and the resulting fatigue and corrosion behaviors were studied. The electrochemical response of the nanotubular oxide surfaces was investigated in Ringer physiological solution through potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The absence of transpassivation in the chloride-containing solution, in addition to the micron-scale values of the passivation current density, indicated the excellent corrosion behavior of the coating and the satisfactory protection against the creation of potential stress concentrators in the surface. Axial fatigue tests were performed in physiological solution on polished and coated conditions, with characterization of the treated surfaces by scanning electron microscopy before and after the tests. The surface modification was not deleterious to the fatigue response of both alloys mainly due to the nano-scale dimension of the nanotubes layer. An estimation based on fracture mechanics revealed that a circumferential crack in the range of 5μm depth would be necessary to affect the fatigue performance, which is far from the thickness of the studied coating, although no cracks were actually observed in the oxide surfaces after the tests.

Preliminary Investigation on Friction Spot Welding of AZ31 Magnesium Alloy

Friction spot welding (FSpW) is a recent solid state welding process developed and patented by GKSS Forschungszentrum (now Helmholtz-Zentrum Geesthacht), Germany. A spot-like connection is produced by means of an especially designed non-consumable tool consisting of pin, sleeve and clamping ring that creates a joint between sheets in overlap configuration through frictional heat and plastic deformation. FSpW offers many advantages over conventional spot joining techniques including high energy efficiency, surface quality and environmental compatibility. Comparing with friction stir spot welding, FSpW produces a weld without keyhole on the surface at the end of the joining process. In the present study, the possibility of joining AZ31 magnesium alloy by FSpW technique was evaluated by using different welding parameters (rotational speed, plunge depth and dwell time), aiming to produce high quality connections. Microstructural features were analyzed by light optical microscope and mechanical performance was investigated by microhardness test and lap shear test. Microstructure analysis revealed that defects free welds could be produced. A slight decrease in grain size of the stir zone was observed causing a slight increase in the microhardness of this region. The preliminary lap shear data demonstrated that the weld strength is comparable to other welding process.

A review on microstructural and mechanical properties of friction spot welds in Al-based similar and dissimilar joints

Friction spot welding (FSpW) is a solid state welding process suitable for spot joining lightweight low melting point materials such as aluminum and magnesium alloys. The process is carried out by plunging a three-piece tool (clamping ring, and rotating sleeve and pin) that creates a connection between sheets in overlap configuration by means of frictional heat and mechanical work. The result is a spot welded lap connection with minimal material loss and a flat surface with no keyhole. The present work presents a summary of results from studies in similar and dissimilar joints such as: AA6181-T4/AA6181-T4, AZ31/AZ31, AZ31/AA6181-T4, DP600/AA6181-T4. Microstructural features of the FSpW joints were analyzed by optical microscopy and SEM; while mechanical performance has been investigated in terms of hardness and shear tensile testing. Aspects of the different fracture modes are reported. Joints with shear strength close to 7 kN were obtained with high reproducibility. The results also have shown that geometric features of the joint play an important role on the fracture mechanism and hence on the mechanical performance of the weldment. The discussion of the results takes into account recent information obtained from material flow studies and modeling work.

Fatigue behavior of Ti-6Al-4V alloy in saline solution with the surface modified at a micro- and nanoscale by chemical treatment.

This work evaluated the influence of the surface modification using acid etching combined with alkaline treatment on the fatigue strength of Ti-6Al-4V ELI alloy. The topography developed by chemical surface treatments (CST) was examined by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). Increased roughness and effective surface area were investigated and compared with the Ti-6Al-4V samples without modification. Surface composition was analyzed by energy dispersive X-ray spectroscopy (EDS). Axial fatigue resistance of polished and modified surfaces was determined by stepwise load increase tests and staircase test method. Light microscopy and SEM were employed to examine the fracture surface of the tested specimens. According to the results, a similar fatigue behavior was found and a negligible difference in the fatigue crack nucleation was observed for the Ti-6Al-4V with CST in comparison to the samples without treatment.

Fatigue resistance, electrochemical corrosion and biological response of Ti-15Mo with surface modified by amorphous TiO2 nanotubes layer: PROPERTIES OF Ti-15Mo MODIFIED BY TiO2 NANOTUBES

The objective of this work was a systemic evaluation of the anodizing treatment in a β-type Ti-15Mo alloy to grow a TiO2 nanostructured layer for osseointegration improvement. The technical viability of the surface modification was assessed based on the resistance to mechanical fatigue, electrochemical corrosion, and biological response. By using an organic solution of NH4 F in ethylene glycol, a well-organized array of 90 nm diameter nanotubes was obtained with a potential of 40 V for 6 h, while undefined nanotubes of 25 nm diameter were formed with a potential of 20 V for 1 h. Nevertheless, the production of the 90 nm diameter nanotubes was followed by micrometer pits that significantly reduced the fatigue performance. The undefined nanotubes of 25 nm diameter, besides the greater cell viability and improved osteoblastic cell differentiation in comparison to the as-polished surface, were not deleterious to the fatigue and corrosion properties. This result strengthens the necessity of an overall evaluation of the anodizing treatment, particularly the fatigue resistance, before suggesting it for the design of implants.

Fatigue Performance of New Developed Biomedical Ti-15Mo Alloy with Surface Modified by TiO2 Nanotubes Formation

In recent years, it was demonstrated that Ti-Mo alloys are promising to be use as orthopedic implants. The presence of TiO2 nanotubes can increase the bioactivity and improve the osseointegration of Ti and its alloys implants, although this modification could lead to a reduction in the dynamic mechanical properties. In this context, the purpose of the present study was to obtain self-organized nanotubes on the surface of biomedical Ti-15Mo alloy and verify whether the fatigue performance was significantly changed. Organized nanotubes were obtained by anodic oxidation using ethylene glycol + NH4F solution. The axial fatigue behavior was characterized by stepwise increases of the applied load in air and in physiological media at 37°C. The results was compared with the as-polished samples in order to compare if the Ti-15Mo alloy fatigue behavior was affected by the surface modification, and it was found that the mechanical performance of the Ti-15Mo alloy was affected by the surface modification, in that specific experimental conditions used to obtain the nanotubes.

Soldagem por ponto no estado sólido de ligas leves

The recent concern about climate change has stimulated research into transport energy efficiency in order to reduce the emission of gases. One of the main solutions is to reduce the structural weight through the application of new materials, such as aluminum and magnesium lightweight alloys. However, new applications are often limited by the difficulty of joining these materials. Friction Stir Welding (FSW) is a solid state joining technique that emerges as a viable alternative to replace or complement the established joining technologies. As a continuous weld is not always requested, two spot welding technologies derived from FSW are under development: Friction Stir Spot Welding (FSSW) and Friction Spot Welding (FSpW). Besides providing almost defect-free and high strength joints, these techniques exhibit high energy efficiency, short welding cycle, ease of automation and environmental compatibility, competing against the conventional spot joining techniques, such as Resistance Spot Welding (RSW) and riveting.