RESUMEN
A novel dual-speed tool for which the shoulder and pin rotation speeds are separately established was utilized to friction stir weld cast magnesium AZ91 with wrought aluminum 6082-T6. To assess the performance and efficacy of the dual-speed tool, baseline dissimilar welds were also fabricated using a conventional FSW tool. Optical microscopy characterized the weld microstructures, and a numerical simulation enhanced the understanding of the temperature and material flow behaviors. For both tool types, regions of the welds contained significant amounts of the AZ91 primary eutectic phase, Al12Mg17, indicating that weld zone temperatures exceeded the solidus temperature of α-Mg (470 °C). Liquation, therefore, occurred during processing with subsequent eutectic formation upon cooling below the primary eutectic temperature (437 °C). The brittle character of the eutectic phase promoted cracking in the fusion zone, and the "process window" for quality welds was narrow. For the conventional tool, offsetting to the aluminum side (advancing side) mitigated eutectic formation and improved weld quality. For the dual-speed tool, experimental trials demonstrated that separate rotation speeds for the shoulder and pin could mitigate eutectic formation and produce quality welds without an offset at relatively higher weld speeds than the conventional tool. Exploration of various weld parameters coupled with the simulation identified the bounds of a process window based on the percentage of weld cross-section exceeding the eutectic temperature and on the material flow rate at the tool trailing edge. For the dual-speed tool, a minimum flow rate of 26.0 cm3/s and a maximum percentage of the weld cross-section above the eutectic temperature of 35% produced a defect-free weld.
RESUMEN
For the friction stir welding (FSW) of AZ91 magnesium alloy, low tool rotational speeds and increased tool linear speeds (ratio 3.2) along with a larger diameter shoulder and pin are utilized. The research focused on the influence of welding forces and the characterization of the welds by light microscopy, scanning electron microscopy with an electron backscatter diffraction system (SEM-EBSD), hardness distribution across the joint cross-section, joint tensile strength, and SEM examination of fractured specimens after tensile tests. The micromechanical static tensile tests performed are unique and reveal the material strength distribution within the joint. A numerical model of the temperature distribution and material flow during joining is also presented. The work demonstrates that a good-quality joint can be obtained. A fine microstructure is formed at the weld face, containing larger precipitates of the intermetallic phase, while the weld nugget comprises larger grains. The numerical simulation correlates well with experimental measurements. On the advancing side, the hardness (approx. 60 HV0.1) and strength (approx. 150 MPa) of the weld are lower, which is also related to the lower plasticity of this region of the joint. The strength (approx. 300 MPa) in some micro-areas is significantly higher than that of the overall joint (204 MPa). This is primarily attributable to the macroscopic sample also containing material in the as-cast state, i.e., unwrought. The microprobe therefore includes less potential crack nucleation mechanisms, such as microsegregations and microshrinkage.
RESUMEN
Free from toxic elements biomaterial potentially applicable for load bearing biomedical implants was obtained for the first time by laser cladding of S520 bioactive glass onto ultrafine-grained commercially pure titanium. The cladding process affected the refined structure of the substrate inducing martensitic transformation near its surface. The α' acicular martensite gradually passes into relatively large grains with increasing distance from the substrate surface, which subsequently are transformed into smaller grains of about 2 µm in diameter. Both the melted zone, where the martensite crystalline structure was found, and the HAZ are characterised by relatively lower hardness in comparison with that of the substrate core indicating increased ductility. Such a combination of zones with different properties may have a synergistic effect and is beneficial for the obtained biomaterial. A characteristic region in the form of about 3 µm width band was formed in the melted zone at about 10 µm below the titanium surface. The results of EDS analysis indicate that several glass elements moved into the region while the titanium content in the same area was decreased. High bioactivity of the coated S520 glass was revealed by in vitro testing with SBF solution and almost complete reduction of P concentration occurred after 14 days.
Asunto(s)
Vidrio , Titanio , Materiales Biocompatibles , Rayos Láser , Ensayo de Materiales , Propiedades de SuperficieRESUMEN
Tissue engineering requires properly selected geometry and surface properties of the scaffold, to promote in vitro tissue growth. In this study, we obtained three types of electrospun poly(methyl methacrylate) (PMMA) scaffolds-nanofibers, microfibers, and ribbons, as well as spin-coated films. Their morphology was imaged by scanning electron microscopy (SEM) and characterized by average surface roughness and water contact angle. PMMA films had a smooth surface with roughness, Ra below 0.3 µm and hydrophilic properties, whereas for the fibers and the ribbons, we observed increased hydrophobicity, with higher surface roughness and fiber diameter. For microfibers, we obtained the highest roughness of 7 µm, therefore, the contact angle was 140°. All PMMA samples were used for the in vitro cell culture study, to verify the cells integration with various designs of scaffolds. The detailed microscopy study revealed that higher surface roughness enhanced cells' attachment and their filopodia length. The 3D structure of PMMA microfibers with an average fiber diameter above 3.5 µm, exhibited the most favorable geometry for cells' ingrowth, whereas, for other structures we observed cells growth only on the surface. The study showed that electrospinning of various scaffolds geometry is able to control cells development that can be adjusted according to the tissue needs in the regeneration processes.