In this paper, the influence of process parameters (tool rotational speed, pin design, and configuration of joined alloys) on the macrostructure and mechanical properties of friction stir welded aluminum alloys 7075-T651 and 5083-H111 was characterized. The tool rotational speed and the alloy placement significantly influenced the formation of the weld (especially the stir zone). Superior mixing of materials was obtained at higher rotational speeds and in the configuration with 5083 on the advancing side and 7075 on the retreating side. However, under these conditions, more defects, such as porosity, voids, or wormholes, were found in the stir zone. Regardless of the pin design and weld configuration, with an increasing tool rotational speed, the mechanical properties decrease. Using the Triflute pin, however, guarantees higher tensile strength and weld efficiency (above 100%). The weld configuration did not influence the mechanical properties. The highest tensile strength (371 MPa) defect-free joint was obtained with 5083 on the advancing side, 7075 on the retreating side, a tool rotational speed of 280 rpm, and the Triflute pin.

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.

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A coupled thermal/material flow model of friction stir welding was developed and applied to the joining of Sc-modified aluminum alloy (7042-T6) extrusions. The model reveals that surface material is pulled from the retreating side into the weld zone where it is interleaved with in situ material. Due to frictional contact with the shoulder, the surface material is hotter than the in situ material, so that the final weld microstructure is composed of bands of material with different temperature histories. For this alloy and the associated FSW heating rates, secondary phase dissolution/precipitation temperatures are in proximity to the welding temperatures. Therefore, depending on the surface and in situ material temperatures in relation to these transformation temperatures, disparate precipitate distributions can develop in the bands of material comprising the weld nugget. Based on the numerical simulation and on thermal analysis data from differential scanning calorimetry, a mechanism for the formation of onion rings within the weld zone is presented.

Dissimilar aluminum alloy plates of 2017A-T451 and 5083-H111 were friction stir welded in a butt joint configuration along the longitudinal direction. Welding trials demonstrated that placing 5083 on the advancing side enhanced material flow and consequently formed a larger weld nugget. Numerical simulation supported this observation through analysis of volumetric flow rates through reference planes surrounding the stir zone. The analysis also suggests that the weld configuration that results in a decreasing temperature-dependent flow stress in the weldment from the leading edge of the tool to the trailing edge will maximize material flow in dissimilar friction stir welding welds. The decreasing flow stress promotes material flow along the retreating side of the tool as flow conditions necessarily become easier from the front to the back. Regardless of its position during welding, however, 2017A alloy dominated the nugget region. In either weld configuration, alternating bands of 2017A and 5083 with similar grain sizes (approximately 10 µm) comprised the weld microstructure. Within the nugget, numerous second-phase particles as well as dislocations occurring as single dislocations or in the form of dislocation tangles or walls (low angle grain boundaries) were present. The relatively high dislocation density observed in both alloys suggested that recrystallization was incomplete. Hardness mapping revealed an asymmetric variation of hardness across the weld centerline that strictly corresponded to the distribution of particular alloys within the nugget. During tensile testing, the AS 5083-RS 2017A configuration failed under ductile shear rupture occurring in the base 5083 material far from the weld. For the opposite configuration, the tensile samples ruptured perpendicular to the load axis exactly on the border between the nugget and the thermomechanically affected zone on the 2017A alloy side.