Friction stir welding (FSW), a mature solid-state joining method, has become a revolutionary welding technique over the past two decades because of its energy efficiency, environmental friendliness ...and high-quality joints. FSW is highly efficient in the joining of Al alloys, Mg alloys, Ti alloys, polymers and other dissimilar materials. Recently, FSW has gained considerable scientific and technological attention in several fields, including aerospace, railway, renewable energy and automobile. To broaden the adoption of FSW in manufacturing fields, three inherent issues—back support, weld thinning and keyhole defects—must be addressed to ensure the structural integrity, safety and service life of the manufactured products. This review covers the recent progress on the control strategies for these inherent issues, which are basically divided into self-supported FSW, non-weld-thinning FSW and friction stir-based remanufacturing. Herein, the aim is to focus on the corresponding technical development, process parameters, metallurgical features and mechanical properties. Additionally, the challenges and future outlooks are emphasized systematically.
The refill friction stir spot welding (refill FSSW) process is a solid-state joining process to produce welds without a keyhole in spot joint configuration. This study presents a thermo-mechanical ...model of refill FSSW, validated on experimental thermal cycles for thin aluminium sheets of AA7075-T6. The temperatures in the weld centre and outside the welding zone at selected points were recorded using K-type thermocouples for more accurate validation of the thermo-mechanical model. A thermo-mechanical three-dimensional refill FSSW model was built using DEFORM-3D. The temperature results from the refill FSSW numerical model are in good agreement with the experimental results. Three-dimensional material flow during plunging and refilling stages is analysed in detail and compared to experimental microstructure and hardness results. The simulation results obtained from the refill FSSW model correspond well with the experimental results. The developed 3D numerical model is able to predict the thermal cycles, material flow, strain, and strain rates which are key factors for the identification and characterization of zones as well for determining joint quality.
Friction spot joining is an alternative technique for joining metals with polymers and composites. This study investigated the fatigue performance of aluminum alloy 2024/carbon-fiber-reinforced ...poly(phenylene sulfide) joints that were produced with friction spot joining. The surface of the aluminum was pre-treated using various surface treatment methods. The joined specimens were tested under dynamic loading using a load ratio of R = 0.1 and a frequency of 5 Hz. The tests were performed at different percentages of the lap shear strength of the joint. Three models—exponential, power law, and wear-out—were used to statistically analyze the fatigue life of the joints and to draw the stress–life (S–N) curves. The joints showed an infinite life of 25–35% of their quasi-static strength at 106 cycles. The joints surpassing 106 cycles were subsequently tested under quasi-static loading, showing no considerable reduction compared to their initial lap shear strength.
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•A novel method of friction surfacing assisted hybrid friction stir welding was proposed to join dissimilar metals.•The tensile load of Ti/Al joints was improved to 12.2kN, reached ...85.3% of the Al base material.•Tool abrasion was totally avoided by introducing a friction surfaced Al-coating layer.•The bonding was combined contribution of nanoscale-IMCs and mechanical interlocking.
A new method of friction surfacing assisted hybrid friction stir welding (FS-HFSW) technique was developed to improve the joint efficiency and avoid the pin abrasion for joining of dissimilar Ti/Al joints. The FSW tool with enlarged head and concave end-face was designed to broaden the lap width and promote material flow. The maximum tensile load reached 12.2kN, representing 85.3% of the parent Al alloy, with a ductile fracture locating at the heat affected zone of base Al. The excellent bonding of Ti and Al was based on the combined effects of nanoscale TiAl3 IMCs layer and complex mechanical inter-locking.
Antimicrobial polymers represent a very promising class of therapeutics with unique characteristics for fighting microbial infections. As the classic antibiotics exhibit an increasingly low capacity ...to effectively act on microorganisms, new solutions must be developed. The importance of this class of materials emerged from the uncontrolled use of antibiotics, which led to the advent of multidrug-resistant microbes, being nowadays one of the most serious public health problems. This review presents a critical discussion of the latest developments involving the use of different classes of antimicrobial polymers. The synthesis pathways used to afford macromolecules with antimicrobial properties, as well as the relationship between the structure and performance of these materials are discussed.
The development of an effective, realistic, and sustainable microbial biorefinery depends on several factors, including as one of the key aspects an adequate selection of microbial strain. The ...oleaginous red yeast
Rhodotorula
sp. has been studied as one powerful source for a plethora of high added-value biomolecules, such as carotenoids, lipids, and enzymes. Although known for over a century, the use of
Rhodotorula
sp. as resource for valuable products has not yet commercialized. Current interests for
Rhodotorula
sp. yeast have sparked from its high nutritional versatility and ability to convert agro-food residues into added-value biomolecules, two attractive characteristics for designing new biorefineries. In addition, as for other yeast-based bioprocesses, the overall process sustainability can be maximized by a proper integration with subsequent downstream processing stages, for example, by using eco-friendly solvents for the recovery of intracellular products from yeast biomass. This review intends to reflect on the current state of the art of microbial bioprocesses using
Rhodotorula
species. Therefore, we will provide an analysis of bioproduction performance with some insights regarding downstream separation steps for the extraction of high added-value biomolecules (specifically using efficient and sustainable platforms), providing information regarding the potential applications of biomolecules produced by
Rhodotorula
sp, as well as detailing the strengths and limitations of yeast-based biorefinery approaches. Novel genetic engineering technologies are further discussed, indicating some directions on their possible use for maximizing the potential of
Rhodotorula
sp. as cell factories.
Key points
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Rhodotorula sp. are valuable source of high value-added compounds.
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Potential of employing Rhodotorula sp. in a multiple product biorefinery.
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Future perspectives in the biorefining of Rhodotorula sp. were discussed.
To develop a suitable keyhole closure process in high-strength AlZnMg(Cu) alloys, refill friction stir spot welding was used to perform repair welds of through holes of 7.5mm diameter in AA 7075-T651 ...plates with 6mm thickness. The thermal cycle and the evolution of microstructural features were investigated in detail. The mechanical performance of the welds was studied based on the changes in microstructure and temperature exposure caused by the welding process.
Thermal cycle measurements revealed high heating rates and peak temperatures of up to 540°C in the weld center. Leftover grains from the base metal that did not recrystallize were determined in the stirred zone of the weld center. The welds showed a W-shaped hardness distribution with a lowest hardness of 70% of base metal values in the heat affected zone. Under quasi-static loading, two failure modes were determined, with mode 1 failure occurring in the heat affected zone and mode 2 failure occurring in the outer regions of the stirred zone with crack initiation in the lower portions of the weld. Post-weld natural aging was proven to be highly significant for the mechanical properties of the welds and is effective for up to 4weeks after welding.
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•Defect-free and highly efficient keyhole closure welds in AA 7075 - T651 are achieved using refill friction stir spot welding.•A comprehensive microstructural analysis revealed partially recrystallized grains in the stirred zone for the first time.•Microstructural features and thermal cycles explain the mechanical properties of the keyhole repair welds including an UTS of 74%.•The applied energy and peak temperatures in the HAZ show a linear dependency on the tool revolutions per mm plunge/retract.•The influence of the process parameter on the mechanical properties are analyzed in detail.
Friction surfacing (FS) is a solid-state layer deposition process for metallic materials at temperatures below their melting point. While the bonding of the deposited layers to the substrate is ...proven suitable for coating applications, so far the mechanical properties of additively manufactured stacks have not been systematically investigated. In particular, the effect of successive deposited FS layers, i.e., repetitive thermo-mechanical loading, on the interface properties as well as anisotropy and strength of the deposited stack is unknown. For this purpose, the mechanical properties of FS deposited multi-layer stacks from dissimilar aluminum alloys have been investigated, characterizing layer-to-layer as well as layer-to-substrate bonding interfaces via micro-flat tensile testing. Furthermore, directional dependencies in the stack and failure mechanisms are analyzed. The results show a homogeneous, fine-grained microstructure with average grain sizes between 4.2 and 4.6
μ
m within the deposited material. The resulting tensile properties with no significant directional dependency present an ultimate tensile strength between 320 and 326 MPa exceeding the strength of the AA5083 H112 consumable base material. No difference was obtained in terms of layer-to-layer or layer-to-substrate interface strength. Furthermore, homogeneous hardness was observed within the deposited structure, which is in the range of AA5083 base material’s hardness of 91 HV. The results indicate that the FS process in conjunction with the material used is suitable for additively generated structures and highlight the potential of this solid-state layer deposition technology.
Astaxanthin (AXT) is a natural xanthophyll with strong antioxidant, anticancer and antimicrobial activities, widely used in the food, feed, pharmaceutical and nutraceutical industries. So far, 95% of ...the AXT global market is produced by chemical synthesis, but growing customer preferences for natural products are currently changing the market for natural AXT, highlighting the production from microbially-based sources such as the yeast Phaffia rhodozyma. The AXT production by P. rhodozyma has been studied for a long time at a laboratory scale, but its use in industrial-scale processes is still very scarce. The optimization of growing conditions as well as an effective integration of upstream-downstream operations into P. rhodozyma-based AXT processes has not yet been fully achieved. With this critical review, we scrutinized the main approaches for producing AXT using P. rhodozyma strains, highlighting the impact of using conventional and non-conventional procedures for the extraction of AXT from yeast cells. In addition, we also pinpointed research directions, for example, the use of low-cost residues to improve the economic and environmental sustainability of the bioprocess, the use of environmentally/friendly and low-energetic integrative operations for the extraction and purification of AXT, as well as the need of further human clinical trials using yeast-based AXT.
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