Numerical simulations on material flow behaviors in whole process of friction stir welding

A finite element model based on solid mechanics was developed with ABAQUS to study the material flow in wholeprocess of friction stir welding （FSW）, with the technique of tracer particles. Simulation results indicate that the flow pattern of thetracer particles around the pin is spiral movement. There are very different flow patterns at the upper and lower parts of the weld. Thematerial on the upper surface has the spiral downward movement that is affected by the shoulder and the lower material has the spiralupward movement that is affected by the pin. The velocity of the material flow on the periphery of the stirring pin is higher than thatat the bottom of the stirring pin. The material can be rotated with a stirring pin a few times, agreeing well with the previousexperimental observation by tungsten tracer particles.

Effects of Sc and Zr microalloying on the microstructure and mechanical properties of high Cu content 7xxx Al alloy

The effects of Sc and Zr microalloying on the microstructure and mechanical properties of a 7xxx Al alloy with high Cu content(7055)during casting,deformation,and heat treatment were investigated.The addition of Sc and Zr not only refined the grains but also transformed theθ-phase into the W-phase in the 7055 alloy.Minor Sc and Zr additions enhanced the hardness and yield strength of the 7055-T6 alloy by strengthening the grain boundaries and Al3(Sc,Zr)precipitates.However,a further increase in the Sc and Zr fractions did not refine the grains but instead resulted in the formation of the large-sized W-phase and primary coarse Al3(Sc,Zr)phase and subsequently deteriorated the mechanical properties of the alloys.The 7055 alloy with 0.25Sc addition exhibited the best mechanical property among the prepared alloys.

Mechanical properties and thermal stability of 7055 Al alloy by minor Sc addition

This study investigated the effect of 0.25 wt%Sc addition on the microstructure and mechanical properties of AA 7055 alloy.The addition of Sc obviously refined the grains of AA 7055 alloy during casting,homogenizing,rolling,solution,and aging treatments due to the formation of primary and precipitate Al3(Sc,Zr)phase.The recrystallization and precipitation of AA 7055 alloy were inhibited during heat treatments by Sc addition.The Sccontaining AA 7055 alloy exhibited higher thermal stability than AA 7055 alloy during homogenizing treatment,because of the grain boundary pinning effect of nano-sized Al3(Sc,Zr)particles.Given its structure characteristics such as fine grains,fineη′phase,and lessηphase,AA 7055 alloy with added Sc showed good mechanical properties after aging at 120℃for 24 h,with an ultimate tensile strength(UTS)of 679 MPa and elongation(EL)of 14%.This work provides an effective strategy to fabricate AlZn-Mg(-Cu)series(7 xxx)alloys with excellent mechanical properties.

Enhancing High-Temperature Strength and Thermal Stability of Al2O3/Al Composites by High-Temperature Pre-treatment of Ultrafine Al Powders

Amorphous Al2 O3-reinforced Al composite(am-Al2 O3/Al) compacted from ultrafine Al powders for high-temperature usages confronts with drawbacks because crystallization of am-Al2 O3 at high temperatures will result in serious strength loss.Aiming at this unsolved problem,in this study,high-temperature Al materials with enhanced thermal stability were developed through introducing more thermally stable nano-sized particles via high-temperature pre-treatment of ultrafine A1 powders.It was found that the pre-treatment at ≤550℃ could introduce a few Al2 O3 in the Al matrix and increase the strength of the composites,but the strength was still below that of am-Al2 O3/Al because without being pinned firmly,grain boundaries(GBs) were softened at high temperature and intergranular fracture happened.When the pre-treatment was carried out at 600℃,nitridation and oxidation processes happened simultaneously,producing large numbers of intergranular(AlN+γ-Al2 O3) particles.GB sliding and intergranular fracture were suppressed;therefore,higher strength than that of am-Al2 O3/Al was realized.Furthermore,the(AIN+γ-Al2 O3)/Al exhibited more superior thermal stability compared to amAl2 O3/Al for annealing treatment at 580℃ for 8 h.Therefore,an effective way to fabricate high-temperature Al composite with enhanced thermal stability was developed in this study.

Mechanical and Damping Behavior of Age-Hardened and Non-age-hardened Al Alloys After Friction Stir Processing

This study investigated the microstructure,mechanical,and damping properties of a non-age-hardened Al alloy(5086)and an age-hardened Al alloy(7075)after friction stir processing(FSP).Microstructural analyses indicate that FSP led the grain refinement of samples,and the grains size decreased with the decrease in the tool rotation rate.Furthermore,FSP with low rotation rate promotes theηphase precipitation in the 7075 alloy,causing the micron-sized particles in the 5086 alloy to break up.After being subjected to FSP with low rotation rate,the 5086 and 7075 alloys exhibited excellent mechanical and damping properties.Such improved properties were ascribed to their equilibrium grain boundaries,fine grain,low density of dislocations,high fraction of high misorientation angle,and uniform particle distribution.

High-Speed Friction Stir Welding of SiC_p/Al–Mg–Si–Cu Composite

A 17 vol%SiCp/Al–Mg–Si–Cu composite plate with a thickness of 3 mm was successfully friction stir welded(FSWed)at a very high welding speed of 2000 mm/min for the first time.Microstructural observation indicated that the coarsening of the precipitates was greatly inhibited in the heat-affected zone of the FSW joint at high welding speed,due to the significantly reduced peak temperature and duration at high temperature.Therefore,prominent enhancement of the hardness was achieved at the lowest hardness zone of the FSW joint at this high welding speed,which was similar to that of the nugget zone.Furthermore,the ultimate tensile strength of the joint was as high as 369 MPa,which was much higher than that obtained at low welding speed of 100 mm/min(298 MPa).This study provides an effective method to weld aluminum matrix composite with superior quality and high welding efficiency.

Origin of Insignificant Strengthening Effect of CNTs in T6-Treated CNT/6061Al Composites

Carbon nanotube（CNT）-reinforced 6061 Al（CNT/6061 Al） composites were fabricated via powder metallurgy combined with friction stir processing（FSP）. CNTs were dispersed after FSP and accelerated the precipitation process of the CNT/6061 Al composites. However, the strengthening effect of CNTs on the T6-treated materials was insignificant,while the composites under the FSP and solution treatment conditions exhibited increased strength compared to the matrix.Precipitate-free zones（PFZs） were detected around CNTs in the T6-treated CNT/6061 Al composites, and a model was proposed to describe the effect of PFZs on strength. The calculations indicated that the strength of PFZs was similar to that of the T6-treated 6061 Al. As a result, the strengthening effect of CNTs on the T6-treated CNT/6061 Al composites was insignificant.

Achieving a High-Strength CoCrFeNiCu High-Entropy Alloy with an Ultrafine-Grained Structure via Friction Stir Processing

High-entropy alloys(HEAs) are a new class of materials with a potential engineering application,but how to obtain ultrafine or nano-sized crystal structures of HEAs has been a challenge.Here,we first presented an equiatomic CoCrFeNiCu HEA with excellent mechanical properties obtained via friction stir processing(FSP).After FSP,the Cu element segregation in the cast CoCrFeNiCu HEA was almost eliminated,and the cast coarse two-phase structure(several micrometers) was changed into an ultrafine-grained single-phase structure(150 nm) with a large fraction of high-angle grain boundaries and nanoscale deformation twins.This unique microstructure was mainly attributed to the severe plastic deformation during FSP,and the sluggish diffusion effect in dynamics and the lattice distortion effect in crystallography for HEAs.Furthermore,FSP largely improved the hardness and yield strength of the CoCrFeNiCu HEA with a value of 380 HV and more than 1150 MPa,respectively,which were> 1.5 times higher than those of the base material.The great strengthening after FSP was mainly attributed to the significant grain refinement with large lattice distortion and nano-twins.This study provides a new method to largely refine the microstructure and improve the strength of cast CoCrFeNiCu HEAs.