In this study, the ultrafine grained (UFG) 6061 Al alloys fabricated by cold rolling were friction stir welded (FSW) with different rotation rates under both air cooling and rapid cooling in water. Low-heat-input parameters of 400 rpm rotation rate in water (400-Water) could effectively inhibit the coarsening of recrystallized grains, reduce the precipitation rate, and retain more dislocations of the UFG 6061 Al parent metal. 400-Water joint showed high lowest-hardness value, narrow low-hardness zone, and high tensile strength, attributing to the effect of dislocation, grain boundary, solid-solution, and precipitation hardening. This work provides an effective strategy to fabricate large-sized bulk UFG Al alloy by cold rolling with large deformation and low-heat-input FSW.
Local melting and the eutectic film and liquation crack formation mechanisms during friction spot welding (FSpW) of Al-Zn-Mg-Cu alloy were studied by both experiment and finite element simulation. Their effects on mechanical properties of the joint were examined. When the welding heat input was high, the peak temperature in the stir zone was higher than the incipient melting temperature of the Al-Zn-Mg-Cu alloy. This resulted in local melting along the grain boundaries in this zone. In the retreating stage of the welding process, the formed liquid phase was driven by the flowing plastic material and redistributed as a “U-shaped” line in the stir zone. In the following cooling stage, this liquid phase transformed into eutectic films and liquation cracks. As a result, a new characteristic of “U” line that consisted of eutectic films and liquation cracks is formed in the FSpW join. This “U” line was located in the high stress region when the FSpW joint was loaded, thus it was adverse to the mechanical properties of the FSpW joint. During tensile shear tests, the “U” line became a preferred crack propagation path, resulting in the occurrence of brittle fracture.
An as-cast Al-Zn-Mg-Sc alloy was friction stir processed varying tool related parameters, yielding microstructures with different grain sizes (0.68, 1.8 and 5.5 μm). Significant increases in room temperature ductility were obtained in these materials with reasonable enhancement in strength. It is demonstrated that the type of microstructure produced by friction stir processing (FSP) has a significant influence on the choice of post-FSP heat treatment design for achieving improved tensile properties. It is also found that the ultrafine grained FSP material could not achieve the desired high strength during the post-FSP heat treatment without grain coarsening, whereas the micro-grained FSP materials could reach such strength levels (>560 MPa) under conventional age hardening heat treatment conditions.
In this study, 20 mm thick AA7075-T6 alloy plates were joined by friction stir welding. The microstructure and mechanical properties of the nugget zone along the thickness direction from the top to the bottom was investigated. The results showed that the microstructure including the grain size, the degree of dynamic recrystallization, the misorientation angle distribution and the precipitation phase containing its size, type and content exhibited a gradient distribution along the thickness direction. The testing results of mechanical properties of the slices showed that the nugget was gradually weakened along the depth from the top to the bottom. The maximum ultimate tensile strength, yield strength and elongation of the slice in the nugget top-middle are obtained, which are 415 MPa, 255 MPa and 8.1%, respectively.
In this study, the effects of main welding parameters (rotation speed (ω) and welding speed (v)) on the microstructure, micro-hardness distribution and tensile properties of friction stir welded (FSW) 2195-T8 Al-Li alloy were investigated. The effects of T6 post-treatments at different solution and aging conditions on the mechanical properties and microstructure characteristics of the FSW joints were also investigated. The results show that with increasing ω and v, both strength and elongation of the joints increase first, and then decrease with further increase of ω and v. All the joints under varied welding parameters show significant strength loss, and the strength reaches only 65% of the base metal. The effect of T6 post-heat treatment on the mechanical properties of the joints depends on the solution and aging conditions. Two heat treatment processes (480 °C × 0.5 h quenching + 180 °C × 12 h, 520 °C × 0.5 h quenching + 180 °C × 12 h aging) are found to increase the joint strength. Furthermore, low temperature quenching (480 °C) is more beneficial to the joint strength. The joint strength can reach 85% of the base metal. Whereas both low temperature aging (140 °C × 56 h) and stepped aging (100 °C × 12 h + 180 °C × 3 h) processes decrease the joint strength. After heat treatment all the joints show decreased ductility due to the obvious grain coarsening in the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ).
The effects of electric field on the evolution of excess quenched-in vacancy as well as solute clustering in Al-4wt%Cu alloy, and on the vacancy migration and formation enthalpy of pure aluminum were investigated, using positron annihilation lifetime spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, transmission electron microscopy, hardness measurement and four-probe electrical resistivity measurement. The results showed that the electric field improved age hardening response obviously and postponed the decay of excess vacancies for 30 min during the early stage ageing of Al-4wt%Cu alloy. A large number of 2-4 nm GP zones with dense distribution were observed after 1 min ageing with an electric field applied. The electric field-assisted-aged sample owned a lower coarsening rate of GP zone, which was about three fifths of that in the aged sample without an electric field, from 1 min to 120 min ageing. The electric field contributed 8% increase of the vacancy migration enthalpy (0.663 ± 0.021 eV) of pure Al, comparing with that (0.611 ± 0.023 eV) of pure Al without an electric field. The increase of vacancy migration enthalpy, induced by the electric field, was responsible for the difference on evolution of quenched-in vacancy, rapid solute clustering and age hardening improvement during the early stage ageing of Al-4wt%Cu alloy.
In this work, refill friction stir spot welding (RFSSW) was used to weld 2 mm-thick 5083-O alloy. The Box-Behnken experimental design was used to investigate the effect of welding parameters on the joint lap shear property. Results showed that a surface indentation of 0.3 mm effectively eliminated the welding defects. Microhardness of the stir zone (SZ) was higher than that of the base material (BM) and the hardness decreased with increasing the heat input during welding. The optimum failure load of 7.72 kN was obtained when using rotating speed of 2300 rpm, plunge depth of 2.4 mm and refilling time of 3.5 s. Three fracture modes were obtained during the lap shear test and all were affected by the hook defect.
Different artificial two-stage ageing behaviors and their effect on stress corrosion cracking (SCC) susceptibility of Al-Zn-Mg alloy have been investigated. The experimental results show that two hardness peaks present on the second-stage ageing-hardening curve when the first-stage ageing is dealt with comparatively lower temperature than the conventional one. The first peak is caused by dispersive and evenly distributed G.P. zones, while η′ phases and coarsened G.P. zones contribute to the second peak. Tensile strength of experimental alloy raises 9.6% (33.2 MPa) and SCC susceptibility decreases 38.9% by applying the second peak ageing regime instead of conventional T73. Al-Zn-Mg alloy obtains high strength and SCC resistance due to its finely dispersive matrix precipitates (MPts), coarsened and discontinuous grain boundary precipitates (GBPs), as well as the narrow precipitate free zone (PFZ) in the second peak ageing condition.
The relationship between the as-cast microstructure and mechanical properties of the Al-12Si-3.5Cu-2Ni-0.8Mg alloys produced by permanent mold casting (PMC) and high pressure die casting (HPDC) is investigated. The alloys in both PMC and HPDC consist of Al, Si, Al5Cu2Mg8Si6, Al3CuNi, and Al7Cu4Ni phase. However, the microstructure of the HPDC alloy is significantly refined. Compared to the PMC alloy, the ultimate tensile strength of the HPDC alloy is significantly increased from 244 MPa to 310 MPa, while the elongation shows a reverse trend at room temperature. At low stress and temperature range, slight variations of stress exponent and activation energy indicate that the minimum creep rate is controlled by the grain boundary creep. Then the minimum creep rate is higher for the specimen with the smaller grain size, where grain boundary creep is the dominant creep mechanism. At high stress region, the stress exponent for the PMC alloy and HPDC alloy is 5.18 and 3.07, respectively. The different stress exponents and activation energies measured at high stress and high temperature range indicates that the creep mechanism varies with the casting technologies.
Influence of Sc content on microstructure and stress corrosion cracking behavior of medium strength Al-Zn-Mg alloy have been investigated by optical microscopy, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy and slow strain rate test. The results indicate that the addition of Sc results in the formation of the quaternary coherent Al3(Sc, Zr, Ti) dispersoids during homogenization treatment, which will inhibit the dynamic recrystallization behavior. The number density of Al3(Sc, Zr, Ti) particles increases with the increase of Sc content, and thus the recrystallization fraction of hot-extruded alloy is reduced and the peak strength in two-stage artificial aging sample is enhanced. At the same time, the wide of precipitation free zone is reduced, and the content of Zn and Mg in grain boundary particles and precipitation free zone is increased with the increase of Sc content. In peak-aged state, the 0.06 wt% Sc added alloy shows the better stress corrosion cracking resistance than the Sc-free alloy because of the reduction of recrystallization fraction and the interrupted distribution of grain boundary precipitates along grain boundary. However, the further addition of Sc to 0.11 wt% will result in the deterioration of stress corrosion cracking resistance due to the increase of electrochemical activity of grain boundary particles and precipitation free zone as well as hydrogen embrittlement.