Grain Structure Evolution in 6013 Aluminum Alloy during High Heat-Input Friction-Stir Welding.

This work was undertaken to evaluate the influence of friction-stir welding (FSW) under a high-heat input condition on microstructural evolution. Given the extreme combination of deformation conditions associated with such an FSW regime (including the highest strain, temperature, and strain rate), it was expected to result in an unusual structural response. For this investigation, a commercial 6013 aluminum alloy was used as a program material, and FSW was conducted at a relatively high spindle rate of 1100 rpm and an extremely low feed rate of 13 mm/min; moreover, a Ti-6Al-4V backing plate was employed to reduce heat loss during welding. It was found that the high-heat-input FSW resulted in the formation of a pronounced fine-grained layer at the upper weld surface. This observation was attributed to the stirring action exerted by the shoulder of the FSW tool. Another important issue was the retardation of continuous recrystallization. This interesting phenomenon was explained in terms of a competition between recrystallization and recovery at high temperatures. Specifically, the activation of recovery should reduce dislocation density and thus retard the development of deformation-induced boundaries.

Tailoring of Dissimilar Friction Stir Lap Welding of Aluminum and Titanium.

An approach was proposed to optimize dissimilar friction stir lap welding of aluminum and titanium alloys. The basic concept of the new technique included (i) the plunging of the welding tool solely into the aluminum part (i.e., no direct contact with the titanium side) and (ii) the welding at a relatively high-heat input condition. It was shown that sound welds could be readily produced using an ordinary cost-effective tool, with no tool abrasion and no dispersion of harmful titanium fragments within the aluminum side. Moreover, the intermetallic layer was found to be as narrow as ~0.1 µm, thus giving rise to excellent bond strength between aluminum and titanium. On the other hand, several important shortcomings were also revealed. First of all, the high-heat input condition provided significant microstructural changes in the aluminum part, thereby resulting in essential material softening. Furthermore, the new approach was not feasible in the case of highly alloyed aluminum alloys due to the relatively low rate of self-diffusion in these materials. An essential issue was also a comparatively narrow processing window.

The Microstructure and Strength of UFG 6060 Alloy after Superplastic Deformation at a Lower Homologous Temperature.

The paper reports on the features of low-temperature superplasticity of the heat-treatable aluminum Al-Mg-Si alloy in the ultrafine-grained state at temperatures below 0.5 times the melting point as well as on its post-deformation microstructure and tensile strength. We show that the refined microstructure is retained after superplastic deformation in the range of deformation temperatures of 120-180 °C and strain rates of 5 × 10-3 s-1-10-4 s-1. In the absence of noticeable grain growth, the ultrafine-grained alloy maintains the strength up to 380 MPa after SP deformation, which considerably exceeds the value (250 MPa) for the alloy in the peak-aged coarse-grain state. This finding opens pathways to form high-strength articles of Al-Mg-Si alloys after superplastic forming.

Peculiarities of DRX in a Highly-Alloyed Austenitic Stainless Steel.

The features of discontinuous dynamic recrystallization (DRX) in a highly-alloyed austenitic stainless steel were studied at temperatures of 800 °C to 1100 °C. Hot deformation accompanied by DRX was characterized by an activation energy of 415 kJ/mol. The frequency of the sequential DRX cycles depended on the deformation conditions; and the largest fraction of DRX grains with small grain orientation spread below 1° was observed at a temperature of around 1000 °C and a strain rate of about 10-3 s-1. The following power law relationships were obtained for DRX grain size (DDRX) and dislocation density (ρ) vs. temperature-compensated strain rate (Z) or peak flow stress (σP): DDRX ~ Z-0.25, ρ ~ Z0.1, σP ~ DDRX-0.9, σP ~ ρ1.4. The latter, i.e., σP ~ ρ1.4, was valid in the flow stress range below 300 MPa and changed to σP ~ ρ0.5 on increasing the stress. The obtained dependencies suggest a unique power law function between the dislocation density and the DRX grain size with an exponent of -0.5.