Parameters controlling microstructure and hardness during friction-stir welding of precipitation-hardenable aluminum alloy 6063

Abstract

The aluminum (Al) alloys 6063-T5 and T4 were friction-stir welded at different tool rotation speeds (R), and then distributions of the microstructure and hardness were examined in these welds. The maximum temperature of the welding thermal cycle rose with increasing R values. The recrystallized grain size of the weld increased exponentially with increasing maximum temperature. The relationship between the grain size and the maximum temperature satisfied the static grain-growth equation. In the as-welded condition, 6063-T5 Al was softened around the weld center, whereas 6063-T4 Al showed homogeneous hardness profiles. Different R values did not result in significant differences in the hardness profile in these welds, except for the width of the softened region in the weld of 6063-T5 Al. Postweld aging raised the hardness in most parts of the welds, but the increase in hardness was small in the stir zone produced at the lower R values. Transmission electron microscope (TEM) observations detected a similar distribution of the strengthening precipitates in the grain interiors and the presence of a precipitation-free zone (PFZ) adjacent to the grain boundaries in all the welds. Microstructural analyses suggested that the small increase in hardness in the stir zone produced at the lower R values was caused by an increase in the volume fraction of PFZs.

Ultrafine-Grained Al-Mg-Sc Alloy via Friction-Stir Processing

“…The heating experiment of Rhodes et al [19] following FSP and the use of static grain growth model to estimate the grain size observed after FSP by Sato et al, [16] Robson and Campbell, [28] and Kumar et al [29] supports the above idea. There are primarily three ways by which grain size development during FSP has been controlled: (1) cooling rate control, (2) peak temperature control, and (3) use of thermally resistant precipitates/dispersoids.…”

“…Of late, there have been some efforts to obtain UFG microstructure by changing the processing parameters such as tool rotational rate (x) at constant tool traverse velocity (m) or the ratio x/m. [12,16,17] Su et al [18] and Rhodes et al [19] demonstrated the possibility of achieving grains as small as 25 to 100 nm by employing special cooling arrangement during FSP. But as mentioned earlier, most of the mean grain sizes tabulated by Mishra and Ma were larger than 1 lm under ordinary processing conditions.…”

“…The control of grain size by manipulating the peak temperature reached during FSP-mentioned as route (2)-has been tried by many researchers either by changing the tool rotation rate (x) or ratio of x and tool traverse speed (m). Sato et al [16] varied x from 800 to 2450 rpm and obtained a mean grain size ranging from 5.9 lm to 17.8 lm. Similarly, by varying x from 560 to 1840 rpm, Kwon et al [12] obtained grains ranging from 0.5 lm to 4 lm in size.…”

Recrystallization mechanisms during friction stir welding/processing of aluminum alloys

“…Modeling results indicate that the nugget/SZ material has experienced large plastic strains at strain rates in the range 10 1 -10 2 s À1 [8]. Peak temperatures in this region are thought to be in the range from 0.6 to 0.95T Melt , depending on the material, tool design and operating conditions, and the upper portions of this region experience heating and deformation effects from the tool shoulder as well as from the tool pin [14][15][16]. Next is the thermomechanically affected zone (TMAZ), where the material experiences lesser strains and strain rates as well as lower peak temperatures.…”

Development of nanocrystalline structure in Cu during friction stir processing (FSP)

“…In this study, the formation of sub-micron and nano-scale crystalline structures reflects a direct transformation by coarsening of the highly refined microband structures formed around the tool pin. It should be noted that several recrystallization theories including continuous dynamic recrystallization, discontinuous dynamic recrystallization, geometric dynamic recrystallization and recrystallization via particle stimulated nucleation, which were developed to account for grain size control during conventional thermo-mechanical processing, have also been used to explain the formation of fine grain structures (usually 1-10 m) during FSW/FSP [5,[41][42][43][44][45][46][47][48]. Common FSW and FSP technologies inherently involve severe plastic deformation (SPD) as well as transients and gradients in strain, strain rate and temperature.…”