Study on Impact of Microstructure and Hardness of Aluminium Alloy After Friction Stir Welding

The effects of friction stir welding (FSW) on the microstructure and hardness of rolled pure aluminium 6061 were investigated. The weld was obtained by varying its tilt angle (2°) and Pin diameter (6mm). Tensile strength & % Elongation was carried out to evaluate the strength of the weld. Optical microscope study was carried out to study the uniform stirring of materials. The stir zone (SZ) contains fine, equiaxed and fully recrystallized grains. Thermo mechanically- affected zone (TMAZ), heat-affected zone (HAZ), and base material (BM) were different. Hardness test indicated that the minimum and maximum hardness values were obtained in the HAZ and BM, respectively.

INTRODUCTION FSW is becoming more popular for joining a wide range of aluminium alloys for numerous applications. One advantage of FSW is that there is far lower heat input during the process compared with conventional welding methods such as TIG or MIG. Therefore, this solid-state process results in to minimal microstructural changes and better hardness and tensile tests than conventional welding [1][2][3]. The FSW process generates three distinct microstructural zones: the nugget zone (NZ), the thermomechanically affected zone (TMAZ) and the heataffected zone (HAZ) [4]. The HAZ is only affected by heat, without plastic deformation. The TMAZ adjacent to the nugget is plastically deformed and heated. The nugget is affected by the highest temperature and the highest plastic deformation, which generally consists of fine equixed grains due to the fully dynamic recrystallization.
A relationship between microstructure and microhardness of each FSW weld zone has been discovered [5].Changes in microharness along the FSW joint are directly related to the precipitation state.

II.
EXPERIMENTAL DETAILS AA 6061 (0.4 % Si, 0.7% Fe, 0.4% Cu, 0.15% Mn, 1.2% Mg, .35% Cr, 0.25% Zn, 0.15% Ti balance Al) plates of 6mm thickwere friction stir welded vertical to the rolling direction with a travel speed, a rotational speed and a shoulder diameter of 20mm/min, 1000 rpm and 25mm. The friction stir pin had a diameter of 4mm height of 4.8mm. A simultaneous rotation and translation motion of the FSW tool generates the formation of an symmetric weld [6]. Welded cross-sections were ground, polished, and etched with Beaker's reagent for optical metallography. Instrumental (digital) Vickers micro hardness measurements were also made throughout the weld zone and into the initial aluminium alloy plate using a 50gf load. Tensile specimens were machined from NZ in parallel (longitudinal) direction from the weld. The tensile properties of the joints were evaluated using three tensile specimens cut from the same joint. MPa, and102.21MPa, respectively. By comparison, the two FSW specimens showed a significant decrease in both tensile and yield strength. Fig 3 also shows that the ductility for tensile specimen P-AA6061 significantly increased. Fig 1, the tensile specimen P-AA6061 contained only recrystallized grains from the NZ. From the hardness results, we know that the hardness values of the NZ were lower than that of the BM, possibly explaining why the longitudinal tensile specimen P-AA6061 exhibited both tensile and yield strength values. When a tensile load was applied to the joint, failure occurred in the weakest regions of the joint [7], which is the HAZ in this work. The hardness curve is a symmetrical with respect to the weld centreline because the plastic flow field in the two sides of the weld centre is not uniform [8]. The larger distorted grains and distortion energy causes the strain-hardness to increase, resulting in the symmetrical microhardness distribution. The minimum hardness of 83.03HV was obtained in the HAZ region. The maximum value 106.34HV was present in the BM. The hardness of the TMAZ was higher than that of the NZ.

Microstructure:
The microstructure of the different regions of the welded similar material is shown in fig . The NZ consists of fine equixed grains due to dynamic recrystallization. The grains in NZ are much smaller than those in other regions. The average grain size in the four zones in follows the order of BM>HAZ>TMAZ>NZ. In the TMAZ which is adjacent to the NZ, the strain and the temperature were lower than in the NZ and the effect of welding on the microstructure was correspondingly smaller. Unlike NZ, the microstructure was recognizably that of the parent material, although significantly deformed and rotated. The grain size of the HAZ was similar to that of the BM. The HAZ was common to all welding processes subjected to a thermal cycle, but it was not deformed during welding.

SEM With EDX analysis:
Elemental analysis of the macro regions in weld zone was performed using a scanning electron microscope (SEM) equipped with an EDX system. This analysis was conducted to gauge the distribution of alloying elements in the FSW zone. SEM image was analyzed at a magnification of 50X, 250X, 500X, 1.50KX  The EDAX analysis depicted in Table 1 revealed that high contents of oxygen and aluminium are present.