The minimum hardness seen is around 100 VHN3kg and 90 VHN3kg for weld produced using 10 mm and 20 mm shoulder diameter, respectively. The hardness of the material in the stirred zone and thermo mechanically affected zone is comparable with base material hardness. The hardness observed in the WN and TMAZ region is slightly lower than the BM. Precipitate dissolution and reforming due to post weld thermal cycle can be the reason for regaining the hardness in the stirred zone and TMAZ. A more detailed analysis is being carried out.
The base material tensile strength was 428.6 MPa and elongation was 16.6. Tensile properties of welds are shown in Table 3. Even though the softest zone occurred away from the weld, the tensile specimens fractured in the weld region for all three welds produced using un-chamfered shoulder with cylindrical flat-end pin. The only exception was the weld made using chamfered shoulder having a frustum shaped rounded endpin with 10 mm shoulder diameter and 6 mm diameter pin. The defects present in the weld make the weld weaker. From the data generated in Table 3, friction stir welding reduces both the tensile strength and ductility of the material. The maximum weld strength for the weld carried out using a tool with 20 mm shoulder diameter and 6mm pin diameter is 395.84 MPa with an elongation of 9.8 per cent. The maximum elongation for 15 mm shoulder diameter and 4mm pin diameter is 14 per cent with tensile strength of 355.41 MPa.
Conclusions
o A defect-free weld is produced in AFNOR 7020 aluminium alloy.
o In the stirred zone, fine equi-axed grains of size ranging from 5 - 20 µm are transformed from the initial pancake shaped 300 to 500µm size parent metal grain structure
o Grain size in the weld nugget varied with the tool geometry
o Two different tools geometries, un-chamfered shoulder with cylindrical flat end pin and chamfered shoulder with frustum shaped rounded end pin, with different dimensions were used. Chamfered shoulder with frustum shaped rounded end pin produces better quality weld than the other
o The experimental results showed that the tensile properties and fracture locations of the joints are significantly affected by tool geometry
o The reduction in strength of weld joint had been significantly attributed to the presence of the defects rather than micro structural changes
o The size of the unfilled region present in the advancing bottom side of the weld decreases as the shoulder diameter is increased or a tilt angle is given
o Friction stir welds of AFNOR 7020 do not show major reduction in hardness in the weld nugget and thermo-mechanically-affected zone. The softest region was formed away from the weld, in the HAZ and the minimum hardness was 90VHN3kg
o The maximum weld strength obtained in this study was 395.84 MPa (92.35 per cent) with 9.8 per cent (59 per cent) elongation is archived in the weld.
Detailed micro-structural analysis using transmission electron microscopy and precipitation kinetic analysis using differential scanning calorimetry is being carried out for further characterisation of weld and to find the possibility of increasing the weld strength.
|
