FRICTION STIR PROCESSING OF POWDER METALLURGICAL Al ALLOYS

Abstract

Friction stir processing (FSP) is used for microstructural modifications in powder metallurgical aluminium alloys. It serves the purpose of grain refinement, reduction in porosity, homogenisation and thus improves the mechanical properties of the alloy. In this case, study has been carried out to determine (i) effect of compaction pressure on FSP of pure aluminium compact, (ii) difference between the properties of green and sintered compacts of pure aluminium after FSP and (iii) the effect of addition of copper into aluminium matrix. The green compacts were synthesized using Cold Isostatic Press (CIP) machine at seven different compaction pressures which were 50 MPa, 100 MPa, 150 MPa, 200 MPa, 250 MPa, 320 MPa and 380 MPa. FSP was carried out on all the green compacts using process parameters of 1525 rpm and 1mm/min. It was found that FSP was possible on all the green compacts. After FSP, the value of density reached almost equivalent to that of theoretical density. Moreover, intense plastic deformation and high heat generation during FSP causes dynamic recrystallization which results in the generation of fine grains. As a result the mechanical properties after FSP were highly enhanced. It was observed that if the sample was prepared using either too high or too low compaction pressure then during FSP the material movement was hindered. In this study, 200 MPa compaction pressure was found to be optimum for proper material movement during FSP as it has shown ultimate tensile strength (UTS) of 153 MPa and ductility of 38% which is highest among all the samples. Sintering was carried out on this sample in an atmosphere of N2 gas and subjected to FSP to find out the difference between the effect of FSP on green and sintered samples. The mechanical properties of the sintered sample was then compared with the green compact. Electron Back Scattered Diffraction (EBSD) was done to find out the reduction in grain size after FSP. Fractography revealed that after FSP, the fracture surface showed predominant dimple morphology to aid to its ductile property. Copper was also added to pure aluminium in three different compositions of Al-1wt%Cu, Al-5wt%Cu and Al-10wt%Cu and its effect on the properties was studied. It was found that the UTS of Al-1wt%Cu and Al-5wt%Cu was more in comparision to pure aluminium but it decreased in case of Al-10%Cu with significant reduction in ductility. The reason for such behaviour is attributed to the formation of intermetallics in excess which acted as crack initiation sites and hence resulted in quick failure.