Friction stir welding of severely plastic deformed aluminum using (Al2O3 + graphite) hybrid powders: grain structure stability and mechanical performance

Welding of severely plastic deformed aluminum sheets, with high stored strain energy, faces a serious concern, which is decrease in strength of weld and heat-affected zone (HAZ) due to instability in microstructure and related grain growth. In this work, two-passes constrained groove pressed (CGPed) Al-1050 sheets were welded via friction stir welding (FSW), without and with addition of a hybrid powder including 50 vol.% α-Al2O3 nanoparticles+50 vol.% graphite micrometric powders (∼1 vol.% of hybrid powders in stir zone). Al2O3 nanoparticles may result in grain refinement in stir zone by grain boundary pinning. Lubricating nature of graphite may reduce welding heat input resulting in lower degree of HAZ softening. Addition of hybrid powders improved thermal stability of grain structure and increased hardness of stir zone and strength of weld after three passes of FSW and at high rotation speed (ω) to traverse speed (v) ratio. With one-pass FSW, a sound weld was not achieved. However, higher weld efficiency (with transition of failure location from stir zone to HAZ) was obtained when three-pass FSW was carried out due to better material flow, superior distribution of powders, and elimination of cavities. Due to presence of graphite, effect of increase in pass number was negligible on HAZ softening. Finer distribution of hybrid powder was achieved at high ω/v ratios. Weld efficiency was improved from ∼50% (strength of 62 MPa) in one-pass FSW at ω/v = 25 r/mm to a maximum of ∼80% (strength of 101 MPa) after conducting three-pass FSW at ω/v = 70 r/mm.