Nanoindentation and Dao analysis of strain rate-sensitive elasto-plastic behavior in similar and dissimilar AZ31B–AZ91 magnesium alloy joints reinforced with graphene

The present investigation aims to evaluate the strain rate dependent nano-mechanical and elasto–plastic deformation behaviour of graphene reinforced similar and dissimilar AZ31B–AZ91D friction stir welded (FSW) joints using nanoindentation and Dao-based inverse analysis. Similar AZ31B+AZ31B, AZ91D+AZ91D and dissimilar AZ31B+AZ91D joints reinforced with 0.3 wt.% graphene were fabricated at a tool rotational speed of 900 rpm and traverse speed of 40 mm/min. Nanoindentation tests were conducted up to a maximum penetration depth of 2000 nm at strain rates ranging from 0.05 to 0.20 s -1 . All joints exhibited increased deformation resistance with increasing strain rate. The AZ91D+AZ91D+0.3 wt.% graphene joint showed the highest indentation resistance, approximately 18–32% higher than the AZ31B+AZ31B joint, while the AZ31B+AZ91D joint exhibited nearly 10–22% improvement. Nanoindentation hardness increased from 1.72 to 2.24 GPa for AZ31B+AZ31B, 2.42 to 3.24 GPa for AZ91D+AZ91D, and 2.08 to 2.82 GPa for AZ31B+AZ91D with increasing strain rate. The corresponding strain rate sensitivity values were 0.08497, 0.08982 and 0.09549, respectively. Nix–Gao analysis confirmed a pronounced indentation size effect, with the AZ91D-based joint exhibiting nearly 35–45% higher H 2 values than the AZ31B-based joint. Dao-based inverse analysis revealed that the yield stress increased from 146–181 MPa for AZ31B+AZ31B and 227–278 MPa for AZ91D+AZ91D with increasing strain rate. The enhanced mechanical response is attributed to graphene reinforcement, grain refinement through dynamic recrystallization, and the presence of β-Mg17Al12 intermetallic phases within the stir zone.