Strain gradient plasticity in AISI A36 plain carbon steel weldment: Comparison of butt and lap joint configurations

Strain gradient plasticity phenomenon in the parent material (PM), heat affected zone (HAZ) and fusion zone (FZ) of butt and lap joint of shielded metal arc welded AISI A36 steel is studied. The deformation mechanisms of the weldment were explained using mechanistic and phenomenological plasticity and dislocation models. The hardness of the FZ and HAZ of the lap weldment were 1.83 GPa and 1.56 GPa, which were slightly higher than the 1.63 GPa and 1.49 GPa of the FZ and HAZ of the butt. Tensile strengths of the butt joint was ∼384 MPa and that of lap was ∼361 MPa, which were higher than the parent material of ∼290 MPa. The difference in strength between the butt and lap joint were due to the weld configuration and the limited fusion of the single build of the lap joint. The material length scale parameters varied due to microstructural transition from the PM to the FZ as a function of temperature. Length scales of ∼34.37 μm and 22.75 μm were observed for the fusion zones of the lap and butt joints, whereas the lowest of 14.76 μm was for the as-received A36 steel. The geometrically necessary and statistically stored dislocations densities were higher in the lap weldment than for the butt. The average shear strain estimations were used to elucidate the underlying dislocation structures and densities. Grain refinement and localised stress and strain states at the HAZ and FZ led to variations in the mechanical properties. The implications of the results for robust design of welds for structural applications are discussed.