Friction stir lap welding (FSLW) is a solid-state welding process that allows joining of similar and dissimilar materials, which is particularly interesting for the mentioned joints, especially for thin sheets. The welding parameters affect the interface integrity, interaction between steel and aluminium, intermetallic compound (IMC) formation, microstructural evolution, the material flow, defects level, residual stresses, distortions, etc. In the present work, stress maps obtained during in-situ lap shear tests on different combinations of dissimilar Al/Steel (5052/1010; 5052/DP1000; 5182/DP1000 and 5182/DP1000) were evaluated using Bragg Edge neutron Imaging. The detection of steel inclusions in the whole length of the welded sample, the hook dimensions, and deformation maps around them were analysed. The monitoring of deformation maps during in-situ lap shear tests allow following the strain concentration and spatial distribution providing valuable information to understand the role of inclusions, hook and IMC at the interface in the deformation and fracture mechanism. This information cannot be obtained during an in-situ test by another technique.