Abstract Local strength of friction stir (FS) welds and FS processed aluminium alloys in heat-treatable aluminium alloys is dominated by precipitation hardening. Strengthening due to stored dislocations is generally limited to 40 MPa, and grain size strengthening is generally less than 10 MPa. Local crystallographic texture can cause yield strength variation on the order of 5%. Published models for strengthening of FS welds make a range of simplifying assumption which can cause uncertainties in the predictions of up to 50 MPa. Possible improvements are explored.