Study on Pretreatment Groove and Multi‐Walled Carbon Nanotubes‐Filled Friction Stir Lap Welding for 6061‐ <scp>T6</scp> Al Alloy and Carbon Fiber‐Reinforced Polymer Composite

ABSTRACT Dissimilar metal/polymer materials align with lightweight design criteria in engineering. Enhancing the mechanical properties of joints reduces the risk of failure and expands the range of applications. This study presents a measure involving the pretreatment groove and the incorporation of multi‐walled carbon nanotubes (MWCNTs) in friction stir lap welding (FSLW) of 6061‐T6 Aluminum (Al) alloy and carbon fiber‐reinforced thermoplastic (CFRTP). Fracture, tensile, and hardness analyses were conducted using scanning electron microscopy (SEM) and optical microscopy (OM). The void defects in the joint were identified through the image thresholding method. Research findings indicate that the pretreatment groove promotes the formation of an inlaid structure within the joint. The inlaid structure significantly enhances welding reliability and tensile‐shear strength. The tensile‐shear strength peaks at a groove depth of 0.2 mm, attaining 38.7 MPa, which is 43.12% higher than that of the untreated joint. The excessive groove depth negatively affects the welding surface morphology and tensile‐shear strength of the joint. MWCNTs enhance heat transfer and joint hardness, reaching a peak value of 135.53 HV. However, the aggregation particle induces microcracks and void defects, reducing the tensile‐shear strength.