In this work, friction spot stir welding (FSSW) is applied to join high-density polyethylene/multi-walled carbon nanotube (HDPE/MWCNTs) composites. Injection-molded coupons were welded with a single lap-shear configuration under different welding conditions (tool rotational speed, plunge depth, and dwell time). By analyzing the lap-shear tensile load and the fracture surface of the welded joints, it is found that the weld attributes (e.g. weld area and maximum lap-shear tensile load) increase with increasing dwell time, tool rotational speed, and plunging depth. The maximum lap-shear tensile load increases with nanotube loading up to a threshold, followed by a decreasing trend at nanotube loading higher than 1.0 wt.%. It is hypothesized that the bonding mechanism for FSSW of HDPE/MWCNT composites is mainly through the co-crystallization across the interface. When more nanotubes are involved in the welding zone (>1.0 wt.%), saturation of nucleation is reached, the positive effect on the crystallization is vanished, and consequently the overall mechanical properties decrease. Interface failure of the welded joints and bulk fracture originated from the upper coupon within the weld nugget perimeter were identified as the two main failure mechanisms.

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