Influences of friction stir welding parameters on morphology and tensile strength of high density polyethylene lap joints produced by double-pin tool

A double-pin tool is adopted in the submerged friction stir lap welding of high density polyethylene sheets in the present investigation. Effects of welding parameters (rotational speed, welding speed, plunge depth) on weld morphology and tensile strength are investigated. The experiment is arranged by taguchi's L9 orthogonal array. Macromorphology and microstructure of three typical welds are observed and compared. Analysis of variance is adopted to assess the importance of each welding parameter to the joint strength. The results show that joint with low tensile strength presents poor weld formation. Voids and cracks can be observed in the nugget by scanning electron microscope. Joint with high strength shows excellent nugget formation and inverted bowl-shaped weld cross section. Joint strength increases with the increase of rotational speed or the decrease of welding speed. Small or large plunge depth is not beneficial to the joint strength. The joint with maximum tensile strength of 15.3 MPa is obtained at rotational speed of 1300 r/min, welding speed of 20 mm/min and plunge depth of 0.1 mm. Welding speed has the most significant effect on the joint strength, about 75.37%, while plunge depth has the least influence.