Effect of preheating on joint quality in the friction stir welding of bimodal high density polyethylene

The effect of process temperatures on joint quality in friction stir butt welding of bimodal high density polyethylene (HDPE) was investigated. Process temperatures were manipulated by varying the rotation speed and welding speed. Further control of the process temperatures was achieved by heating the bottom side of the welded plates using a film heater. Temperatures on the top and bottom surfaces of the welded plates were measured by infrared thermography and thermocouples, respectively. Material flow patterns were also visualized by implementing a novel welding technique where two different colors of HDPE were placed on the advancing and retreating sides, respectively, and the cross section of the welding zone was inspected after welding. It was found that proper welding of this bimodal HDPE takes place when the material is maintained at high enough temperatures in the viscous state throughout its thickness. Under these conditions, proper interphase mixing and fusion of the material takes place. Weld efficiencies in excess of 100% and large elongations in the order of 60% were also obtained at the optimum range of welding temperatures.