The objective of this research is to determine the effects of laser preheating on friction stir lap welding. Laser preheating of the top surface of the material during friction stir lap welding is used to reduce the welding forces and torque, thereby reducing the stiffness requirements on the clamping and FSW tool. Preheating also has the potential to enable higher welding speeds and reduced tool wear particularly when applied to welding of higher melting temperature alloys. A transient three-dimensional finite element heat transfer model of the workpieces was developed to predict the influence of preheating on the temperature distribution and heat flux within the workpieces. The model accounts for conduction in the workpieces, contact resistance between them, laser absorption, frictional heat generation under the FSW tool shoulder, and losses to the surroundings. Predicted temperatures at discrete locations were compared with thermocouple measurements and found to be in good agreement. The model showed that the energy deposited by the laser in the top plate does not penetrate into the bottom plate ahead of the tool due to the contact resistance between the plates. Hence, the thermal contact resistance, inherent to lap welds, controls the effectiveness of preheating. The effect of preheating on FSW of aluminum was investigated by varying the laser power and workpiece material while maintaining constant tool geometry and material. The results of the parametric study are presented and show that while forces and torques are reduced by preheating, not as much as in butt welding because of the contact resistance produced by the nature of the weld. It was also found that the total power consumed in the process, defined as the sum of tool power and absorbed laser power, can decrease with preheating.

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