Experimental analysis of spot friction welding of 6111-T4 aluminum alloy

Spot friction welding (SFW) is solid state joining process using a rotating tool with the pin at the tip plunged into a two overlapping sheets to create plastic flow of the material and create a spot welded joint. Based on literature survey, the following gaps in research are identified and addressed in this thesis: (1) effects of process parameters on specimen temperature, specimen geometry and their impacts on the SFW joint strength and failure mode, (2) effects of stamping lubricant on the SFW joint strength, and (3) improve the joint strength by creating Metal Matrix Composite (MMC) in-situ at the joint during the welding process.;The effects of key process parameters, cycle time, tool rotational speed, and axial force, on the specimen temperature and joint strength is studied to establish a relationship between the specimen temperature to the maximum lap shear load. When the specimen temperature met or exceeded a threshold temperature of 340°C, an acceptable joint strength of 2.4 kN or higher as specified by Society of Automotive Engineers (SAE) is achieved. The specimen temperature greater than 340°C can be accomplished by adjusting cycle time and/or tool speed and/or axial force.;In this study, cross-sectional macrographs of SFW joints are studied to establish a relationship between joint geometric feature and joint failure mode. A geometric feature, identified as the ratio of distance from the point at which metallurgical bond start to free edge of the pin and to free edge of the shoulder, has demonstrated to be a factor to determine the failure mode of SFW joints. When this ratio was below 1.3, the specimens failed by shear fracture and when it was above 1.8, the specimens failed by mixed mode fracture. The SFW joint area is maximized to achieve highest joint strength when this ratio is between 1.4 and 1.8.;This study conducts a design of experiment (DOE) to establish the effect of stamping lubricant on the joint strength. The maximum lap shear load increased by approximately 10% when the lubricant presents at the top surface compared to that of the baseline (no lubricant) whereas the maximum lap shear load decreased by 10% when the lubricant presents in the middle and at the bottom surfaces compared to that of the baseline. The weld nugget sizes of the tested specimens were measured and the average nugget size loosely correlated to the lap shear strength.;The localized particle reinforced MMC in SFW has demonstrated that using the >45 mum size steel particle in 6111-T4 aluminum, the improvement in maximum lap shear load by 25% and the displacement to failure by 55% compared to that of the baseline (homogeneous material). Cross-sectional macrographs of SFW joints show the increase in length of the crack path due to the torturous crack path which leads to the increase in maximum lap shear load for the particle reinforced MMC SFW specimen.;An analytical model was developed based on Ramberg-Osgood equation to calculate the tensile failure stress for the experimental setup with steel particle reinforced SFW in 6111-T4 aluminum. The estimated tensile stress of particle reinforced MMC increased by 14 to 30% compared to that of homogeneous material. The model is applied to provide the guideline for selection of type of particles that can be applied to further improve the strength of MMC reinforced SFW joint. Based on the variables used in the modeling, the SFW joint strength can be increased by increasing the volume fraction, using stronger reinforcement material, distributing the reinforcement particles uniformly, and using smaller particles.