Abstract

In this study, the dissimilar metals 5754 aluminum and DP980 ultra-high strength steel were joined using the friction bit joining (FBJ) process. The friction bits were made using one of three steels: 4140, 4340, or H13. Experiments were performed in lap shear, T-peel, and cross tension configurations, with the 0.070" thick 5754 aluminum alloy as the top layer through which the friction bit cut, and the 0.065" thick DP980 as the bottom layer to which the friction bit welded. All experiments were performed using a computer controlled welding machine that was purpose-built and provided by MegaStir Technologies. Through a series of designed experiments (DOE), weld processing parameters were varied and controlled to determine which parameters had a significant effect on weld strength at a 95% confidence level. The parameters that were varied included spindle rotational speeds, Z-command depths, Z-velocity plunge rates, dwell times, and friction bit geometry. Maximum lap shear weld strengths were calculated to be 1425.4lbf and were to be obtained using a bit tip length at 0.175", tip diameter at 0.245", neck diameter at 0.198", cutting and welding z-velocities at 2.6"/min, cutting and welding RPMs at 550 and 2160 respectively, cutting and welding z-commands at -0.07" and -0.12" respectively, cooling dwell at 500 ms, and welding dwell at 1133.8 ms. These parameters were further refined to reduce the weld creation time to 1.66 seconds. These parameters also worked well in conjunction with an adhesive to form weld bonded samples. The uncured adhesive had no effect on the lap shear strengths of the samples. Using the parameters described above, it was discovered that cross tension and T-peel samples suffered from shearing within the bit that caused the samples to break underneath the flange of the bit during testing. Visual inspection of sectioned welds indicated the presence of cracking and void zones within the bit.

Degree

College and Department

Ira A. Fulton College of Engineering and Technology; Technology

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