The effects of sealants and surface treatments on the faying surface of swept friction stir spot welds
The goals of this project were to determine and document the effects of sealants and surface treatments have on the ultimate strength, fatigue life, and corrosion resistance of swept Friction Stir Spot Welded (FSSW) joints. One sealant, the PRC-DeSoto PR-1432 GP and several surface treatments were examined. The surface treatments attempted were AlClad, Chromic Acid Anodization on bare sheets, and Alodine chemical conversion coating on bare sheets. Bare sheets without a surface treatment were also evaluated to establish a baseline comparison. Ultimate lap shear testing was based on the unguided NASM 1312-4 2-spot weld coupon configuration and the guided NASM 1312-21 4-spot weld coupon configuration. Results indicated that the sealants and surface treatments decreased the joint strength of the individual spot welded joints a relatively small amount compared to the bare material strength. However, the sealant’s adhesive properties can potentially compensate for this loss if there is sufficient sealant coverage. The guided NASM 1312-21 4-spot weld coupon configuration was used for fatigue testing. Uniform amplitude fatigue tests were conducted with a variety of load levels to generate a load-life curve. The sealants and surface treatment seemed to have no effect on fatigue life at medium to low fatigue load levels. However, at high fatigue load levels, the sealants and surface treatments were slightly detrimental. The fatigue lives of the swept FSSW coupons was less than those of the NAS 1097 AD4 riveted coupons at the medium to low loads. However, the swept FSSW coupons were not optimized for fatigue and may improve under such conditions. The results of the corrosion testing indicated that the integrity of the sealant was not compromised by the swept FSSW. Generally, the surface treatments were not affected by the FSSW operation except where the tools displaced the surface and on the anvil side of the coupon in the heat affected zone. Since these areas are more vulnerable to corrosion, some protection should be added after welding. In general, the results of this project show that swept FSSW can successfully weld through sealants and surface treatments with only a minimum loss of the ultimate strength or fatigue life of the joint. The FSSW operation can be performed while maintaining most of the corrosion resistance.
Wichita State University, College of Engineering, Dept. of Aerospace Engineering
Includes bibliographic references (leaves 50-54)