Evaluation of Ballistic Limit Velocity Using Instrumented Indentation Test of 7xxx Aluminum Alloys After Friction Stir Welding

Friction stir welding has the advantages of producing less material deformation and of simple joining of relatively thick materials, but the disadvantages of forming relatively large heat-affected zones around weld areas. In addition, in research on reductions in bulletproof performance in heat-affected zones, it is necessary to evaluate the ballistic limit velocity (V₅₀), which measures bulletproof performance, and this requires specimens of a certain minimum size and has only limited application to local areas such as heat-affected zones. Instrumented indentation testing (IIT), a method of measuring material properties by utilizing load-depth curves measured by a small indenter leaving fine marks on the material, has no specimen size requirements; in addition, it has simple test procedures and is nondestructive. Here a theoretical model is proposed for evaluating V₅₀ of aluminum alloys by ductile hole formation using IIT. Heat-affected zones generated after friction stir welding of for 7000-series aluminum alloys were simulated, and the model was validated through comparison with the conventional V₅₀ test. In addition, there is currently no way to directly evaluate V₅₀ for welded areas in 7000-series aluminum, but here IIT was used to assess V₅₀ for each local part of the welded specimen. The conventional V₅₀ test takes 15 min per shot and cannot be reused of specimens, but the test using IIT takes 1 min to complete one test and has benefits in terms of time, cost and safety.