On the Effect of Multi-Pass Friction Stir Processing on Microstructure-Tensile Deformation Behavior Relationships in Cast Al-7%Si-0.4%Mg Specimens

Specimens from commercial and continuously cast A356 ingots have been friction stir-processed, and tensile deformation has been characterized. These two types of ingots have been found to be damaged in the liquid state, but at different levels. In both cases, the microstructure has been refined and homogenized. FSP has been found to improve structural quality by breaking up bifilms. For the commercial ingot, each FSP pass has progressively improved structural quality, as evidenced by an 18 times increase in elongation (from 1.0 to 18.8% after three passes), whereas in the continuously cast ingot, it has taken only one pass for FSP to improve structural quality by doubling elongation (from 10.9 to 21.1%) after which additional passes have not resulted in further improvement. Analysis of tensile deformation behavior has shown that all FSPed specimens exhibit a distinct Stage III work hardening, as modeled by Kocks and Mecking. Through the analysis of tensile deformation behavior, it has been hypothesized that improvement in elongation and structural quality with FSP may not be solely attributed to the refinement of Si particles.