Strain and strain rate during friction stir welding/processing of Al-7Si-0.3Mg alloy

The maximum strain and strain rate experienced during friction stir welding/processing (FSW/P) has remained quite unclear, despite various efforts. Knowledge of strain and strain rate is important for understanding the subsequent evolution of grain structure, and serves as a basis for verification of various models as well. In the present study, we facilitated the breaking and embedding of the pin into the workpiece of cast Al-Si-Mg alloy during FS to obtain "frozen" samples for analysis. Metallographic evidence has indicated that in the leading transitional zone the strain gradient increased rapidly before entering into the thread space. Analysis of the deformed dendrites has suggested that the strain was similar to 3.5 and the strain rate was similar to 85 s(-1) when the deforming material entered the thread space. The heavily deformed material then formed a rotational zone confined within thread spaces rotating largely with the pin, depositing a large part on the trailing side of the pin. Thus, once into the thread spaces, further strain is low and strain rate should decrease considerably.