Characterization of anomalous tool degradation during friction stir processing of 6061-T6 aluminum alloy extrusions: A failure analysis study

The diameter of a H13 steel tool with M6 threads and a pin diameter of 5.9 mm and a pin length of 5 mm was measured after each 25.4 mm length of friction stir processing (FSP) of 6061-T6 extrusions. The wear of the pin diameter with FSP distance was found to have three distinct regions, the last of which was the steady state, where the pin diameter remained constant. Metallographic analyses of two tools subjected to FSP for 60 and 120 s showed that (i) threads fractured in early stages of FSP, (ii) a built-up layer formed between the threads, and (iii) threads progressively wore with processing time. The metallographic analysis of an embedded tool showed the presence of a fractured piece of the tool in the stir zone, indicating chipping of the tool. The anomalous wear rate and chipping observed in the experiments was attributed to the hardness of the tool which was found to be significantly lower than what is expected of heat-treated H13 steel.