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Title: Effect of Process Parameters on Abnormal Grain Growth during Friction Stir Processing of a Cast Al Alloy

Abstract

The effects of process parameters and friction stir processing (FSP) run configurations on the stability of nugget microstructure at elevated temperatures were evaluated. Cast plates of an Al-7Si- 0.6Mg alloy were friction stir processed using a combination of tool rotation rates and tool traverse speeds. All single pass runs showed some extent of abnormal grain growth (AGG), whereas multi-pass runs were more resistant to AGG. Additionally, higher tool rpm was found to be beneficial for controlling AGG. These effects were analyzed by comparing the result of this work with other published results and AGG models.

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1001104
Report Number(s):
PNNL-SA-73063
Journal ID: ISSN 0921-5093; VT0504000; TRN: US201101%%838
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing, 528(1):189-199; Journal Volume: 528; Journal Issue: 1
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALLOYS; FRICTION; GRAIN GROWTH; MICROSTRUCTURE; PLATES; PROCESSING; ROTATION; STABILITY; Friction Stir Welding; Friction Stir Processing; FSW; abnormal grain growth

Citation Formats

Jana, Saumyadeep, Mishra, Rajiv S., Baumann, John A., and Grant, Glenn J. Effect of Process Parameters on Abnormal Grain Growth during Friction Stir Processing of a Cast Al Alloy. United States: N. p., 2010. Web. doi:10.1016/j.msea.2010.08.049.
Jana, Saumyadeep, Mishra, Rajiv S., Baumann, John A., & Grant, Glenn J. Effect of Process Parameters on Abnormal Grain Growth during Friction Stir Processing of a Cast Al Alloy. United States. doi:10.1016/j.msea.2010.08.049.
Jana, Saumyadeep, Mishra, Rajiv S., Baumann, John A., and Grant, Glenn J. Thu . "Effect of Process Parameters on Abnormal Grain Growth during Friction Stir Processing of a Cast Al Alloy". United States. doi:10.1016/j.msea.2010.08.049.
@article{osti_1001104,
title = {Effect of Process Parameters on Abnormal Grain Growth during Friction Stir Processing of a Cast Al Alloy},
author = {Jana, Saumyadeep and Mishra, Rajiv S. and Baumann, John A. and Grant, Glenn J.},
abstractNote = {The effects of process parameters and friction stir processing (FSP) run configurations on the stability of nugget microstructure at elevated temperatures were evaluated. Cast plates of an Al-7Si- 0.6Mg alloy were friction stir processed using a combination of tool rotation rates and tool traverse speeds. All single pass runs showed some extent of abnormal grain growth (AGG), whereas multi-pass runs were more resistant to AGG. Additionally, higher tool rpm was found to be beneficial for controlling AGG. These effects were analyzed by comparing the result of this work with other published results and AGG models.},
doi = {10.1016/j.msea.2010.08.049},
journal = {Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing, 528(1):189-199},
number = 1,
volume = 528,
place = {United States},
year = {Thu Nov 25 00:00:00 EST 2010},
month = {Thu Nov 25 00:00:00 EST 2010}
}
  • Cast aluminum alloys in general show poor fatigue performance due to presence of defects. Friction stir processing (FSP) can be used as a tool for enhancing mechanical properties of cast alloys by eliminating such defects. In the present study FSP led to a five times improvement in fatigue life of an investment cast Al-7Si-0.6Mg hypoeutectic alloy. The reason for such enhancement was linked to closure of casting porosity which acted as crack nucleation sites in the as-cast condition. Porosity acted as notches in the as-cast alloy and led to an order of magnitude higher crack growth rate. As FSP eliminatedmore » the porosity and refined Si particles, crack growth rate dropped due to the elimination of the notch effect together with increased crack path tortuosity. Finally, short crack behavior was noted in both cast and FSP specimens. The critical crack length where transition from short crack to long crack behavior took place is related to respective microstructural characteristic dimensions.« less
  • Friction stir processing (FSP) is emerging as a promising tool for microstructural modification. The present study assesses the effects of FSP on the microstructure and mechanical properties of an investment cast Al-7Si-Mg alloy. FSP eliminates porosity and significantly refines eutectic Si particles. The extent of particle refinement varied with changes in processing conditions. High tool rotation rate and low to intermediate tool traverse speed generated a higher volume fraction of finer particles. Tensile ductility changed significantly as a result of FSP, whereas UTS improved only marginally. Yield strength was similar in both cast and FSPed samples under various heat treatedmore » conditions, with the highest value obtained after a T6 heat treatment. Further, FSP caused significant grain refinement in the stir zone, subsequently transforming into very coarse grains as abnormal grain growth (AGG) occurred during solution treatment at high temperature.« less
  • The effect of friction stir processing (FSP) on the fatigue life of a cast Al-7Si-0.6Mg alloy at a stress ratio of R=0 was evaluated. Two types of specimen geometry were used for the FSPed condition, through-thickness processed and partial thickness processed. FSP enhanced the fatigue life by a factor of 15 for the through thickness processed samples at lower stress amplitudes. This is different from the FSP specimens tested at R=-1 and similar stress amplitudes where a 5 times improvement in fatigue life was observed. In light of these observations, various closure mechanisms were examined.
  • An Al-15 wt.% Zn alloy was processed by friction stir processing to produce grain sizes of {approx} 0.5 {mu}m, {approx} 1 {mu}m, and {approx} 2 {mu}m. A simple and effective method was developed to determine the true strain by scribing marker lines with scaled division using focused ion beam micromachining prior to deformation. The 'microscopic' grain boundary sliding, with displacements of adjacent grains of the order of a nanometer, can easily be detected by the proposed technique, providing a surface analysis with high accuracy that could be used to observe the changes in relief with increasing strains. Moreover, the occurrencemore » of grain boundary sliding at room temperature was considered a major cause for higher strain rate sensitivity in fine-grained Al-Zn alloys.« less
  • Abnormal coarsening of austenite ({gamma}) grains occurred in low-alloy steels during a seamless pipe hot-rolling process. Often, the grains became several hundred micrometer in diameter. This made it difficult to apply direct quenching to produce high-performance pipes. The phenomenon of grain coarsening was successfully reproduced using a thermomechanical simulator, and the factors which affected grain coarsening were clarified. The mechanism was found to be basically strain-induced grain growth which occurred during reheating at around 930 C. Furthermore, once a pipe temperature decreased to the dual-phase region after the minimal hot working and prior to the reheating process, the grain coarseningmore » was more pronounced. It was understood that the formation of ferrite along grain boundaries had the role of reducing the migration of grain boundaries into neighboring grains, leaving a strain-free, recrystallized region behind. This abnormal grain coarsening was found to be effectively prevented by an addition of Nb, the content of which varied depending on the C content. The effect of the Nb addition was confirmed by an in-line test.« less