Effect of processing parameters of multipass friction stir processing on microstructure and hardness of 7075 aluminum alloy

Yutaka Matsuda1 , Goroh Itoh2 and Yoshinobu Motohashi3

1 Graduate Student, School of Science and Engineering, Ibaraki University
2 Department of Mechanical Engineering, College of Engineering, Ibaraki University
3 The Research Center for Superplasticity, College of Engineering, Ibaraki University
[Present:Emeritus Professor, Ibaraki University]

Friction stir processing, FSP, has been proposed by applying friction stir welding, FSW, as a method for controlling the microstructures to improve mechanical properties. In this study, microstructure of a 7075 aluminum alloy subjected to multipass FSP, MP-FSP, is assessed to obtain fundamental knowledge on MP-FSP. The MP-FSP has been applied to 7075 aluminum alloy plates with T6 and O tempers, and microstructural characterization has been made by means of optical and scanning electron microscopies together with EDX and EBSD analyses.

Figure 1 shows microstructure of the cross section of the processed regions at low magnification for T6 and O tempers. Stir zone, SZ, with a width of about 5mm can be observed in the center of each pass with what is called onion ring structure. Moreover, a zone with a microstructure different from that of SZ can be found between SZs. We call this zone pass boundary zone, PBZ. Processing direction and interval were found to give influence to the width and configuration of PBZ. Figure 2 shows result of EBSD analysis on SZ and PBZ. It is confirmed that SZ consists of fine equiaxed grains. On the other hand, it is to be noted that the microstructure in PBZ is composed of two types of grains: fine equiaxed and coarse elongated. The coarse elongated grain contained many sub-grains.

In addition, micro hardness measurement has been carried out and the results are shown in Fig. 3. Hardnesses in PBZ are decreased and increased in T6 and O, respectively, by MP-FSP, and are intermediate between those in BM (base metal) and SZ in both tempers. The hardness in SZ and PBZ of MP-FSP'ed 7075-T6 aluminum alloy are increased by additional artificial aging at 393K for 24h.

[Published in Journal of Light Metal, Vol. 63 (2013) No. 1, P.2-7]

Fig. 1 Section view perpendicular to the processing direction with processing interval, dFSP, of 8.0mm. (a) T6, Unidirection; (b) O, Unidirection; (c) O, Alternate direction. The FSP was made from left to right.
Fig. 2 Grain boundary maps determined with EBSD analysis for a cross section perpendicular to the processing direction of SZ and PBZ of a T6 specimen FSP’ed with dFSP = 7.0mm in the alternate direction. >15: black(thick), 15> >5: red, 5> >2: black(thin).
Fig.3 Vickers hardness distribution in T6 and O specimens after MP-FSP in the unidirection. P1-P4: diameter and location of the pin in the 1st-4th paths. AAA: additional artificial aging at 393K for 24h. HVs: hardness in SZ.