Effect of friction stir processed microstructure on tensile properties of an Al-Zn-Mg-Sc alloy upon subsequent aging heat treatment

doi:10.1016/j.jmst.2017.10.021

Abstract

Abstract:

An as-cast Al-Zn-Mg-Sc alloy was friction stir processed varying tool related parameters, yielding microstructures with different grain sizes (0.68, 1.8 and 5.5 μm). Significant increases in room temperature ductility were obtained in these materials with reasonable enhancement in strength. It is demonstrated that the type of microstructure produced by friction stir processing (FSP) has a significant influence on the choice of post-FSP heat treatment design for achieving improved tensile properties. It is also found that the ultrafine grained FSP material could not achieve the desired high strength during the post-FSP heat treatment without grain coarsening, whereas the micro-grained FSP materials could reach such strength levels (>560 MPa) under conventional age hardening heat treatment conditions.

Key words: Friction stir processing, Microstructure, Tensile properties, Heat treatment

Indrajit Charit, Rajiv S. Mishra. Effect of friction stir processed microstructure on tensile properties of an Al-Zn-Mg-Sc alloy upon subsequent aging heat treatment[J]. J. Mater. Sci. Technol., 2018, 34(1): 214-218.

Figures/Tables 8

Fig. 1. A schematic illustration shows the difference among the three FSP runs in terms of plunge depth and tool geometry (Run-1, Run-2 and Run-3). Here PD is the abbreviation for plunge depth. All dimensions are in mm. The drawings are not to scale.

Fig. 2. Optical macrographs of the FSP materials {(a) Run-1, (b) Run-2, and (c) Run-3} in the transverse cross-section and their corresponding microstructures {(d), (e) and (f)}: Figs. (d) and (e) are optical micrographs whereas Fig. (f) is a TEM image. The white boxes shown on the optical macrographs indicate the approximate stir zone locations wherefrom TEM and DSC specimens were taken.

Fig. 3. TEM images showing precipitate morphology in (a) Run-1 (1.8 μm grain size), (b) Run-2 (5.5 μm grain size) and (c) Run-3 samples (0.68 μm grain size).

Table 1 Room temperature tensile properties of the as-cast and as-FSP materials.

Material	YS (MPa)	UTS (MPa)	EL (%)
As-cast	-	233 (fracture strength)	0
Run-1 (1.8 μm)	380	482	22.0
Run-2 (5.5 μm)	301	440	26.4
Run-3 (0.68 μm)	383	416	25.9

Fig. 4. Differential scanning calorimetric curves for Al-Zn-Mg-Sc alloys (both parent as-cast material and as-FSP specimens).

Table 2 Tensile properties of Run-1 Al-Zn-Mg-Sc samples (SHT 480 °C for 1 h followed by quenching in cold water plus artificial aging under different conditions).

Aging conditions	YS (MPa)	UTS (MPa)	EL (%)
120 °C/3 h	487	557	27.9
120 °C/6 h	499	578	26.0
120 °C/12 h	568	593	23.9
160 °C/24 h	460	493	30.0

Fig. 5. (a) A TEM bright field image of the post-FSP peak-aged (SHT: 480 °C/1 h plus aging at 120 °C for 12 h) Run-1 Al-Zn-Mg-Sc alloy exhibiting the grain structure. (b) A TEM dark field image of the same specimen at a higher magnification showing very fine precipitates.

Table 3 Compilation of room temperature tensile properties of the Run-3 (UFG) material solution heat treated and aged for different durations at 120 °C.

SHT conditions	Aging conditions	YS (MPa)	UTS (MPa)	EL (%)
-	120 °C/12 h	371	409	21.0
350 °C/1 h	120 °C/3 h	302	323	34.8
350 °C/1 h	120 °C/6 h	326	358	34.6
350 °C/1 h	120 °C/12 h	303	317	33.8
370 °C/1 h	120 °C/24 h	352	384	30.4
405 °C/1 h	120 °C/12 h	523	591	17.0

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