Characterization of fine precipitates evolution in post ageing treatment after friction stir processed 7075 Al Alloy
Abstract
The effect of post ageing treatment (140oC for 2h) on the microstructure and mechanical behaviour of FSPed 7075 Al alloy has been studied by Optical microscopy (OM), Field emission scanning electron microscopy (FESEM), Differential scanning calorimetry (DSC), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), and mechanical properties. Friction stir processing (FSP) is a solid-state surface modification technique to apply for cast aluminium alloys. FSP has a similar metal working principle like FSW (friction stir welding). The alloy has strong agehardening response with scandium (Sc) inoculated Al-Zn-Mg alloy, on the other hand novelty of FSP only few studies have been carried out to the effect of post ageing treatment on the microstructure, size, morphology and fine dispersion of coherent Al3Sc(L12) type precipitates or ή-phases and its mechanical properties of friction stir processed 7075 Al alloy. The FSPed enhances grain boundary (GB) formation and increases suitable sites for the precipitation of nucleation in post aged 7075 Al alloy. Themechanical properties have been evaluated such as proof strength (σ0. 2) of 122. 9 MPa, ultimate tensile strength (σu) of 256. 4 MPa, ductility (δ) of 8. 6%, Vicker’s hardness in stir zone of 101 HV, strain hardening exponent (n) of 1. 82, and heat input during FSPed of 2. 15 kJ/mm, respectively.
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Introduction
The high strength Al-Zn-Mg alloy (7075 series) was widely used due to their spontaneous age-hardening ability, good corrosion resistance and excellent mechanical properties obtained through fine precipitation of a homogeneous distribution of dispersiod particles [1-3]. Further, friction stir processing (FSP) has been adapted for surface modification technology, especially for fabrication process, processing and synthesis of materials. It has great advantages including surface modification for microstructural evaluation, adjusting mechanical properties by optimizing tool design and process parameters [4, 5]. FSP can be exploited not only by controlling process parameters but also by using an Al-Zn-Mg-Sc alloy contains thermally stable precipitates or dispersoids, also can precipitate out such particles during processing thereby retarding the uncontrolled grain growth. Hence, fine-grained microstructure may be obtained by controlling the grain growth or fine distribution of Al3Sc precipitates during FSPed plus post ageing treatment [6-8]. Ma and Mishra et al. [2005] demonstrated the possibility of achieving grains larger than 1 µm under any other processing conditions. As well, Nascimento et al. [2009], Kwon et al. [2003], and Colligan et al. [1999] insight studied on FSP technique successfully to produce fine-grained structure and surface composite of aluminium alloys. More emphasis has been given on the mechanism of dynamic recrystallization in Al-Zn-Mg-Sc alloy and the role of coherent precipitates in the formation of high temperature (450-550oC) FSPed microstructure. In addition, heat input (2. 15 kJ/mm) is the main criterion for the energy transformation during FSPed [9-11]. The characteristics of fine-grained microstructure obtained through FSP are entirely different from any other conventional severe plastic deformation (SPD) techniques. Thus, the major processing parameters are the tool rotation and traverse speed, the axial force, tile angle and the proper tool design have been well documented by several authors. The possible strengthening mechanism can be attributed to formation of fine grain and subgrain structure and dislocation distribution of the modified surface [12-14]. The objective of the present work is to characterize the precipitates in friction stir processed Al-Zn-Mg-Sc alloy then post ageing treatment at 140oC for 2h using OM, FESEM, SEM, DSC, and TEM analysis. To investigate the effects of scandium on mechanical properties of FSPed aluminium alloy after post ageing treatment (140oC for 2h).
Conclusion
In this study, the following conclusions have been summarized below:
1) With minor Sc addition peak of ageing effect achieved earlier for high strength Al-Zn-Mg alloy.
2) After aged at 120oC for 14h, the alloy exhibits maximum age-hardening effect due to formation of high volume fraction of GP zones as well as minor Sc addition and their responsibility for early formation of ή-phase of 7075 Al alloy.
3) FSP is a novel surface modification technique and resulted in substantial grain refinement with numerous commercial applications of 7075 Al alloy. During FSP of aluminium alloy the dispersoids and strengthening particles in the matrix are distributed uniformly throughout the SZ region due to the stirring action of the tool.
4) The optical micrograph revealed very fine grains (5. 31±1. 0 µm) in SZ as well as TMAZ (6. 96±2. 1 µm) region, but several black spots due to the Zn vaporization and creation of hair line cracks for torsional effects in the TMAZ. Therefore, the SZ comprises very fine grains primarily due to severe plastic deformation (SPD) and dynamic recrystallization mechanism.
5) The FESEM micrograph indicated as several white spots which have Sc content of 2. 89 wt. % and Si+Fe of 6. 89 wt. % in major portion of impurities intend to diminish the strength and ductility of FSPed Al alloy.
6) DSC thermogram indicated some distinct exothermic peaks at around 60oC for formation of GP zones and go on exhibiting anti-recrystallization effects or high thermal stability means thermal strength upto 600oC then softening tendency come for endothermic reaction due to dissolution of hardening phases and coarsening effects of Al3Sc particles at around 636oC.
7) The TEM micrographs have been represented as low magnification and for high magnification of studied alloy. The micrograph revealed (500 nm) very fine precipitates (e. g. Al3Sc and MgZn2) with dislocation tangles and seems high angle grain boundaries are dominated in matrix. Also, distinct grain boundary has around 54. 32±16. 28 nm width and PFZ size around 173. 15±7. 36 nmin the matrix. Other micrograph (200 nm) revealed fine precipitates as well as some coarse particles (agglomeration Al3Sc particles or T phases) in the matrix.
8) The overall strengthening is associated with the fine grain strengthening, sub-grain and precipitation strengthening by Al3Sc particles and MgZn2 precipitates. Moreover, mixture of fine and coarse type coherent spherical Al3Sc particles appears after T4+FSP+Aged at 140oC for 2h.