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Processing Fundamentals
Published in Anshuman Patra, Oxide Dispersion Strengthened Refractory Alloys, 2022
Severe plastic deformation (SPD) is a cost-effective process to induce considerable strain without change in the area of the material and on the application of elevated hydrostatic pressure to inhibit crack propagation. Micron (mean grain size < ~1 μm) and nano-sized grains in a tube, a sheet, and several brittle materials such as amorphous glass can be fabricated by SPD [48, 49]. The fine grains formation by SPD occurs by a continuous dynamic recrystallization (high angle grain boundary formation) or a geometric dynamic recrystallization (formation of serrated grain boundary) process [49]. The literature indicates that for a 50% and a 90% decrease in height under uniaxial compression, the effective strains are 0.69 and 2.30, respectively and, for a plane strain compression, the effective strains for the identical decrease in height are 0.80 and 2.66, respectively [49]. The size of the subgrain (d) and dislocation density (ρ) is related as [49]: d=κρ
Friction Stir Welding of Aluminum Alloys
Published in Noor Zaman Khan, Arshad Noor Siddiquee, Zahid A. Khan, Friction Stir Welding, 2017
Noor Zaman Khan, Arshad Noor Siddiquee, Zahid A. Khan
FSW of AA2219 was performed to investigate the deformation mechanism and texture evolution of different weld zones (Chen and Jiang, 2014). Different microstructural features were obtained for different regions: elongated grains were observed in base metal, HAZ, and TMAZ, whereas SZ was characterized by fine equiaxed grains. Dynamic recovery occurred in TMAZ, whereas SZ underwent geometric dynamic recrystallization (GDRX). Fine grains were formed in SZ by strain-induced boundary migration mechanism (Chen and Jiang, 2014). During FSW of Al alloy, CDRX and DDRX occur in SZ due to high stacking fault energy of aluminum. Generally, CDRX takes place in SZ of FSWed aluminum alloy and sometimes DDRX also occur (Humphreys and Hatherly, 2004).
Friction Stir Processing of Metals
Published in B. Ratna Sunil, Surface Engineering by Friction-Assisted Processes, 2019
Microstructural modification during FSP involves principles of severe plastic deformation. Introducing intense plastic deformation coupled with high temperature lead to recrystallization and microstructural modification in FSP [33–37]. If a material is heated to a certain temperature to rearrange the grain boundaries, the corresponding temperature can be called as recrystallization temperature, and the microstructure-modified material is said to be recrystallized. If the recrystallization happens during the plastic deformation of a material, then it is said to be dynamic recrystallization that is the prime mechanism behind the microstructure modification during FSP. Based on the modified microstructure in the processed zone, three distinct regions are denoted as a nugget or stir zone, TMAZ and HAZ. Dynamic recrystallization is seen in the nugget zone, and hence this zone can also be called a dynamically recrystallized zone. Nugget zone contains a high density of sub-grains evolved within the grains and dislocations [38–40]. Usually, dynamic recrystallization results in finer and equiaxed grains. From the works carried out to understand the mechanism dynamic recrystallization in FSP, it was observed that three kinds are known as discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and geometric-dynamic recrystallization (GDRX) [41–43] play as the important reasons behind the grain refinement during FSP. If a new grain is evolved from a high-angle grain boundary, then it can be called as DDRX [41]. DDRX is lower in materials which show higher recovery such as aluminum alloys. However, in the presence of larger secondary phase particles, aluminum may show DDRX [42, 43]. In the case of CDRX, it is believed that subgrains rotate and attain a high orientation change with a slight boundary migration [44–46]. It is also an important observation that the variation in the grain size within the nugget zone that can be attributed to the variation in the temperature in the processed zone [47]. Particularly, in the thickness direction of an FSP region, a variation in the average grain size can be observed. The important reasons are the difference in the amount of heat dissipation in the thickness direction which directly influences the material flow mechanisms.
A review on manufacturing the surface composites by friction stir processing
Published in Materials and Manufacturing Processes, 2021
Shalok Bharti, Nilesh D. Ghetiya, Kaushik M. Patel
FSP helps in the equiaxed as well as fine-grain structure in the stir zone. It may be due to combine effect of dissolution, recrystallization, recovery, etc which happens during the FSP .[262,263] The heat generated during the process helps in the grain softening as well as grain hardening depending upon the amount of heat provided.[264] If the amount of heat produced is less, the grains experience the grain hardening or freezing whereas if the amount of heat produced is high, the grain softening takes place.[181,265] These characteristics of the grain help to determine the mechanical properties of the material. FSP is known to produce a dynamic recrystallized (DRX) grain structure.[162] In FSP, the material can experience different types of DRX such as geometric dynamic recrystallization (GDRX), continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX), In aluminum alloy, the dynamic recovery (DRV) occurs after FSP which helps the material to prevent it from any energy storage during the process and this happens due to high stacking fault energy of aluminum material.[130] In aluminum alloy, dynamic recovery as well as dynamic recrystallization depends upon the temperature, stain, and processing condition of FSP. In the case of composite, the reinforcement particles can affect grain growth as well as nucleation during the process.[266]
Friction stir welding of duplex stainless steels
Published in Welding International, 2018
Tiago Felipe de Abreu Santos, Edwar Andrés Torres, Antonio Jose Ramirez
The difference in size of the austenite and ferrite grains in the SZ and the deformation observed in the TMAZ-AS can be explained on the basis of the mechanical behaviour of the phases that make up the DSSs investigated. Ferrite and austenite have different mechanical performance, as well as mechanism of microstructural restoration at high temperatures. Ferrite is characterized in that it possesses high stacking-fault energy (SFE) and typically exhibits the mechanism of dynamic recovery at high temperatures. On the other hand, austenite possesses low SFE and typically exhibits the mechanism of dynamic recrystallization at high temperatures. Due to its tendency to recover, formation of new ferrite grains may take place by various mechanisms: discontinuous dynamic recrystallization (DDRX), which corresponds to the classical mechanism of recrystallization, by nucleation and grain growth – present in austenite; continuous dynamic recrystallization (CDRX), involving the formation of nuclei by dynamic recovery, with increase in disorientation by rotation due to significant deformation; geometric dynamic recrystallization (GDRX), characterized by marked grain elongation, forming accumulation of dislocations in the interior where they intersect with the high-angle grain boundaries of the original grains, generating a serrated appearance of the grain, so that the undulations get closer until they make contact, dividing the initial grains and thus creating new micrograins [18]. Furthermore, the mechanisms of recrystallization involved when the microstructure is the same – in the case of austenitic or ferritic stainless steels – are different from those observed in the case of duplex structures, such as in the DSSs and SDSSs [19]. For the systems with duplex structures, in the initial stages of deformation at high temperature, the ferrite in the austenite is deformed more severely, owing to the greater resistance of the austenite, which acts as a matrix for the more ductile phase; with the increase in strength through work hardening in the ferrite, the load is transferred to the austenite and the strain gradient decreases as a result of the adaptation arising from the mechanisms of restoration [20,21].
Geometric dynamic recrystallization of austenitic stainless steel through linear plane-strain machining
Published in Philosophical Magazine, 2020
Yaakov Idell, Jörg Wiezorek, Giovanni Facco, Andreas Kulovits, M. Ravi Shankar
Within the elongated grains formed as a result of the high strain rate of the 25 cms−1 tool velocity (≈ 2 × 103 s−1), there are two types of grain morphologies observed: a highly strained grain (Figure 7c) and a nearly strain-free grain (Figure 7d). The highly strained grain is characteristically identified with an elongated morphology, a high density of large intragranular point-to-point disorientations, and a serrated grain boundary on the scale of the sub-grain dislocation cell facets; meanwhile, the strain-free grain is characterised by an aspect ratio closer to an equiaxed condition, a high density of very small intragranular point-to-point disorientations, and grain boundaries with smaller scale serrations. The latter shares morphological characteristics of those reported for geometric dynamic recrystallization (GDRX), which has been observed previously in Al-Mg alloys [59]. Previous reports of GDRX have been observed during hot rolling or friction welding associated thermo-mechanical processes, where temperatures in excess of 900 °C have been reported [60]. The serrated grain boundaries are the result of pinning and migration induced by the sub-grain boundary formation. As the strain rate increases, the grains become increasingly elongated until the grain thickness is about twice the sub-grain diameter. At this critical grain thickness, the magnitudes of the serrations at the grain boundaries are comparable to the grain width and can connect with one-another across the grain cross-section. The grain boundaries with defects containing opposite signs will annihilate, thereby reducing the excess defect energy, and effectively inducing a ‘pinch off’ event that produces two new strain-free grains [59]. Evidence consistent with this phenomenon of GDRX is clearly observed for the microstructures that are attained for the SPD in linear plane strain machining with the 25 cms1 tool velocity. Here some grains are still highly elongated and internally strained (Figure 7d), while other grains have undergone the process of GDRX and exhibit a smaller aspect ratio and strain-free grain interior (Figure 7e). This recrystallization mechanism is consistent with the XRD data where, in comparison to the lower tool velocity processing conditions, a drop-off in non-uniform micro-strain is observed as tool velocity is increased to 25 cms−1, and a mix of highly strained and strain-free grains present in the microstructure.