In this study, AA2519 alloy was initially processed by multi axial forging (MAF) at room and cryogenic temperatures. Subsequently, the microstructure and the mechanical behavior of the processed samples under quasi-static loading were investigated to determine the influence of cryogenic forging on alloys’ subgrains dimensions, grain boundaries interactions, strength, ductility and toughness. In addition, the failure mechanisms at the tensile rupture surfaces were characterized using scanning electron micro-scope (SEM). The results show significant improvements in the strength, ductility and toughness of the alloy as a result of the cryogenic MAF process. The formation of nanoscale crystallite microstructure, heavily deformed grains with high density of grain boundaries and second phase breakage to finer particles were characterized as the main reasons for the increase in the mechanical properties of the cryogenic forged samples. The cryogenic processing of the alloy resulted in the formation of an ultrafine grained material with tensile strength and toughness that are ~41% and ~80% higher respectively after 2 cycles MAF when compared with the materials processed at ambient temperature. The fractography analysis on the tested materials shows a substantial ductility improvement in the cryoforged (CF) samples when compared to the room temperature forged (RTF) samples which is in alignment with their stress-strain profiles. However, extended forging at higher cycles than 2 cycles led only to increase in strength at the expense of ductility for both the CF and RTF samples.
Severe Plastic Deformation (SPD) may be described as a metal forming process in which an ultra-large plastic strain is applied into a bulk metal leading to a gradual formation of nano/ultrafine grained structures [
Despite of the positive SPD impacts on microstructural modifications, the rate of defect storage and structural refinement decreases significantly with increasing strain due to dynamic recovery and recrystallization of grains during the plastic deformation [
UFG materials are structurally identified by a large volume fraction of grain boundaries, which significantly affect their physical and mechanical characteristics such that mechanical strength of metals can be substantially improved by this grain size refinement into the sub-micrometer range [
To the best of the authors’ knowledge, no data is available in literature on the quasi-static mechanical behavior of the UFG AA2519 processed by MAF at cryogenic temperature. Thus, the main objective of this paper is to investigate the microstructure and mechanical properties of the cryoforged (CF) AA2519 using the room temperature forged (RTF) alloy as the reference material. In addition, the microstructure of the fractured surface was characterized to provide an insight into the failure mechanisms in the tested materials. The number of forging cycles is also considered as a variable to determine the effect of further processing cycle on the materials properties.
A commercial AA2519 alloy was used in this study with the chemical compositions summarized in
Elements | Cu | Fe | Mg | Mn | Si | Ti | V | Zn | Zr | Al |
---|---|---|---|---|---|---|---|---|---|---|
wt% | 5.3 - 6.4 | 0.3 | 0.05 - 0.4 | 0.1 - 0.5 | 0.25 | 0.02 - 0.1 | 0.05 - 0.15 | 0.1 | 0.1 - 0.25 | Bal. |
The XRD patterns were collected over 2θ range of 20˚ - 100˚ with a step size of 0.031˚. The subgrains structure and grain boundaries interaction in the UFG samples were studied using a transmission electron microscope (TEM; Model: FEI Technai) with a voltage of 200 kV and also using a Zeiss optical microscope. The fractured surfaces were investigated using SEM to observe the microscopic features of failure in each case. Microhardness measurements were also carried out on the central cross-section surface using a Vickers indenter at a load of 50 g and a dwell time of 10 s. Tensile tests at a strain rate of 1 mm/min were conducted using a UTS-type machine (S-series H25k-S Model) at an ambient temperature based on the ASTM: E8M standard.
XRD scattering pattern results are depicted in
According to
and defects [
The XRD analysis suggested that the intensity of the Al2Cu diffraction peaks in a RTF alloy was a little stronger than that in a CF alloy, indicating that the amount of Al2Cu precipitates after ambient process had increased. Higher thermal energy and increase of local temperature during forging at a room temperature may supply the driving force for the reaction between Al and Cu to form Al2Cu. Furthermore, the broadened Al2Cu peaks with lower intensity in the CF samples are also an indication of the second phase breakage to finer particles as a result of the induced high plastic strain through the forging process at cryogenic temperature. This process exerts a strong plastic deformation leading to the generation of more shear intense stress followed by the breaking down of the second phase particles. This Al2Cu particles breakage can be accompanied by redistribution through the Al matrix, which assured a more homogeneous microstructure and stronger mechanical strength. The fragmented Al2Cu particles in Al matrix after cryogenic deformation in the CF samples shown in
The microstructure of grains/subgrains of the 6 cycles CF AA2519 was investigated through transmission electron micrographs in
The severely deformed cells marked with arrows in
The stress-strain curves for the quasi-static tensile test results are shown in
The stress-strain curves in
Materials | No. of Cycles | Ultimate Tensile Strength (MPa) | Elongation (%) | Toughness (MJ∙m−3) | Hardness (Hv) |
---|---|---|---|---|---|
RTF-AA2519 | 2 | 252 | 6.1 | 18.03 | 98 |
4 | 305 | 3.8 | 15.09 | 130 | |
6 | 334 | 1.1 | 8.77 | 149 | |
CF-AA2519 | 2 | 356 | 9.0 | 32.45 | 165 |
4 | 415 | 6.8 | 30.06 | 198 | |
6 | 449 | 5.4 | 29.14 | 221 |
process. In fact, high volume of accumulated grain boundaries in the deformed materials acts as barriers for the movement of dislocations thereby preventing further deformation [
As expected,
Although, limited amount of changes were obtained by performing the forging process at higher cycles, the tensile profiles show much more significant influences on the mechanical properties of the CF materials than that of the RTF samples. The UTS values indicate that cryogenic processing creates materials with 41% higher strength after 2 cycles of MAF when compared with the samples processed at ambient temperature. In cryoforging, the strain-hardening is preserved up to the extent to which forging is done. This signifies that neither dislocation recovery nor recrystallization occurs during the entire SPD process whereas dynamic recovery and subsequent softening are inevitable in the materials under conventional forging. In other words, climb of dislocations and cross slip are extensively inhibited within the cryogenic process thereby accumulating high dislocation density which is not the case for ambient forging. Consequently, nanocrystalline/UFG microstructure with bi-modal grains is developed in the CF alloy resulting in high mechanical strength which is in consistence with the Hall-Petch strengthening mechanism. Similar increase in the UTS from 315 to 522 MPa was reported by Lee et al. [
It is widely known that the improvement of mechanical strength in SPD processed materials is followed by ductility reduction. According to
It is worth paying attention to the lowering effect of additional forging cycles on the elongation at fracture for both the CF and the RTF alloys as can be seen in the strain-strain curves in
The stress-strain response of the nanocrystalline CF alloys under tension shows a rapid peak and subsequent softening due largely to necking. The reason for this strength drop can be attributed to the intrinsic feature of nanoscale materials. The high surface (grain boundaries) to volume ratio of the nanograins may facilitate their rotation with a lower stress. Plastic deformation makes the nanosized grains to rotate and consequently coalesce along the directions of shear generating larger paths for dislocation movement. This phenomenon is schematically explained in
orientation closer together. This may diminish their boundaries providing a path for more extended dislocation motion and may consequently lead to the softening behavior in the plastic zone [
The toughness per unit volume of the RTF and the CF materials was computed for various forging cycles using the integration method to obtain the areas of the true stress-strain curves and was depicted in
The deformation behavior of the materials was identified by the microstructural characterization of failure mechanisms at the tensile rupture surfaces and diagnosing failure modes using SEM. Fractographs of the materials subjected to tensile deformation are presented in
(marked with circles) and slight brittle fractured regions (showed with arrows) can be recognized while the small micro-dimples and polished morphology are visible on their fractured surfaces. Therefore, a typical ductile failure mode is present in the CF samples. However, less number of dimples with the reduced deep size is a clear proof of ductility reduction in the extended forging cycles which is in consistent with the stress-strain results. Rahmatabadi et al. [
On the other hand, a large area of polished morphology in
The 6 cycle forged RTF samples have experienced a brittle fracture since no apparent plastic deformation took place as a result of tensile stress. Brittle fracture typically involves small energy absorption and occurs at high speeds. Also, a cleavage fracture surface is obvious on a part of this sample which is an indication of the brittle fracture caused by the normal stress to crystallographic planes with low bonding (cleavage planes). It is noted that conventional forging process makes fractured area with finer dimples which indicates lower ductility as compared to the CF materials with coarser and deeper dimples.
The following conclusions can be drawn from this study:
1) An integrated enhancement in mechanical strength, ductility and toughness of the AA2519 aluminum alloy was achieved as the result of cryogenic MAF process. This can be attributed to crystallite size refinement to nanoscales, heavily deformed grains with high density of grain boundaries and second phase breakage to finer particles observed through the XRD pattern and TEM micrographs of the cryogenic forging. Simultaneous SPD as well as the suppression of the dynamic recovery and recrystallization mechanisms during the cryogenic environment made the SPD process rather effective than performing the same process at room temperature.
2) The SEM micrographs of the fractured surfaces showed a significant ductility improvement in the CF materials which are in alignment with their stress-strain profiles. In addition, a slight increase in the tensile strength and a reduction in ductility are obtained in extended forging cycles due to less ability to accumulate dislocations. However, decreasing rate of ductility in the RFT samples was observed to be higher than that of the CF samples once proceeded to the next cycles.
Future study will focus on the effect of thermal treatments on the microstructure and mechanical response of the alloy after cryogenic forging in order to see the possibility of increasing the ductility of the processed alloy without any considerable strength loss.
Financial support for this work was provided through contract # W911NF-15-1-0457 under the direct supervision of Patricia Huff (HBCU/MI Program Manager, ARO).
The authors declare no conflicts of interest regarding the publication of this paper.
Azimi, A., Owolabi, G.M., Fallahdoost, H., Kumar, N. and Whitworth, H. (2019) Microstructure and Quasi-Static Mechanical Behavior of Cryoforged AA2519 Alloy. Materials Sciences and Applications, 10, 137-149. https://doi.org/10.4236/msa.2019.102011