In this research, AA5005-O aluminum-magnesium alloy and St-52 low carbon steel sheets were friction-stir welded in a butt-dissimilar joint design. Effects of different processing parameters including ...tool rotational speed (
w
), traverse velocity (
v
), plunge depth, and offset distance on the solid-state weldability of these dissimilar materials were assessed in terms of formation of intermetallic compound (IMC) layer at the interface. A 3D thermo-mechanical finite element modeling procedure was employed to predict the nucleation and growth of IMC layer. Formation of various FeAl, FeAl
3
, and Fe
2
Al
5
IMCs at the interface, layer morphology, and thickness were experimentally studied as well, by using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis techniques. A good agreement between the simulated thermal results and experimental data was noticed. The results showed that the thickness of IMC layer at the interface as the main controlling parameter in transverse tensile property and fracture behavior of produced dissimilar joints can be varied extremely as a function of processing parameters. By decreasing the heat input and suppressing the kinetics of IMCs layer formation, the tensile performance of dissimilar welded joints is improved, considerably. However, the soundness of these dissimilar welds played another main role as a restriction mechanism against this trend. The maximum joining efficiency is attained around 90% at an optimized working window of
w
= 1200 rpm,
v
= 90 mm/min, and a plunge depth of 0.3 mm with an offset distance of 0.5 mm toward the Al side. The hardness of this optimized dissimilar weld is enhanced even more than the steel base metal caused by the formation of IMC layer at the interface as well as the dispersion of reinforcing intermetallic particles (IMPs).
In this study, multi-step friction-stir processing (FSP) was employed to fabricate an ultra-fine grained (UFG) Al-matrix nanocomposite with simultaneously enhanced indentation hardness and tensile ...properties. For this aim, about 3.5 vol.% of SiC nanoparticles were incorporated within an Al-Mg alloy matrix by applying up to five cumulative overlapping FSP passes. Dispersion of nanoparticles at the stirred zone (SZ) and their interfaces with the aluminum matrix were studied by using scanning and electron backscattered electron microscopy. The results showed that the grain and sub-grain structures of the SZ were refined down to about 1.4µm and less than 1µm respectively, as a result of dynamic recrystallization (DRX) during FSP. The distribution of grains and their orientations was significantly affected by the presence of SiC nanoparticles during FSP. SiC nanoparticles provided both direct and indirect influences on the strengthening of Al-matrix based on the Orowan looping and grain refinement mechanisms, respectively. The morphology and distribution of precipitates were both broken down and partially dissolved during FSP as well. The processed UFGed nanocomposite exhibited drastically improved hardness, yield stress (YS) and ultimate tensile strength (UTS) by up to ~140%, 75% and 60%, respectively, as compared to the annealed Al-Mg alloy. Fractographic features revealed a combined ductile-brittle rupture behavior, while the ductile portion was more significant and preserved the elongation of nanocomposite up to about 30%. Finally, the tensile flow behavior of the processed nanocomposite was described using a dislocation-based model which suggests that grain boundary strengthening is the dominant mechanism involved.
Fine grained (FG) and ultra-fine grained (UFG) materials processed by severe plastic deformation exhibit beneficial hardness and tensile properties. Constrained groove pressing (CGP) were employed ...for fabrication of FG and UFG sheet metals and accomplished into different types of metals and alloys, such as commercial pure aluminum, AA3003 aluminum alloy, commercial pure copper, nickel, titanium and low carbon steels. Tensile and hardness characteristics in the FG and UFG sheets have been assessed with the aim of evaluating the hardness−strength relationship frequently established for coarse-grained metals and alloys (σUTS/HV=3.45). However, it was revealed that the FG and UFG materials do not obey widely used hardness−strength relationships in the conventional coarse grained structures. A new multiplicity factor less than 3, depending on the chemical composition of processed materials, is proposed in this study. This is attributed to different strain hardening response of the FG and UFG materials with slight work hardening before necking instability. In fine grained and ultra-fine grained structures failure does not occur in (or right after) the onset of necking point. That is, tensile deformation sustains significantly up to fracture point due to the role of superplasticity mechanisms.
The unique combination of very large strains, high temperatures and high strain rates inherent to friction stir welding (FSW) and friction stir processing (FSP) and their dependency on the processing ...parameters provides an opportunity to tailor the microstructure, and hence the performance of welds and surfaces to an extent not possible with fusion processes. While a great deal of attention has previously been focused on the FSW parameters and their effect on weld quality and joint performance, here the focus is on developing a comprehensive understanding of the fundamentals of the microstructural evolution during FSW/P. Through a consideration of the mechanisms underlying the development of grain structures and textures, phases, phase transformations and precipitation, microstructural control across a very wide range of similar and dissimilar material joints is examined. In particular, when considering the joining of dissimilar metals and alloys, special attention is focused on the control and dispersion of deleterious intermetallic compounds. Similarly, we consider how FSP can be used to locally refine the microstructure as well as provide an opportunity to form metal matrix composites (MMCs) for near surface reinforcement. Finally, the current gaps in our knowledge are considered in the context of the future outlook for FSW/P.
In this study, by the implementation of multi-pass friction stir processing (FSP) an advanced AA6061-Graphene-TiB2 hybrid surface nanocomposite is produced in contrast to the AA6061-Graphene and ...AA6061-TiB2 single composites. Effects of micro-sized TiB2 (10-30 wt%) and nano-sized graphene (0.5-2 wt%) particles on the microstructure and mechanical property of aluminum alloy are investigated. Moderate chemical composition as ~20 wt% TiB2 and 1 wt% graphene for the hybrid nanocomposite system yielded the best combinations of mechanical property overcome the wear-strength trade-off. Such optimum incorporation of hybrid inclusions mixture led a hardness increasing up to about two times higher, a yield strength improvement up to ~225 MPa (~300% increasing ratio), and an elongation loss down to ~9%. A considerable reduction in the coefficient of friction ~23% is revealed with the dominant contribution of sliding wear fractographic features on the worn-out fracture surface due to the presence of graphene in the nanocomposite structure acting as the lubricant agent.
•An advanced AA6061-Graphene-TiB2 hybrid surface nanocomposite was prepared by friction stir processing.•A hybrid mixture ratio of 20 wt% and 1 wt% optimized for TiB2 and graphene particles.•A fine recrystallized grain structure <1 μm was formed in the SZ.•Tensile strength was increased up to three times higher without drastic elongation loss.•A considerable hardness improvement of up to around two times higher was obtained.•Friction coefficient was reduced down to ~0.37 due to changing in wear mechanism.
The aim of this research is to characterize the unique microstructural features of Al-matrix nanocomposites reinforced by graphene nano-platelets (GNPs), fabricated by multi-pass friction-stir ...processing (FSP). During this process, secondary phase GNPs were dispersed within the stir zone (SZ) of an AA5052 alloy matrix, with a homogenous distribution achieved after five cumulative passes. The microstructural characteristics and crystallographic textures of different regions in the FSPed nanocomposite, i.e., base metal (BM), heat affected zone (HAZ), thermo-mechanical affected zone (TMAZ), and SZ, were evaluated using electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The annealed BM consisted of a nearly random crystal orientation distribution with an average grain size of 10.7μm. The SZ exhibited equiaxed recrystallized grains with a mean size of 2μm and a high fraction of high-angle grain boundaries (HAGBs) caused by a discontinuous dynamic recrystallization (DDRX) enhanced by pinning of grain boundaries by GNPs. The sub-grains and grain structure modification within the HAZ and TMAZ regions are governed by dislocation annihilation and reorganization in the grain interiors/within grains which convert low-angle to high-angle grain boundaries via dynamic recovery (DRV). The FSP process and incorporation of GNPs produced a pre-dominantly {100} cube texture component in the SZ induced by the stirring action of the rotating tool and hindering effect of nano-platelets. Although, a very strong {112} simple shear texture was found in the HAZ and TMAZ regions governed by additional heating and deformation imposed by the tool shoulder. These grain structure and texture features lead to a hardness and tensile strength increases of about 55% and 220%, respectively.
Display omitted
•A new Al-matrix nanocomposite was prepared by friction stir processing.•Improved hardness and strength were attained by incorporation of graphene nano-platelets.•Microstructural changes, restoration mechanisms and textural developments were studied.•The correlation between the microstructural features and textural components was established.
In this research, creep properties and thermal behavior of Sn-3.5Ag-0.7Cu (SAC)/multi-walled carbon nanotubes (MW-CNTs) lead-free nanocomposite solders was studied by conducting the nanoindentation ...testing at different temperatures, in the range of 292–312 K. Solder materials were prepared by mechanical alloying followed by powder consolidation routes. Furthermore, nickel coating was electro-less plated on the surface of nanotubes to enhance their metallurgical compatibility with the SAC solder matrix. The content of Ni-coated MW-CNTs varied in the range of 0–0.2 wt%. Addition of Ni-coated nanotubes resulted in an improvement of the indentation creep behavior of SAC solder alloy in comparison with the non-coated agents. By increasing the fraction of Ni-coated MW-CNTs up to ~0.1 wt%, the creep resistance of solder nanocomposite was continuously enhanced. However, the higher contents of reinforcing agents led to the deterioration of creep behavior due to the aggregation of nanotubes and a considerable heterogeneous refinement of the microstructure. The activation energy for softening during localized deformation behavior of nanocomposite solders was estimated by two approaches based on Dorn constitutive and Lucas-Oliver creep models and found to be in the ranges of 8.8–18.2 kJ/mol and 17.3–48.4 kJ/mol, respectively. By changing in the activation energy for diffusion and sliding of grain boundaries, in dependency with the extent of grain structural refinement and generation of dislocations due to altering the content of nanotubes as the reinforcing agent, the time-dependent under-loading deformation phenomenon in terms of creep can be controlled. The experimental results revealed that modified carbon nanotubes which are located on sub-grain boundaries can reduce their chemical potential tremendously, suppress the sliding-based creep deformation up to an optimized content of ~0.1 wt%.
•The SAC-based lead-free nanocomposite solders prepared by mechanical alloying.•Nanoindentation testing was carried out to elaborate on creep behavior.•A considerable enhancement of creep resistance noted.•The localized creep activation energy estimated in the range of 17.3-48.3 kJ/mole.
In this study, micro- and nano-sized TiB2 and Al2O3 particles were incorporated separately and simultaneously through the AA8026 aluminum base alloy during multi-pass friction stir processing (FSP) ...with 100% overlapping to fabricate metal matrix mono and hybrid nanocomposites. Various FSP conditions including different rotational speeds (w), traverse velocities (v), and processing pass numbers were assessed to attain a homogenous distribution of reinforcing particles through the Al-metal matrix. Moreover, the impacts of size (micro or nano) and type of reinforcement particles (TiB2 and Al2O3) on the process-ability of single and hybrid nanocomposite systems were examined. Microstructures of different zones and distributions of reinforcing ceramic particles through the Al-matrix under various processing conditions were studied and characterized by using optical (OM), scanning (SEM), and transmission electron microscopy (TEM) techniques, respectively. The main mechanical characteristics of the prepared nanocomposites, such as, indentation Vickers hardness, tensile properties, and wear resistance were measured and compared for all of the various processing conditions. By optimization of the FSP parameters, as a rotational speed of 1600rpm and a traverse velocity of 40mm/min after 4 passes, a uniform AA8026-TiB2-Al2O3 hybrid nanocomposite was attained with significant improvements (~70–100%) in the different mechanical properties. As a result, the tensile yield strength of ~270MPa, elongation of ~4.5%, and indentation Vickers hardness of ~141HV were obtained. Also, the average wear rate was reduced from the 21×10−3mg/m value for the AA8026 base alloy down to 2.6×10−3mg/m for the best processed nanocomposite. A direct relationship between the wear rate and the indentation hardness resistance was demonstrated. Finally, effects of FSP processing conditions and reinforcement particles (type and size) on the microstructure and mechanical properties of the FSPed Al-matrix nanocomposites were addressed and discussed.
The tensile flow properties of austenitic (S316-L) and martensitic (S410-L) stainless steel wall structures deposited by powder-fed laser additive manufacturing (LAM) process are evaluated. The ...properties obtained by the LAM process are compared to commercial rolled sheets of these metals. Strain-rate sensitivity, work hardening, and fracture behavior are assessed by conducting uniaxial tensile testing at different strain rates (0.001, 0.01, 0.1, and 1.0 sec−1). Moreover, a correlation between the final microstructure and mechanical properties is established for the LAM products through detailed analyses of grain structures and hardness indentation measurements. The results indicate a strong dependency for the strain rate in martensitic alloys compared to austenitic alloys produced by the LAM process. Interestingly, the tensile strength of commercial rolled martensitic stainless steel sheet doubles (∼100% increase) by increasing the strain rate, while preserving its elongation to failure. Comparing the two manufacturing methods, a lower strain-rate sensitivity factor is recorded for the additive manufacturing material (m of ∼0.0336) compared to the commercial sheet (m of ∼0.0775). This lower sensitivity is attributed to coarser grain structure and greater microstructural heterogeneity of the LAM product, which stems from directional solidification and cooling phenomenon during the layer-upon-layer deposition process. In contrast, the work hardening exponent (n value) varies little (0.1834–0.2854) for the different materials and manufacturing methods. Fractographic studies reveal that the fracture mode varies from ductile rupture towards ductile-brittle with the formation of greater martensitic phases, which is in combination with the failure component changing from shear to tensile at high strain rates.
► Four passes of constrained groove pressing leads to cell size of 230
nm. ► The constrained groove pressing can effectively enhance dislocation density. ► Constrained groove pressing can increase ...the electrical resistivity up to ∼100%.
In this research, constrained groove pressing (CGP) technique is used for imposing severe plastic deformation (SPD) on the low carbon steel sheets. Using transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical microscopy, the microstructural characteristics of produced sheets are investigated. The results show that CGP process can effectively refine the coarse-grained structure to an ultrafine grain range. Dislocation densities of the ultrafine grained low carbon steel sheets are quantitatively calculated and it is found that the CGP can effectively enhance the dislocation density of the sheets. Measurements of their electrical resistivity values show that microstructure refinement and increasing the dislocation density can efficiently increase the electrical resistivity of the CGPed sheets up to ∼100%.
PDF
