Fine Equiaxed Grain Research Articles

Understanding interface evolution during explosive welding of silver foil and Q235 substrate through experimental observation coupled with simulation

In this work, explosive welding was introduced to manufacture Ag coating on Fe substrate. The microstructure evolutions of the Ag/Fe joint interface and the associated governing mechanisms during the collision process, were investigated by a comprehensive study combined advanced characterizations and smoothed particles hydrodynamics simulations. It was concluded that the self-developed explosive welding process enabled to attain high-quality Ag coatings effectively, certified by absence of geometric defects at the Ag/Fe interface and the surface of Ag foil. The simulation accurately reproduced the wavy interface and vortex structure observed in experiment. From this, the evolution of the wave was reconstructed and the formation mechanism of the vortex was revealed. The microstructure analyses revealed a remarkable diversity of the grain structure near the Ag/Fe interface. The equiaxed fine grains related to dynamic recrystallization were formed in the vicinity of the interface and also within the vortex. The grains of the Fe matrix slightly away from the interface became elongated, which reflected well the deformation features. Meanwhile, the columnar grains induced by molten and re-solidified processes were observed at the vortex boundary. The variation of these grain structures was responsible for the inhomogeneity of nanoindentation hardness distribution.

Tensile and Fatigue Properties of Twin-Roll-Cast AZ31 Magnesium Alloy Strips with Different Thicknesses

This study investigates the microstructure, tensile properties, and high-cycle fatigue resistance of twin-roll-cast Mg-3Al-1Zn (wt%) alloy strips with thicknesses of 1 mm, 1.5 mm, and 3 mm. The investigation results reveal that the 1-and 1.5-mm-thick strips show a fully dynamically recrystallized (DRXed) microstructure consisting of fine equiaxed DRXed grains, whereas the 3-mm-thick strip shows a partially DRXed microstructure containing very coarse elongated unDRXed grains because of the insufficient strain imposed during twin-roll casting. The inhomogeneous microstructure of the 3-mm-thick strip leads to a large deviation in its tensile elongation. The average grain size of the strips increases with increasing strip thickness, which results in reductions in both their tensile strength and their ductility because of the weakened grain-boundary hardening effect and the promoted formation of undesirable twins, respectively. The high-cycle fatigue resistance in the stress regime with finite fatigue life is similar for all three strips, but the fatigue strength with infinite fatigue life decreases from 175 MPa to 140 MPa as the strip thickness increases from 1 mm to 3 mm. The fatigue strength (FL) increases linearly with increasing yield strength (YS) according to the relationship FL = -199.5 + 2.03·YS.

Bio-corrosion impacts on mechanical integrity of ZM21 Mg for orthopaedic implant application processed by equal channel angular pressing

The mechanical integrity of rolled ZM21 Mg was improved by equal channel angular pressing (ECAP) to function as a potential biodegradable bone screw implant. Electron backscattered diffraction (EBSD) revealed deformed grains of 45 µm observed in rolled ZM21 Mg. They were transformed to equiaxed fine grains of 5.4 µm after 4th pass ECAP. The yield strength of rolled and ECAPed ZM21 Mg alloys were comparable. In contrast, 4th pass ZM21 Mg exhibited relatively higher elongation when compared to rolled sample. The mechanical properties of rolled and ECAPed ZM21 Mg were dependant on both grain refinement and crystallographic texture. The rolled and 4th pass ECAPed tensile samples exhibited nonlinear deterioration of mechanical properties when tested after 7, 14, 21 and 28 days immersion in Hank’s solution. The evaluation signifies that regardless their processing condition, ZM21 Mg alloys are suitable for surgical areas that requires high mechanical strength. In addition, the 4th pass ECAP samples were viable to MG-63 cells proving themselves to be promising candidates for future in vivo studies.

Study on processing and strengthening mechanisms of mild steel subjected to laser cavitation peening

A method of laser cavitation peening (LCP) was employed to strengthen the Q235 steel. The plastic deformation, residual stress, microhardness, phase, and microstructure of Q235 steel subjected to laser cavitation peening were investigated through a combination of experiments and simulations. The processing and strengthening mechanisms (LCP impact, plastic strain, grain refinement) were analyzed. Laser-induced cavitation bubble experiences three periodic pulsations during the process of LCP. The strengthening of Q235 steel by LCP can be attributed to the impact of the laser shock wave, bubble collapse shock wave, and water-jet. LCP causes plastic deformation in the surface layer of material and thereby introducing compressive residual stress and enhancing the microhardness. Dislocation structures including dislocation tangles, dislocation walls, and dislocation cells generate within the grains and near grain boundaries after LCP impact. Residual stress, microhardness, and dislocation density increase significantly with the increase of impact times. The high-density dislocation tangles and the sharing of dislocation cells refine the original coarse grains into equiaxed fine grains. The process of grain refinement is accompanied by the dissolution of cementite.

Comparative investigation between fiber laser and disk laser: Microstructure feature of 2219 aluminum alloy welded joint using different laser power and welding speed

In this investigation, the 2219 aluminum alloy sheets with a thickness of 1.5 mm are welded by different types of lasers. By analyzing the macro-morphology and microstructure of the weld obtained by two different laser welding methods, the characteristics of welding aluminum alloy by using these different lasers are studied. The results indicate that under the gravity of molten pool and different laser power densities, the weld seams (WS) welded by fiber laser and disc laser are in X-shape and Y-shape respectively. Besides, under the influence of Marangoni convection, the equiaxed fine grains of the weld are brought into the middle part, forming a small and regular equiaxed crystal zone (EQZ). It is found that the widths of the WS and the heat-affected zone (HAZ) obtained by fiber laser are both wider than those welded by disc laser. The grain size of columnar dendrites and equiaxed dendrites in the WS welded by fiber laser is coarser than that welded by disc laser. The tensile strengths of disc-laser-welded joints are also higher. In short, disc lasers are more suitable for high-power laser welding aluminum alloys.

Strength and ductility improvement of an in-situ TiB2/Al–Zn–Mg–Cu composite by elliptical cross-section torsion extrusion

In this study, a novel severe plastic deformation method, i.e., elliptical cross-section torsion extrusion (E-TE), was applied to improve the performance of an in-situ TiB2/Al–Zn–Mg–Cu composite to promote the application of the composite in the aerospace industry. Results evidence that the E-TE can homogenize the particle distribution and refine the composite's microstructure in a one-step process, thus yields a simultaneous improvement of the strength and ductility. Based on the experimental and simulation results, the underlying microstructure evolution during the E-TE process is revealed that the E-TE imposes large shear deformation on the composite, which mechanically fragments large grains and smashes particle bands/clusters; meanwhile, the dispersed reinforced particles and precipitations facilitate grain refinement via both accelerating continuous dynamic recrystallization and particle stimulated nucleation, and impede grain growth by hindering grain boundary migrant. Therefore, the E-TE processed composite exhibits optimized microstructures of uniformly distributed particles and fine-equiaxial grains; and its strength is increased via the grain boundary strengthening, while the ductility is improved due to the damage-generation retardation and crack-propagation impediment resulted from the fine-homogeneous microstructures. However, the precipitation dissolution under high temperatures weakens the particle/matrix interface bonding and results in void formation, leading to less mechanical properties improvement.

Influence of Residual Stress around Constituent Particles on Recrystallization and Grain Growth in Al-Mn-Based Alloy during Annealing.

Effect of the residual stress on the recovery and recrystallization behaviors of the cold-rolled AA3003 aluminum alloy was investigated. The evolution of deformed microstructure and dislocation density characterized by TEM and Synchrotron X-ray measurements found that the change in the ratio between low angle grain boundaries (LAGBs) and high angle grain boundaries (HAGBs) during annealing is varied depending on the initial dislocation density, where the value of HAGB/LAGBs ratio is amounted to be about 0.8 at maximum. The nucleation and growth rate of the recrystallized grains are strongly dependent on the net driving pressure associated with dislocation density increased by the amount of reduction. EBSD analysis revealed that the deformed zone composed of the fine equi-axed grains with large misorientation angles would be formed in the vicinity of the constituent particles, which is consistent with the region of the large residual stress and total displacement predicted by Eshelby inhomogeneity problem under cold rolling condition.

Micromechanical analysis of residual stress around coarse precipitates under cold rolling conditions

Characteristic microstructures of fine equiaxial grains with highly oriented gradients are often found around coarse precipitates in cold-rolled specimens; these microstructures are regarded as recrystallization nucleation sites under cold rolling and subsequent heat treatment. To predict the influence of applied strain and precipitate shape on strain and stress disturbances near a precipitate, the inhomogeneity problem under cold rolling conditions has been solved analytically using interior- and exterior-point Eshelby tensors. It is found that the stress and strain disturbances around the precipitate vary substantially with the aspect ratio of the precipitate, where the region of stress localization is dependent on the precipitate curvature at both ends of the precipitate parallel to the rolling direction. The residual stress predictions strongly suggest that the formation of a rotated zone is caused by the relaxation of disturbed stress under cold rolling.

Tailoring grain morphology in Ti-6Al-3Mo through heterogeneous nucleation in directed energy deposition

A common challenge in direct energy deposition (DED) is eliminating the anisotropy in mechanical performance associated with microstructure and the formation of coarse columnar grains. In this work, a heterogeneous nucleation mechanism was introduced into the melt pool, and, from this mechanism, an almost fully equiaxed grain morphology was obtained in the DED of Ti-6Al-3Mo. Three types of grain morphologies in DED Ti-6Al-3Mo, including full columnar grains, near-equiaxed grains and almost fully equiaxed grains were obtained from premixed and satellite powder blends from Ti, 6 wt.% Al and 3 wt.% Mo, respectively. Combined with the analysis of the interactions between powder particles and the melt pool in DED, the formation mechanism of the equiaxed grains caused by the incomplete melting of high melting point Mo particles was revealed. As the prior-β grains transformed from coarse columnar grains to fine-equiaxed grains, the strong <100> fiber texture along the deposition direction was weakened, while the size of the α-laths in the prior-β grains slightly decreased, and the selection of α-variants was weakened. Due to the transformation of the prior-β grains from coarse columnar grains to fine-equiaxed grains, the tensile strength of the deposited samples increased from 982 MPa to 1082 MPa, while the yield strength increased from 840 MPa to 922 MPa, and the elongation of the as-deposited alloy also increased from 9.0 % to 9.8 %, which confirmed that the presence of fine-equiaxed grains is beneficial to the strength and plasticity of the DED alloy. This work further demonstrates the role that satelliting powders can play in terms of enhancing the columnar to equiaxed transition (CET) behavior associated with DED.

Relationship among solution heating rate, mechanical properties, microstructure and texture of Al−Mg−Si−Cu alloy

Abstract The relationship among heating rate, mechanical properties, microstructure and texture of Al−Mg−Si−Cu alloy during solution treatment was investigated through tensile test, scanning electron microscope, X-ray diffractometer and EBSD technology. The experimental results reveal that there is a non-monotonic relationship among solution heating rate, mechanical properties, microstructure and texture. As the solution heating rate increases, the strength variations are dependent on the tensile direction; work hardening exponent n decreases first, and then increases; plastic strain ratio r increases first, and then decreases, and finally increases. The final microstructure and texture are also affected by heating rate. As heating rate increases, the microstructure transforms from elongated grain structure to equiaxed grain structure, and the average grain size decreases first, and then increases, and decreases finally. Although the texture components including CubeND{001}〈310〉 and P{011}〈122〉 orientations almost have no change with the increase of heating rate, the texture intensity and volume fraction decrease first, and then increase, and finally decrease. Both microstructure and texture evolutions are weakly affected by heating rate. Improving heating rate is not always favorable for the development of fine equiaxed grain structure, weak texture and high average r value, which may be related to the recrystallization behavior.

Superplastic behavior of a severely deformed Mg–6Gd−3Y−0.5Ag alloy

An extruded Mg–6Gd−3Y−0.5Ag magnesium alloy was processed by the simple shear extrusion (SSE) technique at 553 K for 1, 2, 4 and 6 passes to refine the microstructure. The electron back scattered diffraction (EBSD) analysis was used to investigate the microstructural evolutions of the alloy after the SSE processing. The grain orientation spread (GOS) maps revealed that by increasing the SSE passes, the fraction of the fine dynamically recrystallized (DRXed) grains increased accordingly from 3% after 1 pass of SSE to about 81% after 6 passes. The fraction of low angle grain boundaries (LAGBs) was relatively high in the early stages of the SSE processing, due to occurrence of dynamic recovery (DRV), but it began to drop significantly after 4 and 6 SSE passes, as the DRX proceeded. Therefore, continuous dynamic recrystallization (CDRX) was identified as the governing recrystallization mechanism during SSE at 553 K. The shear punch testing (SPT) was carried out at different temperatures and under various shear strain rates to assess the superplastic behavior of the alloys. It was found that only the alloy processed by 6 SSE passes exhibited superplastic flow, for which a maximum strain rate sensitivity (SRS) index of 0.45 and an average activation energy of 112 kJ mol−1 were obtained. Accordingly, grain boundary sliding (GBS) associated with the diffusion of grain boundaries was suggested to be the prevalent deformation mechanism during the superplastic flow of the 6-pass SSEed condition. Moreover, the SPT results revealed that as the SSE pass number increases the maximum SRS indices increase and shift to the lower temperatures indicating the rise in the contribution of the GBS mechanism. The kernel average misorientation (KAM) maps delineated that after SPT at 623 K the fine equi-axed DRXed grains were almost strain-free without any change in their shape for the 6-pass SSEed condition, while the one after 2 SSE passes was comprised of extremely deformed grains along the SPT loading direction.

Influence of Tool Pin Profile on Mechanical Properties and Microstructure of Friction Stir Welded Aluminium AA7075 Alloy

The present investigation describes the effect of different tool pin profiles on microstructure and mechanical properties of friction stir welded aluminium AA-7075 alloy. The tool pin profiles namely taper cylindrical threaded (TT), cylindrical (CT), square (SQ), triangular (TR), pentagonal (PT), hexagonal (HX) with constant shoulder diameter have been selected to make joints. The friction stir welding was done at constant tool rotational speed and traverse speed. From this study, it is noted that the weld joints prepared using taper cylindrical threaded pin profile exhibited good mechanical properties when compared to other pin profiles. It is due to more surface contact of tool pin and production of equiaxed fine grain structure in the weld region.

Dynamic Recrystallization Behavior and Model Study of Equiaxed Fine Grain Structure TC4

Thermal compression test of equiaxed fine grain structure TC4 was conducted by using Glebble-3800 testing machine. The deformation temperature ranged between 800-950 °C, the strain rate ranged between 0.01-10 s−1, and the deformation was 60%. The dynamic recrystallization behavior under this condition was analyzed. The results show that the dynamic recrystallization behavior of equiaxed fine grain structure TC4 is sensitive to temperature and strain rate. The grain size and volume ratio of dynamic recrystallization increases with increasing temperature and decreases with increasing strain rate. In this paper, with the β-phase transition temperature as the boundary, a dynamic recrystallization critical strain model, a dynamic model and a critical size model of the equiaxed fine grain TC4 are established. The predicted value of the established constitutive model is in good agreement with the experimental value, which provides a theoretical basis for the microstructure control of the alloy during plastic processing.

Nanoindentation mechanical properties on spark plasma sintered 48Ti-48Al-2Cr-2Nb alloy

This study aims to investigate the microstructure, plastic (H) properties, elastic (E) properties, reduced elastic (Er) properties the strain-to-break parameter (H/Er), and the resistance to plastic deformation parameter (H3/Er2) of the Ti-48Al-2Cr-2Nb alloy by use of scanning electron microscopy, nanoindentation and micro-indentation techniques. The results show that the sintering parameters had significant effect on the resulting microstructure. Desirable mechanical properties were obtained with the sample sintered at temperature of 1200 °C, pressure of 50 MPa, holding time of 7.5 min and a heating rate of 50 °C/min which had a near lamellar structure, resulting from the grain boundary pinning effect of the fine equiaxed gamma grains and the impartation of ductility due to the coarsened lamellar colonies. The nano-hardness and elastic modulus were observed to be about 4GPa and 31GPa for the near lamellar microstructure, respectively, with the microhardness of about 4.4GPa. While the duplex and the near gamma microstructures possessed the least nano-hardness (3.65–3.78GPa) and elastic modulus (3.6–29.5GPa) with the exception of sample sintered at temperature of 1150 °C, pressure of 50 MPa, holding time of 7.5 min and a heating rate of 100 °C/min., with nano-hardness and elastic modulus of 4.05GPa and 31.25GPa, respectively, however it had the lowest micro-hardness of 2.7GPa. Furthermore, the ratios H/Er and H3/Er2 values were observed to be greater for the same sample suggesting good wear resistance of the alloy.

Effects of dynamic recrystallization and strain-induced dynamic precipitation on the corrosion behavior of partially recrystallized Mg–9Al–1Zn alloys

Abstract The corrosion susceptibility of recrystallized and un-recrystallized grains in equal channel angular pressed (ECAPed) Mg–9Al–1Zn (AZ91) alloys immersed in chloride containing media was investigated through immersion testing and an electrochemical microcell technique coupled with high resolution techniques such as scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). During ECAP, dynamic recrystallization (DRX) and strain-induced dynamic precipitation (SIDP) simultaneously occurred, resulting in a bimodal grain structure of original elongated coarse grains and newly formed equiaxed fine grains with a large volume fraction of β-Mg17Al12 precipitates. Corrosion preferentially initiates and propagates in the DRXed grains, owing to the greater microchemistry difference between the β-Mg17Al12 precipitates formed at the DRXed grain boundaries and the adjacent α-Mg matrix, which induces a strong microgalvanic coupling between these phases. Additionally, the weaker basal texture of the DRXed grains also makes these grains more susceptible to electrochemical reactions than the highly textured un-DRXed grains. The influence of dynamic recrystallization and dynamic precipitation was also studied in ECAPed alloys with different levels of deformation strain through corrosion and electrochemical techniques. Increasing the strain level led to a more uniform corrosion with a shallow penetration depth, lower corrosion rate values, and higher protective ability of the oxide film. Furthermore, higher levels of strain resulted in greater hardness values of the ECAPed alloys. The superior corrosion resistance and strength of the ECAPed alloys with increasing strain level was attributed to the combination of smaller DRXed grain size, higher DRX ratio, and higher volume fraction of uniformly distributed fine β-Mg17Al12 precipitates.

On the microstructure and mechanical properties of AW2099 aluminium lithium alloy joints produced with electron beam welding

The effect of electron beam welding process on the microstructural and mechanical characteristics of newly developed 3rd generation AW2099 heat treatable aluminium lithium (Al-Li) alloy was investigated. Non-dendritic equi-axed zone (EQZ) is formed at the heat affected zone (HAZ) – weld metal (WM) interface. This zone consists of fine equi-axed grains, while the eutectics based on α-aluminium + θ-Al2Cu are preferentially precipitated at the grain boundaries (GB). Increased content of iron, silicon and manganese was detected in some inter-dendrite sites. Furthermore, β-AlLi and γ-Al2Li3 compounds were recorded in the columnar dendritic zone (CDZ). Electron beam welding (EBW) resulted in dissolution of strengthening precipitates, thus the decrease in nano-indentation hardness in EQZ was observed in comparison to HAZ.

Tailoring microstructure and texture of annealed Al-Mn alloy through the variation of homogenization and prior cold deformation strain

The grain structures and crystallographic textures desired for various applications of aluminium alloys are usually modified by recrystallization during annealing. In this study, the interactions between recrystallization and precipitation were investigated using the same Al-Mn alloy but with different homogenization procedures (which gives different microchemistries in terms of solute level and second phase particle state) and prior cold deformation strains, both of which are easy to realize in industrial practice. The results show that recrystallization and precipitation kinetics are both promoted at large deformations but their mutual interactions also exist. Recrystallization is retarded by precipitation through the suppressing of nucleation and pinning of grain boundary migration, while early finish of recrystallization delays precipitation by reducing their nucleation sites. The delicate interplay between recrystallization and precipitation during annealing leads to different combinations of grain structures and textures. An equiaxed fine grain structure can be obtained when recrystallization is not affected by fine particles while the combined effects of a strong fibrous deformation structure, suppressed nucleation of recrystallization and preferential growth lead to a coarse elongated grain structure. Different crystallographic textures desired for typical applications of Al-Mn alloys can be obtained after annealing simply through the different combinations of homogenization procedures and prior cold deformations.

AlCoCrFeNi high-entropy alloy particle reinforced 5083Al matrix composites with fine grain structure fabricated by submerged friction stir processing

In the present work, 5083Al matrix composites reinforced by 10 vol% AlCoCrFeNi high-entropy alloy (HEA) particles were fabricated by submerged friction stir processing (SFSP). It was found that the fabricated composites consist of equiaxed fine grains with the mean size of 1.2 μm due to dynamic recrystallization, particle stimulated nucleation (PSN) and shortened thermal cycle by water cooling. The HEA/5083Al interface showed a two-layer structure, the layer close to the HEA exhibited FCC + T phases with the thickness of approximately100 nm, and the other layer consisted of the Cr-depleted AlCoCrFeNi HEA particles in the size of roughly 100 nm. The SFSPed HEA/5083Al composites showed 25.1% higher yield stress (YS) and 31.9% higher ultimate tensile strength (UTS) in comparison with the base metal while maintaining acceptable ductility (18.9%). Grain refinement, geometrically necessary dislocations and load transfer effect can mainly be responsible for the improved strength. • SFSP were conducted on AlCoCrFeNi high entropy particles reinforced aluminum matrix composites. • CDRX, PSN and shortened thermal cycle by water cooling can be responsible for the grain refinement. • The interface structure was characterized to investigate the interface formation mechanism.

Microstructural changes in a Zn–Al eutectoid alloy modified with 2 wt.% Cu after superplastic deformation

This work focuses on the characterization of microstructural changes in Zn–Al eutectoid alloy modified with 2wt.% Cu when it is superplastically deformed varying the strain rate and temperature. Microstructural characterization was carried out by SEM and AFM techniques and it was shown that increasing the superplastic strain leads to a change of an initial fine equiaxed grain microstructure (0.85μm), formed by a Zn rich (η) and Al rich (α) phases, toward a microstructure consisting on flow bands. These flow bands formed mainly by elongated grains of the η phase, tend to be aligned along the tensile direction. The flow bands formation is promoted by an increase in temperature or a decrease in strain rate during deformation process. The morphologies and dimensions of flow bands were found to strongly depend on the testing conditions. The largest and thinnest flow bands were observed at 10−3s−1 and 513K. Experimental results evidenced that the origin of these microstructural changes, where deformation capability and growth of Zn-rich phase is significant, was not associated with a micro-superplastic phenomenon. Rather, it is associated to a non-homogeneous plastic flow and possible changes in the controlling deformation mechanism of this alloy, where recrystallization assisted by a redistribution of elements by diffusion is significant.

Microstructural and mechanical properties of oxide dispersion strengthened iron aluminides produced by mechanical milling and hot extrusion

Abstract Yttria dispersed Fe3Al alloy with a nominal composition of Fe–13Al–5Cr-0.18Ti-0.35 Y2O3 was produced by high energy ball milling followed by hot extrusion. The extruded and heat treated ODS Fe3Al exhibited both elongated and fine equi-axed grain structure in longitudinal direction, whereas, equi-axed grain structure was observed in transverse direction. Even though, both prealloyed as well as milled powder showed only FeAl phase, D03 structured Fe3Al along with Y3Al5O12 and TiO phases existed in ODS Fe3Al. Presence of fine (7–25 nm corresponding to Y3Al5O12) and coarse (>25 nm, TiO) particles were observed in ODS Fe3Al. The material exhibited yield and ultimate tensile strengths of 1104 and 1308 MPa with 8% elongation. The improved strength and elongation are due to the presence of dispersoids and fine grained structure.