Gradient Microstructure Research Articles

Enhancing microstructure and mechanical properties of laser directed energy deposited AlSi10Mg alloy by friction stir processing

The low strength-plastic properties of AlSi10Mg alloy prepared by laser directed energy deposition (LDED) have limited its application in aerospace and related fields, prompting researchers to explore friction stir processing (FSP) as a viable post-treatment solution. This study investigated the effects of FSP on the microstructure and mechanical properties of AlSi10Mg alloy formed by LDED, revealing that FSP effectively eliminated pores and refined grains, transforming the coarse and uneven grains in base material (BM) into fine and uniform equiaxed grains. Consequently, the thermal mechanical affected zone presented a microstructure gradient due to the uneven distribution of material flow and frictional heat. Additionally, the yield strength (YS), ultimate tensile strength (UTS), and elongation (EL) of LDEDed specimens were determined to 91.98 MPa, 158.57 MPa, and 4.29 %, respectively. YS and UTS of LDED-FSPed specimens were increased about 20 % and 30 %, and EL was greatly improved approximately 2–3 times compared with LDEDed specimens. Consequently, the grain refinement and porosity elimination caused by FSP improve the mechanical properties and microstructure evolution of LDED-FSPed specimens.

Effects of irradiation plus thermal aging on the phase boundary microstructure of austenitic stainless steel welds

Irradiation damage and thermal aging greatly affect the phase boundary microstructure and stress corrosion cracking of austenitic stainless steel weld metals (ASSWMs) in water-cooled nuclear reactors. However, the effects of irradiation plus thermal aging (I + A) on the phase boundary segregation and phase changes remain unclear. Phase changes and elemental segregation at the carbide and carbide-free phase boundaries of 308 L ASSWMs after I + A treatment were investigated using atom probe tomography and transmission electron microscopy. The I + A treatment induced Si depletion at the phase interfaces of δ-ferrite/austenite (δ/γ) and δ-ferrite/carbide (δ/C) and also induced Ni enrichment and Cr depletion with concentrations having the order Ni/Cr(δ/γ) > Ni/Cr(δ/C) > Ni/Cr(γ/C). The solute-defect binding model and the vacancy mechanism were applied to explain the phase boundary segregation. Furthermore, the I + A treatment affected microstructural evolution near the phase boundaries; this reduced spinodal decomposition and inhibited G-phase and Ni/Si-rich-cluster formation. The phase separation of δ-ferrite near δ/γ and δ/C phase boundaries differed with the distance from the boundary, forming a gradient microstructure from phase boundaries to δ-ferrite internally. The gradual precipitation of the G-phase was observed beyond about 15 and 10 nm from the δ/γ and δ/C interfaces, respectively; moreover, the growth of α'-phase in the δ-ferrite was accelerated with increasing distance from the δ/γ and δ/C interfaces.

Contracted functional connectivity profiles in autism

ObjectiveAutism spectrum disorder (ASD) is a neurodevelopmental condition that is associated with atypical brain network organization, with prior work suggesting differential connectivity alterations with respect to functional connection length. Here, we tested whether functional connectopathy in ASD specifically relates to disruptions in long- relative to short-range functional connections. Our approach combined functional connectomics with geodesic distance mapping, and we studied associations to macroscale networks, microarchitectural patterns, as well as socio-demographic and clinical phenotypes.MethodsWe studied 211 males from three sites of the ABIDE-I dataset comprising 103 participants with an ASD diagnosis (mean ± SD age = 20.8 ± 8.1 years) and 108 neurotypical controls (NT, 19.2 ± 7.2 years). For each participant, we computed cortex-wide connectivity distance (CD) measures by combining geodesic distance mapping with resting-state functional connectivity profiling. We compared CD between ASD and NT participants using surface-based linear models, and studied associations with age, symptom severity, and intelligence scores. We contextualized CD alterations relative to canonical networks and explored spatial associations with functional and microstructural cortical gradients as well as cytoarchitectonic cortical types.ResultsCompared to NT, ASD participants presented with widespread reductions in CD, generally indicating shorter average connection length and thus suggesting reduced long-range connectivity but increased short-range connections. Peak reductions were localized in transmodal systems (i.e., heteromodal and paralimbic regions in the prefrontal, temporal, and parietal and temporo-parieto-occipital cortex), and effect sizes correlated with the sensory-transmodal gradient of brain function. ASD-related CD reductions appeared consistent across inter-individual differences in age and symptom severity, and we observed a positive correlation of CD to IQ scores.LimitationsDespite rigorous harmonization across the three different acquisition sites, heterogeneity in autism poses a potential limitation to the generalizability of our results. Additionally, we focussed male participants, warranting future studies in more balanced cohorts.ConclusionsOur study showed reductions in CD as a relatively stable imaging phenotype of ASD that preferentially impacted paralimbic and heteromodal association systems. CD reductions in ASD corroborate previous reports of ASD-related imbalance between short-range overconnectivity and long-range underconnectivity.

Preparing gradient microstructure to achieve enhanced performance in GH4586 superalloy: Experiments and models

In this present study, a combined method of controlling strain deformation (CSD) and gradient-temperature heat treatment (GTHT) was adopted to prepare gradient microstructure in GH4586 superalloy. Consequently, it brought enhanced tensile strength from 1313.1±18.7 MPa at intermediate transition (IT) region to 1506.3±15.7 MPa at high-strain and low-temperature (HS-LT) region, presenting an improvement of 11.6%. And it also enhanced fracture toughness from 77.7±0.3 MPa⋅m1/2 at IT region to 86.7±3.7 MPa⋅m1/2 at low-strain and high-temperature (LS-HT) region, presenting an improvement of 14.7%. The formation of gradient microstructure along the longitudinal direction of CSD+GTHT-treated sample was thoroughly investigated from perspectives of precipitates distribution, activated slip systems and dislocations evolution. During CSD, the size and volume fraction of γ′ precipitates decreased under the effect of strain-induced dissolution behavior. The activation of (1‾ 11)[101] and (11‾ 1)[011] slip systems dominated dislocation movement at each region and there retained increasing dislocation density due to more extensively activated slip systems at HS region. During GTHT, the high-density dislocations promoted recrystallization while the concentratedly distributed γ′ precipitates obstructed movement of recrystallized front, leading to formation of heterogeneous structure with high-density grain boundaries and dislocations at HS-LT region. Finally, a model reflecting the effect of grain boundary strengthening, precipitation strengthening and dislocation strengthening on mechanical property of gradient microstructure was established and successfully applied to property prediction of GH4586 superalloy.

Insight into layer formation during friction surfacing: Relationship between deposition behavior and microstructure

Friction surfacing (FS) is a solid state layer deposition technique with a simple setup, presenting advantages compared to fusion-based approaches. Previous investigations showed microstructural gradients along layer width and thickness. The current study provides new insight into the FS layer formation for aluminum and its relation with the microstructure evolution. Special consumable studs containing two different aluminum alloys were used to visualize the different materials in the resulting deposit. The investigation was performed at different process parameters, revealing some fundamental material flow characteristics. The layer center presents inner stud material, where advancing side and top are formed by outer stud material. The bottom and retreating side present a mixture of inner and outer stud material. The part of the layer that is formed by the outer material, presumably undergoes higher strain rates during deposition, presenting finer grains. The top of FS layers shows a pronounced texture, i.e. shear texture components, compared to the other parts with random texture. This phenomenon can be related to the shearing of the stud material between already deposited material below and the stud at its rear edge. Overall, the FS layer formation characteristics revealed in this study are directly related to local microstructural properties.

Deformation and microstructure radial gradient evolution of AZ31 magnesium alloy bar during three-roll skew rolling

AZ31 magnesium alloy bar with radial gradient microstructure was prepared by three-roll skew rolling process (TRS), and achieving an excellent combination of ductility and strength. To elucidate the formation mechanism of this gradient microstructure, the deformation behavior during rolling was analyzed through experiments and simulations. The simulation results indicated that the metal flow of the bar in a spatial helical trajectory during rolling, with the trajectory inclination angle gradually decreasing from the center to the surface, promoting dynamic recrystallization in the surface and near-surface regions. The radial equivalent strain decreased from 1.5 at the surface to 0.01 at the center, and the shear strain decreased from 0.051 at the surface to 0.00012 at the center. Experimental results showed that the grain size of the bars decreased from 49.77 µm at the center to 19.72 µm at the surface, and the tensile twin volume fraction decreased from 34.1 % to 19.4 %. Dynamic recrystallization predominantly occurred in the outer ring region of the bars, while significant {10−12} tensile twin activation was observed in the central region, with twin-induced dynamic recrystallization observed at the twin boundaries. Mechanical property tests demonstrated that the gradient structure endowed the bars with a favorable combination of strength and elongation, with hardness significantly increasing from 50 HV at the center to 90 HV at the surface. This indicates that the three-roll skew rolling technology effectively promotes dynamic recrystallization at the surface, and the resulting gradient microstructure achieves an excellent combination of strength and plasticity in magnesium alloy bars.

The role of calcium addition through powder sintering process in microstructure and mechanical behavior of a microalloyed magnesium

Calcium effectively strengthens magnesium through grain refinement when added in small quantities using the ingot metallurgy process. The extremely low solubility of calcium in magnesium restricts its addition using traditional powder metallurgy processes. However, the powder metallurgy process provides advantages like reduced material loss, properties control, and net-shape products. Hence, in this study, a diluted magnesium-calcium alloy was processed using the traditional blend-press-sinter (BPS) powder metallurgy process. Commercially pure magnesium was microalloyed with 0.4 wt% calcium. The sintered magnesium-calcium alloy was subjected to high-temperature extrusion. Microstructural characterization of the microalloyed Mg0.4Ca alloy revealed that the elemental calcium diffused into the magnesium particles during the sintering process and developed a gradient granular microstructure. The added initial calcium nanoparticles also acted as nucleation sites for magnesium during the sintering process. The gradient microstructure recrystallized and grew into equiaxed grains during the subsequent extrusion process. Mechanical properties characterization revealed that the addition of calcium as solute atoms increased the hardness, compressive strength, and ductility of magnesium, which was further enhanced by the extrusion process. The presence of calcium as solute atoms increased the tensile strength of extruded magnesium by a reasonable amount, while ductility remained almost unchanged.

Wire arc additive manufacturing NiTi/Nb bionic laminated heterogeneous structure: Microsturcture evolution and mechanical properties

In the aerospace industry and the medical field, there is a demand for materials that combine the functional properties of NiTi with the excellent ductility of Nb. However, creating high-strength interfaces between these two materials has been difficult due to their inherent differences in physical and chemical properties. In this study, a laminated heterogeneous structure (LHS) combining NiTi and Nb was prepared using wire arc additive manufacturing (WAAM) with different layer thickness ratios. The resulting microstructure of the NiTi/Nb LHS components consisted of a large amount of NiTi and β-Nb eutectic structures. This was due to the metallurgical bonding between the NiTi and Nb layers. The NiTi/Nb LHS components exhibited excellent compressive strength, measuring at 2607.6 ± 31 MPa. Additionally, the interface strength of the NiTi/Nb LHS component was remarkable, showing an ultimate tensile strength of 789.3 ± 8 MPa. The enhanced strength of the NiTi/Nb LHS component can be attributed to the gradient microstructure of the NiTi layer, which promoted heterogeneous plastic deformation generation. Furthermore, this study unveiled the relationship between the formation mechanism of the heterogeneous eutectic microstructure and the strength-ductility synergistic mechanism. As a result, this study provides an innovative approach for additive manufacturing in the strengthening of laminated multi-material interfaces.

Carbon Materials With Conductivity Gradients Allow Dynamic Screening of Steep Temperature Differences Along Thin Films

AbstractCarbon materials comprise a wide range of microstructures and excellent electrical and thermal properties while being cost‐effective and readily available. They can be obtained through carbothermal processes at high temperatures, starting from cellulose. Catalytically active compounds, for example, iron salts, strongly influence the carbon microstructure during the graphitization process. Different degrees of structural order can, therefore, be achieved by adjusting the concentration of the iron salts. An infusion withdrawal impregnation approach is used on filter paper to prepare a continuous gradient of the carbon microstructure. This structural change is accompanied by a continuous variation of the closely related electrical and thermal transport properties. Even more, the synergistic interplay of local sheet resistance and thermal diffusivity results in the formation of switchable temperature gradients when an external current is applied. Steady state temperature differences of up to 80 °C are observed along the centimeter‐scaled samples. The controllable temperature gradient formation will be of great interest for applications requiring a fast temperature screening. Furthermore, the temperature gradient can be imposed onto other materials, which will be particularly relevant for advanced thin film characterization applications.

Engineering of textured gradient microstructures using directed energy deposition: The impact of adaptive cooling rate

Textured gradient microstructures can be engineered by tailoring the molten pool cooling rate during additive manufacturing (AM). Here we consider the design of grain structures and crystallographic orientations by controlling the solidification strategy during AM by directed energy deposition (DED). In this paper the textures generated by open loop (fixed scan speed of 100, 200 and 300 mm/min) and closed loop adaptive control to achieve cooling rates of 500, 1100 and 1750 °C/s were compared using electron back scatter diffraction (EBSD). The cooling rate was determined key to eliminating the microstructural gradients or anisotropy for DED parts or to engineer functionality along the desired path. The solidified macro-textures along the building direction were significantly affected by the formation of columnar grains, their growth direction, and morphological transition to equiaxed grains as a result of rapid cooling.

Compressive behavior and deformation mechanisms of gradient microstructures in a dissimilar welded joint

In this study TC4/TC17 titanium alloy dissimilar joints were obtained via vacuum electron beam welding. The compressive behavior of base metals and joint at different strain levels was studied, along with microstructural and fracture surface observations. The true stress–strain curves of base metals and joint at different strains demonstrated smooth and continuous plastic deformation characteristics. The compressive yield strength of the joint (1065 MPa) was basically equivalent to that of TC17 alloy (1196 MPa), which was significantly higher than that of TC4 alloy (920 MPa) for the failed samples. However, the compressive fracture strain of the joint was lower than that of base metals. Comparison of the microstructures of base metals and joint deformed at different strains, it revealed that the equiaxed α phase in TC4 alloy mainly underwent the deformation, while the TC17 alloy mainly relied on the rotation of lamellar grains to resist the deformation. In the dissimilar joint with a large microstructural gradient, the compressive deformation of several zones, including the weld zone, far heat-affected zone and the base metal, was also related to the rotation of finer grains.

A study of fatigue property enhancement of 1045 steel processed by surface mechanical rolling treatment with an emphasis on residual stress influence

Several factors contribute to the observed bulk mechanical properties enhanced by surface mechanical rolling treatment (SMRT), and it is important to distinguish the influences of each major factor. A commonly used engineering carbon steel, 1045 steel, was chosen for the current study because the material does not have stress-induced phase transformation. A fully reversed strain-controlled gradual decreasing amplitude loading spectrum (ENV) was employed to release the residual stresses induced by SMRT. Fatigue experiments were conducted on the 1045 steel specimens with and without ENV to assess the influences of grain refinement and the residual stress on fatigue. It was found that SMRT refined and recombined the microstructures in the surface layer through deformation and fragmentation of brittle cementite, and extrusion of ductile ferrite between adjacent cementite layers. In the high cycle fatigue regime, both the SMRT induced residual stresses and the refined/recombined microstructures contributed to the enhancement of fatigue strength and fatigue ductility but the gradient microstructure contributed significantly more than do the residual stresses. Unlike stainless steels processed by SMRT, the carbon steel does not display a kink point in the strain-life fatigue curve, and all the crack initiations were found to occur on the specimen surface.

Investigations on diamond rotary rolling treatment induced gradient microstructure and its effects on mechanical properties

Surface and subsurface gradient microstructures play a significant role in mechanical properties. To obtain gradient microstructures, a surface strengthening method, namely diamond rotary rolling treatment (DRRT), was proposed in this study. Varied gradient microstructures in 316 L stainless steel were obtained through different DRRT processing parameters. The micro-hardness distribution along the gradient microstructure was tested. The resulting gradient micro/nanostructures in terms of grain size, dislocation density and phase transformation were characterized and evaluated with electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. The corresponding mechanical properties were investigated by slow strain rate test. The results showed that the DRRT could introduce a gradient hardening layer on the surface, accompanied by a gradient distribution of nano-grains, ultrafine grains, fine grains and coarse grains extending from the surface to the material interior. Besides, the gradient microstructure was associated with martensitic phase transformation, slip and dislocation. Compared to milled and untreated specimens, the DRRT-treated specimens exhibited a significant increase in yield strength and reduced ductility. Particularly, a well-designed DRRT process can enhance the yield limit while preserving higher ductility. Additionally, a quantitative relationship between the overall yield strength and the hardness distribution of the gradient microstructure was established.

Enhanced tribological properties and the microstructure evolution of the gradient nanostructured copper alloy

In this work, the effects of the gradient microstructures by surface mechanical attrition treatment (SMAT) on the corresponding tribological properties of the nanoprecipitates strengthened copper alloy was investigated. The nanoprecipitates strengthened copper alloy possesses a decent tribological properties compared with the pure copper and bronze. The SMAT-processed NP&T alloy possesses distinctly lower coefficients of friction (COF) than its coarse-grained counterpart throughout the entire frictions. In particular, under a sliding load of 2 N, NP&T alloy possesses a lower COF of 0.58 ± 0.03, approximately 15% lower than that of its homogeneous counterpart. Additionally, the wear volumes, wear rates, and worn surface roughness were approximately 45% lower in the NP&T alloy, indicating the superior tribological properties. The macro-, micro-, and atomic-scale friction and wear mechanisms in the worn NP&T alloy were systematically determined. The inherent gradient nanostructures could achieve strain delocalization, thereby suppressing surface deformation and improving wear resistance. Wear-induced substructure in the NP&T alloy comprises gradient layers mediated by twinning and stacking faults combined with nanocomposite layers, resulting in novel tribological performance characteristics. Thus, combining inherent and newly formed wear-resistant gradient nanostructures is an effective design strategy for high-performance Cu alloys.

Formation of Symmetric Gradient Microstructure in Carbon Steel Bars

In recent years, severe plastic deformation has attracted the most attention as a way to improve the mechanical properties of steel bars. Obtaining ultrafine grains and nanostructures in such bars leads to a strong increase in strength properties but strongly reduces their plastic properties. This study shows that the formation of a gradient microstructure allows simultaneous improvement in the strength and plastic properties of carbon steel bars, taking into account the symmetry of the microstructure distribution from the center of machining. A new combined technology is proposed to obtain such a microstructure. This technology consists of drawing bars from medium carbon steel on a radial-displacement rolling mill and carrying out subsequent drawing. Steel bars with a diameter of 30 mm were strained in three passes to a diameter of 16 mm at room temperature. The results show that the average value of microhardness in the center, neutral, and surface areas for the three straining cycles were 1890 MPa, 2335 MPa, and 2920 MPa, respectively. This symmetrical distribution of microhardness confirms the gradient microstructure. Strength characteristics also increased almost twofold: the yield strength increased from 330 to 735 MPa, and the ultimate strength increased from 600 MPa to 1025 MPa. Relative elongation decreased from 18 to 14 MPa, and relative reduction decreased from 40 to 31%, but remained at a fairly good level for AISI 1045 steel. The validity of all results was confirmed through numerous experiments using a set of traditional and modern research methods, which included optical, scanning, and transmission microscopy. EBSD analysis allowed precise positioning of the field of vision for studying microstructural changes across the entire bar cross-section. All of these methods used together, including tensile testing of the mechanical properties and the fractographic method, allow us to assess changes in microhardness and the reproduction of results.

МЕХАНИЗМЫ УПРОЧНЕНИЯ ПРИ ИМПЛАНТАЦИИ НАНОЧАСТИЦ В МАТЕРИАЛ С ПОМОЩЬЮ ЛАЗЕРНОЙ УДАРНОЙ ВОЛНЫ

Implantation of nanoparticles into materials using a laser shock wave (INLSW) is a new method of hardening the surfaces of light metals and alloys. The hardening is based on a combination of plastic deformation caused by the well-known laser shock processing technology (LSP) with high mechanical characteristics of solid-phase nanoparticles. In this paper, we study the specific relationships between the hardening mechanisms and the properties of the surface treated with INLSW technology. It has been established that the hardening effect when using the INLSW technology is mainly achieved by two factors: the gradient microstructure caused by plastic deformation and the gradient distribution over the depth of the implanted SiC nanoparticles. It has been established that the hardening effect in the process of injection molding in gradient-reinforced layers is carried out by three competing hardening mechanisms, namely: dispersion hardening with SiC nanoparticles, hardening by grinding grains and dislocation hardening. The contribution of each hardening mechanism to the overall hardening has been studied using a modified Klein computational model.

Corrosion behavior of laser directed energy deposited SS316L/Inconel718 functionally graded materials

Functionally graded material (FGM) can exhibit a variety of excellent properties by controlling layer-by-layer gradients in composition and microstructure. In this paper, SS316L/Inconel718 FGM was fabricated by laser directed energy deposition (LDED) to investigate the microstructures and corrosion behaviors at the transitional interfaces. The results indicate that the microstructures in gradient regions along the build direction include cellular crystals, dendritic structures, columnar crystals, and equiaxed crystals. With increasing mass fraction of IN718, the precipitates within the FGM matrix transition from metal oxides to Laves phases and MC phases, accompanied by elemental segregation of Nb and Mo at the grain boundaries. The enhanced corrosion resistance is attributed to the increased pitting corrosion resistance elements within the FGM matrix. Additionally, the high content of Cr2O3 and NiO in the passive film enhance the corrosion resistance. The SKP and LEIS test results indicate that the LDED-manufactured SS316L/IN718 FGM exhibits no apparent galvanic coupling corrosion and maintains excellent corrosion resistance transition. This investigation offers a reference for the study of corrosion-resistant composite materials.

Gradient microstructures and mechanical properties of WAAMed and aged Al–Cu alloy after laser shock peening

The integration of wire-arc additive manufacture (WAAM) and laser shock peening (LSP) facilitated the study of the gradient microstructure in T6-treated WAAM Al–Cu alloy and the synergistic strengthening mechanisms involving the precipitation phase and LSP parameters. The results indicate that pore closures, increases in kernel average misorientation (KAM), and low angle grain boundary (LAGB) are positively associated with LSP energy density. Residual stress positively correlates with LSP energy density and inversely with spot diameter, where larger spots exacerbate non-uniformity. The values of dislocation density, microhardness and residual stress of the post-LSP specimen gradually increase with the increase of depth, and reach the maximum value at ~100 μm. As the depth continues to increase, the value begins to decrease gradually. Dislocation accumulation near grain boundaries, interacting with θ’-Al2Cu, contributes to strength enhancement. The main strengthening mechanisms enhanced by LSP involve creating a gradient microstructure through significant plastic deformation, pore closure, and the presence of high-density dislocations. Post-T6 treatment, θ’-Al2Cu significantly restricts dislocation slip, pins dislocations, and encourages the emergence of new dislocation sources, thereby enhancing the strengthening effect and stability induced by LSP.

Effect of arc deposition process on mechanical properties and microstructure of TiAlSiN gradient coatings

In order to successfully prepare a new advanced TiAlSiN gradient coating with Ti content gradually decreased but Al content gradually increased from the cemented carbide substrate to the outermost coating surface by arc ion plating, we deeply investigated the influence mechanisms of two important process parameters. These parameters include substrate bias voltage and arc current, which affect the mechanical properties and microstructure of the gradient coating. The results show that bias voltage and arc current directly affect the particle accumulation effect on the coating surface and the re-sputtering effect caused by high-energy ion bombardment, thus affecting the deposition efficiency of the coating and the formation of defects such as macro-particles and pits on the coating surface. The increase of bias voltage is conducive to the increase of the density of columnar crystals and nanocrystals. While the increase of arc current makes the coating with high Al content transform from columnar crystals to nanocrystals. The hardness and interfacial adhesion of the gradient coatings showed a tendency of firstly increasing and then decreasing with the increase of both bias voltage and arc current. When the bias voltage and arc current are -80 V and 160 A respectively, the coating has the best mechanical properties, its surface hardness reaches 36.94 GPa, and the interfacial adhesion exceeds 120 N. In addition, with the increase of bias voltage and arc current, particles are deposited on the surface of the coating with higher energy, which leads to the preferential orientation transition from (111) to (200) plane. The high hardness of TiAlSiN coating is mainly due to the formation of high Al content of fcc-TiAlN crystals and amorphous Si3N4 in the gradient-coated surface layer. The results have important theoretical and practical significance for developing new high-performance gradient coating tools.

Study on microstructure evolution mechanism and inhomogeneous characteristic of 2219 aluminum alloy in hot flow forming

In order to reveal the evolution mechanism of the deformation microstructure of 2219 aluminum alloy tube under compression-shear stress, in this paper, flow forming tests and their numerical simulations were carried out, respectively, at temperatures from 250 °C to 400 °C. Meanwhile, a midrange layer index was proposed to analyze the gradient of deformation microstructure. The results show that multiple recrystallization mechanisms of the 2219 aluminum alloy are presented simultaneously during hot flow forming. The shear stress during forming is the main factor that generated the microstructure gradient along the thickness direction, and the recrystallization level is higher with higher shear strain. In addition, it is beneficial to improve microstructure homogeneity and mechanical properties by increasing thickness reduction or decreasing the temperature. This study provides guidance for microstructure control of the 2219 aluminum alloy tube flow forming process.