Fine Equiaxed Grain Research Articles

Achieving strength-ductility balance in AZ31-WE43 Mg alloys by symmetric extrusion-shear process at various temperatures

The development of composites that simultaneously improve strength and plasticity is a challenging task, especially in Mg alloys. The focus of this study is to create a novel multiscale gradient structure (GS) in Mg laminates to exhibit optimal strength-plasticity synergies through a temperature-controlled extrusion shear (TCES) and extrusion composite process with preset grain sizes. Initially, the gradient structure is formed by a short extrusion process, leading to an increase in the average grain size of the elongated coarse grains from the middle layer of the equiaxed fine grain zone to the elongated coarse grain surface layer. The resulting gradient structure improves the material synergy. The microstructure evolution mechanism of the alloy during symmetric plastic deformation was analyzed. The initial grains underwent DDRX, CDRX, and shear deformation-induced dynamic recrystallization (SDDRX) during the RZ extrusion stage, the grains were significantly refined, and the basal texture intensity was significantly weakened during the SD extrusion stage. The effect of extrusion temperature (400 °C, 450 °C, 500 °C) on the alloys in the forming zone after ES extrusion was further investigated. Tensile tests on Mg laminates prepared by the extrusion method showed that the ultimate tensile strength (UTS) of the samples prepared at 450 °C was 265 MPa and the elongation (EL) was 23.8 %. The significant improvement in the yield tensile strength (YTS) of the composite Mg alloys post-ES processing is primarily attributed to the fine-grained (FG) interior, while the increase in elongation is due to the coarse-grained (CG) exterior. This study presents a novel approach to enhancing the mechanical properties of Mg alloys, offering a promising direction for the development of advanced materials with superior strength and ductility.

Effect of ultrasonic solidification on the microstructure, mechanical and corrosion properties of FeCoCrNiCuAl0.4 high entropy alloy

One-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) ultrasound with 20 kHz frequency were successfully applied into the solidification process of FeCoCrNiCuAl0.4 high entropy alloy (HEA) to investigate their effects on microstructural evolution and application performance improvement. During static solidification, the primary γ1 phase developed into coarse dendrites with strip-like γ2 phase that diffusely distributed in the interdendritic regions. With the increase of ultrasound dimension, a novel heterostructure characterized by γ2 phase growing continuously among the fine equiaxed γ1 grains appeared. Transient cavitation, which intensified by the rise of ultrasound dimension, was analyzed to be the dominant factor in refining γ1 phase by significantly enhancing its nucleation rate, while the increased acoustic flow promoted the uniform distribution of easily segregated Cr and Cu elements by accelerating solute diffusion. These jointly contributed to the formation of such refined network heterostructure. Owing to the grain refinement strengthening and hetero-deformation induced (HDI) strengthening, the yield strength and ultimate strength of 3D ultrasonically solidified alloy were respectively enhanced by 29 % and 47 % without notable sacrifice of ductility. Meanwhile, the self-corrosion current density of the alloy was also reduced with the improvement of corrosion resistance, which mainly arose from the more balanced corrosion potential brought about by the uniform solute distribution and continuous network heterostructure.

Investigation on the effects of deposition strategy on the mechanical characteristics and microstructure of austenitic stainless steel 308LSi manufactured via wire arc additive manufacturing

This study examines the impact of deposition strategy on ASS 308LSi thin-walled structure manufactured via the Wire Arc Additive Manufacturing (WAAM) technique on the microstructural characteristics, tensile strength, microhardness, Charpy impact testing, and fracture morphology of the WAAM 308LSi. The analysis of the microstructure reveals that the deposition strategy promotes transitioning from columnar to equiaxed fine grain structure. The tensile strength results show that specimens with a 45° deposition strategy exhibit lower anisotropy and higher tensile properties compared to those with a 0° deposition strategy, with improvements of 33.1% in the transverse direction and 26.7% in the longitudinal deposition directions, respectively. The microhardness in WAAM SS308LSi demonstrates variations in the bottom, middle, and top regions, with the highest average value observed at a 45° deposition strategy (213.3 ± 6.5 HV, 201.1 ± 10.7 HV, and 191.5 ± 5.2 HV) as well. The impact testing results indicate that the highest absorbed energy occurs at a 45° deposition strategy, with 75 ± 4.2 J and 74 ± 4.0 J for the transverse and longitudinal directions, respectively. The fractures observed during testing exhibit ductile characteristics, with the presence of dimples and particles. This study demonstrates the significant potential of the 45° deposition strategy with the implementation of double-sided substrate deposition, resulting in a refined microstructure, nearly isotropic behavior, and excellent mechanical performance.

Enhancing the mechanical properties of wire arc directed energy deposition Al–Zn–Mg–Cu aluminum alloy through cyclic deep cryogenic treatment

In this paper, the effects of cyclic deep cryogenic treatment (CDCT) on the mechanical properties and the precipitated phases evolution of 7B55 aluminum alloy produced by wire arc directed energy deposition (WA-DED) were systematically investigated. The findings revealed that, under both T6 and CDCT conditions, the microstructure of the 7B55 aluminum alloy exhibited fine-equiaxed grains. Under the T6 condition, the precipitated phases within the grains were primarily composed of clustered GP zones and η′ phases. Meanwhile, the η phases were continuously distributed along the grain boundaries. Upon subjecting the material to three cycles of CDCT, a noticeable reduction in the number of GP zones within the grains was observed. Simultaneously, there was a substantial increase in the number of η′ phases, accompanied by the occurrence of dislocation lines. The distribution of η phases along the grain boundary transitioned to an interrupted pattern. Under the CDCT-Cycle 3 (CDCT-C3) condition, the ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) achieved 658.6 ± 4.5 MPa, 624.2 ± 5.2 MPa, and 8.42 ± 0.18 %, respectively. However, with the progression to four cycles of CDCT, the GP zones within the grains disappeared and the number of η′ phases started to decrease with the formation of a large number of η phases. The η phase along the grain boundary continuously grew by merging and absorbing solute atoms from the Al matrix near the grain boundaries. This led to the formation of coarsen and interrupted η phases and the Precipitate-Free Zone (PFZ) along the grain boundaries. Subsequently, the mechanical properties experienced a substantial decrease after the CDCT-Cycle 4 (CDCT-C4).

Formation mechanism of AlNbTiVZr high entropy alloy/Ti multi-principal element alloy by laser direct energy deposition: Microstructure evaluation and mechanical properties

A novel refractory multi-principal element alloy with high strength and toughness matching characteristics was successfully prepared by mixing TA15 alloy with AlNbTiVZr high-entropy alloy. The effects of the composition ratio of the high-entropy alloy on the microstructure evolution and its influence on properties were investigated in this study. The tensile strength of the 70 wt % AlNbTiVZr HEA/Ti multi-principal element alloy is 1.18 times more than that of TA15 alloy, which has a value of 1068.0 ± 2.1 MPa. The increase in strength is mainly due to the production of equiaxed fine grains and the development of a robust and resilient body-centered cubic (BCC) phase in the alloy following the introduction of the high-entropy alloy. The attainment of the acceptable tensile strain is the result of the combined influence of the BCC matrix phase and the dispersed second-phase particles. The exceptional mechanical capabilities of the 70 wt % AlNbTiVZr/Ti multi-principal element alloy undeniably enhance its potential for use as a structural component in aerospace load-bearing constructions.

Achieving high strength-ductility combination of Al–Mn–Mg-Sc-Zr alloy subjected by laser powder bed fusion and suitable post heat treatment

Post heat treatment is crucial for optimizing the performance of additive manufacturing components, yet it is still challenging to synergistically improve the strength and ductility through simple heat treatment. The present work investigated the microstructure evolution and mechanical properties of laser powder bed fusion (L-PBF) additive manufactured Al–Mn–Mg-Sc-Zr alloy after aging at 300 °C for different durations. All the as-built and aged samples exhibited a typical bi-modal microstructure consisting of coarse columnar and fine equiaxed grain regions. The as-built samples contained a number of primary Al6Mn precipitates distributed along grain boundaries; meanwhile, Mg solutes segregation was visible at grain boundaries, forming “Mg walls”. After aging for 4 h, the Al6Mn and a small content of Al3Sc were precipitated. Two types of Mg-rich precipitates evolved along grain boundaries simultaneously, increasing ultimate tensile strength from 401 MPa to 501 MPa, while the ductility was significantly reduced from 19.6 % to 4.6 %. When increased to 6 h, the Mg-rich precipitates disappeared, and the density of Al6Mn and Al3Sc precipitates reached its peak concurrently, providing a dual-precipitate strengthening effect. As a result, an excellent strength-ductility balance with an ultimate tensile strength of 528 MPa and ductility of 14.5 % was obtained. The Al6Mn precipitates became coarsening with increasing to 7 h, resulting in lower ductility. The strategy of modulating elemental distribution and precipitated phases by simple heat treatment is expected to be further developed for high strength and ductility Al alloys prepared by L-PBF.

Characteristic analysis of high-speed steel powder prepared by gas atomisation

The performance of high-speed steel (HSS) powder is a prerequisite for the preparation of high-performance powder metallurgy HSS. The physical properties, microstructure, and solidification characteristics of HSS powder prepared by nitrogen gas atomisation were systematically studied and analysed in detail. For the atomised TPM558 HSS powder, the morphology is spherical and the particle size distribution is wide. The smooth small particles of satellite powder attach to the surface of large particles. Shrinkage pits caused by solidification shrinkage exist on the surface of large particles. The average cooling rate is between 104 and 106 K·s−1, with the increase of cooling rate, the powder surface tends to be smooth, and the grain structure is transformed in the order of cellular to dendritic to radial. The single powder particle surface shows fine-equiaxed grains, while the internal presents columnar grains. The crystalline phases of the powder are austenite, ferrite, martensite, and carbide, and the carbide includes MC carbide with face-centred structure and M2C carbide with hexagonal structure. The average nano-hardness of the powder is 9.93 GPa, resulting in poor formability, and it can be pressed and formed only after adding binder.

Research on microstructure and mechanical properties of Ti–6Al–4V ELI fabricated by hybrid directed energy deposition with in-situ rolling and annealing

To achieve superior damage tolerance and fatigue properties, it is desirable for the microstructure of Ti-6Al-4V ELI (Extra Low Interstitial) titanium alloy to consist of equiaxed grains with a homogeneous lamellar structure. However, traditional forging and single additive manufacturing techniques require extreme processes such as quasi β heat treatment and hot isostatic pressing. In this paper, a hybrid directed energy deposition method that integrates in-situ rolling with simple annealing is proposed to simultaneously achieve these microstructures. The results indicate that the initial microstructures produced by hybrid directed energy deposition consist of fine equiaxed prior-β grains, inconspicuous interpass bright bands, randomly oriented lamellar structure and numerous small substructures at the TEM scale. Due to the plastic deformation caused by in-situ rolling, the primary β grains underwent significant refinement with a reduction in grain diameter from over 2000 μm to 113.8 μm. Besides, the unique microstructures and thickness of α laths are influenced by coarsening and dissolution mechanisms that are controlled by diffusion during phase transformation. Furthermore, a uniform microstructure without bright bands and excellent comprehensive properties can be achieved through heating at 880 °C for 2 h and natural cooling. These findings validate that hybrid directed energy deposition coupled with simple annealing present a novel and cost-effective approach for the direct manufacturing of uniform titanium alloy forgings.

Wire arc additive manufacturing of the Al-5.55Cu alloy: Insight the effect of intrinsic heat treatment on microstructure and mechanical properties

Al-5.55Cu alloy single wall sample were fabricated by cold metal transfer wire arc additive manufacturing (CMT-WAAM) technique, and the microstructure characteristics and evolution mechanism of the different deposition regions were analyzed. Under the influence of intrinsic heat treatment and remelting process caused by layer-by-layer deposition, the top region exhibits dendritic characteristics, while the middle and bottom are characterized by a non-uniform banding distribution of fine equiaxial grains alternating with columnar grains. According to the non-equilibrium solidification theory and the related nucleation mechanism, the formation of equiaxed grains in the inter-layer region is related to the elements such as Ti and Zr contained in Al-5.55Cu alloy, which can be used as nucleation particles. In addition, the morphology and quantity of the θ'-Al2Cu phase inside the grain change with the increase of deposition height due to the thermal cycling effect. After heat treatment, the tensile strength of the deposited sample is greatly improved and remains isotropic. The ultimate tensile strength of the heat-treated sample is 508.0 ± 18.9 MPa in the horizontal direction and 503.8 ± 6.2 MPa in the vertical direction.

Effect of ultrasonic vibration on microstructural evolution, clad defects, and surface properties in laser direct energy deposition of Inconel 625

Laser direct energy deposition (DED) has some accompanying issues, such as existence of micropores, elemental segregation at grain boundaries, intergranular corrosion, etc. Therefore, the current work aims for a reduction in clad defects and enhancement in surface properties for laser direct deposition of Inconel 625 by implementing ultrasonic vibration. The acoustic streaming and cavitation effect induced by ultrasonic vibration results in the breaking of columnar grains, along with grain refinement and better elemental distribution in the matrix during the solidification process. The investigation is carried out for deposition using a 240 W Yb-fiber laser under the application of ultrasonic vibration with a variable amplitude of 6–13 μm (frequency: 33–28 kHz). A relatively higher vibration amplitude was found more efficient in converting long columnar grains into finer and uniformly distributed equiaxed grains, with a significant reduction in micropores. Further, it resulted in a shorter molten pool lifetime because of the generation of more nucleation centers, leading to better cooling. The above effects resulted in higher microhardness of the deposited layer. Further, the wear and corrosion resistance showed an improvement with the application of vibration, which may be due to the finer equiaxed grains, less porosity, and better elemental distribution at a higher vibration amplitude.

A comparative study on Al-Mg-Sc-Zr alloy fabricated by wire arc additive manufacturing with controlling interlayer temperature and continuous printing: Porosity, microstructure, and mechanical properties

Wire arc additive manufacturing (WAAM) technique is a promising approach to producing large-scale metal components due to high deposition efficiency and low production cost. However, fundamental research about WAAM-processed Al-Mg-Sc-Zr alloy was still fewer. In this study, Al-6.54Mg-0.36Sc-0.11Zr (wt%) components were successfully manufactured by WAAM with an interlayer temperature at 100 °C (named IW) and continuous printing (named CP), and the corresponding porosity, microstructure, and mechanical properties of components were studied in detail. The porosity of components as-deposited was relatively low, about 0.385% and 0.116%, respectively. The microstructures of the two components exhibited the same distribution characteristics in XZ and YZ planes: fine equiaxed grains (FEG) at remelted zone + FEG and coarse equiaxed grain (CEG) alternative distribution at middle zone + FEG at the top zone of the molten pool. The average grain size of component IW was about 10.51 ± 6.01 μm, and that of component CP significantly increased, to about 11.85 ± 5.86 μm. The short-circuit transition mode of cold metal transfer technology and the heterogeneous nucleation effect of primary Al3(Sc, Zr) and Al3(Sc, Zr, Ti) phases together promoted the formation of equiaxed grains and refined the microstructures. After heat treatment at 325 °C and 6 h, nano-Al3Sc precipitated with a size of about 15–50 nm. The yield strength (YS) of components IW and CP increased from 171 ± 3 to 261 ± 1 MPa and 168 ± 7 to 240 ± 17 MPa, respectively. Component IW had the highest ultimate tensile strength, about 400 ± 1 MPa. For WAAM-processed Al-Mg-Sc-Zr alloys, the contribution of the strengthening mechanism to YS was solid solution strengthening > precipitation strengthening > fine grain strengthening > dislocation strengthening.

Effect of Sc contents on the mechanical properties and damping capacity of Al–Zn–Mg–Cu–Zr–Sc alloys with different grain structures

Al–Zn–Mg–Cu–Zr–Sc alloys with different Sc contents were fabricated by casting, deformation, and T6 treatment. Deformation methods including rolling and friction-stir processing (FSP) were used to design their grain structure. A low additive amount (0.1) of Sc cannot refine the grains of the alloy with rolling and T6 treatment, and it instead coarsened the grains. The reason was the non-uniform distribution of nanosize Al3(Sc,Zr) phases that led to the occurrence of abnormal grain growth during homogenization. Meanwhile, the alloy with only 0.1Sc exhibited finer grains after FSP and T6 treatment than the alloys subjected to the same process but with higher Sc additive amount. Alloys with rolling-induced elongated grain structure exhibited better mechanical properties, and alloys with FSP-induced fine equiaxed grain structure exhibited higher high-strain and high-temperature internal friction values. These features are important performance parameters for applications in fields where vibration and noise are sensitive. The optimum additive amounts of Sc for alloys with elongated and fine equiaxed grain structures were 0.25 and 0.1, respectively.

Friction stir processing of AZ31 alloy to improve the mechanical, biodegradation, and in vitro biocompatible characteristics

The aim of the study was to improve the mechanical, biodegradation and in vitro biocompatible characteristics of as received rolled AZ31 (Mg-1%Zn-3%Al) alloy by develop the surface composite of Mg-Titania. The surface composite was developed by friction stir processing (FSP) and it’s used for prosthetic applications. The influence of tool rotation speed and number of FSP passes on grain structure, grain refinement, tensile strength, hardness, in vitro degradation rate and in vitro biocompatibility were explored. The obtained results support the capability of FSP in terms of mechanical strengthening, refinement of grain structure, dispersion of reinforcement (i.e. TiO2) and controlled degradation rate. Grain structure at stir zone, advancing side and retreating side were investigated. The large grains in the alloy were reduced to a fine equiaxial grains of average dimension of ≈ 3 μm by 3rd pass of FSP. The process was performed under two different tool rotation speed, i.e. 1550 and 2260 RPM. The addition of second phase in the matrix significantly improve the grain refinement. The results obtained from tensile test revealed the improvement in the percentage elongation without compromising the strength. The micro hardness of specimens of different parametric combination were evaluated and found ≈ 30–50% improvement in the micro hardness. The degradation rate by immersion test in simulated body fluid (SBF) for 3, 6 and 10 days were performed and found a significant reduction in degradation rate. The MTT assay for the biocompatibility performed by indirect method, the results validate the non-toxic behavior of processed specimen.

Achieving superior strength-ductility synergy in a unique multi-scale heterostructured titanium laminate fabricated by temperature-controlled rolling and subsequent annealing

Here we proposed a low-cost and high-efficiency strategy for fabricating novel multi-scale gradient-structured/heterogeneous lamella structured (GS/HLS) pure titanium laminates with superior strength-ductility synergy, by temperature-controlled rolling and subsequent annealing of as-sintered multilayered pure titanium foils with preset grain size gradient. A preliminary gradient structure with the average grain size decreasing from the center layer (alternating stacking of equiaxed fine grain bands and elongated grain bands) to the surface layer (equiaxed fine grains) is formed by optimizing the rolling process. Generalized Schmidt factor analysis based on electron backscatter diffraction indicated that there were slip-dominated and twin-dominated grain refinement mechanisms during the rolling process. In addition to the prismatic slip, two kinds of contraction twinning could be activated to achieve the grain refinement in hard domains where slip systems were hard to be activated. After subsequent annealing, equiaxed fine grains in the laminates did not significantly coarsen, while the elongated grains in the center layer had partially merged and grew up into the elongated coarse grains. Thus, the microstructure of the center layer displayed a typical heterogeneous lamella structure composed of alternately stacking of elongated coarse grains and fine grain bands, together with the grain size gradient, a unique multi-scale heterostructured titanium laminate was formed. Tensile test indicated that the as-rolled Ti laminates exhibited extremely high ultimate tensile strength (UTS) of 930 MPa with an acceptable total elongation to fracture (ETF) of 12.3 %, while GS/HLS Ti laminates after annealing at 425 °C possessed a superior combination of high UTS (823 MPa) and high ETF (22.4 %). The main reason was that the annealing treatment significantly reduced the dislocation density in the elongated coarse grains of GS/HLS Ti laminates. This meant that more geometrically necessary dislocations (GNDs) could accumulate in the plastic deformation process, resulting in the hetero-deformation induced (HDI) strengthening and an extra HDI hardening caused by the unique GS/HLS structure, therefore, an excellent strength-ductility synergy of final GS/HLS Ti laminates was achieved.

Impact of ECAP processing on dislocation density and subgrain size in a ti6al4V alloy prepared by direct energy deposition

Ti6Al4V alloy billets were additively manufactured by direct energy deposition and then equal channel angularly pressed (ECAPed) at 700 °C. The originally prepared α′ martensite microstructure was completely converted to an equiaxed fine grain structure during ECAP processing. The α′ martensite decomposed into α + β dual phase structure. The dislocation density in the deposited and ECAPed samples was measured by X-ray profile analysis in samples of two orientations. The dislocation density estimated in the deposition plane was found to be higher than that estimated perpendicularly. This difference is probably a consequence of the internal stresses generated in the samples during the deposition process. A similar anisotropy in dislocation density and crystallite size was found in the ECAPed samples. Lattice constants were measured for the α′ martensite and α phase of the ECAPed alloy. The crystallite size distribution was found to be moderately different for samples cut parallel and perpendicular to the extrusion direction.

Tailoring the mechanical properties and thermal stability of additive manufactured micro-alloyed Al-Cu alloy via multi-stage heat treatment

In this study, a novel Sc/Zr/Ti/Er-modified 2219 aluminum alloy was designed and wall-shaped components were then fabricated by wire-arc directed energy deposition (DED). Due to the heterogeneous nucleation promoted by primary Al3X dispersoids, the as-deposited alloy exhibits a highly equiaxed fine grain structure. Two heat treatment schedules were conducted to tailor the strength and thermal stability of the alloy. Compared with the classical T6 heat treatment, the additional homogenization step before the T6 heat treatment (HT6) prefabricated more Al3X dispersoids, which indirectly promoted the transition from θ″ precipitates to θ′ precipitates, resulting in a slight reduction in strength. However, this homogenization step helps to avoid the aggregation of pores by promoting pores ripening and significantly alleviates the anisotropy of ductility. In addition, the better thermal stability of the HT6 heat treated alloy is due to more Al3X dispersoids and finer θ′ precipitates with high coarsening resistance caused by the segregation of Sc and Mn at the θ′/Al interface during thermal exposure. In general, the wire-arc DEDed micro-alloyed 2219 Al alloy exhibits a promising combination of strength and ductility after heat treatment and its mechanical properties are superior to those of the wrought micro-alloyed Al-Cu alloys.

Complete fine-equiaxed β-columnar grains in laser direct energy deposition of Ti–6Al–4V parts

Coarse β-columnar grains are often present in Ti–6Al–4V laser-additive-manufactured samples, resulting in anisotropic and inferior fatigue performance under dynamic loading. Fine-equiaxed grains (FEGs) with a mean grain size of approximately 139 μm were successfully obtained by a novel deposition strategy through coordinated regulation of direct energy deposition technological parameters. Microstructures were transformed from α′ martensite to α'' martensite. As a result, the comprehensive performance was significantly improved. Without the use of an argon atmosphere glovebox and post heat treatment, the tensile strength and elongation in the horizontal direction of the FEGs were 1091 MPa and 4.09%, respectively. The corrosion resistance was 11.71-fold higher than that of the coarse columnar grains (CCGs). Improving the microstructure and performance by optimizing pulsed laser parameters is simple, effective, and conducive to online regulation of the quality of laser additive samples.

Designing ultrastrong and thermally stable FeCrAl alloys with the fine-grained structure

Designing microstructurally stable FeCrAl alloys with excellent strength-ductility synergy is highly desirable for their engineering applications. However, due to the preference nucleation of precipitates at grain boundaries (GBs), the improved precipitation strengthening of these alloys is usually accompanied by intergranular embrittlement. Here, we propose a novel thermomechanical processing route coupled with the Si alloying strategy via precipitation of coherent deformable Laves precipitates inside equiaxed fine-grains to achieve FeCrAl alloys with ultrahigh yield strength over ∼992 MPa, excellent uniform elongation of ∼7.6% at room temperature, and superior thermal stability at temperature ∼1200 °C. The Si alloying not only decreases the stacking fault energy of Laves precipitates favorable for their stacking-fault-mediated deformation but also hinders grain coarsening at 1200 °C due to the Si-enrichment favorable for GB pinning effects. Our results prove the possibility of achieving the collaborative enhancement of mutually exclusive properties in alloys, such as strength-ductility-thermal stability via nanoprecipitation engineering, and offer a promising route to prepare dispersion-strengthened materials.

Effect of laser parameters on fatigue crack growth behavior of AZ31B magnesium alloy TIG welded bionic joint

The purpose of this study was to evaluate the effect of laser parameters on the fatigue crack growth (FCG) behavior of AZ31B Magnesium alloy TIG welded bionic joints with ‘soft-hard’ alternating structures prepared by laser bionic treatment (LBT). The changes of laser parameters promoted the growth of fine equiaxed grain with random texture, and average grain size was 5.12 ± 0.51–8.19 ± 1.02 μm, which significantly increased isotropy and microhardness. The average microhardness was 66.5 ± 1.68–70.85 ± 1.41 HV0.1, which increased by more than 23.11% compared with untreated welded joints. For bionic welded joints, the heat-affected zone (HAZ) had higher FCG resistance and cycle times than weld metal (WM), and the specimens with notch in the HAZ showed obvious crack deflection. It was found that FCG behavior showed significant microstructure sensitivity by comparing the FCG rate curve and crack opening displacement. The Paris parameters had a significant linear relationship and effectively reflect the FCG resistance of the different specimens. The fatigue fracture showed that the crack growth zone was mainly dominated by typical cleavage steps and secondary cracks. The fatigue fracture region contains quasi-cleavage and ductile fracture. A series of strengthening mechanisms were induced by LBT and could significantly affect FCG behavior, which promoted crack deflection and branching to increase the toughness of the joint, while the laser remelting zone increased the strength of the joint. Therefore, the synergistic effect of strength-toughness of the joint was obtained by LBT, which provided a new idea for optimizing the bionic design and improving the fatigue property of welded joints.

The effect of heat treatment on fatigue response of laser powder bed fused Al-Cu-Mg-Ag-TiB2 (A20X) alloy

The paper aims at assessing the effect of heat treatment on the fatigue behavior of a novel laser-powder bed fused (L-PBF) Al-Cu-Mg-Ag-TiB2 composite, otherwise known as A20X alloy. Heat treatments, (i) stress relieving, and (ii) T7 over-aging and stabilizing, were performed on L-PBF A20X materials, followed by advanced microstructural characterizations, mechanical property measurements (utilizing tensile and depth-sensing indentation testing), and force-controlled fatigue testing. Microstructural assessments revealed the ultra-fine and fine equiaxed grain structure of the L-PBF stress-relieved and aged A20X materials. Mechanical properties (yield stress, ultimate strength, and nano-hardness quantities) of the aged material were recorded as higher than the stress-relieved counter materials. By the same token, the aged material showed better low-cycle and high-cycle fatigue performance and higher fatigue strength relative to the stress-relieved conditions. Based on detailed fractography assessments, the life-limiting fatigue failures of the studied materials (stress relived and T7 aged) were primarily governed by sub-surface L-PBF-induced volumetric defects such as pores and/or lack of fusion. The findings of this study present useful information on the role/mechanism of inevitable process-induced interior defects on fatigue behavior of the Al-Cu-Mg-Ag-TiB2 composites and the effect of post-processing (i.e., aging in this research) on fatigue behavior, which is of great significance for the high-demanding and critical applications of lightweight high-strength L-PBF Al-Cu-Mg-Ag-TiB2 composites in the space and aviation sectors.