Overcoming the limitation of in-situ microstructural control in laser additive manufactured Ti-6Al-4V alloy to enhanced mechanical performance by integration of synchronous induction heating

Although a variety of processing routes were developed to in-situ manipulate microstructure for fabricating high-performance Ti-6Al-4 V alloy by directed energy deposition(DED),the in-situ microstructural control ability has been limited and lead to a narrowed mechanical property control range.This work proved the microstructural correlation betweenβ-grains andα-laths resulting from the unique thermal characteristics of DED for the first time and solved such a dilemma through synchronous induction heating assisted laser deposition(SILD)technology.The results confirmed that the laser energy and inductive energy have a different effect on the solidification and solid phase transformation conditions.By adjusting the laser-induction parameters,the microstructural correlation can be tuned;theβ-grains andα-laths can be controlled relatively separately,thereby significantly enhancing the ductility of as-deposited sample(elongation from 14.2%to 20.1%).Furthermore,the mechanical properties of the tuned microstructures are even comparable to that of DED Ti-6Al-4 V with post heat treatment,which indicates that the potential of SILD to be a one-step manufacturing process to fabricate high performance components without post heat treatment.Furthermore,the tensile testing results of the tuned microstructures indicate thatα-lath size is more influential on the mechanical properties than theβ-grain size due to its stronger hindering effect on the slipping of dislocations.This work promotes the understanding of the microstructural formation mechanism in DED titanium alloy and proves that the combination of synchronous induction and laser can expand the ability to control the microstructure and properties of multi-layer deposition.

Precise control of bainite micro-morphology and the effects on the mechanical properties of ultrahigh-strength complex-phase sheet steel

Bainite, the main microstructure of ultrahigh-strength complex-phase(CP) sheet steel, usually exhibits various micro-morphologies when subjected to different austempering treatments.In the current study, conventional austempering treatment at the bainite nose temperature resulted in two bainite types with distinct micro-morphologies: polygonal blocky bainite and acicular bainite, which resulted in large fluctuations in the mechanical properties of CP steel, particularly yield strength and hole expansion ratio.Therefore, the precise control of bainite micro-morphology was studied and applied to separate the two bainite types through austempering optimization.The two bainite types of different micro-morphologies had different effects on the mechanical properties of CP steel: the acicular bainite favored hole expansion and flangeability but deteriorated ductility, while the polygonal blocky bainite favored ductility but deteriorated hole expansion and flangeability.Accordingly, two types of ultrahigh-strength CP steels of different mechanical properties can be stably manufactured through the precise control of bainitic micro-morphology to satisfy the specific demands of vehicle components in terms of the mechanical properties of CP steels.

Temperature Fluctuation Synthesis/Simultaneous Densification and Microstructure Control of Titanium Silicon Carbide (Ti_3SiC_2) Ceramics

A novel temperature fluctuation synthesis/simultaneous densification process was developed for the preparation of Ti3SiC2 bulk ceramics. In this process. Si is used as an in-situ liquid forming phase and it is favorable for both the solid-liquid synthesis and the densification of Ti3SiC2 rainies. The present work demonstrated that the temperature fluctuation synthesis/simultaneous densification process is one of the most effective and simple methods for the preparation of Ti3SiC2 bulk materials providing relatively low synthesis temperature. short reaction time; and simultaneous synthesis and densification. This work also showed the capability to control the microstructure, e.g., the preferred orientation, of the bulk Ti3SiC2 materials simply by applying the hot pressing pressure at different Stages of the temperature fluctuation process. And textured Ti3SiC2 bulk materials with {002} faces of laminated Ti3SiC2 grains normal to the hot pressing axis were prepared.

Modeling of Microstructure Evolution and Mechanical Properties of Steel Plates Produced by Thermo-Mechanical Control Process and Its On-line Application

An integrated metallurgical model was developed to predict microstructure evolution and mechanical properties of low-carbon steel plates produced by TMCP. The metallurgical phenomena occurring during TMCP and mechanical properties were predicted for different process parameters. In the later passes full recrystallization becomes difficult to occur and higher residual strain remains in austenite after rolling. For the reasonable temperature and cooling schedule, yield strength of 30 mm plain carbon steel plate can reach 310 MPa. The first on-line application of prediction and control of microstructure and properties (PCMP) in the medium plate production was achieved. The predictions of the system are in good agreement with measurements.

Microstructural evolution in Al-Zn eutectoid alloy by hot-rolling

The Al-Zn eutectoid alloy has been widely known as a typical superplastic metallic material, where fine-grained microstructure is usually obtained by heat treatment. Recently, thermo-mechanical controlled process has also been reported to provide a fine-grained microstructure. In the present study, Al-Zn alloy ingots of 20 mm in thickness were homogenized and hot-rolled to a thickness of 2 mm under three processes： 1） the specimen was air-cooled after homogenization and hot-rolled; 2） the specimen was water-quenched after homogenization and hot-rolled; 3） the specimen was immediately hot-rolled after homogenization. Microstructural observation showed that, in processes l and 3, lamellar microstructure was formed after homogenization, and became fragmented to fine-grained microstructure as the hot roiling process proceeded. In process 2, fine-grained microstructure without lamellar microstructure was attained throughout the hot-rolling process. A minimum grain size of 1.6 μm was obtained in process 3. Tensile tests at room temperature showed that the elongation to failure was the largest in process 3.

Grain refinement and strength enhancement in Mg wrought alloys: A review

Low absolute strength becomes one major obstacle for the wider applications of low/no rare-earth(RE) containing Mg alloys. This review firstly demonstrates the importance of grain refinement in improving strength of Mg alloys by comprehensively comparing with other strategy, e.g., precipitation strengthening. Dynamic recrystallization(DRX) plays a crucial role in refining grain size of Mg wrought alloys.Therefore, secondly, the DRX models, grain nucleation mechanisms and the related grain refinement abilities in Mg alloys are summarized,including phase boundary, twin boundary and general boundary induced recrystallization. Thirdly, the newly developed low-RE containing Mg alloy, e.g., Mg-Ce, Mg-Nd and Mg-Sm based alloys, and the RE-free Mg alloys, e.g., Mg-Al, Mg-Zn, Mg-Sn and Mg-Ca based alloy,are reviewed, with the focus on enhancing the mechanical properties mainly via the grain refinement strategy. At the last section, the perspectives and outstanding issues concerning high-performance Mg wrought alloys are also proposed. This review is meant to promote the deep understanding on the critical role of grain refinement in Mg alloys and provide reference for the development of other high strength and low-cost Mg alloys which are fabricated by the conventional extrusion/rolling processing.

Metallurgy and Solidification Microstructure Control of Fusion-Based Additive Manufacturing Fabricated Metallic Alloys:A Review

The metal-based additive manufacturing(AM),also referred to as metal 3D printing,has drawn particular interest because it enables direct creation,aided by computationally-directed path design,of intricate components with site-specific compositions and geometrical requirements as well as low buy-to-fly ratios.During the last two decades,the objective of this revolutionary technology has been shifting from only“rapid prototyping”to advanced manufacturing of special high-end products or devices,which,in many aspects,outperform conventional manufacturing technologies.For fusion-based AM,significant progress has been achieved in understanding the processing window of macroscopic scales,non-equilibrium metallurgy of mesoscale scales,and grain evolution of microscopic scales.Although the versatile capacity of AM facilitates new avenues for discovering advanced materials and structures,their potential has still not been fully explored.Given the unique non-equilibrium solidification during the AM process,coarse columnar grains with strong textures are usually developed along the build direction,which downgrades the mechanical performance.To push the limits of this digital manufacturing,this review attempts to provide in-depth and comprehensive overviews of the recent progress in understanding the evolution and control of the as-built microstructure that has been made recently and the challenges encountered during the AM process.

Revitalizing sodium-ion batteries via controllable microstructures and advanced electrolytes for hard carbon

Sodium-ion batteries(SIBs)with low cost and high safety are considered as an electrochemical energy storage technology suitable for large-scale energy storage.Hard carbon,which is inexpensive and has both high capacity and low sodium storage potential,is regarded as the most promising anode for commercial SIBs.However,the commercialization of hard carbon still faces technical issues of low initial Coulombic efficiency,poor rate performance,and insufficient cycling stability,due to the intrinsically irregular microstructure of hard carbon.To address these challenges,the rational design of the hard carbon microstructure is crucial for achieving high-performance SIBs,via gaining an in-depth understanding of its structure-performance correlations.In this context,our review firstly describes the sodium storage mechanism from the perspective of the hard carbon microstructure's formation.We then summarize the state-of-art development of hard carbon,providing a critical overview of emergence of hard carbon in terms of precursor selection,microstructure design,and electrolyte regulation to optimize strategies for addressing practical problems.Finally,we highlight directions for the future development of hard carbon to achieve the commercialization of high-performance SIBs.We believe this review will serve as basic guidance for the rational design of hard carbon and stimulate more exciting research into other types of energy storage devices.

Recrystallization model for hot-rolling of 5182 aluminum alloy

A recrystallization model for hot rolling of 5182 aluminum alloy was presented by means of the fractional softening during double interval deformation. It is found that the recrystallization rate depends on strain rate more sensitively than deformation temperature, and the time for full recrystallization is very short as strain rate is greater than 1?s -1 . Using the recrystallization—time—temperature curves, the desirable hot rolled microstructure can be obtained by controlling the rolling speed, temperature and cooling rate before cooling during the last pass in reversing mill.[

Effect of growth rate on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy

The effect of growth rates (V=2-50 μm·s-1) on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B (at.%) alloy at a constant temperature gradient (G=18 K·mm-1) was investigated. Results indicated that β phase was the primary phase of the directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy. As the growth rate increases, the solid/liquid interface turns from cellular growth to dendric growth. The interlamellar spacing (λs) decreases with the increase of growth rate according to the relationship of λs=3.39V -0.31. The solidification segregation occurs due to the enrichment of β-stabilizing element Nb, Cr in primary β phase during solidification;moreover, the degree of the segregation increases with the growth rate, resulting in the emergence of B2 phase in lamellar colonies at high growth rates. The microhardness (Hv) grows with the growth rate based on the equation of HV=328.69V 0.072, which mainly attributes to the microstructure refinement.

Microstructure and mechanical properties of Al_3Ti-based alloys in Al-Ti-Mo-X quaternary systems

A part of Al-Ti-Mo-Cr quaternary phase diagram is constructed for themicrostructure control of D0_(22)-Al_3Ti or its derivative, L1_2-(Al,Cr)_3Ti, -based alloys. It wasfound that quaternary bcc phase equilibrates with either D0_(22)-Al_3Ti or L1_2-(Al,Cr)_3Ti, orboth, exist in large compositional areas. The mechanical properties is strongly affected byprecipitates appearing, and presumably alloy microstructures.

Selective sintering of magnesia-calcia materials by utilizing hot spots during induction sintering process

Magnesia-calcia refractories are widely used in the production process of clean steel due to their excellent high-tem-perature stability,slag resistance and ability to purify molten steel.However,there are still problems such as difficult sintering and easy hydration.Magnesia-calcia materials with various calcium oxide contents were prepared by using induction sintering,and the sintering property combined with the hydration resistance of the materials was investigated.The experimental results showed that the magnesia-calcia materials prepared under induction field had higher density,microhardness and hydration resistance.In particular,the relative density of induction sintered magnesia-calcia materials with 50 mo1%CaO was greater than 98%,and the average grain size of CaO was 4.56μm,which was much larger than that of traditional sintered materials.In order to clarify the densification and microstructure evolution mechanism of the magnesia-calcia materials,the changes in temperature and magnetic field throughout the sintering process were analyzed by using finite element simulation.The results showed that the larger heating rate and higher sintering temperature under the induction sintering mode were beneficial to the rapid densification.In addition,the hot spots generated within the material due to the difference in high-temperature electric conductivity between MgO and CaO were the critical factor to realize selective sintering in MgO-CaO system,which provides a novel pathway to solve the problem of difficult sintering and control the microstructure of high-temperature composite material used in the field of high-purity steel smelting.

Si-Assisted Solidification Path and Microstructure Control of 7075 Aluminum Alloy with Improved Mechanical Properties by Selective Laser Melting

The construction and application of traditional high-strength 7075 aluminum alloy(Al7075) through selective laser melting(SLM) are currently restricted by the serious hot cracking phenomenon. To address this critical issue, in this study, Si is employed to assist the SLM printing of high-strength Al7075. The laser energy density during SLM is optimized, and the eff ects of Si element on solidification path, relative density, microstructure and mechanical properties of Al7075 alloy are studied systematically. With the modified solidification path, laser energy density, and the dense microstructure with refined grain size and semi-continuous precipitates network at grain boundaries, which consists of fine Si, β-MgSi, Q-phase and θ-AlCu, the hot cracking phenomenon and mechanical properties are eff ectively improved. As a result, the tensile strength of the SLM-processed Si-modified Al7075 can reach 486 ± 3 MPa, with a high relative density of ~ 99.4%, a yield strength of 291 ± 8 MPa, fracture elongation of(6.4 ± 0.4)% and hardness of 162 ± 2(HV) at the laser energy density of 112.5 J/mm~3. The main strengthening mechanism with Si modification is demonstrated to be the synergetic enhancement of grain refinement, solution strengthening, load transfer, and dislocation strengthening. This work will inspire more new design of high-strength alloys through SLM.

Synthesis and characterization of La Ni_xCo_(1–x)O_3: Role of microstructure on magnetic properties

Perovskite-type structures with the composition La Ni x Co1–x O3（x=0.3, 0.5, 0.7） were synthesized by a modified sol-gel method. Using transitional metal elements on the lanthanum base perovskites, properties could be tuned by doping the structure. Thermogravimetric analysis（TGA） evidenced a temperature of 350 °C as the start point of the perovskite-phase formation. Scanning electron microscopy（SEM） images showed the microstructure changes（grain size） of the cobalt-doped perovskite due to composition. In addition, it was shown that magnetic properties of the samples were dependent of cobalt content; experimental results pointed to the existence of disordered spins interactions, which were more evident with the decrease of cobalt content and the existence of ferromagnetic coupling among spins of the samples. These results showed the feasibility of producing a family of compounds with the desired properties, manipulating composition and therefore the microstructure.

Recent developments in plastic forming technology of titanium alloys

Titanium alloys have been used extensively in industry fields including aviation,aerospace and automobile due to their excellent comprehensive properties.Research and development of advanced plastic forming technology are of great importance to manufacturing titanium products of high performance and lightweight with low cost and short cycle.This paper analyzes the development tendencies of titanium alloy forming technology.Recent achievements in precision forming,microstructure control and multi-scale simulation of titanium alloys are reviewed.The forming techniques of large-sale integral complex components are presented.

Using multiple regression analysis to predict directionally solidified TiAl mechanical property

The mechanical properties of TiAl alloy prepared by directional solidification were predicted through a machine learning algorithm model.The composition,input power,and pulling speed were designated as input variables as representative factors influencing mechanical properties,and multiple linear regression analysis was conducted by collecting data obtained from the literature.In this study,the R^(2)value of the tensile strength prediction result was 0.7159,elongation was 0.8459,nanoindentation hardness was 0.7573,and interlamellar spacing was 0.9674.As the R^(2)value of the elongation obtained through the analysis was higher than the R^(2)value of the tensile strength,it was confirmed that the elongation had a closer relationship with the input variables(composition,input power,pulling speed)than the tensile strength.By adding the elongation to the tensile strength as an input variable,it was observed that the R^(2)value was further increased.The tensile test prediction results were divided into four groups:The group with the lowest residual value(predicted value-actual value)was designated as group A,and the group with the largest residual value was designated as group D.When comparing the values of group A and group D,more overpredictions occurred in group A,while more under predictions occurred in group D.Using the residuals and R^(2)values,the cause of the well-prediction was studied,and through this,the relationship between the mechanical properties and the microstructure was quantitatively investigated.

Effect of Deformation Temperature on Deformation Mechanism of Fe-6.5Si Alloys with Different Initial Microstructures

Deformation behaviors and mechanisms under different temperatures for columnar-grained Fe 6.5Si （mass%） alloys fabricated by directional solidification and equiaxed grained Fe-6.5Si alloy fabricated by forging were comparatively investigated. The results showed that, with increasing the deformation temperature from 300℃ to 500℃, the elongation increased from 2.9% to 30.1% for the equiaxed-grained Fe-6.5Si alloy, while from 6.6% to about 51% for the columnar-grained Fe-6.5Si alloy. The deformation mode of equiaxed-grained Fe 6.5Si alloy trans ferred from nearly negligible plastic deformation to large plastic deformation dominated by dislocation slipping. Comparatively, the deformation mode of the columnar grained alloy transferred from nearly negligible plastic deformation to plastic deformation dominated by the twining, and finally to plastic deformation dominated by dislocation slipping. Meanwhile, compared with the alloy with equiaxed grains, it was found that ultimate tensile strength and elongation could be increased simultaneously, which was ascribed for the twinning deformation in columnar-grained Fe-6.5Si al loy. This work would assist us to further understand the plastic deformation mechanism of Fe-6.5Si alloy and pro vide more clues for high-efficiency production of the alloy.