Laser powder bed fusion additive manufacturing of NiTi shape memory alloys: a review

NiTi alloys have drawn significant attentions in biomedical and aerospace fields due to their unique shape memory effect(SME),superelasticity(SE),damping characteristics,high corrosion resistance,and good biocompatibility.Because of the unsatisfying processabilities and manufacturing requirements of complex NiTi components,additive manufacturing technology,especially laser powder bed fusion(LPBF),is appropriate for fabricating NiTi products.This paper comprehensively summarizes recent research on the NiTi alloys fabricated by LPBF,including printability,microstructural characteristics,phase transformation behaviors,lattice structures,and applications.Process parameters and microstructural features mainly influence the printability of LPBF-processed NiTi alloys.The phase transformation behaviors between austenite and martensite phases,phase transformation temperatures,and an overview of the influencing factors are summarized in this paper.This paper provides a comprehensive review of the mechanical properties with unique strain-stress responses,which comprise tensile mechanical properties,thermomechanical properties(e.g.critical stress to induce martensitic transformation,thermo-recoverable strain,and SE strain),damping properties and hardness.Moreover,several common structures(e.g.a negative Poisson’s ratio structure and a diamond-like structure)are considered,and the corresponding studies are summarized.It illustrates the various fields of application,including biological scaffolds,shock absorbers,and driving devices.In the end,the paper concludes with the main achievements from the recent studies and puts forward the limitations and development tendencies in the future.

Work Function Optimization Technology of Indium Tin Oxide Films

Indium tin oxide(In_(2)O_(3)∶Sn)film is one of the most potential materials in the field of semiconductor industry.However,untreated In2O3∶Sn film has a low work function which can result in a high energy barrier that hinders the passage of carriers through the interface,thus leading to poor overall performance of directly prepared devices.In this study,crystalline transparent conductive In_(2)O_(3)∶Sn films were prepared by plasma exposure assisted magnetron sputtering under room temperature.Based on multiple testing methods,it can be found that the low temperature crystallization characteristics of In_(2)O_(3)∶Sn film were enhanced and the work function was effectively improved after Ar^(+)plasma exposure.The increase of the work function of In_(2)O_(3)∶Sn film was due to the increment of Sn⁃O bond on the surface brought by the transition from low oxidation state Sn^(2+)to high oxidation state Sn^(4+)under the action of high exposure.

Strengthening and toughening of additively manufactured Ti-6Al-4V alloy by hybrid deposition and synchronous micro-rolling

To overcome the disadvantages of inhomogeneous microstructures and poor mechanical properties of additively manufactured Ti-6Al-4V alloys,a novel technique of hybrid deposition and synchronous micro-rolling is proposed.The micro-rolling leads to equiaxed prior β grains,thin discontinuous intergranular α,and equiaxed primary α,in contrast to the coarse columnar prior β grains without the application of micro-rolling.The recrystallization by micro-rolling results in discontinuous intergranular α via the mechanism of strain and interface-induced grain boundary migration.The evolution of α globularization,driven by a solute concentration gradient,starts from the sub-boundary until the formation of equiaxed primary α.Simultaneous strengthening and toughening are achieved,which means an increase in yield strength,ultimate tensile strength,fracture elongation,and work hardening rate.The formation of α recrystallization leads to more fine grain boundaries to strengthen the yield strength,and the improvement of ductility is due to the better-coordinated deformation ability of discontinuous intergranular α and equiaxed primary α.As a result,the fracture mode in micro-rolling changes from intergranular type to transgranular type.

Mechanical properties of TiAl fabricated by electron beam melting-A review

As a typical intermetallic material,TiAl is inevitably difficult to process by conventional methods.Additive manufacturing(AM)has recently become a new option for making net-shape TiAl components.Among all AM methods,electron beam melting(EBM)shows the potential to make TiAl components with good mechanical properties and is used for low pressure turbine blades.The mechanical properties,including tensile and compression properties,fracture toughness,fatigue and creep properties of EBM TiAl are reviewed and compared to the conventionally fabricated alloys.Results show that the tensile strength of EBM alloys is higher than cast alloys,and other properties are comparable to the cast/forged alloys.The sensitivity of mechanical properties and microstructure to EBM processing parameters is presented.Issues including layered microstructure,anisotropy in mechanical properties,and fatigue failure from defects are also reviewed.Finally,some opportunities and challenges of EBM TiAl are identified.

Springback prediction of TC4 titanium alloy V-bending under hot stamping condition

In this paper,the springback of TC4 titanium alloy under hot stamping condition was studied by means of experiment and numerical analysis.Firstly,an analytical model was established to predict the V-shaped springback angleΔαunder the stretch-bending conditions.The model took into account of blank holder force,friction,property of the material,thickness of the sheet and the neutral layer shift.Then,the influence of several process parameters on springback was studied by experiment and finite element simulation using a V-shaped stamping tool.In the hot stamping tests,the titanium alloy sheet fractured seriously at room temperature.The titanium alloy has good formability when the initial temperature of the sheet is 750–900°C.However,the springback angle of formed parts is large and decreases with increasing temperature.The springback angleΔαdecreased by 50%from 0.5°to 0.25°,and the angleΔβdecreased by 46.7%from 1.5°to 0.8°when the initial temperature of sheet increased from 750°C to 900°C.The springback angle of titanium alloy sheet increases gradually with the increase of the punch radius,because of the increase of elastic recovery,the complex distribution of stress,the length of forming region and the decreasing degree of stress.Compared with the simulation results,the analytical model can better predict the springback angleΔα.

Effect of precipitate evolution on corrosion behavior of friction stir welded joints of AA2060-T8 alloy

Friction stir welding was used to join two AA2060-T8 plates,and then the effect of precipitate evolution on microstructure and corrosion behavior of the joint was investigated.The evolution of precipitates on the top surface of the joint was characterized by scanning electron microscopy and transmission electron microscopy.The corrosion behaviors of different regions in the joint were investigated by an electrochemistry method and an alternating salt spray exposure.The corrosion was mainly dependent on the nature of precipitates in each region of the joint.The shoulder affected zone had the worst corrosion resistance as a result of the re-dissolved ofθ′(Al2Cu),T1(Al2CuLi)andδ′(Al3Li)phases,the formation of intergranular precipitates and precipitate-free zones.However,the thermomechanically affected zone had a slightly improved corrosion resistance because it had no intergranular precipitates.The heat affected zone and base metal had the best corrosion resistance.

Effect of connection processes on mechanical properties of 7B04 aluminum alloy structures

The static and fatigue properties of 7B04 aluminum alloy structures connected by riveting and refill friction stir spot welding(refill FSSW)were compared and analyzed.Results show that the static compression load of the typical structure connected by riveting and refill FSSW fluctuated in the range of 117-124 kN,and the shear load was in the range of 89-95 kN.Welds spacing had a small influence on the static load of the structures joined by refill FSSW.However,the fatigue life of riveted structures was lower than that joined by refill FSSW.For the welded structure,the heterogeneous microstructures of the welded joint led to the uneven microhardness,and the hook at the lap interface bent upwards at the same time.These factors made the welded structures during the fatigue test failure along the path of sleeve moving.

Bio-Inspired Microwave Modulator for High-Temperature Electromagnetic Protection,Infrared Stealth and Operating Temperature Monitoring

High-temperature electromagnetic(EM) protection materials integrated of multiple EM protection mechanisms and functions are regarded as desirable candidates for solving EM interference over a wide temperature range.In this work,a novel microwave modulator is fabricated by introducing carbonyl iron particles(CIP)/resin into channels of carbonized wood(C-wood).Innovatively,the spaced arrangement of two microwave absorbents not only achieves a synergistic enhancement of magnetic and dielectric losses,but also breaks the translational invariance of EM characteristics in the horizontal direction to obtain multiple phase discontinuities in the frequency range of 8.2-18.0 GHz achieving modulation of reflected wave radiation direction.Accordingly,CIP/C-wood microwave modulator demonstrates the maximum effective bandwidth of 5.2 GHz and the maximum EM protection efficiency over 97% with a thickness of only 1.5 mm in the temperature range 298-673 K.Besides,CIP/C-wood microwave modulator shows stable and low thermal conductivities,as well as monotonic electrical conductivity-temperature characteristics,therefore it can also achieve thermal infrared stealth and working temperature monitoring in wide temperature ranges.This work provides an inspiration for the design of high-temperature EM protection materials with multiple EM protection mechanisms and functions.

Evolution laws of microstructures and mechanical properties during heat treatments for near-αhigh-temperature titanium alloys

Evolution laws of microstructures,mechanical properties,and fractographs after different solution temperatures were investigated through various analysis methods.With the increasing solution temperatures,contents of the primaryαphase decreased,and contents of transformedβstructures increased.Lamellarαgrains dominated the characteristics of transformedβstructures,and widths of secondaryαlamellas increased monotonously.For as-forged alloy,large silicides with equiaxed and rod-like morphologies,and nano-scale silicides were found.Silicides with large sizes might be(Ti,Zr,Nb)_(5)Si_(3) and(Ti,Zr,Nb)_(6)Si_(3).Rod-like silicides with small sizes precipitated in retainedβphase,exhibiting near 45°angles withα/βboundaries.Retainedβphases in as-heat treated alloys were incontinuous.980STA exhibited an excellent combination of room temperature(RT)and 650°C mechanical properties.Characteristics of fracture surfaces largely depended on the evolutions of microstructures.Meanwhile,silicides promoted the formation of mico-voids.

Structure and mechanical property of TC17 linear friction welding joint

TC17 titanium alloy which usually used on the fan and compressor disk were selected.Microstructure and mechanical property of joint were investigated.Microstructure character of different part of joint was analyzed also.Results showed that the joints included three zones,base metal(BM),thermal mechanical affected zone(TMAZ)and weld zone(WZ).The microstructure of joint middle part was equiaxial grains,but the bottom part was the elongated grains.Dynamic recrystallation happened at the weld zone.Tensile test result showed that joint tensile strength equal to that of the TC17 base metal.The average hardness value of the HAZ was 486 HV which was higher than that in the BM and WZ.

An Error Area Determination Approach in Machining of Aero-engine Blade

As a result of the recently increasing demands on high-performance aero-engine,the machining accuracy of blade profile is becoming more stringent. However,in the current profile,precision milling,grinding or near-netshape technology has to undergo a tedious iterative error compensation. Thus,if the profile error area and boundary can be determined automatically and quickly,it will help to improve the efficiency of subsequent re-machining correction process. To this end,an error boundary intersection approach is presented aiming at the error area determination of complex profile,including the phaseⅠof cross sectional non-rigid registration based on the minimum error area and the phaseⅡof boundary identification based on triangular meshes intersection. Some practical cases are given to demonstrate the effectiveness and superiority of the proposed approach.

Collaborative manufacturing technologies of structure shape and surface integrity for complex thin-walled components of aero-engine:Status,challenge and tendency

Presently,the service performance of new-generation high-tech equipment is directly affected by the manufacturing quality of complex thin-walled components.A high-efficiency and quality manufacturing of these complex thin-walled components creates a bottleneck that needs to be solved urgently in machinery manufacturing.To address this problem,the collaborative manufacturing of structure shape and surface integrity has emerged as a new process that can shorten processing cycles,improve machining qualities,and reduce costs.This paper summarises the research status on the material removal mechanism,precision control of structure shape,machined surface integrity control and intelligent process control technology of complex thin-walled components.Numerous solutions and technical approaches are then put forward to solve the critical problems in the high-performance manufacturing of complex thin-wall components.The development status,challenge and tendency of collaborative manufacturing technologies in the high-efficiency and quality manufacturing of complex thin-wall components is also discussed.

A radar-infrared compatible broadband absorbing surface:Design and analysis

A radar-infrared compatible stealth surface is designed and analyzed.Without modifying the radar absorbing material(RAM),the design can theoretically achieve radar-infrared compatibility and broadband radar absorption through surface patterns and structures.A transmission-line-based model(TLM)is developed to analyze the radar absorbing performance of the surface.Optimization of the structure geometries is conducted aiming to maximize the-10 d B absorption bandwidth in 2–18 GHz.Surface with optimized structure geometries exhibits a superior absorption bandwidth,more than twice the bandwidth of the original 1.5 mm RAM slab,while maintaining a relatively low infrared emissivity.

A New Spot Welding Technique for Fuselage

High strength aluminum alloys(series 2XXX and 7XXX)are kinds of precipitation-hardening alloys,which have been widely applied in manufacturing of fuselage for their high strength-to-weight ratio,good fatigue performance and damage tolerance[1-2].These structures mostly adopt lap joint.The traditional joining methods include riveting and bolting,which cost weight-increase and inefficiency.These disadvantages could be overcome by a new joining technique—friction stir spot welding(FSSW)developed from friction stir welding(FSW).However,after wel-ding,FSSW normally leaves the workpiece with a keyhole,which decreases mechanical properties and corrosion resistance of the joint.To fill the keyhole,refill friction stir spot welding(RFSSW,Refill-FSSW)was invented by GKSS in 1999.GKSS designs a special welding tool for RFSSW,consisting of pin,sleeve and clamping ring,which are precisely assembled with indepen-dent driving systems[3].In a typical RFSSW process,the pin and sleeve move in opposite direction at a given speed ratio.Approximately,the speeds are set inverse proportional to their cross-sectional area.In 2011,GKSS performed RFSSW on aluminum alloy and obtained the joint with good strength.Thereafter,RFSSW was highlighted in research,which was expected to apply in fuselage manufacturing and to replace the traditional joining methods.

Micro Hierarchical Structure and Mechanical Property of Sparrow Hawk (Accipiter nisus) Feather Shaf

In this study,the real 3D model of the feather shaft that is composed of medulla and cortex is characterized by X-ray computer tomography,and the structural features are quantitatively analyzed.Compression and tensile tests are conducted to evaluate the mechanical performance of the feather shaft and cortex at different regions.The analysis of the 3D model shows that the medulla accounts for∼70%of the shaft volume and exhibits a closed-cell foam-like structure,with a porosity of 59%.The cells in the medulla show dodecahedron and decahedron morphology and have an equivalent diameter of∼30μm.In axial compression,the presence of medulla enhances the shaft stability.Especially,the combined effect of the medulla and cortex increases the buckling strength of the middle and distal shaft by 77%and 141%,respectively,compared to the calculated value of the shaft using linear mixed rule.The tensile properties of the cortex along the shaft axis are anisotropic because of the different fiber structures.As the fiber orientation gradually becomes uniform in the axial direction,the Young’s modulus and tensile strength of the cortex on the dorsal gradually increase from calamus to the distal shaft,and the fracture mode changes from tortuous fracture to V-shaped fracture.The cortex on the lateral shows the opposite trend,that is the distal shaft becomes weaker due to fiber tangles.

Motion-Planning Algorithm for a Hyper-Redundant Manipulator in Narrow Spaces

In this study,a hyper-redundant manipulator was designed for detection and searching in narrow spaces for aerospace and earthquake rescue applications.A forward kinematics equation for the hyper-redundant manipulator was derived using the homogeneous coordinate transformation method.Based on the modal function backbone curve method and the known path,an improved modal method for the backbone curves was proposed.First,the configuration of the backbone curve for the hyper-redundant manipulator was divided into two parts:a mode function curve segment of the mode function and a known path segment.By changing the discrete points along the known path,the backbone curve for the manipulator when it reached a specified path point was dynamically obtained,and then the joint positions of the manipulator were fitted to the main curve by dichotomy.Combined with engineering examples,simulation experiments were performed using the new algorithm to extract mathematical models for external narrow space environments.The experimental results showed that when using the new algorithm,the hyper-redundant manipulator could complete the tasks of passing through curved pipes and moving into narrow workspaces.The effectiveness of the algorithm was also proven by these experiments.

Multi-scale analyses of phase transformation mechanisms and hardness in linear friction welded Ti17(a+β)/Ti17(β)dissimilar titanium alloy joint

The Ti17(a+β)-Ti17(β)dual alloy-dual property blisk produced using Linear Friction Welding(LFW)is considered as high-performance component in advanced aeroengine.However,up to now,microstructure evolution and relationship between microstructure and micro mechanical properties of LFWed Ti17(a+β)/Ti17(β)dissimilar joint have not been thoroughly revealed.In this work,complex analyses of the phase transformation mechanisms of the joint are conducted,and phase transformations in individual zones are correlated to their microhardness and nanohardness.Results reveal that a dissolution occurs under high temperatures encountered during LFW,which reduces microhardness of the joint to that of Ti17(a+β)and Ti17(β).In ThermoMechanically Affected Zone of Ti17(a+β)(TMAZ-(a+β))side joint,a large number of nanocrystalline a phases form with different orientations.This microstructure strengthens significantly by fine grains which balances partial softening effect of a dissolution,and increases nanohardness of a phase and microhardness of TMAZ-(a+β).Superlattice metastableβphase precipitates from metastableβin Weld Zone(WZ)during quick cooling following welding,because of short-range diffusion migration of solute atoms,especiallyβstabilizing elements Mo and Cr.The precipitation of the superlattice metastableβphase results in precipitation strengthening,which in turn increases nanohardness of metastableβand microhardness in WZ.

In-situ monitoring plume,spattering behavior and revealing their relationship with melt flow in laser powder bed fusion of nickel-based superalloy

Laser powder bed fusion(LPBF)is a highly dynamic and complex physical process,and single-track de-fects tend to accumulate into non-negligible internal defects of parts.The nickel-based superalloy single track was fabricated by LPBF,and its plume and spattering behavior were monitored in situ and recorded in real time based on image recognition and tracking in this study.The relationship among laser energy density,melt flow,plume and spattering behavior during LPBF was discussed.Volumetric energy density had limitations as a design parameter for LPBF.However,we found that plume and spattering behavior can be used as real-time design parameters for the processing of LPBF parts and implemented the initial velocity statistics for LPBF single-track spattering based on the centroid extraction algorithm.The influ-ence of melt flow evolution paths on the spattering and plume behavior in three different melting modes was revealed,and a shift in plume behavior was found in the overlap region of the additive substrate.This study provides a new method for obtaining statistics of spattering-related physical quantities in the melting mode,which is beneficial for the development of processing methods to mitigate the instability of the LPBF process.

Ablative behavior and mechanism of SiC_(f) /SiBCZr composites prepared by PIP process

SiC_(f)/SiBCZr composites were prepared by polymer precursor impregnation and pyrolysis process with near stoichiometric ratio SiC fiber preform as reinforcement phase and SiBCZr multiphase ceramic precursor as impregnating reagent.The results highlighted that the SiC_(f)/SiBCZr composites exhibited excellent ablative properties after ablative tests at 1200℃/3600 s and 1400℃/3600 s,and the strength retention rates of the composites reached 90%and 85%,respectively.This was mainly due to the liquid sealing effect of the ablative products represented by B2O_(3) and SiO_(2)∙B_(2)O_(3),which inhibited the ablative reaction by reducing the diffusion rate of the oxidation medium,and the solid pinning effect of the substances represented by SiO_(2),ZrO_(2),and ZrSiO_(4),which could play high viscosity and high strength characteristics to improve anti-erosion ability.The above-mentioned SiC_(f)/SiBCZr composites with corrosion resistance,oxidation resistance,and ablative resistance provided a solid material foundation and technical support for the development of reusable spacecraft hot-end components.

Effects of Post-Heat Treatment and Carbide Precipitates on Strength-Ductility Balance of GH3536 Superalloy Prepared by Selective Laser Melting

The strength and ductility cannot achieve a good tradeoff for some superalloy(e.g.GH3536)prepared by selective laser melting(SLM),which seriously restricts their industrial applications.This work examined the effect of post-heat treatment(HT)on the microstructure and mechanical properties of GH3536 produced by SLM.In particular,the influence of carbide precipitate morphology and distribution on strength and ductility of the alloy after heat treatment was discussed.After aging at 650°C(denoted as HT1),the Cr23C6 carbides were distributed in chains.The ductility increased by approximately 31%,while the strength slightly decreased.After aging at 745°C(denoted as HT2),the Cr23C6 carbides were distributed in chains.However,the HT2 samples showed an increase in ductility of~58%and no reduction in strength.As the dislocation density of HT2 sample was higher than that of the HT1 sample,the chain carbides could be pinned to the grain boundaries,consequently improving the ductility but no loss in strength as compared with the as-deposited samples.When the aging temperature was increased to 900°C(denoted as HT3),the carbides were distributed in a discontinuous granular form.As a result,the HT3 samples presented the lowest dislocation density which reduced the strength.