Effects of annealing on microstructure and mechanical properties of γ-TiAl alloy fabricated via laser melting deposition

The microstructure evolution and mechanical properties of the as-deposited γ-TiAl-based alloy specimen fabricated via laser melting deposition and as-annealed specimens at different temperatures were investigated.The results show that the microstructure of as-deposited specimen is composed of fineα2(Ti3Al)+γlamellae.With the increase of annealing temperature,the bulk γ m(TiAl)phase gradually changes from single γ phase toγphase+acicularα2 phase,finally small γ phase+lamellar α2+γ phase.Compared with the mechanical properties of as-depositedγ-TiAl alloy(tensile strength 469 MPa,elongation 1.1%),after annealing at 1260℃ for 30 min followed by furnace cooling(FC),the room-temperature tensile strength of the specimen is 543.4 MPa and the elongation is 3.7%,which are obviously improved.

Microstructure and mechanical property of additively manufactured NiTi alloys:A comparison between selective laser melting and directed energy deposition

NiTi shape memory alloy(SMA)with nominal composition of Ni 50.8 at%and Ti 49.2 at%was additively manufactured(AM)by selective laser melting(SLM)and laser directed energy deposition(DED)for a comparison study,with emphasis on its phase composition,microstructure,mechanical property and deformation mechanism.The results show that the yield strength and ductility obtained by SLM are 100 MPa and 8%,respectively,which are remarkably different from DED result with 700 MPa and 2%.The load path of SLM sample presents shape memory effect,corresponding to martensite phase detected by XRD;while the load path of DED presents pseudo-elasticity with austenite phase.In SLM sample,fine grain and hole provide a uniform deformation during tensile test,resulting in a better elongation.Furthermore,the nonequilibrium solidification was studied by a temperature field simulation to understand the difference of the two 3D printing methods.Both temperature gradient G and growth rate R determine the microstructure and phase in the SLM sample and DED sample,which leads to similar grain morphologies because of similar G/R.While higher G×R of SLM leads to a finer grain size in SLM sample,providing enough driving force for martensite transition and subsequently changing texture compared to DED sample.

Effects of WC-Co reinforced Ni-based alloy by laser melting deposition: Wear resistance and corrosion resistance

WC-Co reinforced C276 alloy composite coatings are fabricated on Q235 steel by laser melting deposition.The microstructure,hardness,wear performance,and electrochemical corrosion behavior of composite coating are studied.The results show that WC-Co particles are mostly uniformly distributed in the coating and provide favorable conditions for heterogeneous nucleation.The microstructure of C276/WC-Co composite coatings is composed of γ-Ni solid solution dendrites and MoNi solid solution eutectics.The WC-Co particles can effectively improve the hardness and wear resistance of C276 alloy.The average hardness of the composite coating containing 10-wt% WC-Co(447 HV_(0.2)) are 1.26 times higher than that of the C276 alloy(356 HV_(0.2)).The wear rate of composite coating containing 10-wt% WC-Co(6.95 ×10^(-3) mg/m) is just 3.5% of that of C276 coating(196.23 × 10^(-3) mg/m).However,comparing with Hastelloy C276,the corrosion resistance of C276/WC-Co composite coating decreases.

Simulation and Analysis of Electromagnetic Force in Laser Melting Deposition by Electromagnetic Impact

Magnetic field was introduced in laser melting deposition to reduce the pores in workpieces.Finite 3-D model of the coil-deposition layer-substrate was established.Simulation results show that the electromagnetic force in deposition layer mainly concentrates in the projection area of the coil.Axial electromagnetic force shows repulsion in one cycle.The experimental results indicate that the magnetic field is beneficial for grain refinement,microhardness increasement and decline of quantities and average sizes of pores.

Microstructural evolution and mechanical properties of laser melting deposited Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy

A rectangular plate of Ti-6.5A1-3.5Mo-I.5Zr-0.3Si titanium alloy was fabricated by laser melting deposition （LMD） technology. Macrostructure and microstructure were characterized by optical microscope （OM） and scanning electron microscope （SEM）. Room temperature tensile properties were evaluated. Results indicate that the macro-morphology is dominated by large columnar grains traversing multiple deposited layers. Two kinds of bands, named the wide bands and the narrow bands, are observed. The wide band consists of crab-like a lath and Widmanstatten a colony. The narrow band consists of a lath and transformed ft. The formation mechanism of the two bands was explored. The influence of heat effect caused by subsequent deposition layers on microstructural evolution during deposition process was discussed. The room temperature tensile test demonstrates that the strength of laser deposited Ti-6.5A1-3.5Mo-I.5Zr-0.3Si is comparable to that of wrought bars.

Low cycle fatigue behavior of laser melting deposited TC18 titanium alloy

Low cycle fatigue （LCF） behavior of laser melting deposited （LMD） TC18 titanium alloy was studied at room temperature. Microstructure consisting of fine lamella-like primary α phase and transformed β matrix was obtained by double annealed treatment, and inhomogeneous grain boundaryαphase was detected. Fatigue fracture surfaces and longitudinal sections of LCF specimens were examined by optical microscopy and scanning electron microscopy. Results indicate that more than one crack initiation site can be detected on the LCF fracture surface. The fracture morphology of the secondary crack initiation site is different from that of the primary crack initiation site. When the crack grows along the grain boundaryαphase, continuous grain boundaryαphase leads to a straight propagating manner while discontinuous grain boundaryαphase gives rise to flexural propagating mode.

High-cycle fatigue crack initiation and propagation in laser melting deposited TC18 titanium alloy

This article examines fatigue crack nucleation and propagation in laser deposited TC18 titanium alloy. The Widmanstatten structure was obtained by double-annealing treatment,. High-cycle fatigue （HCF） tests were conducted at room temperature with the stress ratio of 0.1 and the notch concentration factor Kt = 1. Fatigue cracks initiated preferentially at micropores, which had great effect on the HCF properties. The effect decreased with the decrease of pore size and the increase of distance from the pore location to the specimen surface. The crack initiation region was characterized by the cleavage facets of a lamella and the tearing of β matrix. The soft a precipitated-free zone formed along grain boundaries accelerated the crack propagation. Subsurface observation indicated that the crack preferred to propagate along the grain boundary α or border of a lamella or vertical to a lamella.

Microstructure and mechanical properties of laser melting deposited Ti_2Ni_3Si/NiTi Laves alloys

Two Ti2Ni3Si/NiTi Laves phase alloys with chemical compositions of Ni-39Ti-11Si and Ni-42Ti-8Si (%, mole fraction, the same below), respectively, were fabricated by the laser melting deposition manufacturing process, aiming at studying the effect of Ti, Si contents on microstructure and mechanical properties of the alloys. The Ni-39Ti-11Si alloy consisting of Ti2Ni3Si primary dendrites and Ti2Ni3Si/NiTi eutectic matrix is a conventional hypereutectic Laves phase alloy while the Ni-42Ti-8Si alloy being made up of NiTi primary dendrites uniformly distributed in Ti2Ni3Si/NiTi eutectic is a new hypoeutectic alloy. Mechanical properties of the alloys were investigated by nano-indentation test. The results show that the decrease of Si and the increase of Ti contents change the microstructures of the alloys from hypereutectic to hypoeutectic, which influences the mechanical properties of the alloys remarkably. Corrosion behaviors of the alloys were also evaluated by potentiodynamic anodic polarization curves.

Influence of Electroshocking Treatment on Microstructure and Mechanical Properties of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si Thin-Wall Specimen Manufactured by Laser Melting Deposition

In order to improve the properties of titanium alloys manufactured by laser melting deposition(LMD),the electroshocking treatment(EST)was proposed in this work.The effects of EST on microstructure and mechanical properties of LMD Ti-6.5Al-3.5Mo-1.5Zr-0.3Si were investigated.The results showed that the width of the heat affected band decreased and disappeared under the thermal and athermal effects of EST,resulting in the uniform microstructure.In the microstructure,theαlaths became coarser gradually,and the quantity ofα/βinterface was reduced.The reduction of the quantity ofα/βinterface leads to make less resistant to dislocation,resulting in the reduction in hardness and strength.The discontinuous grain boundaryαphase and nucleationαcolony near grain boundary inhibited the crack propagation and improved the ductility.Summary,EST can manipulate the microstructure and improve the mechanical properties of LMD titanium alloys.

Microstructure evolution of laser deposited Ti60A titanium alloy during cyclic thermal exposure

Cyclic thermal exposure tests of infrared heating to 800 ℃ in 120 s followed by compressed air cooling to 150 ℃ in 60 s were performed for the laser deposited Ti60A （Ti5.54Al3.38Sn3.34Zr0.37Mo0.46Si） alloy. The effects of thermal exposure cycles on length ofβphase, area fraction ofαphase and microhardness of alloy were examined by OM, SEM and EDS. The results indicate that thermal exposure cycles have significant effects on length ofβphase, area fraction ofαphase and microhardness of the alloy. The original fine basket-weaveβand 78.5%αtransform to transient wedge-likeβ, finally leaving granularβand 97.6%coarsenedαwith the increased thermal exposure cycles. The formation mechanism of coarsenedαand broken-upβmicrostructure is discussed. The alloy after 750 thermal exposure cycles has the maximum microhardness, 33.3%higher than that of the as-deposited alloy.

Microstructure and mechanical properties of hybrid fabricated 1Cr12Ni2WMoVNb steel by laser melting deposition

Laser melting deposition was carried out to deposit a 1Cr12Ni2WMoVNb steel bar on a wrought bar of same material. Room-temperature tensile properties of the hybrid fabricated 1Cr12Ni2WMoVNb steel sample were evaluated, and microstructure, fracture surface morphology, and hardness profile were analyzed by an optical microscope （OM）, a scanning electron microscope （SEM）, and a hardness tester. Results show that the hybrid fabricated 1Cr12Ni2WMoVNb steel sample consists of laser deposited zone, wrought substrate zone, and heat affected zone （HAZ） of the wrought substrate. The laser deposited zone has coarse columnar prior austenite grains and fine well-aligned dendritic structure, while the HAZ of the wrought substrate has equiaxed prior austenite grains which are notably finer than those in the wrought substrate zone. Besides, austenitic transformation mechanism of the HAZ of the wrought substrate is different from that of the laser deposited zone during the reheating period of the laser deposition, which determines the different prior austenite grain morphologies of the two zones. Microhardness values of both the laser deposited zone and the HAZ of the wrought substrate are higher than that of the wrought substrate zone. Tensile properties of the hybrid fabricated 1Cr12Ni2WMoVNb steel sample are comparable to those of the wrought bar, and fracture occurs in the wrought substrate zone during the tensile test.

Characterization on Laser Melting Deposition of Metallic Components

Laser melting deposition process (LMD) is an advanced computer-aided laser additive manufacturing process (LAM). This process can produce net-shape components with fully dense microstructure and good properties based on a computer aided design (CAD) model by using a high power laser to melt the injected powders and re-solidify them in a layered way. It presents obvious superiority in direct manufacturing components with small-lot, repairing high-value components with good performance, and integrated fabricating new materials and composite structure. Deep understanding the melting process, alloying and reaction within the molten pool, solidification behavior, as well as formation and control of residual stress and defects during deposition are crucial to successful application.

Microstructure Characteristics and Mechanical Properties of Nb-17Si-23Ti Ternary Alloys Fabricated by In Situ Reaction Laser Melting Deposition

Fabrication of ternary Nb-17Si-23Ti alloys was attempted by in situ reaction laser melting deposition（LMD）with dual powder feeding method from Nb-28 at.% Ti powder mixture and pure Si powder.The microstructures of the as-deposited alloys were examined with scanning electronic microscope,and the phase constituents were analyzed by X-ray energy-dispersive spectrometer and X-ray diffraction.Furthermore,the effect of laser power on microstructure charac-teristics,microhardness and indentation fracture toughness was also investigated.The in situ reaction LMD process resulted in remarkable refinement of the microstructure.The as-deposited samples mainly consisted of Nbss,metastable（NbTi）3Si and Ti-rich Nbss.With the increase in the laser power from 1000 to 2000 W,the Nbss morphology changed from discontinuous dendritic to near equiaxed,but the Ti-rich Nbss phase tended to vanish.Furthermore,with the increase in the laser power,the microhardness of as-deposited samples increased from 822 to 951 HV,while the indentation fracture toughness was improved from 12.3 to 14.1 MPa mt/2.The corresponding mechanism is also discussed.

Effect of thermal cycles on microstructure of reduced activation steel fabricated using laser melting deposition

Reduced activation steel was successfully fabricated by laser melting deposition employing a Gaussian and a ring-shaped laser.The microstructure evolution of the reduced activation steel was investigated using the scanning electron microscope,transmission electron microscope and electron backscatter diffraction.The experimental results showed that the grains close to the substrate were smaller than the grains in the upper layers.Compared to those deposited using a Gaussian laser,the samples deposited using a ring-shaped laser showed a more homogeneous microstructure.Furthermore,a finite element analysis(FEA)model was applied to reveal the thermal history during laser melting deposition.The simulation results were well validated with the experimental results.FEA results indicate that the peak temperature increases and the cooling rate decreases,as the layer gets further from the substrate.Additionally,the temperature and the cooling rate resulting from the Gaussian laser model were higher at the midline of the samples and lower around the edges,whereas those of the ring-shaped laser model were consistent with both at the center and around the edges.

Effect of laser power on porosity and mechanical properties of GH4169 fabricated by laser melting deposition

The porosity and mechanical properties of GH4169(a precipitation strengthened nickel-base superalloy)specimens fabricated using the laser melting deposition technique at different laser powers were investigated.The results showed that the dendritic structure with the Laves phase and carbides embedded in the Ni-γ matrix formed in the as-fabricated GH4169 due to the strong temperature gradient and the high cooling rates.Porosity remarkably decreased first and slightly increased subsequently as the laser power increased from 300 to 800 W.The lowest porosity of the specimens characterized by 3D X-ray tomography is 0.28%.The specimens fabricated at 600 W tensiled along the direction perpendicular to the building direction exhibit the average yield strength of 587 MPa,the ultimate tensile strength of 903 MPa,and the elongation at fracture of 13.6%.Furthermore,the fatigue limit of the 600 W fabricated specimens is 173.7 MPa corresponding to the fatigue ratio of 0.1.And the relationship among the porosity,laser power and mechanical properties is discussed.

Characterization of In-situ Titanium Matrix Composites Prepared by Laser Melting Deposition

Preliminary characterization of microstructure and mechanical properties of (TiB+TiC)/TC4 and TiC/Ti60 in-situ titanium matrix composites prepared by laser melting deposition is reported in this paper. The results indicate that in-situ reaction occurred during laser melting deposition of coaxially fed mixed powders from TC4 and B4C with formation of form TiB and TiC reinforcement. For TiC/Ti60 composites, there are some un-melted TiC particles and re-solidified TiC particles appeared as discontinuous chain-like morphology. Reinforcements of TiB and TiC with fraction about 25 vol% were formed with feeding 5 wt% B4C. The morphology of TiB tended to be needle-like and prismatic, while TiC appeared as granular. Small amount of un-reacted B4C with reduced size remained within the composites. A thin skull of reaction product formed around the un-reacted B4C weakened its interface bonding with the titanium alloy matrix, thus resulting in less outstanding properties of the composites. Under 600 ℃, the ultimate tensile strength of the TiCP (5wt%)/Ti60 composites was 60 MPa higher than that of Ti60 alloy, following with decreased elongation.

Solid-state phase transformation of TC11 titanium during unstable thermal cycling in laser melting deposition process

Thermal cycling procedure during laser additive manufacturing (LAM) process causes the appearance of bright and dark patterns on the etched surface of TC11 alloy components. The formation mechanisms of these patterns and the solid-state transformation related to LAM process are systematically investigated with the predication of temperature fields using the finite element software ABAQUS. The results indicate that by increasing subsequent thermal cycles, the peak temperatures for every cycle decrease. When peak temperatures are above Tβ(phase transition temperature of β phase), which is 1010℃ in TC11 alloy, no pattern is observed. Meanwhile, a decrease in peak temperature leads to appearance of an ultrafine basket-weave α+β microstructure (dark contrast) with gradually increased amount of α colonies in the alloy. A special bimodal microstructure with ‘fork-like'α lamella appears in the layer when the peak temperatures of thermal cycles firstly fall into α+β dual-phase region. And this special bimodal microstructure gives a bright contrast and only appears at the region where the peak temperatures are below 970℃, leaving the rest region with a dark contrast. With the continuous increase in thermal cycles in α+β dual-phase region,α lamella gradually coarsens. After five thermal cycles in α+β two-phase region, no further changes in microstructure are observed, and the morphologies of α lamella in dark and bright regions are almost the same but with different amounts of α phase.

A comparative study on microstructure,nanomechanical and corrosion behaviors of AlCoCuFeNi high entropy alloys fabricated by selective laser melting and laser metal deposition

The present study investigated the microstructure,nanomechanics,and corrosion behavior of AlCoCuFeNi high entropy alloys fabricated by selective laser melting(SLM)and laser metal deposition(LMD).The microstructure of SLM-processed specimens was mainly composed of columnar-grained BCC matrix(^90μm in width)and Cu-rich twinned FCC phase.The columnar grains grew epitaxially along the building direction and exhibited a strong{001}texture.In comparison,a coarse columnar-grained BCC matrix(^150μm in width)with a stronger<001>texture,rod-like B2 precipitates,and large core-shell structured FCC phases were formed in the LMD-processed specimens due to the higher heat accumulation effect.Consequently,the LMD-processed specimens showed a lower hardness,wear resistance,and corrosion resistance,but higher creep resistance and reduced Young's modulus than the SLM-processed specimens.Hot cracks occurred in both types of specimens,which could not be completely suppressed due to Cu segregation.

Mechanical and microstructural characterization of additive manufactured Inconel 718 alloy by selective laser melting and laser metal deposition

The direct comparison of the microstructure and tensile properties of Inconel 718 fabricated by selective laser melting (SLM) or laser metal deposition (LMD) has been carried out. In the as-built state, LMD-fabricated specimens show lower tensile yield strength and fracture elongation than SLM-fabricated specimens due to the coarser solidification microstructure, including grains, cellular dendrites and Laves phases. This is mainly because the cooling rate of the LMD process is 2 to 3 orders lower than that of the SLM process. Upon the same heat treatment, both yield strengths of SLMand LMD-fabricated specimens are enhanced significantly. Notably, LMD-fabricated specimens exhibit simultaneous improvement in the strength and ductility, which is mainly attributed to the presence of small granular Laves phases and uniformly distributed nanoscale c00 strengthening phases. The results could serve as a guidance for selecting suitable postheat treatment routes for specific additive manufacturing process to attain excellent strength-ductility synergy.

Unsteady state precipitation of M_(23)C_(6)carbides during thermal cycling in reduced activation steel manufactured by laser melting deposition

The temperature field distribution and thermal history of Fe-9Cr2WVTa reduced activation steel prepared by laser melting deposition(LMD)have been calculated with Gaussian and Ring laser beams,and the nucleation and growth behaviors of M_(23)C_(6)precipitates in the 1st,7th and 19th layers have been calculated using the modified classical nucleation theory and Svoboda Fischer Fratzl Kozeschnik model.The energy distribution shows W-shape with Ring laser beam while it shows V-shape with Gaussian laser beam,which results in the more uniform M_(23)C_(6)size in the same layer with Ring laser beam.Precipitates in the bottom(i.e.,the 1st layer)have the minimum size and the size increases with the layer number with Gaussian and Ring laser beams.The temperature history,the instantaneous nucleation rate and the size evolution of M_(23)C_(6)have been systematically discussed.The results indicate that the nucleation,growth and re-dissolution of precipitates in reduced activation steel depend on the amount of energy absorbed in the thermal cycle during LMD.The continuous accumulation of energy during the thermal cycle leads to larger M_(23)C_(6)at the top area.The unsteady state precipitation dynamics of M_(23)C_(6)carbides during thermal cycling are consistent with the simulation results.