Showing papers on "Equiaxed crystals published in 2014"

Effect of energy input on formability, microstructure and mechanical properties of selective laser melted AZ91D magnesium alloy

Abstract: Selective laser melting (SLM) technology has been used to manufacture the AZ91D magnesium alloy. The relative density, microstructure, microhardness and tensile properties of the deposited AZ91D samples at different laser energy inputs were characterized. The results indicate that laser energy input plays a significant role in determining formation qualities of the SLMed samples. High density samples without obvious macro-defects can be obtained between 83 J/mm 3 and 167 J/mm 3 . The SLMed AZ91D presents a unique layerwise feature in which the fully divorced eutectic β-Mg 17 Al 12 distributing along the boundary of the equiaxed α-Mg matrix. The average size of α-Mg in overlapping regions is a little larger than that in the center of the scanning tracks due to the remelting process though the element distributions of Mg and Al are quite uniform. The microhardness of all samples shows directional independence. The microhardness and tensile strengths of the SLMed AZ91D at room temperature are superior to those of the die-cast AZ91D due to the combined effect of grain refinement and solid solution strengthening.

Microstructure, mechanical and wear properties of laser surface melted Ti6Al4V alloy.

Abstract: Laser surface melting (LSM) of Ti6Al4V alloy was carried out with an aim to improve properties such as microstructure and wear for implant applications. The alloy substrate was melted at 250 W and 400 W at a scan velocity of 5 mm/s, with input energy of 42 J/mm2 and 68 J/mm2, respectively. The results showed that equiaxed α+β microstructure of the substrate changes to mixture of acicular α in β matrix after LSM due to high cooling rates in the range of 2.25×10−3 K/s and 1.41×10−3 K/s during LSM. Increasing the energy input increased the thickness of remelted region from 779 to 802 µm and 1173 to 1199 µm. Similarly, as a result of slow cooling rates under present experimental conditions, the grain size of the alloy increased from 4.8 μm to 154–199 μm. However, the hardness of the Ti6Al4V alloy increased due to LSM melting and resulted in lowest in vitro wear rate of 3.38×10−4 mm3/Nm compared to untreated substrate with a wear rate of 6.82×10−4 mm3/Nm.

Ultrafine Grain Formation in a Ti-6Al-4V Alloy by Thermomechanical Processing of a Martensitic Microstructure

Abstract: In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.

Microstructure and mechanical properties of newly developed aluminum–lithium alloy 2A97 welded by fiber laser

Abstract: The newly developed aluminum–lithium alloy 2A97 was for the first time joined by laser beam welding in order to meet the ever-increased long-term requirements of aerospace, aviation and armament industries. The weld appearance, microstructure, solute segregation, precipitate behavior, and their relationships with mechanical properties of welded joints were investigated. Sound joints with no crack and a few small porosities are obtained under appropriate heat inputs. As a result of heterogeneous nucleation involving the effect of Zr and Li, a non-dendritic equiaxed zone forms between partially melted zone and fusion zone. The crystal morphologies in fusion zone vary from columnar dendrite to equiaxed dendrite, with the increase of constitutional supercooling. Solute segregation leads to the variations of Cu content in grain interior and boundary, as well as the weak ability of re-precipitation of fusion zone. Most precipitates in the base metal dissolve during welding, and fusion zone contains a decreased quantity of δ ′, β ′, θ ′, and T 1 . The ultimate tensile strength of laser welded joints is 83.4% of that of the base metal, and can meet the application requirements from related industries, but the ductility still needs to be improved. Welding defects and loss of solid solution/precipitation hardened structure lead to the degradation of mechanical properties. Tensile fracture occurs in weld with the brittle intergranular dominated mode and premature failure occurs and extends in the equiaxed zone.

Study on the hot deformation behavior of TC4-DT alloy with equiaxed α+β starting structure based on processing map

Abstract: Hot deformation behavior of TC4-DT alloy with an initial equiaxed α+β structure was investigated by conducting isothermal hot compression test in the deformation temperature range of 930–1020 °C and strain rate range of 0.001–10 s −1 with the maximum height reduction of 60%. The hot deformation behavior of TC4-DT alloy was characterized based on the analysis of the stress–strain behaviors, kinetics and processing map for obtaining optimum processing parameters and achieving desired microstructure during hot working. A constitutive equation by which the flow stress was expressed as a function of strain rate and deformation temperature was established and the apparent activation energy of deformation was calculated to be 644.642 kJ mol −1 in α+β phase region and 592.054 kJ mol −1 in β phase region. A processing map was constructed at the true strain of 0.9 based on dynamic materials model (DMM) to delineate the safe and unsafe regions in the axes of temperature and strain rate. Two regions were marked as unsafe with one in α+β phase field and the other in β phase field and both were in strain rate higher than 1 s −1 . These instability domains exhibited localized plastic flow and cracking along grain boundaries which should be taken care of during hot processing. The stable domain covered through the whole temperatures under strain rate of 1 s −1 with peaks in power dissipation of 41% and 39% which indicated superplasticity (SP) and dynamic recrystallization (DRX). Microstructure observation of the deformed specimens validated the applicability of the processing map at obtaining the optimum processing parameters of TC4-DT alloy.

Hot forging characteristic of Ti–5Al–5V–5Mo–3Cr alloy with single metastable β microstructure

Abstract: Hot deformation characteristic of Ti–5Al–5V–5Mo–3Cr alloy with a starting microstructure of an equiaxed single β microstructure is examined in relation to microstructural evolution and the result according to processing map technique. In testing at 700 °C, subgrain formation is dominant at higher strain rate, while superplasticity occurs at lower strain rate. Herein, dynamic α precipitation from supersaturated β phase occurs during deformation and affects the flow behavior. In testing at and above 800 °C, dynamic recovery (DRV) is dominant and continuous dynamic recrystallization (CDRX) also occurs especially in the vicinity of boundaries of prior-β-grains. There are three domains having an optimized power dissipation efficiency (ranging from 40% to 50%) in processing map. These three domains are reasonably explained in relation to microstructural conversion of frequent activations of grain boundary sliding, dynamic recovery and simultaneous occurrence of dynamic recovery and continuous dynamic recrystallization.

Study of crack propagation mechanisms during Charpy impact toughness tests on both equiaxed and lamellar microstructures of Ti-6Al-4V titanium alloy

Abstract: The impact toughness of two highly textured rolled plates of Ti–6Al–4V alloy with an α equiaxed and an α lamellar microstructures has been investigated. The results show a strong anisotropy of the fracture energy for both materials and underline that a coincidence of the prismatic planes with the shear bands at the notch tip is favorable for higher fracture energies. Moreover, it is pointed out, as it was already done by previous studies, that the α lamellar microstructure presents higher fracture energy than the α equiaxed one. Thanks to electron back scattering diffraction, and tensile tests, local microstructure heterogeneities, called macrozones, have been observed and characterized. Their size depends on microstructure element and is larger for α lamellar microstructure than for the α equiaxed. High strain is localized on the macrozones favorably oriented for prismatic slip with respect to the direction of impact and leads to a particular dimple free zone on the fracture surface. The contribution of these macrozones in the fracture behavior, and more precisely on the crack propagation rate was evaluated; thus the effects of the macroscopic texture and of the microstructure element on the impact toughness are discussed separately.

Microstructural evolution, creep, and tensile behavior of a Ti–22Al–25Nb (at%) orthorhombic alloy

Abstract: The microstructure, creep, and tensile deformation behavior of a Ti–22Al–25Nb (at%) alloy were studied. The ring-rolled materials were produced through conventional thermo-mechanical processing techniques comprising nonisothermal forging and rolling. Heat treatments at all temperatures above 1060 °C, followed by water quenching, resulted in fully-B2 microstructures. Above 980 °C, the equiaxed O phase was shown within the B2 grains, while lamellar O-phase precipitated below 980 °C. Tensile-creep experiments were conducted in the temperature range 600–700 °C and stress range 100–200 MPa. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress and microstructure. The tensile strength of the alloy at room temperature and 650 °C was also investigated. Microstructural effects on the tensile properties and creep behavior were also discussed and the data were compared to that for other Ti 2 AlNb-based alloys.

Microstructure and mechanical properties of Ti–22Al–25Nb alloy fabricated by vacuum hot pressing sintering

Abstract: A study has been undertaken to verify the feasibility of using a powder metallurgy (P/M) approach to fabricate Ti–22Al–25Nb alloys. Pre-alloyed powders with a nominal composition of Ti–22Al–25Nb (at%) obtained by argon atomization were sieved to the spherical size less than 180 μm and used for the fabrication of P/M Ti–22Al–25Nb alloys via hot pressing in vacuum. Vacuum hot pressing sintering was carried out in a temperature range of 950–1200 °C with a pressure of 35 MPa for 1 h followed by furnace cooling. Microstructure and phase composition examinations of the as-atomized powders and hot pressed (HP׳ed) samples were conducted by applying optical microscopy, back-scatter electron imaging and X-ray diffraction analysis. Tensile tests were studied at room temperature and 650 °C, respectively. The results showed that all HP׳ed samples were composed of coarse equiaxed B2 grains, fine lamellar O phase inside the B2 grains, and some α2 along B2 grain boundaries. The elongations of HP׳ed samples were less than 3.95%, indicating the bad ductility at room temperature. However, the elongations were improved as the tensile temperature increased to 650 °C. The sample sintered at 1050 °C exhibited a better ductility with the elongation of 7.97% at 650 °C than that of other samples.

A Coupled Thermal, Fluid Flow, and Solidification Model for the Processing of Single-Crystal Alloy CMSX-4 Through Scanning Laser Epitaxy for Turbine Engine Hot-Section Component Repair (Part I)

Abstract: Scanning laser epitaxy (SLE) is a new laser-based additive manufacturing technology under development at the Georgia Institute of Technology. SLE is aimed at the creation of equiaxed, directionally solidified, and single-crystal deposits of nickel-based superalloys through the melting of alloy powders onto superalloy substrates using a fast scanning Nd:YAG laser beam. The fast galvanometer control movement of the laser (0.2 to 2 m/s) and high-resolution raster scanning (20 to 200 µm line spacing) enables superior thermal control over the solidification process and allows the production of porosity-free, crack-free deposits of more than 1000 µm thickness. Here, we present a combined thermal and fluid flow model of the SLE process applied to alloy CMSX-4 with temperature-dependent thermo-physical properties. With the scanning beam described as a moving line source, the instantaneous melt pool assumes a convex hull shape with distinct leading edge and trailing edge characteristics. Temperature gradients at the leading and trailing edges are of order 2 × 105 and 104 K/m, respectively. Detailed flow analysis provides insights on the flow characteristics of the powder incorporating into the melt pool, showing velocities of order 1 × 10–4 m/s. The Marangoni effect drives this velocity from 10 to 15 times higher depending on the operating parameters. Prediction of the solidification microstructure is based on conditions at the trailing edge of the melt pool. Time tracking of solidification history is incorporated into the model to couple the microstructure prediction model to the thermal-fluid flow model, and to predict the probability of the columnar-to-equiaxed transition. Qualitative agreement is obtained between simulation and experimental result.

Effect of microstructures on ballistic impact property of Ti–6Al–4V targets

Abstract: Effect of microstructures on ballistic impact properties of Ti–6Al – 4V targets was investigated. The 20 mm Ti–6Al–4V targets having different microstructures were impacted normally by the 12.7 mm AP. The RHA steel was used as the back target, and the residual depth of penetration in the RHA steel target was measured to evaluate the ballistic performance of the Ti–6Al–4V targets. Experimental results show that the bimodal microstructure with thicker α platelets in the transformed β matrix exhibited better ballistic impact property than the other microstructures. The failure modes of Ti–6Al–4V targets varied with the microstructures. In the equiaxed microstructure and the bimodal microstructures, ductile hole formation was the failure mode; while in the lamellar microstructures, the Ti–6Al–4V targets failed by the type of brittle fragmentation. Post-ballistic metallurgical observations reveal that the propagating features of adiabatic shear bands determined failure modes of Ti–6Al–4V targets having different microstructures. The brittle fragmentation failure mode in the lamellar microstructures was facilitated by net-like propagating features of adiabatic shear bands, while the ductile hole formation failure mode in equiaxed and bimodal microstructures was facilitated by the somewhat regularly spaced propagating features of adiabatic shear bands.

Effect of heating rate on the microstructure, texture and tensile properties of continuous cast AA 5083 aluminum alloy

Abstract: Continuous cast (CC) AA 5083 aluminum alloy was cold rolled to 70% reduction and then annealed at different temperatures for 1 h in an air furnace at a heating rate of 5 °C/min and in a salt bath (~75 °C/s), respectively. The effect of heating rate on the microstructure, texture and tensile properties of CC AA 5083 aluminum alloy was investigated. In the case of slow anneal the CC AA 5083 aluminum alloy exhibited a coarse elongated grain structure and a weak {114}〈1 1 ¯ 0〉 recrystallization texture, while a fine equiaxed grain structure and weaker cube and {114}〈1 1 ¯ 0〉 textures were observed in the case of rapid anneal. The rapid anneal promoted the recrystallization of CC AA 5083 aluminum alloy. After complete recrystallization, annealing in a salt bath resulted in higher yield strength and slightly higher ultimate tensile strength than annealing in an air furnace. Luders elongation phenomenon and serration amplitude of CC AA 5083 aluminum alloy were affected by the annealing temperature and heating rate.