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At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Microstructural Characteristics and Mechanical Properties of an Electron Beam-Welded Ti/Cu/Ni Joint

Electron beam welding experiments of TA15 titanium alloy to GH600 nickel superalloy with and without a copper sheet interlayer were carried out. Surface appearance, microstructure and phase constitution of the joint were examined by optical microscopy, scanning electron microscopy and x-ray diffraction analysis. Mechanical properties of Ti/Ni and Ti/Cu/Ni joint were evaluated based on tensile strength and microhardness tests. The results showed that cracking occurred in Ti/Ni electron beam weldment for the formation of brittle Ni-Ti intermetallics, while a crack-free electron beam-welded Ti/Ni joint can be obtained by using a copper sheet as filler metal. The addition of copper into the weld affected the welding metallurgical process of the electron beam-welded Ti/Ni joint significantly and was helpful for restraining the formation of Ti-Ni intermetallics in Ti/Ni joint. The microstructure of the weld was mainly characterized by a copper-based solid solution and Ti-Cu interfacial intermetallic compounds. Ti-Ni intermetallic compounds were almost entirely suppressed. The hardness of the weld zone was significantly lower than that of Ti/Ni joint, and the tensile strength of the joint can be up to 282 MPa.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Multiple Rejuvenation Cycles on Mechanical Properties and Microstructure of IN-738 Superalloy

The scope of this work is to show the effects of multiple applications of a rejuvenation treatment studied for IN-738 on both the microstructure and the mechanical properties of the creep-damaged superalloy and to check the recovery obtained after one and two rejuvenation cycles through creep and tensile tests, whose results will be compared with the performance of the virgin material. This work will show that this rejuvenation treatment is able to recover the microstructure of creep-damaged specimens after one and two applications and that the mechanical properties of the rejuvenated alloy are very similar to the virgin material even after two rejuvenation cycles.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Densification, Microstructural Evolution, Mechanical Properties and Oxidation Study of CrB{sub 2} + EuB{sub 6} Composite

This paper reports the results of investigation on densification, microstructural evolution, mechanical properties and oxidation study of CrB{sub 2} + EuB{sub 6} composite. CrB{sub 2} + EuB{sub 6} (10 and 20 wt.%) composites have been fabricated by hot pressing at a temperature of 1700 °C and 35 MPa pressure. The hardness and flexural strength were measured in the range of 21.25-24.48 GPa and 171-199 MPa, respectively. The fracture toughness increased from 3.3 to 4.01 MPa m{sup 1/2} by the addition of 20% EuB{sub 6}. Microstructural evolution revealed the uniform distribution of EuB{sub 6} and absence of any reaction product. Fracture surface analysis confirmed the presence of transgranular mode of fracture. Oxidation study at 1200 °C revealed that the developed composites have good oxidation resistance and followed the parabolic rate of oxidation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Multiphase Microstructure in a Metastability-Assisted Medium Carbon Alloy Steel

A medium carbon alloy steel is processed by austenizing at 900 °C for 30 min, then rapid quenching into a patented quenching liquid and holding at 170 °C for 5 min, finally isothermally holding at 250 °C for different times. The morphology and mechanical properties are performed by using optical microscopy and scanning electron microscopy. A multiphase microstructure characterized by a mixture of lenticular prior martensite (PM), fine needle bainitic ferrite and filmy retained austenite (RA) is obtained. It is found that the PM formed firstly upon quenching can accelerate the subsequent bainitic transformation and promote refinement of multiphase colonies. The results show that an optimum mechanical property of a 4000.9 MPa bending strength and a 2030 MPa tensile strength is achieved at 250 °C for 120 min, which is attributed to the multiphase microstructural characteristics and a high product of the volume fraction of RA and the carbon content of austenite.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Multiple Local Repairs on Microstructure and Mechanical Properties of T24 Steel Welded Joint

The effect of multiple local repairs on the microstructure and mechanical properties of the T24 steel welded joints was studied. T24 steel tubes were butt-welded by the GTAW method. Peripheral welded joints were made in four locations of the tube. In order to simulate the repair procedure, the welds were cut off from the root and the first local repair was performed. Other two local repairs were carried out in the same way. After each local repair, the microstructure and mechanical properties of the joints were evaluated. The results of the mechanical tests demonstrate that only two local repairs can be performed on the T24 steel peripheral welded joint. After the third local repair, impact energy of the welded joint was lower than required value, which is attributed to the coarser martensite and the coarser carbide precipitates formed in the heat-affected zone, compared with the weld metal.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Controlled Cooling-Induced Ferrite–Pearlite Microstructure on the Cold Forgeability of XC45 Steel

The combination of hot/warm and cold forging with an intermediate controlled cooling process is a promising approach to saving costs in the manufacture of automobile parts. In this work, the effects of the ferrite–pearlite microstructure, which formed after controlled cooling, on the cold forgeability of a medium-carbon steel were investigated. Different specimens for both normal and notched tensile tests were directly heated to high temperature and then cooled down at different cooling rates, producing different ferrite volume fractions, ranging from 6.69 to 40.53%, in the ferrite–pearlite microstructure. The yield strength, ultimate tensile strength, elongation rate, percentage reduction of area, and fracture strain were measured by tensile testing. The yield strength, indicating deformation resistance, and fracture strain, indicating formability, were used to evaluate the cold forgeability. As the ferrite volume fraction increased, the cold forgeability of the dual-phase ferritic–pearlitic steel improved. A quantitatively relationship between the ferrite volume fraction and the evaluation indexes of cold forgeability for XC45 steel was obtained from the test data. To validate the mathematical relationship, different tensile specimens machined from real hot-forged workpieces were tested. There was good agreement between the predicted and measured values. Our predictions from the relationship for cold forgeability had an absolute error less than 5%, which is acceptable for industrial applications and will help to guide the design of combined forging processes.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural Investigation of VPPA–GMAW Welded 7A52 Aluminum Alloys

In this study, the microstructural features of 7A52 heat-treatable aluminum alloys welded by variable polarity plasma arc (VPPA) combined with gas metal arc welding (GMAW) (VPPA–GMAW) were investigated and compared with those of 7A52 aluminum alloys welded by GMAW. The grain structures and modification of the precipitates were analyzed by optical microscopy and transmission electron microscopy. Complementary to modification of the precipitates, the precipitation evolution in the heat-affected zone and magnesium loss in the fusion zone were measured by differential scanning calorimetry and inductively coupled plasma spectroscopy. In VPPA–GMAW, the weld only needs one pass by depositing on one side of the 10-mm-thick aluminum alloy plates because of the large penetration capability of VPPA and good fluidity of the molten metal in the weld pool. Accordingly, the effect of excessive heat input on the microstructure can be minimized. The results show that the grain growth is not obvious and the magnesium loss is small in the fusion zone of VPPA–GMAW compared with that of GMAW under the condition of equal heat input to the workpiece. Small precipitates inside the grains remain homogeneously distributed, and a large volume fraction of the η′ phase exists. The VPPA–GMAW weld exhibits less tendency to soften; thus, VPPA–GMAW is appropriate for welding thick-plate aluminum alloys with a high production rate.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural Characterization and High Strain Rate Plastic Flow Behavior of SMAW Armox500T Steel Joints from Spherical Indentation Experiments

Static indentation and uniaxial compression tests have been conducted on Armox500T steel and its weldments to predict the constraint factor (CF), i.e., ratio of Meyer’s hardness to uniaxial flow stress. Series of dynamic indentation experiments were carried out at impact velocities ranging from 5 to 300 m/s to estimate dynamic hardness as a function of average strain. Subsequently, dynamic indentation (DI) test data and CF determined under static indentation conditions have been used to study the high strain rate plastic flow behavior of Armox500T weldments in comparison with the base metal. It was observed that flow stress for Armox500T and its weldments under dynamic loading conditions (10{sup 4} s{sup −1}) are significantly higher than flow stress measured under static loading conditions (10{sup −3} s{sup −1}). The plastic flow behavior computed from DI is in good agreement with the data evaluated through conventional split Hopkinson pressure bar technique. Further, the study of the microstructure of base metal and weldments by optical microscopy and SEM revealed a considerable variation in the microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Rolling Temperature on the Microstructure and Properties of 2Cr13/316L/9Cr18MoV Stainless Steel Composite Plate Fabricated by Hot-Rolling Bonding Process

A 15-layer composite plate was developed. The microstructure and mechanical properties were characterized by scanning electron microscopy, Vickers microhardness, tensile, and bending tests. Particularly, the effects of hot-rolling at different rolling temperatures of 1130, 1180, 1250, and 1300 °C on the microstructure and mechanical properties of the 2Cr13/316L/9Cr18MoV composite plates were investigated in detail. The results indicated that, with the increase in the rolling temperature, the tensile strength was first decreased and then slightly increased by the coarsening of grains and the increase in the ferrite content in the 2Cr13 layers. The highest tensile elongation of 12.5% and tensile strength of 958.1 MPa were obtained for the composite plate manufactured with the rolling temperature of 1180 °C. The 2Cr13/316L/9Cr18MoV composite plates with rolling temperatures of 1130 and 1180 °C obtained by hot-rolling processes withstood larger bending deformations, showing only local cracks after the bending process because of their strong combined interfaces. However, during the bending to 130° of specimens fabricated with the rolling temperature of 1250 and 1300 °C, the specimens experienced direct fracture at very low bending displacements.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Nb on the Microstructure and Properties of Ti-Added Hypereutectic High-Cr Cast Iron

Effects of Nb on the microstructure and properties of Ti-added hypereutectic high-Cr cast iron were studied. With the addition of Nb, a new phase (Nb,Ti)C was formed. The microstructure analysis revealed that the amount of primary carbides was decreased from 18.6 to 8.8% and the primary and eutectic carbides were greatly refined. The decrease in the amount of primary carbides was attributed with the shrinking of primary M{sub 7}C{sub 3} phase zone in the phase diagram, while the refinement of primary carbides was attributed to the heterogeneous nucleus of (Nb,Ti)C phase for the primary M{sub 7}C{sub 3} carbides. The macrohardness of the alloy increased from 60.8 to 63.5HRC, and the impact toughness increased from 3.8 to 5.2 J cm{sup −2}. Furthermore, the wear resistance was enhanced with the addition of Nb due to the formation of hard (Nb,Ti)C carbides.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Ratcheting Behavior of Additive Manufactured 4043 Aluminum Alloy

In this paper, 4043 aluminum alloy was printed using additive manufacturing (AM) with robot cold metal transfer (CMT) technology. The microstructure and mechanical properties of different parts, including the bottom, middle and upper portions of the printed AM 4043 aluminum alloy, were analyzed. Ratcheting tests were performed using an asymmetric load, and the tensile properties of the pre-ratcheting and un-ratcheted specimens were tested. The results indicate that the dendrites in the middle region are larger than those at the top and bottom regions and that the coarse microstructure associated with the interlayer junction resulted in the decline of the mechanical properties. The tensile strength of the AM specimens is 165 MPa, and the yield strength is 50 MPa. The printed 4043 aluminum alloy exhibits mechanical properties similar to those for conventional cast and welded aluminum alloys. The ratcheting behavior indicates that an increase in the mean stress or stress amplitude leads to an increase in the ratcheting strain, and the accumulation of ratcheting strain causes plastic damage to the materials. Pre-ratcheting behavior causes the tensile and yield strengths to increase compared to the behavior of the un-ratcheted specimens.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Laser Shock Peening on Residual Stress, Microstructure and Hot Corrosion Behavior of Damage-Tolerant TC21 Titanium Alloy

The effect of laser shock peening (LSP) on the residual stress, microstructure and hot corrosion behavior of damage-tolerant TC21 alloy was investigated. The equilateral scanning pattern was adopted to obtain a large overlapping area for LSP. The hot corrosion experiments in a salt mixture composed of 75 wt.% Na{sub 2}SO{sub 4} + 25 wt.% NaCl at 700 and 800 °C for a total exposure of 25 h were conducted. The results demonstrated that LSP caused a high compressive residual stress with a significant increase in micro-hardness and microstructural changes in the form of crystal defects, i.e., high density dislocations. The crystal defects promoted the outward diffusion of Al, Cr and Ti to form protective mixed oxides (Al, Cr, Ti)O{sub 2}. These oxides improved the adhesion of outer oxide layer and hindered the inward diffusion of O, S and Cl, which was beneficial to improve hot corrosion resistance. The mass gains of LSP-treated specimens after hot corrosion at 700 and 800 °C were, respectively, 3.72 and 4.78 mg/cm{sup 2}, which were much lower than those of untreated specimens (5.65 and 6.81 mg/cm{sup 2}). Therefore, LSP leads to an improvement in resistance to hot corrosion of TC21 alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Initial Microstructure on Properties of Cryorolled Al 5052 Alloy Subjected to Different Annealing Treatment Temperatures

Al 5052 alloy sheets were subjected to different pre-annealing temperatures (150, 200, 250, 300, and 350 °C) prior to cryorolling. The process resulted in Al 5052 alloys with different initial microstructures. The pre-annealed alloy sheets were compared with a cryorolled sample not subjected to pre-annealing. The thicknesses of the alloy samples after cryorolling was reduced by 30%. The pre-annealed cryorolled samples exhibited low crystallite size, and high lattice strain. Among them, the cryorolled sample pre-annealed at 300 °C had the lowest crystallite size, and the highest lattice strain. Changes in initial microstructure of this sample resulted in a significant improvement in its hardness (88 Hv), tensile strength (333 MPa), and corrosion resistance. The sample had the highest corrosion resistance among the cryorolled samples.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Minor Er on the Microstructure and Properties of Al-6.0Mg-0.4Mn-0.1Cr-0.1Zr Alloys

As-cast Al-6.0Mg-0.4Mn-0.1Cr-0.1Zr alloys containing different additions of Er were prepared. Then, the specimens were carried out homogenization heat treatment, hot rolling, cold rolling, and annealing. The effects of minor Er on the microstructure and properties of Al-6.0Mg-0.4Mn-0.1Cr-0.1Zr alloy were investigated. Results show that addition of Er can refine the microstructure and improve the mechanical properties of the alloys. As for the annealed alloys, the addition of 0.33%Er can increase the tensile strength by 29 MPa. Strength increment is mainly attributed to refinement and the precipitation of Al{sub 3}Er and Al{sub 3}(Er, Zr). Additionally, the minor Er can improve the corrosion resistance of the alloys, which can be attributed to the compact composite oxide film of Er-O and Al-O, purification, and modification.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Ti Addition on the Microstructure and Mechanical Properties of Weld Metals in HSLA Steels

Acicular ferrite (AF) can significantly improve the mechanical properties of steel welds. One practical approach to enhance the formation of AF is to provide the heterogonous nucleation sites such as Ti oxides. In this study, Ti was added to different conventional welding processes including shield metal arc welding (SMAW), submerged arc welding (SAW) and tandem SAW (T-SAW). In the SMAW process, TiO{sub 2} particles as a source of Ti were inserted into the weld groove, while in the SAW and the T-SAW processes, the Ti-enriched S2MoTB wire was used as the filler metal. The microstructural evolution of weldments was characterized by employing optical and scanning electron microscopes. In addition, microhardness (Vickers, HV), Charpy impact and tensile tests were carried out to investigate the mechanical properties of weldments. Although the microhardness measurements of all weldments did not vary significantly and were in the range of 205-252 HV, there was a considerable difference in tensile and impact properties of the SAW and the T-SAW weldments. In the SMAW process, the addition of TiO{sub 2} results in no significant enhancement in tensile and impact toughness. This can be attributed to the inhomogeneous distribution of TiO{sub 2} particles as well as the formation of large inclusions in the structure. On the other hand, Ti addition to WM increased the yield strength from 489 to 552 MPa for the SAW process, and in contrast, it decreased the impact toughness from 75 to 33 J. This detrimental effect can be related to the higher deposition of other alloying elements in the WM and the formation of more ferrite side plate phase. By applying the T-SAW process, more Ti in WM led to a higher content of AF in the microstructure and increased both yield strength and impact toughness from 528 to 595 MPa and 100 to 180 J, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Different Post-treatments on the Microstructure of EBM-Built Alloy 718

Electron beam melting (EBM) of Alloy 718 is of rapidly growing interest as it allows cost-effective production of complex components. However, the inherent flaws in the component in as-built state are of concern in view of the severe working conditions in which Alloy 718 components typically operate. The present work entails an investigation of changes in microstructure that accompany some post-treatments that are being widely considered to address defects in EBM processed Alloy 718. The effect of two different post-treatments, namely hot isostatic pressing (HIP) and a combined HIP + heat treatment (HT) carried out inside the HIP vessel, have been studied and results from as-built and post-treated specimens were compared in terms of porosity/lack-of-fusion, microstructure, phase constitution (NbC content, δ-phase) and micro-hardness. Post-treatment resulted in reduction in defect content by more than an order of magnitude. HIPing led to complete dissolution of δ phase. In comparison to as-built material, HIPed specimens exhibited significant drop in hardness. However, a sharp ‘recovery’ of hardness to yield values higher than in as-built condition was observed after HIP + HT and can be attributed to precipitation of γ′′ phase.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructure and Mechanical Property Improvement in Laser-Welded TC4 Titanium Alloy and 301L Stainless Steel Joints Without Filler Metal

Pulsed laser welding of Ti alloy to stainless steel (SS) has been applied without filler metal. When the laser beam was focused on the Ti alloy–SS interface, large volumes of Ti-Fe intermetallics were formed in the joint. These Ti-Fe intermetallics were highly brittle, causing the joint to crack spontaneously after welding. The brittle coarse Ti-Fe intermetallics seriously deteriorates the joint strength of Ti alloy and SS; therefore, it is necessary to control the quantities and distributions of brittle Ti-Fe intermetallics in the joint. The amount of melted base materials was controlled by changing laser offset to control the melted proportion and thus control the quantity and distribution of brittle Ti-Fe intermetallics in weld. When laser beam was on the SS side, melting amount of Ti alloy in weld was decreased. When laser beam was on the Ti alloy side, melting amount of SS in weld was decreased. When the laser offset was 0.35 mm away from SS side, the sample realizes the effective connection between Ti alloy and SS. A compound layer was formed at the SS side with the main microstructure of β-Ti and TiFe{sub 2}. The joint fractured at the compound layer with the maximum tensile strength of 182 MPa. Microstructures and mechanical properties of joints were improved by changing laser offset.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Annealing on the Microstructure and Properties of Electron Beam Welded 4J34 Kovar Alloy

Electron beam welding of 4J34 Kovar alloy was conducted, and the microstructure and mechanical properties of the joints in as-welded and postweld annealing treatment (PWAT) states were investigated. The martensitic transformation occurred and led to the generation of α lath martensite due to the rapid cooling in the as-welded condition. After annealing treatment, part of the α martensite transformed to γ austenite and subsequently developed the bulk γ phase and α/γ lamellar microstructures in the fusion zone (FZ) of the postwelded joint. Grains in the heat-affected zone (HAZ) clearly developed and penetrated into the FZ, causing the disappearance of the fusion line between the FZ and the HAZ. All the joints in both the as-welded and PWAT conditions exhibited ductile fracture. The maximum tensile strength of 505 MPa was obtained in the as-welded joint with the welding speed of 150 mm/min. The linear expansion coefficients of the joint and base metal decreased with the increase in temperature. However, the decreasing rate of PWAT joints was lower than that of the as-welded joints.

36 MATERIALS SCIENCE↗