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At least 1,423 records · Page 79
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Interfacial Microstructure and Mechanical Properties of Friction Stir Welded Joints of Commercially Pure Aluminum and 304 Stainless Steel

In the present investigation, friction stir welding of commercially pure aluminum and 304 stainless steel was carried out at varying tool rotational speeds from 200 to 1000 rpm in steps of 200 rpm using 60 mm/min traverse speed at 2 (degree) tool tilt angle. Microstructural characterization of the interfacial zone was carried out using optical microscope and scanning electron microscope. Energy-dispersive spectroscopy indicated the presence of FeAl{sub 3} intermetallic phase. Thickness of the intermetallic layer increased with the increase in tool rotational speed. X-ray diffraction studies indicated the formation of intermetallic phases like FeAl{sub 2}, Fe{sub 4}Al{sub 13}, Fe{sub 2}Al{sub 5}, and FeAl{sub 3}. A maximum tensile strength of ~ 90% that of aluminum along with ~ 4.5% elongation was achieved with the welded sample at tool rotational speed of 400 rpm. The stir zone showed higher hardness as compared to base metals, heat affected zone, and thermo-mechanically affected zone due to the presence of intermetallics. The maximum hardness value at the stir zone was achieved at 1000 rpm tool rotational speed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Three-Layer TIG-Welded 2219 Aluminum Alloys with Dissimilar Heat Treatments

2219-C10S and 2219-CYS aluminum alloys are 2219 aluminum alloys with different heat treatment processes, and they have been widely used in the aerospace industry. In the present study, 2219-C10S and 2219-CYS aluminum alloys were butt-welded by three-layer tungsten inert gas arc welding (with the welding center of the third layer shifted toward the CYS side), and the microstructure characteristics and mechanical properties of the welded joint were investigated. The lamellar θ′ phases, the bulk or rod θ phases, and the coarse rod-shaped or pancake-shaped Al-Cu-Fe-Mn phases coexisted in the two aluminum alloys. The Cu content of the α-Al matrix and the distribution of eutectic structures of different welding layers in the weld zone (WZ) were varied, implying that the segregation degrees of the Cu element were different due to the different welding thermal cycles in different welding layers. The microhardness values of the CYS side were much higher than those of the C10S side in each region on both sides of the joint. The tensile test deformation was concentrated mainly in the regions of WZ and the over aged zone (OAZ), where the microhardness values were relatively low. The main deformation concentrated region was transferred from the CYS side to the C10S side with the increase in the tensile load during the tensile test. The fracture behavior of the tensile test showed that the macroscopic crack initiated near the front weld toe had gone through the crack blunt region, the shear fracture region of the partially melted zone (PMZ), and the shear fracture region of OAZ. Meanwhile, the fracture characteristics gradually evolved from brittle to ductile. The concentrated stress and the dense eutectic structure in the region near the front weld toe of the C10S side contributed to the fracture of the joint. The shift of the welding center of the third layer to the CYS side resulted in two effects: (i) the microhardness values from the middle layer to the top layer in the PMZ of the CYS side were the most significantly increased and (ii) the distance between the front weld toe and the fusion line of the CYS side was significantly larger.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Low-Activation Fe-Cr-V Multicomponent Single-Phase Alloys

A ternary equiatomic and nearly equiatomic alloy composed of the low-activation elements Fe, Cr, and V was designed for potential application as low-activation structural materials for the first wall of fusion reactors. The optimal composition of the multicomponent alloy was determined by minimizing the Gibbs free energy for solid-solution formation. The microstructure, phase stability, and mechanical properties of the equiatomic and optimized Fe-Cr-V alloys were studied. The equiatomic alloy consisted of a body-centered-cubic (BCC) solid-solution phase with a small amount of a face-centered-cubic (FCC) solid-solution phase, whereas the optimized alloy consisted of a single BCC solid-solution phase in the as-cast state. The crystal grains of both alloys were equiaxial. No phase transition was observed in the optimized alloy after homogenization; however, for the equiatomic alloy, the FCC solid-solution phase disappeared and surface relief was observed. The Vickers microhardness and yield strength of the Fe-Cr-V alloys increased after homogenization compared with those of the as-cast alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure Evolution and Mechanical Properties of High-Speed Friction Stir Welded Aluminum Alloy Thin Plate Joints

Sound friction stir welded (FSW) joints of 6061-T6 aluminum alloy sheets with an 0.8 mm thickness were obtained at conventional speed (2000 rpm, 300 mm/min) and high speed (11,000 rpm, 1500 mm/min). The recrystallization mechanism, precipitate evolution, mechanical properties and fracture behavior were investigated in detail. Microstructure analyses revealed that the grain structure evolution in the nugget zone (NZ) was dominated by continuous dynamic recrystallization. In the process of FSW, high speed facilitates the formation of finer equiaxed recrystallized grains, higher density of dislocations and substructures, and a larger number of precipitates in the NZ compared to the conventional speed, which further significantly improves the hardness and tensile strength of the joints. The maximum tensile strength was obtained with 292.6 MPa, 83.2% for the 6061-T6 aluminum alloy and 122.6% for the conventional-speed FSW joints. This work provides an effective method for preparing FSW aluminum alloy thin plate joints with excellent mechanical properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical Properties and Microstructural Evolution of Yb-Modified Al-20%Si Alloy

In the study, the mechanical properties and microstructural evolution of hypereutectic Al-20%Si-xYb alloys (x = 0, 0.1, 0.3, 0.5 and 0.7 wt.%) were studied. The as-prepared samples were characterized by scanning electron microscopy, optical microscope, x-ray diffraction (XRD) and electron probe microanalysis. The characterization results showed with the addition amount of Yb increasing to 0.5%, the eutectic Si structure was gradually modified from coarse needle-like/platelet-like structure into fine coral-like fibrous structure and the primary Si phases were gradually converted from coarse platelet-like/star-like/polygonal shape into fine blocky shape. In particular, according to the results of WDS and XRD analyses, the Yb-rich intermetallic phase in the Al-20%Si alloy is Al{sub 3}Yb. Tensile test was carried out to study the mechanical properties of Al-20%Si-x%Yb alloys, and results showed that the elongation and ultimate tensile strength increased by 73.6% and 64.5%, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Wear Resistance of FeCrBSi Plasma-Sprayed Coating Remelted by Gas Tungsten Arc Welding Process

Herein, an FeCrBSi coating was fabricated via plasma spray on AISI1045 steel, and subsequently, a gas tungsten arc welding (GTA) process was employed to remelt the coating. The microstructure, microhardness, fracture toughness and surface roughness of the coating before and after remelting were investigated, as well as the wear resistance was tested by a UMT-3-type sliding wear apparatus. The results showed that, upon remelting, most defects in the as-sprayed coating were effectively eliminated, the surface roughness decreased by 43%, and the coating–substrate interface bonding changed from mechanical to metallurgical. The phase composition of the as-sprayed coating was primarily α-Fe and a small amount of hard Fe{sub 3}B phase, while the remelted coating consisted of α-Fe and (Fe,Cr){sub 23}C{sub 6} and a small quantity of CrB. In addition, remelting the coating was found to induce a 287.6% increase in the fracture toughness, a 33.4% increase in the average microhardness, and a 47.5% decrease in the wear volume, while the failure mechanism changed from abrasive wear to fatigue wear upon remelting. Therefore, GTA remelting of plasma-sprayed coating was found to be a feasible method to obtain a coating with good wear resistance.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Rapidly Cooled Friction Stir Welded Ti-6Al-4V Alloys

Two mm-thick Ti-6Al-4V alloy plates were friction stir butt welded under natural- and rapid-cooling conditions. The microstructures and mechanical properties of joints were investigated and compared. The rapid cooling via spraying cooling liquid nitrogen on the plate top surface was beneficial to obtaining a narrower cross section and finer and more uniform grains in the stir zone than those obtained by natural cooling. Joint tensile strength of 1021 MPa and elongation of 1.88% under the rapid-cooling condition respectively reached 102.7 and 150.4% of those under the natural-cooling condition, respectively. The uniform-and-compact equiaxed grains led to equiaxed dimples on the fracture surface, which demonstrated a ductile fracture of the welding joints.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Deep Cryogenic Treatment on Mechanical Properties and Microstructure of the Tool Steel CR7V for Hot Stamping

To solve the failure problem of hot stamping tool, the CR7V steel was deep cryogenically treated at different dwelling times with multiple tempering. The hardness, impact toughness and wear resistance of the CR7V steel were examined, and the effects of deep cryogenic treatment on the microstructure of the CR7V steel were studied by means of optical microscope, scanning electron microscope and energy-dispersive spectroscopy. The results show that deep cryogenic treatment has little effect on the hardness of the tempered CR7V steel, but improves its impact toughness and wear resistance. The improvement in mechanical properties is a result of the combination of dissolution of large carbide particles, formation of small-sized and uniformly distributed carbide particles and fine tempered martensite structure. The cryogenic treatment is optimized as follows: vacuum gas quenching, deep cryogenic treatment at − 196 °C for 6 h and triple tempering at 560 °C for 2 h, which improves the wear resistance by 68% and the impact toughness by 58% compared to the conventional heat treatment.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Friction-Wear Behavior of Multi-arc Ion Plating TiAlNC Ceramic Coating on WC-6%Co Substrate

Nanocomposite coating could be used for machining tools, which can be used for cutting, drilling, milling, whetting, profiling, honing and superfinishing of various materials with high productivity. In this work, the TiAlNC ceramic coating was prepared by multi-arc ion plating method on WC-6%Co substrate. The microstructure and morphology of the coating were observed by scanning electron microscopy and transmission electron microscopy. The phase and chemical composition of the coating were determined by x-ray diffraction and x-ray energy-dispersive spectroscopy, respectively. The adhesive force of the coating was measured by a scratch tester, at the same time the friction-wear behavior of the coating was studied. The results show that there were TiN and (Ti,Al)(N,C) two phases, and there was the coexistence of amorphous and nanocrystalline in the coating. The chemical composition of the coating was composed of 20 ± 9 at.% Ti, 16 ± 5 at.% Al, 39 ± 5 at.% N and 25 ± 10 at.% C. The thickness of the coating was about 4.5 μm. The average root mean square value of the surface roughness was 225 ± 16 nm. A critical force for coating failure in scratch testing was about 25 N. The friction coefficients of the substrate and the coating were 0.55 and 0.35, respectively. The ceramic coating could improve the wear resistance of the substrate.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

High-Temperature Deformation Behavior of Ti-6Al-2Sn-4Zr-2Mo Alloy with Lamellar Microstructure Under Plane-Strain Compression

High-temperature plane-strain compression of a Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy with a lamellar structure was applied by a thermal simulation machine Gleeble 3800 at a temperature of 1223-1370 K and a strain rate of 0.01-10 s{sup −1}. Constitutive relations between the flow stress and strain at different temperatures and strain rates were constructed based on the Arrhenius equation. Processing maps based on the dynamic material model at strains of 0.5, 1.0, and 1.25 were also constructed to analyze the mechanism and instability of high-temperature deformation. Dynamic recrystallization was found to occur at 1223-1313 K and 0.01-0.1 s{sup −1}, with a peak efficiency of power dissipation of 70%. Observations of the microstructure demonstrated that α platelets and dynamic globularization were responsible for dynamic softening in the α + β phase field. Flow instability behaviors occurred at 1263-1343 K/1.78-10 s{sup −1}, 1233-1283 K/0.1-3.16 s{sup −1}, and 1353-1373 K/0.56-1.78 s{sup −1}, and adiabatic shear bands and microcracks were observed in these domains. Thus, for the lamellar Ti-6242 alloy, the recommended hot rolling parameters were 1243-1333 K and 0.01-0.1 s{sup −1}. Electron backscattered diffraction results revealed that the dynamic restoration mechanism in the β phase field was dynamic recovery when the strain rate was as high as 10 s{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructures and Mechanical Properties of Resistance Spot-Welded Steel/Aluminum Alloy Joints with Process Tapes

The microstructure and mechanical property of resistance spot-welded joints of steel and aluminum were studied by using process tapes. The steel/aluminum joint consists mainly of aluminum nugget and interface zone. The liquid nugget solidification has an epitaxial growth characteristic, and the interface zone displays a double-layer structure, Fe{sub 2}Al{sub 5} layer at steel side and Fe{sub 4}Al{sub 13} layer at aluminum nugget side. Compared with joints without process tape, the joint nugget diameter and tensile shear load increased by 35.45 and 10.98%, respectively, and its indentation ratio decreased by 56.25% due to the process tapes increasing heat input, producing larger nugget diameter and continuous interface layer, and protecting against deep indentation. The joining area of intermetallic layer and aluminum nugget is weak link in the joint. It is favorable to use the process tapes for improving welding quality, raising joint strength, and energy saving.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Spray-Formed 7075 Alloy During Retrogression

The mechanical properties, conductivity, microstructure, precipitation behavior and evolution process of elements and phases of spray-formed 7075 alloy during retrogression are studied by the tensile test, electric test, hardness test, transmission electron microscope (TEM), x-ray diffraction (XRD) analysis, electron-dispersive spectroscopy (EDS) and lattice parameter measurements. The results show that the mechanical properties and conductivity of the alloy can be effectively improved when treated at a suitable retrogression temperature and time. According to the TEM observations, XRD analysis, EDS and an accurate measurement of the lattice parameter, the redissolution of elements and precipitated phases can be readily observed. From the optimization selection of the RRA treatment for the 7075 alloy, the regularity of redissolution and precipitation of the elements and phases are evaluated. The ultimate tensile strength, yield strength, hardness, elongation and conductivity of the spray-formed 7075 alloy after preaging at 120 °C for 16 h, retrogression at 200 °C for 8 min and reaging at 120 °C for 24 h are 791, 736 MPa, 181 HBW, 8.5% and 22.6 MS/m, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Electromagnetic Shielding Properties of Mg-Zn-Ce-Y-Zr Alloys

Microstructure and electromagnetic interference shielding properties of extruded Mg-Zn-Ce-Y-Zr alloys were systematically investigated. A bimodal grain structure of fine recrystallized grains and deformed grains was observed in the alloys. Yttrium (Y) and cerium (Ce) refined the recrystallized grains. Mg-Zn-Ce phase, Mg{sub 3}Zn{sub 3}Y{sub 2} phase and a few precipitates containing Zn and Zr were observed. Y and Ce additions changed the texture of the alloys. When the total rare earth content reached 4 wt.%, the texture was remarkably weaker than other studied alloys. The shielding capacity increased first and then decreased with Y contents. The value of shielding effectiveness for the extruded Mg-5Zn-1Ce-2Y-0.6Zr alloys reached 79-118 dB in the frequency range of 30 MHz-1.5 GHz, which was significantly higher than that of some aluminum alloys. The addition of Y and Ce is effective in improving the shielding capacity in Mg alloys. The electrical conductivity and second phases in the alloys were important in enhancing shielding effectiveness. The texture and precipitation exerted significant effect on electrical conductivity. In addition, subsequently aging treatment evidently improved shielding effectiveness.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructural Evolution and Mechanical Behavior of Lead-Free Silicon Brass Manufactured by Low-Pressure Die Casting

Low-pressure die casting (LPDC) has been widely used to process aluminum and magnesium alloys; however, it has yet to be applied to manufacture brass alloys. This study investigates the microstructural evolution and mechanical behavior of a lead-free CuZnSiAl silicon brass via a variable-controlling approach on the basis of a faucet casting by LPDC. The manufactured silicon brass castings are predominantly composed of bcc β-CuZn(Si, Al) and minor fcc α-Cu(Zn, Si) solid solutions. The silicon brass castings also predominantly exhibit short rod-like or granular α phases, which distinctly differ from the acicular α morphology obtained by conventional gravity casting. The manufactured silicon brass castings which contain finer β grains and short rod-like or granular α phase exhibit superior mechanical properties. Their optimal mechanical properties—elongation of 15.0%, yield strength of 359.8 MPa, and ultimate tensile strength of 598.9 MPa—are far superior to those of the typical lead brass HPb59-1. The results obtained in this study provide insight into the manufacture and design of lead-free silicon brass castings by LDPC.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Deformation Mechanism of AZ31 Magnesium Alloy Under Dynamic Strain Rate

A hot-rolled AZ31Mg alloy sheet was subjected to dynamic plastic deformation parallel to the rolling direction, and microstructural evolutions and mechanical properties of the deformed samples were examined. It has been found that dynamic strain rate could facilitate {10-12} twin nucleation and growth and leads to a lower yield stress of about 20 MPa and an early end to twinning characteristic (happening at a strain point of about 6%) shown in the stress–strain curve. {10-12} twinning mechanism dominates the early plastic deformation; but when plastic strain exceeds ~ 9%, dislocation–slip mechanism instead of {10-12} twinning dominates the later plastic deformation. And this premature transformation of the dominant deformation mechanisms from {10-12} twinning to dislocation slip is caused by dynamic strain rate. The effect of dynamic strain rate on the number of twin nucleations remains unclear, and the more systematic researches are needed in the future.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure, Texture, and Mechanical Properties of Ni-W Alloy After Accumulative Roll Bonding

In this study, the microstructure, texture, and mechanical properties evolution of Ni-14W (wt.%) alloy processed up to four cycles of accumulative roll-bonding (ARB) were investigated using electron backscatter diffraction, microhardness measurements, and tensile tests. The initial equiaxed grains, with an average size of 10 μm, underwent a strong refinement after ARB processing. The elongated ultrafine grains were parallel to the rolling direction, with a grain thickness of 0.2 µm. The texture after ARB processing was characterized by the typical rolling components (Copper, S and Brass), which showed a tendency toward stabilization after four cycles. The microhardness increased substantially (+ 86%) and seemed to saturate after three cycles. The tensile tests demonstrated that Ni-14W samples subjected to ARB processing exhibited high strength (> 1200 MPa after three ARB cycles) and very poor ductility.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Rotation Speed on Microstructure and Mechanical Properties of 2060 Al-Cu-Li Alloy in Friction Stir Welding

Microstructural and mechanical properties of 2.0-mm-thick 2060 Al-Cu-Li alloy joints obtained at different rotation speeds were assessed in this study. The rotation speeds ranged from 400 to 1300 rpm, with welding speed kept at a constant value of 100 mm/min. With the increase in rotation speed, grain coarsening occurred while the density of precipitation decreased in the stir zone (SZ), and great amounts of the Al{sub 2}CuLi and Al{sub 2}Cu phases were dissolved while only small amounts of the Al{sub 3}Zr and Al{sub 3}Li phases remained. The lowest hardness was found at the interface between the heat-affected zone (HAZ) and thermo-mechanically affected zone (TMAZ); the average hardness value of SZ increased, while the lowest hardness moved in the direction away from the center of joints. The ultimate strength (UTS) of weld first increased rapidly and then slowly decreased as rotation speed increased, with a maximum value of 443 MPa obtained at a rotation speed of 600 rpm, reaching 83.58% of the base metal (BM) strength. Three fracture modes were established to illustrate the growth mechanism of the crack. Analysis of the fracture surface showed that insufficient materials flowed and coarsened secondary phases caused the joint fracture.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Study on Microstructure and Properties of AZ31/6061 GTAW Butt Joints According to Mg-Cd-Al Ternary System

AZ31 Mg and 6061 Al were successfully joined by GTAW method with Cd as filler metal. The tensile strength of the joints reached 170 MPa through this alloying method according to Mg-Cd-Al ternary system. The microstructure, phases, element distribution and fracture mechanism of alloyed joints were studied. The results suggest that the fusion zone is mainly composed of CdMg compounds and Al-based solid solution. A transitional zone thickness of 40-80 μm with few primary Al{sub 12}Mg{sub 17} and fine Al{sub 12}Mg{sub 17}-MCMS (MCMS represents a mixture of CdMg and CdMg{sub 3} compounds) eutectic is formed between the AZ31 base metal and fusion zone. The fracture surface presents three distinct regions because of the different distribution of Al{sub 12}Mg{sub 17} compounds in the transitional zone. The alloying with Cd filler metal can disperse Al{sub 12}Mg{sub 17} intermetallic compounds and form CdMg ordered compounds with excellent plasticity. Thus, the mechanical properties of alloyed joints have been significantly enhanced.

36 MATERIALS SCIENCE↗