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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Fluid Flow, Dissolution, and Mixing Phenomena in Argon-Stirred Steel Ladles

In the current study, the multiphase fluid flow in argon-stirred steel ladles is simulated using an Eulerian–Lagrangian two-phase approach. The momentum source and the turbulent kinetic energy source due to the motion of the bubble are considered for the liquid phase. Argon bubbles are treated as discrete phase particles, and the interfacial forces between the bubbles and the liquid phase; the dependence of the gas density and the bubble diameter on the temperature and the static pressure; and the bubble size distribution are considered. When the fluid flow reaches the quasi-steady state, the ferroalloy melting and mixing phenomena is also modeled. The melting time and the trajectory length of each ferroalloy particle are recorded using a user-defined function (UDF). Local mixing time is predicted in the entire computational domain by checking the mixing criteria in every cell. The effects of gas flow rate, porous plug location, and separation angle of two porous plugs on the fluid flow and the mixing phenomena are investigated. The results show that the flow intensity increases, and the mixing time decreases with the increasing gas flow rate. The optimal porous plug’s radial position with one porous plug is 0.50R for its best mixing condition. When two porous plugs are adopted, the separation angle of 90 deg is recommended to improve the flow field and mixing phenomena.

42 ENGINEERING↗
DOE OSTI2018

Effect of Partial Substitution of Mn for Ni on Mechanical Properties of Friction Stir Processed Hypoeutectic Al-Ni Alloys

In this study, the mechanical properties of as-cast and FSPed Al-2Ni-xMn alloys (x = 1, 2, and 4 wt pct) were investigated and compared with those of the as-cast and FSPed Al-4Ni alloy. According to the results, the substitution of 2 wt pct Mn for 2 wt pct Ni leads to the formation of fine Mn-rich intermetallics in the microstructure increasing the tensile strength, microhardness, fracture toughness, and specific strength of alloy by 22, 56, 45, and 35 pct, respectively. At higher Mn concentrations, the formation of large Mn-rich platelets in the microstructure reduces the tensile properties. Friction stir processing at 12 mm/min and 1600 rpm significantly enhances both the strength and ductility of the alloy. The tensile strength, yield strength, fracture strain, fracture toughness, microhardness, and specific strength of FSPed Al-2Ni-4Mn alloy improved by 97, 83, 30, 380, 152, and 110 pct, respectively, as compared to those of the as-cast Al-4Ni alloy. This can be attributed to dispersion strengthening of Ni- and Mn-rich dispersoids, formation of ultrafine grains, and elimination of casting defects. The fractography results also show that the brittle fracture mode of the as-cast Mn-rich alloys turns to a more ductile mode, comprising fine and equiaxed dimples in FSPed samples.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Nanostructured Fe-20Cr-10Ni-2Si Layer Produced by Laser Cladding and Friction Stir Processing

Laser cladding of Fe-20Cr-10Ni-2Si alloy usually faces challenge of metallurgical defects like micropores and interdendritic fragile precipitates. In this paper, the laser cladding combined with friction stir processing (FSP) was applied to deal with this problem and prepare the nanostructured Fe-20Cr-10Ni-2Si alloy layer. The coarse dendritic grains (5 ~ 10 μm) of laser processed Fe-20Cr-10Ni-2Si alloy were broken into nanocrystallines which contain uniformly distributed nanoparticles (5 ~ 400 nm) in the deformation zone by FSP. The gradient microstructure with an average grain size of several nanometers on the top surface to several micrometers in the center part was obtained by FSP. Meanwhile, the grain size in the plastic zone decreases as the rotary speed rises. Grain refinement by FSP results in the increase of microhardness from 323 HV up to 583 HV, decreasing the friction coefficient from 0.42 to 0.35 and thus the wear resistance is improved.

36 MATERIALS SCIENCE↗
DOE OSTI2019

A Review on Friction Stir Processing of Titanium Alloy: Characterization, Method, Microstructure, Properties

In the past two decades, friction stir processing (FSP) technology has received considerable attention. FSP can be used to adjust and control the microstructure of materials, including eliminating defects, destroying dendrites and controlling fractions in the second stage, and is therefore widely used in titanium and its alloys for biomedical, aerospace and automotive applications. This article comprehensively reviews the methods of studying FSP, the microstructure evolution of materials and the summary of material properties. It begins with the introduction of the FSW system, characterization, structure and elemental analysis, and simulation and performance testing methods, and then introduces the microstructure evolution mechanism of various titanium materials and discusses in detail the material properties of titanium alloy, namely hardness and wear resistance, elasticity and plasticity, corrosion resistance and biocompatibility. Finally, this review presents unresolved issues and outstanding challenges in FSP technology and reveals the direction of this emerging field of research.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of Local Post Weld Heat Treatment on Tensile Properties in Friction Stir Welded 2219-O Al Alloy

To improve the formability of the aluminum alloy welds and overcome the size limitation of the bulk post weld heat treatment (BPWHT) on large size friction stir welded joints, a local post weld heat treatment method (LPWHT) was proposed. In this method, the resistance heating as the moving heat source is adopted to only heat the weld seam. The temperature field of LPWHT and its influence on the mechanical properties and formability of FSW 2219-O Al alloy joints was investigated. The evaluation of the tensile properties of FSW samples was also examined by mapping the global and local strain distribution using the digital image correlation methodology. The results indicated that the formability was improved greatly after LPWHT, while the hardness distribution of the FSW joint was homogenized. The maximum elongation can reach 1.4 times that of as-welded joints with increase the strength and the strain of the nugget zone increased from 3 to 8% when annealing at 300 °C. The heterogeneity on the tensile deformation of the as-welded joints was improved by the nugget zone showing large local strain value and the reason was given according to the dimple fracture characteristics at different annealing temperatures. The tensile strength and elongation of LPWHT can reach 93.3 and 96.1% of the BPWHT, respectively. Thus, the LPWHT can be advantageous compared to the BPWHT for large size welds.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Analysis on the Fracture of Al-Cu Dissimilar Materials Friction Stir Welding Lap Joint

Friction stir welding (FWS) is regarded as a more plausible alternative to other welding methods for Al-Cu dissimilar joining. However, the structure of an FSW joint is different from others. In this study, lap joints of 6061 aluminum alloy and commercially pure copper were produced by FSW, and the effects of rotation rate on macromorphology, microstructure and mechanical properties were investigated. In addition, a fracture J integral model was used to analyze the effect of microstructure on the mechanical properties. The results revealed that the macrodefect-free joints were obtained at a feed rate of 150 mm/min and 1100 rpm and that the failure load of the joint reached as high as 4.57 kN and only reached 2.91 kN for the 900 rpm, where tunnel defects were identified. Particle-rich zones composed of Cu particles dispersed in an Al matrix, and “Flow tracks” were observed by the EDS. The J integral results showed that the microdefects on the advancing side cause serious stress concentration compared with the microdefects located on the Al-Cu interface, resulting in the fracture of the joints.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure of Friction Stir Welded AlSi9Mg Cast with 5083 and 2017A Wrought Aluminum Alloys

Wrought aluminum alloys 5083 and 2017A were each joined with cast aluminum alloy AlSi9Mg through friction stir welding in butt weld configurations. For each material system, the wrought and cast alloy positions, i.e., the advancing side or the retreating side, were exchanged between welding trials. The produced weldments were free from cracks and discontinuities. For each alloy configuration, a well-defined nugget comprised of alternating bands of the welded alloys characterized the microstructure. The degree of mixing, however, strongly depended on which wrought alloy was present and on its position during processing. In all cases, the cast AlSi9Mg alloy dominated the weld center regardless of its position during welding. Electron backscattered diffraction analysis showed that the grain size in both alloys (bands) constituting the nugget was similar and that the majority of grain boundaries exhibited a high angle character (20°-60°). Regardless of the alloy, however, all grains were elongated along the direction of the material plastic flow during welding. A numerical simulation of the joining process visualized the material flow patterns and temperature distribution and helped to rationalize the microstructural observations. The hardness profiles across the weld reflected the microstructure formed during welding and correlated well with the temperature changes predicted by the numerical model. Tensile specimens consistently fractured in the cast alloy near the weld nugget.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Friction Stir Process Derived Al-TiO{sub 2} Nanocomposite

Aluminum-based composites have many advantages over their conventional counterparts. A major problem in such composites is the clustering of particles in the matrix. Friction stir processing (FSP) can homogenize particle distribution in aluminum-based composites. In this study, unannealed TiO{sub 2} particles were used to prepare Al-TiO{sub 2} nanocomposite using FSP. The TiO{sub 2} particles, about 1 µm, were dispersed into an aluminum matrix by 6 passes of FSP. The TiO{sub 2} particles were fractured by multiple FSP passes, leading to a nano-size particle distribution in the matrix. Nanoscale dispersion was confirmed by scanning electron microscopy and transmission electron microscopy. The fractured TiO{sub 2} particles reacted with the aluminum matrix to form Al{sub 3}Ti intermetallic and Al{sub 2}O{sub 3} ceramic. The progression of the Al-TiO{sub 2} reaction from the fourth to the sixth pass of FSP was revealed by x-ray diffraction. Due to the nanoscale dispersion, the yield and ultimate tensile strength of the composite increased to 97 and 145 MPa, respectively. Ductility of the composite decreased marginally compared to the as-received aluminum. As the dispersed particles pin dislocations, the strain-hardening rate of the composite was considerably increased and the same was seen in the Kocks-Mecking plot. The TiO{sub 2} particles are mechanically activated due to their fracture during FSP, hence leading to reaction with the matrix. The particle refinement and dispersion lead to a homogeneous matrix with higher strength.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Wear Behavior of AZ31/Al{sub 2}O{sub 3} Magnesium Matrix Surface Nanocomposite Fabricated via Friction Stir Processing

The aim of this study was to produce magnesium-based surface nanocomposites via friction stir processing and to investigate the effect of tool rotational speed on the microstructure, hardness and wear behavior. The surface of the nanocomposites was characterized using optical and scanning electron microscopes, as well as through microhardness and wear tests. The results indicated that with the increase in rotational speed, the grain size of the surface nanocomposites increased, but its hardness decreased despite the improved distribution of Al{sub 2}O{sub 3} nanoparticles. It was also found that the wear resistance has a direct relation to the distribution of the Al{sub 2}O{sub 3} nanoparticles. Furthermore, the addition of nano-Al{sub 2}O{sub 3} changed the wear mechanism from the adhesive mode in the as-received AZ31 to the abrasive mode in the nanocomposite specimens. The rotational speed of 1400 rpm was an optimum parameter to achieve a suitable composite layer with the highest wear resistance.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Friction Stir Welding of Low-Carbon AISI 1006 Steel: Room and High-Temperature Mechanical Properties

Friction stir welding (FSW) is an ecologically benign solid-state joining process. In this work, FSW of low-carbon AISI 1006 steel was carried out to study the microstructure and mechanical properties of the resulting joints at both room temperature (RT) and 200 °C. In the parameter space investigated here, a rotational tool speed and translation feed combination of 1200 rpm and 60 mm/min produced a defect-free weld with balanced mechanical properties and a superior Vickers microhardness profile compared to all other conditions and to base metal (BM). At faster translation feeds (100 and 150 mm/min), wormhole defects were observed in the weld microstructure and were attributed to higher strain rate experienced by the weld zone. Under tensile loading, welded material exhibited yield strength that was up to 86 and 91% of the BM at RT and 200 °C, respectively. On the other hand, tensile strength of welded material was nearly similar to that of the base metal at both RT and 200 °C. However, at both temperatures the tensile ductility of the welded joints was observed to be significantly lower than the BM. Annealing of the 1200 rpm and 60 mm/min FSW specimen resulted in tensile strength of 102% compared to base material and 47% increase in the strain at failure compared to the as-welded specimen. The Charpy impact values revealed up to 62 and 53% increase in the specific impact energy for the 1200 rpm and 60 mm/min welded joints as compared with the BM.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanical Properties and Wear Behavior of AA5182/WC Nanocomposite Fabricated by Friction Stir Welding at Different Tool Traverse Speeds

Grain growth inhibition at the heat-affected zone, improved weld strength and superior tribological properties of welds are desirable attributes of modern manufacturing. With the focused on these attributes, tungsten carbide (WC) nanoparticles were employed as reinforcements for the friction stir welding of 5-mm-thick AA5182 aluminum alloy by varying tool traverse speeds. The microstructure, microhardness, ultimate tensile strength, fracture and wear behavior of the resultant WC-reinforced welds were investigated, while unreinforced AA5182 welds were employed as controls for the study. The result shows that the addition of WC nanoparticles causes substantial grain refinement within the weld nugget. A decrease in traverse speed caused additional particle fragmentation, improved hardness value and enhanced weld strength in the reinforced welds. Improved wear rate and friction coefficient of welds were attained at a reduced traverse speed of 100 mm/min in the WC-reinforced welds. This improvement is attributed to the effects of reduced grain size/grain fragmentation and homogeneous dispersion of WC nanoparticles within the WC-reinforced weld nugget.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Characterization of Friction-Stir-Welded Dual-Phase Brass

Friction stir welding (FSW) is an ideal process to join brass to avoid the evaporation of zinc. In the present investigation, 6-mm-thick dual-phase brass plates were joined efficiently using FSW at various tool rotational speeds. The microstructures were studied using optical microscopy, electron backscattered diffraction and transmission electron microscopy. The optical micrographs revealed the evolution of various zones across the joint line. The microstructure of the heat-affected zone was similar to that of base metal. The weld zone exhibited finer grains due to dynamic recrystallization. The recrystallization was inhomogeneous and the inhomogeneity reduced with increased tool rotational speed. The dual phase was preserved in the weld zone due to the retention of zinc. The severe plastic deformation created a lot of dislocations in the weld zone. The weld zone was strengthened after welding. The role of tool rotational speed on the joint strength is further reported.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure and Mechanical Properties of Dissimilar Joints of Al-Mg{sub 2}Si and 5052 Aluminum Alloy by Friction Stir Welding

Al-Mg{sub 2}Si alloy and 5052 Al alloy were welded successfully by friction stir welding (FSW) in this study. The results show that the alloy consists of three distinct zones after FSW: the base material zone (BMZ), the transitional zone, and the weld nugget (WN). The morphologies of the primary Mg{sub 2}Si phases are identified as coarse equiaxed crystals for Al-Mg{sub 2}Si alloys in the BMZ. The WN is a mixture of rich Al-Mg{sub 2}Si and rich 5052 alloy, and a banded structure is formed in the zone. Interestingly, in the WN, the equiaxed crystals changed to polygonal particles with substantially reduced sizes in the rich Al-Mg{sub 2}Si zone. However, in addition to the white rich Mg phase appearing in the rich 5052 zone near the interface, the 5052 alloy does not show obvious changes. The hardness gradually increases from the BMZ of the 5052 to the welded joint to the Al-Mg{sub 2}Si BMZ. In addition, the ultimate tensile strength (UTS) of the welded joint is higher than that of the base material of the Al-Mg{sub 2}Si, whereas it is lower than that of the 5052 base alloy. The results of the elongation are similar to the UTS results. The fracture mechanism is also investigated.

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

Evaluating the Electrochemical Behavior of Friction Stir-Welded AA2219-O Alloy Joint in 3.5% NaCl Solution

Single-pass friction stir welding of AA2219-O aluminum alloy was carried out at high rotation speed (1180 rpm), and transverse travel rate of 33.5 mm/min was selected. The microstructural, mechanical and electrochemical characteristics of the weldment are investigated in this study. The high rotational tool speed resulted in a less uniform distribution of the intermetallic θ-phase (CuAl{sub 2}) in the thermo-mechanically affected zone (TMAZ) than in the weld nugget zone (WNZ). The higher transfer coefficient and lower charge transfer resistance (R{sub ct} + R{sub θ}) were observed in case of TMAZ as evaluated from potentiodynamic polarization and impedance spectroscopy, respectively. These results confirmed that microstructural variations could deteriorate the electrochemical performance of the weldment. TMAZ provided the least corrosion resistance than the base metal (BM) and WNZ due to the high concentration of cathodic intermetallic particles and possibly due to the large mechanical distortion of the grain structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Phase-Field Modeling of Nugget Zone for a AZ31-Mg-Alloy Friction Stir Weld

This work presents simulation of microstructure evolution in the nugget zone (NZ) of a AZ31-Mg-alloy friction stir weld. The process parameters (tool geometrical characteristics, rotational speed, travel speed, applied load) have been correlated with the resulting microstructural features in the NZ of the weld (grain size and population) with the aid of the MICRESS software, which provides the ability to simulate both nucleation and grain growth during dynamic recrystallization phenomena evolving in the NZ during the weld thermal cycle. The input parameters of the developed model include the tool geometry, the welding conditions as well as the recrystallization energy, the grain boundary mobility and specific material properties. NZ microstructure obtained by simulation shows good agreement with experimental measurements for both grain population and size.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fatigue Properties of Ti-6Al-4V Titanium Alloy Friction Stir Welding Joint

Friction stir welding was used to consolidate 2-mm-thick Ti-6Al-4V titanium alloy sheets, and the static and fatigue properties of the joint were investigated. The results reveal that uniform microhardness distribution of the nugget zone (NZ) after post-weld heat treatment was associated with the uniform equiaxed microstructure of the NZ. The joint fatigue limit predicted by the S–N curve reached 640.84 MPa, up to 64.76% of joint tensile strength. Fatigue specimens fractured at the transition zone between the base material and NZ, and the positions of fatigue crack initiation sites were relatively random. The fracture surface of the fatigue crack propagation region was composed of radial strips and striations, and numerous dimples were observed on the final fast fracture surface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of Welding Combined Plastic Forming on Microstructure Stability and Mechanical Properties of Friction Stir-Welded Al-Cu Alloy

A new tailor-welded process, which combines stages of welding, plastic forming, and heat treatment strengthening, was developed to produce a friction stir-welded (FSW) Al-alloy joint. The Al-Cu sheets were FSW under different welding parameters and then were plastic-deformed. The microstructural evolution and mechanical properties of the FSW joint were investigated by scanning electron microscopy and tensile tests. It was found that high heat input during FSW and low solution temperature suppress the abnormal grain growth (AGG) of the joint due to the differential in grain size and grain boundary energy. The microstructure heterogeneity of the FSW joint is effectively improved, as no AGG occurs. The retention of fine equiaxed grains and the increase in the density of precipitates result in excellent mechanical properties. The increase in the strength and micro-hardness of the joint mainly depends on the plastic deformation prior to aging treatment.

36 MATERIALS SCIENCE↗