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NASA NTRS1990

Deformation and fracture of thin sheet aluminum-lithium alloys: The effect of cryogenic temperatures

The objective is to characterize the fracture behavior and to define the fracture mechanisms for new Al-Li-Cu alloys, with emphasis on the role of indium additions and cryogenic temperatures. Three alloys were investigated in rolled product form: 2090 baseline and 2090 + indium produced by Reynolds Metals, and commercial AA 2090-T81 produced by Alcoa. The experimental 2090 + In alloy exhibited increases in hardness and ultimate strength, but no change in tensile yield strength, compared to the baseline 2090 composition in the unstretched T6 condition. The reason for this behavior is not understood. Based on hardness and preliminary Kahn Tear fracture experiments, a nominally peak-aged condition was employed for detailed fracture studies. Crack initiation and growth fracture toughness were examined as a function of stress state and microstructure using J(delta a) methods applied to precracked compact tension specimens in the LT orientation. To date, J(delta a) experiments have been limited to 23 C. Alcoa 2090-T81 exhibited the highest toughness regardless of stress state. Fracture was accompanied by extensive delamination associated with high angle grain boundaries normal to the fatigue precrack surface and progressed microscopically by a transgranular shear mechanism. In contrast the two peak-aged Reynolds alloys had lower toughness and fracture was intersubgranular without substantial delamination. The influences of cryogenic temperature, microstructure, boundary precipitate structure, and deformation mode in governing the competing fracture mechanisms will be determined in future experiments. Results contribute to the development of predictive micromechanical models for fracture modes in Al-Li alloys, and to fracture resistant materials.

Wagner, John A.↗

NASA NTRS1989

Potential applications of MMC and aluminum-lithium alloys in cameras for CRAF spacecraft

In undertaking the design of a 2000-mm focal length camera for the Mariner Mark II series of spacecraft, JPL sought novel materials with the requisite dimensional and thermal stability, outgassing and corrosion resistance, low mass, high stiffness, and moderate cost. Metal-matrix composites and Al-Li alloys have, in addition to excellent mechanical properties and low density, a suitably low coefficient of thermal expansion, high specific stiffness, and good electrical conductivity. The greatest single obstacle to application of these materials to camera structure design is noted to have been the lack of information regarding long-term dimensional stability.

Lane, Marc↗

NASA NTRS1989

NASA-UVA light aerospace alloy and structures technology program

The report on progress achieved in accomplishing of the NASA-UVA Light Aerospace Alloy and Structures Technology Program is presented. The objective is to conduct interdisciplinary graduate student research on the performance of next generation, light weight aerospace alloys and associated thermal gradient structures in close collaboration with researchers. The efforts will produce basic understanding of material behavior, new monolithic and composite alloys, processing methods, solid and fluid mechanics analyses, measurement advances, and a pool of educated graduate students. The presented accomplishments include: research on corrosion fatigue of Al-Li-Cu alloy 2090; research on the strengthening effect of small In additions to Al-Li-Cu alloys; research on localized corrosion of Al-Li alloys; research on stress corrosion cracking of Al-Li-Cu alloys; research on fiber-matrix reaction studies (Ti-1100 and Ti-15-3 matrices containing SCS-6, SCS-9, and SCS-10 fibers); and research on methods for quantifying non-random particle distribution in materials that has led to generation of a set of computer programs that can detect and characterize clusters in particles.

Gangloff, Richard P.↗

NASA NTRS1991

Structure and properties during aging of an Al-Cu-Li-Ag-Mg alloy, Weldalite (tm) 049

An Al-Cu-Li-Ag-Mg alloy, Weldalite (trademark) 049, was recently introduced as an ultra-high strength alloy (7000 MPa yield strength in artificially aged tempers) with good weldability. In addition, the alloy exhibits an extraordinary natural aging response (440 MPa yield strength (YS) in the unstretch condition) and a high ductility reversion condition which may be useful as a cold-forming temper. In contrast to other Al-Li alloys, these properties can essentially be obtained with or without a stretch or other coldworking operation prior to aging. Preliminary studies have revealed that the T4 temper (no stretch, natural age) is strengthened by a combination of GP zones and delta prime (Al3Li). The T6 temper (no stretch, aged at 180 C to peak strength) was reported to be strengthened primarily by T(sub 1) phase (Al2CuLi) with a minor presence of a theta prime like (Al2Cu) phase. On the other hand, a similar but lower solute containing alloy was reported to contain omega, (stoichiometry unknown), theta prime, and S prime in the peak strength condition. The purpose of this study is to further elucidate the strengthening phases in Weldalite (trademark) 049 in the unstretched tempers, and to follow the development of the microstructure from the T4 temper through reversion (180 C for 5 to 45 minutes) to the T6 temper.

Gayle, Frank W.↗

NASA NTRS1991

The effect of zinc additions on the environmental stability of Alloy 8090 (Al-Li-Cu-Mg-Zr)

Stress corrosion cracking (SCC) remains a problem in both Al-Li and conventional Al heat treatable alloys. It has recently been found that relatively small additions (less than or approximately 1 wt-percent) of Zn can dramatically improve the SCC performance of alloy 8090 (Al-Li-Cu-Mg-Zr). Constant load time to failure experiments using cylindrical tensile samples loaded between 30 and 85 percent of TYS indicate improvements of orders of magnitude over the baseline 8090 for the Zn-containing alloys under certain aging conditions. However, the toughnesses of the alloys were noticeably degraded due to the formation of second phase particles which primarily reside on grain and subgrain boundaries. EDS revealed that these intermetallic particles were Cu and Zn rich. The particles were present in the T3 condition and were not found to be the result of quench rate, though their size and distribution were. At 5 hours at 160 C, the alloys displayed the greatest susceptibility to SCC but by 20 hours at 160 C the alloys demonstrated markedly improved TTF lifetimes. Aging past this time did not provide separable TTF results, however, the alloy toughnesses continued to worsen. Initial examination of the alloys microstructures at 5 and 20 hours indicated some changes most notably the S' and delta' distributions. A possible model by which this may occur will be explored. Polarization experiments indicated a change in the trend of E(sub BR) and passive current density at peak aging as compared to the baseline 8090. Initial pitting experiments indicated that the primary pitting mechanism in chloride environments is one occurring at constituent (Al-Fe-Cu) particles and that the Cu and Zn rich boundary precipitates posses a breakaway potential similar to that of the matrix acting neither anodic or cathodic in the first set of aerated 3.5 w/o NaCl experiments. Future work will focus on the identification of the second phase particles, evaluation of K(sub 1SCC) and plateau da/dt via both DCB and slow strain rate techniques. A lower Zn content variant will be examined in the near future in hopes of optimizing toughness, density and SCC performance.

Kilmer, Raymond J.↗

NASA NTRS1991

Hydrogen interactions in aluminum-lithium alloys

A program is described which seeks to develop an understanding of the effects of dissolved and trapped hydrogen on the mechanical properties of selected Al-Li-Cu-X alloys. A proposal is made to distinguish hydrogen (H2) induced EAC from aqueous dissolution controlled EAC, to correlate H2 induced EAC with mobile and trapped concentrations, and to identify significant trap sites and hydride phases (if any) through use of model alloys and phases. A literature review shows three experimental factors which have impeded progress in the area of H2 EAC for this class of alloys. These are as listed: (1) inter-subgranular fracture in Al-Li alloys when tested in the S-T orientation in air or vacuum make it difficult to readily detect H2 induced fracture based on straight forward changes in fractography; (2) the inherently low H2 diffusivity and solubility in Al alloys is further compounded by a native oxide which acts as a H2 permeation barrier; and (3) H2 effects are masked by dissolution assisted processes when mechanical testing is performed in aqueous solutions.

Smith, S. W.↗

NASA NTRS1989

Fatigue crack propagation in aluminum-lithium alloys

The principal mechanisms which govern the fatigue crack propagation resistance of aluminum-lithium alloys are investigated, with emphasis on their behavior in controlled gaseous and aqueous environments. Extensive data describe the growth kinetics of fatigue cracks in ingot metallurgy Al-Li alloys 2090, 2091, 8090, and 8091 and in powder metallurgy alloys exposed to moist air. Results are compared with data for traditional aluminum alloys 2024, 2124, 2618, 7075, and 7150. Crack growth is found to be dominated by shielding from tortuous crack paths and resultant asperity wedging. Beneficial shielding is minimized for small cracks, for high stress ratios, and for certain loading spectra. While water vapor and aqueous chloride environments enhance crack propagation, Al-Li-Cu alloys behave similarly to 2000-series aluminum alloys. Cracking in water vapor is controlled by hydrogen embrittlement, with surface films having little influence on cyclic plasticity.

Rao, K. T. V.↗

NASA NTRS1990

Fabrication of structural components from commercial aluminum alloys using superplastic forming

SPF technology was used to fabricate structural components from the 7475 Al and 8090 Al-Li commercial alloys. Gas-pressurization cycles were established for SPF three-hat stiffener configurations on the basis of uniaxial data and component-geometry considerations. It is established that higher forming rates than the optimum strain rates selected from the uniaxial data for each alloy could be used in the later stages of forming without reducing SPF components' dimensional conformity. Cavitation was precluded through the use of back pressure during forming.

Hales, S. J.↗

NASA NTRS1992

A study of aluminum-lithium alloy solidification using acoustic emission techniques

Physical phenomena associated with the solidification of an aluminum lithium alloy was characterized using acoustic emission (AE) techniques. It is shown that repeatable patterns of AE activity may be correlated to microstructural changes that occur during solidification. The influence of the experimental system on generated signals was examined in the time and frequency domains. The analysis was used to show how an AE signal from solidifying aluminum is changed by each component in the detection system to produce a complex waveform. Conventional AE analysis has shown that a period of high AE activity occurs in pure aluminum, an Al-Cu alloy, and the Al-Li alloy, as the last fraction of solid forms. A model attributes this to the internal stresses of grain boundary formation. An additional period of activity occurs as the last fraction of solid forms, but only in the two alloys. A model attributes this to the formation of interdendritic porosity which was not present in the pure aluminum. The AE waveforms were dominated by resonant effects of the waveguide and the transducer.

Henkel, Daniel P.↗

NASA NTRS1993

Intrinsic fatigue crack growth rates for Al-Li-Cu-Mg alloys in vacuum

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable delta-prime (Al3Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constant K(max) to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent; da/dN varied up to tenfold based on applied Delta-K or Delta-K/E. When compared based on a crack tip cyclic strain or opening displacement parameter, growth rates were equivalent for all alloys except X2095-T8, which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly delta-prime, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear.

Slavik, D. C.↗

NASA NTRS1994

A study of gas flow pattern, undercutting and torch modification in variable polarity plasma arc welding

A study on the plasma and shield gas flow patterns in variable polarity plasma arc (VPPA) welding was undertaken by shadowgraph techniques. Visualization of gas flow under different welding conditions was obtained. Undercutting is often present with aluminum welds. The effects of torch alignment, shield gas flow rate and gas contamination on undercutting were investigated and suggestions made to minimize the defect. A modified shield cup for the welding torch was fabricated which consumes much less shield gas while maintaining the weld quality. The current torch was modified with a trailer flow for Al-Li welding, in which hot cracking is a critical problem. The modification shows improved weldablility on these alloys.

Mcclure, John C.↗

NASA NTRS1994

The effect of fabrication on corrosion in aluminum 2195: Environmental and microstructural considerations

Aluminum alloys containing lithium are particularly attractive to the aerospace structural designer. Lithium's density is only 0.53 g/cc, thus an addition of one weight percent lithium not only increases yield strength, but decreases the density by almost three percent while increasing the modulus by over six percent. The fact that lithium improves these physical properties simultaneously has led to intense study and development of the alloy system. Heretofore, problems in large scale alloy production have retarded commercial development. During the last fifteen years, advances in production technology have rekindled interest in Al-Li alloys, and aluminum suppliers have developed many candidate aerospace materials. However, if these alloys are to be employed successfully, a more complete understanding of their nonequilibrium metallurgy is required. Peel and Starke have each pointed out that an understanding of the weldability of these alloys is a critical step in their implementation. This study addresses the critical lack of information on the environmental compatibility of welded Al 2195 components. Corrosion data for these systems is incomplete, particularly for welded materials exposed to sea water or sea water condensate.

Walsh, Daniel W.↗

NASA NTRS1995

Production of aluminum-lithium near net shape extruded cylinders

In the late 1980's, under funding from the Advanced Launch System Program, numerous near net shape technologies were investigated as a means for producing high quality, low cost Aluminum-Lithium (Al-Li) hardware. Once such option was to extrude near net shape barrel panels instead of producing panels by machining thick plate into a final tee-stiffened configuration (which produced up to 90% scrap). This method offers a reduction in the volume of scrap and consequently reduces the buy-to-fly cost. Investigation into this technology continued under Shuttle-C funding where four Al alloys 2219, 2195, 2096, and RX 818 were extruded. Presented herein are the results of that program. Each alloy was successfully extruded at Wyman Gordon, opened and flattened at Ticorm, and solution heat treated and stretched at Reynolds Metals Company. The first two processes were quite successful while the stretching process did offer some challenges. Due to the configuration of the panels and the stretch press set-up, it was difficult to induce a consistent percentage of cold work throughout the length and width of each panel. The effects of this variation will be assessed in the test program to be conducted at a future date.

Hartley, Paula J.↗

NASA NTRS1995

Determination of Stress-Corrosion Cracking in Aluminum-Lithium Alloy ML377

The use of aluminum-lithium alloys for aerospace applications is currently being studied at NASA Langley Research Center's Metallic Materials Branch. The alloys in question will operate under stress in a corrosive environment. These conditions are ideal for the phenomena of Stress-Corrosion Cracking (SCC) to occur. The test procedure for SCC calls for alternate immersion and breaking load tests. These tests were optimized for the lab equipment and materials available in the Light Alloy lab. Al-Li alloy ML377 specimens were then subjected to alternate immersion and breaking load tests to determine residual strength and resistance to SCC. Corrosion morphology and microstructure were examined under magnification. Data shows that ML377 is highly resistant to stress-corrosion cracking.

Valek, Bryan C.↗

NASA NTRS1998

Characterization of Al-Cu-Li Alloy 2090 Near Net Shape Extrusion

Aluminum-lithium (Al-Li) alloys near net shape extrusions are being evaluated for potential application in launch vehicle structures. The objective of this study was to determine tensile and fracture properties, corrosion resistance, and weldability of integrally stiffened panels of Al-Cu-Li alloy 2090 in the T8 temper. The microstructure was pre-dominantly unrecrystallized. Texture analyses revealed the presence of fiber components in the stiffeners and a combination of fiber and rolling components in the skin. Variations in grain morphology and texture through the extruded cross section were correlated with the tensile, fracture, and corrosion behavior. Tensile strengths at room and cryogenic temperatures of the 2090 extrusions were similar to other 2090 product forms and were higher than 2219-T87, the primary structural material in the Space Shuttle external tank; however, ductilities were lower. The fracture resistance of the 2090 extrusion was lower than 2219-T87 plate at room temperature. At cryogenic temperatures, tensile ductility and fracture behavior of the 2090 extrusion were similar to other 2090 product forms but were lower than 2219-T87 plate. The exfoliation and stress corrosion resistance of the 2090 extrusion compared favorably with the characteristics of other 2090 product forms. The weldability and weldment properties of the extrusions were similar to 2090 and 2219 plates.

Birt, M. J.↗

NASA NTRS1998

Thermal Output of WK-Type Strain Gauges on Various Materials at Cryogenic and Elevated Temperatures

Strain gage apparent strain (thermal output) is one of the largest sources of error associated with the measurement of strain when temperatures and mechanical loads are varied. In this paper, experimentally determined apparent strains of WK-type strain gages, installed on both metallic and composite-laminate materials of various lay-ups and resin systems for temperatures ranging from -450 F to 230 F are presented. For the composite materials apparent strain in both the 0 ply orientation angle and the 90 ply orientation angle were measured. Metal specimens tested included: aluminum-lithium alloy (Al-LI 2195-T87), aluminum alloy (Al 2219-T87), and titanium alloy. Composite materials tested include: graphite-toughened-epoxy (IM7/997- 2), graphite-bismaleimide (IM7/5260), and graphite-K3 (IM7/K3B). The experimentally determined apparent strain data are curve fit with a fourth-order polynomial for each of the materials studied. The apparent strain data and the polynomials that are fit to the data are compared with those produced by the strain gage manufacturer, and the results and comparisons are presented. Unacceptably high errors between the manufacture's data and the experimentally determined data were observed (especially at temperatures below - 270-F).

Kowalkowski, Matthew K.↗

NASA NTRS1999

Assembly of a Full-Scale External Tank Barrel Section Using Friction Stir Welding

A full-scale pathfinder barrel section of the External Tank for the National Aeronautics and Space Administration (NASA) Space Transport System (Space Shuttle) has been assembled at Marshall Space Flight Center (MSFC) via a collaborative effort between NASA/MSFC and Lockheed Martin Michoud Space Systems. The barrel section is 27.5 feet in diameter and 15 feet in height. The barrel was assembled using Super-Light-Weight (SLWT), orthogrid, Al-Li 2195 panel sections and a single longeron panel. A vertical weld tool at MSFC was modified to accommodate FSW and used to assemble the barrel. These modifications included the addition of a FSW weld head and new controller hardware and software, the addition of a backing anvil and the replacement of the clamping system with individually actuated clamps. Weld process 4evelopment was initially conducted to optimize the process for the welds required for completing the assembly. The variable thickness welds in the longeron section were conducted via both two-sided welds and with the use of a retractable pin tool. The barrel assembly was completed in October 1998. Details of the vertical weld tool modifications and the assembly process are presented.

Jones, Chip↗

NASA NTRS1999

Mechanical Properties of Friction Stir Welds in A12195-T8

An extensive study of the mechanical properties of friction stir welded Al-Li 2195 has been conducted by Lockheed Martin Michoud Space Systems under contract to NASA. The study was part of a development program in which weld parameters were defined for using FSW to assemble large-scale aluminum cryogenic tanks. In excess of 300 feet of 0.320 in. gage plate material was welded and tested. The tests include room temperature and cryogenic temperature tensile tests and surface crack tension (SCT) tests, nondestructive evaluation, metallurgical studies, and photostress analysis. The results of the testing demonstrated improved mechanical properties with FSW as compared to typical fusion welding processes. Increases in ultimate tensile strength, cryogenic enhancement and elongation were observed with the tensile test results. Increased fracture toughness was observed with the SCT results. Nondestructive evaluations were conducted on all welded Joints. No volumetric defects were indicated. Surface indications on the root side of the welds did not significantly affect weld strength. The results of the nondestructive evaluations were confirmed via metallurgical studies. Photostress analysis revealed strain concentrations in multi-pass and heat-repaired FSW's. Details of the tests and results are presented.

Kinchen, David G.↗