Marcia S. Domack

Metallurgical Mechanisms Controlling Mechanical Properties of Aluminum Alloy 2219 Produced by Electron Beam Freeform Fabrication

The electron beam freeform fabrication (EBF3) layer-additive manufacturing process has been developed to directly fabricate complex geometry components. EBF3 introduces metal wire into a molten pool created on the surface of a substrate by a focused electron beam. Part geometry is achieved by translating the substrate with respect to the beam to build the part one layer at a time. Tensile properties have been demonstrated for electron beam deposited aluminum and titanium alloys that are comparable to wrought products, although the microstructures of the deposits exhibit features more typical of cast material. Understanding the metallurgical mechanisms controlling mechanical properties is essential to maximizing application of the EBF3 process. In the current study, mechanical properties and resulting microstructures were examined for aluminum alloy 2219 fabricated over a range of EBF3 process variables. Material performance was evaluated based on tensile properties and results were compared with properties of Al 2219 wrought products. Unique microstructures were observed within the deposited layers and at interlayer boundaries, which varied within the deposit height due to microstructural evolution associated with the complex thermal history experienced during subsequent layer deposition. Microstructures exhibited irregularly shaped grains, typically with interior dendritic structures, which were described based on overall grain size, morphology, distribution, and dendrite spacing, and were correlated with deposition parameters. Fracture features were compared with microstructural elements to define fracture paths and aid in definition of basic processingmicrostructure- property correlations.

Texture Evolution within the Thermomechanically Affected Zone of an Al-Li Alloy 2195 Friction Stir Weld

Friction stir welding (FSW) of Al-Li alloy 2195 plate produces strong texture gradients. The microstructural characteristics evolve from the base plate, through the thermomechanically affected zone (TMAZ), to the weld nugget interface. In the current study, electron backscattered diffraction (EBSD) analyses were employed to quantify the spatial distribution of texture gradients associated with the evolution of texture within the TMAZ. The strong texture of the base plate enabled the texture evolution to be characterized as a function of location. Systematic partitioning of EBSD data relative to the degree of lattice rotation at each point accurately captured the crystallographic transitions across the advancing side TMAZ. Over a large section of this region, the texture evolves as a result of continuous rigid body rotations. The rigid body rotations were correlated with the complex material flow patterns commonly associated with the FSW process and prior observations of shear-related textures. Finally, a correlation between texture and fracture in a subscale tensile specimen is observed, where failure occurs within a visible band of low-Taylor factor grains.

Evolution and Control of 2219 Aluminium Microstructural Features through Electron Beam Freeform Fabrication

Electron beam freeform fabrication (EBF3) is a new layer-additive process that has been developed for near-net shape fabrication of complex structures. EBF3 uses an electron beam to create a molten pool on the surface of a substrate. Wire is fed into the molten pool and the part translated with respect to the beam to build up a 3-dimensional structure one layer at a time. Unlike many other freeform fabrication processes, the energy coupling of the electron beam is extremely well suited to processing of aluminum alloys. The layer-additive nature of the EBF3 process results in a tortuous thermal path producing complex microstructures including: small homogeneous equiaxed grains; dendritic growth contained within larger grains; and/or pervasive dendritic formation in the interpass regions of the deposits. Several process control variables contribute to the formation of these different microstructures, including translation speed, wire feed rate, beam current and accelerating voltage. In electron beam processing, higher accelerating voltages embed the energy deeper below the surface of the substrate. Two EBF3 systems have been established at NASA Langley, one with a low-voltage (10-30kV) and the other a high-voltage (30-60 kV) electron beam gun. Aluminum alloy 2219 was processed over a range of different variables to explore the design space and correlate the resultant microstructures with the processing parameters. This report is specifically exploring the impact of accelerating voltage. Of particular interest is correlating energy to the resultant material characteristics to determine the potential of achieving microstructural control through precise management of the heat flux and cooling rates during deposition.

Evaluation of Sc-Bearing Aluminum Alloy C557 for Aerospace Applications

Abstract. The performance of the A1-Mg-Sc alloy C557 was evaluated to assess its potential fora broad range of aerospace applications, including airframe and launch vehicle structures. Ofspecific interest were mechanical properties at anticipated service temperatures and thermalstability of the alloy. Performance was compared with conventional airframe aluminum alloysand with other emerging aluminum alloys developed for specific service environments.Mechanical properties and metallurgical structure were evaluated for commercially rolled sheetin the as-received H 116 condition and after thermal exposures at 107°C. Metallurgical analyseswere performed to define grain morphology and texture, strengthening precipitates, and to assessthe effect of thermal exposure.Introduction. Wrought, non-heat-treatable aluminum-magnesium alloys are potentialcandidates for structural applications because of their low density, good weldability, andexcellent corrosion resistance [1]. The addition of scandium has been shown to increase thestrength while maintaining the ductility of aluminum-magnesium alloys [2, 3]. Additions ofscandium and zirconium to A1-Mg alloys synergystically promote strengthening and result inhigher strengths than either Sc or Zr additions produce alone [4]. In wrought A1-Mg-Sc alloyswith Zr additions, strengthening occurs primarily by development of coherent A13Sc and A13Zrdispersoids. Additional strengthening is achieved by grain refinement, as the dispersoids inhibitrecrystallization during working [4].The moderate strength level achieved in A1-Mg-Sc alloys combined with the inherentcorrosion resistance of the A1-Mg system makes these alloys attractive for airframe structuralapplications. Sc-bearing dispersoids are inherently thermally stable [5], which enablesconsideration for structural applications where extended elevated temperatures are anticipated.However, in alloys with magnesium levels above about 3.5%, precipitation of the 13phase,AlsMgs, can occur during thermal exposure and has been shown to be detrimental to corrosionresistance [5]. Limited data available at cryogenic temperatures [2] suggests that A1-Mg-Scalloys are suitable for service down to liquid hydrogen temperatures.In the current study, the A1-Mg-Sc alloy C557 was evaluated to assess its potential for abroad range of aerospace applications. Particular emphasis is placed on strength-toughnessbehavior at temperatures from -184°C to 107°C to establish service temperature applicability andafter exposures up to 10,000 hours at 107°C to evaluate thermal stability.MaterialsThe material evaluated was produced by Alcoa as a 4,500 kg (10,000 lb) commercial scaleingot. Proprietary thermomechanical processing was used to produce rolled sheets, 1.22 m x3.66 m, both 1.6 mm and 2.3 mm thick, in the H116 condition. The thermomechanicalprocessing schedule was tailored to optimize the sheet for superplastic forming and the H 116heat treatment [1] was chosen to ensure good stress corrosion resistance. The chemistry of therolled sheet, provided in Table 1, is very similar to the Russian alloy 1535 [2]. As shown inTable 1. Chemical Composition of C557 Rolled Sheet (in weight percent).Sheet Thickness Mg Sc Mn Zr Zn Fe Si1.6 mm 3.94 0.22 0.62 0.100 0.016 0.099 0.0522.3 mm 4.02 0.24 0.62 0.096 0.015 0.095 0.062

Effect of Thermomechanical Processing on Texture and Mechanical Properties of Al-Cu-Li Alloy Plate

For 2195-T8 plate, design properties are based on the mechanical properties at the near surface location, corresponding to the load carrying thin membrane location in machined integrally stiffened structure. Mechanical properties at the near surface location are lower than those at the t/4 and t/2 locations. This work examined the effect of alternate temper and hot rolling practices on near surface strength levels. Results showed that alternate hot rolling practice were very effective in raising near surface strengths and improving property uniformity, and were well correlated with crystallographic texture measurements.

Using Powder Cored Tubular Wire Technology to Enhance Electron Beam Freeform Fabricated Structures

Electron Beam Freeform Fabrication (EBF3) is an additive manufacturing technique, developed at NASA Langley Research Center, capable of fabricating large scale aerospace parts. Advantages of using EBF3 as opposed to conventional manufacturing methods include, decreased design-to-product time, decreased wasted material, and the ability to adapt controls to produce geometrically complex parts with properties comparable to wrought products. However, to fully exploit the potential of the EBF3 process development of materials tailored for the process is required. Powder cored tubular wire (PCTW) technology was used to modify Ti-6Al-4V and Al 6061 feedstock to enhance alloy content, refine grain size, and create a metal matrix composite in the as-solidified structures, respectively.

Investigation of Abnormal Grain Growth in a Friction Stir Welded and Spin-Formed Al-Li Alloy 2195 Crew Module

In order to improve manufacturing efficiency and reduce structural mass and costs in the production of launch vehicle structures, NASA is pursuing a wide-range of innovative, near-net shape manufacturing technologies. A technology that combines friction stir welding (FSW) and spin-forming has been applied to manufacture a single-piece crew module using Aluminum-Lithium (Al-Li) alloy 2195. Plate size limitations for Al-Li alloy 2195 require that two plates be FSW together to produce a spin-forming blank of sufficient size to form the crew module. Subsequent forming of the FSW results in abnormal grain growth (AGG) within the weld region upon solution heat treatment (SHT), which detrimentally impacts strength, ductility, and fracture to ughness. The current study seeks to identify microstructural factors that contribute to the development of AGG. Electron backscatter diffraction (EBSD) was used to correlate driving forces for AGG, such as stored energy, texture, and grain size distributions, with the propensity for AGG. Additionally, developmental annealing treatments prior to SHT are examined to reduce or eliminate the occurrence of AGG by promoting continuous or uniform grain growth.

Correlation of Fracture Behavior with Microstructure in Friction Stir Welded, and Spin-Formed Al-Li 2195 Domes

Single-piece, spin-formed domes manufactured from friction stir welded (FSW) plates of Al-Li alloy 2195 have the potential to reduce the cost of fabricating cryogenic propellant tanks. Mechanical properties in the completed domes can be related directly to the final material condition and the microstructures developed. However, these new fabrication techniques have resulted in unexpected material challenges, such as abnormal grain growth in the weld nugget and the propensity for fracture in the adjacent thermo-mechanically affected zone (TMAZ). In this study, the microstructure and texture transformations within the TMAZ are related to fracture location in the vicinity of the weldment. The texture variations in the TMAZ are caused primarily by the varying amounts of shear deformation introduced during the FSW process. Grain morphology and microtexture characteristics are examined as a function of location in the TMAZ via electron backscatter diffraction (EBSD). A strong correlation between fracture location and the presence of texture banding in the TMAZ is observed. The fracture path tends to follow a distinct region of low Taylor Factor (TF) grains.