Bradley A. Lerch

Fretting Wear of Ti-48Al-2Cr-2Nb

Abstract An investigation was conducted to examine the wear behavior of gamma titanium aluminide (Ti-48Al-2Cr-2Nb in atomic percent) in contact with a typical nickel-base superalloy under repeated microscopic vibratory motion in air at temperatures from 296–823 K. The surface damage observed on the interacting surfaces of both Ti-48Al-2Cr-2Nb and superalloy consisted of fracture pits, oxides, metallic debris, scratches, craters, plastic deformation, and cracks. The Ti-48Al-2Cr-2Nb transferred to the superalloy at all fretting conditions and caused scuffing or galling. The increasing rate of oxidation at elevated temperatures led to a drop in Ti-48Al-2Cr-2Nb wear at 473 K. Mild oxidative wear was observed at 473 K. However, fretting wear increased as the temperature was increased from 473–823 K. At 723 and 823 K, oxide disruption generated cracks, loose wear debris, and pits on the Ti-48Al-2Cr-2Nb wear surface. Ti-48Al-2Cr-2Nb wear generally decreased with increasing fretting frequency. Both increasing slip amplitude and increasing load tended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals.

Development, Characterization, and Design Considerations of Ni19.5Ti50.5 Pd25Pt5 High-temperature Shape Memory Alloy Helical Actuators

Shape memory alloys (SMAs) have been used in various applications since their discovery. However, their use as actuation devices in high-temperature environments has been limited due to the temperature constraints of commercially available materials. Recently, SMAs that produce good work characteristics at elevated temperatures have been developed at NASA’s Glenn Research Center. One such alloy, Ni19.5Ti50.5Pd25Pt 5, has shown repeatable strain recovery on the order of 2.5% in the presence of an externally applied stress at temperatures greater than 250°C. Based on these findings, potential applications for this alloy are being explored and further work is being done to assess the use of this alloy in various structural forms. In this article, the characterization of Ni 19.5Ti50.5Pd25Pt5 helical actuators is reported, including their mechanical responses and how variations in their responses correlate to changes in geometric parameters and training loads. Finally, implementation of previously published SMA spring design methodology in future SMA helical actuator development is considered through comparison of the observed and predicted responses.

Investigation of Low-Cycle Bending Fatigue of AISI 9310 Steel Spur Gears

An investigation of the low-cycle bending fatigue of spur gears made from AISI 9310 gear steel was completed. Tests were conducted using the single-tooth bending method to achieve crack initiation and propagation. Tests were conducted on spur gears in a fatigue test machine using a dedicated gear test fixture. Test loads were applied at the highest point of single tooth contact. Gear bending stresses for a given testing load were calculated using a linear-elastic finite element model. Test data were accumulated from 1/4 cycle to several thousand cycles depending on the test stress level. The relationship of stress and cycles for crack initiation was found to be semi-logarithmic. The relationship of stress and cycles for crack propagation was found to be linear. For the range of loads investigated, the crack propagation phase is related to the level of load being applied. Very high loads have comparable crack initiation and propagation times whereas lower loads can have a much smaller number of cycles for crack propagation cycles as compared to crack initiation.

Analysis of Stainless Steel Sandwich Panels with a Metal Foam Core for Lightweight Fan Blade Design

The quest for cheap, low density and high performance materials in the design of aircraft and rotorcraft engine fan and propeller blades poses immense challenges to the materials and structural design engineers. Traditionally, these components have been fabricated using expensive materials such as light weight titanium alloys, polymeric composite materials and carbon-carbon composites. The present study investigates the use of a sandwich foam fan blade made up of solid face sheets and a metal foam core. The face sheets and the metal foam core material were an aerospace grade precipitation hardened 17-4 PH stainless steel with high strength and high toughness. The stiffness of the sandwich structure is increased by separating the two face sheets by a foam core. The resulting structure possesses a high stiffness while being lighter than a similar solid construction. Since the face sheets carry the applied bending loads, the sandwich architecture is a viable engineering concept. The material properties of 17-4 PH metal foam are reviewed briefly to describe the characteristics of the sandwich structure for a fan blade application. A vibration analysis for natural frequencies and a detailed stress analysis on the 17-4 PH sandwich foam blade design for different combinations of skin thickness and core volume are presented with a comparison to a solid titanium blade.

Evaluation of Ti-48Al-2Cr-2Nb Under Fretting Conditions

Kazuhisa Miyoshi, Bradley A. Lerch, Susan L. Draper, and Sai V. RajNational Aeronautics and Space AdministrationGlenn Research CenterCleveland, Ohio 44135SUMMARYAn investigation was conducted to examine the fretting behavior of 7-TiAI (Ti-48AI-2Cr-2Nb) in contact with anickel-base superalloy (Inconel 718) in air at temperatures from 23 to 550 °C. Fretting wear experiments were con-ducted with 9.4-mm-diameter hemispherical Inconel (IN) 718 pins in contact with Ti-48AI-2Cr-2Nb fiats (and thereverse) at loads from 1 to 40 N and fretting frequencies from 50 to 160 Hz with slip amplitudes from 50 to 200 gmfor 1 to 20 million fretting cycles. The results were similar for both combinations of pin and fiat. Reference frettingwear experiments were also conducted with 9.4-ram-diameter hemispherical Ti-6AI-4V pins in contact withIN718 flats.The interfacial adhesive bonds between Ti-48AI-2Cr-2Nb and IN718 in contact were generally stronger than thecohesive bonds in the cohesively weaker Ti-48AI-2Cr-2Nb. The failed Ti-48AI-2Cr-2Nb subsequently transferred tothe IN718 surface at any fretting condition. The wear scars produced on Ti-48AI-2Cr-2Nb contained metallic andoxide wear debris, scratches, plastically deformed asperities, cracks, and fracture pits. Oxide layers readily formedon the Ti-48AI-2Cr-2Nb surface at 550 °C, but cracks easily occurred in the oxide layers. Factors including frettingfrequency, temperature, slip amplitude, and load influenced the fretting behavior of Ti-48AI-2Cr-2Nb in contactwith IN718. The wear volume loss of Ti-48AI-2Cr-2Nb generally decreased with increasing fretting frequency. Theincreasing rate of oxidation at elevated temperatures up to 200 °C led to a drop in wear volume loss at 200 °C.However, the fretting wear increased as the temperature was increased from 200 to 550 °C. The highest tempera-tures of 450 and 550 °C resulted in oxide film disruption with generation of cracks, loose wear debris, and pits onthe Ti-48AI-2Cr-2Nb wear surface. The wear volume loss generally increased as the slip amplitude increased. Thewear volume loss also generally increased as the load increased. Increasing slip amplitude and increasing load bothtended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals.1.0 INTRODUCTIONAdhesion, a manifestation of mechanical strength over an appreciable area, has many causes, including chemi-cal bonding, deformation, and the fracture processes involved in interface failure. A clean metal in contact with aclean metal will fail either in tension or in shear because some of the interfacial bonds are generally stronger thanthe cohesive bonds in the cohesively weaker metal (ref. 1). The failed metal subsequently transfers to the other con-tacting metal. Adhesion undoubtedly depends on the surface cleanliness, the area of real contact, the chemical,physical, and mechanical properties of the interface, and the modes of junction rupture. The environment influencesthe adhesion, deformation, and fracture behaviors of contacting materials in relative motion.Clean surfaces can be created by repeated sliding, making direct contact of the fresh, clean surfaces unavoidablein practical cases (ref. 2). This situation applies in some degree to contact sliding in air, where fresh surfaces arecontinuously produced on interacting surfaces in relative motion. Microscopically small surface-parallel relativemotion, which can be vibratory (in common fretting or false brinnelling) or creeping (in common fretting), producesfresh, clean interacting surfaces and causes junction (contact area) growth in the contact zone (refs. 3 to 5).Fretting wear produced between contacting elements is adhesive wear taking place in a nominally static contactunder normal load and repeated microscopic vibratory motion (refs. 6 to 10). The most damaging effect of frettingis the possibly significant reduction in fatigue capability of the fretted component even though the wear producedby fretting appears to be quite mild (ref. 10). It was reported that the reduction in fatigue strength by fretting ofTi-47AI-2Nb-2Mn with 0.8 vol.% TiB 2 was approximately 20 percent.Fretting fatigue is a complex problem of significant interest to aircraft engine manufacturers (refs. 11 to 14).Fretting failure can occur to a variety of engine components. Numerous approaches, depending on the componentand the operating conditions, have been taken to address the fretting problem. The components of interest in thisinvestigation were the fan and compressor blades. Many existing fan and compressor components have titaniumNASA/TM--2001-210902 i

Development and Hot-fire Testing of Additively Manufactured Copper Combustion Chambers for Liquid Rocket Engine Applications

NASA and industry partners are working towards fabrication process development to reduce costs and schedules associated with manufacturing liquid rocket engine components with the goal of reducing overall mission costs. One such technique being evaluated is powder-bed fusion or selective laser melting (SLM), commonly referred to as additive manufacturing (AM). The NASA Low Cost Upper Stage Propulsion (LCUSP) program was designed to develop processes and material characterization for GRCop-84 (a NASA Glenn Research Center-developed copper, chrome, niobium alloy) commensurate with powder-bed AM, evaluate bimetallic deposition, and complete testing of a full scale combustion chamber. As part of this development, the process has been transferred to industry partners to enable a long-term supply chain of monolithic copper combustion chambers. To advance the processes further and allow for optimization with multiple materials, NASA is also investigating the feasibility of bimetallic AM chambers. In addition to the LCUSP program, NASA has completed a series of development programs and hot-fire tests to demonstrate SLM GRCop-84 and other AM techniques. NASA’s efforts include a 4K lbf thrust liquid oxygen/methane (LOX/CH4) combustion chamber and subscale thrust chambers for 1.2K lbf LOX/hydrogen (H2) applications that have been designed and fabricated with SLM GRCop84. The same technologies for these lower thrust applications are being applied to 25-35K lbf main combustion chamber (MCC) designs. This paper describes the design, development, manufacturing and testing of these numerous combustion chambers, and the associated lessons learned throughout their design and development processes.

Impact of Powder Variability on the Microstructure and Mechanical Behavior of Selective Laser Melted Alloy 718

Powder-bed additive manufacturing processes use fine powders to build parts layer-by-layer. Alloy 718 powder feedstocks for selective laser melting (SLM) additive manufacturing are produced commercially by both gas and rotary atomization and are available typically in the 10–45 or 15–45 µm size ranges. A comprehensive investigation was conducted to understand the impact of powder variability on the microstructure and mechanical behavior of SLM 718 heat treated to Aerospace Material Specification (AMS) 5664. This study included sixteen virgin powders and three once-recycled powders within the 10–45 and 15–45 µm size ranges that were obtained from seven direct source suppliers and one reseller. Although alike as highly regular spheroids, these powders showed distinct differences in composition (especially Al, C and N contents), particle size distributions, and powder features such as degree of agglomeration, fusion and surface roughness. Compositional differences expectedly had the strongest impact on microstructure. High N and C contents formed TiN-nitrides and/or (Ti, Nb, Mo)-C carbides on the grain boundaries, prevented recrystallization during heat treatment, and resulted in retained (001)-scalloped shaped grains that ranged from 19 to 41 µm in average size. In the absence of this particle pinning, the average grain size of the heat treated SLM 718 ranged from 51 to 90 µm. Room temperature tensile and high cycle fatigue (HCF) testing compared as-fabricated (AF) and low stress ground (LSG) surface conditions. Tensile testing revealed consistent behavior between the two surface conditions and amongst the powder lots. The finer grained SLM 718 builds displayed the lowest tensile properties. A SLM 718 build fabricated from a powder with eight times lower C content showed statistically better tensile properties presumably due to enhanced coarsening of δ-Ni3Nb precipitates. The specimens from once-recycled powders had slightly higher tensile strengths and slightly higher ductility compared to their virgin equivalents; once-recycling also did not substantially degrade the mean HCF life. The LSG fatigue lives agreed with conventionally manufactured 718 data, while AF lives exhibited a knock-down due to surface roughness. The fatigue lives of AF specimens were statistically equivalent across powder lots except for one and failures typically initiated at stress concentrators associated with SLM surface asperities. Fatigue testing of low stress ground specimens result in both transgranular and within facet crack initiations. More than half of the cracks initiated from these facets for the machined condition; however, these facets appeared to be within grains that were larger-than-average in size. A nitrogen-atomized powder with fine prior particles of TiN-nitrides and M(Ti, Nb, Mo)C carbides from atomization on powder surfaces resulted in the best fatigue performance with segregation of these particles to the SLM 718 grain boundaries leading to higher resistance to early-stage crack propagation. Typically the fine-grained builds with minor phases along the grain boundaries did not perform well in fatigue, whereas a larger-grain build with lower carbon content and coarser δ-Ni3Nb precipitates showed the next best HCF response. Further details of the build microstructure and its impact on tensile and fatigue behavior was considered.

A study for stainless steel fan blade design with metal foam core

The pursuit for cheap, low-density and high-performance materials in the design of aircraft engine blades raises wide-ranging challenges to the materials and structural design engineers. Traditionally, these components have been fabricated using expensive materials such as lightweight titanium alloys and polymer composite materials composites. The present study investigates the use of a sandwich foam fan blade made of solid face sheets and a metal foam core. The face sheets and the metal foam core material were an aerospace grade precipitation-hardened 17-4 stainless steel with high strength and high toughness. The stiffness of the sandwich structure is increased by separating the two face sheets by a foam core. The resulting structure possesses a high stiffness while being lighter than a similar solid construction. Since the face sheets carry the applied bending loads, the sandwich architecture is a viable engineering concept. The material properties of 17-4 precipitation-hardened metal foam are briefly reviewed to describe the characteristics of the sandwich structure for a fan blade application. Vibration characteristics and design criteria on the 17-4 precipitation-hardened metal foam core sandwich blade design with different combinations of skin thickness and core volume are presented with a comparison to a solid titanium blade.