J. Foyos

Alternative heat treatments for A357-T6 aluminum alloy

Abstract An aluminum foundry experienced frequent failures of A357-T6 aluminum castings. These failures are manifested in attaining tensile stresses and total percentage elongation below the minimum requirements of MIL-A-22180D. Sixteen heat treatments were designed to study the effect of varying the heat treatment parameters on the mechanical properties and consequently to minimize failures due to the heat treatment step.

The effect of off-axis thermomechanical processing on the mechanical behavior of textured 2095 Al-Li alloy

Abstract This investigation was conducted to study the influence of off-axis deformation in the thermomechanical processing on the properties of Al–Li 2095 HT72 alloy sheet. The thermomechanical processing involved several stages. First, the as-received hot rolled sheet was solution heat treated at 510°C for 30 min. Samples were then cut from the sheet and stretched off-axis at 0, 30, 60 and 90° angles (stretch axis) to the original rolling direction. All the specimens were then aged at 180°C for 24 h. Tensile coupons were then machined from each of these aged samples at 0, 30, 45, 60 and 90° (tensile axis) to the rolling direction, then tested to failure. The crystallographic texture and tensile properties of the processed samples were evaluated, and were compared to the solution heat-treated material. In the solution heat-treated condition, the material was highly textured with a very strong Brass component. Off-axis stretch followed by aging had little or no effect on the texture development of the solution heat-treated material. Upon testing, samples with stretch axis at 0° showed pronounced mechanical anisotropy; specimens tested at 45° showed about 40% drop in tensile and yield strength values when compared to those tested at 0°. However, stretching along 60° or 90° directions minimized mechanical anisotropy. The texture characteristics of all tested samples were determined and related to the changes in the mechanical properties. An attempt was made to correlate the data with some texture theories. Analysis of the results and normalization based on the Taylor’s factor showed that that crystallographic texture contributes only partly to the total mechanical anisotropy.

Failure analysis of K-monel 500 (Ni–Cu–Al alloy) bolts

Abstract A bolt manufacturer used several subcontractors to fabricate K-monel 500 bolts. The completed bolts failed mechanical testing, and the manufacturer ascribed the failure to the bar stock. Testing contracted by the supplier of the bar stock showed that it met specifications. Metallographic testing described here shows that the bolts failed because of excessive grain growth and subsequent softening during heat treatment performed on the bolts after they were formed. Alternative heat treatments are discussed.

Evaluation of Advanced Adhesives for Aerospace Structures

Polymer adhesives are finding increased use in panel joining applications in aircraft and aerospace structures where the applied stresses permit their use and where a uniform stress distribution is needed. One such adhesive, Hysol EA-9394™, was compared to three other formulations in this study. The new formulations were Hysol EA-9396, Hysol EA-9396 filled with nickel nanofibers and mixed by machine (Jamesbury Blender), and Hysol EA-9396 filled with nickel nanofibers and hand mixed in the laboratory. The comparison consisted of measuring shear lap strengths of aluminum test pieces bonded together with the candidate adhesives. The mechanical tests were supplemented by a Weibull analysis of the strength data and by a visual inspection of the failure mode (adhesive/cohesive). The lap shear strengths (fracture stress values) of all three Hysol EA-9396 adhesives were greater than that of the baseline Hysol EA-9394 polymer.

Low-cycle fatigue of titanium 6A1-4V surgical tools

Abstract A handle holder is used in heart valve replacement surgery to insert a mechnical flow valve into the patients heart. In order to position the valve correctly, a 0.1″ diameter titanium 6Al4V handle holder shaft is bent until the proper angle is found. This bending of the tool makes it vulnerable to low-cycle fatigue failure. Low cycle fatigue testing of titanium 6A1-4V specimens shows that surface cracks appear approximately 20 cycles before failure occurs. It is recommended that the tool be carefully inspected before each used and replaced when surface cracks appear. The data showed that the life of the tool may be increased by 30% if it is bent in the same direction during its entire use.

Evaluation of the Microstructure and Charpy Impact Toughness of Hot Worked Powder Ti-6Al-4V

Ti-6Al-4V powder, produced by the hydride-dehydride (HDH) process, was hot isostatically pressed (HIP) into three bars. The 10 cm (4 in) diameter bars were hot worked (HW) to three different diameters: 5.1 cm (2 in) (75% reduction in area), 3.8 cm (1.5 in) (86% reduction in area), and 2.5 cm (1 in) (94% reduction in area). Three samples were machined out of each bar along the end, middle and transverse orientations. These samples were ground, polished, and etched. The microstructure of the samples was evaluated at 100X and 200X magnifications. The objective of this experiment was to examine the effect of deformation on the microstructure and properties of hot rolled titanium alloy bar product. Charpy impact samples were also machined out of each of the various diameter bars. Impact testing was used to quantify toughness by correlating the microstructure to the energy absorbed. The tensile properties of the hot bars were determined as well as the crystallographic texture. Scanning electron microscopy (SEM) was performed on the fractured surface of the Charpy impact samples.

The Effect of Processing Method on Microstructure and Mechanical Behavior of Ti-6Al-4V Plate Produced by Powder Metallurgy Technique

Three Ti-6Al-4V plate materials produced by powder metallurgy technique, included pre-alloyed hydride-dehydride (HDH) plate rolled to 75% reduction in thickness, and two blended elemental (BE) powder plates rolled to 75% and 87% reduction were evaluated. The objective of this study was to determine differences in microstructure and toughness between the pre-alloyed HDH and BE Ti-6Al-4V materials processed to the same product form. Heat treatments were performed below the beta transus temperature at 982, 871, 760, and 732°C (1800, 1600, 1400, and 1350°F) for 1, 2, and 4 hours in order to determine differences in heat treating response, and above the beta transus at 1076°C (1970°F) to determine the transformation temperature. The samples were evaluated by optical microscopy and scanning electron microscopy. Charpy impact testing was performed in order to determine differences in the energy absorbed during fracture. Pole figures (0002) of selected conditions were also performed in order to determine any differences in texture between the various conditions.

LOW-CYCLE FATIGUE OF TITANIUM 6A1–4V SURGICAL TOOLS

Abstract A handle holder is used in heart valve replacement surgery to insert a mechanical flow valve into the patients heart. In order to position the valve correctly, a 0.1″ diameter titanium 6A1–4V handle holder shaft is bent until the proper angle is found. This bending of the tool makes it vulnerable to low-cycle fatigue failure. Low cycle fatigue testing of titanium 6A1–4V specimens shows that surface cracks appear approximately 20 cycles before failure occurs. It is recommended that the tool be carefully inspected before each use and replaced when surface cracks appear. The data showed that the life of the tool may be increased by 30% if it is bent in the same direction during its entire use.