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J. Eng. Mater. Technol. 2019;141(4):041001-041001-31. doi:10.1115/1.4042870.

Al 7068-T651 alloy is one of the recently developed materials used mostly in the defense industry due to its high strength, toughness, and low weight compared to steels. The aim of this study is to identify the Johnson–Cook (J–C) material model parameters, the accurate Johnson–Cook (J–C) damage parameters, D1, D2, and D3 of the Al 7068-T651 alloy for finite element analysis-based simulation techniques, together with other damage parameters, D4 and D5. In order to determine D1, D2, and D3, tensile tests were conducted on notched and smooth specimens at medium strain rate, 100 s−1, and tests were repeated seven times to ensure the consistency of the results both in the rolling direction and perpendicular to the rolling direction. To determine D4 and D5 further, tensile tests were conducted on specimens at high strain rate (102 s−1) and temperature (300 °C) by means of the Gleeble thermal–mechanical physical simulation system. The final areas of fractured specimens were calculated through optical microscopy. The effects of stress triaxiality factor, rolling direction, strain rate, and temperature on the mechanical properties of the Al 7068-T651 alloy were also investigated. Damage parameters were calculated via the Levenberg–Marquardt optimization method. From all the aforementioned experimental work, J–C material model parameters were determined. In this article, J–C damage model constants, based on maximum and minimum equivalent strain values, were also reported which can be utilized for the simulation of different applications.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(4):041002-041002-9. doi:10.1115/1.4042955.

The vibration bending fatigue life uncertainty of additively manufactured titanium (Ti) 6Al-4V specimens is studied. In this investigation, an analysis of microscopic discrepancies between ten fatigued specimens paired by stress amplitude is correlated with the bending fatigue life scatter. Through scanning electron microscope (SEM) analysis of fracture surfaces and grain structures, anomalies and distinctions such as voids and grain geometries are identified in each specimen. These data along with previously published results are used to support assessments regarding bending fatigue uncertainty. The understanding gained from this study is important for the future development of a predictive vibration bending fatigue life model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(4):041003-041003-12. doi:10.1115/1.4042662.

The effects of two temper conditions (T4 and T6 heat treatments) upon the stress corrosion cracking (SCC) of AA6061 plates have been investigated in this work. AA6061 alloys were double-side-welded by the tungsten inert gas (TIG) welding method. SCC behavior of both the as-welded and as-received alloys was reported. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to determine the precipitate structure of the thermal-altered zones and the base metal (BM), and also the hardness variations were examined using microhardness testing (Vickers hardness). The small-size precipitate structures in the T6 tempered alloy and the coarser precipitate structures in the T4 tempered alloy were found by microstructural investigations. As a result, T4 temper heat treatment of this alloy considerably reduced its susceptibility to stress corrosion cracks due to relatively coarse and more separate precipitate morphology. In welded specimens, SCC failure occurred in the area between the heat-affected zone (HAZ) and the base metal. Stress corrosion resistance in the fusion zone was strong in both temper conditions. The aim of this work was to obtain the effects of heat treatment and welding on SCC behavior of the age-hardenable aluminum alloy. The authors conclude that a deep insight into the SCC resistance of AA6061 alloy indicates the precipitate particle distributions and they are the key point for AA6061 alloy joints in chloride solution.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(4):041004-041004-12. doi:10.1115/1.4043271.

Much research has been conducted on effective elastic properties of meso-scaled periodic cellular material (MPCM) structures; however, there is only limited research providing guidelines on how to develop improved unit cell (UC) topologies and shapes for a given set of loading requirements and conditions. This paper presents guidelines to improve the shear flexibility of the MPCMs while maintaining the effective shear modules by changing the topology or the shape of a unit cell. The guidelines are intended to use design knowledge for helping engineers by providing recommendations at any stage of the design process. In this paper, the guidelines are developed by changing topology characteristics to achieve a desired effective property of the MPCM structure. The effects of individual members, such as side connection, transverse connection, vertical legs, and curved beams of MPCM structure, when subjected to the in-plane shear loading are investigated through conducting a set of numerical simulation on UCs with similar topology and shape characteristics. Based on the simulation results, the unit cell design guidelines are developed to provide recommendations to engineers on improving the shear flexure of MPCM during the design process. Ultimately, a unit cell design guideline development method is offered and demonstrated by developing two new design guidelines.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(4):041005-041005-7. doi:10.1115/1.4043159.

In this study, the effects of temperature, stress, and type of materials and their interactions on the creep rate and rupture time were investigated by using central composite design (CCD). An experimental plan for CCD with two numerical factors, one categorical factor, and two levels was used to optimize the required number of experiments. Temperatures of 800 and 900 °C and stresses of 250 and 450 MPa were selected as factors for GTD-111 and IN-738LC superalloys, respectively. Experimental and numerical results showed that the main effects of factors and their interactions are significant on the creep rate and rupture time. Among all factors, the effects of temperature and stress dominated other factors. Moreover, it was indicated that the combination between temperature and stress is much more effective on creep rate response than on rupture time. The high creep rate and the low rupture time values were obtained at the highest stress and temperature for IN-738LC. With the same experimental condition, creep rate values were the most and rupture time values were the least for IN-738LC in comparison with GTD-111.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(4):041006-041006-12. doi:10.1115/1.4043342.

Electron beam additive manufacturing (EBAM) is a powder-bed fusion additive manufacturing (AM) technology that can make full density metallic components using a layer-by-layer fabrication method. To build each layer, the EBAM process includes powder spreading, preheating, melting, and solidification. The quality of the build part, process reliability, and energy efficiency depends typically on the thermal behavior, material properties, and heat source parameters involved in the EBAM process. Therefore, characterizing those properties and understanding the correlations among the process parameters are essential to evaluate the performance of the EBAM process. In this study, a three-dimensional computational fluid dynamics (CFD) model with Ti-6Al-4V powder was developed incorporating the temperature-dependent thermal properties and a moving conical volumetric heat source with Gaussian distribution to conduct the simulations of the EBAM process. The melt pool dynamics and its thermal behavior were investigated numerically, and results for temperature profile, melt pool geometry, cooling rate and variation in density, thermal conductivity, specific heat capacity, and enthalpy were obtained for several sets of electron beam specifications. Validation of the model was performed by comparing the simulation results with the experimental results for the size of the melt pool.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(4):041007-041007-10. doi:10.1115/1.4043492.

Constrained groove pressing (CGP) is a severe plastic deformation technique to produce the ultra-fine grained sheet. The inhomogeneous strain distribution and geometry variation induce differential mechanical properties in the processed sheet. The improved mechanical properties of CGP sheets is due to the composite effect of weak and strong regions formed by geometric and strain inhomogeneities. Weaker regions exhibit large strain, lower yield strength, and higher strain hardening compared to stronger regions. The estimation of mechanical properties is influenced by these defects leading to the difference in the mechanical properties along different orientations. Experimental investigation revealed that the commonly used tensile samples cut perpendicular to the groove orientation exhibit variation in thickness along the gauge length affecting the results from tensile tests. To further understand the effect of geometric variation, a typical CGP specimen was reverse engineered and finite element (FE) simulation was performed using the actual geometry of the CGP processed specimen. The strain distribution from FE simulation was validated experimentally using the digital image correlation data. Based on the numerical and experimental studies, miniature specimens were designed to eliminate the geometric effects from the standard parallel specimen. Miniature parallel specimens showed lower yield strength and total elongation compared to the standard specimens. However, the statistical scatter of total elongation of the miniature specimens was much less than that of the standard specimens, indicating better repeatability. Probably this is the first study to quantify the contribution of composite geometric effect in the mechanical properties of CGP.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2019;141(4):041008-041008-8. doi:10.1115/1.4043626.

Cobalt-based γ–γ′ superalloys are novel heat-resistant materials suitable for high-temperature applications, such as components of the turbine engine. These alloys exhibit favorable strength and corrosion resistance at high temperatures owing to the γ–γ′ microstructure, analogous to that of Ni-based superalloys. The aim of this paper is to evaluate the oxidation behavior of basic Co-9Al-9W (at%) and new tungsten-free Co-10Al-5Mo-2Nb (at%) alloys at elevated temperatures. The investigation is concerned with thermogravimetric studies in the temperature range of 40–1200 °C. The oxidized surfaces after high temperature oxidation have been characterized using optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction analysis (XRD).

Commentary by Dr. Valentin Fuster

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