Microstructural characterization of two lithium-containing aluminium alloys

The microstructures of two lithium-containing aluminium alloys have been investigated. The two alloys were an Al-Li-Mn alloy, heat treated to provide an under-aged, peak-aged and an over-aged condition, and a commercial Al-Cu-Li alloy, 2020, heat treated and aged to contain ordered precipitate structures. It was observed that both materials were recrystallized with fairly large grains. The Al-Li-Mn material had a high volume fraction of Al₆Mn dispersoids and the Al-Cu-Li alloy had a substantial volume fraction of coarse intermetallic particles and intermediate size disperoids. The major strengthening precipitates were identified from brightfield and dark-field images and selected-area diffraction patterns taken in the transmission electron microscope. Precipitate-free zones were found to be present in both the Al-Li-Mn and Al-Cu-Li alloys. The results of this study suggest that the peak-aged Al-Cu-Li alloy and the under-aged and peak-aged Al-Li-Mn alloys enhance deformation to occur primarily by planar slip, and the larger particle size and interparticle spacing of the over-aged Al-Li-Mn promotes a combination of planar slip and Orowan looping.     

The non-destructive and nano-microstructural characterization of thermal-barrier coatings

The durability of thermal barrier coatings (TBCs) plays an important role in the service reliability and maintainability of hot-section components in advanced turbine engines for aerospace and utility applications. Photostimulated luminescence spectroscopy (PSLS) and electrochemical impedance spectroscopy (EIS) are being concurrently developed as complimentary nondestructive evaluation (NDE) techniques for quality control and liferemain assessment of TBCs. This paper discusses recent achievements in understanding the residual stress, phase constituents, and electrochemical resistance (or capacitance) of TBC constituents—with an emphasis on the thermally grown oxide. Results from NDE by PSLS and EIS are correlated to the nano- and microstructural development of TBCs. Authors’ Note: More information on the authors’ research and education activities can be obtained from mmae.ucf.edu/∼ysohn and me.udel.edu/karlsson. For more information, contact Y.H. Sohn, University of Central Florida, Advanced Materials Processing and Analysis Center (AMPAC) and Department of Mechanical, Materials and Aerospace Engineering, Orlando, FL 32816-2455, USA; (407) 882-1181; fax (407) 882-1462; e-mail ysohn@mail.ucf.edu, and A.M. Karlsson, Department of Mechanical Engineering, University of Delaware, Newark, DE 19716-3140; (302) 831-6437; fax (302) 831-3619; e-mail karlsson@mde.udel.edu.     

The influence of Al2O3 particulate reinforcement on cyclic stress response and fracture behavior of 6061 aluminum alloy

The cyclic stress response characteristics and cyclic fracture behavior of aluminum alloy 6061 discontinuously reinforced with particulates of Al₂O₃ are presented and discussed. The 6061/Al₂O₃ composite specimens and the unreinforced 6061 aluminum alloy were cyclically deformed using tension-compression loading under constant total strain amplitude control. Both the composite and the unreinforced alloy exhibited softening to failure from the onset of cyclic deformation. The degree of softening was observed to increase at the elevated test temperature for both the composite and the unreinforced counterpart. The intrisic micromechanisms controlling the stress response characteristics during fully-reversed cyclic straining are highlighted and rationale for the observed behavior is discussed. The cyclic fracture behavior of the composite is discussed in terms of the competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic stress response, and test temperature.     

Automation of surface finishing processes

The development of robots capable of interacting dynamically with the environment has proven to be a difficult task. Since a majority of manufacturing tasks require interaction of robots with their environment, this has become an important focus for research in the industry. Solutions both to specific manufacturing operations and to the theoretical body of knowledge within the industry are extremely important. The interaction of the robot with its environment is investigated in this paper for the configuration of a quick actuator and sensor attached to the robot tool. This configuration has been tested in an industrial application for Steinway & Sons, the world renowned piano manufacturer, specifically for the automation of the labor-intensive rubbing and finishing operations. The robot architecture utilizes a combination of macro-micro manipulator to improve its response time. A quick actuator added to the end of the robot arm is the micromanipulator, and the robot arm is the macromanipulator. Force and impedance control laws are executed concurrently by two separate controllers to control the quick actuator and the robot arm respectively. The experimental system proves the ability of this configuration to follow the complex contour of a grand piano rim and to exert a given force while rubbing its surface.     

Laser-welding behaviour of cast Ni3Al intermetallic alloy

Ductile nickel aluminide, Ni₃Al+B, is an intermetallic alloy with high strength and ductility making it a promising structural material for both elevated temperature and cryogenic temperature applications. In order to be able to use this alloy over a spectrum of temperature-critical applications, it must be capable of being joined or welded. The weldability of a cast nickel aluminide alloy containing boron was studied using laser welding. Welding was carried out at laser beam traverse speeds ranging from 42.33–254 mms^–1 in the bead-on-plate and butt-joint configurations. Two types of surface preparation, namely chemical cleaning and mechanical polishing, were used prior to laser welding. The quality of the laser welds was evaluated through mechanical tests (hardness and tensile), X-ray diffraction and microscopical observations. High-magnification examination of the welds revealed fine columnar structures in the weld zone. The hardness of the weld zone was substantially higher than that of the base metal. Microscopic examination also revealed the welds to contain shrinkage cracks. For a constant set of laser parameters, the chemically etched surfaces provided deeper penetration than the mechanically polished surface. The performance of the laser-welded joint is rationalized.