Characterization of hierarchical structures in remelted Ni-Mn-Ga substrates for directed energy deposition manufacturing of single crystals

Magnetic shape-memory alloys (MSMAs) such as Ni-Mn-Ga Heusler alloys have been of great interest in the past decade due to the observed inverse magnetoplastic (IMP) effect, wherein an applied mechanical stress reorients the direction of easy magnetization of the crystal. However, practically large strains achieved with this mechanism may only be generated from single crystals, the manufacturing of which is traditionally time-intensive and delicate. A possible approach for more quickly producing single crystal MSMAs is to use directed energy deposition (DED), a type of laser additive manufacturing. This study focused on the laser remelting of an austenitic Ni51Mn24.4Ga24.6 single crystal substrate without deposition of an additional layer, so as to observe the effects of varying laser power and travel velocity on the resulting melt pool. Using digital microscopy and image analysis, various properties of the melt pool and contained microstructures were measured as functions of laser power and travel velocity. Some trends were suggested that provided insight into which parameter values are conducive to retention of a single crystal; however, a more comprehensive study is needed to confirm these trends

Robust control of systems with output hysteresis and input saturation using a finite time stability approach

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This paper presents a robust control approach for a class of nonlinear dynamic systems consisting of a linear plant connected in series with a hysteresis operator, and affected by control input saturation. Such a class of systems commonly appears in applications concerning smart materials, in particular thermal shape memory alloys wire actuators. The goal of this paper is to design a robust controller, in the form of an output PI law, which ensures set-point regulation with a desired decay rate and, at the same time, accounts for the effects of both hysteresis and input saturation. The resulting controller appears as attractive on the implementation stand-point, since no accurate hysteresis compensator is required. In order to deal with the proposed problem, the hysteretic plant is first reformulated as a linear parameter-varying system. Subsequently, a finite time stability approach is used to impose constraints on the control input. A new set of bilinear matrix inequalities is developed, in order to perform the design with reduced conservatism by properly exploiting some structural properties of the model. The effectiveness of the method is finally validated by means of a numerical case of study. © 2018 IEEE.Peer ReviewedPostprint (author's final draft