In recently developed Additive Manufacturing (AM) technologies, high-energy sources have been used to fabricate metallic parts, in a layer by layer fashion, by sintering and/or melting metal powders. In particular, Electron Beam Additive Manufacturing (EBAM) utilizes a high-energy electron beam to melt and fuse metal powders to build solid parts. EBAM is one of a few AM technologies capable of making full-density metallic parts and has dramatically extended their applications. Heat transport is the center of the process physics in EBAM, involving a high-intensity, localized moving heat source and rapid self-cooling, and is critically correlated to the part quality and process efficiency. In this study, a finite element model was developed to simulate the transient heat transfer in a part during EBAM subject to a moving heat source with a Gaussian volumetric distribution. The developed model was first examined against literature data. The model was then used to evaluate the powder porosity and the beam size effects on the high temperature penetration volume (melt pool size). The major findings include the following. (1) For the powder layer case, the melt pool size is larger with a higher maximum temperature compared to a solid layer, indicating the importance of considering powders for the model accuracy. (2) With the increase of the porosity, temperatures are higher in the melt pool and the molten pool sizes increase in the depth, but decrease along the beam moving direction. Furthermore, both the heating and cooling rates are higher for a lower porosity level. (3) A larger electron-beam diameter will reduce the maximum temperature in the melt pool and temperature gradients could be much smaller, giving a lower cooling rate. However, for the tested electron beam-power level, the beam diameter around 0.4 mm could be an adequate choice.

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a b s t r a c tUltrasonic welding offers ability to weld thin layers of malleable metals at low temperature and low power consumption. During ultrasonic welding, intensive material interactions occur due to the severe plastic deformation (SPD) and frictional heat generation, which leads to the microstructural change. Different grain microstructures have been observed after different ultrasonic welding conditions. Theory of the microstructural evolution was for the first time hypothesized as three regimes, namely SPD, dynamic recrystallization (DRX) and grain growth according to the material thermomechanical loading conditions. A novel metallo-thermo-mechanically coupled model was developed to model the temperature-dependent mechanical deformation and microstructural evolution during the ultrasonic spot welding process. The numerical analysis was carried out with a three-dimensional (3D) finite element model using DEFORM 11.0. The material constitutive model considered cyclic plasticity, thermal softening and acoustic softening. Dynamic recrystallization and grain growth kinetics laws were applied to simulate the microstructural evolution under different welding time durations. The simulation results demonstrated that the essential characteristics of the deformation field and microstructure evolution during ultrasonic welding were well captured by the metallo-thermo-mechanically coupled model. The numerical framework developed in this study has been shown to be a powerful tool to optimize the ultrasonic welding process for its mechanical properties and microstructures.

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In this work, the feasibility to recycle pure magnesium machining chips is first investigated experimentally with a solid-state recycling technique of friction stir extrusion (FSE). Heat generated from frictions among the stirring chips, die, and mold facilitates the extrusion process. Mechanical tests, optical microscopy (OM), and scanning electron microscopy (SEM) analysis are conducted to evaluate the mechanical and metallurgical properties of extruded wires. Mechanical tests show that almost all recycled specimens can achieve higher strength and elongation than original material of magnesium at room temperature. Due to a refined grain microstructure, good mechanical properties are obtained for samples produced by the rotational speed of 250 rpm and plunge rate of 14 mm/min. A metallo-thermo-mechanical coupled analysis is further conducted to understand the effects of process parameters. The analysis is carried out with a multistep two-dimensional (2D) coupled Eulerian–Lagrangian finite-element (FE) method using abaqus. The material constitutive model considers both work hardening and strain softening. Material grain size evolution is modeled by dynamic recrystallization (DRX) kinetics laws. The deformation process and its consequential microstructural attributes of grain size and microhardness are simulated. Physics principles of the microstructure evolution are discussed based on both experimental and numerical analyses.