Microstructure evolution of TI-SN-NB alloy prepared by mechanical alloying

In the present study, Ti-16Sn-4Nb alloy was prepared by mechanical alloying (MA). Optical microscopy, scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX), and X-ray diffraction analysis (XRD) were used to characterise the phase transformation and the microstructure evolution. Results indicated that ball milling to 8 h led to the formation of a supersaturated hcp &alpha;-Ti and partial amorphous phase due to the solid solution of Sn and Nb into Ti lattice. The microstructure of the bulk sintered Ti-16Sn-4Nb alloy samples made from the powders at shorter ball milling times, i.e. 20 min- 2 h, exhibited a primary &alpha; surrounded by a Widmanst&auml;tten structure (transformed &beta;); while in the samples made from the powders at longer ball milling times, i.e. 5- 10 h, the alloy evolved to a microstructure with a disordered and fine &beta; phase dispersed homogeneously within the &alpha; matrix. These results contribute to the understanding of the microstructure evolution in alloys of this type prepared by powder metallurgy.<br /

Upper bound analysis of differential velocity sideways extrusion process for curved profiles using a fan-shaped flow line model

An analytical model for predicting the shapes of rectangular bars with variable curvatures along their lengths through a novel forming method, differential velocity sideways extrusion (DVSE), previously proposed by the authors, has been developed on the basis of the upper bound method. A new flow line function was presented to describe its deformation field. The plastic deformation zone (PDZ) was assumed to be fan-shaped, where the trajectory of the material flow within the PDZ had an elliptic shape. The proposed continuous flow line function was validated using finite element simulations. The flow patterns, extrusion pressure, curvature, and effective strain predicted by the analytical solutions agreed well with modelling results. Compared to the classical discontinuous simple shear model of channel angular extrusion (CAE) with a 90° die, the new approach was shown to predict the effective strain more closely

Delay-Dependent Stability of Single-Loop Controlled Grid-Connected Inverters with LCL Filters

LCL filters have been widely used for grid-connected inverters. However, the problem that how time delay affects the stability of digitally controlled grid-connected inverters with LCL filters has not been fully studied. In this paper, a systematic study is carried out on the relationship between the time delay and stability of single-loop controlled grid-connected inverters that employ inverter current feedback (ICF) or grid current feedback (GCF). The ranges of time delay for system stability are analyzed and deduced in the continuous s-domain and discrete z-domain. It is shown that in the optimal range, the existence of time delay weakens the stability of the ICF loop, whereas a proper time delay is required for the GCF loop. The present work explains, for the first time, why different conclusions on the stability of ICF loop and GCF loop have been drawn in previous studies. To improve system stability, a linear predictor-based time delay reduction method is proposed for ICF, while a time delay addition method is used for GCF. A controller design method is then presented that guarantees adequate stability margins. The delay-dependent stability study is verified by simulation and experiment

Investigation of die designs on welding quality and billet material utilisation for multi-container extrusion of wide stiffened aluminium panels

Wide stiffened aluminium panels are extensively used in aerospace, marine, and civil industries due to their light-weight structure and high stiffness. In this paper, a wide stiffened aluminium panel was manufactured using the principle of the multi-container extrusion, and a comparative study was conducted using two different die designs at the same extrusion condition, in which metal flow behaviour, extrusion force, welding quality, and billet material utilisation have been investigated numerically. Additionally, the effect of extrusion speed on the extrusion process was evaluated with the modified design. It was shown that, compared with the initial design, better metal flow behaviour can be obtained in the modified design. Multi-container extrusion greatly reduces the extrusion force, and the modified design results in a more uniform extrusion force for each extrusion container. The total extrusion force for the modified design is slightly higher compared with the initial die design, due to the increased friction in the upper die channels and the second-step welding chamber. Besides, the modified design of the multi-container extrusion can obtain better welding quality evaluated by different welding criteria, and the extrusion speed has a minor effect on the welding quality. The most notable feature is that the modified design greatly improves the material utilisation, which could save 39.5% material compared to the initial design