Effects of Hot Extrusion and Heat Treatment on Mechanical Properties and Microstructures of AZ91 Magnesium Alloy

AbstractThe effects of heat treatment on the microstructure, tensile property and fracture behavior of as-extruded AZ91 magnesium alloy were studied with OM and SEM. The results show that the grains of as-cast AZ91 alloy are refined by hot extrusion due to dynamic recrystallization, and the mechanical properties are improved obviously. The ductility is significantly enhanced after solution treatment of the as-extruded AZ91 alloy, tensile strength is almost the same as before, and hardness is significantly reduced after solution treatment and artificial aging treatment. The tensile strength reduced and the ductility is significantly enhanced of as-extruded after annealing processes. The fracture surface of as-extruded AZ91 magnesium alloy has the mixed modes of ductile and brittle characteristics. But after T6 or annealing treatment, the dimple number increases evidently

The Mechanism of Ultrasonic Vibration on Grain Refining and Degassing in GTA Spot Welding of Copper Joints

This paper examines the effect of ultrasonic vibration (USV) on grain size and interrupted porosity in Gas Tungsten Arc (GTA) spot-welded copper. Grain size was refined by perpendicularly attaching a transducer to the welded sheet and applying USV to the weld pool for a short time (0, 2, 4, and 6 s) in addition improvements to the degassing process. Results illustrate a significant reduction of grain size (57%). Notably, USV provided interaction between reformations (fragmentation) and provided nucleation points (detaching particles from the fusion line) for grains in the nugget zone and the elimination of porosity in the nugget zone. The GTA spot welding process, in conjunction with USV, demonstrated an improvement in the corrosion potential for a copper spot-welded joint in comparison to the joint welded without assistance of USV. Finally, welding of copper by GTA spot welding in conjunction with ultrasound for 2 s presented significant mechanical properties

Effect of Y Addition on the Semi-Solid Microstructure Evolution and the Coarsening Kinetics of SIMA AZ80 Magnesium Alloy

Semi-solid feedstock of AZ80 magnesium alloy modified by trace rare-earth Y element (0, 0.2, 0.4, 0.8 wt. %) was fabricated by strain-induced melting activation (SIMA) in the form of extrusion and partial remelting. The effect of Y addition on the microstructure evolution of the extruded and isothermally heat treated alloy was observed by using an optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and quantitative analysis. The results show that the Y addition can refine the microstructure and make the β-Mg17Al12 phases agglomerate. During the subsequent isothermal treatment at 570 °C, the average solid grain size, shape factor and liquid fractions increased with the prolonged soaking time. The smaller spheroidal solid grains and larger shape factor were obtained in the semi-solid microstructure due to Y addition. The coalescence and Ostwald ripening mechanism operated the coarsening process of solid grains simultaneously. The coarsening rate constants of AZ80M1 (0.2 wt. % Y addition) of 164.22 μm3 s−1 was approximately four times less than the un-modified AZ80 alloy of 689.44 μm3 s−1. In contrast, the desirable semi-solid structure featured, with fine and well globular solid grains, an appropriate liquid fraction, and shape factor was achieved in AZ80M1 alloy treated at 570 °C for 20–30 min

Effects of semi-solid treatment by electro-magnetic induction on micro-structure evolution and mechanical properties of the Mg-2.4Y-4Nd-0.5Zr-1Ni alloys

The semi-solid billets of Mg-2.4Y-4Nd-0.5Zr-1Ni (WE34-1Ni) alloys are fabricated by electro-magnetic induction heating semi-solid treatment at 2.05 kW and 4.10 kW from 580 °C to 625 °C. In this work, the microstructure evolution and mechanical properties of WE34-1Ni alloys are investigated, and the results reveal that with increasing semi-solid temperatures, the average grain size of the solid globules and liquid fraction at the grain boundary gradually increase while the shape factor fluctuates slightly. Compared with 2.05 kW power, the semi-solid billet with 4.10 kW power at 625 °C has more fine homogeneous grains, the lower average size of the solid globules, more liquid fraction. The semi-solid billet with 4.10 kW at 625 °C obtains ideal semi-solid spheroid structure with the solid grains surrounded by a small amount of liquid pools and the best mechanical properties of the semi-solid process parameters. Besides, the elongation as-extruded of WE34-1Ni alloys increased from 21.4 ± 0.7% to 33.2 ± 0.3% at 4.10 kW power and 625 °C via electro-magnetic induction heating semi-solid treatment

Magnesium application in railway rolling stocks: A new challenge and opportunity for lightweighting

Magnesium (Mg) and their alloys show potential in railway rolling stock applications due to their attractive mechanical properties, developed manufacturing processes, cost efficiency, and affluent resource reserve. In particular, high-strength Mg alloy components are important to lightweighting efforts in the railroad industry. In this review, Mg alloy processing technology for railroad vehicles is discussed with emphasis on energy savings, manipulation convenience, and reduction in friction/wear, vibration, and fatigue damage. It is shown that a Mg alloy train has a theoretical 8.6–12.6% comprehensive weight reduction potential in the equal-strength and equal-stiffness condition, where low-speed trains (metro, light rail train, tram/trolley car, monorail car, suspension train/schwebebahn etc..) exhibit a larger energy-savings than high speed trains. It is evident that the regulations of fabrication, processing, post treatment, assembly/installation, protection, and maintenance/replacement of Mg alloy components for the railroad industry all need further research and development. Keywords: Mg alloys, Lightweight train, Energy saving