Qichi Le

Study on hydrogen removal of AZ91 alloys using ultrasonic argon degassing process.

Argon degassing, ultrasonic degassing and a novel ultrasonic argon degassing treatment were applied for the hydrogen removal of AZ91 magnesium alloy. The hydrogen concentration, microstructures and mechanical properties have also been investigated. AZ91 alloys contains a high hydrogen concentration. The mechanical properties of the as-cast alloy are much improved using degassing process, which should be mainly attributed to the hydrogen removal. Among the three degassing process, the ultrasonic argon treatment is a high efficient process both for hydrogen removal and microstructure refining. One hand, ultrasonic wave could break up the purged argon bubble to improve the degassing efficiency of these bubbles. On the other hand, ultrasound could also generate many cavitation bubbles in the melt, which should account for the microstructure refinement. The ultrasonic argon treatment involves dynamics between the ascending argon bubbles and ultrasonic effects, such as cavitation and streaming, etc.

The role of ultrasound in hydrogen removal and microstructure refinement by ultrasonic argon degassing process

In this work, the role of ultrasound in hydrogen removal and microstructure refinement by the ultrasonic argon degassing has been fully investigated by the experimental work in water and AZ91-0.4Ca magnesium melt, respectively. Ultrasound is able to break up argon gas into numbers of small bubbles and drive them diving deeply to the bottom of water, which are responsible for the efficient degassing regime of ultrasonic argon process. The argon flowrate plays a dominant role in promoting hydrogen removal effect. Meanwhile, the increasing argon flowrate can suppress the microstructure refinement, due to the subdued ultrasonic cavitation under a large argon flowrate. Mechanical properties of AZ91-0.4Ca alloy can be much promoted by the ultrasonic argon degassing process. Ultrasound is the key to achieve not only efficient degassing regime, but also microstructure refinement as well as mechanical properties promotion.

Effect of ultrasonic power on grain refinement and purification processing of AZ80 alloy by ultrasonic treatment

Ultrasound with different powers was applied to treat AZ80 alloy melt to attain grain refinement or purification processing of the alloy. The influence of ultrasonic powers from 0 W to 1400 W on microstructures of the AZ80 alloy with ultrasonic grain refinement treatment was investigated. The average grain size of the alloy could be decreased from 387 μm to 147 μm after the ultrasound with the optimal power 600 W was applied to treat the melt. The effect of ultrasonic powers from 0 W to 230 W on the inclusion distribution in the ingot that was treated by ultrasonic purification processing was also studied. The optimal ultrasonic power in the ultrasonic purification processing was 80 W. In order to gain insight into the mechanism by which ultrasonic power affected the microstructure of the alloy or inclusion distribution in the ingot, numerical simulations were carried out and the ultrasonic field propagation in the melt was characterized.

Electrical resistivity behaviors of liquid Pb-Sn binary alloy in the presence of ultrasonic field.

Electrical resistivity behaviors of liquid Pb-Sn alloys have been investigated in the presence of ultrasonic field. The process demonstrated significantly that electrical resistivity could reveal the precise influence caused by ultrasound. Details revealed by applying the resistivity measuring approach to the liquid Pb-Sn alloy show that the short ordered structures in the liquid could be modified by ultrasonic irradiation, and the resistivity approach could have application value in the ultrasonic irradiation process on the specific liquid metals and alloys.

Effect of a low frequency electromagnetic field on the direct-chill (DC) casting of AZ80 magnesium alloy ingots

Abstract The effects of a low frequency electromagnetic field applied during semi-continuous direct-chill casting on AZ80 ingots were investigated. The experiments were performed both in the absence and in the presence of the electromagnetic field. In contrast to the ingot cast in the conventional process, the microstructure of the ingot was significantly refined by the electromagnetic field and a high degree of grain refinement was realized when the operating frequency was at 22Hz. The ingot exhibited a relatively increased and uniform distribution of Vickers hardness and decreased segregation of Al when the operating frequency of the electromagnetic field was in the range of 15–30Hz.

Effects of radiator shapes on the bubble diving and dispersion of ultrasonic argon process

In this work, three ultrasonic radiators in different shapes have been designed in order to investigate the effects of radiator shapes on the argon bubble dispersion and diving as well as the degassing efficiency on magnesium melt. The radiator shape has a strong influence on the bubble diving and dispersion by ultrasound. A massive argon bubble slowly flows out from the radiator with the hemispherical cap, due to the covering hemispherical cap. Using a concave radiator can intensively crush the argon bubbles and drive them much deep into the water/melt, depending on the competition between the argon flow and opposite joint shear force from the concave surface. The evolution of wall bubbles involves the ultrasonic cavities carrying dissolved gas, migrating to the vessel wall, and escaping from the liquid. Hydrogen removal can be efficiently achieved using a concave radiator. The hydrogen content can be reduced from 22.3 μg/g down to 8.7 μg/g. Mechanical properties are significantly promoted, due to the structure refinement and efficient hydrogen removal.

Influence of Gd, Nd and Ce Additions on Microstructures and Mechanical Properties of Ultra-light Dual Phase Mg-9Li-0.4Zr Alloys

Effects of various rare-earth elements (RE: Gd, Nd and Ce) additions on microstructures and mechanical properties of ultra-light dual phase Mg-9Li-0.4Zr alloy were investigated. The results indicated that Mg-9Li-5RE-0.4Zr alloys exhibited α-Mg, β-Li and RE-containing intermetallic compound structures. 5 wt.% Gd, Nd and Ce additions enhanced the strength properties of as-extruded Mg-9Li-0.4Zr alloy, but greatly deteriorated the ductility. The final mechanical properties of Mg-9Li-5RE-0.4Zr alloys were 161MPa∼167MPa in ultimate strength, 141MPa∼156MPa in yield strength and 20.8%∼32.5% in elongation. Heat treatment was hardly able to improve the mechanical properties for Mg-9Li-5RE-0.4Zr alloys.

Dual-frequency ultrasonic treatment on microstructure and mechanical properties of ZK60 magnesium alloy

Compared with other dual-frequency acoustic applications, melt-treatment with dual-frequency ultrasound was less researched, especially in magnesium field. In this present work, traditional single-frequency ultrasonic field (SUF) treatment and dual-frequency ultrasonic field (DUF) treatment were used to refine the as-cast microstructure and improve the mechanical properties of the ZK60 (Mg-Zn-Zr) magnesium alloy. The influences of DUF on the microstructure evolution and mechanical properties were systematically investigated, and the cavitation bubbles dynamic behaviors were investigated by numerical simulation. α-Mg grains and second phases were dramatically refined by introduced ultrasound, and DUF showed higher refinement efficiency than SUF. The DUF treatment promoted the formation of small α-Mg globular grains and changed the distribution and morphology of MgZn2 phases. Mechanical properties of the as-cast alloy were much promoted with DUF. Yield strength, ultimate tensile strength and elongation increased to 153MPa, 239MPa and 13.9% respectively after 1400W DUF treatment, which were 30.8%, 42.3% and 58.0% higher than the values obtained from untreated samples and 20.5%, 20.7% and 30.0% higher than 1200W SUF treated samples. The DUF can generate more and larger cavitation bubbles, and make more bubbles into instantaneous bubbles, improving refinement efficiency.

The thermoelectric power of Al-0.99 wt.% Fe alloys in the AC magnetic field

The melt structure of Al-0.99 wt.% Fe alloys in the AC magnetic field have been studied with thermoelectric power by the four-point probe technique and microstructure with the liquid quenching method. The melt temperature is in the range of 913 K-1013 K. The thermoelectric power increases due to the AC magnetic field and decreases after the AC magnetic field stops, then keeps stable. Some characteristic parameters of thermoelectric power in the recovery process are used to represent the variation of melt structure. The α-Al phase refinement in the AC magnetic field is attributed to the persistent variation of melt structure. The persistent variation of thermoelectric power can be used to characterize the variation of the α-Al phase size. The hardness increases and the diffraction peaks of some planes reduce, which can reflect the uniform and disorder melt structure in the AC magnetic field.

Preparation of Large Scale Magnesium Alloy Billets by LFEC Processing

In this paper, in-situ temperature measurement was carried out in Low-Frequency Electromagnetic Casting (LFEC) processing for AZ80-1Y alloy to investigate the heat transfer behavior during solidification. The effects of the electromagnetic conditions (frequency and the intensity), casting velocity, cooling water flow, together with the casting temperature on the sump and the mushy zone were investigated in detail.