Papers by Author: Wei Min Mao

Abstract: The high-strength Al-Zn-Mg-Cu alloy has broad application prospects in high-end equipment manufacturing fields, such as aerospace, national defense and military industry, and transportation. However, the grains of Al-Zn-Mg-Cu alloy are coarse, and the grains further grow after heat treatment, which seriously reduces the mechanical property of alloy castings. Therefore, the study on reducing the grain size of the cast alloy was carried out by adding Zr element and applying electromagnetic stirring treatment technology, as well as suppressing the recrystallization of the aluminum alloy during the heat treatment process. The results show that adding Zr element reduced the grain size by 30%, the tensile strength increased by 50 MPa, and the elongation increased by more than 10%.

Abstract: High-strength aluminum alloy with large-scale and thin-walled complex castings have broad application prospects in aerospace, weapons, electronics, defense and military industries. However, due to the uneven thickness of the plate, the casting defects are inevitable by the ordinary casting method, and it is impossible to accurately control the shape and performance of the casting in the casting process. Previous studies have found that the semi-solid rheological extrusion casting technology with short process and near-end type can help solve this technical problem. Therefore, this paper studies the semi-solid rheological extrusion casting process of thin-walled complex casting of ZL114A aluminum alloy. The combination of numerical simulation and experimental research is used to simulate and optimize the filling and solidification process of thin-walled specimens. Based on this, a semi-solid rheological extrusion casting test was conducted. The result showed that, (1) The optimized model can well reflect the filling and solidification process under different rheological extrusion casting parameters, and obtain defect-free castings through process optimization. (2) The thin-walled parts of the thin plate casting produced by semi-solid rheology extrusion have excellent mechanical property and ductility.

Abstract: Stir casting is a near net shape process that can cast the composite components directly. Fluidity is an important factor for mold filling in casting process, but the fluidity of composite slurry is poor due to the addition of SiCp. In this research, SiCp/A357 composites with 20wt.% SiCp were manufactured by mechanical stir casting. Effect of mechanical stirring and air pressure on the fluidity of SiCp/A357 composites were investigated with eight thin flow channels. The fluidity was compared at different rotating speed and air pressure. The results showed that the fluidity increased with rotating speed, which was more obvious in semi-solid interval. It was noticed that the fluidity decreased with air pressure, the gas involving and the cooling speed were aggravated with air pressure increasing, which reduced the fluidity.

Abstract: Semisolid A356 alloy feedstock was generated by reversed cone channel pouring. The influence of pouring temperatures and the reversed cone angle on the semisolid microstructure were investigated, and the microstructure of the slurry after soaking was also studied. The results indicate that the suitable pouring temperature for obtaining semisolid feedstock is not above 660°C and the semisolid microstructure can be improved by increasing the reversed cone angle, but proper reversed cone angle should be considered for avoiding solid shell in the condition of big reversed cone angle.

Abstract: The possible mechanisms concerning abnormal growth of Goss grains in grain oriented electrical steels were investigated. The density of inhibitor particles near sheet surface, where the Goss grains located, was lower than that in center layer before secondary recrystallization, and the grains near surface could grow more easily because of reduced pinning effect. Few Goss grains could survive the growth competition and reach the sheet surface, after which the inhibitor particles inside the Goss grains coarsened slower. The phenomenon resulted in easy growth of the Goss grains at the expense of smaller neighboring grains while they could hardly be consumed by larger neighboring grains during the high temperature secondary recrystallization. Very large final size of the Goss grains was then obtained. The mechanisms were discussed based on the hot rolling characteristics and the elastic anisotropy of the ferrite matrix.

Abstract: The evolution of microstructural morphology in semisolid A356 alloy was researched under the mechanical stirring. The results showed that there was the effect of the stirring time on the microstructural morphology of semisolid A356 alloy, in which the microstructural morphology became better with the stirring time prolonging. There was also the effect of the stirring process on the microstructural morphology, in which the microstructural morphology obtained from the melt cooling to the reset solid volume fraction to start stirring was coarser than that obtained from the stirring above the liquidus temperature, but the microstructural morphology rapidly changes under the shearing force.

Abstract: The semi-solid A356 alloy slurry is prepared by slightly electromagnetic stirring with Ti-based refiner. The effects of the refiner on the morphology and the grain size of the primary phase in the slurry are researched. The results indicate that the slurry with particle-like and rosette-like primary phases can be prepared by slightly electromagnetic stirring with the refiner. Compared with the A356 alloys without the refiner, the grain size and particle morphology of primary phase as well as the distribution of the grain with particle-like or rosette-like along radial in the ingot in A356 are markedly improved by the refiner.

Abstract: The semi-solid A356 alloy slurry is prepared by compound process, and the effect of the compound process on morphology and size of primary α-Al in A356 alloy is researched. The results indicate that the compound process remarkably affects the morphology and the size of primary α-Al. Primary α-Al with particle-like is distributed uniformly in A356 alloy, and there is no the transient area of change in structure morphology. Compared with the samples prepared by low superheat pouring and slightly electromagnetic stirring, the nucleation rate, morphology and grain size of primary α-Al in A356 alloy are markedly improved by the compound process. The mechanism of refining grain in the compound process is probed.

Abstract: The apparent viscosity model of the semi-solid A356 aluminum alloy, based on fitting of experimental data obtained by the Couette type viscometer, was developed in the paper. The commercial package CastSoft6.0 coupled with the model was used to simulate the mould filling of the semi-solid A356 aluminum alloy in the key-shaped component with iron cores. The simulation results showed that the position of the iron cores has an important effect on the filling of the semisolid slurry, and it is easy to obtain the completely filled key-shaped component when the iron cores were near to the inlet. The filling tests verified that the simulation results have good agreement with the experimental results. The fitting results indicated that the developed apparent viscosity model is practical and feasible and it can be used to simulate the mould filling process of the semisolid A356 aluminum alloy slurry. Also the parameters were optimized and the optimum parameters are as follows: the inject pressure is more than 15MPa, the inlet velocity is more than 1.73m/s and the forming temperature is over 585 °C.

Abstract: The study on new rheoforming technologies of semisolid alloys has recently been one main subject interesting many professors, experts and industrials, and so many new methods about preparing semisolid alloy slurry and rheoforming process have been put forward. The semisolid slurry of AlSi7Mg alloy was prepared by being poured at a low superheat and stirred by an electromagnetic field at a low power for a short time, and then the slurry was further soaked for proper time and finally rheoformed into a key-shaped die cavity in this paper. The influence of slurry temperature, injection specific pressure and injection speed on the rheoforming process was also studied. The experimental results show that if AlSi7Mg alloy melt is poured at 630°C or 650°C and meanwhile stirred by an electromagnetic field at a low power for a short time, the most primary α-Al grains solidified in the slurry are spherical and only a few are rosette-like. If the slurry after made is then soaked in the liquidus-solidus range for some time, the ripening of primary α-Al grains is developed further and they become more spherical and larger. The experimental results also show that the slurry temperature, injection pressure and speed have a great influence on filling ability. The higher the slurry temperature, injection pressure and speed, the higher the slurry filling ability, and the shorter the core distance from the ingate, the more easily the die cavity is filled. With the given key-shaped casting, if only the slurry temperature should be equal to or more than 585,, or the injection pressure is equal to or more than 20 MPa, or the injection speed should be equal to or more than 1.73m/s, the key-shaped die cavity can be filled completely. The microstructural distribution in the casting is very homogeneous and it shows that the slurry prepared by the new method is very suitable for rheoforming and helpful to obtaining high quality castings.