Strain-induced Melt Activation Method Research Articles

Effect of isothermal process parameters on semi-solid microstructure of chip-based Al–Cu–Mn–Ti alloy prepared by SIMA method

A typical Al–Cu–Mn–Ti aluminum alloy chip was adopted to prepare semi-solid billets by a Strain-Induced Melt Activation (SIMA) method, and the effects of isothermal process parameters on the semi-solid microstructure evolution of the alloy were investigated in this work. The result showed that semi-solid billets with highly spheroidal and homogeneous fine grains could be prepared from chips by the SIMA method. With the increase of isothermal temperature, the finer and near-spherical grains are obtained, the grains coarsen and became ellipse at 903 K because of the coarsening mechanisms of coalescence and Ostwald ripening. The relationship of isothermal holding time and grains size followed the LSW theory well, and more spherical microstructure can be brought by prolonging the holding time until 3000 s. Thus, the optimal isothermal treatment temperature is 893 K and holding time is 3000 s, the corresponding average size and roundness of grains are 137 [Formula: see text]m and 1.108, respectively.

Thixoforming of semi-solid AZ91D alloy with high solid fraction prepared by the RUE-based SIMA process

In the present study, semi-solid billets of AZ91D magnesium alloy were prepared by employing the repetitive upsetting-extrusion (RUE) based strain induced melt activation (SIMA) process. Thixoforming experiments were performed using semi-solid alloys experiencing various RUE temperatures and semi-solid reheating temperatures. The microstructure characterization and microhardness of the thixoformed products were investigated. Microstructure examination results indicated that semi-solid globular microstructure ensuring good thixotropic filling performance was prepared by applying the RUE-based SIMA method. Thixoformed components with good surface quality were obtained. Compared with the original as-cast alloy, significant grain refinement and microhardness improvement occurred in the thixoformed components. And the mean grain sizes and microhardness values were measured to be in the ranges of 11.81–23.84 μm and Hv80.67–Hv90.65, respectively. By means of further comparison of microstructure and microhardness, the grain coarsening and microhardness reduction of the thixoformed components were verified by elevating the RUE deformation temperature and semi-solid remelting temperature. Furthermore, strengthening mechanisms during thixoforming involving the application of the RUE-based SIMA route were discussed in detail. The results showed that the comprehensive effect of fine grain strengthening, dispersion strengthening, and solid solubility strengthening should be responsible for microhardness improvement.

STUDY OF ROLLING-REMELTING SIMA PROCESS FOR PREPARING THE SEMI-SOLID BILLET OF ZCuSn10 ALLOY

Semi-solid billet of ZCuSn10 alloy is prepared by strain induced melt activation(SIMA) method which included the rolling and remelting process. Firstly, ZCuSn10 alloy is cast, and samples are cut from ingot casting. Secondly, the samples are rolled with 2~4 passes after holding at 450 ℃ for 15 min, then the new samples are cut from deformed alloy. Lastly, the new samples are reheated up to 850 ℃ or 875 ℃ for 15 min, then water quenching. Semi-solid microstructure is observed and compared with microstructure of ZCuSn10 alloy directly reheated after casting. The distribution of Sn element in microstructure under different conditions is measured by using EDS function of SEM, and the microstructure changes during the SIMA process are observed by means of OM and TEM. Based on the experiments, the microstructure evolution is synthetically analyzed and explained during the course of semi-solid billet of ZCuSn10 alloy prepared by SIMA method. The results indicate that semi-solid microstructure of ZCuSn10 alloy by rolling- remelting SIMA process is equal-fine grain, and spheroidization of solid particle is well. The optimum semi- solid microstructure is obtained when alloy deformed 19.7% is remelted at 875 ℃ for 15 min, the average grain diameter is 75.8 μm, shape factor is 1.62, and volume fraction of liquid phase is 17.28%. Deformation process plays a crucial role in grain refinement and spheroidization during SIMA process for preparing the semi-solid billet of ZCuSn10 alloy, as deformation and remelting temperature increases, the size and shape of solid phase in semi solid microstructure are smaller and more round, volume fraction of liquid phase increases. The main mechanism of SIMA process preparing semi-solid billet of ZCuSn10 alloy is that predeformation breaks dendrites and stores energy of deformation into dendrites, and promotes dendrites melting through remelting process. Meanwhile, liquid phase occupies sharp corners of solid particles by Sn element diffusing from liquid phase into a solid phase, so that fine, uniform and roundness a solid particles are gained.

Microstructure of AZ91D magnesium alloy semi-solid billets prepared by SIMA method from chips

AZ91D magnesium alloy chips were adopted to prepare semi-solid billets. The chips were subjected to a series of isothermal treatments for various holding times at 783-843 K after being compressed into billet at 523 K. The semi-solid microstructure of AZ91D magnesium alloy containing spherical solid particles was studied. The effects of reheating temperature and holding time on microstructures were investigated. And the semi-solid forming mechanism was discussed. The result shows that semi-solid billets with highly spheroidal and homogeneous grains can be prepared from chips by strain induced melt activation(SIMA) method. Meanwhile, it is found that increasing the heating temperature can accelerate the spheroidizing process and reduce the solid volume fraction. With the increase of the holding time, the solid particles become more globular, the grains grow slowly and the solid volume fraction slightly changes. At the same time, owing to the decrease of interfacial energy, the intragranular liquid phases form by the diffusion of solute atoms, the grain boundaries melt and grains separate from each other during the isothermal treatment. The grains gradually spheroidize and begin to merge with a further increase of the holding time. It is considered that the semi-solid forming process includes three stages: the recrystallization and the growth of grain stage, the semi-solid microstructure forming stage controlled by the diffusion of solute, and the spheroidization of solid particle stage controlled by the liquid-solid interface tension.

Microstructure of semi-solid ADC12 aluminum alloy adopting new SIMA method

A semi-solid microstructure of ADC12 aluminum alloy containing spherical solid particles was studied. A new strain-induced melt activation (SIMA) process was proposed. In the treatment, chips were cut from ADC12 ingot by lathe machining and the plastic deformation was produced through cutting. The chips were put into a metal mold and compressed into a billet at 473 K. The microstructures of chip and billet were studied. The effect of the isothermal treatment on the semi-solid microstructure evolution of ADC12 aluminum alloy prepared by the new method was analyzed. A series of heating treatments were carried out from 823 to 838 K. The effects of heating temperature on microstructures were studied. The experimental results show the ADC12 aluminum alloy prepared by the new method can reap homogeneous and spherical grains at 828 K. The average grain size is about 82 μm. Also, the grain microstructure obtained by the present process is better than the traditional one. These results prove that the ADC12 aluminum alloy can be applied to semi-solid processing with the new SIMA method.

Formation process of AZ31B semi-solid microstructures through strain-induced melt activation method

A semi-solid microstructure of AZ31B magnesium alloy containing spherical solid particles was studied. This material was subjected to a series of spheroidizing treatments for various holding times at 863–893 K after being compressed by 28–30% in length at 573 K. The effects of heating temperature and holding time on microstructures were studied and the surface composition and line scans of the samples were investigated by energy-dispersive X-ray spectroscopy. It was found that increasing the heating temperature could accelerate the spheroidizing process, resulting in small spherical semi-solid microstructures. With a longer holding time, the solid content slightly changed and the solid particles became irregular. The concentrations of Al and Zn in the liquid phase were higher than those in the spheroidal solid phase and the peak intensity simultaneously changed. The macula observed through the microscope were thought to be possibly a Mn compound. Finally, the mechanism of formation of the semi-solid microstructure was discussed.

Microstructural evolution and flow stress of semi-solid type 304 stainless steel

Thixoforming or semi-solid metal forming offers many advantages in comparison with casting and conventional forging. However, because of the high-melting temperature and other related difficulties, there is relatively less amount of experimental data and investigations available on steels. The present study is aiming to provide the basic deformation characteristics of stainless steel under semi-solid state, which is affected by the change in morphologies of microstructure at elevated temperature. Microstructural evolution and flow stress of semi-solid type 304 stainless steel using strain-induced melt activation method is investigated, and different morphologies of microstructure of type 304 steel are observed at different forming temperatures in the semi-solid state. Hot compression tests for varied combination of deformation rate and deformation temperature around solidus temperature are conducted to obtain flow stress curves of type 304 stainless steel for two different semi-solid states and solid state. Flow stress curves around solidus temperature exhibit abrupt change according to the change in morphologies of microstructure, which is related to the phase change from delta-ferrite/austenite to liquid/delta-ferrite via liquid/delta-ferrite/austenite. The characteristics of deformation in two semi-solid states of type 304 stainless steel are discussed referring the change in flow stress and micrographs of quenched microstructure obtained by the experiments.