In the present research, semisolid billet of 7005 aluminum alloy was fabricated by using recrystallization and partial remelting (RAP), then thixoformed at different isothermal temperatures, preheating temperatures and load routes. Mechanical properties and microstructure of the thixoformed product were investigated. The results showed that microstructure achieved by three-step induction heating warm extruded 7005 aluminum alloy consists of a uniform and spheroidal microstructure suitable for thixoforming. Preheating temperature of the die affected significantly the filling status of semisolid billet of 7005 aluminum alloy. Complete filling status with good surface quality was obtained at a preheating temperature of 365 °C. Thixoformed microstructures consisting of relatively spheroidal grains illustrate the dependence of filling process on the sliding and rotating of solid grains rather than plastic deformation of solid grains. A non-uniform distribution of liquid phase was found in the different regions of the thixoformed product due to the slower adjustable velocity of solid grains as compared with liquid phase. Increase of isothermal temperatures led to a slight decrease of mechanical properties of the thixoformed product due to coarsening of solid grains. The highest yield strength, ultimate tensile strength and elongation of thixoformed components with T6 heat treatment are 237 MPa, 361 MPa and 16.8%, respectively, which were achieved at the isothermal temperature of 605 °C. Load route has a significant effect on mechanical properties and microstructure of the thixoformed product. Defects, such as crack and microporosity occurred in the microstructure of the thixoformed product obtained under load route 2. It led to an obvious reduction of mechanical properties as compared with route 1. A better compatibility of deformation caused by more liquid fraction at the isothermal temperature of 612 °C is beneficial to reducing non-uniformity of liquid phase in the different regions of the thixoformed product.

Semisolid billets of 7075 aluminum alloy were fabricated by strain induced melting activated (SIMA) and recrystallization and partial remelting (RAP). Effect of isothermal temperature and soaking time on microstructure of fabricated 7075 aluminum alloys was analyzed. The results show that average grain sizes of 7075 aluminum alloys fabricated by SIMA and RAP soaked for 20 min firstly increased and then decreased with increasing isothermal temperature. As to SIMA, when isothermal temperature increased from 580 °C to 610 °C, grain size increased and an isothermal temperature above 610 °C led to a decrease of solid grain size. For RAP, increasing isothermal temperatures from 580 °C to 600 °C led to an increase of grain size and further increase led to a decrease. Larger spheroidal grains, lower spheroidization temperature and better spheroidization effect were obtained in the RAP microstructure in comparison to the SIMA microstructure. Coarsening of grains occurred in the microstructure while extending soaking time at 590 °C and 600 °C. Increasing isothermal temperature reduced spheroidization time. An optimal soaking time of 20 min is required for RAP and SIMA to fabricate high-quality semisolid slurry. Coalescence of grains and Ostwald ripening operated simultaneously in coarsening of grains. The coarsening rate constants are 316 μm3 s−1 and 402 μm3 s−1 respectively in the SIMA microstructure fabricated at 590 °C and 600 °C. The coarsening rate constants of 329 μm3 s−1 and 393 μm3 s−1 were achieved in the RAP microstructure. The RAP microstructure exhibited a slightly higher coarsening rate at 590 °C, but a slightly lower coarsening rate at 600 °C as compared to SIMA.

The 7075 aluminium matrix composite reinforced with nano-sized SiC particles was fabricated by ultrasonic assisted semisolid stirring method. The compression mechanical behaviour of the fabricated composite in semisolid state was investigated. The results show that the microstructure of the composite before semisolid compression consists of fine and spheroidal solid grains surrounded by liquid phase. Semisolid compression led to a nonuniform plastic deformation of solid grains. A slight plastic deformation occurred in the locations near the free surface due to the dependence of deformation on liquid flow and flow of liquid incorporating solid grains. However, obvious plastic deformation occurred in the central location and location contacting to die due to the contribution of plastic deformation of solid grains. The true stress–strain curve of the sample compressed at 500 °C consists of rapid increase of true stress and steady stage. However, rapid increase of true stress and decrease of true stress and steady stage are involved in the true stress–strain curves of the samples compressed at 550, 560, 570, 580 and 590 °C. The true stress–strain curve at 600 °C is similar to that at 500 °C. Apparent viscosity decreases with an increase of shear rate, indicating a shear thinning occurrence. When soaking time increases from 5 min to 15 min, the peak stress and steady stress decrease significantly. A further increase of the soaking time led to a slight change. Peak stress and steady stress increase with increasing volume fraction of SiC particles. A sudden increase or decrease of compression velocity led to a significant increase or decrease of the steady stress. The destruction of the samples compressed at solid state temperature mainly depends on cracks parallel to compression direction. However, the destruction forms of the samples compressed at semisolid temperatures consist of cracks parallel to compression direction and partial collapse. Increasing soaking time led to an obvious change of the destruction forms. Compression velocity affects slightly the macro appearance of the sample compressed at semisolid temperatures.

An innovative method so-called ultrasonic-assisted semisolid stirring was proposed to fabricate semisolid slurries of the nano-sized SiC/7075 aluminum matrix composite. In this method, ultrasonic treatment and semisolid stirring were combined together to disperse the nano-sized SiC and break the primary dendrites of the matrix. The microstructure and mechanical properties of the semisolid slurries and rheoformed cylinder component were investigated. The results show that ultrasonic treatment dispersed the nano-sized SiC particles well due to the effect of transient cavitation and acoustic streaming. With increase of the stirring time the amount of the spheroidal grains of semisolid slurries increases, indicating achieving a better semisolid slurry. When stirring temperature is 620 °C, the 20 min stirring time led to a desirable semisolid microstructure consisting of a large number of spheroidal grains surrounded by liquid phase. High-quality semisolid slurries were only achieved at the stirring temperatures of 615 °C and 620 °C upon the 20 min stirring time. Liquid fraction in the regions near to lateral surface is larger than those in the top, bottom and central regions due to main dependence on liquid flow and flow of liquid incorporating solid grains. Yield strength and ultimate tensile strength of rheoformed cylinder components of the nano-sized SiC/7075 aluminum matrix composite increased as compared with those of the 7075 matrix. Yield strength, ultimate tensile strength and elongation of the rheoformed cylinder components of the nano-sized SiC/7075 aluminum matrix composite without T6 are 264 MPa, 357 MPa and elongation of 7.5%, respectively. T6 heat treatment led to a significant improvement of yield strength, ultimate tensile strength and elongation. The mechanical properties of the rheoformed cylinder components with T6 of the nano-sized SiC/7075 aluminum matrix composite involve yield strength of 381 MPa, ultimate tensile strength of 478 MPa and elongation of 8.5%.

Thin-wall and high-rib components of AM60B Mg alloy were formed by double control forming (DCF) under the optimal conditions. Numerical simulation of high-speed filling process was carried out on Flow-3D software. The microstructure and properties of the components formed by DCF under the optimal conditions were investigated. The results showed that the design of the overflow launders and runners were feasible. The velocity magnitude in the component region was less than that in the regions of the runners and overflow launders. The temperature decreased gradually from the runners to overflow launders. Finer and more uniform microstructure without defects such as microcrack and microporosity was found in the components formed by DCF as compared to die casting. The reason was due to the dual effect of high pressure on increasing the nucleation rate and removing the defects. The mechanical properties of the components formed by DCF under the optimal conditions were significantly improved in comparison to die casting and DCF under non-optimal conditions. Fine and uniform microstructure without defects led to the improvement of mechanical properties. The fracture morphology of the components formed by die casting was characterized by brittle cleavage fracture and some microporosities, deteriorating the mechanical properties. Ductile fracture morphology with a large number of dimples was found in the fracture microstructure of the components formed by DCF under the optimal conditions, enhancing the mechanical properties.Highlights► Microstructure of DCF-formed part under optimal conditions was fine and uniform. ► Properties of DCF-formed part under optimal conditions were greatly improved. ► The high-quality microstructure of DCF-formed part led to improved properties.

A set of novel forming die combining the advantages of dies casting and forging was designed, by which double control forming idea was firstly proposed. The motorcycle wheel made of AM60B alloy was used as the typical component to demonstrate advantages of the double control forming. The effect of pressure on the mechanical properties and microstructure of the parts formed by double control forming was investigated. The results showed that high mechanical properties and complex shape were achieved in the parts formed by double control forming. Compared to die casting, the mechanical properties of the formed part significantly increased and the microstructure changed from the coarse dendrites to fine equiaxed grains. The shrinkage voids and microcracks in the formed parts were obviously reduced or even completely eliminated with the increase of pressure. When a pressure of 4000 kN was applied, the optimal mechanical properties such as ultimate tensile strength of 265.6 MPa and elongation of 21% were achieved and the microstructure was characterized by fine and uniform equiaxed grains due to the large undercooling degree caused by the high pressure.

A double control forming technology combining the die casting and forging was firstly proposed for the production of Mg alloy components with enhanced properties. In this technology, high-speed filling of liquid melt and high-pressure forging of partially solidified melt were performed by using injection and forging systems of a double control forming device. Some Mg alloy motorcycle wheel components were produced by die casting and double control forming to verify the improvement of the mechanical properties of components formed by double control forming. The results showed that double control forming was an alternative technology for producing the complex Mg alloy components with enhanced properties. Average tensile strength and elongation of Mg alloy components produced by double control forming were greatly improved in comparison with die casting. The average tensile strength was enhanced from 126.8 MPa to 213 MPa and elongation was improved from 3.5% to 7.2%. The optimal process parameters were obtained according to the results of orthogonal experiments, which involved pouring temperature of 675 °C, injection speed of 2 m/s and die temperature of 210 °C. The improved nucleation frequency in the melt caused by the forging pressure led to successful grain refinement of the microstructure of the component produced by double control forming. The defects were removed from the microstructure due to plastic deformation caused by the forging pressure. A refined and densified microstructure led to an enhancement of mechanical properties of Mg alloy component produced by double control forming.Highlights► A double control forming (DCF) technology was firstly developed. ► DCF technology combines the filling of diecasting and strengthening of forging. ► The complex components with enhanced mechanical properties were formed by DCF.

AM60 magnesium alloy semisolid slurry was prepared by new strain induced melt activated (new SIMA) and its microstructure was observed under optical microscope and then analyzed via digital image analysis system. The results showed that microstructure was well refined to many fine equiaxed grains with the average size of 8 μm after the process of as-cast AM60 magnesium alloy by equal channel angular extrusion (ECAE). And fine and spheroidal semisolid slurry with average grain size ranging from 9.9 μm to 17.3 μm was prepared by new SIMA and the used time was also shorter, as compared with semisolid isothermal treatment (SSIT). LSW theory of Ostwald ripening was demonstrated to be able to be used for the description of the coarsening process of semisolid slurry in new SIMA, instead of coalescence of solid grains in SSIT. When the isothermal temperature was increased, primary solid grains showed an initial increase and a following decrease of an average grain size.

Influence of equal channel angular extrusion on room temperature mechanical properties of cast Mg–9Al–Zn alloy was investigated. The results show that room temperature mechanical properties of Mg–9Al–Zn alloy, such as yield strength, ultimate tensile strength and elongation, can be improved heavily by equal channel angular extrusion. Processing routes, processing temperature and extrusion passes have important influence on room temperature mechanical properties of processed Mg–9Al–Zn alloy by equal channel angular extrusion. The optimum room temperature mechanical properties such as yield strength of 209 MPa, ultimate tensile strength of 339 MPa and elongation of 14.1%, can be obtained when Mg–9Al–Zn alloy was processed by equal channel angular extrusion for 6 passes at route BC at 498 K. Large bulk materials of Mg–9Al–Zn alloy with average grain size of 4 μm and high mechanical properties can be prepared.