Papers by Author: Li Kai Shi

Abstract: A new rheo-extrusion technique has been developed to produce extruded profiles. The AZ31 slurry is fabricated by a twin-screw mechanism, and has spheroidal solid particles with a grain size of about 50 to 80μm. The slurry was introduced into an extrusion container with an inner diameter of 95mm, and then squeezed to a billet before extruding into a round bar with a diameter of 22mm using a 500kN extrusion machine. It is found that the new technique has a lower deformation resistance than that of the conventionally hot extrusion technique in the same extrusion velocity. The microstructure and mechanical properties of the rheo-extruded bars were examined in detail. The results illustrated that grains of the bar were very fine with the average grain size about 2.3μm，and the mechanical properties were improved substantially compared with those of the same alloy produced by the conventional hot extrusion.

Abstract: A new slurry making method for rheocasting termed “Melt Spreading and Mixing Technique” (MSMT), based on forced uniform solidification theory, has been proposed, by which the bulk metal is dispersed into thin liquid film that is cooled rapidly and uniformly to allow copious nucleation. With a model alloy Al-wt%6.5Si to conduct a series of experiments, feasibility of this method and influence of process parameters upon slurry microstructure were studied. The results show that semisolid slurry for mass production with uniformly fine nondendritic microstructures can be obtained consistently and continuously by optimizing process parameters.

Abstract: An advanced rheo-diecasting technique, based on a modified multi-electromagnetic stirring continuous preparation (MSCP) process was developed for manufacturing near-net shape Al-alloys components with high integrity. The modified MSCP process innovatively combines noncontact electromagnetic stirring and an annular chamber with specially designed profiles to in situ make high quality semisolid slurry, and intensive forced shearing and self-wiping action can be achieved under high shear rate and high intensity of turbulence inside the annular chamber. Three Al-Si alloys with hypoeutectic, eutectic and hypereutectic compositions were tested in this study. The results indicate that, compared with those produced by conventional high-pressure diecasting (HPDC) process, rheo-diecasting samples based on the modified MSCP process have finer and more uniform microstructure.

Abstract: The effect of friction stir welding (FSW) parameters on microstructure and properties of 15vol.%SiCp/2009Al composite welds was investigated at tool rotation rates of 400-1000 rpm for a constant welding speed of 50 mm/min. 8mm thick defect-free welds were produced at higher tool rotation rates of 600-1000 rpm, whereas an unwelded seam was distinctly visible at the root of plates at a lower tool rotation rate of 400 rpm. FSW resulted in generation of fine and equiaxed recrystallized grains of ~6 μm and significant improvement of SiC particle distribution in the weld nugget. Under as-FSW condition, the hardness of the weld nugget was significantly higher than that of as-extruded parent material. Furthermore, tensile and yield strengths of as-FSW composite welds in both the longitudinal and transverse directions were superior to those of as-extruded parent material. Post-weld T4 temper resulted in limited grain growth due to the pinning of SiC particles. Under the T4 condition, the tensile strength of the welds along the longitudinal and transverse directions reached 82 and 95% of the parent material, respectively.

Abstract: The billet of magnesium AZ31B was made by twin-screw rheo-cast, and the specimen cut from the rheo-cast billet and normal cast billet were compressed in Gleeble-3000 hot simulator. The deformation temperature was 300, 350
, and the strain rates were 0.001~1s-1. The relationship among flow stress, deformation temperature and strain rates were analyzed. The variation of the microstructure was observed and compared after the specimen was hot compressed. The results show that the surface of the rheo-cast specimen is much more smooth than the normal cast specimen after the compression. The dynamic recrystallization had taken place during the compression and the recrystallized grain of rheo-cast sample is more tiny than normal cast specimen.

Abstract: Production of semi-solid slurry with fine and uniform microstructures is one of the basic and key problems for the industrial application of semi-solid processing. The common disadvantages of existing techniques are as follows: continuous production difficulty, high process controlling requirement and low quality microstructure. In order to overcome those problems, the MSCP (Multielectromagnetic Stirring Continuous Preparation) process has been developed by GRINM. Experimental results show that annular chamber for feedstock production can improve the efficiency of electromagnetic stirring and reduce the difficulty of temperature grad control. Compared with single electromagnetic stirring, multi- electro magnetic stirring can achieve more uniform stirring. Strict temperature grad control in annular chamber is the key of continuous preparation of high quality semi-solid slurry. Semi-solid slurry of different solid fractions was obtained by MSCP.

Abstract: An application of the semi-solid alloy Al-6Si-2Mg to produce automobile components was carried out for farther development of this new alloy. The experiments were done in a self-developed SSM trial line composed of semi-continuous caster combined with electromagnetic stirring for feedstock billet production, 6-station induction reheating equipment for restoring thixotropy of SSM billet and modified die-casting unit for semi-solid thixoforming. The results show that Al-6Si-2Mg alloy has such a suitable fraction solid and low temperature sensitivity of fraction solid that the controllability of SSM process is increased greatly. The billets have uniformly fine microstructure with a higher degree of sphericity and also retain good thixotropic properties. During thixoforming, the die is filled completely, the microstructure of components is dense, and the primary phases are near spherical, and the mechanical property after heat treatment has good performance.

Abstract: Heat-resistant Al-Fe-V-Si aluminum alloys enhanced by in-situ TiC particles have been prepared by spray forming process with suitable process parameters. Research results show that the microstructure of as-deposited alloy is fine and homogeneous. In-situ TiC particles prevent the unsteady phases from coming into being. On the other hand, the TiC particles increase the volume fraction of heat-resistant phases. So the mechanical properties of the enhanced alloy by in-situ TiC particles are better than that of Al-Fe-V-Si alloy without TiC particles. The hot extrusion temperature is also an important parameter to understand. Under the permission, it is better to extrude the alloy at lower temperature. The tensile strength of the alloy without TiC particles is about 435MPa at room temperature and is about 204MPa at 350°C. However, when the alloy is enhanced by in-situ TiC particles, the strength of alloy is about 482MPa at room temperature and is about 224MPa at 350°C temperature.

Abstract: An ultra-high strength Al-11Zn-2.9Mg-1.7Cu alloy has been prepared by spray forming process. The microstructures of as-cast and as-deposited alloys have been studied. XRD analysis result shows that the phases of spray formed Al-11Zn-2.9Mg-1.7Cu alloy consist of a-Al, MgZn2, Al2Cu and Al2CuMg. T6 heat treatment process is used to strengthen the alloy. The ultimate tensile strength reach up to 810MPa, and the elongation is about 9~11%. This kind of aluminum alloy is an ideal structural material for the aerospace and automobile industries.