| In this study, A356-B4C composite strips were processed in short and efficient process by compound of semi-solid and rheo-rolling. At the semi-solid temperature, B4C particles were compounded with A356 alloy through intense shearing/stirring effects, then, this semisolid metal slurry directly entered the broadside-restricted rolling slot for rheo-rolling, so this method improved the process parameters. Pretreatment method for B4C particles had been given, and the microstructure evolution, the disciplinarians of mechanical properties and wear behavior under different process parameters were studied. Many results are achieved as follows:(1) The optimal process parameters of pretreatment for B4C particles were obtained which contained acid pickling and oxidation, the acid pickling parameters:solution is 4vol.%HCl and the time is 5h; the oxidation parameters:temperature is 410℃ and the time is 5h;(2) Dispersion mechanism of B4C particles under the shearing/stirring has been obtained, and dispersion model is set up based on the analysis. Experimental results proved the dispersion mechanism:a). The results reveal that the eddy-current was produced inside the melt during the process of semi-solid shearing/stirring, and negative pressure created by the eddy-current forced the B4C particles to disperse into the melt; b). The centrifugal forces of different B4C particles were different on the same track, and the B4C particles which had the bigger mass were detached from the cluster firstly; c). The effects of the shear stress of alloy fluid layer do good to the forming of α-Al grain.d). With the action of shearing/stirring, the collision happening between B4C particles and mixing blade or among B4C particles can make the edge of B4C particles be jagged, which could increasing the interfacial area;(3) Effects of experimental parameters on microstructures of A356-B4C composite strips prepared by compound of semi-solid and continuous rheo-rolling were investigated and obtained, and the experimental parameters have been optimized. The stirring rate is 500r/min/min, and the stirring time is 20min, and the stirring temperature is 580℃, and the vibration of plate is 80Hz, and the flow rate of cooling water is 1.5L/min and the linear speed of working roll is 0.25 m·s-1;(4) The phases between the interface of B4C particles and A356 alloy were analysed, and the growth modes of Al12Mg17and Al3BC phases were obtained. Al2O3 and Al3BC phases could prevent the movement of dislocation inside grain crystal, and restrain the generation and diffusion of cracks.(5) The A356-B4C composite strips are produced using this method, and the microstructure mainly include spherical and rose shape α-Al grain, and the distributing of the B4C particles is even. The ultimate hardness is 109HV and tensile strength of the A356-10wt.% B4C strip is 197 MPa, showing an 81.7% and 35.86% increase respectively coMpared with the traditional casting of A356 alloy.(6) The fracture morphologies of A356-B4C composite plates prepared by the different processes had been obtained. It can be seen that fracture is mainly intergranular fracture which was prepared with the original B4C particles, and the fracture shows river pattern, tear ridges and very few dimples; The tesile fracture is mixture of transgranular fracture and intergranular fracture prepared with the pretreatment B4C particles, and a certain amount of dimples can be seen in the section;(7) The wear mechanism of A356-B4C composite plate is analyzed. The patterns of the effect of different content of B4C, friction load and speed on friction coefficient are obtained: The friction coefficient decreases with the increase of content of B4C within the range of 0 to 10%, friction load from ON to 60N and friction speed between 30r/min and 80r/min. The phases of H3BO3 and B2O3 play an important part for these patterns because they could acts as a solid lubricant. However, the wear of materials will be increased when it is beyond the scope of these conditions, because H3BO3 phase can be decomposed into B2O3 which can increase the friction coefficient obviously above 200℃. |
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